JP2011049535A - Distributed-feedback semiconductor laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a distributed-feedback semiconductor laser capable of suitably providing wavelength selectivity by a diffractive grating, and increasing the output of laser light. <P>SOLUTION: This distributed-feedback semiconductor laser 1A is formed with: an active layer 20 formed on a semiconductor substrate 10 for generating light; an optical guide layer 21 formed on one surface of the active layer 20; and a clad layer 25 formed on a surface of the optical guide layer 21 on the opposite side to the active layer 20, having a refractive index lower than that of the optical guide layer 21, and having a diffractive grating 24 formed on an interface with the optical guide layer 21. The optical guide layer 21 is formed with a first optical guide layer 22 on the active layer 20 side, and a second optical guide layer 23 on the clad layer 25 side. A low-refractive-index layer 30 having a refractive index lower than that of the optical guide layer is formed between the first and second optical guide layers 22, 23. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a distributed feedback semiconductor laser.

  A semiconductor laser is a light emitting element utilizing light emission in an active layer and generation of laser light by light resonance. In an ordinary semiconductor laser, a Fabry-Perot (FP) resonator using a cleavage plane is used as an optical resonator structure for resonating laser light. On the other hand, a distributed feedback (DFB) semiconductor laser is a laser element having a structure in which a diffraction grating is provided for a waveguide structure of an optical resonator. In the DFB laser, oscillation in a single mode can be realized by the wavelength selectivity of the diffraction grating in such a waveguide structure (see, for example, Patent Documents 1 and 2).

Japanese Examined Patent Publication No. 4-46477 JP 2002-57405 A

  In the above-described waveguide structure of the DFB laser, a light guide layer and a clad layer are usually provided on the active layer, and a diffraction grating is formed by an uneven structure formed at the interface between the light guide layer and the clad layer. This diffraction grating realizes characteristics such as single mode oscillation. However, in such a configuration, there is a problem that it is difficult to increase the output of the laser beam while obtaining the wavelength selectivity for the oscillation of the laser beam.

  The present invention has been made to solve the above-described problems, and provides a distributed feedback semiconductor laser capable of suitably obtaining wavelength selectivity by a diffraction grating and increasing the output of laser light. With the goal.

  In order to achieve such an object, a distributed feedback semiconductor laser according to the present invention includes (1) an active layer formed on a semiconductor substrate and generating light, and (2) formed on one surface of the active layer. And (3) formed on the surface of the light guide layer opposite to the active layer, having a lower refractive index than the light guide layer, and forming a diffraction grating at the interface with the light guide layer (4) The light guide layer includes a first light guide layer on the active layer side and a second light guide layer on the clad layer side, and the first light guide layer and the second light guide layer. A low refractive index layer having a lower refractive index than that of the light guide layer is provided between the guide layer and the guide layer.

  In the distributed feedback semiconductor laser (DFB laser) described above, a diffraction grating is formed on one side (upper side or lower side) of the active layer in the waveguide structure including the light guide layer and the clad layer provided for the active layer. The light guide layer thus formed is divided into two layer portions, a first light guide layer and a second light guide layer. The refractive index is lower than that of the surrounding light guide layer at a position between these layer portions and between the diffraction grating at the interface between the second light guide layer and the clad layer and the active layer. A low refractive index layer configured as described above is provided.

  In such a configuration, since the refractive index of the low refractive index layer is lower than the refractive indexes of the first and second light guide layers, the low refractive index layer is applied to light spreading from the active layer to the outside of the waveguide structure. It functions as a barrier layer, and the light intensity distribution is attenuated in the low refractive index layer. In the DFB laser configured as described above, the coupling coefficient κ of the laser beam forward and backward waves reciprocating in the optical resonator is adjusted and reduced by the attenuation of the light in the low refractive index layer. As a result, a DFB laser capable of suitably obtaining wavelength selectivity by the diffraction grating and capable of increasing the output of the laser light is realized.

  In the DFB laser having the above-described configuration, the waveguide structure of the optical resonator including the active layer, the optical guide layer, the low refractive index layer, the cladding layer, and the diffraction grating is adjusted for the coupling coefficient κ by the low refractive index layer. It is preferable that κL be 0.6 or more and 0.8 or less, where κ is the coupling coefficient and L is the resonator length of the optical resonator. Thereby, it is possible to suitably achieve both wavelength selectivity by the diffraction grating and high output of the laser beam.

