JP2723888B2 - Semiconductor laser device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structure of a semiconductor laser device, and more particularly to a semiconductor laser device having a low astigmatism and suitable as a light source for a compact disk / video disk. [Prior Art] A gain-guided longitudinal multi-mode semiconductor laser can reduce noise, and is suitable for a compact and light source for video disks. However, the gain guided laser has a large astigmatism and a low quantum efficiency due to a loss of laser cavity, so that a high output stable operation cannot be obtained. To reduce astigmatism,
The reduction is achieved by making the end face of the semiconductor laser curved. In addition, a refractive index waveguide structure is effective for increasing the quantum efficiency and stabilizing a high output. Applied Physics Letter Vol. 47 No. 5 (1985)
Page 445 (Appl. Phys. Lett. Vol. 47, No. 5 (1985) P. 445)
Describes that a semiconductor laser with a curved end face has achieved a reduction in threshold current and a high output by means of a refractive index waveguide structure, but sufficient consideration has been given to reducing astigmatism. Not realized. Also Applied
Physics Letter Vol. 47, No. 10 (1985) No. 923 (A
ppl. Phys. Lett. Vol. 46, No. 10 (1985) P. 923) discusses the reduction of astigmatism of a semiconductor laser, but fails to obtain a good characteristic result. This is because the stripe width of the semiconductor laser is large and the lateral mode control is not sufficient, and the position of the laser beam waist determined by the relationship between the radius of curvature of the curved end face and the laser cavity length is a condition for reducing astigmatism of the semiconductor laser. It is considered that sufficient results have not been obtained. Further, a conventional semiconductor laser has a waveguide provided by utilizing a groove-like structure provided in a GaAs substrate, as shown in FIG. 15, for example. In this structure, the distance between the active layer of the laser and the GaAs substrate is different between the region above the groove and the other region, causing a difference in the effective refractive index between the two regions and causing the laser light to reach the groove region. Was trapped. When the refractive index difference between the two regions is sufficiently large, the semiconductor laser oscillates in a single longitudinal mode, and since the coherence of the laser is strong, the oscillation state becomes unstable due to return light from the disk surface. A problem of return light noise has occurred. On the other hand, when the difference in refractive index is small, light confinement due to the gain distribution of the active layer caused by the difference in current injection density becomes the main waveguide mechanism, longitudinal multimode oscillation occurs, and the optical noise becomes strong. The laser wavefront was bent by the effect of the gain distribution, and astigmatism occurred. As a means for obtaining an element that is resistant to return light noise and has small astigmatism, a method in which the refractive index difference is set to an intermediate region between the single mode and the multimode by optimizing the distance between the substrate and the active layer outside the groove. However, astigmatism cannot be completely eliminated. [Problems to be Solved by the Invention] In the above prior art, the laser end face is a curved semiconductor laser, and a high efficiency and high output stable operation is achieved as a refraction waveguide type structure, and the laser beam spreads at the time of single mode high output. However, it has not been sufficiently considered to reduce astigmatism by a laser structure, and there has been a problem that the laser beam is so large at low output that the laser beam cannot be narrowed down. An object of the present invention is to provide a gain-guided structure near the end face of a laser while taking advantage of the characteristics of high-efficiency and high-output stable operation of a semiconductor laser having a refractive index-guided structure, and to take into account the stripe width. To provide a semiconductor laser having a small astigmatism by controlling a curved end face shape in a longitudinal mode, and to provide a semiconductor laser having no astigmatism while performing multi-mode oscillation by solving the above-described problems of the semiconductor laser. It is in. [Means for Solving the Problems] The above-mentioned object is to consider the stripe width in a range where a single transverse mode can be obtained by using a refractive index waveguide structure at the center of the semiconductor laser and a gain waveguide structure near the end face. This is achieved by providing a curved surface shape having the center of an arc at the boundary point where the laser beam propagates from the refractive index waveguide to the gain waveguide on the laser end face. In a semiconductor laser having a gain-guided structure, the laser light wavefront is curved, and the apparent waist of the emitted beam is inside the element, causing astigmatism. This astigmatism is
