JP2013021023A - Semiconductor laser element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor laser element which can reduce variation in an oscillation wavelength and suppress wavelength chirping even in the case where a modulation current is injected to an active layer.SOLUTION: A semiconductor laser element 1A comprises an n-type clad layer 15, a p-type clad layer 19, a p-type contact layer 20 and an active layer 17. The semiconductor laser element 1A further comprises: a diffraction grating 24 including a diffraction grating layer 13 and an adhesion layer 14 provided on the diffraction grating layer 13, and provided on a location sandwiching the n-type clad layer 15 with the active layer 17; a cathode electrode 21 contacting the n-type clad layer 15; and an anode electrode 22 contacting the p-type contact layer 20. The active layer 17 composes a first optical guide and the diffraction grating layer 13 and the adhesion layer 14 compose a second optical guide. The first optical guide and the second optical guide are optically connected via the n-type clad layer 15 to compose a laser resonator.

Description

  The present invention relates to a semiconductor laser device.

  Patent Document 1 describes a distributed feedback laser (DFB laser) formed on a semiconductor substrate. In this distributed feedback semiconductor laser, a p-type InP cladding layer, a light guide layer, a strained quantum well active layer, and an n-type InP cladding layer are formed in this order on a p-type InP substrate. A diffraction grating is formed at the interface between the p-type InP cladding layer and the light guide layer. The refractive indexes of the p-type InP cladding layer and the n-type InP cladding layer are smaller than those of the light guide layer and the strained quantum well active layer. That is, this distributed feedback semiconductor laser has a single optical waveguide composed of a p-type InP cladding layer, an optical guide layer, a strained quantum well active layer, and an n-type InP cladding layer. The well active layer functions as the core region of this optical waveguide. The light generated in the active layer is confined in the light guide layer and the strained quantum well active layer sandwiched between the p-type InP cladding layer and the n-type InP cladding layer.

JP-A-7-249829

  The conventional DFB laser having a structure as shown in Patent Document 1 has the following problems. When a modulation current is applied to the DFB laser, a modulated optical signal is output. By the way, the refractive indexes of the active layer and the light guide layer of the DFB laser change according to the magnitude of the current flowing through the active layer and the light guide layer. Therefore, when a high-frequency modulation current is applied to the semiconductor laser element, the refractive indexes of the active layer and the light guide layer also change according to the change in the amount of current. By changing the refractive indexes of the active layer and the light guide layer, the effective refractive index of the optical waveguide changes. Since the diffraction grating is formed at the interface between the p-type InP clad layer and the light guide layer, the reflection wavelength of the diffraction grating changes when the effective refractive index of the optical waveguide including the light guide layer changes. Along with this, so-called wavelength chirping in which the oscillation wavelength of the DFB laser changes occurs. This wavelength chirping limits the transmission distance and transmission speed in the optical communication system. That is, the conventional DFB type semiconductor laser device is limited to use as a light source for optical communication having a small transmission speed at a relatively short distance.

  The present invention has been made in view of such problems, and a semiconductor laser device capable of reducing a change in oscillation wavelength and suppressing wavelength chirping even when a modulation current is injected into an active layer. The purpose is to provide.

  In order to solve the above-described problems, a semiconductor laser device according to the present invention includes a first conductive semiconductor layer and a second conductive semiconductor layer stacked in a predetermined direction, and a first conductive semiconductor layer and a second conductive layer in a predetermined direction. An active layer provided between the conductive semiconductor layer, a diffraction grating layer having convex portions arranged at a predetermined period along the light propagation direction, and a convex portion of the diffraction grating provided on the diffraction grating layer A diffraction grating provided at a position sandwiching the first conductive type semiconductor layer between the active layer, a first electrode in contact with the first conductive type semiconductor layer, and a second conductive type semiconductor A second electrode in contact with the layer, the active layer constitutes the first optical waveguide, the diffraction grating layer and the buried layer constitute the second optical waveguide, and the first optical waveguide and the second optical waveguide A laser resonator optically coupled to the waveguide through the first conductive type semiconductor layer; And said that you configure.

  This semiconductor laser device includes a first optical waveguide centered on the active layer and a second optical waveguide centered on the diffraction grating layer and the buried layer. The first optical waveguide and the second optical waveguide are optically coupled via the first conductivity type semiconductor layer to constitute a laser resonator. The diffraction grating layer has convex portions arranged at a predetermined period along the light propagation direction. The buried layer provided on the diffraction grating layer is formed by embedding the convex portion of the diffraction grating layer. In such a configuration, light generated in the active layer propagates in the first optical waveguide. On the other hand, apart from the waveguide mode of light propagating in the first optical waveguide (hereinafter referred to as the active layer mode), the waveguide mode of light propagating in the second optical waveguide including the diffraction grating layer (hereinafter referred to as the active layer mode). The diffraction grating layer mode). The dispersion curve of the diffraction grating layer mode is different from the dispersion curve of the active layer mode. The active layer mode and the diffraction grating layer mode are guided close to each other in the laser resonator composed of the first and second optical waveguides. At this time, the phase matching condition for laser oscillation is satisfied at the wavelength at which the dispersion curve of the active layer mode and the dispersion curve of the diffraction grating layer mode intersect. Laser oscillation occurs at a specific wavelength (single wavelength) that satisfies this phase matching condition. Here, the wavelength of light propagating in the diffraction grating layer mode is determined by the period of the diffraction grating. The first and second optical waveguides are provided spatially separated via the first conductive type semiconductor layer. Therefore, when a current is injected into the active layer, the dispersion curve of the active layer mode propagating through the first optical waveguide changes with a change in the refractive index of the active layer, but the refractive index of the diffraction grating layer hardly changes. In addition, the dispersion curve of the diffraction grating layer mode propagating through the second optical waveguide and the wavelength of light hardly change. Therefore, according to the semiconductor laser device described above, even when a modulation current is injected into the active layer, the change in the oscillation wavelength can be reduced and the wavelength chirping can be suppressed.

The semiconductor laser element may be characterized in that the diffraction grating layer is made of a semiconductor and the buried layer is made of a dielectric. Since the refractive index of the dielectric is much smaller than the refractive index of the semiconductor, according to such a configuration, the difference in refractive index between the two materials constituting the diffraction grating can be increased. This increases the coupling coefficient κ between the forward wave and the backward wave in the diffraction grating layer mode. By increasing the coupling coefficient κ, it is possible to reduce the wavelength change corresponding to the change of the propagation constant in the dispersion curve of the diffraction grating layer mode. As a result, it is possible to further suppress the influence on the oscillation wavelength caused by the change in the refractive index of the active layer due to current injection. Therefore, according to this semiconductor laser device, the wavelength chirping can be further reduced. The dielectric preferably contains at least one of SiO ₂ , SiN, Al ₂ O ₃ , and TiO.

  Further, when the buried layer is made of a dielectric, the diffraction grating layer is preferably formed by etching amorphous silicon deposited on the surface of the first conductivity type semiconductor layer opposite to the surface facing the active layer. Can be formed. The buried layer can be preferably formed by depositing a dielectric so as to cover the diffraction grating layer.

  The semiconductor laser element may be characterized in that the diffraction grating layer is made of a semiconductor and the buried layer is made of polyimide resin or BCB resin. Since the refractive index of a resin such as a polyimide resin or a BCB resin is much smaller than the refractive index of a semiconductor, according to such a configuration, the difference in refractive index between the two materials constituting the diffraction grating can be increased. . As a result, the coupling coefficient κ between the forward wave and the backward wave in the diffraction grating layer mode increases, so that the wavelength change according to the change in the propagation constant in the dispersion curve of the diffraction grating layer mode can be reduced. As a result, it is possible to further suppress the influence on the oscillation wavelength caused by the change in the refractive index of the active layer due to current injection, so that the wavelength chirping can be further reduced. In this way, when the buried layer is made of polyimide resin or BCB resin, the diffraction grating layer and the first conductivity type semiconductor layer can be bonded via the buried layer.

