JPS625355B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor laser having a structure effective for controlling oscillation mode.

Converting semiconductor lasers into fundamental modes is necessary for the practical application of broadband, long-distance transmission and low-distortion analog modulation in optical fiber communications.

Initially, control of the oscillation mode was achieved with so-called electrode stripe lasers, structures that confine optical energy and injected current to specific regions that minimize optical loss and wasteful recombination. Since then, various striped lasers have been developed and are still available today, but all of them have their own drawbacks and are unsatisfactory in terms of characteristics.

For example, when only the current distribution is confined in the lateral direction, such as in an electrode stripe laser, the laser light is mainly guided in the stripe direction by the gain distribution, but the waveguide effect due to this gain is unstable, and the current is increased. As a result, high-order transverse mode oscillation easily occurs, and furthermore, current-light output characteristics are often distorted. This is because the electrode stripe laser does not have a structure that confines carriers and light in the lateral direction of the active layer.

In order to compensate for the above drawbacks, a so-called rib guide or stripe laser, in which a waveguide mechanism is structurally built inside a semiconductor laser, has been proposed. This structure attempts to obtain stable fundamental mode oscillation by forming a light guide and a rib-structured waveguide in the light emitting region.

As a prior art prior to the present invention, this rib guide stripe laser should be mentioned, and first, the structure, mechanism, etc. of this type of GaAs-AlGaAs semiconductor laser will be briefly explained. FIG. 1 is a schematic perspective view thereof. Reference numeral 1 denotes an n-type GaAs substrate in which a band-shaped groove 10 is formed, and the following layers are laminated thereon. A light confining layer 2 _of n-type Al _0.3 Ga _0.7 As, a light guide and carrier confining layer 3 (hereinafter referred to as light guide ₎ made _of n-type Al _0.3 Ga _0.9 As and thickened in the center _. layer), an active layer 4 _of GaAs, and a layer 5 for confining light and carriers of the p-type Al _{0.3 Ga 0.7 As} layer. Because of the groove 10 formed, it has a concave, bowl-like shape. Electrodes 7 and 9 are in contact with the semiconductor substrate 1 and the SiO ₂ film 8 formed on the p-type GaAs 6 as the electrode facilitating layer, respectively. With such a structure, a portion of the light generated in the active layer 4 leaks into the light guide layer 3.
Since the light guide layer 3 is sufficiently transparent to the light generated in the active layer 4, there is no loss of oscillated light within this layer. There, the light spreads and propagates between the guide layer 3 and the active layer 4. Furthermore, these semiconductor layers 3,
4 is sandwiched between a layer 2 with a low refractive index that confines light and a layer 5 that confines light and carriers, so that light is guided in the vertical direction by the layers 2 and 5 with a low refractive index and is confined in the regions of layers 3 and 4. It will be done. In addition, in the lateral direction, the effective refractive index of the light guide layer 3 is maximized at the center of the groove 9 region and becomes thinner toward the outside of the groove 10. is confined in the central part of the raised light guide layer 3. Therefore, this structure is equivalent to providing an optical waveguide mechanism with a refractive index distribution in the lateral direction of the light emitting region, and is therefore characterized by stable fundamental mode oscillation even at high optical output. However, this rib guide stripe laser has the following drawbacks due to insufficient lateral carrier confinement.

That is, the carriers injected into the active layer 4 spread in the lateral direction of the active layer 4 due to the diffusion effect, and the effective light emitting area becomes considerably wider than the actual electrode width. For example, in a striped laser with an electrode width of 4 .mu.m, the light emitting region of the active layer spreads from just below the electrode to a width of about 20 to 30 .mu.m on one side, with its intensity attenuating exponentially in the lateral direction. Carriers other than directly below the electrode do not directly contribute to oscillation, so
This lateral current component increases the operating current, and the lateral spreading current component is several times larger than the effective oscillation current component as the electrode width becomes narrower than 10 μm, making it a narrow stripe laser. Lateral confinement of the carrier is an important issue. In addition to increasing the operating current, the temperature of the active layer also increases, reducing the differential quantum efficiency and making high-output operation impossible.

Furthermore, when the optical output exceeds a certain level, the output end face of the semiconductor laser is damaged by the light, causing the oscillation to stop. The extent to which this end face is damaged varies depending on the material of the active layer crystal and the amount of absorption of oscillated light by the active layer. The semiconductor laser with a light guide layer having the above structure is less likely to be damaged at the end face when compared with a normal semiconductor laser without a light guide layer. However, as long as the end face of the active layer exists, operation becomes difficult under high power.

The purpose of this invention is to provide a semiconductor laser structure that eliminates the drawbacks of conventional semiconductor lasers, allows easy fundamental mode oscillation, has a low oscillation threshold current, has high differential quantum efficiency, and can achieve high output. It is to be.

The present invention is solved by the structure of a semiconductor laser as described below.

The main structure of the semiconductor laser of the present invention is as follows.

A light confinement layer, a light guide and carrier confinement layer (hereinafter abbreviated as light guide layer), an active layer, and a light and carrier confinement layer are successively grown and formed on a semiconductor substrate by a common epitaxial growth method. has been done. The light and carrier confinement layer is formed in the shape of a bowl with a striped central portion on one side, and is completely buried in the light guide layer, thereby providing the light guide layer with a bowl-shaped guide region.

