Patent Publication Number: US-2022231478-A1

Title: Semiconductor laser device

Description:
TECHNICAL FIELD 
     The present application relates to a semiconductor laser device. 
     BACKGROUND ART 
     In a semiconductor laser device in which an electro-absorption (EA) modulator and a distributed feedback laser are integrated, a structure in which a diffraction grating is formed in the EA modulator has been reported (for example, refer to Patent Document 1). In the semiconductor laser device of Patent Document 1, the diffraction grating is formed in an entire waveguide of an EA modulator region, and the diffraction grating is also formed in a separation region where a modulator electrode and a laser electrode are separated, that is, a region between the distributed feedback laser region and the EA modulator region. 
     CITATION LIST 
     Patent Document 
     
         
         Patent Document 1: Japanese Patent Application Laid-open No. H4-198913 (FIG. 1) 
       
    
     SUMMARY OF INVENTION 
     Problems to be Solved by Invention 
     In the semiconductor laser device of Patent Document 1, the diffraction grating is formed from an emission end face from which laser light of the distributed feedback laser region is emitted up to an emission end face of the EA modulator region. Since light is reflected in the diffraction grating, the semiconductor laser device of Patent Document 1 has a problem in that light output of the emitted laser light is reduced by the diffraction grating formed in the EA modulator region and the region between the distributed feedback laser region and the EA modulator region. 
     A technology disclosed in the present application aims to increase an optical output of laser light in a semiconductor laser device in which an EA modulator and a distributed feedback laser that are formed with diffraction gratings are integrated. 
     Means for Solving Problems 
     In an example of a semiconductor laser device disclosed in the present application, a distributed feedback laser part and an electro-absorption modulator part are formed on the same semiconductor substrate, and laser light emitted from the laser part is emitted from an emission end face of the modulator part. The laser part includes a first diffraction grating formed to extend in a direction of an optical axis of the laser light, and the modulator part at least partially includes a second diffraction grating formed to extend in the direction of the optical axis of the laser, wherein a non-diffraction grating region in which a diffraction grating is not formed is interposed between the second diffraction grating of the modulator part and an emission end face of the laser part from which the laser light is emitted to the modulator part. 
     Effect of Invention 
     In an example of the semiconductor laser device disclosed in the present application, since the non-diffraction grating region in which the diffraction grating is not formed is interposed between the second diffraction grating of the modulator part and the emission end face of the laser part from which the laser light is emitted to the modulator part, light output of the laser light can be increased. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view showing a semiconductor laser device according to Embodiment 1. 
         FIG. 2  is a cross-sectional view of a mesa stripe of the first example in the semiconductor laser device of  FIG. 1 . 
         FIG. 3  is a diagram showing a diffraction grating in a modulator part of  FIG. 1 . 
         FIG. 4  is a cross-sectional view of a laser part in the semiconductor laser device of  FIG. 1 . 
         FIG. 5  is a cross-sectional view of the modulator part in the semiconductor laser device of  FIG. 1 . 
         FIG. 6  is a cross-sectional view of the modulator part in the semiconductor laser device of  FIG. 1 . 
         FIG. 7  is a cross-sectional view of a separation part in the semiconductor laser device of  FIG. 1 . 
         FIG. 8  is a cross-sectional view of a mesa stripe of the second example in the semiconductor laser device of  FIG. 1 . 
         FIG. 9  is a cross-sectional view of a mesa stripe of the third example in the semiconductor laser device of  FIG. 1 . 
         FIG. 10  is a cross-sectional view of a mesa stripe of the fourth example in the semiconductor laser device of  FIG. 1 . 
         FIG. 11  is a perspective view showing a semiconductor laser device according to Embodiment 2. 
         FIG. 12  is a cross-sectional view of a mesa stripe in the semiconductor laser device of  FIG. 11 . 
         FIG. 13  is a diagram showing a diffraction grating in a modulator part of  FIG. 11 . 
         FIG. 14  is a cross-sectional view of the modulator part in the semiconductor laser device of  FIG. 11 . 
         FIG. 15  is a perspective view showing a semiconductor laser device according to Embodiment 3. 
         FIG. 16  is a diagram showing a diffraction grating of the first example in a modulator part of  FIG. 15 . 
         FIG. 17  is a cross-sectional view of the modulator part in the semiconductor laser device of  FIG. 15 . 
         FIG. 18  shows a diagram showing a diffraction grating of the second example in the modulator part of  FIG. 15 . 
         FIG. 19  is a diagram showing a diffraction grating of the third example in the modulator part of  FIG. 15 . 
     
    
    
     MODES FOR CARRYING OUT INVENTION 
     Embodiment 1 
     A semiconductor laser device  50  of Embodiment 1 will be described referring to the drawings. The same or corresponding components are denoted by the same reference numerals, and repetitive description may be omitted. In other embodiments, the same or corresponding components are denoted by the same reference numerals, and repetitive description may be omitted.  FIG. 1  is a perspective view showing the semiconductor laser device according to Embodiment 1.  FIG. 2  is a cross-sectional view of a mesa stripe of the first example in the semiconductor laser device of  FIG. 1 , and  FIG. 3  is a diagram showing a diffraction grating in a modulator part of  FIG. 1 .  FIG. 4  is a cross-sectional view of a laser part in the semiconductor laser device of  FIG. 1 , and  FIG. 5  and  FIG. 6  each are a cross-sectional view of a modulator part in the semiconductor laser device of  FIG. 1 .  FIG. 7  is a cross-sectional view of a separation part in the semiconductor laser device of  FIG. 1 .  FIG. 8  is a cross-sectional view of a mesa stripe of the second example in the semiconductor laser device of  FIG. 1 ,  FIG. 9  is a cross-sectional view of a mesa stripe of the third example in the semiconductor laser device of  FIG. 1 , and  FIG. 10  is a cross-sectional view of a mesa stripe of the fourth example in the semiconductor laser device of  FIG. 1 .  FIG. 1  also shows a waveguide cross section  33  parallel to the z-direction, which is the emission direction of the laser light and the optical axis direction.  FIG. 4  to  FIG. 7  show cross sections perpendicular to the z-direction. 
