JP2863340B2 - Semiconductor laser device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser device capable of high-output operation, having high reliability and excellent low noise.

[0002]

2. Description of the Related Art Semiconductor laser devices have been attracting attention as light sources since the discovery that quartz-based optical fibers can be used as transmission media in optical communication systems. Strongly promoted.
At present, GaAs / AlGaAs system and InP
/ InGaAsP-based semiconductor laser devices are used in commercial optical communication lines. Semiconductor laser devices are widely used in many fields other than optical communication, such as measurement control using light and optical information processing.

However, when a semiconductor laser device is used as a light source, in the field of optical communication, a part of the emitted laser light is reflected by a repeater, a duplexer, or the like to become return light, and this return light is induced. Noise is a problem.

[0004] In an information recording / reproducing apparatus such as an optical disk apparatus for optically recording and reproducing information, a part of the emitted laser light is reflected by the disk surface and re-enters the cavity of the semiconductor laser element. Come back. The return light from the disk surface combines with light inside the element to generate noise, which causes an error when recording and reproducing information.

[0005] Therefore, especially in such an optical information processing field, a semiconductor laser device used as a light source is
It is required that the relative noise intensity be -130 dB / Hz or less. In addition, the information recording / reproducing apparatus as described above requires a high output operation of 50 mW or more and a high reliability at the time of writing information.

Conventionally, for a high-power semiconductor laser device usable for an information recording / reproducing device such as an optical disk device, the reflectivity of the light emitting end face is reduced to about 5%.
Extraction of high light output has been performed. However, when the reflectance at the light emitting end face is reduced, the return light from the disk surface is more likely to be combined with the light inside the element, so that the noise induced by the return light increases.

In order to reduce the noise induced by such return light, for example, a multi-longitudinal mode oscillation is performed by applying a high frequency superimposed current to a drive current of a semiconductor laser device. When multiple longitudinal mode oscillations occur, the temporal coherence is reduced, so that the influence of the return light is reduced and the noise is reduced. Also, a method of reducing the return light itself by using an optical isolator, or setting the semiconductor laser device to an oscillation mode control system intermediate between a gain guided type and a refractive index guided type, and causing the semiconductor laser device to self-oscillate. A method to make it work is considered.

[0008]

However, in the method of applying a high-frequency superimposed current to a semiconductor laser device, it is necessary to externally provide a circuit for applying the high-frequency superimposed current in addition to a normal semiconductor laser drive circuit. . In general, high-frequency circuits are more complicated to handle than ordinary electronic circuits. When such circuits are incorporated, strict shielding is required to suppress unnecessary radiation of high-frequency waves to the outside of the equipment. Yes, this leads to higher costs compared to systems that do not use high frequency superposition.

In a method using an optical isolator, it is necessary to extend the optical path by an amount corresponding to the insertion of the optical isolator, as compared with a normal optical system. This increases the size of the optical system, making it difficult to reduce the size of the device. Moreover, the use of an optical isolator leads to an increase in cost.

In a method in which the semiconductor laser device is set to an oscillation mode control system intermediate between the gain waveguide type and the refractive index waveguide type and the semiconductor laser device is self-excited, astigmatism of the semiconductor laser device is large. Therefore, it is not suitable for applications requiring a small condensed spot (for example, the light source of the information recording / reproducing apparatus described above). In addition, the relationship between the drive current and the optical output is broken at a relatively low output, and a high output operation cannot be performed.

In a normal semiconductor laser device,
It is possible to reduce the noise induced by the return light by increasing the reflectivity of the light emitting end face. However, when high output operation is performed, the light density inside the device increases, and the reliability decreases or the light This leads to destruction of the emission end face.

The present invention solves the above-mentioned conventional problems. The object of the present invention is to provide a light source for an information recording / reproducing apparatus, such as an optical disk apparatus, because it has excellent low noise characteristics. It is therefore an object of the present invention to provide a semiconductor laser device which does not require a high-frequency circuit or an optical isolator, can operate at high output, and exhibits excellent reliability even under high output operation.

[0013]

According to the present invention, there is provided a semiconductor laser device comprising a laminated structure including an active layer formed on a semiconductor substrate, and a laser cavity end face formed by side surfaces of the semiconductor substrate and the laminated structure. A semiconductor layer formed on the laser resonator end face on the light emitting end face side and having a larger forbidden band width than the active layer; and a dielectric reflection film formed on the semiconductor layer; and a reflectance of the light emitting end face . Is 20% or more, the above object is achieved.

