KR0128711B1 - Semiconductor lasers - Google Patents

Abstract

500Å, d₂=0.2~0.7㎛로 선정된 것이며, 모드호핑노이즈가 없고, 또한 이득가이드형에 비해 노이즈 및 Ith의 저감화, 화필드 패턴의 개선을 도모함으로써 예를 들면 비디오디스크, 디지탈오디오디스크의 기입, 독출광원으로서 이용하여 해상도 및 S/N의 향상과 함께 광학계의 구성의 간략화를 도모할 수 있는 등 많은 이익이 있다.BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser, in which a first absorbing layer, an active layer, a second cladding layer, and a current path are restricted, and a light absorbing layer for light emitted from the active layer is sequentially disposed in contact with the light absorbing layer. The stripe-shaped removal portion forming a current path is formed in the width W, the width W of the removal portion, the thickness d ₁ of the active layer, and the distance d ₂ of the active layer and the light absorption layer are W = 1 to 4 μm, d ₁ , respectively.

500 Å, d ₂ = 0.2 ~ 0.7㎛, it has no mode hopping noise, and compared with gain guide type, it aims to reduce noise and Ith and improve the field pattern, for example of video disc and digital audio disc. There are many advantages, such as being able to use as a writing and reading light source, to improve the resolution and S / N, and to simplify the structure of the optical system.

Description

Semiconductor laser

1 is an enlarged cross sectional view of a conventional semiconductor laser device.

2 is a curve diagram showing the noise level and power with respect to the forward current of the semiconductor laser.

3 is a schematic enlarged cross-sectional view of an example of a semiconductor laser of the present invention.

4 is a curve diagram showing the relationship between the threshold current density and the width of the removal portion of the light absorption layer.

5 is a distribution curve of the exudation of light from the active layer.

* Explanation of symbols for main parts of the drawings

31 gas 32,34 first and second cladding layers

33): active layer 35: light absorption layer

35a: Remover 36: Cap Layer

37,38: electrode.

The present invention relates to a semiconductor laser. 2. Description of the Related Art A conventional semiconductor laser is roughly classified into a refractive index guide (index guide) type and a gain guide (gain guide type) type by a closing mode of light and carrier in its longitudinal mode.

As a refractive index guide type semiconductor laser, for example, one having a structure shown in FIG. 1 is proposed. This semiconductor laser comprises, for example, a first clad layer 2 made of N-type Al _z Gal- _z As and an P-type or N-type Al _x Gal _- on an N-type GaAs substrate 1. A second cladding layer 4 composed of an active layer 3 made of _x As, a P-type Al _z Gal _-z As, and an N-type Al _y Gal _-y As embedded in the second cladding layer 4. It consists of a light absorption layer (5) consisting of a P-type high impurity concentration cap layer (6). The light absorbing layer 5 has, for example, a stripe shape having a width W extending in a direction orthogonal to the surface in FIG. 1 at the center thereof, and has a removing part 5a from which the light absorbing layer 5 is removed. Is done. The light absorption layer 5 has a prohibition band smaller than that of the active layer 3, so that the refractive index of light emitted from the light emitting region of the active layer 3 is higher than that of the light emitting region, that is, the active layer 3. The composition is selected and made. That is, xy is selected in the composition of each layer 3 and 5 mentioned above. In the figure, (7) and (8) represent electrodes deposited on the cap layer 6 and the substrate 8 by ohmic.

n가 형성된다. 이 경우, 광흡수층(5)의 스트라이프형의 제거부(5a)의 폭 W은 5~8㎛로 선정되며, 전술한 거리 d₂의 선정과 함께 활성층(3)에 있어서의 제거부(5a)와 정면대응하는 부분(이하 중앙부분이라고 함)의 겉절을 n₁과 그 양측부의 굴절율 n₂과의 차

n=n₁-n₂a가 예를 들면 +10^-2~10^-3이 얻어지도록 한다. 이와 같이하여 활성층(3)에 있어서, 제거부(5a)에 대향하는 중앙부분에 횡발진형태의 폐입의 효과가 생기게 하여, 여기에 발광영역의 규제가 생기도록 하고 있다.In such a configuration, when a predetermined forward voltage is applied between the electrodes 7 and 8, light is emitted in the active layer 3 enclosed by the first and second cladding layers 2 and 4. . In this case, the distance d ₂ between the active layer 3 and the light absorbing layer 5 is an example of the distance that light emitted from the active layer 3 can reach the light absorbing layer 5, for example, d ₂ = 0.3 In this case, since light from the active layer 3 extruded to the light absorption layer 5 is absorbed in this light absorption layer 5, this light absorption layer 5 is selected. The difference between the effective refractive indices, that is, the refractive index adjusted in the transverse direction, in the portion where the light absorbs in the portion under the portion and the portion corresponding to the center stripe-shaped removal portion 5a where the light absorption layer 5 does not exist is hardly generated. Car

n is formed. In this case, the width W of the stripe-shaped removal section 5a of the light absorption layer 5 is selected from 5 to 8 µm, and the removal section 5a in the active layer 3 is selected with the above-described distance d ₂ . Is the difference between n ₁ and the refractive index n ₂ of both sides

