Patent Publication Number: US-2012044965-A1

Title: Semiconductor laser apparatus and optical apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The priority application number JP2010-184617, Semiconductor Laser Apparatus and Optical Apparatus, Aug. 20, 2010, Shinichiro Akiyoshi et al., upon which this patent application is based is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor laser apparatus and an optical apparatus, and more particularly, it relates to a semiconductor laser apparatus and an optical apparatus each comprising a base including a first upper surface and a second upper surface having different heights from each other. 
     2. Description of the Background Art 
     A semiconductor laser apparatus (optical apparatus) mounted with a plurality of semiconductor laser devices on a base including a first upper surface and a second upper surface having different heights from each other is known in general, as disclosed in Japanese Patent Laying-Open No. 2000-222766, for example. 
     FIG. 7 in Japanese Patent Laying-Open No. 2000-222766 discloses a semiconductor laser apparatus (optical apparatus) comprising a submount (base) including a first upper surface and a second upper surface located above the first upper surface, a first semiconductor laser chip bonded onto the first upper surface, including a first light-emitting region located on a side (upper side) opposite to a side bonded to the first upper surface and a second semiconductor laser chip bonded onto the second upper surface, including a second light-emitting region located on a side (lower side) bonded to the second upper surface. In this optical apparatus, the first light-emitting region of the first semiconductor laser chip and the second light-emitting region of the second semiconductor laser chip are greatly separated from each other in a height direction in a state where the submount is horizontally arranged. In Japanese Patent Laying-Open No. 2000-222766, a beam emitted from the first light-emitting region of the first semiconductor laser chip and a beam emitted from the second light-emitting region of the second semiconductor laser chip are reflected by a wavelength selective film and a reflective device, whereby an optical axis of the laser beam from the first semiconductor laser chip and an optical axis of the laser beam from the second semiconductor laser chip are aligned on the same optical axis and the respective light-emitting regions of the laser chips are displaced on the optical axis. 
     In the optical apparatus disclosed in Japanese Patent Laying-Open No. 2000-222766, however, a height position of the first light-emitting region of the first semiconductor laser chip and a height position of the second light-emitting region of the second semiconductor laser chip are greatly separated from each other, and hence if this structure is applied to a structure in which the laser beam from the first semiconductor laser chip and the laser beam from the second semiconductor laser device are incident upon a lens without the wavelength selective film and the reflective device, for example, an application position (spot) of the laser beam from the first semiconductor laser chip and an application position of the laser beam from the second semiconductor laser chip are disadvantageously greatly deviated from each other in the height direction. 
     SUMMARY OF THE INVENTION 
     A semiconductor laser apparatus according to a first aspect of the present invention comprises a base including a step portion, a first upper surface on a lower side of the step portion and a second upper surface on an upper side of the step portion, a first semiconductor laser device bonded onto the first upper surface, including a first light-emitting region on an upper side thereof, and a second semiconductor laser device bonded onto the second upper surface, including a second light-emitting region on a lower side thereof, wherein the first light-emitting region is located above the second upper surface in a state where the base is horizontally arranged. 
     An optical apparatus according to a second aspect of the present invention comprises a semiconductor laser apparatus including a base having a step portion, a first upper surface on a lower side of the step portion and a second upper surface on an upper side of the step portion, a first semiconductor laser device bonded onto the first upper surface, having a first light-emitting region on an upper side thereof and a second semiconductor laser device bonded onto the second upper surface, having a second light-emitting region on a lower side thereof, and an optical system controlling a laser beam emitted from the semiconductor laser apparatus, wherein the first light-emitting region is located above the second upper surface in a state where the base is horizontally arranged. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top plan view of a two-wavelength semiconductor laser apparatus according to a first embodiment of the present invention; 
         FIG. 2  is a front elevational view of the two-wavelength semiconductor laser apparatus according to the first embodiment of the present invention, as viewed from a laser beam emitting direction; 
         FIGS. 3 to 5  are sectional views for illustrating a manufacturing process of the two-wavelength semiconductor laser apparatus according to the first embodiment of the present invention; 
         FIG. 6  is a front elevational view of a three-wavelength semiconductor laser apparatus according to a second embodiment of the present invention, as viewed from a laser beam emitting direction; 
         FIGS. 7 and 8  are sectional views for illustrating a manufacturing process of the three-wavelength semiconductor laser apparatus according to the second embodiment of the present invention; 
         FIG. 9  is a schematic diagram showing a structure of an optical pickup according to a third embodiment of the present invention; and 
         FIG. 10  is a front elevational view of a two-wavelength semiconductor laser apparatus according to a modification of the first embodiment of the present invention, as viewed from a laser beam emitting direction. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention are hereinafter described with reference to the drawings. 
     First Embodiment 
     A structure of a two-wavelength semiconductor laser apparatus  100  according to a first embodiment of the present invention is now described with reference to  FIGS. 1 and 2 . The two-wavelength semiconductor laser apparatus  100  is an example of the “semiconductor laser apparatus” in the present invention. 
     The two-wavelength semiconductor laser apparatus  100  according to the first embodiment of the present invention comprises a heat radiation substrate  10  made of AlN having insulating properties, a blue-violet semiconductor laser device  20  having a lasing wavelength of about 405 nm and a red semiconductor laser device  30  having a lasing wavelength of about 650 nm both bonded to the heat radiation substrate  10 , and a base portion  40  supporting the heat radiation substrate  10  from below (from a Z 2  side), as shown in  FIGS. 1 and 2 . The base portion  40  is formed to horizontally arrange the heat radiation substrate  10  through a bonding layer  50  (see  FIG. 2 ). The base portion  40  is connected to a cathode terminal (not shown). The heat radiation substrate  10  is an example of the “base” in the present invention. The blue-violet semiconductor laser device  20  is an example of the “first semiconductor laser device” in the present invention, and the red semiconductor laser device  30  is an example of the “second semiconductor laser device” in the present invention. 
     The heat radiation substrate  10  includes a step portion  11   c  and upper surfaces  11   a  and  11   b  formed at heights different from each other (in a direction Z) through the step portion  11   c . Specifically, the upper surface  11   a  is located on a lower side (Z 2  side) of the step portion  11   c  and formed at a height H 1  upward from (on a Z 1  side of) a lower surface  12  of the heat radiation substrate  10 . The upper surface  11   b  is located on an upper side (Z 1  side) of the step portion  11   c  and formed at a height H 2  upward from the lower surface  12  of the heat radiation substrate  10 . The height H 2  is larger than the height H 1 . The blue-violet semiconductor laser device  20  and the red semiconductor laser device  30  are bonded onto the upper surfaces  11   a  and  11   b , respectively. The upper surfaces  11   a  and  11   b  and the lower surface  12  of the heat radiation substrate  10  are formed to be flat. The upper surfaces  11   a  and  11   b  are examples of the “first upper surface” and the “second upper surface” in the present invention, respectively. 
     As shown in  FIG. 1 , the upper surface  11   a  of the heat radiation substrate  10  is located on one side (X 1  side) in a direction (direction X) orthogonal to a direction Y, which is emitting directions of laser beams from the blue-violet semiconductor laser device  20  and the red semiconductor laser device  30  described later, and the upper surface  11   b  of the heat radiation substrate  10  is located on the other side (X 2  side). 
     The step portion  11   c  is formed to extend along the emitting direction (direction Y) of the laser beam from the blue-violet semiconductor laser device  20  and the emitting direction (direction Y) of the laser beam from the red semiconductor laser device  30 . The step portion  11   c  is formed to extend from one end of the heat radiation substrate  10  on a Y 1  side to the other end thereof on a Y 2  side. The step portion  11   c  is formed to extend vertically upward (in a direction Z 1 ) from the upper surface  11   a  on the lower side and reach the upper surface  11   b  on the upper side, as shown in  FIG. 2 . In other words, a height of the step portion  11   c  in a vertical direction is a difference between the heights of the upper surfaces  11   a  and  11   b  (H 2 −H 1 ). 