     The semiconductor laser device  50  of Embodiment 1 includes a laser part  21 , a modulator part  22 , and a separation part  23 . A distributed feedback laser is formed in the laser part  21 . The modulator part  22  is an electro-absorption (EA) modulator monolithically formed on an n-InP semiconductor substrate  1 , and the semiconductor laser device  50  is an electro-absorption (EA) modulator integrated semiconductor laser. The laser part  21  is formed in a region between the dashed lines  31   a  and  31   b , the separation part  23  is formed in a region between the dashed lines  31   b  and  31   c , and the modulator part  22  is formed in a region between the dashed lines  31   c  and  31   d . In the semiconductor laser device  50  of Embodiment 1, the laser part  21  of the distributed feedback type and the modulator part  22  of the electro-absorption type are formed on the same semiconductor substrate  1 , and the laser light emitted from the laser part  21  is emitted from an emission end face  32  of the modulator part  22 . 
     The laser part  21  comprises a semiconductor substrate  1 , a first cladding layer  2  of n-InP, a diffraction grating material layer  16  in which a diffraction grating  17  is formed, a second cladding layer  3  of n-InP, an active layer  4 , a third cladding layer  6  of p-InP, an InP buried layer  8 , a contact layer  7  of p-InGaAs, an insulating film  12  of SiO 2 , a cathode electrode  11  formed on a rear surface of the semiconductor substrate  1 , and a first anode electrode  13  formed on the front side of the semiconductor substrate  1  and connected to the contact layer  7 . The modulator part  22  comprises the semiconductor substrate  1 , the first cladding layer  2  of n-InP, the diffraction grating material layer  16  having a diffraction grating  18  partially formed therein, the second cladding layer  3  of n-InP, an absorption layer  5 , the third cladding layer  6  of p-InP, the InP buried layer  8 , the contact layer  7  of p-InGaAs, the insulating film  12  of SiO 2 , the cathode electrode  11  formed on the rear surface of the semiconductor substrate  1 , and a second anode electrode  14  formed on the front side of the semiconductor substrate  1  and connected to the contact layer  7 . The separation part  23  is a structure in a region (separation region, isolation region) for separating the laser part  21  and the modulator part  22 . The separation part  23  comprises the semiconductor substrate  1 , the first cladding layer  2  of n-InP, the diffraction grating material layer  16  in which no diffraction grating is formed, the second cladding layer  3  of n-InP, the absorption layer  5 , the third cladding layer  6  of p-InP, the InP buried layer  8 , the insulating film  12  of SiO 2 , and the cathode electrode  11  formed on the rear surface of the semiconductor substrate  1 . In  FIG. 1 , the z-direction is the direction of the optical axis of the laser light emitted by the laser part  21 , the x-direction is the direction perpendicular to the z-direction, in which the layers provided in the semiconductor laser device  50  extend, and the y-direction is the direction perpendicular to the x-direction and the x-direction, in which the layers provided in the semiconductor laser device  50  are stacked. 
     The active layer  4  is composed of an InGaAsP multiple quantum well. The absorption layer  5  is composed of an InGaAsP multiple quantum well. In the laser part  21 , the diffraction grating  17  having a recess  29  formed in the diffraction grating material layer  16  of InGaAsP is provided in a lower layer under the active layer  4 , that is, between the first cladding layer  2  and the second cladding layer  3  on the side to the semiconductor substrate  1 . In the modulator part  22 , a diffraction grating material layer  16  of the same InGaAsP as the laser part  21  is formed, and the diffraction grating  18  in which recesses  29   a ,  29   b ,  29   c  and  29   d  are formed is provided partially in the diffraction grating material layer  16 .  FIG. 1  and  FIG. 2  show an example in which the diffraction grating  18  is provided only on the side of the emission end face  32  opposite to the laser part  21 . The separation part  23  is also provided with the diffraction grating material layer  16  of InGaAsP, which is the same as that in the laser part  21 , but is not provided with the diffraction grating with the recess  29  being formed. 
     A mesa stripe  15 , which is a mesa extending in the direction (z-direction) of the optical axis of the laser light emitted by the laser part  21 , is formed in the laser part  21 . The mesa stripe  15  of the laser part  21  is composed of the first cladding layer  2 , the diffraction grating material layer  16  in which the diffraction grating  17  is formed, the second cladding layer  3 , the active layer  4 , and a part of the third cladding layer  6 . A part of the third cladding layer  6  constituting the mesa stripe  15  is a portion between the active layer  4  and the surface position of the buried layer  8  opposite side of the semiconductor substrate  1 . The mesa stripe  15  is continuously formed in the laser part  21 , the separation part  23 , and the modulator part  22 . Although the mesa stripe  15  is continuously formed, the mesa stripe  15  in the laser part  21 , the separation part  23 , and the modulator part  22  may be described separately as a first mesa stripe, a third mesa stripe, and a second mesa stripe, respectively. The laser part  21  includes the diffraction grating  17  which is a first diffraction grating formed to extend in the direction of the optical axis of the laser light. 
