Abstract:
An optical semiconductor module including a base having installed on an optical fiber and an optical semiconductor element, and a package which houses the base on a bottom thereof and has a first side wall with an optical section through which the optical fiber is led and a second side wall facing the first side wall, where the base is cut off to form a curved surface with respect to the bottom at a lower corner on a side of the base facing the second side wall of the housing, and a ratio of r/t is from 0.4 to 1.0, where t is a thickness of the base, and r is a curvature radius of the curved surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of U.S. Ser. No. 12/147,060, filed Jun. 26, 2008, which is a division and claims the benefit under 35 U.S.C. §120 from parent application, U.S. Ser. No. 11/837,084, (U.S. Pat. No. 7,477,810), filed Aug. 10, 2007, of which the entire contents of both are incorporated herein by reference. Application Ser. No. 11/837,084 is a continuation of U.S. Ser. No. 10/460,405 (U.S. Patent Publication No. 2003/0235377 A1) filed Jun. 13, 2003, and claims the benefit of priority under 35 U.S.C. §119 from Japanese Patent Application No. 2002-174969, filed Jun. 14, 2002. 
    
    
     BACKGROUND OF THE INVENTION 
     1) Field of the Invention 
     The present invention relates to an optical semiconductor module having an optical semiconductor element and an optical fiber that are used for optical communications. 
     2) Description of the Related Art 
       FIG. 23A  is a plan view of a part of a conventional semiconductor laser diode (hereinafter, “LD”) module, and  FIG. 23B  is a longitudinal cross-sectional view of the part. The LD module includes a base  6  onto which a heat sink  3  and a carrier  5  are soldered. On the heat sink  3 , an LD element  1  and a thermistor  2  are mounted, and on the carrier  5 , a photo diode (hereinafter, “PD”) element  4  is mounted. Further, a metal ferrule  7   a  supporting an optical fiber  7  is fixed onto the base  6  via two support members  6   a  by YAG laser welding or with solder. The welding points, in the drawings, are denoted by solid circles P wd . The optical fiber  7  is precisely aligned so as to be optically coupled with a laser beam emitted from the LD element  1 , and is fixed onto the support member  6   a.    
       FIG. 24  is a longitudinal cross-sectional view of the conventional LD module, where the base  6  is accommodated within a package  8 , and is fixed onto a bottom plate  8   e  of the package  8 . The package  8  is sealed with a lid  9  at the upper opening, thereby to complete the assembly of the LD module. A lensed fiber, one end of which being formed in a spherical or a wedged shape to serve as a lens portion, is used as the optical fiber  7  in order to ensure a high coupling efficiency between the optical fiber  7  and the laser beam emitted from the LD element  1 . The other end of the optical fiber  7  is led out of the package  8  through a longitudinal hole  8   a  of a snout  8   b  prior to mounting of the base  6  onto the bottom plate  8   e  of the package  8 . A portion between the optical fiber  7  and the internal wall of the snout  8   b  is hermetically sealed with sealant S like solder or synthetic resin. 
     The LD module, particularly one using the lensed fiber, requires that the LD element  1  and the lensed fiber are coupled with an extremely high precision, and that the optical fiber  7  and the support member  6   a  are precisely positioned. 
     In the LD module shown in  FIG. 24 , the base  6  is directly fixed onto the bottom plate  8   e , with no Peltier module for cooling interposed. The longitudinal hole  8   a  in  FIG. 24  is formed through a front wall  8   c  of the package  8  at a position far down from the upper end of the front wall  8   c . Such position of the longitudinal hole  8   a  makes the mounting of the base  6  difficult. 
