JPH05183232A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To attain the stabilized crystal, the strangulated outgoing pattern and the reduced absorption loss by adopting a structure not provided with a diffraction grating at all in a light emitting part. CONSTITUTION:The title semiconductor laser formed on the same substrate 1 and containing at least two mutually coupled optical waveguides is composed of one optical waveguide for a light emitting part A not provided with a diffraction grating 2 at all as well as the other optical waveguide for a wavelength control part B provided with the diffraction grating 2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength tunable single-wavelength semiconductor laser device having a plurality of optical waveguides on the same substrate, each optical waveguide having a light emitting function or a wavelength control function. ..

[0002]

2. Description of the Related Art A semiconductor laser used as a light source for optical communication is required to oscillate at a single wavelength and to be able to change the wavelength in a wide range in order to cope with an increase in communication capacity.

(Prior Art 1) Conventionally, as a wavelength tunable single wavelength laser, there is a semiconductor laser as shown in FIG.
This semiconductor laser has a light emitting section A having an active layer 3 and a diffraction grating 2 below or above it via a guide layer 5.
And a wavelength control unit B having an active layer and a guide layer provided on the extension of the active layer 3 of the light emitting unit A. In this semiconductor laser, the optical waveguide is formed as a single piece through the light emitting section and the wavelength control section located on the extension thereof. In order to separately control the light emitting section A and the wavelength control section B, separate electrodes 11a and 11b are provided on the upper portion.

When a current is injected into the light emitting portion A, the current concentrates on the active layer 3. Then, when the current is made larger than the oscillation threshold value, laser oscillation occurs, and laser light of a single wavelength that follows the period of the diffraction grating 2 is emitted. Next, a current smaller than the oscillation threshold is injected into the wavelength controller B. Then, the current is changed to change the carrier densities of the active layer 3 and the guide layer 5 of the wavelength control section B to change the equivalent refractive index in a state where laser oscillation is not reached. The effective length of the optical waveguide changes due to the change of the equivalent refractive index of the wavelength control unit B, and the wavelength of the internal standing wave changes, so that the wavelength of the light emitted from the cleavage planes at both ends changes. ..

(Problem of Prior Art 1) However, in this semiconductor laser, the wavelength control is performed without changing the refractive index of the optical waveguide located in the light emitting portion for emitting the laser beam, that is, while keeping the Bragg wavelength constant. Since only the refractive index of the optical waveguide located in the portion is changed, there is a problem that the wavelength controllable range is poor because the wavelength change limit is about 10 Å which is the internal mode interval depending on the resonator length.

(Prior Art 2) In order to solve this problem, the following semiconductor laser has been proposed (Japanese Patent Laid-Open No. 63-605).
88). That is, this semiconductor laser is formed on the same substrate and has a plurality of optical waveguides, which are an optical waveguide forming a light emitting section and an optical waveguide forming a wavelength control section optically coupled to the optical waveguide at the side. A semiconductor laser device,
The optical waveguide forming the light emitting portion has a diffraction grating.

The semiconductor laser device will be described below with reference to FIG. FIG. 4 is a front view (view seen from the emission end face direction) and a side cross-sectional view of a semiconductor laser device according to Related Art 2. Two optical waveguides are present on a substrate 1 made of n-type indium phosphide (hereinafter referred to as InP), and respectively form a light emitting section and a wavelength control section. In FIG. 4, the right side is described as the light emitting unit A and the left side is described as the wavelength control unit B. In each part, photoluminescence (hereinafter,
A guide layer 5a or 5b of indium gallium arsenide phosphide (hereinafter referred to as InGaAsP) having a wavelength of 1.3 μm (referred to as PL) is formed. Then, the PL wavelength 1.
InGaAsP active layers 3a and 3b having a thickness of 55 μm are formed to form optical waveguides. A p-type InP current blocking layer 8 and an n-type InP current blocking layer 9 are formed in such a manner that the side surfaces are buried, and a p-type InP clad layer 6 is formed thereon, and electrodes 11a and 11b are formed on the clad layer 6. Board 1
The electrode 12 is attached to the lower surface of the semiconductor laser device of the conventional example. In this semiconductor laser device, the light emitting unit A
Is the diffraction grating 2 at the interface between the substrate 1 and the guide layers 5a, 5b.
A distributed feedback type (hereinafter referred to as DFB) including a and 2b.
A portion that forms a laser oscillator and has an optical waveguide that is optically coupled to the optical waveguide included in the light emitting unit A at the side forms the wavelength control unit B. The diffraction grating 2b in the wavelength control unit B may or may not be present.

