JPS59147479A - Semiconductor laser element - Google Patents

Abstract

PURPOSE:To remove the effect of re-incident light by inserting an intermediate layer between a first weir layer and an active layer and controlling the refractive index of the intermediate layer. CONSTITUTION:An N type current stopping layer 12 is grown on a P type substrate 11, and a groove 17 penetrating up to the substrate 11 in a stripe shape from the surface of the layer 12 is formed. A first weir layer 13 of an n1 refractive index is grown on the groove, and a P type intermediate layer 20 of an n2 refractive index, an active layer of an n3 refractive index, an N type second weir layer 15 of an n4 refractive index and an N type cap layer 16 are laminated on the layer 13 in succession. The refractive indices of each layer are set in order of n3>n2>n1>=n4. A P side electrode 18 and an N side electrode 19 are each formed on the back of the substrate 11 and the layer 16. When n2<=1/3(n1+2n3) is satisfied in the constitution, reactive currents increase, and a self-oscillation phenomenon is generated under the state the which effective refractive-index difference is formed between an oscillation region and a non- oscillation region. Consequently, the spectral width of a uniaxial mode is extended, and the effect of re-incident light is removed. Accordingly, an element, the linearity of current-to-output characteristics thereof is excellent and noises therefrom an small, is obtained.

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 an output laser beam is reduced.

<Prior Art> Conventionally, when a semiconductor laser element is used as a light source in an information processing device such as a video disk or an audio disk, the output laser beam is reflected from the disk surface through the optical system of the device and returned to the semiconductor laser. When the re-incoming light enters the device, 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-incoming light with the output light. ), and still shows curve 1 in Figure 2. As shown in Figure 2, the noise of the output light increases, which may make it impossible to put it to practical use. Incidentally, when there is as much re-incident light as the curve t1 in FIG. 2, the curve t2 represents the operating temperature vs. S/N ratio characteristic curve when there is no re-incident light. Semiconductor is a way to solve this problem.

The current injection width of the laser device, that is, the stripe width, is set to 1
Carrier diffusion length in the active layer from 0 to 15 μm, that is, 2
Attempts have been made to narrow it down to about 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 the spontaneous emission light in the laser mode increases, and the injection current increases. Since the density 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, carriers injected into the active layer diffuse laterally, causing carrier oscillation between the laser oscillation region and the non-oscillation 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 the increase and decrease of carriers, and the oscillation spectrum width is expanded to about IA. This shortens the coherent length of the laser beam and makes the phase of the light reflected from the optical system and re-injected into the semiconductor laser element disordered, eliminating interference noise with the re-incoming light. The influence of re-incident light is reduced.

However, in general, in gain-guided lasers, the near-field image changes due to injection current, changes over time, etc., making the coupling with the optical system unstable, and the coupling efficiency with the optical system decreases due to large astigmatism. It has disadvantages such as: 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 device is formed on a p-GaAs substrate 1.
- Depositing a GaAs current blocking layer 2 and forming striped grooves 7
P-Gao, , Ato, 3As in the groove 7 and other regions
Using the difference in the layer thickness of the first weir layer 3, Ga0.95 At
0.05 A difference is provided in the amount of laser light generated in the As active layer 4 that oozes out into the current blocking layer 2 and the substrate 1, and an effective refractive index difference is provided between the oscillation region and the non-oscillation region. This realizes refractive index waveguide. Furthermore, on the active layer 4 there is an n-G
A□, 7A7o, 3As second weir layer 5 is clad to form a double heterojunction, and further an n-GaAs cap layer 6 is deposited. In addition, 8 is a P-side electrode, and 9 is an n-side electrode 2. Reflecting the fact that this semiconductor laser device is a refractive index waveguide type, stable oscillation can be obtained in both near-field and far-field images, and it is still not stable. Since there is almost no point aberration, the coupling efficiency with the optical system is high. 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 narrow stripe width 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 first active layer 4, and it is possible to cause the above-mentioned self-oscillation phenomenon. As a result of increasing the thickness, the seepage of light from the active layer 3 to the substrate 1 is reduced.
There is no effective refractive index difference between the laser oscillation region and the non-laser region, and the device approximates a gain waveguide type laser device.

