JPH0797687B2 - Semiconductor laser device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser, and more particularly, to an aberration-free and low-noise semiconductor laser required for consumer use.

[Background of the Invention]

In a conventional semiconductor laser, it is effective to induce self-oscillation in order to suppress the noise due to the returning light. See Hayashi et al., Technical Report of IEICE, 84-30, page 65. Has been done. However, this method has a problem that astigmatism is large (about 10 μm). Further, in order to reduce the astigmatism and suppress the noise due to the returning light, the above-mentioned document discloses a method of making the end surface of the index-guided semiconductor laser have a high reflectance. Relative noise intensity (R / N) is 10 ^-12 Hz when the light exceeds 1%
However, there was a problem that the optical power density increased to ^-1 and the optical power density inside became high, and the reliability was lowered.

[Object of the Invention]

It is an object of the present invention to provide a highly reliable semiconductor laser with less aberration of laser light, low noise.

[Outline of Invention]

The present inventors formed a multilayered dielectric film on two reflecting surfaces of a quantum well semiconductor laser formed so that the change in the refractive index with respect to the change in the carrier density is small and the laser oscillation is performed at a low threshold current. In this case, the aberration of the laser light is small, and the noise of the returned light is about 10% (R / N <10 ^-14 Hz
^-1 ) We have found that a semiconductor laser can be obtained. It was also found that the effect becomes remarkable when the reflectance of the reflecting surface is set to 60% or more.

The thickness of the dielectric film is λ / 4n, where λ is the wavelength of the laser light and n is the refractive index of the dielectric.
It was also found that the dielectric film is composed of a two-layer film. The structure of this two-layer film is the _first with a refractive index of n ₁ and a film thickness of λ / 4n ₁ .
With a dielectric film of n ₂ and a refractive index of n ₂ larger than n ₁ and a film thickness of λ / 4
It consists of a second dielectric film of n ₂ .

It was also found that stacking two or more of the two-layer film has an effect of increasing the strength of the film.

Needless to say, the higher the reflectance of the reflecting surface according to the present invention, the better, but the effect of the present invention was obtained when the reflectance was 60% or more.

Example of Invention

 Examples of the present invention will be described below.

FIG. 1 (a) is a side view of a semiconductor laser device according to the present invention. FIG. 1 (b) is an enlarged view of a part 5 ′ of the quantum well layer 5.

An n-type Ga _0.55 Al _0.45 As cladding layer 4 and a p-type GaAs cap layer 3 were sequentially formed on the n-type GaAs substrate 7 by MOCVD. Next, after forming the P-type electrode 8 and the n-side electrode 9, the semiconductor laser having a cavity length of 300 μm was obtained by the cleavage method.
Next, SiO 2 is used as the first dielectric film 1 on both reflecting surfaces.
₂ (n ₁ : 1.45) was formed to a film thickness of 134 nm, and an amorphous Si (n ₂ : 3.3) film having a film thickness of 59 nm was formed thereon as the second dielectric film 2. By forming this two-layer film, the reflectance of the reflecting surface is
It has reached 75%.

The quantum well layer is a Ga _0.9 Al _0.1 As layer 5b with a thickness of 100 A.
This is a layer in which Ga _0.7 Al _0.3 As layers 5a having a thickness of 30 A and b layers are alternately laminated.

The stripe structure was set for lateral mode control, and the stripe width was 5 μm.

The departure wavelength of the laser of this example was 780 nm, and the threshold current was 30 mA. In addition, Fig. 2 shows the measured values of relative noise intensity (R / N) for return light at an optical output of 5 mW and a temperature of 50 ° C. When the amount of return light is 5% or less, R / N is 10 ^-1 Hz ^-1
It was about.

Further, the stripe width was effective in the range of 3 to 15 μm.

The reflectance of the end surface coated with the dielectric film depends on the material, manufacturing conditions,
It becomes 40 to 85% depending on the film thickness, etc., but when this reflectance is 50% or more, there was a noise reduction effect.

Embodiment 2 FIG. 3 shows a cross section of a refractive index guided quantum well semiconductor laser. In this embodiment, a dielectric film similar to that of the embodiment is applied to the end face of a semiconductor laser of this structure and a reflectance of 75 is obtained. % Reflective film was formed. Since the current is confined by the n-type GaAs light absorption layer 11, the threshold current of the semiconductor laser of this embodiment is 5 m.
As shown in Fig. 4, the relative noise intensity (R / N) with respect to the amount of return light at a temperature of 10 to 50 ° C is as shown in Fig. 4. Ten
% Was less than 10 ^-5 Hz ^-1 . Astigmatism is 1μ
It was less than m.

Embodiment 3 FIG. 5 shows the structure of another index guided quantum well semiconductor laser. Similar to the first embodiment, a dielectric film is applied to both end faces of the semiconductor laser having this structure to improve the reflectivity of the end faces to 75% to reduce noise. In this structure, the active region 5 is limited by the action of current confinement by the Zn diffusion region 16.

This semiconductor laser has a threshold current value of 10 mA and a current of 40 mA.
The optical output of 5mW was obtained. The relative noise intensity at an optical output of 5 mW was the same as in Example 2. The astigmatism was 2 μm or less.

The lifetime of the semiconductor laser in the example of the present invention was 5,000 hours on average at 70 ° C. and an optical output of 5 nW.