This can be reduced by making the end face of the semiconductor laser a curved surface. However, a semiconductor laser having a curved laser end face has a high threshold current and may have low efficiency. The astigmatism and efficiency of a semiconductor laser having a curved end surface shape are related to the radius of curvature of the curved end surface and the cavity length of the laser, and by taking these into consideration, forming an appropriate curved end surface curvature radius and cavity length. , It is possible to reduce astigmatism of the output beam. 6 (a) and 6 (b) are examples of a conventional structure, and FIGS. 6 (c) and (d) are examples of the present invention. Further, as means for achieving the above object, in the present invention, the end face of the semiconductor laser is made to be a curved surface, and an area where the effective refractive index can be adjusted and controlled is provided near the end face, and this area and a normal waveguide area are provided. This is achieved by optimizing the curvature of this curved surface by studying that the end surface has a convex lens effect by making the interface of the curved surface a curved surface. [Function] The stripe width of the semiconductor laser is increased to 10 μm or more,
The multi-longitudinal mode laser having the gain waveguide structure has excellent low noise characteristics, but is inferior in low threshold current and high efficiency. In a semiconductor laser having a refractive index guided structure with a small laser cavity loss, high-efficiency, high-output stable operation can be expected. Therefore, by combining the laser end face portion with a gain waveguide structure and the central portion with a refractive index waveguide structure, efficiency can be improved and high output operation can be performed. The efficiency ratio between a laser having a curved end surface and a laser cleaved is determined by the ratio between the laser cavity length and the radius of curvature of the curved surface. That is, in order to improve the efficiency of a laser having a curved end face, it is necessary to reduce the laser cavity length and increase the radius of curvature of the curved surface. Further, in a semiconductor laser having a curved end surface shape, the beam spread may be reduced during high-power operation. The astigmatism of a semiconductor laser having a gain waveguide structure is determined by the spot width of the beam and the radius of curvature of the wavefront. This astigmatism can be reduced by making the laser end face into a curved end face shape.
That is, the beam spot becomes smaller on the convex end face, and the radius of curvature of the wavefront can be reduced by reducing the radius of curvature of the curved surface. Also, the beam spot becomes large on the concave end surface, and the wavefront radius of curvature can be reduced by reducing the radius of curvature of the curved surface. At this time, if the radius of curvature of the curved surface is determined so as to correspond to the interval of the gain waveguide type structure at the laser end face, the beam waist can be placed at the boundary point of the waveguide structure, so that from the end of the refractive index waveguide type structure. It is possible for the beam to occur. Further, according to the present invention, a semiconductor laser having a completely flat radiation wavefront while performing multimode oscillation resistant to return light noise was obtained. In addition, in the case of this structure, since the divergence angle of the radiation beam is narrower than that of a normal semiconductor laser, there is an advantage that the lens aperture when applied to a system can be reduced. Embodiment Next, an embodiment of the present invention will be described with reference to the drawings. Example 1 Example 1 will be described with reference to FIG. Among the semiconductor laser structures, the first in the channeled stripe planar (CSP) type laser
As shown in FIG. 3A, a refractive index waveguide structure is provided at the center of the laser, and a gain waveguide structure is provided at the end face. That is, n-type Ga
Etching is performed on the As substrate 6 (thickness: 100 μm) using an appropriate mask to provide a ridge-shaped 1 μm step at the center, and a channel of 1 μm is provided extending in the stripe direction from one end face of the resonator to the other. Thereby, n-type
The distance between the GaAs substrate 6 and the active layer 8 formed on the substrate is narrowed at the center of the laser from the laser end face, and the effective refractive index difference between the channel and the active layer formed on both sides of the channel is reduced by the laser end face. The former is configured as a gain waveguide structure and the latter is configured as a refractive index waveguide structure so that the laser beam becomes larger at the center of the laser. Thereafter, an n-type Ga _1-x Al _x As cladding layer 7 (thickness 0.5 μm, x = 0.
5), undoped Ga _1-y Al _y As active layer 8 (thickness 0.1 μm, y =
0.14), p-type Ga _1-x Al _x As clad layer 7 (thickness 1.5 μm, x =
0.5), a p-type GaAs cap layer 10 (1 μm thick) is grown. Thereafter, a p-type Zn diffusion region 13 is formed using a mask to provide a stripe-shaped current injection region. Next, p-type
A Mo-Aup electrode 12 is deposited on the GaAs cap layer, and an AuGeNi / Cr / Au electrode 11 is deposited on the n-type GaAs substrate. Further, a concave curved surface as shown in FIG. 1A is formed on the laser end surface by reactive ion beam etching using a suitable resist mask on the p-electrode. 1 (b) and 1 (c) are cross-sectional views of the laser device of FIG. 1 (a) cut out along the lines bb 'and cc'. The stripe width is 10 μm or less, which is suitable for a single transverse mode. At this time, a laser cavity length of 200 μm is suitable for a curved surface with a radius of curvature of 20 to 50 μm.
