JP2569087B2 - Semiconductor laser device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser device used for pickup of an optical disk such as a video disk.

[Conventional technology]

When a semiconductor laser is used as a pickup for an optical disk such as a video disk, for example, in a so-called single mode semiconductor laser in which an oscillation spectrum (transverse mode) oscillates at a single wavelength, good coherency is an obstacle. It is known that the optical output becomes unstable due to the return light to the semiconductor laser and noise is generated. Therefore, a semiconductor laser having relatively poor coherency is desired as a semiconductor laser used as an optical disk pickup.

An example of a semiconductor laser that achieves the above characteristics is the one discussed in the Proceedings of the Japan Society of Applied Physics 9P-K-7p.170 (Autumn 1981), and sets the structural parameters appropriately. As a result, a self-excited oscillation (pulsation) phenomenon occurs, and a multi-axial mode oscillation having a wide spectral line width, that is, a modestly poor coherency is obtained.

[Problems to be solved by the invention]

However, the above prior art does not take into consideration the light emission angle of the semiconductor laser, particularly the direction horizontal to the active layer, and has a problem in connection with an optical disk pickup optical system. In the optical disk pickup optical system, a semiconductor laser device having a horizontal beam divergence angle of about 12 degrees or less in the active layer is required for the active layer due to the relationship between the lens opening angle and the like. The beam divergence angle of a semiconductor laser is closely related to the spot size of the semiconductor laser, and therefore to the shape and refractive index distribution of the optical waveguide. However, in the above-described conventional semiconductor laser device, when the structural parameters are set to narrow the beam divergence angle, the pulsation phenomenon is suppressed, and as a result, the noise of the semiconductor laser increases.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a low-noise semiconductor laser which solves the above-mentioned problems of the prior art and performs a pulsation operation, has a moderately poor coherency, and has a narrow beam divergence angle in the horizontal direction.

[Means for solving the problem]

In order to achieve the above object, the present invention provides the following semiconductor laser structure.

In the semiconductor laser of the present invention, a semiconductor cladding layer is formed on both sides of a flat semiconductor active layer, and the thickness of one of the cladding layers is thicker in the optical waveguide than in the outside. A semiconductor layer for current constriction having a stripe-shaped convex shape or an inverted convex shape and having an opening along the optical waveguide portion stripe-shaped region; Is wide at least in the vicinity of the reflection surface on the light beam extraction side of the semiconductor laser, and the change is smooth.

FIG. 2 is a schematic view of the optical waveguide region of the semiconductor laser of the present invention. FIG. 2 is a schematic plan view, and a dotted line shows a stripe-shaped optical waveguide region. Central portion of the present invention is a laser in the width W ₁ of the optical waveguide section, this region is a structure as Paruseshiyon phenomenon effectively occurs, W ₂ at least one of the reflecting surface near widen> W ₁ And the beam spread angle is corrected in this area. Also, the transition from width W ₁ to W ₂ is slowly carried out.

[Action]

In the semiconductor laser of the present invention, it is possible to set the structural parameters of the internal region other than the reflection surface vicinity portion to a structure in which the pulsation phenomenon easily occurs, and
In the vicinity of the reflection surface, the optical waveguide structure can be expanded, and the laser spot size in the waveguide can be increased. By expanding the size of the laser spot near the reflecting surface, the divergence angle of the beam emitted from the reflecting surface can be made narrower than before. The divergence angle of the beam is arbitrarily set by changing the width W ₂ of the optical waveguide in the vicinity of the reflection surface, that is, the width of the stripe-shaped convex (or inverted convex) provided in the semiconductor cladding layer. It is possible. Further, in the present invention, the change in the stripe-shaped convex shape (or the reverse convex shape) between the inside and the vicinity of the reflection surface is made gentle, and the active layer is made almost flat over the entire region. , It is possible to smooth the light propagation during this time, there is no harmful light scattering and reflection loss at the joint,
There is no increase in the oscillation threshold. Therefore, according to the present invention, a low-noise semiconductor laser device is obtained in which the pulsation phenomenon is effectively generated, the coherency is moderately low, and the light beam emitted from the laser is both narrow.

〔Example〕

Hereinafter, an embodiment of the present invention will be described with reference to FIG. FIG. 1 is a structural exploded view of a semiconductor laser device according to an embodiment of the present invention. 1 is a p-type GaAs substrate (Zn doped, p ~
1 × 10 ¹⁸ cm ⁻³ ) and an n-type GaAs current confinement layer 2 (Te
A dope, n-4 × 10 ¹⁸ cm ⁻³ , about 1 μm thick) was formed by a liquid phase growth method. Next, a stripe-shaped groove 3 was formed on the GaAs current confinement layer 2 so as to reach the p-type GaAs substrate 1 by using both photolithography and wet etching. Here, it is an important point of the present invention that the stripe-shaped groove 3 is about 5 μm in the vicinity of one resonator reflection surface and about 3.7 μm inside the element. The connection between them was loosely connected. Next, the liquid phase growth is performed again, and the p-type Ga _0.55 Al _0.45 As cladding layer 4 (Zn doped, p〜6 × 10 ¹⁷ cm ^-3 , thickness, 0.35 μm outside the groove), undoped Ga _0.86 Al _0.14 As active layer 5 about 0.08 μm thick, formed flat over the entire area), n-type Ga _0.55 Al _0.45 As clad layer 6 (Te doped, nn8 × 10 ¹⁷ cm ^-3 , about 1.8 μm thick) , N
-Type GaAs cap layer 7 (Te-doped, n~4 × 10 ¹⁸ cm ^-3, about 4μm thick) were formed. Reference numeral 8 denotes an n-side ohmic electrode formed by a vacuum evaporation method. On the back, GaAs substrate 1 is approximately
After polishing and etching to 80 μm, a p-side ohmic electrode 9 was formed by a vacuum evaporation method. After electrode formation, resonator length approx.
It was chipped to 250 μm and assembled. This laser oscillated at a threshold current value of about 50 mA and a wavelength of about 780 nm, a pulsation phenomenon was obtained with a good yield up to an optical output of 6 mW, and a low-noise laser with a relative noise intensity of 10 ^-14 Hz ^-1 was obtained. The beam divergence angle θ _″ in the horizontal direction of the active layer was about 10 °, which was suitable for an optical disc pickup optical system.

