Abstract:
A semiconductor laser diode includes, on a substrate, a first cladding layer; an active layer formed on the first cladding layer; a second cladding layer formed on the active layer and having a ridge stripe for injecting a current into the active layer; and a light emitting portion formed on both sides of the ridge stripe and having a current blocking layer for confining the current in the ridge stripe. A distance from a lower face of the current blocking layer to an upper face of the active layer is within a given range. Also, the current spreads beyond a width of the ridge stripe after passing the ridge stripe and before reaching the active layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority under 35 U.S.C. §119 on Patent Application No. 2005-348065 filed in Japan on Dec. 1, 2005, the entire contents of which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a semiconductor laser diode, and more particularly, it relates to a semiconductor laser diode whose longitudinal mode is multi-longitudinal mode including self sustained pulsation.  
         [0003]     Recently, a semiconductor laser diode is frequently used as an optical pickup light source for use in an optical recording device or reading device for a recording medium such as an optical disk or a magneto-optical recording disk. Such semiconductor laser diodes are widely applied to a recorder, a PC, a vehicle and the like, and the market of optical disks is being increasing. In particular, there is a large demand for on-vehicle application typified by a car navigation system, and a demand for optical pickup device capable of reproducing all disks including CDs and DVDs is increasing.  
         [0004]     What an on-vehicle optical pickup device is strongly required are (1) compactness, (2) wide operation temperature guarantee for operating in a wide temperature range from a low temperature to a high temperature and (3) suppression of signal degradation (noise reduction).  
         [0005]     First, for (1) compactness of an optical pickup device, it is effective to simplify the device by reducing optical components, and in one method for this purpose, a red semiconductor laser of a 650 nm band for a DVD and an infrared semiconductor laser of a 780 nm band for a CD are integrally provided on one semiconductor substrate as a monolithic semiconductor laser. Thus, the semiconductor laser itself can be integrated as one component as well as optical components such as a collimator lens and a beam splitter can be shared by the red semiconductor laser and the infrared semiconductor laser, and hence, this method is useful for the compactness.  
         [0006]     Also, for (2) wide operation temperature guarantee, it is necessary to improve the temperature characteristic of the semiconductor laser diode itself. As one method for this purpose, technique disclosed in Japanese Laid-Open Patent Publication No. 2000-174385 (hereinafter referred to as Patent Document 1) is known. Patent Document 1 describes that when a taper stripe structure in which the width of a ridge is changed along an optical. path direction is employed, a lower current density can be attained than in a general straight stripe structure and the temperature characteristic can be improved by suppressing heat generation of the diode through reduction of differential resistance owing to a large ridge width.  
         [0007]     Next, with respect to (3) suppression of signal degradation (noise reduction), a factor to cause noise will be first examined.  
         [0008]     There is little light returning from an optical disk, an optical recording medium or an optical system to a semiconductor laser diode. Therefore, in the case where the laser beam has high coherence, light obtained within a resonator and the returned light affect each other, resulting in causing noise in an output of the semiconductor laser diode.  
         [0009]     As a countermeasure for such noise derived from the returned light, a method in which the semiconductor laser diode is subjected to fast modulation, a method in which multi-longitudinal mode is employed for the oscillation mode of the semiconductor laser diode, a method in which the semiconductor laser diode itself is placed in a pulse oscillation state or the like is employed.  
         [0010]     In the method in which the semiconductor laser diode is subjected to the fast modulation, however, a high-frequency superposed module is used and hence the number of components is increased. Therefore, this method is disadvantageous in the compactness and the cost of the optical pickup. Furthermore, equipment using a high frequency (such as an ETC (electronic toll collection system)) apart from the high-frequency superposed module for the fast modulation of the semiconductor laser diode is frequently mounted on a recent vehicle. Therefore, resonance may be caused between the frequencies of these equipment so as to cause a problem of malfunction of the equipment. Accordingly, the method in which the semiconductor laser diode is subjected to the fast modulation cannot be the best method.  
         [0011]     On the other hand, as the method in which the multi-longitudinal mode is employed for the oscillation mode, a gain guide structure used for the optical waveguide mechanism is well known. Since the threshold current is increased when the gain guide structure is used, however, the operation power is increased and this method is disadvantageous for the temperature characteristic.  
         [0012]     Also, as the method in which the semiconductor laser diode is pulse oscillated, the spread of a current is made narrower than the spread of light so as to form a saturable absorber in an active layer. As one means for forming a saturable absorber, technique disclosed in Japanese Patent Publication No. 3183692 (hereinafter referred to as Patent Document 2) is known. In the technique disclosed in Patent Document 2, the resistance of a semiconductor layer disposed between a current blocking layer and an active layer is set to be higher than that of the active layer. Thus, a current can be allowed to reach the active layer while suppressing the spread of the current to be equivalent to the width of a ridge stripe, and light is not supplied in a portion not having the stripe width because no current passes this portion, and hence, the laser beam is absorbed in this portion so as to form a saturable absorber. As a result, self sustained pulsation can be performed.  
         [0013]     In such a semiconductor laser diode, the thickness of the semiconductor layer disposed between the current blocking layer and the active layer is approximately 0.45 μm through 0.65 μm when it is an infrared laser diode and is approximately 0.25 μm through 0.4 μm when it is a red laser diode.  
       SUMMARY OF THE INVENTION  
       [0014]     In the above-described structure, since the current spread is equivalent to the stripe width, the self sustained pulsation can be performed stably at room temperature. However, when the temperature is high, since the current spread is suppressed, the current injected into the active layer is concentrated in a portion directly below the stripe, so as to increase the operation current density. Therefore, a leakage current is increased and the heat generation in the diode is increased, so as to lower the emission efficiency. Accordingly, when the temperature is high, there arises a problem that the optical output is thermally saturated due to the heat generation in the diode in a current-optical output characteristic (I-L characteristic), which largely affects an on-vehicle laser diode necessary to guarantee a wide operation temperature range.  
         [0015]     Accordingly, the present invention provides a semiconductor laser diode in which a longitudinal mode can stably keep multi-longitudinal mode oscillation (including self sustained pulsation) characteristics and temperature characteristics in a wide temperature range from a low temperature to a high temperature.  
         [0016]     The first semiconductor laser diode of this invention includes a light emitting portion on a substrate, and the light emitting portion includes a first cladding layer; an active layer formed on the first cladding layer; a second cladding layer formed on the active layer and having a ridge stripe for injecting a current into the active layer; and a current blocking layer formed on both sides of the ridge stripe for confining the current in the ridge stripe, and a distance from a lower face of the current blocking layer to an upper face of the active layer is within a given range, and the current spreads beyond a width of the ridge stripe after passing the ridge stripe and before reaching the active layer.  
         [0017]     At this point, a full width at half maximum of a current distribution is regarded as the current spread. Specifically, in the first semiconductor laser diode, the full width at half maximum of a current distribution on the active layer of the current having passed the ridge stripe spreads beyond the width of the ridge stripe. Also, expressions such as a width and lateral spread are herein used for a distance along the parallel direction.  
         [0018]     In the first semiconductor laser diode, since the distance (remaining thickness) from the lower face of the current blocking layer to the upper face of the active layer is not less than a given value, the current having passed the ridge stripe spreads in the lateral direction (the parallel direction) before reaching the active layer, and hence, the full width at half maximum of the current distribution is larger than the width of the ridge stripe. However, an optical field distribution spreads further beyond this current spread, and a sufficient saturable absorber is formed as a result, so as to stably cause multi-longitudinal mode oscillation. Furthermore, since the remaining thickness has an upper limit, higher-order lateral mode oscillation is suppressed so as to cause oscillation in the fundamental lateral mode alone.  
         [0019]     The given range is preferably set to have a lower limit corresponding to the distance (that is, the distance from the lower face of the current blocking layer to the upper face of the active layer, i.e., the remaining thickness) obtained at a point where increase of lateral spread of the current becomes gentle against increase of the distance and an upper limit corresponding to the distance obtained at a point where increase of a full width at half maximum of an NFP (Near Field Pattern) becomes gentle against the increase of the distance.  
