JPH0550158B2 - - Google Patents

Description

Detailed Description of the Invention <Technical Field> The present invention relates to a semiconductor laser element having properties intermediate between a gain waveguide mechanism and a refractive index waveguide mechanism.

<Prior Art> Conventional semiconductor laser devices are classified into gain waveguide type and refractive index waveguide type based on waveguide structure. In the former case, oscillation is likely to occur in longitudinal multi-mode, the beam waist in the direction parallel to the junction is located about 20 to 40 μm deep into the cavity from the end face of the resonator, and the transverse mode is likely to become unstable due to the injected current. When the stripe for current injection is narrowed to about 4 μm, it becomes a so-called leaky mode, and the far-field pattern parallel to the junction becomes a bimodal pattern.
It has the following properties. On the other hand, in the latter case, the beam waist in the direction parallel to the junction is located near the resonant end face, oscillates in a single longitudinal mode, the transverse mode is stable, the far-field pattern is single-peaked, and the peak shift is small, etc. It has a nature.

FIG. 1A shows a cross-sectional view of an oxide film stripe type laser element as an example of a gain waveguide type double heterojunction semiconductor laser, and FIG. 1B and C show far-field images in a direction parallel to the junction. FIG. 1B is a far-field image when the stripe lateral S of the current injection stripe structure is wide at about 10 μm, and FIG. 1C is a far-field image when S is set to be narrow at about 4 μm. Also, Figure 1D
shows an example of vertical mode. In Figure 1A,
1 is a substrate, 2 is a first cladding layer, 3 is an active layer, 4
5 is a second cladding layer, 5 is a cap layer, 6 and 6' are electrodes, 7 is an oxide film, and 8 is a current injection stripe, that is, a current path.

Figure 2A shows a cross-sectional view of a CSP laser element as an example of a refractive index waveguide type double heterojunction semiconductor laser, Figure 2B shows a far-field pattern in the direction parallel to the junction, and Figure 2C shows an example of a longitudinal mode. . In the figure, the same reference numerals as in FIG. 1 represent the same contents. Further, 9 is a current injection stripe (width S) formed by diffusing impurities of a conductivity type opposite to that of the cap layer 5, and 10 is a substrate 1.
This is a groove (width W) formed in the groove. By allowing the light generated in the active layer 3 to reach the substrate outside the groove 10, the effective refractive index within the active layer is lowered at the outside of the groove 10 based on the light seepage effect, forming a refractive index waveguide. are doing. Since the first cladding layer 2 is thinner outside the groove 10, the light generated in the active layer 3 leaks into the substrate 1.

From the above-mentioned properties, it seems that the refractive index guided semiconductor laser is superior, but this laser element also has the following drawbacks. That is,
If the laser emission output changes, the element temperature changes, or the emitted laser beam is reflected by an optical component, etc., and even a small portion of it is returned to the element itself, the situation is shown in Figure 2C. The single mode transferred to other wavelengths causes noise. This is what is called mode competition noise. This noise has a relatively low frequency ranging from several MHz (megahertz) to 10 MHz, and the signal-to-noise ratio decreases to about 70 dB (decibels), making it a major hindrance when used as a light source for video discs and the like. It is known that this mode competition noise rarely occurs in multimode lasers. However, as mentioned above, gain-guided semiconductor lasers having multi-modes have drawbacks such as instability in transverse modes, which poses problems in practical use.

As described above, the emergence of a refractive index waveguide type multimode semiconductor laser device is currently awaited as a laser light source with stable transverse mode characteristics.

<Objective of the Invention> The present invention provides a semiconductor laser device that has the structure and properties of both a gain waveguide mechanism and a refractive index waveguide mechanism, that is, can perform stable fundamental transverse mode operation and longitudinal multimode oscillation at the same time. The purpose is to provide

<Description of structure and effects> Regarding the semiconductor laser device of the present invention, FIG.
This will be explained with reference to B and C. Two stripe-shaped grooves are formed in parallel on a substrate 1. Next, a current blocking layer 11 having a conductivity type opposite to that of the substrate 1 (or consisting of a layer having a high resistance value) is deposited on the substrate 1 including the groove to impart a current blocking function to the substrate. A concave groove 1 with a width W is formed by etching a portion corresponding to the chevron between the two grooves from the surface.
3 to expose the surface of this chevron. Board 1
The portion corresponding to the chevron-shaped portion becomes a current path from which the current blocking layer 11 is removed, that is, an internal stripe for current injection (stripe width S) 12. Current blocking layer 11
By processing and forming the groove 13 in the first cladding layer 2 laminated thereon, the concave groove 13 and the groove 13 are formed.
An optical waveguide is formed in which a layer thickness distribution is provided between the first cladding layer 2 and the other portions, and the effective refractive index of the active layer 3 superimposed on the first cladding layer 2 changes in accordance with this layer thickness distribution. A second cladding layer 4 and a cap layer 5 are superimposed on the active layer 3. Electrodes 6 and 6' are formed on the substrate 1 and the cap layer 5 to obtain the basic structure of a semiconductor laser device. The width S of the internal stripe 12 for current injection can be made considerably smaller by utilizing the side etch effect during etching. Now, if the width W of the groove 13 is set sufficiently wider than S, the effect of refractive index waveguiding by the groove 13 becomes small, and the refractive index distribution is caused by minority carriers injected from the stripe 12 and accumulated in the active layer 3. It is dominated by refractive index changes (the more carriers there are, the smaller the refractive index is). That is, the gain waveguide laser shown in FIG. 1 is obtained. When W is made to be the same as or narrower than S, the refractive index distribution is almost unaffected by changes in the refractive index due to minority carriers injected from the stripes 12 and accumulated in the active layer 3, and is completely suppressed by the grooves 13. A refractive index waveguide is formed. That is, the refractive index guided laser shown in FIG. 2 is obtained. As W becomes wider within the range of W > 2S,
The characteristics gradually change from those shown in FIG. 2 B and C to those shown in FIG. 1 C and D. In this case, the far-field image and longitudinal mode in the direction parallel to the junction are shown in Figures 3B and C, respectively.
Shown below. Furthermore, as W is made wider than S, the beam waist in the direction parallel to the joint gradually moves inward from the end face. If W>5S, instability of the transverse mode, which is a property of the gain waveguide, will occur, so it is desirable that 2S<W<5S. Also,
In order to suppress higher-order modes, it is better to narrow W to 8 μm or less. In this way, a semiconductor laser that oscillates in a stable fundamental transverse mode and simultaneously in longitudinal multi-modes is realized.

