Abstract:
An optical transmitter module includes light-emitting devices with coaxial type packaging. Coplanar anode and cathode electrodes of one light-emitting device are mounted on a substrate so that heat generated from the light-emitting device can be effectively dissipated through the substrate. Furthermore, the direct electric connection between the light-emitting device and the substrate eliminates the requirement of wire boding for electric connection, increasing the performance of the optical transmitter module in high speed signal operation.

Description:
This Non-provisional application claims priority under 35 U.S.C. § 119(a) on patent application Ser. No(s). 092137233 filed in Taiwan on Dec. 26, 2003, the entire contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The present invention relates to an optical transmitter module. In particular, the invention relates to an optical transmitter module constructed with coaxial type packaging. 
   2. Related Art 
   The optical communication industry has rapidly progressed to meet the demand of high-speed communication. A high-speed transmission system based on an optical-fiber requires a high-quality transceiver module. Techniques for optical transceiver module including high-speed devices technique, optical sub-modules technique, alignment technique and packaging technique. In the present state of the art, an optical sub-module is conventionally constructed with so-called coaxial type packaging, e.g. TO-CAN (transistor outline can) packaging. 
   Recently, the signal-processing speed of the optical sub-module has become increasingly higher. Because electrical connection in the convention TO-CAN packaging devices is achieved by wire bonding. The signal transmission speed in the conventional coaxial type or TO-CAN packaging device is usually limited to 2.5 Gbps. When the signal transmitting speed is up to 10 Gbps or higher, the coaxial type packaging device cannot efficiently work due to parasitic inductance of wire bonding and capacitance of package in high-frequency operation. The higher the signal transmitting speed, the more serious is the degradation of the signal quality, due to parasitic inductance, and capacitance of package and poor heat dissipation. Therefore, the reduction of parasitic inductance, and capacitance of package and better heat dissipation are critical to the future of coaxial type packaging products. 
   U.S. patent application Ser. No. 2002/0141142 discloses a coaxial type packaging, e.g. TO-CAN packaging device, that reduces the parasitic effects by adding a capacitor. However, the device is still need wire bonding, which also has the same prior problem in high speed signal transmission up to 10 Gpbs. 
   EP Patent No. 1267459 teaches mounting a light-emitting device on a large heat sink. The direct contact of the device with the heat sink increases the heat dissipation. Furthermore, conductive traces are formed on the heat sink to increase high-frequency features. However, forming traces on an edge of the heat sink is very difficult, and the electrical connection of the light-emitting device is still achieved by wire bonding, which adversely affects the yield and high-speed signal transmission. 
   SUMMARY OF THE INVENTION 
   Objects of the invention is to overcome the disadvantages of the prior art and provide an optical transmitter module having a plurality of devices mounted on a substrate with coaxial type packaging, e.g. TO-CAN packaging, which can be operated in high speed signal transmission up to 10 Gbps. To reduce signal transmitting path and parasitic inductance and capacitance of package in high frequency operation, a light-emitting device (such as laser, LED and so on) is bonded to the substrate by flip die method. And the heat produced by the device can be effectively dissipated through the substrate to outside. Signal transmission performance thereby is increased in high-frequency operation. 
   The optical transmitter module with coaxial type or TO-CAN packaging is used to convert an electric signal to an optical signal and then transmit the optical signal. The optical transmitter module includes a light-emitting device, a substrate, a base that has a plurality of pins and a cap. The light-emitting device is electrically connected via the substrate to the pins of the base. The cap is mounted on the base over the substrate to protect the light-emitting device. The cap includes a transparent region where light emitted from the light-emitting device can transmit through. Other devices such as optical detection devices or passive components are mounted on the substrate also and can be connected electrically to conductive traces on the substrate to reduce wire bonding. 
