JPH07106697A - Distributed feedback semiconductor laser - Google Patents

Abstract

PURPOSE:To obtain a gain coupled distributed feedback semiconductor laser without forming an irregular plane. CONSTITUTION:A quantum well layer 6 sandwiched by a buffer layer 5 and a clad layer 8 in the vicinity of an active layer 4 exhibits light absorbancy at the oscillation wavelength of the active layer 4. The quantum well layer 6 comprises a first region disordered at a period for applying the distributed feedback along the optical axis of induced emission, and a second region having same conductivity as the buffer layer 5 and the clad layer 8, wherein the first region has a light absorbance lower than that of the second region.

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 in the fields of optical communication, optical measurement and optical information processing, and in particular, a distributed feedback type semiconductor laser capable of obtaining a spectrum of a single longitudinal mode, so-called DFB ( Distributed FeedBack) Laser.

[0002]

2. Description of the Related Art Distributed feedback semiconductor lasers (hereinafter referred to as DFB)
It is called a laser. ) Is to provide a periodic structure along the traveling direction in the optical waveguide, and oscillate light having a wavelength satisfying the Bragg condition determined by the period by the feedback of light using the Bragg reflection. The return method is
There are a refractive index coupled type that changes the refractive index periodically and a gain coupled type that changes the gain periodically.

In the refractive index coupling type DFB laser, a non-light emitting region called a stop band exists in a region centered on the Bragg wavelength due to interference of lights having different traveling directions.
The stimulated emission light has two or more longitudinal modes degenerated on both sides of this region, and stable single longitudinal mode oscillation is difficult. As a means to solve this problem,
A DFB laser provided with a phase shift region in the diffraction grating is considered to be promising, but a high technology is required to give a predetermined phase shift to a predetermined position of the diffraction grating, which results in an increase in cost and a reduction in yield. It is the cause. The phase shift also causes spatial hole burning, and
This causes non-linearity of current characteristics and increases in line width.

On the other hand, in the case of a gain-coupling type DFB laser in which the gain is changed periodically, there exists a longitudinal mode that exactly matches the wavelength according to the period of the gain, and single longitudinal mode oscillation is performed. As a method of periodically changing the gain, there is a method of alternately providing a gain region and an absorption region along the optical axis. In this method, the perturbation amount of the gain coefficient is larger than the average value of the gain coefficient due to the alternating existence of the gain region and the absorption region, as compared with the case where one of the gain coefficient and the absorption coefficient has a periodic change. There is an advantage that it becomes large and a large gain difference between modes can be obtained.

As a method of alternately providing the gain region and the absorption region, there is a method of forming an uneven surface in which the conductivity and the absorption coefficient change in the same cycle as shown in FIG. -55686). This is the active layer 24
In a semiconductor distributed feedback type laser device having a periodic structure for performing distributed light feedback on the stimulated emission light from the active layer 24 arranged along the periodic structure, the periodic structure makes the conductivity periodic along the optical axis direction of the laser. The absorption coefficient for the stimulated emission light is changed in the same period as the periodic structure. That is, a current blocking layer that absorbs the light generated by the active layer 24 and blocks the current injected into the active layer 24 is periodically provided above or below the active layer 24 that generates stimulated emission light. A current density distribution is generated in the active layer 24, and a periodic distribution of the light absorption coefficient is also generated. as a result,
It is said that a stronger gain coupling property can be obtained, the current utilization efficiency can be further improved, and a lower threshold current operation can be expected.

The disclosed method for obtaining this structure is to provide two semiconductor layers 21 and 22 having different conductivity from each other across the active layer 24, and at least one of the two semiconductor layers, It is said that a conductive diffraction grating 27 different from that of the semiconductor layer is embedded. A semiconductor layer having substantially the same composition as the active layer 24 is provided on the active layer 24 with the buffer layer 28 interposed therebetween, and the semiconductor layer is etched to form a diffraction grating 27. The conductivity of the layer is said to be different from the conductivity of the layer in which it is embedded (see FIG. 5).

[0007]

However, this Japanese Unexamined Patent Application Publication No.
In the distributed feedback laser described in the -55686 publication,
The growth is interrupted once and a step of marking unevenness by etching is required. Re-growth on an uneven surface is difficult to grow with good crystallinity as compared with the case of re-growth on a flat surface, and the characteristics as a semiconductor laser are inferior. In order to improve the crystallinity of the epitaxially regrown layer on the uneven surface, there are various restrictions on the crystal orientation, the crystal composition, etc., and high technology is required. Further, the shape of the unevenness is deteriorated by the regrowth after the unevenness marking. In order to prevent this deterioration, a complicated manufacturing process such as a multi-step temperature rising process while switching the atmosphere gas and the introduction of a deterioration preventing thin film layer is required.

