Patent Publication Number: US-2022216673-A1

Title: Semiconductor Laser

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national phase entry of PCT Application No. PCT/JP2019/019490, filed on May 16, 2019, which application is hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a semiconductor laser used for a light source for an optical transmitter or the like. 
     BACKGROUND 
     Various wavelength multiplexing light sources have been developed in order to perform transmission of a large amount of information using wavelength division multiplexing (WDM). In WDM, single-mode oscillation and oscillation wavelength control of a laser as a light source are important. For example, as a technique for realizing single-mode oscillation, there is a phase shift distributed feedback (DFB) laser (see NPL 1). 
     A phase shift DFB laser has a structure in which a diffraction grating inverts a phase thereof on the way (phase shift) and can oscillate at a Bragg wavelength of the diffraction grating. The Bragg wavelength is determined by a period of the diffraction grating. By manufacturing the diffraction grating using electron beam lithography technology, the period of the diffraction grating can be controlled with high accuracy. However, since oscillating light is emitted from both ends of an element in the above-mentioned phase shift DFB laser, the light emitted from one end portion thereof may not be used, and in this case, half of the light will be lost. 
     In order to solve the above-mentioned problems, a distributed reflector (DR) laser that is configured by connecting a distributed Bragg reflector (DBR) having a high reflectance to one end of a phase shift DFB laser to emit light from the other end thereof has been proposed (see NPL 2). 
     A DR laser in which DBRs are provided at both end portions of a phase shift DFB laser, a reflectance of one DBR is lowered with respect to the other DBR, and light is emitted from the one DBR has also been proposed (see NPL 3). In the configuration in which DBRs are provided on both sides, as compared with a configuration in which a DBR is provided on only one side, an oscillation threshold gain can be lowered, which is advantageous for oscillation of a short resonator laser having a large loss. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL1—Japanese Patent Application Publication No. 2019-12769 
       
    
     Non Patent Literature 
     
         
         NPL 1—K. Utaka et al., “λ/4-Shifted InGaAsP/InP DFB Lasers”, IEEE Journal of Quantum Electronics, Vol. QE-22, No. 7, pp. 1042-1051, 1986. 
         NPL 2—K. Ohira et al., “GaInAsP/InP distributed reflector laser with phase-shifted DFB and quantum-wire DBR sections”, IEICE Electronics Express, Vol. 2, No. 11, pp. 356-361, 2005. 
         NPL 3—K. Otsubo et al., “Low-Driving-Current High-Speed Direct Modulation up to 40 Gb/s Using 1.3-um Semi-Insulating Buried-Hetero structure AlGaInAs-MQW Distributed Reflector (DR) Lasers”, Proc./OFC/NFOEC, Paper OThT6, 2009. 
       
    
     SUMMARY 
     Technical Problem 
     Incidentally, an attempt to introduce WDM according to a wavelength multiplexing light source using the above-mentioned semiconductor laser technology not only in a metro network but also in an optical interconnect for short-distance communication such as between chips is being studied. In a case in which it is applied to this optical interconnect between chips, as is well known, low power consumption is important. However, no light source suitable for such an optical interconnect between chips has been reported at present. In the above-mentioned waveguide type laser, it is effective to shorten a length of an active region (active layer) in a waveguide direction for low power consumption, and this configuration is being anticipated. 
     In order for a laser having a short active layer length to satisfy oscillation conditions, it is important to increase a reflectance in a diffraction grating. In order to increase the reflectance, a coupling coefficient of the diffraction grating should be increased, but in a case in which a phase shift is applied to the diffraction grating to control an oscillation wavelength, spatial hole burning causes instability of an oscillation mode and deterioration of modulation characteristics. For that reason, a double-sided DR laser that can make an oscillation threshold gain lower than that of a DFB laser or a one-sided DR laser with the same coupling coefficient is advantageous for oscillation of a laser having a short active layer length. 
