JPS62173786A - Distributed feedback type semiconductor laser - Google Patents

Abstract

PURPOSE:To obtain a laser of extremely stable single longitudinal mode oscillation by setting the band gap of an absorption layer provided periodically along a light propagating direction for absorbing a light from an active layer to the band gap or less of the active layer. CONSTITUTION:An N-type clad layer 2 and an active layer 3 are sequentially formed on an N-type substrate 1. A block layer 4 for enclosing carrier in the layer 3 is formed on the layer 3, and a guide layer 5 for introducing a light from the layer 3 to a diffraction grating 7 is formed on the layer 4. Then, after a GaAs layer having a band gap equal to or smaller than that of the layer 3 is formed, a diffraction grating 7 and periodically divided absorption layers 6 are formed by a holographic exposure method. Then, after a clad layer 8 having a refractive index smaller than that of the layer 5 is formed, an electrode 10 is formed through a cap layer 9, and other electrode 11 is formed on the back surface of the substrate 1. With this structure, a single mode oscillation near a black wavelength can be performed.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a distributed feedback semiconductor laser.

[Summary of the invention]

The present invention provides stable distributed feedback semiconductor lasers by providing a periodic light absorption layer near the active layer, periodically changing the laser gain, and periodically modulating the imaginary part of the refractive index. It is designed to perform single wavelength operation.

[Conventional technology j Distributed feedback semiconductor laser (hereinafter abbreviated as DFB laser)
Since it is easy to operate at a single wavelength, it is expected to be used as a light source for long-distance, large-capacity optical fiber communications. However, a conventional DFB laser that uses a periodic change in only the real part of the refractive index given by the periodic irregularities of a diffraction grating has a possibility of bimodal oscillation. In order to make this a single mode oscillation, it is necessary to introduce a λ/4 shift into the center of the diffraction grating.

By the way, a DFB laser in which a periodic change is also introduced in the imaginary part of the refractive index becomes single-longitudinal mode oscillation without any other special measures. Conventionally, as a proposal regarding this method, one is a method of periodically dividing the active layer (Japanese Patent Application Laid-Open No. 10278-1989).
8), and others include a method of forming periodic active layers during growth (Japanese Unexamined Patent Publication No. 102789/1989).

[Problem that the invention seeks to solve]

However, since the former method described above includes a step of periodically dividing or scraping the active layer, it is expected that non-radiative recombination will increase and the lifetime will be shortened. Furthermore, in the latter method, it is difficult to stabilize the transverse mode.

In view of the above points, the present invention provides a DF that operates at a single wavelength by periodically modulating the imaginary part of the refractive index without damaging the active part.
B laser.

[Means for solving problems]

The present invention provides a DFB laser having a cladding layer (21 (81), an active layer (3), and a guide layer (5) for guiding light from the active layer (3) to a diffraction grating). 3) is provided with an absorption layer (6) having a bandgap equal to or smaller than the bandgap of the active Jif (31).

The absorption layer (6) absorbs the light from the active layer (3), and is provided periodically along the direction in which the light travels.

Further, a diffraction grating (7) is formed from periodic uneven portions including this absorption layer (6).

[Effect]

According to the above configuration, the light seeping out from the active layer (3) is absorbed by the periodically existing absorption layer (6), and the laser gain of the absorbed portion is reduced. As a result, D
The laser gain seen in the longitudinal direction of the FB laser changes periodically, and the imaginary part of the refractive index changes periodically. Therefore, in addition to the periodic change in the real part of the refractive index due to the diffraction grating, the imaginary part also changes periodically, and the single-longitudinal mode oscillation near the black wavelength becomes r+J function.

With this configuration, the active layer is not damaged, so an increase in non-radiative recombination and a shortening of the lifetime can be avoided.

〔Example〕

Hereinafter, embodiments of the DFB laser according to the present invention will be described with reference to the drawings.

FIG. 1 shows an example of a GaAs-based DFB laser.

In the wavelength range of 750 nm to 900 nm, the material is GaAs-based. In this example, an n-GaAs substrate (1) is
- Crat layer (2) made of AlGaAs and GaAs
An active layer (3) of (or AIGaAs) is sequentially formed.

The kraft layer (2) and the active layer (3) have a normal structure, and the cladding layer (2) has a high hel ratio. A thin p-^lGaA3 block layer (4) with a slightly higher Hel ratio is formed on this active layer (3) in order to confine carriers in the active layer (3). For example, an InGaAsP guide f# (5) is formed to guide light from the layer (3) to a diffraction grating (7) to be described later.

