JPH0548203A - Semiconductor laser device of distributed feedback type - Google Patents

Abstract

PURPOSE:To obtain a large gain coupling coefficient kg, in a semiconductor laser device of a distributed feedback type, which has the structure for obtaining gain coupling by the spatial periodic variation of its carrier density. CONSTITUTION:In the same waveguide, two active layers 7, 12 are provided. In proximity to the active layer 7, provided is a diffraction grating layer 5, which gives a spatial periodic distribution to an injected current. The gain of a laser device is controlled by the currents respectively injected into the active layers 7, 12, and by the current injected into the active layer 7, the magnitude of gain coupling is controlled.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is used to generate laser light. In particular, it relates to an improvement in the structure of a semiconductor distributed feedback laser device using distributed feedback by gain coupling.

[0002]

2. Description of the Related Art As a semiconductor distributed feedback laser device which realizes distributed feedback by gain coupling, the inventors of the present invention have invented a structure for periodically changing the carrier density in an active layer and a manufacturing method thereof, and have already issued a patent. Filed (Japanese Patent Application No. 2-235235). In the case of this structure, since the active layer having a flat structure can be used, the component of refractive index coupling can be considerably reduced, which is suitable for realizing a pure gain-coupled semiconductor distributed feedback laser device.

[0003]

It is considered that the characteristics such as the gain difference between modes in the gain coupled semiconductor distributed feedback laser device are improved as the gain coupling coefficient κ _g is increased. In the structure shown in the above-mentioned prior application, the gain coupling coefficient κ _g increases as the difference in height of carrier density in the active layer increases. Therefore, in order to obtain a large gain coupling coefficient κ _g , it is better that the carriers injected into the active layer do not diffuse.

As one method of preventing carrier diffusion, it is considered that the active layer is doped with a high density to shorten the carrier life. But with this method,
The threshold current for laser oscillation becomes high.
It is also possible to shorten the carrier life by increasing the amount of current injected into the active layer. This method is described in detail in "Semiconductor Laser and Optical Integrated Circuit", edited by Suematsu, published by Ohmsha. But even with this method,
Since the threshold carrier density of the entire active layer increases, it is difficult to realize a large gain coupling coefficient κ _g at an appropriate threshold value. In addition, when trying to adjust the value of the gain coupling coefficient κ _g , there is no choice but to change the film thickness of the active layer and the like, which makes the device design difficult.

The present invention solves the above problems and provides a semiconductor distributed feedback laser device having a large gain coupling coefficient κ _{g with} a structure for gain coupling by periodically changing the carrier density of the active layer. With the goal.

[0006]

SUMMARY OF THE INVENTION A semiconductor distributed feedback laser device of the present invention comprises a first active layer which emits light by current injection and a first active layer which is provided with a periodic change in conductivity. In a semiconductor distributed feedback laser device having a diffraction grating layer for giving a spatial periodic distribution to the injected current in a semiconductor waveguide, a semiconductor distributed feedback laser device is provided in the semiconductor waveguide, and light is mutually emitted between the first active layer and the semiconductor active layer. A second active layer is provided at a position where is coupled, gain is controlled by the current injected into the second active layer and the current injected into the first active layer, and the current is injected into the first active layer. It is characterized in that a means for controlling the magnitude of gain coupling is provided.

It is desirable to have a heavily doped layer in or in contact with the first active layer. Specifically, the entire first active layer may be heavily doped with impurities, and such a layer may be provided in contact with the first active layer.

[0008]

It is considered that the limit for increasing the gain coupling coefficient κ _g is to obtain gain coupling and gain in the same active layer. Therefore, in the present invention, an active layer for gain coupling and an active layer for gain gain are separately provided. In the vicinity of the active layer for obtaining gain coupling, that is, the first active layer, a layer provided with a periodic change in conductivity is arranged as a diffraction grating, and distributed feedback by gain coupling is realized. The structure of the active layer for gaining gain, that is, the second active layer is the same as that of conventional refractive index coupling. These two active layers are closely arranged so as to be integrated in terms of light intensity distribution, and form one optical waveguide. The current is injected into the two active layers independently or divided into appropriate ratios.

Conventionally, in a variable frequency laser or the like,
A structure having two current injection layers such as an active layer and a modulation layer in one waveguide has been realized. That is, by controlling the refractive index or absorptivity of one injection layer, the oscillation wavelength and gain of the other current injection layer are controlled, and the oscillation wavelength is changed by using two current injection layers having different band gaps. Things are known. However, these structures are intended for modulation and do not consider distributed feedback due to gain coupling.

[0010]

1 and 2 are views showing a first embodiment of the present invention. FIG. 1 is a sectional view of an active layer region portion in a direction parallel to a stripe, and FIG. 2 is a sectional view in a direction perpendicular to the stripe. The figure is shown. In the following description, “upper” means a direction away from the substrate, that is, a crystal growth direction.