     The mesa stripe  15  of the modulator part  22  is composed of the first cladding layer  2 , the diffraction grating material layer  16  including a portion where the diffraction grating  18  is formed, the second cladding layer  3 , the absorption layer  5 , and the part of the third cladding layer  6 . The mesa stripe  15  of the separation part  23  is composed of the first cladding layer  2 , the diffraction grating material layer  16  in which no diffraction grating is formed, the second cladding layer  3 , the absorption layer  5 , and the part of the third cladding layer  6 . The part of the third cladding layer  6  constituting the mesa stripe  15  is a portion between the absorption layer  5  and the surface position of the buried layer  8  opposite side of the semiconductor substrate  1 . The modulator part  22  includes the diffraction grating  18 , which is a second diffraction grating extended and formed in the direction of the optical axis of the laser light. Grooves  19   a  and  19   b  are formed on both sides of the mesa stripe  15  of the laser part  21 , the separation part  23 , and the modulator part  22  in the x-direction so as to penetrate through the buried layer  8  and the third cladding layer  6  and reach the semiconductor substrate  1 . In the laser part  21  and the modulator part  22 , the grooves  19   a  and  19   b  also penetrate through the contact layer  7 . In the laser part  21  and the modulator part  22 , the insulating film  12  is formed on the surface of the contact layer  7  and the grooves  19   a  and  19   b . In the separation part  23 , the insulating film  12  is formed on the surface of the third cladding layer  6  and the grooves  19   a  and  19   b . The first anode electrode  13  of the laser part  21  is connected to the contact layer  7  through an opening  35   a  formed in the insulating film  12 . The second anode electrode  14  of the modulator part  22  is connected to the contact layer  7  through an opening  35   b  formed in the insulating film  12 . 
     In the semiconductor laser device  50  of Embodiment 1, the film thickness ta of the diffraction grating material layer  16  between the first cladding layer  2  and the second cladding layer  3  is the same in the laser part  21 , the separation part  23  and the modulator part  22 . In  FIG. 3 , a dashed line  34   a  indicates a rear surface of the diffraction grating material layer  16  (the face on the side to the semiconductor substrate  1 ), and a dashed line  34   b  indicates a front surface of the diffraction grating material layer  16  (the face on the side opposite to the semiconductor substrate  1 ). As shown in  FIG. 2 , in the mesa stripe  15 , the modulator part  22  includes a diffraction grating region  24  in which the diffraction grating  18  is formed and a non-diffraction grating region  25  in which the diffraction grating  18  is not formed. In the modulator part  22 , the diffraction grating region  24  is the region from a dashed line  31   f  to a dashed line  31   d , and the non-diffraction grating region  25  is the region from the dashed line  31   c  to the dashed line  31   f . As described above, the diffraction grating  18  is formed on the side of the emission end face  32 , and the non-diffraction grating region  25  is provided on the side to the laser part  21  in the modulator part  22 . As shown in  FIG. 3 , the diffraction grating  18  includes four protrusions  28   a ,  28   b ,  28   c , and  28   d  and four recesses  29   a ,  29   b ,  29   c , and  29   d . The protrusions  28   a ,  28   b ,  28   c , and  28   d  of the modulator part  22  protrude in a direction perpendicular to the semiconductor substrate  1 , and the recesses  29   a ,  29   b ,  29   c , and  29   d  of the modulator part  22  are recessed toward the side to the semiconductor substrate  1  from the faces of the protrusions  28   a ,  28   b ,  28   c ,  28   d  of the modulator part  22  on the side opposite to the semiconductor substrate  1 . In  FIG. 2  and  FIG. 3 , the thicknesses of the protrusions  28   a ,  28   b ,  28   c  and  28   d  are the same, and the depths of the recesses  29   a ,  29   b ,  29   c  and  29   d  formed by removing the diffraction grating material layer  16  are the same as the film thickness ta of the diffraction grating material layer  16 . The reference numeral  28  is collectively used for the protrusions, and  28   a ,  28   b ,  28   c , and  28   d  are used for distinction. The reference numeral  29  is collectively used for the recesses, and  29   a ,  29   b ,  29   c  and  29   d  are used for distinction. 
     In  FIG. 3 , the interval between the dashed line  31   f  and the end of the protrusion  28   a  on the side to the emission end face  32  is za, the interval between the end of the protrusion  28   a  on the side to the emission end face  32  and the end of the protrusion  28   b  on the side to the emission end face  32  is zb, the interval between the end of the protrusion  28   b  on the side to the emission end face  32  and the end of the protrusion  28   c  on the side to the emission end face  32  is zc, and the interval between the end of the protrusion  28   c  on the side to the emission end face  32  and the end of the protrusion  28   d  on the side to the emission end face  32  is zd. Note that, the end of the protrusion  28   d  on the side to the emission end face  32  coincides with the emission end face  32 . The interval za, the interval zb, the interval zc, and the interval zd each are the interval in the protrusion  28  and the recess  29  that are adjacent in a pair, which are the same.  FIG. 3  shows an example in which the width of the protrusion  28  in the z-direction and the width of the recess  29  in the z-direction are the same. In the diffraction grating  18  of Embodiment 1, the period in the protrusion  28  and the recess  29  is the same over the entire diffraction grating. That is, in the diffraction grating  18  of Embodiment 1, the period in the protrusion  28  and the recess  29  is uniform. 
     In the laser part  21 , the diffraction grating  17  is formed over the entire mesa stripe  15  in the z-direction, that is, between the dashed line  31   a  and the dashed line  31   b . That is, in the mesa stripe  15 , the laser part  21  does not include the non-diffraction grating region  25  in which no diffraction grating is formed but includes only the diffraction grating region  24  in which diffraction gratings are formed totally. The diffraction grating  17  of the laser part  21  includes a plurality of protrusions  28  and a plurality of recesses  29 . The protrusions  28  of the laser part  21  extend in the direction perpendicular to the semiconductor substrate  1 , and the recesses  29  of the laser part  21  are recessed toward the side to the semiconductor substrate  1  from the faces of the protrusions  28  of the laser part  21  on the side opposite to the semiconductor substrate  1 .  FIG. 2  shows an example in which the depths of the recesses  29  formed by removing the diffraction grating material layer  16  are the same as the film thickness ta of the diffraction grating material layer  16 . The interval in the protrusion  28  and recess  29  that are adjacent in a pair in the diffraction grating  17  of the laser part  21  is the same as the interval in the protrusion  28  and recess  29  that are adjacent in a pair in the diffraction grating  18  of the modulator part  22 . In the diffraction grating  17  of Embodiment 1, the period in the protrusion  28  and the recess  29  is the same over the entire diffraction grating. That is, in the diffraction grating  17  of Embodiment 1, the period in the protrusion  28  and the recess  29  is uniform. 
