JPH04155987A - Semiconductor distribution feedback laser device and its manufacture - Google Patents

Abstract

PURPOSE:To suppress the cyclic perturbation of refractive index and perform light distribution feedback mainly consisting of the cyclic perturbation of gain coefficient by providing a low refractive index layer at each apex of the irregularity provided in an active layer, and providing a layer, whose refractive index is between that of active layer and that of the low refractive index layer, in contact with the irregularity. CONSTITUTION:Each layer of double hetero junction structure is epitaxially grown separately in two stages. In the first stage, a clad layer 3, an active layer 5, and a low refractive index layer 6 are crystal-grown in order on a substrate 1. Next, by interfering exposure method and chemical etching, a diffraction grating (irregularity) is marked in a low refractive index layer 6 and an active layer 5. In the second stage, a middle refractive index layer 7 is grown on the active layer 5 and the low refractive index layer 6. At this time, the top of the middle refractive index layer 7 is flattened. Further thereon, a clad layer 8 and a contact layer 9 are grown in order continuously to complete double hetero junction structure. Next, an SiO2 insulating layer 12 is stacked on the contact layer 9, and a stripe-shaped window is made, and then electrode layers 11 and 10 are deposited. Furthermore, this is cleaved to complete individual semiconductor laser elements.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a semiconductor distributed feedback laser device used as an electro-optical conversion element.

The present invention relates to a long-distance large-capacity optical communication device, an optical information processing device,
Suitable for use as a light source for optical recording devices, optical measurement devices, and local electronic devices.

〔overview〕

The present invention provides a semiconductor distributed feedback laser device in which an active layer is provided with a periodic uneven shape as a diffraction grating. By providing a layer with a refractive index intermediate to the low refractive index layer, periodic perturbations in the refractive index are suppressed and light distribution feedback is performed mainly based on periodic perturbations in the gain coefficient.

[Conventional technology]

Semiconductor distributed feedback laser devices, which generate stimulated emission light by performing distributed feedback of light to the active layer using a diffraction grating provided near the active layer, generally have a relatively simple configuration and generate stimulated emission with excellent oscillation spectrum characteristics. Since light can be obtained, a lot of research and development has been carried out in the past, and long-distance, high-capacity optical communication,
It is expected to be useful as a light source device suitable for use in 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, a diffraction grating with a triangular wave cross section is formed on the interface of a transparent waveguide layer in close proximity to the active layer on the side far from the active layer, and light distribution is achieved by periodically changing the waveguide refractive index. Research and development aimed at implementing repatriation is currently being pursued.

However, in such optical distribution feedback using refractive index coupling, appropriate feedback regarding the optical phase is not performed for light having a Bragg wavelength that is reflected in accordance with the period of layer thickness change of the leading waveguide layer. For this reason, stable laser oscillation cannot be obtained, and there is a high possibility that longitudinal mode oscillations of two wavelengths symmetrically spaced above and below the Bragg wavelength will occur simultaneously. Furthermore, even when only one of these two wavelengths of longitudinal mode oscillation occurs, it is difficult to select in advance which of the two wavelengths to cause longitudinal mode oscillation. , the accuracy of setting the oscillation wavelength will be significantly impaired.

In other words, in optical distribution feedback using refractive index coupling based on periodic perturbations of the refractive index in the leading waveguide layer, in principle,
The problem of dual-wavelength longitudinal mode oscillation degeneracy arises, and it is difficult to avoid this problem.

Of course, various means for solving such difficulties have been studied in the past. 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 becomes complicated, and a manufacturing process just to eliminate degeneracy needs to be added. First, it was necessary to form an antireflection film on the end face of the laser element.

On the other hand, if distributed optical feedback is performed using refractive index coupling as described above, an oscillation stop band will occur in the Bragg wavelength region, but if distributed optical feedback is performed using gain coupling based on periodic perturbations of the gain coefficient, then the oscillation stop band will be generated in the Bragg wavelength region. The fundamental theory that states that complete single-wavelength longitudinal mode oscillation should be obtained without the appearance of any 23rd
Pages 27 to 2335 (“Coupled-Wav
eTheory of Distributed
Feedback La5ers”.

