Patent Publication Number: US-8111726-B2

Title: Semiconductor laser device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor laser device. 
     2. Background Art 
     Various types of semiconductor laser devices are conventionally known, including those for DVD writing or information processing, solid state lasers such as Nd-doped YAG (Nd:YAG) lasers and Yb-doped YAG (Yb:YAG) layers, pumping source lasers such as Yb-doped fiber lasers and Er-doped fiber amplifiers, and lasers for optical communications. J. Sebastian et al. disclose a semiconductor laser device including a structure in which a GaAsP active layer is sandwiched between Al x Ga 1-x As optical guiding layers having the same thickness and composition. (See J. Sebastian et al., “High-Power 810-nm GaAsP-AlGaAs Diode Lasers With Narrow Beam Divergence,” IEEE J. Select. Topics In Quantum Electron., vol. 7, No. 2, pp. 334-339, 2001.) This structure allows the semiconductor laser device to operate at high output power. Other prior art includes Japanese Laid Open Patent Publication Nos. 11-233882 (1999), 11-233883 (1999), 11-243259 (1999), 2006-32437, and 11-163458 (1999). 
     SUMMARY OF THE INVENTION 
     In recent years, there has been an increasing need for a semiconductor laser device having a high electrical-to-optical power conversion efficiency in order to reduce power consumption. However, we are already approaching the point where it is no longer possible to improve the slope efficiency of semiconductor laser devices by increasing the thickness of the guiding layers and thereby reducing light absorption. That is, prior art techniques do not seem to be adequate to further reduce the power consumption of semiconductor laser devices. The present inventors have earnestly conducted research in order to overcome this problem and have come up with a semiconductor laser device having a high electrical-to-optical power conversion efficiency. 
     The prevent invention has been made to solve the above problem. It is, therefore, an object of the present invention to provide a semiconductor laser device having a high electrical-to-optical power conversion efficiency. 
     According to a first aspect of the present invention, a semiconductor laser device includes: an n-type cladding layer, a p-type cladding layer, an active layer located between the n-type cladding layer and the p-type cladding layer, an n-side guiding layer located on the same side of the active layer as the n-type cladding layer, and a p-side guiding layer located on the same side of the active layer as the p-type cladding layer. The n-side guiding layer, the active layer, and the p-side guiding layer are undoped or substantially undoped. The sum of the thicknesses of the n-side guiding layer, the active layer, and the p-side guiding layer is not less than 0.5 times the lasing wavelength of the semiconductor laser device and is not more than 2 μm. The p-side guiding layer is thinner and has a lower refractive index than the n-side guiding layer. 
     According to a second aspect of the present invention, a semiconductor laser device includes: an n-type cladding layer; a p-type cladding layer; an active layer located between the n-type cladding layer and the p-type cladding layer; an n-side guiding layer located on the same side of the active layer as the n-type cladding layer; and a p-side guiding layer located on the same side of the active layer as the p-type cladding layer. The n-side guiding layer, the active layer, and the p-side guiding layer are undoped or substantially undoped. The sum of the thicknesses of the n-side guiding layer, the active layer, and the p-side guiding layer is not less than 0.5 times the lasing wavelength of the semiconductor laser device and is not more than 2 μm. The p-side guiding layer is thinner than the n-side guiding layer, and the p-type cladding layer has a lower refractive index than the n-type cladding layer. 
     According to a third aspect of the present invention, a semiconductor laser device includes: an n-type cladding layer, a p-type cladding layer, an active layer located between the n-type cladding layer and the p-type cladding layer, an n-side guiding layer located on the same side of the active layer as the n-type cladding layer, and a p-side guiding layer located on the same side of the active layer as the p-type cladding layer. The n-side guiding layer, the active layer, and the p-side guiding layer are undoped or substantially undoped. The sum of the thicknesses of the n-side guiding layer, the active layer, and the p-side guiding layer is not less than 0.5 times the lasing wavelength of the semiconductor laser device and is not more than 2 μm. The p-side guiding layer is thinner and has a lower refractive index than the n-side guiding layer. The p-type cladding layer has a lower refractive index than the n-type cladding layer. 
     Other and further objects, features and advantages of the invention will appear more fully from the following description. 
     According to a fourth aspect of the present invention, a semiconductor laser device includes: an n-type cladding layer; a p-type cladding layer; an active layer located between the n-type cladding layer and the p-type cladding layer; an n-side guiding layer located on the same side of the active layer as the n-type cladding layer; and a p-side guiding layer located on the same side of the active layer as the p-type cladding layer. The n-side guiding layer, the active layer, and the p-side guiding layer are undoped or substantially undoped. The sum of the thicknesses of the n-side guiding layer, the active layer, and the p-side guiding layer is not less than 0.5 times the lasing wavelength of the semiconductor laser device and is not more than 2 μm. The p-side guiding layer is thinner than the n-side guiding layer. The n-side guiding layer includes a first layer located on a side of the active layer, a second layer located on a side of the n-type cladding layer, and a third layer disposed between the first layer and the second layer. The third layer has a higher refractive index than the first layer and the second layer. 
     According to a fifth aspect of the present invention, a semiconductor laser device includes: an n-type cladding layer; a p-type cladding layer; an active layer located between the n-type cladding layer and the p-type cladding layer; an n-side guiding layer located on the same side of the active layer as the n-type cladding layer; and a p-side guiding layer located on the same side of the active layer as the p-type cladding layer. The n-side guiding layer, the active layer, and the p-side guiding layer are undoped or substantially undoped. The sum of the thicknesses of the n-side guiding layer, the active layer, and the p-side guiding layer is not less than 0.5 times the lasing wavelength of the semiconductor laser device and is not more than 2 μm. The p-side guiding layer is thinner than the n-side guiding layer. The p-type cladding layer has a lower refractive index than said n-type cladding layer. The n-side guiding layer includes a first layer located on a side of the active layer, a second layer located on a side of the n-type cladding layer, and a third layer disposed between the first layer and the second layer. The third layer has a higher refractive index than the first layer and the second layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a semiconductor laser device used to illustrate features of a first embodiment of the present invention; 
         FIG. 2  shows the percentage of light confined to the undoped region of the semiconductor laser device as a function of the thickness of the undoped region; 
         FIG. 3  is a perspective view of a semiconductor laser device according to a first practical example of the first embodiment of the present invention; 
         FIG. 4  shows near field intensity distribution and the carrier distributions according to a first practical example of the first embodiment of the present invention; 
         FIG. 5  shows a comparative semiconductor laser device; 
         FIG. 6  schematically shows the carrier distribution and the optical intensity distribution according to the comparative semiconductor laser device; 
         FIG. 7  is a perspective view of a semiconductor laser device according to a second practical example of the first embodiment of the present invention; 
         FIG. 8  is a perspective view of a semiconductor laser device according to a third practical example of the first embodiment of the present invention; 
         FIG. 9  is a perspective view of a semiconductor laser device according to a fourth practical example of the first embodiment of the present invention; 
         FIG. 10  is a cross-sectional perspective view of a semiconductor laser device according to a fifth practical example of the first embodiment of the present invention; 
         FIG. 11  is a perspective view of a semiconductor laser device according to a sixth practical example of the first embodiment of the present invention; 
         FIG. 12  is a cross-sectional perspective view of a semiconductor laser device according to a seventh practical example of the first embodiment of the present invention; 
         FIG. 13  is a perspective view of a semiconductor laser device according to a second embodiment of the present invention; 
         FIG. 14  is a perspective view of a semiconductor laser device according to a second practical example of the second embodiment of the present invention; 
         FIG. 15  is a perspective view of a semiconductor laser device according to a third practical example of the second embodiment of the present invention; 
         FIG. 16  is a perspective view of a semiconductor laser device according to a fourth practical example of the second embodiment of the present invention; 
         FIG. 17  is a cross-sectional perspective view of a semiconductor laser device according to a fifth practical example of the second embodiment of the present invention; 
         FIG. 18  is a perspective view of a semiconductor laser device according to a sixth practical example of the second embodiment of the present invention; 
         FIG. 19  is a cross-sectional perspective view of a semiconductor laser device according to a seventh practical example of the second embodiment of the present invention; 
         FIG. 20  is a perspective view of a semiconductor laser device according to a first practical example of a third embodiment of the present invention; 
         FIG. 21  is a perspective view of a semiconductor laser device according to a second practical example of the third embodiment of the present invention; 
         FIG. 22  is a perspective view of a semiconductor laser device according to a third practical example of the third embodiment of the present invention; 
         FIG. 23  is a perspective view of a semiconductor laser device according to a fourth practical example of the third embodiment of the present invention; 
         FIG. 24  is a cross-sectional perspective view of a semiconductor laser device according to a fifth practical example of the third embodiment of the present invention; 
         FIG. 25  is a perspective view of a semiconductor laser device according to a sixth practical example of the third embodiment of the present invention; 
         FIG. 26  is a cross-sectional perspective view of a semiconductor laser device according to a seventh practical example of the third embodiment of the present invention; 
         FIG. 27  is a perspective view of a semiconductor laser device according to a first practical example of a fourth embodiment of the present invention; 
         FIG. 28  is a perspective view of a semiconductor laser device according to a second practical example of the fourth embodiment of the present invention; and 
         FIG. 29  is a perspective view of a semiconductor laser device according to a first practical example of a fifth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       FIG. 1  is a perspective view of an 810 nm semiconductor laser device used to illustrate features of a first embodiment of the present invention. With reference to  FIG. 1 , the following describes the advantages obtained when the undoped region of the device, which includes the guiding and active layers, etc., has a thickness at least 0.5 times the oscillation or lasing wavelength. 
