Patent Publication Number: US-2022236506-A1

Title: Optical transceiver module for optical transceiver and optical transceiver

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority to Japanese Patent Application No. 2021-010955, filed on Jan. 27, 2021, the entire content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments described herein generally relate to an optical transceiver module for an optical transceiver and an optical transceiver. 
     2. Description of the Related Art 
     With the increase in the amount of information transmitted on the Internet and the like, it is desired to improve the transmission speed of information on the transmission paths using optical fibers. In accordance with the demand for improving the transmission speed, it has been a crucial issue to increase the transmission capacity per rack in data centers, and it is desired to reduce the sizes of the optical transceivers that transmit and receive optical signals. For example, United States Patent Application Publication No. 2020/0150366 discloses a method for implementing an integrated coherent transmit-receive optical sub assembly (IC-TROSA) in a quad small form factor pluggable double density (QSFP-DD), which is one of the small form factors. 
     In a case where an optical transceiver module such as the IC-TROSA or the like is implemented in the QSFP-DD, not only photoelectric conversion components including a photoelectric conversion circuit and optical components but also control components included in control circuits for controlling the photoelectric conversion circuit are required to be accommodated in a QSFP-DD housing. Also, in the optical transceiver module, the photoelectric conversion components are arranged according to an electric interface defined by a multi-source agreement (MSA), and the optical components are arranged according to the positions of the photoelectric conversion components and the optical interface. 
     Therefore, the photoelectric conversion components, the optical components, and the control components are arranged in a mixed manner in the optical transceiver module, the area in which the control components are arranged is limited, and it is necessary to implement the control components with a high density in a vacant area. If a problem occurs in the control components in the optical transceiver module in which the components are implemented with a high density, it is difficult to perform rework such as replacement of the control components. Also, the optical transceiver module including the photoelectric conversion components and the optical components are hermetically sealed. If a problem occurs with components in the hermetically sealed optical transceiver module, it is difficult to rework the components. In a case where the components cannot be reworked, there would be no choice but to discard the optical transceiver module that has a problem. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention provide an optical transceiver module that includes a package having a first surface and a second surface and containing a light receiving element, a light emitting element, and an optical modulator configured to modulate light that is output from the light emitting element, a rigid circuit board having a first surface and a second surface and including a control circuit provided on the rigid circuit board, the control circuit being configured to control at least one of the light receiving element, the light emitting element, or the optical modulator, and a flexible circuit board including a plurality of signal wires, wherein the rigid circuit board is connected to the package via the flexible circuit board, with the first surface of the rigid circuit board facing the first surface of the package, and the at least one of the light receiving element, the light emitting element, or the optical modulator is electrically connected to the control circuit via the plurality of signal wires. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a perspective view illustrating an example of a configuration of an optical transceiver including an optical transceiver module according to a first embodiment; 
         FIG. 2  is an exploded perspective view illustrating the optical transceiver of  FIG. 1 ; 
         FIG. 3  is a perspective view illustrating an IC-TROSA of  FIG. 1 ; 
         FIG. 4  is an exploded perspective view of  FIG. 3 ; 
         FIG. 5  is a functional block diagram illustrating a circuit configuration of IC-TROSA of  FIG. 3 ; 
         FIG. 6  is an exploded perspective view before an IC-TROSA package is hermetically sealed; 
         FIG. 7  is an explanatory diagram illustrating a step of cutting unnecessary portions of a flexible circuit board after a rigid board is soldered to the flexible board; 
         FIG. 8  is a partial cross-sectional view illustrating an example of a rigid board on which control components and a spacer are mounted; 
         FIG. 9  is a perspective view illustrating an example of a flex-rigid circuit board of an optical transceiver module according to a second embodiment; and 
         FIG. 10  is a perspective view illustrating the flex-rigid circuit board of  FIG. 9  as seen from the back surface. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments described herein provide an optical transceiver module in which a control circuit for controlling a photoelectric conversion circuit can be easily reworked. 
     Description of Embodiments of the Present Disclosure 
     First, embodiments of the present disclosure are explained. 
     [1] According to an aspect of the present disclosure, an optical transceiver module includes a package having a first surface and a second surface and containing a light receiving element, a light emitting element, and an optical modulator configured to modulate light that is output from the light emitting element, a rigid circuit board having a first surface and a second surface and including a control circuit provided on the rigid circuit board, the control circuit being configured to control at least one of the light receiving element, the light emitting element, or the optical modulator, and a flexible circuit board including a plurality of signal wires, wherein the rigid circuit board is connected to the package via the flexible circuit board, with the first surface of the rigid circuit board facing the first surface of the package, and the at least one of the light receiving element, the light emitting element, or the optical modulator is electrically connected to the control circuit via the plurality of signal wires. 