Journal of Applied Phys.
ics, 1972 Vol, 43°pp 2327-
2335). However, Kogernitsutta's paper is just the result of a theoretical study, and does not provide any concrete structure.

Some of the inventors of this application invented a new semiconductor laser device applying the basic theory of Kogelnitsu et al. and filed the following patent application.

Japanese Patent Application No. 1-189593, filed on July 30, 1988, Japanese Patent Application No. 1-168729, filed on June 30, 1999, Japanese Patent Application No. 1-185001 to 185005, filed on July 18 of the same year.

The structures shown in the specifications and drawings of each of these patent applications made it possible to realize distributed light feedback by gain coupling.

[Problem to be solved by the invention]

Many of the above-mentioned patent applications provide periodic uneven shapes on the surface of the active layer and utilize periodic perturbations in the gain coefficient due to changes in thickness at this time.

Because of the need to confine light, the active layer typically has a different refractive index than its surrounding layers. In this case, forming an uneven shape in one active layer inevitably causes a periodic change in the refractive index. That is, in the structure in which the active layer is provided with a concavo-convex shape, the optical distribution feedback is not obtained only by gain coupling, but the effect of perturbation like refractive index coupling remains.

The present inventors have shown that it is possible to suppress this and design only gain-coupled perturbations, as described in Applied Physics Letters, 1990, Vol.
pleddistributedfeedback
Semiconductor 1asers”.

Applied Physics Letters,
1990. Vol, 56. pp, 1620-1622)
It was shown to.

However, since this structure depends on the shape of the active layer grown on the uneven surface, it has been difficult to precisely control the refractive index coupling component.

SUMMARY OF THE INVENTION An object of the present invention is to solve such problems and provide a semiconductor distributed feedback laser device that suppresses distributed optical feedback due to refractive index coupling and obtains distributed optical feedback mainly based on gain coupling.

[Means to solve the problem]

The semiconductor distributed feedback laser device of the present invention includes a low refractive index layer having a lower refractive index than the active layer on each top of the uneven shape provided as a diffraction grating on the surface of the active layer. It is characterized by comprising an intermediate refractive index layer having a higher refractive index than the refractive index layer and a lower refractive index than the active layer.

The refractive index of the active layer, the intermediate refractive index layer, and the low refractive index layer, the depth of the uneven shape, and the thickness of the intermediate refractive index layer are determined based on the periodicity of the real part of the refractive index caused by the active layer and the intermediate refractive index layer. It is desirable that the change and the periodic change in the real part of the refractive index caused by the low refractive index layer and the intermediate refractive index layer cancel each other out.

To manufacture such a laser device, an active layer and a low refractive index layer having a lower refractive index than the active layer are grown on a substrate, and a periodic uneven shape as a diffraction grating is formed on the low refractive index layer and the active layer. Further, an intermediate refractive index layer having a higher refractive index than the low refractive index layer and lower than the active layer is grown.

Here, by adjusting the refractive index and thickness of the intermediate refractive index layer according to the shape of the diffraction grating imprinted on the low refractive index layer and the active layer, the refractive index coupling component can be precisely controlled.

In this specification, "upper" refers to the same direction as the direction of crystal growth during manufacturing. Moreover, "down" refers to the opposite direction.

[For production]

By periodically changing the thickness of the active layer, a periodic perturbation of the gain coefficient is obtained. At this time, if a low refractive index layer is provided on each top of the uneven shape on the surface of the active layer, and an intermediate refractive index layer is further provided in contact with the uneven shape, the active layer-intermediate refractive index layer passes through the groove portion of the uneven shape. index layer, active layer, intermediate refractive index layer...
A periodic structure is formed, and a periodic structure of a low refractive index layer, an intermediate refractive index layer, a low refractive index layer, an intermediate refractive index layer, etc. is formed through the top. Looking at this in terms of refractive index, it becomes: Groove side: high (active layer) - middle one, high one, middle -...Top side: low - middle one, low, one middle... That is, the periodic structures on the groove side and the top side have opposite phases, and the periodic changes in the real part of the refractive index can be canceled out.

That is, periodic perturbations in the refractive index are suppressed, optical distribution feedback is performed mainly based on periodic perturbations in the gain coefficient due to periodic changes in the thickness of the active layer, and stable single mode oscillation is obtained.