   The substrate has conductive traces through which the light-emitting device is electrically connected to other devices on the substrate. Furthermore, the conductive traces of the substrate are configured with adequate impedance to reduce parasitic impedance. The light-emitting device which having coplanar anode and cathode electrodes is electrically connected to the substrate by flip die method to reduce parasitic effects induced by wire bonding. An active region of the light-emitting device is proximity to the substrate so that heat generated by the active region can be dissipated through the substrate efficiently, to increase the performance of the signal transmission of the module at high temperature. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given herein below illustration, and thus are not limitative of the present invention, and wherein: 
       FIG. 1  is a schematic view of an optical transmitter module according to a first embodiment of the invention; and 
       FIG. 2  is a schematic view of an optical transmitter module according to a second embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the invention, all optical devices are integrally mounted on a substrate to overcome disadvantages of the prior art described above and improving heat dissipation at the same time. A laser diode, having coplanar anode and cathode electrodes, is used as a light-emitting device in the invention. The light-emitting device is bonded to the substrate by flip die method and is electrically connected through the substrate via conductive traces. Thereby, the limitation of parasitic inductance of wire bonding can be reduced and then signal transmission can be protected and high-speed performance of the optical transmitter module can be increased. 
     FIG. 1  is a schematic view of an optical transmitter module according to one embodiment of the invention. A laser diode  110 , having coplanar anode and cathode electrodes, is bonded on a substrate  100  and is electrically connected to conductive traces  120 . A monitor diode  130  is mounted on the same surface of the substrate  100  and electrically connected to the conductive traces  120 . The electrical connection between the monitor diode  130  and the conductive traces  120  can be achieved by flip die method or wire bonding. The parasitic impedance of conductive traces  120  can be controlled easily to instead of parasitic impedance of wire bonding. A base  200  includes at least one stand  211  where the substrate  100  can be bonded on. The base  200  further has a plurality of pins  210 . The laser diode  110  electrically connects to the pins  210  via the substrate  100  so that high-frequency driving signals can be transmitted to the laser diode  110  from the outside. The contact area between the substrate  100  and the stands  211  is large enough to conduct heat from the substrate to the outside of the optical transmitter module. Since all the devices are mounted on the same substrate, all the signals can be transmitted via the conductive traces of the substrate without additional wires, and then signal loss caused by parasitic impedance can be reduced when high-frequency signal transmission is implemented. 
   Furthermore, the laser diode  110 , having coplanar anode and cathode electrodes, is used as the light-emitting device and mounted on the substrate by flip die method. Because a light-emitting region of the light-emitting device generating a substantial amount of heat in operation is proximity to the substrate, the heat can dissipate away from the device by heat conduction to the substrate efficiently. Therefore, the device is not damaged as a result of poor heat dissipation, and the light-emitting efficiency of the module is not reduced. In addition, a cap (not shown) which may be made of metal can be mounted on the base to complete the TO-CAN packaging. The cap includes a transparent region through which light generated from the laser diode can penetrate. The transparent region is made of a transparent material or can be a lens. 
   Alignment keys can be further formed on the substrate for precise alignment of the laser diode. The cap and the base can be also precisely aligned relative to each other by means of these alignment keys. Thereby, the assembly time of the optical transmitter module can be reduced with increased yield. 
   In order to reduce cross-talk effects, an equipotential plane  300  can be further formed on the substrate. The laser diode  110  and the monitor diode  130  can electrically connect to the equipotential plane  300  and then connect to the base via a ground hole of the substrate  100 . Then the metal cap is mounted on the base to protect the devices mounted on the substrate  100  and implements a well shielding to prevent the cross-talk effects or electromagnetic influence. 
   In another embodiment of the invention, optical detection devices can be formed on another substrate coupled with the base or mounted directly on a carrier  220  connected to one of pins  210  electrically. Referring to  FIG. 2 , the laser diode  110  is mounted on a first substrate  100  by flip die method and electrically connected to the conductive traces  120 . The first substrate  100  is mounted to the stand  211  of the base  200 . The monitor diode  130  is mounted on a second substrate  101  and electrically connects to the first substrate  100  by flip die method or wire bonding. The second substrate  101  is mounted on the carrier  220  placed on the base  200  and electrically connects to the pins  210  of the base  200 . Thereby, high-frequency electric signals can be transmitted to the laser diode  110  from the outside of the module. Although the devices are separately formed on the first and second substrates  100 ,  101 , parasitic impedance can still be reduced when a high-frequency signal is transmitted because least wire bonding used. 
   Knowing the invention thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.