         [0020]     Since the given range is thus defined, effects to stabilize the multi-longitudinal mode oscillation and to cause oscillation in the fundamental lateral mode can be definitely attained. It is noted that the points where the increase of the current lateral spread and the increase of the full width at half maximum of the NFP become gentle against the increase of the remaining thickness can be experimentally obtained.  
         [0021]     Specifically, the given range is preferably 0.65 μm or more and 1.2 μm or less. When the remaining thickness is within this range, oscillation of the fundamental lateral mode and the multi-longitudinal mode can be realized.  
         [0022]     In the first semiconductor laser diode, the current having passed the current blocking layer spreads laterally, and hence, a current density can be prevented from increasing because of the current concentrated in the active layer. Therefore, occurrence of a leakage current and heat generation can be suppressed in the semiconductor laser diode, so that it can be stably operated even at a high temperature.  
         [0023]     In this manner, in the first semiconductor laser diode, wide operation temperature guarantee and oscillation of the fundamental lateral mode and the multi-longitudinal mode can be both attained.  
         [0024]     The active layer is preferably made of Al x Ga 1−x As (wherein 0≦x≦1), and the first cladding layer and the second cladding layer are preferably made of an AlGaInP-based material.  
         [0025]     When such a structure is employed, the semiconductor laser diode can definitely attain both of the wide operation temperature guarantee and the oscillation of the fundamental lateral mode and the multi-longitudinal mode.  
         [0026]     It is noted that the AlGaInP-based material means a material having a composition expressed as (Al w Ga 1−w ) 2 In 1−z P (wherein 0&lt;w&lt;1 and 0&lt;z&lt;1).  
         [0027]     The width of the ridge stripe is preferably 1 μm or more and 4 μm or less.  
         [0028]     Thus, since the width of the ridge stripe is 4 μm or less, a far field pattern (FFP) along the horizontal direction can be prevented from having a double-humped property. Also, when the width of the ridge stripe is too small, the resistance is increased so as to generate heat, resulting in degrading the temperature characteristic. For preventing this, the width of the ridge stripe is preferably 1 μm or more. Also, the width of 1 μm or more is an example of the minimum width realized for forming a ridge stripe in consideration of the process.  
         [0029]     Also, the given range is preferably 0.4 μm or more and 0.7 μm or less. Furthermore, the active layer is preferably made of Ga y In 1−y P (wherein 0&lt;y&lt;1), and the first cladding layer and the second cladding layer are preferably made of an AlGaInP-based material. Moreover, the width of the ridge stripe is preferably 2.5 μm or more and 5.5 μm or less.  
         [0030]     Also in these cases, the effects of the semiconductor laser diode of this invention can be definitely attained.  
         [0031]     Furthermore, the ridge stripe preferably has side faces vertical to a principal plane of the substrate.  
         [0032]     When such a vertical ridge structure is used, differential resistance Rs can be reduced as compared with the case where a ridge stripe having a smaller width on the top than on the bottom is used, and hence, the heat generation in the diode can be suppressed. As a result, the semiconductor laser diode can realize wider operation temperature guarantee.  
         [0033]     The second semiconductor laser diode of this invention includes, on a substrate, at least a first light emitting portion and a second light emitting portion, and each of the first light emitting portion and the second light emitting portion includes a first cladding layer; an active layer formed on the first cladding layer; a second cladding layer formed on the active layer and having a ridge stripe for injecting a current into the active layer; and a current blocking layer formed on both sides of the ridge stripe for confining the current in the ridge stripe, a distance from a lower face of the current blocking layer to an upper face of the active layer is a value in a given range in both of the first light emitting portion and the second light emitting portion, and the current spreads beyond a width of the ridge stripe after passing the ridge stripe and before reaching the active layer in both of the first light emitting portion and the second light emitting portion.  
         [0034]     Also in the second semiconductor laser diode, a full width at half maximum is regarded as current spread as in the first semiconductor laser diode.  
         [0035]     In the second semiconductor laser diode, the distance (remaining thickness) from the lower face of the current blocking layer to the upper face of the active layer is defined in each of the first and second light emitting portions, and hence, oscillation of the findamental lateral mode and the multi-longitudinal mode can be stably caused. In addition, since the current spreads in the active layer, increase of the current density is avoided, and hence, the second semiconductor laser diode is a monolithic two-wavelength laser diode stably operating even at a high temperature.  
         [0036]     Specifically, the given range in the first light emitting portion is preferably 0.65 μm or more and 1.2 μm or less, and the given range in the second light emitting portion is preferably 0.4 μm or more and 0.7 μm or less.  
         [0037]     When the remaining thicknesses are thus defined, wide operation temperature guarantee and stability of the oscillation mode can be realized in each of the light emitting portions respectively emitting different kinds of lasers.  
         [0038]     The ridge stripe is preferably simultaneously formed in the first light emitting portion and the second light emitting portion.  
         [0039]     Thus, a distance between light emitting points of the first and second light emitting portions (namely, a distance between their ridge stripes) can be more highly precisely set than in the case where the ridge stripes are independently formed. This is very advantageous in mounting a laser diode and an optical component in an optical pickup device or the like. Specifically, if the distance between light emitting points is varied, the optical component should be mounted in accordance with the variation, which increases the difficulty in assembling and causes characteristic degradation such as lowering of laser beam utilization efficiency. However, such problems can be avoided when a plurality of ridge stripes are simultaneously formed. In addition, the number of procedures can be reduced as compared with the case where the ridge stripes are independently formed.  
         [0040]     The ridge stripe preferably has side faces vertical to a principal plane of the substrate in at least one of the first light emitting portion and the second light emitting portion.  
         [0041]     Thus, the differential resistance Rs can be reduced as compared with the case where a ridge stripe having a smaller width on the top than on the bottom is used, and hence the heat generation in the diode can be suppressed. As a result, the semiconductor laser diode can attain wider operation temperature guarantee.  
         [0042]     The active layer of the first light emitting portion is preferably made of Al x Ga l−x As (wherein 0≦x≦1), the active layer of the second light emitting portion is preferably made of Ga y In 1−y P (wherein 0&lt;y&lt;1), and the first cladding layer and the second cladding layer are preferably made of an AlGaInP-based material.  
         [0043]     Thus, the wide operation temperature guarantee and the stability of the oscillation mode can be definitely realized in both of the light emitting portions.  
         [0044]     The width of the ridge stripe of the first light emitting portion is preferably 1.0 μm or more and 4.0 μm or less, and the width of the ridge stripe of the second light emitting portion is preferably 2.5 μm or more and 5.5 μm or less.  
         [0045]     Thus, since the width of the ridge stripe is 4 μm or less in the first light emitting portion and 5.5 μm or less in the second light emitting portion, an FFP along the horizontal direction can be prevented from having a double-humped property. Also, the lower limit of the width in the first light emitting portion (i.e., 1 μm) is an example of the minimum width in consideration of the process, and the lower limit of the width in the second light emitting portion (i.e., 2.5 μm) is a value for preventing external differential efficiency from having non-linearity in the I-L characteristic.  
         [0046]     The ridge stripe preferably has a taper stripe structure having a changing width in at least one of the first light emitting portion and the second light emitting portion.  
         [0047]     When such a taper stripe structure is employed, the stripe width can be larger than in employing a straight stripe structure, and hence, the differential resistance Rs can be reduced so as to suppress the heat generation in the diode, and thus, the temperature characteristic can be improved.  
         [0048]     The taper stripe structure preferably has a width gradually increased from a side of an emitting facet for emitting light toward a side of a rear facet opposing the emitting facet.  
         [0049]     Alternatively, the taper stripe structure preferably has a width gradually reduced from a center thereof toward a side of an emitting facet for emitting light and toward a side of a rear facet opposing the emitting facet.  