     Electrodes  13   a  and  13   b  are formed on the upper surfaces  11   a  and  11   b  of the heat radiation substrate  10 , respectively. The blue-violet semiconductor laser device  20  is bonded to the electrode  13   a  through a solder layer  14   a , and the red semiconductor laser device  30  is bonded to the electrode  13   b  through a solder layer  14   b . The electrodes  13   a  and  13   b  are separated from each other in the direction X (device width direction) and the direction Z (height direction) by the step portion  11   c . The electrodes  13   a  and  13   b  are examples of the “first electrode” and the “second electrode” in the present invention, respectively. 
     The blue-violet semiconductor laser device  20  is made of a nitride-based semiconductor. Specifically, in the blue-violet semiconductor laser device  20 , an n-type cladding layer  22  made of n-type AlGaN is formed on an upper surface of an n-type GaN substrate  21 , as shown in  FIG. 2 . An active layer  23  having a multiple quantum well (MQW) structure in which quantum well layers (not shown) made of InGaN and barrier layers (not shown) made of GaN are alternately stacked is formed on an upper surface of the n-type cladding layer  22 . Luminous characteristics of the active layer  23  made of a nitride-based semiconductor are easily deteriorated due to accumulation of thermal stress if heat of about 300° C. is applied in bonding the blue-violet semiconductor laser device  20  onto the upper surface  11   a  of the heat radiation substrate  10 . A p-type cladding layer  24  made of p-type AlGaN is formed on an upper surface of the active layer  23 . A material constituting the n-type cladding layer  22 , the active layer  23  and the p-type cladding layer  24  is an example of the “nitride-based semiconductor” in the present invention. 
     As shown in  FIG. 1 , a ridge portion (projecting portion)  25  extending along the direction Y is formed on the p-type cladding layer  24  in a substantially central portion of the blue-violet semiconductor laser device  20  in the direction X. The laser beam is emitted from a light-emitting surface  20   a , which is a surface of the blue-violet semiconductor laser device  20  on one end (on a Y 1  side) in the emitting direction (direction Y). At this time, the laser beam is emitted from a position of the active layer  23  corresponding to the ridge portion  25  on the light-emitting surface  20   a , as shown in  FIG. 2 . In other words, a light-emitting region  20   b  (region surrounded by a broken line) of the blue-violet semiconductor laser device  20  is located in a position corresponding to the ridge portion  25  formed in the substantially central portion of the blue-violet semiconductor laser device  20  in the direction X at a height of the active layer  23 . The light-emitting region  20   b  is an example of the “first light-emitting region” in the present invention. The ridge portion  25  is an example of the “first ridge portion” in the present invention. 
     A p-side ohmic electrode  26  in which a Pt layer, a Pd layer and a Pt layer are stacked successively from a side closer to the p-type cladding layer  24  is formed in an upper portion of the ridge portion  25  of the p-type cladding layer  24 . A current blocking layer  27  made of SiO 2  is formed on an upper surface of the p-type cladding layer  24  other than the ridge portion  25 , both side surfaces of the ridge portion  25  and both side surfaces of the p-side ohmic electrode  26 . A p-side pad electrode  28  made of Au or the like is formed on upper surfaces of the p-side ohmic electrode  26  and the current blocking layer  27 . An n-side electrode  29  in which an Al layer, a Pd layer and an Au layer are stacked successively from a side closer to the n-type GaN substrate  21  is formed on a substantially entire region of a lower surface of the n-type GaN substrate  21 . This n-side electrode  29  is electrically connected to the electrode  13   a  and the base portion  40  through the solder layer  14   a . An upper surface (on a Z 1  side) of the p-side pad electrode  28  in the blue-violet semiconductor laser device  20  is an example of the “first surface” in the present invention. 
     The n-side electrode  29  formed on the lower surface of the n-type GaN substrate  21  and the upper surface  11   a  of the heat radiation substrate  10  are bonded onto each other, whereby the blue-violet semiconductor laser device  20  is bonded onto the upper surface  11   a  such that the active layer  23  and the ridge portion  25  are located above (on a Z 1  side of) the n-type GaN substrate  21 . In other words, the blue-violet semiconductor laser device  20  is bonded onto the upper surface  11   a  in a junction-up system, so that the light-emitting region  20   b  is located on a side (upper side (Z 1  side)) opposite to a side bonded to the upper surface  11   a . A lower surface of the n-side electrode  29  bonded onto the upper surface  11   a  is an example of the “second surface” in the present invention. 
     According to the first embodiment, a height H 3  from the lower surface  12  of the heat radiation substrate  10  to the active layer  23  in the vertical direction (direction Z) is larger than the height H 2  from the lower surface  12  of the heat radiation substrate  10  to the upper surface  11   b  in the vertical direction (H 3 &gt;H 2 ). Thus, the active layer  23  is located above (on a Z 1  side of) the upper surface  11   b  on the upper side of the step portion  11   c , whereby the light-emitting region  20   b  of the blue-violet semiconductor laser device  20  is located above the upper surface  11   b  on the upper side of the step portion  11   c.    
     The red semiconductor laser device  30  is made of a GaInP-based semiconductor and is a semiconductor laser device where a larger amount of heat is generated than in the blue-violet semiconductor laser device  20 . Specifically, in the red semiconductor laser device  30 , an n-type cladding layer  32  made of AlGaInP is formed on a lower surface of an n-type GaAs substrate  31 , as shown in  FIG. 2 . An active layer  33  having an MQW structure in which quantum well layers (not shown) made of GaInP and barrier layers (not shown) made of AlGaInP are alternately stacked is formed on a lower surface of the n-type cladding layer  32 . Luminous characteristics of the active layer  33  made of a GaInP-based semiconductor are hardly deteriorated because thermal stress is hardly accumulated as compared with the active layer  23  of the blue-violet semiconductor laser device  20 , even if heat of about 300° C. is applied in bonding the red semiconductor laser device  30  onto the upper surface  11   b  of the heat radiation substrate  10 . A p-type cladding layer  34  made of AlGaInP is formed on a lower surface of the active layer  33 . A material constituting the n-type cladding layer  32 , the active layer  33  and the p-type cladding layer  34  is an example of the “GaInP-based semiconductor” in the present invention. As shown in  FIG. 1 , a ridge portion (projecting portion)  35  extending along the direction Y is formed on the p-type cladding layer  34  in a substantially central portion of the red semiconductor laser device  30  in the direction X. The laser beam is emitted from a light-emitting surface  30   a , which is a surface of the red semiconductor laser device  30  on one end (on the Y 1  side) in the emitting direction (direction Y). At this time, the laser beam is emitted from a position of the active layer  33  corresponding to the ridge portion  35  on the light-emitting surface  30   a , as shown in  FIG. 2 . In other words, a light-emitting region  30   b  (region surrounded by a broken line) of the red semiconductor laser device  30  is located in a position corresponding to the ridge portion  35  formed in the substantially central portion of the red semiconductor laser device  30  in the direction X at a height of the active layer  33 . The light-emitting region  30   b  is an example of the “second light-emitting region” in the present invention. The ridge portion  35  is an example of the “second ridge portion” in the present invention. 
     A current blocking layer  37  made of SiO 2  is formed on a lower surface of the p-type cladding layer  34  other than the ridge portion  35  and both side surfaces of the ridge portion  35 . A p-side electrode  38  made of Au or the like is formed on lower surfaces of the ridge portion  35  and the current blocking layer  37 . This p-side electrode  38  is connected to the electrode  13   b  and a lead terminal (on an anode side) (not shown) through the solder layer  14   b . An n-side electrode  39  in which an AuGe layer, an Ni layer and an Au layer are stacked successively from a side closer to the n-type GaAs substrate  31  is formed on a substantially entire region of an upper surface of the n-type GaAs substrate  31 . An upper surface (on a Z 1  side) of the n-side electrode  39  in the red semiconductor laser device  30  is an example of the “fourth surface” in the present invention. 