The structure of the above-mentioned semiconductor laser device is not particularly limited, but includes a double hetero structure (DH structure), a quantum well structure (QW structure), and an isolated confinement hetero structure (SCH structure).
Etc. can be adopted. Further, a mechanism for confining the laser beam in the lateral direction may be used in combination with these structures. For example, VSIS (V-channeled Substr.)
An inner stripe type referred to as an “ate Inner Stripe” structure can be employed.

The substrate and the laminated structure of the semiconductor laser device may be formed of a conventionally used semiconductor material, for example, GaAs, AlGaAs, InP, InP.
GaAlP, InGaAsP or the like is used. The method of forming a stacked structure including an active layer on the semiconductor substrate is as follows.
A conventionally used crystal growth method may be used, for example, a liquid phase epitaxial growth method (LPE method), a metal organic chemical vapor deposition method (MOCVD method), a molecular beam epitaxial growth method (MBE method), or the like.

The semiconductor layer formed on the light emitting end face may be formed of a semiconductor material having a larger forbidden body width than the active layer. For example, AlGaAs, InGaAlP, I
nGaAsP, ZnSe, ZnS or the like is used. The method of forming the semiconductor layer may be any growth method capable of forming the semiconductor layer on the end face of the laser cavity.
For example, metal organic vapor phase epitaxy (MOCVD), molecular beam epitaxy (MBE), atomic layer epitaxy (ALE), metal organic molecular epitaxy (MOMBE) and the like are used.

The thickness of the semiconductor layer depends on its material. For example, when using AlGaAs,
It is set within the range of nm to 3 μm. Within such a range, a window effect appears because a thin semiconductor layer having good crystallinity is formed with little influence of distortion caused by lattice mismatch between the semiconductor substrate and the laminated structure.

The reflection film formed on the semiconductor layer is
It is sufficient that the reflectivity of the light emitting end face is 20% or more, and the material is not particularly limited. As a material for the reflection film, for example, a dielectric material such as Al ₂ O ₃ , SiO ₂ , or AlN is used. As a method for forming the reflective film, for example, a vacuum evaporation method or the like is used. The reflectance of the reflective film,
By changing the thickness depending on the material and setting it appropriately, the reflectance of the light emitting end face is controlled to 20% or more.

[0019]

According to the above structure, when a semiconductor layer having a larger bandgap than the active layer is formed on the end face of the laser cavity on the light emission side, the thickness of the semiconductor layer is in the range of 0.2 nm to 3 μm. Therefore, the effect of distortion of the semiconductor layer due to lattice mismatch with the semiconductor substrate and the laminated structure is small, and a semiconductor layer having good crystallinity is formed. For this reason,
Since a good interface is formed between the semiconductor layer and the cavity facet, non-radiative recombination does not occur near the laser cavity facet, a good window effect appears, and deterioration of the cavity facet is prevented. . Moreover, the reflectance of the light emitting end face is 20% or more,
Since it is much larger than several percent of the conventional high-power semiconductor laser element, the laser light emitted from the semiconductor laser element is prevented from being combined with the return light reflected by the optical disk or the like. As a result, noise induced by return light is reduced, and is required to be -130 dB / Hz when used as a light source of an information recording / reproducing device such as an optical disk device.
The following relative noise intensity is realized, for example, 50 mW
High reliability can be obtained even under the high output operation described above.

[0020]

Embodiments of the present invention will be described below.

In this embodiment, an internal stripe type semiconductor laser device having a VSIS structure was manufactured. FIG. 1 shows the structure of this semiconductor laser device. FIG. 2 is a cross-sectional view of FIG.
FIG. 3 is a partial cross-sectional view taken along line A ′, showing a structure near a laser resonator end face on a light emission side. The VSIS structure is described in, for example, S.D. Yamamoto et al., App.
l. Phys. Lett. , Vol. 40, no. 5,
pp. 372-374 (1982). Such a semiconductor laser device was manufactured as follows.

First, n-GaAs is formed on a p-GaAs substrate 100 by a liquid phase epitaxial growth method (LPE method).
After growing the s current blocking layer 101, a V-shaped stripe groove 102 penetrating the n-GaAs current blocking layer 101 and reaching the p-GaAs substrate 100 was formed by photolithography and chemical etching. Next, p-Al _0.45 Ga _0.55 As first is filled by a liquid phase epitaxial growth method (LPE method) so as to fill the V-shaped stripe groove 102.
After growing the cladding layer 103, p-A
l _0.15 Ga _0.85 As active layer 104, n-Al _0.45 Ga
_{A 0.55} As second cladding layer 105 and an n-GaAs contact layer 106 were sequentially grown.

