JPH06244493A - Distributed feedback type semiconductor laser and its manufacture - Google Patents

Abstract

PURPOSE:To provide a structure for decreasing non-radiative recombination at the interface between a diffraction grating and a luminous layer, and preventing joining efficiency with an optical fiber from deteriorating, and to provide its manufacturing method. CONSTITUTION:An active layer 1 for obtaining laser oscillation has a multilayer structure composed of a diffraction grating waveguide region 1a whose luminous layer is cut periodically by a built-in diffraction grating 3, and active regions 16 containing individual luminous layers provided on the upside and underside of this diffraction grating waveguide regions 1a. Especially, each of the diffraction grating waveguide region 1a and the active regions 1b has quantum well structure by the lamination of more than one well layer 6 to be a luminous layer, or lamination structure having more than one luminous layer composed of a plurality of double heterojunctions.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser used as a light source in the fields of optical communication, optical information processing (optical integrated circuit), etc., and more particularly to a structure of a distributed feedback semiconductor laser and a manufacturing method thereof. .

[0002]

2. Description of the Related Art A conventional distributed feedback semiconductor laser is a refraction device that forms a diffraction grating whose refractive index is periodically changed with respect to the emission direction of the laser light and returns the light in a distributed manner to obtain oscillation. A rate-coupling type and a gain-coupling type in which distributed feedback is performed by a diffraction grating whose gain or loss is periodically changed in the emission direction of laser light have been proposed. Note that this distributed feedback semiconductor laser is a laser in which light emitted from the active layer is selected in the oscillation longitudinal mode with good monochromaticity by a diffraction grating located above or below the active layer.

Among the distributed feedback semiconductor lasers described above, as shown in FIG. 3, the refractive index coupled type has an active layer 1 serving as a light emitting layer sandwiched between clad layers 2 and electrodes 4a and 4b on the upper and lower sides. It is configured by attaching.

In particular, the diffraction grating 3 formed on the upper side of the active layer 1 (may be formed on the lower side) has irregularities periodically formed in the emitting direction of the laser light generated in the active layer 1. It refers to the interface between the clad layer 2b and the clad layer 2a formed on the clad layer 2b having a different refractive index, and has a structure in which the refractive index changes periodically.

The above-mentioned characteristic of the refractive index coupling type distributed feedback semiconductor laser is that it oscillates in two longitudinal modes in principle. Therefore, in order to obtain a single longitudinal mode, it is necessary to further form irregularities having a quarter period of the oscillation wavelength in the central portion of the semiconductor laser or to change the reflectance of the end face of the semiconductor laser between the front surface and the rear surface. However, it is technically difficult to form unevenness of 1/4 cycle, and in the method of changing the reflectance of both end faces of the semiconductor laser, the quality of the single longitudinal mode depends on the phase of the unevenness at the end face, Since it is difficult to control the phase, there is a problem that a ratio of a semiconductor laser having a high single longitudinal mode property is small.

On the other hand, a characteristic of the gain-coupled distributed feedback semiconductor laser is that single longitudinal mode oscillation can be obtained by periodically changing the gain or loss with respect to the emission direction of the laser light. Since the laser oscillates in a single longitudinal mode in principle, there is a problem that, as in the above-described refractive index coupling type, it is necessary to form unevenness of 1/4 period in the center of the semiconductor laser or change the reflectance of both end surfaces. Absent.

As a first conventional example of the gain coupling type distributed feedback semiconductor laser, for example, WTTsang, etc.
m wavelength InGaAs / InGaAsP distributed feedback m
ulti-quantum well laser grown by chemical beam epit
axy "(Applied Physics Letters, 1991, vol.59, No. 19, pp.
2375-2377), an example in which a good single longitudinal mode is obtained by periodically changing the loss is shown. In this report, by forming a diffraction grating in an optical waveguide layer having a multiple quantum well structure, it is configured to change the absorptance, that is, the loss, in the emission direction of laser light.

According to this structure, since the loss ratio can be easily adjusted by changing the multiple quantum well structure of the optical waveguide layer, it is possible to easily select the optimum ratio of the coupling between the light and the diffraction grating. it can.