         [0050]     The specific structure of the taper stripe structure may be one of the above. It is noted that the width of the ridge stripe of such a taper stripe structure is an average width calculated by assuming that the taper stripe structure is a straight stripe structure.  
         [0051]     As described so far, in the semiconductor laser diode of this invention, the distance from the lower face of the current blocking layer to the upper face of the active layer is defined within the given range, so as to control the lateral spread of the current in the active layer. Thus, the oscillation of the fundamental lateral mode and the multi-longitudinal mode can be realized, and the occurrence of a leakage current and the heat generation are suppressed because the current density is suppressed in the active layer, and therefore, the semiconductor laser diode can be stably operated even at a high temperature. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0052]      FIGS. 1A, 1B  and  1 C are diagrams for showing the structure of a semiconductor laser diode according to Embodiment 1 of the invention, and specifically,  FIG. 1A  is a cross-sectional view thereof,  FIG. 1B  is a detailed cross-sectional view of an active layer and  FIG. 1C  is a plan view of a stripe portion;  
         [0053]      FIG. 2A  is a diagram for showing the relationship between a distance (remaining thickness) d 1  from a lower face of a current blocking layer to an upper face of an active layer and the full width at half maximum of an oscillation spectrum and  FIGS. 2B, 2C  and  2 D are diagrams of spectra corresponding to various distances d 1 ;  
         [0054]      FIG. 3A  is an NFP image of a threshold current of the semiconductor laser diode of Embodiment 1 and  FIG. 3B  is a diagram for showing a laterally spread current and the full width at half maximum of the NFP against the remaining thickness d 1 ;  
         [0055]      FIG. 4A  is an NFP image obtained when a current of 10 mA is allowed to pass through the semiconductor laser diode of Embodiment 1 and  FIG. 4B  is a diagram for showing the relationship between a stripe width and the full width at half maximum of the NFP;  
         [0056]      FIG. 5A  is a diagram for showing the relationship between the stripe width and the full width at half maximum of an oscillation spectrum obtained in the semiconductor laser diode of Embodiment 1 and  FIGS. 5B, 5C  and  5 D are diagrams of oscillation spectra corresponding to various stripe widths;  
         [0057]      FIG. 6A  is a diagram for showing I-L characteristics obtained in Embodiment 1 and a conventional diode,  FIG. 6B  is a diagram of oscillation spectra obtained at 25° C. and 85° C. in Embodiment 1 and  FIG. 6C  is a diagram of oscillation spectra obtained at 25° C. and 85° C. in the conventional diode;  
         [0058]      FIGS. 7A, 7B  and  7 C are diagrams for showing the structure of the semiconductor laser diode of Embodiment 1 in which a vertical ridge structure is formed;  
         [0059]      FIGS. 8A, 8B  and  8 C are diagrams for showing the structure of a semiconductor laser diode according to Embodiment 2 of the invention, and specifically,  FIG. 8A  is a cross-sectional view thereof,  FIG. 8B  is a detailed cross-sectional view of an active layer of a red laser section and  FIG. 8C  is a plan view of stripe portions;  
         [0060]      FIG. 9A  is a diagram for showing the relationship, in the red laser section of Embodiment 2, between a distance (remaining thickness) d 2  from a lower face of a current blocking layer to an upper face of an active layer and the full width at half maximum of an oscillation spectrum and  FIGS. 9B, 9C  and  9 D are diagrams of spectra corresponding to various distances d 2 ;  
         [0061]      FIG. 10A  is an NFP image of a threshold current of the red laser section of the semiconductor laser diode of Embodiment 2 and  FIG. 10B  is a diagram for showing a laterally spread current and the full width at half maximum of the NFP against the remaining thickness d 2 ;  
         [0062]      FIG. 11A  is an NFP image obtained when a current of 10 mA is allowed to pass through the red laser section of the semiconductor laser diode of Embodiment 2 and  FIG. 11B  is a diagram for showing the relationship between a stripe width and the full width at half maximum of the NFP;  
         [0063]      FIG. 12A  is a diagram for showing the relationship between the stripe width and the full width at half maximum of an oscillation spectrum obtained in the red laser section of the semiconductor laser diode of Embodiment 2 and  FIGS. 12B, 12C  and  12 D are diagrams of oscillation spectra corresponding to various stripe widths;  
         [0064]      FIG. 13A  is a diagram for showing I-L characteristics obtained in the red laser section of Embodiment 2 and a conventional diode,  FIG. 13B  is a diagram of oscillation spectra obtained at 25° C. and 85° C. in the red laser section of Embodiment 2 and  FIG. 13C  is a diagram of oscillation spectra obtained at 25° C. and 85° C. in the conventional diode;  
         [0065]      FIGS. 14A, 14B  and  14 C are diagrams for showing the structure of the semiconductor laser diode of Embodiment 2 in which a vertical ridge structure is formed; and  
         [0066]      FIGS. 15A, 15B  and  15 C are diagrams for showing the structure of a semiconductor laser diode according to Embodiment 3 of the invention, and specifically,  FIG. 15A  is a cross-sectional view thereof,  FIG. 15B  is a detailed cross-sectional view of an active layer and  FIG. 15C  is a plan view of stripe portions. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0067]     Preferred embodiments of the invention will now be described with reference to the accompanying drawings.  
       EMBODIMENT 1  
       [0068]     A semiconductor laser diode according to Embodiment 1 of the invention will now be described.  FIGS. 1A, 1B  and  1 C are diagrams of the semiconductor laser diode of this embodiment. First,  FIG. 1A  shows the cross-sectional view thereof.  
         [0069]     The semiconductor laser diode has a structure in which an n-type GaAs buffer layer  102 , an n-type (AlGa)InP cladding layer  103 , an active layer  104 , a p-type (AlGa)InP first cladding layer  105 , a p-type GalnP etching stopper layer  106 , a p-type (AlGa)InP second cladding layer  107 , a p-type GaInP intermediate layer  108  and a p-type GaAs contact layer  110  are successively stacked in this order in the upward direction on an n-type GaAs substrate  101 . Thus, the semiconductor laser diode has a double hetero structure in which the active layer  104  is sandwiched between the two cladding layers, namely, the n-type (AlGa)InP cladding layer  103  and the p-type (AlGa)InP first cladding layer  105 .  
         [0070]     At this point, the active layer  104  is a quantum well active layer having three well layers as shown in  FIG. 1B . Specifically, three GaAs well layers  1045   w ,  1043   w  and  1041   w  are successively formed in the upward direction so as to sandwich two (AlGa)InP barrier layers  1044   b  and  1042   b  among them, and this five-layered structure is sandwiched between two (AlGa)InP guide layers  1040   g  and  1046   g . As a result, the layers  1046   g ,  1045   w ,  1044   b ,  1043   w ,  1042   b ,  1041   w  and  1040   g  are stacked in this order from the lower side (namely, the side of the n-type (AlGa)InP cladding layer  103 ). It is noted that the p-type (AlGa)InP first cladding layer  105  is disposed on the upper most (AlGa)InP guide layer  1040   g.    
         [0071]     Furthermore, as shown in  FIG. 1A , the p-type (AlGa)InP second cladding layer  107 , the p-type GaInP intermediate layer  108  and the p-type GaAs contact layer  110  are formed as a ridge stripe in the shape of a mesa stripe, so as to construct a stripe portion  111  having a larger width on its bottom than on its top. Moreover, an n-type GaAs current blocking layer  109  is formed on both sides of the stripe portion  111  so as to bury the stripe portion  111 . The stripe portion  111  and the n-type GaAs current blocking layer  109  thus formed on the p-type (AlGa)InP first cladding layer  105  construct a current confining structure for confining a region of a current injected into the active layer  104 .  
         [0072]     In  FIG. 1A , a distance (remaining thickness) from the lower face of the n-type current blocking layer  109  (namely, the lower face of the ridge stripe) to the upper face of the active layer  104  is shown as a distance d 1 .  