[0008] As described above, the technique requiring the formation and regrowth of the uneven surface requires a complicated manufacturing process, resulting in an increase in cost, and there is a possibility that the characteristics as a semiconductor laser are not stable, and a defective product is There are problems in reliability such as easy occurrence, and there are various problems in practical use.

In order to obtain a narrow spectral linewidth, which is an important characteristic of a single mode laser, it is necessary to reduce the linewidth enhancement coefficient determined by the laser structure. For this purpose, in addition to the gain coupling component, It is known that a large effect can be obtained by positively utilizing the refractive index coupling component. However, in the distributed feedback laser described in Japanese Patent Laid-Open No. 5-55686, a pure gain coupling coefficient is obtained. Therefore, a material and a shape that cancel the periodic distribution of the refractive index generated in the absorption layer and the active layer are selected, and no consideration is given to reducing the line width increase coefficient.

[0010]

Therefore, in the distributed feedback semiconductor laser of the present invention, a periodic structure for performing distributed feedback is provided in the vicinity of the active layer 4, and this periodic structure is regarded as a disordered region. A difference in light absorption was introduced between the non-disordered region and the non-disordered region so that an uneven structure due to etching was not generated in the production of this periodic structure. That is, an active layer 4 that generates stimulated emission light, a buffer layer 5 having a first conductivity provided on the active layer 4,
The quantum well layer 6 having optical absorption at the oscillation wavelength of the active layer 4 provided on the buffer layer 5, and the clad layer 8 having the same conductivity as the buffer layer 5 provided on the quantum well layer 6.
And the quantum well layer 6 is disordered at a period for performing distributed feedback along the optical axis direction of the stimulated emission light and is between the first region 9 having the first conductivity and the regions. And a second region 10 having a conductivity opposite to that of the first region, and the first region 9 has a light absorption rate lower than that of the second region 10.

[0011]

The operation of the present invention will be described with reference to FIGS. 1, 2 and 3. FIG. 1 is a sectional view of a distributed feedback semiconductor laser of the present invention cut along the optical axis direction. Figure 2
Is an enlarged sectional view of the vicinity of the active layer 4 on the same plane as FIG.
It is the figure which showed the relationship of the gain and loss in the said part, and the change of the refractive index. FIG. 3 is a diagram showing the relationship between wavelength and light absorption in each case of a bulk structure, a so-called disordered structure which is disordered by impurity diffusion, and a quantum well structure. As described above, the light absorption in the disordered region is lower than that in the quantum well structure. This fact will be explained below.

First, the buffer layer 5 having the first conductivity is provided on the active layer 4 for generating stimulated emission light, and the buffer layer 5 is provided with light at the wavelength of the stimulated emission light from the active layer 4. An absorptive quantum well layer 6 is provided. The quantum well layer 6 has a conductivity opposite to that of the buffer layer 5. Then, the clad layer 8 having the same conductivity as the buffer layer 5 is provided on the quantum well layer 6. Impurities having different conductivity in the quantum well are diffused into the quantum well layer 6 at a period required for distributed feedback. By diffusing the impurities, the region, that is, the first region 9 is disordered, and the light absorption is made lower than that of the region in which the impurities are not diffused, that is, the second region 10. Second region 10 in which impurities are not diffused
The light absorption is unchanged.

The first region 9 thus impurity-diffused
Has the same conductivity as the upper clad layer 8 and the lower buffer layer 5, so that current flows only in that portion,
Gain is obtained only in the active layer 4 below this region. That is, as a result, alternately in the traveling direction of light,
Light is amplified in the active layer 4 below the disordered first region 9, and light is absorbed in the other second region 10 to form gain coupling. As described above, since the gain regions and the absorption regions are alternately present on the active layer 4, a large inter-mode gain difference can be obtained as compared with the case where either the gain coefficient or the absorption coefficient has a periodic change.
As a result, the current utilization efficiency is increased and the device operates at a lower threshold current.

In addition, the disordered first quantum well layer 6 is formed.
Since the region 9 has a low refractive index and a low refractive index region, and the second region 10 in which impurities are not diffused has a high refractive index region, the gain and the refractive index have opposite phases.
The line width of the laser light can be reduced by the so-called equivalent phase shift in which the position of the antinode of the standing wave of light in the resonator shifts as compared with the case of periodic change of gain only. As a result, it is possible to obtain a DFB laser capable of stably performing the single longitudinal mode oscillation of only the intended light.

[0015]

EXAMPLE A procedure for producing the DFB laser of the present invention will be described with reference to FIGS. First, the n-type InP substrate 1 (1
0) n-type InP clad layer 2 with a thickness of 1 μm
(Carrier concentration 1 × 10 ₁₈ cm −3), active layer 4, thickness 3
00 angstrom p-type InP buffer layer 5 (carrier concentration 1.5 × 10 ₁₈ cm −3), n-type multi-structure quantum well layer 6 (carrier concentration 1 × 1) used for distributed feedback.
0 ₁₈ cm −3) is grown sequentially.

