Patent Publication Number: US-10326257-B2

Title: Semiconductor laser device and manufacturing method of the same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation-in-part application of U.S. patent application Ser. No. 14/696,102, filed Apr. 24, 2015, which claims priority to Japanese Patent Application No. 2014-091042, filed Apr. 25, 2014, and Japanese Patent Application No. 2015-028474, filed Feb. 17, 2015; this present application also claims priority to Japanese Patent Application No. 2017-172330, filed Sep. 7, 2017, all of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to a semiconductor laser device and a manufacturing method of the semiconductor laser device. 
     2. Background Arts 
     Japanese Patent Application Laid-Open No. H5-029703 discloses a semiconductor laser device including a diffraction grating. As a method for forming a diffraction grating, an electron beam exposure is known. When a diffraction grating is formed on a wafer using the electron beam exposure, a portion where no diffraction grating is lithographed is provided between adjacent diffraction gratings along an optical waveguide of the diffraction grating. When the wafer is cleaved to form respective devices and the optical grating extends in the cleaved facet of the device, the optical grating terminated at the cleaved facet degrades the performance of the whole device. On the other hand, when a portion where no diffraction grating is lithographed is provided between the diffraction gratings, conditions to grow semiconductor layers on the diffraction grating become inhomogeneous between an area where the diffraction grating is formed and another area where no diffraction grating is formed, which may cause or arise defects arising from a boundary between two areas. When such defects are arose in the device, luminescent efficiency of the semiconductor laser device degrades. 
     SUMMARY 
     One aspect of the present application relates to a semiconductor laser device that comprises a first diffraction grating provided on a substrate, a second diffraction grating continuous to one end of the first diffraction grating, the first diffraction grating and the second diffraction grating being provided along an optical waveguide direction, and an active layer provided above the first diffraction grating. A feature of the semiconductor laser device of the present application is that the second diffraction grating has a pitch 1.05 times or greater, or 0.95 times or smaller of the pitch of the first diffraction grating. 
     Another aspect of the present application relates to a method of manufacturing a semiconductor laser device that comprises: forming a first diffraction grating and a second diffraction grating on a wafer continuously by the continuous electron beam exposure; and forming an active layer above the first diffraction grating and the second diffraction grating. A feature of the method is that a pitch of the first diffraction grating is different from a pitch of the second diffraction grating. 
     Still another aspect of the present application relates to a method of manufacturing a semiconductor laser device that comprises: forming a third diffraction grating and a fourth diffraction grating continuous to the third diffraction grating on a wafer by an electron beam exposure, the third diffraction grating having a stripe width and the fourth diffraction grating having a width wider than the stripe width of the third diffraction grating; forming an active layer above the third diffraction grating and the fourth diffraction grating; and forming a mesa stripe by etching the third diffraction grating, the fourth diffraction grating, and the active layer, the mesa stripe in a portion corresponding to the third diffraction grating having a stripe width equal to or narrower than the stripe width in another portion corresponding to the third diffraction grating. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other purposes, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which: 
         FIG. 1  is a diagram illustrating a cross-section of a semiconductor laser device according to the present embodiment, the cross-section perpendicular to an optical waveguide direction; 
         FIG. 2  is a cross-sectional view taken along a line II-II of  FIG. 1 ; 
         FIG. 3  is a diagram illustrating a manufacturing step of the semiconductor laser device; 
         FIG. 4  is a diagram illustrating a manufacturing step of the semiconductor laser device; 
         FIG. 5  is a diagram illustrating a manufacturing step of the semiconductor laser device; 
         FIG. 6  is a diagram illustrating a manufacturing step of the semiconductor laser device; 
         FIG. 7  is a diagram illustrating a manufacturing step of the semiconductor laser device; 
         FIG. 8  is a diagram illustrating a manufacturing step of the semiconductor laser device; 
         FIG. 9  is a diagram illustrating a manufacturing step of the semiconductor laser device; 
         FIG. 10  is a diagram illustrating a manufacturing step of the semiconductor laser device; 
         FIG. 11  is a diagram illustrating a manufacturing step of the semiconductor laser device; 
         FIG. 12  is a diagram illustrating a manufacturing step of the semiconductor laser device; 
         FIG. 13  is a diagram illustrating a manufacturing step of the semiconductor laser device; 
         FIG. 14  is a cross-sectional view of a semiconductor laser device  18  according to a comparative example; 
         FIG. 15  is a diagram illustrating an optical gain of a MQW structure designed so that a laser oscillation wavelength becomes 1.5 μm, and a Bragg wavelength of 1.3 μm by a diffraction grating; 
         FIG. 16  is a cross-sectional view of a semiconductor laser device according to a second embodiment; 
         FIG. 17  is a diagram of a wafer viewed from a direction perpendicular to a principal surface of the wafer in a step of forming diffraction gratings on the wafer according to a third embodiment; 
         FIG. 18  magnifies a portion to be a semiconductor device in the wafer illustrated in  FIG. 17 ; 
         FIG. 19  is a diagram illustrating a cross-section of the semiconductor laser device according to the third embodiment, the cross-section perpendicular to an optical waveguide direction; 
         FIG. 20  is a diagram of the wafer viewed from the direction perpendicular to the principal surface of the wafer in a step for forming diffraction gratings on the wafer according to a fourth embodiment; 
         FIG. 21  magnifies a portion to be a semiconductor device later in the wafer illustrated in  FIG. 20 ; 
         FIG. 22  magnifies a part of a first diffraction grating according to the comparative example; 
         FIG. 23  is a diagram of the wafer viewed from the direction perpendicular to the principal surface of the wafer in a step of forming diffraction gratings on the wafer according to a fifth embodiment; 
         FIG. 24  magnifies a portion to be a semiconductor device later in the wafer illustrated in  FIG. 23 ; 
         FIG. 25  is a diagram illustrating a cross-section of a semiconductor laser device according to a sixth embodiment; 
         FIG. 26  is a diagram illustrating a manufacturing step of the semiconductor laser device according to the sixth embodiment; 
         FIG. 27  is a diagram illustrating a manufacturing step of the semiconductor laser device according to the sixth embodiment; 
         FIG. 28  is a diagram illustrating a manufacturing step of the semiconductor laser device according to the sixth embodiment; 
         FIG. 29  is a diagram illustrating a manufacturing step of the semiconductor laser device according to the sixth embodiment; 
         FIG. 30  is a diagram illustrating a manufacturing step of the semiconductor laser device according to the sixth embodiment; 
         FIG. 31  is a microscope photo for which a diffraction grating of a semiconductor laser device according to the comparative example is photographed; 
         FIG. 32  is a microscope photo for which diffraction gratings  14  and  15  of a semiconductor laser device  1 A according to the sixth embodiment are photographed; 
         FIG. 33  is a microscope photo illustrating a case where a first diffraction grating  14  and a second diffraction grating  15  are not separated from each other (are made continuous); 
         FIG. 34  is a microscope photo illustrating a case where the first diffraction grating  14  and the second diffraction grating  15  are separated from each other; 
         FIG. 35  is a diagram illustrating a cross-section of a semiconductor laser device according to a seventh embodiment; 
         FIG. 36  is a diagram of the wafer viewed from the direction perpendicular to the principal surface of the wafer in a step for forming diffraction gratings on the wafer according to an eighth embodiment; and 
         FIG. 37  magnifies a portion to be a semiconductor device later in the wafer illustrated in  FIG. 36 . 