Furthermore, the conductivity type of the active layer corresponding to the bowl-shaped guide region is converted into a striped shape except for at least the region near the light output end face. Figure 2,
The basic principle of the present invention will be explained with reference to Fig. 3. Fig. 2 is a typical example of a semiconductor laser when the present invention is applied to a GaAs-AlGaAs semiconductor, and Fig. 3 is a schematic perspective view thereof. Figure 3A shows the cross section of the -' part (shown in Figure 2), Figure 3B shows the cross section of the -' part, and Figure 3C shows the cross section of the -' part. shows. Semiconductor substrate 1
1 is n-type GaAs, optical confinement layer 12 is n-type
The Al _0.3 Ga _0.7 As layer and the optical guide layer 13 _are also n-type _.
Al ₀ . ₁ Ga ₀ . ₉ As layer, GaAs active layer 14,
The light and carrier confinement layer 15 of _{Al 0.3 Ga 0.7 As} is initially of n-type conductivity.

The optical guide layer 13 in the groove 18 portion provided in the semiconductor substrate 11 is in contact with the optical confinement layer 12 which is arched inwardly toward the semiconductor substrate 11, and has a plano-convex shape with a convex center and a flat surface. It has a cross section of
Furthermore, an active layer 14 and a light and carrier confinement layer 15
The portions facing the plano-convex guide regions are striped p-type regions 19 and 19, respectively, except for the vicinity of the end faces.
It has been converted to 20. The n-type electrode 16 of the electrodes 16 and 17 is attached to the semiconductor substrate, and the p-type electrode is attached to the p-type region of the p-type region.
They are provided so as to be in contact with the _{surface of the Al 0.3 Ga 0.7} As layer ₂₀ , respectively. Typical layer thicknesses are 0.8 .mu.m for layer 12, 0.4 .mu.m for layer 13, 0.1 .mu.m for layer 14 and 1.5 .mu.m for layer 15 at the center of groove 18, respectively.

In operation, by applying a positive voltage to the electrode 17 and a negative voltage to the electrode 16, the rectifying junction 21 is forward biased. _{The rectifying junction 21 is an Al 0.3 Ga 0.7} As homojunction _,
It consists of a GaAs homojunction and a GaAs-Al ₀ . ₁ Ga ₀ . ₉ As heterojunction in parallel. Therefore, most of the injected current flows from the junction where the diffusion potential is low. Active layer 14
Since the forbidden band width of is the narrowest, current flows in both the GaAs homojunction and the GaAs-Al ₀ . ₁ Ga ₀ . ₉ As heterojunction. Therefore, laser oscillation occurs in the p-type region 19 of the active layer 14. The carriers injected into the p-type region active layer 19 are transmitted to each adjacent semiconductor 1 in the vertical direction.
3, 15 and the GaAs homojunction in the lateral direction, it is completely confined within this p-type region 19 without diffusion. Lasing occurs from p-type region 19 when the gain overcomes the loss with sufficient injection carriers. This laser light is applied to the light guide layer 1
3 and is guided by the light confinement layer 12 with a low refractive index and the light and carrier confinement layer 15. The thickness of the central part of the light guide layer 13 is different from that of the outside thereof. Therefore, the lateral refractive index distribution of the light guide layer 13 is larger at the central part than at the outside. In other words, an optical waveguide mechanism is built into the light emitting region, and due to the lateral confinement of light,
Stable fundamental mode oscillation.

Since the non-excited active layer near the end face also has an optical waveguide mechanism in which the refractive index changes in both the vertical and horizontal directions, the laser light can propagate without changing its fundamental mode.

Since carrier injection into the active region is performed by a confinement effect utilizing the difference in diffusion potential of the rectifying junction, there is an advantage that current injection efficiency is increased and luminous efficiency is also increased.

This has the advantage of requiring less operating current since there is no need to flow extra current.

Another feature is that when forming a p-n junction in the active layer 14, the carrier concentration n〓3×
10 ¹⁸ cm ^-3 , carrier concentration p in the p region 5×10 ¹⁸ cm
When controlled within the range of ^-3 , the n-type region near the end face can be made transparent to the laser beam due to the density difference. The excitation region end face of the active layer is not exposed,
Being surrounded by crystals that are transparent to laser light makes it difficult for edge damage to occur and can withstand high output power.

When forming the striped p-type regions 19 and 20, even if the p-type region penetrates the active layer and a p-n junction is provided in the n-type optical guide layer 13, the width of the striped p-type region If the diffusion length of the carrier is the same, the effect obtained is the same, but there is an advantage in that the difficulty in manufacturing the p-type region is eliminated.

As described in detail above, the semiconductor laser according to the present invention has advantages such as easy fundamental mode control, high output operation, and long life.

[Brief explanation of the drawing]

FIG. 1 is a schematic perspective view of a conventional semiconductor laser.
FIG. 2 is a schematic perspective view of a semiconductor laser according to an embodiment of the present invention, and FIG. 3 is a schematic cross-sectional view of its main parts. In the drawings, 1, 11...semiconductor substrate, 2,
12... Light confinement layer, 3, 13... Light guide layer, 4, 14... Active layer, 5, 15... Light and carrier confinement layer, 6... Electrode facilitation layer, 8...
SiO ₂ film, 7, 16... n-type electrode, 9, 17...
p-type electrode, 10, 18...groove, 19, 20...p
A type conversion region, 21... rectifying junction is shown, respectively.

Claims (1)

[Claims] 1. A two-layer structure in which a first conductivity type light guide and carrier confinement layer having a smaller refractive index and a wider forbidden band width than the active layer are formed adjacent to the first conductivity type active layer. It has a layer structure sandwiched between a first conductivity type optical confinement layer with a small refractive index and a first conductivity type optical and carrier confinement layer, and the layer thickness of the light guide and carrier confinement layer is striped in the center. furthermore, at least a portion of the active layer and the light and carrier confinement layer at a position corresponding to the thicker portion of the light guide and carrier confinement layer is located near the light output end face and the light output end face. 1. A semiconductor laser characterized in that the semiconductor laser is converted into a semiconductor region having a second conductivity type in a stripe shape except for a region.