     The reference numerals and corresponding components in  FIG. 1  are as follows:  1 , an n-electrode;  2 , an n-type GaAs substrate;  3 , an n-type Al x Ga 1-x As cladding layer with a thickness of 1.5 μm (where the Al mole fraction x=0.55);  101 , an n-side Al y Ga 1-y As outer guiding layer with a thickness of t/2 nm (where y=0.35);  6 , an Al w Ga 1-w As active layer with a thickness of 10 nm (where w=0.10);  102 , a p-side Al s Ga 1-s As guiding layer with a thickness of t/2 nm (where s=0.35);  8 , a p-type Al t Ga 1-t As cladding layer with a thickness of 1.5 μm (where t=0.55);  9 , a p-type GaAs contact layer;  10 , a p-electrode; and  11 , proton implantation regions. 
     The n-side Al y Ga 1-y As outer guiding layer  101  and the p-side Al s Ga 1-s As guiding layer  102  are undoped or substantially undoped; no intentional doping is performed on these layers. 
       FIG. 2  shows the percentage of light confined to the undoped region of the semiconductor laser device of  FIG. 1  as a function of the thickness of the undoped region, wherein the thickness of the undoped region is changed by changing the thickness (t/2) of the n-side Al y Ga 1-y As outer guiding layer  101  and the thickness (t/2) of the p-side Al s Ga 1-s As guiding layer  102 . When the undoped region has a small thickness of, e.g., 100 nm, the percentage of light confined in the undoped region is approximately 28%. The remaining 72% of the light is present in the p-type doped and n-type doped cladding layers, meaning that the loss in the semiconductor laser device depends substantially on the free carrier absorption in the cladding layers. 
     When the undoped region has a substantial thickness of at least 0.5 times the lasing wavelength of the semiconductor laser, on the other hand, 80% or more of the light is present in the undoped region. Therefore, the loss in the semiconductor laser depends substantially on the free carrier absorption in the undoped region. 
     The illustrations in  FIGS. 1 and 2  are by way of example only. Any semiconductor laser device is constructed such that the guiding layers have a higher refractive index than the cladding layers in order to confine light to the guiding layers. Further, the degree of optical confinement is determined substantially solely by the normalized frequency, which in turn is determined by the difference in refractive index between the guiding layers and the cladding layers, the thickness of the undoped region, and the lasing wavelength. Therefore, if the undoped region has a thickness at least 0.5 times the lasing wavelength, then most of the light is confined to the undoped region. 
     According to the present embodiment, the undoped region is limited to a maximum thickness of 2-3 μm (corresponding to the electron and hole diffusion lengths). 
     Configuration of First Practical Example of First Embodiment 
       FIG. 3  is a perspective view of an 810 nm semiconductor laser device according to a first practical example of the first embodiment. Reference numerals and corresponding components in  FIG. 3  are as follows:  4 , an n-side Al y Ga 1-y As outer guiding layer with a thickness of 650 nm (where y=0.35);  5 , an n-side Al z Ga 1-z As inner guiding layer with a thickness 50 nm (where z=0.30); and  7 , a p-side Al s G 1-s As guiding layer with a thickness 300 nm (where s=0.35). 
     The n-side Al y Ga 1-y As outer guiding layer  4 , the n-side Al z Ga 1-z As inner guiding layer  5 , and the p-side Al s Ga 1-s As guiding layer  7  are undoped or substantially undoped; no intentional doping is performed on these layers. That is, these layers are not intentionally doped with impurities in the crystal growth process and in the wafer process, so that they are undoped or substantially undoped. Further, in this example, the undoped layers of the device, which include the guiding layers  4 ,  5 , and  7  and the active layer, have a combined thickness Dudp of 1010 nm, which is approximately 1.24 times the lasing wavelength (see  FIG. 3 ). 
     It should be noted that in the following description, a guiding layer located on the same side of the active layer as the p-type cladding layer is referred to simply as a “p-side guiding layer.” In the present embodiment, the p-side Al s Ga 1-s As guiding layer  7  is a p-side guiding layer. Likewise, a guiding layer located on the same side of the active layer as the n-type cladding layer is referred to simply as an “n-side guiding layer.” In the present embodiment, the n-side Al y Ga 1-y As outer guiding layer  4  and the n-side Al z Ga 1-z As inner guiding layer  5  are n-side guiding layers. 
     Operation and Optical Intensity Distribution in First Practical Example 
     The semiconductor laser device is forward biased so that electrons are injected from the n-type Al x Ga 1-x As cladding layer  3  into the Al w Ga 1-w As active layer  6  through the n-side Al y Ga 1-y As outer guiding layer  4  and the n-side Al z Ga 1-z As inner guiding layer  5  and so that holes are injected from the p-type Al t Ga 1-t As cladding layer  8  into the active layer  6  through the p-side Al s Ga 1-s As guiding layer  7 . 
       FIG. 4  schematically shows the near field pattern (NFP), or near field intensity distribution, which is determined by the carrier distributions (electron and hole distributions) and refractive index distributions in the n-side Al y Ga 1-y As outer guiding layer  4 , the n-side Al z Ga 1-z As inner guiding layer  5 , and the p-side Al s Ga 1-s As guiding layer  7 . 