  The semiconductor laser device may be characterized in that a relative refractive index difference (n1−n0) / n1 between the refractive index n1 of the diffraction grating layer and the refractive index n0 of the buried layer is 0.15 or more. If the relative refractive index difference between the diffraction grating layer and the buried layer is 0.15 or more, the coupling coefficient κ of the forward wave and the backward wave in the diffraction grating layer mode is sufficiently large, and is due to a change in the refractive index of the active layer. The influence on the oscillation wavelength can be effectively suppressed. That is, according to this semiconductor laser element, chirping can be reduced more effectively.

  Further, the semiconductor laser element may be characterized in that the diffraction grating layer has a thickness of 200 nm or more. As described above, since the diffraction grating layer is thick, the diffraction grating layer mode can be suitably generated.

  The semiconductor laser element may be characterized in that the thickness of the first conductivity type semiconductor layer is not less than 50 nm and not more than 500 nm. When the first conductivity type semiconductor layer has such a thickness, the first conductivity type semiconductor layer functions as a relatively thin clad. Since the active layer and the diffraction grating layer are arranged with the relatively thin clad interposed therebetween, the first and second optical waveguides can be effectively separated, so that the active layer mode and the diffraction grating mode Can be suitably generated and these modes can be coupled to each other.

  According to the semiconductor laser device of the present invention, even when a modulation current is injected into the active layer, the change in the oscillation wavelength can be reduced and the wavelength chirping can be suppressed.

FIG. 1 is a cross-sectional view showing the structure of the semiconductor laser device according to the first embodiment. FIG. 1A shows a cross section perpendicular to the light propagation direction of the semiconductor laser element. FIG. 1B shows a cross section taken along line II in FIG. 1A (ie, a cross section along the light propagation direction). FIG. 2A is a diagram showing a refractive index distribution in the stacking direction of the semiconductor laser elements. FIG. 2B is a diagram showing a waveguide mode generated in the semiconductor laser element. FIG. 3 is a graph showing the relationship between the frequency of propagation light generated in the semiconductor laser element and the propagation constant. FIG. 4 shows a relative refractive index difference (n1−n0) / n1 between a refractive index n1 of a material having a large refractive index and a refractive index n0 of a material having a small refractive index among two materials constituting the diffraction grating. The relationship between the speed and the relative refractive index difference (n1-n0) / n1 and the frequency difference Δf2 is shown. FIG. 5A to FIG. 5C are cross-sectional views showing respective steps of the method for fabricating the semiconductor laser device according to the first embodiment. FIG. 6A is a cross-sectional view showing one step of the method of manufacturing the semiconductor laser element according to the first embodiment. FIG. 6B is a cross-sectional view taken along the line II-II in FIG. 6A and shows a cross section perpendicular to the light propagation direction. FIG. 7A and FIG. 7B are cross-sectional views showing respective steps of the method for manufacturing the semiconductor laser device according to the first embodiment. FIG. 8A and FIG. 8B are cross-sectional views showing the respective steps of the semiconductor laser device manufacturing method according to the first embodiment. FIG. 9 is a cross-sectional view showing the structure of the semiconductor laser device according to the second embodiment. FIG. 9A shows a cross section perpendicular to the light propagation direction of the semiconductor laser element. FIG. 9B shows a cross section taken along line IV-IV in FIG. 9A (that is, a cross section along the light propagation direction). FIG. 10A and FIG. 10B are cross-sectional views showing the respective steps of the semiconductor laser device fabrication method according to the second embodiment. FIG. 11A and FIG. 11B are cross-sectional views showing the respective steps of the semiconductor laser device manufacturing method according to the second embodiment. FIG. 12A and FIG. 12B are cross-sectional views showing the respective steps of the semiconductor laser device manufacturing method according to the second embodiment. FIG. 13A and FIG. 13B are cross-sectional views showing the respective steps of the semiconductor laser device fabrication method according to the second embodiment. FIG. 14A is a cross-sectional view showing one step in the method for fabricating the semiconductor laser device according to the second embodiment. FIG. 14B shows a cross section taken along the line VV in FIG. FIG. 15A is a cross-sectional view showing a step of the method of manufacturing the semiconductor laser device according to the second embodiment. FIG.15 (b) has shown the cross section along the VI-VI line of Fig.15 (a). FIG. 16 is a cross-sectional view showing the structure of the semiconductor laser device according to the third embodiment. FIG. 16A shows a cross section perpendicular to the light propagation direction of the semiconductor laser element. FIG. 16B shows a cross section taken along line VII-VII in FIG. 16A (that is, a cross section along the light propagation direction).

  Embodiments of a semiconductor laser device according to the present invention will be described below in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted.

(First embodiment)
FIG. 1 is a cross-sectional view showing the structure of a semiconductor laser device 1A according to the first embodiment of the present invention. FIG. 1A shows a cross section perpendicular to the light propagation direction of the semiconductor laser element 1A. FIG. 1B shows a cross section taken along line II in FIG. 1A (ie, a cross section along the light propagation direction).

  The semiconductor laser element 1 </ b> A includes an n-type cladding layer 15. The n-type cladding layer 15 is the first conductivity type semiconductor layer in the present embodiment. The n-type cladding layer 15 is composed of a III-V group compound semiconductor such as n-type InP, for example, and is epitaxially grown on a group III-V compound semiconductor substrate (not shown) and then separated from the III-V group compound semiconductor substrate. It is a semiconductor layer. A suitable thickness of the n-type cladding layer 15 is relatively thinner than that of a normal cladding layer, for example, not less than 50 nm and not more than 500 nm, and in one embodiment is 250 nm.

  The semiconductor laser element 1A further includes a p-type cladding layer 19 and a p-type contact layer 20. The p-type cladding layer 19 and the p-type contact layer 20 are the second conductivity type semiconductor layers in this embodiment. The n-type cladding layer 15, the p-type cladding layer 19 and the p-type contact layer 20 are laminated in a predetermined direction (the thickness direction of these layers 15, 19 and 20). The p-type cladding layer 19 is made of, for example, a III-V group compound semiconductor such as p-type InP, and is epitaxially grown on the upper optical confinement layer 18 described later. A suitable thickness of the p-type cladding layer 19 is, for example, 1.5 μm. The p-type contact layer 20 is made of a III-V group compound semiconductor such as p-type GaInAs, and is epitaxially grown on the p-type cladding layer 19. A suitable thickness of the p-type contact layer 20 is, for example, 200 nm.

  The semiconductor laser element 1A further includes a lower light confinement layer 16, an active layer 17, and an upper light confinement layer 18. The lower optical confinement layer 16, the active layer 17, and the upper optical confinement layer 18 constitute a first optical waveguide. In other words, the lower optical confinement layer 16, the active layer 17, and the upper optical confinement layer 18 are high refractive index regions provided between the n-type cladding layer 15 and the p-type cladding layer 19, and the first optical waveguide. It functions as the core area.

  The refractive indexes of the lower optical confinement layer 16 and the upper optical confinement layer 18 are larger than the refractive indexes of the n-type cladding layer 15 and the p-type cladding layer 19. The band gap energy of the lower optical confinement layer 16 and the upper optical confinement layer 18 is smaller than the band gap energy of the n-type cladding layer 15, and the band gap wavelength is 1.2 μm, for example. The band gap energy Eg (eV) and the band gap wavelength λg (μm) have a relationship of Eg = 1.24 / λg. The lower optical confinement layer 16 and the upper optical confinement layer 18 are made of a III-V group compound semiconductor such as undoped InAlGaAs. A suitable thickness of the lower optical confinement layer 16 and the upper optical confinement layer 18 is, for example, 100 nm, respectively. The lower optical confinement layer 16 is formed by epitaxial growth on the n-type cladding layer 15. The upper optical confinement layer 18 is epitaxially grown on the active layer 17.

