Abstract:
A conventional semiconductor laser diode is small in optical power at a constant operating current and limited in ridge width when integrated with an optical device, which forces the integration to be performed by lowering the original characteristic and makes it difficult to reduce cost and power consumption. In a semiconductor laser diode, widening of the ridge width is made possible by lowering the difference in refractive indexes between the ridge and other components, diffusion current and increase in the difference of refractive indexes are prevented by forming approximately vertical grooves along both sides of the ridge, and deterioration in characteristics due to regrowth is prevented by forming a diffraction grating on the ridge. The semiconductor laser diode is integrated with an optical device such as electroabsorption type optical modulator without increase of growth cycles and without restriction of the ridge width by using a tapered waveguide.

Description:
The present application claims priority from Japanese application JP 2006-044887 filed on Feb. 22, 2006, the content of which is hereby incorporated by reference into this application. 
     FIELD OF THE INVENTION 
     The present invention relates to a semiconductor laser diode and an integrated optical waveguide device in which the semiconductor laser diode and an optical modulator are integrated. 
     BACKGROUND OF THE INVENTION 
     Recently, it is becoming increasingly important to reduce power consumption and cost of a semiconductor laser diode and an integrated semiconductor optical waveguide device in which a semiconductor laser diode and an electroabsorption type optical modulator are integrated. For the semiconductor laser diode, there are mainly ridge type and buried heterostructure type, of which the ridge type is advantageous for cost reduction because of its easier fabrication and smaller number of growth steps and is actively developed for use in both information and communication. The ridge type semiconductor laser diode is formed by laminating a lower clad layer, multiple or single well layer, upper clad layer, and ridge on a substrate formed of n-type semiconductor. An integrated semiconductor optical waveguide device (EA/DFB) is constructed by integrating this ridge type semiconductor laser diode and an electroabsorption type optical modulator with ridge on a common substrate. 
     In the field of information, high speed recording of information is demanded in accordance with an increase in the amount of information to be recorded, resulting in an increasing need for a higher power semiconductor laser diode. Although this may be met by increasing an operating current, it is disadvantageous for reduction in power consumption. On the other hand, in the field of communication, the mainstream transmission speed in current backbone network and metro network is 2.5 Gbps or 10 Gbps. Therefore, a structure of integrated semiconductor optical waveguide device in which an optical modulator is monolithically integrated for high speed modulation of a transmitter is advantageous for reduction in cost. However, power consumption increases with an increase in transmission distance as well as with the use of higher bit rate. With the aim of reducing the power consumption, the development of an electroabsorption modulator integrated distributed feedback laser (EA/DFB) that does not require temperature control between −5 degree C. and 85 degree C. has been pursued (Non-patent document 1: OFCNFOEC OFC POSTDEADLINE PAPERS Thursday, Mar. 10, 2005 PDP14). In order to achieve this, further enhancement in optical power of a semiconductor laser diode under a constant operating current is needed. Thus, the enhancement in optical power under a constant operating current is required for the reduction in power consumption of a semiconductor laser diode for use in both information and communication. 
     One method of power enhancement of a semiconductor laser diode is to widen its ridge width. Since the amount of heat generated becomes larger in general as the operating current in a semiconductor laser diode is raised, optical power is saturated at a certain current level, thereby making it impossible to obtain enough output. On the other hand, an electric resistance at the time of current injection into a semiconductor laser diode whose ridge width is widened is lowered, the amount of heat generation is correspondingly suppressed, and the saturation current is enhanced. As the result, the saturation output is also enhanced, and the optical power at a constant operating current is increased. The widening of the ridge width can be realized by forming an upper buffer layer between an upper clad layer and the ridge. An average refractive index difference in the lamination direction between the portion including the ridge and the portion not including the ridge becomes smaller by forming the upper buffer layer compared to when the upper buffer layer is not provided, and so-called cut-off width referred in the slab waveguide is increased, which makes it possible to widen the ridge width in lateral single mode. 
     As another method of the power enhancement, there is a method to suppress a rise of threshold current of a semiconductor laser diode. When the threshold current is low, optical power at a constant operating current rises, and thus the power enhancement of a semiconductor laser diode can be realized. As the method to suppress the rise of the threshold current, for example, there is a method disclosed in JP-A No. 214372/2004 (Patent document 1). In this method, a cover layer injected with Fe is formed by regrowth in both side directions of the ridge of a conventional ridge type semiconductor laser diode formed of InP series such as InP, InGaAsP and InGaAlAs, and this is used as an Fe supply source to an upper clad layer. The upper clad layer is made insulative, thereby suppressing diffusion of current injected from the ridge in the upper clad layer and a rise of the threshold current. When these lasers are made to function as a distributed feedback (DFB) type, a diffraction grating has been conventionally fabricated in an upper portion of n-substrate, a multiple well layer, or an upper buffer layer. 
     On the other hand, as for EA/DFB, for example, a semiconductor optical waveguide device in which a buried heterostructure type semiconductor laser diode and a ridge type electroabsorption type optical modulator are integrated differs in mode expansion in each portion, and therefore, a method of integrating the buried heterostructure type and the ridge type by tapering each joint portion is proposed in JP-A No. 78792/1996 (Patent document 2). Further, a method in which the light emitting side of a semiconductor optical waveguide device is tapered to make light coupling to fiber better is proposed in JP-A No. 66046/2000 (Patent document 3). However, no semiconductor optical waveguide device integrated with a high power laser in which ridge is widened or threshold current is lowered as described above has been disclosed. 
     As described above, the insertion of the upper buffer layer between the upper clad layer and the ridge is effective for power enhancement of a semiconductor laser diode, whereas there has been a problem that lateral diffusion of carrier becomes larger particularly on the p-side and the threshold current is increased. Further, since the average refractive index difference between the portion including the ridge and the portion not including the ridge becomes smaller, mode shape laterally expands, and far field pattern expansion becomes markedly different between in the horizontal direction and in the vertical direction, resulting in being asymmetrical. This causes an increase in loss of coupling to an exterior such as fiber. 