     According to the optical transceiver module, the control circuit, the light receiving element, the light emitting element, and the optical modulator can be provided in areas that are independent from each other, and therefore, in a case where a problem occurs with the control circuits, rework such as replacement of the control components included in the control circuit can be performed easily. Accordingly, the optical transceiver module in which the control circuit for controlling the photoelectric conversion circuit can be reworked easily can be provided. 
     [2] In the above-described [1], the flexible circuit board may include a main body portion connected to the rigid circuit board, a first end portion provided on one side of the main body portion, and a second end portion provided on an opposite side from the one side of the main body portion, and the first end portion and the second end portion may be connected to the package. Accordingly, the implementation area of the control circuit implemented on the first surface of the rigid circuit board can be secured as a single area. 
     [3] In the above-described [2], the package may include a first side surface and a second side surface that cross the first surface of the package, the first end portion may be connected to the first side surface by solder, and the second end portion may be connected to the second side surface by solder. Accordingly, the first end portion and the second end portion are soldered to the first side surface and second side surface, respectively, of the package, so that the rigid circuit board can be rigidly fixed to the package. 
     [4] In the above-described [2], the optical transceiver module may include a plurality of spacers disposed between the first surface of the rigid circuit board and the first surface of the package, wherein a height of each of the plurality of spacers may be greater than a height of a control component included in the control circuit implemented on the first surface of the rigid circuit board. Accordingly, the control component Implemented on the first surface of the rigid circuit board can be prevented from being short-circuited with the package. Furthermore, the control component implemented on the back surface of the rigid circuit board can be prevented from coming into pressurized contact with the package and being damaged. 
     [5] In the above-described [4], the main body portion may include an opening portion on an inner side. Accordingly, a spacer can be brought into contact with the package without colliding with the flexible circuit board, and the rigid circuit board can be reliably supported on the package. 
     [6] In the above-described [5], the plurality of spacers may penetrate through the opening portion. Accordingly, because the plurality of spacers penetrate through the opening portion, the spacers can be brought into contact with the package without colliding with the flexible circuit board. 
     [7] In the above-described [1], the rigid circuit board and the flexible circuit board may be formed integrally. Accordingly, the area where components can be implemented on the back surface of the circuit board can be made larger than the rigid circuit board, and the efficiency of implementation of the control components can be improved. Furthermore, the degree of flexibility in arrangement and wiring of the control components implemented on the circuit board increases, and therefore, the board design can be facilitated. 
     [8] According to an aspect of the present disclosure, an optical transceiver in which the optical transceiver module of the above-described [1] may be implemented. According to the optical transceiver, the control circuit, the light receiving element, the light emitting element, and the optical modulator can be provided in areas that are independent from each other, and therefore, in a case where a problem occurs with the control circuits, rework such as replacement of the control components included in the control circuit can be performed easily. Accordingly, the optical transceiver module in which the control circuit for controlling the photoelectric conversion circuit can be reworked easily can be provided. 
     Embodiment of the Present Disclosure 
     Specific examples of an optical transceiver module for an optical transceiver and an optical transceiver according to the present disclosure are described below with reference to the drawings. The embodiments are not limited to the following explanations. In the following explanation, signal lines for transmitting information such as signals are denoted with the reference numerals as the reference numerals for denoting signal names. Unless otherwise noted, lines with arrowheads in the drawings indicate transmission paths of signals or information. Also, signal lines represented as single lines in the drawings may have multiple bits. 
     First Embodiment 
     [Overall Configuration of Optical Transceiver] 
       FIG. 1  is a perspective view illustrating an example of a configuration of an optical transceiver including an optical transceiver module according to a first embodiment. For example, an optical transceiver  100  as illustrated in  FIG. 1  includes an IC-TROSA  200 , a host board  300 , and a housing  400  conforming to the QSFP-DD standard in which the IC-TROSA  200  and the host board  300  are accommodated. In  FIG. 1 , a part of the housing  400  is illustrated as transparent so that the IC-TROSA  200  and the host board  300  can be seen. 
     The IC-TROSA  200  includes: a photoelectric conversion circuit including an optical modulator, a light receiving element, a tunable laser, and the like; and a control circuit configured to control the photoelectric conversion circuit. The IC-TROSA  200  is an example of an optical transceiver module. The optical transceiver module implemented on the optical transceiver  100  is not limited to the IC-TROSA  200 . The host board  300  has a terminal unit  302  connected to a connector of a host apparatus, not illustrated. 
     On one of the surfaces of the IC-TROSA  200  (i.e., a lower side of  FIG. 1 ), the housing  400  includes a heat dissipation unit  402  for dissipating heat that is generated from the IC-TROSA  200 . On the opposite side from the terminal unit  302 , the housing  400  includes a socket unit  404  into which optical cables are inserted. An example of the IC-TROSA  200  is explained later with reference to  FIG. 3  and subsequent drawings. 
