WO2018134940A1

WO2018134940A1 - Optical modulator integrated semiconductor laser

Info

Publication number: WO2018134940A1
Application number: PCT/JP2017/001699
Authority: WO
Inventors: 和久高木; 崇柳楽; 斎藤　健
Original assignee: 三菱電機株式会社
Priority date: 2017-01-19
Filing date: 2017-01-19
Publication date: 2018-07-26
Also published as: JPWO2018134940A1; JP6146554B1

Abstract

A semiconductor laser (1) and an electroabsorption optical modulator (2) are integrated with each other. The semiconductor laser (1) outputs laser light. The electroabsorption optical modulator (2) modulates the laser light. In the electroabsorption optical modulator (2), a first conductivity-type semiconductor layer (4), an optical waveguide core layer (10), and a second conductivity-type semiconductor layer (6) are laminated in this order. A first electrode (8) is formed below the first conductivity-type semiconductor layer (4). Just above the optical waveguide core layer (10), a second electrode (11) is formed on the second conductivity-type semiconductor layer (6). The optical absorption coefficient of the optical waveguide core layer (10) changes due to a voltage applied between the first electrode (8) and the second electrode (11). The width of the optical waveguide core layer (10) is gradually reduced toward the light output end of the electroabsorption optical modulator (2).

Description

Optical modulator integrated semiconductor laser

The present invention relates to an optical modulator integrated semiconductor laser.

An optical modulator integrated semiconductor laser is used as a light source for transmission in an optical fiber communication device (see, for example, Patent Documents 1 to 4). In addition to the electroabsorption optical modulator (EAM) and the integrated semiconductor laser (EML), there is also an optical modulator integrated semiconductor laser in which a light beam diameter of the emitted light, that is, a spot size converter (SSC) for expanding the spot size is integrated. It has been proposed (see Non-Patent Document 1, for example). In this laser, a transparent waveguide core layer having a spot size conversion function is connected to the emission end of the core layer of the optical modulator by a butt joint.

Japanese Unexamined Patent Publication No. 10-308556 Japanese Unexamined Patent Publication No. 2005-234319 Japanese Unexamined Patent Publication No. 2016-156928 Japanese Unexamined Patent Publication No. 2015-045789

However, the value of the imaginary part of the effective refractive index of the optical waveguide does not match at the connection between the optical modulator and the spot size converter. For this reason, reflected return light to the semiconductor laser is generated, and transmission characteristics are deteriorated.

The present invention has been made to solve the above-described problems, and an object thereof is to obtain an optical modulator integrated semiconductor laser capable of improving transmission quality.

An optical modulator integrated semiconductor laser according to the present invention comprises: a semiconductor laser that emits laser light; and an electroabsorption optical modulator that is integrated with the semiconductor laser and modulates the laser light, the electroabsorption optical modulation And a first conductive semiconductor layer, an optical waveguide core layer and a second conductive semiconductor layer, a first electrode formed under the first conductive semiconductor layer, and the optical waveguide core. A second electrode formed on the second conductive type semiconductor layer immediately above the layer, and the optical waveguide core is applied with a voltage applied between the first electrode and the second electrode. The optical absorption coefficient of the layer changes, and the width of the optical waveguide core layer gradually decreases toward the light exit end of the electroabsorption optical modulator.

In the present invention, the width of the optical waveguide core layer of the electroabsorption optical modulator gradually decreases toward the output end of the electroabsorption optical modulator. Thus, the electroabsorption optical modulator operates as a spot size converter that modulates light and simultaneously enlarges the spot size. As a result, reflected return light caused by the difference in the imaginary part of the refractive index does not occur, so that transmission quality can be improved.