     To suppress the above-described rise of the threshold current, the application of the method disclosed in Patent document 1 is conceivable. However, this method requires crystal regrowth to form a cover layer. Therefore, it is disadvantageous in terms of cost reduction, and further, the application of the method is limited to InP-substrate based laser diodes, making it impossible to apply to semiconductor laser diodes formed of other materials such as GaAs-substrate based laser diodes. 
     For a high power semiconductor laser diode, it is effective to insert the upper buffer layer between the upper clad layer and the ridge to widen the ridge width. The application of this method to an integrated semiconductor waveguide device such as EA/DFB created another problem. For example, for power enhancement of a semiconductor laser diode, when the ridge width of a semiconductor laser diode is set to 2 μm, the capacitance increases and the band decreases in an electroabsorption type optical modulator portion. On the other hand, when the ridge width of the semiconductor laser diode is set to 1.4 μm in accord with the ridge width of 1.4 μm of the electroabsorption type optical modulator portion, the thermal characteristic of the semiconductor laser diode deteriorates and high output cannot be obtained. Therefore, as a trade-off value, the ridge width of EA/DFB has been set to ca.1.6 μm for integration which deviates from an original optimal ridge width and at which an overall characteristic deteriorates but each of the semiconductor laser diode and the electroabsorption type optical modulator can fulfill its function with ease. 
     Further, the insertion position of a diffraction grating also affects laser characteristics. A conventional position for fabrication of a diffraction grating has been in an upper portion of n-substrate, a multiple well layer, or an upper buffer layer. When the diffraction grating is inserted into the multiple well layer or the upper buffer layer, regrowth is carried out after forming the diffraction grating. However, a change in carrier concentration occurs at the regrowth interface, resulting in trapping of carrier, which causes deterioration of a characteristic in respect of power enhancement. On the other hand, when the diffraction grating is fabricated in the upper portion of n-substrate, the above problem can be ignored but wavelength controllability deteriorates because the diffraction grating has been formed before a multiple well layer is formed. 
     As described above, the above methods have not yet reached a point where reduction in power consumption and reduction in cost are compatible with each other. In addition, the design of a semiconductor laser diode and an electroabsorption type optical modulator, particularly ridge width thereof, is subject to limitation for integration, and each device has not been able to be integrated under optimal conditions. 
     SUMMARY OF THE INVENTION 
     The present invention accomplishes a high power semiconductor laser diode. Further, the present invention achieves reduction in power consumption of an integrated semiconductor optical waveguide device in which the semiconductor laser diode enhanced in output power is integrated with an electroabsorption type optical modulator as well as reduction in cost of the integration without deteriorating each characteristic of the semiconductor laser diode and the electroabsorption type optical modulator. 
     First, the semiconductor laser diode is formed from a lower clad layer, a multiple or single well layer, an upper clad layer, an upper buffer layer, and a ridge on an n-type semiconductor substrate. Further, insulative grooves with a low refractive index that are cut into the upper buffer layer along both sides of the ridge are formed, thereby suppressing lateral diffusion of current injected from the ridge. Owing to this, a rise of threshold current is suppressed. 
     In the semiconductor laser diode constructed from the lower clad layer, the multiple or single well layer, the upper clad layer, the ridge, and the like on the n-type semiconductor substrate, a diffraction grating is formed, in the ridge, of a semiconductor material having a refractive index higher than that of a semiconductor material mainly constituting the ridge. In this way, the ridge is manufactured without characteristic deterioration caused by contamination of impurities and a change in carrier concentration due to regrowth after forming the diffraction grating, compared to when the diffraction grating is formed on the upper clad layer. 
     On the other hand, in an integrated semiconductor optical waveguide device such as EA/DFB, the respective ridges of the semiconductor laser diode and an electroabsorption type optical modulator provided with a ridge are connected by a waveguide having a ridge in a tapered form. 
     According to the present invention, a semiconductor laser diode having a high output power and a low threshold current can be realized. At the same time, in an integrated semiconductor optical waveguide device such as EA/DFB, the semiconductor laser diode and the electroabsorption type optical modulator can not only be made to exhibit their respective characteristics maximally but also be integrated at low cost while suppressing anisotropy in the expansion of the far field pattern of emitting light in the vertical direction and the horizontal direction with low loss of light and without increasing the growth cycle at the time of integration. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram to explain the first half of a fabrication process of a semiconductor laser diode having an upper buffer layer as the premise of the present invention, where  FIG. 1A ,  FIG. 1B ,  FIG. 1C , and  FIG. 1D  show successive steps of the process; 
         FIG. 2  is a diagram to explain the second half of the fabrication process of the semiconductor laser diode having the upper buffer layer as the premise of the present invention, where  FIG. 2A ,  FIG. 2B , and  FIG. 2C  show successive steps of the process, and  FIG. 2C  represents a completed state of the semiconductor laser diode; 
         FIG. 3  is a graph showing a comparison result of operation characteristic of a semiconductor laser diode without the upper buffer layer constructed in almost the same structure as that of the semiconductor laser diode as the premise of the present invention; 
         FIG. 4  is a diagram to explain the first half of a fabrication process of a semiconductor laser diode of Embodiment 1 of the present invention using the structure in  FIG. 2A  as a starting point, where  FIG. 4A ,  FIG. 4B ,  FIG. 4C , and  FIG. 4D  show successive steps of the process; 