  The active layer 17 is made of a III-V compound semiconductor and is provided between the lower light confinement layer 16 and the upper light confinement layer 18. Preferably, the active layer 17 has a multiple quantum well structure in which quantum well layers and barrier layers are alternately stacked. The quantum well layer and the barrier layer included in the multiple quantum well structure are made of undoped InAlGaAs having different compositions, for example. The thickness of the quantum well layer is, for example, 5 nm. The thickness of the barrier layer is 8 nm, for example. The band gap energy of the semiconductor layer constituting the barrier layer is larger than the band gap energy of the semiconductor layer constituting the quantum well layer. The band gap wavelength of the semiconductor layer constituting the well layer is, for example, 1.6 μm. The band gap wavelength of the semiconductor layer constituting the barrier layer is, for example, 1.25 μm. The emission wavelength of the active layer 17 is, for example, 1.55 μm. The quantum well layer has a compressive strain (for example, 0.8%). In one embodiment, seven quantum well layers are provided and eight barrier layers are provided.

In the present embodiment, the lower optical confinement layer 16, the active layer 17, and the upper optical confinement layer 18 are provided on a region extending in the optical waveguide direction on the n-type cladding layer 15 and constitute a mesa structure 32. Yes. The mesa structure 32 has side surfaces 32a and 32b along the optical waveguide direction. The p-type cladding layer 19 and the p-type contact layer 20 are provided on a region extending in the optical waveguide direction on the upper optical confinement layer 18 and constitute a mesa structure 33. The mesa structure 33 has side surfaces 33a and 33b along the optical waveguide direction. The interval between the side surfaces 33a and 33b is narrower than the interval between the side surfaces 32a and 32b. The distance between the side surfaces 33a and 33b is, for example, 1.5 μm, and the distance between the side surfaces 32a and 32b is, for example, 6 μm. An insulating film 23 for protecting the semiconductor layer is provided from both side surfaces 33 a and 33 b of the mesa structure 33 over the upper optical confinement layer 18 and the side surfaces 32 a and 32 b of the mesa structure 32. The insulating film 23 is made of an insulating silicon compound such as SiO ₂ .

  The semiconductor laser device 1A further includes a cathode electrode 21 and an anode electrode 22. The cathode electrode 21 and the anode electrode 22 are electrodes for supplying current to the active layer 17. The cathode electrode 21 is the first electrode in the present embodiment, and the anode electrode 22 is the second electrode in the present embodiment. The cathode electrode 21 is provided on the n-type cladding layer 15 exposed from the insulating film 23 around the lower optical confinement layer 16, the active layer 17, and the upper optical confinement layer 18, and is in ohmic contact with the n-type cladding layer 15. Is made. The cathode electrode 21 includes, for example, AuGe. The anode electrode 22 is provided on the mesa structure 33 and is in ohmic contact with the p-type contact layer 20 through an opening formed in the insulating film 23. The anode electrode 22 is made of, for example, Ti / Pt / Au or Ti / Au.

  The semiconductor laser device 1A further includes a substrate 11, a cladding layer 12, a diffraction grating layer 13, and an adhesive layer (embedded layer) 14. The substrate 11 has a main surface 11a. The substrate 11 of this embodiment is made of, for example, silicon (Si). In addition, the constituent material of the board | substrate 11 is not restricted to this, It can consist of various materials which can deposit the clad layer 12 and the diffraction grating layer 13 on the main surface 11a.

The clad layer 12 is a layer made of a dielectric, and is made of, for example, a silicon compound such as SiO ₂ . The thickness of the cladding layer 12 is 1 μm, for example. The diffraction grating layer 13 has convex portions arranged at a predetermined period along the light propagation direction. In this embodiment, the diffraction grating layer 13 includes a plurality of parts divided in the light propagation direction at the period. The area consists of The diffraction grating layer 13 is made of a material having a higher refractive index than that of the cladding layer 12, and is made of, for example, Si that is a semiconductor. When the cladding layer 12 and the diffraction grating layer 13 are made of SiO ₂ and Si, respectively, the diffraction grating layer 13 is preferably formed by etching, for example, the surface Si layer of an SOI (Silicon on Insulator) substrate.

  The preferable thickness of the diffraction grating layer 13 is 200 nm or more, which is much thicker than the thickness (about 10 nm) of the diffraction grating layer of a general DFB type semiconductor laser element. In one example, the thickness of the diffraction grating layer 13 is 300 nm. The band gap wavelength of the diffraction grating layer 13 is, for example, 1.25 μm, which is longer than the band gap wavelength (1.1 μm) of the diffraction grating layer of a general DFB type semiconductor laser element. In other words, the band gap energy of the diffraction grating layer 13 is smaller than the band gap energy of the diffraction grating layer of a general DFB type semiconductor laser element. The bandgap energy of the diffraction grating layer 13 is smaller than the bandgap energy of the cladding layer 12 located immediately below it.

  The adhesive layer 14 is an embedding layer in the present embodiment, and is provided on the diffraction grating layer 13 to embed a convex portion of the diffraction grating layer 13. The adhesive layer 14 is made of a resin such as polyimide or BCB (Benzocyclobutene) having a refractive index smaller than that of the diffraction grating layer 13, and in one embodiment, is made of a polymer such as DVS-BCB (Divinyl-siloxane-bis-BCB). Polyimide or BCB is suitable as a material for the adhesive layer 14 because it is transparent to the wavelength of light generated in the active layer 17 and has low optical loss. The adhesive layer 14 constitutes a diffraction grating 24 together with the diffraction grating layer 13. As described above, since the refractive index of the adhesive layer 14 is smaller than the refractive index of the diffraction grating layer 13, the refractive index of the diffraction grating 24 is periodic along the optical waveguide direction with the period of the convex portion of the diffraction grating layer 13. It has a structure that changes to The diffraction grating 24 of the present embodiment is configured by the diffraction grating layer 13 divided at the above-described period, and the adhesive layer 14 provided between the diffraction grating layers 13 and on the diffraction grating layer 13. Yes.

  In addition, it is desirable to select the material of the adhesive layer 14 so that the refractive index difference between the diffraction grating layer 13 and the adhesive layer 14 constituting the diffraction grating 24 becomes large. As will be described later, the relative refractive index difference (n1−n0) / n1 between the refractive index n1 of the diffraction grating layer 13 and the refractive index n0 of the adhesive layer 14 is preferably 0.15 or more. That is, the refractive index n0 of the adhesive layer 14 is preferably 85% or less of the refractive index n1 of the diffraction grating layer 13. In this embodiment, the refractive index of polyimide or BCB is about 1.5, which is much smaller than about 3.2, which is the refractive index of the diffraction grating layer 13. Therefore, the relative refractive index difference (n1-n0) / n1 can be easily set to 0.15 or more.

  Here, the diffraction grating layer 13 and the adhesive layer 14 constitute a second optical waveguide. That is, the diffraction grating layer 13 which is a high refractive index region sandwiched between the cladding layer 12 and the adhesive layer 14 functions as a core region of the second optical waveguide. The first optical waveguide constituted by the lower optical confinement layer 16, the active layer 17 and the upper optical confinement layer 18, and the second optical waveguide constituted by the diffraction grating layer 13 and the adhesive layer 14 are an n-type cladding layer. 15 and optically coupled. The clad layer 12 and the diffraction grating layer 13 are bonded to the n-type clad layer 15 by an adhesive layer 14.

  The emission wavelength of the semiconductor laser element 1 </ b> A is determined by the period of the diffraction grating 24. The period of the diffraction grating 24 is, for example, 0.24 μm, and the duty ratio of the diffraction grating 24 (ratio of the diffraction grating layer 13 and the adhesive layer 14 in which the diffraction grating layer 13 is embedded in one period of the diffraction grating 24) is, for example, 0. 71. The period of the diffraction grating 24 at this time is preferably set so that first-order diffraction occurs with respect to the wavelength generated in the active layer 17. The light generated in the active layer 17 is reflected by the diffraction grating 24 and can propagate in the second optical waveguide in the laser resonator length direction (direction parallel to the main surface 11a of the substrate 11).

  In a general DFB type semiconductor laser element, the diffraction grating layer is provided immediately below the lower optical confinement layer. On the other hand, the diffraction grating layer 13 of this embodiment is provided below the n-type cladding layer 15. Therefore, in the semiconductor laser device 1A according to the present embodiment, the distance between the diffraction grating layer 13 and the active layer 17 is larger than that of a general DFB laser.