1 is a plan view showing a semiconductor device according to a first embodiment of the present invention. FIG. 2 is a cross-sectional view taken along the line II of FIG. FIG. 4 is a cross-sectional view taken along the line III-IV in FIG. 1. FIG. 5 is a cross-sectional view taken along line V-VI in FIG. 1. FIG. 7 is a cross-sectional view taken along the line VII-VIII in FIG. It is a top view which shows the semiconductor device which concerns on a comparative example. FIG. 7 is a cross-sectional view taken along line V-VI in FIG. 6. FIG. 7 is a cross-sectional view taken along the line VII-VIII in FIG. It is a top view which shows the semiconductor device which concerns on Embodiment 2 of this invention. FIG. 10 is a sectional view taken along line V-VI in FIG. 9. FIG. 10 is a sectional view taken along line VII-VIII in FIG. It is a top view which shows the semiconductor device which concerns on Embodiment 3 of this invention. FIG. 13 is a sectional view taken along line V-VI in FIG. 12. FIG. 13 is a cross-sectional view taken along the line VII-VIII in FIG.

An optical modulator integrated semiconductor laser according to an embodiment of the present invention will be described with reference to the drawings. The same or corresponding components are denoted by the same reference numerals, and repeated description may be omitted.

Embodiment 1 FIG.
FIG. 1 is a plan view showing a semiconductor device according to Embodiment 1 of the present invention. A semiconductor laser 1 and an electroabsorption optical modulator 2 are integrated. The semiconductor laser 1 emits laser light, and the electroabsorption optical modulator 2 modulates the laser light.

FIG. 2 is a cross-sectional view taken along the line II of FIG. In the semiconductor laser 1, an n-InP clad layer 4, an active layer 5, a p-InP clad layer 6, and a p-InGaAsP contact layer 7 are sequentially stacked on an n-InP substrate 3. The active layer 5 is, for example, InGaAsP-MQW or AlGaInAs-MQW and includes a diffraction grating layer. A cathode electrode 8 is formed under the n-InP substrate 3. An anode electrode 9 is formed on the p-InGaAsP contact layer 7 immediately above the active layer 5.

FIG. 3 is a cross-sectional view taken along the line III-IV in FIG. FIG. 4 is a cross-sectional view taken along the line V-VI in FIG. FIG. 5 is a sectional view taken along line VII-VIII in FIG. In the electroabsorption optical modulator 2, an n-InP cladding layer 4, an optical waveguide core layer 10, a p-InP cladding layer 6, and a p-InGaAsP contact layer 7 are sequentially stacked on an n-InP substrate 3. The optical waveguide core layer 10 is, for example, AlGaInAs-MQW or InGaAsP-MQW.

An anode 11 is formed on the p-InGaAsP contact layer 7 directly above the optical waveguide core layer 10. Except for the anode electrodes 9 and 11, the p-InGaAsP contact layer 7 is covered with a SiO ₂ insulating film 12. A semi-insulating InP layer 13 and an n-InP current blocking layer 14 embed both sides of the active layer 5 and the optical waveguide core layer 10.

The light absorption coefficient of the optical waveguide core layer 10 changes depending on the voltage applied between the cathode electrode 8 and the anode electrode 11. The electroabsorption optical modulator 2 has a portion where the width of the optical waveguide core layer 10 is constant, and a portion where the width of the optical waveguide core layer 10 gradually decreases toward the light emitting end of the electroabsorption optical modulator 2. . The anode electrode 11 is also formed immediately above the portion where the width gradually decreases. The same modulation signal is applied to both the constant width portion and the gradually decreasing portion via the common anode electrode 11 to operate as an optical modulator. The portion where the width of the optical waveguide core layer 10 gradually decreases along the propagation direction of the laser light operates not only as an optical modulator but also as a spot size converter portion.

Subsequently, the effect of the present embodiment will be described in comparison with a comparative example. FIG. 6 is a plan view showing a semiconductor device according to a comparative example. FIG. 7 is a sectional view taken along line V-VI in FIG. FIG. 8 is a sectional view taken along line VII-VIII in FIG. 6 is the same as FIG. 2, and the cross-sectional view along III-IV in FIG. 6 is the same as FIG.