         FIG. 5  is a diagram to explain the second half of the fabrication process of the semiconductor laser diode of Embodiment 1 of the present invention, where  FIG. 5A ,  FIG. 5B ,  FIG. 5C , and  FIG. 5D  show successive steps of the process; 
         FIG. 6  is a diagram representing a completed state of the semiconductor laser diode of Embodiment 1 of the present invention; 
         FIG. 7A  is a graph to explain characteristics of the semiconductor laser diode of Embodiment 1; 
         FIG. 7B  is a characteristic graph showing evaluated relation of groove width in the upper buffer layer and cut-off width; 
         FIG. 8  is a diagram showing a fabrication process of a semiconductor laser diode of Embodiment 2 of the present invention realized by a structure starting with an n-type GaAs semiconductor substrate, where  FIG. 8A ,  FIG. 8B , and  FIG. 8C  shows main successive steps of the process, and  FIG. 8C  represents a completed state of the semiconductor laser diode; 
         FIG. 9  is a diagram showing the first half of a fabrication process of an integrated optical waveguide device of Embodiment 3 of the present invention, where  FIG. 9A ,  FIG. 9B ,  FIG. 9C ,  FIG. 9D , and  FIG. 9E  show successive steps of the process; 
         FIG. 10  is a diagram showing the second half of the fabrication process of the integrated optical waveguide device of Embodiment 3 of the present invention, where  FIG. 10A ,  FIG. 10B ,  FIG. 10C , and  FIG. 10D  show successive steps of the process; 
         FIG. 11  represents a completed state of the integrated optical waveguide device of Embodiment 3 of the present invention; 
         FIG. 12  is a graph showing evaluation results of characteristics of the integrated optical waveguide device of Embodiment 3 of the present invention when the ridge widths of semiconductor laser diodes were 2.0 μm and 2.5 μm, respectively, and the ridge width of an electroabsorption type optical modulator was 1.4 μm, where the waveguide length was taken on the horizontal axis and the transmittance was taken on the vertical axis; and 
         FIG. 13  is a diagram representing a completed state of an integrated optical waveguide device of Embodiment 4, where a Mach-Zehnder type optical modulator was employed in place of the electroabsorption type optical modulator in the integrated optical waveguide device of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention are explained with Embodiments 1 to 5 with reference to related drawings. 
     Embodiment 1 
     Embodiment 1 in which the present invention was applied to a 1.5 μm-wavelength band ridge waveguide type semiconductor laser diode is explained first. It should be noted that the figure size and the scale described in Embodiment 1 do not necessarily correspond proportionately. A semiconductor laser diode having an upper buffer layer as the premise of the present invention is explained using  FIGS. 1 and 2 , and then an embodiment of a semiconductor laser diode of the present invention in which insulative grooves with a low refractive index that were cut into the upper buffer layer along both sides of the ridge were formed is explained. 
     As shown in  FIG. 1A , on an n-type InP semiconductor substrate  1  (thickness 2 mm), an n-type InP buffer layer  2  (thickness 0.15 μm), a lower clad layer  3  (thickness 0.13 μm) formed of n-type InGaAsP series, a multiple-quantum well active layer  4  (thickness 0.114 μm) in which a well layer formed of InGaAsP (thickness 7 nm, compositional wavelength 1.3 μm) having 1.0% compressive strain and a barrier layer formed of InGaAsP (thickness 12 nm, compositional wavelength 1.3 μm) having 0.5% tensile strain were laminated by 6 cycles, an upper clad layer  5  (thickness 0.1 μm) formed of InGaAsP series, an upper buffer layer  6  (thickness 0.2 μm) formed of p-type InP, an etching stop layer  7  (thickness 5 nm) formed of InGaAsP series, a lower spacer layer  8  (thickness 20 nm) formed of InP, and a diffraction grating layer  9  (thickness 30 nm) formed of InGaAsP series were laminated by metal organic chemical vapor deposition (MOCVD). In this embodiment, the emission wavelength of the multiple-quantum well active layer was about 1.5 μm. 
     Next, as shown in  FIG. 1B , a diffraction grating was formed on the diffraction grating layer  9  by a known interference exposure method and subsequent etching using a phosphoric acid based solution. Since in Embodiment 1, the InP spacer layer  8  was provided between the etching stop layer  7  and the diffraction grating layer  9 , the diffraction grating became a floating type and could be fabricated with high precision even if etching time varied somewhat in every fabrication. 
     Subsequently, as shown in  FIG. 1C , a p-type InP layer  10  (thickness 2.0 μm) and a contact layer  11  (thickness 0.3 μm) comprising InGaAsP (compositional wavelength 1.3 μm) and InGaAs were laminated, by MOCVD, on the diffraction grating layer  9  with the diffraction grating formed. 
     Then, as shown in  FIG. 1D , formation of a ridge  12  that comprises the lower spacer layer  8 , the diffraction grating layer  9 , the p-type InP layer  10 , and the contact layer  11  was carried out by etching up to the etching stop layer  7  leaving a ridge (width 2.8 μm). 
     Subsequently, as shown in  FIG. 2A , a silicon oxide film  13  (thickness 0.1 μm) was formed on the entire surface upper than the etching stop layer  7  by thermo-chemical vapor deposition (T-CVD). Then, as shown in  FIG. 2B , the insulating film (silicon oxide film  13 ) on the contact layer  11  in the upper portion of the ridge  12  was removed. Although the silicon oxide film  13  was employed as the insulating film in Embodiment 1, it is also possible to use a silicon nitride film and the like. As shown in  FIG. 2C , a polyimide resin layer  14  was provided on the insulating film  13  on both sides of the ridge  12 , and the wafer surface was flattened. After a p-electrode  15  and an n-electrode  16  were formed on the upper portion of the ridge  12  and on the backside of the n-type InP substrate  1 , respectively, a device having a cavity length of 300 μm was cut out by a cleavage step, and a reflection film having 95% reflectance and a low reflection film having 0.1% reflectance were coated on the rear end face and the front end face thereof, respectively. 
     When the semiconductor laser diode shown in  FIG. 2C  was operated in the operating current range of up to 300 mA at from −5 degree C. to 85 degree C., a lateral single mode was confirmed. In addition, excellent lasing characteristics with a threshold current of 15 to 25 mA and lasing efficiency of 0.3 to 0.4 W/A were shown under the conditions of room temperature and continuous lasing. Further, a threshold current of ca. 35 mA and a lasing efficiency of 0.15 to 0.2 W/A were obtained at an operating temperature of 85 degree C. 
