Semiconductor laser device

Abstract: In a semiconductor laser device having an active layer (3) and clad layers (2, 4), lens-like portions (7, 10) which magnify radiation are formed in the active layer (3) and/or the clad layer (4) where the radiation leaks, thereby making it possible to enlarge a stripe width and to produce a funda­ mental mode oscillation of high output. Further, a lens-like portion which condenses the radiation is formed at an end face of an emission port for a laser beam, thereby making it pos­ sible to narrow the radiant angle of the laser beam.

Description: Background of the Invention: The present invention relates to the structure of a semiconductor laser device which oscillates in the fundamental mode at a high output. As shown in Fig. 7 by way of example, a prior-art semiconductor laser has guided laser radiation by establishing the difference of effective refractive indices inside and outside a laser stripe in such a way that a region 6 exhibitive of an absorptivity for the laser radiation is disposed at a position spaced several hundred nm from the active layer 3 of the semiconductor laser. In case of the laser having such a structure, in a region of intense laser radiation, carriers within the active layer are dissipated by the stimulated emission of the laser radiation, and hence, the refractive index of the active layer increases. Especially the index increase at the part of intense radiation within the active layer becomes greater than at the other parts. Therefore, the self-focusing in which the radiation concentrates on the particular part more and more takes place to lead to the self-filamentation. Under the condition of the self-filamentation, the laser beam of the semiconductor laser shifts with the movement of a filament until the oscillation thereof shifts to a mode of higher order. For the purpose of preventing this drawback and attaining the fundamental mode oscillation, the width of the stripe needs to be reduced. When the stripe width is made smaller, the laser beam cannot shift, and the higher-order mode oscillation is cut off. With the narrow stripe, however, the light density of the laser heightens, and unfavorably the crystal thereof becomes liable to optical damages. It can accordingly be said that the reduction of the stripe width is demeritorious for the fabrication of a high-output laser. The literature of the prior art forming the background of the present invention is, for example, "Applied Physics Letters," Vol. 30, No. 12, 1977, page 649. A semiconductor laser device comprising the features contained in the first part of claim 1 is disclosed in "Applied Physics Letters", vol. 47, No. 3, 1985, pages 188-190. A lens-like portion is formed there at the light emitting end face of the laser device, which converges the emitted laser beam without requiring a separate optical component. Summary of the Invention: An object of the present invention is to prevent the self-focusing in the prior art and to broaden the stripe width of a semiconductor laser, thereby to lower the light density of the laser and to heighten the output thereof. When the stripe width is broadened, an identical output is attained with a lower light density, and a higher optical output is attained for an identical light density. This object is accomplished by the semiconductor laser device as specified in claim 1. That is, the semiconductor laser device of the present invention consists in that lenses which magnify the radiation enough to exceed the increase of the refractive index ascribable to the dissipation of carriers are formed in the active layer and/or the clad layer into which the radiation leaks, whereby the concentration of the radiation is hindered, so as to prevent the self-focusing. It is needless to say that the portion having the refractive index distribution exhibitive of the lens effect can be disposed in the active layer. In recent semiconductor lasers, however, the active layer is as thin as 0.1 µm or less, and the radiation leaks into the clad layer. Therefore, when the refractive index distribution exhibiting the lens effect exists at the part of the clad layer within 0.5 - 0.7 µm from the active layer, the progression of the radiation is affected, and the refractive index distribution can be formed in the clad layer. Alternatively, the index distribution may be formed in both the active layer and the clad layer. In