JPS6358389B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a striped semiconductor laser device that oscillates in longitudinal multi-mode, and particularly to a structure for improving astigmatism.

In recent years, semiconductor laser devices have made remarkable progress, and in particular, not only transverse single mode but also longitudinal single mode oscillation has been realized with various striped structures having refractive index guides, and they are entering a period of practical use in fields such as optical communications. However, in longitudinal single mode oscillation, noise is likely to occur due to interference of returned light.
Additionally, there are problems such as significant mode competition noise occurring during longitudinal mode transitions due to temperature changes, especially in the consumer field where video disks, audio disks, etc. are used over a wide temperature range.
It is said that a semiconductor laser device that performs longitudinal multi-mode oscillation, in which these phenomena are less likely to occur, is more suitable than a longitudinal single-mode laser.

On the other hand, as structures of semiconductor laser devices that perform longitudinal multimode oscillation, striped laser devices having various structures without a refractive index guide are known. However, in such a striped laser device, the light is guided by a gain guide in the direction parallel to the active layer (hereinafter referred to as a "gain guide laser"), so the wavefront is curved and the light is guided in the direction parallel to the active layer. A so-called astigmatism occurs in which the virtual radiation position is shifted compared to the laser beam that is emitted and spread in the direction. This astigmatism is undesirable for applications such as video disks and digital audio disks in which laser light is focused by a lens. This is because astigmatism requires the use of a special lens such as a cylindrical lens in order to effectively focus the light. Moreover, even if a cylindrical lens is used, astigmatism slightly depends on the manufacturing conditions of the element and has poor reproducibility, so it is necessary to prepare lenses of various standards, and in practice, astigmatism is caused by a cylindrical lens. It was difficult to compensate for aberrations.

This invention was made in view of the above points, and includes a gain guide type stripe laser device.
The purpose is to realize a semiconductor device with small astigmatism by integrally forming a refractive index guide configured by changing the impurity concentration in the laser light emission path in a direction parallel to the laser active layer. It is said that

FIG. 1 is a perspective view showing an embodiment of this invention.
1 is an n-type gallium arsenide (GaAs) substrate, 2, 3
and ₄ are n-type aluminum gallium arsenide ( _Al type Al _y Ga _1-y As active layer, and p-type Al _x Ga _1-x As layer, where 1>x
>y, n-type Al _x Ga _1-x As layer 2 and p-type Al _x
x and y such that the refractive index of the Ga _1-x As layer 4 is sufficiently smaller than the refractive index of the p-type Al _y Ga _1-y As active layer 3.
The value of is selected. 5 is p-type Al _x Ga _1-x As layer 4
n-type GaAs layer formed on top, 6 is n-type GaAs
Grooves 7 are formed in stripes in a part of the surface of the layer 5 shown as A in the figure, so as not to reach the p-type Al _x Ga _1-x As layer 4 . Illustration A
8 is a shallow p-type diffusion layer formed by diffusion of zinc (Zn) including the inner surface of the groove 7, and 8 is an extension of the above-mentioned striped groove 6 in a portion B slightly spaced from the illustrated portion A. n-type GaAs layer 5 except for the
A p ⁺ -type diffusion layer formed by diffusing Zn at a high concentration so as to reach at least the n-type Al _x Ga _1-x As layer 2 from the surface of the n-type GaAs substrate 1; It is an electrode.

Figure 2 is a sectional view taken along the - line in Figure 1.
As shown in the figure, the p-type diffusion layer 7 is in contact with the p-type Al _x Ga _1-x As layer 4 at the bottom of the trench 6 . In addition, 10
is a p-side electrode formed inside the groove 6 and over the upper surface of the p-type diffusion layer 7. In addition, in FIG. 1, the p-side electrode 10 is omitted to avoid complexity.

Now, in this example device, the p-side electrode 10
When a voltage is applied between the n-side electrode 9 and the n-side electrode 9 so that the former has positive polarity, the current flows concentrated only directly under and in the vicinity of the striped groove 6, as shown by the broken line arrow in FIG. This current causes electrons to
Type Al _x Ga _1-x As layer 2 to p-type Al _y Ga _1-y As active layer 3
is injected only in the vicinity directly below the striped groove 6,
In this part, electrons and holes recombine and emit light. Therefore, in such a structure, if the striped grooves 6 are designed to have an appropriate width and the current is sufficiently increased, gain-guided laser oscillation will occur in the direction parallel to the active layer 3. It is well known that this occurs. In the gain-guided laser oscillation mode, generally, if the stripe width is made sufficiently narrow, the transverse mode becomes a single mode, but the longitudinal mode becomes a multi-mode oscillation.