FIG. 3 shows the relationship between the groove width W ₁ (see FIG. 2) at the center of the device and the incidence of the pulsation device in the semiconductor laser of the present invention. This result indicates that when the width of the central portion of the element is increased to 5 μm or more, the occurrence rate of the pulsation element is remarkably reduced, and it is difficult to obtain a low-noise laser. It was but connexion, the groove width W ₁ is preferably set to about 3.5 ± 1 [mu] m.

FIG. 4 shows the correlation between the groove width W ₂ (see FIG. 2) near the reflection surface of the device and the beam divergence angle θ _″ in the horizontal direction in the active layer in the semiconductor laser of the present invention. It can be seen from this that the larger the value of W ₂ , the smaller θ _″ can be. Θ _″ suitable for optical disk pickup (12
In order to obtain ゜), it is desirable that W ₂ be about 4 μm or more.

〔The invention's effect〕

According to the present invention, the internal structure of the semiconductor laser device is such that the optical waveguide portion width W _{1 is} such that the pulsation phenomenon effectively occurs.
The width W ₂ of the optical waveguide near the reflection surface on the light extraction side can be designed so as to obtain a beam divergence angle θ _″ suitable for the external optical system. region changes from W ₁ to W _2, by changing the slowly width, can significantly reduce the loss in light propagation in the meantime, almost no such increase in the threshold current. in addition, active in the resonator entire By forming the layer to be substantially flat, the propagation loss of light in the region where the width of the optical waveguide portion changes can be significantly reduced, and according to the present invention, the pulsation phenomenon effectively occurs. As a result, it is possible to obtain a very excellent low-noil semiconductor laser device having a moderately low coherency, a narrow beam divergence angle suitable for coupling with an optical system, a low threshold current value, and a very excellent low-noil semiconductor laser device. . The present invention not only GaAlAs-based semiconductor laser device, other material systems, for example also be applied to InGaAsP system, the technical effect is very large.

[Brief description of the drawings]

FIG. 1 is a structural exploded view of a semiconductor laser device showing one embodiment of the present invention. FIG. 2 is a schematic plan view showing an optical waveguide region of the semiconductor laser of the present invention. FIG. 3 shows a semiconductor laser of the present invention, in which a groove width at the center of the device is shown.
Is a diagram showing the relationship between W ₁ and Paruseshiyon incidence of elements. Figure 4 is a semiconductor laser of the present invention, showing the correlation between the horizontal beam divergence angle theta _"the groove width W ₂ and the active layer of the reflective surface near the device. 1 ...... p-type GaAs substrate ..., N-type GaAs current confinement layer, 3.
... Striped groove for optical waveguide, 4 ... P-type Ga _0.55 Al _0.45 As
Clad layer, 5 ... Ga _0.86 Al _0.14 As active layer, 6 ... n-type
Ga _0.55 Al _0.45 As cladding layer, 7 ... n-type GaAs cap layer, 8 ... n-side ohmic electrode.

Claims (3)

(57) [Claims] 1. A semiconductor active layer having a flat shape on both sides,
The forbidden band width is larger than the active layer, and it has a structure sandwiched by a semiconductor cladding layer having a small refractive index, and the thickness of at least one of the cladding layers is
It has a stripe-shaped convex shape or an inverted convex shape that is thicker than the outside, and has a conductive type opposite to the clad layer in a region outside the stripe-shaped convex shape or the inverted convex shape of the clad layer. In the semiconductor laser device having the semiconductor current confinement layer of the above, the width of the stripe-shaped convex shape or the reverse convex shape is wide at least near the reflection surface on the light beam extraction side of the semiconductor laser device, and the change is smooth. A semiconductor laser device, characterized in that: 2. After forming a first semiconductor current confinement layer of the second conductivity type on a semiconductor substrate of the first conductivity type, a single semiconductor current confinement layer reaching the semiconductor substrate along a portion to be an optical waveguide portion. In a semiconductor laser device in which a second semiconductor cladding layer of the first conductivity type, a third semiconductor active layer, and a fourth semiconductor of the second conductivity type are formed thereon, 2. The device according to claim 1, wherein the width is wide at least in the vicinity of the reflection surface on the light beam extraction side of the laser element, and the active layer has a substantially flat shape over the entire area. 14. The semiconductor laser device according to claim 1. 3. The groove width of the central portion of the semiconductor laser device is 3.5 ± 1.
3. The semiconductor laser device according to claim 2, wherein the groove width in the vicinity of the reflection surface on the light beam extraction side is 4 μm or more.