     
    
    
     DETAILED DESCRIPTION 
     Description of Embodiments 
     First of all, embodiments of the invention of the present application will be described. 
     A semiconductor laser device according to an embodiment of the present invention comprises a first diffraction grating provided on a substrate along an optical waveguide direction, a second diffraction grating provided on the substrate and continuous to one end of the first diffraction grating, where the second diffraction grating extends along the optical waveguide direction, and an active layer provided above the first diffraction grating and generating light with a wavelength determined by the first diffraction grating. The second diffraction grating has a pitch 1.05 times or greater, or 0.95 times or smaller of the pitch of the first diffraction grating. 
     According to this semiconductor laser device, because the second diffraction grating is continuous to the one end of the first diffraction grating, defects are not likely to occur in semiconductor layers grown on the first diffraction grating corresponding to the end of the first diffraction grating. It is therefore possible to prevent the defects from spreading within the active layer. Further, because the pitch of the second diffraction grating is set 1.05 times or greater, or 0.95 times or smaller of the pitch of the first diffraction grating, it is possible to prevent the second diffraction grating from affecting the laser oscillation of the semiconductor laser device. 
     The second diffraction grating may have a cleaved surface in another end thereof opposite to the one end. 
     The active layer may be provided on the second diffraction grating. 
     The semiconductor laser device may further comprise a waveguide layer provided above the second di action grating. The waveguide layer may be configured to guide light generated in the active layer. 
     The semiconductor laser device may further comprise a modulation region provided above the second diffraction grating. The modulation layer may be configured to modulate light generated in the active layer. 
     The active layer may further comprise an n-type cladding layer, and a p-type cladding layer each made of InP, where the active layer may be made of AlInGaAs, and provided between the n-type cladding layers and the p-type cladding layer. 
     A method of manufacturing the semiconductor laser device according to an embodiment of the present invention comprises steps of: forming a first diffraction grating and a second diffraction grating on a wafer continuously by a continuous electron beam exposure, where the first diffraction grating has a pitch different from a pitch of the second diffraction grating; and forming an active layer above the first diffraction grating and the second diffraction grating. 
     Because the second diffraction grating is continuously formed in one end of the first diffraction grating, defects are not likely to be arose in layers grown on the first diffraction grating in a portion corresponding to the end of the first diffraction grating. It is therefore possible to prevent the defects from spreading into the active layer. 
     The method may further comprise steps of etching the active layer in a portion formed above the second diffraction grating; and forming a waveguide layer above the second diffraction grating. 
     The method may further comprise steps of etching the active layer in a portion formed above the second diffraction grating; and forming a modulation region above the second diffraction grating. 
     In the method, the step of forming the second diffraction grating may include a step to set the pitch thereof to be 1.05 times or greater, or 0.95 times or smaller of the pitch of the first diffraction grating. Accordingly, the second diffraction grating may be prevented from affecting the laser oscillation of the semiconductor laser device. 
     The method may further comprise a step of cleaving the wafer at a portion of the second diffraction grating. 
     An abnormal growth may occur in semiconductor layers embedding the diffraction grating which is exposed to electron beams several times. The abnormal growth may degrade the luminescent efficiency of the semiconductor laser device. To reduce such an abnormal growth, the step of forming the first diffraction grating and the second diffraction grating may include: forming a grating layer on the wafer; forming a plurality of exposed regions on the grating layer each including at least one first pattern and one second pattern continuous to the first pattern along an optical waveguide direction, where the first pattern corresponds to the first diffraction grating and the second pattern corresponds to the second diffraction grating; and etching the grating layer by the exposed regions as a mask. A feature of the method is that the exposed regions are discretely arranged along the optical waveguide direction. Because the exposed regions do not overlap with each other along the optical waveguide direction, no portion exposed to the electron beams several times. Accordingly, the abnormal growth in the first diffraction grating and the second diffraction grating may be suppressed. 
     The step of forming the first diffraction grating and the second diffraction grating may further comprise steps of forming a grating layer on a substrate; and forming a plurality of exposed regions by an electron beam exposure, each exposed regions including a first pattern with a first width and a second patterns putting the first pattern therebetween, where the first pattern corresponds to the first diffraction grating and the second pattern corresponds to the second diffraction gratings, the second pattern involved in one exposed region in a portion continuous to the second pattern involved in another exposed region next to the one exposed region being exposed to the electron beam at least twice and having a width greater than the first width of the first pattern. The method may further comprise a step, after the step of forming the active layer, etching the first diffraction grating, the second diffraction grating, and the active layer to form a mesa stripe with a width equal to or narrower than the first width of the first pattern. 
     A method of manufacturing the semiconductor laser device according to another embodiment of the present invention comprises steps of: forming a third diffraction grating and a fourth diffraction grating continuous to the third diffraction grating on a wafer continuously by an electron beam exposure, where the third diffraction grating has a stripe width and the fourth diffraction grating has a width wider than the strip width of the third diffraction grating; forming an active layer above the third diffraction grating and the fourth diffraction grating; and forming a mesa stripe by etching the third diffraction grating, the fourth diffraction grating, and the active layer, where the mesa stripe in has a stripe width equal to or narrower than the width of the third diffraction grating. 
     Even if defects are arose in semiconductor layers grown on the diffraction gratings, especially in a boundary between the fourth diffraction gratings along; the defects are localized at corners of the boundary. Most of the defects are removed by removing the corners of the boundary by etching. It is therefore possible to prevent the defects from spreading into the active layer. 
     The method may further comprise a step of cleaving the wafer in the fourth diffraction grating. 
     The method may further comprise steps of etching the active layer formed above the fourth diffraction grating; and forming a waveguide layer above the fourth diffraction grating. 
     The method may further comprise steps of etching the active layer formed above the fourth diffraction grating; and forming a modulation region above the fourth diffraction grating. 
