Patent Publication Number: US-7899283-B2

Title: Optical device and method for manufacturing the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation Application of a PCT international application No. PCT/JP2008/055113 filed on Mar. 19, 2008 in Japan, the entire contents of which are incorporated by reference. 
    
    
     FIELD 
     The embodiments discussed herein are related to a waveguide optical device having a buried diffraction grating and a method for manufacturing the same. 
     BACKGROUND 
     One type of a waveguide optical device having a buried diffraction grating includes a DFB laser made from compound semiconductors, for example. 
     In recent years, there are techniques to improve the laser characteristics of a DFB laser by varying the coupling coefficient, which determines the amount of feedback of a diffraction grating, along the direction of a cavity. 
     For example, there is technique to improve the stability of the longitudinal mode upon a higher power optical output, by reducing the coupling coefficient toward the center of the cavity, thereby reducing the hole burning in the longitudinal direction. 
     There are other techniques to prevent occurrence of the hole burning. For example, there is a technique to gradually reduce the width of the buried diffraction grating toward the center of the cavity. Also, there is a technique to gradually increase the width of the buried diffraction grating toward the center of the cavity. Furthermore, there is a technique to gradually increase the height of the buried diffraction grating toward the center of the cavity. Also, there is a technique to gradually reduce the height of the buried diffraction grating toward the center of the cavity. 
     In addition, there is a technique to increase the threshold gain difference or the gain difference between the main and side modes, using a structure wherein the coupling coefficient is increased at the center of the cavity but is reduced at ends, as compared to the center. 
     Furthermore, there are a techniques to narrow the spectral line width by increasing the length of the cavity, when a DFB laser is used as an FM modulation light source, by dividing the drive electrode into three parts along the direction of the cavity, and modulating the injection current of the center electrode. 
     SUMMARY 
     According to one aspect of the embodiment, an optical device includes: an optical waveguide; and a plurality of diffraction grating layers provided along the optical waveguide, wherein each of the diffraction grating layers comprises a diffraction grating, each diffraction grating comprising a discontinuous first semiconductor layer and a second semiconductor layer burying the first semiconductor layer, the first and second semiconductor layers having different refractive indices, the plurality of diffraction grating layers comprise at least two diffraction grating layers being different from each other in terms of the length of a region where the diffraction grating is provided, and the diffraction gratings in an overlap region of the plurality of diffraction grating layers have the same phase and period. 
     In addition, according to another aspect of the embodiment, a method for manufacturing an optical device includes: stacking a plurality of layers above a substrate, a plurality of layers comprising a first layer and a second layer; forming a first mask having a diffraction grating pattern on the plurality of layers; transferring the diffraction grating pattern to the first layer by etching the first layer using the first mask; forming a second mask so as to cover a part of the first mask; transferring the diffraction grating pattern to the second layer by etching the second layer using the first and second masks; removing the first and second masks; and forming a plurality of diffraction grating layers by burying a third layer. 
     The object and advantages of the embodiment will be realized and attained by means of the elements and combinations particularly pointed out in the claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a structure of an optical device (DFB laser) according to a first embodiment; 
         FIGS. 2A-2E  are schematic perspective views illustrating a method for manufacturing one exemplary configuration of the optical device (DFB laser) according to the first embodiment; 
         FIGS. 3A-3E  are schematic perspective views illustrating the method for manufacturing one exemplary configuration of the optical device (DFB laser) according to the first embodiment; 
         FIGS. 4A-4E  are schematic perspective views illustrating the method for manufacturing one exemplary configuration of the optical device (DFB laser) according to the first embodiment; 
         FIG. 5  is a diagram illustrating a structure of an optical device (DFB laser) according to a second embodiment; 
         FIGS. 6A-6E  are schematic perspective views illustrating a method for manufacturing one exemplary configuration of the optical device (DFB laser) according to the second embodiment; 
         FIGS. 7A-7E  are schematic perspective views illustrating the method for manufacturing one exemplary configuration of the optical device (DFB laser) according to the second embodiment; 
         FIGS. 8A-8E  are schematic perspective views illustrating the method for manufacturing one exemplary configuration of the optical device (DFB laser) according to the second embodiment; 
         FIG. 9  is a diagram illustrating another exemplary structure of an optical device (DFB laser) according to the second embodiment; 
         FIG. 10  is a diagram illustrating a structure of an optical device (DFB laser) according to a third embodiment; 
         FIGS. 11A-11E  are schematic perspective views illustrating a method for manufacturing one exemplary configuration of the optical device (DFB laser) according to the third embodiment; 
         FIGS. 12A-12E  are schematic perspective views illustrating the method for manufacturing one exemplary configuration of the optical device (DFB laser) according to the third embodiment; 
         FIGS. 13A-13E  are schematic perspective views illustrating the method for manufacturing one exemplary configuration of the optical device (DFB laser) according to the third embodiment; and 
         FIG. 14  is a diagram illustrating another exemplary structure of an optical device (DFB laser) according to the first embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In a DFB laser having upper and lower buried diffraction gratings with an active layer sandwiched therebetween, when one of the buried diffraction grating is provided in the vicinity of the front end face of the laser so as to reflect a reflected returning light of the laser light from the outside, and the upper and lower diffraction gratings have different phases so that the oscillation wavelength of a single wavelength is obtained, a precise fabrication while synchronizing the phases of the upper and lower diffraction gratings is difficult. 
     When varying the coupling coefficient of the diffraction grating with the cavity, in order to improve the device characteristics, it is required to increase the difference in the coupling coefficient between a region with an increased coupling coefficient and a region with a reduced coupling coefficient (the contrast of the coupling coefficient is required to be increased). 
     However, for increasing the coupling coefficient difference between the increased coupling coefficient region and the reduced coupling coefficient region, by using a surface diffraction grating that is fabricated by forming grooves on the surface of an InP substrate and burying them with a semiconductor layer, formation of quite shallow diffraction grating is necessary at the reduced coupling coefficient region. 
     Since precise fabrication of such very shallow diffraction grating is difficult, the variation in the coupling coefficient may occur, resulting in deviation in the device characteristics (i.e., the threshold current of the laser, in this case). Additionally, the yield is low. 
     In addition, if the width of the buried diffraction grating is varied, the width of the diffraction grating in the region with the largest coupling coefficient is set to the half of the period of the diffraction grating (a duty ratio of 50%), and the diffraction grating in the reduced coupling coefficient region is formed by widening (greater than a duty ratio of 50%) or narrowing (smaller than a duty ratio of 50%) the width of the diffraction grating. 
     However, for increasing the coupling coefficient difference between the increased and reduced coupling coefficient regions, the width of diffraction grating in the reduced coupling coefficient region is required to be widened or narrowed significantly. 
     For significantly increasing the width of the diffraction grating, the size of openings forming in a mask for forming the diffraction grating is reduced significantly, and formation of the diffraction grating by etching becomes difficult. On the other hand, for significantly reducing the width of the diffraction grating, the width of an etching mask should also be reduced significantly, making stable and precise formation of a mask with a very narrow width [the duty ratio of several percents (%)] difficult. Supposing formation of the diffraction grating with a very narrow width would be possible, oftentimes, the diffraction grating with the very narrow width may disappear after buried, making reliable fabrication of a buried diffraction grating difficult. Naturally, the yield is low. 
     In view of the above issues, in the optical device having a structure wherein the coupling coefficient of the diffraction grating is varied within the cavity, it is desirable to fabricate the diffraction gratings precisely and reliably, thereby improving the yield. In addition, it is desirable to increase the difference in the coupling coefficient between the increased and reduced coupling coefficient regions (the contrast of the coupling coefficient is increased), thereby improving the device characteristics. 
     Hereinafter, an optical device according to embodiments and a method for manufacturing the same will be described with reference to the drawings. 
     First Embodiment 
     An optical device and a method for manufacturing the same according to a first embodiment will be described with reference to  FIGS. 1-4E . 
     The optical device according to this embodiment is a distributed feed-back (DFB) laser (laser device; waveguide optical device; active optical device; light emitting device) having a structure wherein the coupling coefficient of diffraction grating is varied within the cavity, for example, and includes an optical waveguide  1  and a plurality of (two, in this example) diffraction grating layers  2  and  3  provided along the optical waveguide  1 , as depicted in  FIG. 1 . 
     As depicted in  FIG. 1 , the first diffraction grating layer  2  and the second diffraction grating layer  3  are provided as a plurality of diffraction grating layers, and the diffraction grating layers  2  and  3  are provided below the optical waveguide  1  (the substrate side with respect to the optical waveguide  1 ; one side of the optical waveguide  1 ) . Note that the diffraction grating layers  2  and  3  are disposed in the vicinity of the optical waveguide  1 . 
     The diffraction grating layers  2  and  3  include diffraction gratings (buried diffraction gratings; buried-type diffraction grating)  2 A and  3 A having discontinuous first semiconductor layers  102  and  104 , and second semiconductor layers  103  and  107  burying the first semiconductor layers  102  and  104 , respectively, as depicted in  FIG. 1 . Here, the first semiconductor layers  102  and  104  have a refractive index different from that of the second semiconductor layers  103  and  107 . The discontinuously remained portions of the first semiconductor layer  102  and the portions of the second semiconductor layer  103  formed in the grooves of the first semiconductor layer  102  constitute the diffraction grating  2 A. The continuously remained portions of the first semiconductor layer  102  do not constitute the diffraction grating  2 A. That is, the diffraction grating layer  2  includes a grating region where the diffraction grating  2 A constituted of the first semiconductor layer  102  and the second semicondoctor layer  103  is provided, and an non-grating region where the diffraction grating  2 A is not provided and which is constituted of the first semiconductor layer  102 . In addition, the discontinuously remained portions of the first semiconductor layer  104  and the portions of the second semiconductor layer  107  formed in the grooves of the first semiconductor layer  104  constitute the diffraction grating  3 A. That is, the diffraction grating layer  3  includes a grating region where the diffraction grating  3 A constituted of the first semiconductor layer  104  and the second semicondoctor layer  107  is provided. 
     The diffraction gratings  2 A and  3 A in the diffraction grating layers  2  and  3  are provided to overlap each other, and the diffraction gratings  2 A and  3 A at an overlap region (a corresponding region) have the same phase, period, and duty ratio. Here, the term “duty ratio” means the ratio of the remained portion after etching with respect to the period of the diffraction grating. Note that, in this embodiment, the duty ratio of each of the diffraction gratings  2 A and  3 A provided in the diffraction grating layers  2  and  3  is constant. 
     In this embodiment, as depicted in  FIG. 1 , the diffraction grating  2 A in the first diffraction grating layer  2  is provided only at the center region in the direction along the optical waveguide  1  (the direction of the length of the cavity). In other words, the region of the first diffraction grating layer  2  where the diffraction grating  2 A is provided, is the center region in the direction along the optical waveguide  1 . 
     In addition, as depicted in  FIG. 1 , the diffraction grating  3 A in the second diffraction grating layer  3  is formed along the entire length of the direction along the optical waveguide  1 . In other words, the region where the diffraction grating  3 A in the second diffraction grating layer  3  is provided, is the entire region of the direction along the optical waveguide  1 . 
     Thus, in this embodiment, the length of the region of the first diffraction grating layer  2  where the diffraction grating  2 A is provided in the direction along the optical waveguide  1 , is shorter than the length of the region of the second diffraction grating layer  3  where the diffraction grating  3 A is provided in the direction along the optical waveguide  1 . The lengths of the regions where the diffraction gratings  2 A and  3 A are provided, are different between the first diffraction grating layer  2  and the second diffraction grating layer  3 . In this case, the overlap region of the diffraction grating layers  2  and  3  is the center region in the direction along the optical waveguide  1 . 
