JPH0252868B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Technical Field> The present invention relates to a semiconductor laser device in which interference noise caused by return light of output laser light is reduced.

<Prior art> Conventionally, when a semiconductor laser device is used as a light source in an information processing device such as a video disk or an audio disk, the return light of output laser light due to reflection from the disk surface via the optical system of the device is transmitted to the semiconductor laser device. When the re-incident light enters the laser beam again, the linearity of the injection current vs. optical output characteristic line of the semiconductor laser device changes as shown by the solid line in Figure 1 due to the interference of the re-incident light with the output light compared to when there is no re-incident light (indicated by the broken line). In addition, as shown by curve _l1 in FIG. 2, the noise of the output light increases, which may make it impossible to put it to practical use. Note that the curve _l1 in FIG. 2 represents the operating temperature vs. S/N ratio characteristic curve when there is re-incident light, and the curve _l2 represents the operating temperature versus S/N ratio characteristic curve when there is no re-incident light. As a means to solve this problem, attempts have been made to narrow the current injection width, ie, the stripe width, of the semiconductor laser device from the usual 10 to 15 μm to about the carrier diffusion length in the active layer, that is, about 2 to 4 μm. In a semiconductor laser device with such a narrow stripe width, the optical distribution of the laser beam is determined by the gain distribution, but as the cavity volume becomes smaller, the involvement of spontaneous emission light in the laser mode increases and the injection current density increases. Since the gain is large, the spectral width of the gain is expanded, and as a result, the laser oscillates in a multi-axis mode, and the influence of re-incident light is reduced. Furthermore, in the semiconductor laser device described above, the carriers injected into the active layer diffuse in the lateral direction, causing vibrations of the carriers between the laser oscillation region and the non-laser region, and as a result, the laser light output resonates at the relaxation oscillation frequency. This causes a high-frequency self-oscillation phenomenon. Therefore, the laser light output is FM modulated by AM modulation and a change in the refractive index in response to an increase or decrease in carrier, and the oscillation spectrum width is expanded to about 1 Å. This shortens the coherent length of the laser beam and disorder the phase of the light that is reflected from the optical system and re-enters the semiconductor laser element, eliminating interference noise with the re-entering light and re-entering the laser beam. Light effects are reduced.

However, in general, in gain-guided lasers, the near-field image changes due to the injection current and changes over time, making the coupling with the optical system unstable, and the large astigmatism reduces the coupling efficiency with the optical system. It has drawbacks such as lowering the temperature. Therefore, in consideration of the stability of the output laser beam and the coupling between the laser beam and the optical system, a refractive index waveguide type laser element is required.

As an example of a refractive index guided laser device, a V-channel semiconductor laser device having an internal current confinement structure is shown in FIG. This semiconductor laser element is P-
An n-GaAs current blocking layer 2 is deposited on a GaAs substrate 1, a striped groove 7 is formed, and _this portion is used as a current path _. Utilizing the difference in the layer thickness of the first weir layer 3, a difference is created in the "seepage" of the laser light generated in the Ga _0.95 Al _0.05 As active layer 4 into the current blocking layer 2 and the substrate 1, thereby separating the oscillation region and non-oscillation region. This achieves refractive index waveguide by providing an effective refractive index difference between the regions. A second weir layer 5 of n-Ga _0.7 Al _0.3 As is clad on the active layer 4 to form a double heterojunction, and an n-GaAs cap layer 6 is further deposited. Further, 8 is a P-side electrode, and 9 is an n-side electrode. Since this semiconductor laser element is a refractive index waveguide type, stable oscillation can be obtained in both near-field and far-field patterns, and there is almost no astigmatism, so the coupling efficiency with the optical system is high. have characteristics. However, when the returned light of the output laser light enters the element again, the problem of interference noise caused by the returned light as explained in FIG. 1 occurs. This is because in the laser element structure shown in FIG. 3, the internal current confinement part is close to the active layer 4, and the reactive current (current that does not contribute to oscillation) flowing into the non-oscillation region is extremely small. This is because the self-oscillation phenomenon seen in laser devices does not occur.
As a measure to improve this, it is conceivable to increase the layer thickness of the first weir layer 3 as shown in FIG. 4 in order to increase the distance between the internal current confinement portion and the active layer 4. In such a laser element, it is possible to increase the reactive current flowing into the non-emitting region of the active layer 4, and it is possible to generate the above-mentioned self-oscillation phenomenon, but on the other hand, the thickness of the first weir layer 3 is increased. As a result, the leakage of light from the active layer 3 to the substrate 1 is reduced, and as a result, the effective refractive index difference between the laser oscillation region and the non-laser region disappears, and the device approximates a gain waveguide type laser device. .