       FIG. 2  is an exploded perspective view illustrating the optical transceiver  100  of  FIG. 1 . The housing  400  includes an upper housing  410  including an accommodation space in which the IC-TROSA  200  and the host board  300  are accommodated; and a lower housing  420 . The optical transceiver  100  is connected to a host apparatus, not illustrated, with the heat dissipation unit  402  being on the upper side. Accordingly, the heat generated from the IC-TROSA  200  can be released to the upper side (to the lower side in  FIG. 2 ) via the heat dissipation unit  402 . 
     [Overall Configuration of Optical Transceiver Module] 
       FIG. 3  is a perspective view illustrating the IC-TROSA  200  of  FIG. 1 . The IC-TROSA  200  includes: a package  210  in a box shape in which the Photoelectric conversion circuit is included; and a rigid circuit board  220  in a rectangular shape on which a control circuit is implemented. Also, the IC-TROSA  200  includes a flexible circuit board  230  that electrically connects, with each other, the circuit included in the package  210  and the circuit implemented on the rigid circuit board  220 . For example, the package  210  including the photoelectric conversion circuit is formed by a ceramic or the like, and the rigid circuit board  220  is formed by a glass epoxy board including multiple wiring layers, although the package  210  and the rigid circuit board  220  are not limited thereto. 
     Sleeves  242  and  244  are attached to one end of the package  210  in a length direction L. Ferrules of optical cables, not illustrated are inserted into the sleeves  242  and  244 . A flexible circuit board  250  is connected to the other end of the package  210  in the length direction L. The flexible circuit board  250  is configured to connect to the host board  300  of  FIG. 2 . The maximum external dimensions of the IC-TROSA  200  excluding the flexible circuit board  250  and the sleeves  242  and  244  are determined by the MSA. The external dimensions of the IC-TROSA  200  determined by the MSA are a width of up to 15.1 mm (the size in the width direction W), a length of up to 30 mm (the size in the length direction L), and height of up to 6.5 mm. 
     Radio frequency signals are transmitted and received between the photoelectric conversion circuit implemented in the package  210  and the host board  300 . Therefore, the terminals of the package  210  and the terminals of the flexible circuit board  250  are soldered. Also, the terminals of the flexible circuit board  250  and the terminals of the host board  30 C, not illustrated, are soldered. 
     Control components such as a microcomputer  222  and the like for monitoring the operation state of the photoelectric conversion circuit in the package  210  and controlling the photoelectric conversion circuit, other control components  224 , and a connector  226  are implemented on the rigid circuit board  220 . The microcomputer  222  implemented on the rigid circuit board  220  is an example of a control circuit. 
     The connector  226  is attached to the other end of the rigid circuit board  220  in the length direction L. The other end of the flexible circuit board  240  (illustrated in  FIG. 2 ), one end of which is connected to the host board  300 , is connected to the connector  226  in such a manner that the flexible circuit board  240  can be inserted into and removed from the connector  226 . Various kinds of control components are implemented on not only the front surface (upper side of  FIG. 3 ) but also the back surface of the rigid circuit board  220 . 
     Multiple terminals (not illustrated) of the rigid circuit board  220  are connected to multiple terminals (not illustrated) provided on the main body portion  232  of the flexible circuit board  230  facing the back surface side of the rigid circuit board  220 . The connection between the rigid circuit board  220  and the flexible circuit board  230  is explained with reference to  FIG. 4  and  FIG. 7 . 
     The flexible circuit board  230  includes the main body portion  232  that faces the back surface of the rigid circuit board  220  and has a shape corresponding to the rectangular shape of the rigid circuit board  220  and the rectangular shape of the front surface (the upper surface of  FIG. 3 ) of the package  210 . Also, the flexible circuit board  230  includes a pair of protruding portions  234  and  236  that protrude to the opposite side from the front surface of the flexible circuit board  230 . 
     The protruding portions  234  and  236  are provided on a pair of sides along the length direction L on both sides of the main body portion  232  in the width direction W. The protruding portions  234  and  236  are examples of a first end portion and a second end portion, respectively. The protruding portions  234  and  236  include multiple terminals arranged in the length direction L. The terminals of the protruding portions  234  and  236  are soldered to the terminals provided on side surfaces  212  and  214  that are on both sides of the package  210  in the width direction W and that cross (e.g., substantially perpendicular to) the front surface of the package  210 . The flexible circuit board  230  includes multiple signal wires that connect multiple terminals of the main body portion  232  and multiple terminals of the protruding portions  234  and  236 . The side surfaces  212  and  214  are examples of a first side surface and a second side surface, respectively. 