       FIG. 3  is a graph showing a comparison result of the operation characteristic of a semiconductor laser diode constructed in almost the same structure as that shown in  FIG. 2C  except that the upper buffer layer  6  was not provided. In  FIG. 3 , the horizontal axis represents operating current, and the vertical axis represents optical power. The solid line  21  shows a characteristic of the semiconductor laser diode provided with the upper buffer layer  6  at an operating temperature of 85 degree C., and the broken line  22  shows a characteristic of the semiconductor laser diode not provided with the upper buffer layer  6  at the same temperature, respectively. When compared at the operating current of 150 mA, the optical power of the semiconductor laser diode provided with the upper buffer layer  6  increased by approximately 20% compared to that of the semiconductor laser diode not provided with the upper buffer layer  6 . At this time, the threshold current was 5 to 10 mA higher in the semiconductor laser diode provided with the upper buffer layer  6 . 
     In the above structure, although the lasing wavelength of laser, i.e. the emission wavelength in the multiple-quantum well layer was set to 1.5 μm, a similar effect could also be obtained when the emission wavelength was set to 1.3 μm band. Further, a similar effect was also obtained with a distributed Bragg reflection type and a Fabry-Perot type in place of a distributed feedback type. Furthermore, a laser diode having a similar characteristic could also be obtained even if InGaAlAs series was used instead of InGaAsP series. Still further, the upper buffer layer  6  might be formed of InGaAsP series and InGaAlAs series in place of InP. 
     Although it was understood that widening of the ridge width by inserting the upper buffer layer  6  between the upper clad layer  5  and the ridge  12  is effective for power enhancement of a semiconductor laser diode, there is a problem that the threshold current increases as described in BACKGROUND OF THE INVENTION and in the explanation with reference to  FIG. 3 . To solve this problem, in Embodiment 1 of the present invention, grooves cut into the upper buffer layer  6  along both side faces of the ridge  12  were provided to suppress diffusion of current flowing into the multiple-quantum well active layer  4 , thereby preventing the increase in the threshold current. 
       FIG. 4A  is a diagram showing a structure in the fabrication process in which the silicon oxide film  13  was formed on the entire surface upper than the etching stop layer  7  and which is the same as in  FIG. 2A  and represents the starting point for constructing the semiconductor laser diode of Embodiment 1. 
     After forming the silicon oxide film  13 , the silicon oxide film  13  was removed in a dry etching step. At this time, when the silicon oxide film  13  was made thicker at the portion of the contact layer in the upper portion of the ridge  12 , the silicon oxide film  13  was left on the ridge  12  as shown in  FIG. 4B , producing a configuration that the ridge  12  covered with the silicon oxide film  13  is on the upper face of the etching stop layer  7 . 
       FIG. 4C  shows a state that a photoresist  17  was formed on the upper face of the etching stop layer  7  avoiding the ridge  12 . 
       FIG. 4D  shows a state that the silicon oxide film  13  was subsequently removed in a photo lithography step while leaving the photoresist  17 . As is apparent from the figure, this state represents a state in which the ridge  12  was on the etching stop layer  7  and only the photoresist  17  surrounding the ridge  12  was removed in a groove shape. 
       FIG. 5A  shows a state that the etching stop layer  7  was first removed in a groove shape by a phosphoric acid based solution using the photoresist  17  removed in the groove shape as a mask from the state shown in  FIG. 4D , followed by etching the upper buffer layer  6  in the groove shape with a hydrochloric acid based solution. At this time, the depth of the groove can be adjusted by controlling the etching time. In Embodiment 1, a groove having a depth of 0.1 μm was formed with respect to the film thickness of 0.2 μm of the upper buffer layer  6 . Further, since the film thickness of the silicon oxide film  13  was set to 0.1 μm, the width of the groove formed on the buffer layer  6  also became 0.1 μm. The groove could be formed approximately perpendicularly. More specifically, a groove having a width from more than 0 to 200 nm and an angle of 90 degrees ±10 degrees from the substrate surface. 
     In the embodiment illustrated, the groove was formed up to one half of the thickness of the upper buffer layer  6 , but the groove may be formed through the entire thickness of the upper buffer layer  6 . In this case, there is an effect that a rise in threshold current by providing the upper buffer layer  6  can be prevented, whereas there is a possibility of affecting a pattern of light emitted from the semiconductor laser diode. Accordingly, how deep the groove is made should be considered depending on each case. 
       FIG. 5B  shows a state that the photoresist  17  was removed. As is apparent from comparison with  FIG. 1D , this state is the same as that in  FIG. 1D  except that grooves are formed in the etching stop layer  7  and the upper buffer layer  6  along the ridge  12 . 
       FIG. 5C  shows a state that a silicon oxide film  23  (thickness 0.1 μm) was formed on the entire surface upper than the etching stop layer  7  by T-CVD as explained in  FIG. 2A . At first glance, this state appears to be the same as the silicon oxide film  13  in  FIG. 4A . However, the silicon oxide film  13  in  FIG. 4A  was provided to form grooves along the ridge  12  and removed after finishing its use, whereas the silicon oxide film  23  was provided to form an insulating layer including the formed grooves. 
     In  FIG. 5D , the insulating film (silicon oxide film  23 ) on the contact layer  11  in the upper portion of the ridge  12  was removed as explained in  FIG. 2B . Although the silicon oxide film  23  was employed here as the insulating film in Embodiment 1, it is also possible to use a silicon nitride film and the like. 
       FIG. 6  shows a state that a polyimide resin layer  24  was formed on the upper face of the silicon oxide film  23  to flatten the wafer surface, the p-electrode  15  was formed on the upper portion of the ridge  12 , and the n-electrode  16  was formed on the backside of the n-type InP substrate  1 . 
