JPH06232499A - Distribution feedback type semiconductor laser and its manufacture - Google Patents

Abstract

PURPOSE:To provide the structure of a distribution feedback type semiconductor laser which improves coupling efficiency to optical fiber and its manufacture. CONSTITUTION:A diffraction lattice layer 4 which has a periodic structure which allows periodic refraction index variation or gain variation is provided between a guide layer 2a formed on the one of the surfaces of an activating layer 1 and a clad layer 3a. A flat area 5 which does not have such periodic structure is provided in the vicinity of the output edge of the diffraction lattice layer 4, and symmetrical refraction index distribution in the vertical direction to the activating layer 1 is allowed in the vicinity of the edge.

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 variation on either the upper side or the lower side of the active layer that becomes the optical waveguide layer. Light is distributed in a distributed manner to obtain oscillation by forming a grating layer, and a technique for forming a periodic structure of this diffraction grating layer is essential.

The period of this periodic structure is usually extremely short. For example, in order to obtain laser oscillation in the 1.3 μm band,
It becomes about 2 μm. In order to obtain such a fine periodic structure, it is common to use an interference exposure method. In this interference exposure method, a resist is applied to the surface of a semiconductor substrate having an optical waveguide layer. After that, the periodic structure of the diffraction grating is formed on the entire surface by exposing with an interference pattern formed by irradiating a laser beam from two directions, and developing and etching using this grating resist.

On the other hand, in a general semiconductor laser, since the light output from the active layer is used by being coupled to an optical fiber serving as a transmission line, the light intensity distribution at the output end is vertically and horizontally symmetrical, and The closer it is to the same circle as the fiber, the more efficient the coupling.

Therefore, its 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 in the vertical direction with respect to the active layer 1 as shown in FIG. , The light intensity distribution on the output 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.

[0008]

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 in the vertical direction about the active layer. Therefore, as shown in (c) in the figure, the light intensity distribution is also asymmetric in the vertical direction about the active layer. There was a problem that could not be efficiently combined with.

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

[0010]

A distributed feedback semiconductor laser according to the present invention has a periodical refractive index variation formed between a guide layer and a cladding layer formed on one surface side of an active layer. Alternatively, by providing a flat region without this periodic structure in the vicinity of the output end of the diffraction grating layer having a periodic structure that obtains gain variation, the refractive index distribution in the vertical direction with respect to the active layer becomes symmetrical near this output end. It is characterized by the structure. The diffraction grating layer may be arranged above or below the active layer.

In particular, the flat region provided in the vicinity of the output end of the diffraction grating layer has a length in the waveguide direction that is sufficiently shorter than the length in the waveguide direction of the distributed feedback semiconductor laser, and It is a length that does not affect the characteristics of the distributed feedback semiconductor laser itself.
10 to 20μ for a laser length (length in the waveguide direction) of μm
Built in about m.

Further, the manufacturing method is to eliminate the periodic structure by exposing a predetermined area to be the flat area over a predetermined width in advance of the interference exposure method, or the entire surface of the substrate on which the optical waveguide layer is preliminarily formed. After the periodic structure is formed by the interference exposure method, the periodic structure is eliminated by etching a predetermined region over a certain width.

[0013]

The distributed feedback semiconductor laser according to the present invention is
By providing a flat region without a periodic structure near the output end of the diffraction grating layer to obtain periodical refractive index fluctuations or gain fluctuations, the refractive index distribution in the vertical direction centered on the active layer in the part with this periodic structure Is asymmetric, but since the refractive index distribution in the vertical direction with respect to the active layer is symmetrical near the output end, the light intensity distribution at the laser output end face is guaranteed to be symmetric with respect to the active layer. .

In particular, the flat region provided in the vicinity of the output end of the diffraction grating layer has a length in the waveguide direction of 10 to 20 μm.
Since it is about m, which is sufficiently shorter than the length of the distributed feedback semiconductor laser in the waveguide direction of 300 to 1000 μm, the characteristics of the distributed feedback semiconductor laser itself are not impaired.

Further, the manufacturing method is such that a predetermined area to be the flat area is exposed over a predetermined width in advance of the interference exposure method, or the whole surface of the substrate in which the optical waveguide layer is previously formed is subjected to the cycle by the interference exposure method. After forming the structure, a predetermined portion is etched over a certain width to eliminate the periodic structure of the refractive index fluctuation or the gain fluctuation at a desired position in the diffraction grating layer.

[0016]

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. 1 is a perspective sectional view showing the structure of a distributed feedback semiconductor laser according to the present invention. This distributed feedback semiconductor laser has guide layers 2a and 2b on both surfaces of an active layer 1 serving as an optical waveguide layer. And a diffraction grating layer 4 having a periodic refractive index variation or gain variation is formed between the guide layer 2a and the cladding layer 3a on either the upper side or the lower side of the active layer 1. It is embedded (in addition,
Here, the diffraction grating layer 4 includes the guide layer 2a and the cladding layer 3a.
It refers to the part that contains the diffraction grating built into the interface with).

The diffraction grating layer 4 is not formed on the entire surface of the active layer 1 in the waveguide direction, but the flat region 5 having no periodic structure is provided near the output end.