     Details of Embodiments 
     Specific examples of a semiconductor laser device and a manufacturing method of a semiconductor laser device of the present invention will be described below with reference to the accompanying drawings. It should be noted that the present invention is not limited to these examples but shown in the claims, and it is intended that all modifications that come within the meaning and range of equivalence to the claims should be embraced herein. In the description, the same elements or elements having the same function are denoted with the same reference signs, and an overlapping description will be omitted. 
     First Embodiment 
       FIG. 1  is a diagram illustrating a cross-section of a semiconductor laser device  1  according to the present embodiment, where the cross-section is perpendicular to an optical waveguide.  FIG. 2  is a cross-sectional view along line II-II denoted in  FIG. 1 . The semiconductor laser device  1  includes an n-type InP substrate  2 , an n-type InP cladding layer  3 , an active layer  4 , a p-type InP cladding layer  5 , a p-type InP blocking layer  6 , an n-type InP blocking layer  7 , a p-type InP layer  8 , a contact layer  9 , a protective film  10 , a p-type electrode  11  and an n-type electrode  12 . 
     The n-type InP cladding layer  3 , the active layer  4  and the p-type InP cladding layer  5  are sequentially grown on the n-type InP substrate  2 . These n-type InP cladding layer  3 , the active layer  4  and the p-type InP cladding layer  5  have a mesa stripe (mesa). The height of this mesa is, for example, 2.0 μm. The mesa is formed in a center of the n-type InP substrate  2  in a lateral direction perpendicular to an optical waveguide direction. 
     The n-type InP substrate  2  is doped with Si (silicon) by a concentration of 1.0×10 18 /cm 3 . The n-type InP cladding layer  3  is doped with Si (silicon) by a concentration of 1.0×10 18 /cm 3 . The thickness of the n-type InP cladding layer  3  is, for example, 0.5 μm. The active layer  4  has, for example, a multi quantum well (MQW) structure including AlInGaAs. The p-type InP cladding layer  5  is doped with Zn by a concentration of 1.0×10 18 /cm 3 . The thickness of the p-type InP cladding layer  5  is, for example, 0.2 μm. 
     A first diffraction grating  14  and a second diffraction grating  15  are formed inside the n-type InP cladding layer  3 . The first diffraction grating  14  and the second diffraction grading  15  include, for example, AlInGaAs, or the like. Refractive indices of the first di action grating  14  and the second diffraction grating  15  are different from a refractive index of the n-type InP cladding layer  3 . The second diffraction gratings  15  are adjacent to respective ends of the first diffraction grating  14  in the optical waveguide direction. That is, the second diffraction gratings  15  put the first diffraction grating  14  therebetween in the optical waveguide direction. An end opposite to an end facing the first diffraction grating  14  forms a cleaved face (exit face)  13 . A pitch of the second diffraction grating  15  is 1.05 times or greater, or 0.95 times or smaller of a pitch of the first diffraction grating  14 . The pitch of the second diffraction grating  15  may be preferably 0.8 times or smaller, or 1.2 times or greater of the pitch of the first diffraction grating  14 . For example, when the pitch of the first diffraction grating  14  is designed to be 0.24 μm, a laser oscillation wavelength becomes 1.500 μm. Therefore, the pitch of the second diffraction grating may be 0.228 μm or smaller, or 0.252 μm or greater. The pitch of the second diffraction grating  15  may be further preferably 0.192 μm or smaller, or 0.288 μm or greater, preferably. While  FIG. 2  illustrates an example where an active layer  4  is formed above the first diffraction grating  14  and the second diffraction grating  15 , a waveguide layer for guiding light generated in the active layer  4  may be formed above the second diffraction grating  15 . This waveguide layer may have an MQW structure or a bulk structure as long as the waveguide layer does not provide an optical gain different from that of the active layer  4 . 
     The p-type InP blocking layer  6  and the n-type InP blocking layer  7  are sequentially formed on the n-type InP substrate  2  so as to embed the mesa stripe. The p-type InP blocking layer  6  is doped with Zn by a concentration of 4.0×10 17 /cm 3 . The thickness of the p-type InP blocking layer  6  is, for example, 3.0 μm. The n-type InP blocking layer  7  is doped with Si by a concentration of 1.0×10 19 /cm 3 . The thickness of the n-type InP blocking layer  7  is, for example, 0.4 μm. 
     The p-type InP layer  8  and the contact layer  9  are provided so as to cover the p-type InP cladding layer  5  and the p-type InP blocking layer  7 . The p-type InP layer  8  and the contact layer  9  are sequentially formed. The p-type InP layer  8  is doped with Zn by a concentration of 1.2×10 18 /cm 3 . The thickness of the p-type InP layer  8  is, for example, 2.0 μm. The contact layer  9  is, for example, made of an InGaAs layer doped with Zn by a concentration of 1.2×10 19 /cm 3 . The thickness of the contact layer  9  is, for example, 0.5 μm. The contact layer  9  has band-gap energy smaller than band-gap energy of the p-type InP layer  8 . The p-type InP layer  8  functions as a part of the p-type InP cladding layer  5 . 
     The protective film  10  is formed on the contact layer  9 . The protective film  10  exposes a portion of the contact layer  9  above the mesa stripe. The protective film  10  is an insulating film and is made of, for example, SiO 2 , or the like. The p-type electrode  11  covers a region where the protective film  10  is not provided, and the top of the protective film  10 . The p-type electrode  11  is, for example, a metal stack of titanium (Ti), platinum (Pt) and gold (Au). The n-type electrode  12  is formed on a back surface of the n-type InP substrate  2 . The n-type electrode  12  is another metal stack of, for example, gold (Au), germanium (Ge) and nickel (Ni). 
     A manufacturing method of the semiconductor laser device  1  according to the present embodiment will be described next. 
       FIGS. 3 to 13  are diagrams illustrating manufacturing steps of the semiconductor laser device.  FIG. 3 ,  FIG. 4 ,  FIG. 7  and  FIG. 8  are cross-sectional views of the semiconductor laser device taken along the optical waveguide direction thereof.  FIG. 5  and  FIG. 6  are external appearances of the semiconductor laser device viewed from the direction perpendicular to the principal surface of a wafer  16 .  FIGS. 9 to 13  are cross-sectional views of the semiconductor laser device taken along the direction perpendicular to the optical waveguide direction. The wafer  16  becoming part of the semiconductor laser device  1  is referred to as the n-type InP substrate  2 . 