     In  FIG. 4 , the broken line indicates the band structure of the conduction band, the thick solid lines indicate the carrier concentrations (electron and hole concentrations), and the chain line indicates the optical intensity distribution. At each point within the optical guiding layers the electron and hole densities are equal, since charge neutral conditions exist within these layers. In  FIG. 4 , the active layer extends along the x-axis (i.e., perpendicular to the y-axis), and its center is located at the origin of the coordinate system. The carrier density is minimized at the junction between the active layer and each adjacent guiding layer, and increases toward each cladding layer  3 ,  8 . Let μ n  and μ p  denote the electron mobility and hole mobility, respectively. The slope of the carrier distribution curve within the p-side guiding layer is μ n /μ p  times the slope of the carrier distribution curve within the n-side guiding layers. 
     It should be noted that the optical intensity distribution is asymmetrical. Specifically, the peak of the optical intensity distribution is located on the same side of the active layer  6  as the n-side inner guiding layer  5 , which has a high refractive index. 
     Description of Advantages of First Embodiment by Way of Comparison with Comparative Example 
       FIG. 5  shows a comparative semiconductor laser device having a symmetrical structure. In  FIG. 5 , reference numeral  400  denotes an n-side Al y Ga 1-y As guiding layer with a thickness of 500 nm (where y=0.35) and reference numeral  700  denotes a p-side Al s Ga 1-s As guiding layer with a thickness of 500 nm (where s=0.35).  FIG. 6  schematically shows the carrier distribution and the optical intensity distribution. 
     As shown in  FIG. 6 , the carrier density is minimized at the junction between the active layer and each guiding layer, and gradually increases toward each cladding layer  3 ,  8 , as in the first embodiment. However, in the comparative example, a large number of carriers are present in the p-type guiding layer, since the slope of the carrier distribution curve within the p-side guiding layer is μ n /μ p  times the slope of the carrier distribution curve within the n-side guiding layer. Further, the optical intensity distribution is symmetrical relative to the active layer. 
     In the present embodiment, on the other hand, the carrier density in the p-side guiding layer is significantly reduced as compared to the comparative example, since the thickness of the p-side guiding layer is reduced and the combined thickness of the n-side guiding layers is greater than the n-side guiding layer of the comparative example. This reduces the light absorption by carriers within the guiding layers, thereby increasing the slope efficiency. 
     Further, the combined refractive index of the two n-side guiding layers (i.e., the n-side Al y Ga 1-y As outer guiding layer  4  and the n-side Al z Ga 1-z As inner guiding layer  5 ) is higher than the refractive index of the p-side guiding layer. As a result, the optical intensity distribution in the device is such the optical intensity in and around the active layer is higher than when the combined refractive index of the n-side guiding layers is equal to the refractive index of the p-side guiding layer. This increases the optical confinement factor, i.e., the percentage of light confined to the active layer, allowing the semiconductor laser device to have a high electrical-to-optical power conversion efficiency without increase in the threshold current. 
     In the present embodiment, the p-side guiding layer, the active layer, and the n-side guiding layers are undoped or substantially undoped; no intentional doping is performed on these layers. Further, the sum of the thicknesses of the p-side guiding layer, the active layer, and the n-side guiding layers is at least 0.5 times the lasing wavelength. As a result, most of the laser light exists within these layers, and the free carrier absorption in the cladding layers is reduced. 
     These features enable the semiconductor laser device to have a high electrical-to-optical power conversion efficiency. Further, environmental advantages of the present embodiment include extended useful life (i.e., extended durability) and reduced energy consumption (energy saving). 
     Other Practical Examples of First Embodiment 
     Second Practical Example 
       FIG. 7  is a perspective view of a 980 nm semiconductor laser device according to a second practical example of the present embodiment. Reference numerals and corresponding components in  FIG. 7  are as follows:  12 , an n-type Al x Ga 1-x As cladding layer with a thickness of 1.5 μm (where the Al mole fraction x=0.30);  13 , an n-side Al y Ga 1-y As outer guiding layer with a thickness of 650 nm (where y=0.05);  14 , an n-side GaAs inner guiding layer with a thickness of 50 nm;  15 , an In x Ga 1-x As active layer with a thickness of 10 nm (where z=0.20);  16 , a p-side Al s Ga 1-s As guiding layer with a thickness of 300 nm (where s=0.05); and  17 , a p-type Al t Ga 1-t As cladding layer with a thickness of 1.5 μm (where t=0.30). 
     The n-side Al y Ga 1-y As outer guiding layer  13 , the n-side GaAs inner guiding layer  14 , and the p-side Al s Ga 1-s As guiding layer  16  are undoped or substantially undoped; no intentional doping is performed on these layers. In this example, the undoped layers of the device, which include the guiding layers  13 ,  14 , and  16  and the active layer  15 , have a combined thickness Dudp of 1010 nm, which is approximately 1.03 times the lasing wavelength (see  FIG. 7 ). 
     Third Practical Example 
       FIG. 8  is a perspective view of an 810 nm semiconductor laser device according to a third practical example of the present embodiment. Reference numerals and corresponding components in  FIG. 8  are as follows:  18 , an n-type (Al x Ga 1-x ) 0.51 In 0.49 P cladding layer with a thickness of 1.5 μm (where the Al mole fraction x=0.50);  19 , an n-side InGaP outer guiding layer with a thickness of 275 nm;  20 , an n-side In 1-y Ga y As z P 1-z  inner guiding layer with a thickness of 25 nm (where z=0.10 and y=0.56);  21 , a GaAs 1-w P w  active layer with a thickness of 10 nm (where w=0.12);  22 , a p-side InGaP guiding layer with a thickness of 100 nm; and  23 , a p-type (Al x Ga 1-x ) 0.51 In 0.49 P cladding layer with a thickness of 1.5 μm (where x=0.50). 
     The n-side InGaP outer guiding layer  19 , the n-side In 1-y Ga y As z P 1-z  inner guiding layer  20 , and the p-side InGaP guiding layer  22  are undoped or substantially undoped; no intentional doping is performed on these layers. In this example, the undoped layers of the device, which include the guiding layers  19 ,  20 , and  22  and the active layer  21 , have a combined thickness Dudp of 410 nm, which is approximately 0.51 times the lasing wavelength (see  FIG. 8 ). 
     Further, the As mole fraction z and the Ga mole fraction y of the n-side InGaAsP inner guiding layer are such that this layer is lattice matched to GaAs. 
     Fourth Practical Example 
       FIG. 9  is a perspective view of a 400 nm semiconductor laser device according to a fourth practical example of the present embodiment. The reference numerals and corresponding components in  FIG. 9  are as follows:  24 , an n-electrode;  25 , an n-type GaN substrate;  26 , an n-type Al x Ga 1-x N cladding layer with a thickness of 0.5 μm (where the Al mole fraction x=0.14);  27 , an n-side GaN outer guiding layer with a thickness of 300 nm;  28 , an n-side In y Ga 1-y N inner guiding layer with a thickness of 25 nm (where the In mole fraction y=0.05);  29 , In z Ga 1-z N active layers with a thickness of 3.5 nm (where the In mole fraction z=0.15);  30 , GaN barrier layers with a thickness of 7 nm;  31 , a p-side GaN guiding layer with a thickness of 150 nm;  32 , a p-type Al x Ga 1-x N cladding layer with a thickness of 0.5 μm (where the Al mole fraction x=0.14);  33 , a p-type GaN contact layer;  34 , SiN insulating films; and  35 , p-electrodes. 
     Thus the semiconductor laser of this example has a triple quantum well structure including three In z Ga 1-z N active layers with a thickness of 3.5 nm (where the In mole fraction z=0.15) to lase at a wavelength of approximately 400 nm. 