〔Example〕

FIG. 1 shows the structure of a semiconductor distributed feedback laser device according to an embodiment of the present invention.

This laser device includes an active layer 5 that generates stimulated emission light, and a diffraction grating that performs optical distribution feedback on the stimulated emission light generated by the active layer 5 is formed as an uneven shape on the surface of the active layer 5. .

Here, the feature of this embodiment is that a low refractive index layer 6 having a lower refractive index than the active layer 5 is provided on each top of the uneven shape. This is because the intermediate refractive index layer 7 has a high refractive index and a lower refractive index than the active layer 5.

The structure of this laser device will be explained in more detail along with the manufacturing method. Here, an example of an InP system, that is, a case where each layer is lattice matched to fnP, will be explained.

First, each layer of a double heterojunction structure is epitaxially grown in two stages on a highly doped n-type 1nP substrate 1.

Each layer is lattice matched to the InP substrate 1.

In the first stage of epitaxial growth, for example, an n-type 1nP cladding layer 3 with a thickness of 11tr+1 and an 0.
．． 12μm thick low purity concentration 1no, 63Gao, a
Js active layer 5 and 4Qnm thick p-type 1nP low refractive index layer 6
and are sequentially grown as crystals. Next, by interference exposure method and chemical etching, the low refractive index layer 6 and the active layer 5 are etched with a period of 256.
A diffraction grating (uneven shape) with a depth of 80 nm and a depth of 80 nm is imprinted.

In the second stage of epitaxial growth, p-type ino, 72Ga0.2sAso, 81P0.3 with an average thickness of 5 Qnm are deposited on the active layer 5 with the diffraction grating imprinted thereon and the low refractive index layer 6.
9. Grow intermediate refractive index layer 7. At this time, the upper surface of the intermediate refractive index layer 7 is made flat. Furthermore, on top of this
A p-type 1nP cladding layer 8 with a thickness of 1 and a high concentration p-type Ino, S, Gao, 4ff^S contact layer 9 with a thickness of 0.5p are successively grown in order to complete a double heterojunction structure. .

Here, since the side surfaces of the active layer 5 are exposed during crystal growth after etching, it is necessary to perform a very slight etching process immediately before to prevent defects from occurring. It has been reported that the problem of defect generation does not occur in the InP system if it is treated appropriately (Journal of Crystal Growth, Vol. 93, 1988, No. 36).
Pages 5 to 369 (J, Crystal, Growth,
93 (1988) pp, 365-369)).

After the second epitaxial growth is completed, a 5102 insulating layer 12 is deposited on the top surface of the contact layer 9 to form a striped window, e.g.
and 10 are deposited. Furthermore, this is folded to complete individual semiconductor laser devices.

As growth conditions for metal organic vapor phase epitaxy, for example, [Raw material] Phosphine PH3 Arsin AsH.

Triethyl indium (C2H5) 31n triethyl gallium (C2H5) 3Ga dimethyl zinc (
CH,) 2Zn hydrogen sulfide H,S [Conditions] Pressure: 76 Torr Total flow rate: 5 sin Substrate temperature: 700°C (first time), 650°C (second time).

The conductivity type and composition of each layer described above are summarized in a table.

(Hereinafter, this page margin) Figure 2 shows the layer structure in the vicinity of the active layer of the above-mentioned embodiment, and the right side of Figure 3 and Figure 4 are lines 3-3, 4-4 in Figure 2, respectively.
It shows the refractive index distribution along.

The perturbation in the real part of the refractive index caused by the active layer 5 on which the diffraction grating is imprinted and the intermediate refractive index layer 7 filling in between is the phase caused by the cut low refractive index layer 6 and the intermediate refractive index layer 7 filling in between. is canceled by the perturbation of the opposite real part of the refractive index. The composition and thickness described above are examples designed so that the perturbation of the real part of the refractive index is approximately zero.

The purpose of making the upper surface of the intermediate refractive index layer 7 flat is to simplify design calculations.If any unevenness remains on this upper surface, the perturbation of the real part of the refractive index is also taken into consideration, and the overall Try to cancel the perturbation.