     In the separation part  23 , no diffraction grating is formed over the entire z-direction, that is, between the dashed lines  31   b  and  31   c . That is, in the mesa stripe  15 , the separation part  23  does not include the diffraction grating region  24  in which the diffraction grating is formed and but includes the non-diffraction grating region  25  in which the diffraction grating is not formed totally. In the semiconductor laser device  50  shown in  FIG. 2 , the non-diffraction grating region  25  in which no diffraction grating is formed is interposed between the diffraction grating  18  of the modulator part  22  and the emission end face (end portion indicated by the dashed line  31   b ) of the laser part  21  from which the laser light is emitted to the modulator part  22 . 
     A method of manufacturing the semiconductor laser device  50  will be described. The first cladding layer  2  and the diffraction grating material layer  16  are sequentially crystal-grown on the surface of the semiconductor substrate  1  by a metal-organic chemical vapor deposition (MOCVD) method, and the recesses  29  are formed in the diffraction grating material layer  16  to form the diffraction gratings  17  and  18 . The second cladding layer  3 , the active layer  4 , the absorption layer  5 , and the part of the third cladding layer  6  are sequentially crystal-grown by MOCVD method on the surface of the diffraction grating material layer  16  including diffraction gratings  17 ,  18 , and mesa stripe  15  is formed by dry etching using an SiO 2  mask. Then, the buried layer  8  is crystal-grown on the semiconductor substrate  1  exposed on both sides of the mesa stripe  15 . 
     The SiO 2  mask is removed, the third cladding layer  6  and the contact layer  7  are sequentially crystal-grown on the surface of the buried layer  8  and the mesa stripe  15 , and the contact layer  7  on the surface of the separation part  23  is removed by wet etching using a photoresist mask. After the photoresist mask is removed, the grooves  19   a  and  19   b  are formed by wet etching using the photoresist mask. The photoresist mask is removed, and the insulating film  12  is formed on the surfaces of the third cladding layer  6 , the contact layer  7 , and the grooves  19   a  and  19   b  of the separation part  23 . The openings  35   a  and  35   b  in the insulating film  12  are formed using the photoresist mask. The photoresist mask is removed, metal layers are formed on the front and rear surfaces of the semiconductor laser device  50 , and the cathode electrode  11 , the first anode electrode  13 , and the second anode electrode  14  are formed. Thereafter, the insulating film  12  is thickly formed between the first anode electrode  13  and the second anode electrode  14 . 
     In the semiconductor laser device (the semiconductor laser device of the comparative example) in which the EA modulator and the distributed feedback laser are integrated and no diffraction grating is formed in the region of the EA modulator, reflected light of the laser light from the emission end face (the end face on the side of the modulator part) is incident on the distributed feedback laser, thereby causing an adiabatic chirp in which the oscillation wavelength of the distributed feedback laser varies. The adiabatic chirp also affects a transmission characteristic of the optical signal as it is transmitted through an optical fiber. In the semiconductor laser device of the comparative example, since the diffraction grating is not formed in the region of the EA modulator, the phase of the reflected light which is the return light reflected on the emission end face of the laser light (the end face on the side of the modulator part) changes for each semiconductor laser device in accordance with the cleavage position of the emission end face. In contrast, in the semiconductor laser device  50  of Embodiment 1, the diffraction grating  18  is provided in the modulator part  22 . Therefore, in the semiconductor laser device  50  of Embodiment 1, since the phase of the reflected light of the laser light generated in the forward side of the element of the semiconductor laser device  50 , that is, in the emission end face  32 , is determined by a structural factor such as a starting position of the diffraction grating  18  on the side to the laser part  21  in the modulator part, it is possible to greatly suppress the phase variation of each of the semiconductor laser devices in which the phase of the reflected light from the forward side of the element varies for each semiconductor laser device. As a result, the semiconductor laser device  50  of Embodiment 1 can suppress variation in the amount of adiabatic chirp for each semiconductor laser device and can avoid a decrease in the transmission yield. 
     In the semiconductor laser device  50  according to Embodiment 1, it is not necessary to add the waveguide having the diffraction grating in the forward side of the modulator part  22 , that is, on the side of the emission end face  32 . Therefore, the semiconductor laser device  50  of Embodiment 1 is easy in the manufacturing process and has a smaller element size than the semiconductor laser device having the structure in which the waveguide having the diffraction grating is added, so that the cost can be reduced. 
     In the semiconductor laser device of Patent Document 1, the diffraction grating is formed in the entire waveguide of the EA modulator region, and the diffraction grating is also formed in the separation region where the modulator electrode and the laser electrode are separated. Since light is reflected by the diffraction grating, a problem arises in that the larger the area of the diffraction grating, the lower the optical output of the laser light from the semiconductor laser device. In contrast, in the semiconductor laser device  50  of Embodiment 1, as shown in  FIG. 2  and  FIG. 3 , no diffraction grating is formed in the separating part  23  and on the side to the laser part  21  in the modulator part  22 ; that is, at least no diffraction grating is formed on the separation part  23 , so that the optical output of the laser light can be increased as compared with the semiconductor laser device of Patent Document 1. 
