JPH07118563B2 - Semiconductor distributed feedback laser device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor distributed feedback laser device used as an electro-optical conversion element. INDUSTRIAL APPLICABILITY The present invention is suitable for use as a light source of a long-distance large-capacity optical communication device, an optical information processing device, an optical recording device, an optical application measuring device, and other optoelectronic devices.

[0002]

2. Description of the Related Art A semiconductor distributed feedback laser device which generates stimulated emission light by performing distributed feedback of light to the active layer by a diffraction grating provided in the vicinity of the active layer is generally excellent in oscillation due to its relatively simple structure. Since stimulated emission light with spectral characteristics can be obtained, many researches and developments have been made in the past.
Its usefulness is expected as a suitable light source device used for long-distance and large-capacity optical communication, optical information processing and recording, optical applied measurement, and the like.

Such a semiconductor distributed feedback laser device employs an optical waveguide structure in which the active layer is surrounded by a transparent heterojunction semiconductor layer or the like to efficiently generate stimulated emission light. In particular, by forming a diffraction grating with a triangular wave-shaped cross-section on the interface of the transparent waveguide layer, which is very close to the active layer, on the side far from the active layer, and periodically changing the waveguide refractive index, Research and development in the direction of returning home is being carried out exclusively.

However, in such a distributed light feedback by the refractive index coupling, an appropriate feedback about the optical phase is performed with respect to the light of the Bragg wavelength which is reflected corresponding to the period of the layer thickness change of the optical waveguide layer. I don't know. For this reason, stable laser oscillation cannot be obtained, and there is a high possibility that longitudinal mode oscillation of two wavelengths vertically symmetrically separated from the Bragg wavelength will occur at the same time. Further, even when only one of the two longitudinal mode oscillations is generated, it is difficult to select in advance which wavelength of the two wavelengths the longitudinal mode oscillation is to be performed. However, the accuracy of setting the oscillation wavelength is significantly impaired.

That is, in the distributed optical feedback utilizing the refractive index coupling based on the periodic perturbation of the refractive index in the optical waveguide layer, in principle, the problem of degeneracy of two-wavelength longitudinal mode oscillation occurs, which should be avoided. It is difficult.

Of course, various means for solving such difficulties have been studied. However, in any case, for example, a structure in which the phase is shifted by a quarter wavelength at approximately the center of the diffraction grating, the structure of the laser device is complicated, and it is necessary to add a manufacturing process only for eliminating degeneracy. In addition, it is necessary to form an antireflection film on the end face of the laser element.

On the other hand, when the distributed optical feedback is performed by the refractive index coupling as described above, an oscillation stop band is generated in the Bragg wavelength region. However, if the distributed optical feedback is performed by the gain coupling based on the periodic perturbation of the gain coefficient, the oscillation prevention is performed. The fundamental theory that a single-wavelength longitudinal mode oscillation should be obtained by suppressing the appearance of a band is Kogelnik et al., “Coupling wave theory of distributed feedback lasers”, Journal of Applied Physics, 1972, Vol. 43, pp. 2327 to 233.
Page 5 ("Coupled-Wave Theory of Distributed Feedback
Lasers ", Journal of Applied Physics, 1972 Vol.43, p.
p.2327-2335). This paper is the result of theoretical examination only, and does not show the concrete structure.

The inventors of the present invention invented a new semiconductor laser device to which the basic theory of Kogelnik et al. Is applied, and applied for some patents. (1) Japanese Patent Application No. Sho 63-189593, July 30, 1988
A structure in which a semiconductor opaque layer is provided in the vicinity of an active layer, and a diffraction grating is formed in the opaque layer to perform distributed feedback based on periodic perturbation in a gain coefficient or a loss coefficient of the opaque layer, and a manufacturing method thereof. . (2) JP-A-3-34489 (Japanese Patent Application No. 1-16872)
9, filed on June 30, 1991) A structure in which the thickness of an active layer is periodically changed by utilizing the growth on a diffraction grating pattern, and a method for manufacturing the structure. (3) Japanese Patent Application No. 2-235235, filed on September 5, 1990 A structure in which a periodic gain distribution is obtained by providing a layer whose conductivity is periodically changed in the vicinity of the active layer. (4) Japanese Patent Application No. 2-282698, filed October 19, 1990 An optical absorption layer is provided on each top of the uneven shape corresponding to the period of the diffraction grating, and an active layer is grown on the light absorption layer via a buffer layer. Therefore, the structure that combines the periodic distribution of gain and the periodic distribution of light absorption. There is.