     Then, the device was cut out by a cleavage step, and the reflection film having 95% reflectance and the low reflection film having 0.1% reflectance were coated on the rear end face and the front end face thereof, respectively, to complete a semiconductor laser diode. 
       FIG. 7A  is not only a graph explaining the characteristic of the semiconductor laser diode of Embodiment 1 shown in  FIG. 6  but also a graph showing comparison results of the operation characteristics of a semiconductor laser diode not having the upper buffer layer  6  and semiconductor laser diodes having the upper buffer layer  6  in which grooves reaching one half of the upper buffer layer  6  are present or absent, respectively. As in  FIG. 3 , the horizontal axis represents operating current, and the vertical axis represents optical power. The solid line  21 ′ shows the characteristic of the semiconductor laser diode of Embodiment 1 at an operating temperature of 85 degree C., and the broken line  22  shows the characteristic of the semiconductor laser diode not provided with the upper buffer layer  6  at the same temperature, respectively. The thin solid line  21  shows the characteristic of the case shown in  FIG. 3  where the grooves were not provided on the upper buffer layer  6 . As is apparent from comparison of the solid line  21  and the broken line  22 , their threshold currents are comparable, according to Embodiment 1, and an increase in threshold current caused by provision of the upper buffer layer could be suppressed. Further, as is apparent from comparison of the solid line  21 ′ and the solid line  21 , optical power could also be enhanced. 
       FIG. 7B  is a characteristic graph showing evaluated relation of groove width in the upper buffer layer  6  and cut-off width, where the groove width is shown on the horizontal axis and the cut-off width is shown on the vertical axis. As mentioned earlier, a high power semiconductor laser diode can be realized by widening the ridge width. On the other hand, the widening of the ridge width becomes disadvantageous to the single mode condition. In the present invention, the upper buffer layer  6  was provided and the grooves to prevent current diffusion in the upper buffer layer  6  were provided as shown in Embodiment 1, thereby realizing effective widening of the ridge width. The result of evaluation of the relation between the groove width and the cut-off width under the condition satisfying the single mode is shown in  FIG. 7B . Although a cut-off width (ridge width) of 2.8 μm could be realized at a groove width of 0.1 μm as described above, a cut-off width (ridge width) of 2.8 μm could be realized even at a groove width of 0.2 μm. However, it was found that when the groove width became 0.2 μm or wider, the cut-off width became smaller. In addition, a study result obtained when the insulating layer was formed by regrowth as disclosed in JP-A No. 214372/2004 mentioned above is also depicted. When InP, InGaAsP or InGaAlAs was regrown, due to an anisotropy of the crystal growth rate, the groove angle became 126 degrees or more. Thus, the groove width of 0.1 μm could not be formed and the single mode could not be satisfied at a large cut-off width as obtained in the present invention. 
     When the semiconductor laser diode of Embodiment 1 was operated in the range up to the operating current of 300 mA at from −5 degree C. to 85 degree C., the operation characteristic shown in  FIG. 7A  was obtained, and a lateral single mode was confirmed. In addition, it was found from the result in  FIG. 7B  that the groove width is desirable to be equal to or smaller than 0.8 μm in consideration of fabrication error and the like. 
     When the far field pattern was evaluated, expansion of the far field pattern in the semiconductor laser diode having the upper buffer layer  6  but not having the grooves formed along the side faces of the ridge  12  was 45 degrees in the perpendicular direction and 20 degrees in the horizontal direction with respect to the substrate  1 . On the other hand, in the semiconductor laser diode of Embodiment 1, the expansion of the far field pattern was 45 degrees in the perpendicular direction and 25 degrees in the horizontal direction with respect to the substrate  1 , and the anisotropy in the expansion was confirmed to be lessened. 
     Further, the semiconductor laser diode of Embodiment 1 exhibited excellent lasing characteristics with a threshold current of 10 to 20 mA and lasing efficiency of 0.3 to 0.4 W/A under the conditions of room temperature and continuous lasing. At an operating temperature of 85 degree C., a threshold current of ca. 20 to 30 mA and a lasing efficiency of 0.15 to 0.2 W/A were obtained. At a temperature of 85 degree C., a threshold current approximately equal to that for the semiconductor laser diode having no upper buffer layer, and an operating current of 150 mA, the optical power was increased by approximately 20% to 40%. 
     Although the lasing wavelength of laser, i.e. the emission wavelength of the multiple-quantum well active layer, was set to 1.5 μm in Embodiment 1, similar effects could also be obtained when the wavelength was set to 1.3 μm band or when a distributed Bragg reflection type or a Fabry-Perot type was used in place of the distributed feedback type. It is needless to say that similar effects were obtained as long as grooves having a similar angle could be formed even when the fabrication method of grooves differed. In addition, InGaAlAs series may be used in place of InGaAsP series. Although the silicon oxide film was used as the insulating film, it is also possible to use a silicon nitride film and the like. 
     In Embodiment 1, there are advantages that fabrication of a diffraction grating to select an appropriate lasing wavelength in accord with the wavelength corresponding to the absorption-edge energy of the optical modulator region becomes possible by fabricating the diffraction grating on the upper clad layer and that keeping constant the difference (ΔH) between the wavelength of lasing light of the distributed feedback type semiconductor laser diode and the wavelength corresponding to the absorption-edge energy of the optical modulator region becomes possible. 
     Embodiment 2 
     A semiconductor laser diode applied with the present invention can also be realized by a structure starting with an n-type GaAs semiconductor substrate in place of the structure starting with the n-type InP semiconductor substrate.  FIGS. 8A to 8C  are diagrams showing as Embodiment 2 the semiconductor laser diode realized by the structure starting with the n-type GaAs semiconductor substrate. 
     The semiconductor laser diode shown in Embodiment 2 is different in materials and part of the manufacturing process because the starting substrate is different but can be constructed by steps similar to those in Embodiment 1, and therefore explained in a simplified manner. 