     The p-side electrode  38  formed below (on a Z 2  side of) the n-type GaAs substrate  31  and the upper surface  11   b  of the heat radiation substrate  10  are bonded onto each other, whereby the red semiconductor laser device  30  is bonded onto the upper surface  11   b  such that the active layer  33  and the ridge portion  35  are located below (on the Z 2  side of) the n-type GaAs substrate  31 . In other words, the red semiconductor laser device  30  is bonded onto the upper surface  11   b  in a junction-down system, so that the light-emitting region  30   b  is located on a side (lower side (Z 2  side)) bonded to the upper surface  11   b . A lower surface of the p-side electrode  38  bonded onto the upper surface  11   b  is an example of the “third surface” in the present invention. 
     According to the first embodiment, a height H 4  from the lower surface  12  of the heat radiation substrate  10  to the active layer  33  of the red semiconductor laser device  30  in the vertical direction (direction Z) is substantially equal to the height H 3  from the lower surface  12  of the heat radiation substrate  10  to the active layer  23  of the blue-violet semiconductor laser device  20  in the vertical direction. Thus, the light-emitting region  20   b  of the blue-violet semiconductor laser device  20  and the light-emitting region  30   b  of the red semiconductor laser device  30  are located at the heights substantially equal to each other and arranged such that height positions of at least portions thereof overlap each other. In this state, the light-emitting region  20   b  and the light-emitting region  30   b  are arranged along the emitting directions (direction Y) of the laser beams at the heights equal to each other or close to each other. The height (H 2 −H 1 ) of the step portion  11   c  in the vertical direction is adjusted such that the light-emitting region  20   b  of the blue-violet semiconductor laser device  20  and the light-emitting region  30   b  of the red semiconductor laser device  30  are located at the heights substantially equal to each other. 
     According to the first embodiment, a distance L 1  from the step portion  11   c  to the ridge portion  25  (light-emitting region  20   b ) of the blue-violet semiconductor laser device  20  in a horizontal direction (direction X) is substantially constant along the emitting direction (direction Y) of the laser beam, as shown in  FIGS. 1 and 2 . Similarly, a distance L 2  from the step portion  11   c  to the ridge portion  35  (light-emitting region  30   b ) of the red semiconductor laser device  30  in the horizontal direction (direction X) is substantially constant along the emitting direction (direction Y) of the laser beam. In other words, the step portion  11   c  is formed such that the horizontal distance from the step portion  11   c  to each of the ridge portions (waveguides) of the laser devices is substantially constant along an extensional direction of a cavity.  FIG. 1  shows that the distances L 1  and L 2  are from the step portion  11   c  to respective centerlines (dashed lines) of the ridge portions of the semiconductor laser devices. 
     The electrode  13   a  formed on the heat radiation substrate  10  and the base portion  40  are electrically connected with each other through a wire  60 . The electrode  13   b  formed on the heat radiation substrate  10  and the lead terminal (on the anode side) (not shown) are electrically connected with each other through a wire  61 . The p-side pad electrode  28  of the blue-violet semiconductor laser device  20  and a lead terminal (on the anode side) (not shown) are electrically connected with each other through a wire  62 . The n-side electrode  39  of the red semiconductor laser device  30  and the base portion  40  are electrically connected with each other through a wire  63 . The wires  60  and  61  are examples of the “bonding wire” in the present invention. 
     A manufacturing process of the two-wavelength semiconductor laser apparatus  100  according to the first embodiment is now described with reference to  FIGS. 2 to 5 . 
     As shown in  FIG. 3 , a prescribed region of an upper surface  11  of a plate-like heat radiation substrate  10  on the X 1  side is first etched by a prescribed depth (H 2 −H 1 ) in the vertical direction (direction Z), thereby forming the heat radiation substrate  10  having the upper surfaces  11   a  and  11   b  and the step portion  11   c . At this time, the height (H 2 −H 1 ) (the quantity of etching) of the step portion  11   c  in the vertical direction is adjusted such that the light-emitting region  20   b  of the blue-violet semiconductor laser device  20  and the light-emitting region  30   b  of the red semiconductor laser device  30  are located at the heights substantially equal to each other when the blue-violet semiconductor laser device  20  and the red semiconductor laser device  30  are bonded onto the upper surfaces  11   a  and  11   b  of the heat radiation substrate  10  in a later process. 
     Then, the electrodes  13   a  and  13   b  are formed on the upper surfaces  11   a  and  11   b  of the heat radiation substrate  10 , respectively, as shown in  FIG. 4 . Thereafter, the solder layers  14   a  and  14   b  are formed on the electrodes  13   a  and  13   b , respectively. 
     The blue-violet semiconductor laser device  20  and the red semiconductor laser device  30  are formed through prescribed manufacturing processes. The p-side pad electrode  28  of the blue-violet semiconductor laser device  20  is grasped from above (from a Z 1  side) with a collet  70  such that the n-side electrode  29  of the blue-violet semiconductor laser device  20  and the solder layer  14   a  are opposed to each other. Then, the n-side electrode  29  of the blue-violet semiconductor laser device  20  and the electrode  13   a  are bonded to each other through the solder layer  14   a  melted by applying heat of about 300° C. At this time, the blue-violet semiconductor laser device  20  is bonded onto the upper surface  11   a  (electrode  13   a ) of the heat radiation substrate  10  in a junction-up system, so that the light-emitting region  20   b  is located on the side (upper side (Z 1  side)) opposite to the side bonded to the upper surface  11   a . The blue-violet semiconductor laser device  20  is bonded onto the upper surface  11   a  such that the height from the lower surface  12  of the heat radiation substrate  10  to the active layer  23  of the blue-violet semiconductor laser device  20  in the vertical direction (direction Z) is H 3  (see  FIG. 5 ). 
     Thereafter, the n-side electrode  39  of the red semiconductor laser device  30  is grasped from above (from the Z 1  side) with the collet  70  such that the p-side electrode  38  of the red semiconductor laser device  30  and the solder layer  14   b  are opposed to each other, as shown in  FIG. 5 . Then, the p-side electrode  38  of the red semiconductor laser device  30  and the electrode  13   b  are bonded to each other through the solder layer  14   b  melted by applying heat of about 300° C. At this time, the red semiconductor laser device  30  is bonded onto the upper surface  11   b  (electrode  13   b ) of the heat radiation substrate  10  such that the height from the lower surface  12  of the heat radiation substrate  10  to the active layer  33  of the red semiconductor laser device  30  in the vertical direction (direction Z) is H 4  (see  FIG. 2 ). Thus, the light-emitting region  20   b  of the blue-violet semiconductor laser device  20  and the light-emitting region  30   b  of the red semiconductor laser device  30  are located at the heights substantially equal to each other and arranged such that the height positions of at least the portions thereof overlap each other. The red semiconductor laser device  30  is bonded onto the upper surface  11   b  of the heat radiation substrate  10  in a junction-down system, so that the light-emitting region  30   b  is located on the side (lower side (Z 2  side)) bonded to the upper surface  11   b.    
     Thereafter, the heat radiation substrate  10  is bonded to the base portion  40  through the bonding layer  50 , as shown in  FIG. 2 . At this time, the upper surfaces  11   a  and  11   b  and the lower surface  12  of the heat radiation substrate  10  are horizontally arranged. Then, the electrode  13   a  and the base portion  40  are connected with each other through the wire  60 . The electrode  13   b  and the lead terminal (on the anode side) (not shown) are connected with each other through the wire  61 . The p-side pad electrode  28  and the lead terminal (on the anode side) (not shown) are connected with each other through the wire  62 . The n-side electrode  39  and the base portion  40  are connected with each other through the wire  63 . Thus, the two-wavelength semiconductor laser apparatus  100  is formed. 
