Abstract:
According to an aspect of an embodiment, a flexible circuit board for connecting a first device and a second device, the flexible circuit board comprises: a base material comprising a flexible material having a first end adapted to connect with the first apparatus, a second end adapted to connect with the second apparatus and a hollow arranged between the first end and the second end; a signal line arranged on a surface of the base material, the signal line capable of electrically connecting the first apparatus and the second apparatus, the signal line having a constant characteristic impedance along the signal line in association with the base material; and a line arranged on the base material and over the hollow, the line capable of electrically connecting the first apparatus and the second apparatus.

Description:
FIELD OF THE INVENTION 
     The present art relates to a flexible substrate, an optical device, optical transmitter, and/or an optical receiver. 
     An optical transceiver including the optical transceiver and the transmitter used for an optical communication system advances in small size and low costs. For example, in the case of an optical transceiver of 10 Gbps, the standard of optical transceiver generally includes compact size and low consumption power, typically, such as X2 and XFP (10 Gigabit Small Form Factor Pluggable). 
     In accordance with this, even in the case of an electro/optic converting unit and an optic/electro converting unit used in the optical transceiver, the standards of small size, e.g., TOSA (Transmitter Optical SubAssembly) called an XMD-MSA (Miniature Device Multi Source Agreement) and ROSA (Receiver Optical Sub Assembly) type (receptacle type), are general. On the other hand, the increase in velocity is required for the optical communication system, an optical transceiver of 40 Gbps is developed and 100 G Ethernet (registered trademark) is discussed. 
     The conventional optical transmitter is a TOSA having a driver IC and a light emission element in a casing. The driver IC generates a drive signal that drives the light emission element on the basis of an electrical signal input from an electrical input portion arranged to the casing. As for the rest, the conventional optical transmitter driving method well known a direct driving of light emission element method and an Electro-Absorption Modulator Integrated Laser Diode (EML) driving method. The driver IC outputs the generated drive signal to the light emission element or the EML via a signal line. A mismatching portion (wire) with characteristic impedance of a signal line for passing the drive signal needs to be reduced. 
     Since the TOSA having the light emission element and an Electro-Absorption (EA) modulator needs to control temperature, the TOSA is arranged on a Peltier element, thereby thermally (actually, spatially) insulating the TOSA from another part (casing). In particular, for Electro-Absorption Modulator Integrated Laser Diode (EML) formed by integrating the EA and the light emission element, it is heated by itself at the operating time and various heat releasing structures are examined (e.g., Japanese Laid-open Patent Publication No. 2003-222826). 
     Further, such a structure for reducing the connection distance of a signal line with a holder shaped to bridge the elements is disclosed (e.g., specification of Japanese Patent No. 3353718). In addition, a structure for connecting a signal line with a flexible substrate is disclosed (e.g., Japanese Laid-open Patent Publication No. 2006-338018). 
     However, with the above-mentioned conventional arts, there is a problem that the positional deviation between an electrical input unit and an optical output unit in a casing causes that between the driver IC and the light emission element. This positional deviation is corrected with a wire and the driver IC is connected to the light emission element. Then, the connection distance with the wire is increased and high-frequency characteristics cannot be kept. 
     In the case of reducing the size of the optical transmitter, the distance between the light emission element (modulator) and Peltier element and the casing is close to each other. Therefore, there is a problem that the heat of the light emission element is not sufficiently released and the temperature control of the light emission element is not possible. 
     For example, the EA (Electro-Absorption) depends on the temperature, and has a problem that the modulation is not operated and an optical output is reduced when the temperature changes. In addition, the EML (EA Modulator integrated Laser diode: EA modulator integrated semiconductor laser) has a problem that the wavelength of an optical signal for light emission changes, in addition to the same problem of that of the EA. 
     In an optical transmitter 1500 in which a casing 1510 includes a driver IC 1520, the temperature control of a light emission element 1530 needs a long distance between the driver IC 1520 and the light emission element or EML 1530. On the other hand, if the distance between the driver IC 1520 and the light emission element 1530 is long, there is a problem that the high-frequency characteristics cannot be kept and an optical transmitter 1500 cannot be compact. 