     Note that, although two diffraction grating layers  2  and  3  are provided as a plurality of diffraction grating layers and the lengths of the regions of the diffraction grating layers  2  and  3  where the diffraction gratings  2 A and  3 A are provided, are different from each other, these are not limiting. For example, a third diffraction grating layer may be add for increasing the coupling coefficient at the center region in the direction along the optical waveguide  1 . Here, the length of the region of the third diffraction grating layer where the diffraction grating is provided in the direction along the optical waveguide  1  is the same as that of the first diffraction grating layer  2 . Thus, it is suffice that the plurality of diffraction grating layers include at least two diffraction grating layers being different from each other in terms of the length of the region where the diffraction grating is provided. 
     As described above, in this embodiment, using buried diffraction gratings, a plurality of diffraction grating layers including region where a buried diffraction grating is provided, are stacked. At the region where maximizing the coupling coefficient is desired, a buried diffraction grating is provided in all of the stacked diffraction grating layers (two layers, i.e., the first diffraction grating layer  2  and the second diffraction grating layer  3 , in this example). At the region where a reduced coupling coefficient is desired, a diffraction grating is provided only in a part of the stacked diffraction grating layers (the second diffraction grating layer  3 , in this example). 
     More specifically, as depicted in  FIG. 1 , the second diffraction grating layer  3  is stacked above the first diffraction grating layer  2 , and the diffraction gratings  2 A and  3 A in the diffraction grating layers  2  and  3  are stacked at the center region in the direction along the optical waveguide  1 . In other words, the number of stacked diffraction gratings is varied in the direction along the optical waveguide  1 . As a result, the coupling coefficient at the center region in the direction along the optical waveguide  1  is increased, whereas the coupling coefficient is decreased at the remaining region (closer end regions) as compared to the center region. 
     The DFB laser (optical semiconductor device) is a DFB laser oscillating at the 1.55 μm wavelength band. As depicted in  FIG. 1 , this DFB laser includes, above an n-doped InP substrate  101 , the first diffraction grating layer  2 , the second diffraction grating layer  3 , and the optical waveguide  1  including a quantum well active layer  108  as the waveguide core layer. The first diffraction grating layer  2  includes the buried diffraction grating  2 A, which is formed by burying an n-doped GaInAsP layer (e.g., with a composition wavelength of about 1.25 μm and a thickness of about 25 nm)  102 , which is discontinuous at the center region, with an n-doped InP layer (e.g., with a thickness of about 15 nm; burying layer)  103 , wherein the n-doped GaInAsP layer  102  and the n-doped InP layer  103  have different refractive indices. The second diffraction grating layer  3  includes the buried diffraction grating  3 A, which is formed by burying an n-doped GaInAsP layer (e.g., with a composition wavelength of about 1.15 μm and a thickness of about 20 nm)  104  that is discontinuous at the entire region, with an n-doped InP layer  107 , wherein the n-doped GaInAsP layer  104  and the n-doped InP layer  107  have different refractive indices. 
     As described above, in the specific exemplary configuration of this embodiment, the diffraction grating  2 A in the first diffraction grating layer  2  is formed from the n-doped GaInAsP layer  102  having a composition wavelength of about 1.25 μm and the n-doped InP layer  103 , and then the diffraction grating  3 A in the second diffraction grating layer  3  is formed from the n-doped GaInAsP layer  104  having a composition wavelength of about 1.15 μm and the n-doped InP layer  107 , so that the refractive index difference between the semiconductor layers  102  and  103  constituting the diffraction grating  2 A in the first diffraction grating layer  2  becomes greater than the refractive index difference between the semiconductor layers  104  and  107  constituting the diffraction grating  3 A in the second diffraction grating layer  3 . In other words, the coupling coefficient difference between the increased and reduced coupling coefficient regions is increased by increasing the coupling coefficient without increasing the thickness of the first diffraction grating layer  2 . 
     More specifically, the portions of the n-type InP burying layer  103  burying between the discontinuous n-type GaInAsP layer  102  constitute the diffraction grating  2 A, and the portions of the n-type InP burying layer  107  burying between the discontinuous n-type GaInAsP layer  104  constitute the diffraction grating  3 A. 
     Note that the portion of the n-type InP burying layer  103  formed above the n-type GaInAsP layer  102  constitutes a spacer layer  4  between the first diffraction grating layer  2  and the second diffraction grating layer  3 , and the portion of the n-type InP burying layer  107  formed above the n-type GaInAsP layer  104  constitutes a spacer layer (cladding layer)  5  between the second diffraction grating layer  3  and the active layer  108 . Note that minimizing the thickness of the spacer layer  4  is preferred, as long as the variation of an etching depth can be permissible. 
     In addition, in the specific exemplary configuration of this embodiment, the thickness of the n-doped GaInAsP layer  102  constituting the diffraction grating  2 A in the first diffraction grating layer  2  is about 25 nm, and the thickness of the n-doped GaInAsP layer  104  constituting the diffraction grating  3 A in the second diffraction grating layer  3  is about 20 nm, so that the first diffraction grating layer  2  and the second diffraction grating layer  3  have different thicknesses. 
     Furthermore, the refractive index difference between the semiconductor layers  102  and  103  constituting the diffraction grating  2 A in the first diffraction grating layer  2  is made greater than the refractive index difference between the semiconductor layers  104  and  107  constituting the diffraction grating  3 A in the second diffraction grating layer  3 , so that the first diffraction grating layer  2  and the second diffraction grating layer  3  have different refractive index differences. 
     Therefore, in the first diffraction grating layer  2  and the second diffraction grating layer  3 , although the diffraction gratings  2 A and  3 A have the same duty ratio, the coupling coefficients of the diffraction gratings  2 A and  3 A are different from each other. 
     In this manner, according to the configuration of this embodiment, since the contrast of the coupling coefficient can be increased by stacking the diffraction grating layers  2  and  3 , devices satisfying desired needs can be achieved, with improved the device characteristics. 
     Unlike this embodiment, one may attempt to increase the contrast of the coupling coefficient simply by using a surface diffraction grating (i.e., a diffraction grating that is made by forming grooves on a substrate surface and then burying a semiconductor layer therebetween, or a diffraction grating which is formed by forming grooves on a surface of a first semiconductor layer and then burying a second semiconductor layer therebetween), without using a stack of the diffraction grating layers, for example. In such a case, since, a depth of a diffraction grating in the increased coupling coefficient region should be made to a depth of about 17 nm while a depth of a diffraction grating in the reduced coupling coefficient region should be made to a depth of about 7 nm. In such a case, the coupling coefficient value may deviate by about 4 cm −1  even when the depths of both of the increased and reduced coupling coefficient regions in the diffraction grating deviate only by about 1 nm. 
     That is to say, if one attempts to form such a diffraction grating by a reactive ion etching using a mixed gas of ethane and hydrogen, for example, the time duration for etching about 7 nm becomes quite short, about 7.6 seconds, according to an etch rate of 55 nm/min, reported by M. Matsuda et al., “Reactively Ion Etched Nonuniform-Depth Grating for Advanced DFB Lasers”, 3rd International Conference on Indium Phosphide and Related Materials, Apr. 8-11, 1991, TuF.4, the entire content of which is incorporated herein by reference. However, controlling the time duration for generating the etching plasma, and the off timing of the high-frequency switch in the order of 0.1 second are almost impossible. 
     Thus, the deviation of an approximately ±1 second of the etch duration may be assumed, which causes the variation of the etching depth of about ±1 nm, in this case, resulting in the variation of the coupling coefficient of about ±4 cm −1 . 
     Assuming this processing precision in this hypothetical example, if the coupling coefficient within the entire cavity varies deviates to become smaller, the threshold gain is increased to about 1.4 times of the threshold gain without any deviation, causing an increase in the threshold current of the laser. 
     In contrast, according to this embodiment, since the contrast of the coupling coefficient can be increased by stacking the diffraction grating layers  2  and  3 , any significant reduction in the depth (thickness) of the diffraction gratings is not required for increasing the contrast of the coupling coefficient. Thus, an increase in the threshold current of the laser can be reduced by setting the thicknesses of the diffraction gratings so that the variation of the coupling coefficient falls within the allowable range. 
     Furthermore, if one attempts to increase the contrast of the coupling coefficient solely with a surface diffraction grating, for example, since very deep grooves and very shallow grooves are burried with a burying layer, a planarized surface cannot be obtained without increasing the thickness of the burying layer. However, a thicker burying layer cannot provide a desired coupling coefficient. 
     In contrast, according to this embodiment, since the contrast of the coupling coefficient can be increased by stacking the diffraction grating layers  2  and  3 , an increasing contrast of the coupling coefficient is obtained without burying very deep grooves and very shallow grooves with a burying layer. Accordingly, a planer surface can be obtained with a thinner burying layer, and thus a desired coupling coefficient can be achieved. 
     In addition, since this embodiment employs the buried diffraction gratings  2  and  3  formed by burying the discontinuous GaInAsP layers  102  and  104  with the InP layers  103  and  107 , respectively, the diffraction grating can be precisely and reliably fabricated and the yield can be improved. 
     Firstly, with regard to a surface diffraction grating, a precise control of the coupling coefficient is difficult since the depth of the diffraction grating is determined by the timing to stop the etching at some midpoint of the substrate or the semiconductor layer. 
     In contrast, with regard to a buried diffraction grating formed by burying a discontinuous first semiconductor layer with a second semiconductor layer, the etching to divide the first semiconductor layer is stopped at the timing when the entire first semiconductor layer is removed and an underlying semiconductor layer is removed to some midpoint thereof. Any variation in the etching depth is compensated by burying with a second semiconductor layer made from the same semiconductor material as the underlying semiconductor layer. Since, in this case, the depth of the diffraction grating is determined by the thickness of the first semiconductor layer, the coupling coefficient can be precisely controlled and the yield can be improved. 
     Secondly, typically, in case of a surface diffraction grating, since grooves are formed on the surface of the InP substrate and then the GaInAsP layer which is quaternary mixed crystal is grown thereon, compositional modulation and a crystal defect tend to be increased as the depth of the diffraction grating increases. Furthermore, the grooves formed on the InP substrate may deform due to mass transport when exposed to an elevated temperature during the crystal growth. As a result of those, it becomes difficult to obtain a coupling coefficient according to the design. 
     Furthermore, since a deeper diffraction grating is unrealistic, for obtaining a higher coupling coefficient, a quaternary mixed crystal semiconductor layer is required which provide a refractive index difference between InP as great as possible. In such a case, when a semiconductor material having a composition longer than a composition wavelength of about 1.3 μm is used, light absorption is increased, resulting in deterioration of the laser characteristics. 
     In contrast, with regard to the buried diffraction gratings  2  and  3  formed by burying the discontinuous GaInAsP layers  102  and  104  with the InP layer (burying layer)  103  and  107 , since the GaInAsP quaternary mixed crystal layer is buried with the InP buried layer, substantially no deformation of the discontinuous GaInAsP layers  102  and  104  occurs. 
     Furthermore, unlike a surface diffraction grating, no compositional modulation (refractive index modulation) occurs even if the grooves formed in the GaInAsP layers  102  and  104  are deep since they are buried with the InP buried layers  103  and  107 . 