short, the semiconductor laser device of the present invention is so constructed that the part thereof through which the laser radiation passes is formed with the portions(hereinbelow, termed "lenses") each of which has the refractive index distribution exhibitive of the lens effect of magnifying the radiation. In a case where the refractive index inside the lens is lower than the refractive index of the layer formed with the lenses (the refractive index outside the lens), the lens is shaped convex, and in a case where the former is higher than the latter, the lens is shaped concave. The lenses are arrayed at pitches d within the stripe of the laser device. The one of the effective refractive indices of the layer formed with the lenses, inside and outside the lenses is denoted by n₁ and the other of them n₂. The symbol k₁ is let denote an effective refractive index at the well-known apex of a self-focusing region exhibiting a parabolic distribution, and k₂ is let denote the coefficient of a square term in a formula indicating the parabolic distribution. Further, R is let denote the radius of curvature of each lens (here, R has a plus value when the center of the curvature lies on the side of a material exhibiting the refractive index n₁, and R has a minus value when the center lies on the side of a material exhibiting the refractive index n₂). Then, the values of the radius R and the pitch d may be determined so as to satisfy an inequality mentioned below. By the way, the effective refractive index is an apparent refractive index for the progressing laser radiation as is well known. Since the radiation leaks into the clad layer, the effective refractive index takes a value different from the refractive index of the layer itself where the lenses exist, but a method of determining the effective index is known. In a semiconductor laser employing GaAs- and AlGaAs-system compounds, all the values n₁, n₂ and k₁ are about 3.4, the absolute value of a difference (n₂ - n₁) is 5 x 10⁻³ to 2 x 10⁻², and the value k₂ is about 4 x 10⁻⁵. The values of the indices n₁ and n₂ are almost equal, and may be the same in approximation. However, they have the slight difference (n₂ - n₁), the value of which is an important factor greatly influential on the determination of the radius R and the pitch d . Excepting the formation of the lenses, the structure of the semiconductor device of the present invention may be the structure of the prior art as it is. In addition, the thicknesses of respective semiconductor layers constituting the laser device may be the same as in the prior art. The stripe width, however, can be increased much. In practical use, the stripe width is set in consideration of the intended application of the laser device, the light intensity thereof, a processing accuracy for the fabrication of the lens portion, etc. Using the present invention, the self-focusing of a semiconductor laser can be prevented, so that a laser of broad stripe width can be oscillated in the fundamental mode, and an output several times higher than those of conventional semiconductor lasers can be attained. Further, a lens-like refractive index distribution effective to concentrate radiation is formed at the end face of an exit port for the laser radiation, whereby the radiant angle of the laser radiation can be narrowed. Brief Description of the Drawings: Fig. 1 is an explanatory view showing the plan arrayal of the lens portion of a semiconductor laser device in each of the embodiments of the present invention; Fig. 2 is a perspective view of a semiconductor laser structure in the first embodiment of the present invention; Fig. 3 is a perspective view of a semiconductor laser structure in the second embodiment of the present invention; Fig. 4 is a perspective view of a semiconductor laser structure in the third embodiment of the present invention; Fig. 5 is a perspective view of a semiconductor laser structure in the fourth embodiment of the present invention; Fig. 6 is a perspective view of a semiconductor laser structure in the fifth embodiment of the present invention; and Fig. 7 is a sectional view of a semiconductor laser structure in a prior art. Description of the Preferred Embodiments: Embodiment 1: Fig. 2 is a perspective view showing a semiconductor laser device in this embodiment, in which a part of a portion including lenses is cut open. Incidentally, also in each of Figs. 