       FIG. 25  is a longitudinal cross-sectional view of the conventional LD module in the mounting step. In the mounting step, the base  6  is accommodated onto the bottom plate  8   e , maintaining a state that the optical fiber  7  passes through the longitudinal hole  8   a . In such a situation, a position of the metal ferrule  7   a  holding the optical fiber  7  is limited by the position of the longitudinal hole  8   a , causing often an interference between a lower portion of the base  6  and a rear wall  8   d  of the package  8  when mounting the base  6 , depending on sizes of the package  8  and the base  6 . And while the base  6  is being mounted onto the bottom plate  8   e  of the package  8  so as to avoid the interference, stress is often applied to the optical fiber  7  and thereby the optical fiber  7  is bent, pressed, or pulled. In this way, this stress often causes misalignment between the LD element  1  and the optical fiber  7 . 
     Further, in the mounting step, the optical fiber  7  is often excessively bent at a portion A encircled by a dashed line in  FIG. 25 , by contact with the snout  8   b . In some cases, the optical fiber  7  is coated with metal (e.g., gold), in order to facilitate the hermetic sealing of the optical fiber  7  inside the inner space of the snout  8   b  with solder S or other sealant, or in order to facilitate the soldering of the optical fiber  7  to the base  6 . Such metal-coated optical fibers have a minimum allowable bending curvature radius larger than that of non-coated optical fibers and are more repulsive against the bending deformation. Therefore, the optical fiber  7  needs to be handled so as not to be bent too much. 
     The similar situation may be present not only in optical semiconductor modules in general of butterfly-type, including receiver modules (hereinafter, “PD modules”) having a PD element mounted on the base  6 , but also in the LD module of so-called DIL (Dual In Line) type which utilizes a small-sized package. 
     SUMMARY OF THE INVENTION 
     The optical semiconductor module according to one aspect of the present invention includes an optical semiconductor element; an optical fiber optically coupled with the semiconductor element; a base having an upper surface and a lower surface, the optical fiber and the optical semiconductor element being mounted on the upper surface; and a package having a bottom plate on which the base is directly mounted, a front wall having a hole through which the optical fiber is inserted, and a rear wall opposite to the front wall, wherein the base has a rear face formed on an end portion opposite to the front wall, the rear face being positioned above the lower surface. 
     The optical semiconductor module according to another aspect of the present invention includes an optical semiconductor element; an optical fiber optically coupled with the semiconductor element; a base having an upper surface and a lower surface, the optical fiber and the optical semiconductor element being mounted on the upper surface; a package having a bottom plate on which the base is directly mounted, and a front wall having a hole through which the optical fiber is inserted; and a lid having a first portion disposed above the base and a second portion facing the hole, the first portion and the second portion of the lid being combined with each other to cover the package. 
     The other features and advantages of the present invention are specifically set forth in or will become apparent from the following detailed descriptions of the invention when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a longitudinal cross-sectional view of a LD module according to a first embodiment of the present invention; 
         FIG. 2  is a top plan view of a package of the LD module shown in 
         FIG. 1 ; 
         FIG. 3  is a longitudinal cross-sectional view of a part of the LD module shown in  FIG. 1 ; 
         FIG. 4  is a longitudinal cross-sectional view of the LD module in the mounting step; 
         FIG. 5A  and  FIG. 5B  are longitudinal cross-sectional views of other examples of a base of the LD module; 
         FIG. 6A  is a top plan view of other example of the base, and  FIG. 6B  is a longitudinal cross-sectional view of the base shown in  FIG. 6A ; 
         FIG. 7  is a longitudinal cross-sectional view of a LD module according to one example of a second embodiment of the present invention; 
         FIG. 8  is a longitudinal cross-sectional view of the LD module shown in  FIG. 7  in the mounting step; 
         FIG. 9  is a longitudinal cross-sectional view of a LD module according to another example of the second embodiment in the mounting step; 
         FIG. 10  is a longitudinal cross-sectional view of a LD module according to still another example of the second embodiment in the mounting step; 