     For the above reasons, the designed coupling coefficient can be obtained, and the yield can be improved. In addition, since the contrast of the coupling coefficient can be increased by stacking the diffraction grating layers  2  and  3 , no significant increase in the depth of the grooves formed in the GaInAsP layer is required for increasing the contrast of the coupling coefficient. Therefore, a cristal defect is unlikely to occur. Furthermore, no deterioration of the laser characteristics occurs since no a material having a significantly greater refractive index difference between InP is required. 
     In addition, since diffraction grating can be formed without any deformation, compositional modulation, crystal defect, or the like of the diffraction grating, a desired coupling coefficient can be obtained even with a thicker buried layer of about 50 nm or greater, and the surface of the burying layer can be planarized so as to be able to grow the active layer having a satisfactorily crystallinity crystallographically acceptable level when growing the active layer  108  on the surface of the InP burying layer  107 . 
     Furthermore, the value of the overall coupling coefficient can be fine-tuned by modifying the thickness of the InP burying layer  107  (the thickness of the spacer layer  5 ; the spacing between the active layer  108  and the second diffraction grating layer  3 ). 
     Now, a method for manufacturing the DFB laser according to the specific exemplary configuration of this embodiment will be described with reference to  FIGS. 2A-4E . 
     Firstly, as depicted in  FIG. 2A , on an n-doped InP substrate  101 , an n-doped GaInAsP layer (e.g., with a composition wavelength of about 1.25 μm and a thickness of about 25 nm; a layer having a refractive index different from that of the substrate  101 )  102 , an n-doped InP layer (e.g., with a thickness of about 15 nm; a layer having the same refractive index as the substrate  101 )  103 , and an n-doped GaInAsP layer (e.g., with a composition wavelength of about 1.15 μm and a thickness of about 20 nm; a layer having a refractive index different from the substrate  101 )  104  are sequentially stacked, using a metal organic chemical vapor deposition (MOVPE), for example. Note that an n-doped InP cladding layer may be formed between the n-doped InP substrate  101  and the n-doped GaInAsP layer  102 . 
     Next, as depicted in  FIG. 2B , a mask  105  having a diffraction grating pattern and made from an electron beam resist (ZEP520 available from Zeon Corporation) is formed on the surface of the n-type GaInAsP layer  104  by electron beam exposure technique, for example. 
     Note that the diffraction grating pattern formed in the mask  105  includes a pattern for forming a phase shift having a phase of n radian (λ/4 phase shift) at the center of the cavity for each device. 
     Subsequently, the n-type GaInAsP layer  104  and a portion of n-type InP layer  103  are removed using the mask  105  by a reactive ion etching (RIE) using a mixed gas of ethane and hydrogen, for example. 
     In this example, as depicted in  FIG. 2C , the etching is stopped at some midpoint of the n-type InP layer  103  after the n-type GaInAsP layer  104  is divided. As a result, the diffraction grating pattern is transferred to the entire surface of the n-type GaInAsP layer  104 , and the n-type GaInAsP layer  104  is divided. 
     Next, as depicted in  FIG. 2D , a positive photoresist (OFPR8600 available from Tokyo Ohka Kogyo., Ltd.; e.g., with a thickness of about 300 nm)  106  is applied on the surface, for example. Note that no deformation of the mask  105  occurs since the electron beam resists forming the mask  105  and the positive photoresist  106  are immiscible. 
     Next, as depicted in  FIG. 2E , a portion of the positive photoresist  106  (the center portion of the cavity; the center region in the direction along the optical waveguide, in this example) is removed using a conventional photolithographic technique to form a positive photoresist mask  106 A, covering the end portions in the direction along the optical waveguide (covering the surface of a portion of the mask  105 ; covering the surface of the regions corresponding to the end regions of the optical waveguide). 
     Thereafter, using the electron beam resist mask  105 , which is exposed to the surface again, and the positive photoresist mask  106 A, the remained portion of the n-type InP layer  103 , the n-type GaInAsP layer  102 , and a portion of the n-type InP substrate  101  are removed by a reactive ion etching (RIE) using a mixed gas of ethane and hydrogen, for example. 
     In this example, as depicted in  FIG. 3A , the etching is stopped at some midpoint of the n-type InP substrate  101  (at an etching depth of about 10 nm, in this example) after the n-type InP layer  103  and the n-type GaInAsP layer  102  are divided. 
     As a result, the diffraction grating pattern is transferred to a part of the n-type GaInAsP layer  102  (the center portion of the cavity; the center region in the direction along the optical waveguide, in this example), and the n-type GaInAsP layer  102  is divided. 
     In this case, since the diffraction grating pattern provided in the n-type GaInAsP layer  104  and the diffraction grating pattern provided in the n-type GaInAsP layer  102  are formed using the same mask  105 , the diffraction gratings  2 A and  3 A in the overlap region of a first diffraction grating layer  2  and a second diffraction grating layer  3 , which are to be formed as will be described later, will have the same phase, period, and duty ratio. 
     Note that, in this example, the thickness and the refractive index of the n-type GaInAsP layer  102  (the thickness of and the refractive index difference in the first diffraction grating layer  2 ) are different from the thickness and the refractive index of the n-type GaInAsP layer  104  (the thickness of and the refractive index difference in the second diffraction grating layer  3 ). As a result, the coupling coefficient of the diffraction grating  2 A provided in the first diffraction grating layer  2  is different from the coupling coefficient of the diffraction grating  3 A provided in the second diffraction grating layer  3 . 
     Each of the diffraction grating layers  2  and  3  has a constant duty ratio within the layer. In this case, since the width of the diffraction grating pattern of the etching mask (mask pattern) does not require any modification, the processing precision of the diffraction grating is stabilized. Since each of the diffraction grating layers  2  and  3  has a constant thickness and refractive index difference within the layer, the coupling coefficient of the diffraction grating within the layer is constant. 
     As depicted in  FIG. 3B , the mask  105  and the mask  106 A are removed from the surface using a conventional resist removal technique. 
     Subsequently, as depicted in  FIG. 3C , an n-doped InP layer (a layer having the same refractive index as the substrate  101 )  107  is grown over the entire surface, using MOVPE, for example. The grooves formed by stopping the etching at some midpoint of the n-type InP layer  103  and the grooves formed by stopping the etching at some midpoint of the n-type InP substrate  101  are buried with the n-type InP layer  107 . 
     As a result, the discontinuous the n-type GaInAsP layer  102  is buried with the n-type InP layer  107 , thereby the first diffraction grating layer  2  is formed, a portion (the center portion of the cavity; the center region in the direction along the optical waveguide, in this example) of which is provided with the diffraction grating  2 A. In addition, the discontinuous the n-type GaInAsP layer  104  is buried with the n-type InP layer  107 , thereby the second diffraction grating layer  3  is formed, the entirety (the entire length along the optical waveguide) of which is provided with the diffraction grating  3 A. 
     Furthermore, the spacer layer  4  is formed between the first diffraction grating layer  2  and the second diffraction grating layer  3  by the n-type InP layer  103  and the a portion of n-type InP layer  107 . In addition, the spacer layer  5  is formed above the second diffraction grating layer  3  by a portion of the n-type InP layer  107 . 
     Next, as depicted in  FIG. 3D , a quantum well active layer  108  and a p-type doped InP cladding layer (e.g., with a thickness of about 250 nm)  109  are stacked sequentially by MOVPE, for example. 
     In this example, the quantum well active layer  108  is made using a GaInAsP-based compound semiconductor material. In other words, the quantum well active layer  108  includes an undoped GaInAsP quantum well layer (e.g., with a thickness of about 5.1 nm and a compressive strain amount of about 1.0%), and an undoped GaInAsP barrier layer (e.g., with a composition wavelength of about 1.2 μm and a thickness of about 10 nm), the layer number of the quantum well layer being six, and the emission wavelength thereof being about 1560 nm. 
     Note that undoped GaInAsP-separate confinement heterostructure (SCH) layers (light guide layers; e.g., with a wavelength of about 1.15 μm and a thickness of about 20 nm) may be provided above and below the quantum well active layer  108 , sandwiching the quantum well active layer  108 . 
     Thereafter, as depicted in  FIG. 3E , a mask (e.g., a stripe etching mask having a thickness of about 400 nm and a width of about 1.6 μm)  110  made from SiO 2  is formed over the semiconductor surface using a conventional chemical vapor deposition (CVD) and photolithography technique. 
     As depicted in  FIG. 4A , the resultant semiconductor stack structure is etched such that the n-type InP substrate  101  is etched by about 0.7 μm, for example, using dry etching, for example, to form a stripe mesa structure (mesa stripes). 
     Next, as depicted in  FIG. 4B , current blocking layers, made from a p-type InP layer  111 , an n-type InP layer  112 , and a p-type InP layer  113 , are grown on opposite of the mesa structure using MOVPE, for example, and the etching mask  110  is removed with hydrofluoric acid, for example. Thereafter, as depicted in  FIG. 4C , a p-type InP cladding layer (e.g., with a thickness of about 2.2 μm)  114  and a p-type GaInAs contact layer (e.g., with a thickness of about 300 nm)  115  are sequentially grown using MOVPE, for example. 
     As depicted in  FIG. 4D , after forming a p-side electrode  116  and an n-side electrode  117 , as depicted in  FIG. 4E , anti-reflection coatings  118  and  119  are formed on the both end face sides of the device, thereby obtaining a completed device. 
     Thus, according to the optical device (DFB laser) and the method for manufacturing the same of this embodiment, in the optical device having a structure wherein the coupling coefficient of the diffraction grating is varied within the cavity, the diffraction grating can be precisely and reliably fabricated, thereby improving the yield. In addition, the difference in the coupling coefficient can be increased between the increased and reduced coupling coefficient regions (the contrast of the coupling coefficient can be increased), thereby improving the device characteristics. 
     Furthermore, since the diffraction grating can be precisely and reliably fabricated, any variation in the device characteristics can be minimized, thereby improving the device characteristics. The controllability of the coupling coefficient and flexibility in designing the coupling coefficient are also improved. 
     Especially, according to the configuration of the above-described embodiment, since any of the refractive index difference between the two semiconductor layers constituing each diffraction grating layer, the thickness of each diffraction grating layer, and each spacing between respective diffraction grating layers (thickness of the spacer layer) can be desirably set, a diffraction grating having coupling coefficient of a significantly wider range, ranging from several cm −1  to several hundreds of cm −1 , for example, can be fabricated precisely. In this case, the range of coupling coefficient that can be designed is widened as the layer number of the diffraction grating layers is increased. 
     In addition, in the manufacturing method of the above-described embodiment, the lower diffraction grating layer of the plurality of diffraction grating layers have a region where the diffraction grating is not provided. The range of coupling coefficient that can be designed is increased as more diffraction grating layers are disposed on the substrate side with respect to the optical waveguide. 
     Note that the DFB laser that are constructed as described above can also be used as an FM modulated light source, for example, by dividing the drive electrode into three parts along the direction of the cavity and modulating the injection current into the center electrode, as disclosed in S. Ogita et al., “FM Response of Narrow-Linewidth, Multielectrode λ/4 Shift DFB Laser”, IEEE Photonics Technology Letters, vol. 2, no. 3, March 1990, pp. 165-166, the entire content of which is incorporated herein by the reference, or Japanese Patent No. 2966485, the entire content of which is incorporated herein by the reference. 