[0009]

However, there are still some problems to be solved in the structures shown in these applications. For example, in the structure shown in the application (1), the threshold current is increased by adding the absorption layer. In the structure shown in the application of (2), since the gain coupling coefficient is determined by the threshold current density, it is difficult to obtain a coupling coefficient larger than a certain level. It is difficult to obtain a sufficient gain coupling when operating with low injection only by periodical conductivity reversal like the structure shown in the application of (3). Further, in the structure shown in the application of (4), current utilization efficiency may be reduced due to uniform current injection over the entire surface,
It is considered that low threshold current operation is difficult.

It is an object of the present invention to provide a semiconductor distributed feedback laser device having a new structure that solves the above problems.

[0011]

In the semiconductor distributed feedback laser device of the present invention, the conductivity periodically changes along the laser optical axis direction parallel to the active layer, and at the same cycle as this periodic change. It is characterized by having a structure in which the absorption coefficient for the stimulated emission light generated by the active layer changes. In particular,
Two semiconductor layers having different conductivity from each other are provided with the active layer sandwiched therebetween, and a diffraction grating having a conductivity different from that of the semiconductor layer is embedded in at least one of the two semiconductor layers.

As one method for obtaining such a structure, a semiconductor layer having substantially the same composition as that of the active layer is provided on the active layer with a buffer layer interposed therebetween, and the semiconductor layer is etched to form a diffraction grating. To form. The conductivity of the semiconductor layer in which the diffraction grating is formed is made different from the conductivity of the layer in which it is embedded. Thereby, the absorptivity and the electrical conductivity simultaneously change periodically.

As another method, a buffer layer is grown on a semiconductor layer on which uneven shapes corresponding to the period of the diffraction grating are imprinted so that the periodic shapes corresponding to the uneven shapes remain on the upper surface, and When the active layer is grown so that a thickness change corresponding to the periodical shape of the upper surface thereof occurs, the conductivity is different from that of the surrounding layers and the absorptivity is provided on each top of the uneven shape of the semiconductor layer. You may provide the semiconductor layer which it has. That is, a diffraction grating is formed in a periodic uneven shape along the laser optical axis direction, a semiconductor layer having an absorptive composition is arranged on the top of this uneven shape, and the thickness of the active layer is corresponding to this uneven shape. Is provided with a periodic change.
In this case, the buffer layer is for preventing various crystal defects introduced into the substrate or the epitaxial growth layer at the time of engraving the diffraction grating from propagating to the active layer.

The diffraction grating may have a shape in which a semiconductor layer filling the diffraction grating and a layer having a different conductivity are periodically cut. It is preferable that the conductivity type of the diffraction grating is different from the conductivity type of the semiconductor layer in which the diffraction grating is embedded.

The semiconductor layer in which the diffraction grating is embedded preferably has a refractive index distribution canceling structure for canceling the periodic distribution of the refractive index due to the diffraction grating. Also, apart from this,
When the thickness of the active layer is periodically changed to correspond to the unevenness of the diffraction grating, the periodic distribution of the refractive index due to the diffraction grating is canceled by the periodic distribution of the refractive index due to the thickness distribution of the active layer. In addition, it is desirable to select the material and shape of the diffraction grating and the active layer.

The change in conductivity and the change in conductivity can be realized by one layer, but can also be realized by two layers which are in contact with each other or two layers which are adjacent to each other.

To give a periodic change in conductivity, ion implantation or diffusion can be used in addition to forming the concavo-convex shape with a layer having different conductivity from the surroundings.

As used herein, the term "upper" means the direction of crystal growth, that is, the direction away from the substrate.