         FIG. 11  is a longitudinal cross-sectional view of a LD module according to still another example of the second embodiment in the mounting step; 
         FIG. 12  is a longitudinal cross-sectional view of a LD module according to still another example of the second embodiment in the mounting step; 
         FIG. 13  is a longitudinal cross-sectional view of a PD module according to a third embodiment of the present invention; 
         FIG. 14A  is a top plan view of one example of a base used for the PD module shown in  FIG. 13 , and  FIG. 14B  is a side view of the base shown in  FIG. 14A ; 
         FIG. 15  is a longitudinal cross-sectional view of the PD module shown in  FIGS. 14A and 14B  in the mounting step; 
         FIG. 16  is a side view of a LD module according to a fourth embodiment of the present invention; 
         FIG. 17  is a top plan view of the LD module shown in  FIG. 16 ; 
         FIG. 18  is a longitudinal cross-sectional view of a LD module according to a fifth embodiment of the present invention; 
         FIG. 19  is a longitudinal cross-sectional view of the LD module shown in  FIG. 18  in the mounting step; 
         FIG. 20  is a longitudinal cross-sectional view of another example of the LD module according to the fifth embodiment, having a different package from that shown in  FIG. 18 ; 
         FIG. 21  is a longitudinal cross-sectional view of still another example of the LD module according to the fifth embodiment, having a different form of a snout from that shown in  FIG. 18 ; 
         FIGS. 22A to 22C  are longitudinal cross-sectional views of still other examples of the LD module according to the fifth embodiment, having different forms of a snout and different forms of a base from the LD module shown in  FIG. 18 ; 
         FIG. 23A  is a plan view of a part of a conventional LD module, and  FIG. 23B  is a side view of the part; 
         FIG. 24  is a longitudinal cross-sectional view of the conventional LD module; and 
         FIG. 25  is a longitudinal cross-sectional view of the conventional LD module in the mounting step. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the optical semiconductor module relating to the present invention will be explained in detail below with reference to the accompanying drawings. In the following embodiments, constituent parts identical with those of the LD module shown in  FIG. 24  are designated with like reference symbols. 
     An optical semiconductor module according to a first embodiment of the present invention is an LD module.  FIG. 1  is a longitudinal cross-sectional view of the LD module. This LD module  10  differs from the conventional LD module shown in  FIG. 24 , in that a base  11  is fixed onto the bottom plate  8   e  instead of the base  6 . On the base  11  are provided, the heat sink  3  mounting thereon the LD element  1  and the thermistor  2 , the carrier  5  mounting thereon the PD element  4 , and two pieces of support members  11   a  supporting the metal ferrule  7   a  holding the optical fiber  7 . The metal ferrule  7   a  is fixed to the base  11  through the two support members  11   a  by YAG laser-welding or soldering. The optical fiber  7  is arranged to pass through the longitudinal hole  8   a  of the snout  8   b . The portion between the optical fiber  7  and the internal wall of the snout  8   b  is hermetically sealed with a sealant S like solder. The package  8  is sealed with the lid  9  at the upper opening. The package  8  also has a front wall  8   c  and a rear wall  8   d  that faces the front wall  8   c.    
       FIG. 2  is a top plan view of the package  8 . The package  8  is of a butterfly type having a plurality of leads L d  protruding from the package  8  at both sides. The snout  8   b  is covered with a rubber covering B g . A part of the optical fiber  7  that is led out of the package  8  is protected by a protective tube T g . The package  8  has a length L not larger than 16 millimeter, a width W not larger than 10 millimeter, and a height H not larger than 8 millimeter, as shown in  FIGS. 1 and 2 . 
       FIG. 3  is a longitudinal cross-sectional view of a part of the LD module shown in  FIG. 1 , and specifically illustrates the base  11 . The base  11  differs from the base  6  shown in  FIGS. 23A ,  23 B,  24 , and  25 , in the shape of the rear end portion to be faced to the rear wall  8   d  of the package  8 . As shown in  FIG. 3 , the base  11  has a rear end portion E having an upper edge F U  and an lower edge F L . The lower edge F L  is positioned nearer to the front end portion of the base  11  than the upper edge F U . In other words, the base  11  has a taper  11   b  that is inclined from the vicinity of the upper edge F U  toward the lower edge F L , at the rear end portion E. As a result, the formation of the taper  11   b  is equivalent to elimination of the rear lower corner of the base  11 . 
