Optical communication module

An optical communication module includes a module board housed in a casing, a VCSEL and a driving IC mounted on a mounting surface of the module board, a lens holder mounted on the mounting surface of the module board, a lens block held by the lens holder, a plurality of thermal vias passing through the module board, and a first fixing screw and a second fixing screw passing through the module board to be screwed into the casing so as to press a back surface of the module board against a bottom surface of the casing, and the first fixing screw and the second fixing screw are each arranged in a region between the plug connector and the lens holder and on either outer side of the lens holder.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese Patent Application No. 2016-091766 filed on Apr. 28, 2016, the content of which is hereby incorporated by reference into this application.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to an optical communication module having a photoelectric conversion function.

BACKGROUND OF THE INVENTION

A general optical communication module has a board and a casing that houses the board. A board included in an optical communication module is generally called a “module board” and is distinguished from a board (generally called a “host board”) included in a communication device, a transmission apparatus, etc. to which a communication module is connected. In the following description, in accordance with the above-mentioned distinction, the board included in the optical communication module may be referred to as a “module board,” and the board included in the communication device or the transmission apparatus may be referred to as a “host board” in some cases. Also, a communication device, a transmission apparatus, and the like to which a communication module is connected may be collectively referred to as a “communication device.”

A plurality of elements are mounted on the module board. For example, a light emitting element and a driving element driving the light emitting element are mounted on the module board, and a light receiving element and an amplifying element amplifying electric signals output from the light receiving element are mounted thereon. Further, a lens optically coupling the light emitting element and the light receiving element with an optical fiber is provided on the module board.

Various elements mounted on the module board emit heat during the operation, and in particular, the driving element generates a large amount of heat. In view of this, a technique has been proposed in which heat generated from an element mounted on the module board is transferred to the casing by a thermal via provided in the module board so as to be radiated from the surface of the casing (Japanese Patent Application Laid-Open Publication No. 2015-92524).

In addition, a technique has been proposed in which a board on which an electronic component is mounted and a casing that houses the board are fastened by a bolt screw to reduce a contact thermal resistance between the board and the casing (Japanese Patent Application Laid-Open Publication No. 2014-36033). To be specific, Japanese Patent Application Laid-Open Publication No. 2014-36033 discloses a way of pressing the board against the casing while applying pressure to the board by tightening a bolt screw.

SUMMARY OF THE INVENTION

When a module board is fixed to a casing with screws and the module board is pressed against the casing, reduction in thermal resistance between the module board and the casing can be expected.

However, when the module board is fixed to the casing with the screws, a position of the screw becomes a fixed point of the module board with respect to the casing. In this case, when force is applied to the module board, stress concentrates on the fixed point. For example, when a plug connector provided on the module board is inserted into or removed from a receptacle connector provided on a host board, force is applied to the module board provided with the plug connector. At this time, when the module board is fixed to the casing with the screws, stress concentrates at the fixed point (screw position or its vicinity).

Meanwhile, the module board constituting the optical communication module is thinner than the circuit board constituting the power semiconductor or the like, and there is a possibility that the board is distorted or tilted even by small force. In addition, the module board constituting the optical communication module is provided with a lens optically coupling the light emitting element and the light receiving element with an optical fiber, and the lens is held by a holding member fixed to the module board. The lens optically coupling the light emitting element and the light receiving element with the optical fiber is positioned with high accuracy with respect to the light emitting element, the light receiving element, and the optical fiber. In other words, the respective optical axes of the lens, the light emitting element, the light receiving element, and the optical fiber are aligned with one another with high accuracy.

Therefore, there is a concern that, when force is applied to the module board, if distortion or tilting of the module board occurs at the position where the holding member that holds the lens is located or in the vicinity thereof, the optical axes of the lens, the light emitting element, the light receiving element, and the optical fiber may be shifted. Further, when the holding member is adhered to the module board, there is a possibility that the mating surface may be peeled off.