     Further, the ROSA  1125  also has a problem that the positional deviation between an input unit and an electrical output unit in the casing causes the positional deviation between the electrical output unit and light receiving device. This positional deviation is corrected by the wire and the electrical output unit is simultaneously connected to the light receiving device. Then, there is a problem that the connection distance is long and the high-frequency characteristics cannot be kept. 
     SUMMARY 
     According to an aspect of an embodiment, a flexible circuit board for connecting a first device and a second device, the flexible circuit board comprises: a base material comprising a flexible material having a first end adapted to connect with the first apparatus, a second end adapted to connect with the second apparatus and a hollow arranged between the first end and the second end; a signal line arranged on a surface of the base material, the signal line capable of electrically connecting the first apparatus and the second apparatus, the signal line having a constant characteristic impedance along the signal line in association with the base material; and a line arranged on the base material and over the hollow, the line capable of electrically connecting the first apparatus and the second apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view showing an optical transmitter according to an embodiment. 
         FIG. 2  is a cross-sectional view of an A-A line shown in  FIG. 1 . 
         FIG. 3  is a perspective view showing a flexible substrate (No. 1) of the optical transmitter according to the embodiment. 
         FIG. 4  is a perspective view showing a flexible substrate (No. 2) of the optical transmitter according to the embodiment. 
         FIG. 5  is a perspective view showing a flexible substrate (No. 3) of the optical transmitter according to the embodiment. 
         FIG. 6  is a cross-sectional view showing the flexible substrate of the optical transmitter according to the embodiment. 
         FIG. 7  is a cross-sectional view showing a flexible substrate of an optical transmitter according to a first modification of the embodiment. 
         FIG. 8  is a cross-sectional view showing a flexible substrate of an optical transmitter according to a second modification of the embodiment. 
         FIG. 9  is a plan view showing the optical receiver according to the embodiment. 
         FIG. 10  is a cross-sectional view showing a C-C line shown in  FIG. 9 . 
         FIG. 11  is a perspective view and a cross-sectional view showing an optical transceiver according to the embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     These embodiments provide a flexible substrate for an optical device, an optical transmitter, and an optical receiver, in which high-frequency characteristics to be kept and the size of an optical apparatus to be reduced with the structure. 
     Hereinbelow, a description will be given of a flexible substrate, an optical part, an optical transmitter, and an optical receiver according to a preferred embodiment with reference to the drawings. 
       FIG. 11  is a perspective view and a cross-sectional view showing an optical communication system and an optical transceiver according to an embodiment. As shown in the perspective view in  FIG. 11 , an optical communication system  1110  comprises a plurality of ports  1111  to  1114 , and an optical transceiver  1120  is inserted into the port  1111 . An optical fiber  1130  is connected to the optical transceiver  1120 . The optical transceiver  1120  transmits and receives optical signals to/from another communication system via the optical fiber  1130 . As shown in the cross-sectional view in  FIG. 11 , the optical transceiver  1120  comprises: an electrical connector  1121 ; a recovery circuit  1122 ; a driver IC  1123 ; a TOSA  1124 ; and an ROSA  1125 . 
     Upon inserting the optical transceiver  1120  to the port  1111  in the optical communication system  1110 , the electrical connector  1121  is connected to the port  1111  of the optical communication system  1110 . The electrical connector  1121  outputs the electrical signal output from the communication system  1110  to the circuit  1122 . Further, the electrical connector  1121  outputs the electrical signal output from the recovery circuit  1122  to the communication system  1110 . 
     The recovery circuit (CDR: Clock and Data Recovery)  1122  extracts a data signal and a clock signal from the electrical signal output from the electrical connector  1121 , and outputs the resultant signals to the driver IC  1123 . Further, the recovery circuit  1122  extracts a data signal and a clock signal from the electrical signal output from the ROSA  1125 , and outputs the resultant signals to the electrical connector  1121 . 