     Although an example of the semiconductor laser device  50  in which the diffraction grating  18  is formed on the side of the emission end face  32  of the modulator part  22  has been described so far, the example of the arrangement of the diffraction grating  18  is not limited thereto. A non-diffraction grating region  25  should be interposed between the end (the emission end face) at which the laser light of the laser part  21  is emitted, that is, the end indicated by the dashed line  31   b , and the diffraction grating  18  formed in the modulator part  22 . In this case, since the non-diffraction grating region  25  in which the diffraction grating  18  is not formed exists between the laser part  21  and the emission end face  32 , the light output of the laser light can be increased as compared with the semiconductor laser device of Patent Document 1 in which the diffraction grating is formed from the emission end face from which the laser light of the distributed feedback laser region is emitted, to the emission end face of the EA modulator region. The semiconductor laser device  50  including the mesa stripe  15  of the second example shown in  FIG. 8  is an example in which the diffraction grating  18  is formed on the side to the laser part  21  in the mesa stripe  15  of the modulator part  22 . In the modulator part  22  and the separation part  23  in the mesa stripe  15  of the second example shown in  FIG. 8 , a region between the dashed line  31   c  and the dashed line  31   f  is the diffraction grating region  24  in which the diffraction grating  18  is formed, and a region between the dashed line  31   f  and the dashed line  31   d  and a region between the dashed line  31   b  and the dashed line  31   c  is the non-diffraction grating region  25  in which no diffraction grating  18  is formed. 
     The semiconductor laser device  50  including the mesa stripe  15  of the third example shown in  FIG. 9  is an example in which the diffraction grating  18  is formed in the center in the mesa stripe  15  of the modulator part  22 . In the modulator part  22  and the separation part  23  in the mesa stripe  15  of the third example shown in  FIG. 9 , a region between the dashed line  31   f  and the dashed line  31   g  is the diffraction grating region  24  in which the diffraction grating  18  is formed, and a region between the dashed line  31   b  and the dashed line  31   f  and a region between the dashed line  31   g  and the dashed line  31   d  are the non-diffraction grating region  25  in which no diffraction grating  18  is formed. The semiconductor laser device  50  including the mesa stripe  15  of the fourth example shown in  FIG. 10  is an example in which the diffraction grating  18  is formed over the entire mesa stripe  15  of the modulator part  22 . Even in the semiconductor laser device  50  including the mesa stripe  15  of the fourth example shown in  FIG. 10 , since the non-diffraction grating region  25  in which the diffraction grating  18  is not formed exists between the emission end face of the laser part  21  and the emission end face  32  in the mesa stripe  15 , the light output of the laser light can be increased as compared with the semiconductor laser device of Patent Document 1 in which the diffraction grating is formed from the emission end face from which the laser light of the distributed feedback laser region is emitted, to the emission end face of the EA modulator region. 
     Note that, the examples in which the separation part  23  is formed between the laser part  21  and the modulator part  22  has been described. However, even in the semiconductor laser device  50  in which the separation part  23  is not formed, the mesa stripe  15  should have a structure in which the non-diffraction grating region  25  in which the diffraction grating  18  is not formed exists between the emission end face of the laser part  21  and the emission end face  32 . In this case, since the non-diffraction grating region  25  in which the diffraction grating  18  is not formed exists between the emission end face of the laser part  21  and the emission end face  32  in the mesa stripe  15 , the light output of the laser light can be increased as compared with the semiconductor laser device of Patent Document 1 in which the diffraction grating is formed from the emission end face from which the laser light of the distributed feedback laser region is emitted, to the emission end face of the EA modulator region. The semiconductor laser device  50  in which the separation part  23  is not formed between the laser part  21  and the modulator part  22  is a device in which the contact layer  7  is not separated between the laser part  21  and the modulator part  22 , as in the semiconductor laser device of Patent Document 1, for example (first example without the separation part). Further, the semiconductor laser device  50  in which the separation part  23  is not formed between the laser part  21  and the modulator part  22  is a device in which a step is provided between the laser part  21  and the modulator part  22  to separate the contact layer  7  and the first anode electrode  13  of the laser part  21  from the contact layer  7  and the second anode electrode  14  of the modulator part  22  (second example without the separation part). Note that, in the case of the first example without the separation part and the second example without the separation part, the structure of the mesa stripe  15  of the fourth example shown in  FIG. 10  cannot be applied thereto. 
     As described above, the semiconductor laser device  50  of Embodiment 1 is the semiconductor laser device in which the laser part  21  of the distributed feedback type and the modulator part  22  of the electro-absorption type are formed on the same semiconductor substrate  1 , and the laser light emitted from the laser part  21  is emitted from the emission end face  32  of the modulator part  22 . In the semiconductor laser device  50  of Embodiment 1, the laser part  21  includes the first diffraction grating (diffraction grating  17 ) formed to extend in the direction of the optical axis of the laser light, and the modulator part  22  includes at least partially the second diffraction grating (diffraction grating  18 ) formed to extend in the direction of the optical axis of the laser light. In the semiconductor laser device  50  of Embodiment 1, the non-diffraction grating region  25  in which no diffraction grating is formed is interposed between the second diffraction grating (diffraction grating  18 ) of the modulator part  22  and the emission end face (end portion indicated by the dashed line  31   b ) of the laser part  21  from which the laser light is emitted to the modulator part  22 . In the semiconductor laser device  50  of Embodiment 1, with this configuration, since the non-diffraction grating region  25  in which no diffraction grating is formed is interposed between the second diffraction grating (diffraction grating  18 ) of the modulator part  22  and the emission end face (end portion indicated by the dashed line  31   b ) of the laser part  21  from which the laser light is emitted to the modulator part  22 , the light output of the laser light can be increased. 
     Embodiment 2 
       FIG. 11  is a perspective view showing a semiconductor laser device according to Embodiment 2, and  FIG. 12  is a cross-sectional view of a mesa stripe in the semiconductor laser device of  FIG. 11 .  FIG. 13  is a diagram showing a diffraction grating in a modulator part of  FIG. 11 , and  FIG. 14  is a cross-sectional view of the modulator part in the semiconductor laser device of  FIG. 11 . The semiconductor laser device  50  of Embodiment 2 is different from the semiconductor laser device  50  of Embodiment 1 in that a diffraction grating  26  having a non-uniform period is formed in the diffraction grating region  24  of the modulator part  22 . Apart different from the semiconductor laser device  50  of Embodiment 1 will mainly be described. 