  As shown in FIG. 1B, antireflection films (AR films) 35a and 35b are provided on both end faces 34a and 34b of the semiconductor laser device 1A in the light propagation direction. In addition, a laser resonator is configured by the first optical waveguide including the active layer 17 and the second optical waveguide including the diffraction grating layer 13 provided between the end surfaces 34a and 34b. An interval (laser resonator length) between the end face 34a and the end face 34b is, for example, 250 μm.

  The operation of the semiconductor laser device 1A having the above configuration will be described. FIG. 2A is a diagram showing a refractive index distribution in the stacking direction of the semiconductor laser element 1A. 2A, the horizontal axis of the graph indicates the refractive index, and the vertical axis indicates the position in the thickness direction of the semiconductor laser element 1A. FIG. 2B is a diagram showing a waveguide mode generated in the semiconductor laser element 1A. In FIG. 2B, the horizontal axis of the graph indicates the intensity of the guided light, and the vertical axis indicates the position in the thickness direction of the semiconductor laser element 1A. As shown in FIG. 2, in the semiconductor laser device 1A, an active layer mode M1 that is a waveguide mode centered on the active layer 17 and a diffraction grating layer mode M2 that is a waveguide mode centered on the diffraction grating layer 13. Two waveguide modes are generated. The diffraction grating layer mode M2 has a property as a standing wave. The diffraction grating layer mode M2 is noticeably generated when the diffraction grating layer 13 is thick and the diffraction grating layer 13 and the active layer 17 are separated from each other as in the present embodiment. The thickness of the diffraction grating layer 13 is, for example, 200 nm or more. Further, since the active layer mode M1 and the diffraction grating layer mode M2 are guided in parallel while being close to each other in the semiconductor laser device 1A, these modes M1 and M2 can be coupled to each other. . In other words, the internal structure of the semiconductor laser element 1A can be an optical coupler that couples the active layer mode M1 and the diffraction grating layer mode M2.

Here, FIG. 3 shows the frequency and propagation of propagating light generated in the semiconductor laser device 1A when the semiconductor laser device 1A is modulated by a digital signal current including a high level (1 level) and a low level (0 level). It is a graph which shows the relationship with a constant. In FIG. 3, the vertical axis indicates the normalized frequency, and the horizontal axis indicates the normalized propagation constant. Here, the frequency f of the laser beam has a relationship of f = c ₀ / λ with respect to the wavelength λ in the vacuum of the laser beam. The normalized frequency is defined by f / (a · c ₀ ), and the normalized propagation constant is a parameter defined by β · a / (2π). Here, a represents the period of the diffraction grating, β represents the propagation constant, and c ₀ represents the speed of light in vacuum. Note that the normalized propagation constant is normalized so that the laser beam frequency becomes the Bragg frequency when the value is 0.5. Further, a graph G11 shown in the figure shows an active layer mode M1 when the semiconductor laser element 1A is driven with a high-level signal current (that is, when the transmission light from the semiconductor laser element 1A is at a high level). The dispersion curve is shown. On the other hand, the graph G12 shows a dispersion curve of the active layer mode M1 when the semiconductor laser element 1A is driven with a low-level signal current (that is, when the transmission light from the semiconductor laser element 1A is at a low level). ing. Graph G13 shows the dispersion curve of diffraction grating layer mode M2. In this embodiment, since no current flows through the diffraction grating layer 13, the dispersion curve of the diffraction grating layer mode M2 does not change depending on the high level / low level of the transmitted light.

  A normal DFB laser having only an active layer mode oscillates at a Bragg frequency. This indicates that the operating point of the normal DFB laser is the point at which the normalized propagation constant β · a / (2π) becomes 0.5 in the dispersion curve of the active layer mode of FIG. In addition, the Bragg frequency varies with a change in refractive index caused by a change in current injected into the active layer. Specifically, when the transmission light is at a high level, laser oscillation occurs at a point A1 in the figure, and when the transmission light is at a low level, laser oscillation occurs at a point A2 in the figure. In this case, a difference Δf1 between the normalized frequency at the point A1 and the normalized frequency at the point A2 appears as a large change (chirping) of the oscillation frequency.

  On the other hand, when the active layer mode M1 and the diffraction grating layer mode M2 are coupled, oscillation occurs at an operating point where the phase matching conditions of the modes M1 and M2 match. That is, when the transmitted light is at a high level, oscillation occurs at the intersection (the point B1 in the figure) between the graph G11 and the graph G13. Further, when the transmitted light is at a low level, it oscillates at the intersection (the point B2 in the figure) between the graph G12 and the graph G13. Therefore, the difference Δf2 between the normalized frequency at the point B1 and the normalized frequency at the point B2 appears as a change in the oscillation frequency.

This frequency difference Δf2 becomes smaller as the slope of the graph G13, which is the dispersion curve of the diffraction grating layer mode M2, becomes gentler. That is, the frequency difference Δf2 decreases as the group velocity vg decreases. Here, FIG. 4 represents the relationship between the relative refractive index difference between the two materials constituting the diffraction grating and the group velocity (graph G21). Here, regarding the refractive indexes of two materials, the refractive index of a material having a large refractive index is n1, and the refractive index of a material having a small refractive index is n0. At this time, the relative refractive index difference between the two materials is represented by (n1-n0) / n1. Further, FIG. 4 also shows the relationship (graph G22) between the relative refractive index difference (n1-n0) / n1 and the frequency difference Δf2. In FIG. 4, the horizontal axis indicates the relative refractive index difference (n1-n0) / n1. Also, the left vertical axis indicates the normalized group velocity (vg / c ₀ ) normalized by the light velocity c ₀ in vacuum, while the right vertical axis indicates the frequency difference Δf2. Here, the frequency difference Δf2 in FIG. 4 is plotted by converting the normalized frequency difference Δf2 in FIG. 3 into an actual frequency difference.

  As shown in FIG. 4, as the refractive index difference between the two materials constituting the diffraction grating increases, the group velocity tends to decrease and the frequency difference Δf2 tends to decrease. In general, the group velocity of light propagating through an optical waveguide having a diffraction grating decreases as the coupling coefficient κ of the diffraction grating layer mode propagating through the optical waveguide increases. That is, in FIG. 4, the group velocity of propagating light is increased by increasing the refractive index difference between the two materials constituting the diffraction grating and increasing the coupling coefficient κ of the forward wave and the backward wave of the diffraction grating layer mode. It is shown that the frequency difference Δf2 can be reduced by reducing the frequency. In the semiconductor laser device 1A of the present embodiment, the difference in refractive index between the two materials constituting the diffraction grating is increased, and the depth of the unevenness formed in the diffraction grating layer 13 is further increased, so that the diffraction grating layer mode is improved. The coupling coefficient κ of the forward wave and the backward wave is increased. The diffraction grating layer 13 constituting the diffraction grating is made of a semiconductor such as Si, and the adhesive layer 14 is made of a resin such as BCB. When the adhesive layer 14 is made of BCB, the refractive index is about 1.5. When the diffraction grating layer 13 is made of Si, its refractive index is about 3.5. Therefore, since the coupling coefficient κ between the forward wave and the backward wave in the diffraction grating layer mode can be increased, the frequency difference Δf2 is reduced and chirping can be effectively reduced.

  Further, from FIG. 4, in order to reduce chirping, the relative refractive index difference between the two materials constituting the diffraction grating is preferably 0.15 or more. In that case, the coupling coefficient κ between the forward wave and the backward wave in the diffraction grating layer mode becomes sufficiently large, and the frequency difference Δf2 can be made 15 GHz or less. Further, the relative refractive index difference is more preferably 0.4 or more, and in that case, the frequency difference Δf2 can be 6 GHz or less. For example, in this embodiment, when the diffraction grating layer 13 is made of Si and the adhesive layer 14 is made of BCB, the relative refractive index difference is about 0.57, and the frequency difference Δf2 can be made sufficiently small. In a normal DFB type semiconductor laser element, the frequency difference Δf1 representing chirping is about 20 GHz.