Furthermore, as in Japanese Patent Application Laid-Open No. 57-153489, an intermediate layer is provided between the active layer and the cladding layer to create a structure in which the distance between the internal current confinement portion and the active layer is increased to provide a refractive index difference ΔN. There are also semiconductor laser devices in which not only the layer thickness but also the intermediate layer thickness is changed in the lateral direction. When the layer thickness of the intermediate layer is changed in the lateral direction in this way, a refractive index difference ΔN is created in the lateral direction due to the layer thickness difference alone, and the magnitude of this difference is, for example, between the center of the channel with a layer thickness of 0.6 μm and the outside of the channel with a layer thickness of 0.1 μm. The value is approximately 0.01. However, although the magnitude of ΔN is sufficient to stabilize the transverse mode, it is too large to cause self-oscillation, and even if the refractive index of the intermediate layer itself is controlled, the effect of ΔN based on the layer thickness difference takes priority, making it difficult to control the occurrence of self-oscillation. In addition, the intermediate layer is generally grown in a liquid phase, but the growth rate is different between the inside of the channel (thick part) and the outside of the channel (flat part), so the layer thickness inside and outside the channel of the intermediate layer must be controlled as designed. It is not easy to make.

Further, as in Japanese Patent Application Laid-Open No. 57-89286, a downwardly convex n-type cladding layer is provided to form a stripe, and a region other than the stripe portion of the cladding layer is formed as a Zn diffusion region to form a Zn diffusion region. There are also semiconductor laser devices in which a Pn junction is provided between the upper n-type layer and the upper n-type layer. By forming a P-type Zn diffusion region in a region other than the stripe portion of the n-type cladding layer in this way, the current can be reliably narrowed within the stripe, thereby reducing reactive current and lowering the internal stripe shape with a low threshold. It becomes possible to obtain a semiconductor laser element. However, the n-type layer on the clad layer
The Pn junction formed by the P-type diffusion region of Zn becomes a remote junction, leading to a decrease in the luminous efficiency of the device.

<Objective of the Invention> The present invention fundamentally solves the above-mentioned problems in conventional semiconductor laser devices, and has an internal current confinement mechanism, a refractive index waveguide mechanism, and a single-axis mode oscillation. A new and useful semiconductor that eliminates the influence of re-incident light by expanding the spectral width of the single-axis mode, exhibits current-light output characteristics with good linearity, and prevents an increase in noise in semiconductor lasers. The purpose is to provide a laser device.

<Example> In the present invention, an intermediate layer with a refractive index of n ₂ is inserted between a first weir layer with a refractive index of n ₁ and an active layer with a refractive index of n ₃ , and the layer thickness and refractive index of this intermediate layer are By controlling the reactive current in the active layer and the effective refractive index difference between the oscillation region and the non-oscillation region, noise due to returned light can be reduced even in index-guided laser devices. The laser element structure has been established. The intermediate layer is provided adjacent to the active layer on the substrate side thereof, and has two meanings in suppressing noise due to returned light. The first is by introducing an intermediate layer between the weir layer and the active layer.
The distance between the internal current confinement portion and the active layer increases, and the reactive current that does not contribute to oscillation and flows into the non-oscillation region increases. As a result, it becomes possible to cause the self-oscillation phenomenon seen in the narrow stripe width laser device described above also in the refractive index waveguide type laser device in which the internal current confinement structure is formed. As a result of experiments, it has been verified that the expansion of the oscillation spectrum width due to the increase in reactive current occurs when the sum of the layer thicknesses of the first weir layer and the intermediate layer outside the groove is 0.3 μm or more. The second method is to control the amount of laser light generated in the active layer seeping into the substrate by providing an intermediate layer, thereby making it possible to cause a self-oscillation phenomenon. In index-guided laser elements, this "seepage" is created by creating an effective refractive index difference (△N) between the oscillation region and the non-oscillation region by utilizing the seepage effect of laser light into the substrate. ΔN is determined in response to the amount. By introducing an intermediate layer adjacent to the active layer, the laser beam electric field intensity distribution in the stacking direction centered on the active layer is changed,
It becomes possible to control the amount of "seepage" into the substrate and change ΔN. On the other hand, in the active layer, the injected carriers oscillate temporally due to consumption of the injected carriers and lateral diffusion of the carriers due to oscillation.
Correspondingly, the refractive index difference between the oscillating region and the non-oscillating region also oscillates temporally at a frequency of Δn. In a normal index-guided laser element, △n is sufficiently small compared to △N, so no vibration effect is observed.
Self-oscillation phenomenon was also discovered in index-guided laser devices by introducing an intermediate layer and controlling its layer thickness and refractive index to reduce ΔN by reducing the "seepage" of light into the substrate. The spectral width of the single-axis mode expands.