     As illustrated in  FIG. 3 , a diffraction grating layer  30  is grown on the principal surface of the wafer  16 . Then, as illustrated in  FIG. 4 , the first diffraction grating  14  and the second diffraction grating  15  are formed using an electron beam exposure.  FIG. 5  is a diagram of the wafer  16  illustrated in  FIG. 4 , viewed from a direction perpendicular to the principal surface of the wafer  16 .  FIG. 6  magnifies a primary portion of the semiconductor laser device  1  illustrated in  FIG. 5 .  FIG. 7  is a view of the semiconductor laser device illustrated in  FIG. 6 , which is viewed from a lateral direction perpendicular to the optical waveguide direction. 
     As illustrated in  FIG. 4  to  FIG. 7 , the second diffraction grating  15  having a pitch different from a pitch of the first diffraction grating  14  is formed between the first diffraction gratings  14  so as to continue the first diffraction gratings  14 . The pitch of the first diffraction grating  14  and that of the second diffraction grating  15  are set as follows. That is, the pitch of the second diffraction grating  15  is set 0.95 times or smaller, or 1.05 times or greater of the pitch of the first diffraction grating  14 . The pitch of the second diffraction grating  15  may be further preferably set to be 0.8 times or smaller, or 1.2 times or greater for the pitch of the first diffraction grating  14 . The pitch of the first diffraction grating  14  is designed, when the oscillation wavelength of the semiconductor laser device becomes 1.500 μm, to be 0.24 μm. In this case, the pitch of the second diffraction grating is set 0.228 μm or smaller, or 0.252 μm or greater. The pitch of the second diffraction grating  15  may be further preferably 0.192 μm or smaller, or 0.288 μm or greater. 
     Process for forming patterns of the first diffraction grating  14  and the second diffraction grating  15  will be specifically described as follows. The wafer  16  on which the diffraction grating layer  30  is formed is heated prior to resist works (hereinafter, referred to as “pre-heating”). After pre-heating, a resist is applied on the diffraction grating layer  30 , then, the wafer  16  and the resist are heated (hereinafter, referred to as “pre-baking”). At this time, a temperature of the pre-heating may be, for example, 180° C., and the thickness of the applied resist may be, for example, 1500 Å. A temperature of the pre-baking may be, for example, 140° C., and a time of the pre-baking may be, for example, three minutes. Then, fine patterns are lithographed in the resist for forming the first diffraction grating  14  and the second diffraction grating  15  using a probe current of the electron beam exposure. At this time, the probe current may be, for example, between 0.1 and 1.0 nA (nano-ampere). Then, the wafer  16  on which the resist is provided is immersed in a developer. At this time, a time during which the wafer  16  is immersed in the developer may be, for example, thirty seconds (30 sec.). Then, the wafer  16  taken out from the developer is heated (hereinafter, referred to as “post-baking”). A temperature of the post-baking may be, for example, 140° C., a time of the post-baking may be, for example, three minutes. These processes form the striped patterns by the resist on the wafer  16 . The striped patterns have a first pitch and a second pitch. Subsequently, an etching process is performed on part of the diffraction grating layer  30  using the patterned resist as a mask. Then, after the remaining resist is removed, the first diffraction grating  14  and the second diffraction grating  15  having pitches different from each other are formed on the substrate  2 . A nano-imprinting method may be used instead of the electron beam exposure. 
     After the first diffraction grating  14  and the second diffraction grating  15  are formed, as illustrated in  FIG. 8 , the n-type InP cladding layer  3 , the active layer  4  and the p-type InP cladding layer  5  are sequentially grown on the wafer  16 . At this time, the first diffraction grating  14  and the second diffraction grating  15  are embedded by the n-type InP cladding layer  3 . The n-type InP cladding layer  3  is, for example, doped with Si (silicon) by a concentration of 1.0×10 18 /cm 3 . The thickness of the n-type InP cladding layer  3  is, for example, 0.5 μm. The active layer  4  has, for example, a multi quantum well (MQW) structure including AlInGaAs. The p-type InP cladding layer  5  is, for example, doped with Zn by a concentration of 1.0×10 18 /cm 3 . The thickness of the p-type InP cladding layer  5  is, for example 0.2 μm. 
     Subsequently, as illustrated in  FIG. 9 , a mask  17  is formed on the p-type InP cladding layer  5  in a region where a mesa stripe comprising the n-type InP cladding layer  3 , the active layer  4  and the p-type InP cladding layer  5  is to be formed. A width of the first diffraction grating  15  is 10 μm, for example. The mask  17  is, for example, an SiO 2  film having a thickness of 0.5 μm. A width of the mask  17  is 3.0 μm, for example. 
     Subsequently, as illustrated in  FIG. 10 , the dry etching is carried out for the p-type InP cladding layer  5 , the active layer  4 , the n-type InP cladding layer  3 , and a part of the wafer  16  using the mask  17  as an etching mask. The dry etching forms a mesa stripe on the wafer  16 . An RIE (Reactive Ion Etching) using SiCl 4  as an etching gas is available for the dry etching. The thickness of the mesa stripe except for the mask  17  is, for example, 2.0 μm. The width of the mesa stripe is, for example, 1.5 μm. 
     Subsequently, as illustrated in  FIG. 11 , the p-type InP blocking layer  6  and the n-type InP blocking layer  7  are sequentially grown so as to bury both sides of the mesa stripe. The p-type InP blocking layer  6  is, for example, doped with Zn by a concentration of 4.0×10 17 /cm 3 . The thickness of the p-type InP blocking layer  6  is, for example, 3.0 μm. The n-type InP blocking layer  7  is, for example, doped with Si by a concentration of 1.0×10 19 /cm 3 . The thickness of the n-type InP blocking layer  7  is, for example, 0.4 μm. 
     Subsequently, as illustrated in  FIG. 12 , the mask  17  is removed using HF (hydrofluoric acid), or the like. Then, the p-type InP layer  8  is grown so as to cover an upper surface of the p-type InP cladding layer  5  and that of the n-type InP blocking layer  7 . The grown p-type InP layer  8  continues to the p-type InP cladding layer  5  and operates as a part of the p-type InP cladding layer  5 . The p-type InP layer  8  is, for example, doped with Zn by a concentration of 1.2×10 18 /cm 3 . The thickness of the p-type InP layer  8  is, for example, 2.0 μm. 