     The driver IC (Integrated Circuit)  1123  is a driver amplifier that outputs a drive signal for driving the TOSA  1124  to the TOSA  1124  on the basis of the electrical signal output from the recovery circuit  1122 . The TOSA  1124  is an optical transmitter that transmits an optical signal via the optical fiber  1130  on the basis of the drive signal output from the driver IC  1123 . The ROSA  1125  is an optical receiver that receives the optical signal via the optical fiber  1130 . The ROSA  1125  outputs an electrical signal based on the received optical signal to the recovery circuit  1112 . 
       FIG. 1  is a plan view showing an optical transmitter according to the embodiment. Referring to  FIG. 1 , an optical transmitter  100  according to the embodiment comprises a driver IC  120 , a light emission element  130 , and a flexible substrate  140  in a casing  110 . The casing  110  comprises a plurality of electrical input units  111   a  to  111   f  and an optical output unit  112 . 
     The electrical input units  111   a  to  111   f  exist at one end of the casing  110 . The electrical input units  111   a  to  111   f  receive an electrical signal and power. For example, the electrical input units  111   a  to  111   f  are ceramic terminals in which a metallic material is arranged on ceramics. An optical output unit  112  is arranged to the other end of the casing  110 . Herein, the optical output unit  112  is a receptacle sequentially connected to a connector of another optical device. The optical output unit  112  outputs the output signal output from the casing  110  to another optical device. 
     The driver IC  120  is a drive circuit that outputs a drive signal for driving the light emission element  130 . The driver IC  120  generates the drive signal on the basis of an electrical signal input from the electrical input unit  111   a . The driver IC  120  outputs the drive signal generated via a flexible substrate  140  to the light emission element  130 . The driver IC  120  is an amplifier that amplifies the electrical signal input from the electrical input unit  111   a  and sets the drive signal. 
     The driver IC  120  is connected to the electrical input unit  111   a  via a wire. In order to keep the high-frequency characteristics in the connection between the electrical input unit  111   a  and the driver IC  120 , the driver IC  120  is arranged to match the position of the electrical input unit  111   a . For example, the driver IC  120  is arranged at a position where a connection distance  261  to the electrical input unit  111   a  is within 375 μm. Herein, the driver IC  120  is arranged on one end of the flexible substrate  140 . 
     The light emission element  130  generates an optical signal from the driver IC  120  on the basis of the drive signal output via the flexible substrate  140 . The light emission element  130  outputs the generated optical signal to the outside via the optical output unit  112 . The light emission element  130  is, e.g., an EA modulator or an EML chip. 
     Incidentally, an optical coupling unit  150  is provided on the transmitting side of the optical transmitter  100 . The light emission element  130  outputs the generated optical signal via the optical coupling unit  150  and the optical output unit  112 . Herein, the optical coupling unit  150  comprises a lens  151 , a window  152 , and a lens  153 . The lens  151  is arranged between the light emission element  130  and the casing  110 , and passes the optical signal output from the light emission element  130 . 
     The window  152  is arranged to the casing  110 , and has a function of an optical isolator that passes an optical signal only in the direction from the light emission element  130  to the optical output unit  112 . The lens  153  is included in the optical output unit  112 , and passes the optical signal that is output from the light emission element  130  and passes through the lens  151  and the window  152 . 
     The light emission element  130  is arranged to match the position of the optical output unit  112  so as to output the optical signal to the optical output unit  112 . For example, the light emission element  130  is arranged so that the precision of the positional deviation from the optical output unit  112  is within 100 μm. As mentioned above, the driver IC  120  is arranged to match the position of the electrical input unit  111   a . On the other hand, the light emission element  130  is arranged to match the position of the optical output unit  112 . 