     Such a laser light source can be used, for example, for coherent optical transmission. Although improvement in the modulation efficiency requires increasing the range of the modulation current into the center electrode, excessive increase in the range of the modulation current may amplify the influence of the hole burning in the longitudinal direction, impairing the stability of the single mode. Accordingly, for improving the efficiency in the FM modulation, the structure as described above, namely, the structure wherein the coupling coefficient is increased at the center of the cavity and the coupling coefficient is reduced at the ends as compared to the center can be used, as a structure to suppress any reduction in the gain difference between the main and side modes even when the hole burning in the longitudinal direction is increased (see M. Ohashi et al., “Mode Analysis of DFB Laser Diode with Nonuniform Coupling Coefficients”, Fall 1989, the Annual Meeting of the Japan Society of Applied Physics, 30p-ZG-13, the entire content of which is incorporated herein by the reference). 
     According to this structure, the difference in the normalized threshold gain between the main and side modes in the longitudinal mode becomes about 1.7, which is about 2.4 times with respect to about 0.72 of a conventional λ/4 shifted DFB laser. Thus, unlike conventional laser, since the minimal normalized threshold gain difference of about 0.2, which is required for maintaining single mode operation, can be maintained even when the normalized threshold gain difference is reduced due to the influence of the hole burning in the longitudial direction during a high current injection (see H. Shoji et al., “Theoretical Analysis of λ/4 shifted DFB Lasers with Nonuniform-Depth Grating”, Fall 1991, the Annual Meeting of the Japan Society of Applied Physics, 10p-ZM-17, the entire content of which is incorporated herein by the reference), the efficiency in the FM modulation can be improved while maintaining a stable single longitudinal mode operation (see Y. Kotaki et al., “MQW-DFB Laser with Nonuniform-Depth Grating, Spring 1991, the Annual Meeting of the Japan Society of Applied Physics, 29p-D-7, the entire content of which is incorporated herein by the reference). 
     In the DFB laser of the above-described embodiment, the difference in the normalized threshold gain between the main and side modes in the longitudinal mode of abput 1.7 is obtained, which is about 2.4 times with respect to about 0.72 of a conventional λ/4 shifted DFB laser, by designing such that the normalized coupling coefficient κL at the center region, where the diffraction grating is two layers, becomes about 5, the normalized coupling coefficient κL at the end regions, where the diffraction grating is a single layer, becomes about 2, the ratio of the end regions, where the diffraction grating is a single layer, to the entire cavity length becomes about 0.18. Accordingly, a further stable single longitudinal mode operation can be achieved. 
     Second Embodiment 
     Now, an optical device and a method for manufacturing the same according to a second embodiment will be described with reference to  FIGS. 5-8E . 
     The optical device (DFB laser) and the method for manufacturing the same according to this embodiment are different from the above-described first embodiment in that three diffraction grating layers  20 ,  30 , and  40  are provided, that a quantum well active layer  211  is formed using an AlGaInAs-based compound semiconductor material, and that a semi-insulating buried heterostructure (SI-BH structure; high-resistance buried structure) is used, as depicted in  FIG. 5 ,  FIG. 8E . 
     In other words, the optical device according to this embodiment is a distributed feed-back (DFB) laser (laser device; waveguide optical device; active optical device; light emitting device; device for code division multiplexing communication) having a structure wherein the coupling coefficient of diffraction grating is varied within the cavity, for example, and includes an optical waveguide  1  and a plurality of (three, in this example) diffraction grating layers  20 ,  30 , and  40  provided along the optical waveguide  1 , as depicted in  FIG. 5 . 
     As depicted in  FIG. 5 , the first diffraction grating layer  20 , the second diffraction grating layer  30 , and the third diffraction grating layer  40  are provided as a plurality of diffraction grating layers, and the diffraction grating layers  20 ,  30 , and  40  are provided below the optical waveguide  1  (the substrate side with respect to the optical waveguide  1 ; one side of the optical waveguide  1 ). 
     The diffraction grating layers  20 ,  30 , and  40  include diffraction gratings (buried diffraction gratings; buried-type diffraction grating)  20 A,  30 A, and  40 A having discontinuous first semiconductor layers  202 ,  204 , and  206 , and second semiconductor layers  203 ,  207 , and  210  burying the first semiconductor layers  202 ,  204 , and  206 , respectively, as depicted in  FIG. 5 . Here, the first semiconductor layers  202 ,  204 , and  206  have a refractive index different from that of the second semiconductor layers  203 ,  207 , and  210 . The discontinuously remained portions of the first semiconductor layer  202  and the portions of the second semiconductor layer  203  formed in the grooves of the first semiconductor layer  202  constitute the diffraction grating  20 A. The continuously remained portions of the first semiconductor layer  202  do not constitute the diffraction grating  20 A. That is, the diffraction grating layer  20  includes a grating region where the diffraction grating  20 A constituted of the first semiconductor layer  202  and the second semicondoctor layer  203  is provided, and an non-grating region where the diffraction grating  20 A is not provided and which is constituted of the first semiconductor layer  202 . In addition, the discontinuously remained portions of the first semiconductor layer  204  and the portions of the second semiconductor layer  205  formed in the grooves of the first semiconductor layer  204  constitute the diffraction grating  30 A. The continuously remained portions of the first semiconductor layer  204  do not constitute the diffraction grating  30 A. That is, the diffraction grating layer  30  includes a grating region where the diffraction grating  30 A constituted of the first semiconductor layer  204  and the second semicondoctor layer  205  is provided, and an non-grating region where the diffraction grating  30 A is not provided and which is constituted of the first semiconductor layer  204 . Furthermore, the discontinuously remained portions of the first semiconductor layer  206  and the portions of the second semiconductor layer  210  formed in the grooves of the first semiconductor layer  206  constitute the diffraction grating  40 A. That is, the diffraction grating layer  40  includes a grating region where the diffraction grating  40 A constituted of the first semiconductor layer  206  and the second semicondoctor layer  210  is provided. 
     The diffraction gratings  20 A,  30 A, and  40  A in the diffraction grating layers  20 ,  30 , and  40  are provided to overlap each other, and the diffraction gratings  20 A,  30 A, and  40 A at an overlap region have the same phase, period, and duty ratio. Here, the term “duty ratio” means the ratio of the remained portion after etching with respect to the period of the diffraction grating. Note that, in this embodiment, the duty ratio of each of the diffraction gratings  20 A,  30 A, and  40 A provided in the diffraction grating layers  20 ,  30 , and  40  is constant. 
     In this embodiment, as depicted in  FIG. 5 , the diffraction grating  20 A in the first diffraction grating layer  20  is provided only at the closer center region in the direction along the optical waveguide  1  (the direction of the length of the cavity). In other words, the region where the diffraction grating  20 A in the first diffraction grating layer  20  is provided, is the closer center region in the direction along the optical waveguide  1 . 
     In addition, as depicted in  FIG. 5 , the diffraction grating  30 A in the second diffraction grating layer  30  is provided only at the center region in the direction along the optical waveguide  1  (the direction of the length of the cavity). In other words, the region where the diffraction grating  30 A in the second diffraction grating layer  30  is provided, is the center region in the direction along the optical waveguide  1 . 
     Furthermore, as depicted in  FIG. 5 , the diffraction grating  40 A in the third diffraction grating layer  40  is formed along the entire length of the direction along the optical waveguide  1 . In other words, the region where the diffraction grating  40 A in the third diffraction grating layer  40  is provided, is the entire region of the direction along the optical waveguide  1 . 
     As described above, in this embodiment, as depicted in  FIG. 5 , the length of the region where the diffraction grating  20 A in the first diffraction grating layer  20  is provided in the direction along the optical waveguide  1 , is shorter than the length of the region of the second diffraction grating layer  30  where the diffraction grating  30 A is provided in the direction along the optical waveguide  1 . In addition, the length of the region where the diffraction grating  30 A in the second diffraction grating layer  30  is provided in the direction along the optical waveguide  1  is shorter than the length of the region of the third diffraction grating layer  40  where the diffraction grating  40 A is provided in the direction along the optical waveguide  1 . In other words, the lengths of the regions where the diffraction gratings  20 A,  30 A, and  40 A are provided, are different among the first diffraction grating layer  20 , the second diffraction grating layer  30 , and the third diffraction grating layer  40 . In this case, the overlap region of the diffraction grating layers  20 ,  30 , and  40  is the closer center region in the direction along the optical waveguide  1 . 
     Note that, although three diffraction grating layers  20 ,  30 , and  40  are provided as a plurality of diffraction grating layers and the lengths of the regions of the diffraction grating layers  20 ,  30 , and  40  where the diffraction gratings  20 A,  30 A, and  40 A are provided, are different from each other, these are not limiting. For example, a forth diffraction grating layer may be add for increasing the coupling coefficient at the center region or the closer center region in the direction along the optical waveguide  1 . Here, the length of the region of the forth diffraction grating layer where the diffraction grating is provided in the direction along the optical waveguide  1  is the same as that of the first diffraction grating layer  20  or the second diffraction grating layer  30 . Thus, it is suffice that the plurality of diffraction grating layers include at least two diffraction grating layers being different from each other in terms of the length of the region where the diffraction grating is provided. 
     As described above, in this embodiment, using buried diffraction gratings, a plurality of diffraction grating layers including region where a buried diffraction grating is provided, are stacked. At the region where maximizing the coupling coefficient is desired, a buried diffraction grating is provided in all of the stacked diffraction grating layers (three layers, i.e., the first diffraction grating layer  20 , the second diffraction grating layer  30 , and the third diffraction grating layer  40 , in this example). At the region where a reduced coupling coefficient is desired, a diffraction grating is provided in a step-wise manner only in part of the stacked diffraction grating layers (the second diffraction grating layer  30 , or the second diffraction grating layer  30  and the third diffraction grating layer  40 , in this example). 
     In this embodiment, as depicted in  FIG. 5 , the second diffraction grating layer  20  and the third diffraction grating layer  30  are stacked above the first diffraction grating layer  20 , and the diffraction gratings  20 A,  30 A, and  40 A in the diffraction grating layers  20 ,  30 , and  40  are stacked at the closer center region in the direction along the optical waveguide  1 , and the diffraction gratings  30 A and  40 A in the diffraction grating layers  30  and  40  are stacked at the center region in the direction along the optical waveguide  1 . In other words, the number of stacked diffraction gratings is varied in the direction along the optical waveguide  1 . As a result, the coupling coefficient at the closer center region in the direction along the optical waveguide  1  is increased, whereas the coupling coefficient is reduced in a step-wise manner toward the center region and the closer end regions. 
     More specifically, as depicted in  FIG. 5 , this DFB laser (optical semiconductor device) includes, above an n-doped InP substrate  201 , the first diffraction grating layer  20 , the second diffraction grating layer  30 , the third diffraction grating layer  40 , and the optical waveguide  1  including a quantum well active layer  211  as the waveguide core layer. The first diffraction grating layer  20  includes the buried diffraction grating  20 A, which is formed by burying an n-doped GaInAsP layer (e.g., with a composition wavelength of about 1.25 μm and a thickness of about 15 nm)  202 , which is discontinuous at the closer center region, with an n-doped InP layer (e.g., with a thickness of about 15 nm; burying layer)  203 , wherein the n-doped GaInAsP layer  202  and the n-doped InP layer  203  have different refractive indices. The second diffraction grating layer  30  includes the buried diffraction grating  30 A, which is formed by burying an n-doped GaInAsP layer (e.g., with a composition wavelength of about 1.25 μm and a thickness of about 20 nm)  204  that is discontinuous at the centre region, into an n-doped InP layer  205  (e.g., with a thickness of about 15 nm; burying layer), wherein the n-doped GaInAsP layer  204  and the n-doped InP layer  205  have different refractive indices. The third diffraction grating layer  40  includes the buried diffraction grating  40 A, which is formed by burying an n-doped GaInAsP layer (e.g., with a composition wavelength of about 1.15 μm and a thickness of about 15 nm)  206  that is discontinuous at the entire region, with an n-doped InP layer  210 , wherein the n-doped GaInAsP layer  206  and the n-doped InP layer  210  have different refractive indices. 