         [0073]     The plane structure of such a current confining structure is schematically shown in  FIG. 1C . In this case, a shape  111   a  of the lower face of the stripe portion  111  is shown, and the shape  111   a  has the same width over its whole length from an emitting facet A to a rear facet B. This width of the lower face of the stripe portion  111  is herein designated as a stripe width Ws. The rest of the plane shape corresponds to a shape  109   a  of the lower face of the n-type current blocking layer  109 .  
         [0074]     Furthermore, although not shown in the drawings, a p-type electrode is formed on the p-type GaAs contact layer  110  and the n-type current blocking layer  109 , and an n-type electrode is formed on the lower face of the n-type GaAs substrate  101 . The semiconductor laser diode has such a structure and is a laser diode for emitting infrared laser.  
         [0075]     As for the material of each layer, for example, with respect to the n-type (AlGa)InP cladding layer  103 , the p-type (AlGa)InP first cladding layer  105  and the p-type (AlGa)InP second cladding layer  107 , an exemplified composition ratio is (Al 0.7 Ga 0.3 ) 0.51 In 0.49 P. Also, with respect to the (AlGa)InP guide layers  1040   g  and  1046   g  and the (AlGa)InP barrier layers  1042   b  and  1044   b , an exemplified composition ratio is (Al 0.4 Ga 0.6 ) 0.51 In 0.49 P.  
         [0076]      FIG. 2A  shows the behavior of the multi-longitudinal mode property (the full width at half maximum of an oscillation spectrum) against the change of the remaining thickness d 1  in the semiconductor laser diode having the aforementioned structure. It is assumed, in this case, that the stripe width Ws is set to a constant value of 1.5 μm and that the measurement is performed at room temperature and at 4 mW.  
         [0077]     As shown in  FIG. 2A , when the remaining thickness d 1  is increased, the full width at half maximum of the oscillation spectrum is increased at first, but when the remaining thickness d 1  is 1 μm or more, the full width at half maximum of the oscillation spectrum is substantially constant.  FIGS. 2B, 2C  and  2 D show the actual spectrum waveforms obtained when the remaining thickness d 1  is 0.6 μm, 0.95 μm and 1.45 μm. The oscillation spectrum is close to a single peak in the case shown in  FIG. 2B  where the remaining thickness d 1  is 0.6 μm, is multi-longitudinal mode with a large full width at half maximum in the case shown in  FIG. 2C  where the remaining thickness d 1  is 0.95 μm, and starts to exhibit a double-humped property in the case shown in  FIG. 2D  where the remaining thickness d 1  is 1.45 μm.  
         [0078]     This can be explained by using an effective refractive index difference Δn between a portion corresponding to the stripe portion  111  and portions corresponding to the both sides of the stripe portion  111 .  
         [0079]     In the case where the remaining thickness d 1  is 0.6 μm, the difference Δn is approximately 1×10 −3 . Owing to this comparatively large difference Δn, light cannot spread to the side of the ridge, and hence, a saturable absorber is difficult to form. As a result, the full width at half maximum of the oscillation spectrum is small.  
         [0080]     On the contrary, in the case where the remaining thickness d 1  is 1.45 μm, the difference Δn is as small as approximately 1×10 −5 . Therefore, light can spread to the side of the ridge, but a current also excessively spreads because the remaining thickness is too large. As a result, the diode has a gain in a wide range of wavelength, and hence, the multi-longitudinal mode oscillation can be easily caused and higher-order lateral mode oscillation can be caused. Since the higher-order lateral mode and fundamental lateral mode are different in the propagation constant, two oscillation spectra respectively corresponding to the higher-order lateral mode and the fundamental lateral mode are both generated. This seems to be the cause of the double-humped spectrum shown in  FIG. 2D .  
         [0081]     It is confirmed, based on the aforementioned findings, that the range of the remaining thickness d 1  for keeping the stable lateral mode typified by the spectrum shown in  FIG. 2C  and for enabling the multi-longitudinal mode oscillation is approximately 0.65 μm through 1.2 μm.  
         [0082]     In a conventional general semiconductor laser diode, a remaining thickness d 1  for the self sustained pulsation of infrared laser is 0.45 μm through 0.65 μm (corresponding to a difference Δn of 3×10 −3  through 1×10 −3 ). In contrast, although the remaining thickness d 1  is as large as 0.65 μm through 1.2 μm (corresponding to a difference Δn of 1×10 −3  through 5×10 −5 ) in this embodiment, the multi-longitudinal mode oscillation (including the self sustained pulsation) can be performed. The reason is as follows:  
         [0083]      FIG. 3A  shows a near field pattern (NFP) image obtained in a threshold current state of a sample with a constant stripe width Ws of 1.5 μm and a remaining thickness d 1  of 0.9 μm. The NFP image shows an optical field distribution (a distribution of light intensity), and the degree of light spread can be expressed by using a full width at half maximum obtained from the optical field distribution.  
         [0084]      FIG. 3B  shows the full width at half maximum of an NFP image obtained in a threshold current state normalized on the basis of a remaining thickness d 1  of 0.6 μm (shown with broken lines) and laterally spread current (calculated values) normalized on the basis of the remaining thickness d 1  of 0 μm (shown with solid lines). In other words, the full width at half maximum and the laterally spread current are shown by using ratios to the base values. The full width at half maximum of the NFP image is larger at first as the remaining thickness d 1  is larger, and when the remaining thickness d 1  exceeds approximately 1.2 μm, it is gently increased and becomes almost constant. On the other hand, the laterally spread current is increased as the remaining thickness d 1  is increased to approximately 0.65 μm but is almost saturated (namely, is gently increased) thereafter.  
         [0085]     In this manner, in the range of the remaining thickness d 1  of 0.65 through 1.2 μm, the laterally spread current is almost saturated and the optical field distribution (spread) is increased, and therefore, this range can be regarded as a region where a saturable absorber is increased. On the contrary, when the remaining thickness d 1  is 1.2 μm or more, the optical field distribution (spread) and the laterally spread current are both substantially constant, the saturable absorber is not increased, and as a result, the full width at half maximum of the oscillation spectrum is substantially constant. Accordingly, a region where a saturable absorber can be easily formed is a region where the remaining thickness d 1  is 0.65 μm or more. The above-described region where the oscillation spectrum does not exhibit the double-humped property is the region where the remaining thickness d 1  is 1.2 μm or less, and therefore, a preferable range of the remaining thickness d 1  for enabling the stable multi-longitudinal mode oscillation is 0.65 μm through 1.2 μm.  
         [0086]     In this manner, in the semiconductor laser diode of this embodiment, since the remaining thickness is larger than in the conventional technique, the lateral spread of the current is larger, but a light emitting portion is further increased. As a result, a saturable absorber is sufficiently formed, so that the stable fundamental lateral mode can be kept and that the multi-longitudinal mode oscillation can be performed.  
         [0087]     Next, the degree of the current spread against the stripe width Ws will be described by using an NFP image obtained before laser oscillation. Since the NFP image obtained before the laser oscillation is strongly correlated with a density distribution of a current injected into an active layer, it can be used for evaluating the current spread.  
         [0088]      FIG. 4A  shows an NFP image obtained by allowing a current of 10 mA to pass a laser diode with a stripe width Ws of 1.7 μm and a remaining thickness d 1  of 0.9 μm, and the NFP image can be regarded also as a current distribution because of the aforementioned strong correlation. As shown in  FIG. 4A , since the remaining thickness d 1  is 0.9 μm and comparatively large, the current is widely spread in the lateral direction, and this spread is 5.8 μm when expressed by using the full width at half maximum. The stripe width Ws is 1.7 μm, and hence, the current is spread three or more times as large as the stripe width Ws.  
         [0089]      FIG. 4B  shows the behavior of the current spread expressed by using the full width at half maximum obtained when the stripe width Ws is changed. According to  FIG. 4B , when the stripe width Ws is 1 μm, the full width at half maximum is 5.7 μm, and the current spread becomes larger as the stripe width Ws is increased, and when the stripe width Ws is 5 μm, the full width at half maximum is 8 μm or more.  