     The flexible circuit board  230  is soldered to the rigid circuit board  220  and is also soldered to the package  210 . Accordingly, the rigid circuit board  220  is fixed to the package  210 , with the back surface (one surface) of the rigid circuit board  220  and the front surface (one surface) of the package  210  being arranged opposite to each other. The photoelectric conversion circuit included in the package  210  is electrically connected via multiple signal wires to the control circuit implemented on the rigid circuit board  220 . 
     As illustrated in  FIG. 3 , the control component included in the control circuit implemented on the rigid circuit board  220  is exposed to the outside of the package  210 . The control components and the photoelectric conversion components are provided in areas that are independent from each other, so that rework such as replacement of the control components can be performed easily in a case where a problem occurs with control components (the control circuit). The control components are provided outside of the package  210 , so that noise generated by operations of the control circuit implemented on the control component can be alleviated from affecting the photoelectric conversion circuit in the package  210 . 
     The external dimensions of the rigid circuit board  220  may be approximately the same as the external dimensions of the package  210 . For example, the external dimensions of the rigid circuit board  220  may be the maximum width (15.1 mm) and the maximum length (30 mm) of the IC-TROSA defined by the MSA. Therefore, the degree of flexibility in arrangement of the control component and the degree of flexibility in wires formed on the rigid circuit board  220  can be improved, which can facilitate the implementation design (the wire layout). 
     The rigid circuit board  220  of which the external dimensions are large can be used, and therefore, general-purpose control components can be used, and the cost of the IC-TROSA  200  can be reduced, so that the development period can be shortened. Furthermore, it is not necessary to provide control components in the package  210 , and therefore, the degree of flexibility in the layout of the photoelectric conversion components and the optical components can be improved, which can facilitate the implementation design. 
     In contrast, in a case where control components (control circuits) are implemented in the package of the IC-TROSA, for example, it is desired to integrate multiple control components into an application specific integrated circuit (ASIC) and the like to be made into a single chip. In this case, the cost of the IC-TROSA greatly increases. Normally, the package on which the photoelectric conversion circuit is implemented is hermetically sealed, and therefore, in a case where a problem is found with the circuit implemented in the package, it is difficult to rework components including the circuit in which a problem is found. For this reason, in a case where a problem is found with the circuit implemented in the package, it is necessary to discard the package. 
     Furthermore, in a case where the rigid circuit board on which the control component is implemented is accommodated in the package of the IC-TROSA, the size of the rigid circuit board is smaller than the rigid circuit board  220 . Therefore, the efficiency of implementation of the control components decreases. Furthermore, it is difficult to arrange and solder wires for connecting the rigid circuit board accommodated in the package of the IC-TROSA and the photoelectric conversion components in the package. 
       FIG. 4  is an exploded perspective view of  FIG. 3 . The flexible circuit board  230  includes multiple terminals arranged along the length direction L on both sides of the width direction W of the main body portion  232  such that the multiple terminals are arranged to face the back surface (one surface) of the rigid circuit board  220 . The multiple terminals of the main body portion  232  of the flexible circuit board  230  are soldered to the terminals provided on the back surface of the rigid circuit board  220 . As explained with reference to  FIG. 7 , in the actual implementation, the flexible circuit board  230  is soldered to the rigid circuit board  220 , and thereafter, unnecessary portions are cut off. 
     The length in the length direction L is larger than the length in the width direction W in the rigid circuit board  220 , the main body portion  232  of the flexible circuit board  230 , and the package  210 . Accordingly, the soldering terminals for connecting the rigid circuit board  220  and the flexible circuit board  230  are formed along the length direction L, so that, as compared with the case where the terminals are formed in the width direction W, more terminals can be formed. 
     Therefore, in a case where the same number of terminals are formed in the length direction L and the width direction W, the size of the terminals formed along the length direction L can be increased and can be soldered more reliably. Furthermore, the terminal intervals can be increased, and therefore, solder bridging between neighboring terminals can be provided. 
     Furthermore, the soldering terminals are formed along the length direction L on both sides in the width direction W, so that, as illustrated in  FIG. 7 , the implementation area for the control components on the back surface of the rigid circuit board  220  can be secured as a single area. Furthermore, the protruding portions  234  and  236  of the flexible circuit board  230  are formed in the length direction L in which the length is longer than in the width direction W, so that the rigid circuit board  220  can be fixed reliably and rigidly on the package  210 . 
     Multiple circuit components  216  such as an optical modulator, a light receiving element, a tunable laser, and the like, and optical components are implemented in the package  210 . The respective circuit components  216  are connected to the terminals provided in the terminal unit  219  on the rear side of the package  210 , and are connected to the terminals of the flexible circuit board  250  as illustrated in  FIG. 3 . After the circuit components  216  are implemented, the package  210  is hermetically sealed by placing a lid  218  in a plate shape. 
