Abstract:
An optical module includes an optical modulator that includes a cutout portion and a first terminal projecting to the inside of the cutout portion, and is configured to perform optical modulation by using an electrical signal input to the first terminal; a driver, at least a part of the driver being housed inside the cutout portion, that is configured to generate an electrical signal; an electrode pattern that extends from the driver inside the cutout portion, and is configured to transmit the electrical signal generated by the driver; and a flexible board having flexibility, one end of the flexible board being electrically connected with the first terminal inside the cutout portion, another end of the flexible board extending in the direction away from the driver, the flexible board being connected with the electrode pattern and configured to input the electrical signal transmitted by the electrode pattern to the first terminal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2015-206776, filed on Oct. 20, 2015, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are related to an optical module. 
       BACKGROUND 
       [0003]    Conventionally, a Mach-Zehnder interferometer has been used for an optical modulator that modulates light emitted from a light source in some cases. In such an optical modulator, signal electrodes and grounding electrodes are arranged along optical waveguides parallel to each other. In recent years, an optical modulation system has been diversified, and the optical modulator has been provided with a plurality of Mach-Zehnder interferometers in many cases. In this case, the Mach-Zehnder interferometers are integrated into one chip, thus enabling the size of the optical modulator to be reduced. 
         [0004]    The optical modulator provided with the Mach-Zehnder interferometers inputs therein a plurality of electrical signals that are different from each other, thus enabling generation of a multi-level modulation signal. That is, the electrical signals that are different from each other are input to the respective signal electrodes corresponding to the respective Mach-Zehnder interferometers from the outside of the optical modulator, thus enabling optical modulation by a multi-level modulation system, such as a differential quadrature phase shift keying (DQPSK) system, for example. 
         [0005]    There may be a case in which connectors are provided to an input part from which electrical signals are input to the optical modulator. However, if connectors are provided for respective electrical signals, the optical modulator becomes larger in size thus increasing a mounting area for the optical modulator. To address this issue, there may be a case in which a flexible printed circuit board (FPC) having flexibility is used for the input part from which electrical signals are input to reduce the size of a device including the optical modulator. 
         [0006]    To be more specific, a plurality of circuit patterns corresponding to the respective signal electrodes of the optical modulator are printed on the FPC, and an electrical signal output from a driver is input to the optical modulator via each circuit pattern printed on the FPC. One end on the optical-modulator side of the FPC is inserted into a cutout portion formed in the optical modulator, and each circuit pattern is, for example, soldered onto a coaxial terminal projecting to the inside of the cutout portion so as to be electrically connected with the optical modulator. On the other hand, one end on the driver side of the FPC, each circuit pattern of which is, for example, soldered onto an electrode pattern for transmitting an electrical signal from the driver, is electrically connected with the driver. 
         [0007]    In terms of the reduction in the size of the device, such a structure may be adopted that arranges the optical modulator and the driver hierarchically by using boards that are different from each other to connect the optical modulator and the driver that are hierarchically arranged with each other, by using the FPC (see Japanese Laid-open Patent Publication No. 2005-128440). 
         [0008]    However, in the structure in which the optical modulator and the driver that are hierarchically arranged are connected with each other by using the FPC, the arrangement space of the optical modulator and the arrangement space of the driver are separated from each other and hence, there exists the possibility that the entire mounting area of the device increases. Consequently, the structure in which the optical modulator and the driver are hierarchically arranged is unpractical. 
         [0009]    It is also possible to adopt such a structure that a part of the driver is housed inside the cutout portion included in the optical modulator to reduce the mounting area corresponding to the driver. However, in this case, the coaxial terminal projecting to the inside of the cutout portion and the driver are arranged in close proximity to each other thus giving rise to a sharp flexure of the FPC that connects the coaxial terminal and the electrode pattern extending from the driver. When the FPC is sharply flexed, unintended stress is applied to the FPC and hence, there exists the possibility that disconnection occurs in the connection portion between the coaxial terminal and the FPC or in the connection portion between the electrode pattern and the FPC. 
