JPH0613705A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To form the longitudinal multimode oscillation in low output and to lower noise level by a method wherein the film thickness of a current limiting layer is specifically prescribed in an AlGaAs self-matching type double hetero- structure semiconductor laser having a complex refractive index waveguide mechanism. CONSTITUTION:Using a molecular beam epitaxial method, a clad layer 3, an active layer 4, the lower clad layer 5 and a current-limiting layer 6 are laminated successively on an N-type GaAs substrate 2. Then, a stripe groove is formed by conducting an etching treatment to the depth at which the GaAs of the current-limiting layer 6 is left in 1000 angstrom. Then, the GaAs on the bottom of the stripe groove is evaporated by projecting an arsenic molecular beam thereon while the substrate is being heated up, and after the lower clad layer has been exposed, a P-type upper clad layer 7 and a cap layer 8 are laminated. After the upper and the lower electrode films 1 and 9 have been formed, the substrate is cut and separated into chips. In this case, the film thickness of the current-limiting layer 6 is set at 2000 to 6000 angstrom, optically induced noise is sharply reduced, and self-matching type longitudinal multimode oscillation can be accomplished.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a semiconductor laser.
More specifically, it is used as a light source for pickups of compact discs and magneto-optical discs, and is capable of a single transverse mode up to high-power operation, and is a self-aligned high-frequency oscillator that has multi-longitudinal mode oscillation at low-power operation. The present invention relates to an output semiconductor laser.

[0002]

2. Description of the Related Art Recently, there has been a strong demand for a high-power semiconductor laser having a light output of 30 mW or more as a light source for disk writing of a magneto-optical disk.

Characteristics required for a semiconductor laser for a magneto-optical disk are (1) single transverse mode oscillation up to high power operation, (2) low astigmatic difference, (3) low ellipticity, (4) Low noise and (5) high reliability.

Among these, in order to obtain the single transverse mode oscillation up to the high output operation of (1), in the self-aligned double hetero structure semiconductor laser, for example, the active layer is made thin, or the active layer and the current are reduced. By designing the film thickness, such as shortening the distance from the limiting layer, a refractive index waveguide structure is used (Ryoichi Ito et al. “Semiconductor laser”, Baifukan (1989)).

[0005]

However, when the refractive index guided structure is used, a low output operation (for example, an optical output of 3 mW) is obtained.
Even in the case of single longitudinal mode oscillation becomes easy and coherence becomes high. It is known that such a semiconductor laser is apt to generate return light induced noise, and the low noise of the item (4) cannot be satisfied.

[0006] As a conventional countermeasure, it has been attempted to reduce noise by creating a multi-longitudinal mode by superimposing a high frequency (see "Electronics of D. Welford" and "A. Moradian"). Observation of linewidth broadening in (GaAl) As diode due to number fluctuations
diode lasers duevto electron number fluctuation
s) ”(Applied Physics Letter (Appl.Phys.L
(Et.), Vol. 40, p. 560, 1982)), and it was difficult to sufficiently reduce the noise level even if the high frequency was superposed if the longitudinal mode unity before the high frequency superposition was strong.

On the other hand, as a means for making a multi-longitudinal mode,
A method using a gain-guided structure is known (Ryoichi Ito et al. “Semiconductor laser”, Baifukan (1989)), but in the gain-guided structure, it is necessary to maintain single transverse mode oscillation up to high output operation. However, there is a problem in that the astigmatism also increases.

As a result of intensive studies made by the present inventors in view of the above circumstances, when the thickness of the current limiting layer was reduced from 8000 to 10000 Å in the past, when the film thickness became 6000 Å or less, a single lateral The present invention has been completed by discovering that a multi-longitudinal mode is started while maintaining the mode. That is, an object of the present invention is to provide a self-aligned high-index type structure that is a multi-longitudinal mode oscillation in a low output operation (for example, an optical output of 3 mW) while having a refractive index guided structure capable of a single transverse mode up to a high output operation. An object is to provide an output semiconductor laser.