<Example> A case will be described in which an example of the semiconductor laser device of the present invention is manufactured using a GaAs-AlGaAs-based compound semiconductor.

As shown in FIG. 4A, two parallel stripe grooves 15,
15' is formed. Both groove width 4μm, depth 0.8μm
m, and the interval between them (that is, the internal stripe 16, which will become a current injection path to be described later), was 2 μm.
As shown in FIG. 4B, a current blocking layer 17 made of n-type GaAs with a carrier concentration of 3×10 ¹⁸ cm ^-3 is formed on the substrate 14 by liquid phase epitaxial growth to a thickness of 0.8 μm outside the groove. to form. Next, as shown in FIG. 4C, a concave groove 18 having a width W=8 μm is formed by etching so that the stripe 16 is in the center, and the current blocking layer 17 is removed to expose the surface of the stripe 16. Thereafter, a multilayer crystal structure having a double heterojunction as shown in FIG. 3A is formed by liquid phase epitaxial growth again to constitute a laser oscillation operating section. The composition and thickness of each layer of the multilayer crystal structure are as follows: first cladding layer 2 (P-Al _0.45
Ga _0.55 As, 0.15 μm), active layer 3 (P-Al _0.15 Ga _0.85
As, 0.08 μm), second cladding layer 4 (n-Al _0.45
Ga _0.55 As, 1.0 μm) and cap layer 5 (n-
GaAs, 3.0 μm). Au-Ge-Ni was deposited on the surface of the cap layer 5 as the n-side electrode 6, and Au-Zn was deposited on the back surface of the substrate as the p-side electrode 6'.
A current injection electrode layer is formed by alloying at 450°C.

As described above, a semiconductor laser having a concave groove width W=8 μm formed by removing the current blocking layer and a current injection internal stripe width S=2 μm is manufactured.

The laser of the above example oscillated at a threshold current of 60 mA and a wavelength of 780 μm, and operated in a stable fundamental transverse mode up to an emission output of 20 mW. It was also vertical multi-mode up to 15mW. When this semiconductor laser device was used as a signal light source for a video disk, good images were obtained without mode competition noise or return light noise.

Incidentally, the semiconductor laser device of the present invention has the above-mentioned characteristics.
It is not limited to GaAs-AlGaAs system,
Can be applied to InP-InGaAsP system and other heterojunction laser devices

[Brief explanation of drawings]

FIGS. 1A, B, C, and D are diagrams illustrating the configuration and characteristics of a conventional oxide film stripe structure semiconductor laser device. FIGS. 2A, B, and C are diagrams showing the configuration and characteristics of a conventional CSP laser device. Figure 3A,
B and C are a configuration diagram and a characteristic explanatory diagram of a semiconductor laser device used to explain the present invention. Figure 4 A, B,
C is a process diagram of an internal stripe structure of a semiconductor laser device illustrating one embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... Substrate, 2... Second cladding layer, 3... Active layer, 4... Second cladding layer, 5... Cap layer, 6, 6'... Electrode, 15... Stripe groove,
16... Internal stripe, 17... Current blocking layer.

Claims (1)

[Scope of Claims] 1. A substrate on which two parallel stripe grooves are formed, a current blocking layer laminated on the substrate, and a layer of the laminated current blocking layer from the substrate surface between the stripe grooves. a concave groove formed in the current blocking layer by removing the concave groove, and a double heterojunction layer consisting of a first clad layer, an active layer and a second clad layer, which covers the concave groove and is formed on the current blocking layer. The surface of the substrate from which the current blocking layer has been removed is used as an internal stripe for current injection, and a layer thickness distribution is provided between the concave groove portion of the first cladding layer and a portion other than the groove portion to form an optical waveguide. A semiconductor laser device characterized in that the width S of the internal stripe for current injection and the width W of the concave groove formed in the current blocking layer are set to satisfy 2S<W<5S.