JPH06232498A - Distribution feedback type semiconductor laser - Google Patents

Abstract

PURPOSE:To provide the structure of a distribution feedback type semiconductor laser which improves coupling efficiency to an optical fiber. CONSTITUTION:A diffraction lattice layer 4 which allows periodic refraction index variation and allows oscillation by distributively feeding back light is formed between a first guide layer 2a and a first clad layer 3a. A third guide layer 5, which has almost the same thickness as the diffraction lattice layer 4 and almost same refraction index distribution as the average refraction index distribution of the diffraction lattice layer 4 in a direction vertical to the activating layer, is provided on one plane side of the activating layer 1, which is the opposite side to the side whereupon the diffraction lattice layer 4 is formed, and the symmetry of light intensity distribution having the activating layer 1 as the center is ensured.

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) and the like, and more particularly to the structure of a distributed feedback semiconductor laser.

[0002]

2. Description of the Related Art Conventionally, distributed feedback semiconductor lasers have excellent single mode characteristics and are suitable for high speed operation. Therefore, in the field of using light for information transmission such as optical communication and optical information processing (optical integrated circuit), It is used as a light source.

This distributed feedback semiconductor laser is, for example, KA
ZUO SAKAI etc. "1.5μm Range InGaAsP / InP Distribute
d Feedback Lasers ", IEEE J. of Quantum Electronics
As shown in vol.QE-18 No.8, pp.1272-1278, 1982, diffraction with a periodic refractive index fluctuation on either the upper side or the lower side of the active layer that becomes the optical waveguide layer. By creating a lattice layer, light is distributed and returned to obtain oscillation.

On the other hand, since a general semiconductor laser is used by coupling the light output from the active layer to an optical fiber that serves as a transmission line, the light intensity distribution at the output end of the laser is symmetrical vertically and horizontally, Also, the closer to the same circular shape as the optical fiber, the more efficient the coupling.

Therefore, the structure is, for example, as shown in FIG.
As shown in FIG.
By providing the guide layers 2a and 2b, and the first and second cladding layers 3a and 3b, a refractive index distribution symmetrical with respect to the active layer 1 as shown in FIG. The light intensity distribution on the end face is symmetrical as shown in FIG.

On the other hand, in the structure of the conventional distributed feedback semiconductor laser, as shown in FIG. 5A, the first guide provided on the upper side of the active layer 1 (may be on the lower side). Layer 2
Oscillation is obtained by providing a diffraction grating layer 4 between a and the first cladding layer 3a and returning light in a distributed manner at the diffraction grating layer 4.

[0007]

Since the conventional distributed feedback semiconductor laser has a structure in which the diffraction grating layer is provided on either the upper side or the lower side of the active layer as described above, this laser output The refractive index distribution on the end face is shown in Fig. 5.
As shown in (b), it is not symmetrical about the active layer,
Therefore, as shown in FIG. 6C, the light intensity distribution is also asymmetric in the vertical direction with the active layer as the center, and there is a problem that it cannot be efficiently coupled to the optical fiber.

The present invention has been made to solve the above problems, and an object thereof is to provide a structure of a distributed feedback semiconductor laser which improves the coupling efficiency with an optical fiber.

[0009]

A distributed feedback semiconductor laser according to the present invention has a periodic refractive index variation between a first guide layer and a first cladding layer, and distributes light in a distributed manner. On the other surface side of the active layer, which is the side opposite to the side on which the diffraction grating layer for returning to obtain oscillation is formed, and has a thickness substantially the same as the thickness of this diffraction grating layer, and It is characterized in that a third guide layer of the diffraction grating layer having a refractive index distribution substantially the same as the average refractive index distribution in the direction perpendicular to the active layer is provided.

The diffraction grating layer may be arranged above or below the active layer.

The third guide layer has a refractive index distribution that continuously changes in the direction perpendicular to the active layer, and is substantially the same as the refractive index distribution of the diffraction grating layer in the direction perpendicular to the active layer. A single layer having a uniform refractive index distribution or a single layer having a uniform refractive index and having a uniform refractive index substantially the same as the average refractive index of the diffraction grating layer.

The third guide layer is formed by laminating a plurality of films having different refractive indexes, each of which has a uniform refractive index distribution, in addition to the single layer as described above. The diffraction grating layer has a multilayer structure having a refractive index distribution that is substantially the same as the average refractive index distribution in the direction perpendicular to the active layer.

