JPH05291688A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To provide a semiconductor laser which has a long life and excellent reliability, and can be increased in the productivity. CONSTITUTION:A semiconductor laser consisting of an active layer 24 with a stripe structure, and clad layers on both sides of the active layer. At least one quantum well layer 24b is introduced into the active layer, and the active layer connects at least two stripes of different widths. In addition, the damping ratio of articulation, a scale of the coherency of a laser beam to be emitted, is 0.5 or lower.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a semiconductor laser.
More specifically, it is a self-excited oscillation semiconductor laser that is used as a basic device in the field of optoelectronics and is particularly suitable in the fields of optical recording and optical communication.
The present invention relates to a semiconductor laser having excellent reliability and capable of improving productivity.

[0002]

2. Description of the Related Art In various technical fields utilizing semiconductor lasers, noise caused by the coherency of semiconductor laser light has often become a problem. Particularly in the field of optical communication, when semiconductor laser light is propagated in a multimode optical fiber, noise (modal noise) generated by interference competition between modes at the emitting end of the optical fiber has become a problem (for example, Yoneda, Suto: "Application of consumer laser diode to optical communication", IEICE, January 1989, p. 60).

As a means for avoiding such a problem, by using a semiconductor laser which causes self-pulsation (self-pulsation) which periodically oscillates at a high frequency even in a continuous steady oscillation operation, interference between modes is prevented. Attempts have been made to remove it.

Conventionally, a semiconductor laser which causes such self-sustained pulsation is characterized in that a supersaturated absorption layer is provided in a cladding layer (for example, Japanese Laid-Open Patent Publication No. 61-61160).
-171186 and JP-A-63-202083), characterized in that the stripe width and the cladding layer thickness are optimized (for example, JP-A-61-101089), the active layer is Characterized by having a quantum well structure (for example, JP-A-2-72688 and JP-A-2-72688)
2-78290), characterized in that the electrodes for current injection have a separated structure (see, for example, Japanese Patent Laid-Open No.
62-5680), in the cross section of the semiconductor laser,
Characterized by having various structures for suppressing carrier diffusion in the lateral direction (for example, JP-A-62-133).
789, JP-A-62-97389, JP-A-62-97385 and JP-A-62-86783) are known.

[0005]

However, the above semiconductor laser has a problem that the device life is shortened due to heat generation in the supersaturated absorption layer.
Further, with regard to the semiconductor laser, there is a problem that the stripe width and the clad layer thickness have to be accurately controlled, and the productivity is lowered. In addition, as long as the structure has a single stripe width, the well width and the number of wells must be strictly controlled in order to form a quantum well structure capable of causing suitable self-pulsation. In addition, it is necessary to slow down the growth rate of the crystal in order to form a good quantum well, and there is a problem in that the productivity is reduced as well. In addition, in the above, since the electrodes have the separated structure, a new fine processing step, that is, 2
It is necessary to introduce the resist process for the second time, and there is a problem that productivity is reduced. Further, all of the various semiconductor lasers proposed in (1) have a complicated structure, and there has been a problem that productivity is lowered due to the complicated production process.

[0006]

In order to solve the problems of the above-mentioned prior art, that is, the problems of reduced reliability or reduced productivity, the present inventors have manufactured semiconductor lasers having various structures. As a result of repeated intensive studies, the structure of the present invention was found.

The semiconductor laser of the present invention is a semiconductor laser composed of an active layer having a stripe structure and a clad layer sandwiching the active layer, and at least one quantum well layer is introduced into the active layer. The active layer connects stripes having at least two different widths, and further, the attenuation ratio of clarity, which is a measure of coherency of emitted laser light, is 0.5 or less.

That is, in the semiconductor laser of the present invention, a quantum well layer is introduced into an active layer having a stripe structure, and the active layer is generally formed as proposed in the above-mentioned known example. It is characterized in that stripes having at least two different widths are connected to each other in the resonance direction of laser light, rather than having a well-known single width. At the same time, it is also characterized in that the attenuation ratio of clarity, which is a measure of the coherency of the emitted laser light, is 0.5 or less.

The coherency of the laser light is obtained by separating the emitted laser light into two in an interferometer known as a Michelson interferometer or a Mach-Zehnder interferometer while changing the optical path length difference between the two. The interference fringes are measured again with one line, and it is indicated by the attenuation of "clarity V" expressed by the following equation obtained from the light intensity (Imax) and the light intensity (Imin) of the valleys of the interference fringes. Known (for example, Koichi Shimoda "Introduction to Laser Physics", Iwanami Shoten, (1983) p. 27).