As a technique for periodically changing the gain, for example, as shown in FIG. 4A, a diffraction grating 3 is formed at the interface between the active layer 1 and the cladding layer 2 to form a periodic structure. Forming technology (second conventional example), namely Toshio Higashi and others "Analysis of DFB laser with periodic gain and refractive index" (IEICE Thesis, OQE91-31, pp.79-84) This is a technology that has been adopted, and for example, as shown in FIG.
A technique for forming the quantum wires 5 in the active layer 1 (third conventional example) or a technique disclosed in Japanese Patent Laid-Open No. 4-165686, for example, as shown in FIG. A technique has been proposed in which a diffraction grating is formed at the interface between the active layer 1 (6 in the figure is a well layer that becomes a light emitting layer) and the cladding layer 2 of the structure.

[0010]

In the conventional distributed feedback semiconductor laser, for example, in the case of the first conventional example (FIG. 4A), as shown in FIG. 5, the diffraction grating 3 has an active layer 1 and a cladding. Since it is formed at the interface with the layer 2 (indicated by a thick line in the figure), non-radiative recombination is increased at this interface.
There is a problem that it is difficult to put into practical use because the threshold current becomes high and the luminous efficiency becomes poor.

In both the refractive index coupling type and the gain coupling type, the conventional distributed feedback type semiconductor laser has a diffraction grating formed on either the upper side or the lower side of the active layer, so that the laser light is emitted. Since the refractive index distribution in the direction perpendicular to the direction is asymmetrical about the active layer, the far-field pattern is also asymmetrical, and there is a problem that the coupling efficiency with the optical fiber deteriorates.

The present invention has been made to solve the above problems, and reduces non-radiative recombination at the interface between the diffraction grating and the light emitting layer, and also deteriorates the coupling efficiency with the optical fiber. It is an object of the present invention to provide a structure of a distributed feedback semiconductor laser that prevents the occurrence of the above and a manufacturing method thereof.

[0013]

In a distributed feedback semiconductor laser according to the present invention, an active layer for obtaining laser oscillation is provided with a diffraction grating waveguide region in which a light emitting layer is periodically cut by a built-in diffraction grating. The multi-layer structure is characterized in that active regions including a light emitting layer are provided on the upper side and the lower side of the diffraction grating waveguide region.

In particular, the diffraction grating waveguide region and each active region have a quantum well structure in which one or more well layers to be light emitting layers are laminated, or one or more light emitting layers composed of a plurality of double heterojunctions. However, each active layer may be composed of a single layer.

Further, the diffraction grating waveguide region and each active region in the active layer may be a combination of the above quantum well structure and a laminated structure forming a double heterojunction portion, in which case the quantum well structure is used. The obtained laser oscillation wavelength and the laser oscillation wavelength obtained in the laminated structure having the double hetero junction are configured to be substantially the same.

As a method of manufacturing a distributed feedback semiconductor laser according to the present invention, in the first step, a quantum well structure or a laminated structure in which a double hetero junction is formed on a clad layer formed on a semiconductor substrate. An active region is formed, and in a second step, a diffraction grating is formed by an interference exposure method on a layer surface having a quantum well structure further formed on the active region or a laminated structure forming a double heterojunction portion, This diffraction grating portion is embedded with a semiconductor material having a bandgap energy larger than that of the oscillation laser light to form a diffraction grating waveguide region, and in the third step, a quantum well structure is formed on the diffraction grating waveguide region. Alternatively, an active region having a laminated structure composed of a double heterojunction is formed, and a clad layer is further formed on the active region. It is characterized in that granulation.

[0017]

The distributed feedback semiconductor laser according to the present invention is
A diffraction grating waveguide region having a built-in diffraction grating has a quantum well structure in which one or more well layers to be a light emitting layer are stacked, or a stack having one or more light emitting layers composed of a plurality of double hetero junctions. Since the structure is adopted, the interface between the light emitting layer and the diffraction grating, in which non-radiative recombination that is cut by the diffraction grating occurs, is only the cross section of the light emitting layer cut by the diffraction grating. Therefore, the interface between the light emitting layer and the diffraction grating is significantly reduced as compared with the conventional one, and non-radiative recombination at the interface is dramatically reduced.