3 - 6 which are perspective views showing semiconductor lasers in other embodiments, a part of a portion including lenses is cut open. On an n-GaAs substrate 1, and using MOCVD (organometallic chemical vapor deposition) method, there were successively grown an n-Ga 0.5 Al 0.5 As clad layer 2 having a thickness of 1.5 µm, a Ga 0.86 Al 0.14 As active layer 3 having a thickness of 0.07 µm, a p-Ga 0.5 Al 0.5 As clad layer 4 having a thickness of 0.3 µm, a p-Ga 0.7 Al 0.3 As layer 5 having a thickness of 0.2 µm, and an n-GaAs light absorption layer 6 having a thickness of 1.0 µm. Thereafter, a striped SiO₂ pattern was formed using conventional photolithography, and a selected part of the n-GaAs light absorption layer 6 was removed by reactive ion etching. Further, by performing a photolithographic step again, the p-Ga 0.7 Al 0.3 As layer 5 was processed into a configuration shown in Fig. 1 in which lens-like pit portions 7 recurred along the stripe. Subsequently, the burying growth of two layers consisting of a p-Ga 0.5 Al 0.5 As layer 8 at a thickness of 1.5 µm and a p-GaAs cap layer 9 at a thickness of 0.5 µm was carried out into a structure as shown in Fig. 2. Thus, the material of the p-Ga 0.5 Al 0.5 As layer 8 was buried as lenses into the pit portions 7 of the p-Ga 0.7 Al 0.3 As layer 5. In addition, the p-Ga 0.5 Al 0.5 As layer 8 served as a clad layer continuous to the p-Ga 0.5 Al 0.5 As clad layer 4. The p-Ga 0.7 Al 0.3 As layer 5 served to dispose the lenses for affording a refractive index variation, and the lenses were formed on the basis of the difference between the refractive indices of the part where the material of this layer exists and the parts where it does not exist, namely, the pit portions. Meanwhile, one period (distance d indicative of the pitch of the lenses) of such a lens-like structure (having a radius of curvature R) subjects the position and direction of laser radiation to conversion as expressed by the following matrix: On the other hand, in a case where a radiation focusing effect ascribable to the dissipation of carriers by the laser radiation occurs between the lenses, the matrix becomes as follows on the assumption that the propagation constant of the radiation can be approximated by k = k₁ - 1/2 k₂ x² where x denotes a variable indicating a position and corresponds to a distance from a predetermined point: The parameters of the matrix may satisfy the following inequality lest the above-stated system should cause the focusing effect of the laser: In the present embodiment, the values of the refractive indices n₁, n₂ and k₁ were set at 3.4, the value of the coefficient k₂ was set at 4 x 10⁻⁵, and the value of the difference of the indices n₂ and n₁ was set at about 1 x 10⁻². Thus, R = 15 µm and d = 10 µm were set on the basis of the aforementioned calculation. Then, a semiconductor laser which oscillated in a stable fundamental lateral mode up to an optical output of 200 mW was obtained with a device having a stripe width of 10 µm. By the way, also in the ensuing embodiments, the values of n₁, n₂ and k₁ were set at 3.4, the value of k₂ was set at 4 x 10⁻⁵, and the value of the difference between n₂ and n₁ was set at about 1 x 10⁻². Besides, although only one lens is seen in Fig. 2, the large number of lenses are actually arrayed as illustrated in Fig. 1. Also in the embodiments of Figs. 3 - 6, a large number of lenses are actually arrayed. Embodiment 2: As the second embodiment of the present invention, a semiconductor laser having a structure as shown in Fig. 3 was manufactured by way of example. On an n-GaAs substrate 1, and using MOCVD method, there were successively grown an n-Ga 0.5 Al 0.5 As clad layer 2, a Ga 0.86 Al 0.14 As active layer 3, a p-Ga 0.5 Al 0.5 As clad layer 4, and an n-GaAs light absorption layer 6. Thereafter, a striped SiO₂ pattern was formed using conventional photolithography, and a selected part of the n-GaAs light absorption layer 6 was removed by reactive ion etching. Further, by performing a photolithographic step again, the p-Ga 0.5 Al 0.5 As clad layer 4 was processed into the configuration shown in Fig. 1 in which lens-like pit portions 7 recurred along the stripe. Subsequently, the burying growth of two layers consisting of a p-Ga 0.7 Al 0.3 As layer 10 at a thickness of 1.5 µm and a p-GaAs cap layer 9 was carried out into the structure as shown in Fig. 3. The thicknesses of the respective