In other words, when the module board is fixed to the casing with the screws, reduction in thermal resistance between the module board and the casing can be expected, but another problem specific to the optical communication module such as the misalignment of the optical axes may arise depending on the fixed position.

An object of the present invention is to reduce the thermal resistance between the module board and the casing by pressing the module board against the casing while preventing distortion and tilting of the module board at the position where the holding member is located or in the vicinity thereof.

An optical communication module according to the present invention has a photoelectric conversion function and includes a casing made of metal, a board housed in the casing and having an electrical connector at one end thereof, a light emitting element mounted on a front surface of the board, a driving element mounted on the front surface of the board and driving the light emitting element, a holding member mounted on the front surface of the board so as to cover the light emitting element and the driving element, an optical fiber optically coupled with the light emitting element, an optical member held by the holding member and optically coupling the light emitting element with the optical fiber, a plurality of thermal vias which pass through the board and each of which has a first end face exposed on the front surface of the board and a second end face exposed on a back surface of the board, and a first fixing screw and a second fixing screw which pass through the board and are screwed into the casing and which press the back surface of the board against a bottom surface of the casing. At least one of the thermal vias has the first end face thermally connected to the driving element and the second end face thermally connected to the bottom surface of the casing, and the first fixing screw and the second fixing screw are each disposed in a region between the electrical connector and the holding member and on either outer side of the holding member.

According to an aspect of the present invention, the first fixing screw and the second fixing screw are disposed line-symmetrically with respect to a center line of the board as a symmetry axis.

According to another aspect of the present invention, the holding member includes a rear face to which an optical connector provided at an end of the optical fiber is connected, a front face located opposite to the rear face, and a first side face and a second side face each intersecting the rear face and the front face. Then, the first fixing screw is disposed in a region between the electrical connector and the front face and in an outer region than the first side face, and the second fixing screw is disposed in a region between the electrical connector and the front face and in an outer region than the second side face.

According to another aspect of the present invention, the optical communication module includes a heat conductive material interposed between the back surface of the board and the bottom surface of the casing.

According to another aspect of the present invention, the optical communication module includes a light receiving element mounted on the front surface of the board, and an amplifying element mounted on the front surface of the board and amplifying an electric signal output from the light receiving element. The holding member is mounted on the front surface of the board so as to cover the light emitting element, the driving element, the light receiving element, and the amplifying element. The plurality of thermal vias include at least one thermal via in which the first end face is thermally connected to the amplifying element and the second end face is thermally connected to the bottom surface of the casing.

According to the present invention, thermal resistance between the module board and the casing can be reduced by pressing the module board against the casing while distortion and tilting of the module board at the position where the holding member is located or in the vicinity thereof is prevented.

DESCRIPTIONS OF THE PREFERRED EMBODIMENT

Hereinafter, an embodiment of the present invention will be described by way of example. An optical communication module1shown inFIG. 1is connected to a host board provided in a communication device or a transmission apparatus such as a server or a network switch. The optical communication module1has a photoelectric conversion function so as to convert an optical signal into an electric signal and convert an electric signal into an optical signal. An electrical connector (plug connector2) is provided at the tip of the optical communication module1, and the plug connector2is connected to an electrical connector (receptacle connector) provided on the host board. That is, the optical communication module1according to the present embodiment includes the plug connector2that can be inserted into and removed from the receptacle connector provided in the host board, and when the plug connector2is inserted into the receptacle connector, a module board5and the host board are connected. That is, the optical communication module1and the communication device are connected. Conversely, when the plug connector2is removed from the receptacle connector, the connection between the module board5and the host board is released. That is, the connection between the optical communication module1and the communication device is released. Further, the optical communication module1connected to the communication device performs parallel communication at high speed (25 Gbit/s or higher).

A semiconductor chip for communication is mounted on the host board of the communication device to which the optical communication module1is connected, and the optical communication module1connected to the communication device is connected to the semiconductor chip for communication via a signal wire formed on the host board. In addition, a plurality of receptacle connectors are provided on the host board, and the plurality of optical communication modules1are connected to the semiconductor chip for communication via the respective receptacle connectors.