It could be 10 μm or less. In addition, stable operation with a threshold current of 30 mA and an output of 50 mV was realized in the multi-vertical mode. Embodiment 2 Another embodiment of the present invention will be described with reference to FIG. Among the semiconductor laser structures, a self-aligned structure (SAS) type laser is provided with a structure having the same effect as in the first embodiment.
In other words, the n-type-GeAs current pinching layer 14 is made of n-type Ga as shown in FIG. 2 (a) so that the laser center portion has a refractive index waveguide structure and the end surface has a gain waveguide type structure. _{A 1-x} Al _x As clad layer 9 is formed in a ridge shape by utilizing an etching step.
Other crystal layer growth can be performed in the same manner as in the first embodiment. Also,
In addition to the liquid phase growth method, a metal organic vapor phase growth method can also be used. FIGS. 2 (b) and 2 (c) respectively show b in FIG.
It is sectional drawing cut out by the line of -b ', cc'. In this example, the same result as that of Example 1 was obtained. Embodiment 3 Another embodiment of the present invention will be described with reference to FIG. Among the semiconductor laser structures, a structure having the same effect as that of the first embodiment is provided to a buried structure (BH) type laser. That is,
In order to make the laser center part a refractive index waveguide structure and the end face part a gain waveguide structure, after creating a normal double hetero structure, the laser center part is removed by etching using an appropriate mask. The buried layer 15 is formed by liquid crystal growth or metal organic chemical vapor deposition. Other crystal layer growth can be performed in the same manner as in the first embodiment. 3 (b) and 3 (c) are cross-sectional views cut along the lines bb 'and cc' in FIG. 3 (a), respectively. In this example, the same result as that of Example 1 was obtained. Embodiment 4 Another embodiment of the present invention will be described with reference to FIGS. In the above embodiment, the active layer is a flat layer, and the refractive index waveguide is weakened at the end face, as a method of forming the central part of the laser with the refractive index guided structure, and the both sides with the gain guided structure. The refractive index waveguide layer was not formed on the end face. In this embodiment, the same effect as in the above embodiment was obtained by forming the active layer of the laser such that a step was formed between the center portion and the end face portion. 4 and 5, the active layer 8 can be realized by crystal growth exactly the same as in the above embodiment, except that the active layer 8 is formed using etching so that a step is formed between the center and the end face. FIG. 4 shows an example in which the present invention is applied to the CSP type laser shown in FIG. 1, and (a) and (b) in FIG. 4 correspond to (b) and (c) in FIG. FIG. 5 shows an example in which the present invention is applied to the SAS type laser shown in FIG. 2, and FIGS. 5 (a) and 5 (b) correspond to FIGS. 2 (b) and 2 (c). In addition to the present embodiment, a laser having a window structure in which a transparent non-absorbing region n-type Ga _1-x Al _x As layer is provided on the laser end face is also conceivable. This embodiment is inferior to the above embodiments in terms of the threshold current and high output of the laser, but has similar results in terms of noise and astigmatism. Example 5 In a semiconductor laser having a curved end surface, the radius of curvature R of the curved surface
By setting the parameter of the laser cavity length L to an appropriate value, astigmatism can be reduced and efficiency can be improved. By controlling the radius of curvature R of the curved surface and the cavity length L of the laser for the straight stripe and the laser whose stripe width changes at the end face as shown in FIG. As a result, astigmatism was reduced and efficiency was improved. Laser efficiency and astigmatism are related to the ratio of the laser cavity length L to the radius of curvature R of the curved surface. The efficiency of the laser could be increased by decreasing L and increasing R. The astigmatism is L, R
By increasing both, the reduction can be achieved. However, when R is increased, the far-field image of the output beam is widened. To reduce the beam spread as much as possible, a practical laser cavity length of 200 μm is required.