layers were the same as in Embodiment 1, except that the p-Ga 0.7 Al 0.3 As layer 10 was set at the above thickness of 1.5 µm. In the present embodiment, the material of the p-Ga 0.7 Al 0.3 As layer 10 was buried as lenses into the pit portions of the p-Ga 0.5 Al 0.5 As clad layer 4. In addition, the p-Ga 0.7 Al 0.3 As layer 10 served as a part of the clad layer portion. The lens-like portions as stated above were designed on the basis of the same calculation as in the case of Embodiment 1. When R = 15 µm and and d = 10 µm were set, a semiconductor laser which oscillated in a stable fundamental lateral mode up to an optical output of 200 mW was obtained with a device having a stripe width of 10 µm. Embodiment 3: As the third embodiment of the present invention, there was manufactured by way of trial a semiconductor laser in which, in order to attain a higher output, the stripe width in the structure of Embodiment 1 or 2 was expanded, and lens-like portions 11 were formed into a two-dimensional array as shown in Fig. 4. Conditions for preventing the occurrence of self-filamentation were evaluated by the same calculation as in Embodiment 1. On the basis of this calculation, ten lenses of R = 15 µm and d = 10 µm each having a width of 10 µm were arranged within the stripe 100 µm wide so as to form the two-dimensional lens array. Then, a semiconductor laser oscillating in a stable fundamental lateral mode up to an optical output of 1 W was obtained. Embodiment 4: As the fourth embodiment of the present invention, a semiconductor laser of a structure as shown in Fig. 5 was manufactured by way of trial. On an n-GaAs substrate 1, and using MOCVD method, there were successively grown an n-Ga 0.5 Al 0.5 As clad layer 2, a Ga 0.86 Al 0.14 As active layer 3, a p-Ga 0.5 Al 0.5 As clad layer 4, a p-Ga 0.7 Al 0.3 As layer 5, a p-GaAs quantum well layer 12, a p-Ga 0.7 Al 0.3 As layer 13, and an n-GaAs light absorption layer 6. Thereafter, a striped SiO₂ pattern was formed using conventional photolithography, and a selected part of the n-GaAs light absorption layer 6 was removed by reactive ion etching. Further, by performing a photolithographic step again, the p-Ga 0.7 Al 0.3 As layer 13 was processed into the configuration shown in Fig. 1 in which lens-like pit portions recurred along the stripe. Subsequently, the burying growth of three layers consisting of a p⁺-Ga 0.5 Al 0.5 As layer 14 doped with Zn, a p-Ga 0.5 Al 0.5 As layer 8 and a p-GaAs cap layer 9 was carried out into the structure as shown in Fig. 5. The thicknesses of the respective layers were the same as in Embodiment 1 except that the p-GaAs quantum well layer 12 was made 0.2 µm thick, that the p-Ga 0.7 Al 0.3 As layer 13 was made 0.2 µm thick, and that the p⁺-Ga 0.5 Al 0.5 As layer 14 was made 0.3 µm thick. Next, the resultant structure was heat-treated at 850 °C for 60 minutes while an As pressure was kept applied in an MOCVD equipment. As a result, Zn contained in the p⁺-Ga 0.5 Al 0.5 As layer 14 was rediffused into the p-GaAs quantum well layer 12 through the lens-like pit portions provided in the p-Ga 0.7 Al 0.3 As layer 13, and the parts of the p-GaAs quantum well layer 12 corresponding to the lens-like pits of the p-Ga 0.7 Al 0.3 As layer 13 were brought into a mixed crystal in the shape of lenses. A lens-like structure similar to that of any of Embodiments 1 - 3 was formed owing to the difference between the refractive indices of a quantum well rendered the mixed crystal and a quantum well not rendered it. In the present embodiment, the p-Ga 0.7 Al 0.3 As layer 13 served also as a continuous clad layer, and the p⁺-Ga 0.5 Al 0.5 As layer 14 served as the diffusion source of Zn for the p-GaAs quantum well layer 12. When R = 15 µm and d = 10 µm were set on the basis of the calculation in Embodiment 1, a semiconductor laser oscillating in a stable fundamental lateral mode up to an optical output of 200 mW was obtained with a device having a stripe width of 10 µm. Incidentally, the rediffusion technique in the present invention is effective, not only for establishing the refractive index distribution, but also for improving the electric connection of a regrown boundary. Even when the rediffusion technique is applied to a conventional self-aligned structure having no lens portions, the effect of lowering the series resistance of an element is achieved. Embodiment 5: As the fifth embodiment of the present