  In addition, the light guide layer, the low refractive index layer, the cladding layer, and the diffraction grating are preferably provided on the opposite side of the active layer from the semiconductor substrate (upper side of the active layer). Such a configuration is effective, for example, in suitably forming a semiconductor laminated structure including a diffraction grating in a DFB laser manufacturing process. In such a configuration, it is preferable that a normal light guide layer and a cladding layer except for the low refractive index layer and the diffraction grating are provided on the lower side of the active layer.

  In this case, the low refractive index layer includes the first low refractive index layer on the first light guide layer side and the second light guide layer side, and the refractive index is changed from the first low refractive index layer to the second light guide layer. And a second low refractive index layer having a graded structure that changes toward the surface. As described above, by configuring the upper layer portion of the low refractive index layer with the graded layer, a semiconductor multilayer structure including the low refractive index layer and each semiconductor layer thereabove can be suitably formed. . Note that the refractive index of the first low refractive index layer is preferably substantially constant.

  In the DFB laser, it is preferable that the upper semiconductor laminated portion including the second light guide layer and the clad layer has a ridge structure in which a low refractive index layer is formed as an etch stop layer. Thus, by using the low refractive index layer as an etch stop layer, a ridge structure for confining light in the lateral direction can be suitably formed.

  According to the distributed feedback semiconductor laser of the present invention, in the waveguide structure including the light guide layer and the clad layer on the active layer, the light guide layer is divided into two layer portions, the first and second light guide layers. By providing a low refractive index layer at a position between the active layer and the diffraction grating at the interface between the second light guide layer and the clad layer, the wavelength selectivity by the diffraction grating is suitable. And a DFB laser capable of increasing the output of the laser beam.

It is side surface sectional drawing which shows the structure of one Embodiment of a distributed feedback semiconductor laser. FIG. 2 is a perspective view showing a configuration of the distributed feedback semiconductor laser shown in FIG. 1. It is a schematic diagram showing a light intensity distribution in a waveguide structure of a distributed feedback semiconductor laser. It is side surface sectional drawing which shows the structure of other embodiment of a distributed feedback type semiconductor laser. It is a figure shown about an example of the manufacturing method of a distributed feedback type semiconductor laser. It is a perspective view which shows an example of a structure of the conventional distributed feedback semiconductor laser. It is a graph which compares the characteristic of the distributed feedback semiconductor laser of a conventional structure, and the characteristic of a Fabry-Perot type semiconductor laser. It is a graph which compares the characteristic of the distributed feedback semiconductor laser of a novel structure, and the characteristic of a Fabry-Perot type semiconductor laser. It is a graph which shows the wavelength dependence of the output light intensity of a laser beam. It is a figure shown about the 1st structural example of a distributed feedback semiconductor laser. 11 is a chart showing characteristics of the semiconductor laser shown in FIG. It is a figure shown about the 2nd structural example of a distributed feedback semiconductor laser. 13 is a chart showing characteristics of the semiconductor laser shown in FIG. It is a figure shown about coupling coefficient (kappa) of a diffraction grating.

  Hereinafter, preferred embodiments of a distributed feedback semiconductor laser according to the present invention will be described in detail with reference to the drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. Further, the dimensional ratios in the drawings do not necessarily match those described.

  FIG. 1 is a side sectional view showing a configuration of an embodiment of a distributed feedback semiconductor laser (DFB laser) according to the present invention. In FIG. 1, FIG. 1A shows a semiconductor multilayer structure in the DFB laser by a side sectional view along the light resonance direction in the waveguide structure of the optical resonator of the DFB laser. FIG. 1B shows the refractive index distribution in the stacking direction in the waveguide structure of FIG. FIG. 2 is a perspective view showing the configuration of the DFB laser shown in FIG.

  In the DFB laser 1A according to the present embodiment, the lower clad layer 15, the lower light guide layer 11, the active layer 20, the upper light guide layer 21, the upper clad layer 25, and the contact layer 28 are laminated on the semiconductor substrate 10 in this order. Configured. The active layer 20 is constituted by a single quantum well (SQW) active layer or a multiple quantum well (MQW) active layer, and generates light for generating laser light by resonance in the optical resonator of the DFB laser 1A. Let

  On the lower surface side of the active layer 20, the lower light guide layer 11 having a refractive index lower than that of the active layer and having a thickness X0 is provided. A lower cladding layer 15 having a lower refractive index than the light guide layer is provided on the opposite side of the lower light guide layer 11 from the active layer. On the other hand, an upper light guide layer 21 having a refractive index lower than that of the active layer is provided on the upper surface side of the active layer 20. An upper cladding layer 25 having a lower refractive index than that of the light guide layer is provided on the opposite side of the upper light guide layer 21 from the active layer. In the present DFB laser 1A, a waveguide structure of an optical resonator is configured by such a semiconductor laminated structure.

