JPH04326788A - Distribution feedback type semiconductor laser - Google Patents

Abstract

PURPOSE:To enable the stable single longitudinal mode oscillation of only the objective wavelength light by providing a light absorbing layer, where the quantum effect changes cyclically in the direction of light wave guide. CONSTITUTION:A first clad layer 2 consisting of AlGaAs or the like, an active layer 4 consisting of GaAs or the like, and a guide layer 5 consisting of AlGaAs or the like are epitaxially grown on a substrate 1 consisting of GaAs or the like. A diffraction gratings 10 at required pitches, which consist of parallel- arranged grooves where the sectional form in the direction of light wave guide is saw-toothed, is made on the guide layer 5 by etching. A light absorbing layer 6 consisting of AlGaAs or the like and the second clad layer 7 consisting of AlGaAs or the like are grown epitaxially in order thereon to have such thickness as to bring about quantum effect, that is, quantum well structure. A diffraction grating 10 is provided in the vicinity of the active layer 4, and hereon a light absorbing layer 6 is provided, which performs light absorption in cycles by quantum effect. Hereby, periodical gain coupling is performed, and single longitudinal mode oscillation becomes possible.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a distributed feedback semiconductor laser, so-called DFB (Distributed Feedb).
ack) Related to lasers.

0002

BACKGROUND OF THE INVENTION A DFB laser is a laser diode that incorporates a diffraction grating to enable selective oscillation of a specific wavelength, that is, a single longitudinal mode. Research and development in various fields is remarkable for the application of transmission.

FIG. 3 shows a schematic enlarged cross-sectional view of the main parts of an example of a conventional DFB laser. As shown in FIG. 3, in this case, an active layer 22 and a p-type guide layer 23 are formed on the n-type first cladding layer 21, and the surface of the p-type guide layer 23 is etched, for example, with a sawtooth cross section. A diffraction grating 24 having a predetermined pitch Λ is formed, and a p-type second cladding layer 25 is epitaxially grown thereon to form an index-coupled DFB structure. In this case, the pitch Λ of the diffraction grating 24 is selected depending on the Bragg reflection condition, for example the holographic exposure (
The unevenness is formed by etching using a photoresist formed by a two-beam interference exposure method (two-beam interference exposure method) as a mask. 26
is an end face formed by cleavage or the like.

In this way, it is possible to achieve a single wavelength by providing the diffraction grating 24, but in this case, the laser beam may not have a single wavelength due to the use of Bragg reflection, and both end faces In a state where the reflectance of 26 is eliminated, there are two modes and a plurality of side modes, as shown in a schematic oscillation spectrum diagram in FIG. Therefore, in such an index-coupled DFB, a phase shift structure is introduced to stably obtain a laser that oscillates in the desired wavelength mode, but the manufacturing process becomes complicated and production slows down. There are problems such as being inferior in gender.

On the other hand, a DFB laser having a gain-coupled structure has also been proposed in order to more effectively perform oscillation in a single wavelength mode. This gain-coupled configuration changes the gain periodically by periodically changing the thickness of the active layer, and because it does not use Bragg reflection, it can reduce noise and is also affected by the effects of returned light. There are advantages such as less In this gain-coupled semiconductor laser, for example, as shown in a schematic enlarged cross-sectional view in FIG. By etching an uneven surface 47 in the form of a shape, and epitaxially growing a first guide layer 43 and then an active layer 44 on the uneven surface so as to fill the unevenness, an active layer having periodically different thicknesses is formed. 44 is formed, and a second guide layer 45 and a cladding layer 46 are epitaxially grown.

The pitch Λ of the diffraction grating formed by the above-mentioned uneven surface 47 can be determined by, for example, forming a photoresist mask by exposure by photolithography or by X-ray exposure through the mask, and performing etching using this as a mask. However, in this case, since the pitch is on the submicron order of about 2000 Å, it is difficult to form an uneven surface with a uniform groove inclination and period. It is difficult to obtain 44 periodic thickness control with good reproducibility.

Furthermore, in this case, since the uneven surface 47 is formed by etching, its crystallinity is poor. Therefore, the guide layer 43 and the active layer 44 that are epitaxially grown on this layer cannot be grown with good crystallinity, so there is a risk that the characteristics of the semiconductor laser will not be stable, and that there will be problems with reliability, such as the possibility of defective products. There are still problems in terms of performance, and there are various problems in practical application.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to obtain a distributed feedback type semiconductor laser in which a gain-coupled type semiconductor laser that can stably obtain single longitudinal mode oscillation occurs predominantly.

[0009]

[Means for Solving the Problems] FIG. 1 shows a schematic enlarged perspective view of an example of the distributed feedback semiconductor laser of the present invention. As shown in FIG. 1, the present invention is constructed by providing a light absorption layer 6 near the active layer 4 in which the quantum effect changes periodically in the optical waveguide direction.