       FIG. 4  is a longitudinal cross-sectional view of the LD module in the mounting step. When the base  11  is mounted into the package  8  in a direction of an arrow mark shown in the  FIG. 4 , no interference occurs between the base  11  and the rear wall  8   d  of the package  8 . This is because of the elimination of the rear lower corner of the base  11  by the taper  11   b . Therefore, the base  11  can be easily mounted onto the bottom plate  8   e  of the package  8 . Consequently, no significant misalignment between the LD element  1  and the optical fiber  7  can occur in the mounting step and it is possible to suppress a reduction in the coupling efficiency between the laser beam emitted from the LD element  1  and the optical fiber  7 . 
     Moreover, the prevention of interference between the base  11  and the rear wall  8   d  results in a prevention of an excessive bend of the optical fiber  7  at the portion A encircled by the dashed line in  FIG. 4 . Consequently, the optical fiber  7  can be maintained at a large radius of bending curvature during the mounting step into the package  8 , and is free from a breakage arising from the excessive bending. 
     The shape of the rear end portion E of the base  11  is not limited to that shown in  FIG. 3 . The rear end portion E may be formed into various shapes to avoid an interference against the rear wall  8   d  in the mounting step. For example, the rear end portion may be formed into a curved face  11   c  with a curvature radius of r as shown in  FIG. 5A , or a step  11   d  with a height h 2  and a length L 1  as shown in  FIG. 5B . 
     In the LD module  10  according to the present embodiment, the base  11  is mounted into the package  8  in the state that the rear end portion E is slightly inclined from a horizontal state such that the optical fiber  7  led from the snout  8   b  to the outside of the package is not excessively bent. When the base  11  is formed into the taper  11   b , for example, a height h 1  of the base  11  from its bottom surface to a line of intersection between the taper  11   b  and the rear end surface of the base  11  is set to a range of about 0.4≦h 1 /t≦1.0, where t is a thickness of the base  11 , as shown in  FIG. 3 . While an inclination angle of the taper face  11   b  is suitably set according to a size of the package  8 , an inclination angle θ within a side cross-sectional surface of the base  11  parallel to an optical axis is set to a range of about 20°≦θ≦60°. On the other hand, when the base  11  is formed to have the curved face  11   c  having a cylindrical surface or the step  11   d , the radius r of the cylindrical surface (see  FIG. 5A ) or the height h 2  and the length L 1  of the step (see  FIG. 5B ) are set to ranges of about 0.4≦r/t, L 1 /t≦1.0, and 0.4≦h 2 /t≦0.7, where t is a thickness of the base  11 . 
     The optical fiber  7  may be directly fixed with solder Sd, onto a support member  11   e  that is fixed to the base  11 , with no metal ferrule  7   a  employed, as shown in  FIGS. 6A and 6B . It is needless to mention that the rear end portion E of the base  11  may also be formed into the curved face  11   c  or the step  11   d  as shown in  FIG. 5A  or  FIG. 5B . The support member  11   e  may be made of nonmetal such as ceramic, for example. When the optical fiber  7  is fixed onto the support member  11   e  with an adhesive or a synthetic resin, an optical fiber of which external periphery is not metal-coated may also be used. 
       FIG. 7  is a longitudinal cross-sectional view of an LD module according to the second embodiment of the present invention. The LD module  15  differs from the conventional LD module, in the shape of its package. The LD module  15  is configured as an optical semiconductor module having a butterfly-type package. As shown in  FIG. 7 , a snout  108   b  of a package  108  consists of a first part  108   b   1  and a second part  108   b   2 . The second part  108   b   2  has a larger diameter than that of the first part  108   b   1 , and is positioned between the first part  108   b   1  and a front wall  108   c  of the package  108 . These first part  108   b   1  and the second part  108   b   2  have a longitudinal hole  108   a  through them, for guiding the optical fiber  7  to the outside. The longitudinal hole  108   a  consists of a first longitudinal hole  108   a   1  of the first part  108   b   1 , and a second longitudinal hole  108   a   2  of the second part  108   b   2 . The longitudinal hole  108   a  is, concretely, formed such that an upper boundary F UI  of the second longitudinal hole  108   a   2  is located higher than an upper boundary F UO  of the first longitudinal hole  108   a   1 . As a result, the optical fiber  7 , when led to the outside of the package  108  through the longitudinal hole  108   a , is prevented from being brought into contact with the internal surface of the second longitudinal hole  108   a   2 . Therefore, an excessive bending deformation of the optical fiber  7  is suppressed. 