For the active layer 4, the guide layer 39 is formed to have a thickness of 500 angstroms 0.99 and 1.08 μm, respectively.
A two-stage distributed refractive index type separate confinement heterostructure (GRIN-SCH structure) having an m composition is used, and the quantum well layer 40 inside the active layer 4 is 45 angstroms thick, 1.38 μm.
Composition of well layers, and 85 Å thickness,
The barrier layer having a composition of 08 μm has a 10-period structure, and the oscillation wavelength is 1.31 μm.

In this embodiment, the quantum well layer 6 used for distributed feedback also has a multiple quantum well structure. The quantum well layer 6 has a thickness of 45 Å, a well layer having a composition of 1.45 μm, and a thickness of 85 Å, 1.08.
The barrier layer having a composition of μm has a two-period structure. Next, 1.3
A diffusion mask is formed at a pitch of 2050 angstroms for Bragg reflection of light having a wavelength of 1 μm, Zn is selectively diffused as a p-type impurity, and then the diffusion mask is removed. The diffusion depth is set to 300 Å so as to stop in the p-type InP buffer layer 5 below the quantum well layer 6. The periodic structure required for distributed feedback can be obtained by using an ion implantation method in addition to the impurity diffusion method.

A first region 9 disordered by impurity diffusion and a second region 10 between them are formed, and a gain is obtained only in the active layer 4 below the first region 9. In addition, the first region 9 becomes a region that transmits light due to disordering of the quantum well layer 6. Further, the first region 9 becomes a low refractive index region due to disordering of the quantum well layer 6, and the second region 10 forms a high refractive index region.

After that, a p-type InP cladding layer 8 (carrier concentration: 1.5 × 10 ₁₈ cm −3) having a thickness of 2.5 μm is grown. And 1.5μ so that lateral mode control is performed.
An m-width mesa waveguide is formed by etching, and p-type InP and n-type InP current blocking layers are grown on both sides of the mesa waveguide (not shown). Finally, on the substrate side n
A p-type electrode 12 is formed on the clad layer 8 on the active layer 4, and a p-type electrode 12 is formed on the clad layer 8.

In this embodiment, an example in which it is formed on an n-type substrate is shown, but the same effect can be obtained even when it is formed on a p-type substrate.

Further, in the present invention, as in another embodiment shown in FIG. 4, a clad layer 8 is grown on the substrate 1, a quantum well layer 6 is provided thereon, and a buffer layer 5 is interposed. A structure in which the active layer 4 is provided is also effective. In the present invention, since the distributed feedback structure is provided by diffusing impurities, the surface of the quantum well layer 6 is smooth, and growth on the quantum well layer 6 does not adversely affect the growth of the active layer or the like. .

[0022]

The distributed feedback semiconductor laser of the present invention has the following effects. (1) First, in the DFB laser of the present invention, a periodic structure required for distributed feedback is formed by impurity diffusion to form a flat structure without unevenness. Therefore, not only the crystallinity of the epitaxial regrowth is improved, but also the structure is simplified, so that the manufacturing is simplified and the yield and the reliability are improved. (2) Next, the gain and the refractive index have opposite phases, that is, the high gain region has a low refractive index and the low gain region has a high refractive index, so that the line width of the laser light can be reduced. (3) Further, since the conductivity of the quantum well layer, the light absorption coefficient and the periodical change of the refractive index are simultaneously formed by diffusion, the manufacturing becomes extremely easy.

[Brief description of drawings]

FIG. 1 is a sectional view of a distributed feedback semiconductor laser of the present invention.

FIG. 2 is an enlarged cross-sectional view in the vicinity of an active layer of a distributed feedback semiconductor laser of the present invention, showing the relationship between gain and loss and the change in refractive index in that portion.

FIG. 3 is a diagram showing a relationship between wavelength and light absorption in an active layer.

FIG. 4 is a sectional view of another embodiment of the present invention.

FIG. 5 is a diagram showing a conventional distributed feedback semiconductor laser.

[Explanation of symbols]

 4 Active layer. 5 Buffer layer. 6 Quantum well layer. 8 Clad layer. 9 First area. 10 Second area.

Claims (1)

[Claims] 1. An active layer (4) for generating stimulated emission light, a buffer layer (5) having a first conductivity provided on the active layer, and an active layer provided on the buffer layer. A quantum well layer (6) having a light absorption property at an oscillation wavelength of, and a clad layer (8) having the same conductivity as the buffer layer provided on the quantum well layer. A first region (9) which is disordered at a period for performing distributed feedback along the optical axis direction of the stimulated emission light and which has the first conductivity, and the first conductivity between the regions. And a second region (10) having a conductivity opposite to that of the first region, wherein the first region has a light absorption rate lower than that of the second region.