With the structure as described above, the refractive index distribution in the direction perpendicular to the active layer 1 of the distributed feedback semiconductor laser is shown in the portion having this periodic structure (the portion indicated by AA 'in FIG. 1). As shown in FIG. 2A, it is asymmetric with respect to the active layer 1. On the other hand, since the portion having no periodic structure (the portion indicated by BB ′ in FIG. 1) is symmetrical with respect to the active layer 1 as shown in FIG. 2B, the light intensity distribution on the output end face is Is assured of vertical symmetry with respect to the active layer 1.

Next, a method of manufacturing the distributed feedback semiconductor laser having the above structure will be described with reference to FIG.

First, in the first step, on the surface of the semiconductor substrate 6 in which the optical waveguide layer is previously formed (the guide layers 2a, 2b and the cladding layer 3b are already formed in this semiconductor substrate 6). A resist 7 is applied, and a predetermined portion is exposed using a mask 8 over a certain width (see FIG. 3).
(A)).

Then, the entire surface of the semiconductor substrate 6 is exposed with an interference pattern by the interference exposure method (FIG. 3).
(B)).

Next, the surface of the semiconductor substrate 6 is developed and etched using the exposed grating-shaped resist 7 to form the diffraction grating layer 4 having the periodic structure of the diffraction grating and the flat region 5 having no periodic structure. (FIG.3 (c)).

Then, the cladding layer 3 is formed on the surface of the semiconductor substrate 6 on which the periodic structure of the diffraction grating is partially formed as described above.
A distributed feedback semiconductor laser according to the present invention is manufactured by forming a layer to be a and cutting it in the direction perpendicular to the active layer 1 using the flat region 5 having no periodic structure of the diffraction grating as an output end face (FIG. 3). (D)).

In the above embodiment, as another means for removing the periodic structure on the surface of the semiconductor substrate 6 over a certain width, the mask 8 is used for exposure, and then the interference pattern is further exposed by the interference exposure method. Even if a periodic structure is formed on the entire surface of the semiconductor substrate 6 by an interference pattern by the interference exposure method and only a predetermined portion is etched again to eliminate the periodic structure, the same effect can be obtained.

[0026]

As described above, according to the present invention, a periodic refractive index variation or gain variation formed between the guide layer formed on one surface side of the active layer and the cladding layer is obtained. By providing a flat region without this periodic structure in the vicinity of the output end of the diffraction grating layer having a periodic structure, and in the vicinity of this output end, by making the refractive index distribution in the direction perpendicular to the active layer symmetrical, Since the symmetry of the light intensity distribution centering on the active layer is guaranteed and the coupling efficiency with the optical fiber is improved by this, compared with the conventional distributed feedback semiconductor laser (for example, FIG. 5) having the same performance, A large amount of light is coupled to the optical fiber, which has substantially the same effect as increasing the light output.

Furthermore, 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 is a perspective sectional view showing a configuration of an embodiment of a distributed feedback semiconductor laser according to the present invention.

FIG. 2 is a diagram showing the refractive index distribution in the direction perpendicular to the active layer in each part of the distributed feedback semiconductor laser according to the present invention.

FIG. 3 is a diagram for explaining each step in the method of manufacturing the distributed feedback semiconductor laser according to the present invention.

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]

DESCRIPTION OF SYMBOLS 1 ... Active layer, 2a ... 1st guide layer, 2b ... 2nd guide layer, 3a ... 1st clad layer, 3b ... 2nd clad layer, 4 ... Diffraction grating layer, 5 ... Flat area, 6 ... Semiconductor substrate, 7 ... Resist, 7 ... Mask.

Claims (4)

[Claims] 1. A periodic structure for obtaining a periodic refractive index variation or gain variation is provided between a guide layer and a cladding layer formed on one surface side of an active layer to return light in a distributed manner. In a distributed feedback semiconductor laser in which a diffraction grating layer for obtaining oscillation is formed, a flat region without the periodic structure is provided near the output end of the diffraction grating layer. laser. 2. The refractive index distribution in the direction perpendicular to the active layer near the output end where the flat region is provided is:
The distributed feedback semiconductor laser according to claim 1, wherein the distributed feedback semiconductor laser is symmetrical with respect to the active layer. 3. A first step in which a resist is applied to the surface of a semiconductor substrate on which an optical waveguide layer is previously formed, a predetermined portion is exposed over a certain width, and then the entire surface is exposed by an interference pattern by an interference exposure method. And a second step of developing and etching the surface of the semiconductor substrate with an exposed resist to form a region having a periodic structure of the diffraction grating and a region having no periodic structure, and the partially forming the diffraction grating. A distributed feedback semiconductor including a third step in which a clad layer is formed on the surface of a semiconductor substrate on which a periodic structure is formed, and a region without the periodic structure of the diffraction grating is used as an output end face and cut in a direction perpendicular to the active layer. Laser manufacturing method. 4. A semiconductor substrate surface on which an optical waveguide layer is previously formed is coated with a resist, the entire surface is exposed with an interference pattern by an interference exposure method, and development is performed using the exposed grid-shaped resist. A first step of forming a periodic structure of the diffraction grating by etching; and a second step of eliminating the periodic structure by etching a predetermined portion of a surface of the semiconductor substrate on which the periodic structure of the diffraction grating is formed. A step of forming a cladding layer on the surface of the semiconductor substrate on which the periodic structure of the diffraction grating is partially formed, and cutting in a direction perpendicular to the active layer, with the region without the periodic structure of the diffraction grating as an output end face; A method of manufacturing a distributed feedback semiconductor laser, which comprises the step of 3.