     According to the first embodiment, as hereinabove described, the light-emitting region  20   b  on an upper side (Z 1  side) of the blue-violet semiconductor laser device  20  bonded onto the upper surface  11   a  on the lower side of the step portion  11   c  is located above (on the Z 1  side of) the upper surface  11   b  on the upper side of the step portion  11   c , onto which the red semiconductor laser device  30  is bonded, in a state where the heat radiation substrate  10  is horizontally arranged, whereby the light-emitting region  20   b  located on the upper side of the blue-violet semiconductor laser device  20  can be rendered closer to the light-emitting region  30   b  located on a lower side (Z 2  side) of the red semiconductor laser device  30  bonded onto the upper surface  11   b . Thus, the height (H 3 ) of the light-emitting region  20   b  in the blue-violet semiconductor laser device  20  and the height (H 4 ) of the light-emitting region  30   b  in the red semiconductor laser device  30  can be rendered close to each other in the structure having the blue-violet semiconductor laser device  20  and the red semiconductor laser device  30  mounted on the same heat radiation substrate  10 . 
     According to the first embodiment, the height (H 4 ) from the lower surface  12  of the heat radiation substrate  10  to the active layer  33  of the red semiconductor laser device  30  in the vertical direction (direction Z) is rendered substantially equal to the height (H 3 ) from the lower surface  12  of the heat radiation substrate  10  to the active layer  23  of the blue-violet semiconductor laser device  20  in the vertical direction, whereby the light-emitting region  20   b  of the blue-violet semiconductor laser device  20  and the light-emitting region  30   b  of the red semiconductor laser device  30  are located at the heights substantially equal to each other and arranged such that the height positions of at least the portions thereof overlap each other. Thus, the height (H 3 ) of the light-emitting region  20   b  and the height (H 4 ) of the light-emitting region  30   b  can be reliably rendered close to each other. 
     According to the first embodiment, the light-emitting regions  20   b  and  30   b  extend along the emitting directions of the laser beams from the blue-violet semiconductor laser device  20  and the red semiconductor laser device  30 . 
     The light-emitting regions  20   b  and  30   b  are arranged along the emitting directions (direction Y) of the laser beams at the heights equal to each other or close to each other. Thus, an optical axis of the laser beam in the blue-violet semiconductor laser device  20  and an optical axis of the laser beam in the red semiconductor laser device  30  can be aligned in the substantially same direction (direction Y). 
     According to the first embodiment, the height (H 2 −H 1 ) of the step portion  11   c  in the vertical direction is adjusted such that the light-emitting region  20   b  of the blue-violet semiconductor laser device  20  and the light-emitting region  30   b  of the red semiconductor laser device  30  are located at the heights substantially equal to each other, whereby the heights of the light-emitting regions  20   b  and  30   b  can be adjusted by simply adjusting the height of the step portion  11   c  of the heat radiation substrate  10 . Thus, the semiconductor laser apparatus  100  can be easily manufactured employing the versatile blue-violet semiconductor laser device  20  and the versatile red semiconductor laser device  30  both formed through the normal manufacturing processes. 
     According to the first embodiment, the lower surface of the n-side electrode  29  of the blue-violet semiconductor laser device  20  is bonded onto the upper surface  11   a  through the solder layer  14   a . Thus, the blue-violet semiconductor laser device  20  is bonded to the heat radiation substrate  10  in a junction-up system, and hence the light-emitting region  20   b  of the blue-violet semiconductor laser device  20  can be easily arranged above the upper surface  11   b  of the heat radiation substrate  10 . 
     According to the first embodiment, the lower surface of the p-side electrode  38  of the red semiconductor laser device  30  is bonded onto the upper surface  11   b  through the solder layer  14   b . Thus, the red semiconductor laser device  30  is bonded to the heat radiation substrate  10  in a junction-down system, and hence the light-emitting region  30   b  of the red semiconductor laser device  30  can be easily rendered close to the light-emitting region  20   b  of the blue-violet semiconductor laser device  20  located above the upper surface  11   b  of the heat radiation substrate  10 . 
     According to the first embodiment, the amount of heat generation in the red semiconductor laser device  30  is larger than the amount of heat generation in the blue-violet semiconductor laser device  20 . Thus, the red semiconductor laser device  30  where a larger amount of heat is generated is bonded to the heat radiation substrate  10  in a junction-down system, and hence heat generated in the red semiconductor laser device  30  can be easily radiated to the heat radiation substrate  10 . 
     According to the first embodiment, the upper surface (on the Z 1  side) of the n-side electrode  39  of the red semiconductor laser device  30  is located above the upper surface (on the Z 1  side) of the p-side pad electrode  28  of the blue-violet semiconductor laser device  20 . Thus, the red semiconductor laser device  30  can be easily bonded onto the upper surface  11   b  of the heat radiation substrate  10  to which the blue-violet semiconductor laser device  20  is previously bonded without influence of a height of the blue-violet semiconductor laser device  20  in the manufacturing process. 
     According to the first embodiment, the step portion  11   c  is formed to extend in the direction Y along the emitting directions of the laser beams from the blue-violet semiconductor laser device  20  and the red semiconductor laser device  30 , whereby the laser beam from the red semiconductor laser device  30  bonded onto the upper surface  11   b  is not blocked by the upper surface  11   a  or the blue-violet semiconductor laser device  20  bonded onto the upper surface  11   a , dissimilarly to a case where the step portion  11   c  extends in a direction intersecting with the emitting directions (direction Y). Thus, a range to which the red semiconductor laser device  30  can emit the laser beam can be inhibited from decrease. 
     According to the first embodiment, the distance L 1  from the step portion  11   c  to the ridge portion  25  of the blue-violet semiconductor laser device  20  in the horizontal direction is substantially constant along the emitting direction (direction Y) of the laser beam. The distance L 2  from the step portion  11   c  to the ridge portion  35  of the red semiconductor laser device  30  in the horizontal direction is substantially constant along the emitting direction (direction Y) of the laser beam. Thus, the optical axis of the laser beam emitted from the blue-violet semiconductor laser device  20  and the optical axis of the laser beam emitted from the red semiconductor laser device  30  can be aligned as much as possible with reference to the step portion  11   c.    
     According to the first embodiment, the blue-violet semiconductor laser device  20  made of a nitride-based semiconductor is employed as the first semiconductor laser device, and the red semiconductor laser device  30  made of a GaInP-based semiconductor is employed as the second semiconductor laser device. According to the first embodiment, the light-emitting region  20   b  of the blue-violet semiconductor laser device  20  is located on the upper side (the side opposite to the side bonded to the upper surface  11   a ), and hence even the blue-violet semiconductor laser device  20  made of a nitride-based semiconductor easily influenced by heat in bonding can be inhibited from being influenced by the heat in bonding when the blue-violet semiconductor laser device  20  is bonded onto the upper surface  11   a  of the heat radiation substrate  10 . Thus, deterioration of luminous characteristics due to the heat in bonding can be inhibited. Further, the light-emitting region  30   b  of the red semiconductor laser device  30  is located on the side bonded to the upper surface  11   b  (a side closer to the heat radiation substrate  10  (lower side)), and hence heat generated in the light-emitting region  30   b  when the laser beam is emitted from the red semiconductor laser device  30  can be easily radiated to the heat radiation substrate  10 . 
     According to the first embodiment, the heat radiation substrate  10  made of AlN having insulating properties is employed. The heat radiation substrate  10  comprises the electrode  13   a  formed on the upper surface  11   a  of the step portion  11   c  and the electrode  13   b  formed on the upper surface  11   b  of the step portion  11   c . Thus, power can be easily supplied to the blue-violet semiconductor laser device  20  and the red semiconductor laser device  30  employing the electrode  13   a  formed on the upper surface  11   a  and the electrode  13   b  formed on the upper surface  11   b  even on the heat radiation substrate  10  including the step portion  11   c . Further, deviation in the height direction between an application position of the laser beam from the blue-violet semiconductor laser device  20  and an application position of the laser beam from the red semiconductor laser device  30  can be easily inhibited from increase by effectively employing the heat radiation substrate  10  constituting the two-wavelength semiconductor laser apparatus  100 . 