  The lower light guide layer 11 and the upper light guide layer 21 are preferably formed of the same semiconductor material and refractive index. Similarly, the lower cladding layer 15 and the upper cladding layer 25 are preferably formed of the same semiconductor material and refractive index. Further, at the interface between the upper light guide layer 21 and the upper cladding layer 25, a concavo-convex structure serving as a diffraction grating 24 for constituting the DFB laser 1A is formed in the layer portion having the thickness X3.

  In the present embodiment, the upper light guide layer 21 includes a first light guide layer 22 having a thickness X1 on the active layer 20 side (lower side) and a second light guide layer having a thickness X2 on the clad layer 25 side (upper side). 23 and two layer portions. Further, between the first light guide layer 22 and the second light guide layer 23, a low refractive index layer 30 having a thickness D having a refractive index lower than that of the light guide layers 22 and 23 is provided. As shown in the configuration example of FIG. 1, the low refractive index layer 30 is preferably configured to have a refractive index lower than that of the cladding layer 25.

  As shown in the perspective view of FIG. 2, the second light guide layer 23, the upper clad layer 25, and the contact layer 28 above the low refractive index layer 30 are etched to resonate laser light in the optical resonator. A ridge structure extending along the direction is formed. The low refractive index layer 30 described above is preferably configured to function as an etch stop layer in the formation of such a ridge structure.

  The effect of the DFB laser 1A according to the present embodiment will be described.

  In the DFB laser 1A shown in FIG. 1 and FIG. 2, in the waveguide structure formed by the light guide layers 11 and 21 and the clad layers 15 and 25 provided for the active layer 20, it is diffracted on one side of the active layer 20. The upper light guide layer 21 on which the grating 24 is formed is divided into two layer portions, a first light guide layer 22 and a second light guide layer 23. The refractive index is between the layer portions 22 and 23 and between the diffraction grating 24 at the interface between the second light guide layer 23 and the upper cladding layer 25 and the active layer 20. A low refractive index layer 30 configured to be lower than the light guide layers 22 and 23 is provided.

  In such a configuration, the refractive index of the low refractive index layer 30 is lower than the refractive indexes of the first and second light guide layers 22 and 23 as shown in the graph of the refractive index distribution in FIG. The low refractive index layer 30 functions as a barrier layer against light spreading from the active layer 20 to the outside of the waveguide structure, so that light is attenuated. In the DFB laser 1A having the above-described configuration, the coupling coefficient κ of the laser beam forward and backward waves reciprocating in the optical resonator is adjusted and reduced by the attenuation of light in the low refractive index layer 30. Thereby, the wavelength selectivity by the diffraction grating 24 is suitably obtained, and the DFB laser 1A capable of increasing the output of the laser light is realized.

  Here, FIG. 3 is a schematic diagram showing the light intensity distribution in the waveguide structure of the optical resonator of the DFB laser. In FIGS. 3A and 3B, only the formation range of the diffraction grating 24 is indicated by a broken line for the sake of simplicity of illustration. Further, regarding the light intensity distribution in the waveguide structure, the light intensity distribution in the vertical direction (semiconductor stacking direction in the laser element) perpendicular to the resonance direction in the optical resonator is shown.

  FIG. 3A shows the light intensity distribution A2 in the waveguide structure of the DFB laser 2 having the conventional structure in which the low refractive index layer 30 is not provided. The configuration of the DFB laser 2 is the same as that of the DFB laser 1A shown in FIG. 1 except that the low refractive index layer 30 is not provided. FIG. 3B shows a light intensity distribution A1 in the waveguide structure of the DFB laser 1A having the low refractive index layer 30 shown in FIG.

  As shown in FIG. 3A, in the DFB laser 2 having the conventional structure, the light intensity distribution A2 in the waveguide structure of the optical resonator has a Gaussian function distribution that is substantially vertically symmetrical about the active layer 20. It becomes. Further, in such a light intensity distribution A2, the coupling coefficient κ for the resonant laser light is the size of the overlapping portion B2 between the light intensity distribution A2 and the diffraction grating 24 (the ratio Γ of the overlapping portion B2 to the entire light intensity distribution A2). (Grating) = Γ (G)).

  On the other hand, in the DFB laser 1A having a novel structure in which the low refractive index layer 30 is provided in the upper light guide layer 21, as shown in FIG. 3B, the low refraction that acts as a barrier layer in the light intensity distribution A1. The light intensity attenuates at the rate layer 30. As a result, the size of the overlapping portion B1 between the light intensity distribution A1 and the diffraction grating 24 is reduced, the light reaching the diffraction grating 24 is reduced, and the coupling coefficient κ is reduced. At this time, the light intensity distribution in the resonance direction becomes uniform and approaches the case of a Fabry-Perot (FP) laser, so that it is possible to achieve high output of laser light.