     As shown in  FIG. 8A , on an n-type GaAs semiconductor substrate  31  (thickness 2 mm), an n-type GaAs buffer layer  32  (film thickness 0.5 μm), a lower clad layer  33  (film thickness 2.5 μm) formed of n-type (Al x Ga 1-x ) 0.5 In 0.5 P (x=0.60), a multiple-quantum well active layer  34  (thickness 0.024 μm) in. which a well layer formed of GaInP (thickness 6 nm) having 1.1% compressive strain and a barrier layer (thickness 6 nm) formed of (Al x Ga 1-x ) 0.5 In 0.5 P (x=0.45) having 0.7% tensile strain were laminated by 2 cycles, an upper clad layer  35 . (film thickness 0.02 μm) formed of p-type (Al x Ga 1-x ) 0.5 In 0.5 P (x=0.50), an upper buffer layer  36  (film thickness 0.3 μm) formed of (Al x Ga 1-x ) 0.5 In 0.5 P, a layer  40  (film thickness 2.0 μm) formed of (Al x Ga 1-x ) 0.5 In 0.5 P (x=0.60), and a contact layer  41  (film thickness 0.2 μm) formed of GaAs were laminated by MOCVD. In this embodiment, the emission wavelength of the multiple-quantum well active layer was about 0.66 μm. 
     Then, in dry etching step, etching was performed until the surface of the upper buffer layer  36  was exposed as shown in  FIG. 8B  to form a ridge  42 . The width of the ridge  42  was 1.7 μm. 
     After going through the steps similar to those in  FIGS. 4A to 4D  and  FIGS. 5A to 5D , the semiconductor laser diode shown in  FIG. 8C  could be realized by the process starting with the n-type GaAs semiconductor substrate. Here, the reference numeral  43  denotes a silicon oxide film and represents an insulating layer corresponding to the silicon oxide film  23  in Embodiment 1. The reference numeral  44  represents a polyimide resin, which was provided on the upper face of the silicon oxide film  43  to flatten the wafer surface. This corresponds to the polyimide resin  24  in Embodiment 1. The reference numerals  45  and  46  represent p- and n-electrodes, respectively, which correspond to the p- and n-electrodes  15  and  16  in Embodiment 1. The device was cut out by a cleavage step, and a reflection film having 92% reflectance and a low reflection film having 7% reflectance were coated on the rear end face and the front end face thereof, respectively. The semiconductor laser diode of Embodiment 2 serves as a Fabry-Perot type laser, and therefore it is not necessary to form a diffraction grating layer in the ridge  42 . 
     When grooves are formed by etching along the ridge  42 , it is possible to adjust the groove depth by controlling the etching time in Embodiment 2 as well. In Embodiment 2, grooves with a width of 0.1 μm and a depth of 0.1 μm were formed with respect to the film thickness of 0.3 μm of the upper buffer layer  36 . The grooves having an approximately vertical angle could be formed. 
     When the semiconductor laser diode of Embodiment 2 was operated in the range up to the operating current of 450 mA at from −10 degree C. to 80 degree C., a lateral single mode was confirmed. Even when grooves were formed as described above, it was confirmed that ridge width could be widened. Further, the semiconductor laser diode of Embodiment 2 exhibited excellent lasing characteristics with a threshold current of 40 to 55 mA and lasing efficiency of 1.0 to 1.2 W/A under the conditions of room temperature and continuous lasing. At a threshold current approximately equal to that for the semiconductor laser diode having no upper buffer layer and an operating current of 400 mA, the optical power was increased by about 20% to 40%. 
     Although the lasing wavelength of the semiconductor laser diode, i.e. the emission wavelength of the multiple-quantum well active layer, was set to 0.66 μm in Embodiment 2, similar effects could be obtained even when set to other wavelengths. The semiconductor laser diode fabricated as described above can be applied to laser diode (LD) for digital versatile disc (DVD). 
     Embodiment 3 
     An example of the integrated semiconductor optical waveguide device to which the present invention was applied is explained in Embodiment 3 with reference to  FIGS. 9A to 9E ,  FIGS. 10A to 10E , and  FIG. 11 . It should be noted that the figure is used strictly for the purpose of explaining the present embodiment and the figure size and the scale described in the present embodiment do not necessarily correspond proportionately. 
     As shown in  FIG. 9A , on an n-type InP semiconductor substrate  51  (thickness 2 mm), an n-type InP buffer layer  52  (film thickness 0.5 μm), a lower clad layer of electroabsorption type optical modulator  53  (film thickness 0.1 μm) formed of n-type InGaAlAs (compositional wavelength 0.92 μm), a multiple-quantum well active layer  54  in which a well layer (film thickness 7 nm, compositional wavelength 1.5 μm) formed of InGaAlAs having 0.6% compressive strain and a barrier layer (film thickness 10 nm, compositional wavelength 1.35 μm) formed of InGaAlAs having 0.6% tensile strain were laminated by 9 cycles, and an upper clad layer of electroabsorption type optical modulator  55  (film thickness 0.1 μm) formed of InGaAlAs (compositional wavelength 0.92 μm) were laminated. 
     As shown in  FIG. 9B , etching was performed up to the surface of the n-type InP buffer layer  52  leaving the width (for example, 300 μm) corresponding to an electroabsorption type optical modulator. 
     As shown in  FIG. 9C , a waveguide lower clad layer  56  (film thickness 0.1 μm) formed of n-type InGaAsP (compositional wavelength 1.10 μm), a waveguide core  57  (film thickness 0.16 μm) formed of InGaAsP (compositional wavelength 1.3 μm), and a waveguide upper clad layer  58  (film thickness 0.1 μm) formed of InGaAsP (compositional wavelength 1.15 μm) were laminated. 
     As shown in  FIG. 9D , etching was performed up to the surface of the n-type InP buffer layer  52  leaving the electroabsorption type optical modulator in a length of 300 μm and a waveguide in a length of 150 μm. 