     As shown in  FIG. 13 , the diffraction grating  26  includes four protrusions  28   a ,  28   b ,  28   c , and  28   d  and four recesses  29   a ,  29   b ,  29   c , and  29   d .  FIGS. 12 and 13  show an example in which the depths of the recesses  29   a ,  29   b ,  29   c  and  29   d  formed by removing the diffraction grating material layer  16  are the same as the film thickness ta of the diffraction grating material layer  16 . Note that, the reference numeral  28  is collectively used for the protrusions, and  28   a ,  28   b ,  28   c , and  28   d  are used for distinction. The reference numeral  29  is collectively used for the recesses, and  29   a ,  29   b ,  29   c  and  29   d  are used for distinction. 
     In  FIG. 13 , the interval between the dashed line  31   f  and the end of the protrusion  28   a  on the side to the emission end face  32  is za, the interval between the end of the protrusion  28   a  on the side to the emission end face  32  and the end of the protrusion  28   b  on the side to the emission end face  32  is zb, the interval between the end of the protrusion  28   b  on the side to the emission end face  32  and the end of the protrusion  28   c  on the side to the emission end face  32  is zc, and the interval between the end of the protrusion  28   c  on the side to the emission end face  32  and the end of the protrusion  28   d  on the side to the emission end face  32  side is zd. Note that, the end of the protrusion  28   d  on the side to the emission end face  32  coincides with the emission end face  32 . The interval za, the interval zb, the interval zc, and the interval zd each are the interval in the protrusion  28  and the recess  29  that are adjacent in a pair, which are different. In  FIG. 13 , the intervals za, zb, zc, and zd have the relationship of formula (1), and an example is shown in which the width in the z-direction of the protrusion  28  and the width in the z-direction of the recess  29  are the same. 
       zd&gt;zc&gt;zb&gt;za  (1)
 
     In the diffraction grating  26  of Embodiment 2, the interval in the protrusion  28  and the recess  29  is non-uniform in the z-direction which is the emission direction of the laser light. That is, in the diffraction grating  26  of Embodiment 2, the period in the protrusion  28  and the recess  29  is non-uniform. 
     In the diffraction grating, which wavelength of light is reflected depends on the period of the diffraction grating. Therefore, in the diffraction grating  26  of Embodiment 2 in which the period of the diffraction grating is non-uniform, the wavelength band of light reflection that occurs at the diffraction grating  26  of the modulator part  22  can be expanded as compared with the case where the diffraction grating has a uniform period. That is, the semiconductor laser device  50  of Embodiment 2 provided with the diffraction grating  26  having the non-uniform period in the modulator part  22  can expand the wavelength band of the light reflection that occurs at the diffraction grating as compared with the semiconductor laser device  50  of Embodiment 1 provided with the diffraction grating  18  having the uniform period in the modulator part  22 . Thus, in the semiconductor laser device  50  of Embodiment 2, even when the drive conditions of the laser part  21  and the modulator part  22  change or when the element size of the semiconductor laser device  50  changes due to manufacturing variation, the output light generated from the laser part  21  can be reliably reflected by the diffraction grating  26  of the modulator part  22  toward the side of the laser part  21 . That is, in the semiconductor laser device  50  of Embodiment 2, the effects of the semiconductor laser device  50  of Embodiment 1 that the variation of the amount of adiabatic chirp for each semiconductor laser device can be suppressed and the decrease in the transmission yield can be avoided can be obtained even in one or both of the case where the drive conditions change and the case where the manufacturing variation occurs as compared with the semiconductor laser device  50  of Embodiment 1. That is, the semiconductor laser device  50  of Embodiment 2 is more resistant to the change in the drive conditions and the manufacturing variation than the semiconductor laser device  50  of Embodiment 1. 
     The semiconductor laser device  50  of Embodiment 2 has the same structure as the semiconductor laser device  50  of Embodiment 1 except for the period of the diffraction grating of the modulator part  22 . Therefore, since the semiconductor laser device  50  of Embodiment 2 has the structure in which at least the diffraction grating  26  is not formed in the separation part  23  or the structure in which the non-diffraction grating region  25  in which the diffraction grating  26  is not formed exists between the emission end face of the laser part  21  and the emission end face  32 , as in the semiconductor laser device  50  of Embodiment 1, it is possible to increase the optical output of the laser light as compared with the semiconductor laser device of Patent Document 1. 
     The semiconductor laser device  50  of Embodiment 2 is provided with the diffraction grating  26  having the non-uniform period in the mesa stripe  15  of the modulator part  22 , so that the reflected light that occurs at the forward side of the element of the semiconductor laser device  50 , that is, at the emission end face  32 , is directed to a position away from the absorption layer  5  and the active layer  4  by the diffraction grating  26  of the modulator part  22  in which the wavelength band of light reflection is expanded. Therefore, the propagation of the reflected light from the forward side of the element to the active layer  4  of the laser part  21  can be suppressed, the variation of the amount of adiabatic chirp for each semiconductor laser device can be suppressed, and the decrease in the transmission yield can be avoided. 
     In the semiconductor laser device  50  of Embodiment 2, similarly to the semiconductor laser device  50  of Embodiment 1, it is not necessary to add the waveguide provided with the diffraction grating in the forward side of the modulator part  22 , that is, on the side of the emission end face  32 . Therefore, the semiconductor laser device  50  according to Embodiment 2 is easy in the manufacturing process and has a smaller element size than the semiconductor laser device having the structure in which the waveguide provided with the diffraction grating is added, so that the cost can be reduced. 