  In the DFB laser 1A having the above-described configuration, the adjustment of the coupling coefficient κ by the low refractive index layer 30 includes the active layer 20, the light guide layers 11 and 21, the low refractive index layer 30, the cladding layers 15 and 25, and the diffraction grating 24. The waveguide structure of the included optical resonator may be configured such that κL is 0.6 or more and 0.8 or less when the coupling coefficient is κ and the resonator length of the optical resonator is L. preferable.

  By setting the coupling coefficient κ and the resonator length L in this way, the wavelength selectivity by the diffraction grating 24 and the high power output of the laser light from the DFB laser 1A are preferably compatible, and the characteristics are as a whole. Can be improved. As for the coupling coefficient κ, the refractive indexes of the light guide layer 21, the low refractive index layer 30, and the cladding layer 25, the thicknesses X1 and X2 of the first and second light guide layers 22 and 23, and the low refractive index. By controlling the design value of each parameter such as the thickness D of the layer 30, it can be set and adjusted to a desired value.

  In the above embodiment, the first and second light guide layers 22 and 23, the low refractive index layer 30, the cladding layer 25, and the diffraction grating 24 are opposite to the semiconductor substrate 10 with respect to the active layer 20, that is, It is provided on the upper side of the active layer 20. Such a configuration is effective, for example, in suitably forming a semiconductor laminated structure including the diffraction grating 24 in the manufacturing process of the DFB laser 1A. In such a configuration, the lower side of the active layer 20 is also provided with a normal light guide layer 11 and a cladding layer 15 excluding a low refractive index layer and a diffraction grating, as shown in FIG. It is preferable that the waveguide structure of the optical resonator is constituted as a whole. In addition, the laminated portion including the light guide layer, the low refractive index layer, the cladding layer, and the diffraction grating can generally be provided below the active layer 20 (on the semiconductor substrate 10 side with respect to the active layer 20). It is.

  Further, in such a DFB laser 1A, as shown in the perspective view of FIG. 2, in the upper semiconductor laminated portion including the second light guide layer 23 and the upper cladding layer 25, the low refractive index layer 30 is used as the etch stop layer. It is preferable to have a ridge structure formed as: As described above, by using the low refractive index layer 30 as an etch stop layer when manufacturing the laser element, a ridge structure for confining light in the lateral direction can be suitably formed.

  Further, as described above, in the configuration in which the semiconductor laminated structure including the low refractive index layer 30 and the diffraction grating 24 is provided on the upper side with respect to the active layer 20, the low refractive index layer 30 has the first low refractive index on the lower side. It is good also as a structure which has a layer and the 2nd low-refractive-index layer which has a graded structure on the upper side. Here, FIG. 4 is a side sectional view showing the configuration of another embodiment of the DFB laser. In FIG. 4, FIG. 4A shows a semiconductor laminated structure in the DFB laser by a side sectional view along the light resonance direction. FIG. 4B shows a refractive index distribution in the stacking direction in the waveguide structure. In the DFB laser 1B of the present embodiment, the configuration other than the low refractive index layer 30 is the same as the configuration of the DFB laser 1A shown in FIG.

  In the DFB laser 1B shown in FIG. 4, the low refractive index layer 30 is on the first light guide layer 22 side and the first low refractive index layer 31 having a substantially constant refractive index and on the second light guide layer 23 side. The second low refractive index layer 32 having a graded structure in which the refractive index changes from the first low refractive index layer 31 toward the second light guide layer 23. As described above, by configuring the upper layer portion of the low refractive index layer 30 with the graded layer 32, a semiconductor multilayer structure including the low refractive index layer 30 and the semiconductor layers thereabove is preferably formed. be able to.

  Here, Patent Document 1 (Japanese Patent Publication No. 4-46477) describes a configuration in which a barrier layer is inserted between an active layer and a light guide layer. However, the barrier layer described in Document 1 is for reducing the reactive current flowing into the light guide layer, and is a low refractive index layer introduced to adjust the coupling coefficient κ in the DFB laser of the present application. Is different. In the configuration of Document 1, since the barrier layer is formed directly on the active layer, the coupling coefficient κ cannot be adjusted appropriately.