Let n = n ₁ -n ₂ a be obtained, for example, from +10 ^-2 to 10 ^-3 . In this way, in the active layer 3, the effect of closing in the form of lateral oscillation is generated in the center portion of the active layer 3 opposite to the removal portion 5a, and the restriction of the light emitting area is caused here.

As the refractive index guide type semiconductor laser, not only the structure in which the light absorption layer 5 is embedded, but also as disclosed in Japanese Patent Application Laid-Open No. 52-143,787 known as CSP (Channeled Substrate Planer), For example, even when employing a structure in which the base 1 is a light absorbing layer, the above-described operation is explained.

ne이 조정된 굴절율의 변화분

n 에 비해 지배적으로 되도록 하여, 활성층(3)의 중앙부와 양측부와의 실효굴절율의 차

(여기서

≡

n-

ne)가 -10^-2~10^-3정도로 되도록 하면, 이득가이드형 구성으로 된다.As mentioned above, although the refractive index guide type semiconductor laser in the structure of FIG. 1 is comprised, you may comprise a gain guide type in this structure. That is, in the structure of FIG. 1, since the light absorption layer 5 is selected to be of a different conductivity type than the second cladding layer 4, the die of the PNPN in the portion where the light absorption layer 5 is present is present. Since the Lister structure is formed to have the effect of restricting the passage of the current, the light absorbing layer 5 can be used as a current limiting region to obtain a gain guide type configuration. In other words, qualitatively, the width W of the removal portion 5a is made smaller than the refractive index guide type described above, so that a strong current concentration is generated in the stripe portion, or the thickness d ₁ of the active layer 3 is increased. In addition, the distance d ₂ between the active layer 3 and the current limiting region (ie, the light absorbing layer) 5 is increased to reduce the absorption of light from the active layer 3 by the light absorbing layer 5. Negative refractive index change by injection carrier

change in refractive index with ne adjusted

Difference in effective refractive index between the center and both sides of the active layer 3 so as to be dominant compared to n

(here

≡

n-

If ne) is set to about -10 ^-2 to 10 ^-3 , a gain guide type configuration is obtained.

However, the above-mentioned refractive index guide semiconductor lasers and gain guide semiconductor lasers each have advantages and disadvantages, and both have problems, for example, when used as a write and read light source of a video disc.

In other words, the refractive index guide type has a drawback that mode hopping noise due to feedback light occurs when the longitudinal mode is a single mode, for example, when used as a light source for writing or reading in an optical video disc. . FIG. 2 shows the results of measuring the noise of the forward current with respect to the semiconductor laser, and the curve 21 in the figure is that of the refractive index guide type semiconductor laser. As a result, mode hopping noise is generated. On the other hand, this refractive index guide type has a so-called beam waist position in the vicinity of the optical cross section of the light emitting region, so that the far field and so-called far field patterns in actual use are used. pattern) is bilaterally symmetrical and likewise has a merit that it is easy to obtain a spot shape with small distortion, for example, as reading or writing light in actual use. On the other hand, in the above-described gain guide type semiconductor laser, the beam waist position is located at about 20 µm inward from the optical end face of the light emitting area, and the field pattern is often asymmetrical, and astigmatism is also caused. 결) is large and spot distortion is relatively large. As the gain guide semiconductor laser shows its noise characteristics in the second middle curve 22, the noise level is higher than that of the refractive index guide type by the curve 21, but the longitudinal mode is multimode. This has the advantage that no mode hopping noise is generated by the feedback light.

The present invention provides a semiconductor laser having the intermediate characteristics of the above-mentioned refractive index guide type laser and gain guide type semiconductor laser, taking advantage of both of them, and complementing the drawbacks of both, thereby achieving particularly low noise, For example, it is suitable for use as a writing or reading light source such as an optical video disk or a digital audio disk.