     According to the first embodiment, the electrodes  13   a  and  13   b  are separated from each other by the step portion  11   c  and connected with the wires  60  and  61 , respectively. Thus, the electrodes  13   a  and  13   b  can be easily isolated from each other by effectively employing the step portion  11   c . Further, the wires  60  and  61  are bonded at heights different from each other, and hence contact between the wires  60  and  61  can be easily inhibited. 
     Second Embodiment 
     A second embodiment is described with reference to  FIGS. 6 to 8 . In a three-wavelength semiconductor laser apparatus  200  according to this second embodiment, a two-wavelength semiconductor laser device  280  having a red semiconductor laser device  230  and an infrared semiconductor laser device  290  monolithically formed on the same GaAs substrate  281  is employed in place of the red semiconductor laser device  30  of the first embodiment. In the figures, a structure similar to that of the two-wavelength semiconductor laser apparatus  100  according to the first embodiment is denoted by the same reference numerals. The three-wavelength semiconductor laser apparatus  200  is an example of the “semiconductor laser apparatus” in the present invention. 
     A structure of the three-wavelength semiconductor laser apparatus  200  according to the second embodiment of the present invention is now described with reference to  FIG. 6 . 
     The three-wavelength semiconductor laser apparatus  200  according to the second embodiment comprises a heat radiation substrate  10 , a blue-violet semiconductor laser device  220  having a lasing wavelength of about 405 nm, the two-wavelength semiconductor laser device  280  having the red semiconductor laser device  230  with a lasing wavelength of about 650 nm and the infrared semiconductor laser device  290  with a lasing wavelength of about 780 nm monolithically formed and a base portion  40 , as shown in  FIG. 6 . The blue-violet semiconductor laser device  220  is bonded onto an upper surface  11   a  of the heat radiation substrate  10  on an X 1  side, and the two-wavelength semiconductor laser device  280  is bonded onto an upper surface  11   b  of the heat radiation substrate  10  on an X 2  side. The blue-violet semiconductor laser device  220  is an example of the “first semiconductor laser device” in the present invention, and the red semiconductor laser device  230  and the infrared semiconductor laser device  290  are an example of the “second semiconductor laser device” in the present invention. 
     A ridge portion  225  formed on a p-type cladding layer  224  of the blue-violet semiconductor laser device  220  deviates to a step portion  11   c  (X 2  side) from a center of the blue-violet semiconductor laser device  220  in a direction X (horizontal direction). In other words, a light-emitting region  220   b  of the blue-violet semiconductor laser device  220  deviates to the step portion  11   c  (X 2  side) from the center of the blue-violet semiconductor laser device  220  in the direction X (horizontal direction). A p-side ohmic electrode  226 , a current blocking layer  227  and a p-side pad electrode  228  are formed to correspond to the ridge portion  225 . The ridge portion  225  is an example of the “first ridge portion” in the present invention. 
     Electrodes  213   b  and  213   c  are formed on the upper surface  11   b  of the heat radiation substrate  10 . The electrode  213   b  is formed on a side (X 1  side) closer to the step portion  11   c , and the electrode  213   c  is formed on a side (X 2  side) farther from the step portion  11   c . The red semiconductor laser device  230  of the two-wavelength semiconductor laser device  280  is bonded onto the electrode  213   b  through a solder layer  214   b , and the infrared semiconductor laser device  290  of the two-wavelength semiconductor laser device  280  is bonded onto the electrode  213   c  through a solder layer  214   c . The electrodes  213   b  and  213   c  are examples of the “second electrode” in the present invention. 
     In the two-wavelength semiconductor laser device  280 , the red semiconductor laser device  230  and the infrared semiconductor laser device  290  are monolithically formed on the common (same) n-type GaAs substrate  281 . The red semiconductor laser device  230  is formed on the X 1  side on a lower surface of the n-type GaAs substrate  281 , and the infrared semiconductor laser device  290  is formed on the X 2  side on the lower surface of the n-type GaAs substrate  281 . The red semiconductor laser device  230  and the infrared semiconductor laser device  290  are separated from each other through a groove portion  282  formed in a substantially central portion of the lower surface of the n-type GaAs substrate  281  in the direction X. The n-type GaAs substrate  281  is an example of the “substrate” in the present invention. 
     The red semiconductor laser device  230  is formed with an n-type cladding layer  32 , an active layer  33 , a p-type cladding layer  234 , a current blocking layer  237  and a p-side electrode  238  on the X 1  side on the lower surface of the n-type GaAs substrate  281 . The current blocking layer  237  is formed integrally with a current blocking layer  297  of the infrared semiconductor laser device  290  described later. 
     A ridge portion  235  formed on the p-type cladding layer  234  of the red semiconductor laser device  230  deviates to the step portion  11   c  (X 1  side) from a center of the red semiconductor laser device  230  in the direction X (horizontal direction). In other words, a light-emitting region  230   b  of the red semiconductor laser device  230  deviates to the step portion  11   c  (X 1  side) from the center of the red semiconductor laser device  230  in the direction X (horizontal direction). The current blocking layer  237  and the p-side electrode  238  are formed to correspond to the ridge portion  235 . The ridge portion  235  is an example of the “second ridge portion” in the present invention. The infrared semiconductor laser device  290  is made of a GaAs-based semiconductor. Specifically, the infrared semiconductor laser device  290  is formed with an n-type cladding layer  292  made of AlGaAs on the X 2  side on the lower surface of the n-type GaAs substrate  281 . An active layer  293  having an MQW structure in which quantum well layers made of AlGaAs having a lower Al composition and barrier layers made of AlGaAs having a higher Al composition are alternately stacked is formed on a lower surface of the n-type cladding layer  292 . Luminous characteristics of the active layer  293  made of a GaAs-based semiconductor are hardly deteriorated because thermal stress is hardly accumulated as compared with an active layer  23  of the blue-violet semiconductor laser device  220 , even if heat of about 300° C. is applied in bonding the infrared semiconductor laser device  290  (two-wavelength semiconductor laser device  280 ) onto the upper surface  11   b  of the heat radiation substrate  10 . A p-type cladding layer  294  made of AlGaAs is formed on a lower surface of the active layer  293 . A material constituting the n-type cladding layer  292 , the active layer  293  and the p-type cladding layer  294  is an example of the “GaAs-based semiconductor” in the present invention. 
     A ridge portion (projecting portion)  295  extending along a direction Y is formed on a portion of the p-type cladding layer  294  deviating to the step portion  11   c  (X 1  side) from a center of the infrared semiconductor laser device  290  in the direction X (horizontal direction). A laser beam is emitted from a light-emitting surface  290   a , which is a surface of the infrared semiconductor laser device  290  on one end (on a Y 1  side) in an emitting direction (direction Y). At this time, the laser beam is emitted from a position of the active layer  293  corresponding to the ridge portion  295  on the light-emitting surface  290   a , as shown in  FIG. 2 . In other words, a light-emitting region  290   b  (region surrounded by a broken line) of the infrared semiconductor laser device  290  is located in a position corresponding to the ridge portion  295  deviating to the step portion  11   c  (X 1  side) from the center of the infrared semiconductor laser device  290  in the direction X (horizontal direction) at a height of the active layer  293 . The light-emitting region  290   b  is an example of the “second light-emitting region” in the present invention. The ridge portion  295  is an example of the “second ridge portion” in the present invention. 
     The current blocking layer  297  formed integrally with the current blocking layer  237  of the red semiconductor laser device  230  is formed on a lower surface of the p-type cladding layer  294  other than the ridge portion  295  and both side surfaces of the ridge portion  295 . A p-side electrode  298  made of Au or the like is formed on lower surfaces of the ridge portion  295  and the current blocking layer  297 . This p-side electrode  298  is connected to the electrode  213   c  and a lead terminal (on an anode side) (not shown) through the solder layer  214   c.    