invention, a semiconductor laser of a structure as shown in Fig. 6 was manufactured by way of trial. On an n-GaAs substrate 1, and using MOCVD method, there were successively grown an n-Ga 0.5 Al 0.5 As clad layer 2, a Ga 0.86 Al 0.14 As active layer 3, a p-Ga 0.5 Al 0.5 As clad layer 4, and an n-GaAs light absorption layer 6. Thereafter, a striped SiO₂ pattern was formed using conventional photolithography, and a selected part of the n-GaAs light absorption layer 6 was removed by reactive ion etching. Further, by performing a photolithographic step again, the Ga 0.86 Al 0.14 As active layer 3 and the p-Ga 0.5 Al 0.5 As layer 4 were processed into the configuration shown in Fig. 1 in which lens-like portions 7 recurred along the stripe. Subsequently, the burying growth of two layers consisting of a p-Ga 0.7 Al 0.3 As layer 10 and a p-GaAs cap layer 9 was carried out into the structure as shown in Fig. 6. The thicknesses of the respective layers were the same as in each of the preceding embodiments. Such lens-like portions were designed on the basis of the same calculation as in the case of Embodiment 1. When R = 15 µm and d = 10 µm were set, a semiconductor laser oscillating in a stable fundamental lateral mode up to an optical output of 200 mW was obtained with a device having a stripe width of 10 µm. In the prior art, the stripe width of which the highest optical output can be expected is 3 - 5 µm, whereas in each of the embodiments, the stripe width is 10 µm. Besides, it has been verified that there is an effect even when the stripe width in the semiconductor laser of the present invention is enlarged to 40 µm. As compared with a narrow stripe, a broadened stripe can produce an identical optical output at a lower light density and can remarkably heighten an optical output at an identical light density. With the prior art, when the stripe width is enlarged, it is difficult to produce a high output under a stable oscillation. In contrast, even when the stripe width in the semiconductor laser of the present invention is 10 µm or greater, a high output and a stable oscillation can be attained, and the output of a single fundamental mode can be increased double or more in comparison with that in the pior art.

Claims: A semiconductor laser device comprising an active layer (3), clad layers (2, 4) and a lens-like portion (7, 11) having a refractive index distribution which causes a lens effect, characterised in that lens-like portions (7, 11) are formed in said active layer (3) and/or in that part of at least one of said clad layers (2, 4) into which light leaks from said active layer, wherein said refractive index distribution causes a magnifying lens effect. The device of claim 1, wherein, when the refractive index in said lens-like portions (7, 11) is lower than that of the layer (2...4) where they are formed, each said lens-like portion is convex, and when the former refractive index is higher than the latter, each said lens-like portion is concave. The device of claim 2, wherein the radius of curvature R of each said lens-like portion (7, 8) and the pitch d of said lens-like portions satisfy the following inequality: where n₁ denotes the effective refractive index inside said lens-like portions (7, 11) and n₂ denotes the effective refractive index outside said lens-like portions (7, 11) in the layer in which said lens-like portions are formed, k₁ denotes the effective refractive index at the apex of a self-focusing region having a parabolic distribution, and k₂ denotes a coefficient in a square term of an equation indicative of the parabolic distribution. The device of claim 3, wherein said active layer (3) and said clad layers (2, 4) are made of a GaAs system compound including Ga x Al 1-x As, and the values of the effective refractive indices n₁, n₂ and k₁ are all about 3.4, while the value of the coefficient k₂ is 4 x 10⁻⁵. The device of claim 4, wherein the absolute value of the index difference (n₂-n₁) is 5 x 10⁻³ to 2 x 10⁻². The device of any of claims 1 to 5, further comprising a refractive index distribution which has a lens effect of condensing the light and which is formed at an end face of an emission port for a laser beam of said laser device.

Inventors: KAJIMURA TAKASHI, NAKATSUKA SHINICHI, KAJIMURA, TAKASHI, NAKATSUKA, SHINICHI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1988
Main classification: H01S3
IPC codes: H01S5, H01S3