It is appropriate to set the radius of curvature of the curved surface to 20 to 50 μm, and the spread of the far-field image can be suppressed to 20 ° with astigmatism of about 10 μm. Further, the shape of the end face is arc-shaped, but may be another curve, or a combination of a straight line and a curve. Furthermore, GaAlAs
/ GaAs material was used as an example, but InGaAsP / InP, InGaAlP /
Needless to say, the same effect can be expected with other materials such as GaAs. Embodiment 6 FIG. 7 shows the structure of the embodiment. As shown in FIG. 7 (a), this structure comprises a curved end face (21) serving as a mirror of a laser resonator, and a stripe-shaped current injection region (22) sandwiched between the curved end faces. The cross-sectional structure of the current injection region is MOCVD as shown in FIG.
With current Kyo窄structure by forming the Zn diffusion twice growth by law _{p-Ga 0.5 Al 0.5 As (} 26) / Ga 0.86 Al 0.14 As (25) /
It has an n-Ga _0.5 Al _0.5 As (24) double heterostructure. In the current injection region, the optical waveguide mechanism is a so-called gain guide, and the laser performs multi-mode oscillation. However, a wavefront bending which causes astigmatism which is a characteristic of the gain guide occurs. However, instead of the conventional structure having a cleaved face as an end face, when a curved end face is formed by using RIE (reactive ion etching), a laser beam emitted from the end face becomes astigmatic due to a lens effect of the end face. A plane wave having no aberration can be obtained. FIG. 8 shows the curvature 1 / R of the end face, the curvature of the wave front of the radiation beam (FIG. 8 (a)) and the radiation to obtain the shape of the curved end face necessary to obtain such a plane wave. The relationship between the angles (FIG. 8 (b)) is an experimental result obtained for current confinement widths of 2, 5, and 10 μm. As shown in FIG. 2, 1 / R is obtained for a 2 μm stripe.
Is 22.6 mm ¹ , 5 / μm stripe, 1 / R = 12.3 mm, 10
In the case of a μm stripe, the curvature of the wavefront becomes zero around 1 / R = 5.4 mm, and the beam radiation angle becomes minimal. From the above results, the curvature of the end face was set to the ninth with respect to the stripe width of the laser.
It is considered appropriate to set the range in the shaded area in the figure. Based on the above analysis, a curved end face laser having an optimized end face curvature was produced. As a result, the wave front of the emitted beam was hardly bent, and a narrow beam having an emission angle of 1 to 2 degrees was obtained. Example 7 A structure having a built-in lens structure on the end face as shown in FIG. 10 was devised. When this structure is viewed from above, a current injection region 22 as shown in FIG. 10, a lens region 32 having an effective refractive index different from that of the current injection region, and a mirror of a resonator, which are in contact with this region with a curved interface, are provided. From the end face of the semiconductor. The structure of the current injection region may be any structure as long as the current can be limited only in the laser stripe. In the present embodiment, the eleventh embodiment in which the current is confined by a semi-insulating (GaAl) As layer by ion implantation is used. The sectional structure was as shown in the figure. The laser structure is the same as that of the first embodiment, p-Ga _0.5 Al _0.5 As (26) / Ga _0.86 Al _0.14 As (25) / n-G
a _0.5 Al _0.5 As (24) Double hetero structure. The change in the refractive index of the end face part for forming the lens area of the end face is:
The substrate was provided with a curved step in the direction intersecting with the stripe, and the laser light was deviated from the active layer in the end face region. In order to improve the verticality of the wall surface of the step, RIBE (reactive ion e
tching) method was used. The effective refractive index of the waveguide region is about 3.2
Since the refractive index of the cladding layer is 3.4, a difference in the refractive index of about 0.2 occurs. In this case, the relationship between the curvature of the area interface, the curvature of the wavefront of the radiation beam, and the beam radiation angle is respectively 13th.
The results are as shown in FIGS. From these results, it is considered that the condition that the curvature of the wavefront of the radiation beam is small and the radiation angle is small is the shaded region in FIG. In general, if the refractive index of the two media forming the lens was n ₁ n _2, the medium and the air having a refractive index n ₁ to obtain the effect of lens equivalent formed by the medium and air refractive index n ₁ the curvature of the n ₁ / a lens formed (n ₂ -n ₁₎ is required times curvature. When a semiconductor laser having this structure was experimentally manufactured, a narrow beam having an emission angle of 1 to 2 degrees was obtained, as in Example 1, with almost no bending of the wavefront of the output beam.