[0019]

By periodically providing, above or below the active layer for generating stimulated emission light, a layer that absorbs the light generated by the active layer and blocks the current injected into the active layer, (1) A current density distribution, that is, a carrier density distribution is generated in the active layer, and a periodic distribution of the light absorption coefficient is also generated. (2) When the active layer has a thickness distribution, a periodic change in the gain coefficient due to the change in the thickness can be obtained. (3) Therefore, the gain coefficient for the light wave propagating in the cavity axis direction changes at a period that matches the period of the diffraction grating,
Distributed feedback is realized by gain coupling. (4) By combining these mechanisms for generating gain coupling, stronger gain coupling can be obtained, and further, current utilization efficiency can be improved and lower Threshold current operation can be expected.

Further, in the case of a GaAs / AlGaAs laser device, that is, a device having a structure in which GaAs is lattice-matched, the diffraction grating can be formed of a material having a small Al composition, and the quality of the regrown crystal can be improved. To be enhanced.

[0021]

1 is a perspective view showing the structure of a semiconductor distributed feedback laser device according to a first embodiment of the present invention.

In this embodiment, a periodic current blocking structure is provided on an active layer, and a buffer layer 2, a cladding layer 3, an active layer 4 and a buffer layer 5 are epitaxially formed on a substrate 1. A diffraction grating 7 is provided on the buffer layer 5. A clad layer 8 and a contact layer 9 are further epitaxially formed on the diffraction grating 7. The contact layer 9 is connected to the electrode layer 11 through a window opened in the insulating layer 10. An electrode layer 12 is provided on the back surface of the substrate 1.

The feature of this embodiment is that the diffraction grating 7 is formed by the absorptive current blocking layer 6, and the conductivity of the diffraction grating 7 is parallel to the laser optical axis direction parallel to the active layer 4.
And the absorption coefficient for the stimulated emission light generated by the active layer 4 changes in the same cycle as this cyclic change.

To manufacture this laser device, each layer of a semiconductor laser device having a double heterojunction structure is continuously epitaxially grown on the substrate 1 in two steps. That is, the buffer layer 2, the cladding layer 3, the active layer 4, the buffer layer 5, and the absorptive current blocking layer 6 are epitaxially grown in this order on the substrate 1. Then, the interference exposure method and the dry etching capable of selective etching are applied to imprint the diffraction grating 7 on the absorptive current blocking layer 6 which is the uppermost layer of the growth layer. At this time, in the etched portion, the absorptive current blocking layer 6
Should be completely removed. Then, the upper clad layer 8 is formed on the absorptive current blocking layer 6 on which the diffraction grating 7 is engraved.
Then, the contact layer 9 is epitaxially grown in this order to complete the double heterojunction structure. Then, the insulating layer 10 is deposited on the upper surface of the contact layer 9 to form a striped window, and then the negative electrode layer 11 is vapor-deposited on the entire surface. Further, the positive electrode layer 10 is vapor-deposited on the back surface of the substrate 1. Finally, the semiconductor block thus manufactured is cleaved to complete individual semiconductor laser devices.

Examples of the conductivity type, composition, and thickness of each layer are shown below. Substrate 1 p ⁺ -GaAs buffer layer 2 p ⁺ -GaAs, thickness 0.5 μm clad layer 3 p-AlGaAs, thickness 1 μm active layer 4 GRIN-SCH-SQW buffer layer 5 n-Al _0.45 Ga _0.55 As, thickness 0.1 μm absorptive current blocking layer 6 p-Al _0.03 Ga _0.97 As, thickness 0.1 μm diffraction grating 7 periods 260 nm clad layer 8 n-Al _0.45 Ga _0.55 As, thickness 1 μm contact layer 9 n ⁺ -GaAs, thickness 0.5 μm insulating layer 10 SiO ₂ (width of a striped window is about 10 μm) Electrode layer 11 Au / Au-Ge Electrode layer 12 Au / Cr

Although a single quantum well is used as the active layer 4 in this embodiment, a multiple quantum well or a bulk crystal may be used. In the structure of this embodiment, the active layer 4 is flat, and the advantage can be effectively utilized by using a single quantum well or multiple quantum wells. Further, the present invention can be implemented by inverting the conductivity types of all semiconductor layers including the substrate 1. Although not shown here, it is desirable that the cladding layer 8 be provided with a refractive index distribution canceling structure that cancels the periodic distribution of the refractive index due to the diffraction grating 7. As such a structure, a patent application by the applicant of the present application, Japanese Patent Application No. 3-1812
As shown in FIG. 09, a structure in which layers having different refractive indexes are combined can be used.