       SUMMARY 
       [0010]    According to an aspect of an embodiment, an optical module includes a wiring board; an optical modulator arranged on the wiring board, the optical modulator having a cutout portion and a first terminal projecting to the inside of the cutout portion, the optical modulator being configured to perform optical modulation by using an electrical signal input to the first terminal; a driver arranged on the wiring board, at least a part of the driver being housed inside the cutout portion, the driver being configured to generate an electrical signal; an electrode pattern on the wiring board, the electrode pattern extending from the driver inside the cutout portion, the electrode pattern being configured to transmit the electrical signal generated by the driver; and a flexible board having flexibility, one end of the flexible board being electrically connected with the first terminal inside the cutout portion, another end of the flexible board extending in the direction away from the driver, the flexible board being electrically connected with the electrode pattern and configured to input the electrical signal transmitted by the electrode pattern to the first terminal. 
         [0011]    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. 
         [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 invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]      FIG. 1  is a schematic plan view illustrating a constitution of an optical module according to a first embodiment; 
           [0014]      FIG. 2  is a schematic cross sectional view illustrating the constitution of the optical module according to the first embodiment; 
           [0015]      FIG. 3  is a schematic cross sectional view illustrating a constitution of an optical module according to a second embodiment; and 
           [0016]      FIG. 4  is a schematic plan view illustrating a constitution of an optical module according to a modification. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0017]    Preferred embodiments of the present invention will be explained with reference to accompanying drawings. A technique disclosed herein is not limited to the embodiments. 
       [a] First Embodiment 
       [0018]      FIG. 1  is a schematic plan view illustrating a constitution of an optical module according to the first embodiment. An optical module  100  illustrated in  FIG. 1  has a printed circuit board (PCB)  110 , an optical modulator  120 , a driver  130 , an electrode pattern  140 , and an FPC  150 . 
         [0019]    The PCB  110  is a glass epoxy board or the like, and mounts thereon various kinds of parts that constitute the optical module  100 . The PCB  110  is one example of a wiring board. 
         [0020]    The optical modulator  120  modulates and outputs light emitted from a light source that is not illustrated in the drawings. In this case, the optical modulator  120  performs optical modulation based on an electrical signal output from the driver  130 . To be more specific, the optical modulator  120  has, as illustrated in  FIG. 1 , a package  125 , a modulator chip  121  arranged inside the package  125 , and a relay board  122 . The optical modulator  120  may also have a plurality of direct current (DC) terminals  123  extending from the package  125  to the outside of the package  125 . 
         [0021]    The modulator chip  121  is constituted of optical waveguides parallel to each other, a signal electrode, and a grounding electrode, and performs optical modulation based on an electrical signal supplied to the signal electrode while the light emitted from the light source is propagating through the optical waveguides. To be more specific, the optical waveguide is formed by thermally diffusing a metal film made of titanium (Ti) or the like, the metal film being formed on a part of a crystal substrate using electro-optic crystals, such as lithium niobate (LiNbO 3  (LN)) or lithium tantalate (LiTaO 2 ). Furthermore, the optical waveguide may be formed by proton exchange processing using benzoic acid after patterning. On the other hand, each of the signal electrode and the grounding electrode is a coplanar electrode formed along the corresponding optical waveguide. In  FIG. 1 , four sets of optical waveguides parallel to each other are formed in the modulator chip  121 , and the signal electrode and the grounding electrode corresponding to each optical waveguide are formed. The signal electrode and the grounding electrode are, for example, formed on each optical waveguide by patterning. Furthermore, in order to prevent the light propagating in the optical waveguide from being absorbed by the signal electrode and the grounding electrode, a buffer layer is formed between the crystal substrate and the signal electrode (grounding electrode). As the buffer layer, silicon dioxide (SiO2), the layer of which has a thickness of approximately 0.2 to 2 μm, can be used, for example. 
