JPS6317356B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a distributed feedback semiconductor laser that can oscillate in a single longitudinal mode even during high-speed modulation. Semiconductor lasers used as light sources for optical communications are required to be stable even when the oscillation wavelength and oscillation mode are modulated at high speed. Therefore, as a semiconductor laser that meets these requirements, a so-called distributed feedback semiconductor laser (hereinafter referred to as a DFB laser) is used.
is proposed. This DFB laser forms a periodic structure by periodically changing the thickness of the semiconductor layer (active layer or optical guide layer formed in contact with the active layer) that forms the optical waveguide along the direction of light oscillation. However, this periodic structure introduces a periodic change in refractive index into the waveguide, and this change in refractive index provides a light feedback function to enable laser oscillation. However, since the optimum period of the above-mentioned periodic structure is as fine as 0.2 to 0.3 μm, it is difficult to process the periodic structure with high precision.Therefore, it is difficult to obtain a periodic structure with high light feedback efficiency and good oscillation characteristics. It has been difficult to manufacture DFB lasers with high yields. The present invention improves these conventional drawbacks, and its purpose is to provide a DFB laser that can obtain high feedback efficiency even if the machining accuracy of the periodic structure is the same as the conventional one. . Examples will be described in detail below. 1 and 2 are cross-sectional views of a DFB laser according to an embodiment of the present invention. FIG. 1 is a cross-sectional view perpendicular to the light extraction direction, and FIG. 2 is a cross-sectional view of the central part of the element along the light extraction direction. FIG. In each figure, 1 is a p-type semiconductor substrate such as a p-type Inp substrate, and 2 is a p-type or n-type GaInAsp4.
Active layer consisting of original mixed crystal layer etc. 3 is n-type GaInAsp4
4 is a current blocking layer; 5 is a diffraction grating formed by the optical guide layer and current blocking layer 4; 6 and 7 are n-type InP layers, etc.;
8 is a p-type semiconductor layer such as a p-type InP layer,
9 is a surface formed by a method such as chemical etching to be inclined with respect to the active layer 2 and the light guide layer 3; 10 is a light extraction surface formed by a method such as cleavage or chemical etching; 11 is a p-type ohmic electrode; 12 is a surface formed by a method such as cleavage or chemical etching. An n-type ohmic electrode 13 is a laser beam to be emitted. The active layer 2 and the optical guide layer 3 serve as optical waveguide layers, and the p-type semiconductor substrate 1 and the n-type semiconductor layer 6 serve as optical confinement layers. The difference between the DFB laser of this embodiment and the conventional DFB laser is that the conventional DFB laser processes the surface of the light guide layer 3 into irregularities to form a region corresponding to the irregularities including the current blocking layer 4 shown in FIG. In contrast to the n-type semiconductor layer 6 formed thereon, in this example, the surface of the light guide layer 3 is processed to be uneven and a current blocking layer 4 is provided on the convex portion, and this current blocking layer 4 and the light The point is that an n-type semiconductor layer is formed on the guide layer 3. Generally, in a DFB laser, the diffraction grating 5
The period Λ of It is the effective refractive index of the wave path. Laser oscillation of the DFB laser is caused at a wavelength that satisfies
The higher the diffraction efficiency, the better. Diffraction efficiency is determined by the shape of the diffraction grating and the difference between the refractive index of the convex portion of the optical confinement layer 6 and the equivalent refractive index of the current blocking layer 4 and the light guide layer 3 in the concave portion of the optical confinement layer 6 with the diffraction grating as the boundary. Since it is determined by In this example, as described above, the light guide layer 3
A current blocking layer 4 is formed on the top of the convex portion (bottom surface of the concave portion of the optical confinement layer). In general, in a semiconductor crystal, the refractive index decreases as the number of injected carriers increases, so if such a structure is used, the injected current will concentrate on the convex portion of the optical confinement layer 6, resulting in a decrease in the refractive index of that portion, resulting in the above-mentioned refraction. The index difference becomes larger than before, and the diffraction efficiency increases. Furthermore, as the impurity concentration of the semiconductor crystal increases, its refractive index decreases, so if the impurity concentration of the optical confinement layer 6 is made higher than before, the refractive index difference becomes larger and the diffraction efficiency further increases. The current blocking layer 4 is preferably one that completely blocks the current, but may also be one that suppresses the current to some extent, such as a high-resistance n-type semiconductor (InP, GaInAsP, etc.) whose specific resistance is larger than the convex portion of the optical confinement layer 6; An insulating layer or a p-type semiconductor layer having a conductivity type opposite to that of the optical confinement layer 6 can be used. <Specific Example> A DFB laser was manufactured in which each layer in FIGS. 1 and 2 had the values shown below. P-type semiconductor substrate 1 Zn-doped (100) p-type InP substrate, thickness 80 μm, carrier density 5×10 ¹⁸ /cm ³ , EPD (etch pit density) 5×10 ³ /cm ² Active layer 