         [0090]     In this manner, although the current lateral spread is equivalent to the stripe width in the conventional semiconductor laser diode, the current spread is larger than the stripe width in the semiconductor laser diode of this embodiment because of the large remaining thickness d 1 .  
         [0091]     Furthermore,  FIG. 5A  shows the behavior of the multi-longitudinal mode property (the full width at half maximum of an oscillation spectrum) obtained with the remaining width d 1  set to be a constant value of 0.85 μm and the stripe width changed. In particular, the oscillation spectra obtained when the stripe width Ws is 1.3 μm, 2.3 μm and 4.2 μm are respectively shown in  FIGS. 5B, 5C  and  5 D. As shown in  FIGS. 5B through 5D , as the stripe width Ws is larger, the full width at half maximum of the oscillation spectrum is reduced.  
         [0092]     This seems for the following reason: First, as the stripe width Ws is larger, the diffusion distance in the lateral direction of the current below the ridge stripe portion is larger. Therefore, the volume of the active layer into which the current is injected is larger below the ridge stripe portion. As a result, the volume of a saturable absorber formed in the active layer below the current blocking layer is relatively small as compared with the volume of the active layer into which the current injected. Accordingly, the self sustained pulsation is difficult to cause.  
         [0093]     Furthermore, when the stripe width Ws is 4.2 μm, a phenomenon that a far field pattern (FFP) along the horizontal direction has a double-humped property and a kink is caused in the vicinity of 9 mW is observed. This reveals that the upper limit of the stripe width Ws is approximately 4 μm in consideration of the characteristics of the diode. Also, since a stripe width of 1 μm is the process limit in forming a stripe, the lower limit is 1 μm. When the stripe width is smaller than the lower limit, the differential resistance is too high to generate heat, which harmfully affects the temperature characteristic.  
         [0094]     On the basis of the aforementioned results, the temperature characteristic is compared between the structure of the semiconductor laser diode of this embodiment and the conventional structure. As the results of the comparison,  FIG. 6A  shows the temperature dependency of the I-L characteristic and  FIGS. 6B and 6C  respectively show the temperature dependency of the oscillation spectrum in this embodiment and in the conventional technique. At this point, the remaining thickness d 1  is 0.83 μm in the structure of this embodiment, and the remaining thickness is 0.5 μm in the conventional structure. When the remaining thickness d 1  is converted into a difference Δn, the different Δn is 4×10 −4  in the structure of this embodiment and is 2.5×10 −3  in the conventional structure. It is noted that the stripe width Ws is 2.7 μm in the both structures and the composition ratios are set to the above-described values.  
         [0095]     As shown in  FIG. 6A , in the I-L characteristic obtained at 25° C., the conventional structure has a lower threshold current. However, in the I-L characteristic obtained at 85° C., the structure of this embodiment has a lower threshold current. This seems for the following reason: At a temperature of 25° C., the current lateral spread is smaller in the conventional structure than in the structure of this embodiment because of the smaller remaining thickness, and an unavailable current not related to the oscillation is smaller, and hence, the current is efficiently converted into light. Also, one factor of the good I-L characteristic seems to be that the waveguide loss in the active layer is reduced because the difference Δn is comparatively large. On the other hand, at a temperature of 85° C., the current injected into the active layer is concentrated in a portion directly below the stripe so as to increase the current density in the conventional structure, and a leakage current is caused in the diode so as to generate heat, which degrades the temperature characteristic. In this manner, the semiconductor laser diode of this embodiment has wider operation temperature guarantee than the conventional diode.  
         [0096]     Furthermore, as shown in  FIGS. 6B and 6C , the full width at half maximum is larger in the structure of this embodiment than in the conventional structure at both temperatures of 25° C. and 85° C., and hence, good self sustained pulsation seems to be caused in this embodiment.  
         [0097]     As described so far, when the remaining thickness d 1  is defined, a semiconductor laser diode with a good temperature characteristic can be realized while stably keeping the multi-longitudinal mode characteristics with a large full width at half maximum of the oscillation spectrum. Specifically, the remaining thickness d 1  is set in a range where the current spread is substantially constant against the increase of the remaining thickness d 1  and where the lateral spread of light in the active layer is remarkably increased against the increase of the remaining thickness d 1 .  
         [0098]     For this purpose, the lower limit of the remaining thickness d 1  is set to a value where the current spread starts to be substantially constant against the increase of the remaining thickness d 1 , and the remaining thickness is set to a region where the optical field distribution (the full width at half maximum of the NFP image) is not more than approximately three times as large as the stripe width.  
         [0099]     As specific dimensions, as described above, the distance (remaining thickness) d 1  from the lower face of the n-type current blocking layer  109  to the upper face of the active layer  104  is set to 0.65 through 1.2 μm (corresponding to a difference Δn of 1×10 −3  through 5×10 −5 ) and the stripe width Ws is set to 1.0 through 4.0 μm.  
         [0100]     It is noted that the material of each cladding layer preferably has resistance of 0. Ωcm or more and specifically, is preferably AlGaInP. In the case where the resistance of the cladding layer is too low (for example, when the resistance is less than 0.1 Ωcm with AlGaAs used), the current is excessively spread in the lateral direction and hence a saturable absorber cannot be formed. Therefore, in order to definitely form a saturable absorber, the cladding layer preferably has the aforementioned resistance.  
         [0101]     Also, in this embodiment, an inclined ridge, namely, the stripe portion  111  having a larger width on the bottom than on the top as shown in  FIG. 1A , is used as the ridge stripe. Instead, a vertical ridge structure having the same width on the bottom and the top may be used. The structure obtained in such a case is shown in  FIGS. 7A through 7C , which respectively correspond to  FIGS. 1A through 1C . As shown in  FIG. 7A , a stripe portion  211  having the same width on the bottom and the top is constructed by a p-type (AlGa)InP second cladding layer  107 , a p-type GaInP intermediate layer  108  and a p-type GaAs contact layer  110 . Also in this case, a current blocking layer  209  is formed so as to cover the side face of the stripe portion  211 .  
         [0102]     The rest of the structure is the same as that of the semiconductor laser diode shown in  FIGS. 1A through 1C . For example, the structure of an active layer  104  shown in  FIG. 7B  is the same as that shown in  FIG. 1B . Also,  FIG. 7C  schematically shows the plane structure of the stripe portion  211 , and specifically, a shape  211   a  of the lower face of the stripe portion  211  and a shape  209   a  of the lower face of the n-type current blocking layer  209  are shown. Furthermore, like reference numerals are used to refer to like elements so as to omit detailed description.  
         [0103]     In the case where the vertical ridge structure is used in this manner, the width on the top of the stripe portion  211  is larger than in using the inclined ridge shown in  FIG. 1A , and therefore, the differential resistance Rs can be reduced. As a result, the heat generation in the diode is suppressed, so as to improve the temperature characteristic.  
       EMBODIMENT 2  
       [0104]     A semiconductor laser diode according to Embodiment 2 will now be described.  
         [0105]      FIGS. 8A, 8B  and  8 C show the semiconductor laser diode of this embodiment.  FIG. 8A  is a cross-sectional view thereof. The semiconductor laser diode is a monolithic two-wavelength laser diode in which an infrared laser section  700  and a red laser section  730  are built on one n-type GaAs substrate  701 .  
         [0106]     First, the infrared laser section  700  has the same structure as that of the semiconductor laser diode of Embodiment 1. Specifically, an n-type GaAs buffer layer  702 , an n-type (AlGa)InP cladding layer  703 , an active layer  704 , a p-type (AlGa)InP first cladding layer  705 , a p-type GaInP etching stopper layer  706 , a p-type (AlGa)InP second cladding layer  707 , a p-type GaInP intermediate layer  708  and a p-type GaAs contact layer  710  are stacked in this order in the upward direction on the n-type GaAs substrate  701  shared with the red laser section  730  independently of the red laser section  730 . Thus, the infrared laser section  700  has the double hetero structure in which the active layer is sandwiched between the two cladding layers.  