500Å, d₂=0.2~0.7㎛로 선정하는 것이다.In the present invention, the first cladding layer, the active layer, the second cladding layer, and the current path are restricted, and the light absorbing layer for light emitted from the active layer is sequentially disposed in contact with each other. The removal portion was formed, and the width W of the removal portion, the thickness d ₁ of the active layer, and the distance d ₂ between the active layer and the light absorbing layer were W = 1 to 4 µm and d ₁ , respectively.

500 Å, d ₂ = 0.2 ~ 0.7㎛.

Next, embodiments of the present invention will be described in detail with reference to the drawings.

An example of the present invention will be described with reference to FIG. This example is similar to that illustrated in FIG. 1, and the first cladding layer 32 and P made of N-type Al _z Gal- _z As on one conductivity-type, for example, N-type GaAs base 31. A second cladding layer made of P-type Al _z Gal _-z As of another conductivity type different from the active layer 33 made of Al type or N type Al _x Gal _-x As and the first cladding layer 32 ( 34) and Al _y Gal _-y As, which have the same effect as limiting the current path and simultaneously absorb light from the active layer 33 and the second cladding layer 34 with the same conductivity type. P-type high impurity concentration cap layer 36 is disposed.

The light absorption layer 35 has a different conductivity type than the second cladding layer 34, in this example N type, and faces the active layer 33 at a distance d ₂ through the cladding layer 34. For example, it is formed to be embedded in the clad layer 34. For example, the removal part 35a in which the light absorption layer 35 is removed has a stripe shape of a width extending in the direction perpendicular to the surface in FIG. 3, for example. To form.

The cladding layers 32 and 34 are selected to zx so that their bar width is larger than that of the active layer 33. In addition, the light absorption layer 35 is smaller than that of the prohibitively wide active layer 33, and its composition is such that the refractive index of light emitted from the light emitting region of the active layer 33 is higher than that of the light emitting region, that is, the active layer 33. This is done by selection. That is, xy is selected in the composition of each active layer 33 and light absorption layer 35 mentioned above. The electrodes 37 and 38 are deposited ohmic on the cap layer 36 and the base 31, respectively.

500Å , 바람직하게는 1500Åd₁700Å로 선정하고, 활성층(33)과 광흡수층(35)과의 간격 d₂을 d₂=0.2~0.7㎛, 바람직하게는 0.3~0.5㎛로 선정한다.In this structure, the width W of the removal portion 35a of the light absorption layer 35 is selected to W = 1 to 4 µm, preferably 2 to 4 µm, and the thickness d ₁ of the active layer 33 is d. _One

500 kPa, preferably 1500 kD ₁ 700 kPa, and the distance d ₂ between the active layer 33 and the light absorbing layer 35 is set to d ₂ = 0.2-0.7 µm, preferably 0.3-0.5 µm.

In the semiconductor laser having this configuration, the noise characteristic of the forward current I between the positive electrodes 37 and 38 when the forward voltage is applied between the electrodes 37 and 38 is calculated in the second middle curve 23. As shown by The second curve 24 shows the relationship between the current I and the power P. FIG.

가 종래의 굴절율가이드형이나 이득가이드형의 각 반도체레이저에 비해, 플러스 또는 마이너스의 매우 작은 값의 영역에서 동작하고 있다고 생각된다.As is apparent from comparing the curve 23 with the curve 22, the noise level of the semiconductor laser according to the present invention is reduced as compared with the gain guide type semiconductor laser. In other words, considering the case of using a power of 5 mW, for example, the difference between the curves 22 and 23 in the current I ₁ passed at this time is about 5 dBm. That is, according to the present invention at the time of use at an output of 5 mW, noise of about 5 dBm can be reduced compared to the gain guide type configuration. In the semiconductor laser according to the present invention, as apparent from the curve 23, generation of mode hopping noise such as that generated in the curve 21 can be seen. That is, in the case of the configuration of the present invention, it is considered that self-excited oscillation occurs due to the closing mode of light and carrier intermediate between the conventional refractive index guide type and the gain guide type. That is, in the semiconductor laser according to the present invention,

Compared with the conventional refractive index guide type and gain guide type semiconductor lasers, it is considered that they operate in the region of very small value of plus or minus.

In addition, the semiconductor laser according to the present invention can improve the symmetry of the field field pattern compared to the gain guide type, and thus the beam spot diameter is reduced, thereby substantially reducing the threshold current. Also with respect to these, the semiconductor laser according to the present invention can be understood as being an intermediate operation between the refractive index guide type and the gain guide type.