     One end of the flexible substrate (FPC: Flexible Printed Circuits)  140  is connected to the electrical input units  111   b  to  111   f . Further, the flexible substrate  140  is connected to the driver IC  120 , and has a signal line  140   a  that outputs the drive signal output by the driver IC  120  to the light emission element  130 . The signal line  140   a  is a high-frequency signal line, and impedance of the signal line  140   a  is kept to 50Ω. The FPC  140  has at least one hollow portion  401  around a remaining portion  402 . The remaining portion  402  is narrower than other portion of the FPC  140  which is arranged on the Peltier element  203  and the driver IC  120 . The hollow portion  401  is a part of the cutting out of the FPC  140 . Therefore hollow portion  401  is a cutout portion. Also FPC  140  has a ground plane  620  arranged on opposite side of the signal line and the ground plane  620  extends from the driver IC  120  side end to the light emission element  130  side end via the remaining portion  402 . The ground plane  620  is formed in accordance a base material of the FPC 140 . Therefore, the signal line  140   a  has constant characteristic impedance along itself in association with the base material. 
     One of the flexible substrate  140  is arranged so that the connection distance between the signal line  140   a  and the driver IC  120  is within, e.g., 375 μm. As a consequence, the high-frequency characteristics between the signal line  140   a  and the driver IC  120  can be kept. Further, the other end of the flexible substrate  140  is arranged so that the connection distance between the signal line  140   a  and the light emission element  130  is within, e.g., 375 μm. Thus, the high-frequency characteristics between the signal line  140   a  and the light emission element  130  can be kept. 
     Herein, the flexible substrate  140  comprises a plurality of DC lines  140   b  to  140   f  in addition to the signal line  140   a . The DC lines  140   b  to  140   f  are connected to the electrical input units  111   b  to  111   f , respectively. For example, the DC line  140   b  and the DC line  140   c  are power supply lines that supply power input from the electrical input unit  111   b  and the electrical input unit  111   c  to the driver IC  120 . 
     Further, the DC line  140   d  is a power supply line that supplies the power input from the electrical input unit  111   d  to a Peltier element (which will be described later). Furthermore, the DC line  140   e  is a signal line that outputs the electrical signal output from a PD (Photo Detector)  160  to the electrical input unit  111   e.    
     Herein, the PD  160  is a PD for monitoring an optical signal that partly receives the optical signals generated by the light emission element  130  and outputs an electrical signal based on the received optical signal. Further, the DC line  140   f  is a power supply line that supplies the power input from the electrical input unit  111   f  to the light emission element  130 . 
     Furthermore, the flexible substrate  140  may have a plurality of the signal lines  140   a . In this case, the flexible substrate  140  performs parallel transfer. 
       FIG. 2  is a cross-sectional view of an A-A line shown in  FIG. 1 . Referring to  FIG. 2 , the same structure as that shown in  FIG. 1  is designated by the same reference numerals, and a description thereof is omitted. Referring to  FIG. 2 , the driver IC  120  is fixed to the casing  110  by a fixing member  201 . Specifically, one end of the flexible substrate  140  is fixed onto the fixing member  201  fixed to the casing  110 , and the driver IC  120  is fixed to one end of the flexible substrate  140 . The signal line is arranged on a top face of the flexible substrate  140 . A ground plane is arranged on a bottom face of the flexible substrate  140 . 
     The light emission element  130  is fixed to the casing  110  by a fixing member  202  and a Peltier element  203 . Specifically, the Peltier element  203  is fixed onto the fixing member  202  fixed to the casing  110 . The other end of the flexible substrate  140  is fixed onto the Peltier element  203 . The light emission element  130  is fixed onto the other end of the flexible substrate  140 . 
     The Peltier element  203  is driven by power supplied by the DC line  140   d . The fixing member  202  contains, e.g., aluminum nitride (AIN). A portion to which the fixing member  201  and the fixing member  202  are fixed in the casing  110  and the fixing member  201  contain, e.g., copper tungsten (CuW). Here, the lens  151  in the optical coupling unit  150  is arranged on the fixing member  202 . Position of the above parts may flexibly arrange in the casing. For example, the fixing member  202  is fixed onto the Peltier element  203  fixed to the casing  110 , and the lines  151  and the lines  153  may flexibly arrange in optical pass of the casing  110 . 