  Further, as in this embodiment, the thickness of the n-type cladding layer 15 is preferably 50 nm or more and 500 nm or less. Since the active layer 17 and the diffraction grating layer 13 are disposed with the relatively thin n-type cladding layer 15 having a thickness of 500 nm or less interposed therebetween, the first and second optical waveguides can be effectively separated, so that the active layer The mode M1 and the diffraction grating layer mode M2 can be suitably generated, and these modes M1 and M2 can be coupled to each other. Further, since the thickness of the n-type cladding layer 15 is 50 nm or more, a current can be efficiently supplied to the cathode electrode 21.

  Further, the semiconductor laser device 1A of the present embodiment has a semiconductor laminated structure including the n-type cladding layer 15, the active layer 17, and the p-type contact layer 20, and the cathode electrode 21 is in contact with the n-type cladding layer 15. The p-type contact layer 20 has a so-called longitudinal current injection type configuration in which the anode electrode 22 is in contact with the p-type contact layer 20. In contrast, for example, a mesa structure including an active layer is formed, an n-type semiconductor is disposed along one side surface of the mesa structure, and a p-type semiconductor is disposed along the other side surface. An injection type configuration is also conceivable.

  However, such a lateral current injection type configuration has problems that the threshold is high and the photoelectric conversion efficiency is suppressed. That is, in the lateral current injection type configuration, current flows from one side of the active layer to the other side, but there is a limit to reducing the width of the active layer, and the current path in the active layer is the diffusion of carriers. It becomes longer than the length. Therefore, the carrier distribution in the active layer becomes non-uniform, and the light emission efficiency tends to be suppressed or unstable. Further, if the energy discontinuity of the band structure is large at the interface between the undoped active layer and the p-type semiconductor, the resistance increases and the current injection efficiency decreases. In the configuration of the lateral current injection type, it is difficult to cope with such a stepwise change in doping concentration and semiconductor composition in order to suppress such energy discontinuity.

  In response to the above-described problem, the semiconductor laser device 1A of the present embodiment has a vertical current injection type configuration, and therefore the carrier distribution can be easily made uniform by adjusting the thickness of the active layer. In addition, it is easy to change the doping concentration and the semiconductor composition stepwise in order to keep energy discontinuity small above and below the active layer. Therefore, according to the semiconductor laser device 1A of the present embodiment, the threshold can be lowered and the photoelectric conversion efficiency can be increased.

  Here, FIG. 5 to FIG. 8 are cross-sectional views for explaining an example of the manufacturing method of the semiconductor laser device 1A according to the present embodiment described above. 5A to 5C and 6A show cross sections along the light propagation direction. FIG. 6B is a cross-sectional view taken along the line II-II in FIG. 6A and shows a cross section perpendicular to the light propagation direction. 7 (a), 7 (b), 8 (a), and 8 (b) show cross sections perpendicular to the light propagation direction.

In this manufacturing method, first, as shown in FIG. 5A, an SOI substrate 10 in which a SiO ₂ layer (clad layer 12) and a Si layer 41 are provided on a silicon substrate 11 is prepared. Next, a mask having a pattern corresponding to the diffraction grating 24 is formed on the Si layer 41 by using a normal photolithography technique, and the Si layer 41 is etched through this mask. As a result, as shown in FIG. 5B, the diffraction grating layer 13 having convex portions arranged at a predetermined cycle along the light propagation direction is formed. In one example, the etching depth reaches the cladding layer 12 in this etching process. As a result, the diffraction grating layer 13 is divided at the above period. Thus, the first substrate product 40 having the silicon substrate 11, the cladding layer 12, and the diffraction grating layer 13 is produced. Note that the form of the diffraction grating is not limited to this embodiment. For example, when etching is stopped in the middle of the Si layer 41, the diffraction grating layer 13 has an upper surface (that is, a diffraction film on the side opposite to the surface facing the cladding layer 12). A diffraction grating having periodic convex portions may be formed on the surface of the grating layer 13.

  On the other hand, apart from the manufacturing process of the first substrate product 40, a semiconductor substrate 43 is prepared as shown in FIG. 5C, and the p-type contact layer 20, p is formed on the main surface 43a of the semiconductor substrate 43. The type cladding layer 19, the upper optical confinement layer 18, the active layer 17, the lower optical confinement layer 16, and the n-type cladding layer 15 are epitaxially grown in this order. The semiconductor substrate 43 is, for example, an InP substrate. Thus, the second substrate product 50 having the semiconductor substrate 43 and the respective semiconductor layers 15 to 20 is produced.

  Subsequently, as shown in FIGS. 6A and 6B, the first substrate product 40 and the second substrate product 50 are bonded to each other through the adhesive layer 14. Specifically, a resin such as BCB or polyimide is sandwiched between the diffraction grating layer 13 of the first substrate product 40 and the n-type clad layer 15 of the second substrate product 50, and the resin is cured to make the first. The substrate product 40 and the second substrate product 50 are joined. At this time, part of the resin enters gaps between the plurality of convex portions of the diffraction grating layer 13 and embeds the diffraction grating layer 13. Thus, the adhesive layer 14 is formed as an embedded layer that embeds the convex portion of the diffraction grating layer 13. The distance D1 between the upper surface of each convex portion of the diffraction grating layer 13 and the n-type cladding layer 15 is preferably thin for good coupling between the active layer mode M1 and the diffraction grating layer mode M2 described above, For example, 50 nm.

  Subsequently, as shown in FIG. 7A, the semiconductor substrate 43 on the p-type contact layer 20 is removed. This removal is preferably performed, for example, by etching the semiconductor substrate 43. Subsequently, as shown in FIG. 7B, mesa structures 32 and 33 are formed. First, a mask having a pattern corresponding to the planar shape of the mesa structure 33 is formed on the p-type contact layer 20, and the p-type contact layer 20 and the p-type cladding layer 19 are etched through this mask. This etching stops when it reaches the upper optical confinement layer 18. Thus, the mesa structure 33 having the side surfaces 33a and 33b is formed. Next, a mask having a pattern corresponding to the planar shape of the mesa structure 32 is formed on the p-type contact layer 20 and the upper optical confinement layer 18, and the upper optical confinement layer 18, the active layer 17, and the lower portion are formed through this mask. The optical confinement layer 16 is etched. This etching stops when it reaches the n-type cladding layer 15. Thus, the mesa structure 32 having the side surfaces 32a and 32b is formed.

Subsequently, as illustrated in FIG. 8A, the insulating film 23 is formed from the side surfaces 33 a and 33 b of the mesa structure 33 to the side surfaces 32 a and 32 b of the mesa structure 32 through the upper optical confinement layer 18. Specifically, first, an insulating material (SiO ₂ or the like) is deposited on the entire surface of the silicon substrate 11. Then, a mask having openings on the mesa structure 33 and the peripheral region of the mesa structure 32 is formed on the insulating material by using a normal photolithography technique. The insulating film 23 is formed by etching the insulating material through this mask.

  Subsequently, as shown in FIG. 8B, the cathode electrode 21 is formed on the n-type cladding layer 15 and the anode electrode 22 is formed on the mesa structure 33. Specifically, a mask having an opening corresponding to the planar shape of the cathode electrode 21 is formed using a normal photolithography technique, and then the metal material of the cathode electrode 21 is deposited on the entire surface of the silicon substrate 11. Then, the metal material excluding the cathode electrode 21 is removed by removing the mask (lift-off), and an annealing process is performed. The anode electrode 22 can also be formed in the same manner. Thereafter, the end surfaces 34a and 34b shown in FIG. 1B are formed by cleavage, and antireflection films 35a and 35b are formed on the end surfaces 34a and 34b. Thus, the semiconductor laser device 1A shown in FIG. 1 is completed.

(Second Embodiment)
FIG. 9 is a cross-sectional view showing the structure of a semiconductor laser device 1B according to the second embodiment of the present invention. FIG. 9A shows a cross section perpendicular to the light propagation direction of the semiconductor laser element 1B. FIG. 9B shows a cross section taken along line IV-IV in FIG. 9A (that is, a cross section along the light propagation direction).