The optical communication module1has a casing4to which one end side of an optical fiber cable3is connected, and a board (module board5) housed in the casing4. The module board5is a multilayer board, and the plug connector2is an edge connector provided on one side of the module board5. Note that the casing4is composed of a lower case4athat is illustrated and an upper case that is not illustrated. The lower case4aand the upper case are made of metal and constitute the casing4having a space capable of housing the module board5by being butted to each other.

A photoelectric conversion unit6is provided on the module board5. Although not shown inFIG. 1, the photoelectric conversion unit6is composed of a light emitting element, a driving element, a light receiving element, an amplifying element, and the like mounted on a front surface5aof the module board5. In the following description, the front surface5aof the module board5on which the photoelectric conversion unit6is provided may be referred to as a “mounting surface5a.”

As shown inFIG. 2, a vertical cavity surface emitting laser (VCSEL)10as the light emitting element, a driving IC11as the driving element, a photodiode (PD)20as the light receiving element, and a transimpedance amplifier (TIA)21as the amplifying element are mounted on the mounting surface5aof the module board5. Further, a lens block30as an optical member optically coupling the VCSEL10and the PD20with an optical fiber3awhich is the core wire of the optical fiber cable3(FIG. 1) is provided on the mounting surface5a. The lens block30has a plurality of lens elements and a reflecting surface and is held by a holding member40made of a resin and mounted on the mounting surface5aof the module board5. The holding member40holds the lens block30at a predetermined position on the module board5and is adhesively fixed to the mounting surface5aof the module board5. In the following description, the holding member40may be referred to as a “lens holder40” in some cases.

The lens holder40is mounted on the mounting surface5aso as to cover the VCSEL10, driving IC11, PD20, and TIA21mounted on the mounting surface5a, and the optical fiber3ais connected to a rear face41of the lens holder40. In other words, one surface to which the optical fiber3ais connected is the rear face41of the lens holder40. Specifically, an optical connector50provided at the end of the optical fiber3ais butted to a joint surface31of the lens block30exposed on the rear face41of the lens holder40, and the VCSEL10and the PD20are optically coupled with the optical fiber3avia the lens block30. More specifically, the optical connector50is a mechanically transferable (MT) connector, and a tip surface51of the optical connector50is butted to the joint surface31of the lens block30. Two guide holes are provided in the tip surface51of the optical connector50while two guide pins protrude from the joint surface31of the lens block30, and each guide pin is inserted into a corresponding guide hole. However, a connecting structure between the lens block30and the optical fiber3ais not limited to the above structure, and any other known or new connecting structure can be adopted arbitrarily.

A plurality of signal wires are provided on the mounting surface5aof the module board5. For example, a signal wire electrically connecting the driving IC11and the plug connector2, a signal wire electrically connecting the TIA21and the plug connector2, and the like are provided on the mounting surface5a. In addition, the driving IC11and the VCSEL10are mounted on the mounting surface5aby bare-chip mounting, and the driving IC11and the VCSEL10are electrically connected via a bonding wire. Similarly, the TIA21and the PD20are mounted on the mounting surface5aby bare-chip mounting, and the TIA21and the PD20are electrically connected via a bonding wire. Here, the bare-chip mounting means a mounting structure or a mounting method in which elements (VCSEL10, PD20, etc.) and ICs (driving IC11, TIA21, etc.) are mounted on a board in a state of a chip as it is and electrodes on the chip and electrodes on the board are connected via bonding wires or the like.

The electric signal input to the plug connector2is transmitted to the driving IC11via the signal wire provided on the mounting surface5a. The driving IC11to which the electric signal is input outputs an electric signal (drive signal) in response to the input electric signal. The drive signal output from the driving IC11is transmitted to the VCSEL10via the bonding wire. The VCSEL10to which the drive signal is input outputs an optical signal in response to the input drive signal. The traveling direction of the optical signal output from the VCSEL10is changed by the lens block30, and the optical signal is made incident on the optical fiber3a.