  Patent Document 2 (Japanese Patent Laid-Open No. 2002-57405) describes that an InGaAsP etching stop layer is provided in an InP cladding layer. However, in such a configuration, since the InGaAsP layer has a higher refractive index than the InP layer, it cannot function as a low refractive index layer for adjusting the coupling coefficient κ.

  On the other hand, in the DFB lasers 1A and 1B according to the above embodiment, the low refractive index layer 30 is not directly formed on the active layer 20, but the light guide layer 21 is replaced with the first and second light guide layers 22, 23, the low refractive index layer 30 is provided on the active layer 20 via the first light guide layer 22. According to such a configuration, by appropriately setting the respective refractive indexes of the first and second light guide layers 22 and 23 and the low refractive index layer 30, the thicknesses of the semiconductor layers, and the like, the waveguide It is possible to suitably adjust and set the light intensity distribution in the structure, the coupling coefficient κ of the diffraction grating 24, and the resulting element characteristics of the DFB laser.

  An example of a method for manufacturing a DFB laser according to the present invention will be described with reference to FIG. 5 together with a specific configuration example of a semiconductor multilayer structure in the DFB laser. In addition, here, as shown in FIG. 4, as an example, the low refractive index layer 30 includes the lower first low refractive index layer 31 and the upper graded layer 32. Will be described.

In this example, first, as shown in FIG. 5A, in the first crystal growth, an n-type Al _0.43 Ga _0.57 As lower cladding having a thickness of 1.5 μm is formed on the n-type GaAs substrate 10. Layer 15, 500 nm thick non-doped Al _0.36 Ga _0.64 As lower light guide layer 11, AlInGaAs quantum well active layer 20, 150 nm thick non-doped Al _0.36 Ga _0.64 As first light guide layer 22 A non-doped Al _0.70 Ga _0.30 As low refractive index layer 31 having a thickness of 20 nm, a non-doped Al _{0.70 → 0.36} Ga _{0.30 → 0.64} As graded layer 32 having a thickness of 50 nm, and a thickness A non-doped Al _0.36 Ga _0.64 As second light guide layer 23 having a thickness of 350 nm is formed in order, and then taken out from the crystal growth furnace.

  Next, a photoresist film is applied onto the second light guide layer 23 on the outermost surface in the semiconductor multilayer structure formed by the first crystal growth, and a diffraction grating pattern of the photoresist is formed with a period of about 240 nm by interference exposure. . Then, as shown in FIG. 5B, using this photoresist as a mask, grooves are formed by chemical etching to form a diffraction grating 24 having a depth of about 50 nm, and then the photoresist film is removed.

Subsequently, the crystal structure in which the diffraction grating 24 is formed is again introduced into the growth furnace, and in the second crystal growth, the p-type Al _0.43 Ga _0.57 As upper clad layer 25 having a thickness of 1.0 μm. And a p-type GaAs contact layer 28 are formed. Further, the low refractive index layer 30 is used as an etch stop layer, and a ridge structure is formed by wet etching. Thereby, the DFB laser having the structure shown in FIGS. 1, 2, and 4 is obtained. As for the ridge structure, by setting the ridge width to about 5 μm, for example, the light can be sufficiently confined in the lateral direction to realize single transverse mode oscillation.

  Next, the characteristics of the distributed feedback semiconductor laser (DFB laser) according to the present invention having the above-described configuration will be described in comparison with the characteristics of a conventional DFB laser or a Fabry-Perot semiconductor laser (FP laser). FIG. 6 is a perspective view showing an example of the configuration of a conventional DFB laser. The DFB laser 3 shown in FIG. 6 is different from the DFB laser 1A having the new structure shown in FIGS. 1 and 2 in the configuration of the upper light guide layer 21, the upper cladding layer 25, and the low refractive index layer 35.

  That is, in the DFB laser 3 of FIG. 6, the upper light guide layer 21 is configured by a single semiconductor layer. The upper clad layer 25 includes a first clad layer 26 on the light guide layer 21 side and a second clad layer 27 on the contact layer 28 side, and a low refractive index layer 35 is provided therebetween. . The second cladding layer 27 and the contact layer 28 above the low refractive index layer 35 are formed to have a ridge structure.

  In the DFB laser 3 having such a conventional structure, since the low refractive index layer 35 is located above the diffraction grating 24, the coupling coefficient κ cannot be reduced. At this time, since the light intensity distribution in the resonance direction becomes larger than that of the FP laser, there may be a problem that the light output characteristic is deteriorated or the characteristic variation for each element becomes large.