     Spacers  260  are fixed to locations in proximity to the four corners on the back surface of the rigid circuit board  220 . In the example as illustrated in  FIG. 4 , the spacers  260  have a rectangular parallelepiped shape, but are not limited to the rectangular parallelepiped shape. The spacers  260  may have a cylindrical shape or the like. One end of the spacer  260  (a side of the spacer  260  on the opposite side from the rigid circuit board  220 ) is in contact with one surface of the package  210  (i.e., the lid  218 ). Note that although the spacers  260  are preferably fixed to the four locations on the periphery of the rigid circuit board  220 , the positions and the number of spacers  260  are not limited the example illustrated in  FIG. 4 . 
     The rigid circuit board  220  is supported on the package  210  via the spacers  260 , so that the control components implemented on the back surface of the rigid circuit board  220  can be prevented from coming into contact with the package  210 . Accordingly, the control components implemented on the back surface of the rigid circuit board  220  can be prevented from being short-circuited with the package  210 . Furthermore, the control components implemented on the back surface of the rigid circuit board  220  can be prevented from coming into pressurized contact with the package  210  (the lid  218 ) and being damaged. 
     The base film and the signal wires of the flexible circuit board  230  are not provided at the positions corresponding to the spacers  260 . Specifically, an opening portion  237 , through which the spacers  260  penetrate, are provided on the inner side (i.e., a central portion) of the main body portion  232  of the flexible circuit board  230 . Therefore, the spacers  260  can be brought into contact with the package  210  without colliding with the flexible circuit board  230 , and the rigid circuit board  220  can be reliably supported on the package  210 . 
     Also, as illustrated in  FIG. 3 , the back surface (one surface) of the rigid circuit board  220  is fixed to the package  210  in such a state that the back surface is arranged to face the upper surface (one surface) of the package  210  via the flexible circuit board  230 . Accordingly, components such as a light emitting element, an optical modulator, a light receiving element, and the like implemented in the package  210  are electrically connected to the circuit of the rigid circuit board  220  via multiple signal wires provided on the flexible circuit board  230 . 
     As illustrated in  FIG. 4 , with the IC-TROSA  200  according to this embodiment, both of the rigid circuit board  220  on which the control circuit is implemented and the package  210  in which the photoelectric conversion circuit is implemented can be produced, and each of them can be inspected individually. The rigid circuit board  220  and the package  210 , which are inspected individually, are assembled via the flexible circuit board  230 , and the IC-TROSA  200  is manufactured, so that the defect rate in the final inspection after assembly can be reduced. As a result, for example, rework due to defect of the control circuit that is found after assembly can be reduced. Note that the control components included in the control circuit are implemented on the rigid circuit board  220 , and therefore, the rework can be facilitated. 
     [Circuit Configuration of Optical Transceiver Module] 
       FIG. 5  is a functional block diagram illustrating a circuit configuration of the IC-TROSA  200  of  FIG. 3 . In  FIG. 5 , arrows of thick broken lines represent transmission paths of optical signals. In  FIG. 5 , arrows of solid lines represent electric signals (control signals). A single control signal in  FIG. 5  may include multiple control signals in the actual implementation. 
     A tunable laser  11 , a MMI/PD module  12  in which a multi-mode interference (MMI) element and a photodiode PD are integrated, a transimpedance amplifier (TIA)  13 , a driver  14 , and an optical modulator  15  are included in the package  210  of the IC-TROSA  200 . A thermo-electric cooler (TEC)  16  for cooling the tunable laser  11  and a TEC  17  for cooling the optical modulator  15  are included in the package  210 . 
     The rigid circuit board  220  includes components such as a microcomputer  21 , an electric current output DAC  22 , and a voltage output DAC  23 . Hereinafter, the electric current output DAC  22  is also referred to as an IDAC  22 , and the voltage output DAC  23  is also referred to as a VDAC  23 . The microcomputer  21  corresponds to the microcomputer  222  of  FIG. 3 . The microcomputer  21 , the IDAC  22 , and the VDAC  23  are an example of a control circuit for controlling the photoelectric conversion circuit included in the package  210 . The host board  300  includes a TEC controller  31 , a central processing unit (CPU)  32 , and a digital signal processor (DSP)  33 . 
     The microcomputer  21  operates according to a control signal received from the CPU  32  of the host board  300 . The microcomputer  21  receives a temperature monitor signal TH 1  indicating the temperature of the TEC  16  from the TEC  16 , and outputs the received temperature monitor signal TH 1  to the TEC controller  31  via the flexible circuit board  240 . The microcomputer  21  receives the temperature monitor signal TH 2  indicating the temperature of the TEC  17  from the TEC  17 , and outputs the received temperature monitor signal TH 2  to the TEC controller  31 . Alternatively, the temperature monitor signals TH 1  and TH 2  transmitted to the rigid circuit board  220  may be directly provided to the TEC controller  31  without the intervention of the microcomputer  21 . 
