Abstract:
The present invention provides an optical module with effective thermal dissipation characteristic. The present optical module comprises a semiconductor optical device, a stem, plural lead terminals, a substrate and a base. The stem mounts the semiconductor optical device. The lead terminals extend along a predetermined axis from the stem. The base mounts the substrate on which an electronic circuit is provided. The edge portion of the base adjacent to the stem is made of a material containing the same ingredient as that contained in the stem, or the stem and the base are formed in unity.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of Invention  
           [0002]    This invention relates to an optical module and an optical network using the same.  
           [0003]    2. Related Prior Art  
           [0004]    A semiconductor laser with a can-type package is widely applied in an optical communication system. Such can-type package  82 , as shown in FIG. 5, is connected to a substrate  86  via lead terminals  84  and constitutes a light-emitting module  80 . The light-emitting module thus formed is installed into a chassis and forms an optical transmitting module disclosed in the Japanese patent laid open 2001-296458. In this patent, an optical transceiver is also disclosed, in which not only the optical-transmitting module but also an optical-receiving module are provided in the chassis.  
           [0005]    In the present and future optical communication system, further speed and capacity must be requested and accordingly thermally stable operation will be required to the optical-module used in such high-speed and high-capacity optical system. To realize the stable operation in the optical module, heat generated in the can-type package and generated by electrical components mounted on the substrate must be effectively dissipated outside the package. Therefore, an object of the present invention is to provide an optical module with superior thermal dissipation characteristic and an optical communication system using such optical modules.  
         SUMMARY OF THE INVENTION  
         [0006]    According to one aspect of the present invention, an optical module comprises a semiconductor optical device, a stem, a plurality of lead terminals, a substrate and a base. The stem mounts the semiconductor optical device. The plurality of lead terminals extends along a predetermined axis from the stem. The base mounts the substrate on which an electronic circuit is provided. The electronic circuit is electrically connected to the semiconductor optical device via lead terminals.  
           [0007]    Since the optical module thus configured provides the base extending along the predetermined axis from the stem and the substrate is mounted on the base, heat generated by the semiconductor optical device is transmitted to the base via the stem and heat generated by electronic components constituting the electronic circuit and mounted on the substrate is directly transmitted to the base, thereby facilitating heat dissipation.  
           [0008]    In the present optical module, the stem may be made of a first material, and the base may be made of a second material. Conventional module has a stem made of CuW, which is comparatively costly material. Since the present invention has the base made of different material to the stem, the base may be made of relatively economical material, such as aluminum, thereby reducing the cost of the module without fang the effective heat dissipation.  
           [0009]    In the optical module, an edge portion of the base may contain the same material as that constituting the stem. When the stem and the base are made of different material, a thermal stress between the lead terminals and the substrate, which originates in different thermal expansion co-efficient of respective materials, may bring an electrical defect therebetween. By containing the same material as the stem in a edge portion of the base, a stress induced between the lead terminals and the substrate may relax, thereby enhancing the reliability of the module.  
           [0010]    The ingredient of the first material to the second material at the edge portion may gradually decrease from the stem to the portion apart therefrom. This configuration further relaxes the discrepant thermal expansion co-efficient.  
           [0011]    Sintering preferably forms the base, which is most suitable technique to form the edge portion of the base because the ingredient thereof is varied continuously.  
           [0012]    The stem and the base are preferably made of the same material and are formed in unity. This enhances the thermal dissipation characteristic without taking the thermal expansion co-efficient into account.  
           [0013]    The lead terminals and the base preferably sandwich the substrate therebetween, which facilitates the positioning of the substrate and effects the manufacturing. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0014]    [0014]FIG. 1 is a plan view showing an embodiment of the present invention;  
         [0015]    [0015]FIG. 2 is a side view of the embodiment;  
         [0016]    [0016]FIG. 3 shows a structure of the package with removing the substrate;  
         [0017]    [0017]FIG. 4 shows a configuration of the optical network; and  
         [0018]    [0018]FIG. 5 is a plan view showing the conventional light-emitting module.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0019]    Preferred embodiments of the present invention will be described as referring to accompany drawings. In the description of drawings, elements identical to each other will be referred to with numerals identical to each other without overlapping explanations.  