[0009]

The semiconductor laser of the present invention is an AlGaAs self-aligned double-heterostructure semiconductor laser having a complex index guiding mechanism, in which the current limiting layer has a film thickness of 6000 to 2000Å. It is characterized by being.

[0010]

According to the present invention, the thickness of the current limiting layer is 6000-20.
Since it is set to 00Å, the longitudinal mode oscillation at low output is multi-valued and the noise level can be sufficiently reduced.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the semiconductor laser of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a sectional explanatory view of an embodiment of the semiconductor laser of the present invention. In FIG. 1, 1 is an N-side electrode, 2 is an N-type GaAs substrate, 3 is an N-type Al _0.6 Ga _0.4 As clad layer, 4 is an N-type Al _0.15 Ga _0.85 As active layer, and 5 is P.
-Type Al _0.6 Ga _0.4 As lower cladding layer, 6 is N-type GaA
s current limiting layer, 7 is a P-type Al _0.6 Ga _0.4 As upper cladding layer, 8 is a P-type GaAs cap layer, and 9 is a P-side electrode.

To manufacture a semiconductor laser having this structure,
First, by the first MBE (Molecular Beam Epitaxy) method, an N-type Al _0.6 Ga _0.4 As layer as the cladding layer 3 and an N-type Al as the active layer 4 are formed on the N-type GaAs substrate 2.
Each of the _0.15 Ga _0.85 As active layer, the P-type Al _0.6 Ga _0.4 As as the lower cladding layer 5 and the N-type GaAs layer as the current limiting layer 6 (these layers are referred to as the first growth layer) is sequentially grown as a crystal growth material and a dopant material. Laminate by changing the molecular beam intensity of. As the dopant metal, Si or Sn is used when the N type is used, and Be is used when the P type is used. Next, by the etching process, a stripe groove is dug from the surface of the first growth layer to a depth in which GaAs of the current limiting layer 6 remains about 1000 liters.

The substrate in which the stripe groove has been dug in this way is again placed in the MBE apparatus, and the surface of the substrate is first irradiated with an arsenic molecular beam while the substrate is being heated.
After evaporating aAs to expose the lower cladding layer, 2
A P-type Al _0.6 Ga _0.4 As as the upper cladding layer 7 and a P-type GaAs layer as the cap layer 8 are laminated by the MBE process of the second time. Finally, after the upper and lower electrode films 1 and 9 are formed, the substrate is cut to separate each chip.

A feature of the present invention is that the film thickness of the current limiting layer 6 is specified. When the film thickness is 6000 to 2000Å, more preferably 3000 to 5000Å, a single transverse mode up to high output operation is obtained. A self-aligned high-power semiconductor laser capable of multi-longitudinal mode oscillation in a low-power operation (for example, a light output of 3 mW) can be obtained in spite of a possible index-guided structure.

That is, the thickness of the current limiting layer 6 was changed variously, and the change of the relative intensity noise (RIN) when the returning light was received was examined.

The thickness of the current limiting layer 6 can be easily changed by controlling the time when the GaAs layer is grown by the above-mentioned MBE apparatus, for example, Ga.
When the growth rate of As is 1.0 μm / h, it takes about 60 minutes to form a crystal with a thickness of 10,000 Å, and to form 6000 Å, it takes about 36 minutes to grow a crystal. available. By this method, the current limiting layer 6 whose thickness (unit: Å) was changed to 10000, 7000, 6000, 4500, 3500, 2000 was produced, and for each of them, Po = 3 mW, fc = 1
The relative intensity noise in the case of MHz and a return light amount of 1% was investigated.

The results are shown in FIG. As can be seen from FIG. 2, the return light induced noise becomes smaller as the thickness of the current limiting layer 6 becomes thinner.