     As described above, in the specific exemplary configuration of this embodiment, the diffraction grating  20 A in the first diffraction grating layer  20  is formed from the n-doped GaInAsP layer  202  having a composition wavelength of about 1.25 μm and the n-doped InP layer  203 , and then the diffraction grating  40 A in the third diffraction grating layer  40  is formed from the n-doped GaInAsP layer  206  having a composition wavelength of about 1.15 μm and the n-doped InP layer  210 , so that the refractive index difference between the semiconductor layers  202  and  203  constituting the diffraction grating  20 A in the first diffraction grating layer  20  becomes greater than the refractive index difference between the semiconductor layers  206  and  210  constituting the diffraction grating  40 A in the third diffraction grating layer  40 . In this manner, the coupling coefficient difference between the increased and reduced coupling coefficient regions is increased by increasing the coupling coefficient without increasing the thickness of the first diffraction grating layer  20 . 
     In addition, the portions of the n-type InP burying layer  203  burying between the discontinuous n-type GaInAsP layer  202  constitute the diffraction grating  20 A, the portions of the n-type InP burying layer  205  burying between the discontinuous n-type GaInAsP layer  204  constitute the diffraction grating  30 A, and the portions of the n-type InP burying layer  210  burying between the discontinuous n-type GaInAsP layer  206  constitute the diffraction grating  40 A. Additionally, the portion of the n-type InP burying layer  203  formed above the n-type GaInAsP layer  202  constitutes a spacer layer  41  between the first diffraction grating layer  20  and the second diffraction grating layer  30 , the portion of the n-type InP burying layer  205  formed above the n-type GaInAsP layer  204  constitutes a spacer layer  41  between the second diffraction grating layer  30  and the third diffraction grating layer  40 , and the portion of the n-type InP burying layer  210  formed above the n-type GaInAsP layer  206  constitutes a spacer layer (cladding layer)  51  between the third diffraction grating layer  40  and the active layer  211 . Note that minimizing the thickness of the spacer layers  41  and  42  is preferred, as long as the variation of an etching depth can be permissible. 
     In the specific exemplary configuration of this embodiment, the thickness of the n-doped InP layer  203  is about 15 nm; the thickness of the n-doped InP layer  205  is about 15 nm; the thicknesses of the spacer layer  41  and the spacer layer  42  are equal; and the first diffraction grating layer  20 , the second diffraction grating layer  30 , and the third diffraction grating layer  40  are spaced apart with the same distance. 
     In addition, in the specific exemplary configuration of this embodiment, the thickness of the n-doped GaInAsP layer  202  constituting the diffraction grating  20 A in the first diffraction grating layer  20  is about 15 nm, the thickness of the n-doped GaInAsP layer  204  constituting the diffraction grating  30 A in the second diffraction grating layer  30  is about 20 nm, and the thickness of the n-doped GaInAsP layer  206  constituting the diffraction grating  40 A in the third diffraction grating layer  40  is about 15 nm, so that the first diffraction grating layer  20  or the third diffraction grating layer  40 , and the second diffraction grating layer  30  have different thicknesses. In other words, the plurality of diffraction grating layers include diffraction grating layers having different thicknesses, a part of the diffraction grating layers have different thicknesses. 
     Furthermore, the refractive index difference between the semiconductor layers  202  and  203  constituting the diffraction grating  20 A in the first diffraction grating layer  2 , or the refractive index difference between the semiconductor layers  204  and  205  constituting the diffraction grating  30 A in the second diffraction grating layer  30  is made greater than the refractive index difference between the semiconductor layers  206  and  210  constituting the diffraction grating  40 A in the third diffraction grating layer  40 , so that the first diffraction grating layer  20  or the second diffraction grating layer  30 , and the third diffraction grating layer  40  have different refractive index differences. In other words, the plurality of diffraction grating layers include diffraction grating layers having different refractive index differences, and two semiconductor layers constituting the diffraction grating in a part of the diffraction grating layers have different refractive index differences. 
     Therefore, in the first diffraction grating layer  20 , the second diffraction grating layer  30 , and the third diffraction grating layer  40 , although the diffraction gratings  20 A,  30 A, and  40 A have the same duty ratio, the diffraction gratings  20 A,  30 A, and  40 A have different coupling coefficients. 
     As described above, similar to the above-described first embodiment, according to the configuration of this embodiment, since the contrast of the coupling coefficient can be increased by stacking the diffraction grating layers  20 ,  30 , and  40 , devices satisfying desired needs can be achieved, with improved the device characteristics. 
     The details of other elements are similar to the configuration and the specific exemplary configuration of the above-described first embodiment, and descriptions thereof will be omitted. 
     Now, a method for manufacturing the DFB laser according to the specific exemplary configuration of this embodiment will be described with reference to  FIGS. 6A-8E . 
     Firstly, as depicted in  FIG. 6A , on an n-doped InP substrate  201 , an n-doped GaInAsP layer (e.g., with a composition wavelength of about 1.25 μm and a thickness of about 15 nm; a layer having a refractive index different from that of of the substrate  201 )  202 , an n-doped InP layer (e.g., with a thickness of about 15 nm; a layer having the same refractive index as the substrate  201 )  203 , an n-doped GaInAsP layer (e.g., with a composition wavelength of about 1.25 μm and a thickness of about 20 nm; a layer having a refractive index different from that of the substrate  201 )  204 , an n-doped InP layer (e.g., with a thickness of about 15 nm; a layer having the same refractive index as the substrate  201 )  205 , and an n-doped GaInAsP layer (e.g., with a composition wavelength of about 1.15 μm, a thickness of about 15 nm; a layer having a refractive index different from that of the substrate  201 )  206  are sequentially stacked, using a metal organic chemical vapor deposition (MOVPE), for example. Note that an n-type InP cladding layer may be formed between the n-type InP substrate  201  and the n-type GaInAsP layer  202 . 
     Next, as depicted in  FIG. 6B , a mask  207  having a diffraction grating pattern and made from an electron beam resist (ZEP520 available from Zeon Corporation) is formed on the surface of the n-type GaInAsP layer  206  by electron beam exposure technique, for example. 
     Note that the diffraction grating pattern formed in the mask  207  includes a pattern for forming a phase shift having a phase of n radian (λ/4 phase shift) at the center of the cavity for each device. 
     Subsequently, the n-type GaInAsP layer  206  and a portion of the n-doped InP layer  205  are removed using the mask  207  by a reactive ion etching (RIE) using a mixed gas of ethane and hydrogen, for example. In this example, as depicted in  FIG. 6C , the etching is stopped at some midpoint of the n-type InP layer  205  after the n-type GaInAsP layer  206  is divided. As a result, the diffraction grating pattern is transferred to the entire surface of the n-type GaInAsP layer  206 , and the n-type GaInAsP layer  206  is divided. 
     Next, as depicted in  FIG. 6C , a positive photoresist (OFR8600 available from Tokyo Ohka Kogyo., Ltd.; having a thickness of 300 nm)  208  is applied on the surface, for example. Note that no deformation of the mask  207  occurs since the electron beam resists forming the mask  207  and the positive photoresist  208  are immiscible. 
     Next, as depicted in  FIG. 6D , a portion of the positive photoresist  208  (the center portion of the cavity; the center region in the direction along the optical waveguide, in this example) is removed using a conventional photolithographic technique to form a positive photoresist mask  208 A, covering the regions at the closer end portions in the direction along the optical waveguide (covering the surface of a portion of the mask  207 ; covering the surface of the regions corresponding to the end regions of the optical waveguide). 
     Thereafter, using the electron beam resist mask  207 , which is exposed to the surface again, and the positive photoresist mask  208 A, the remained portion of the n-type InP layer  205 , the n-type GaInAsP layer  204 , and a part of the n-type InP layer  203  are removed by a reactive ion etching (RIE) using a mixed gas of ethane and hydrogen, for example. 
     In this example, as depicted in  FIG. 6E , the etching is stopped at some midpoint of the n-type InP layer  203  after the n-type InP layer  205  and the n-type GaInAsP layer  204  are divided. 
     As a result, the diffraction grating pattern is transferred to a part of the n-type GaInAsP layer  204  (the center portion of the cavity; the center region in the direction along the optical waveguide, in this example), and the n-type GaInAsP layer  204  is divided. 
     Next, as depicted in  FIG. 7A , a negative photoresist (OMR85 available from Tokyo Ohka Kogyo., Ltd.; having a thickness of 300 nm)  209  is applied on the surface, for example. Note that no deformation of the mask  207  occurs since the electron beam resists forming the mask  207 , the positive photoresist  208  forming the mask  208 A, and the negative photoresist  209  are immiscible. 
     Next, as depicted in  FIG. 7B , a portion of the negative photoresist  209  (the closer center portion of the cavity included in the center portion of the cavity; the closer center region included at the center region in the direction along the optical waveguide, in this example) is removed using a conventional photolithographic technique to form a negative photoresist mask  209 A, covering the end portions in the direction along the optical waveguide (covering the surface of a portion of the positive photoresist  208 ; covering the surface of the regions corresponding to the end regions of the optical waveguide). 
     Thereafter, using the electron beam resist mask  207 , which is exposed to the surface again, the positive photoresist mask  208 A, and the negative photoresist mask  209 A, the n-type InP layer  203 , the n-type GaInAsP layer  202 , and a portion of the n-type InP substrate  201  are removed by a reactive ion etching (RIE) using a mixed gas of ethane and hydrogen, for example. 
     In this example, as depicted in  FIG. 7C , the etching is stopped at some midpoint of the n-type InP substrate  201  (at an etching depth of about 10 nm, in this example) after the n-type InP layer  203  and the n-type GaInAsP layer  202  are divided. 
     As a result, the diffraction grating pattern is transferred to a part of the n-type GaInAsP layer  202  (the closer center portion of the cavity; the closer center region in the direction along the optical waveguide, in this example), and the n-type GaInAsP layer  202  is divided. 
     In this case, since the diffraction grating pattern provided in the n-type GaInAsP layer  202 , the diffraction grating pattern provided in the n-type GaInAsP layer  204 , and the diffraction grating pattern provided in the n-type GaInAsP layer  206  are formed using the same mask  207 , the diffraction gratings  20 A,  30 A, and  40 A, in the overlap region of a first diffraction grating layer  20 , a second diffraction grating layer  30 , and a third diffraction grating layer  40 , which are to be formed as will be described later, will have the same phase, period, and duty ratio. 
     Note that, in this example, although the thickness of the n-type GaInAsP layer  202  (the thickness of the first diffraction grating layer  20 ) and the thickness of the n-type GaInAsP layer  206  (the thickness of the third diffraction grating layer  40 ) are the same, the refractive index of the n-type GaInAsP layer  202  (the refractive index difference in the first diffraction grating layer  20 ) and the refractive index of the n-type GaInAsP layer  206  (the refractive index difference in the third diffraction grating layer  40 ) are different from each other. As a result, the coupling coefficient of the diffraction grating  20 A provided in the first diffraction grating layer  20  is different from the coupling coefficient of the diffraction grating  40 A provided in the third diffraction grating layer  40 . 
