Abstract:
The present invention provides a new arrangement of the stem and the cap resistance-welded to the stem, in which the deformation induced by the welding in a portion adjacent to the projection does not cause the failure for the assembly of the module. The cap and the stem of the present module provided first and second portions. The diameter of the second portion is greater than that of the first portion such that, even the welding deforms the first portions of the cap and the stem; the deformed portion is inside of respective second portions.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an optical module that provides a body with a stem where a semiconductor optical device is mounted thereon and extends a plurality of lead pins therefrom, and a cap resistance-welded to the stem to form a cavity where the semiconductor optical device is air-tightly sealed. 
   2. Related Prior Art 
   An optical module generally provides the body, which includes the stem that is made of metal, mounts the semiconductor optical device and holds the lead pins air-tightly sealed by the seal glass, and the metal cap with a glass plate or a lens in a top thereof to pass the optical signal therethrough. The cap is typically resistance-welded to the metal stem. 
   In the resistance-welding, at least one of the cap and the stem forms a rib or interposes a member with a projection between the cap and the stem, and a quite large current is supplied and concentrated on the tip portion of the projection to melt the projection as pressing the cap and the stem to each other to fix both members. 
   A Japanese Patent Application published as JP-H06-140644A has disclosed an optical module that provides a lens holder covering the cap and providing a V-groove into which members for the resistance-welding are set. This arrangement can narrower the are coming in contact at the welding, which enables to increase the current density for the welding, that is, the total current may be reduced and the excess heating of the devices installed in the optical module may be escaped. However, the arrangement of the lens holder mentioned above covers the side of the stem where the projection is also provided. Although it reliably secures the air-tightness, the arrangement inevitably widens the module. 
   The resistance-welding is necessary to press the members to be welded to each other, which causes the deformation of the welded portions because the mechanical stress is applied to a portion where the Joule heat is generated at the welding. For the optical module, as shown in the prior document mentioned above, the cap is resistance-welded to the stem. The portion adjacent and periphery to the projection of the cap or the stem may be stuck out compared to those before the welding. 
   Another Japanese Patent Application published as JP-2003-320462A has suggested an arrangement that enables to escape the module from the above subject, in which the projection accompanies with hollows in both sides of the projection. The melted and deformed portion of the projection may be set within the hollows. However, this arrangement of the projection may be unfavorable from the viewpoint of the airtightness after the resistance-welding. 
   The present invention is to provide an arrangement for the resistance-welding that enables, even when the welding accompanies the mechanical deformation and the large Joule heat, to absorb the deformation and to escape the device in the module from the heating. 
   SUMMARY OF THE INVENTION 
   An optical module according to the present invention has a metal cap and a metal stem that constitute, what is called, a co-axial package, where the metal cap is fixed to the stem by the resistance-welding. The cap of the invention provides first and second portions with first and second diameters, respectively. The second diameter is greater than the first diameter by an amount such that an end portion of the cap deformed by the resistance welding is within the outer most surface of the second portion. Similarly, the stem provides first and second portions with third and fourth diameters, respectively. The fourth diameter is greater than the third diameter by an amount such that an end of the second portion of the stem deformed by the resistance-welding is within the outermost surface of the second portion of the stem. 
   In another embodiment of the present invention, an optical module with the co-axial package comprises a metal stem and a metal cap fixed to the stem by the resistance-welding. The stem provides a groove in a primary surface thereof to which the metal cap is to be resistance-welded. The groove is formed along an inner edge of a ring rib formed in an end of the cap such that the groove may separates the ring rib from the lead pin extending from the stem. The groove may have a rectangular cross section or a triangular cross section, and may be divided in a plurality of arched portion such that each portion has a same radius to each other and separates the corresponding lead pin from the ring rib of the metal cap. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1A  is a perspective view showing an optical module according to the first embodiment of the present invention, and  FIG. 1B  is a cross section of the module; 
       FIG. 2  magnifies a portion to be resistance-welded of the optical module shown in  FIG. 1 ; 
       FIG. 3A  is a cross section of the module whose cap is welded to the stem, and  FIG. 3B  magnifies the welded portion of the cap and the stem; 
       FIG. 4A  is a cross section of the cap and the stem to be welded to each other according to the second embodiment of the invention, and  FIG. 4B  is a plan view of the stem shown in  FIG. 4A ; 
       FIG. 5  magnifies the welded portion of the cap and the stem shown in  FIGS. 4A and 4B ; 
       FIG. 6A  illustrates a thermal distribution at the welding for the arrangement of the second embodiment shown in  FIGS. 4A and 4B , while,  FIG. 6B  shows a thermal distribution of the conventional arrangement; and 
       FIG. 7A  is a cross section of the cap and the stem to be welded to each other according to the modification of the second embodiment shown in  FIGS. 4A and 4B , and  FIG. 7B  is a perspective view of the stem of the modified embodiment. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
   Next, preferred embodiments of the present invention will be described as referring to accompanying drawings. In the description, the same numerals or the same symbols will refer to the same elements without overlapping explanations. 