<Purpose 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, and a single-axis laser diode. Newly useful semiconductor lasers that emit mode oscillations eliminate the influence of re-incident light by expanding the spectral width of the single-axis mode, exhibiting current-optical output characteristics with good linearity, and preventing noise increase. Embodiment The present invention aims to provide a semiconductor laser device with a refractive index of n2 and an intermediate layer of refractive index of n2 between a first weir layer of refractive index of ni and an active layer of refractive index of n3, By controlling the layer thickness and refractive index of the intermediate layer, the reactive current in the active layer is controlled, and the effective refractive index difference between the oscillation region and the non-oscillation region is controlled, thereby creating an index-guided laser. A laser element structure with less noise caused by returned light has been established in the element. The intermediate layer is provided on the substrate side adjacent to the active layer, and has two meanings in suppressing noise due to returned light. The first is that by introducing an intermediate layer between the weir layer and the active layer, the distance between the internal current confinement part and the active layer increases, increasing the reactive current that does not contribute to oscillation and flows into the non-oscillation region. That's true. As a result, it is 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 in a region other than the groove where the sum of the layer thicknesses of the first weir layer and the intermediate layer 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 an index-guided laser element that uses the effect of laser light seeping into the substrate to create an effective outer refractive index difference (ΔN) between the oscillation region and the non-oscillation region, ΔN is determined in response to the amount. By introducing an intermediate layer adjacent to the active layer, it is possible to change the laser beam electric field intensity distribution in the stacking direction centering on the active layer, control the amount of N leaking into the substrate, and change ΔN. Become. On the other hand, in the active layer, the injected carriers oscillate over time 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, but by introducing an intermediate layer and controlling its layer thickness and refractive index, light seeps into the substrate. In a refractive index waveguide type laser device, by reducing ΔN by reducing 〃.

As self-oscillation phenomena are discovered, the spectral width of the uniaxial mode expands.

Hereinafter, a detailed explanation will be given according to a semiconductor laser device according to an embodiment of the present invention shown in FIG.

G used for the semiconductor laser device r/″iP type substrate in Figure 5
Zn having a carrier concentration of 0 1 x 1018 cm 3 indicating an aAs-GaAtAs double heterojunction semiconductor laser device
A Te-doped n-QaAs current blocking layer 12 having a carrier concentration of 5 x 1018 cnt'' is formed on a GaAs substrate 11 using a liquid phase epitaxial growth method.
After growing to a thickness of 0.8 μm, grooves 17 penetrating from the surface of the current blocking layer 12 to the PGaAs substrate 11 in a striped pattern are formed. This groove 17 is the current II layer 12.
The current path is removed. The stripe width of the groove 17 is 3 μm. By liquid phase epitaxial growth method again,
Zn-doped P is used as the multilayer crystal structure for this upward tilt laser operation.
- Grow the first weir layer 13 made of Gao 9.7AAo, 3As and have a refractive index nl to a thickness of 0.2 μm on the surface of the outside of the groove, and then sequentially grow Zll doped P-G a 1.
The intermediate M20 with a refractive index n2 consisting of x A tx A s (0 <
a (1g5A t o, .Refractive index n3 made of 5As
The active layer 14 has a layer thickness of 0. I Am, Te doped n
cao, 5AtO, 5AS with refractive index n4
A weir layer 15 with a thickness of 1 μm and a gap layer 16 made of Te-doped n-GaAs with a thickness of 3 μm are laminated on the first weir layer 13. The refractive index of each layer is set to n3>n2>old)n4.