[0013]

The distributed feedback semiconductor laser according to the present invention is
On the other surface side, which is the opposite side of one surface of the active layer on which the diffraction grating layer is formed, has the same thickness as the thickness of this diffraction grating layer, and the average refractive index of this diffraction grating layer. Since the third guide layer having the same refractive index as
The asymmetry of the refractive index distribution is relaxed, and the symmetry of the light intensity distribution around the active layer is guaranteed.

The third guide layer newly provided for ensuring the symmetry of the light intensity distribution centered on the active layer is obtained by changing the composition ratio of the material in the direction perpendicular to the active layer. Although it is desirable that the refractive index distribution is a single layer (the refractive index distribution is substantially the same as the average refractive index distribution of the diffraction grating layer in the direction perpendicular to the active layer), This diffraction grating is composed of a single layer having a refractive index substantially equal to the average refractive index and a uniform refractive index distribution, or a plurality of films having different refractive indexes each having a uniform refractive index distribution. The multilayer structure having a refractive index distribution substantially the same as the average refractive index distribution in the direction perpendicular to the active layer approximates the refractive index distribution of the diffraction grating layer, and Alleviates asymmetry.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 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. 1A is a sectional view showing the structure of the distributed feedback semiconductor laser according to the first embodiment of the present invention.

In the first embodiment, the first guide layer 2a and the first cladding layer 3a formed on the upper side of the active layer 1 are formed.
And a diffraction grating layer 4 having a periodic refractive index variation and for returning the light generated in the active layer 1 in a distributed manner to obtain oscillation.

Below the active layer 1, a second guide layer 2b having substantially the same thickness and refractive index as the first guide layer 2a and substantially the same as the first cladding layer 3a are provided. Of the diffraction grating layer 4 and the second cladding layer 3b having the same refractive index and the same thickness as the diffraction grating layer 4 and perpendicular to the active layer of the diffraction grating layer 4. Guide layer 5 having a refractive index distribution substantially the same as the average refractive index distribution in the direction
Has a structure.

As a result, the refractive index distribution in the vertical direction centered on the active layer 1 can alleviate the asymmetry as shown in FIG. 2B, and as shown in FIG. It is possible to obtain a symmetrical light intensity distribution centered on the active layer 1.

Specifically, when a 1.3 μm band distributed feedback semiconductor laser is to be produced, a semiconductor substrate is usually I.
nP is used, and the first and second cladding layers 3a and 3b are used.
Is made of InP, the first and second guide layers 2a and 2b, and the active layer 1 are made of InGaAsP having different compositions.

Here, the third guide layer 5 continuously changes its refractive index distribution in the direction perpendicular to the active layer 1 by changing the composition ratio of the material InGaAsP (the refractive index of the third guide layer 5 has been changed from the conventional one). It is known that the index depends on the composition ratio of InGaAsP which is a material), and the refractive index distribution in the direction perpendicular to the active layer is made to be substantially the same as the average refractive index distribution of the diffraction grating layer 4. .

When the refractive index of each portion in the third guide layer 5 changes with the distance from the active layer 1, it is necessary to change the composition ratio of the materials with the distance. Can be manufactured with high precision by, for example, the MOCVD method.

Next, the structure of the distributed feedback semiconductor laser according to the second embodiment of the present invention is shown in FIG.

In the second embodiment, the position and the film thickness of the third guide layer 6 are the same as those in the first embodiment described above, but the third guide layer 6 has a uniform refractive index. It is characterized by a single layer having a distribution.

That is, since the vertical refractive index distribution can alleviate the asymmetry about the active layer 1 as shown in FIG. 2B, the active layer 1 is centered as shown in FIG. As a result, an almost symmetrical light intensity distribution is obtained.

According to the second embodiment, the first
Since it is not necessary to continuously change the refractive index distribution of the third guide layer 5 as in the embodiment described above, it is possible to easily manufacture the third guide layer 6.

Next, the structure of the distributed feedback semiconductor laser according to the third embodiment of the present invention is shown in FIG.

In the third embodiment, the position where the third guide layer 7 is provided and the overall film thickness are the same as those in the first and second embodiments described above, but the third guide layer 7 is ,
It is a multi-layer structure formed by laminating a plurality of films each having a uniform refractive index, and is characterized in that the respective refractive indices gradually differ.

In particular, in the third embodiment, the third guide layer 7 is formed by laminating a plurality of single layers having gradually different refractive indexes in the direction perpendicular to the active layer 1 (FIG. 1 (a)). ), The average refractive index of the entire film is approximately the same as the average refractive index of the diffraction grating layer.

As a result, the vertical refractive index distribution in the third embodiment is as shown in FIG.
Since the asymmetry with respect to the center can be relaxed, a substantially symmetrical light intensity distribution can be obtained about the active layer 1 as shown in FIG.