     However, the oscillation characteristics of double-sided DR laser is more strongly affected by manufacturing errors such as a deviation of an active layer that becomes a gain region than that of the DFB laser and the single-sided DR laser. Especially, in a case in which an active layer and an optical waveguide portion are made of different materials in order to make manufacturing simple, it is more strongly affected by manufacturing errors due to a difference in equivalent refractive index, which affects single mode products. In order to deal with this problem, a structure in which a position of a phase shift is devised has been proposed (see PTL 1). 
     However, the above-mentioned technique has a problem that an increase in threshold gain due to a diffraction grating loss becomes larger. In order for a laser having a short active layer length to satisfy oscillation conditions, it is necessary to increase a coupling coefficient of a diffraction grating, but generally, the loss increases as a coupling coefficient of a diffraction grating becomes larger. For this reason, in a laser having a short active layer length, it is important to inhibit the increase in threshold gain due to the diffraction grating loss. 
     Embodiments f the present invention have been made to solve the above problems, and an object thereof is to provide a phase shift distributed feedback laser having distributed Bragg reflectors provided on both sides, which can inhibit influences of manufacturing errors and enable stable single-mode oscillation without increasing a threshold gain. 
     Means for Solving the Problem 
     A semiconductor laser according to embodiments of the present invention includes: an active layer formed on a substrate; a distributed feedback active region which is formed along the active layer and includes a first diffraction grating including a phase shift portion that shifts a phase of a diffraction grating; and a first distributed Bragg reflector and a second distributed Bragg reflector which are disposed continuously with the distributed feedback active region with the distributed feedback active region interposed therebetween, in which the first distributed Bragg reflector includes a first core layer which is formed continuously with the active layer in a waveguide direction and has a refractive index different from that of the active layer, and a second diffraction grating formed along the first core layer, the second distributed Bragg reflector includes a second core layer which is formed continuously with the active layer in the waveguide direction on a side opposite to the first core layer with the active layer interposed therebetween and has a refractive index different from that of the active layer, and a third diffraction grating formed along the second core layer, and a length L 1  of the first distributed Bragg reflector in the waveguide direction, a length L 2  of the distributed feedback active region in the waveguide direction, a length L 3  of the second distributed Bragg reflector in the waveguide direction, and a position x ps  of the phase shift portion in the waveguide direction with an end portion thereof on the first distributed Bragg reflector side set as an origin are set to satisfy correlations of x ps =L 1 +L 2 ×α, L 2 (1−a)+L 3 &gt;x ps , and 0.5&lt;α&lt;1. 
     In one configuration example of the above semiconductor laser, the distributed feedback active region includes a p-type semiconductor layer and an n-type semiconductor layer formed in contact with the active layer, an n-type electrode connected to the n-type semiconductor layer, and a p-type electrode connected to the p-type semiconductor layer. 
     In one configuration example of the above semiconductor laser, the p-type semiconductor layer and the n-type semiconductor layer are formed on the substrate in contact with a side surface of the active layer in a direction perpendicular to the waveguide direction. 
     In one configuration example of the above semiconductor laser, the p-type semiconductor layer and the n-type semiconductor layer are formed to interpose the active layer from above and below. 
     Effects of Embodiments of the Invention 
     As described above, according to embodiments of the present invention, because the length L 1  of the first distributed Bragg reflector, the length L 2  of the distributed feedback active region, the length L 3  of the second distributed Bragg reflector, and the position x ps  of the phase shift portion are set to satisfy the correlations of x ps =L 1 +L 2 ×α, L 2 (1−a)+L 3 &gt;x ps , and 0.5&lt;α&lt;1, the phase shift distributed feedback laser having the distributed Bragg reflectors provided on both sides can inhibit influences of manufacturing errors and perform stable single-mode oscillation without increasing the threshold gain. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a configuration diagram showing a configuration of a semiconductor laser according to an embodiment of the present invention. 
         FIG. 2A  is a perspective view showing a more detailed configuration of the semiconductor laser according to the embodiment of the present invention. 
         FIG. 2B  is a cross-sectional view showing the configuration of the semiconductor laser according to the embodiment of the present invention. 
         FIG. 3A  is a configuration diagram showing a state during manufacture of the semiconductor laser according to the embodiment of the present invention. 
