JPS63111689A - Distributed feedback type semiconductor laser - Google Patents

Abstract

PURPOSE:To obtain a distributed feedback type semiconductor laser of surface luminescence which has a small light-emitting spot and an excellent light emission efficiency, by making it possible to take out a laser beam from an optical waveguide layer in the direction not parallel to this layer by a high-order diffraction grating. CONSTITUTION:A primary diffraction grating 12a and a secondary diffraction grating 12b are formed on an n-InP substrate 11, and thereafter an n-InGaAsP guide layer 13, an InGaAsP active layer 14, a P-InP clad layer 15 and a P-InGaAsP cap layer 16 are made to grow in crystal. Then, an electrode 17 is formed on the P-InGaAsP cap layer 16 except for a part corresponding to a part wherein the secondary diffraction grating 12b is formed, and an electrode is formed on the n-InP substrate 11. When a current is injected into a semiconductor laser having such a construction, a light generated in the InGaAsP active layer 14 is turned into a laser beam uniform in phase by the diffraction grating 12a and 12b. On the occasion, a component vertical to the guide layer 13 is generated in the secondary diffraction grating 12b, while only a component parallel to the guide layer 13 is present in the primary diffraction grating 12a, and thus a part of the laser beam is taken out from the secondary diffraction grating 12b as a laser beam 19 vertical to the surface of the P-InGaAsP layer 16.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a distributed feedback semiconductor laser used as a light source for, for example, optical fiber communication or optical measurement.

[Conventional technology]

Figure 4 is a magazine (D, R, 5cifres et al.
, 8pp1. Phys.

Lett,, vol, 26. p48-50 (1975
)) is a perspective view showing the conventional distributed feedback (DFB) type semiconductor laser shown in FIG.
aAs substrate, (2) P-Gao.

Ale, , As cladding layer, (3) is a diffraction grating corresponding to the second-order Bragg reflection condition, (4) is a P-GaAs layer, (5) is a 1-GaAs layer, (6) is a The provided Au reflective film (7) is a laser beam. Here, the P-GaAs layer (4) and the n-GaAs layer (5) act as an active layer and an optical waveguide layer.

Next, the operation will be explained.

First, regarding the operation of a diffraction grating, a magazine (D, R.

5cifres et al., Appl., Phys.
Lett-+vol-26+ p48-50 (197
5) This will be explained using FIG. 5 with reference to the description in ).

The light (21) traveling rightward and parallel to the diffraction grating is reflected at an angle θ that satisfies the Bragg reflection condition. Since the phases of the wavefronts (22) of the reflected lights must be aligned, the optical path length difference of each reflected light is equal to the wavelength in the medium λ
. /n, is an integer multiple. Therefore β ′ λ.

b ten people -- (A' = 0.1, 2...) −
・−・−−−−−(1) is satisfied. 8 is the period of the diffraction grating, λ. is the wavelength of light in vacuum, n9 is the refractive index of the medium, and b is the length shown in FIG. By the way, b−ΔSinθ −・−・−
-----(2). However, θ is the difference between the wavefront (22) and the plane of the diffraction grating (3). Equation (1) and equation (2
), n98 is obtained. Here, if we consider a second-order diffraction grating (3), Δ is λ. /n*. As a result, equation (3) can be rewritten as follows.

Sinθ=x'-1(7!'=0.1.2)
----------(41A'=O (θ=-1) propagates as it is in the traveling direction, β'=2 (θ=-) indicates the reflected light that returns in the opposite direction to the incident light.A'- 1 becomes θ=0 and is reflected perpendicularly to the surface of the optical waveguide, that is, the diffraction grating (3).

Next, the configuration and operation of FIG. 4 will be explained.

First, an S cladding layer (2) is grown on a P-GaAs substrate (11), and a diffraction grating (3) corresponding to the second-order Bragg reflection condition is formed.Next A P-GaAs layer (4) and an n-GaAs layer (5) are grown to form a branch feedback (DFB) type semiconductor laser having a single heterojunction.

When current is injected from P and n electrodes (not shown), P-G
Light is emitted at the P-n junction of the aAs layer (4) and the n-GaAs layer (5), and from the second (3), not only the component parallel to the plane of the diffraction grating (3) but also the upper n-GaAs layer ( 5), a vertical laser beam can be extracted from the laser beam, and it can function as a surface-emitting type semiconductor laser. For this reason, n-Ga
A portion of the electrode provided on the As layer (5) is removed to serve as a laser beam extraction section. In addition, from 2 (3),
It is coupled into layer (5) but is not extracted to the outside due to total internal reflection. In addition, in order to prevent laser light from being extracted from separated end faces, an Au reflective film (6) is placed on both end faces.
is provided. This surface-emitting type semiconductor laser has a theoretically smaller divergence angle of 0.21° than an edge-emitting type laser.

[Problem that the invention seeks to solve]

Conventional surface-emitting distributed feedback semiconductor lasers are configured as described above, and because they emit light vertically over the entire surface of the diffraction grating, the emission spot becomes large, making coupling with external optical devices difficult. In addition, the laser light is also reflected at the part where the electrode is provided, but this laser light is not extracted to the outside and becomes a loss, resulting in a problem that the luminous efficiency deteriorates.

The present invention was made to solve the above-mentioned problems, and an object of the present invention is to obtain a surface-emitting distributed feedback semiconductor laser with a small light emission spot and high light emission efficiency.