     An n-side electrode  283  in which an AuGe layer, an Ni layer and an Au layer are stacked successively from a side closer to the n-type GaAs substrate  281  is formed on a substantially entire region of an upper surface of the n-type GaAs substrate  281 . 
     The infrared semiconductor laser device  290  is bonded onto the upper surface  11   b  such that the active layer  293  and the ridge portion  295  are located below (on a Z 2  side of) the n-type GaAs substrate  281 . In other words, the infrared semiconductor laser device  290  is bonded onto the upper surface  11   b  in a junction-down system, so that the light-emitting region  290   b  is located on a side (lower side (Z 2  side)) bonded to the upper surface  11   b.    
     According to the second embodiment, a height from a lower surface  12  of the heat radiation substrate  10  to the active layer  33  of the red semiconductor laser device  230  in a vertical direction (direction Z) and a height from the lower surface  12  of the heat radiation substrate  10  to the active layer  293  of the infrared semiconductor laser device  290  in the vertical direction are substantially equal to each other, and the heights each are a height H 4 . Further, the height H 4  is substantially equal to a height H 3  from the lower surface  12  of the heat radiation substrate  10  to the active layer  23  of the blue-violet semiconductor laser device  220  in the vertical direction. Thus, the light-emitting region  220   b  of the blue-violet semiconductor laser device  220 , the light-emitting region  230   b  of the red semiconductor laser device  230  and the light-emitting region  290   b  of the infrared semiconductor laser device  290  are located at the heights substantially equal to each other and arranged such that height positions of at least portions thereof overlap each other. A height (H 2 −H 1 ) of the step portion  11   c  in the vertical direction is adjusted such that the light-emitting region  220   b  of the blue-violet semiconductor laser device  220 , the light-emitting region  230   b  of the red semiconductor laser device  230  and the light-emitting region  290   b  of the infrared semiconductor laser device  290  are located at the heights substantially equal to each other. 
     The electrode  213   b  formed on the heat radiation substrate  10  and a lead terminal (on the anode side) (not shown) are electrically connected with each other through a wire  61 . The p-side pad electrode  228  of the blue-violet semiconductor laser device  220  and a lead terminal (on the anode side) (not shown) are electrically connected with each other through a wire  62 . The n-side electrode  283  of the two-wavelength semiconductor laser device  280  and the base portion  40  are electrically connected with each other through a wire  63 . The electrode  213   c  formed on the heat radiation substrate  10  and the lead terminal (on the anode side) (not shown) are electrically connected with each other through a wire  264 . 
     The remaining structure of the three-wavelength semiconductor laser apparatus  200  according to the second embodiment is similar to that of the two-wavelength semiconductor laser apparatus  100  according to the first embodiment. 
     A manufacturing process of the three-wavelength semiconductor laser apparatus  200  according to the second embodiment is now described with reference to  FIGS. 3 and 6  to  8 . 
     As shown in  FIG. 3 , the heat radiation substrate  10  having the upper surfaces  11   a  and  11   b  and the step portion  11   c  are first formed. Then, an electrode  13   a  is formed on the upper surface  11   a  of the heat radiation substrate  10 , as shown in  FIG. 7 . The electrodes  213   b  and  213   c  are formed on the X 1  and X 2  sides, respectively, on the upper surface  11   b  of the heat radiation substrate  10 . Thereafter, solder layers  14   a ,  214   b  and  214   c  are formed on the electrodes  13   a ,  213   b  and  213   c , respectively. 
     The blue-violet semiconductor laser device  220  in which the ridge portion deviates to one side from the center in the direction X orthogonal to the emitting direction (direction Y) and the two-wavelength semiconductor laser device  280  having the red semiconductor laser device  230  and the infrared semiconductor laser device  290  monolithically formed in which the ridge portions deviate to one side from the centers in the direction X perpendicular to the emitting direction (direction Y) are formed through prescribed manufacturing processes. Then, an n-side electrode  29  of the blue-violet semiconductor laser device  220  and the electrode  13   a  are bonded to each other through the solder layer  14   a  melted by applying heat of about 300° C. The blue-violet semiconductor laser device  220  is bonded such that the ridge portion  225  deviates to the step portion  11   c  (X 2  side) from the center of the blue-violet semiconductor laser device  220  in the direction X (horizontal direction). At this time, the blue-violet semiconductor laser device  220  is bonded onto the upper surface  11   a  of the heat radiation substrate  10  in a junction-up system, so that the light-emitting region  220   b  is located on the side (upper side (Z 1  side)) opposite to the side bonded to the upper surface  11   a . The blue-violet semiconductor laser device  220  is bonded onto the upper surface  11   a  such that the height from the lower surface  12  of the heat radiation substrate  10  to the active layer  23  of the blue-violet semiconductor laser device  220  in the vertical direction (direction Z) is H 3  (see  FIG. 8 ). 
     Thereafter, the n-side electrode  283  of the two-wavelength semiconductor laser device  280  is grasped from above (from a Z 1  side) with a collet  70  such that the p-side electrode  238  of the red semiconductor laser device  230  and the solder layer  214   b  are opposed to each other while the p-side electrode  298  of the infrared semiconductor laser device  290  and the solder layer  214   c  are opposed to each other, as shown in  FIG. 8 . Then, the p-side electrode  238  of the red semiconductor laser device  230  and the electrode  213   b  are bonded to each other through the solder layer  214   b  melted by applying heat of about 300° C. The red semiconductor laser device  230  is bonded such that the ridge portion  235  deviates to the step portion  11   c  (X 1  side) from the center of the red semiconductor laser device  230  in the direction X (horizontal direction). The p-side electrode  298  of the infrared semiconductor laser device  290  and the electrode  213   c  are bonded to each other through the solder layer  214   c  melted by applying heat of about 300° C. simultaneously with the bonding of the red semiconductor laser device  230 . The infrared semiconductor laser device  290  is bonded such that the ridge portion  295  deviates to the step portion  11   c  (X 1  side) from the center of the infrared semiconductor laser device  290  in the direction X (horizontal direction). 
     At this time, the red semiconductor laser device  230  and the infrared semiconductor laser device  290  are bonded onto the upper surface  11   b  of the heat radiation substrate  10  such that the height from the lower surface  12  of the heat radiation substrate  10  to the active layer  33  of the red semiconductor laser device  230  in the vertical direction (direction Z) and the height from the lower surface  12  of the heat radiation substrate  10  to the active layer  293  of the infrared semiconductor laser device  290  in the vertical direction are H 4  (see  FIG. 6 ). Thus, the light-emitting region  220   b  of the blue-violet semiconductor laser device  220 , the light-emitting region  230   b  of the red semiconductor laser device  230  and the light-emitting region  290   b  of the infrared semiconductor laser device  290  are located at the heights substantially equal to each other and arranged such that the height positions of at least the portions thereof overlap each other. The red semiconductor laser device  230  and the infrared semiconductor laser device  290  of the two-wavelength semiconductor laser device  280  are bonded onto the upper surface  11   b  of the heat radiation substrate  10  in a junction-down system, so that the light-emitting regions  230   b  and  290   b  are located on the side (lower side (Z 2  side)) bonded to the upper surface  11   b.    
     Thereafter, the heat radiation substrate  10  is bonded to the base portion  40  through a bonding layer  50 , as shown in  FIG. 6 . At this time, the upper surfaces  11   a  and  11   b  and the lower surface  12  of the heat radiation substrate  10  are horizontally arranged. Then, the electrode  13   a  and the base portion  40  are connected with each other through a wire  60 . The electrode  213   b  and the lead terminal (on the anode side) (not shown) are connected with each other through the wire  61 . The p-side pad electrode  228  and the lead terminal (on the anode side) (not shown) are connected with each other through the wire  62 . The n-side electrode  283  and the base portion  40  are connected with each other through the wire  63 . The electrode  213   c  and the lead terminal (on the anode side) (not shown) are connected with each other through the wire  264 . Thus, the three-wavelength semiconductor laser apparatus  200  is formed. 