         FIG. 3B  is a configuration diagram showing a state during manufacture of the semiconductor laser according to the embodiment of the present invention. 
         FIG. 3C  is a configuration diagram showing a state during manufacture of the semiconductor laser according to the embodiment of the present invention. 
         FIG. 3D  is a configuration diagram showing a state during manufacture of the semiconductor laser according to the embodiment of the present invention. 
         FIG. 3E  is a configuration diagram showing a state during manufacture of the semiconductor laser according to the embodiment of the present invention. 
         FIG. 3F  is a configuration diagram showing a state during manufacture of the semiconductor laser according to the embodiment of the present invention. 
         FIG. 4  is a configuration diagram showing a state of deviation of the semiconductor laser according to the embodiment of the present invention. 
         FIG. 5  is a configuration diagram showing a state of deviation of the semiconductor laser according to the embodiment of the present invention. 
         FIG. 6  a characteristic diagram showing a relationship between an amount of deviation of a position of an active layer in a waveguide direction from a design value and a difference in threshold mode gain in a case in which a length of the active layer in the waveguide direction is formed to be 500 nm shorter than the design value in total. 
         FIG. 7  is a characteristic diagram showing a relationship between a diffraction grating loss and the threshold mode gain. 
         FIG. 8  is a characteristic diagram showing a relationship between an inclination of a graph in  FIG. 7  and a that defines a position x ps  of a phase shift portion. 
         FIG. 9  is a cross-sectional view showing another configuration of the semiconductor laser according to the embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Hereinafter, a semiconductor laser according to an embodiment of the present invention will be described with reference to  FIG. 1 . This semiconductor laser includes a distributed feedback active region  131 , a first distributed Bragg reflector  132   a  and a second distributed Bragg reflector  132   b  that are disposed continuously with the distributed feedback active region  131 . This semiconductor laser is a so-called DR laser. The first distributed Bragg reflector  132   a  and the second distributed Bragg reflector  132   b  are disposed continuously with the distributed feedback active region  131  with the distributed feedback active region  131  interposed therebetween in a waveguide direction. 
     The distributed feedback active region  131  includes an active layer  103  on which a first diffraction grating  121  is formed. Further, the active layer  103  extends a predetermined length in a light emitting direction, and the first diffraction grating  121  is formed on the active layer  103  in the distributed feedback active region  131  in the direction in which it extends. The first diffraction grating  121  is formed along the active layer  103  and includes a phase shift (λ/4 shift) portion  121   a  that shifts a phase of the diffraction grating. The phase shift portion  121   a  is set such that a Bragg wavelength of the first diffraction grating  121  becomes uniform. Also, in the present example, the first diffraction grating  121  is formed on the active layer  103 . 
     Further, the first distributed Bragg reflector  132   a  includes a first core layer  113   a  on which a second diffraction grating  122   a  is formed. The first core layer  113   a  is formed continuously with the active layer  103  in the waveguide direction. 
     Further, the second diffraction grating  122   a  is formed along the first core layer  113   a . Similarly, the second distributed Bragg reflector  132   b  includes a second core layer  113   b  on which a third diffraction grating  122   b  is formed. The second core layer  113   b  is formed continuously with the active layer  103  in the waveguide direction on a side opposite to the first core layer  113   a  with the active layer  103  interposed therebetween. Also, the third diffraction grating  122   b  is formed along the second core layer  113   b.    
     The first core layer  113   a  and the second core layer  113   b  have refractive indexes different from that of the active layer  103 . In addition, in the present example, the second diffraction grating  122   a  is formed on (an upper surface of) the first core layer  113   a , and the third diffraction grating  122   b  is formed on (an upper surface of) the second core layer  113   b . Further, in the semiconductor laser, a non-reflective film (not shown) is formed on an output end surface on the first distributed Bragg reflector  132   a  side. 