     The microcomputer  21  receives from the tunable laser  11  a miscellaneous monitor signal PD 1  such as a power monitor signal, a wavelength monitor signal, and the like indicating the state of the tunable laser  11 . The microcomputer  21  receives a miscellaneous monitor signal PD 2  indicating the state of the MMI/PD module  12  from the MMI/PD module  12 . The microcomputer  21  receives a miscellaneous monitor signal PD 3  indicating the state of the optical modulator  15  from the optical modulator  15 . 
     The microcomputer  21  outputs a control signal for controlling the TIA  13  and the drive  14  via a signal line SPI 1  corresponding to the serial peripheral interface (SPI). Also, the microcomputer  21  outputs a control signal for controlling the IDAC  22  and the VDAC  23  via a signal line SPI 2  corresponding to an SPI interface. 
     The IDAC  22  outputs, to the tunable laser  11 , an electric current LD for oscillating the tunable laser  11  and a control signal l for controlling the wavelength of the tunable laser  11 , according to the control signal from the microcomputer  21 . The VDAC  23  outputs a Mach-Zehnder bias MZV for controlling the optical modulator  15  to the optical modulator  15 , according to the control signal from the microcomputer  21 . 
     The tunable laser  11  generates an optical signal of a predetermined wavelength according to the control signal HT 1  received from the IDAC  22 , and outputs an optical signal, split by a beam splitter, not illustrated, to the MMI/PD module  12  and the optical modulator  15 . The tunable laser  11  is an example of a light emitting element. For example, the light emitting element may be a semiconductor laser such as DFB laser and EML (Electro-absorption Modulator integrated with DFB Laser). 
     For example, with a 90-degree hybrid coupler by the MMI element, the MMI/PD module  12  separates, with respect to the polarization, a phase-modulated and polarization-multiplexed optical signal received from an optical cable connected to the sleeve  244 . After the optical signal separated with respect to the polarization is superimposed with the optical signal from the tunable laser  11  to cause interference, the MMI/PD module  12  detects, with the photodiode PD, an in-phase component I and a quadrature component Q of an X polarization and an in-phase component I and a quadrature component Q of a Y polarization. The photodiode PD converts the in-phase component I and the quadrature component Q of the X polarization and the in-phase component I and the quadrature component Q of the Y polarization, which have been detected, into respective electric current signals, and outputs the converted electric current signals to the TIA  13 . The photodiode PD is an example of a light receiving element. For example, the light receiving element may be a photodetector that does not include a diode structure. 
     The TIA  13  operates according to the control signal from the microcomputer  21 . The TIA  13  generates respective voltage signals by amplifying the electric current signal of the in-phase component I and the electric current signal of the quadrature component Q of each of the X polarization and the Y polarization received from the photodiode PD of the MMI/PD module  12 . The TIA  13  outputs the generated voltage signal to the DSP  33 . 
     The driver  14  operates according to the control signal from the microcomputer  21 , and drives the optical modulator  15  according to the signal of the in-phase component I and the signal of the quadrature component Q of each of the X polarization and the Y polarization received from the DSP  33 . 
     The optical modulator  15  is, for example, a Mach-Zehnder-type modulator, and operates by receiving the Mach-Zehnder bias MZV for the in-phase component I and the quadrature component Q of each of the X polarization and the Y polarization from the VDAC  23 . The optical modulator  15  generates a phase-modulated and polarization-multiplexed optical signal by combining the signal of the in-phase component I and the signal of the quadrature component Q with respect to the polarization by using the optical signal that is output from the tunable laser  11 . The generated optical signal is output to the optical cable connected to the sleeve  242 . 
     The TEC controller  31  outputs to the TEC  16  a control signal TEC 1  for controlling the TEC  16  according to the temperature monitor signal TH 1  received via the microcomputer  21 . The TEC controller  31  outputs to the TEC  17  a control signal TEC 2  for controlling the TEC  17  according to the temperature monitor signal TH 2  received via the microcomputer  21 . The CPU  32  controls the IC-TROSA  200 , and controls the DSP  33 . 
     The DSP  33  receives the voltage signal of the in-phase component I and the quadrature component Q of the X polarization and the voltage signal of the in-phase component I and the quadrature component Q of the Y polarization from the TIA  13  via the flexible circuit board  250 . The DSP  33  generates parallel high-speed reception data signals (digital signals), and outputs the generated high-speed reception data signal to a host apparatus, not illustrated, according to the received voltage signal. 
     Also, the DSP  33  receives parallel high-speed transmission data signals (digital signals) from the host apparatus. The DSP  33  converts the received parallel high-speed transmission data signals into signals of the in-phase component I and the quadrature component Q of the X polarization and signals of the in-phase component I and the quadrature component Q of the Y polarization. The DSP  33  outputs the converted signals of the in-phase component I and the converted signals of the quadrature component Q of the X polarization and the Y polarization to the driver  14 . 