     Next, a description will be given of absorption operation with the above-mentioned structure. The flexible substrate  140  with flexibility is used, thereby keeping the high-frequency characteristics irrespective of the positional deviation between the driver IC  120  and the light emission element  130  and connecting the driver IC  120  to the light emission element  130 . 
     For example, referring to  FIG. 2 , when the height of the driver IC  120  is deviated from that of the light emission element  130 , the deviation of height can be absorbed by the flexible substrate  140 . As a consequence, the connection distance of the connection portion using the wire can be 375 μm or less, and the high-frequency characteristics can be kept. 
     Therefore, it is possible to improve the tolerance with respect to the positional deviation between the driver IC  120  and the light emission element  130 . Further, it is possible to improve the tolerance with respect to the positional deviation between the electrical input unit  111   a  and the optical output unit  112 , resulting in the positional deviation between the driver IC  120  and the light emission element  130 . 
     Next, a description will be given of the operation of heat conduction with the above-mentioned structure. The fixing member  201  has a function as a heat releasing member for diffusing the heat generated by the driver IC  120  to the casing  110 . The heat generated by the driver IC  120  is conducted and diffused to the fixing member  201  and the casing  110 . 
     The Peltier element  203  conducts the heat generated by the light emission element  130  to the fixing member  202  with the Peltier advantage. The fixing member  202  also has a function as a heat releasing member for diffusing the heat conducted by the Peltier element  203 . The heat generated by the light emission element  130  is conducted to the Peltier element  203 , the fixing member  202 , and the casing  110 , and is diffused. 
     Herein, if increasing a distance  220  between the fixing member  201  to which the driver IC  120  is arranged and the Peltier element  203  to which the light emission element  130  is arranged, the high-frequency characteristics can be kept with the flexible substrate  140  and the driver IC  120  can be connected to the light emission element  130 . 
     As a consequence, the high-frequency characteristics between the driver IC  120  and the light emission element  130  can be kept and a space  210  can be also assured. For example, it is possible to set, to 1.5 mm or more, the distance  220  between the fixing member  201  for diffusing the heat of the driver IC  120  and the Peltier element  203  for diffusing the heat of the light emission element  130 . Therefore, the driver IC  120  can be sufficiently separated from the light emission element  130  in view of the heat, and the driver IC  120  and the light emission element  130  can be stably operated. 
     Further, since the flexible substrate  140  can keep the high-frequency characteristics, the driver IC  120  is sufficiently separated from the light emission element  130  in view of the heat. Therefore, the size of the optical transmitter  100  can be reduced. Further, since the driver IC  120  is included in the casing  110 , the size of the optical transceiver including the optical transmitter  100  can be reduced. 
     In addition, since the flexible substrate  140  enables the direct connection between the driver IC  120  and the light emission element  130 , the connection portion using the wire can be reduced. Therefore, it is easy to keep the high-frequency characteristics between the driver IC  120  and the light emission element  130 , and it is possible to improve the high-frequency characteristics. Further, the reduction in connection portion using the wire facilitates a manufacturing process of the optical transmitter  100 . 
     Incidentally, the embodiment can also be applied to the case in which the driver IC  120  is not included in the casing  110  (refer to  FIG. 11 ). In this case, the electrical input unit  111   a  is directly connected to the light emission element  130  with the signal line  140   a  in the flexible substrate  140 . The flexible substrate  140  with flexibility is used, thereby connecting the electrical input unit  111   a  to the light emission element  130  while keeping the high-frequency characteristics, irrespective of the positional deviation between the electrical input unit  111   a  and the light emission element  130 . 
     For example, as shown in  FIG. 2 , even if the height of the electrical input unit  111   a  is deviated from that of the light emission element  130 , the deviation of height can be absorbed by the flexible substrate  140 . As a consequence, the connection distance of the connection portion using the wire can be 375 μm or less, and the high-frequency characteristics can thus be kept. Therefore, it is possible to improve the tolerance with respect to the positional deviation between the electrical input unit  111   a  and the light emission element  130 . Further, it is possible to improve the tolerance with respect to the positional deviation between the electrical input unit  111   a  and the optical output unit  112 , resulting in the positional deviation between the electrical input unit  111   a  and the light emission element  130 . 