     The remaining manufacturing process of the three-wavelength semiconductor laser apparatus  200  according to the second embodiment is similar to that of the two-wavelength semiconductor laser apparatus  100  according to the first embodiment. 
     According to the second embodiment, as hereinabove described, the light-emitting region  220   b  of the blue-violet semiconductor laser device  220  is formed at a position deviating to the step portion  11   c  (X 2  side) from the center of the blue-violet semiconductor laser device  220  in the direction X while the light-emitting region  230   b  of the red semiconductor laser device  230  is formed at a position deviating to the step portion  11   c  (X 1  side) from the center of the red semiconductor laser device  230  in the direction X and the light-emitting region  290   b  of the infrared semiconductor laser device  290  is formed at a position deviating to the step portion  11   c  (X 1  side) from the center of the infrared semiconductor laser device  290  in the direction X. Thus, the light-emitting region  220   b  and the light-emitting regions  230   b  and  290   b  can be rendered closer to the step portion  11   c , and hence the light-emitting region  220   b  of the blue-violet semiconductor laser device  220  and the light-emitting regions  230   b  and  290   b  of the red and infrared semiconductor laser devices  230  and  290  can be rendered close to each other in the horizontal direction (direction X). 
     According to the second embodiment, the red semiconductor laser device  230  and the infrared semiconductor laser device  290  are monolithically formed on the same n-type GaAs substrate  281 , whereby in the three-wavelength semiconductor laser apparatus  200  comprising the blue-violet semiconductor laser device  220  bonded onto the upper surface  11   a  and the two-wavelength semiconductor laser device  280  including the red semiconductor laser device  230  and the infrared semiconductor laser device  290  both bonded onto the upper surface  11   b  and monolithically formed on the same n-type GaAs substrate  281 , the height (H 3 ) of the light-emitting region  220   b  in the blue-violet semiconductor laser device  220  and the heights (H 4 ) of the light-emitting regions  230   b  and  290   b  in the red and infrared semiconductor laser devices  230  and  290  can be rendered close to each other. Further, the red semiconductor laser device  230  and the infrared semiconductor laser device  290  are formed on the common n-type GaAs substrate  281 , whereby deviation between a height position of the light-emitting region  230   b  of the red semiconductor laser device  230  and a height position of the light-emitting region  290   b  of the infrared semiconductor laser device  290  can be inhibited when the red semiconductor laser device  230  and the infrared semiconductor laser device  290  are bonded to the heat radiation substrate  10 . 
     According to the second embodiment, the two-wavelength semiconductor laser device  280  includes the light-emitting regions  230   b  and  290   b , and the height positions of at least the portions of the light-emitting region  220   b  of the blue-violet semiconductor laser device  220  and each of the light-emitting regions  230   b  and  290   b  of the two-wavelength semiconductor laser device  280  overlap each other in a state where the heat radiation substrate  10  is horizontally arranged. Thus, the height position of the light-emitting region  220   b  and the height positions of the light-emitting regions  230   b  and  290   b  plurally provided can be reliably rendered close to each other, and hence deviation in a height direction between an application position of a laser beam from the blue-violet semiconductor laser device  220  and application positions of a plurality of laser beams from the two-wavelength semiconductor laser device  280  can be reliably inhibited from increase. 
     According to the second embodiment, the red semiconductor laser device  230  and the infrared semiconductor laser device  290  having the different lasing wavelengths from each other are bonded onto the upper surface  11   b  through the groove portion  282 . The light-emitting regions  230   b  and  290   b  of the red and infrared semiconductor laser devices  230  and  290  are arranged at the positions deviating to the step portion  11   c  from the centers of respective device bodies in a state where the heat radiation substrate  10  is horizontally arranged. Thus, the light-emitting regions  230   b  and  290   b  of the red and infrared semiconductor laser devices  230  and  290  can be rendered closer to the step portion  11   c  also when forming the three-wavelength semiconductor laser apparatus  200 , and hence optical axes of the laser beams in the respective semiconductor laser devices can be easily aligned. The remaining effects of the second embodiment are similar to those of the first embodiment. 
     Third Embodiment 
     An optical pickup  300  according to a third embodiment of the present invention is now described with reference to  FIGS. 6 and 9 . The optical pickup  300  is an example of the “optical apparatus” in the present invention. 
     The optical pickup  300  according to the third embodiment of the present invention comprises a can-type three-wavelength semiconductor laser apparatus  310  mounted with the three-wavelength semiconductor laser apparatus  200  according to the second embodiment, an optical system  320  adjusting laser beams emitted from the three-wavelength semiconductor laser apparatus  310  and a light detection portion  330  receiving the laser beams, as shown in  FIG. 9 . 
     The optical system  320  has a polarizing beam splitter (PBS)  321 , a collimator lens  322 , a beam expander  323 , a λ/4 plate  324 , an objective lens  325 , a cylindrical lens  326  and an optical axis correction device  327 . 
     The PBS  321  totally transmits the laser beams emitted from the three-wavelength semiconductor laser apparatus  310 , and totally reflects the laser beams fed back from an optical disc  340 . The collimator lens  322  converts the laser beams emitted from the three-wavelength semiconductor laser apparatus  310  and transmitted through the PBS  321  to parallel beams. The beam expander  323  is constituted by a concave lens, a convex lens and an actuator (not shown). The actuator has a function of correcting wave surface states of the laser beams emitted from the three-wavelength semiconductor laser apparatus  310  by varying a distance between the concave lens and the convex lens. 
     The λ/4 plate  324  converts the linearly polarized laser beams, substantially converted to the parallel beams by the collimator lens  322 , to circularly polarized beams. Further, the λ/4 plate  324  converts the circularly polarized laser beams fed back from the optical disc  340  to linearly polarized beams. A direction of linear polarization in this case is orthogonal to a direction of linear polarization of the laser beams emitted from the three-wavelength semiconductor laser apparatus  310 . Thus, the PBS  321  substantially totally reflects the laser beams fed back from the optical disc  340 . The objective lens  325  converges the laser beams transmitted through the λ/4 plate  324  on a surface (recording layer) of the optical disc  340 . An objective lens actuator (not shown) renders the objective lens  325  movable. 
     The cylindrical lens  326 , the optical axis correction device  327  and the light detection portion  330  are arranged to be along optical axes of the laser beams totally reflected by the PBS  321 . The cylindrical lens  326  provides the incident laser beams with astigmatic action. The optical axis correction device  327  is constituted by a diffraction grating and so arranged that spots of zero-order diffracted beams of blue-violet, red and infrared laser beams transmitted through the cylindrical lens  326  coincide with each other on a detection region of the light detection portion  330  described later. 
     The light detection portion  330  outputs a playback signal on the basis of intensity distribution of the received laser beams. Thus, the optical pickup  300  comprising the three-wavelength semiconductor laser apparatus  310  is formed. 
     In this optical pickup  300 , the three-wavelength semiconductor laser apparatus  310  can independently emit blue-violet, red and infrared laser beams from the blue-violet semiconductor laser device  220 , the red semiconductor laser device  230  and the infrared semiconductor laser device  290  (see  FIG. 6 ). The laser beams emitted from the three-wavelength semiconductor laser apparatus  310  are adjusted by the PBS  321 , the collimator lens  322 , the beam expander  323 , the λ/4 plate  324 , the objective lens  325 , the cylindrical lens  326  and the optical axis correction device  327  as described above, and thereafter applied onto the detection region of the light detection portion  330 . 