     In addition, although the refractive index of the n-type GaInAsP layer  202  (the refractive index difference in the first diffraction grating layer  20 ) and the refractive index of the n-type GaInAsP layer  204  (the refractive index difference in the second diffraction grating layer  30 ) are the same, the thickness of the n-type GaInAsP layer  202  (the thickness of the first diffraction grating layer  20 ) and the thickness of the n-type GaInAsP layer  204  (the thickness of the second diffraction grating layer  30 ) are different from each other. As a result, the coupling coefficient of the diffraction grating  20 A provided in the first diffraction grating layer  20  is different from the coupling coefficient of the diffraction grating  30 A provided in the second diffraction grating layer  30 . 
     In addition, the thickness and the refractive index of the n-type GaInAsP layer  204  (the thickness of and the refractive index difference in the second diffraction grating layer  30 ) and the thickness and the refractive index of the n-type GaInAsP layer  206  (the thickness of and the refractive index difference in the third diffraction grating layer  40 ) are different from each other. As a result, the coupling coefficient of the diffraction grating  30 A provided in the second diffraction grating layer  30  is different from the coupling coefficient of the diffraction grating  40 A provided in the third diffraction grating layer  40 . 
     Each of the diffraction grating layers  20 ,  30 , and  40  has a constant duty ratio within the layer. In this case, since the width of the diffraction grating pattern of the etching mask (mask pattern) does not require any modification, the processing precision of the diffraction grating is stabilized. Since each of the diffraction grating layers  20 ,  30 , and  40  has a constant thickness and refractive index difference within the layer, the coupling coefficient of the diffraction grating within the layer is constant. 
     As depicted in  FIG. 7D , the mask  207 , the mask  208 A, and the mask  209 A are removed from the surface using a conventional resist removal technique. 
     Subsequently, as depicted in  FIG. 7E , an n-doped InP layer (a layer having the same refractive index as the substrate  201 )  210  is grown over the entire surface, using MOVPE, for example. The grooves formed by stopping the etching at some midpoint of the n-type InP layer  205 , the grooves formed by stopping the etching at some midpoint of the n-type InP layer  203 , and the grooves formed by stopping the etching at some midpoint of the n-type InP substrate  201  are buried with the n-type InP layer  210 . 
     As a result, the discontinuous the n-type GaInAsP layer  202  is buried with the n-type InP layer  210 , thereby the first diffraction grating layer  20  is formed, a portion (the closer center portion of the cavity; the closer center region in the direction along the optical waveguide, in this example) of which is provided with the diffraction grating  20 A. In addition, the discontinuous the n-type GaInAsP layer  204  is buried with the n-type InP layer  210 , thereby the second diffraction grating layer  30  is formed, a portion (the center portion of the resonator; the center region in the direction along the optical waveguide, in this example) of which is provided with the diffraction grating  30 A. Furthermore, the discontinuous the n-type GaInAsP layer  206  is buried with the n-type InP layer  210 , thereby the third diffraction grating layer  40  is formed, the entirety (the entire length along the optical waveguide) of which is provided with the diffraction grating  40 A. 
     Furthermore, the spacer layer  41  is formed between the first diffraction grating layer  20  and the second diffraction grating layer  30  by the n-type InP layer  203  and a portion of the n-type InP layer  210 . In addition, the spacer layer  42  is formed between the second diffraction grating layer  30  and the third diffraction grating layer  40  by the n-type InP layer  205  and a portion of the n-type InP layer  210 . Furthermore, the spacer layer  51  is formed above the third diffraction grating layer  40  by a portion of the n-type InP layer  210 . 
     In this example, the thickness of the spacer layer  41  between the first diffraction grating layer  20  and the second diffraction grating layer  30  is determined by the thickness of the n-type InP layer  203  (the thickness of a portion formed above the n-type GaInAsP layer  202 ), and the thickness of the spacer layer  42  between the second diffraction grating layer  30  and the third diffraction grating layer  40  is determined by the thickness of the n-type InP layer  205  (the thickness of a portion formed above the n-type GaInAsP layer  204 ), both of which are about 15 nm. Accordingly, the diffraction grating layer  20 ,  30 ,  40  are spaced apart with the same distance. 
     Next, as depicted in  FIG. 8A , a quantum well active layer  211 , a p-type doped InP cladding layer (e.g., with a thickness of about 2.5 μm)  212 , and a p-type GaInAs contact layer  213  (e.g., with a thickness of about 300 nm) are stacked sequentially by MOVPE, for example. 
     In this example, the quantum well active layer  211  includes an undoped AlGaInAs quantum well layer (e.g., with a thickness of about 6.0 nm and a compressive strain amount of about 1.0%), and an undoped AlGaInAs barrier layer (e.g., with a composition wavelength of about 1.05 μm and a thickness of about 10 nm), the layer number of the quantum well layer being ten, and the emission wavelength thereof being about 1310 nm. 
     Note that undoped AlGaInAs-SCH layers (e.g., with a wavelength of about 1.0 μm and a thickness of about 20 nm) may be provided above and below the quantum well active layer  211 , sandwiching the quantum well active layer  211 . 
     Thereafter, as depicted in  FIG. 8B , a mask (e.g., a stripe etching mask having a thickness of about 400 nm and a width of about 1.3 μm)  214  made from SiO 2  is formed over the semiconductor surface using a conventional chemical vapor deposition (CVD) and photolithography technique. 
     As depicted in  FIG. 8C , the resultant semiconductor stack structure is etched such that the n-type InP substrate  201  is etched by about 0.7 μm, for example, using dry etching, for example, to form a stripe mesa structure (mesa stripes). 
     Next, as depicted in  FIG. 8D , current blocking layers  215  made from an Fe-doped InP are grown on opposite sides of the mesa structure using MOVPE, for example, and the etch mask  214  is removed with hydrofluoric acid, for example. Thereafter, as depicted in  FIG. 8E , after forming a p-side electrode  216  and an n-side electrode  217 , anti-reflective coatings  218  and  219  are formed on the both end face sides of the device, thereby obtaining a completed device. 
     Accordingly, similar to the first embodiment set forth above, according to the optical device (DFB laser) and the method for manufacturing the same of this embodiment, in the optical device having a structure wherein the coupling coefficient of the diffraction grating is varied within the cavity, the diffraction grating can be precisely and reliably fabricated, thereby improving the yield. In addition, the difference in the coupling coefficient can be increased between the increased and reduced coupling coefficient regions (the contrast of the coupling coefficient can be increased), thereby improving the device characteristics. 
     Furthermore, since the diffraction grating can be precisely and reliably fabricated, any variation in the device characteristics can be minimized, thereby improving the device characteristics. The controllability of the coupling coefficient and flexibility in designing the coupling coefficient are also improved. 
     Especially, the device of this embodiment has an advantage in that an even higher efficiency in the FM modulation is obtained and the single mode oscillation is stabilized since the coupling coefficient is made even higher at the center, as compared to the first embodiment set forth above. 
     Note that although the diffraction grating layer  20 ,  30 ,  40  are spaced apart with the same distance in the above-described embodiment, this is not limiting. For example, in the configuration of the above-described embodiment, as depicted in  FIG. 9 , the spacing between the first diffraction grating layer  20  and the second diffraction grating layer  30  (the thickness of the spacer layer  41 ; the thickness of the n-type InP layer  203  above the n-type GaInAsP layer  202  maybe set to about 25 nm, for example, and the spacing between the second diffraction grating layer  30  and the third diffraction grating layer  40  (the thickness of the spacer layer  42 ; the thickness of the n-type InP layer  205  above the n-type GaInAsP layer  204 ) maybe set to about 10 nm, for example, such that the diffraction grating layer  20 ,  30 ,  40  may be spaced apart with different distances. In addition, when more than three diffraction grating layers are provided, the diffraction grating layers may be spaced apart with the same distance, may be spaced apart with different distances, or a part of the diffraction grating layers may be spaced apart with different distances. 
     Third Embodiment 
     Now, an optical device and a method for manufacturing the same according to a third embodiment will be described with reference to  FIGS. 10-13E . 
     The optical device (DFB laser) and the method for manufacturing the same according to this embodiment are different from the above-described first embodiment in that the regions, where diffraction gratings  21 A and  31 A included in a plurality of diffraction grating layers  21  and  31  are stacked, are end regions of the direction along the optical waveguide  1 , that the thicknesses of the plurality of diffraction grating layers  21  and  31  are the same, and that the spacings between the plurality of diffraction grating layers  21  and  31  are different from each other. 
     In other words, the optical device according to this embodiment is a distributed feed-back (DFB) laser (laser device; waveguide optical device; active optical device; light emitting device; device for code division multiplexing communication) having a structure wherein the coupling coefficient of diffraction grating is varied within the cavity, for example, and includes an optical waveguide  1  and a plurality of (two, in this example) diffraction grating layers  21  and  31  provided along the optical waveguide  1 , as depicted in  FIG. 10 . Here, the first semiconductor layers  302  and  304  have a refractive index different from that of the second semiconductor layers  303  and  307   
     As depicted in  FIG. 10 , the first diffraction grating layer  21  and the second diffraction grating layer  31  are provided as a plurality of diffraction grating layers, and the diffraction grating layers  21  and  31  are provided below the optical waveguide  1  (the substrate side with respect to the optical waveguide  1 ; one side of the optical waveguide  1 ). 
     The diffraction grating layers  21  and  31  include diffraction gratings (buried diffraction gratings; buried-type diffraction grating)  21 A and  31 A having discontinuous first semiconductor layers  302  and  304 , and second semiconductor layers  302  and  307  burying the first semiconductor layers  302  and  304 , respectively, as depicted in  FIG. 10 . Here, the first semiconductor layers  303  and  304  have a refractive index different from that of the second semiconductor layers  303  and  307 . The discontinuously remained portions of the first semiconductor layer  302  and the portions of the second semiconductor layer  303  formed in the grooves of the first semiconductor layer  302  constitute the diffraction grating  21 A. The continuously remained portions of the first semiconductor layer  302  do not constitute the diffraction grating  21 A. That is, the diffraction grating layer  21  includes a grating region where the diffraction grating  21 A constituted of the first semiconductor layer  302  and the second semicondoctor layer  303  is provided, and an non-grating region where the diffraction grating  21 A is not provided and which is constituted of the first semiconductor layer  302 . In addition, the discontinuously remained portions of the first semiconductor layer  304  and the portions of the second semiconductor layer  307  formed in the grooves of the first semiconductor layer  304  constitute the diffraction grating  31 A. That is, the diffraction grating layer  31  includes a grating region where the diffraction grating  31 A constituted of the first semiconductor layer  304  and the second semicondoctor layer  307  is provided. 
     The diffraction gratings  21 A and  31 A in the diffraction grating layers  21  and  31  are provided to overlap each other, and the diffraction gratings  21 A and  31 A at an overlap region have the same phase, period, and duty ratio. Here, the term “duty ratio” means the ratio of the remained portion after etching with respect to the period of the diffraction grating. Note that, in this embodiment, the duty ratio of each of the diffraction gratings  21 A and  31 A provided in the diffraction grating layers  21  and  31  is constant. 
     In this embodiment, as depicted in  FIG. 10 , the diffraction grating  21 A in the first diffraction grating layer  21  is provided only at the end regions of the direction along the optical waveguide  1  (the direction of the length of the cavity). In other words, the region where the diffraction grating  21 A in the first diffraction grating layer  21  is provided, is the end regions of the direction along the optical waveguide  1 . 