In this semiconductor distributed feedback laser device, the first active layer 7 which generates light by current injection and the current injected into the active layer 7 having a periodic change in conductivity are spatially cycled. A diffraction grating layer 5 for giving a distribution is provided in the semiconductor waveguide. A buffer layer 6 is provided between the active layer 7 and the diffraction grating layer 5.
Are provided, and the cladding layers 8 and 13 are provided so as to sandwich these.

The feature of this embodiment is that a second active layer 12 is further provided in the semiconductor waveguide at a position where light is coupled with the active layer 7, and this active layer 12 is provided.
As a means for controlling the gain by the current injected into the active layer 7 and the current injected into the active layer 7 and controlling the magnitude of the gain coupling by the current injected into the active layer 7, the currents are separately injected into the active layers 7 and 12. The structure for doing so is provided. That is, the active layer 12 is formed on the clad layer 3 provided on the substrate 1, and a part of the clad layer 3 and the active layer 1 are formed.
2, clad layer 8, active layer 7, buffer layer 6, diffraction grating layer 5
By burying the clad layer 13 with the buried layer 14, a current path from the substrate 1 to the buried layer 14 via the clad layer 3, the active layer 12, and the clad layer 8 and the clad layer 13 to the active layer 7 are formed. And a current path to the buried layer 14 via the cladding layer 8 is formed. An electrode 10 is provided on the back surface of the substrate 1, an electrode 11 is connected to the cladding layer 13 via a contact layer 9, and an electrode 16 is connected to the buried layer 14 via a contact layer 15.

The peripheral structure of the active layer 7 is equivalent to that shown in the above-mentioned previous application, and the diffraction grating layer 5 is formed with a conductivity type different from that of the buffer layer 6 and the cladding layer 13. In order to eliminate the refractive index coupling component, it is desirable to use a composition in which the cladding layer 13 and the diffraction grating layer 5 have the same refractive index. Further, the buffer layer 6 is preferably as thin as possible in order to reduce current diffusion. The injection current to the active layer 7 flows between the diffraction grating layers 5 and
Immediately below, the injection current is reduced and a periodic perturbation of gain occurs.

Further, since it is not necessary to make a gain in the active layer 7, 10 ¹⁸ c is added to the active layer 7 in order to shorten the carrier life.
It is desirable to dope a high concentration impurity exceeding m ⁻³ .

The structure around the active layer 12 is an ordinary double hetero structure. The clad layer 8 is set to a thickness such that the two active layers 7 and 12 are optically coupled and function as one optical waveguide.

Examples of conductivity type and composition in this embodiment are as follows:
Similarly to the case where p-type InP is used for the substrate 1, p-type G
The results obtained using aAs are shown in Table 1 and Table 2, respectively.

Table 11 Substrate p ⁺ -InP 3 Cladding layer p-InP 12 Active layer i-In _0.53 Ga _0.47 As and InGaAsP (λg = 1.3 μm) multiple quantum well structure (equivalent λg = 1. 55 μm) 8 clad layer n-InP 7 active layer i-InGaAsP (λg = 1.55 μm) 6 buffer layer p-InP 5 diffraction grating layer n-InP 13 clad layer p-InP 9 contact layer p ⁺ -In _0.53 Ga _0.47 As 14 buried layer n-InP 15 contact layer n-In _0.53 Ga _0.47 As However, the quaternary mixed crystal is lattice-matched to InP. Also, λ
g is the wavelength of light corresponding to the forbidden band width.

[0018]

Here, an example using an InP-based multiple quantum well structure and an example using a GaAs-based single quantum well structure as the active layer 12 are shown. InP-based and GaAs-based single quantum well structures are shown, respectively. The present invention can be similarly implemented by using a structure or a multiple quantum well structure, and the present invention can be similarly implemented by using a mixed crystal active layer. Alternatively, the active layer 7 may have a quantum well structure.

Although the basic structure has been shown in the above description,
It is desirable to jointly use a structure for preventing an extra current to the area other than the active layer, for example, an internal current confinement layer or a structure having a high resistance by ion implantation.

In order to prevent oscillation in the Fabry-Perot mode, it is desirable to provide antireflection coatings on both end faces to reduce the reflectance.

FIG. 3 is a diagram showing a second embodiment of the present invention, which is a sectional view in a direction parallel to the stripe of the active region portion.

This embodiment has a structure in which the positions of the first active layer 7 and the second active layer 12 in the first embodiment are exchanged. That is, the clad layer 3 and the active layer 7 are formed on the substrate 1, and the diffraction grating layer 5 is provided thereon via the buffer layer 6. A clad layer 8 is provided on the diffraction grating layer 5.
Are formed, and the active layer 12 is provided thereon. A clad layer 13 is provided on the active layer 12, and the electrode 11 is connected via the contact layer 9.

FIG. 4 is a view similar to FIG. 3 showing a modification of the second embodiment. In the case of this example, the buffer layer 6 is also provided with irregularities and is flattened by the diffraction grating layer 5. In this case, the diffraction grating layer 5 does not have to be divided into a grating shape, and may be continuous as a thin layer. When the diffraction grating layer 5 is a continuous thin layer, the current injected into the active layer 7 flows from the cladding layer 3 through the buffer layer 6 to the buried layer 14.