         [0107]     Also, the active layer  704  is, similarly to the active layer  101  of the semiconductor laser diode of Embodiment 1 shown in  FIG. 1B , a quantum well active layer including three well layers.  
         [0108]     Furthermore, as shown in  FIG. 8A , the p-type (AlGa)InP second cladding layer  707 , the p-type GaInP intermediate layer  708  and the P-type GaAs contact layer  710  are formed as a ridge stripe in the shape of a mesa stripe, so as to construct a stripe portion  711  having a larger width on the bottom than on the top. An n-type GaAs current blocking layer  709  is formed on the both sides of the stripe portion  711 , thereby constructing a current confining structure for confining a region of a current injected into the active layer  704 . Such a structure is the same as that shown in  FIG. 1A .  
         [0109]     Also, a distance (remaining thickness) from the lower face of the current blocking layer  709  to the active layer  704  is shown as a distance d 1 .  
         [0110]     As for the material of each layer of the infrared laser section  700 , for example, with respect to the n-type (AlGa)InP cladding layer  703 , the p-type (AlGa)InP first cladding layer  705  and the p-type (AlGa)InP second cladding layer  707 , an exemplified composition ratio is (Al 0.7 Ga 0.3 ) 0.5 In 0.49 P. Also, with respect to (AlGa)InP barrier layers  1042   b  and  1044   b  included in the active layer  704 , an exemplified composition ratio is (Al 0.4 Ga 0.6 ) 0.5 In 0.49 P (see  FIG. 1B ).  
         [0111]     Next, the red laser section  730  basically has the same structure as the infrared laser section  700 . Specifically, an n-type GaAs buffer layer  732 , an n-type (AlGa)InP cladding layer  733 , an active layer  734 , a p-type (AlGa)InP first cladding layer  735 , a p-type GaInP etching stopper layer  736 , a p-type (AlGa)InP second cladding layer  737 , a p-type GaInP intermediate layer  738  and a p-type GaAs contact layer  740  are stacked in this order in the upward direction on the n-type GaAs substrate  701  shared with the infrared laser section  700  independently of the infrared laser section  700 . Thus, the red laser section  730  has the double hetero structure in which the active layer is sandwiched between the two cladding layers.  
         [0112]     However, the active layer  734  is a quantum well active layer including five well layers as shown in  FIG. 8B . Specifically, five GaInP well layers  7349   w ,  7347   w ,  7345   w ,  7343   w  and  7341   w  are successively formed in this order in the upward direction so as to sandwich four (AlGa)InP barrier layers  7348   b ,  7346   b ,  7344   b  and  7342   b  among them, and this multilayered structure including the nine layers is vertically sandwiched by two (AlGa)InP guide layers  7350   g  and  7340   g . As a result, the layers  7350   g ,  7349   w ,  7348   b ,  7347   w ,  7346   b ,  7345   w ,  7344   b ,  7343   w ,  7342   b ,  7341   w  and  7340   g  are successively stacked in this order from the lower side (namely, the side of the n-type (AlGa)InP cladding layer  733 ).  
         [0113]     Furthermore, as shown in  FIG. 8A , similarly to the infrared laser section  700 , the p-type (AlGa)InP second cladding layer  737 , the p-type GaInP intermediate layer  738  and the p-type GaAs contact layer  740  are formed (as a stripe portion  741 ) in the shape of a mesa stripe, and the n-type GaAs current blocking layer  709  is formed on the both sides of the stripe portion  741 , so as to construct a current confining structure.  
         [0114]     Moreover, a distance (remaining thickness) from the lower face of the current blocking layer  709  (namely, the lower face of the ridge stripe) to the upper face of the active layer  734  is shown as a distance d 2 .  
         [0115]     As for the material of each layer of the red laser section  730 , for example, with respect to the n-type (AlGa)InP cladding layer  733 , the p-type (AlGa)InP first cladding layer  735  and the p-type (AlGa)InP second cladding layer  737 , an exemplified composition ratio is (Al 0.71 Ga   0.3 ) 0.51 In 0.49 P. Also, with respect to the (AlGa)InP guide layers  7340   g  and  7350   g  and the (AlGa)InP barrier layers  7342   b ,  7344   b ,  7346   b  and  7348   b , an exemplified composition ratio is (Al 0.5 Ga 0.5 ) 0.51 In 0.49 P. Furthermore, with respect to the GaInP well layers  7349   w ,  7347   w ,  7345   w ,  7343   w  and  7341   w , an exemplified composition ratio is Ga 0.45 In 0.57 P.  
         [0116]     Next,  FIG. 8C  shows the plane shapes of the stripe portion  711  of the infrared laser section  700  and the stripe portion  741  of the red laser section  730 . At this point, the lower face of the stripe portion  711  is in a shape  711   a  having the same width over its whole length from an emitting facet A to a rear facet B similarly to that shown in  FIG. 1C . On the contrary, the lower face of the stripe portion  741  of the red laser section  730  is in a shape  741   a  of a taper stripe structure having a width increasing from the emitting facet A to the rear facet B. In this embodiment, the stripe width on the emitting facet A is indicated as a stripe width Ws of the red laser section  730 .  
         [0117]     The stripe portion  711  of the infrared laser section  700  and the stripe portion  741  of the red laser section  730  are simultaneously formed. Also, the current blocking layer  709  is formed simultaneously in the infrared laser section  700  and the red laser section  730 . Moreover, although not shown in the drawings, p-type electrodes are formed on the p-type GaAs contact layers  710  and  740  and the n-type current blocking layer  709 , and n-type electrodes are formed on the lower face of the n-type GaAs substrate  701 . These p-type electrodes and n-type electrodes are also simultaneously formed in the infrared laser section  700  and the red laser section  730 .  
         [0118]     As described so far, the semiconductor laser diode of this embodiment is a monolithic two-wavelength laser diode including the infrared laser section  700  and the red laser section  730 . The behavior of the multi-longitudinal mode property (the full width at half maximum of an oscillation spectrum) obtained when the remaining thickness is changed in this semiconductor laser diode is shown in  FIG. 9A  in the same manner as in Embodiment 1. However, the behavior in the infrared laser section  700  is herein omitted because it is the same as that of the semiconductor laser diode of Embodiment 1, and the behavior in the red laser section  730  alone is shown.  
         [0119]     In this case, it is assumed that the stripe width is 3 μm on the emitting facet A and 5 μm on the rear facet B and that the measurement is performed at room temperature and at 3.5 mW.  
         [0120]     As shown in  FIG. 9A , as the remaining thickness d 2  is increased, the full width at half maximum of the oscillation spectrum is increased at first, and when the remaining thickness d 2  exceeds 0.45 μm, the full width at half maximum of the oscillation spectrum becomes substantially constant.  FIGS. 9B, 9C  and  9 D respectively show the actual spectrum waveforms obtained when the remaining thickness d 2  is 0.39 μm, 0.47 μm and 0.72 μm. The oscillation spectrum is close to a single peak in the case shown in  FIG. 9B  where the remaining thickness d 2  is 0.39 μm and is a multi-longitudinal mode spectrum with a large full width at half maximum in the cases shown in  FIGS. 9C and 9D  where the remaining thickness d 2  is 0.47 μm and 0.72 μm.  
         [0121]     This can be explained by using an effective refractive index difference Δn between a portion corresponding to the stripe portion  741  and portions corresponding to the both sides of the stripe portion  741 .  
         [0122]     In the case where the remaining thickness d 2  is 0.39 μm, the difference Δn is approximately 1.2×10 −3 . Owing to this comparatively large difference Δn, light cannot spread to the side of the ridge, and hence, a saturable absorber is difficult to form. As a result, the full width at half maximum of the oscillation spectrum is small.  