Further, since the active layer has a laminated structure in which the active regions are provided on the upper side and the lower side of the diffraction grating waveguide region, the refractive index distribution in the direction perpendicular to the laser emission direction can be made substantially symmetrical, whereby the output end face is formed. Since the light intensity distribution can be made substantially symmetrical with respect to the active layer, a far field image with good symmetry can be obtained.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present invention will be described below with reference to FIGS. In the figure, the same parts are designated by the same reference numerals and the description thereof will be omitted.

FIG. 1 is a diagram showing the structure of a typical embodiment of the distributed feedback semiconductor laser according to the present invention. Hereinafter, the case of manufacturing the embodiment shown in FIG. A description will be given along the manufacturing process.

First, in the first step, the active region 1b is formed on the cladding layer 2 formed on the semiconductor substrate via the guide layer 1c. Particularly, in FIG. 1A, the active region 1b is formed. 3 shows the structure of an example in which a quantum well structure in which one or more light emitting layers 6 are stacked via barrier layers (semiconductor layers having a bandgap energy larger than the energy of oscillation laser light) is used.

FIG. 1B shows an embodiment in which a laminated structure is adopted as the light emitting layer of the active layer region 1b.
(C) is an example in which a similar quantum well structure is adopted as the active region 1b.

Next, in the second step, the diffraction grating waveguide region 1a is formed on the active region 1b of the quantum well structure formed as described above via the barrier layer. In the diffraction grating waveguide region, first, the light emitting layer 6 is formed through the respective barrier layers.
A quantum well structure in which one or more layers are laminated is formed, and the surface of this layer is etched by an interference exposure method to form a diffraction grating, and then the surface is flattened by a barrier layer.

At this time, since the interface portion between the light emitting layer 6 and the diffraction grating 3 is only the cross section (A1, A2, A3, A4 in the figure) cut by the diffraction grating 3 as shown in FIG.
Compared with the conventional interface part (the part shown by the thick line in FIG. 5), it can be dramatically reduced.

FIG. 1B shows the diffraction grating waveguide region 1
This is an example in which a laminated structure having one or more light emitting layers composed of a plurality of double heterojunction portions is adopted in a, and FIG. 1C shows a similar quantum well structure as the diffraction grating waveguide region 1a. This is an example adopted, and when different structures are formed in the active layer 1, the number of layers, the layer thickness, the combination of constituent elements, or the composition ratio of constituent elements is changed so that the obtained laser oscillation wavelengths are substantially equal. There is a need to.

In the third step, the above steps are reversed so that the refractive index becomes substantially symmetrical in the direction perpendicular to the laser light emitting direction. That is, the active region 1 of the quantum well structure is formed on the diffraction grating waveguide region 1a formed as described above.
Then, the cladding layer 2 is formed on the active layer 1 via the guide layer 1c.

FIG. 1B shows the diffraction grating waveguide region 1
This is an example in which a laminated structure having one or more light emitting layers formed of a plurality of double heterojunction portions is adopted as an active region 1b on a, and FIG. 1C shows an active region on the diffraction grating waveguide region 1a. This is an example in which a similar quantum well structure is adopted as the region 1b. With the above configuration, the refractive index distribution in the direction perpendicular to the laser light oscillation direction is made substantially symmetrical with the diffraction grating waveguide region 1a as the center. Therefore, it is possible to symmetrically correct the output light intensity distribution on the emission end face.

In addition to the above embodiment, a structure in which several active layers 1b in the active layer 1 are combined with a quantum well structure and a laminated structure having a double heterojunction portion can be considered. Regarding the lattice waveguide region 1a, some combinations of a quantum well structure and a laminated structure having a plurality of double heterojunctions are possible.

Further, a laminated structure or a quantum structure having a plurality of double hetero structures of a diffraction grating waveguide region 1a in which a diffraction grating is formed and active regions 1b provided above and below the diffraction grating waveguide region 1a. The well layer structure can be easily selected by optimizing the coupling efficiency with the optical fiber by changing the number of layers, layer thickness, combination of constituent elements, or composition ratio of constituent elements.