  FIG. 7 is a graph comparing the characteristics of the DFB laser having the conventional structure shown in FIG. 6 with the characteristics of the FP laser. Here, the structure of the FP laser is assumed to be a structure in which only the diffraction grating 24 is removed from the DFB laser shown in FIG. 6 (conventional FP laser). In the graph of FIG. 7, the horizontal axis represents the injection current (mA), and the vertical axis represents the optical output (mW). Three graphs indicated by C1 indicate current-light output characteristics in the FP laser having the conventional structure. In addition, three graphs indicated by C2 indicate current-light output characteristics in a DFB laser having a conventional structure.

In these graphs C1 and C2, the slope efficiency of the FP laser is about 0.72 (W / A), whereas the slope efficiency of the DFB laser is reduced to about 0.35 (W / A). Yes. In the conventional structure shown in FIG. 6, the coupling coefficient κ is estimated to be about 10 to 30 cm ⁻¹ from the laminated structure, and this value is considered too large to be suitable for high output. However, if the coupling coefficient κ is too small, the diffraction grating 24 does not function and single-wavelength oscillation is not realized. Therefore, it is required to set the coupling coefficient κ to an appropriate value.

  FIG. 8 is a graph comparing the characteristics of the new structure DFB laser shown in FIG. 4 (see FIG. 2 for the ridge structure) and the characteristics of the FP laser. Here, the structure of the FP laser is assumed to be a structure (new structure FP laser) in which only the diffraction grating 24 is removed from the DFB laser shown in FIG.

  In the graph of FIG. 8, the graph indicated by D1 indicates the current-light output characteristics of the FP laser having a new structure. Further, three data plots indicated by D2 indicate current-light output characteristics in a DFB laser having a new structure in which the total thickness of the light guide layer is 600 nm, 800 nm, and 1000 nm, respectively. In the above three types of DFB lasers, the thickness of the first light guide layer 22 corresponding to the distance between the active layer 20 and the low refractive index layer 30 is fixed at 150 nm.

In each of these graphs, in the graph D1, the slope efficiency of the FP laser having a new structure is about 0.93 (W / A). In contrast, in the three data plots D2, the low refractive index layer 30 is provided in the light guide layer 21 even in the DFB structure in which the diffraction grating 24 is provided at the interface between the light guide layer 21 and the cladding layer 25. Thus, it can be seen that the slope efficiency equivalent to that of the FP structure is obtained. In this new structure, the coupling coefficient κ is suppressed to about 4 cm ⁻¹ , and even if the total thickness of the light guide layer is changed as described above, there is no significant change in the characteristics.

  Here, in the DFB laser having the conventional structure in which the low refractive index layer 30 is not provided, the coupling coefficient κ greatly changes as the thickness of the light guide layer changes. On the other hand, in the DFB laser having a new structure in which the low refractive index layer 30 is provided between the active layer 20 and the diffraction grating 24, the overlapping portion between the light intensity distribution in the waveguide structure and the diffraction grating 24 is reduced. . At the same time, even if the thickness of the light guide layer is changed as described above, the value of the coupling coefficient κ settles to the same level, and the light confinement coefficient to the quantum well active layer 20 is also controlled to the same level. Due to these characteristics and effects, the DFB laser having a new structure can be expected to have a great effect on the productivity of the laser device, such as suppressing variations in characteristics and improving the manufacturing yield.

  FIG. 9 is a graph showing the wavelength dependency of the output light intensity of the laser light output from the semiconductor laser (the wavelength spectrum of the output laser light). In the graph of FIG. 9, the horizontal axis indicates the wavelength (nm) and the vertical axis indicates the output light intensity (dBm). The graph E0 shows the output light spectrum of the FP laser, the graph E1 shows the output light spectrum of the DFB laser with a new structure at an optical output of 100 mW, and the graph E2 shows the DFB laser with a new structure at an optical output of 200 mW. The output light spectrum of is shown.

  Regarding the output characteristics of the DFB laser, in the graph E1 with an optical output of 100 mW, the center wavelength is 810.72 nm. In the graph E2 with an optical output of 200 mW, the center wavelength is 810.96 nm. Further, in these graphs E1 and E2, the peak width is sufficiently narrower than the FP laser graph E0 FWHM <0.15 nm, and the wavelength selectivity by the diffraction grating 24 is sufficient up to an optical output of about 200 mW. It can be seen that single wavelength oscillation is realized.

  The configuration and characteristics of the DFB laser according to the present invention will be further described with reference to FIGS.

  FIG. 10 is a diagram illustrating a first configuration example of the DFB laser. This configuration example shows a configuration of a DFB laser using an AlGaAs-based material when the semiconductor substrate 10 is a GaAs substrate. In the present DFB laser, as shown in FIG. 10A, the lower light guide layer 11 and the first and second light guide layers 22 and 23 of the upper light guide layer are made of AlGaAs (Al = 36%), and are clad layers. 15 and 25 are made of AlGaAs (Al = 43%), and the low refractive index layer 30 is made of AlGaAs (Al = 70%).