From this result, the thickness of the current limiting layer was 6000Å
It was found that the return light induced noise is significantly reduced and multi-longitudinal mode oscillation can be obtained by the following. Also, from the viewpoint of multi-longitudinal mode oscillation, the thinner the current limiting layer 6 is, the more preferable data is obtained.
The function as the current limiting layer 6 cannot be sufficiently exhibited, the current region is widened, and the luminous efficiency is reduced, which is not preferable. Therefore, it is preferable to set the thickness to 6000 to 2000Å. Further, in order to further reduce the return light induced noise and improve the luminous efficiency, it is more preferable to set it to 5000 to 3000Å.

Further, in order to obtain stability of the transverse mode and low astigmatic difference, the distance between the active layer and the current limiting layer (thickness of the p-type lower cladding layer) is preferably 2500 to 3500Å. In any case, by setting the thickness of the current limiting layer of the present invention to 6000 to 2000Å, the multi-longitudinal mode in low output operation can be realized. Therefore, the distance between the active layer and the current limiting layer is 2500
~ 3500Å, and the thickness of the current limiting layer is 6000 ~ 2000Å
Thus, the stability of the transverse mode at high output and the low astigmatic difference can be satisfied, and the multi-longitudinal mode at low output can be achieved, and the noise level can be sufficiently reduced.

Next, FIG. 3 shows the result of measuring the characteristics of the longitudinal oscillation mode at an optical output of 3 mW, which is a low output of one embodiment of the semiconductor laser of the present invention, in comparison with the characteristics of the semiconductor laser of the conventional structure. 3A shows the characteristics of the semiconductor laser according to this embodiment, and FIG. 3B shows the structure of the conventional semiconductor laser for comparison. In the structure of both semiconductor lasers, the thickness of the current limiting layer 6 is 4000Å in this embodiment.
However, the conventional example is different in that it is 10000 Å. The other film thicknesses are common to both, and the N-type cladding layer 3
Then 13000Å, active layer 4 is 500Å, lower clad layer 5 is 30
00Å, P-type upper clad layer 7 is 10000 Å, cap layer 8 is
It was 30,000 Å. In addition, a SAM (Self Aligned Structure by MBE) structure that uses the MBE method for the crystal growth method (Haruo Tanaka, "Self-Aligned Structure Laser by MBE" (Japanese Journal of Applied Physics, 24, 89)
Page, 1985)).

The front end face (emission end face) of each semiconductor laser is coated with Al ₂ O ₃ in a thickness of λ / 4 (reflectance of 10%).
The following), and Al ₂ O ₃ having a thickness of λ / 4, a-
Four layers of Si were coated with a thickness of λ / 4 (reflectance of about 90%).
From the comparison of FIG. 3, it can be seen that the semiconductor laser of the present invention has multiple longitudinal modes.

[0023]

As described above, in the semiconductor laser of the present invention, the thickness of the current limiting layer is 6000 to 2000Å, so that the following effects are obtained.

(1) A self-aligned high-power semiconductor that has a multi-longitudinal mode oscillation in a low-power operation (for example, an optical output of 3 mW) while having a refractive index guided structure capable of a single transverse mode up to a high-power operation. You can get a laser.

(2) Other characteristics (for example, low astigmatic difference) required for the magneto-optical disk described above can be conventionally satisfied.

(3) The production process is not complicated and the production yield is not reduced.

(4) Since the current limiting layer is thin, the time required for crystal growth can be shortened and the production efficiency can be improved.

[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 diagram showing changes in relative intensity noise (RIN) when the thickness of a current limiting layer is changed.

3A is a diagram showing a longitudinal oscillation mode at an optical output of 3 mW of the semiconductor laser of the present embodiment shown in FIG. 1, and FIG.
FIG. 6 is a diagram showing a similar waveform obtained by a conventional semiconductor laser.

[Explanation of symbols]

 1, 9 electrode 2 substrate 3 clad layer 4 active layer 5 lower clad layer 6 current limiting layer 7 upper clad layer 8 cap layer

Claims (1)

[Claims] 1. An AlGaA having a complex index guiding mechanism.
An s-based self-aligned double heterostructure semiconductor laser, wherein the current-limiting layer has a film thickness of 6000 to 2000Å.