     Further, the space  210  can be ensured while keeping the high-frequency characteristics between the electrical input unit  111   a  and the light emission element  130 . Therefore, the Peltier element  203  that diffuses the heat of the light emission element  130  can sufficiently be separated from the casing  110  in view of the heat, and the light emission element  130  can stably be operated. In addition, since the high-frequency characteristics can be kept by the flexible substrate  140 , the casing  110  is sufficiently separated from the light emission element  130  in view of the heat. Thus, the internal structure of the casing  110  is not necessarily complicated. Therefore, the size of the optical transmitter  100  can be reduced. 
       FIG. 3  is a perspective view showing a flexible substrate (No. 1) of the optical transmitter according to the embodiment. Referring to  FIG. 3 , the DC line  140   b  and the DC line  140   c  shown in  FIG. 1  are omitted (similarly in  FIGS. 4 and 5 ). As shown in  FIG. 3 , the flexible substrate  140  in the optical transmitter  100  according to the embodiment has the flexibility in the Z axis direction in the drawing. Therefore, the flexible substrate  140  can absorb the deviation between the driver IC  120  and the light emission element  130  in the height direction (refer to  FIG. 2 ). 
     Further, the flexible substrate  140  may have flexure  301  between the driver IC  120  and the light emission element  130 . As a consequence, the flexible substrate  140  has the stretch property in the Y axis direction in the drawing. Thus, the flexible substrate  140  can absorb the deviation between the driver IC  120  and the light emission element  130  in the connection direction therebetween (refer to  FIGS. 1 and 2 ). Even if the heat expansion changes the distance between the driver IC  120  and the light emission element  130 , the flexure  301  of the flexible substrate  140  enables the absorption of the deviation. 
       FIG. 4  is a perspective view showing a flexible substrate (No. 2) in the optical transmitter according to the embodiment. Referring to  FIG. 4 , the flexible substrate  140  has the cutout portion or hollow portion  401  between the driver IC  120  and the light emission element  130 . Herein, the cutout or hollow portion  401  is formed in the Y axis direction in the drawing (in the direction perpendicular to the connection direction between the driver IC  120  and the light emission element  130 ). Cutout portion or hollow portion  401  can form to cut out sides of the base material of the flexible substrate  140  in  FIG. 3 . The hollow or cutout portion  401  makes the remaining portion  402  on the flexible substrate  140 . As a consequence, the remaining portion  402  to be narrowed by the hollow  401  on the flexible substrate  140  can be flexibly bent. Sides of the remaining portion  402  have the hollow portions  401 . 
     Thus, it is possible to increase the flexibility in the Z axis direction and the stretch property in the Y direction in the drawing on the flexible substrate  140 . Further, with the cutout or hollow  401 , the flexible substrate  140  has the flexibility in the X axis direction in the drawing. Therefore, the flexible substrate  140  can absorb the deviations between the driver IC  120  and the light emission element  130  in the height direction of the driver IC  120  and the light emission element  130  and in the direction perpendicular to the connection direction (refer to  FIG. 1 ). 
     Further, the flexible substrate  140  has the flexibility with respect to the twisting of the rotational direction with the axis, as center, in the Y direction in the drawing. Therefore, if the driver IC  120  is not in parallel with the light emission element  130  (refer to  FIG. 2 ) and both of them are arranged at different angles, the flexible substrate  140  can absorb the deviation in angle. 
     Herein, the halfway portions of the DC line  140   d , the DC line  140   e , and the DC line  140   f  have a flying-lead structure in which an electrode remains and a basic material is deleted. The cutout or hollow  401  formed by deleting the basic material enables the flexible substrate  140  to have the above-mentioned flexibility. Further, the basic material remains at the forming portion of the signal line  140   a  on the flexible substrate  140 , thereby keeping the high-frequency characteristics of the signal line  140   a.    