In this way, it is possible to suppress the perturbation of the real part of the refractive index and realize optical distribution feedback in which the imaginary part of the refractive index, that is, the perturbation of the gain coefficient, is mainly caused by the active layer 5 on which the diffraction grating is imprinted, and the period of the gain coefficient. A semiconductor distributed feedback laser device that performs single mode oscillation at a Bragg wavelength corresponding to .

〔Effect of the invention〕

As described above, in the semiconductor distributed feedback laser device of the present invention, the overall change in the real part of the refractive index is small, and periodic perturbations in the refractive index are suppressed. As a result, optical distribution feedback mainly based on periodic perturbations in the gain coefficient due to periodic changes in the thickness of the active layer is performed, making it possible to oscillate in a single mode.

In addition, since distributed optical feedback is achieved through gain coupling, unlike conventional index-coupled semiconductor distributed feedback laser devices, longitudinal mode oscillation of a single wavelength is performed, which is different from conventional devices. It is thought that there is no uncertainty in the oscillation wavelength. However, although it is possible to achieve a completely single longitudinal mode with conventional semiconductor distributed feedback laser devices, in both cases the configuration of the semiconductor laser device becomes complicated, and in addition, it is necessary to form an anti-reflection film on the end face of the laser element, etc. In contrast to the increase in the number of manufacturing steps, the device of the present invention can easily realize a complete single longitudinal mode with almost no changes to the conventional manufacturing steps and without the need for anti-reflection measures.

Furthermore, since optical distribution feedback using gain coupling is used, interference noise induced by reflected return light from the near end or far end, etc., is reduced compared to the case using conventional refractive index coupling. It is expected that it will become significantly smaller.

In addition, in the semiconductor distributed feedback laser device of the present invention, since the resonator has a periodic distribution of gain due to current injection, it is possible to generate ultrashort pulses in high-speed current modulation, and the chirping of the oscillation wavelength is also small. Be expected.

Therefore, the semiconductor distributed feedback laser device of the present invention has the following characteristics:
Gas lasers are not only promising as high-performance light sources necessary for long-distance optical communications and wavelength division multiplexing communications, but also have traditionally been used in fields such as optical information processing and recording, optical applied measurement, and light sources for high-speed optical phenomena. It is expected to be used as a high-performance, compact light source that can replace solid-state laser devices.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing the structure of a semiconductor distributed feedback laser device according to an embodiment of the present invention. FIG. 2 is a cross-sectional view showing the layer structure near the active layer. FIG. 3 is a diagram showing the refractive index distribution along line 3-3 in FIG. FIG. 4 is a diagram showing the refractive index distribution along line 4--4 in FIG. DESCRIPTION OF SYMBOLS 1... Substrate, 3.8... Clad layer, 5... Active layer, 6... Low refractive index layer, 7... Intermediate refractive index layer, 9...
...Contact layer, 10.11...Electrode layer, 12...
・Insulating layer.

Claims (1)

[Claims] 1. An active layer that generates stimulated emission light, and a diffraction grating that performs optical distribution feedback on the stimulated emission light generated by the active layer, and the diffraction grating is formed by adjusting the irregularities on the surface of the active layer. In the semiconductor distributed feedback laser device formed as a shape, a low refractive index layer having a lower refractive index than the active layer is provided at each top of the uneven shape, and in contact with the uneven shape, the layer has a lower refractive index than the low refractive index layer. A semiconductor distributed feedback laser device comprising an intermediate refractive index layer having a high refractive index and a lower refractive index than the active layer. 2. The refractive index of the active layer, the intermediate refractive index layer, and the low refractive index layer, the depth of the uneven shape, and the thickness of the intermediate refractive index layer are determined based on the refractive index effect produced by the active layer and the intermediate refractive index layer. 2. The semiconductor distributed feedback laser device according to claim 1, wherein the periodic change in the real part of the refractive index and the periodic change in the real part of the refractive index caused by the low refractive index layer and the intermediate refractive index layer cancel each other out. 3. A step of growing an active layer and a low refractive index layer having a lower refractive index than the active layer on the substrate; and a step of imprinting a periodic uneven shape as a diffraction grating on the low refractive index layer and the active layer. . A method for manufacturing a semiconductor distributed feedback laser device, comprising: growing an intermediate refractive index layer having a higher refractive index than the low refractive index layer and a lower refractive index than the active layer.