[Means for solving problems]

The distributed feedback semiconductor laser according to the present invention includes a first-order diffraction grating that satisfies the Bragg reflection condition provided in a part of the optical waveguide layer, and a second-order diffraction grating provided in the other part of the optical waveguide layer.
It is composed of a diffraction grating of a higher order than the next order, and the high-order diffraction grating makes it possible to extract laser light from an optical waveguide layer in a direction that is not parallel to the optical waveguide layer.

[Effect]

The diffraction grating in this invention is composed of a first-order diffraction grating and a higher-order diffraction grating, and the laser beam is extracted in a direction that is not parallel to the optical waveguide layer only at the higher-order diffraction grating.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. 1st
In the figure, (Ill is an n4nP substrate, (12a) is an n4nP substrate,
A first-order diffraction grating formed on a part of the -1nP substrate, (
12b) is a higher-order, e.g., second-order, diffraction grating formed elsewhere on the n-1nP substrate αυ; A crystal grown optical waveguide layer, for example n-1nG
aAsP guide layer, 04+ is the active layer grown on this guide layer 01, InGaAsP active layer, α9 is P-1
An nP cladding layer, Q61 is a P-1nGaAsP cap layer, αD is a P electrode, a mark is an n electrode, and 0 is a laser beam.

Next, the operation of this invention will be explained.

First, a first-order diffraction grating (12a
) to form a second-order diffraction grating (12b), and then n-1
nGa-AsP guide layer 01% InGaAsP active layer α0, P-InPn layer 50, P-1nGaAs
The P camp layer Q61 is crystal grown to form a second-order diffraction grating (
An electrode αη is formed on the P-1nGaAsP cap layer 00 except for a portion corresponding to the portion where 12b) is formed, thereby forming an α-type electrode on the n-InP substrate.

When a current is injected into the semiconductor laser made in this way, the light emitted from the InGaAsP active layer becomes laser light with the same phase due to the diffraction gratings (12a) and (12b), but from equation (3), Although it does not exist in the first-order diffraction grating (12a), there is a vertical component in the second-order diffraction grating (12b). Therefore, a part of the laser beam is extracted from the second-order diffraction grating (12b) as a laser beam α wave perpendicular to the P-1nGaAsP N(16) plane.

The ratio of light extracted perpendicularly to light propagating through the guide layer 0 can be changed depending on the length and shape of the regions in which the diffraction gratings (12a) and (12b) are formed.

As described above, in this embodiment, the laser beam extracted in the vertical direction is extracted by the second-order diffraction grating (12b), and the part that is not extracted in the vertical direction is extracted by the first-order diffraction grating (12b).
12a), it is possible to prevent a decrease in luminous efficiency.

Laser light extracted from the vertical plane can be used, for example, as monitor light.

In another embodiment shown in FIG. 2, a reflective film (2 m) using an interference film is attached to both end faces of the semiconductor laser, and all the laser output can be taken out from the P-1nGaAsP camp layer αe.This reflective film C!0 For example, SiO□
and Ti1t, each with a thickness of 174 wavelengths, are laminated alternately, and the same < A I! , zos and Si
This can be realized by using ng.

In general, distributed feedback semiconductor lasers tend to oscillate at two wavelengths that are close to each other and are unstable, but in order to oscillate at a single stable wavelength, it is necessary to use a second-order diffraction grating (12b) as shown in Figure 3. It is sufficient to shift the phase by a quarter wavelength with respect to the first-order diffraction grating (12a).

In the above embodiment, an example using a second-order diffraction grating is shown, but a second-order or higher-order diffraction grating may be used.

Further, in the above embodiment, an InP-based semiconductor laser was used, but a GaAs-based or other material-based semiconductor laser may be used.

〔Effect of the invention〕

As described above, according to the present invention, the diffraction grating is a first-order diffraction grating that satisfies the Bragg reflection condition provided in a part of the optical waveguide layer, and a second-order or higher-order diffraction grating provided in another part of the optical waveguide layer. The high-order diffraction grating makes it possible to extract laser light from the optical waveguide layer in a direction that is not parallel to the optical waveguide layer, thereby creating a surface with a small light-emitting spot and high light-emitting efficiency. This has the effect of making it possible to obtain a distributed feedback type semiconductor laser for emitting light.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional side view showing a distributed feedback semiconductor laser according to one embodiment of the invention, FIG. 2 is a cross-sectional side view showing another embodiment of the invention, and FIG. 3 is a cross-sectional side view showing another embodiment of the invention. FIG. 4 is a perspective view showing a conventional semiconductor laser, and FIG. 5 is an explanatory view showing the operation of the diffraction grating. (12a)...1st-order diffraction grating, (12b)...higher-order diffraction grating, 03)...optical waveguide layer, 04)...active layer, 00...laser light. In addition, in the figures, the same reference numerals indicate the same or equivalent parts.

Claims (3)

[Claims] (1) In a distributed feedback semiconductor laser having an active layer, an optical waveguide layer, and a diffraction grating, the diffraction grating includes a first-order diffraction grating that satisfies the Bragg reflection condition provided in a part of the optical waveguide layer, and It is composed of a second-order or higher order diffraction grating provided with the other part of the waveguide layer, and the high-order diffraction grating makes it possible to extract laser light from the optical waveguide layer in a direction that is not parallel to the optical waveguide layer. A distributed feedback semiconductor laser characterized by: (2) Claim 1, characterized in that a high reflection film is provided on one or both end faces of the optical waveguide layer.
Distributed feedback semiconductor laser as described in . (3) The distributed feedback semiconductor laser according to claim 1 or 2, wherein the phases of the first-order diffraction grating and the higher-order diffraction grating are shifted by a quarter wavelength.