  The semiconductor laser device 1B includes an n-type cladding layer 15, a lower optical confinement layer 16, an active layer 17, an upper optical confinement layer 18, a p-type cladding layer 19, a p-type contact layer 20, a cathode electrode 21, an anode electrode 22, and an insulation. A film 23 is provided. These configurations are the same as those in the first embodiment described above. However, in the present embodiment, the thickness of the n-type cladding layer 15 is, for example, 400 nm.

  The semiconductor laser element 1B further includes a diffraction grating layer 53 and a buried layer 54. The diffraction grating layer 53 has convex portions arranged at a predetermined period along the light propagation direction. In the present embodiment, the diffraction grating layer 53 has a plurality of parts divided in the light propagation direction at the period. The area consists of The diffraction grating layer 53 is made of a material having a refractive index larger than that of the n-type cladding layer 15, and is made of, for example, amorphous silicon that is an amorphous semiconductor. The preferred thickness of the diffraction grating layer 53 is 200 nm or more, and in one embodiment is 300 nm. The band gap wavelength of the diffraction grating layer 53 is, for example, 1.25 μm, which is longer than the band gap wavelength (1.1 μm) of the diffraction grating layer of a general DFB type semiconductor laser element. In other words, the band gap energy of the diffraction grating layer 53 is smaller than the band gap energy of the diffraction grating layer of a general DFB semiconductor laser element. The band gap energy of the diffraction grating layer 53 is smaller than the band gap energy of the n-type cladding layer 15 located immediately above it.

The buried layer 54 is provided on the diffraction grating layer 53 and buryes the convex portion of the diffraction grating layer 53. The buried layer 54 can be formed by depositing a dielectric having a refractive index smaller than that of the diffraction grating layer 53 so as to cover the diffraction grating layer 53. The dielectric that forms the diffraction grating layer 53 includes at least one material of SiO ₂ , SiN, Al ₂ O ₃ , and TiO. The buried layer 54 constitutes a diffraction grating 64 together with the diffraction grating layer 53. As described above, since the refractive index of the buried layer 54 is smaller than the refractive index of the diffraction grating layer 53, the refractive index of the diffraction grating 64 is periodic along the optical waveguide direction with the period of the convex portion of the diffraction grating layer 53. Has a structure that changes with time. The diffraction grating 64 of the present embodiment includes a diffraction grating layer 53 divided at the above-described period, and a buried layer 54 provided between the diffraction grating layers 53 and on the diffraction grating layer 53. ing.

It should be noted that the material of the buried layer 54 is preferably selected so that the refractive index difference between the diffraction grating layer 53 constituting the diffraction grating 64 and the buried layer 54 becomes large. As described in the first embodiment, the relative refractive index difference (n1−n0) / n1 between the refractive index n1 of the diffraction grating layer 53 and the refractive index n0 of the buried layer 54 is 0.15 or more. Is preferred. That is, the refractive index n0 of the buried layer 54 is preferably 85% or less of the refractive index n1 of the diffraction grating layer 53. In one embodiment, the buried layer 54 is made of SiO ₂ as a dielectric. In this case, the refractive index of the buried layer 54 is about 1.5, which is much smaller than the refractive index 3.2 of the diffraction grating layer 53. The relative refractive index difference (n1-n0) / n1 in this example is about 0.53.

  Here, the diffraction grating layer 53 and the buried layer 54 constitute a second optical waveguide. That is, the diffraction grating layer 53 that is a high refractive index region sandwiched between the n-type cladding layer 15 and the buried layer 54 functions as a core region of the second optical waveguide. The first optical waveguide constituted by the lower optical confinement layer 16, the active layer 17 and the upper optical confinement layer 18, and the second optical waveguide constituted by the diffraction grating layer 53 and the buried layer 54 are n Optically coupled through the mold cladding layer 15. Note that the buried layer 54 of the present embodiment is preferably sufficiently thicker than the diffraction grating layer 53. A distance D2 between the interface between the diffraction grating layer 53 and the buried layer 54 and the surface of the buried layer 54 (the back surface of the semiconductor laser element 1B) is, for example, 0.4 μm. By providing the buried layer 54 relatively thick as described above, the buried layer 54 preferably functions as a cladding layer in the second optical waveguide.

  As shown in FIG. 9B, antireflection films (AR films) 75a and 75b are provided on both end faces 74a and 74b of the semiconductor laser element 1B in the light propagation direction. In addition, a laser resonator is configured by the first optical waveguide including the active layer 17 and the second optical waveguide including the diffraction grating layer 53 provided between the end surfaces 74a and 74b. The distance (laser resonator length) between the end surface 74a and the end surface 74b is, for example, 250 μm.

  The operation of the semiconductor laser device 1B having the above configuration will be described. In the semiconductor laser element 1B, as in the semiconductor laser element 1A according to the first embodiment, an active layer mode that is a waveguide mode centered on the active layer 17 and a waveguide mode that centers on the diffraction grating layer 53 are provided. Two guided modes occur, such as a diffraction grating layer mode. In the semiconductor laser device 1B, the active layer mode and the diffraction grating layer mode are guided in parallel while being close to each other, so that these modes can be coupled to each other. In other words, the internal structure of the semiconductor laser device 1B can be an optical coupler that couples the active layer mode and the diffraction grating layer mode.

As shown in FIG. 3, when the active layer mode and the diffraction grating layer mode are coupled, oscillation occurs at an operating point where the phase matching conditions of these modes match. The difference Δf2 between the oscillation frequency when the transmitted light is at a high level and the oscillation frequency when the transmission light is at a low level becomes smaller as the slope of the dispersion curve of the diffraction grating layer mode becomes gentler. Furthermore, as shown in FIG. 4, the frequency difference Δf <b> 2 decreases as the refractive index difference between the two materials constituting the diffraction grating increases. In the present embodiment, the diffraction grating layer 53 constituting the diffraction grating is made of a semiconductor such as amorphous silicon, and the buried layer 54 is made of a dielectric such as SiO ₂ . When the buried layer 54 is made of SiO ₂ , the refractive index is about 1.5. When the diffraction grating layer 53 is made of amorphous silicon, the refractive index is about 3.5. Therefore, the frequency difference Δf2 is reduced, and chirping can be effectively reduced.

As shown in FIG. 4, in order to reduce chirping, the relative refractive index difference between the two materials constituting the diffraction grating 64 is preferably 0.15 or more. In that case, the coupling coefficient κ between the forward wave and the backward wave in the diffraction grating layer mode becomes sufficiently large, and the frequency difference Δf2 can be made 15 GHz or less. Further, the relative refractive index difference is more preferably 0.4 or more, and in that case, the frequency difference Δf2 can be 6 GHz or less. For example, in this embodiment, when the diffraction grating layer 53 is made of amorphous silicon and the buried layer 54 is made of SiO ₂ , the relative refractive index difference is about 4.2, and the frequency difference Δf2 can be made sufficiently small.

  10 to 15 are cross-sectional views for explaining an example of the manufacturing method of the semiconductor laser device 1B according to the present embodiment described above. 10 to 13, 14 (a), and 15 (a) show cross sections perpendicular to the light propagation direction. 14B is a cross-sectional view taken along the line VV in FIG. 14A, and FIG. 15B is a cross-sectional view taken along the line VI-VI in FIG. Each shows a cross section along the light propagation direction.

  In this manufacturing method, first, as shown in FIG. 10A, a semiconductor substrate 43 is prepared, and a p-type contact layer 20, a p-type cladding layer 19, and an upper light are formed on a main surface 43 a of the semiconductor substrate 43. The confinement layer 18, the active layer 17, the lower optical confinement layer 16, and the n-type cladding layer 15 are epitaxially grown in this order. The semiconductor substrate 43 is, for example, an InP substrate. Next, as shown in FIG. 10B, the n-type cladding layer 15 and the first temporary substrate 44 are bonded to each other via an adhesive 45. The first temporary substrate 44 can be made of various materials capable of maintaining mechanical strength, and in one embodiment is a Si substrate.

  Subsequently, as shown in FIG. 11A, the semiconductor substrate 43 on the p-type contact layer 20 is removed. This removal is preferably performed, for example, by etching the semiconductor substrate 43. Then, as shown in FIG. 11B, a mesa structure 32 having side surfaces 32a and 32b and a mesa structure 33 having side surfaces 33a and 33b are formed. The method for forming the mesa structures 32 and 33 is the same as that in the first embodiment.