     Consequently, in the LD module  15 , in the mounting step of the base  11  into the package  108 , it is possible not only to maintain a state of a large bending curvature radius of the optical fiber  7  by the taper  11   b  but also to prevent the optical fiber  7  from being brought into contact with the internal surface of the second longitudinal hole  108   a   2  of the second part  108   b   2  of the snout  108   b , as shown in  FIG. 8 . Consequently, in the LD module  15 , it possible to prevent an excessive bending deformation of the optical fiber  7  arising from the interference between the optical fiber  7  and the internal surface of the longitudinal hole  108   a  that would otherwise occur at a portion A encircled by a dashed line. 
     The LD module  15  according to the second embodiment has an effect that the second part  108   b   2  provided in the snout  108   b  prevents the optical fiber  7  from being excessively bent, in addition to the effect that the taper  11   b  of the base  11  facilitates the mounting of the base  11  into the package  108 . Therefore, in the LD module  15 , it is possible to suppress a breakage of the metal-coated optical fiber  7  in the mounting step more surely than in the LD module  10  according to the first embodiment. Note that the longitudinal hole  108   a  may have a plurality of diameters so as to have three or more steps. The snout  108   b  may take a shape other than that shown in  FIG. 7 , as far as it is possible to suppress the bending deformation of the optical fiber  7  in the mounting step. For example, the second part  108   b   2  of the snout  8   b  is inclined toward the upper end portion of the front wall  108   c , and the upper boundary F UI  of the longitudinal hole  108   a  is positioned higher than the upper boundary F UO . 
     A package  208  may be used in place of the package  108  (see  FIG. 9 ). The package  208  has a front wall  208   c , a rear wall  208   d , a bottom plate  208   e , and a snout  208   b . An external diameter of the snout  208   b  is uniform along the longitudinal direction. The snout  208   b  has a longitudinal hole  208   a  whose diameter is linearly small toward the outside. In other words, the longitudinal hole  208   a  has taper at its internal surface along the longitudinal direction. The taper may not necessarily be formed over the entire of the internal surface along its longitudinal direction but may be formed at least on a part thereof. For example, the taper is formed on only a part of the internal surface near the front wall  208   c.    
     Further, a package  308  may be used in place of the package  108  (see  FIG. 10 ). The package  308  has a front wall  308   c , a rear wall  308   d , a bottom plate  308   e , and a snout  308   b  having a longitudinal hole  308   a . The snout  308   b  has a first part  308   b   1  and a second part  308   b   2 . A part of the longitudinal hole  308   a  on the first part  308   b   1  has a uniform diameter along the longitudinal direction, and a part of longitudinal hole  308   a  on the second part  308   b   2  has diameter that is linearly small toward the first part  308   b   1 . External diameters of the snout  308   b  are values corresponding to the diameters of the longitudinal hole  308   a  of the first part  308   b   1  and of the second part  308   b   2 . 
     Further, a package  408  may be used in place of the package  108  (see  FIG. 11 ). The package  408  has a front wall  408   c , a rear wall  408   d , a bottom plate  408   e , and a snout  408   b  having a longitudinal hole  408   a . The external diameter of snout  408   b  and the diameter of the longitudinal hole  408   a  are linearly small from the front wall  408   c  toward the outside of the package  408 . 