     The n-side GaN outer guiding layer  27 , the n-side In y Ga 1-y N inner guiding layer  28 , the GaN barrier layers  30 , and the p-side GaN guiding layer  31  are undoped or substantially undoped; no intentional doping is performed on these layers. In this example, the undoped layers of the device, which include the guiding layers  27 ,  28 , and  31 , the barrier layers  30 , and the active layers  29 , have a combined thickness Dudp of 499.5 nm, which is approximately 1.25 times the lasing wavelength (see  FIG. 9 ). 
     Fifth Practical Example 
       FIG. 10  is a cross-sectional perspective view of a 1310 nm semiconductor laser device according to a fifth practical example of the present embodiment. The reference numerals and corresponding components in  FIG. 10  are as follows:  36 , an n-electrode;  37 , an n-type InP substrate;  38 , an n-type In 1-x Ga x As y P 1-y  cladding layer with a thickness of 500 nm (where the Ga mole fraction x=0.183 and the As mole fraction y=0.40);  39 , an n-side In 1-z Ga z As w P 1-w  outer guiding layer with a thickness of 600 nm (where the Ga mole fraction z=0.262 and the As mole fraction w=0.568);  40 , an n-side In 1-s Ga s As t P 1-t  inner guiding layer with a thickness of 50 nm (where the Ga mole fraction s=0.348 and the As mole fraction t=0.750);  41 , In 1-u Ga u As v P 1-v  active layers with a thickness of 7.5 nm (where the Ga mole fraction u=0.443 and the As mole fraction v=0.950);  42 , In 1-q Ga q As r P 1-r  barrier layers with a thickness of 23 nm (where the Ga mole fraction q=0.262 and the As mole fraction r=0.568);  43 , a p-side In 1-z Ga z As w P 1-w  guiding layer with a thickness of 300 nm (where the Ga mole fraction z=0.262 and the As mole fraction w=0.568);  44 , a p-type In 1-x Ga x As y P 1-y  cladding layer with a thickness of 500 nm (where the Ga mole fraction x=0.183 and the As mole fraction y=0.40);  45 , a p-type InP contact layer;  46 , SiO 2  insulating films; and  47 , p-electrodes. 
     Thus the semiconductor laser of this example has a triple quantum well structure including three In 1-u Ga u As v P 1-v  active layers with a thickness of 7.5 nm (where the Ga mole fraction u=0.443 and the As mole fraction v=0.950) to lase at a wavelength of about 1310 nm. 
     The n-side In 1-z Ga z As w P 1-w  outer guiding layer  39 , the n-side In 1-s Ga s As t P 1-t  inner guiding layer  40 , the In 1-q Ga q As r P 1-r  barrier layers  42 , and the p-side In 1-z Ga z As w P 1-w  guiding layer  43  are undoped or substantially undoped; no intentional doping is performed on these layers. In this example, the undoped layers of the device, which include the guiding layers  39 ,  40 , and  43 , the barrier layers  42 , and the active layers  41 , have a combined thickness Dudp of 1018.5 nm, which is approximately 0.78 times the lasing wavelength (see  FIG. 10 ). 
     Sixth Practical Example 
       FIG. 11  is a perspective view of a 660 nm semiconductor laser device according to a sixth practical example of the present embodiment. Reference numerals and corresponding components in  FIG. 11  are as follows:  48 , an n-type (Al x Ga 1-x ) 0.51 In 0.49 P cladding layer with a thickness of 1.5 μm (where the Al mole fraction x=0.70);  49 , an n-side (Al y Ga 1-y ) 0.51 In 0.49 P outer guiding layer with a thickness of 600 nm (where the Al mole fraction y=0.50);  50 , an n-side (Al z Ga 1-z ) 0.51 In 0.49 P inner guiding layer with a thickness of 50 nm (where the Al mole fraction z=0.30);  51 , Ga 0.51 In 0.49 P active layers with a thickness of 5.5 nm;  52 , (Al w Ga 1-w ) 0.51 In 0.49 P barrier layers with a thickness of 5 nm (where the Al mole fraction w=0.50);  53 , a p-side (Al y Ga 1-y ) 0.51 In 0.49 P guiding layer with a thickness of 300 nm (where the Al mole fraction y=0.50); and  54 , a p-type (Al x Ga 1-x ) 0.51 In 0.49 P cladding layer with a thickness of 1.5 μm (where the Al mole fraction x=0.70). 
     The n-side (Al y Ga 1-s ) 0.51 In 0.49 P outer guiding layer  49 , the n-side (Al x Ga 1-z ) 0.51 In 0.49 P inner guiding layer  50 , the (Al w Ga 1-w ) 0.51 In 0.49 P barrier layers  52 , and the p-side (Al y Ga 1-y ) 0.51 In 0.49 P guiding layer  53  are undoped or substantially undoped; no intentional doping is performed on these layers. In this example, the undoped layers of the device, which include the guiding layers  49 ,  50 , and  53 , the barrier layers  52 , and the active layers  51 , have a combined thickness Dudp of 976.5 nm, which is approximately 1.45 times the lasing wavelength (see  FIG. 11 ). 
     Seventh Practical Example 
       FIG. 12  is a cross-sectional perspective view of a 1550 nm semiconductor laser device according to a seventh practical example of the present embodiment. Reference numerals and corresponding components in  FIG. 12  are as follows:  55 , an n-type InP cladding layer with a thickness of 0.75 μm;  56 , an n-side In 1-x Ga x As y P 1-y  outer guiding layer with a thickness of 600 nm (where the Ga mole fraction x=0.800 and the As mole fraction y=0.175);  57 , an n-side In 1-z Ga z As w P 1-w  inner guiding layer with a thickness 50 nm (where the Ga mole fraction z=0.277 and the As mole fraction w=0.600);  58 , In 1-s Ga s As t P 1-t  active layers with a thickness of 8 nm (where the Ga mole fraction s=0.329 and the As mole fraction t=0.710);  59 , In 1-u Ga u As v P 1-v  barrier layers with a thickness of 20 nm (where the Ga mole fraction u=0.80 and the As mole fraction v=0.175);  60 , a p-side In 1-x Ga x As y P 1-y  guiding layer with a thickness of 300 nm (where the Ga mole fraction x=0.800 and the As mole fraction y=0.175); and  61 , a p-type InP cladding layer with a thickness of 0.75 μm. 
     Thus the semiconductor laser device of this example has a five quantum well structure including five In 1-s Ga s As t P 1-t  active layers with a thickness of 8 nm (where the Ga mole fraction s=0.329 and the As mole fraction t=0.710) to lase at a wavelength of about 1550 nm. 
     The n-side In 1-x Ga x As y P 1-y  outer guiding layer  56 , the n-side In 1-z Ga z As w P 1-w  inner guiding layer  57 , the In 1-u Ga u As v P 1-v  barrier layers  59 , and the p-side In 1-x Ga x As y P 1-y  guiding layer  60  are undoped or substantially undoped; no intentional doping is performed on these layers. In this example, the undoped layers of the device, which include the guiding layers  56 ,  57 , and  60 , the barrier layers  59 , and the active layers  58 , have a combined thickness Dudp of 1070 nm, which is approximately 0.69 times the lasing wavelength (see  FIG. 12 ). 
     Second Embodiment 
     First Practical Example of Second Embodiment 
       FIG. 13  is a perspective view of an 810 nm semiconductor laser device according to a second embodiment of the present invention. Reference numerals and corresponding components in  FIG. 13  are as follows:  62 , an n-side Al y Ga 1-y As guiding layer with a thickness of 700 nm (where y=0.35); and  63 , a p-type Al t Ga 1-t As cladding layer with a thickness of 1.5 μm (where t=0.60). This semiconductor laser device is substantially similar to that of the first practical example of the first embodiment (shown in  FIG. 3 ) except the configuration of an inner guiding layer. 