         [0020]    [0020]FIG. 1 is a plan view of a light-emitting module  10  according to the present embodiment and FIG. 2 is a side view of the light-emitting module  10 . In FIGS. 1 and 2, a package  12  of the light-emitting module is partially cross-sectional view to show the inside configuration.  
         [0021]    As shown in FIG. 1 and FIG. 2, the light-emitting module  10  comprises a package  12  and a substrate  14 . The package  12  has a primary portion  18  for enclosing a semiconductor laser  16  and a base  20  for mounting the substrate  14 .  
         [0022]    The primary  18  includes a stem  22 , the semiconductor laser  16 , a plurality of lead terminals  24  and a casing  26  of a can type. The stem  22  is made of a metal such as CuW, Kovar and iron, and has a shape of a disk. The stem  22  has a primary surface  22   a , where the semiconductor laser  16  is mounted via a sub-mount  32 , and another surface  22   b  opposite to the primary surface  22   a . The stem  22  is arranged such that the primary surface  22   a  intersects an optical axis with a right angle.  
         [0023]    The semiconductor laser  16  is a type of Fabry-Perot (FP) laser diode and/or a type of distributed feedback (DFB) laser diode. The semiconductor laser  16  is mounted on the primary surface  22   a  of the stem. Namely, as shown in FIG. 2, the laser diode  16  is mounted on a sub-mount  28  made of a material with high thermal conductivity such as aluminum nitride (AlN), and the sub-mount  28  is disposed on the Peltier element  30 . Further, The Peltier element  30  is disposed on a heat sink  32 , which is made of a material with a high thermal conductivity such as CuW, and the heat sink  32  is mounted on the primary surface  22   a  of the stem  22 . In one side of the semiconductor laser opposite to a direction to which the light is emitted, a light-receiving device  34  is disposed to monitor the operation of the semiconductor laser  16 .  
         [0024]    The plurality of lead terminals  24  pierce the stem  22  and extends along the optical axis X. One of the lead terminals electrically connects to the semiconductor laser  16  via a bonding-wire, which is not shown in drawings. Another lead terminal is connected to the light-receiving device  34 .  
         [0025]    The casing  26  has a cylindrical shape. Inner surface of the casing  26  has a disk shaped projection  36  with an opening in the center thereof. Within the opening, a spherical lens  38  is secured. The casing  26  covers the primary surface  22   a  of the stem  22 , thereby enclosing the semiconductor laser  16 , the light-receiving device  34 , the sub-mount  28 , the Peltier element  30  and the head sink  32  therein.  
         [0026]    Thus, the primary portion  18  of the package  12  is constituted, which forms a can-package similar to a conventional package. However, the package according to the present invention further includes the base  20  for mounting the substrate  14 . The base  20  may be formed independent to the stem  22  and may be assembled mechanically later, or may be integrally formed with the stem  22  from the thermal characteristic viewpoint.  
         [0027]    [0027]FIG. 3 shows the package  12  as the substrate  14  is removed from the base  20 . As shown in FIG. 3, the base  20  extends continuously from the surface  22   b  of the stem  22  along the optical axis X. The base has a shape of substantially rectangle sheet and has a surface  20   a  for securing the substrate  14  thereon.  
         [0028]    The substrate  14  is made of ceramics, on which an electronic circuit is formed for driving the semiconductor laser  16 . The substrate  14  is mounted on the surface  20   a  of the base and connected to lead terminals  24 .  
         [0029]    The base  20  of the package  12  is made of metal such as aluminum or copper, which is different metal of the stem  22 . The edge portion  20   b  of the base  20  may contain same materials, for example CuW, which is included in the stem  22 . Further, the content of such material in the edge portion  20   b  gradually preferably decreases as the position is apart from the stem  22  along the optical axis X. Where, the gradual decrease includes not only continuous decreasing but also stairs-like decreasing.  
         [0030]    Such edge portion  20   b  of the stem may be formed by the sintering, namely, metals in powder form are shaped to a predetermined form and hot-pressed. The sintering is not necessary for melting and casting of source metals. Therefore, the sintering is suitable to form the edge portion  20   b  of the base  20 . A length of the edge portion is preferably between from 10% to 20% of the total length of the base  20  measured along the optical axis X. Moreover, the substrate  14  of the present embodiment is preferably sandwiched between lead terminals  24  and the base  20 .  
