JPH0587997B2 - - Google Patents

Description

[Detailed description of the invention] Industrial applications The present invention relates to semiconductor lasers.

BACKGROUND ART AND PROBLEMS Conventional semiconductor lasers are broadly classified into index guide type and gain guide type depending on the light confinement mechanism.

For example, a structure shown in FIG. 1 has been proposed as a refractive index guided semiconductor laser. This semiconductor laser includes, for example, a first cladding layer 2 made of N-type Al _z Ga _1-z As and a P-type or N-type Al _x Ga _1-x As on an N-type GaAs substrate 1. An active layer 3, a second cladding layer 4 made of P-type Al _z Ga _1-z As, and a light composed of N-type Al _y Ga _1-y As embedded in the second cladding layer 4. Absorbent layer 5
and a P-type cap layer 6 with a high impurity concentration. The light-absorbing layer 5 has, for example, a cut-out portion 9 in the center thereof in which the light-absorbing layer 5 is removed in the form of a stripe having a width W in the direction perpendicular to the plane of the paper in FIG.
It consists of The light absorption layer 5 has a bandgap width smaller than that of the active layer 3.
The composition is selected so that the refractive index of light oscillated from the light emitting region is equivalently higher on the light emitting region, that is, on the active layer 3 side. That is,
In the compositions of the layers 3 and 5 described above, x>y is selected. In the figure, numerals 7 and 8 indicate the cap layer 6 and the electrodes ohmicly applied to the substrate 8, respectively.

In such a configuration, when a predetermined forward voltage is applied between the electrodes 7 and 8, light is emitted in the active layer 3 confined by the first and second cladding layers 2 and 4. In this case, the distance d between the active layer 3 and the light absorption layer 5 is selected to be a distance that allows the light oscillated from the active layer 3 to reach the light absorption layer 5. is the light absorption layer 5
The light from the active layer 3 seeped out is absorbed in the light absorption layer 5, so that the light absorption layer 5
There is a difference in effective refractive index between the lower part and the part corresponding to the central striped cutout 9 where the light absorption layer 5 does not exist, where almost no light is absorbed. By forming the built-in refractive index difference Δn, the light emitting region is restricted to the central portion of the active layer 3 corresponding to the cutout 9 of the light absorption layer 5. That is, the effect of confining the transverse oscillation state occurs in areas other than this light emitting region. In this case, the difference Δn=n ₁ −n ₂ between the refractive index n ₁ of the central portion and the equivalent refractive index n ₂ of both sides thereof is, for example, +10 ⁻² to +10 ⁻³ .

On the other hand, _a gain _- guided semiconductor laser, as shown in FIG. The active layer 3 is made of _{Al x} Ga 1 _{- x} As or N-type Al For example, in the center thereof, a striped P-type region 10 with a width W _G extending in a direction perpendicular to the plane of the paper in FIG. It is formed by selective diffusion of Zn. Reference numeral 11 denotes an insulating layer formed on the cap layer 6, and the electrode 7 is ohmicly applied on the current concentration region 10 through a striped window 12 formed in the insulating layer.

In the semiconductor laser having such a structure, the operating current is concentrated in the striped region 10, and the operating current injected directly under the stripe causes laser oscillation within the active layer.

The index-guided semiconductor laser and gain-guided semiconductor laser described above each have advantages and disadvantages. In other words, the refractive index guide type has a single longitudinal mode, so it has the disadvantage that it is susceptible to noise due to returned light when used as a light source for writing or reading in, for example, an optical video disk. On the other hand, since the so-called beam waist position exists near the optical end face of the light emitting region, it has the advantage that it is easy to set the focal position in actual use. Furthermore, a far-field pattern in a cross-section parallel to the bond, the so-called far field pattern.
This has the advantage that the pattern is symmetrical and that it is easy to obtain a spot shape with little distortion, for example, as a reading or writing light in actual use. In contrast, in the gain-guided semiconductor laser described above, the beam waist position is located approximately 20 μm inside the optical end face of the light emitting region, and furthermore, the far-field pattern is often asymmetrical, and The drawback is that astigmatism is large and spot distortion is relatively large. However, this gain-guided semiconductor laser has the advantage that its longitudinal mode is multi-mode and is resistant to noise caused by the above-mentioned return light.

Purpose of the Invention The present invention utilizes the advantages of both the index-guided semiconductor laser and the gain-guided semiconductor laser described above, and compensates for the drawbacks of both, for example, in writing or recording on optical video disks or digital audio disks. The object of the present invention is to provide a semiconductor laser that is excellent in its spot shape as a readout light source, facilitates the design of optical lenses, etc., and allows an even more excellent beam spot shape to be easily obtained.