     [Configuration of Package of Optical Transceiver Module] 
       FIG. 6  is an exploded perspective view before the package  210  of the IC-TROSA  200  is hermetically sealed. Multiple circuit components  216  such as the tunable laser  11 , the MMI/PD module  12 , the TIA  13 , the driver  14 , the optical modulator  15 , the TECs  16  and  17 , and the like as illustrated in  FIG. 5  are implemented in the accommodation space provided in the package  210 . 
     After the circuit components  216  are implemented in the package  210 , the lid  218  is placed on the opening portion of the package  210 , and the package  210  is hermetically sealed. It is difficult to detach the lid  218  from the package  210  without damaging the hermetically sealed package  210 . Therefore, it is difficult to rework the circuit component  216  of the package  210  that is once hermetically sealed. 
     [Connection Between Rigid Circuit Board and Flexible Circuit Board] 
       FIG. 7  is an explanatory diagram illustrating an example of cutting off unnecessary portions of the flexible circuit board  230  after the flexible circuit board  230  is soldered to the rigid circuit board  220 . The unnecessary portions of the main body portion  232  of the flexible circuit board  230  may be cut off and removed. 
     For example, various kinds of control components  271  and  272 , the spacers  260 , and the terminals of the flexible circuit board  230  are soldered to the back surface of the rigid circuit board  220 . As a result, a state as illustrated on the left side of  FIG. 7  is obtained. Note that the flexible circuit board  230  and the various kinds of control components  271  and  272  may be soldered to the rigid circuit board  220  at a same time, or the flexible circuit board  230  and the various kinds of control components  271  and  272  may be soldered to the rigid circuit board  220  at different times. 
     Thereafter, unnecessary portions of the flexible circuit board  230  that are located on either side of the rigid circuit board  220  in the length direction L are cut off. The unnecessary portions that are cut off are areas  231 A and  231 B that do not include the terminals soldered to the terminals of the rigid circuit board  220  and that extend in the width direction W. The areas  231 A and  231 B are cut off and removed, so that the main body portion  232  is separated by terminal areas  238  and  239  including the terminals soldered to the terminals of the rigid circuit board  220 . 
     After the flexible circuit board  230  including the connected protruding portions  234  and  236  is soldered to the rigid circuit board  220 , the unnecessary portions of the flexible circuit board  230  are cut off, so that the accuracy in the position of the terminals of the protruding portions  234  and  236  are prevented from decreasing. In this case, the accuracy in the position of the terminals includes the shift in position in the length direction L and the shift in the interval between the protruding portions  234  and  236  in the width direction W. In contrast, in a case where two respective flexible circuit boards that include the protruding portions  234  and  236  are soldered to the rigid circuit board  220 , the accuracy in the position of the terminals of the protruding portions  234  and  236  may decrease. 
     [Arrangement of Spacers with Respect to the Rigid Circuit Board] 
       FIG. 8  is a partial cross-sectional view illustrating an example of the rigid circuit board  220  on which the control components  271  and  272  and the spacer  260  are implemented. For example, the spacers  260  are designed so that a height H 1  of the spacers  260  soldered to the rigid circuit board  220  is greater than any of the heights of all the control components implemented on the back surface of the rigid circuit board  220  (the upper side of  FIG. 8 ). 
     In the example of  FIG. 8 , the height H 1  of the spacer  260  is greater than a height H 2  of the control component  271  that is the tallest among the control components implemented on the back surface of the rigid circuit board  220 . Accordingly, the control components implemented on the back surface of the rigid circuit board  220  can be prevented from being short-circuited with the package  210 . Furthermore, the control components implemented on the back surface of the rigid circuit board  220  can be prevented from coming into pressurized contact with the package  210  and being damaged. 
     As described above, in this embodiment, the control circuit for controlling the photoelectric conversion circuit is implemented on the rigid circuit board  220  provided outside of the package  210 , so that in a case where a problem occurs with the control circuits, rework such as replacement of the control components implemented on the control circuit can be performed easily. Therefore, the optical transceiver module  200  and the optical transceiver  100  in which the control circuit for controlling the photoelectric conversion circuit can be easily reworked can be provided. Furthermore, the control components are provided outside of the package  210 , so that noise generated by operations of the control circuit implemented on the control component can be alleviated from affecting the photoelectric conversion circuit in the package  210 . 
     As the rigid circuit board  220  is provided outside of the package  210 , the size of the rigid circuit board  220  can be increased. As a result, the degree of flexibility in arrangement of the control components in which the control circuit is Implemented and the degree of flexibility in wires formed on the rigid circuit board  220  can be improved, which can facilitate the implementation design (the wire layout). Because general-purpose control components can be used, the cost of the IC-TROSA  200  can be reduced, so that the development period can be shortened. 