         [0022]    The relay board  122  relays an electrical signal output from the driver  130  to the modulator chip  121 , and inputs the electrical signal to the signal electrode of the modulator chip  121 . In  FIG. 1 , the relay board  122  has four circuit patterns corresponding to the respective four signal electrodes formed in the modulator chip  121 . Furthermore, the relay board  122  has four coaxial terminals  202  that are electrically connected to the respective four circuit patterns. To consider the case where a plurality of signal electrodes formed in the modulator chip  121  input respective electrical signals thereto, when all input parts from which electrical signals are input are aligned on one side of the optical modulator  120 , the optical modulator  120  is easily mounted on the PCB  110  with a small mounting area. Accordingly, in the present embodiment, the relay board  122  is arranged in the optical modulator  120 , and the relay board  122  relays the electrical signal input from one side of the optical modulator  120  to the modulator chip  121 . 
         [0023]    The DC terminal  123  is a terminal for a control signal that controls the modulator chip  121 , and is arranged on the side surface from which the driver  130  is exposed, out of the side surfaces of the optical modulator  120 . To consider the case where the DC terminals  123  input thereto respective control signals that control the modulator chip  121 , when all input parts from which control signals are input are aligned on one side of the optical modulator  120 , the optical modulator  120  is easily mounted on the PCB  110  with a small mounting area. Accordingly, in the present embodiment, the DC terminals  123  are arranged on the side surface from which the driver  130  is exposed, and the DC terminal  123  inputs the control signal input from one side of the optical modulator  120  to a DC electrode of the modulator chip  121 . 
         [0024]    The driver  130  generates an electrical signal for modulating light emitted from the light source. That is, the driver  130  generates a high frequency electrical signal with an amplitude and a phase each corresponding to transmission data, and drives the optical modulator  120  by the electrical signal. A part of the driver  130  is housed in a cutout portion  201  of the package  125  (the cutout portion  201  of the optical modulator  125 ), the cutout portion  201  being formed in the vicinity of the PCB  110 . Consequently, a mounting area corresponding to the driver  130  is reduced. 
         [0025]    The electrode pattern  140  is an electrode pattern printed on the PCB  110 . In the present embodiment, four electrode patterns  140  corresponding to respective four circuit patterns of the relay board  122  are printed on the PCB  110 . The electrode pattern  140  and the circuit pattern of the FPC  150  are soldered to each other. Furthermore, the electrode pattern  140  printed on the PCB  110  extends from the driver  130  inside the cutout portion  201  formed in the optical modulator  120 , and transmits the electrical signal output from the driver  130  to the FPC  150 . 
         [0026]    The FPC  150  is a flexible board having flexibility, and supplies the electrical signal output from the driver  130  to the optical modulator  120 . That is, one end of the FPC  150  is electrically connected with the relay board  122  of the optical modulator  120 , and the other end of the FPC  150  is electrically connected with the driver  130  via the electrode pattern  140  printed on the PCB  110 . The FPC  150  forms a circuit pattern that propagates an electrical signal on the PCB  110 -side surface thereof. In the present embodiment, four circuit patterns connected to the respective four circuit patterns printed on the relay board  122  are formed on the FPC  150 . 
         [0027]    Next, with reference to  FIG. 2 , the explanation is made with respect to electric connections among the optical modulator  120 , the driver  130 , and the FPC  150 .  FIG. 2  is a schematic cross sectional view illustrating the constitution of the optical module according to the first embodiment. First of all, the connection portion between the optical modulator  120  and the FPC  150  is explained. 
         [0028]    As illustrated in  FIG. 2 , the cutout portion  201  is formed in the vicinity of the PCB  110  of the package  125  included in the optical modulator  120 , and the coaxial terminal  202  projects from the upper surface of the cutout portion  201  to the inside of the cutout portion  201 . One end of the FPC  150  is inserted into the cutout portion  201  formed in the optical modulator  120 , and electrically connected with the coaxial terminal  202  of the optical modulator  120  inside the cutout portion  201 . That is, the coaxial terminal  202  and the circuit pattern of the FPC  150  are soldered to each other and hence, the FPC  150  and the optical modulator  120  are electrically connected with each other. 
         [0029]    The coaxial terminal  202  penetrates the relay board  122  in the optical modulator  120  and the upper surface of the cutout portion  201 , and projecting to the inside of the cutout portion  201  from the optical modulator  120 . The coaxial terminal  202  further penetrates a through hole formed in the FPC  150 , and connected with a circuit pattern by way of a solder  203  on the PCB  110 -side surface of the FPC  150 . Consequently, the optical modulator  120  and the FPC  150  are electrically connected with each other. 