Summary of the Invention The present invention has a light emitting region in a striped pattern, and maintains a required interval at positions corresponding to both sides of the light emitting region so that the equivalent refractive index of light emitted from the light emitting region is that of the light emitting region. A refractive index-guided oscillation section is formed by forming a region that lowers the equivalent refractive index along the length direction of the resonator, and the interval between the regions that lowers the equivalent refractive index is controlled so that the interval is small and gain-guided operation is dominant. An oscillation section in which a refractive index guide type and a gain guide type coexist is formed by forming a region.

That is, in the present invention, for example, in the refractive index guided semiconductor laser illustrated in FIG.
When reducing the width W of the striped cutout portion 9 of the light absorption layer 5 (hereinafter referred to as the interval between the light absorption layers on both sides) that forms the built-in refractive index difference Δn, Δn~ is expressed as Δn~≡( When defined as (built-in refractive index change) - (negative refractive index change due to injection carrier), Δn~ becomes -10 ^-3 to -10 ^-4 , and if current is concentrated in this stripe part, this causes This is based on the investigation that light waveguide based on gain distribution, that is, gain-gain type operation is dominant, and that this semiconductor laser exhibits the characteristics of a gain-gain type semiconductor laser. For example, in the refractive index guided semiconductor laser explained in FIG. 5 - Lower layer of second cladding layer 4 and active layer 3 - N-type first cladding layer 2 and base 1 constitute an N-P-N-P thyristor, so that When the voltage is below a predetermined voltage, the thyristor blocks the current path. Therefore, in this structure as well, the current is concentrated in the central stripe portion, and as mentioned above, by narrowing the distance W between the light absorption layers 5 on both sides, Δn~ can be reduced. Gain-guided operation can be achieved.

Therefore, in the present invention, the spacing between the light absorption layers to form a region in which the equivalent refractive index of light oscillated from the active layers on both sides of the stripe portion is lower than that of the active layer is different at least in part. In this way, at least a portion in the resonator direction constitutes an oscillation section in which a refractive index guide type and a gain guide type are mixed.

Embodiment An example of a semiconductor laser according to the present invention will be described with reference to FIGS. 3 and 4. FIG. 3 is an enlarged schematic plan view thereof, and FIG. 4 is an enlarged sectional view taken along the line AA in FIG. 3. In this example, the structure basically corresponds to the refractive index guide type explained in FIG. 1, and in FIGS. 3 and 4, parts corresponding to those in FIG. shown below.
That is, also in this example of the present invention, for example, N
A first cladding layer 2 made of N-type Al _z Ga _1-z As and a P-type or N-type GaAs substrate 1 are formed on a GaAs substrate 1 .
Active layer 3 consisting of Al _x Ga _1-x As and P-type Al _z Ga _1-z
A second cladding layer 4 made of As and an N-type Al _y Ga _1-y As embedded in the second cladding layer 4.
A light absorption layer 5 consisting of a P-type cap layer 6 having a high impurity concentration is provided. As described above, the light absorption layer 5 has a bandgap width smaller than that of the active layer 3, and is made such that the equivalent refractive index for light oscillated from the light emitting region of the active layer 3 is lower than that of the light emitting region, that is, the active layer 3. Its composition is selected based on the following. That is, in the composition of each layer 3 and 5 described above,
x>y is selected. Also in the present invention, a striped cutout 9 is formed in the center of the light absorption layer 5.
However, in particular in the present invention, the striped cutouts 9, in other words, the intervals between the light absorbing layers 5 on both sides are not made uniform in each part, but for example in the center, as shown in FIG. As explained in , the refractive index guide is set such that the width W is, for example, 3 μm or more, so that the built-in difference Δn in the refractive index due to light absorption due to the presence of the light absorption layer 5 becomes, for example, +10 ^-2 to +10 ^-3 . The waveguide mechanism is defined by the operation of the mold, and the spacing at both ends of the stripe, that is, at both optical end faces and their vicinity, is defined as a small width Wn such that Δn~ is -10 ^-4 or less. for example
By selecting a thickness of 3 μm or less, the operation is not only of the refractive index guided type but also of the gain guided type, that is, the presence of the light absorption layer 9 produces a gain distribution due to current concentration based on the thyristor structure, so that both the refractive index guided and the gain guided are mixed. Do what you want.

In this case as well, the distance d between the light absorption layer 9 and the active layer 3 is selected to be such that light from the light emitting region of the active layer 3 can reach the light absorption layer 9.