     Subsequently, as illustrated in  FIG. 13 , the contact layer  9  is grown on the p-type InP layer  8 . The contact layer  9  is, for example, made of p-type InGaAs layer doped with Zn by a concentration of 1.2×10 19 /cm 3 . The thickness of the contact layer  9  is, for example, 0.5 μm. Then, a protective film  10  is formed on the contact layer  9  except for at least a region above the mesa stripe. A p-type electrode  11  is formed so as to cover the contact layer  9  exposed from the protective film  10 , and the protective film  10  in a portion peripheral of the exposed contact layer  9 . An n-type electrode  12  is formed on a bottom surface of the wafer  16 . The protective film  10  is an insulating film and may be made of, for example, SiO 2 , or the like. The p-type electrode  11  is, for example, a metal stack of titanium (Ti), platinum (Pt), and gold (Au). The n-type electrode  12  is, for example, a metal stack including gold (Au), germanium (Ge), and nickel (Ni). The above-described method forms a plurality of the semiconductor laser devices  1  illustrated in  FIG. 1  on the wafer  16 . Subsequently, to manufacture the semiconductor laser device  1  independently, the wafer  16  is cleaved in a region of the second diffraction grating  15 . This cleavage forms a plurality of semiconductor laser devices  1 . 
     Performing the processes described above, the semiconductor laser device  1  is completed. Advantages of the semiconductor laser device  1  according to the present embodiment will be described while comparing with a comparative example. 
       FIG. 14  is a cross-sectional view of a semiconductor laser device  18  according to the comparative example. The semiconductor laser device  18  has portions  19  where a diffraction grating is not formed, in respective ends of the first diffraction grating  14 . Other arrangements of the semiconductor laser device  18  are the same as those of the semiconductor laser device  1 . To form the semiconductor laser device  18 , when the wafer  16  is cleaved, the portions  19  where no diffraction grating is formed are cleaved. That is, the portions  19  are set as cleaved positions of the wafer  16 . If such portions  19  are formed in advance, growth conditions become different at both ends of the first diffraction grating  14  from those for a center portion thereof. Specifically, the density of periodic patterns for the gratings is different in the end portions from the center portion. Due to this difference, defects may be caused or arose from the both ends of the first diffraction grating  14 , which results in a degraded crystal quality. The defects may invade in the active layer  4  to reduce the efficiency of the laser oscillation. On the other hand, in the semiconductor laser device  1  according to the present embodiment, the first diffraction grating  14  and the second diffraction grating  15  are continuously arranged along the optical waveguide direction. Therefore, the growth conditions are not likely to differ at both ends of the first diffraction grating  14 . It is therefore possible to suppress the defects arose at both ends of the first diffraction grating  14  and to prevent the defects from invading within the active layer  4 . 
     The difference of the growth conditions may be primarily reflected in the crystal quality of the n-type InP cladding layer  3  between regions near both ends and rest regions thereof. The defects arising from both ends of the first diffraction grating  14  are primarily arose at least in the n-type InP cladding layer  3 , and do not restrict only to those arose in the ends. 
     If the second diffraction grating  15  has a pitch which may generate laser light, that is, the pitch thereof corresponds to a wavelength where a substantial laser gain is left, a laser oscillation may occur at a wavelength other than a desired wavelength determined by the first diffraction grating. To prevent such an unexpected laser oscillation, the pitch of the second diffraction grating  15  may fall within a range where the second diffraction grating  15  does not cause laser light, that is, a range showing substantially no laser gain is left. Specifically, the pitch of the second diffraction grating  15  may be 0.95 times or smaller, or 1.05 times or greater of the pitch of the first diffraction grating  14 . The pitch of the second diffraction grating  15  may be preferably 0.8 times or smaller, or 1.2 times or greater of the pitch of the first diffraction grating  14 . When the pitch of the second diffraction grating  15  falls within these ranges, the second diffraction grating  15  hardly causes laser light. Therefore, by designing the second diffraction grating  15  so that the pitch thereof falls within the above-described ranges, it is possible to prevent the second diffraction grating  15  from affecting the laser oscillation. 
       FIG. 15  is a diagram illustrating a laser gain G 1  of a MQW structure designed so that a laser oscillation occurs at 1.500 μm, and a Bragg wavelength G 2  for the second diffraction grating  15  of 1.300 μm. As illustrated in  FIG. 15 , when the MQW structure is designed so that the laser oscillation occurs at 1.500 μm, the laser device shows a substantial optical gain in a wavelength range between approximately 1.500 μm and 1.700 μm. In this case, even if the second diffraction grating  15  has a pitch causing a laser oscillation at 1.300 μm or smaller and the diffraction spectrum caused thereby is extremely cute as the behavior G 2  in  FIG. 15 , light having a wavelength of 1.300 μm or shorter is scarcely hard to be generated because the laser device has no optical gain in those wavelengths. Thus, the influence of the second diffraction grating  15  on the laser oscillation is extremely small. 
     The pitch of the first diffraction grating  14  designed so that the laser oscillation occurs at 1.500 μm is 0.24 μm, and 0.8 times thereof is 0.192 μm. This value is smaller than 0.20 m which is the pitch of the first diffraction grating  14  designed so that the laser oscillation wavelength is 1.300 μm. By setting the pitch of the second diffraction grating  15  to be 0.192 μm, the second diffraction grating  15  will act only on light having a wavelength further shorter than 1.300 μm. In the present embodiment in which a gain is not provided at the above-described wavelength band at all, the influence of the second diffraction grating  15  on the laser oscillation is extremely small or substantially ignorable. 
     Second Embodiment 
       FIG. 16  is a cross-sectional view of a semiconductor laser device  20  according to a second embodiment. The semiconductor laser device  20  further includes a modulation region  21  in addition to the elements of the semiconductor laser device  1  of the first embodiment. The modulation region  21  which is provided above the second diffraction grating  15  has an optical waveguide  22 , a contact layer  23  provided on the p-type InP cladding layer  5 , and the p-type electrode  24  provided on the contact layer  23 . The optical waveguide  22  may include a quantum well structure. The p-type InP cladding layer  5  in the modulation region  21  continues from the p-type InP cladding layer  5  provided on the active layer  4 . In the semiconductor laser device  20 , a modulation signal is applied between the p-type electrode  24  and the n-type electrode  12 , so that laser light generated at the active layer  4  is modulated while passing through the optical waveguide  22 . The optical waveguide  22  is formed using, for example, the following method. First, the active layer  4  illustrated in  FIG. 8 , or the like is selectively etched to the second diffraction grating  15 . Then, a layer which becomes the optical waveguide  22  is selectively formed in a region where the active layer  4  is etched. The growth of the p-type InP cladding layer  5 , or the like, after the optical waveguide  22  is formed is the same as that of the first embodiment. 
     In the semiconductor laser device  20  of the present embodiment, similar to the first embodiment, the first diffraction grating  14  and the second diffraction grating  15  are continuously disposed. Growth conditions for semiconductor layers, in particular, for the lower cladding layer  3 , are not likely to change at both ends of the first diffraction grating  14 . It is therefore possible to suppress crystal defects from being arose at the both ends of the first diffraction grating  14 . 