         [0030]    The cutout portion  201  has a first surface  201   a  facing the driver  130  and a second surface  201   b  facing the electrode pattern  140 , the first surface  201   a  and the second surface  201   b  constituting the upper surface of the cutout portion  201 . The second surface  201   b  is raised toward the electrode pattern  140  from the first surface  201   a  set as a reference, and the coaxial terminal  202  projects to the inside of the cutout portion  201  from the second surface  201   b.  Furthermore, one end of the FPC  150  is inserted into the cutout portion  201  formed in the package  125  of the optical modulator  120 , and electrically connected with the coaxial terminal  202  of the optical modulator  120  on the second surface  201   b.  In this manner, the one end of the FPC  150  is connected with the coaxial terminal  202  of the optical modulator  120  on the second surface  201   b  that is close to the electrode pattern  140 , thus suppressing the flexure of the FPC  150  that connects the coaxial terminal  202  and the electrode pattern  140 . 
         [0031]    Next, the connection portion between the driver  130  and the FPC  150  is explained. As illustrated in  FIG. 2 , a part of the driver  130  is housed inside the cutout portion  201  formed in the package  125  of the optical modulator  120 , and the electrode pattern  140  formed on the PCB  110  extends from the driver  130  inside the cutout portion  201 . 
         [0032]    The other end opposite to one end on the coaxial terminal  202 -side of the FPC  150  (hereinafter referred merely to the “other end”) extends in the direction away from the driver  130 , and electrically connected with the electrode pattern  140  on the PCB  110 . That is, the circuit pattern of the FPC  150  and the electrode pattern  140  on the PCB  110  are connected with each other by way of a solder  204 . Consequently, the driver  130  and the FPC  150  are electrically connected with each other. In the example illustrated in  FIG. 2 , the other end of the FPC  150  extends in the direction away from the driver  130 , and electrically connected with the electrode pattern  140  on the PCB  110  at a position outside the cutout portion  201 . In this manner, the other end of the FPC  150  connected with the coaxial terminal  202  at the one end of the FPC  150  extends in the direction away from the driver  130 , and connected with the electrode pattern  140  on the PCB  110  thus suppressing the flexure of the FPC  150  that connects the coaxial terminal  202  and the electrode pattern  140 . 
         [0033]    The driver  130  and the electrode pattern  140  are electrically connected with each other by way of a lead pin  205  projecting from the driver  130 , the lead pin  205  being soldered onto the electrode pattern  140 . That is, the lead pin  205  projecting from the driver  130  is electrically connected to the electrode pattern  140  by way of a solder  206 . 
         [0034]    In this manner, according to the present embodiment, a part of the drivers is housed in the cutout portion formed in the optical modulator, one end of the FPC is connected with the coaxial terminal of the optical modulator inside the cutout portion, and the other end of the FPC extends in the direction away from the driver and connected with the electrode pattern extending from the driver. Due to such a constitution, even when the coaxial terminal projecting to the inside of the cutout portion and the driver of which a part is housed in the cutout portion are arranged close to each other, the flexure of the FPC that connects the coaxial terminal and the electrode pattern extending from the driver is suppressed, and an excessive stress is not applied to the FPC. As a result, it is possible to suppress disconnection in the connection portion between the coaxial terminal and the FPC or the connection portion between the electrode pattern and the FPC while reducing the mounting area corresponding to the driver. 
       [b] Second Embodiment 
       [0035]    A technical feature of the second embodiment lies in that a coaxial terminal projecting from a side surface of an optical modulator is provided, one end of an FPC is connected with the coaxial terminal on the side surface of the optical modulator, and the other end of the FPC extending along the side surface of the optical modulator is connected with an electrode pattern extending from a driver. 
         [0036]    The constitution of an optical module  100  according to the second embodiment is identical with the case of the first embodiment, and the explanation is omitted. The second embodiment differs from the first embodiment in respect to a position at which the coaxial terminal projects, and the manner of connection between the coaxial terminal and the electrode pattern by way of the FPC. 
