JPH09307184A - Ridge wave guide type semiconductor laser diode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ridge wave guide type semiconductor laser diode keeping the linear relationship in light output/implantation current characteristics until a very high light output. SOLUTION: In this semiconductor laser diode, a specific thickness portion out of clad layers is a mesa structure and the remaining clad layers of a thickness D are stacked on the entire surface of an activation layer. In this embodiment, when a width of a spot size with a strength 1/e<2> of a near visual field image in a vertical direction to activation layers 22 to 26 is indicated by W, this diode has clad layers 28 to 34 stacked on the entire surface of the activation layers having a remaining thickness so that D satisfies 0.5×W<=D.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser diode, and more particularly to a ridge waveguide type semiconductor laser diode having high output and good laser characteristics.

[0002]

2. Description of the Related Art A GaAs quantum well laser diode using an InGaAs strained quantum well active layer has been actively researched and developed for practical use. To put it to practical use, first
It is necessary to increase the optical output of the laser diode. For example, when the laser diode is used at an oscillation wavelength of 0.98 μm as an excitation light source for an erbium-doped fiber used as an optical fiber amplifier in the 1.5 μm band, Further, when a laser diode is used at an oscillation wavelength of 1.02 μm as an excitation light source for a praseodymium-doped fiber which is a 1.3 μm band fiber amplifier, the laser diode is required to have an extremely high optical output of about 200 mW. .

Here, with reference to FIG.
An As-based quantum well laser diode will be described with an example. A layer structure 10 of a conventional ridge waveguide type GaAs laser diode is, for example, a buffer layer 14 and a lower clad layer 16 which are sequentially formed on a n-type GaAs substrate 12 and are made of GaAs or AlGaAs compound semiconductor layers. , First optical confinement layer 18, second optical confinement layer 20, first strained quantum well layer 22, barrier layer 24, second strained quantum well layer 2
6, a third optical confinement layer 28, a fourth optical confinement layer 30, an upper first cladding layer 32, an etching stop layer 34, an upper second cladding layer 36, a cap layer 38, a laminated structure,
The upper second cladding layer 36 and the cap layer 38 on the etching stop layer 34 have a mesa structure.

[0004]

By the way, the most important factor limiting the increase in the light output of a GaAs quantum well laser diode is the instantaneous end face destruction (COD) in which the light emitting surface of the laser diode is instantly destroyed. Is. When the light output increases and reaches a certain value, a characteristic instantaneous end face destruction (COD) occurs in the GaAs laser diode, and at that time, the function of the laser diode stops. So CO
In order to prevent the occurrence of D, conventionally, a wide width, for example, 4μ
The m-th mesa structure is adopted to reduce the light density in the active layer. However, in the case of the wide mesa structure, the optical gain of the laser diode becomes non-uniform in the direction parallel to the active layer. Spatial hole burning occurs, which easily causes the beam steering phenomenon. The beam steering phenomenon is a phenomenon in which a light beam travels back and forth in a direction parallel to the active layer, and this causes a so-called kink, which is a non-linear optical output / injected current characteristic of the semiconductor laser diode, and Is significantly impaired. Non-linear means external differential quantum efficiency η (= dL / d
(I-I _TH ), where L is the optical output, I is the injection current value at the optical output L, and I _TH is the threshold current value) are not constant, and in an extreme case In some cases, η may be close to zero. Therefore, the signal conversion by the injection current cannot be performed and the laser characteristics are significantly impaired. This phenomenon is particularly pronounced when coupling laser diodes to fibers.

In order to prevent the beam steering phenomenon from occurring even when the light output is increased, various effects intricately entangled with each other, including the guided mode control of the active layer, must be taken into consideration. In the conventional GaAs quantum well laser diode, this problem has not been solved. Therefore, the object of the present invention is to obtain a very high light output,
It is an object of the present invention to provide a ridge waveguide type semiconductor laser diode whose optical output / injection current characteristics maintain a linear relationship.

[0006]

The inventor has found that the light density of the active layer is inversely proportional to the product of the spot size in the vertical direction and the spot size in the horizontal direction, and that the active layer is laminated on the entire surface. The remaining thickness D of the clad layer, for example, in the layer structure 10 of FIG. 1, the total thickness of the third and fourth optical confinement layers 28, 30, the upper first clad layer 36, and the etching stop layer 34 We focused on lateral light confinement, that is, control of the lateral spot size. Further, in the case of the ridge waveguide type semiconductor laser diode adopting the wide mesa structure to reduce the light density, the present inventor further found that between the width W of the vertical spot size and the remaining thickness D of the cladding layer. Therefore, it is thought that the beam steering phenomenon does not occur even when the light output is high, that is, the external quantum efficiency η can be maintained at a constant state when the constant relation holds. Therefore, the present inventor conducted a number of experiments described below, and as a result, kink generation output and W
I found a relationship between D and D. In the present specification, the kink generation output means the optical output at the injection current when the external quantum efficiency η is not constant, for example, the optical output at the injection current at which the rate of change of η is 20%.