  Subsequently, as illustrated in FIG. 12A, the insulating film 23 is formed from the side surfaces 33 a and 33 b of the mesa structure 33 to the side surfaces 32 a and 32 b of the mesa structure 32 through the upper optical confinement layer 18. Then, as shown in FIG. 12B, the cathode electrode 21 is formed on the n-type cladding layer 15 and the anode electrode 22 is formed on the mesa structure 33. In addition, the formation method of the insulating film 23, the cathode electrode 21, and the anode electrode 22 is the same as that of 1st Embodiment.

  Subsequently, as shown in FIG. 13A, the first temporary substrate 44 and the second temporary substrate 47 are bonded with an adhesive 46 so that the mesa structures 32 and 33 and the second temporary substrate 47 face each other. Are joined to each other. Thereafter, as shown in FIG. 13B, the first temporary substrate 44 is peeled from the n-type cladding layer 15. Then, as shown in FIG. 14, a diffraction grating layer 53 is formed on the n-type cladding layer 15. Specifically, after depositing an amorphous silicon film on the surface of the n-type clad layer 15 opposite to the surface facing the active layer 17, the n-type cladding layer 15 corresponds to the diffraction grating 64 using a normal photolithography technique. A mask having a pattern is formed on the amorphous silicon film. Then, the amorphous silicon film is etched through this mask. As a result, as shown in FIG. 14B, a diffraction grating layer 53 having convex portions arranged at a predetermined period along the light propagation direction is formed.

  Subsequently, as shown in FIG. 15, a buried layer 54 is deposited on the n-type cladding layer 15. At this time, part of the buried layer 54 enters the gaps between the plurality of convex portions of the diffraction grating layer 53 and embeds the diffraction grating layer 53. Thereafter, the second temporary substrate 47 is peeled off, and the end faces 74a and 74b shown in FIG. 9B are formed by cleavage, and antireflection films 75a and 75b are formed on the end faces 74a and 74b. Thus, the semiconductor laser device 1B shown in FIG. 9 is completed.

  Unlike the semiconductor laser device 1A of the first embodiment, when the semiconductor laser device 1B of this embodiment is manufactured as in the manufacturing method described above, a structure having an active layer and a structure having a diffraction grating layer are different from the semiconductor laser device 1A of the first embodiment. There is no need for a process of bonding objects together. Therefore, it is not necessary to control the thickness of the adhesive layer, and the semiconductor laser element can be easily manufactured.

(Third embodiment)
FIG. 16 is a cross-sectional view showing the structure of a semiconductor laser device 1C according to the third embodiment of the present invention. FIG. 16A shows a cross section perpendicular to the light propagation direction of the semiconductor laser element 1C. FIG. 16B shows a cross section taken along line VII-VII in FIG. 16A (that is, a cross section along the light propagation direction).

  The semiconductor laser device 1A includes an n-type cladding layer 85. The n-type cladding layer 85 is the second conductivity type semiconductor layer in the present embodiment. The n-type cladding layer 85 is made of a III-V group compound semiconductor such as n-type InP, for example, and has a main surface 85a and a back surface 85b. A suitable thickness of the n-type cladding layer 85 is, for example, 100 μm.

  The semiconductor laser element 1 </ b> C further includes a p-type cladding layer 89. The p-type cladding layer 89 is the first conductivity type semiconductor layer in the present embodiment. The p-type cladding layer 89 is made of, for example, a III-V group compound semiconductor such as p-type InP, and is epitaxially grown on an upper optical confinement layer 88 described later. A suitable thickness of the p-type cladding layer 89 is, for example, not less than 50 nm and not more than 500 nm, and in one embodiment is 400 nm.

  The semiconductor laser device 1C further includes a lower light confinement layer 86, an active layer 87, and an upper light confinement layer 88. The lower optical confinement layer 86, the active layer 87, and the upper optical confinement layer 88 constitute a first optical waveguide. That is, the lower optical confinement layer 86, the active layer 87, and the upper optical confinement layer 88 are high refractive index regions provided between the n-type cladding layer 85 and the p-type cladding layer 89, and the first optical waveguide. It functions as the core area.

  The refractive indexes of the lower optical confinement layer 86 and the upper optical confinement layer 88 are larger than the refractive indexes of the n-type cladding layer 85 and the p-type cladding layer 89. The band gap energy of the lower optical confinement layer 86 and the upper optical confinement layer 88 is smaller than the band gap energy of the n-type cladding layer 85, and the band gap wavelength is, for example, 1.2 μm. The lower optical confinement layer 86 and the upper optical confinement layer 88 are made of a III-V group compound semiconductor such as undoped InAlGaAs. A suitable thickness of the lower optical confinement layer 86 and the upper optical confinement layer 88 is, for example, 100 nm. The lower optical confinement layer 86 is formed by epitaxial growth on the n-type cladding layer 85. The upper optical confinement layer 88 is epitaxially grown on the active layer 87.

  The active layer 87 is made of a III-V group compound semiconductor and is provided between the lower optical confinement layer 86 and the upper optical confinement layer 88. The internal configuration of the active layer 87 is the same as that of the active layer 17 of the first embodiment described above.

The lower optical confinement layer 86, the active layer 87, and the upper optical confinement layer 88 are provided on a region extending in the optical waveguide direction on the n-type cladding layer 85 and constitute a mesa structure 92. The mesa structure 92 has side surfaces 92a and 92b along the optical waveguide direction. The p-type cladding layer 89 is provided on a region extending in the optical waveguide direction on the upper optical confinement layer 88 and constitutes a mesa structure 93. The mesa structure 93 has side surfaces 93a and 93b along the optical waveguide direction. The interval between the side surfaces 93a and 93b is narrower than the interval between the side surfaces 92a and 92b. The distance between the side surfaces 93a and 93b is, for example, 1.5 μm, and the distance between the side surfaces 92a and 92b is, for example, 3.0 μm. An insulating film 83 for protecting the semiconductor layer is provided from the side surfaces 92 a and 92 b of the mesa structure 92 to the n-type cladding layer 85. The insulating film 83 is made of an insulating silicon compound such as SiO ₂ .

  The semiconductor laser element 1C further includes an anode electrode 81 and a cathode electrode 82. The anode electrode 81 and the cathode electrode 82 are electrodes for supplying current to the active layer 87. The anode electrode 81 is the first electrode in this embodiment, and the cathode electrode 82 is the second electrode in this embodiment. The anode electrode 81 is provided on the upper optical confinement layer 88 along the side surfaces 93 a and 93 b of the mesa structure 93, and is in ohmic contact with the side surface of the p-type cladding layer 89. The anode 81 is made of, for example, Ti / Pt / Au or Ti / Au. The cathode electrode 82 is provided on the entire surface on the back surface 85 b of the n-type cladding layer 85 and is in ohmic contact with the n-type cladding layer 85. The cathode electrode 82 is made of, for example, Ti / Pt / Au or Ti / Au.

  The semiconductor laser element 1C further includes a diffraction grating layer 113 and a buried layer 114. The diffraction grating layer 113 has convex portions arranged at a predetermined period along the light propagation direction. In this embodiment, the diffraction grating layer 113 includes a plurality of parts divided in the light propagation direction at the period. The area consists of The diffraction grating layer 113 is made of a material having a refractive index larger than that of the p-type cladding layer 89, and is made of, for example, amorphous silicon that is an amorphous semiconductor. A suitable thickness of the diffraction grating layer 113 is 200 nm or more, and in one embodiment is 300 nm. The band gap wavelength of the diffraction grating layer 113 is, for example, 1.25 μm, which is longer than the band gap wavelength (1.1 μm) of the diffraction grating layer of a general DFB type semiconductor laser element. In other words, the band gap energy of the diffraction grating layer 113 is smaller than the band gap energy of the diffraction grating layer of a general DFB type semiconductor laser element. The band gap energy of the diffraction grating layer 113 is smaller than the band gap energy of the p-type cladding layer 89 located immediately below it.