[0030]

As described above, according to the present invention, the diffraction grating waveguide region in which the diffraction grating is formed has a quantum well structure in which at least one well layer serving as a light emitting layer is laminated, or a plurality of double well layers. Since the laminated structure has one or more light emitting layers composed of heterojunction portions, the interface between the light emitting layer and the diffraction grating, which causes non-radiative recombination that is cut by the diffraction grating, is significantly reduced compared to the conventional case. However, since the non-radiative recombination at the interface portion is drastically reduced, there is an effect that a threshold current is small and a distributed feedback semiconductor laser having a high luminous efficiency can be obtained.

Further, since the active layer has a laminated structure in which the active regions are provided on the upper side and the lower side of the diffraction grating waveguide region, the light intensity distribution (perpendicular to the laser emitting direction) at the laser emitting end face is centered on the active layer. Since it can be made substantially symmetrical, there is an effect that a far field image with good symmetry can be obtained.

Further, according to the present invention, the coupling efficiency with the optical fiber is changed by changing the number of layers, the layer thickness, the combination of the constituent elements, or the composition ratio of the constituent elements in the structure of each region constituting the active layer. Has an effect that it can be easily selected to be optimum.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing the configuration of each embodiment of a distributed feedback semiconductor laser according to the present invention.

FIG. 2 is a cross-sectional view for explaining the operation of the distributed feedback semiconductor laser according to the present invention.

FIG. 3 is a perspective sectional view showing a configuration of a conventional distributed feedback semiconductor laser (refractive index coupling type).

FIG. 4 Conventional distributed feedback semiconductor laser (gain-coupled type)
3 is a cross-sectional view showing the configuration of FIG.

FIG. 5: Conventional distributed feedback semiconductor laser (gain-coupled type)
FIG. 6 is a cross-sectional view for explaining the problem.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Active layer, 1a ... Diffraction grating waveguide region, 1b ... Active region, 1c ... Guide layer, 2 ... Clad layer, 3 ... Diffraction grating,
6 ... Well layer (light emitting layer).

Claims (7)

[Claims] 1. A distributed feedback semiconductor laser including an active layer serving as a light emitting layer, wherein a diffraction grating having periodic refractive index fluctuations or gain fluctuations is formed, wherein the active layer is formed by the diffraction grating. A multilayer structure in which a light emitting layer is periodically cut by the diffraction grating, and active regions including a light emitting layer are provided above and below the diffraction grating waveguide region, respectively. A distributed feedback semiconductor laser. 2. The diffraction grating waveguide region has a quantum well structure in which one or more well layers to be a light emitting layer are laminated, or a laminated structure having one or more light emitting layers composed of a plurality of double heterojunctions. The distributed feedback semiconductor laser according to claim 1, wherein 3. The distribution according to claim 2, wherein the quantum well structure in the diffraction grating waveguide region or the laminated structure forming a double heterojunction portion is periodically cut by the diffraction grating. Feedback semiconductor laser. 4. The active region provided on the upper side and the lower side of the diffraction grating waveguide region has a quantum well structure in which one or more well layers to be a light emitting layer are stacked, or a plurality of double hetero junctions. 3. The distributed feedback semiconductor laser according to claim 1, which has a laminated structure having one or more light emitting layers. 5. In the active layer, the diffraction grating waveguide region or each active region having the quantum well structure is oscillated in the diffraction grating waveguide region or each active region having a laminated structure forming the double heterojunction. The distributed feedback semiconductor laser according to claim 1, wherein laser oscillation is obtained at a wavelength substantially the same as the wavelength. 6. The active layer has a refractive index distribution that is substantially symmetrical in a direction perpendicular to a laser light emission direction, according to any one of claims 1, 2, 3, 4, and 5. The distributed feedback semiconductor laser according to item 1. 7. A first step of forming an active region of a quantum well structure or a laminated structure forming a double heterojunction part on a clad layer formed on a semiconductor substrate, and further forming on the active region. A semiconductor material having a bandgap energy larger than the energy of oscillation laser light by forming a diffraction grating by an interference exposure method on the surface of a layer having a stacked quantum well structure or a double heterojunction structure. And a second step of forming a diffraction grating waveguide region by embedding with the above, and forming an active region of a quantum well structure or a laminated structure forming a double heterojunction on the diffraction grating waveguide region, and further activating the active region. A method of manufacturing a distributed feedback semiconductor laser, comprising a third step of forming a cladding layer on a region portion.