     In the rigid circuit board  220  and the flexible circuit board  230 , the soldering terminals are formed in the length direction L on both sides in the width direction W, so that the rigid circuit board  220  can be fixed reliably and rigidly on the package  210 . 
     The rigid circuit board  220  is supported on the package  210  via the spacers  260 , so that the control components implemented on the back surface of the rigid circuit board  220  can be prevented from coming into contact with the package  210 . Accordingly, the control components implemented on the back surface of the rigid circuit board  220  can be prevented from being short-circuited with the package  210 , and can be prevented from coming into contact with the package  210  and being damaged. The opening portion  237  is provided in the flexible circuit board  230 , so that the spacers  260  can be brought into contact with the package  210  without colliding with the flexible circuit board  230 , and the rigid circuit board  220  can be reliably supported on the package  210 . 
     Second Embodiment 
     [Configuration Example with Flex-Rigid Circuit Board] 
       FIG. 9  is a perspective view illustrating an example of a flex-rigid circuit board in an optical transceiver module according to a second embodiment. Substantially the same elements as those in the above-described first embodiment are denoted with the same reference numerals. 
     In the second embodiment, instead of the rigid circuit board  220  and the flexible circuit board  230  as illustrated in  FIG. 4 , the optical transceiver module is formed by using the flex-rigid circuit board  280  as illustrated in  FIG. 9 . The flex-rigid circuit board  280  is a hybrid circuit board in which the flexible circuit board is formed in one of multi-level wiring layers of the rigid circuit board. Specifically, the flex-rigid circuit board  280  is achieved by integrally forming the rigid circuit board  220  and the flexible circuit board  230  of  FIG. 4 . 
     The configuration of the optical transceiver module except for the flex-rigid circuit board  280  is substantially the same as in  FIG. 1  to  FIG. 6 . Specifically, the optical transceiver module using the flex-rigid circuit board  280  is substantially the same as in  FIG. 3  to  FIG. 6 , and the optical transceiver including the optical transceiver module using the flex-rigid circuit board  280  is substantially the same as in  FIG. 1  and  FIG. 2 . 
     The flex-rigid circuit board  280  includes a rigid circuit board unit  220 A and a flexible circuit board unit  230 A. The sizes of the rigid circuit board unit  220 A in the length direction L and the width direction W are substantially the same as the size of the rigid circuit board  220  as illustrated in  FIG. 3 . The control components ( 222 ,  224 , and the like) and the connector  226  implemented on the rigid circuit board unit  220 A are the same as in  FIG. 3 . 
     Like the flexible circuit board  230  as illustrated in  FIG. 3  and  FIG. 4 , the flexible circuit board unit  230 A includes a pair of protruding portions  234  and  236 . Like  FIG. 3 , the terminals provided on the protruding portions  234  and  236  are soldered to the terminals provided on both side surfaces  212  and  214  of the package  210 , not illustrated. 
       FIG. 10  is a perspective view illustrating the flex-rigid circuit board  280  of  FIG. 9  as seen from the back surface. As can be understood by comparing  FIG. 10  and  FIG. 7 , in the flex-rigid circuit board  280 , the soldering terminals that electrically and mechanically connect the rigid circuit board unit  220 A and the flexible circuit board unit  230 A can be made unnecessary. Accordingly, in the flex-rigid circuit board  280 , the area where components can be implemented on the back surface of the circuit board can be made larger than in  FIG. 7 , and the efficiency of implementation of the control components onto the flex-rigid circuit board  280  can be improved. Furthermore, the degree of flexibility in arrangement and wiring of control components implemented on the flex-rigid circuit board  280  increases, and therefore, the board design can be facilitated. 
     As described above, in this embodiment, substantially the same effects as in the above-described first embodiment can be obtained. For example, an optical transceiver module for an optical transceiver and an optical transceiver in which the control circuit for controlling the photoelectric conversion circuit can be reworked easily can be provided. Furthermore, in the second embodiment, the efficiency of implementation of the control components to the flex-rigid circuit board  280  can be improved, as compared with the efficiency of implementation of the control components to the rigid circuit board  220  of  FIG. 7 . Furthermore, the degree of flexibility in arrangement and wiring of the control components implemented on the flex-rigid circuit board  280  increases, and therefore, the board design can be facilitated. 
     According to embodiments described herein, an optical transceiver module in which a control circuit for controlling a photoelectric conversion circuit can be easily reworked is provided. 
     Although specific embodiments have been described above, the present disclosure is not limited to the above-described embodiments. Variations, modifications, substitutions, additions, omissions, and combinations can be made to the described subject matter without departing from the scope of the present invention, and it is to be understood that such variations, modifications, substitutions, additions, omissions, and combinations obviously belong in the technical scope of the present invention.