Therefore, according to the third embodiment, it is not necessary to continuously change the refractive index distribution of the third guide layer 5 as in the case of the first embodiment. Easy to make.

In the first embodiment, the distributed feedback semiconductor laser using InGaAsP as the third guide layer 5 in the wavelength band of 1.3 μm has been described. The present invention is not limited to the feedback type semiconductor laser, and similarly, the refractive index distribution can be controlled and manufactured by changing the composition ratio of the material forming the third guide layer 5.

Further, the present invention has another structure of the distributed feedback semiconductor laser, for example, the presence / absence of a λ / 4 shift structure, the presence / absence of stripe width modulation, the end face reflectance, the difference in the stripe forming method and the buried structure, and the structure of the active layer. It is obvious that the present invention can be applied regardless of the above, and has the same effects as those of the above-mentioned respective embodiments.

[0034]

As described above, according to the present invention, there is a periodical refractive index fluctuation between the first guide layer and the first cladding layer, and the light is oscillated by distributed feedback. On the other surface side of the active layer, which is the side opposite to the side on which the diffraction grating layer is formed, the active layer of the diffraction grating layer having a thickness substantially the same as the thickness of this diffraction grating layer is formed. By providing the third guide layer having a refractive index distribution that is substantially the same as the average refractive index distribution in the vertical direction, the symmetry of the light intensity distribution around the active layer is guaranteed, and Since the coupling efficiency is improved, the amount of light coupled to the optical fiber is larger than that of the conventional distributed feedback semiconductor laser having the same performance (for example, FIG. 5), and an effect substantially equivalent to the increase of the optical output is obtained. is there.

Further, in various optical communication systems,
The substantial increase in the optical output has the effects of improving the S / N ratio and dramatically increasing the transmission distance.

[Brief description of drawings]

FIG. 1 shows a first distributed feedback semiconductor laser according to the present invention.
FIG. 6 is a cross-sectional view showing a configuration according to the example of FIG. 6, a view showing a refractive index distribution, and a view showing a light intensity distribution.

FIG. 2 is a second view of the distributed feedback semiconductor laser according to the present invention.
FIG. 6 is a cross-sectional view showing a configuration according to the example of FIG. 6, a view showing a refractive index distribution, and a view showing a light intensity distribution.

FIG. 3 is a third view of the distributed feedback semiconductor laser according to the present invention.
FIG. 6 is a cross-sectional view showing a configuration according to the example of FIG. 6, a view showing a refractive index distribution, and a view showing a light intensity distribution.

FIG. 4 is a cross-sectional view showing the structure of a general semiconductor laser.

FIG. 5 is a cross-sectional view showing the structure of a conventional distributed feedback semiconductor laser.

[Explanation of symbols]

1 ... Active layer, 2a ... 1st guide layer, 2b ... 2nd guide layer, 3a ... 1st cladding layer, 3b ... 2nd cladding layer, 4 ... Diffraction grating layer, 5, 6, 7 ... 3 guide layers.

Claims (4)

[Claims] 1. A first layer formed on one surface of an active layer
Distributed feedback semiconductor laser having a periodic refractive index variation between the first guide layer and the first cladding layer and having a diffraction grating layer for feedbacking light in a distributed manner to obtain oscillation. In the above, the second guide layer formed on the other surface side of the active layer and having the substantially same thickness and the same refractive index as the first guide layer and the first cladding layer, and An average refractive index distribution of the diffraction grating layer in a direction perpendicular to the active layer, having a thickness substantially the same as that of the diffraction grating layer, between the second cladding layers having a refractive index. A distributed feedback semiconductor laser, comprising a third guide layer having a refractive index distribution substantially the same as 2. The third guide layer has a refractive index profile that continuously changes in the vertical direction with respect to the active layer, and the average of the diffraction grating layers in the vertical direction with respect to the active layer. 2. The distributed feedback semiconductor laser according to claim 1, wherein the distributed feedback semiconductor laser is a single layer having a refractive index distribution substantially the same as the refractive index distribution. 3. The third guide layer is a single layer having a uniform refractive index distribution and a refractive index substantially the same as the average refractive index of the diffraction grating layer. The distributed feedback semiconductor laser described. 4. The third guide layer is formed by laminating a plurality of films each having a uniform refractive index distribution and having different refractive indexes, and is perpendicular to the active layer of the diffraction grating layer. 2. The distributed feedback semiconductor laser according to claim 1, wherein the distributed feedback semiconductor laser has a multilayer structure having a refractive index distribution substantially the same as the average refractive index distribution in the direction.