Guide I! For (5), one piece of AlGaAs may be used, but here, rnGaAs which is lattice matched to AlGaAs is used.
PJ- was used. This is effective for suppressing oxidation as much as possible when forming a diffraction grating in the next holographic exposure method.

Next, Ga is used as an absorption layer having a bandgap equal to or smaller than the bandgap of the active layer (3).
After forming Asjffl, a diffraction grating (7) is formed by a holographic exposure method, and at the same time an absorption layer (6) having periodic portions is formed. The blood exposed to the air during the formation of this diffraction grating (7) is A to suppress oxidation as much as possible.
A tsue that does not include I is desirable. At this point, in this example, G
aAs absorption layer (mu) and InGaAsP guide Pt ff
Since only 1 liter is exposed to air, interfacial oxidation problems are not reduced. Absorption IN (61 GaAs1t#
may be n-type or p-type, but in this example n-Ga
It showed As1m. Next, a p-AlGaAs cladding layer (8) is formed as usual. The refractive index of this cladding layer (8) needs to be smaller than the refractive index of InGaAsP. Next, one electrode (10) is formed through the GaAs gap layer (9), and the other electrode (11) is formed on the back surface of the n-GaAs substrate (1) to form a DFB laser (12).
Configure.

According to this configuration, light from the active layer (3) is absorbed by the periodically arranged absorption layers (6), thereby reducing the laser gain in that portion. Therefore, the laser gain seen in the longitudinal direction of the DFB laser changes periodically, and the imaginary part of the refractive index changes periodically.

Therefore, the feedback of light as a DFB laser is determined by the diffraction grating (
In addition to refractive index type coupling that utilizes periodic changes in the real part of the refractive index as described in 7), gain type coupling utilizes periodic changes in the imaginary part of the refractive index, and a single-longitudinal mode near the black wavelength is generated. The release becomes 1 and IJ ability.

In the configuration shown in FIG. 1, when an n-type absorber layer (6) is used, the absorber layer (6) acts as a periodic narrowing layer (that is, a guide) @ (51 and If the layer (4) is made sufficiently thin, the current injected into the active layer (3) becomes periodic, forming a periodic gain distribution, which strengthens the effect of the periodic absorption layer (6). It fits.

FIG. 2 is an example of a DFB laser using InP thread.

In the wavelength range of 1.3 μm to 1.5 μm, it becomes TnP type.

In this example, the n-InP substrate (21) is
A cladding layer (22) is formed, and an active layer (26) is formed on this.
p-type or n-type, in this example p-1, with a bandgap smaller than or equal to the bandgap of
An nGaAsP absorption layer (23) of 1 m is grown, and then a diffraction grating (24) is formed by holographic exposure. This causes the absorption 15 (23) to be divided periodically. Next, the n-1nGaAsP guide layer (25), rnG
aAsP active layer (26) and p-1nP cladding! @
(27) is grown to form electrodes (28) and (29) to constitute a DFB laser (30). In addition, the diffraction grating (
24), the absorption layer (23) may be formed on the p side from the active 1'1 (2B).

In the case of this configuration as well, the laser gain is changed periodically by the periodic absorption layer (23) in the same way as explained in FIG.
In addition to the real part of the refractive index, the imaginary part also changes periodically, allowing single-longitudinal mode oscillation.

〔Effect of the invention〕

According to the present invention described above, in a DFB laser, by having a light absorption layer having a band gap equal to or smaller than the band gap of the active layer in the vicinity of the active layer, the refractive index can be adjusted without damaging the active layer. It is possible to give a periodic change IM to the imaginary part of
A DFB laser with longitudinal mode oscillation can be realized.

Description of Drawings FIG. 1 is a sectional view showing one embodiment of the DFB laser according to the present invention, and FIG. 2 is a sectional view showing another embodiment of the DFB laser according to the present invention.

(1) is the substrate, (2) is the cladding layer, (3) is the active layer,
(4) is a block layer, (5) is a guide layer, (6) is an absorption layer, (7) is a diffraction grating, (8) is a cladding layer, (10)
, (11) is an electrode.

Claims (1)

[Claims] A distributed feedback semiconductor laser comprising a cladding layer, an active layer, a guide layer, and an absorption layer, wherein the bandgap of the absorption layer is equal to or smaller than the bandgap of the active layer. laser.