V = (Imax-Imin) / (Imax + Imin) Regarding the envelope of this interference fringe, the peak intensity when the optical path difference between the two laser beams is 0 is Ia, and the peak intensity that appears next is Ib. At this time, the ratio γ = Ib / Ia generally represents the amount of attenuation of clarity.
The coherency of the semiconductor laser light can be easily measured as a γ value using an interferometric measuring instrument such as an optical spectrum analyzer or a wavelength meter which is generally commercially available.

The present inventors have proposed a self-excited oscillation semiconductor laser
As a result of various examinations of manufacturing with a manufacturing method excellent in productivity, the following has been found out. That is, in the case of a structure in which stripes having at least two different widths are connected in the resonance direction of laser light, which is a constituent feature of the present invention, self-excited oscillation occurs when γ is 0.5 or less, and γ Is 0.
It has been found that if it exceeds 5, it may cause relaxation oscillations, but does not cause permanently held periodic oscillations.

In order to obtain a structure in which stripes having at least two different widths are connected to each other in the resonance direction of laser light, it suffices to prepare a photomask in which the stripes are drawn, which complicates the semiconductor laser manufacturing process. There is no need for precise control, increase in the number of steps, or precise control. Further, in order to set γ to 0.5 or less, it suffices to install a commercially available interferometric measuring instrument in the manufacturing process and perform automatic measurement, without lowering production efficiency. Moreover, since the structure in which the stripes having at least two different widths are connected in the resonance direction of the laser light is not a heat absorption structure, the reliability of coping with the reduction in life due to heat absorption in the supersaturated absorption layer is high. It does not cause a problem.

The geometrical shape of the structure in which at least two stripes having different widths are connected in the resonance direction of the laser beam is not particularly limited as long as γ is 0.5 or less. But, preferably,
In stripes having at least two different widths, the ratio of the maximum width a to the minimum width b is preferably b / a <0.87 so that γ is 0.5 or less.

In the conventional stripe structure having a single width, as described above, it is necessary to strictly control the well width and the number of wells in order to cause self-sustained pulsation. Therefore, it is necessary to slow down the crystal growth rate in order to form the crystals, which causes a decrease in productivity. Since the semiconductor laser of the present invention has a structure in which stripes having at least two different widths are connected in the resonance direction of laser light, it is not necessary to form a strict quantum well structure as in the conventional case, and productivity is reduced. It is possible to introduce a quantum well without doing so. Moreover, since the quantum well is introduced into the active layer, the differential gain of laser oscillation is increased, and self-excited oscillation with a high frequency becomes possible due to the synergistic effect of the stripe structure and the quantum well structure.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The semiconductor laser of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 is a sectional explanatory view of an embodiment of a semiconductor laser of the present invention, FIG. 2 is a detailed explanatory view of an active layer in the semiconductor laser shown in FIG. 1, and FIG. 3 is a diagram showing a shape of a stripe groove. It is a plane explanatory view of the semiconductor laser shown in FIG.

First, the sectional structure shown in FIG. 1 will be described.

This chip comprises a lower clad layer 26 made of N-type Al _x Ga _1-x As (Al composition x = 0.55), an active layer having a quantum well structure on a substrate 28 made of N-type GaAs.
24, a first upper cladding layer 22 made of P _- type Al _x Ga _1-x As (Al composition x = 0.55), and P-type GaAs, and has a bandgap equal to or higher than that of the active layer 24 due to the quantum effect. Protective layer 20, N type A
AlG composed of l _x Ga _1-x As (Al composition x = 0.55)
aAs layer 18, current limiting layer 16 made of N-type GaAs, P
A second upper clad layer 14 made of Al _x Ga _1-x As type (Al composition x = 0.55) and a cap layer 12 made of P ⁺ type GaAs are laminated. However, AlGaAs
The layer 18 and the current limiting layer 16 are removed in a stripe shape in the central portion, and the second upper clad layer 14 is laminated on the protective layer 20 in that portion (stripe groove 32). An N electrode 30 is provided below the substrate 28, and a P electrode 10 is provided above the cap layer 12.

FIG. 2 is a detailed explanatory view of the active layer 24 shown in FIG. 1, in which a barrier layer 24a made of Al _y Ga _1-y As (Al composition y = 0.22) and a quantum well layer 24b made of GaAs are combined. Has a quantum well structure.