     In addition, a length L 1  of the first distributed Bragg reflector  132   a  in the waveguide direction, a length L 2  of the distributed feedback active region  131  in the waveguide direction, a length L 3  of the second distributed Bragg reflector  132   b  in the waveguide direction, and a position x ps  of the phase shift portion  121   a  are set to satisfy correlations of x ps =L 1 +L 2 ×α, L 2 (1−a)+L 3 &gt;x ps , and 0.5&lt;α&lt;1. Further, the position x ps  is a position of the phase shift portion  121   a  in the waveguide direction with an end portion thereof on the first distributed Bragg reflector  132   a  side set as an origin. For example, the position x ps  in a case of α=1 is a position adjacent to the other end of the distributed feedback active region  131 , in other words, a boundary between the first diffraction grating  121  and the third diffraction grating  122   b . Also, for example, the position x ps  in a case of α=0.5 becomes a central portion of the distributed feedback active region  131 . 
     As will be described later, when the position of the phase shift portion  121   a  is set to the boundary (α=1) between the second distributed Bragg reflector  132   b  and the distributed feedback active region  131 , influences of manufacturing errors can be inhibited, but an oscillation threshold gain increases due to a diffraction grating loss. On the other hand, when the position of the phase shift portion  121   a  is set to a center (α=0.5) of the distributed feedback active region  131 , it is possible to inhibit an increase in the oscillation threshold gain due to the diffraction grating loss, but there is a risk of multi-mode oscillation due to the influences of manufacturing errors. 
     Because of this, it is preferable that the position of the phase shift portion  121   a  be close to the center of the distributed feedback active region  131  within a range in which the influences of manufacturing errors can be sufficiently inhibited, and an optimum location thereof differs depending on a resonator length and a coupling coefficient of a diffraction grating, but the range is 0.5&lt;α&lt;1. For example, it can be set to α=0.75. 
     Further, when the phase shift portion  121   a  is closer to the second distributed Bragg reflector  132   b  than the center of the distributed feedback active region  131 , an electric field is localized in the phase shift portion  121   a , and as a result, light is emitted from the second distributed Bragg reflector  132   b  side. In a case in which light is emitted from the first distributed Bragg reflector  132   a  side, the length of the second distributed Bragg reflector  132   b  in the waveguide direction is set sufficiently large that a correlation of L 2 (1−x)+L 3 &gt;x ps  is satisfied, and light is extracted only from the first distributed Bragg reflector  132   a.    
     Hereinafter, the semiconductor laser according to the embodiment will be described in more detail with reference to  FIGS. 2A and 2B . Also,  FIG. 2A  shows a cross-section of the distributed feedback active region  131  perpendicular to the waveguide direction. Further,  FIG. 2B  shows a cross-section of the first distributed Bragg reflector  132   a  perpendicular to the waveguide direction. 
     The distributed feedback active region  131 , the first distributed Bragg reflector  132   a , and the second distributed Bragg reflector  132   b  are formed on the same substrate  101 . The distributed feedback active region  131  includes an n-type semiconductor layer  105  and a p-type semiconductor layer  106  formed in contact with the active layer  103 . 
     In the present example, the n-type semiconductor layer  105  and the p-type semiconductor layer  106  are disposed in a plane direction of the substrate  101 , and these are formed on the substrate  101  in contact with side surfaces of the active layer  103  in a direction perpendicular to the waveguide direction. Further, an n-type electrode  107  electrically connected to the n-type semiconductor layer  105  and a p-type electrode  108  electrically connected to the p-type semiconductor layer  106  are provided. In the present example, a current is injected in the plane direction (lateral direction) of the substrate  101 . In addition, the n-type electrode  107  can also be formed on the n-type semiconductor layer  105  via an n-type contact layer in which n-type impurities are introduced at a higher concentration. Similarly, the p-type electrode  108  can be formed on the p-type semiconductor layer  106  via a p-type contact layer in which p-type impurities are introduced at a higher concentration. 