         [0123]     On the contrary, in the cases where the remaining thickness d 2  is 0.47 μm and 0.72 μm, the difference Δn is as small as approximately 3.8×10 −4  and 3.6×10 −5 , respectively. Therefore, light can spread to the side of the ridge, and a saturable absorber is sufficiently formed, so that the multi-longitudinal mode oscillation can be performed. However, when the remaining thickness d 2  is 0.72 μm, although not shown in the drawing, a phenomenon that an FFP along the horizontal direction has a double-humped property is observed. Therefore, the range of the remaining thickness d 2  for realizing both the stable multi-longitudinal mode oscillation and fundamental lateral mode oscillation is approximately 0.4 μm through 0.7 μm.  
         [0124]     In a conventional general semiconductor laser diode, the remaining thickness d 2  for the self sustained pulsation of red laser is 0.25 μm through 0.4 μm (corresponding to a difference Δn of 3×10 −3  through 1×10−3). In contrast, although the remaining thickness d 2  is as large as 0.4 μm through 0.7 μm (corresponding to a difference Δn of 1×10 −3  through 5×10 −5 ) in this embodiment, the multi-longitudinal mode oscillation (including the self sustained pulsation) can be performed. The reason is as follows:  
         [0125]      FIG. 10A  shows an NFP image obtained in a threshold current state with a stripe width set to 3 μm on the emitting facet A and 5 μm on the rear facet B and with a remaining thickness d 2  set to 0.47 μm. The degree of light spread can be expressed by using a full width at half maximum obtained from the optical field distribution of the NFP image in the same manner as in Embodiment 1.  
         [0126]      FIG. 10B  shows the full width at half maximum of an NFP image obtained in a threshold current state normalized on the basis of a remaining thickness d 2  of 0.39 μm (shown with broken lines) and laterally spread current (calculated values) normalized on the basis of the remaining thickness d 2  of 0 μm (shown with solid lines). The full width at half maximum of the NFP image is larger at first as the remaining thickness d 2  is larger, and when the remaining thickness d 2  exceeds approximately 0.7 μm, it becomes almost constant. On the other hand, the laterally spread current is increased as the remaining thickness d 2  is increased to approximately 0.4 μm but is almost saturated thereafter.  
         [0127]     In this manner, in the range of the remaining thickness d 2  of 0.4 through 0.7 μm, the lateral current spread is almost saturated and the optical field distribution (spread) is increased, and therefore, this range can be regarded as a region where a saturable absorber is increased. On the contrary, when the remaining thickness d 2  is 0.7 μm or more, the optical field distribution (spread) and the lateral current spread are both substantially constant, and the saturable absorber is not remarkably increased, and as a result, the full width at half maximum of the oscillation spectrum is substantially constant. Accordingly, a region where a saturable absorber can be easily formed is a region where the remaining thickness d 2  is 0.4 μm or more. The above-described region where the FFP does not exhibit the double-humped property is the region where the remaining thickness d 2  is 0.7 μm or less, and therefore, a range of the remaining thickness d 2  for enabling the stable multi-longitudinal mode oscillation is 0.4 μm through 0.7 μm.  
         [0128]     Next, the degree of the current spread against the stripe width will be described by using an NFP image obtained before laser oscillation.  FIG. 11A  is an NFP image obtained by allowing a current of 10 mA to pass when the stripe width is 3.3 μm on the emitting facet A and 6 μm on the rear facet B and the remaining thickness d 2  is 0.43 μm. Since this NFP image is strongly correlated with a density distribution of a current injected into the active layer, it can be regarded as a current distribution. As shown in  FIG. 11A , since the remaining thickness d 2  is comparatively large and 0.43 μm, the current is widely spread in the lateral direction, and this spread is 5.9 μm when expressed by using the full width at half maximum. The stripe width Ws on the emitting facet A is 3.3 μm, and hence, the current is spread almost twice as large as the stripe width Ws.  
         [0129]      FIG. 11B  shows the behavior of the full width at half maximum of the NFP obtained when the stripe width Ws on the emitting face A is changed with the stripe width on the rear facet B fixed to 6 μm. According to  FIG. 11B , when the stripe width Ws on the emitting facet A is 2.9 μm, the current spread (full width at half maximum) is 5.3 μm, and the current spread is larger as the stripe width is increased, and when the stripe width Ws is 5 μm, the full width at half maximum is 8 μm or more. In this manner, when the remaining thickness d 2  is larger than in the conventional technique, the current spread becomes larger than the stripe width.  
         [0130]     Next,  FIG. 12A  shows the behavior of the multi-longitudinal mode property (the full width at half maximum of an oscillation spectrum) obtained with the remaining width d 2  set to a constant value of 0.45 μm, the stripe width on the rear facet B fixed to 6 μm and the stripe width Ws on the emitting facet A changed. The oscillation spectra obtained when the stripe width Ws on the emitting facet A is 2.2 μm, 4.2 μm and 5.8 μm are respectively shown in  FIGS. 12B, 12C  and  12 D. As shown in  FIGS. 12B through 12D , as the stripe width Ws on the emitting facet A is larger, the full width at half maximum of the oscillation spectrum is reduced. This seems for the same reason as that described in Embodiment 1. Specifically, as the stripe width Ws on the emitting facet A is larger, the volume of the active layer into which the current is injected is larger below the ridge stripe portion. As a result, the volume of a saturable absorber formed in the active layer below the current blocking layer is relatively small. Accordingly, the self sustained pulsation is difficult to cause.  
         [0131]     Furthermore, when the stripe width Ws is 5.8 μm, a phenomenon that higher-order lateral mode oscillation is caused in an FFP along the horizontal direction and that a kink is caused in the vicinity of 7 mW is observed. This reveals that the upper limit of the stripe width Ws is approximately 5.5 μm in consideration of the characteristics of the diode.  
         [0132]     Moreover, when the stripe width is 2.2 μm, non-linearity is caused in external differential efficiency Se in the I-L characteristic. This is because waveguide loss is abruptly reduced when the volume of a saturable absorber is increased and the saturable absorber becomes transparent. Such an I-L characteristic is practically unpreferred because an APC (auto power control) operation is difficult. Accordingly, the lower limit of the stripe width is approximately 2.5 μm.  
         [0133]     On the basis of the aforementioned results, the temperature characteristic is compared between the structure of the semiconductor laser diode of this embodiment and the conventional structure. As the results of the comparison,  FIG. 13A  shows the temperature dependency of the I-L characteristic and  FIGS. 13B and 13C  respectively show the temperature dependency of the oscillation spectrum in this embodiment and in the conventional technique. At this point, the remaining thickness d 2  is 0.42 μm in the structure of this embodiment, and the remaining thickness is 0.33 μm in the conventional structure. When the remaining thickness d 2  is converted into a difference Δn, the different Δn is 5×10 −4  in the structure of this embodiment and is 1.4×10−3 in the conventional structure. It is noted that the stripe width Ws is 3.2 μm on the emitting facet A and 5.2 μm on the rear facet B in the both structures and the composition ratios are set to the above-described values.  
         [0134]     As shown in  FIG. 13A , in the I-L characteristic obtained at 25° C., the conventional structure has a lower threshold current. However, in the I-L characteristic obtained at 85° C., the structure of this embodiment has a lower threshold current. This seems for the same reason as that described in Embodiment 1. Specifically, at a temperature of 25° C., the current does not laterally spread widely in the conventional structure because of the smaller remaining thickness, and the unavailable current not related to the oscillation is smaller, and hence, the current is efficiently converted into light. Also, one factor of the good I-L characteristic seems to be that the waveguide loss in the active layer is reduced because the difference Δn is comparatively large. On the other hand, at a temperature of 85° C., the current injected into the active layer is concentrated in a portion directly below the stripe so as to increase the current density in the conventional structure, and a leakage current is caused in the diode so as to generate heat, which degrades the temperature characteristic. In the semiconductor laser diode of this embodiment, the current lateral spread is larger than in the conventional structure, and hence, the current density is lower than in the conventional structure, so as to suppress the occurrence of a leakage current. Thus, the semiconductor diode of this embodiment has wider operation temperature guarantee.  