     Third Embodiment 
       FIG. 17  is a diagram of the wafer  16  viewed from a direction perpendicular to the principal surface of the wafer  16  in a step of forming diffraction gratings on a wafer  16  according to a third embodiment.  FIG. 18  magnifies a portion of a semiconductor device among the wafer  16  illustrated in  FIG. 17 . 
       FIG. 17  and  FIG. 18  illustrate the wafer  16 , a third diffraction grating  25  and a fourth diffraction grating  26 . The third diffraction grating  25  and the fourth diffraction grating  26  have the same pitch, and this pitch is the same as that of, for example, the first diffraction grating  14  of the first embodiment. The third diffraction grating  25  is formed in a stripe shape. On the other hand, the fourth diffraction grating  26  has a trapezoidal envelope, when viewed from a direction perpendicular to the principal surface of the wafer  16 . Therefore, the end  27  has a width in a direction perpendicular to the optical waveguide direction two to three times wider than a width of the other end continuous to the third diffraction grating  25 . 
     The width above described and a depth will be explained. In the pattern, which includes the end  27  and the other end, constituting the third diffraction grating  25  and the fourth diffraction grating  26 , when viewed from the direction perpendicular to the principal surface of the wafer  16 , a length in a direction along the optical waveguide direction is defined as the depth, and a length in a direction perpendicular to the optical waveguide direction is defined as the width. The third diffraction grating  25  corresponds to an aggregate in which the patterns constituting the third diffraction grating  25  are periodically and linearly arranged along the optical waveguide direction. The widths of the patterns constituting the third diffraction grating  25  are uniform or substantially uniform. A width of the third diffraction grating  25  corresponds to the widths of the respective patterns constituting the third diffraction grating  25 . The widths of the respective patterns constituting the fourth diffraction grating  26  are gradually lengthened from the boundary to the third diffraction grating  25  toward an opposite side of the third diffraction grating  25 , when viewed from the direction perpendicular to the principal surface of the wafer  16 . Therefore, the width of the end  27  which is one of the patterns constituting the fourth diffraction grating  26  is greater than the width of the third diffraction grating  25 . 
       FIG. 19  is a diagram illustrating a cross-section of the semiconductor laser device  1  according to the third embodiment, the cross-section perpendicular to the optical waveguide direction. As illustrated in  FIG. 19 , a width W 1  of a mesa stripe  29  which overlaps with the third diffraction grating  25  is equal to or narrower than a width W 2  of the third diffraction grating  25 . This mesa stripe  29  is constituted by the etched n-type InP cladding layer  3 , the etched active layer  4  and the etched p-type InP cladding layer  5  above the third diffraction grating  25  and the fourth diffraction grating  26 . 
     Manufacturing steps of the semiconductor laser device according to the present embodiment is the same as the manufacturing steps of the semiconductor laser device  1  according to the first embodiment except steps to form a diffraction grating illustrated in  FIG. 4  to  FIG. 6  of the first embodiment. That is, while, in the first embodiment, the first diffraction grating  14  and the second diffraction grating  15  are formed in the steps illustrated in  FIG. 4  to  FIG. 6 . However, in the present embodiment, the third diffraction grating  25  and the fourth diffraction grating  26  are formed by steps different from those for the first diffraction grating  14  and the second diffraction grating  15 . 
     In the process for manufacturing the semiconductor laser device according to the present embodiment, defects may be arose from the end  27  of the fourth diffraction grating  26  during epitaxial growth of the lower cladding layer  3 . The defects are eccentrically located at both corners  28  of the end  27 . Therefore, after the mask  17  illustrated in  FIG. 9  to form the mesa stripe  29  is formed so as not to overlap with the corners  28  by etching the layers,  3  to  5 , as those illustrated in  FIG. 10 , most of the defects will be removed. It is therefore possible to suppress the defects from invading into the active region in the active layer  4 . 
     Fourth Embodiment 
       FIG. 20  is a diagram of the wafer  16  viewed from a direction perpendicular to the principal surface of the wafer  16  in a step of forming diffraction gratings on the wafer  16  according to a fourth embodiment.  FIG. 21  is a magnified view of a portion to be a semiconductor laser device later. 
     As illustrated in  FIG. 20  and  FIG. 21 , regions  41  each including at least one first diffraction grating  14  and one second diffraction grating  15  continuous to the one first diffraction grating  14  are discretely provided on the wafer  16  along the optical waveguide direction. When the first diffraction grating  14  and the second diffraction grating  15  are formed, the respective positions of these regions  41  are irradiated with electron beams. Accordingly, the regions  41  are referred to as the exposed regions which are exposed to the electron beams. 
       FIG. 22  magnifies the first diffraction grating according to the comparative example. As illustrated in  FIG. 22 , the second diffraction grating  15  is not provided in the semiconductor laser device according to the comparative example. Also, a plurality of regions  141  exposed to electron beams are continuously provided so as to couple with each other along the optical waveguide direction. In this case, a coupling portion  142  in the boundary of two regions  141  is irradiated with electron beams at least twice. Therefore, the first diffraction grating  14  overlapping with the boundary  142  is likely to irregular growth of the lower cladding layer  3 . Particularly, corners  14   a  of the first diffraction grating  14  is more likely to grow the lower cladding layer  3  irregularly. When the irregular growth appears in the lower cladding layer  3  and/or the layers thereon in the first diffraction grating  14 , the shape of the mesa stripe in the first diffraction grating  14  degrades, which may deteriorate luminescent efficiency of the semiconductor laser device. 
     On the other hand, in the present embodiment, regions  41  including at least one first diffraction grating  14  and one second diffraction grating  15  which continues to the first diffraction grating  14  are provided so as to provide a space without any gratings against the next region  41  along the optical waveguide direction. Because the regions  41  do not overlap with each other in the optical waveguide direction, no first diffraction grating  14  and no second diffraction grating  15  are exposed to electron beams a plurality of times. By this means, the irregular growth of the semiconductor layers,  3  to  4 , in the first diffraction grating  14  and the second diffraction grating  15  is suppressed; accordingly, a semiconductor laser device may show favorable luminescent efficiency. 
     Fifth Embodiment 
       FIG. 23  is a diagram of the wafer  16  viewed from a direction perpendicular to the principal surface of the wafer  16  in a step of forming diffraction gratings on the wafer  16  according to a fifth embodiment.  FIG. 24  is an enlarged view of a portion to be a semiconductor device later among the wafer  16  illustrated in  FIG. 23 . 