In the light emitting section A which is a DFB laser oscillator, when a current is passed from the electrode 11a to the electrode 12,
The current intensively flows into the active layer 3a, and when the value of the current exceeds a certain value (threshold value), laser oscillation occurs, and a laser having a single wavelength of about 1.55 μm corresponding to the Bragg wavelength of the diffraction grating 2a. Emits light. The wavelength control unit B is also the light emitting unit A.
Has a structure of a double heterojunction laser oscillator similar to
The carrier density changes depending on the magnitude of the injected current flowing from the electrode 11b to the electrode 12, and the equivalent refractive index as an optical waveguide changes. Since the optical waveguides of the light emitting section A and the wavelength control section B are optically coupled to each other at their side portions,
The equivalent refractive index of the entire optical waveguide including is affected by the change in the equivalent refractive index of the optical waveguide of the wavelength control unit B, and the value changes. Since the oscillation wavelength of the laser is proportional to the equivalent refractive index, the oscillation wavelength changes as a result, and the oscillation wavelength can be controlled by adjusting the injection current value of the wavelength control unit B.

In the above device, the problem described in the prior art 1, that is, the problem that the controllable range of the wavelength is narrow, corresponds to the change of the equivalent refractive index of the wavelength control unit B and the equivalent refraction of the light emitting unit A. It was solved by changing the rate.

[0010]

The problems to be solved by the present invention will be described below. In the device of Prior Art 2, since the diffraction grating is formed in the optical waveguide forming the light emitting portion, the guide layer and the active layer are grown on the diffraction grating, so that the crystal lattice of these layers is structurally stable. It lacks in properties and becomes more likely to deteriorate when used for a long time. Further, forming an active layer on a diffraction grating by a crystal growth method such as a vapor phase growth method is an extremely advanced crystal growth technique, which is a major obstacle to practical use. Further, when the diffraction grating is formed in the optical waveguide of the light emitting section A, a guide layer is required between the active layer and the active layer. As a result, the area of the emission port of the laser light emitted by the thickness of the guide layer is reduced. It becomes large and causes a decrease in coupling efficiency with the optical fiber. Furthermore, since the band gap between the guide layer of the light emitting portion and the active layer above it is close, absorption loss due to the guide layer is also a problem.

[0011]

SUMMARY OF THE INVENTION The above-mentioned problem is that the optical waveguide forming the light emitting portion is not provided with a diffraction grating, and the optical waveguide in the wavelength controlling portion B optically side-coupled with the optical waveguide of the light emitting portion A is provided. This can be solved by providing the diffraction grating 2 only on one of the waveguides.

[0012]

The light emitting portion A in the present invention has a structure that does not have a diffraction grating for unifying the emission wavelengths, so that it has crystal stability that can withstand a wider range of use conditions, and has a narrow emission pattern. Emits light. In addition, the active layer can be easily grown by the vapor phase growth method. On the other hand, the injection current that flows in the wavelength control unit changes the carrier density of the optical waveguide of the wavelength control unit and induces the change of the equivalent refractive index, so that the equivalent of the entire device including the light emitting unit optically coupled with these optical waveguides is obtained. Change the refractive index. Since the emission wavelength is proportional to the equivalent refractive index, the emission wavelength changes. Furthermore, since the current injected into the wavelength control unit is sufficiently small, it is extremely unlikely that the structure will be affected even when flowing through the crystal formed on the diffraction grating.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. (Structure) First, the structure of the semiconductor laser device of the present invention is shown in FIG.
Will be explained. FIG. 1 is a front view (a view seen from the emission end face direction) and a side sectional view of a laser device having one optical waveguide in each of a light emitting section and a wavelength control section in an embodiment of the present invention. In FIG. 1, the right side is described as the light emitting unit A and the left side is described as the wavelength control unit B. The side sectional view in FIG.
A cross section of a position crossing the optical waveguide in the light emitting portion A is shown, and a structure of an optical waveguide portion in the wavelength control portion B such as a diffraction grating is shown by a broken line. The boundary between the cladding layers 4, 6 and 7 in FIG.
Since these layers have exactly the same composition and are completely integrated after growth, they are indicated by a dashed line rather than a solid line. The wavelength control unit B is a portion that controls the wavelength having the diffraction grating 2 at the interface between the substrate 1 and the guide layer 5, and the light emitting unit A is a portion that emits light without the diffraction grating. Here, unlike the guide layer of Prior Art 2, the guide layer 5 does not intervene between the active layer and the diffraction grating, but guides light on the wavelength control section B side. When a current larger than the threshold value of laser oscillation is passed from the electrode 11a to the electrode 12, the optical waveguide in the light emitting section A causes laser oscillation and emits laser light from the end face.
The wavelength of this light is unified according to the Bragg wavelength of the diffraction grating 2 in the wavelength controller B optically coupled. Further electrode 1
When a current is passed from 1b to the electrode 12, the carrier density in the optical waveguide in the wavelength control section B changes, and the equivalent refractive index of the waveguide changes. Since the light oscillated in the light emitting section A seeps out into the optical waveguide in the wavelength control section B as well, it undergoes this change in the refractive index, and its wavelength changes compared to when there is no current injection into the wavelength control section B. .