0010

[Function] As described above, in the distributed feedback semiconductor laser of the present invention, the quantum effect changes periodically in the optical waveguide direction in the vicinity of the active layer 4, that is, the bandgap changes periodically, causing the light from the active layer to change periodically. A structure in which the absorption of light changes periodically and the gain changes periodically can be obtained, thereby making it possible to obtain a distributed feedback semiconductor laser that can stably perform single longitudinal mode oscillation of only light of the desired wavelength. can.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An example of the distributed feedback semiconductor laser of the present invention will be explained in detail below with reference to FIGS. 1 and 2. In this case, A
When applied to an lGaAs-based semiconductor laser, as shown in FIG.
A first cladding layer 2 made of As or the like, an active layer 4 made of GaAs or the like, and a guide layer 5 made of AlGaAs or the like are epitaxially grown in sequence by, for example, MOCVD (metal organic chemical vapor deposition), and this guide layer 5, a diffraction grating 10 having a required pitch is formed by etching, for example, a parallel array groove having a serrated cross-sectional shape in the optical waveguide direction, using a well-known method such as a holographic explosion. 150 Å thick enough to produce a quantum effect, that is, a quantum well structure.
A light absorption layer 6 made of AlGaAs or the like and a second cladding layer 7 made of AlGaAs or the like are successively epitaxially grown to have an average film thickness of about 100% or less.

FIG. 2 shows a schematic enlarged cross-sectional view of the main parts of a distributed feedback semiconductor laser having such a configuration. In FIG. 2, t indicates the thickness of the guide layer 5, and is selected from about 1000 Å to 3000 Å depending on the setting of the coupling efficiency. Further, d indicates the depth between the top portion 6T and the valley portion 6V of the diffraction grating 10, and is, for example, 1000 Å. Also, this diffraction grating 1
The pitch Λ of 0 is, for example, 2000 Å.

The thickness of the light absorption layer 6 is set to be about 150 Å or less to produce a quantum effect, but when epitaxially growing on the diffraction grating 10 by the MOCVD method or the like, the thickness at the top portion 6T is t1 and the thickness at the valley 6V is t2, then t1 < t2
Therefore, if the band gaps between the top 6T and the valley 6V are Eg1 and Eg2, respectively, Eg1
>Eg2.

Therefore, by appropriately selecting the composition of the light absorption layer 6, the active layer 4, and the guide layer 5 on the active layer 4, that is, the Al content in this case, the band gap of the active layer 4 can be set to Eg3. When the band gap of the guide layer 5 is Eg4, Eg1 >Eg4 >Eg3 >
Eg2.

Therefore, the light leaking from the active layer 4 to the guide layer 5 is absorbed only in the troughs 6V of the light absorption layer 6 which are periodically formed above the guide layer 5. Thus, a distributed feedback semiconductor laser capable of performing periodic gain coupling can be obtained.

In this way, the diffraction grating 1 is placed near the active layer 4.
By providing a light absorbing layer 6 on which light absorption layer 6 periodically absorbs light due to a quantum effect, periodic gain coupling can be dominantly performed. It is possible to perform mode oscillation.

Further, as described above, after the active layer 4 is epitaxially grown, the diffraction grating 10 is formed on the guide layer 5 thereon.
By providing the light absorption layer 5 after the formation of the active layer 4, the crystallinity of the active layer 4 can be maintained well.

Furthermore, when the light absorption layer 6 is epitaxially grown on the diffraction grating 10 as described above, the thickness can be made to change spontaneously and periodically without any particular control. The process is simple, and productivity can be improved compared to conventional distributed feedback semiconductor lasers having a phase shift structure.

The present invention is not limited to the above-mentioned AlGaAs-based semiconductor laser, but can be applied to various distributed feedback semiconductor lasers such as InGaAsP-based semiconductor lasers. Further, the present invention is not limited to the structure according to the above-mentioned manufacturing method, but may be any structure in which the quantum effect of the light absorption layer 6 changes periodically in the optical waveguide direction.
In addition, for example, if the guide layer has a multilayer structure and the refractive index of each layer is appropriately selected so that the average refractive index is made uniform in each part, the influence of the index coupling type can be further avoided. It is possible to achieve a more single wavelength.

[0020]

As described above, according to the distributed feedback semiconductor laser of the present invention, single longitudinal mode oscillation can be performed reliably and effectively by gain coupling.

Furthermore, since the active layer can be constructed with good crystallinity, reliability can be improved, that is, generation of defective products can be suppressed, and yield can be improved.

Furthermore, by forming the diffraction grating 10 and epitaxially growing the light absorption layer 6 thereon as described above, it is possible to simplify the manufacturing process and improve productivity.

[Brief explanation of the drawing]

FIG. 1 is a schematic enlarged perspective view of an example of a distributed feedback semiconductor laser according to the present invention.

FIG. 2 is a schematic enlarged cross-sectional view of a main part of an example of the distributed feedback semiconductor laser of the present invention.

FIG. 3 is a schematic enlarged cross-sectional view of an example of a conventional distributed feedback semiconductor laser.

FIG. 4 is an oscillation spectrum diagram of a distributed feedback semiconductor laser.

FIG. 5 is a schematic enlarged cross-sectional view of an example of a conventional distributed feedback semiconductor laser.

[Explanation of symbols]

1 Board 2 First cladding layer 4 Active layer 5 Guide layer 6 Light absorption layer 7 Second cladding layer 10 Diffraction grating

Claims (1)

[Claims] 1. A distributed feedback semiconductor laser characterized in that a light absorption layer in which a quantum effect changes periodically in the optical waveguide direction is provided in the vicinity of an active layer.