     Embodiment 3 
       FIG. 15  is a perspective view showing a semiconductor laser device according to Embodiment 3,  FIG. 16  is a diagram showing a diffraction grating of the first example in a modulator part of  FIG. 15 , and  FIG. 17  is a cross-sectional view of the modulator part in the semiconductor laser device of  FIG. 15 .  FIG. 18  is a diagram showing a diffraction grating of the second example in the modulator part of  FIG. 15 , and  FIG. 19  is a diagram showing a diffraction grating of the third example in the modulator part of  FIG. 15 . The semiconductor laser device  50  of Embodiment 3 is different from the semiconductor laser device  50  of Embodiment 1 in that a diffraction grating  27  in which the depth of the recess  29  or/and the thickness of the protrusion  28  are non-uniform is formed in the diffraction grating region  24  of the modulator part  22 . A part different from the semiconductor laser device  50  of Embodiment 1 will mainly be described. 
     As shown in  FIG. 16 , the diffraction grating  27  includes six protrusions  28   a ,  28   b ,  28   c ,  28   d ,  28   e , and  28   f  and six recesses  29   a ,  29   b ,  29   c ,  29   d ,  29   e , and  29   f . Note that, the reference numeral  28  is collectively used for the protrusions, and  28   a ,  28   b ,  28   c ,  28   d ,  28   e  and  28   f  are used for distinction. The reference numeral  29  is collectively used for the recesses, and  29   a ,  29   b ,  29   c ,  29   d ,  29   e , and  29   f  are used for distinction.  FIG. 16  shows an example in which the protrusion  28  is the same in thickness as the film thickness ta of the diffraction grating material layer  16  and the depths of the recesses  29   a ,  29   b ,  29   c ,  29   d ,  29   e ,  29   f  formed by removing the diffraction grating material layer  16  are not the same. That is, in the diffraction grating  27  of the first example in the modulator part  22  shown in  FIG. 16 , the example is such that the thickness of the protrusion  28  is uniform, and the depth of the recess  29  formed by removing the diffraction grating material layer  16  is non-uniform. 
     In  FIG. 16 , the interval between the dashed line  31   f  and the end of the protrusion  28   a  on the side to the emission end face  32  is zp, the same as the interval between the end of the protrusion  28   e  on the side to the emission end face  32  and the end of the protrusion  28   f  on the side to the emission end face  32 . Note that, the end of the protrusion  28   f  on the side to the emission end face  32  coincides with the emission end face  32 . The interval between the end of the protrusion  28   a  on the side to the emission end face  32  and the end of the protrusion  28   b  on the side to the emission end face  32 , the interval between the end of the protrusion  28   b  on the side to the emission end face  32  and the end of the protrusion  28   c  on the side to the emission end face  32 , the interval between the end of the protrusion  28   c  on the side to the emission end face  32  and the end of the protrusion  28   d  on the side to the emission end face  32 , and the interval between the end of the protrusion  28   d  on the side to the emission end face  32  and the end of the protrusion  28   e  on the side to the emission end face  32  each are zp. The interval zp is the interval in the protrusion  28  and recess  29  that are adjacent in a pair, which is the same.  FIG. 16  shows an example in which the width of the protrusion  28  in the z-direction and the width of the recess  29  in the z-direction are the same. In the diffraction grating  27  of Embodiment 3, the period in the protrusion  28  and the recess  29  is the same over the entire diffraction grating. That is, in the diffraction grating  27  of Embodiment 3, the period in the protrusion  28  and the recess  29  is uniform. However, in the diffraction grating  27  of Embodiment 3, the depth of the recess  29  as described above or/and the thickness of the protrusion  28  are non-uniform in the diffraction grating region  24  of the modulator part  22 . 
     When the depth of a groove, that is, the recess  29  of the diffraction grating or/and the thickness of the portion in which the groove is not formed, that is, the protrusion  28 , is changed, an equivalent refractive index of the diffraction grating changes. A diffraction grating having a non-uniform equivalent refractive index with respect to the z-direction expands the wavelength band of light reflection of the diffraction grating as compared with the diffraction grating having a uniform equivalent refractive index. That is, the semiconductor laser device  50  of Embodiment 3 provided with the diffraction grating  27  in which the equivalent refractive index is non-uniform with respect to the z-direction in the modulator part  22  can expand the wavelength band of light reflection that occurs at the diffraction grating as compared with the semiconductor laser device  50  of Embodiment 1 provided with the diffraction grating  18  in which the equivalent refractive index is uniform with respect to the z-direction in the modulator part  22 . Thus, in the semiconductor laser device  50  of Embodiment 3, even when the drive conditions of the laser part  21  or/and the modulator part  22  change or when the element size of the semiconductor laser device  50  changes due to the manufacturing variation, the output light generated from the laser part  21  can be reliably reflected by the diffraction grating  27  of the modulator part  22  toward the laser part  21 . That is, in the semiconductor laser device  50  of Embodiment 3, the effects of the semiconductor laser device  50  of Embodiment 1 that the variation of the amount of adiabatic chirp for each semiconductor laser device can be suppressed and the decrease in the transmission yield can be avoided can be obtained even in one or both of the case where the drive conditions change and the case where the manufacturing variation occurs as compared with the semiconductor laser device  50  of Embodiment 1. That is, the semiconductor laser device  50  of Embodiment 3 is more resistant to the change in the drive conditions and the manufacturing variation than the semiconductor laser device  50  of Embodiment 1. 
     The semiconductor laser device  50  of Embodiment 3 has the same structure as the semiconductor laser device  50  of Embodiment 1 except for the diffraction grating of the modulator part  22 . Therefore, since the semiconductor laser device  50  of Embodiment 3 has the structure in which at least the diffraction grating  27  is not formed in the separation part  23  or the structure in which the non-diffraction grating region  25  in which the diffraction grating  27  is not formed exists between the emission end face of the laser part  21  and the emission end face  32 , as in the semiconductor laser device  50  of Embodiment 1, it is possible to increase the optical output of the laser light as compared with the semiconductor laser device of Patent Document 1. 