Next, on the back surface of the substrate 11 and on the cap layer 16,
P-side electrode 18 made of u-Zn, Au-Ge-Nj
An n-side electrode 19 is formed. The resonator end facets are formed by the arm-opening method, and a double heterojunction semiconductor laser device having a resonance length of 250 μm and a device width of 300 μm is constructed.

When a DC voltage is applied through the Pf, 11 electrode 18 and the n-side electrode 19, a 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.

The layer thickness of the intermediate layer 20 is fixed at 0.3 μm, and its refractive index n
We prototyped multiple laser elements with variations in 2.

When I investigated its oscillation spectrum, I found that n2 is 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;
Expansion of the oscillation spectrum width based on the self-oscillation phenomenon described above was not observed. As n2 decreased, the 11 seepage effect weakened, and a self-oscillation phenomenon was observed in the region of n2<3 (n++2r+3). FIG. 6 is a characteristic diagram explaining this, and the horizontal axis is the G a l of the intermediate layer 20.
The mixed crystal ratio X of X A tx A s, and the vertical axis indicates the refractive index. The shaded area in the figure is n2, < (n++2n3
). FIG. 7 shows a conventional index-guided semiconductor laser device (broken line) and the intermediate layer 2 shown in the above embodiment.
FIG. 2 is a characteristic diagram showing the current-optical output characteristics and the oscillation spectrum characteristics at an output of 3 mW of a refractive index waveguide type semiconductor laser device (solid line) having a refractive index of 0. In the index-guided semiconductor laser device including the intermediate layer 20, the oscillation threshold current increases because the inflow of reactive current into the non-oscillation region increases, but the injected carriers between the lasing region and the non-oscillation region vibration occurs,
As a result, it was confirmed that the single-axis mode spectrum width SW1 was as wide as IAA. On the other hand, in a conventional laser device without the intermediate layer 20, the uniaxial mode spectrum width S W
2 k'! It is about 0.001 A.

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-GaAAAs laser element as an example, it is possible to apply the present invention to semiconductor laser elements made of other materials. In addition, although the present invention limits the layer thickness of the first weir layer or the intermediate layer and 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 something you do.

<Effects of the Invention> According to the present invention, as 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. As a result, the spectral width of the uniaxial mode is expanded, and the influence of re-incident light is removed. Therefore, it is possible to obtain a semiconductor laser device with good linearity of current vs. 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 8/N ratio of a conventional semiconductor laser device. 3 and 4 are block diagrams showing a conventional semiconductor laser device. FIG. 5 is a block diagram of a semiconductor laser device showing an embodiment of the present invention. FIG. 6 is an explanatory diagram illustrating the relationship between the mixed crystal ratio X and the refractive index of Ga and As. FIG. 7 (d) 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. 011/Substrate which is an explanatory diagram showing the laser beam width in the direction parallel to bonding.
12... Current blocking layer 13... First weir layer 14... Active layer 15... Second weir layer 16 Capacity input table No. 1t'XI? f1.2 Figure 3 Figure 4 Figure 51 Figure 6 Figure 6 511λ9 (mA) Shin 7 Close 421- Figure 8

Claims (1)

[Claims] 1. A first weir layer with a refractive index of nl, an intermediate layer with a refractive index of n2, and a refractive index of n3 on a growth surface having a striped current confinement mechanism.
A multilayer crystal structure for laser operation is constructed by sequentially stacking an active layer of , a second weir layer with a refractive index of n4, and a refractive index of each layer of n3)n2>n+)n<n2<-! A semiconductor laser device characterized by satisfying the following relationship: -(nl+213)3. 2. Each layer constituting the multilayer crystal structure for laser operation is made of Ga.
The semiconductor laser device according to claim 1, comprising 1-xA4xAs (0≦X×1). 3. The semiconductor laser device according to claim 2, wherein the sum of the layer thicknesses of the weir layer and the intermediate layer is set to 0.3 μm or more.