     Also, a lower clad layer  102  is formed on the substrate  101 , and the active layer  103  is formed on the lower clad layer  102 . The first core layer  113   a  is also formed on the lower clad layer  102 . Further, the active layer  103  is interposed between a semiconductor layer  104   a  and a semiconductor layer  104   b  in a vertical direction when viewed from the substrate  101 . Also, a laminated structure of the semiconductor layer  104   a , the active layer  103 , and the semiconductor layer  104   b  is interposed between the n-type semiconductor layer  105  and the p-type semiconductor layer  106 . The p-type semiconductor layer  106  and the n-type semiconductor layer  105  are formed to interpose the active layer  103  in a direction parallel to the plane of the substrate  101 . Further, in the present example, the first diffraction grating  121  is formed on an upper surface of the semiconductor layer  104   b.    
     Here, the active layer  103  is formed in contact with the upper portion of the semiconductor layer  104   a , and the semiconductor layer  104   b  is formed in contact with the upper portion of the active layer  103 . Further, the n-type semiconductor layer  105  and the p-type semiconductor layer  106  are formed in contact with a side portion of the laminated structure of the semiconductor layer  104   a , the active layer  103 , and the semiconductor layer  104   b . Also, the n-type semiconductor layer  105  and the p-type semiconductor layer  106  are not formed in the first distributed Bragg reflector  132   a.    
     In the distributed feedback active region  131  according to the embodiment, a current is injected into the active layer  103  in a direction parallel to the plane of the substrate  101 . Also, the n-type electrode  107  and the p-type electrode  108  are not formed in the first distributed Bragg reflector  132   a  and the second distributed Bragg reflector  132   b.    
     The substrate  101  is made of, for example, silicon. The lower clad layer  102  is made of, for example, silicon oxide (SiO 2 ) and has a thickness of 2 μm. Further, the active layer  103  has, for example, a quantum well structure having a thickness of 150 nm in which well layers and barrier layers made of InGaAsP are alternately laminated. Also, a width of the active layer  103  is about 0.7 μm. Also, a total thickness of the semiconductor layer  104   a , the active layer  103 , and the semiconductor layer  104   b  is 250 nm. In addition, the n-type semiconductor layer  105  and the p-type semiconductor layer  106  also both have a thickness of 250 nm. A light-emitting wavelength of the active layer  103  having the quantum well structure is 1.55 μm. Also, the Bragg wavelength of the first diffraction grating  121  is 1.55 μm. 
     Further, for example, the semiconductor layer  104   a  and the semiconductor layer  104   b  are composed of undoped InP (i-InP). In addition, one n-type semiconductor layer  105  interposing the active layer  103  is composed of n-type InP (n-InP) doped with Si by about 1×10 18  cm −3 , and the other p-type semiconductor layer  106  is composed of p-type InP (p-InP) doped with Zn by about 1×10 18  cm −3 . 
     Also, the first core layer  113   a  and the second core layer  113   b  are composed of undoped InP (i-InP), have a width of about 1.5 μm, and have a thickness of 250 nm. Further, although not shown, the n-type contact layer and the p-type contact layer can be composed of, for example, InGaAs. 
     In the above-mentioned semiconductor laser, the lower clad layer  102  made of silicon oxide having a low refractive index is formed in a lower portion of a layer of InP having a high refractive index, and air having a low refractive index is formed in an upper portion thereof. As a result, strong light confinement in the active layer  103 , the first core layer  113   a , and the second core layer  113   b  is realized, which is advantageous for low power operation of the laser. Further, since the diffraction grating is formed to have a high refractive index difference between the InP layer and the air layer, a high coupling coefficient exceeding 1000 cm −1  can be realized. In addition, according to the above configuration, optical waveguides of the first distributed Bragg reflector  132   a  and the second distributed Bragg reflector  132   b  do not need to be regrown and can be easily manufactured. In this case, a difference in effective refractive index between the distributed feedback active region  131  and the distributed Bragg reflectors is different, but, as will be described later, stable single-mode oscillation can be realized even in consideration of manufacturing errors. 
     The first distributed Bragg reflector  132   a  and the second distributed Bragg reflector  132   b  can also be made of embedded waveguides. In this case, for example, a core is composed of InGaAsP having a composition of a non-gain medium, and a clad covering the core is composed of InP. 