The buried layer 114 is provided on the diffraction grating layer 113 and buryes the convex portion of the diffraction grating layer 113. The buried layer 114 can be made of a dielectric having a refractive index smaller than that of the diffraction grating layer 113. The dielectric that forms the diffraction grating layer 113 includes at least one material of SiO ₂ , SiN, Al ₂ O ₃ , and TiO. The buried layer 114 forms a diffraction grating 124 together with the diffraction grating layer 113. Since the refractive index of the buried layer 114 is smaller than the refractive index of the diffraction grating layer 113 as described above, the refractive index of the diffraction grating 124 is periodic along the optical waveguide direction with the period of the convex portion of the diffraction grating layer 113. Has a structure that changes with time. The diffraction grating 124 of the present embodiment is configured by a diffraction grating layer 113 divided at the above-described period, and a buried layer 114 provided between the diffraction grating layers 113 and on the diffraction grating layer 113. Yes.

It should be noted that the material of the buried layer 114 is preferably selected so that the refractive index difference between the diffraction grating layer 113 and the buried layer 114 constituting the diffraction grating 124 is increased. As described in the first embodiment, the relative refractive index difference (n1−n0) / n1 between the refractive index n1 of the diffraction grating layer 113 and the refractive index n0 of the buried layer 114 is 0.15 or more. Is preferred. That is, the refractive index n0 of the buried layer 114 is preferably 85% or less of the refractive index n1 of the diffraction grating layer 113. In one embodiment, the buried layer 114 is made of SiO ₂ as a dielectric. In this case, the refractive index of the buried layer 114 is about 1.5, which is much smaller than the refractive index 3.2 of the diffraction grating layer 113. The relative refractive index difference (n1-n0) / n1 in this example is about 0.46.

  Here, the diffraction grating layer 113 and the buried layer 114 constitute a second optical waveguide. That is, the diffraction grating layer 113 which is a high refractive index region sandwiched between the p-type cladding layer 89 and the buried layer 114 functions as a core region of the second optical waveguide. The first optical waveguide configured by the lower optical confinement layer 86, the active layer 87, and the upper optical confinement layer 88, and the second optical waveguide configured by the diffraction grating layer 113 and the buried layer 114 are p Optically coupled through the mold cladding layer 89.

  As shown in FIG. 16B, antireflection films (AR films) 135a and 135b are provided on both end faces 134a and 134b of the semiconductor laser device 1C in the light propagation direction. In addition, a laser resonator is constituted by the first optical waveguide including the active layer 87 and the second optical waveguide including the diffraction grating layer 113 provided between the end surfaces 134a and 134b. The distance (laser resonator length) between the end surface 134a and the end surface 134b is, for example, 250 μm.

  In the semiconductor laser device 1 </ b> C having the above configuration, the active layer mode that is a waveguide mode centering on the active layer 87 and the diffraction grating layer 113 are centered, as in the semiconductor laser device 1 </ b> A according to the first embodiment. Two guided modes such as a diffraction grating layer mode, which is a guided mode, are generated. In the semiconductor laser device 1C, the active layer mode and the diffraction grating layer mode are guided in parallel while being close to each other, so that these modes can be coupled to each other. In other words, the internal structure of the semiconductor laser element 1C can be an optical coupler that couples the active layer mode and the diffraction grating layer mode.

As shown in FIG. 3, when the active layer mode and the diffraction grating layer mode are coupled, oscillation occurs at an operating point where the phase matching conditions of these modes match. The difference Δf2 between the oscillation frequency when the transmitted light is at a high level and the oscillation frequency when the transmission light is at a low level becomes smaller as the slope of the dispersion curve of the diffraction grating layer mode becomes gentler. Furthermore, as shown in FIG. 4, the frequency difference Δf <b> 2 decreases as the refractive index difference between the two materials constituting the diffraction grating increases. In the present embodiment, the diffraction grating layer 113 constituting the diffraction grating is made of a semiconductor such as amorphous silicon, and the buried layer 114 is made of a dielectric such as SiO ₂ . When the buried layer 114 is made of SiO ₂ , the refractive index is about 1.5. When the diffraction grating layer 113 is made of amorphous silicon, the refractive index is about 3.5. Therefore, the frequency difference Δf2 is reduced, and chirping can be effectively reduced.

  Further, when manufacturing the semiconductor laser device 1C of the present embodiment, unlike the first embodiment and the second embodiment described above, a step of attaching a substrate is not required. Therefore, a structure including the buried layer 114 made of a dielectric can be manufactured relatively easily.

  The semiconductor laser device according to the present invention is not limited to the above-described embodiment, and various other modifications are possible. For example, in the above-described embodiment, the semiconductor laser element whose active layer or the like is made of a III-V group compound semiconductor is exemplified, but the present invention can also be applied to a semiconductor laser element made of another kind of semiconductor.

  While the principles of the invention have been illustrated and described in the preferred embodiments, it will be appreciated by those skilled in the art that the invention may be modified in detail without departing from such principles. We therefore claim all modifications and changes that come within the scope and spirit of the following claims.

  DESCRIPTION OF SYMBOLS 1A-1C ... Semiconductor laser element, 10 ... SOI substrate, 11 ... Substrate, 12 ... Cladding layer, 13 ... Diffraction grating layer, 14 ... Adhesive layer, 15 ... N-type cladding layer, 16 ... Lower light confinement layer, 17 ... Active Layer 18, Upper optical confinement layer 19 P-type cladding layer 20 P-type contact layer 21 Cathode electrode 22 Anode electrode 23 Insulating film 24 64 Diffraction grating 32 33 Mesa Structure: 35a, 35b, 75a, 75b ... Antireflection film, 40 ... First substrate product, 43 ... Semiconductor substrate, 44, 47 ... Temporary substrate, 45, 46 ... Adhesive, 50 ... Second substrate product, 53 ... diffraction grating layer, 54 ... buried layer.

Claims (9)

A first conductivity type semiconductor layer and a second conductivity type semiconductor layer laminated in a predetermined direction;
An active layer provided between the first conductive semiconductor layer and the second conductive semiconductor layer in the predetermined direction;
A diffraction grating layer having convex portions arranged at a predetermined period along a light propagation direction; and an embedded layer provided on the diffraction grating layer and embedding the convex portions of the diffraction grating layer, A diffraction grating provided at a position sandwiching the first conductivity type semiconductor layer between
A first electrode in contact with the first conductivity type semiconductor layer;
A second electrode in contact with the second conductivity type semiconductor layer,
The active layer constitutes a first optical waveguide;
The diffraction grating layer and the buried layer constitute a second optical waveguide;
A semiconductor laser element, wherein the first optical waveguide and the second optical waveguide are optically coupled via the first conductive type semiconductor layer to constitute a laser resonator.   2. The semiconductor laser device according to claim 1, wherein the diffraction grating layer is made of a semiconductor, and the buried layer is made of a dielectric. It said _dielectric, SiO 2, SiN, characterized in that it comprises at least one of _Al 2 _{O 3,} and TiO, a semiconductor laser device according to claim 2. The diffraction grating layer is formed by etching amorphous silicon deposited on a surface of the first conductivity type semiconductor layer opposite to the surface facing the active layer,
4. The semiconductor laser device according to claim 2, wherein the buried layer is formed by depositing the dielectric so as to cover the diffraction grating layer.   2. The semiconductor laser device according to claim 1, wherein the diffraction grating layer is made of a semiconductor, and the buried layer is made of a polyimide resin or a BCB resin.   6. The semiconductor laser device according to claim 5, wherein the diffraction grating layer and the first conductivity type semiconductor layer are joined via the buried layer.   7. The relative refractive index difference (n1-n0) / n1 between the refractive index n1 of the diffraction grating layer and the refractive index n0 of the buried layer is 0.15 or more. The semiconductor laser device according to any one of the above.   The semiconductor laser device according to claim 1, wherein a thickness of the diffraction grating layer is 200 nm or more.   9. The semiconductor laser device according to claim 1, wherein a thickness of the first conductivity type semiconductor layer is not less than 50 nm and not more than 500 nm.

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