       FIG. 5  is a perspective view showing a flexible substrate (No. 3) in the optical transmitter according to the embodiment. Referring to  FIG. 5 , the flexible substrate  140  may have a device hole  501  and a device hole  502 . The device hole  501  is a hole that is arranged to one end of the flexible substrate  140  to set the driver IC  120  at the connection position to the signal line  140   a.    
     The device hole  502  is a hole that is arranged to the other end of the flexible substrate  140  to set the light emission element  130  at the connection position to the signal line  140   a . The device hole  501  and the device hole  502  enable the height adjustment of the driver IC  120 , the light emission element  130 , and the signal line  140   a , thereby easily keeping the high-frequency characteristics. More specifically, the device hole  501 &#39;s depth is according to height of the driver IC  120 , and the device hole  502 &#39;s depth is according to height of the light receiving element  910 . 
       FIG. 6  is a cross-sectional view showing the flexible substrate in the optical transmitter according to the embodiment.  FIG. 6  is a cross-sectional view of a B-B line shown in  FIG. 5 . Referring to  FIG. 6 , the signal line  140   a  is formed onto a surface of a base film  610  such as polymide or liquid crystal polymer. Further, the signal line  140   a  is a micro strip line in association with a ground plane  620  being arranged to opposite side of the signal line on the base film  610 . 
     As a consequence, the signal line  140   a  can keep the high-frequency characteristics from the driver IC  120  to the light emission element  130 . Further, on the B-B line in  FIG. 5 , the base film  610  at the forming portion of the DC line  140   d , the DC line  140   e , and the DC line  140   f  and the ground plane  620  are cut out (cutout or hollow portion  401 ). 
       FIG. 7  is a cross-sectional view showing a flexible substrate of the optical transmitter according to a first modification of the embodiment. Referring to  FIG. 7 , the signal line  140   a  may shape a coplanar line sandwiched by a ground plane  710  and a ground plane  720  formed onto the surface of the base film  610  instead of the ground plane  620  on the bottom face of the flexible substrate  140  of the  FIG. 6 . The ground plane  710  and the ground plane  720  are formed along the signal line  140   a  while keeping a constant distance to the signal line  140   a.    
     As a consequence, the signal line  140   a  can keep the high-frequency characteristics from the driver IC  120  to the light emission element  130 . In this case, on the B-B line in  FIG. 5 , the base film  610  at the forming position of the DC line  140   d , the DC line  140   e , and the DC line  140   f  is cut out. 
       FIG. 8  is a cross-sectional view showing a flexible substrate of the optical transmitter according to a second modification of the embodiment. Referring to  FIG. 8 , the signal line  140   a  may shape a grounded coplanar line sandwiched by the ground plane  710  and the ground plane  720  formed onto the surface of the base film  610  with the ground plane  620  arranged to the back surface of the base film  610 . Herein, the ground plane  710  and the ground plane  720  are connected to the ground plane  620  via a  810  and a via  820 . 
     Thus, the signal line  140   a  can keep the high-frequency characteristics from the driver IC  120  to the light emission element  130 . In this case, on the B-B line in  FIG. 5 , the base film  610  at the forming position of the DC line  140   d , the DC line  140   e , and the DC line  140   f  and the ground plane  620  are also cut out. 
       FIG. 9  is a plan view showing an optical receiver according to the embodiment. Referring to  FIG. 9 , the same structure as that shown in  FIG. 1  is designated by the same reference numeral and a description thereof is omitted. As shown in  FIG. 9 , an optical receiver  900  according to the embodiment comprises a light receiving element  910  and the flexible substrate  140  in the casing  110 . Herein, the electrical input units  111   a  to  111   f  are set as electrical output units  111   a  to  111   f , and the optical output unit  112  is set as an optical input unit  112 . 
     The optical receiver  900  receives an optical signal by using the optical input unit  112 . The optical coupling unit  150  passes through the optical signal input from the optical input unit  112  and enables the light receiving element  910  to couple the passing signals. Herein, the window  152  has a function of an optical isolator that passes through the optical signal only in the direction from the optical input unit  112  to the light receiving element  910 . 