     In addition, as depicted in  FIG. 10 , the diffraction grating  31 A in the second diffraction grating layer  31  is formed along the entire length of the direction along the optical waveguide  1 . In other words, the region where the diffraction grating  31 A in the second diffraction grating layer  31  is provided, is the entire region of the direction along the optical waveguide  1 . 
     Thus, in this embodiment, the length of the region where the diffraction grating  21 A in the first diffraction grating layer  21  is provided in the direction along the optical waveguide  1 , is shorter than the length of the region of the second diffraction grating layer  31  where the diffraction grating  31 A is provided in the direction along the optical waveguide  1 . The lengths of the regions where the diffraction gratings  21 A and  31 A are provided, are different between the first diffraction grating layer  21  and the second diffraction grating layer  31 . In this case, the overlap region of the diffraction grating layers  21  and  31  is the end regions of the direction along the optical waveguide  1 . 
     Note that, although two diffraction grating layers  21  and  31  are provided as a plurality of diffraction grating layers and the lengths of the regions of the diffraction grating layers  21  and  31  where the diffraction gratings  21 A and  31 A are provided, are different from each other, these are not limiting. For example, a third diffraction grating layer may be add for increasing the coupling coefficient at the end regions of the direction along the optical waveguide  1 . Here, the length of the region of the third diffraction grating layer where the diffraction grating is provided in the direction along the optical waveguide  1  is the same as that of the first diffraction grating layer  21 . Thus, it is suffice that the plurality of diffraction grating layers include at least two diffraction grating layers being different from each other in terms of the length of the region where the diffraction grating is provided. 
     As described above, in this embodiment, using buried diffraction gratings, a plurality of diffraction grating layers including region where a buried diffraction grating is provided, are stacked. At the region where maximizing the coupling coefficient is desired, a buried diffraction grating is provided in all of the stacked diffraction grating layers (two layers, i.e., the first diffraction grating layer  21  and the second diffraction grating layer  31 , in this example). At the region where a reduced coupling coefficient is desired, a diffraction grating is provided only in a part of the stacked diffraction grating layers (the second diffraction grating layer  31 , in this example). 
     More specifically, as depicted in  FIG. 10 , the second diffraction grating layer  31  is stacked above the first diffraction grating layer  21 , and the diffraction gratings  21 A and  31 A in the diffraction grating layers  21  and  31  are stacked at the end regions in the direction along the optical waveguide  1 . In other words, the number of stacked diffraction gratings is varied in the direction along the optical waveguide  1 . As a result, the coupling coefficient at the end regions of the direction along the optical waveguide  1  is increased, whereas the coupling coefficient is decreased at the remaining region (center region) as compared to the end regions. 
     More specifically, as depicted in  FIG. 10 , this DFB laser (optical semiconductor device) includes, above an n-doped InP substrate  301 , the first diffraction grating layer  21 , the second diffraction grating layer  31 , and the optical waveguide  1  including a quantum well active layer  308  as the waveguide core layer. The first diffraction grating layer  21  includes the buried diffraction grating  21 A, which is formed by burying an n-doped GaInAsP layer (e.g., with a composition wavelength of about 1.15 μm and a thickness of about 20 nm)  302 , which is discontinuous at the end regions, with an n-doped InP layer (e.g., with a thickness of about 20 nm; burying layer)  303 , wherein the n-doped GaInAsP layer  302  and the n-doped InP layer  303  have different refractive indices. The second diffraction grating layer  31  includes the buried diffraction grating  31 A, which is formed by burying an n-doped GaInAsP layer (e.g., with a composition wavelength of about 1.15 μm and a thickness of about 20 nm)  304  that is discontinuous at the entire region, with an n-doped InP layer  307 , wherein the n-doped GaInAsP layer  304  and the n-doped InP layer  307  have different refractive indices. 
     In addition, in the specific exemplary configuration of this embodiment, as depicted in  FIG. 10 , the portions of the n-type InP burying layer  303  burying between the discontinuous n-type GaInAsP layer  302  constitute the diffraction grating  21 A, and the portions of the n-type InP burying layer  307  burying between the discontinuous n-type GaInAsP layer  304  constitute the diffraction grating  31 A. 
     Note that the portion of the n-type InP burying layer  303  formed above the n-type GaInAsP layer  302  constitutes a spacer layer  43  between the first diffraction grating layer  21  and the second diffraction grating layer  31 , and the portion of the n-type InP burying layer  307  formed above the n-type GaInAsP layer  304  constitutes a spacer layer (cladding layer)  52  between the second diffraction grating layer  31  and the active layer  308 . In addition, minimizing the thickness of the spacer layer  43  is preferred, as long as the variation of an etching depth can be permissible. 
     In addition, in the specific exemplary configuration of this embodiment, the thickness of the n-doped GaInAsP layer  302  constituing the diffraction grating  21 A in the first diffraction grating layer  21  is about 20 nm, and the thickness of the n-doped GaInAsP layer  304  defining the diffraction grating  31 A in the second diffraction grating layer  31  is about 20 nm, so that the first diffraction grating layer  21  and the second diffraction grating layer  31  have the same thickness. 
     Furthermore, the refractive index difference between the semiconductor layers  302  and  303  constituing the diffraction grating  21 A in the first diffraction grating layer  21  is made the same as the refractive index difference between the semiconductor layers  304  and  307  constituing the diffraction grating  31 A in the second diffraction grating layer  31 , so that the first diffraction grating layer  21  and the second diffraction grating layer  31  have the same refractive index difference. 
     Therefore, in the first diffraction grating layer  21  and the second diffraction grating layer  31 , since the diffraction gratings  21 A and  31 A also have the same duty ratio, the coupling coefficients of the diffraction gratings  2 A and  3 A are the same. 
     In this manner, according to the configuration of this embodiment, since the contrast of the coupling coefficient can be increased by stacking the diffraction grating layers  21  and  31 , devices satisfying desired needs can be achieved, with improved the device characteristics. 
     The details of other elements are similar to the configuration and the specific exemplary configuration of the above-described first embodiment, and descriptions thereof will be omitted. 
     Now, a method for manufacturing the DFB laser according to the specific exemplary configuration of this embodiment will be described with reference to  FIGS. 11A-13E . 
     Firstly, as depicted in  FIG. 11A , on an n-doped InP substrate  301 , an n-doped GaInAsP layer (e.g., with a composition wavelength of about 1.15 μm and a thickness of about 20 nm; a layer having a refractive index different from that of the substrate  301 )  302 , an n-doped InP layer (e.g., with a thickness of about 20 nm; a layer having the same refractive index as the substrate  301 )  303 , and an n-doped GaInAsP layer (e.g., with a composition wavelength of about 1.15 μm and a thickness of about 20 nm; a layer having a refractive index different from that of the substrate  301 )  304  are sequentially stacked, using a metal organic chemical vapor deposition (MOVPE), for example. Note that an n-type InP cladding layer may be formed between the n-type InP substrate  301  and the n-type GaInAsP layer  302 . 
     Next, as depicted in  FIG. 11B , a mask  305  having a diffraction grating pattern and made from an electron beam resist (ZEP520 available from Zeon Corporation) is formed on the surface of the n-type GaInAsP layer  304  by electron beam exposure technique, for example. Note that the diffraction grating pattern formed in the mask  305  includes a pattern for forming a phase shift having a phase of n radian (λ/4 phase shift) at the center of the cavity for each device. 
     Subsequently, the n-type GaInAsP layer  304  and a portion of n-type InP layer  303  are removed using the mask  305  by a reactive ion etching (RIE) using a mixed gas of ethane and hydrogen, for example. 
     In this example, as depicted in  FIG. 11C , the etching is stopped at some midpoint of the n-type InP layer  303  after the n-type GaInAsP layer  304  is divided. As a result, the diffraction grating pattern is transferred to the entire surface of the n-type GaInAsP layer  304 , and the n-type GaInAsP layer  304  is divided. 
     Next, as depicted in  FIG. 11D , a positive photoresist (OFPR8600 available from Tokyo Ohka Kogyo., Ltd.; e.g., with a thickness of about 300 nm)  306  is applied on the surface, for example. Note that no deformation of the mask  305  occurs since the electron beam resists forming the mask  305  and the positive photoresist  306  are immiscible. 
     Next, as depicted in  FIG. 11E , a portion of the positive photoresist  306  (the end portions of the cavity; the end regions of the direction along the optical waveguide, in this example) is removed using a conventional photolithographic technique to form a positive photoresist mask  306 A, covering the center portion in the direction along the optical waveguide (covering the surface of a portion of the mask  305 ; covering the surface of the region corresponding to the center region of the optical waveguide). 
     Thereafter, using the electron beam resist mask  305 , which is exposed to the surface again, and the positive photoresist mask  306 A, the remained portion of the n-type InP layer  303 , the n-type GaInAsP layer  302 , and a portion of the n-type InP substrate  301  are removed by a reactive ion etching (RIE) using a mixed gas of ethane and hydrogen, for example. 
     In this example, as depicted in  FIG. 12A , the etching is stopped at some midpoint of the n-type InP substrate  301  (at an etching depth of about 10 nm, in this example) after the n-type InP layer  303  and the n-type GaInAsP layer  302  are divided. 
     As a result, the diffraction grating pattern is transferred to parts of the n-type GaInAsP layer  302  (the end portions of the cavity; the end regions of the direction along the optical waveguide, in this example), and the n-type GaInAsP layer  302  is divided. 
     In this case, since the diffraction grating pattern provided in the n-type GaInAsP layer  304  and the diffraction grating pattern provided in the n-type GaInAsP layer  302  are formed using the same mask  305 , the diffraction gratings  21 A and  31 A in the overlap region of a first diffraction grating layer  21  and a second diffraction grating layer  31 , which are to be formed as will be described later, will have the same phase, period, and duty ratio. 
     Note that, in this example, the thickness and the refractive index of the n-type GaInAsP layer  302  (the thickness of and the refractive index difference in the first diffraction grating layer  21 ) are the same as the thickness and the refractive index of the n-type GaInAsP layer  304  (the thickness of and the refractive index difference in the second diffraction grating layer  31 ). As a result, the coupling coefficient of the diffraction grating  21 A provided in the first diffraction grating layer  21  is the same as the coupling coefficient of the diffraction grating  31 A provided in the second diffraction grating layer  31 . 
     Each of the diffraction grating layers  21  and  31  has a constant duty ratio within the layer. In this case, since the width of the diffraction grating pattern of the etching mask (mask pattern) does not require any modification, the processing precision of the diffraction grating is stabilized. Since each of the diffraction grating layers  21  and  31  has a constant thickness and refractive index difference within the layer, the coupling coefficient of the diffraction grating within the layer is constant. 
     As depicted in  FIG. 12B , the mask  305  and the mask  306 A are removed from the surface using a conventional resist removal technique. 
     Subsequently, as depicted in  FIG. 12C , an n-doped InP layer (a layer having the same refractive index as the substrate  301 )  307  is grown over the entire surface, using MOVPE, for example. 
     The grooves formed by stopping the etching at some midpoint of the n-type InP layer  303  and the grooves formed by stopping the etching at some midpoint of the n-type InP substrate  301  are buried with the n-type InP layer  307 . 