     Since the AlGaAs active layer  6  is located closer to the p-type AlGaAs cladding layer  63  than to the n-type AlGaAs cladding layer  3 , fewer carriers are present within the guiding layers, as compared to when the active layer is spaced equally from the n-type and p-type cladding layers. This reduces the free carrier absorption and the resulting loss in the device, thereby increasing the slope efficiency. 
     Further, since the Al mole fraction t of the p-type Al t Ga 1-t As cladding layer  63  is greater than the Al mole fraction x of the n-type Al x Ga 1-x As cladding layer  3 , the p-type AlGaAs cladding layer  63  has a lower refractive index than the n-type AlGaAs cladding layer  3 . This means that the difference in refractive index between the p-side AlGaAs guiding layer  7  and the p-type AlGaAs cladding layer  63  is greater than that between the n-side AlGaAs guiding layer  62  and the n-type AlGaAs cladding layer  3 . As a result, the optical intensity distribution in the p-side AlGaAs guiding layer  7  is such that the optical intensity drastically decreases toward the p-type AlGaAs cladding layer  63 . This increases the optical confinement factor to the AlGaAs active layer  6 , thereby reducing the threshold current. 
     Further, this semiconductor laser device differs from that of the first practical example of the first embodiment (shown in  FIG. 3 ) substantially only in that it includes only one n-side guiding layer, namely, the n-side Al y Ga 1-y As guiding layer  62  (i.e., it does not include an n-side inner guiding layer), as described above. That is, in this embodiment, as in the first embodiment, the p-side guiding layer, the active layer, and the n-side guiding layer are undoped or substantially undoped; no intentional doping is performed on these layers. Further, the sum of the thicknesses of the p-side guiding layer, the active layer, and the n-side guiding layer is at least 0.5 times the lasing wavelength. As a result, most of the laser light exists within these layers, and the free carrier absorption in the cladding layers is reduced. 
     These features enable the semiconductor laser device to have a high electrical-to-optical power conversion efficiency. 
     Other Practical Examples of Second Embodiment 
     Second Practical Example 
       FIG. 14  is a perspective view of a 980 nm semiconductor laser device according to a second practical example of the present embodiment. In  FIG. 14 , reference numeral  64  denotes an n-side Al y Ga 1-y As guiding layer with a thickness of 700 nm (where y=0.05), and reference numeral  65  denotes a p-type Al t Ga 1-t As cladding layer with a thickness of 1.5 μm (where t=0.35). This semiconductor laser device is substantially similar to that shown in  FIG. 7  except the configuration of an inner guiding layer. The n-side Al y Ga 1-y As guiding layer  64  and the p-side Al s Ga 1-s As guiding layer  16  are undoped or substantially undoped; no intentional doping is performed on these layers. 
     Third Practical Example 
       FIG. 15  is a perspective view of an 810 nm semiconductor laser device according to a third practical example of the present embodiment. In  FIG. 15 , reference numeral  66  denotes an n-side InGaP guiding layer with a thickness of 300 nm, and reference numeral  67  denotes a p-type (Al x Ga 1-x ) 0.51 In 0.49 P cladding layer with a thickness of 1.5 μm (where x=0.55). This semiconductor laser device is substantially similar to that shown in  FIG. 8  except the configuration of an inner guiding layer. The n-side InGaP guiding layer  66  and the p-side InGaP guiding layer  22  are undoped or substantially undoped; no intentional doping is performed on these layers. 
     Fourth Practical Example 
       FIG. 16  is a perspective view of a 400 nm semiconductor laser device according to a fourth practical example of the present embodiment. In  FIG. 16 , reference numeral  68  denotes an n-side GaN guiding layer with a thickness of 350 nm, and reference numeral  69  denotes a p-type Al x Ga 1-x N cladding layer with a thickness of 0.5 μm (where the Al mole fraction x=0.16). This semiconductor laser device is substantially similar to that shown in  FIG. 9  except the configuration of an inner guiding layer. The n-side GaN guiding layer  68 , the GaN barrier layers  30 , and the p-side GaN guiding layer  31  are undoped or substantially undoped; no intentional doping is performed on these layers. 
     Fifth Practical Example 
       FIG. 17  is a cross-sectional perspective view of a 1310 nm semiconductor laser device according to a fifth practical example of the present embodiment. In  FIG. 17 , reference numeral  70  denotes an n-side In 1-z Ga z As w P 1-w  guiding layer with a thickness of 650 nm (where the Ga mole fraction z=0.262 and the As mole fraction w=0.568), and reference numeral  71  denotes a p-type In 1-x Ga x As y P 1-y  cladding layer with a thickness of 500 nm (where the Ga mole fraction x=0.160 and the As mole fraction y=0.350). This semiconductor laser device is substantially similar to that shown in  FIG. 10  except the configuration of an inner guiding layer. The n-side In 1-z Ga z As w P 1-w  guiding layer  70 , the In 1-q Ga q As r P 1-r  barrier layers  42 , and the p-side In 1-z Ga z As w P 1-w  guiding layer  43  are undoped or substantially undoped; no intentional doping is performed on these layers. 
     Sixth Practical Example 
       FIG. 18  is a perspective view of a 660 nm semiconductor laser device according to a sixth practical example of the present embodiment. In  FIG. 18 , reference numeral  72  denotes an n-side (Al y Ga 1-y ) 0.51 In 0.49 P guiding layer with a thickness of 650 nm (where the Al mole fraction y=0.50), and reference numeral  73  denotes a p-type (Al x Ga 1-x ) 0.51 In 0.49 P cladding layer with a thickness of 1.5 μm (where the Al mole fraction x=0.80). This semiconductor laser device is substantially similar to that shown in  FIG. 11 , except the configuration of an inner guiding layer. The n-side (Al y Ga 1-y ) 0.51 In 0.49 P guiding layer  72 , the (Al w Ga 1-w ) 0.51 In 0.49 P barrier layers  52 , and the p-side (Al y Ga 1-y ) 0.51 In 0.49 P guiding layer  53  are undoped or substantially undoped; no intentional doping is performed on these layers. 
     Seventh Practical Example 
       FIG. 19  is a cross-sectional perspective view of a 1550 nm semiconductor laser device according to a seventh practical example of the present embodiment. In  FIG. 19 , reference numeral  74  denotes an n-type In 1-x Ga x As y P 1-y  cladding layer with a thickness of 1.0 μm (where the Ga mole fraction x=0.023 and the As mole fraction y=0.050);  75 , an n-side In 1-z Ga z As w P 1-w  guiding layer with a thickness of 650 nm (where the Ga mole fraction z=0.800 and the As mole fraction w=0.175);  76 , In 1-s Ga s As t P 1-t  active layers with a thickness of 8 nm (where the Ga mole fraction s=0.329 and the As mole fraction t=0.710);  77 , In 1-u Ga u As v P 1-v  barrier layers with a thickness of 20 nm (where the Ga mole fraction u=0.800 and the As mole fraction v=0.175);  78 , a p-side In 1-z Ga z As w P 1-w  guiding layer with a thickness of 300 nm (where the Ga mole fraction z=0.800 and the As mole fraction w=0.175); and  79 , a p-type InP cladding layer with a thickness of 1.0 μm. 