         [0037]      FIG. 3  is a schematic cross sectional view illustrating a constitution of the optical module according to the second embodiment. In  FIG. 3 , parts identical with those illustrated in  FIG. 1  and  FIG. 2  are given same numerals. 
         [0038]    As illustrated in  FIG. 3 , a coaxial terminal  302  projects from a side surface of a package  125 . One end of an FPC  150  is electrically connected with a coaxial terminal  302  projecting from a side surface  125   a  of the package  125 . That is, the coaxial terminal  302  and a circuit pattern of the FPC  150  are soldered to each other and hence, the FPC  150  and an optical modulator  120  are electrically connected with each other. 
         [0039]    The coaxial terminal  302  is soldered onto a relay board  122  arranged inside the optical modulator  120 , penetrates a side wall of the package  125 , and projects from the side surface  125   a  of the optical modulator  120  in the lateral direction. The coaxial terminal  302  further penetrates a through hole formed in the FPC  150 , and connected with the circuit pattern by way of a solder  303  on an opposite surface of the FPC  150  to the optical modulator  120 . Consequently, the optical modulator  120  and the FPC  150  are electrically connected with each other. 
         [0040]    Furthermore, the other end opposite to one end on the coaxial terminal  302 -side of the FPC  150  (hereinafter, merely referred to as the “other end”) extends along the side surface  125   a  of the package  125 , and electrically connected with an electrode pattern  140  on a PCB  110 . That is, the circuit pattern of the FPC  150  and the electrode pattern  140  on the PCB  110  are connected with each other by way of a solder  304 . Consequently, the driver  130  and the FPC  150  are electrically connected with each other. In the example illustrated in  FIG. 3 , the other end of the FPC  150  extends along the side surface  125   a  of the package  125 , and electrically connected with the electrode pattern  140  on the PCB  110  in a state that the FPC  150  is inserted into a through hole T formed in the PCB  110 . In this manner, the other end of the FPC  150  connected with the coaxial terminal  302  at one end of the FPC  150  extends along the side surface  125   a  of the package  125 , and connected with the electrode pattern  140  on the PCB  110  thus suppressing the flexure of the FPC  150  that connects the coaxial terminal  302  and the electrode pattern  140 . 
         [0041]    In this manner, according to the present embodiment, a part of the driver is housed in the cutout portion formed in the optical modulator, one end of the FPC is connected with the coaxial terminal projecting from the side surface of the package  125 , and the other end of the FPC extends along the side surface of the package  125 , and connected with the electrode pattern extending from the driver. Due to such a constitution, the flexure of the FPC that connects the coaxial terminal and the electrode pattern extending from the driver is suppressed, and an excessive stress is not applied to the FPC. As a result, it is possible to suppress disconnection in the connection portion between the coaxial terminal and the FPC or the connection portion between the electrode pattern and the FPC while reducing the mounting area corresponding to the driver. 
         [0042]    Furthermore, the other end of the FPC extending along the side surface of the optical modulator is connected with the electrode pattern on the PCB in a state that the other end of the FPC is inserted into the through hole formed in the PCB and hence, and thus it is possible to extend the FPC in the vertical direction, and reduce a stress applied to the FPC. 
         [0043]    In each embodiment mentioned above, a part of the driver  130  is housed in the cutout portion  201  formed in the package  125 . However, the driver  130  may be entirely housed in the cutout portion  201  formed in the optical modulator  120 . That is, at least a part of the driver  130  may be housed in the cutout portion  201  formed in the package  125 . 
         [0044]    Furthermore, in each embodiment mentioned above, the DC terminal  123  is arranged on the side surface from which the driver  130  is exposed, out of the side surfaces of the package  125 . However, as illustrated, for example, in  FIG. 4 , the DC terminal  123  may be arranged on a side surface opposite to the side surface from which the driver  130  is exposed, out of the side surfaces of the package  125 . Due to such a constitution, it is possible to avoid the interference of wiring connected to the DC terminal  123  with the driver  130 , and improve the optical module in degree of freedom of design. 
         [0045]    According to one aspect of the optical module disclosed in the present application, it is possible to achieve the advantageous effect that the disconnection is suppressed while reducing the mounting area. 
         [0046]    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 embodiments of the present invention have 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.