     As a result, the discontinuous the n-type GaInAsP layer  302  is buried with the n-type InP layer  307 , thereby the first diffraction grating layer  21  is formed, portions (the end portions of the cavity; the end regions of the direction along the optical waveguide, in this example) of which are provided with the diffraction grating  21 A. In addition, the discontinuous the n-type GaInAsP layer  304  is buried with the n-type InP layer  307 , thereby the second diffraction grating layer  31  is formed, the entirety (the entire length along the optical waveguide) of which is provided with the diffraction grating  31 A. 
     Furthermore, the spacer layer  43  is formed between the first diffraction grating layer  21  and the second diffraction grating layer  31  by the n-type InP layer  303  and a portion of the n-type InP layer  307 . In addition, the spacer layer  52  is formed above the second diffraction grating layer  31  by a portion of the n-type InP layer  307 . 
     Next, as depicted in  FIG. 12D , a quantum well active layer  308  and a p-type doped InP cladding layer (e.g., with a thickness of about 250 nm)  309  are stacked sequentially by MOVPE, for example. 
     In this example, the quantum well active layer  308  includes an undoped GaInAsP quantum well layer (e.g., with a thickness of about 5.1 nm and a compressive strain amount of about 1.0%), and an undoped GaInAsP barrier layer (e.g., with a composition wavelength of about 1.2 μm and a thickness of about 10 nm), the layer number of the quantum well layer being six, and the emission wavelength thereof being about 1560 nm. 
     Note that undoped GaInAsP-SCH layers (e.g., with a wavelength of about 1.15 μm and a thickness of about 20 nm) may be provided above and below the quantum well active layer  308 , sandwiching the quantum well active layer  308 . 
     Thereafter, as depicted in  FIG. 12E , a mask (e.g., a stripe etching mask having a thickness of about 400 nm and a width of about 1.6 μm)  310  made from SiO 2  is formed over the semiconductor surface using a conventional chemical vapor deposition (CVD) and photolithography technique. 
     As depicted in  FIG. 13A , the resultant semiconductor stack structure is etched such that the n-type InP substrate  301  is etched by about 0.7 μm, for example, using dry etching, for example, to form a stripe mesa structure (mesa stripes). 
     Next, as depicted in  FIG. 13B , current blocking layers, made from a p-type InP layer  311 , an n-type InP layer  312 , and a p-type InP layer  313 , are grown on opposite sides of the mesa structure using MOVPE, for example, and the etching mask  310  is removed with hydrofluoric acid, for example. Thereafter, as depicted in  FIG. 13C , a p-type InP cladding layer (e.g., with a thickness of about 2.2 μm)  314  and a p-type GaInAs contact layer (e.g., with a thick of about 300 nm)  315  are sequentially grown using MOVPE, for example. 
     As depicted in  FIG. 13D , after forming a p-side electrode  316  and an n-side electrode  317 , as depicted in  FIG. 13E , anti-reflective coatings  318  and  319  are formed on the both end face sides of the device, thereby obtaining a completed device. 
     Accordingly, similar to the first embodiment set forth above, according to the optical device (DFB laser) and the method for manufacturing the same of this embodiment, in the optical device having a structure wherein the coupling coefficient of the diffraction grating is varied within the cavity, the diffraction grating can be precisely and reliably fabricated, thereby improving the yield. In addition, the difference in the coupling coefficient can be increased between the increased and reduced coupling coefficient regions (the contrast of the coupling coefficient can be increased), thereby improving the device characteristics. 
     Furthermore, since the diffraction grating can be precisely and reliably fabricated, any variation in the device characteristics can be minimized, thereby improving the device characteristics. The controllability of the coupling coefficient and flexibility in designing the coupling coefficient are also improved. 
     Especially, in the device of this embodiment, since the coupling coefficient is increased on device end face sides, the hole burning in the longitudinal direction can be reduced even if the cavity is fabricated longer. As a result, a stable single longitudinal mode operation can be maintained with a higher optical output, and the oscillation linewidth of the laser can be further narrowed. 
     Therefore, for example, the device of this embodiment may be applied to a device having the structure as disclosed in G. Morthier et al., “A New DFB-Laser Diode with Reduced Spatial Hole Burning”, IEEE Photonics Technology Letter, vol. 2, no. 6, June 1990, pp. 388-390, namely, the structure wherein the coupling coefficient is reduced toward the center of the cavity, thereby preventing the hole burning in the longitudinal direction, and improving the stability of the longitudinal mode upon a higher power optical output. 
     Such a device may be used as a laser light source for a coherent optical transmission system, or a system which requires a laser light source with a very narrow oscillation linewidth of about 100 to about 500 kHz, such as a multilevel modulation optical communication system, for example. 
     In order to narrow the oscillation linewidth of a laser, the laser should be able to operate in a single longitudinal mode, like a DFB laser, and the cavity length of the laser should be increased to further narrow the linewidth. For example, a linewidth of about 1 MHz or less can be achieved with a laser having a cavity length of about 1000 μm or more. 
     Furthermore, since the linewidth is proportional to the inverse of the optical output of the laser, a narrower linewidth can be achieved by operating the laser with a higher optical output. Accordingly, an even narrower linewidth can be achieved by further increasing the cavity length of the laser and operating the laser at an optical output as high as possible. However, increasing the cavity length excessively may reduce the gain difference between the main and side modes due to the influence of the hole burning in the longitudinal direction in a DFB laser during a high optical output operation, which may deteriorate the single mode stability, leading to a sudden broadening of the linewidth. Accordingly, the profile that can reduce the influence of the hole burning in the longitudinal direction during a high optical output operation, as set forth above, is effective in such cases. 
     Others 
     Note that the present invention is not limited to the embodiments described above and the configurations described in other sections, and may be modified in various manners without departing from the sprit of the present invention. 
     Although, in the above-described embodiments and variations thereof, each diffraction grating layer has a constant duty ratio of the diffraction grating within the layer, this is not limiting. 
     For example, as depicted in  FIG. 14 , in the configuration of the above-described first embodiment, the second diffraction grating layer  3  may be configured such that the duty ratio (50%, in this example) of the diffraction grating  3 A at the overlap region, which overlap with the region of the first diffraction grating layer  2  where the diffraction grating  2 A is provided, may be different from the duty ratio (25%, in this example) of the diffraction grating  3 A at the remaining regions (i.e., the end regions in the direction along the optical waveguide). In other words, one of a plurality of diffraction grating layers may be configured as a diffraction grating layer having a diffraction grating with different duty ratios. Note that such a variation may also be applied to the above-described second and third embodiments. 
     In addition, for example, a plurality of diffraction grating layers have the same duty ratio in the diffraction gratings provided in the overlap region, but the duty ratio of the diffraction grating in each diffraction grating layer may be varied within the layer, as long as the duty ratio is not too high or not too low. 
     As described above, a plurality of diffraction grating layers may have at least one diffraction grating layer having a diffraction grating with different duty ratios. 
     In addition, in the above-described embodiments and variations thereof, the plurality of diffraction grating layers have the same duty ratio in the diffraction gratings provided at the overlap region, this is not limiting. For example, the cross-section of a diffraction grating may be trapezoidal, rather than rectangular, when the diffraction grating is formed by etching, and the duty ratio of the diffraction grating provided in the overlap region may be varied accordingly. Even in such cases, the same advantageous effects as those of the above-described embodiments and variations thereof can be obtained, as long as the phase and period of the diffraction grating provided in the overlap region are the same. 
     In addition, although the above-described embodiments and variations thereof have been described in the structures in which the coupling coefficient is increased toward the center of the cravity (first and second embodiments), or the coupling coefficient is decreased toward the center of the cravity (third embodiment), these are not limiting. Any structure wherein the coupling coefficient is varied within the cavity may be used, and the structure can be flexibly provided according to the design of an optical device. 
     For example, the above-described embodiments and variations thereof have been described in the structure in which the coupling coefficient profile is symmetric with respect to the center of the cavity along the direction of the cavity. However, the coupling coefficient profile may be asymmetric with respect to the center of the cavity along the direction of the cavity. For example, a structure wherein the coupling coefficient is increased at the laser front end face so as to enhance the returning light resistance, or a structure wherein the coupling coefficient is reduced at the laser front end face so as to increase the optical output and so forth can be adopted. 
     In addition, the above-described embodiments and variations thereof have been described in the structure in which the plurality of diffraction grating layers are provided below the optical waveguide (the substrate side with respect to the optical waveguide), this is not limiting. For example, the diffraction grating layers are provided above the optical waveguide (the side opposite to the substrate with respect to the optical waveguide), and the same advantageous effects as those of the above-described embodiments can be obtained in this case. 
     In addition, the above-described embodiments and variations thereof have been described in the structure in which only a single phase shift with phase n is present at the center of the cavity, this is not limiting. For example, a structure without phase shift, or a structure with a plurality of phase shifts may be possible, and the shift amount of one or more phase shifts may be arbitrary set. 
     Furthermore, the above-described embodiments and variations thereof have been described in the structure in which quantum well active layers using a GaInAsP-based compound semiconductor material (first and third embodiments) or an AlGaInAs-based compound semiconductor material (second embodiment) are formed above an n-type InP substrate to construct a DFB laser, these are not limiting. The present invention can be applied to a wide range of devices (optical devices) having a diffraction grating in the vicinity of an optical waveguide. 
     For example, quantum well active layers may be formed using an AlGaInAs-based compound semiconductor material in the configuration of the first or third embodiment, or quantum well active layers may be formed using a GaInAsP-based compound semiconductor material in the configuration of second embodiment. In addition, quantum well active layers may be formed using other compound semiconductor materials, such as a GaInNAs-based compound semiconductor material. In this case, the same advantageous effects as those of the above-described embodiments can be obtained. 
     In addition, any materials that can be used for optical devices (semiconductor lasers) maybe used for devices, for example. For example, other compound semiconductor materials may be used. In addition to semiconductor materials, organic or inorganic materials may also be used. In this case, the same advantageous effects as those of the above-described embodiments can be obtained. 
     In addition, any substrates may be used, such as substrates having p-type conductivity or semi-insulating substrates. When such a substrate is used, the conductivity of every layer formed above the substrate is inversed. In this case, the same advantageous effects as those of the above-described embodiments can be obtained. 
     In addition, a GaAs substrate may be used, and each layer may be formed from any semiconductor material that can be crystallized (e.g., epitaxially grown) on the GaAs substrate. In this case, the same advantageous effects as those of the above-described embodiments can be obtained. 
     Furthermore, the layers may be formed above a silicon substrate using the substrate bonding technique. In this case, the same advantageous effects as those of the above-described embodiments can be obtained. 
     In addition, other active layer structures may be used, such as bulk active layers using a bulk semiconductor material or quantum dot active layers. In this case, the same advantageous effects as those of the above-described embodiments can be obtained. 
     In addition, a pn-buried structure or an SI-BH structure is adapted as a waveguide structure in the above-described embodiments and variations thereof, this is not limiting. For example, other buried structures may be used, and a ridge waveguide structure and the like may be used. 
     In addition, the present invention may be applied to other semiconductor lasers, such as a distributed Bragg reflector (DBR) laser or a distributed reflector (DR) laser. Furthermore, in addition to active optical devices, such as semiconductor lasers, the present invention may also be applied to passive optical devices, such as optical filters. In these cases, the same advantageous effects as those of the above-described embodiments can be obtained. 
     In addition, the above-described embodiments and variations thereof have been described in the context in which anti-reflection coatings are provided on the end faces, this is not limiting. Any combination of end face structures, such as anti-reflection, cleaved facets, and/or high-reflection structures may be used. 
     All the examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment(s) has (have) been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the gist and scope of the invention.