500Å(바람직하게는 1500Åd₁700Å), d₂=0.2~0.7㎛(바람직하게는 d₂=0.3~0.5㎛)로 함으로써 얻어진다. 다음에, 이들에 대해 고찰하면, 이제 광흡수층(35)의 제거부(35a)의 폭 W에 대하여 보면, 전술한 바와 같이 W가 크다고 하면 전류집중이 약해지므로 캐리어의 주입에 의한 굴정율변화┃

ne┃가 작아지며, 또한 W를 크게 하면, 조정된 굴절율의 변화가 지배적으로 되어 굴절율가이드형의 동작으로 되지만, 이 W는 반도체레이저의 발진의 한계치전류 Ith를 결정하는 한 인자로도 되어 있다. 즉, 한계치전류 Ith는 한계치전류 밀도 Jth와 발광영역의 면적의 적(積)으로서 주어지므로, W가 커질 때 발광영역의 폭, 따라서 면적이 커져서, Ith를 커지게 하지만, 한편 Jth와 W는 제4도에 나타낸 바와 같이 W가 커지면, Jth가 작은 값을 나타내므로, W가 어떤 범위에서 한계치전류 Ith를 작게 억제할 수 있는 것이며, 여기서 Ith의 저감화에 있어서도 W의 선정은 중요한 인자로 된다.In addition, the above-mentioned mode hopping noise reduction, low noise reduction, limit current reduction, and fire field pattern improvement include the width W of the removal portion 35a of the light absorption layer 35, the active layer 33 and the light absorption layer 35, By simultaneously specifying three characters of the distance d ₂ , the thickness d ₁ of the active layer 33, that is, W = 1 to 4 μm, preferably W = 2 to 4 μm, d ₁

500Å (preferably d ₂ = 0.3 ~ 0.5㎛) (preferably _{1500Åd 1 700Å), d 2 =} 0.2 ~ 0.7㎛ obtained by a. Next, when these are considered, the width W of the removal portion 35a of the light absorption layer 35 now shows that if W is large as described above, the current concentration is weakened.

When ne ┃ becomes small and W is made large, the change in the adjusted refractive index becomes dominant and becomes a refractive index guide type operation, but this W also serves as a factor for determining the threshold current Ith of oscillation of the semiconductor laser. That is, since the threshold current Ith is given as the product of the threshold current density Jth and the area of the light emitting area, when W is large, the width of the light emitting area, and therefore the area, becomes large, thereby increasing Ith, while Jth and W are zero. As shown in Fig. 4, when W increases, the value of Jth is small, so that W can suppress the threshold current Ith small in a certain range, where W is an important factor in reducing Ith.

In addition, when the thickness d ₁ of the active layer 33 is increased, it causes an increase in the Ith as described above by causing an increase in the immunity of the light emitting region. Therefore, it is desired that this is not too large in reducing the Ith. And now with respect to the outflow of light from the active layer 33 to the cladding layers 32 and 34, this is a steep slope as indicated by the curve 51 in this figure when d ₁ is small as shown in FIG. When d ₁ is large, as shown by the curve 52 in this figure, it becomes a distribution of mild inclination. Therefore, when d ₁ becomes large, the effect of light absorption by the light absorption layer 35 becomes small, so that the gain guide type operation is performed. That is, generation of mode favorable noise can be avoided.

The distance d ₂ between the active layer 33 and the light absorbing layer 35 also determines the effect of absorbing light from the active layer 33 by the light absorbing layer 35. When d ₂ is small, the light absorbing effect is reduced. The larger the refractive index guide type is, the larger the d ₂ becomes and the absorption effect decreases or disappears, and the gain guide type becomes.

From this, the above-described W, d ₁ and d ₂ are simultaneously specified, resulting in no occurrence of the mode hopping noise described above, and showing a lower noise level than the gain guide type, and the threshold current Ith is that of the gain guide type. It is lower than Ith, and the field pattern can be improved.

As described above, according to the configuration of the present invention, there is no mode hopping noise, and compared with the gain guide type, the reduction of noise and Ith and the improvement of the field pattern are achieved, for example, writing and reading of a video disc and a digital audio disc. The use as a light source brings about a lot of benefits, such as the improvement of the resolution S / N and the simplification of the structure of the optical system.

Claims (1)

The first cladding layer, the active layer, the second cladding layer, and the current path are restricted, and a light absorbing layer for light emitted from the active layer is sequentially disposed in contact with the light absorbing layer. And the width W of the removal portion, the thickness d ₁ of the active layer, and the distance d ₂ between the active layer and the light absorbing layer are W = 1 to 4 µm and d ₁ , respectively.

500 GHz, d ₂ = 0.2 ~ 0.7㎛, characterized in that the semiconductor laser.

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