     As shown in  FIG. 9E , a lower clad layer,  59  (thickness 0.13 μm) formed of n-type InGaAsP (compositional wavelength 1.10 μm), a multiple-quantum well active layer  60  in which a well layer formed of InGaAsP (film thickness 7 nm, compositional wavelength 1.5 μm) having 1.0% compressive strain and a barrier layer formed of InGaAsP (film thickness 12 nm, compositional wavelength 1.3 μm) having 0.5% tensile strain were laminated by 5 cycles, an upper clad layer  61  (film thickness 0.10 μm) formed of InGaAsP (compositional wavelength 1.10 μm), an upper buffer layer  62  (film thickness 0.2 μm) formed of p-type InP, an etching stop layer  63  (film thickness 0.005 μm) formed of InGaAsP (compositional wavelength 1.3 μm), a lower spacer layer  64  (film thickness 0.02 μm) formed of p-type InP, and a diffraction grating layer  65  (film thickness 0.03 μm) formed of InGaAsP (compositional wavelength 1.3 μm) were laminated. The emission wavelength of the multiple-quantum well active layer was about 1.5 μm. 
     As shown in  FIG. 10A , a diffraction grating was formed on the diffraction grating layer  65  by a known interference exposure method and subsequent etching using a phosphoric acid based solution. Further, a window  66  was formed on the electroabsorption type optical modulator. This diffraction grating serves as a floating type diffraction grating because the lower spacer layer  64  formed of InP is provided between the etching stop layer  63  and the diffraction grating layer  65 . Accordingly, the diffraction grating could be accurately fabricated even when etching time varied somewhat. 
     Subsequently, as shown in  FIG. 10B , a p-type InP layer  67  (film thickness 2.0 μm) and a contact layer  68  (film thickness 0.3 μm) formed of InGaAsP (compositional wavelength 1.3 μm) and InGaAs were laminated, by MOCVD, on the diffraction grating layer  65  with a diffraction grating formed, the upper clad layer  58 , and the n-type InP buffer layer  52  with the window  66  formed. Since the p-type InP layer  67  with the film thickness of 2.0 μm was exceptionally thicker compared to other layers, the laminated surface was practically flattened. 
     As shown in  FIG. 10C , etching was performed up to the upper clad layer of the electroabsorption type optical modulator  55 , the waveguide upper clad layer  58 , and the etching stop layer  63  of the laser diode to form a ridge  69 . At this time, the ridge width of the laser diode was 2.5 μm, and the ridge width of the electroabsorption type optical modulator was 1.5 μm. The ridge width of the waveguide that connects the laser diode portion to the electroabsorption type optical modulator portion was tapered so as to continuously change from the former toward the latter. 
     As shown in  FIG. 10D , the contact layer  68  of the waveguide was removed to separate the contact layer of the laser diode and that of the electroabsorption type optical modulator. 
       FIG. 11  is a diagram showing the completed state of the integrated optical waveguide device of Embodiment 3. This could be obtained by implementing the following steps after the structure shown in  FIG. 10D . As explained in  FIG. 5C , a silicon dioxide film  70  (thickness 0.1 μm) was formed, by T-CVD, on the entire surfaces upper than the upper clad layer of the electroabsorption type optical modulator  55 , the waveguide upper clad layer  58 , and the etching stop layer  63  of the laser diode. Then, as explained in  FIG. 5C , the insulating film on the contact layer  68  in the upper portion of the ridge  69  of the semiconductor laser diode and the electroabsorption type optical modulator was removed (at this time, the silicon dioxide film  70  remained on the upper portion of the waveguide). Although, in Embodiment 3, the silicon dioxide film was used as the insulating film, it is also possible to use silicon nitride film and the like. Next, as explained in  FIG. 6 , the wafer surface was flattened with a polyimide resin  71 . Finally, a p-electrode  72  was formed on the contact layer  68  of the semiconductor laser diode, a p-electrode  73  was formed on the contact layer  68  of the electroabsorption type optical modulator, and an n-electrode  74  was formed on the backside of the n-type InP substrate  51 . Then, the device was cut out by a cleavage step, and a reflection film having 95% reflectance and a low reflection film having 0.1% reflectance were coated on the rear end face and the front end face thereof, respectively. 
     In Embodiment 3, since the diffraction grating  65  is formed over the etching stop layer  63  in contrast to forming the diffraction grating in the upper clad layer  61  or the upper buffer layer  62 , integration is possible by four growth steps. It should be noted that the growth order of the electroabsorption type optical modulator, the waveguide, and the semiconductor laser diode is not limited to this. Further, as the material for the semiconductor laser diode, InGaAlAs series can be employed in place of InGaAsP series as explained in Embodiment 2. Furthermore, as the material for the electroabsorption type optical modulator, InGaAsP series may be used in place of InGaAlAs series. 
     In Embodiment 3, the ridge width in the region of the semiconductor laser diode was 2.0 μm, and the ridge width in the region of the electroabsorption type optical modulator was 1.4 μm. The waveguide connecting the both regions was 150 μm in length and tapered. As the result, the ridge of the semiconductor laser diode and the ridge of the electroabsorption type optical modulator could be coupled with little optical loss. 
       FIG. 12  is a graph showing evaluation results of characteristics when the ridge widths of the semiconductor laser diode were 2.0 μm and 2.5 μm, respectively, and the ridge width of the electroabsorption type optical modulator was 1.4 μm, where the waveguide length was taken on the horizontal axis and the transmittance was taken on the vertical axis. As is evident from the figure, practically the same characteristics were obtained in the range of the waveguide length of 10 μm to 150 μm. Therefore, as long as both are coupled with an appropriate difference in width and length, the tapering inclination has little influence on the coupling. Further, provision of the window  66  facilitates optical coupling to optical fiber, thereby enabling coupling loss to be suppressed to 3 dB or lower. 