Hereinafter, a semiconductor laser device according to an embodiment of the present invention shown in FIG. 5 will be explained in detail.

The semiconductor laser device shown in Figure 5 was used on a P-type substrate.
This figure shows a GaAs—GaAlAs double heterojunction semiconductor laser device.

A Zn-doped P-GaAs substrate ¹¹ having a carrier concentration of 5×10 18 cm -3 is grown using a liquid phase epitaxial growth method on a ^Zn -doped P-GaAs substrate 11 having a carrier concentration of 1×10 ¹⁸ cm ^{-3 .}
After growing the Te-doped n-GaAs current blocking layer 12 to a thickness of 0.8 μm, grooves 1 penetrate from the surface of the current blocking layer 12 to the P-GaAs substrate 11 in a stripe pattern.
form 7. This groove 17 becomes a current path from which the current blocking layer 12 is removed. The stripe width of the groove 17 is 3 μm. By using the liquid phase epitaxial growth method again, a multilayer crystal structure for laser operation was formed on top of this.
A first weir layer 13 made of Zn-doped P--Ga _0.7 Al _0.3 As and having a refractive index n ₁ is grown with a layer thickness of 0.2 μm outside the groove to have a flat surface, and then Zn-doped P-- _{Ga 1-X} Al <X
<1) with a layer thickness of the intermediate layer 20 with a refractive index n ₂
The active layer 14 is made of undoped Ga _0.95 Al _0.05 As and has a refractive index n ₃ with a thickness of 0.1 μm and a Te doped n
-Second weir layer 15 with refractive index n ₄ made of Ga _0.5 Al _0.5 As
A cap layer 16 made of Te-doped n-GaAs is laminated to a thickness of 3 μm on the first weir layer 13. The refractive index of each layer is set to n ₃ > n ₂ > n ₁ > n ₄ . Next, a P-side electrode 18 made of Au--Zn and an Au--
An n-side electrode 19 made of Ge--Ni is formed. The cavity end faces are formed by the cleavage method, and a double heterojunction semiconductor laser device with a resonance length of 250 μm and a device width of 300 μm is constructed.

When a DC voltage is applied through the P-side electrode 18 and the N-side electrode 19, current is injected in a stripe pattern using the groove 17 as a current path, and laser oscillation is started from the active layer 14.

When we investigated the oscillation spectra of several laser devices in which the thickness of the intermediate layer 20 was fixed at 0.3 μm and the refractive index n ₂ was varied, we found that n ₂ was the same as that of the active layer 1.
When the refractive index n3 of 4 is close to _n3 , the "seepage" of the laser light generated in the active layer 14 into the substrate 1 is still sufficiently large, and the expansion of the oscillation spectrum width due to the self-oscillation phenomenon described above is not observed. Ta. As n ₂ decreased, the "seepage" effect weakened, and a self-oscillation phenomenon was observed in the region of n ₂ <1/3 (n ₁ +2n ₃ ). FIG. 6 is a characteristic diagram explaining this, in which the horizontal axis shows the mixed crystal ratio x of Ga _1-X Al _X As of the intermediate layer 20, and the vertical axis shows the refractive index. The shaded area in the figure is the part that satisfies n ₂ 1/3 (n ₁ +2n ₃ ). FIG. 7 shows the current-light output characteristics and optical output characteristics of a conventional index-guided semiconductor laser device (dashed line) and a refractive index-guided semiconductor laser device equipped with the intermediate layer 20 shown in the above embodiment (solid line).
FIG. 3 is a characteristic diagram showing oscillation spectrum characteristics at 3 mW output. In the index-guided semiconductor laser device equipped with the intermediate layer 20, the oscillation threshold current increases because the inflow of reactive current into the non-oscillation region increases, but the injection carrier between the lasing region and the non-oscillation region increases. oscillations occur, resulting in a uniaxial mode spectrum axis
It was confirmed that SW ₁ is about 1 Å wide. On the other hand, in a conventional laser device without the intermediate layer 20, the uniaxial mode spectrum width SW ₂ is about 0.001 Å.