Meanwhile, the optical signal emitted from the optical fiber3ais made incident on the light receiving element20after its traveling direction is changed by the lens block30. The light receiving element20to which the optical signal is input outputs an electric signal in response to the input optical signal. The electric signal output from the light receiving element20is transmitted to the TIA21via the bonding wire. The TIA21to which the electric signal is input amplifies the input electric signal and outputs the amplified signal. The electric signal output from the TIA21is transmitted to the plug connector2via the signal wire.

Note that the signal wire described here is apart of the signal wire provided on the module board5. Signal wires and ground wires (ground layers) other than the above-mentioned signal wires are provided on the mounting surface5aof the module board5as needed, and signal wires and ground wires (ground layer) are provided on the inner layer of the module board5as needed.

FIG. 1is referred to again. The module board5is fixed to the casing4by a first fixing screw61and a second fixing screw62passing through the module board5. The first fixing screw61and the second fixing screw62pass through the module board5and are screwed into the bottom of the casing4. As shown inFIG. 3, a stage63higher than the periphery is integrally formed at the bottom of the casing4(lower case4a), and the module board5is placed on the stage63. A screw hole64having a female screw is formed in the stage63, and the first fixing screw61is screwed into the screw hole64. Though not shown inFIG. 3, a screw hole corresponding to the second fixing screw62(FIG. 2) is also formed in the stage63, and the second fixing screw62is screwed into this screw hole. That is, two screw holes are formed in the stage63, the first fixing screw61is screwed into one screw hole, and the second fixing screw62is screwed into the other screw hole.

Therefore, when the first fixing screw61and the second fixing screw62shown inFIGS. 2 and 3are tightened, the module board5is sandwiched between heads61aand62aof the respective fixing screws61and62and the stage63, and aback surface5bof the module board5is pressed against an upper surface63aof the stage63which is a part of the bottom surface of the casing4. It is apparent from the drawing that the back surface5bof the module board5pressed against the upper surface63aof the stage63is one surface of the module board5opposite to the mounting surface5a.

As shown inFIG. 3, a plurality of thermal vias70passing through the module board5are formed in the module board5. A first end face70aof each thermal via70is exposed on the mounting surface5aof the module board5, and a second end face70bof each thermal via70is exposed on the back surface5bof the module board5. In other words, one end face of the thermal via70exposed on the mounting surface5aof the module board5is the first end face70a, and the other end face of the thermal via70exposed on the back surface5bof the module board5is the second end face70b.

As shown inFIG. 3, the first end face70aof at least one thermal via70is in contact with a bottom surface of the driving IC11and is thermally connected to the driving IC11. Also, the second end face70bof the thermal via70thermally connected to the driving IC11is thermally connected to the upper surface63aof the stage63. In addition, the first end face70aof at least one different thermal via70is in contact with a bottom surface of the VCSEL10and is thermally connected to the VCSEL10. Also, the second end face70bof the thermal via70thermally connected to the VCSEL10is thermally connected to the upper surface63aof the stage63. Here, a heat radiation sheet71as a heat conductive material is interposed between the back surface5bof the module board5and the upper surface63aof the stage63. As a result, the second end face70bof each thermal via70is thermally connected to the upper surface63aof the stage63via the heat radiation sheet71. Note that a thickness of the heat radiation sheet71in the present embodiment is 100 μm or less.

As described above, when the first fixing screw61and the second fixing screw62shown inFIGS. 2 and 3are tightened, the back surface5bof the module board5is pressed against the upper surface63aof the stage63. In the present embodiment in which the heat radiation sheet71is interposed between the back surface5bof the module board5and the upper surface63aof the stage63, the back surface5bof the module board5and the second end face70bof each thermal via70are pressed against the heat radiation sheet71. As a result, thermal resistance between the back surface5bof the module board5and the second end face70bof each thermal via70, and the upper surface63aof the stage63is reduced, and heat radiation effect is improved. That is, thermal resistance between the module board5and the casing4is reduced, and heat radiation effect of heat emitted from the element mounted on the module board5is improved.