     When data recorded in the optical disc  340  is play backed, the laser beams emitted from the blue-violet semiconductor laser device  220 , the red semiconductor laser device  230  and the infrared semiconductor laser device  290  are controlled to have constant power and applied to the recording layer of the optical disc  340 , so that the playback signal outputted from the light detection portion  330  can be obtained. When data is recorded in the optical disc  340 , the laser beams emitted from the blue-violet semiconductor laser device  220  and the red semiconductor laser device  230  (infrared semiconductor laser device  290 ) are controlled in power and applied to the optical disc  340 , on the basis of the data to be recorded. Thus, the data can be recorded in the recording layer of the optical disc  340 . Thus, the data can be recorded in or played back from the optical disc  340  with the optical pickup  300  comprising the three-wavelength semiconductor laser apparatus  310 . 
     According to the third embodiment, as hereinabove described, the optical pickup  300  comprises the three-wavelength semiconductor laser apparatus  200  according to the second embodiment, whereby deviation in a height direction between an application position (spot) of the laser beam from the blue-violet semiconductor laser device  220 , an application position of the laser beam from the red semiconductor laser device  230  and an application position of the laser beam from the infrared semiconductor laser device  290  can be inhibited from increase when the laser beams are applied to the optical disc  340  through the optical system  320 . 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims. 
     For example, while the ridge portion  25  is formed in the substantially central portion of the blue-violet semiconductor laser device  20  in the direction X, and the ridge portion  35  is formed in the substantially central portion of the red semiconductor laser device  30  in the direction X in the aforementioned first embodiment, the present invention is not restricted to this. In the present invention, a light-emitting region  420   b  (ridge portion  425 ) of a blue-violet semiconductor laser device  420  may deviate to the step portion  11   c  (X 2  side) from a center of the blue-violet semiconductor laser device  420  in the direction X (horizontal direction), and a light-emitting region  430   b  (ridge portion  435 ) of a red semiconductor laser device  430  may deviate to the step portion  11   c  (X 1  side) from a center of the red semiconductor laser device  430  in the direction X (horizontal direction) as in a two-wavelength semiconductor laser apparatus  400  according to a modification of the first embodiment shown in  FIG. 10 . Alternatively, either the ridge portion of the blue-violet semiconductor laser device or the ridge portion of the red semiconductor laser device may deviate to the step portion, and either the ridge portion of the red semiconductor laser device or the ridge portion of the blue-violet semiconductor laser device may be formed in the substantially central portion of a device body. The two-wavelength semiconductor laser apparatus  400 , the blue-violet semiconductor laser device  420  and the red semiconductor laser device  430  are examples of the “semiconductor laser apparatus”, the “first semiconductor laser device” and the “second semiconductor laser device” in the present invention, respectively. The ridge portions  425  and  435  are examples of the “first ridge portion” and the “second ridge portion” in the present invention, respectively. 
     While the two-wavelength semiconductor laser apparatus  100  includes the blue-violet semiconductor laser device  20  bonded onto the upper surface  11   a  of the heat radiation substrate  10  and the red semiconductor laser device  30  bonded onto the upper surface  11   b  of the heat radiation substrate  10  in the aforementioned first embodiment, and the three-wavelength semiconductor laser apparatus  200  includes the blue-violet semiconductor laser device  220  bonded onto the upper surface  11   a  of the heat radiation substrate  10  and the two-wavelength semiconductor laser device  280  having the red semiconductor laser device  230  and the infrared semiconductor laser device  290  both bonded onto the upper surface  11   b  of the heat radiation substrate  10  and monolithically formed in the aforementioned second embodiment, the present invention is not restricted to this. In the present invention, a green semiconductor laser device or a blue semiconductor laser device made of a nitride-based semiconductor may be employed in place of the blue-violet semiconductor laser device in each of the aforementioned first and second embodiments. An infrared semiconductor laser device may be employed in place of the red semiconductor laser device in the aforementioned first embodiment. The three-wavelength semiconductor laser apparatus of the aforementioned second embodiment may include the red semiconductor laser device, a green semiconductor laser device and a blue semiconductor laser device. Thus, the three-wavelength semiconductor laser apparatus having three primary colors of RGB can be formed. At this time, the green semiconductor laser device and the blue semiconductor laser device are preferably arranged on the upper surface  11   a  of the heat radiation substrate  10 , and the red semiconductor laser device is preferably arranged on the upper surface  11   b  of the heat radiation substrate  10 . 
     While the first light-emitting region (the light-emitting regions  20   b  and  220   b ) of the first semiconductor laser device (the blue-violet semiconductor laser devices  20  and  220 ) and the second light-emitting region (the light-emitting regions  30   b ,  230   b  and  290   b ) of the second semiconductor laser device (the red semiconductor laser devices  30  and  230  and the infrared semiconductor laser device  290 ) are located at the heights substantially equal to each other and arranged such that the height positions of at least the portions thereof overlap each other in the aforementioned first and second embodiments, the present invention is not restricted to this. In the present invention, the height position of the first light-emitting region and the height position of the second light-emitting region may not overlap each other as long as the first light-emitting region of the first semiconductor laser device and the second light-emitting region of the second semiconductor laser device are located at heights close to each other. 
     While the height (H 2 −H 1 ) of the step portion  11   c  in the vertical direction is adjusted such that the light-emitting regions  20   b  and  220   b  of the blue-violet semiconductor laser devices  20  and  220 , the light-emitting regions  30   b  and  230   b  of the red semiconductor laser devices  30  and  230  and the light-emitting region  290   b  of the infrared semiconductor laser device  290  are located at the heights substantially equal to each other in the aforementioned first and second embodiments, the present invention is not restricted to this. In the present invention, the height position of the first light-emitting region in the first semiconductor laser device (the blue-violet semiconductor laser device) or the height position of the second light-emitting region in the second semiconductor laser device (the red semiconductor laser device and the infrared semiconductor laser device) may be adjusted without adjusting the height of the step portion in the vertical direction. Alternatively, thicknesses, in the vertical direction, of the electrode and the solder layer formed on the upper surface (first upper surface) on which the first semiconductor laser device is arranged may be adjusted, or thicknesses, in the vertical direction, of the electrode and the solder layer formed on the upper surface (second upper surface) on which the second semiconductor laser device is arranged may be adjusted. 
     While the blue-violet semiconductor laser devices  20  and  220  are bonded onto the upper surface  11   a  (first upper surface) in a junction-up system, and the red semiconductor laser devices  30  and  230  and the infrared semiconductor laser device  290  are bonded onto the upper surface  11   b  (second upper surface) located above the upper surface  11   a  (first upper surface) in a junction-down system in the aforementioned first and second embodiments, the present invention is not restricted to this. In the present invention, the red semiconductor laser device and the infrared semiconductor laser device may be bonded onto the first upper surface in a junction-up system, and the blue-violet semiconductor laser device may be bonded onto the second upper surface located above the first upper surface in a junction-down system. At this time, at least the light-emitting region of the red semiconductor laser device and the light-emitting region of the infrared semiconductor laser device must be located above the second upper surface. 
     While the heat radiation substrate  10  is made of AlN having insulating properties in each of the aforementioned first and second embodiments, the present invention is not restricted to this. The heat radiation substrate may be made of undoped Si having insulating properties, for example. 
     While the current blocking layers  27  and  37 ,  227 ,  237  and  297  are made of SiO 2  in the aforementioned first and second embodiments, the present invention is not restricted to this. In the present invention, another insulating material such as SiN or a semiconductor material such as AlInP or AlGaN may be employed as the current blocking layers. 
     While the aforementioned three-wavelength semiconductor laser apparatus  200  according to the second embodiment is mounted on the can-type three-wavelength semiconductor apparatus  310  in the aforementioned third embodiment, the present invention is not restricted to this. In the present invention, the aforementioned three-wavelength semiconductor laser apparatus  200  according to the second embodiment may be mounted on a frame-type three-wavelength semiconductor laser apparatus having a plate-like planar structure.