     Hereinafter, a method for manufacturing the semiconductor laser according to the embodiment will be briefly described with reference to  FIGS. 3A to 3F .  FIGS. 3A to 3F  are configuration diagrams showing a state during manufacture of the semiconductor laser according to the embodiment, and schematically show a cross-section of the distributed feedback active region  131 . 
     For example, first, the substrate (silicon substrate)  101  having the lower clad layer  102  made of silicon oxide is prepared. For example, the lower clad layer  102  is formed by thermally oxidizing a main surface of the substrate  101 . 
     On the other hand, a sacrificial layer made of InGaAs, a compound semiconductor layer  204   b  made of undoped InP, a compound semiconductor layer  203  serving as the active layer  103 , a compound semiconductor layer  204   a  made of undoped InP, and a compound semiconductor layer serving as the first core layer  113   a  and the second core layer  113   b  are epitaxially grown on an InP substrate. For example, each layer may be grown using a well-known metal organic vapor phase growth method. 
     Next, the uppermost surface of the epitaxially grown substrate and a surface of the lower clad layer  102  of the substrate  101  described above are directly bonded using a known wafer bonding technique, and then the InP substrate and the sacrificial layer are removed. As a result, as shown in  FIG. 3A , in the distributed feedback active region  131 , the lower clad layer  102 , the compound semiconductor layer  204   a , the compound semiconductor layer  203 , and the compound semiconductor layer  204   b  are formed on the substrate  101 . 
     Next, by performing wet etching, dry etching, and the like using a resist pattern as a mask, which is produced using a known photolithography technique, the compound semiconductor layer  204   a , compound semiconductor layer  203 , compound semiconductor layer  204   b , which are grown, and the like are patterned, and as shown in  FIG. 3B , a striped structure of the distributed feedback active region  131  including the active layer  103  is formed. Also, at this point, the semiconductor layer  204   a  for regrowth is formed over the entire area of the lower clad layer  102 . Further, the compound semiconductor layer  204   b  remains on the active layer  103 . As shown in  FIG. 3C , the first distributed Bragg reflector  132   a  and the second distributed Bragg reflector  132   b  are in a state without the active layer  103 . Also, after each pattern is formed, the resist pattern is removed. 
     Next, as shown in  FIG. 3D , a compound semiconductor layer  205  made of undoped InP is regrown from the semiconductor layer  204   a  around the active layer  103 . By the regrowth, the compound semiconductor layer  204   b  on the active layer  103  becomes integrated with the compound semiconductor layer  205 . The first distributed Bragg reflector  132   a  and the second distributed Bragg reflector  132   b  are in a state in which the compound semiconductor layer  205  is formed on the compound semiconductor layer  204   a , as shown in  FIG. 3E . 
     Next, for example, using an ion implantation method, n-type impurities and p-type impurities are selectively introduced into regions on both sides of the active layer  103 , and thus in the distributed feedback active region  131 , as shown in  FIG. 3F , the n-type semiconductor layer  105  and the p-type semiconductor layer  106  are formed, and the semiconductor layer  104   a  and the semiconductor layer  104   b  are formed. At this stage, the compound semiconductor layer  205  remains in the regions interposing the distributed feedback active region  131  (not shown) in the waveguide direction. 
     Next, the first diffraction grating  121  is formed on the surface of the semiconductor layer  104   b . For example, the first diffraction grating  121  may be formed by using a resist pattern as a mask, which is formed through lithography using electron beam exposure, and patterning using predetermined etching. Similarly, the second diffraction grating  122   a  and the third diffraction grating  122   b  are formed in the regions of the first distributed Bragg reflector  132   a  and the second distributed Bragg reflector  132   b  of the compound semiconductor layer  205  in the regions interposing the distributed feedback active region  131  (not shown) in the waveguide direction. At this stage, the first core layer  113   a  and the second core layer  113   b  are not formed. 