As shown in FIG. 3, the semiconductor laser of this embodiment is characterized in that the stripe groove 32 is not uniform in the longitudinal direction but has a plurality of narrow width portions 34a and 34b. .. These narrow width portions 34a, 34b are provided inside the stripe groove 32 excluding both longitudinal end portions 32a, 32b.

Next, a method of manufacturing this chip will be briefly described. First, the first MBE (molecular beam crystal growth method)
By the process, each layer from the lower clad layer 26 to the current limiting layer 16 (these are referred to as a first growth layer) is formed on the substrate 28. Next, by the etching process, the stripe groove 32 is dug from the surface of the first growth layer to the depth at which the surface of the protective layer 20 is exposed. At this time, the shape of the opening of the resist is shown in FIG.
The stripe groove 32 of the present embodiment is formed by forming a stripe shape having two narrow width portions 34a and 34b (hereinafter, referred to as a partial narrow width shape) as shown in FIG.

The substrate in which the stripe groove 32 has been dug in this way is again placed in the MBE apparatus, and first, while the substrate is being heated, the surface is irradiated with an arsenic molecular beam to remove impurities attached in the etching process (thermal cleaning process). ),
Next, a second MBE process is performed to form a second upper clad layer.
14 and the cap layer 12 (these are referred to as a second growth layer) are laminated. Finally, after forming the upper and lower electrode layers 10 and 30,
The substrate is cut into individual chips. The detailed manufacturing method is described in JP-A-61-163688.

In the thus obtained semiconductor laser, the ratio of the two stripe widths (a, b) is b / a = 0.
The relationship between γ and the self-excited oscillation frequency at 75 is shown in FIG. Further, FIG. 5 shows a state in which the thus obtained semiconductor laser is oscillating by itself. It can be seen from FIG. 5 that good self-oscillation occurs.

Further, as shown in FIG. 6, with respect to the stripe shape in which the number of narrow portions is 3, various ratios of b and a were changed to examine whether or not the laser oscillated. The results are shown in Table 1. In FIG. 6, c: d = 1: 1.

[0025]

[Table 1]

Further, various studies were conducted on the ratio of the length c of the wide portion (see FIG. 6) to the length d of the narrow portion, but the range of c: d = 1: 1.3 to 1: 0.7 is preferable. I understood it. When the ratio is smaller than 1: 1.3, self-excited oscillation is difficult to occur, while when it is larger than 1: 0.7, self-excited oscillation is easy to occur, but the temperature characteristic is deteriorated.

The number of the narrow portions b is not particularly limited in the present invention, but it is preferably 2 to 3.
With one, self-excited oscillation is hard to occur, and with six or more, self-excited oscillation is easy to occur, but the operating current rises sharply.

[0028]

As described above, according to the present invention,
(1) It is possible to obtain a self-excited oscillation semiconductor laser having excellent reliability that does not cause a reduction in life caused by heat absorption, and (2) complicates the semiconductor laser manufacturing process or increases the number of processes. Alternatively, it is possible to obtain an effect that a self-excited oscillation semiconductor laser excellent in productivity can be obtained without requiring precise control. This effect can be greatly utilized in the fields of optical recording and optical communication.

[Brief description of drawings]

FIG. 1 is a sectional explanatory view of an embodiment of a semiconductor laser of the present invention.

FIG. 2 is a detailed explanatory view of an active layer in the semiconductor laser shown in FIG.

3 is a plan view of the semiconductor laser shown in FIG.

FIG. 4 is a diagram showing the relationship between γ and the self-excited oscillation frequency of an embodiment of the semiconductor laser of the present invention.

FIG. 5 is a diagram showing how a semiconductor laser according to an embodiment of the present invention oscillates by itself.

FIG. 6 is a diagram showing an example of a stripe shape in the present invention.

[Explanation of symbols]

 14 Second upper clad layer 22 First upper clad layer 24 Active layer 24b Quantum well layer 26 Lower clad layer 32 Stripe grooves 34a, 34b Narrow width part

Claims (2)

[Claims] 1. A semiconductor laser comprising an active layer having a stripe structure and a clad layer sandwiching the active layer, wherein at least one quantum well layer is introduced into the active layer, and At least 2 layers
A semiconductor laser in which stripes having three different widths are connected to each other, and an attenuation ratio of clarity, which is a measure of coherency of emitted laser light, is 0.5 or less. 2. The semiconductor laser according to claim 1, wherein in the stripes having at least two different widths, the ratio of the maximum width a to the minimum width b is b / a <0.87.