     The light receiving element  910  receives the optical signal input from the optical input unit  112  and passing through the optical coupling unit  150 . The light receiving element  910  generates an electrical signal on the basis of the received optical signal. The light receiving element  910  outputs the generated electrical signal to the electrical output unit  111   a  via the signal line  140   a  on the flexible substrate  140 . The light receiving element  910  is, e.g., a PD (Photo Detector). 
     The light receiving element  910  is arranged at the matching position of the optical input unit  112  so as to receive the optical signal output from the optical input unit  112 . For example, the light receiving element  910  is arranged so that the precision of the positional deviation from the optical input unit  112  is within 100 μm. The signal line  140   a  on the flexible substrate  140  connects the light receiving element  910  to the electrical output unit  111   a . As mentioned above, one end of the signal line  140   a  is connected to the electrical output unit  111   a  and, on the other hand, the light receiving element  910  is arranged at the matching position of the optical input unit  112 . 
       FIG. 10  is a cross-sectional view of a C-C line shown in  FIG. 9 . Referring to  FIG. 10 , the same structure as that shown in  FIGS. 2 and 9  is designated by the same reference numeral and a description thereof is omitted. As shown in  FIG. 10 , the light receiving element  910  is fixed to the casing  110  by using the fixing member  202 . Specifically, the other end of the flexible substrate  140  is fixed onto the fixing member  202  fixed to the casing  110 . Further, the light receiving element  910  is fixed onto the other end of the flexible substrate  140 . 
     The various flexible substrates  140  shown in  FIGS. 3 to 8  can be applied to the flexible substrate  140 . By using the flexible substrate  140  with flexibility, the high-frequency characteristics can be kept and the electrical output unit  111   a  can be also connected to the light receiving element  910 , irrespective of the positional deviation between the electrical output unit  111   a  and the light receiving element  910 . 
     For example, referring to  FIG. 10 , even if the height of the electrical output unit  111   a  is deviated from that of the light receiving element  910 , the deviation in height can be absorbed by the flexible substrate  140 . Accordingly, the connection distance of the connection portion using the wire can be within 375 μm or less, and the high-frequency characteristics can be kept. 
     As a consequence, it is possible to improve the tolerance with respect to the positional deviation between the electrical output unit  111   a  and the light receiving element  910 . Further, it is possible to improve the tolerance with respect to the positional deviation between the electrical output unit  110  and the optical input unit  112 , resulting in the positional deviation between the electrical output unit  111   a  and the light receiving element  910 . 
     As mentioned above, the flexible substrate according to the embodiment has the notch portion in the middle of the flexible substrate and the flexibility is therefore high. Thus, even if the size of the flexible substrate is small, the flexibility can sufficiently be obtained. Further, the high-frequency characteristics of the signal line can be kept. Furthermore, another line other than the signal line has a flying-lead structure for passage though the notch portion, thereby forming the long notch portion. The flexibility can sufficiently be obtained. 
     In addition, with the flexible substrate and the optical transmitter according to the embodiment, the driver IC is connected to the light emission element on the flexible substrate, thereby keeping the high-frequency characteristics irrespective of the positional deviation between the driver IC and the light emission element. Further, the driver IC and the light emission element can sufficiently be separated in view of the heat without the conventional casing having a complicated internal structure. Thus, with the flexible substrate optical part, the optical transmitter, and the optical receiver according to the embodiment, the high-frequency characteristics can be kept and the size of the optical device can be reduced. 
     Incidentally, the optical transmitter and the optical receiver as described above according to the embodiment can be applied to optical communication apparatuses, such as an optical transmitting and receiving apparatus, a relay apparatus, and an OADM (Optical Add Drop Multiplexer) forming the optical communication system. 
     Above description of the embodiments, advantageously, the high-frequency characteristics can be kept and the size of the optical apparatus can be also reduced. 
     More specifically, the flexible substrate, optical part, optical transmitter, and optical receiver according to the embodiment are advantageous for a compact optical transceiver, and are particularly suitable to transmission and reception of an optical signal at a high velocity.