     Further, when the far field pattern of the integrated semiconductor optical waveguide device of Embodiment 3 was measured, the expansion was 45 degrees in the perpendicular direction and 35 degrees in the horizontal direction with respect to the substrate in the integrated semiconductor optical waveguide device described in Embodiment 3, whereas the expansion was 45 degrees in the perpendicular direction and 20 degrees in the horizontal direction in the semiconductor laser diode alone. Thus it was confirmed that anisotropy in expansion was lessened in the former. The operating current of the semiconductor laser diode was in the range of 70 to 150 mA when operated at from −5 degree C. to 85 degree C. By controlling the offset bias optimally and making modulation amplitude voltage equal to or lower than 2.5 V at from −5 degree C. to 85 degree C. with respect to the voltage applied to the p-electrode  73  of the electroabsorption type optical modulator, an optical power equal to or higher than 1 dB, a dynamic quenching ratio equal to or higher than 10 dB, and a band equal to or higher than 10 Gbps could be obtained. Owing to this, it became possible to obtain a good eye pattern at a bit rate of 10 Gbps and a transmission distance of 40 km or more without a need for temperature control between −5 degree C. and 85 degree C. 
     Although the semiconductor laser diode of the integrated semiconductor optical waveguide device of Embodiment 3 had the same structure as that of the semiconductor laser diode as the premise of the present invention that was explained in  FIGS. 1 and 2 , it may be structured so as to have grooves shown in  FIG. 6  explained in Embodiment 1. 
     Embodiment 4—Fabrication Method of Mach-Zehnder (MZ) Version of Integrated Device 
     As Embodiment 4 of the integrated semiconductor optical waveguide device applied with the present invention, an example of the integrated semiconductor optical waveguide device in which a Mach-Zehnder type optical modulator was employed in place of the electroabsorption type optical modulator is explained using  FIG. 13 . It should be noted that the figure is used only to explain the present embodiment and the figure size and the scale described in the present embodiment do not necessarily correspond proportionately. 
       FIG. 13  is a diagram showing the structure of the integrated optical waveguide device of Embodiment 4. In the optical waveguide device of Embodiment 4, a semiconductor laser diode LD, a waveguide WG, a Mach-Zehnder type optical modulator MZ, and a window  66  were formed on an n-type InP substrate  1  and InP buffer layer  2  and were in cascade connection. The semiconductor laser diode LD had the same structure as that of the semiconductor laser diode as the premise of the present invention that was explained with reference to  FIGS. 1 and 2 , and the same reference numerals were attached. Although the waveguide WG was the same as that in the structure of the integrated semiconductor optical waveguide device of Embodiment 2 that was explained with reference to  FIGS. 9 to 11 , the reference numerals were omitted because the illustration becomes complicated. The Mach-Zehnder type optical modulator MZ was not only connected to the waveguide WG but also split into two optical paths  81  and  82 , which were again joined together to be connected to the window  66 . On one optical path  81  of the two optical paths  81  and  82 , the contact layer  11  was formed. As in the case of the waveguide WG, the waveguide lower clad layer  56 , the waveguide core  57  formed of InGaAsP (compositional wavelength 1.3 μm), and the waveguide upper clad layer  58  formed of InGaAsP (compositional wavelength 1.15 μm) were also laminated for the two optical paths  81  and  82 . 
     To describe the fabrication process briefly, first, semiconductor laser diode LD was laminated in a manner similar to that shown in  FIG. 1C , and then etching of the regions for the waveguide WG and the Mach-Zehnder type optical modulator MZ was performed up to the surface of the InP buffer layer  2 , followed by laminating the above waveguide parts  56  to  58  thereon. After forming the window  66 , a p-type InP layer and the contact layer  11  were laminated. This state was similar to that shown in  FIG. 10B . Then, in a manner similar to that shown in  FIGS. 10C and 10D , the structure shown in  FIG. 13  was obtained. Finally, in a manner similar to that shown in  FIG. 11 , the integrated semiconductor optical waveguide device of Embodiment 4 was completed, though the illustration thereof was omitted. 
     Here, optical path lengths of the two optical paths  81  and  82  are made to differ from each other by one half of the wavelength of oscillation frequency of the semiconductor laser diode LD. When the voltage applied between the electrode connected to the contact layer  11  of the optical path  81  and the electrode provided on the backside of the substrate  1  becomes a predetermined value, the respective optical path lengths of the optical paths  81  and  82  are, for example, made to become equal by the change in the refractive index of the optical path  81  and the resulting change in transmitted optical path length. As the result, when there is no voltage between those electrodes, no optical signal is output, and when the voltage is applied between the electrodes, an optical signal is output. 
     In Embodiment 4 as well, the ridge of the waveguide WG was tapered, for example with the ridge width of the semiconductor laser diode LD being 2.0 μm and the ridge width of the Mach-Zehnder type optical modulator MZ being 1.0 μm, such that both might be coupled with little optical loss. Further, optical coupling to optical fiber was facilitated by the window  66 , and coupling loss could be suppressed to 3 dB or lower. The operating current of the semiconductor laser diode LD was in the range of 70 to 150 mA when operated at from −5 degree C. to 85 degree C. By controlling the offset bias optimally and making modulation amplitude voltage equal to or lower than 2.5 V at from −5 degree C. to 85 degree C. with respect to the voltage applied to the electrode  11  of the Mach-Zehnder type optical modulator, an optical power equal to or higher than 3 dB, a dynamic quenching ratio equal to or higher than 10 dB, and a band equal to or higher than 10 Gbps could be obtained. Owing to this, it became possible to obtain a good eye pattern at a bit rate of 10 Gbps and a transmission distance of 40 km or more without a need for temperature control between −5 degree C. and 85 degree C. 
     It should be noted that the Mach-Zehnder type optical modulator MZ is not limited to this but may be replaced by an optical waveguide device provided with functions comparable to this. Further, integration with an optical amplifier in addition to an optical modulator is also possible.