     Next, the compound semiconductor layer  205  in the regions interposing the distributed feedback active region  131  in the waveguide direction is patterned in the same manner as described above, and thus the first core layer  113   a  and the second core layer  113   b  are formed at portions at which the second diffraction grating  122   a  and the third diffraction grating  122   b  are formed. According to this configuration, since the first core layer  113   a  and the second core layer  113   b  of the first distributed Bragg reflector  132   a  and the second distributed Bragg reflector  132   b  are formed with the compound semiconductor layer  205  used for forming the n-type semiconductor layer  105  and the p-type semiconductor layer  106  for current injection, the process can be simplified. Then, the n-type electrode  107  is formed on the n-type semiconductor layer  105 , and the p-type electrode  108  is formed on the p-type semiconductor layer  106 . 
     Hereinafter, the position of the phase shift portion  121   a  and the length of the second distributed Bragg reflector  132   b  in the waveguide direction will be described in more detail. First, problems that may occur due to manufacturing errors will be described with reference to  FIGS. 4 and 5 . Also, in  FIGS. 4 and 5 , for convenience of explanation, the lengths of the first distributed Bragg reflector  132   a  and the second distributed Bragg reflector  132   b  in the waveguide direction are shown to be about the same. 
     For example, as a manufacturing error  1 , as shown in  FIG. 4 , an active layer  103   a  having a length in the waveguide direction shorter than that of the distributed feedback active region  131  (first diffraction grating  121 ) may be formed. In this case, a part of the first core layer  113   a  and the second core layer  113   b  enters a region of the first diffraction grating  121  (distributed feedback active region  131 ). In this way, in the part of the first core layer  113   a  and the second core layer  113   b , which are non-gain media, entering the distributed feedback active region  131 , light reflection on the second diffraction grating  122   a  and the third diffraction grating  122   b  does not occur. As a result, when the manufacturing error as described above occurs, a phase change occurs and the oscillation mode becomes unstable. 
     Further, as a manufacturing error  2 , as shown in  FIG. 5 , the active layer  103   b  may be formed at a position deviated in the waveguide direction. In this case, the position of the first diffraction grating  121  with respect to the active layer  103   b  deviates, and the position of the phase shift portion changes. In this state, since an electric field distribution in a resonator changes, the oscillation mode becomes unstable depending on a state of deviation. 
     In actual manufacturing of the laser, it is necessary to consider that both manufacturing error  1  and manufacturing error  2  occur. Here,  FIG. 6  shows a relationship between an amount of deviation from a design value of the position of the active layer in the waveguide direction and a difference between threshold mode gains in a case in which the length of the active layer in the waveguide direction is formed to be 500 nm shorter in total than the design value with respect to the center of the distributed feedback active region in the waveguide direction evenly toward the first distributed Bragg reflector side and the second distributed Bragg reflector side, respectively. 
     Further, the difference between the threshold mode gains is a threshold mode gain difference “Δg m =g m (2)−g m (1)” between the smallest mode and the second smallest mode. Also, in  FIG. 6 , the position of the phase shift portion is indicated by using α. α=4/8 indicates that it is in the center of the distributed feedback active region, and α=8/8 indicates that it is at the boundary between the distributed feedback active region and the second distributed Bragg reflector. In addition, the length of the distributed feedback active region in the waveguide direction is set to 20 μm, the length of the first distributed Bragg reflector in the waveguide direction is set to 10 μm, and the length of the second distributed Bragg reflector in the waveguide direction is set to 50 μm. Further, a state in which the active layer protrudes toward the second distributed Bragg reflector side is defined as a plus. 
     Further, equivalent refractive indexes of the first distributed Bragg reflector and the second distributed Bragg reflector are 2.5, and an equivalent refractive index of the distributed feedback active region is 2.7. Also, the coupling coefficient of the diffraction gratings in the first distributed Bragg reflector and the second distributed Bragg reflector is 1100 cm −1 . Also, the coupling coefficient of the diffraction grating in the distributed feedback active region is 1000 cm −1 . In addition, the Bragg wavelengths are calculated as 1550 nm. A phase shift amount of the phase shift portion is set to λ/4. 
     As shown in  FIG. 6 , in a case in which the active layer deviates 300 nm toward the first distributed Bragg reflector (—300), the difference is Δg m &lt;50 cm −1  in the case of α&lt;6/8, and thus there is a risk of multi-mode oscillation. For this reason, it can be seen that by setting α≥6/8, stable single-mode oscillation can be realized against manufacturing errors. 