2 Non-doped Ga _0.42 In _0.58 As _0.88 P _0.12 Quaternary mixed crystal active layer, thickness 0.13 μm Optical guide layer 3 Sn-doped Ga _0.26 In _0.76 As _0.56 P _0.44 Quaternary mixed crystal waveguide layer, thickness 0.2 μm, carrier density 7×10 ¹⁷ /cm ³ Current blocking layer 4 Non-doped InP crystal layer, thickness 0.1 μm, carrier concentration 2×10 ¹³ /cm ³ N-type semiconductor layer 6 Sn-doped n-type InP crystal layer, thickness 2.5 μm, carrier density 5×10 ¹⁸ /cm ³ N-type semiconductor layer 7 Sn-doped n-type InP crystal layer, 2 μm thick, carrier density 4×10 ¹⁷ /cm ³ P-type semiconductor layer 8 Zn-doped p-type InP crystal layer, 1 μm thick, carrier density 3×10 ¹⁷ /cm 3 cm ³ These crystal layers were grown by a conventional liquid phase epitaxial growth method using the so-called slide boat method, and the growth temperature of each crystal layer was between 950°C and 605°C. The manufacturing procedure is as follows. (1) An active layer 2, a light guide layer 3, and a current blocking layer 4 were successively grown on the p-type InP substrate. (2) A photoresist was applied to a thickness of 500 Å on the substrate surface, and interference fringes along the <110> direction were exposed using two-beam interferometry. After development,
_1MolK2Cr2O7 :HBr: _{CH3COOH = 3} :1:
1 to etch the crystal layers 3 and 4 to a depth of
A 0.2 μm diffraction grating 5 was formed. (3) After growing the n-type semiconductor layer 6, a SiO ₂ film is formed on the crystal surface using the RF sputtering method, and a stripe pattern of the SiO ₂ film with a width of 9 μm is formed along the <110> direction using the photo technique. Mesa etching was performed to a depth of 6 μm using a methanol bromine solution. (4) After crystal growth of buried layers was performed in the order of n-type semiconductor layer 7 and p-type semiconductor layer 8, the SiO ₂ film was removed. (5) Au-Zn alloy as p-type electrode 11 on the substrate side;
Au-Ge- as an n-type electrode 12 on the crystal growth layer side.
A Ni alloy was formed by vapor deposition and then sintered. (6) Using photoresist as a mask, along the <110> direction, the width is 30 μm and the depth is 10 μm at intervals of 400 μm.
1NolK ₂ Cr ₂ O ₇ :HBr:CH ₃ COOH=
It was formed using a 1:1:1 etching solution.
This formed surface 9 and obtained θ ₁ =54.5°. (7) Separate the central part of the stripe by cleavage (θ ₂
= 90°) and a length of 200 μm. When a direct current was passed through the thus manufactured DFB laser with the p-type electrode 11 as the positive electrode and the n-type electrode 12 as the negative electrode, the laser oscillated in the distributed feedback mode at 25° C. with a threshold value of 55 mA. The oscillation spectrum was found to be a single longitudinal mode from the oscillation threshold to more than three times the oscillation threshold. In addition, a 65mA DC current was passed through this DFB laser, and further
When we observed the spectrum when a 400 MHz sinusoidal current (I _pp = 20 mA) was applied, we observed laser oscillation in the distributed feedback mode of a single longitudinal mode at a wavelength of around 1.516 μm. FIG. 3 is a diagram showing an example of the spectrum observed at that time. Note that the threshold value of the conventional DFB laser formed by the above process excluding the current blocking layer 4 was about 1.4 times that of the device manufactured by the above process.
Moreover, the single longitudinal mode was maintained only up to 1.8 times the threshold current. In the above embodiment, a p-type substrate 1 is used, but if an n-type substrate is used, the conductivity types of the other semiconductor layers may be reversed from those described above. Also,
A periodic structure of convexes and convexities may be provided on the substrate side, and when a light guide layer is not interposed between the confinement layer and the active layer, the thickness of the active layer may be changed periodically. Furthermore, the semiconductor materials used are:
In addition to GaInAsP/InP, GaAs/GaAlAs,
Possible examples include GaSb/GaAlAsSb system. As can be seen from the above description, the present invention provides a current blocking layer on the bottom surface of the concave portion of the optical confinement layer, which has a periodic uneven structure along the light extraction direction on the surface facing the active layer. As a result of the current flowing concentrated in the convex part of the optical confinement layer, the refractive index of that part decreases, and the difference in refractive index between the concave and convex parts becomes large, so even if the apparent concave and convex structures are the same, the efficiency of distributed feedback can be increased. It is possible to improve the laser oscillation characteristics in DFB mode.

[Brief explanation of the drawing]

1 and 2 are cross-sectional views of an element according to an embodiment of the present invention, and FIG. 3 is a diagram showing an example of an oscillation spectrum. 1 is a p-type semiconductor substrate, 2 is an active layer, 3 is a light guide layer, 4 is a current blocking layer, 5 is a diffraction grating, 6 and 7 are n-type semiconductor layers, 8 is a p-type semiconductor layer, 11 is a p-type ohmic layer The electrode 12 is an n-type ohmic electrode.

Claims (1)

[Claims] 1. In a distributed feedback semiconductor laser that enables distributed feedback of light by periodically changing the thickness of the optical waveguide layer along the light extraction direction, there is a periodicity along the light extraction direction on the surface facing the active layer. an optical confinement layer having a concavo-convex structure; a current blocking layer formed on the bottom surface of the concave portion of the optical confining layer; A distributed feedback semiconductor laser characterized by comprising an optical waveguide layer whose thickness changes.