The thickness of the quantum well layer is 10 to 20A, and the thickness of the barrier layer is 1.
Similar results were obtained in any combination of 0 to 100 A, the molar ratio W of the quantum well layer Al was 0 to 0.3, and the molar ratio B of Al of the barrier layer was 0.15 to 0.8 (where W <B). .

Furthermore, as a quantum well structure, the GRIN-SCH structure (GIN-SCH structure in which the refractive index and the forbidden band width of the optical guide layer are distributed in the thickness direction
raded−Index−Separrate−confinement−Heterostruct
ure) etc. can be similarly applied.

Although the case where SiO ₂ is used as the first dielectric film has been described above, similar effects have been confirmed when using Al ₂ O ₃ , BeO, Si ₃ N ₄ as the dielectric film.

Further, in the present embodiment, the reflectance of the end face is improved to about 75% by using one set of the two-layer film of the first dielectric film and the second dielectric film, but even if two or more sets are used. Similar effects were obtained. The present invention can also be applied to a semiconductor laser using an InGaAsP system or an InGaP system as a semiconductor. The structure of the laser is not limited to the one based on the three-layer waveguide shown in the above-mentioned embodiment, and the LOC (L (L
arge Optical Caving) structure or SCH (Graded-Index-Se) structure in which optical guide layers are provided adjacent to both sides of the active layer.
The present invention can also be applied to a parate-confinement-Heterostructure) structure.

Similar results were also obtained in the structure in which the waveguide types were all reversed in the above-mentioned embodiment (the structure in which p was replaced by n and n was replaced by p).

[Advantages of the Invention] According to the present invention, since a semiconductor laser with little aberration of laser light, low noise and high reliability is put into practical use, it is possible to develop optical application technologies such as optical recording technology and optical communication technology. There is an effect of contributing, for example, there is a remarkable effect in simplifying the optical system in the optical disk device and improving the S / N ratio at the time of reading information.

[Brief description of drawings]

FIGS. 1, 3 and 5 are structural views of a laser showing an embodiment of the present invention, and FIGS. 2 and 4 are diagrams showing measurement examples of return light induced noise. 1 ... 1st dielectric film, 2 ... 2nd dielectric film, 3 ...
p-GaAs cap layer, 4 ... p-GaAlAs cladding layer, 5
...... Multiple quantum well layer, 5b …… Well layer, 5a …… Barrier layer,
6 ... n-GaAlAs cladding layer, 7 ... n-GaAs substrate, 8
... p-side electrode, 9 ... n-side electrode, 10 ... n-GaAs buffer layer, 11 ... n-GaAs light absorption layer, 12 ... p-GaAlAs embedded cladding layer, 13 ... p-GaAs substrate , 14 …… p-Ga
AlAs optical guide layer, 15 ... Layer with disordered quantum wells, 16
...... Zn diffusion region, 17 …… n-GaAs cap layer, 18 …… Si
O ₂ film.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor, Fukuzawa, 1-280 Higashi Koigakubo, Kokubunji, Tokyo, Central Research Laboratory, Hitachi, Ltd. (72) Inventor, Yoshimitsu Sasaki 1-280, Higashi Koigakubo, Kokubunji, Tokyo Hitachi, Ltd. Central Research Laboratory (56) References JP 59-171186 (JP, A) JP 59-145588 (JP, A)

Claims (8)

[Claims] 1. A semiconductor laser having at least one quantum well layer having a thickness equal to or less than the Dow-Broglie wavelength of an electron wave, wherein each of two reflecting surfaces constituting an optical resonator having a resonance wavelength of λ has a refractive index. Is n ₁ and the film thickness is λ / 4n _{1 and} the second dielectric film is n ₂ (where n ₂ > n ₁ ) and the film thickness is λ / 4n ₂ .
A semiconductor laser device having a multilayer film in which the above dielectric film is laminated, and the reflectance of each of the two reflecting surfaces is 50% or more. 2. The semiconductor laser device according to claim 1, wherein the quantum well layer has a multiple quantum well structure composed of a plurality of quantum well layers. 3. The multi-quantum well structure has a GRIN-SCH (Grade
d-Index-Separate-Confinement-Heterostructure)
The semiconductor laser device according to claim 2, wherein the semiconductor laser device is a mold. 4. The first dielectric film is a SiO ₂ film, an Al ₂ O ₃ film, B
4. The eO film or the Si ₃ N ₄ film, and the second dielectric film is an amorphous Si film, according to any one of claims 1 to 3. Semiconductor laser device. 5. The multilayer film comprises a first dielectric film and a second dielectric film.
5. The semiconductor laser device according to claim 1, wherein the semiconductor laser device has one set of two-layer films in which the above dielectric film is laminated. 6. The multilayer film comprises a first dielectric film and a second dielectric film.
The semiconductor laser device according to any one of claims 1 to 4, wherein the semiconductor laser device has two sets of two-layer films in which the above-mentioned dielectric film is laminated. 7. The reflectance of each of the two reflecting surfaces is 60%.
It is above, The semiconductor laser device in any one of Claim 1 to 6 characterized by the above-mentioned. 8. The semiconductor laser device according to claim 1, wherein the optical waveguide mechanism of the semiconductor laser device is a refractive index waveguide type.