FIG. 5 is a diagram showing a third embodiment of the present invention, and FIG. 6 is a diagram showing a fourth embodiment of the present invention. These figures show sectional views in a direction parallel to the stripes of the active region portion. These examples differ from the first example and the second example in that the buffer layer 6 and the diffraction grating layer 5 are on the lower side of the active layer 7, that is, on the side of the substrate 1. In the case of such a structure, it is possible to provide the active layer 7 with a concavo-convex shape and to introduce gain coupling due to a change in film thickness.

In the above description, the case where the substrate is a p-type has been described. However, even if the conductivity type of each layer is exchanged so that the directions of current injection into the two active layers are reversed using an n-type substrate. The invention can be similarly implemented. In addition, the material composition is I
Although one example is shown for each of nP type and GaAs type, the present invention can be similarly applied to the case of using other compositions or materials other than InP type and GaAs type. Furthermore, a structure that can optimize the distribution of gain coupling coefficient κ _g by dividing the electrode that injects current into the active layer on the side contributing to gain coupling into two or more and making the amount of injection in the cavity direction non-uniform You can also

[0027]

According to the semiconductor distributed feedback laser device of the present invention, by increasing the carrier injection amount of the first active layer, that is, the active layer on the side contributing to gain coupling, or by increasing the concentration of high concentration in the active layer. The carrier life can be shortened by performing the impurity doping. For this reason, carrier diffusion in the active layer can be suppressed, the period distribution of gain can be effectively formed, and the gain coupling coefficient κ _g can be increased. Further, as shown in the above-mentioned prior application, the first active layer has a flat structure, so that the change in refractive index is small and pure gain coupling can be realized. On the other hand,
The second active layer does not have a structure that shortens the carrier life, and the gain required for oscillation can be obtained with a normal carrier injection amount. This allows the gain coupling coefficient κ to be increased without significantly increasing the threshold current.
You can increase _g .

Further, if the second active layer has a quantum well structure, it is possible to realize a laser device having both the advantage of gain coupling and the advantage of quantum well structure, and the selection of TE mode and TM mode and the point of line width. Is advantageous.

The semiconductor distributed feedback laser device of the present invention can also optimize the gain coupling coefficient κ _g by selecting and using the optimum operating point. This is a feature greatly different from the fact that the coupling coefficient is determined at the time of manufacturing the device as in the conventional distributed feedback laser, and the flexibility in use is enhanced.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment of the present invention and is a cross-sectional view in a direction parallel to a stripe of an active region portion.

FIG. 2 is a view showing a first embodiment of the present invention and is a sectional view in a direction perpendicular to a stripe.

FIG. 3 is a diagram showing a second embodiment of the present invention and is a cross-sectional view in a direction parallel to the stripe of the active region portion.

FIG. 4 is a diagram showing a modification of the second embodiment.

FIG. 5 is a diagram showing a third embodiment of the present invention and is a cross-sectional view in a direction parallel to the stripe of the active region portion.

FIG. 6 is a view showing a fourth embodiment of the present invention and is a cross-sectional view of the active region portion in a direction parallel to the stripe.

[Explanation of symbols]

 1 Substrate 3 Cladding Layer 5 Diffraction Grating Layer 6 Buffer Layer 7 Active Layer 8 Cladding Layer 9 Contact Layer 10, 11, 16 Electrode 12 Active Layer 13 Cladding Layer 14 Buried Layer 15 Contact Layer

[Procedure amendment]

[Submission date] October 23, 1991

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] 0029

[Correction method] Change

[Correction content]

The semiconductor distributed feedback laser device of the present invention can also optimize the gain coupling coefficient kg by selecting and using the optimum operating point. This is a feature greatly different from the fact that the coupling coefficient is determined at the time of manufacturing the device as in the conventional distributed feedback laser, and the flexibility in use is enhanced. The semiconductor distributed feedback laser of the present invention has a structure particularly aimed at optimizing the gain coefficient, but since it has a structure having two active layers, it can also be operated as a variable frequency laser. That is, Yamamoto et al., IEICE Technical Research Report 0QE91-41, 1991, 5th
As shown in pages 5 to 60, if the structure of the active layer is selected appropriately, the frequency can be changed even when one of the active layers causes gain coupling as in the present invention.

Claims (2)

[Claims] 1. A first active layer which generates light by current injection, and a diffraction grating which is provided with a periodic change in conductivity and gives a spatial periodic distribution to the current injected into the first active layer. In the semiconductor distributed feedback laser device including a layer in a semiconductor waveguide, a second active layer is further provided in the semiconductor waveguide at a position where light is coupled with the first active layer. Means for controlling the gain by the current injected into the second active layer and the current injected into the first active layer, and controlling the magnitude of gain coupling by the current injected into the first active layer A semiconductor distributed feedback laser device comprising: 2. The semiconductor distributed feedback laser device according to claim 1, further comprising a layer doped with a high concentration of impurities in or in contact with the first active layer.