     Thus the semiconductor laser device of this example has a five quantum well structure including five In 1-s Ga s As t P 1-t  active layers with a thickness of 8 nm (where the Ga mole fraction s=0.329 and the As mole fraction t=0.710) to lase at a wavelength of about 1550 nm. 
     The n-side In 1-z Ga z As w P 1-w  guiding layer  75 , the In 1-u Ga u As v P 1-v  barrier layers  77 , and the p-side In 1-z Ga z As w P 1-w  guiding layer  78  are undoped or substantially undoped; no intentional doping is performed on these layers. 
     Third Embodiment 
     First Practical Example of Third Embodiment 
       FIG. 20  is a perspective view of an 810 nm semiconductor laser device according to a third embodiment of the present invention. In  FIG. 20 , reference numeral  80  denotes a p-type Al t Ga 1-t As cladding layer with a thickness of 1.5 μm (where t=0.60). This semiconductor laser device is similar to that shown in  FIG. 3 , except that the p-type cladding layer  8  is replaced by the p-type Al t Ga 1-t As cladding layer  80 . Since the AlGaAs active layer  6  is located closer to the p-type AlGaAs cladding layer  80  than to the n-type AlGaAs cladding layer  3 , fewer carriers are present within the guiding layers, as compared to when the active layer is spaced equally from the n-type and p-type cladding layers. This reduces the free carrier absorption and the resulting loss in the device, thereby increasing the slope efficiency. 
     Further, the n-side inner guiding layer has a higher refractive index than the p-side guiding layer, by forming the n-side guiding layer having two layers. As a result, the peak of the optical intensity distribution is close to the active layer, and the optical confinement factor, i.e., the percentage of light confined to the active layer, of the present embodiment is higher than that of the configuration which the n-side inner guiding layer and the p-side guiding layer have the same refractive index. 
     Further, since the Al mole fraction t of the p-type Al t Ga 1-t As cladding layer  80  is greater than the Al mole fraction x of the n-type Al x Ga 1-x As cladding layer  3 , the p-type AlGaAs cladding layer  80  has a lower refractive index than the n-type AlGaAs cladding layer  3 . This means that the difference in refractive index between the p-side AlGaAs guiding layer  7  and the p-type AlGaAs cladding layer  80  is greater than that between the n-side AlGaAs guiding layer  4  and the n-type AlGaAs cladding layer  3 . As a result, the optical intensity distribution in the p-side AlGaAs guiding layer  7  is such that the optical intensity drastically decreases toward the p-type AlGaAs cladding layer  80 . This increases the optical confinement factor to the AlGaAs active layer  6 , thereby reducing the threshold current. 
     Further, this semiconductor laser device differs from that of the first practical example of the first embodiment (shown in  FIG. 3 ) only in that the p-type cladding layer  8  is replaced by the p-type Al t G 1-t As cladding layer  80 , as described above. That is, in this embodiment, as in the first embodiment, the p-side guiding layer, the active layer, and the n-side guiding layers are undoped or substantially undoped; no intentional doping is performed on these layers. Further, the sum of the thicknesses of the p-side guiding layer, the active layer, and the n-side guiding layers is at least 0.5 times the lasing wavelength. As a result, most of the laser light exists within these layers, and the free carrier absorption in the cladding layers is reduced. 
     These features enable the semiconductor laser device to have a high electrical-to-optical power conversion efficiency. 
     Other Practical Examples of Third Embodiment 
     Second Practical Example 
       FIG. 21  is a perspective view of a 980 nm semiconductor laser device according to a second practical example of the present embodiment. In  FIG. 21 , reference numeral  81  denotes a p-type Al t Ga 1-t As cladding layer with a thickness of 1.5 μm (where t=0.35). This semiconductor laser device is similar to that shown in  FIG. 7 , except that the p-type cladding layer  17  is replaced by the p-type Al t Ga 1-t As cladding layer  81 . The n-side Al y Ga 1-y As outer guiding layer  13 , the n-side GaAs inner guiding layer  14 , and the p-side Al s Ga 1-s As guiding layer  16  are undoped or substantially undoped; no intentional doping is performed on these layers. 
     Third Practical Example 
       FIG. 22  is a perspective view of an 810 nm semiconductor laser device according to a third practical example of the present embodiment. In  FIG. 22 , reference numeral  82  denotes a p-type (Al x Ga 1-x ) 0.51 In 0.49 P cladding layer with a thickness of 1.5 μm (where x=0.55). This semiconductor laser device is similar to that shown in  FIG. 8 , except that the p-type cladding layer  23  is replaced by the p-type (Al x Ga 1-x ) 0.51 In 0.49 P cladding layer  82 . The n-side InGaP outer guiding layer  19 , the n-side In 1-y Ga y As z P 1-z  inner guiding layer  20 , and the p-side InGaP guiding layer  22  are undoped or substantially undoped; no intentional doping is performed on these layers. 
     Fourth Practical Example 
       FIG. 23  is a perspective view of a 400 nm semiconductor laser device according to a fourth practical example of the present embodiment. In  FIG. 23 , reference numeral  83  denotes a p-type Al x Ga 1-x N cladding layer with a thickness of 0.5 μm (where the Al mole fraction x=0.16). This semiconductor laser device is similar to that shown in  FIG. 9 , except that the p-type cladding layer  32  is replaced by the p-type Al x Ga 1-x N cladding layer  83 . The n-side GaN outer guiding layer  27 , the n-side In y Ga 1-y N inner guiding layer  28 , the GaN barrier layers  30 , and the p-side GaN guiding layer  31  are undoped or substantially undoped; no intentional doping is performed on these layers. 
     Fifth Practical Example 
       FIG. 24  is a cross-sectional perspective view of a 1310 nm semiconductor laser device according to a fifth practical example of the present embodiment. In  FIG. 24 , reference numeral  84  denotes a p-type In 1-x Ga x As y P 1-y  cladding layer with a thickness of 500 nm (where the Ga mole fraction x=0.160 and the As mole fraction y=0.35). This semiconductor laser device is similar to that shown in  FIG. 10 , except that the p-type cladding layer  44  is replaced by the p-type In 1-x Ga x As y P 1-y  cladding layer  84 . The n-side In 1-z Ga z As w P 1-w  outer guiding layer  39 , the n-side In 1-s Ga s As t P 1-t  inner guiding layer  40 , the In 1-q Ga q As r P 1-r  barrier layers  42 , and the p-side In 1-z Ga z As w P 1-w  guiding layer  43  are undoped or substantially undoped; no intentional doping is performed on these layers. 
     Sixth Practical Example 
       FIG. 25  is a perspective view of a 660 nm semiconductor laser device according to a sixth practical example of the present embodiment. In  FIG. 25 , reference numeral  85  denotes a p-type (Al x Ga 1-x ) 0.51 In 0.49 P cladding layer with a thickness of 1.5 μm (where the Al mole fraction x=0.80). This semiconductor laser device is similar to that shown in  FIG. 11 , except that the p-type cladding layer  54  is replaced by the p-type (Al x Ga 1-x ) 0.51 In 0.49 P cladding layer  85 . The n-side (Al y Ga 1-y ) 0.51 In 0.49 P outer guiding layer  49 , the n-side (Al x Ga 1-z ) 0.51 In 0.49 P inner guiding layer  50 , the (Al w Ga 1-w ) 0.51 In 0.49 P barrier layers  52 , and the p-side (Al y Ga 1-y ) 0.51 In 0.49 P guiding layer  53  are undoped or substantially undoped; no intentional doping is performed on these layers. 