Experimental Example Each compound semiconductor layer having the following composition was sequentially epitaxially grown on the n-type GaAs substrate 12 by the organic vapor phase epitaxy method to fabricate the layer structure 10 of the GaAs semiconductor laser diode shown in FIG. . Buffer layer 14: n-GaAs layer Lower cladding layer 16: n-Al _0.3 Ga _0.7 As layer First optical confinement layer 18: Al _0.2 Ga _0.8 As layer Second optical confinement layer 20: GaAs _0.94 P _0.06 layer First strain quantum Well layer 22: In _0.2 Ga _0.8 As layer Barrier layer 24: GaAs _0.94 P _0.06 layer Second strained quantum well layer 26: In _0.2 Ga _0.8 As layer Third optical confinement layer 28: GaAs _0.94 P _0.06 layer Fourth optical confinement layer 30: Al _0.2 Ga _0.8 As layer Upper first cladding layer 32: p-Al _0.3 Ga _0.7 As layer Etching stop layer 34: p-In _0.5 Ga _0.5 P layer Upper second cladding layer 36: p-Al _0.3 Ga _0.7 As Layer Cap layer 38: p-GaAs layer

Next, a SiN film is formed on the cap layer 38, and the upper second clad layer 36 of the layer structure 10 is etched by the wet etching of citric acid using the film as a mask to stop at the etching stop layer 34. The second clad layer 36 was formed in a mesa structure having a width of 4 μm. Then, a SiN film was formed again on the entire surface of the layer structure 10 having the mesa structure. Then, the SiN film is removed from the cap layer 38 to expose the cap layer 38, and the exposed cap layer 3
8 was formed on the substrate 8, and an n-electrode was formed on the lower side of the substrate 12. Furthermore, a dielectric film having a reflectance of 10% is formed on the end face on the low reflection side of the laser structure, and a reflectance of 95% is formed on the end face on the high reflection side.
% Dielectric film was coated to fabricate a sample semiconductor laser diode used in this experiment.

In manufacturing the sample semiconductor laser diode, the film thicknesses of the first optical confinement layer 18, the second optical confinement layer 20, the third optical confinement layer 28, the fourth optical confinement layer 30, and the upper first cladding layer 32 are formed. Was changed as described below, and the film thickness of each of the other layers was fixed as follows. Buffer layer 14: 0.5 μm Lower cladding layer 16: 2.0 μm Etching stop layer 34: 3 nm Upper second cladding layer 36: 2.0 μm Cap layer 38: 0.5 μm

On the other hand, the first light confinement layer 18 and the fourth light confinement layer 30 have the same thickness, the thickness d1 and the second light confinement layer 20 and the third light confinement layer 28 have the same thickness. As shown in Table 1, the values of d1 and d2 were set to different values for each combination of lowercase letters a to i, with the thickness thereof being d2. By combining the values of d1 and d2 with various changes, the first and second optical confinement layers 18 and 2 are combined.
0, the first strained quantum well layer 22, the barrier layer 24, the second strained quantum well layer 26, the third and fourth optical confinement layers 28, 30, the near-field image spot size width W of the active layer can be changed. it can.

[Table 1] Furthermore, independently of the combination of the set values of d1 and d2,
The sum D of the film thicknesses of the clad layer portion laminated on the entire surface of the active layer, that is, the third optical confinement layer 28, the fourth optical confinement layer 30, and the upper first clad layer 32 is set to a different value for each number shown in Table 2. did.

[Table 2]

In this experimental example, Table 3 shows the respective values of d1 and d2 set for each combination a to i and the value of the film thickness D of the upper first cladding layer 32 set for each number 1 to 15. Various combinations, as shown. For example, the combination of the set values of d1 and d2 is a, that is, d1 = 100n
m, d2 = 10 nm, the set value of D is number 6, that is, D =
Make 0.77 μm a-6 combination, d1, d2
A sample semiconductor laser diode a-6 was manufactured so that the values of D and D would be that value. The same applies to other combinations, sample semiconductor laser diodes a-1 to i-6.

Next, each sample semiconductor laser diode was operated to increase the spot size width W and the light output to measure the kink output (mW) when a kink occurred, and the results are shown in Tables 3 to 6. Tables 3 to 6 further show D set values and D / W calculated values for the combinations.