   First Embodiment 
     FIGS. 1A and 1B  show an optical module according to an embodiment of the present invention. The optical module shown in  FIGS. 1 and 2  provides the cap  2  and the body  3  constituting a co-axial package, where the cap  2  is fixed to the body  3  by the resistance-welding. The cap  2  provides two portions, one of which has relatively larger diameter, while, the other has the smaller diameter. The cap  2  also provides a step between these two portions and a lens. The cap may be made of meal such as stainless steel and fixes a lens  4  on the top thereof. 
   The body  3  includes a stem  3 A with a disk shape and a plurality of lead pins  3 B extending from the stem  3 A. The stem  3 A provides a first portion  31  with relatively smaller diameter and a primary surface  30 , where a semiconductor optical device is to be mounted thereon, and a second portion with relatively larger diameter, from which the lead pins  3 B extend to a direction opposite to the primary surface  30 . These cap  2  and the body, in particular the step  3 A, forms a cavity that is air-tightly sealed and encloses the semiconductor optical device therein. 
     FIG. 2  magnifies a portion where the cap  2  is welded to the stem  3 A before the welding. The end of the cap  2  provides, as illustrated in  FIG. 2 , a first lib  21 B with a rectangular cross section, which has a height of L and faces the primary surface  30  of the stem  3 A. The first lib  21 B is deformed when the cap  2  is fixed to the stem  3 A as pressing the cap against the stem  3 A. Accordingly, the height L may be determined so as not to project from the outermost surface  22  of the cap  2 . 
   The cap  2  further provides a second rib  20 A with a triangular cross section on the top  20  of the first rib  21 B. The force applied at the resistance-welding concentrates on the tip of the triangle; accordingly, this tip of the triangle constitutes a melting portion at the welding. 
   The stem  3 A, which has a disk shape and is made of metal such as iron (Fe) or Kovar, includes a first portion with a relatively smaller diameter and a second portion with a relatively larger diameter. The first portion mounts the semiconductor optical device on the primary surface  30  thereof. The first portion  31 , similar to the end of the cap  2 , has a thickness S and provides a room such that, when the cap  2  is welded to the stem  3 A, the deformed portion of the cap  2  and that of the stem  3 A do not project from the outermost surface of the stem  3 A. The first portion may be lathed by a depth corresponding to the step  31 A in  FIG. 2  from the second portion with the larger diameter. The step  31 A may be smaller than a sum of the step  21 D and the step  21 C in the rib structure of the cap  2 , and may be larger or substantially equal to the step  21 D. That is, the outer surface of the first portion  31  of the stem  3 A positions inside of the outer surface  21 B of first rib  21  in the cap  2 , and outside of the outer root of the second rib  20 A. The outer diameter of the cap  2  may be slightly larger than or substantially equal to that of the body  3 . 
   In the present invention for the optical module, where the cap is fixed to the body by resistance-welding, each welded surface,  20  or  30 , provides the portion,  21  or  31 , with relatively smaller diameter. Accordingly, even the deformed portions γ project outward when the cap  2  is welded to the body  3  as pressing against the body  3 , such deformed portion γ may be prevented, as shown in  FIGS. 3A and 3B , to project from the outermost surface of the cap  2  or the stem  3 A. 
   The present invention does not restrict the shape of the first portion, in which the cap and the stem in the embodiment above mentioned provides two portions with different diameters and the step therebetween, another shape, such as a slope connecting the portion with the largest diameter to the other portion with the smallest diameter, may be applicable. 