  As shown in the chart of FIG. 10B, in this configuration example, the oscillation wavelength is set to 810 nm, the refractive index of the quantum well active layer 20 is set to 3.7, and the width of the quantum well is set to 10 nm. Examples of the semiconductor material of the active layer 20 in this case include AlGaAs and InAlGaAs. The refractive index of each other semiconductor layer, the order of the diffraction grating, and the diffraction grating duty ratio (%) are as shown in FIG.

  FIG. 11 is a chart showing the characteristics of the DFB laser of the AlGaAs material shown in FIG. Here, the thickness of the lower light guide layer 11 is fixed at X0 = 500 nm, and the thicknesses of the upper first and second light guide layers 22 and 23 are set so as to satisfy X1 + X2 = 450 nm. Yes.

In the charts (a) and (b) of FIG. 11, the thicknesses X1 and X2 of the first and second light guide layers 22 and 23 and the set values of the thickness D of the low refractive index layer 30 are respectively shown. , The ratio of optical confinement to the quantum well active layer 20 Γ (QW), the ratio of the overlapping portion with the diffraction grating 24 to the entire light intensity distribution in the waveguide structure Γ (G) (see FIG. 3), the coupling coefficient κ (cm ^-1), and shows the results obtained by simulation the value of κL when the resonator length of the optical resonator was L = 0.2 cm.

  Here, Γ (QW) indicating optical confinement in the quantum well active layer 20 is a parameter that determines various characteristics of the device, such as an optical threshold damage (COD) mechanism related to the oscillation threshold, efficiency, and lifetime of the laser element. This change in Γ (QW) greatly affects the device characteristics. Further, κL is a parameter that effectively affects the characteristics of the DFB laser, and it is said that about κL = 2 to 3 is suitable for a general DFB laser.

  The chart (a) of FIG. 11 shows changes in the characteristics of the DFB laser when the thickness of the low refractive index layer 30 is fixed at D = 50 nm and the position of the low refractive index layer 30 in the light guide layer is changed. Show. Also, the chart (b) of FIG. 11 shows changes in the characteristics of the DFB laser when the position of the low refractive index layer 30 in the light guide layer is fixed and the thickness D of the low refractive index layer 30 is changed. ing.

  In these configurations, it has been found that κL is preferably 0.6 or more from the viewpoint of the stability of single wavelength oscillation. Moreover, it is predicted that the upper limit of κL is about 0.8 based on the experimental results of the DFB laser having the conventional structure in which the low refractive index layer 30 is not introduced. When κL is larger than 0.8, the wavelength stability is improved while the output is reduced.

  Also, from the experimental results of a normal FP laser, it was found that Γ (QW) is preferably about 2 from the viewpoint of characteristics and lifetime. Considering these points, the waveguide structure of the optical resonator in the above-described DFB laser having a new structure is preferably configured so that κL is 0.6 or more and 0.8 or less. In the charts (a) and (b) of FIG. 11, such ranges of κL are indicated by R1 and R2, respectively.

  FIG. 12 is a diagram illustrating a second configuration example of the DFB laser. This configuration example shows a configuration of a DFB laser using an AlInGaP-based material when the semiconductor substrate 10 is a GaAs substrate. In this DFB laser, as shown in FIG. 12A, the lower light guide layer 11 and the first and second light guide layers 22 and 23 of the upper light guide layer are made of InGaP, and the cladding layers 15 and 25 are made of AlInGaP ( Al = 15%), and the low refractive index layer 30 is made of AlInGaP (Al = 80%).

  12B, in this configuration example, the oscillation wavelength is set to 808 nm, the refractive index of the quantum well active layer 20 is set to 3.6, and the width of the quantum well is set to 10 nm. In this case, the semiconductor material of the active layer 20 is, for example, InGaAsP. The refractive index of each other semiconductor layer, the order of the diffraction grating, and the diffraction grating duty ratio (%) are as shown in FIG.

  FIG. 13 is a chart showing the characteristics of the AlInGaP-based material DFB laser shown in FIG. Here, similarly to FIG. 11, the thickness of the lower light guide layer 11 is fixed at X0 = 500 nm, and the thicknesses of the upper first and second light guide layers 22 and 23 are set to X1 + X2 = 450 nm. It is set to meet.