[Table 3]

[Table 4]

[Table 5]

[Table 6]

Since a kink output of at least 200 mW is required for a practical GaAs quantum well laser diode, the kink output of Table 3 is 200 mW.
The above combination is OK, and conversely, the kink output is 200.
NG was evaluated for combinations of mW or less. From the above results, if D / W ≧ 0.5, the evaluation is OK. Therefore, it was confirmed that by setting D / W ≧ 0.5, a practical high-output GaAs quantum well laser diode can be realized.

Based on the results of the above experimental example, in the ridge waveguide type semiconductor laser diode according to the present invention, a portion of the clad layer having a predetermined thickness has a mesa structure, and the remaining clad layer having a thickness D is activated. In a ridge waveguide type semiconductor laser diode formed by stacking all over the layer, when the width of the spot size at which the intensity of the near-field image in the direction perpendicular to the active layer is 1 / e ² is W, D is It is characterized in that the clad layer laminated on the entire surface of the active layer has a residual thickness D satisfying D ≧ W × 0.5. The above e is the base of natural logarithm. In the present invention, the clad layer is not limited to the upper clad layer but includes a layer having the same function as the clad layer laminated on the entire surface of the active layer, such as an optical confinement layer.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in more detail based on embodiments with reference to the accompanying drawings. Example First, a desired value is set for the width W of the spot size of the near-field image in the direction perpendicular to the active layer. The spot size width W is, for example, W = λ / (πtan (θ / (2ln) where θ is the full-width half-maximum angle of the far field image (FFP) in the vertical direction.
2) It can be ^{calculated by 0.5} ). Where λ is the wavelength,
In is a natural logarithm. Then, the required remaining thickness D of the upper cladding layer is obtained from the correlation of D ≧ W × 0.5.
For example, if the spot size W is 1.3 μm, the required D is D ≧ 1.3 × 0.5, that is, a value of 0.65 μm or more, and the remaining thickness D of the upper cladding layer is 0.65 μm.
The layer structure 10 shown in FIG. 1 is formed so as to have a length of m or more. In FIG. 1, D is a layer between the second strained quantum well layer 26 and the etching stop layer 34, that is, the third optical confinement layer 2
8, the fourth optical confinement layer 30, the upper first cladding layer, and the etching stop layer 34.

Further, in this embodiment, in order to control the remaining thickness D of the cladding layer from the active layer, In _0.5 Ga _0.5 P
Although the etching stop layer made of is used, an etching stop layer using another material may be used, and the remaining thickness D can be controlled by another method instead of the etching stop layer.

[0017]

According to the structure of the present invention, a ridge waveguide type semiconductor is formed by forming a mesa structure in a predetermined thickness portion of the cladding layer and laminating the remaining cladding layer having a thickness D on the entire surface of the active layer. In the laser diode, when the width of the spot size in which the intensity of the near-field image in the direction perpendicular to the active layer is 1 / e ² is W, the relationship specified in the present invention is maintained between D and W. As a result, the kink phenomenon of the optical output / injection current characteristic due to the beam steering phenomenon is improved, and a ridge waveguide type semiconductor laser diode in which the optical output / injection current characteristic has a linear relationship up to an extremely high optical output is realized. There is.

[Brief description of drawings]

FIG. 1 is a layer structure sectional view showing a layer structure of an example of a ridge waveguide type semiconductor laser diode.

[Explanation of symbols]

10 Layer structure of ridge waveguide type GaAs laser diode 12 Substrate 14 Buffer layer 16 Lower cladding layer 18 First optical confinement layer 20 Second optical confinement layer 22 First strained quantum well layer 24 Barrier layer 26 Second strained quantum well Layer 28 Third optical confinement layer 30 Fourth optical confinement layer 32 Upper first clad layer 34 Etch stop layer 36 Upper second clad layer 38 Cap layer

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takeshi Gyotani 2-6-1, Marunouchi, Chiyoda-ku, Tokyo Furukawa Electric Co., Ltd.

Claims (1)

[Claims] 1. A ridge waveguide type semiconductor laser diode in which a predetermined thickness portion of the clad layer has a mesa structure and the remaining clad layer having a thickness D is laminated on the entire surface of the active layer. Assuming that the width of the spot size at which the intensity of the near-field image in the vertical direction is 1 / e ² is W, the remaining thickness D such that D satisfies D ≧ W × 0.5 is formed on the entire surface of the active layer. A ridge waveguide type semiconductor laser diode having a laminated clad layer.