In the illustrated example, the distance between the light absorption layers 9 on both sides is set to be narrow at both ends of the stripe, that is, at both ends in the cavity length direction, in other words, at the optical end face, so that the refractive index guide and the gain guide are formed at both ends. In this case, as shown in Fig. 5, the interval is set to a narrow interval Wn in the center, and the area where both the refractive index guide and the gain guide operations coexist at the same time. It is also possible to provide a wide spacing W at both ends so that only the refractive index guiding action occurs.

In addition, in the examples shown in FIGS. 3 and 5, the distance between the light absorption layers 9 on both sides is gradually changed from the center toward both ends, but this is done in stages in one or more parts. The configuration can be changed in various ways, such as by alternately providing refractive index guide operation sections and sections in which both refractive index guide and gain guide operations coexist.

In addition, the above-described semiconductor laser can be manufactured by forming, for example, a first cladding layer 2 - an active layer 3 - a second cladding layer on a substrate 1.
The lower part of the cladding layer 4 and the light absorption layer 5 are sequentially and continuously grown by vapor phase growth using organometallic gas.
MOCVD (Metal Oxide Chemical Vapor)
The light absorbing layer 5 is selectively etched at its center to form a cutout 9.
After that, the upper layer of the second cladding layer 4 including the inside of the cutout part 9 - the cap layer 6 can be formed by epitaxially growing the upper layer of the second cladding layer 4 by the MOCVD method.

Furthermore, in each of the above-mentioned examples, the basic configuration is a refractive index guide type structure in which a built-in refractive index difference is formed by the light absorption layer 5, but various other refractive index guide type structures may be used. It can also be a basic configuration. For example, as shown in FIG. 6, the above-mentioned light absorption layer may be omitted, or the light absorption layer may be provided and the substrate 1 may be provided with striped recesses 12 or, contrary to the illustration, striped protrusions. It is also possible to provide a structure in which a bent portion is provided in the layer 3 to achieve confinement based on the difference in refractive index between the first and second cladding layers 2 and 4. In this case, the width of the concave or convex portions may be changed at each portion, and current concentration may be achieved by, for example, providing electrodes in the form of stripes or by providing the current path region 10 described in FIG. 2. A refractive index guide type operating section and a mixed section of a gain guide and a refractive index guide are formed in the section.

In FIG. 6, parts corresponding to those in FIG. 4 are designated by the same reference numerals, and redundant explanation will be omitted.

Further, in the above example, the active layer 3 is of P type, but it can also be of N type, and each of the other substrates 1 and layers 2, 4, 5, and 6 are It will be clear that it is also possible to have the opposite conductivity type.

Effects of the Invention As described above, according to the semiconductor laser according to the present invention, a part of the light emitting region has a refractive index guide operating mechanism, and the other part has a gain guide operating mechanism and a refractive index guide operating mechanism. Since the mixed portion is provided, the characteristics of the refractive index guide type, that is, in the present invention, the beam waist position can be obtained at the end face of the light emitting region, and furthermore, the symmetry of the far field pattern is excellent, and the spot with small astigmatism can be obtained. It is possible to construct a semiconductor laser with low distortion, easy focusing, and easy optical design, and at the same time, it is possible to construct a semiconductor laser with gain-guided characteristics, that is, with low return light noise.

In this way, in order to combine the features of the refractive index guide type and the features of the gain guide type, for example, simply changing the width of the cutout 9 of the light absorption layer 5,
Since it can be configured by simply changing the width of the bent portion of the active layer 3, the manufacturing method is similar to that used to obtain a refractive index guided semiconductor laser without reducing productivity such as increasing the number of manufacturing steps. It can be manufactured by.

[Brief explanation of drawings]

1 and 2 are respectively schematic enlarged cross-sectional views of examples of conventional semiconductor lasers, and FIGS. 3 and 4 are enlarged schematic plan views of examples of semiconductor lasers according to the present invention, and their A- 5 is an enlarged schematic plan view of another example of the semiconductor laser according to the present invention, and FIG. 6 is an enlarged schematic cross-sectional view of still another example of the semiconductor laser according to the present invention. . 1 is a substrate, 2 and 4 are first and second cladding layers, 3 is an active layer, 5 is a light absorption layer, and 6 is a cap layer.

Claims (1)

[Scope of Claims] 1. A light-emitting region having a striped pattern, which is arranged at a position corresponding to both sides of the light-emitting region at a required interval so that the equivalent refractive index for light oscillated from the light-emitting region is the same as the light emitting region. A refractive index-guided oscillation section is formed by forming a region along the length direction of the resonator that lowers the equivalent refractive index than that of the other region, and controls the interval between the regions that lowers the equivalent refractive index so that the interval is small to form a gain guide. A semiconductor laser characterized in that a region where type operation is dominant is formed to constitute an oscillation part in which a refractive index guide type and a gain guide type are mixed.