By cleaving the wafer thus obtained, laser resonator end faces 107 and 108 were formed (resonator length was 450 μm). Then, on these end faces, a metal organic chemical vapor deposition (MOCVD) method is used.
High resistance Al _0.5 Ga _0.5 As layers (carrier density 10 ¹⁷ / cm ³ or less, thickness 1 μm) 109 and 110
Grew. The high resistance Al _0.5 Ga _0.5 As layer 10
9 and 110 are p-Al _0.15 Ga _0.85 As active layers 1
It has a band gap greater than 04. Then, p-GaA
A p-side electrode 111 is formed on the back surface of the s substrate 100,
An n-side electrode 112 is provided on the surface of the GaAs contact layer 106.
Was formed.

Further, on the surface of the high-resistance Al _0.5 Ga _0.5 As layer 109 on the laser resonator end face 107 on the light emitting side, an Al ₂ O ₃ dielectric reflection film 113 is formed by vacuum evaporation.
(Thickness: 55 nm). In addition, the reflectance of the light emitting end face was controlled to 20%. On the other hand, the laser cavity facet 108
The surface of the high resistance Al _0.5 Ga _{0.5 As} 110
₂ O ₃ dielectric film (122 nm thick) and Si film (50 n thick)
m) to form a multilayer reflective film 114. The reflectivity of the end face other than the light exit side was controlled to about 95%. The semiconductor laser device obtained in this way, the end surface reflectivity of 20% of the light emitting end face and mounted on a heat sink as a light emitting side, it was applied to the driving current, and emits a laser beam having a wavelength of 780 nm. Then, the drive current-optical output characteristics, relative noise intensity, and the like of this semiconductor laser device were examined.

FIG. 3 shows the relationship between the drive current of the semiconductor laser device and the optical output. The solid line is data on the semiconductor laser device obtained in this example, and the dotted line is a conventional semiconductor laser device (for example, a high-resistance Al _0.5 Ga _0.5 As layer 109 and 110
Except that it does not have the same structure as the semiconductor laser device of the present embodiment). While the conventional semiconductor laser device was destroyed at an optical output of 130 mW, the semiconductor laser device of the present embodiment oscillated stably up to an optical output of 380 mW, and reached thermal saturation. Therefore, in the semiconductor laser device of this embodiment, the laser cavity facet 10
7 and the high-resistance Al _0.5 Ga _0.5 As layer 109, and between the laser cavity facet 108 and the high-resistance Al _0.5 Ga _0.5 As layer 110, good interfaces are formed. It can be seen that non-radiative recombination due to light absorption does not occur and a good window effect is obtained. Note that the semiconductor laser device of the present example operated stably for 8,000 hours or more under driving conditions of 50 ° C. and 100 mW, and showed extremely good reliability.

Next, the intensity noise of the output light of the semiconductor laser device obtained in this embodiment was measured. As for the noise measurement conditions, the driving method is automatic light output control (APC) 5 m.
W, room temperature, center frequency 1MHz, bandwidth 30kHz,
Numerical aperture (NA) of collimating lens 0.3, optical path length 60
mm. In the semiconductor laser device of this example, when the return light was changed from 0.01% to 10% without applying the high frequency superimposed current, the maximum relative noise intensity of -130 dB / Hz was obtained, and the The values that can be practically used as the light source of the information recording / reproducing device are shown. Furthermore, the light emitting end
The relationship between the reflectance of the surface and the relative noise intensity was investigated. FIG. 4 shows the results. The relative noise intensity shown in this figure is the maximum value when the return light is changed from 0.01% to 10%.
As is apparent from this figure, when the reflectance of the light emitting end face is increased, the relative noise intensity is improved. Generally, in order to apply a semiconductor laser element as a light source of an information recording / reproducing device such as an optical disk device without using a high-frequency circuit or an optical isolator, the relative noise intensity is -130 dB / H.
must be less than or equal to z. Therefore, light emission from FIG.
It is understood that the reflectivity of the firing end face should be set to 20% or more.