   Second Embodiment 
     FIG. 4  illustrates another embodiment of the present invention, where the cap provides a ring shaped rib  20 B in the top surface of the first portion  21 , while, the primary surface of stem  3 A provides a discontinued groove  35  each having an arched shape in positions corresponding to the ring rib  20 B. The diameter of the arched groove  35  is slightly smaller than the diameter of the rib  20 B, that is, the groove  35  positions inside of the ring rib  20 B. Moreover, the lead pin  3 B positions between two grooves  35 . The lead pins  3 B and the seal glass  3 C surrounding each lead pin  3 B are positioned within the circle α with a radius R 1  connecting the center of the stem  3 A to the inner surface of the groove  35 . 
   While, the ring rib  20 B positions in a doughnut region with the inner radius R 2  connecting the center of the stem  3 A to the outer surface of the groove  35  and the outer radius substantially identical with the edge of the first portion  31  of the stem  3 A. 
   The cap  2  and the stem  3 A are thus formed, the obtained module after the resistance-welding of the cap  2  to the stem  3 A deforms respective first portions,  21  and  31 , and the groove  35  as illustrated in  FIG. 5 , which prevents the welded portion from projecting outward from the outermost surface of the cap  2  and that of the stem  3 A. Moreover, the groove  35  prevents the stem  3 A from rushing inward at the resistance-welding. Moreover, the groove  35  may also prevent the heat applied at the welding from conducting inward to the seal glass  3 C. The areas δ cross-hatched in  FIGS. 6A and 6B  denote the region where the heat at the welding conducts. Accordingly, the semiconductor device mounted on the center portion of the stem  3 A may be escaped from the heat at the welding. 
   When the stem  3 A provides no groove in the primary surface  30  thereof, the heat may easily conduct to the seal glass  3 C and the lead pin  3 B as illustrated by the cross hatched area δ in  FIG. 6B . Accordingly, it may be effective that the stem  3 A provides the groove  35  in a position corresponding only to the seal glass  3 C and the lead pin  3 B. 
   As an modification of the present embodiment, the stem  3 A provides a groove with a triangular cross section, as shown in  FIGS. 7A and 7B , whose diameter is slightly smaller than that of the ring rib  20 A in the end surface  20  of the cap  2 . The groove  30 A prevents the heat at the welding from conducting to the inner area of the stem  3 A. Thus, similar to the second embodiment, the arrangement shown in  FIGS. 7A and 7B  may prevent the heat at the welding from affecting to the semiconductor device mounted on the center portion of the stem  3 A. 
   In the first embodiment, the cap and the stem welded to each other provide first portions,  21  and  31 , with relatively smaller diameter and second portions,  22  and  32 , with relatively larger diameter. And the diameter of each first portion may be so determined that the portions deformed by the welding and extruding outward do not project from the outermost surface of the module, which is the outer surface of the second portions,  22  and  32 . The extent of the deformed portion depends on (1) the materials of the cap  2  and the stem  3 A, and (2) the position of the ring rib  20 A, that is, how far the ring rib  20 A from the outer surface of the first portions,  21  and  31 . In the present embodiment, the rib  20 A is apart from the outer surface by 250 μm at the top of the rib  20 A. Therefore, when the ring rib  20 A provides the base of 250 μm and the height of 200 μm in the cross section thereof, the step between the first portion,  21  or  31 , and the second portion,  22  or  32 , of at least 150 μm is enough for the deformed portion not to project from the outermost surface of the module. 
   Conventional optical module such as transmitter optical subassembly (TOSA) or receiver optical sub assembly (ROSA), where the cap is resistance-welded to the stem, projects the portion deformed at the resistance-welding over several tenth micron-meter. As a result, it is occasionally encountered that such optical module is unable to set within the housing of the optical transceiver due to such deformed portion becomes out of the design. The optical module according to the present invention does not cause such irregularity. 
   Moreover, the optical module may further provide the groove in the inner side of the stem, in addition to provide the first and the second portion with different diameters. The width of the groove may be determined by a similar manner to that of the first embodiment, that is, when the ring rib  20 A in the top surface  20  of the cap provides the bottom of 250 μm and the height of 200 μm, the width of about 150 μm for the groove  35  may be enough and the depth thereof may be 400 μm, which is about twice of the height of the ring rib  20 A. 
   Thus, the present invention may prevent not only the device mounted on the primary surface of the stem from sliding at the welding but also the heat at the welding from conducting to the seal glass and the lead pin, which also prevents the module from degrading the air-tightly sealing. 
   Numerous other embodiments can be envisaged without departing from the spirit and scope of the invention, which is defined in the claims.