     Furthermore, a package  508  may be used in place of the package  108  (see  FIG. 12 ). The package  508  has a front wall  508   c , a rear wall  508   d , a bottom plate  508   e , and a snout  508   b  having a longitudinal hole  508   a . The external and internal faces of snout  508   b  are curved such that an external diameter of the snout  508   b  and a diameter of the longitudinal hole  508   a  become gradually small from the front wall  508   c  toward the outside of the package  508 . 
       FIG. 13  is a longitudinal cross-sectional view of a PD module according to a third embodiment of the present invention. The PD module differs from the LD module shown in  FIG. 1 , in that the base  11  having the tapered rear end portion E is used for the PD module. The PD module  20  is an optical semiconductor module having a butterfly-type package. A carrier  5  on which a PD element  4  is provided and a metal support member  11   e  are mounted on the base  11 . The metal support member  11   e  is fixed onto the base  11  with solder. The support member  11   e  may be made of nonmetal, such as ceramic, for example. The optical fiber  7  coated with metal is directly fixed onto the metal support member  11   e  with solder Sd, as shown in  FIGS. 13 ,  14 A, and  14 B. A portion between the optical fiber  7  and the internal wall of the snout  8   b  is hermetically sealed with a sealant S like solder or synthetic resin. An upper portion of the package  8  is sealed with a lid  9 . 
     The base  11  has the rear end portion E with the tapered face  11   b , like that shown in  FIG. 1 . The PD module  20  is formed as explained above. Therefore, as shown in  FIG. 15 , in the mounting step of the base  11  into the package  8 , it is possible to prevent an interference between the base  11  and the rear wall  8   d  of the package  8 . Accordingly, the base  11  can be easily mounted into the package  8 . Consequently, in the mounting step, there occurs no misalignment between the PD element  4  and the optical fiber  7 . As a result, it becomes possible to avoid a reduction in the optical coupling efficiency between the PD element  4  and the optical fiber  7 . 
     Since the rear end portion E of the base  11  has a taper, the optical fiber  7  is free from excessive bending at a portion A encircled by a dashed line in  FIG. 15  when mounting the base  11  into the package  8  with the optical fiber  7  being inserted through the longitudinal hole  8   a . Since the optical fiber  7  maintains a state of a large bending curvature radius, a breakage of the optical fiber  7  is avoided. 
     The rear end portion E of the base  11  may be formed into various shapes, such as, for example, the curved face  11   c  or the step  11   d  as shown in  FIG. 5A  or  FIG. 5B . 
       FIG. 16  is a longitudinal cross-sectional view of a package of a LD module according to a fourth embodiment. The LD module  30  differs from the LD module  10  according to the first embodiment in the type of package. Concretely, in the LD module  30 , a package  608  closed with a lid  609  is a DIL-type.  FIG. 17  is a top plan view of the package of the LD module  30 . 
     As shown in  FIGS. 16 and 17 , the LD module  30  has a plurality of leads Ld extending downward from the side surfaces of the package  608 . An unillustrated snout of the package  608  is covered with a rubber covering B g , and a part of the optical fiber  7  that is led out of the package  608  is protected by a protective tube T g . The package  608  has a length L not larger than 16 millimeter, a width W not larger than 10 millimeter, and a height H not larger than 8 millimeter. The base  11  shown in one of  FIG. 3  and  FIGS. 5A to 6B  is mounted into the package  8 . 
     Therefore, although the LD module  30  is very small, the base  11  can be easily mounted into the package  608 . Consequently, it is possible to avoid a reduction in the coupling efficiency between the laser beam emitted from the LD element  1  to the optical fiber  7 . 
     Since a rear end portion E of the base  11  is formed into the taper  11   b , a curved face  11   c , or a step  11   d , the optical fiber  7  is not excessively bent in the mounting step. As a result, the optical fiber  7  can be maintained at a large bending curvature radius, and thus free from breakage in the mounting step. 
     Note that if a longitudinal hole through which the optical fiber  7  passes is formed to have a larger size at the inside than at the outside of the package  608  as shown in  FIGS. 7 to 12 , it is possible to more surely suppress an excessive bending deformation of the optical fiber  7  in the mounting step, thereby to surely avoid a breakage of the optical fiber  7 . 