     Seventh Practical Example 
       FIG. 26  is a cross-sectional perspective view of a 1550 nm semiconductor laser device according to a seventh practical example of the present embodiment. In  FIG. 26 , reference numeral  86  denotes an n-side In 1-x Ga x As y P 1-y  outer guiding layer with a thickness of 600 nm (where the Ga mole fraction x=0.800 and the As mole fraction y=0.175), and reference numeral  87  denotes an n-side In 1-z Ga z As w P 1-w  inner guiding layer with a thickness of 500 nm (where the Ga mole fraction z=0.277 and the As mole fraction w=0.600). This semiconductor laser device is similar to that shown in  FIG. 19 , except that the n-side guiding layer  75  is replaced by the n-side In 1-x Ga x As y P 1-y  outer guiding layer  86  and the n-side In 1-z Ga z As w P 1-w  inner guiding layer  87 . The n-side In 1-x Ga x As y P 1-y  outer guiding layer  86 , the n-side In 1-z Ga z As w P 1-w  inner guiding layer  87 , the In 1-u Ga u As v P v-1  barrier layers  77 , and the p-side In 1-z Ga z As w P 1-w  guiding layer  78  are undoped or substantially undoped; no intentional doping is performed on these layers. 
     Fourth Embodiment 
     First Practical Example of Fourth Embodiment 
       FIG. 27  is a perspective view of an 810 nm semiconductor laser device according to a fourth embodiment of the present invention. Reference numerals and corresponding components in  FIG. 27  are as follows:  201  denotes an n-type (Al x Ga 1-x ) 0.51 In 0.49 P cladding layer with a thickness of 1.5 μm (where the Al mole fraction x=0.30);  202  denotes an n-side InGaP outer guiding layer with a thickness of 630 nm;  203  denotes an n-side In 1-y Ga y As z P 1-z  center guiding layer with a thickness of 100 nm (where z=0.10 and y=0.56);  204  denotes an n-side InGaP inner guiding layer with a thickness of 20 nm;  205  denotes a GaAs 1-w P w  active layer with a thickness of 12.5 nm (where w=0.12);  206  denotes a p-side InGaP guiding layer with a thickness of 450 nm; and  207  denotes a p-type (Al x Ga 1-x ) 0.51 In 0.49 P cladding layer with a thickness of 1.5 μm (where x=0.30). 
     In the above described configuration, the GaAsP active layer  205  is sandwiched by the n-side guiding layers and the p-side guiding layer, and the total thickness of the n-side guiding layer is larger than the thickness of the p-side guiding layer. That is, the GaAsP active layer  205  is located so as to be close to the p-type AlGaInP cladding layer  207 . According to this configuration, carriers residing within the guiding layers are reduced, as compared to a case where the active layer locates at the middle between the n-type and p-type cladding layers. This reduces the loss due to the free carrier absorption, thereby improving the slope efficiency. 
     Further, in the fourth embodiment, the n-side guiding layer consists of three layers, and the center layer of the three layers has the highest refractive index of the three layers. Thereby, in the fourth embodiment, the n-side guiding layer has a higher refractive index than the p-side guiding layer. This results in that the portion in which the optical intensity distribution is maximized is close to the active layer. The optical confinement factor, i.e. the percentage of light confined within the active layer is approximately 1.7% in a case where the refractive index of the inner layer of the n-side guiding layer is equal to that of the outer layers of the n-side guiding layer. On the other hand, the optical confinement factor of the fourth embodiment is approximately 1.8%. The semiconductor laser device according to the fourth embodiment can obtain increased optical confinement factor. 
     Further, the n-side InGaP inner guiding layer  204 , which contacts to the GaAs 1-w P w  active layer  205 , has a greater band gap energy than the n-side In 1-y Ga y As z P 1-z  center guiding layer. This suppresses leakage of carriers (electrons or holes) to the n-side guiding layer, thereby confining carriers in the GaAs active layer effectively. As a result, a high electrical-to-optical power conversion efficiency can be achieved. 
     In the fourth embodiment, as in the first embodiment, the p-side guiding layer, the active layer, and the n-side guiding layers are made to be undoped or substantially undoped by performing no intentional doping on these layers. Further, the sum of the thicknesses of the p-side guiding layer, the active layer, and the n-side guiding layers is at least 0.5 times the lasing wavelength. That is, the thickness indicated by Dudp in  FIG. 27  is equal to or larger than 0.5 times the lasing wavelength. As a result, most of the laser light exists within these layers, and the free carrier absorption in the cladding layers is reduced. 
     These features enable the semiconductor laser device to have a high electrical-to-optical power conversion efficiency. 
     Other Practical Examples of Fourth Embodiment 
     Second Practical Example 
       FIG. 28  is a perspective view of a 915 nm semiconductor laser device according to a second practical example of the fourth embodiment. Reference numerals and corresponding components in  FIG. 28  are as follows:  208  denotes an n-type In 0.49 Ga 0.51 P cladding layer with a thickness of 1.5 μm;  209  denotes an n-side In 1-s Ga s As t P 1-t  outer guiding layer with a thickness of 600 nm (where t=0.6 and s=0.8);  210  denotes an n-side In 1-y Ga y As z P 1-z  center guiding layer with a thickness of 100 nm (where z=0.7 and y=0.85);  211  denotes an n-side In 1-s Ga s As t P 1-t  inner guiding layer with a thickness of 50 nm (where t=0.6 and s=0.8);  212  denotes a In w Ga 1-w As active layer with a thickness of 12.5 nm (where w=0.05);  213  denotes a p-side In 1-s Ga s As t P 1-t  guiding layer with a thickness of 450 nm (where t=0.6 and s=0.8); and  214  denotes a p-type In 0.49 Ga 0.51 P cladding layer with a thickness of 1.5 μm. 
     The n-side In 1-s Ga s As t P 1-t  outer guiding layer  209 , the n-side In 1-y Ga y As z P 1-z  center guiding layer  210 , the n-side In 1-s Ga s As t P 1-t  inner guiding layer  211 , the In w Ga 1-w As active layer  212 , and the p-side In 1-s Ga s As t P 1-t  guiding layer  213  are made to be undoped or substantially undoped by performing no intentional doping on these layers. Further, the thickness indicated by Dudp in  FIG. 28  is equal to or larger than 0.5 times the lasing wavelength, as in the first practical example. 
     In the fourth embodiment, description is made about an 810 nm semiconductor laser device and a 915 nm semiconductor laser device. It is to be understood, however, that the feature of the fourth embodiment may be applied to other semiconductor lasers made of other materials or other wavelength semiconductor lasers. 
     Fifth Embodiment 
     First Practical Example of Fifth Embodiment 
       FIG. 29  is a perspective view of an 810 nm semiconductor laser device according to a fifth embodiment of the present invention. Reference numeral and corresponding component in  FIG. 29  is as follows:  215  denotes a p-type (Al x Ga 1-x ) 0.51 In 0.49 P cladding layer with a thickness of 1.5 μm (where x=0.40). Other components shown in  FIG. 29  are same as the components of the semiconductor laser shown in  FIG. 27 . 
     In the fifth embodiment, the GaAsP active layer  205  is sandwiched by the n-side guiding layer and the p-side guiding layer, and the total thickness of the n-side guiding layer is larger than the thickness of the p-side guiding layer as in the fourth embodiment. That is, the GaAsP active layer  205  is located so as to be close to the p-type AlGaInP cladding layer  215 . According to this configuration, fewer carriers are present within the guiding layers, as compared to a case where the active layer locates at the middle between the n-type and p-type cladding layers. This reduces the loss due to free carrier absorption in the device, thereby increasing the slope efficiency. 