     As illustrated in  FIG. 23  and  FIG. 24 , similar to the fourth embodiment, regions  41  including at least one first diffraction grating  14  and one second diffraction grating  15  continuous to the first diffraction grating  14  are provided on the wafer  16 . In the present embodiment, the regions  41  are formed continuously along the optical waveguide direction. A boundary  42  between the regions  41  is provided so as to overlap with a diffraction grating  15   a  which is a part of the second diffraction gratings  15 . The diffraction grating  15   a  has a width W 4  which is wider than a width (first width) W 3  of the first diffraction grating  14 . The boundary  42  is provided so as to overlap with the diffraction grating  15   a  having the width W 4 . This boundary  42  is irradiated with electron beams at least twice when the first diffraction grating  14  and the second diffraction grating  15  are formed. 
     In the present embodiment, similar to the third embodiment, a mesa stripe having a width equal to or narrower than the width W 3  of the first diffraction grating  14  is formed by etching the lower cladding layer  3 , the active layer  4 , and the upper cladding layer  5 . By the etching, in the width direction of the mesa stripe, the lower cladding layer  3 , the active layer  4 , and the upper cladding layer  5  outside the mesa stripe are removed. 
     In the present embodiment, the boundary  42  where the regions  41  are coupled to each other and exposed to electron beams at least twice is provided to overlap with the second diffraction grating  15 . The width W 4  of the diffraction grating  15   a  overlapping with the boundary  42  in the second diffraction grating  15  is wider than the width W 3  of the first diffraction grating  14 . Accordingly, portions causing the abnormal growth of the lower cladding layer  3  and so on are eccentrically located outside the mesa stripe in the width direction of the mesa stripe, that is, the width direction of the first diffraction grating  14 . The portions of the lower cladding layer  3 , which are abnormally grown and localized in the boundary, are removed by the subsequent etching to form the mesa stripe. Influence of the abnormal growth in the lower cladding layer  3  on the property of the mesa stripe is suppressed or substantially prevented, so that a semiconductor laser device having favorable luminescent efficiency may be available. 
     The semiconductor laser device and the manufacturing method of the semiconductor laser device according to the present invention are not limited to the above-described embodiments, and can be modified in various ways. For example, while, in the above-described embodiments, the second diffraction gratings are adjacent to both ends of the first diffraction grating, the second diffraction grating may be adjacent to one end of the first diffraction grating. In the first embodiment and the second embodiment, the stripe width of the mesa stripe may be equal to or smaller than the width W 3  of the first diffraction grating  14  as in the third embodiment. Other embodiments (for example, the third embodiment) can be applied to an embodiment (for example, the first embodiment) as appropriate within the technically possible range. 
     Sixth Embodiment 
       FIG. 25  is a cross-sectional view of a semiconductor laser device according to the sixth embodiment. A semiconductor laser device  1 A in the sixth embodiment includes at least the n-type InP substrate  2 , the first diffraction grating  14  having a corrugation with a pitch on the n-type InP substrate  2 , and the second diffraction grating  15  having a corrugation with a pitch on the n-type InP substrate  2 . The pitch of the corrugation of the second diffraction grating  15  has 1.05 times of that of the corrugation of the first diffraction  14  or higher, or has 0.95 times of that of the corrugation of the first diffraction  14  or lower. In a semiconductor laser device  1 A, for example, a pitch of the first diffraction grating  14  designed so that the laser oscillation wavelength is 1.3 μm is 0.20 μm. As illustrated in  FIG. 25 , the first diffraction grating  14  and the second diffraction grating  15  are separated from each other along the optical waveguide direction. An interval D between one end of the first diffraction grating  14  and an end of the second diffraction grating  15  is 1.0 μm or shorter, the end of the second diffraction grating  15  being the closest to the one end of the first diffraction grating  14 . The interval D corresponds to a distance between the first diffraction grating  14  and the second diffraction grating  15  along the optical waveguide direction. The interval D may be 0.6 μm or shorter. When the pitch of the first diffraction grating  14  is 0.20 μm, the interval D may be five pitches or shorter, or may be three pitches or shorter for example. The interval D may be 0.4 μm or longer, or may be 0.5 μm or longer for example. When the pitch of the first diffraction grating  14  is 0.20 μm, the interval D may be two pitches or longer for example. 
     An end of a diffraction grating means an end on an outer side of a pair positioned at an end part of the diffraction grating in the optical waveguide direction, among the plurality of pairs formed of a high refractive index region (AlInGaAs or the like, for example) and a low refractive index region (InP or the like for example) configuring a pitch structure of the diffraction grating. 
     In the sixth embodiment, when the pitch of the first diffraction grating  14  is 0.20 μm, the pitch of the second diffraction grating  15  may be 0.21 μm or longer, or 0.19 μm or shorter. The pitch of the second diffraction grating  15  may be 0.24 μm or longer, or may be 0.16 μm or shorter. 
     Next, a manufacturing method of the semiconductor laser device in the sixth embodiment will be described. Hereinafter, parts different from the first embodiment will be specifically described.  FIG. 26  to  FIG. 30  are diagrams illustrating manufacturing steps of the semiconductor laser device.  FIG. 26 ,  FIG. 29  and  FIG. 30  are cross-sectional views in the optical waveguide direction of the semiconductor laser device being manufactured.  FIG. 27  and  FIG. 28  are external appearances of the semiconductor laser device being manufactured, viewed from an upper direction perpendicular to the optical waveguide direction. 
     As illustrated in  FIG. 26  to  FIG. 30 , the first diffraction grating  14  and the second diffraction grating  15  are formed on the wafer  16  by the electron beam exposure method. Between the first diffraction gratings  14  adjacent to each other, the second diffraction grating  15  having a pitch different from that of the first diffraction grating  14  is formed. In the sixth embodiment, as described above, the first diffraction grating  14  and the second diffraction grating  15  are separated from each other along the optical waveguide direction. The interval D between the end of the first diffraction grating  14  and the end of the second diffraction grating  15  is 1.0 μm or shorter. In addition, the interval D is 0.5 μm or longer for example. 
     In the sixth embodiment, fine patterns are lithographed in the resist for forming the first diffraction grating  14  and the second diffraction grating  15  using a probe current of an electron beam exposure apparatus similarly to the first embodiment. At the time, a pattern for the diffraction grating which is one of the first diffraction grating  14  and the second diffraction grating  15  is lithographed in the resist first. Then, after an electron exposure dose (dosage) of the electron beam exposure apparatus is changed, a pattern for the other diffraction grating is lithographed in the resist. Thus, patterned resist including stripes having a first pitch and stripes having a second pitch is obtained. By using the patterned resist and etching the diffraction grating layer  30 , the first diffraction grating  14  and the second diffraction grating  15  are formed. 