FIG. 2 is a perspective view showing the structure of a semiconductor distributed feedback laser device according to the second embodiment of the present invention.

In this embodiment, a periodic current blocking structure is combined with a periodic change in the thickness of the active layer. A buffer layer 22, a cladding layer 23, and pattern supply layers 33 and 34 are epitaxially formed on the substrate 21. And the bottom of the groove is formed in the pattern supply layers 33 and 34.
To reach, that is, to cut the pattern supply layer 34 is formed at the period of the diffraction grating. A buffer layer 25 is formed on the irregular shape, leaving a periodic shape corresponding to the irregular shape on the upper surface, and a change in thickness corresponding to the periodic shape of the upper surface of the buffer layer 25 is formed thereon. An active layer 24 having A clad layer 28 and a contact layer 29 are epitaxially formed on the active layer 24. The contact layer 29 is connected to the electrode layer 31 through a window opened in the insulating layer 30. An electrode layer 32 is provided on the back surface of the substrate 21.

The feature of this embodiment is that the pattern supply layer 34 is formed of an absorptive current blocking layer. That is, the conductivity type of the pattern absorption layer 34 is set to be different from that of the substrate 21 or the pattern supply layer 33, and the composition of the pattern supply layer 34 is set so as to absorb the stimulated emission light from the active layer 24. ing. In addition, the periodic distribution of the refractive index due to the diffraction grating formed by etching the pattern supply layer 34 is offset by the periodic distribution of the refractive index due to the thickness distribution of the active layer 24.
The composition and shape of the pattern supply layers 33 and 34, the active layer 24, and the buffer layer 25 are selected.

In the above embodiments, the case where the present invention is applied to the GaAs system has been described, but the present invention can also be applied to other material systems such as InP. When the structure of the first embodiment is implemented with an InP system, the conductivity type, composition, and
An example of the thickness is shown below. Substrate 1 (100) n-InP buffer layer 2, cladding layer 3 n-InP, thickness 0.5 μm active layer 4 InGaAsP and InGaAsP (λ _g = 1.3 μm) multiple quantum well structure (equivalent λ _g = 1.55 μm) Buffer layer 5 p-InP, thickness 0.1 μm Absorbing current blocking layer 6 n-InGaAsP (λ _g = 1.55 μm), thickness 0.1 μm Cladding layer 8 p-InP, thickness 1 μm Contact layer 9 p ⁺ -InGaAs, thickness 0.5 μm However, InGaAs and InGaAsP are lattice-matched to InP. λ _g represents the wavelength of light corresponding to the band gap.

Even in the case of InP type, the present invention can be carried out by changing the conductivity type. Further, a bulk mixed crystal can be used for the active layer.

In the above embodiments, the absorption current blocking layer 6 has been described as having a different conductivity type of p and n from the surrounding layers. However, when the conductivity types are different, the conductivity types may be different. A similar effect can be obtained. For example I
In the case of the nP type, InGaAsP whose resistance is increased by Fe doping can be used as the absorptive current blocking layer.

[0033]

The semiconductor distributed feedback laser device utilizing the distributed optical feedback by gain coupling is different from the conventional refractive index coupled semiconductor distributed feedback laser device in that the longitudinal mode oscillation of a single wavelength is completely generated. Therefore, it is considered that the uncertainty of the oscillation wavelength as in the conventional device is not seen. However,
The conventional semiconductor distributed feedback laser device can also be used for complete single longitudinal mode, but in both cases, the structure of the semiconductor laser device is complicated, and in addition, it is necessary to form an antireflection film on the end face of the laser element. In contrast to the increase in the number of steps, the device of the present invention can easily realize the complete single longitudinal mode without changing the conventional manufacturing process and without the need for antireflection measures. Further, when distributed optical feedback is achieved by gain coupling, even if interference noise induced by reflected return light from the near end or far end occurs, it becomes much smaller than in the case of conventional refractive index coupling. It is expected. Furthermore, it is expected that ultra-short pulses can be generated in high-speed current modulation and chirping of the oscillation wavelength is small because the resonator is caused by the period distribution of gain generated by current injection.