     Further, in the fifth embodiment, the n-side guiding layer consists of three layers, and the center layer of the three layers has the highest refractive index of the three layers. Thereby, in the fifth embodiment, the n-side guiding layer has a higher refractive index than the p-side guiding layer. This results in that the portion in which the optical intensity distribution is maximized is close to the active layer. 
     Further, in the fifth embodiment, the p-type cladding layer  215  has a lower refractive index than the n-type cladding layer  201 . As a result, the optical intensity distribution in the p-side InGaP guiding layer  206  is such that the optical intensity drastically decreases toward the p-type AlGaInP cladding layer  215 . This increases the optical confinement factor to the GaAsP active layer  205   
     Further, then-side InGaP inner guiding layer  204 , which contacts to the GaAsP active layer  205 , has a greater band gap energy than the n-side InGaAsP center guiding layer. This suppresses leakage of carriers (electrons or holes) to the n-side guiding layer, thereby confining carriers in the GaAs active layer effectively, resulting in reduction of the threshold current. 
     In the fifth embodiment, as in the first embodiment, the p-side guiding layer, the active layer, and the n-side guiding layer are made to be undoped or substantially undoped by performing no intentional doping on these layers. Further, the sum of the thicknesses of the p-side guiding layer, the active layer, and the n-side guiding layer is at least 0.5 times the lasing wavelength. That is, the thickness indicated by Dudp in  FIG. 29  is equal to or larger than 0.5 times the lasing wavelength. As a result, most of the laser light exists within these layers, and the free carrier absorption in the cladding layers is reduced. 
     These features enable the semiconductor laser device to have a high electrical-to-optical power conversion efficiency. In the fifth embodiment, description is made about an 810 nm semiconductor laser device. It is to be understood, however, that the feature of the fifth embodiment may be applied to other semiconductor lasers made of other materials or other wavelength semiconductor lasers. 
     In the above-described embodiments, proton implantation or the formation of insulating film stripes is used to confine the current and thereby enhance the lasing efficiency. It is to be understood, however, that in other embodiments other current confining methods may be used. It is obvious that the current can also be confined by forming a waveguide (e.g., a ridge waveguide) or by forming a buried current blocking layer such as a buried n-GaAs semiconductor layer while still retaining the advantages of the present invention. It should be further noted that the thicknesses and compositions of the layers in the embodiments given above are by way of example only. The present invention is not limited to these particular thicknesses and compositions unless explicitly so stated. 
     The features and advantages of the present invention may be summarized as follows: 
     According to the first aspect of the present invention, the p-side guiding layer, the active layer, and the n-side guiding layer are undoped or substantially undoped; no intentional doping is performed on these layers. Further, the sum of the thicknesses of the p-side guiding layer, the active layer, and the n-side guiding layer is not less than 0.5 times the lasing wavelength of the semiconductor laser device and is not more than 2 μm. As a result, most of the laser light exists within these layers, and the free carrier absorption in the cladding layers is reduced. Further, the active layer is located closer to the p-type cladding layer than to the n-type cladding layer and that the n-side guiding layer has a higher refractive index than the p-side guiding layer. This reduces the light absorption in the guiding layers while avoiding a reduction in the optical confinement factor to the active layer, thereby allowing the semiconductor laser device to have a high electrical-to-optical power conversion efficiency. 
     According to the second aspect of the present invention, the p-side guiding layer, the active layer, and the n-side guiding layer are undoped or substantially undoped; no intentional doping is performed on these layers. Further, the sum of the thicknesses of the p-side guiding layer, the active layer, and the n-side guiding layer is not less than 0.5 times the lasing wavelength of the semiconductor laser device and is not more than 2 μm. As a result, most of the laser light exists within these layers, and the free carrier absorption in the cladding layers is reduced. Further, the active layer is located closer to the p-type cladding layer than to the n-type cladding layer and that the p-type cladding layer has a lower refractive index than the n-type cladding layer. This reduces the light absorption in the guiding layers while avoiding a reduction in the optical confinement factor to the active layer, thereby allowing the semiconductor laser device to have a high electrical-to-optical power conversion efficiency. 
     According to the third aspect of the present invention, the p-side guiding layer, the active layer, and the n-side guiding layer are undoped or substantially undoped; no intentional doping is performed on these layers. Further, the sum of the thicknesses of the p-side guiding layer, the active layer, and the n-side guiding layer is not less than 0.5 times the lasing wavelength of the semiconductor laser device and is not more than 2 μm. As a result, most of the laser light exists within these layers, and the free carrier absorption in the cladding layers is reduced. Further, the active layer is located closer to the p-type cladding layer than to the n-type cladding layer, that the n-side guiding layer has a higher refractive index than the p-side guiding layer, and that the p-type cladding layer has a lower refractive index than the n-type cladding layer. This reduces the light absorption in the guiding layers while avoiding a reduction in the optical confinement factor to the active layer, thereby allowing the semiconductor laser device to have a high electrical-to-optical power conversion efficiency. 
     According to the fourth aspect of the present invention, the p-side guiding layer, the active layer, and the n-side guiding layer are made to be undoped or substantially undoped by performing no intentional doping on these layers. Further, the sum of the thicknesses of the p-side guiding layer, the active layer, and the n-side guiding layer is not less than 0.5 times the lasing wavelength of the semiconductor laser device and is not more than 2 μm. As a result, most of the laser light exists within these layers, and the free carrier absorption in the cladding layers is reduced. Further, the active layer is located closer to the p-type cladding layer than to the n-type cladding layer, and the n-side guiding layer has an inside portion having a higher refractive index than the outside of the inside portion. This reduces the light absorption in the guiding layers while avoiding a reduction in the optical confinement factor to the active layer. Further, since the inside portion of the n-side guiding layer has the higher refractive index than the outside of the inside portion, the outside of the inside portion has a greater band gap energy than the inside portion. This makes it possible to effectively confine carriers (electrons or holes) in the active layer, thereby allowing the semiconductor laser device to have a high electrical-to-optical power conversion efficiency. 
     According to the fifth aspect of the present invention, the p-side guiding layer, the active layer, and the n-side guiding layer are made to be undoped or substantially undoped by performing no intentional doping on these layers. Further, the sum of the thicknesses of the p-side guiding layer, the active layer, and the n-side guiding layer is not less than 0.5 times the lasing wavelength of the semiconductor laser device and is not more than 2 μm. As a result, most of the laser light exists within these layers, and the free carrier absorption in the cladding layers is reduced. Further, the active layer is located closer to the p-type cladding layer than to the n-type cladding layer and the n-side guiding layer has an inside portion having a higher refractive index than the outside of the inside portion. In addition, the p-type cladding layer has a lower refractive index than the n-type cladding layer. This reduces the light absorption in the guiding layers while avoiding a reduction in the optical confinement factor to the active layer. Further, since the inside portion of the n-side guiding layer has the higher refractive index than the outside of the inside portion, the outside of the inside portion has a greater band gap energy than the inside portion. This makes it possible to effectively confine carriers (electrons or holes) in the active layer, thereby allowing the semiconductor laser device to have a high electrical-to-optical power conversion efficiency. 
     Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described. 
     The entire disclosure of a Japanese Patent Application No. 2008-275846, filed on Oct. 27, 2008 and a Japanese Patent Application No. 2009-90934, filed on Apr. 3, 2009 including specification, claims, drawings and summary, on which the Convention priority of the present application is based, are incorporated herein by reference in its entirety.