       FIG. 18  is a longitudinal cross-sectional view of a LD module according to a fifth embodiment of the present invention. The LD module  35  differs from the optical semiconductor modules according to the first through the fourth embodiments, in a base and a lid of a package. In the first through the fourth embodiment, the rear end portion of the base  11  or a longitudinal hole  708   a  provided in the snout  708   b  of a package  708  was processed in various ways. On the other hand, in the LD module  35 , a butterfly-type or DIL-type package  708  fails to have a rear wall at a side opposite to the snout  708   b , or it has a rear wall with at least a part thereof removed. 
     For example, in the LD module  35  shown in  FIG. 18 , the butterfly-type or DIL-type package  708  has no rear wall opposite to a front wall  708   c . Instead, the wall that faces the front wall  708   c  is replaced by a perpendicular plate  709   a  that is integrated with a lid  709 . 
     Therefore, as shown in  FIG. 19 , the package  708  is open at the rear side thereof (i.e., at the left side in  FIG. 18  and  FIG. 19 ) when mounting the base  11  onto the bottom plate  708   e . Hence, it is possible to insert the base  11  into the package  708  from the left without causing interference of the optical fiber  7  with the inner wall of the longitudinal hole  708   a  at its end exposed on the front wall  708   c . It is substantially unnecessary to curve the optical fiber  7  at a portion A of the package  708  encircled by a dashed line. Since the optical fiber  7  can be maintained at a large bending curvature radius, a breakage of the optical fiber  7  is avoided. Further, it becomes possible to avoid a reduction in the optical coupling efficiency between the LD element  1  and the optical fiber  7 . 
     Another package and lid in place of the package  708  and the lid  709  may be used if the package is open at the side opposite to the snout before closing with the lid. For example, as shown in  FIG. 20 , a package  808  in place of the package  708  may be used for an LD module  45 , wherein the upper portion of a rear wall  808   d  opposite to a front wall  808   c  is removed, and a perpendicular plate  809   b  provided integrally with the lid  809  may be located at this removed portion. 
     Since the optical fiber  7  can be inserted horizontally in this embodiment, a snout may be formed to have a longitudinal hole  908   a  with a diameter that is uniform in the longitudinal direction, as shown in  FIG. 21 . 
     These LD modules shown in  FIGS. 20 and 21  make the mounting step easy, without substantially bending the optical fiber  7 , in a similar manner to that shown in  FIG. 19 . Further, the rear end portion E of the base  11  may have a taper, a curved face or a step, like that shown in  FIG. 3 ,  FIG. 5A , or  FIG. 5B , whereby it is possible to prevent the optical fiber  7  from being excessively bent due to the interference between the rear end portion E of the base  11  and the residual rear wall  808   d , in the mounting step. 
     In addition, a snout of the package may have forms other than those shown in  FIGS. 18 and 21 . For example, LD modules  65 ,  75 , and  85  shown in  FIGS. 22A to 22C  have packages  1008 ,  1108 , and  1208 , respectively. A snout  1008   b  of the package  1008  shown in  FIG. 22A  corresponds to the snout  208   b  shown in  FIG. 9 , a snout  1108   b  of the package  1108  shown in  FIG. 22B  corresponds to the snout  308   b  shown in  FIG. 10 , and a snout  1208   b  of the package  1208  shown in  FIG. 22C  corresponds to the snout  408   b  shown in  FIG. 11 . 
     These package may be used for not only the LD module but also the PD module according to the third embodiment. 
     Advantages derived from the present invention may include one or more of the following. 
     According to one or more embodiment of the present invention, it is easy to mount the base into the package. 
     According to one or more embodiment of the present invention, it is possible to avoid a reduction in the coupling efficiency between the optical fiber and the optical semiconductor element. 
     According to one or more embodiment of the present invention, it is possible to prevent the optical fiber from being excessively bent, thereby to maintain the state of a large radius of bending curvature and prevent a breakage of the optical fiber. 
     Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.