     A semiconductor laser device according to the sixth embodiment comprises: a semiconductor substrate; a first diffraction grating above the semiconductor substrate, the first diffraction grating having a corrugation with a first pitch; a second diffraction grating above the substrate, the second diffraction grating having a corrugation with a second pitch that is 1.05 times greater than the first pitch, or is 0.95 times lower than the first pitch; and an active layer above the first diffraction grating, wherein the first diffraction grating and the second diffraction grating are separated from each other along an optical wave guide direction by a distance, the distance being 1.0 μm or shorter. 
     In addition, a method for manufacturing a semiconductor laser device according to the sixth embodiment, the method comprises steps of: forming a first diffraction grating and a second diffraction grating on a wafer by an electron beam exposure, the first diffraction grating having a corrugation with a first pitch, the second diffraction grating having a corrugation with a second pitch that is 1.05 times greater than the first pitch, or is 0.95 times lower than the first pitch; and forming an active layer above the first diffraction grating and the second diffraction grating, wherein the first diffraction grating and the second diffraction grating are separated from each other along an optical waveguide direction by a distance, the distance being 1.0 μm or shorter. 
     Operational advantages of the semiconductor laser device and the manufacturing method thereof according to such a sixth embodiment will be described while comparing with the comparative example.  FIG. 31  is a microscope photo for which the diffraction grating of the semiconductor laser device  18  according to the comparative example is photographed.  FIG. 32  is a microscope photo for which the diffraction gratings  14  and  15  of the semiconductor laser device according to the sixth embodiment are photographed. The comparative example is already described in the first embodiment. 
     As illustrated in  FIG. 32 , in the semiconductor laser device according to the sixth embodiment, the first diffraction grating  14  and the second diffraction grating  15  are lined along the optical waveguide direction. The interval D of the first diffraction grating  14  and the second diffraction grating  15  in the optical waveguide direction is as extremely short as 1.0 μm or shorter. In this case, during epitaxial growth, growth conditions are not likely to differ at both ends of the first diffraction grating  14  in the optical waveguide direction. Therefore, it is possible to suppress generation of crystal defects of an epitaxial layer growing from both ends of the first diffraction grating  14 , and suppress a phenomenon that the crystal defects invade an active region within the active layer  4 . As a result, influence on laser oscillation efficiency can be suppressed. 
     In addition, in the step for forming the first diffraction grating  14  and the second diffraction grating  15 , as described above, the pattern for one diffraction grating is lithographed by the electron beam exposure. Next, after the lithography is tentatively stopped, the electron exposure dose (dosage) of the electron beam exposure apparatus is changed. Then, the pattern for the other diffraction grating is lithographed. When the first diffraction grating  14  and the second diffraction grating  15  are continuously provided, due to an error of alignment after the dosage is changed, the pattern of one diffraction grating and the pattern of the other diffraction grating sometimes overlap with each other. Then, the resist mask is doubly exposed at a part where the patterns overlap. When the diffraction grating is formed by using such the resist mask, abnormal growth tends to occur at the doubly exposed part. Due to the occurrence of the abnormal growth, there is a risk that luminescent efficiency of the semiconductor laser device degrades. 
     In order to reduce the occurrence of such abnormal growth, in the sixth embodiment, the first diffraction grating  14  and the second diffraction grating  15  are separated from each other. In this case, since the pattern for the first diffraction grating  14  and the pattern for the second diffraction grating  15 , which are respectively exposed regions, do not overlap with each other in the optical waveguide direction, the occurrence of the abnormal growth in the first diffraction grating  14  and the second diffraction grating  15  is reduced, and the semiconductor laser device  1 A having excellent luminescent efficiency can be obtained. 
       FIG. 33  is a microscope photo illustrating the case where the first diffraction grating  14  and the second diffraction grating  15  are not separated from each other (are made continuous). Referring to  FIG. 33 , a situation where the first diffraction grating  14  and the second diffraction grating  15  overlap with each other is recognized. In addition,  FIG. 34  is a microscope photo illustrating a case where the first diffraction grating  14  and the second diffraction grating  15  are separated from each other. Referring to  FIG. 34 , a situation where the first diffraction grating  14  and the second diffraction grating  15  are separated without overlapping with each other is recognized. 
     As in the sixth embodiment, the interval D of the first diffraction grating  14  and the second diffraction grating  15  may be 0.5 μm or longer. Thus, it is possible to reliably avoid overlapping of the pattern (exposed region) for the first diffraction grating  14  and the pattern (exposed region) for the second diffraction grating  15  with each other when the electron beam exposure is performed. 
     Seventh Embodiment 
       FIG. 35  is a cross-sectional view of a semiconductor laser device according to the seventh embodiment. A semiconductor laser device  20 A further includes a modulation region  21  in addition to a configuration of the semiconductor laser device  1 A of the sixth embodiment. In the modulation region  21 , the second diffraction grating  15  is located. Also in the semiconductor laser device  20 A of the seventh embodiment, similarly to the sixth embodiment, the first diffraction grating  14  and the second diffraction grating  15  are lined at the interval D of 1.0 μm or shorter. Therefore, it is possible to suppress the generation of the crystal defects of the epitaxial layer growing from both ends of the first diffraction grating  14 . In addition, since the pattern for the first diffraction grating  14  and the pattern for the second diffraction grating  15  do not overlap with each other in the optical waveguide direction, the occurrence of the abnormal growth in the first diffraction grating  14  and the second diffraction grating  15  is reduced, and the semiconductor laser device  20 A having the excellent luminescent efficiency can be obtained. 
     Eighth Embodiment 
       FIG. 36  is a diagram of the wafer viewed from the direction perpendicular to the principal surface of the wafer in a step for forming the diffraction gratings on the wafer among manufacturing steps of the semiconductor laser device according to the eighth embodiment.  FIG. 37  is a diagram that magnifies a portion to be a semiconductor device later in the wafer illustrated in  FIG. 36 . 
     As illustrated in  FIG. 36  and  FIG. 37 , on the wafer  16 , the plurality of regions  41  each including at least one first diffraction grating  14  and one second diffraction grating  15  that are continuously provided are set discretely along the optical waveguide direction. The first diffraction grating  14  and the second diffraction grating  15  in the region  41  are separated from each other similarly to the sixth embodiment. 
     The present invention is not limited to the above-described embodiments. The above-described embodiments may be freely combined. In addition, the second diffraction grating may be provided discretely only on one end of the first diffraction grating in the optical waveguide direction.