         [0031]    Next, a function and a way of the present light-emitting module will be described hereinbelow.  
         [0032]    The light-emitting module  10  has the base  20  that extends along the optical axis X from the surface  22   b  of the stem  22 . The substrate  14  is mounted on the surface  20   a  of the base  20 . Therefore, heat generated by the semiconductor laser  16 , and by the electronic part when the driving circuit is involved in the package, is transmitted to the base  20  via the stem  22 . The heat generated by electronic parts mounted on the substrate  14  is directly transmitted to the base  20 . Thus, the heat can be transmitted through wide area of the base  20 , which enhances efficiency of the heat dissipation of the module  10 . Thicker stem  22  along the optical axis X also enhances the heat dissipation from the package  12  because the surface area of the package increases. In this case, care must be paid so as not to enlarge the size of the module  10 . The present light-emitting module  10  has the base  20  extending along the substrate  14 , which does not bring the enlargement of the module  10 .  
         [0033]    Generally, the stem  22  of the conventional module is made by comparably costly material such as CuW. However in the present embodiment, since the base  20  is made of comparably low-priced metal such as aluminum, the heat dissipation and the low cost can be compatible.  
         [0034]    As described previously, the edge portion  20   b  of the base  20  may contain same materials as the stem  22  contains, for example when the stem  22  is made of CuW and the base  20  is made of aluminum, the edge portion  20   b  is made of an alloy of CuW and aluminum. This configuration relaxes the difference in thermal expansion coefficient of respective materials, thereby decreasing the inferior electrical contact between the lead terminals and the wiring on the substrate.  
         [0035]    Further, since the present light-emitting module  10  has the configuration that the base  20  is fixed mechanically and electrically to the stem  22 , a stray inductance can be reduced, thereby stabilizing the ground line.  
         [0036]    Although descriptions above are solely concentrated to the light-emitting module  10 , the concept and the spirit of the present invention can be applied to a light-receiving module which comprises a semiconductor light-receiving device such as photo diode instead of the semiconductor laser. In such light-receiving module, heat generated by the light-receiving device and, by a pre-amplifier if the module contains therein, can be transmitted to the base  20  via the stem  22 . The heat generated by electrical components mounted on substrate  14  transmits to the base  20 , thereby facilitating the heat dissipation through a wider area compared to a conventional module. In the light-receiving module, the light-receiving device  34  for monitoring the optical output from the light-emitting device is not necessary.  
         [0037]    These light-emitting module and light-receiving module described above are installed in a chassis, not shown in drawings, and constitutes an optical-transmitting module or optical-receiving module, respectively, or both the light-emitting module and the light-receiving module are installed in a chassis and constitute an optical transceiver.  
         [0038]    The optical network  50  includes a plural optical-transmitting module  52 , an optical multiplexer  56 , an optical transmission line  56 , an optical amplifier  58 , an optical de-multiplexer  60  and a plural optical-receiving module  62 . Each optical-transmitting modules  52  and optical-receiving modules  62  include the light-emitting module or the light-receiving module of the present invention. The optical multiplexer  56  multiplexes optical signals with respective wavelengths output from optical-transmitting modules  52 . The optical de-multiplexer  60  divides optical signals transmitted through the optical transmission line  56  into optical signals with respective specific wavelength output to optical-receiving modules  62 . Since the light-emitting module or the light-receiving module of the present invention has a superior thermal dissipation configuration, the optical network shown in FIG. 4 develops a thermal stability even in a high-speed and a high-capacity transmission.  
         [0039]    From the invention thus described, it will be obvious that the invention may be varied in many ways. For example, the light-emitting module  10  or the light-receiving module has the configuration that the stem  22  and the base  20  are formed independently and the mechanically secured to each other. However, the both members  20  and  22  may be formed in unity. In such configuration, the thermal dissipation efficiency can be enhanced without taking the difference of the thermal coefficient of both members into account. The manufacturing of such unified member can be also simplified.  
         [0040]    Further, although a “point to point” configuration for the optical network is shown in FIG. 4, the network does not restricted to such configuration. For example, the present invention is applicable to a network with a ring configuration or a mesh configuration without any modification of the invention.