Further, the thickness of the semiconductor layer formed on the end face of the laser resonator was examined. Generally, as long as such a semiconductor layer has a forbidden band width larger than the active layer, in principle, it is a single molecular layer (about 0.2 n in the case of an AlGaAs layer).
If the thickness is more than m), a window effect appears. However,
When the thickness of the semiconductor layer is increased to some extent, strain is generated in the semiconductor layer due to lattice mismatch with the semiconductor substrate and the semiconductor material constituting the laminated structure, and the crystallinity is reduced. Therefore, it is considered that the upper limit of the thickness of the semiconductor layer formed on the end face of the laser resonator is a critical thickness at which such a decrease in crystallinity is problematic.

Actually, in the semiconductor laser device of the present embodiment, the maximum optical output when the thickness of the high-resistance Al _0.5 Ga _0.5 As layer 109 was changed was determined. The result is shown in FIG. As is apparent from this figure, when the thickness exceeds 3 μm, the maximum light output starts to decrease. Therefore, when the semiconductor layer formed on the end face of the laser resonator is made of AlGaAs, its thickness may be set in the range of 0.2 nm to 3.0 μm.

[0029]

According to the present invention, high output operation is possible, and particularly, high reliability is obtained even under high output operation.
A semiconductor laser device having excellent low noise characteristics can be obtained. In particular, when a semiconductor layer having a larger forbidden band width than the active layer is formed on the end face of the laser resonator on the light emission side, the thickness of the semiconductor layer is set in the range of 0.2 nm to 3 μm. In addition, the effect of distortion of the semiconductor layer due to lattice mismatch with the stacked structure can be reduced, and a semiconductor layer having good crystallinity can be formed. For this reason, a good interface can be formed between the semiconductor layer and the cavity facet, so that non-radiative recombination does not occur near the laser cavity facet, a good window effect appears, and the cavity facet is formed. Deterioration can be prevented. Moreover, since the reflectivity of the light emitting end face is 20% or more, the relative noise intensity of the return light noise is-
130 dB / Hz or less. If such a semiconductor laser element is used as a light source of an information recording / reproducing device such as an optical disk device, it is possible to prevent errors during writing and reading without using a high-frequency circuit or an optical isolator. Therefore, it is possible to reduce the size of the information recording / reproducing apparatus and reduce the manufacturing cost.

[Brief description of the drawings]

FIG. 1 is a perspective view showing the structure of a semiconductor laser device according to one embodiment of the present invention.

FIG. 2 is a partial cross-sectional view taken along line AA ′ of FIG.

FIG. 3 is a graph showing the relationship between drive current and light output for the semiconductor laser device of FIG. 1 and a conventional semiconductor laser device.

4 is a graph showing the relationship between the reflectance of the light emitting end face of the semiconductor laser device of FIG. 1 and the relative noise intensity.

5 is a graph showing the relationship between the thickness of a semiconductor layer formed on the laser resonator end face on the light emission side of the semiconductor laser device of FIG. 1 and the maximum light output.

[Explanation of symbols]

Reference Signs List 100 GaAs substrate 101 n-GaAs current blocking layer 102 V-shaped stripe groove 103 p-Al _0.45 Ga _0.55 As first cladding layer 104 p-Al _0.15 Ga _0.85 As active layer 105 n-Al _0.45 Ga _0.55 As second cladding layer 106 n-GaAs contact layer 107 Laser resonator end face on light emission side 108 Other laser resonator end face 109, 110 High-resistance Al _0.5 Ga _0.5 As semiconductor layer 113 Al ₂ O ₃ dielectric reflection film 114 Al ₂ O ₃ dielectric film Reflective film of Si and Si film

Continued on the front page (72) Inventor Masaki Kondo 22-22 Nagaikecho, Abeno-ku, Osaka City Inside Sharpe Co., Ltd. (72) Inventor Saburo Yamamoto 22-22 Nagaikecho, Abeno-ku, Osaka City Sharpe Co., Ltd. (56) Reference References JP-A-1-166586 (JP, A) JP-A-64-19787 (JP, A) 1991 (Heisei Era) The 38th Spring Meeting of the Society of Applied Chemistry 29p-D-15 p. 971 Proceedings of the 35th Annual Meeting of the Society for Natural Sciences, Spring 1988 (Showa 63) 30p-ZQ-9 p. 900 (58) Field surveyed (Int.Cl. ⁶ , DB name) H01S 3/18

Claims (1)

(57) [Claims] 1. A laminated structure including an active layer formed on a semiconductor substrate, and a laser cavity formed on a side of the semiconductor substrate and the laminated structure formed on a laser cavity end face on a light emitting end face side. A semiconductor layer having a larger forbidden band width than the active layer; and a dielectric reflection film formed on the semiconductor layer, wherein the light emitting end face has a reflectance of 20% or more. Laser element.