Further, as shown inFIG. 1, the first fixing screw61and the second fixing screw62are disposed between the plug connector2and the photoelectric conversion unit6. To be more specific, as shown inFIG. 2, the first fixing screw61and the second fixing screw62are each disposed in a region between the plug connector2and the lens holder40and on either outer side of the lens holder40. Further, the first fixing screw61and the second fixing screw62are disposed line-symmetrically with respect to a center line X of the module board5, which divides one side of the module board5on which the plug connector2is provided into two, as the symmetry axis. Hereinafter, positions of the first fixing screw61and the second fixing screw62on the module board5will be described in more detail.

In addition to the rear face41, the lens holder40is provided with a front face42positioned on the side opposite to the rear face41. Further, the lens holder40includes a first side face43and a second side face44which intersect the rear face41and the front face42, respectively, and are parallel to each other. The first fixing screw61is disposed in a region between the plug connector2and the front face42of the lens holder40and in an outer region than the first side face43of the lens holder40. On the other hand, the second fixing screw62is disposed in a region between the plug connector2and the front face42of the lens holder40and in an outer region than the second side face44of the lens holder40. As a result, an extension line of the first side face43of the lens holder40passes through a region on the inner side of the head61aof the first fixing screw61, and an extension line of the second side face44of the lens holder40passes through a region on the inner side of the head62aof the second fixing screw62.

As described above, since the module board5is pressed against the casing4by the first fixing screw61and the second fixing screw62in the present embodiment, the thermal resistance between the module board5and the casing4is reduced, and the heat radiation effect of the heat emitted from the element mounted on the module board5is improved. Improvement of the heat radiation effect is effective in parallel communication that generates a large amount of heat and is particularly effective in high-speed parallel communication with a larger amount of heat generation (25 Gbit/s or more).

Further, in the present embodiment, the first fixing screw61and the second fixing screw62which fix the module board5to the casing4are disposed between the plug connector2and the lens holder40. In other words, the fixed point of the module board5with respect to the casing4is located between the plug connector2and the lens holder40. Therefore, the force applied to the module board5when the plug connector2is inserted or removed is prevented from reaching the position of the lens holder40, or the force reaching the position of the lens holder40is at least reduced. Hence, occurrence of distortion and tilting of the module board5at the position where the lens holder40is located and its vicinity is prevented or suppressed, and the optical axis misalignment and peeling-off of the lens holder40are prevented, and thus, a degree of accuracy of the optical system is improved as a whole. Improvement in degree of accuracy of such an optical system is particularly effective in parallel communication which is easily affected by distortion of the optical system. Also, prevention or suppression of occurrence of distortion and tilting of the module board5is suitable for bare-chip mounting in which objects to be mounted are extremely small and precise mounting is required.

Further, the first fixing screw61and the second fixing screw62which press the module board5against the casing4are disposed line-symmetrically with respect to the center line X of the module board5as the symmetry axis. Accordingly, pressure caused by tightening the first fixing screw61and pressure caused by tightening the second fixing screw62equally act on the module board5. Therefore, since the module board5is not distorted or tilted due to the tightening of the first fixing screw61and the second fixing screw62, no optical axis misalignment occurs in the photoelectric conversion unit6, and no gap is generated between the back surface5bof the module board5and the heat radiation sheet71. In addition, when the back surface5bof the module board5and the upper surface63aof the stage63are adhered with a thermally conductive adhesive or the like, separation between the back surface5bof the module board5and the upper surface63aof the stage63is prevented.

The present invention is not limited to the above-described embodiment, and various modifications can be made within a range not changing its essential features. For example, third and fourth fixing screws can be added, provided that the first fixing screw and the second fixing screw are disposed in the above-mentioned region.