     The semiconductor laser device  50  of Embodiment 3 is provided with the diffraction grating  27  having the non-uniform equivalent refractive index with respect to the z-direction in the mesa stripe  15  of the modulator part  22 , so that reflected light that occurs at the forward side of the element of the semiconductor laser device  50 , that is, at the emission end face  32 , is directed to a position away from the absorption layer  5  and the active layer  4  by the diffraction grating  27  of the modulator part  22  in which the wavelength band of light reflection is expanded. Therefore, the propagation of the reflected light from the forward side of the element to the active layer  4  of the laser part  21  can be suppressed, the variation in the amount of adiabatic chirp for each semiconductor laser device can be suppressed, and the decrease in the transmission yield can be avoided. 
     In the semiconductor laser device  50  of Embodiment 3, similarly to the semiconductor laser device  50  of Embodiment 1, it is not necessary to add the wave guide provided with the diffraction grating in the forward side of the modulator part  22 , that is, on the side of the emission end face  32 . Therefore, the semiconductor laser device  50  of Embodiment 3 is easy in the manufacturing process and has a smaller element size than the semiconductor laser device having the structure in which the waveguide provided with the diffraction grating is added, so that the cost can be reduced. 
     The diffraction grating  27  whose equivalent refractive index is non-uniform with respect to the z-direction is not limited to the structure of  FIG. 16 . As in the diffraction grating  27  of the second example in the modulator part  22  shown in  FIG. 18 , the thickness of the protrusion  28  may be non-uniform, and the depth of the recess  29  formed by removing the diffraction grating material layer  16  may be uniform. Further, as in the diffraction grating  27  of the third example in the modulator part  22  shown in  FIG. 19 , the thickness of the protrusion  28  may be non-uniform, and the depth of the recess  29  formed by removing the diffraction grating material layer  16  may be non-uniform.  FIG. 18  shows an example in which the protrusion  28  increases in thickness in accordance with the position in the positive direction of the z-direction and the depth of the recess  29  is uniform.  FIG. 19  shows an example in which the protrusion  28  of the two in a pair increases in thickness in accordance with the position on the positive side in the z-direction, and the recess  29  of the two in a pair has a deep recess and a shallow recess. 
     The thicknesses of the protrusions  28   a ,  28   b ,  28   c ,  28   d ,  28   e  and  28   f  are ha, hb, hc, hd, he and hf, respectively, and the depths of the recesses  29   a ,  29   b ,  29   c ,  29   d ,  29   e  and  29   f  are da, db, dc, dd, de and df, respectively. The thickness of the protrusion  28  is the thickness on the positive side (upper side in  FIG. 18  and  FIG. 19 ) in the y-direction with respect to the dashed line  34   a . The depth of the recess  29  is the depth on the negative side (lower side in  FIG. 18  and  FIG. 19 ) in the y-direction with respect to the dashed line  34   b . For the diffraction grating  27  of the second example in the modulator part  22  shown in  FIG. 18 , a formula (2) and a formula (3) are satisfied, and for the diffraction grating  27  of the third example in the modulator part  22  shown in  FIG. 19 , a formula (4) and a formula (5) are satisfied. 
       hf&gt;he&gt;hd&gt;hc&gt;hb&gt;ha  (2)
 
       df=de=dd=dc=db=da  (3)
 
       ( hf=he )&gt;( hd=hc )&gt;( hb=ha )  (4)
 
       ( df=dd=db )&lt;( de=dc=da )  (5)
 
     In the formula (4) and the formula (5), the equal thickness or depth is enclosed by parentheses, and the relationship between the sizes for each of the parentheses is clarified. 
     In  FIGS. 18 and 19 , similarly to  FIG. 16 , the interval between the dashed line  31   f  and the end of the protrusion  28   a  on the side to the emission end face  32  is zp, similarly to the interval between the end of the protrusion  28   e  on the side to the emission end face  32  and the end of the protrusion  28   f  on the side to the emission end face  32 . Note that, the end of the protrusion  28   f  on the side to the emission end face  32  coincides with the emission end face  32 . The interval between the end of the protrusion  28   a  on the side to the emission end face  32  and the end of the protrusion  28   b  on the side to the emission end face  32 , the interval between the end of the protrusion  28   b  on the side to the emission end face  32  and the end of the protrusion  28   c  on the side to the emission end face  32 , the interval between the end of the protrusion  28   c  on the side to the emission end face  32  and the end of the protrusion  28   d  on the side to the emission end face  32 , and the interval between the end of the protrusion  28   d  on the side to the emission end face  32  and the end of the protrusion  28   e  on the side to the emission end face  32  each are zp. The interval zp is the interval in the protrusion  28  and recess  29  that are adjacent in a pair, which is the same.  FIG. 18  and  FIG. 19  show an example in which the width of the protrusion  28  in the z-direction and the width of the recess  29  in the z-direction are the same. 
     Note that, although various exemplary embodiments and examples are described in the present application, various features, aspects, and functions described in one or more embodiments are not inherent in a particular embodiment and can be applicable alone or in their various combinations to each embodiment. Accordingly, countless variations that are not illustrated are envisaged within the scope of the art disclosed herein. For example, the case where at least one component is modified, added or omitted, and the case where at least one component is extracted and combined with a component in another embodiment are included. 
     DESCRIPTION OF REFERENCE NUMERALS AND SIGNS 
       1 : semiconductor substrate,  4 : active layer,  5 : absorption layer,  11 : cathode electrode,  13 : first anode electrode,  14 : second anode electrode,  15 : mesa stripe,  16 : diffraction grating material layer,  17 : diffraction grating,  18 : diffraction grating,  21 : laser part,  22 : modulator part,  23 : separation part,  25 : non-diffraction grating region,  26 : diffraction grating,  27 : diffraction grating,  28 ,  28   a ,  28   b ,  28   c ,  28   d ,  28   e ,  28   f : protrusion,  29 ,  29   a ,  29   b ,  29   c ,  29   d ,  29   e ,  29   f  recess,  32 : emission end face,  50 : semiconductor laser device, za, zb, zc, zd: interval