In addition, in the case of this structure, since the difference in the refractive index of the lens portion is smaller than in the first embodiment, there are advantages such as less scattering of light due to surface roughness during curved surface processing and better controllability of the focal length of the lens. There is. [Effects of the Invention] According to the present invention, a multi-longitudinal mode can be realized in a single transverse mode in a semiconductor laser, and high output stable operation up to a threshold current of 30 mA and 50 mW with an astigmatism of 10 μm or less was confirmed. Further, the relative noise intensity is 1 × 10 ^-13 Hz ^-1 within 5% of the returning light quantity.
The following low noise characteristics (temperature 0 to 50 ° C.) were obtained. As a result, there is an effect that a semiconductor laser having a low aberration which can satisfy a condition required as a compact and light source for a video disk, and can sufficiently narrow a beam with low noise can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1, 2 and 3 show an embodiment of the present invention, wherein (a) is a perspective view of a semiconductor laser device, (b) is a longitudinal sectional view in the stripe direction,
(C) is a vertical cross section of the stripe, and FIGS.
Another embodiment related to FIGS. 1 and 2 is shown, and FIGS.
(B) is a diagram corresponding to (b) and (c) in FIGS.
FIG. 7 is a diagram showing a curved end surface shape and an optical waveguide structure (an example of a stripe shape) of a semiconductor laser. FIG. 7 is a diagram showing Example 6.
FIG. 8 is a view showing the beam spread at the end face, FIG. 9 is a view showing the relationship between the current confinement width and an appropriate end face curvature, FIG. FIG. 12 is a cross-sectional structural view of a current injection region in the structure of the device of Example 2, FIG. 12 is a structural diagram of a lens region of the structure of the device in Example 2, and FIG. FIG. 14 is a diagram showing the relationship between the current pinching width and an appropriate end face curvature, and FIG. 15 is a diagram showing the structure of a conventional semiconductor laser. 1... Concave laser end face, 2... Convex laser end face, 3
... A straight stripe, 4... A stripe having a width wider than the center at the end face, 5... A stripe having a width narrower than the center at the end face, 6.
As _{substrate, 7 ...... n-Ga 0.5 Al} 0.5 As Kuratsudo layer, 8 ...... undoped Ga _1-x Al _x As active _{layer, 9 ...... P-Ga 0.5 Al} 0.5 As Kuratsudo layer, 10 ...... P-GaAs cap Layer, 11 n-electrode (AuGeNi / Cr / Au), 12 P-electrode (Au / Mo), 13 P
+ -Zn diffusion layer, 14 ... n-GaAs current pinching layer, 15 ... n-
Ga _0.5 Al _0.5 As buried _{layer, 16 ...... Al 2 O 3 -SiO} 2 insulating layer,
21: curved end face, 22: current injection area, 23: GaAs
Substrate, 24 ... n-GaAlAs clad layer, x = 0.5, 25 ... G
aAlAs active layer, x = 0.14, 26 ... p-GaAlAs cladding layer, x = 0.5, 27 ... n-GaAs layer, 28 ... p-GaAs buried layer, 29 ... p electrode, 30 ... n electrode , 32 ... lens area,
33 ... hydrogen ion implantation area, 34 ... substrate step.

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Shinichi Nakatsuka               1-280 Higashi Koigabo, Kokubunji-shi               Central Research Laboratory, Hitachi, Ltd. (72) Inventor Shigeo Yamashita               1-280 Higashi Koigabo, Kokubunji-shi               Central Research Laboratory, Hitachi, Ltd. (72) Inventor Akemi Yamanaka               190 Komogi-shi               In the Komoro branch factory of Hitachi, Ltd. (72) Inventor Shun Kajimura               1-280 Higashi Koigabo, Kokubunji-shi               Central Research Laboratory, Hitachi, Ltd.                (56) References JP-A-53-80185 (JP, A)                 JP-A-55-107289 (JP, A)                 JP-A-61-253881 (JP, A)                 JP-A-60-66890 (JP, A)                 JP-A-61-112392 (JP, A)

Claims (1)

(57) [Claims] A resonator structure for oscillating laser light in a stripe direction including an active layer and a stripe-shaped current injection region for injecting a current into the active layer; an end face of the resonator structure is formed in a curved surface shape; The resonator structure includes a first region including the curved end face in the stripe direction and a second region sandwiched between the first regions, and a portion of the active layer facing the stripe-shaped current injection region. A semiconductor laser device wherein an effective refractive index difference between the first region and the second region is larger than the first region.