       FIG. 7  shows a relationship between the diffraction grating loss and the threshold mode gain (g m ). It is assumed that the diffraction grating loss is evenly distributed over the distributed feedback active region, the first distributed Bragg reflector, and the second distributed Bragg reflector. Further,  FIG. 8  shows a relationship between an inclination of the graph in  FIG. 7  and a that defines the position x ps  of the phase shift portion. As shown in  FIG. 8 , it can be seen that the larger a is, the higher the threshold gain is due to the diffraction grating loss. 
     From the results shown in  FIG. 6 , it can be seen that the larger a is (the closer the position of the phase shift portion is to the second distributed Bragg reflector side), the more the influences of the manufacturing errors can be inhibited. On the other hand, as shown in  FIGS. 7 and 8 , the larger a is, the stronger the influence of the diffraction grating loss is. Considering these results, α=6/8 is set such that the influences of the manufacturing errors can be sufficiently inhibited, whereby multi-mode oscillation due to the influences of manufacturing errors can be prevented while inhibiting an increase in the threshold gain due to the diffraction grating loss. 
     Incidentally, although the case in which the current is injected in the direction parallel to the plane of the substrate has been described as an example in the above description, the present invention is not limited thereto and may be configured such that the current is injected in the direction perpendicular to the plane of the substrate. For example, as shown in  FIG. 9 , in the distributed feedback active region  331 , the width of the active layer  302  can be formed wider than that of the core layer  312   a , and the p-type semiconductor layer  303  can be formed to have the same width as the core layer  312   a . In this case, a diffraction grating is not formed in the active layer  302 , but the first diffraction grating  321  is formed on the side surface of the p-type semiconductor layer  303  in the waveguide direction. Also, in this case, although not shown, a phase shift portion is provided on the first diffraction grating  321 . Further, an n-type electrode  304  is formed on a back surface of the substrate  301  made of an n-type semiconductor, and a p-type electrode  305  is formed on the p-type semiconductor layer  303 . In this case, the p-type semiconductor layer  303  and the substrate (n-type semiconductor layer)  301  are formed to interpose the active layer  302  from above and below. 
     As described above, according to the present invention, the length of the first distributed Bragg reflector L 1 , the length of the distributed feedback active region L 2 , the length of the second distributed Bragg reflector L 3 , and the position x ps  of the phase shift portion are set to satisfy the correlations of x ps =L 1 +L 2 ×α, L 2 (1−α)+L 3 &gt;x ps , and 0.5&lt;α&lt;1, and thus the phase shift distributed feedback laser having the distributed Bragg reflectors provided on both sides can inhibit influences of manufacturing errors and perform stable single-mode oscillation without increasing the threshold gain. 
     Further, it is clear that the present invention is not limited to the embodiments described above, and many modifications and combinations can be carried out by those having ordinary knowledge in the art within the technical idea of the present invention. For example, the waveguide structure can be applied to a ridge type and a high mesa type waveguide structures. Further, the substrate is composed of InP in the above description, but the present invention is not limited to this, and the substrate may be composed of semiconductors such as GaAs and GaN. In addition, the active layer is not limited to InGaAsP and can be composed of semiconductors such as InGaAlAs, AlGaAs, and InGaN. 
     REFERENCE SIGNS LIST 
     
         
         
           
               101  Substrate 
               102  Lower clad layer 
               103  Active layer 
               104   a  Semiconductor layer 
               104   b  Semiconductor layer 
               105  n-type semiconductor layer 
               106  p-type semiconductor layer 
               107  n-type electrode 
               108  p-type electrode 
               113   a  First core layer 
               113   b  Second core layer 
               121  First diffraction grating 
               121   a  Phase shift portion 
               122   a  Second diffraction grating 
               122   b  Third diffraction grating 
               131  Distributed feedback active region 
               132   a  First distributed Bragg reflector 
               132   b  Second distributed Bragg reflector.