         [0135]     Furthermore, as shown in  FIGS. 13B and 13C , the full width at half maximum of the oscillation spectrum is larger in the structure of this embodiment than in the conventional structure at both temperatures of 25° C. and 85° C., and hence, a good multi-longitudinal mode characteristic is attained.  
         [0136]     As described so far, in the same manner as in Embodiment 1, when the remaining thickness d 2  is defined, a semiconductor laser diode with a good temperature characteristic can be realized while stably keeping the multi-longitudinal mode characteristic with a large full width at half maximum of the oscillation spectrum in this embodiment. Specifically, the remaining thickness d 2  is set in a range where the current spread is substantially constant against the increase of the remaining thickness d 2  and where the lateral spread of light is increased against the increase of the remaining thickness d 2 .  
         [0137]     For this purpose, the lower limit of the remaining thickness d 2  is set to a value where the current spread starts to be substantially constant against the increase of the remaining thickness d 2 , and the remaining thickness is set to a region where the optical field distribution (the full width at half maximum of the NFP) is not more than approximately twice as large as the stripe width.  
         [0138]     As specific dimensions, as described above, the distance (remaining thickness) d 2  from the lower face of the n-type current blocking layer  709  to the upper face of the active layer  734  is set to 0.4 through 0.7 μm (corresponding to a difference Δn of 1×10 −3  through 5×10 −5 ) and an average stripe width (i.e., an average width calculated by assuming that the taper stripe structure is a straight stripe structure) is set to 2.5 through 5.5 μm. In addition, the infrared laser section  700  has the same structure as the semiconductor laser diode of Embodiment 1. Thus, stable characteristics can be attained as the monolithic two-wavelength laser.  
         [0139]     It is noted that the material of each cladding layer preferably has resistance of 0.1 Ωcm or more and specifically, is preferably AlGaInP.  
         [0140]     Also, in this embodiment, an inclined ridge, namely, the stripe portion  711  having a larger width on the bottom than on the top as shown in  FIG. 8A , is used as the ridge stripe. Instead, a vertical ridge structure having the same width on the bottom and the top may be used in the same manner as described in Embodiment 1. The structure obtained in such a case is shown in  FIGS. 14A through 14C , which respectively correspond to  FIGS. 8A through 8C . As shown in  FIG. 14A , a stripe portion  811  having the same width on the bottom and the top is constructed by a p-type (AlGa)InP second cladding layer  707 , a p-type GaInP intermediate layer  708  and a p-type GaAs contact layer  710 . Furthermore, a stripe portion  841  having the same width on the bottom and the top is constructed by a p-type (AlGa)InP second cladding layer  737 , a p-type GaInP intermediate layer  738  and a p-type GaAs contact layer  740 . Also in this case, a current blocking layer  809  is formed so as to cover the side faces of the stripe portions  811  and  841 .  
         [0141]     The rest of the structure is the same as that of the semiconductor laser diode shown in  FIGS. 8A through 8C , and hence, like reference numerals are used to refer to like elements so as to omit detailed description.  
         [0142]     In the case where the vertical ridge structure is used in this manner, the width on the top of the stripe portions  811  and  841  is larger than in using the inclined ridge shown in  FIG. 8A , and therefore, the differential resistance Rs can be reduced. As a result, the heat generation in the diode is suppressed, so as to improve the temperature characteristic.  
         [0143]     In this embodiment, the monolithic two-wavelength laser diode in which the infrared laser section  700  equivalent to the semiconductor laser diode of Embodiment 1 and the red laser section  730  are formed on the shared n-type GaAs substrate  701  is described. Needless to say, a laser diode including the red laser section  730  alone as a light emitting portion for laser oscillation can be formed. Also in this case, it goes without saying that a laser diode capable of stably operating in a wider temperature range than in the conventional technique can be obtained when the red laser section has the remaining thickness described in this embodiment.  
       EMBODIMENT 3  
       [0144]     A semiconductor laser diode according to Embodiment 3 of the invention will now be described.  
         [0145]      FIGS. 15A, 15B  and  15 C show the semiconductor laser diode of this embodiment. This semiconductor laser diode has substantially the same structure as the semiconductor laser diode of Embodiment 2 except for one difference. Therefore, the difference of the structure alone will be herein described in detail. Like reference numerals are used in  FIGS. 13A through 13C  to refer to like elements used in the semiconductor laser diode of Embodiment 2 shown in  FIG. 8A through 8C .  
         [0146]     The semiconductor laser diode of this embodiment is different from the semiconductor laser diode of Embodiment 2 in the plane shapes of a stripe portion  911  including a p-type (AlGa)InP second cladding layer  707 , a p-type GaInP intermediate layer  708  and a p-type GaAs contact layer  710  and a stripe portion  941  including a p-type (AlGa)InP second cladding layer  737 , a p-type GaInP intermediate layer  738  and a p-type GaAs contact layer  740 .  
         [0147]     Specifically, each of the stripe portions  911  and  941  is in a shape in which its stripe width is gradually increased inwardly from an emitting facet A, is uniform as a straight stripe at the center and is gradually reduced toward a rear facet B as shown in  FIG. 15C . In other words, each stripe portion has an inner part having a constant stripe width and tapered parts having widths gradually reduced toward the emitting facet A and toward the rear facet B and continuously disposed on the sides of the inner part.  
         [0148]     In such a taper stripe structure, the stripe width can be increased than in the straight stripe structure, and hence, the differential resistance Rs can be reduced to suppress the heat generation of the diode, so as to improve the temperature characteristic.  
         [0149]     However, an average of the width of the stripe portion  911  of the infrared laser section  700  (an average width calculated by assuming the taper stripe structure is a straight stripe structure) should be 1 through 4 μm, and an average of the width of the stripe portion  941  of the red laser section  730  should be 2.5 through 5.5 μm. These dimensional ranges are the same as those described in Embodiments 1 and 2. When the average widths are out of these ranges, there arise problems that a kink is caused, that the oscillation spectrum or the FFP exhibits a double-humped property and that the fabrication yield of the laser diode is lowered.  
         [0150]     In each of Embodiments 1 through 3, an n-type GaAs layer is used as the current blocking layer. However, a metal film such as a Ti/Au film, a semiconductor film such as an AlGaAs, AlInP or α-Si film, or an insulating film such as a SiN x  or SiO x  film may be used instead. Furthermore, the invention is not limited to the composition ratios such as (Al 0.5 Ga 0.5 ) 0.51 In 0.49 P.  
         [0151]     Moreover, although the number of well layers included in the active layer is three in the infrared laser section and five in the red laser section, good characteristics can be attained as far as the number is three through five in the infrared laser section and four through seven in the red laser section.  
         [0152]     Furthermore, the three stripe structures including the straight stripe structure, the taper stripe structure in which the width on the emitting facet A is smaller and the width on the rear facet B is larger and the stripe structure in which the width is larger at the center and is gradually reduced toward the facets are herein described. The invention is not limited to these stripe structures, but a structure in which the width is smaller on the rear facet B and larger on the emitting facet A, a structure in which the width is smaller at the center and is gradually increased toward the facets or the like may be employed. However, an average stripe width (namely, an average width calculated by assuming that a taper stripe structure is a straight stripe structure) should be 1 through 4 μm in the infrared laser section and 2.5 through 5.5 μm in the red laser section.  
         [0153]     In addition, it goes without saying that the present invention is applicable to various lasers including an AlGaAs/GaAs-based laser, an AlGaN/InGaN-based laser and a ZnMgSSe/ZnS-based laser.  
         [0154]     The semiconductor laser diode of the present invention described so far performs the fundamental lateral mode and the multi-longitudinal mode oscillation stably even at a high temperature and exhibits a good temperature characteristic. Therefore, it is useful as a semiconductor laser diode required to guarantee a wide operation temperature range, and specifically, is useful as a laser light source or the like in the field of optical disk systems.