Particularly, in the semiconductor distributed feedback laser device of the present invention, the periodic light absorption layer is provided, and the conductivity is made periodic so that the carriers are selectively injected into the antinode portion of the electric field. A large gain coupling can be obtained.
Further, since the waste of the injection current is small, high efficiency can be expected.

When the GaAs / AlGaAs system is used, the aluminum composition at the regrowth interface can be reduced and the crystal quality can be improved.

Therefore, the semiconductor distributed feedback laser device of the present invention is not only promising as a high-performance light source required for long-distance optical communication, wavelength-multiplexed optical communication, etc., but also optical information processing, optical information recording, and optical information recording. It is expected to be used as a high-performance and small-sized light source that can replace the gas laser device and the solid-state laser device that have been conventionally used in the fields of applied measurement, light source of high-speed optical phenomenon, and the like.

[Brief description of drawings]

FIG. 1 is a perspective view showing the structure of a semiconductor distributed feedback laser device according to a first embodiment of the present invention.

FIG. 2 is a perspective view showing the structure of a semiconductor distributed feedback laser device according to a second embodiment of the present invention.

[Explanation of symbols]

 1, 21 Substrate 2, 22 Buffer layer 3, 8, 23, 28 Cladding layer 4, 24 Active layer 5, 25 Buffer layer 6 Absorptive current blocking layer 7 Diffraction grating 9, 29 Contact layer 11, 12, 31, 32 Electrode Layer 10, 30 Insulation layer 33, 34 Pattern supply layer

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takeshi Inoue 2-11-13 Nakamachi, Musashino City, Tokyo Photometer Measurement Technology Development Co., Ltd. (72) Inventor Machiko Tsuchihashi 2-11 Nakamachi, Musashino City No. 13 Photometer Measurement Technology Development Co., Ltd. (56) Reference JP-A-3-35581 (JP, A)

Claims (7)

[Claims] 1. A induced an active layer for generating emitted light, arranged along the active layer induced emission light against absorption coefficient parallel laser light to the active layer generated in the active layer
In a semiconductor distributed feedback laser device including a periodic structure that periodically changes along the axial direction, the periodic structure causes a periodic change in gain of the active layer.
Therefore, the conductivity changes in the same cycle as the periodic change of absorption coefficient.
A semiconductor distributed feedback laser device including a structure that changes periodically . 2. A diffraction grating having two semiconductor layers different in conductivity from each other with an active layer interposed therebetween, and the periodic structure having a conductivity different from that of the one semiconductor layer embedded in at least one of the two semiconductor layers. The semiconductor distributed feedback laser device according to claim 1, further comprising: 3. The diffraction grating is formed in a periodic uneven shape along the laser optical axis direction, and includes a semiconductor layer having an absorptive composition at the top of the uneven shape, and the active layer corresponding to this uneven shape. 3. The semiconductor distributed feedback laser device according to claim 2, wherein the thickness of the laser is periodically changed. 4. The semiconductor distributed feedback laser device according to claim 2, wherein the diffraction grating has a shape in which a semiconductor layer filling the diffraction grating and a layer having a different conductivity are periodically cut. 5. The semiconductor distributed feedback laser device according to claim 2, wherein the conductivity type of the diffraction grating is different from that of the semiconductor layer in which the diffraction grating is embedded. 6. The semiconductor distributed feedback laser device according to claim 2, wherein the semiconductor layer in which the diffraction grating is embedded is provided with a refractive index distribution canceling structure for canceling the periodic distribution of the refractive index due to the diffraction grating. 7. The material and shape of the diffraction grating and the active layer are selected so that the periodic distribution of the refractive index by the diffraction grating is offset by the periodic distribution of the refractive index by the thickness distribution of the active layer. Semiconductor distributed feedback laser device.