(Manufacturing Method) Next, the manufacturing process of this element will be described with reference to FIG. A diffraction grating 2 having a target Bragg wavelength is formed on an n-type InP substrate 1 (see FIG. 2).
a). The side on which this diffraction grating is formed becomes the wavelength control unit B. InGaAs with PL wavelength of 1.55 μm on this substrate
An active layer 3 of P and a clad layer 4 of p-type InP are sequentially grown (FIG. 8B). Next, the portion grown on the diffraction grating is removed by etching (FIG. 6C), and a new InGaAsP guide layer 5 having a PL wavelength of 1.3 μm and a p-type I are newly added.
After the nP clad layer 6 is sequentially grown, a p-type InP clad layer 7 having the same composition as the clad layer 6 is grown on the entire surface (FIG. 8D). Width of about 1 μm
(E in the same figure), the p-type InP current blocking layer 8, the n-type InP current blocking layer 9 and the InGaAsP contact layer 10 for improving the ohmic property of the electrode are sequentially grown to form a mesa. Then, the metal electrodes 11a, 11b and 12 are finally formed by vapor deposition and alloying (see FIG. 1).

The embodiment shown in FIGS. 1 and 2 is an example in which there is only one optical waveguide in the wavelength control section B, but it is also possible to use a plurality of optical waveguides that optically couple with each other. .. In this case, the wavelength tunable width can be further increased by increasing the number of optical waveguides whose equivalent refractive index changes. Further, although the buried type laser element is used in the embodiment, it is obvious that it can also be realized by a ridge waveguide type laser element. Further, it should be noted that the present invention is applicable not only to InP-based / InGaAsP-based materials but also to all semiconductor light emitting devices.

[0016]

In the structure of the present invention, since the diffraction grating is not provided on the side of the light emitting portion, the guide layer is unnecessary, and the area of the laser light emission port is reduced by that amount.
The far-field pattern of the emitted light could be reduced by about 20% in the vertical angular spread as compared with the conventional technique. As a result,
The coupling efficiency with the spherical fiber, which was 5 dB, was improved to -3 dB. Also, in the life test, the DFB laser device shown in the above-mentioned example obtained the same numerical value as that of the Fabry-Perot type device having no diffraction grating. Further, since it is not necessary to grow the active layer on the diffraction grating, it becomes easy to form the active layer having the multiple quantum well structure by the vapor phase growth method, and the output of the laser light is increased and the laser oscillation is reduced. Thresholding becomes possible. In addition, again
Since the guide layer is not provided on the active layer of the light emitting portion, the effect of reducing absorption loss can be obtained.

[0017]

[Brief description of drawings]

FIG. 1 is a structural diagram of an embodiment of the present invention.

FIG. 2 is a diagram showing a manufacturing process according to an embodiment of the present invention.

FIG. 3 is a diagram showing a configuration of a semiconductor laser of prior art 1.

FIG. 4 is a diagram showing a configuration of a semiconductor laser device according to Related Art 2.

[Explanation of reference numerals] 1 substrate. 2 diffraction grating. 2a diffraction grating. 2b diffraction grating. 3 Active layer. 3a Active layer. 3b Active layer. 4 Clad layer. 5 guide layers. 5a guide layer. 5b guide layer. 6 Clad layer. 7 Clad layer. 8 Current blocking layer. 9 Current blocking layer. 10 contact layer. 11a Metal electrode. 11b Metal electrode. 12 Metal electrodes. A light emitting part. B wavelength control unit.

Claims (1)

[Claims] 1. A semiconductor laser device comprising at least two optical waveguides formed on the same substrate (1) and optically coupled to each other, at least one of said optical waveguides having an active layer (3). And a light emitting section (A) having no diffraction grating, and at least one of the optical waveguides constitutes a wavelength control section (B) having a diffraction grating (2). Laser device.