FIG. 8 is an explanatory diagram showing the results of measuring the beam waist position in a direction parallel to the junction of a refractive index waveguide semiconductor laser device having an intermediate layer. It was confirmed that the beam waist position coincided with the resonator end face.

Although the present invention has been described above using a GaAs--GaAlAs-based laser element as an example, it is possible to apply the present invention to semiconductor laser elements made of other materials. Further, although the present invention limits the layer thickness of the first weir layer or the intermediate layer or the refractive index of the intermediate layer, the scope of application is limited to the layer thickness or composition value of each layer used in the above examples. It's not a thing.

<Effects of the Invention> According to the present invention, since the distance between the internal current confinement portion and the active layer is increased, the reactive current increases and an effective refractive index difference is formed between the oscillation region and the non-oscillation region. In this state, a self-oscillation phenomenon occurs. At this time, since there is no layer thickness distribution in the intermediate layer provided between the internal current confinement part and the active layer, the magnitude of the refractive index difference becomes appropriate, making it easy to control the self-oscillation phenomenon.
In addition, local non-uniformity in growth rate is eliminated, and layer thickness can be easily controlled by controlling growth time and the like. Furthermore, since the first dam layer and the intermediate layer have the same conductivity type regardless of whether they are inside or outside the stripe, it is possible to prevent deterioration of luminous efficiency due to remote bonding. This expands the spectral width of the uniaxial mode and eliminates the influence of re-incident light.
Therefore, it is possible to obtain a semiconductor laser device with good linearity of current versus light output characteristics and low noise.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the injection current versus light output characteristic of a conventional semiconductor laser device. FIG. 2 is a characteristic diagram of operating temperature versus S/N ratio of a conventional semiconductor laser device. FIGS. 3 and 4 are configuration diagrams showing a conventional semiconductor laser device. FIG. 5 is a configuration diagram of a semiconductor laser device showing one embodiment of the present invention. Figure 6 is
FIG. 2 is an explanatory diagram illustrating the relationship between the mixed crystal ratio x and the refractive index of Ga _1-X Al _X As. FIG. 7 is an explanatory diagram showing the injection current vs. optical output characteristics and oscillation spectra of the semiconductor laser device shown in FIG. 5 and the conventional semiconductor laser device. FIG. 8 is an explanatory diagram showing the laser beam width in a direction parallel to the junction of the semiconductor laser device shown in FIG. 5. DESCRIPTION OF SYMBOLS 11... Substrate, 12... Current blocking layer, 13... First weir layer, 14... Active layer, 15... Second weir layer, 16... Cap layer, 17... Groove, 18... P side electrode, 19... N side electrode, 20...middle class.

Claims (1)

[Scope of Claims] 1: a first conductivity type substrate; a second conductivity type current blocking layer formed on the first conductivity type substrate and having a striped groove reaching the substrate; A first conductivity type Ga _1-x1 Al _X1 As (0x ₁ <
1) a first weir layer; a first conductivity type Ga _1-X Al _X As (0X<1) intermediate layer formed on the first weir layer and having a substantially flat upper surface; Ga _1-X2 Al _X2 As(0X ₂
<1) Active layer and Ga _1-X3 Al _X3 As (0X ₃
<1) A second weir layer, wherein the sum of the layer thicknesses of the first weir layer and the intermediate layer is set to 0.3 μm or more, and the first weir layer. Middle class. Active layer. and the refractive index of the second weir layer is n ₁ .n ₂ .n ₃ . and n ₄ respectively, then the refractive index of each layer is n ₃ > n ₂ > n ₁ n ₄ n ₂ 1/3 (n ₁ + 2n ₃ ) A semiconductor laser device characterized by satisfying the following relationship.