  The chart (a) in FIG. 13 shows changes in the characteristics of the DFB laser when the thickness of the low refractive index layer 30 is fixed at D = 50 nm and the position of the low refractive index layer 30 in the light guide layer is changed. Show. Further, the chart (b) of FIG. 13 shows changes in the characteristics of the DFB laser when the position of the low refractive index layer 30 in the light guide layer is fixed and the thickness D of the low refractive index layer 30 is changed. ing. Further, in the charts (a) and (b) of FIG. 13, the above-described preferable κL ranges are indicated by R3 and R4, respectively.

  As shown in the first and second configuration examples in FIGS. 10 to 13, the coupling coefficient κ of the diffraction grating 24 in the DFB laser is an overlapping portion with the diffraction grating 24 with respect to the entire light intensity distribution in the waveguide structure. Depends on the ratio Γ (G) (%). This Γ (G) varies depending on the semiconductor material system used for the DFB laser, the oscillation wavelength, and the like.

FIG. 14 is a diagram illustrating the coupling coefficient κ. In the graph (a) of FIG. 14, the horizontal axis indicates the overlapping ratio Γ (G) (%) between the light intensity distribution and the diffraction grating 24, and the vertical axis indicates the coupling coefficient κ (cm ⁻¹ ). . The graph F1 shows the correlation between Γ (G) and the coupling coefficient κ at a wavelength λ = 810 nm in an AlGaAs-based DFB laser. A graph F2 shows a correlation between Γ (G) and a coupling coefficient κ at a wavelength λ = 808 nm in a DFB laser made of an AlInGaP-based material.

  The chart (b) in FIG. 14 shows data corresponding to the graph F1 in FIG. 14 (a). Further, the chart (c) in FIG. 14 shows data corresponding to the graph F2 in FIG. As shown in these data, in the DFB laser having the above structure, Γ (G) and the coupling coefficient κ have a substantially linear correlation. Therefore, the value of Γ (G) is obtained by the low refractive index layer 30. By adjusting, the coupling coefficient κ of the diffraction grating 24 can be adjusted.

  The distributed feedback semiconductor laser according to the present invention is not limited to the above-described embodiments and configuration examples, and various modifications are possible. For example, as for the material of each semiconductor layer constituting the semiconductor multilayer structure of the DFB laser, various materials are specifically used as long as the above refractive index distribution in the waveguide structure of the optical resonator can be realized. Good.

  INDUSTRIAL APPLICABILITY The present invention can be used as a distributed feedback semiconductor laser capable of suitably obtaining wavelength selectivity by a diffraction grating and increasing the output of laser light.

  DESCRIPTION OF SYMBOLS 1A, 1B ... Distributed feedback type semiconductor laser (DFB laser), 10 ... Semiconductor substrate, 11 ... Lower light guide layer, 15 ... Lower clad layer, 20 ... Active layer, 21 ... Upper light guide layer, 22 ... First light guide Layer, 23 ... second light guide layer, 24 ... diffraction grating, 25 ... upper cladding layer, 28 ... contact layer, 30 ... low refractive index layer, 31 ... first low refractive index layer, 32 ... second low refractive index layer (Graded layer).

Claims (5)

An active layer formed on a semiconductor substrate and generating light;
A light guide layer formed on one surface of the active layer;
A cladding layer formed on a surface of the light guide layer opposite to the active layer, having a refractive index lower than that of the light guide layer, and having a diffraction grating formed at an interface with the light guide layer; Prepared,
The light guide layer is constituted by a first light guide layer on the active layer side and a second light guide layer on the cladding layer side, and between the first light guide layer and the second light guide layer. A distributed feedback semiconductor laser comprising a low refractive index layer having a refractive index lower than that of the light guide layer.   The waveguide structure of the optical resonator including the active layer, the light guide layer, the low refractive index layer, the clad layer, and the diffraction grating has a coupling coefficient κ and a resonator length of the optical resonator L The distributed feedback semiconductor laser according to claim 1, wherein κL is configured to be not less than 0.6 and not more than 0.8.   3. The light guide layer, the low refractive index layer, the clad layer, and the diffraction grating are provided on the side opposite to the semiconductor substrate with respect to the active layer. Distributed feedback semiconductor laser.   The low refractive index layer includes a first low refractive index layer on the first light guide layer side and a refractive index on the second light guide layer side from the first low refractive index layer to the second light guide layer. 4. The distributed feedback semiconductor laser according to claim 3, comprising a second low refractive index layer having a graded structure that changes toward.   5. The laminated portion including the second light guide layer and the clad layer has a ridge structure formed using the low refractive index layer as an etch stop layer. Distributed feedback semiconductor laser.