By the way, in the embodiment of the present invention, as shown in FIG. 1, the gain guide type laser structure is not formed over the entire area of the base, but is formed to have a length that does not increase the threshold current of laser oscillation. It is limited to the illustrated part A. Of the remaining parts, part B has p ⁺ -type diffusion layers 8 formed on both sides thereof in a shape that leaves an extension of the resonator of the gain-guided laser in part A described above. As a result, the lateral p-type impurity concentration distribution in the active layer 3 in this region has a p ⁺ -p-p ⁺ structure.
Figures 3a and 3b schematically show the lateral impurity concentration distribution and the lateral refractive index distribution in the active layer 3 at the cut plane taken along the - line in Figure 1, respectively. It is a diagram. If the impurity concentration of the p ⁺ type part is set to about 1 x 10 ¹⁹ cm ^-3 and the impurity concentration of the p type part (active layer itself) to about 1 x 10 ¹⁷ cm ^-3 , then the refractive index α of the p ⁺ type part The difference Δα between the refractive index α ₁ and the refractive index α ₂ of the p-type portion is approximately 0.02. Therefore, p ⁺ −p
The -p ⁺ structure sufficiently functions as a refractive index guide and has a light trapping effect even in the direction parallel to the active layer 3, so that the p-type region functions as a waveguide.

FIG. 4 is a partial plan view illustrating the effect of this refractive index guide portion B on improving astigmatism, and FIG.
4b shows the wavefront of laser light and the spread of light near the resonator end face in a conventional gain guide type laser device without the refractive index guide portion B, and FIG. It shows the wavefront of laser light and the spread of light reaching the vicinity of the wavepath output. As can be seen from Figure 4a, in the conventional gain guide type laser device, the wavefront (indicated by the broken line) gradually curves near the cavity end face, and the light confinement effect is weaker, and the light emitted from far deeper than the end face also becomes laser light. Therefore, the laser light appears to be emitted from a distance d deeper than the cavity end face. However, as shown in FIG. 4b, in the structure of the embodiment of the present invention, the light with a curved wavefront that enters the refractive index guide part B from the gain guide type laser part A partially leaks out of the guide. In this part, the refractive index is smaller and the speed of light is faster than in the inside of the guide, so the phase front is gradually and directly corrected.
Therefore, the wavefront becomes gradually straighter even inside the refractive index guide B, and if the length of the refractive index guide portion B is appropriately designed, the wavefront can be straightened quickly near the end face. Furthermore, in this example device, unlike a conventional gain guide type laser device,
Light emitted diagonally from far deeper than the end face is p ⁺
It may be absorbed in the mold region 8 and not come out to the outside, so the laser beam radiation source position is near the end face of the refractive index guide part B (waveguide) (for example, e in the figure).
difference in degree).

On the other hand, since the direction perpendicular to the active layer 3 is guided by a double heterostructure refractive index guide, the radiation source position of the laser beam is still at the end face of the waveguide, so it is parallel to the direction perpendicular to the active layer 3. The directions and the positions of the radiation sources almost match, and no astigmatism occurs.

In the above embodiment, an example using a GaAs-AlGaAs-based material was shown, but this method can also be applied to a semiconductor laser device using an indium phosphide (InP)-indium gallium arsenic phosphorus (InGaAsP) material. The invention is applicable. Further, the present invention is applicable to all laser devices having a gain guide type stripe structure other than those using the current confinement method as in the embodiment.

As detailed above, the semiconductor laser device of the present invention includes a gain guide type striped laser region that performs longitudinal multi-mode oscillation in one semiconductor substrate, and a curved shape of the light oscillated and emitted from this striped laser region. Since it is integrated with the refractive index guide region that corrects and straightens the wavefront, it is possible to create a new semiconductor laser device that has less transition noise during temperature fluctuations, less astigmatism, and can effectively extract laser oscillation output. .

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing an embodiment of the present invention, FIG. 2 is a sectional view taken along the - line in FIG. 1, and FIGS. 3 a and b are cross-sectional views taken along the - line in FIG. A diagram schematically showing the lateral impurity concentration distribution in the active layer on the surface and the lateral refractive index distribution accompanying this, and FIG. 4 is a part explaining the effect of the refractive index guide portion on improving astigmatism. In the plan view, FIG. 4a shows the wavefront of the laser beam and the spread of light near the cavity end face in a conventional gain-guided laser device without a refractive index guide part, and FIG. 4b shows an embodiment of the present invention. The wavefront of the laser beam and the spread of the light from the vicinity of the resonator end face to the vicinity of the waveguide exit are shown in FIG. In the figure, 1 is an n-type GaAs substrate (semiconductor substrate), 2 is an n-type Al _x Ga _1-x As layer (first semiconductor layer),
3 is a p-type Al _y Ga _1-y As layer (second semiconductor layer), 4 is a p-type Al _x Ga _1-x As layer (third semiconductor layer), A is a striped semiconductor laser region, and B is a striped semiconductor laser region. This is a refractive index guide region. Note that the same reference numerals in the figures indicate the same or corresponding parts.

Claims (1)

[Claims] 1 an n-type first semiconductor layer on an n-type semiconductor substrate, an n-type semiconductor layer having a smaller forbidden band width than the first semiconductor layer;
or a p-type second semiconductor layer, an n- or p-type third semiconductor layer having a larger forbidden band width than the second semiconductor layer;
a gain-guide semiconductor laser region formed in a first part of the substrate which does not include one end facet in a substrate in which semiconductor layers are sequentially formed; and a second part of the substrate which includes the one end facet thereof Optical excitation consisting of a p ⁺ -p-p ⁺ structure formed in the second semiconductor layer and formed by p-type impurity diffusion having the central axis of the waveguide as an extension of the resonator of the gain-guided semiconductor laser region. 1. A semiconductor laser device comprising a refractive index guide region having no function.