In the comparative example, the width of the optical waveguide core layer 10 of the electroabsorption optical modulator 2 is constant, and the transparent waveguide core layer 16 of the spot size converter 15 having a spot size conversion function is formed at the emission end thereof. Connected by. The extinction ratio is Ext, the absorption coefficient of the electroabsorption optical modulator 2 is α, the modulator length is L, the optical wavelength is λ, the imaginary part of the refractive index of the optical waveguide core layer 10 is k, and the real part of the refractive index is n. When the reflectance is R, the following relational expression is established.
Ext = exp (−αL)
k = αλ / 4π
R = (k / sqrt (n ² + k ² ) + n) ²

As an example, when the extinction ratio Ext is 11 dB, the modulator length L is 180 μm, and the oscillation wavelength λ of the semiconductor laser 1 is 1.55 μm, the fluctuation of the imaginary part k of the refractive index is 7.5E-4. When the effective refractive index n of the real part of both the optical waveguide core layer 10 and the transparent waveguide core layer 16 is 3.2, the fluctuation R of the reflectance due to the fluctuation of the imaginary part of the refractive index is 1.9E-16%. It becomes. In this way, the value of the imaginary part of the effective refractive index does not match at the connection part between the electroabsorption optical modulator 2 and the spot size converter 15. For this reason, the light whose intensity is changed by the semiconductor laser 1 returns, disturbs the light intensity inside the active layer 5, and deteriorates the transmission characteristics.

On the other hand, in the present embodiment, the width of the optical waveguide core layer 10 of the electroabsorption optical modulator 2 gradually decreases toward the emission end of the electroabsorption optical modulator 2. Thereby, the electroabsorption optical modulator 2 operates as a spot size converter that modulates light and simultaneously enlarges the spot size. Thereby, the reflected return light caused by the difference in the imaginary part of the refractive index as in the comparative example does not occur, so that the transmission quality can be improved.

Further, when the window structure is provided, the return light becomes large due to the extremely large difference in refractive index between the optical waveguide core layer 10 of the electroabsorption optical modulator 2 and the buried layer of the window structure. The effect of cannot be obtained. Therefore, in order to reduce light reflection at the light emitting end, it is preferable that the optical waveguide core layer 10 extends to the light emitting end.

Embodiment 2. FIG.
FIG. 9 is a plan view showing a semiconductor device according to the second embodiment of the present invention. FIG. 10 is a sectional view taken along line V-VI in FIG. FIG. 11 is a sectional view taken along line VII-VIII in FIG. 9 is the same as FIG. 2, and the cross-sectional view along III-IV in FIG. 9 is the same as FIG.

A transparent waveguide core layer 16 is formed around the optical waveguide core layer 10 and is in contact with the optical waveguide core layer 10. The transparent waveguide core layer 16 is, for example, InGaAsP or AlGaInAs. The anode electrode 11 is not formed immediately above the transparent waveguide core layer 16 or is not in contact with the p-InGaAsP contact layer 7 by being formed on the SiO ₂ insulating film 12. The refractive index of the transparent waveguide core layer 16 is a value between the refractive index of the semi-insulating InP layer 13 and the n-InP current blocking layer 14 and the refractive index of the optical waveguide core layer 10. The band gap energy of the transparent waveguide core layer 16 is larger than the band gap energy of the optical waveguide core layer 10.

Thus, by providing the transparent waveguide core layer 16, the extinction characteristic and the spot size of the electroabsorption optical modulator 2 can be designed independently. Other configurations and effects are the same as those of the first embodiment.

Embodiment 3 FIG.
FIG. 12 is a plan view showing a semiconductor device according to the third embodiment of the present invention. FIG. 13 is a sectional view taken along line V-VI in FIG. FIG. 14 is a sectional view taken along the line VII-VIII in FIG. 12 is the same as FIG. 2, and the cross-sectional view along III-IV in FIG. 12 is the same as FIG.

The total width of the optical waveguide core layer 10 and the transparent waveguide core layer 16 gradually decreases toward the emission end of the electroabsorption optical modulator 2. Thereby, the spot size can be enlarged as compared with the second embodiment. Other configurations and effects are the same as those of the first and second embodiments.

1 semiconductor laser, 2 electroabsorption optical modulator, 4 n-InP cladding layer (first conductivity type semiconductor layer), 6 p-InP cladding layer (second conductivity type semiconductor layer), 8 cathode electrode (first electrode) ), 11 anode electrode (second electrode), 10 optical waveguide core layer, 13 semi-insulating InP layer (buried layer), 14 n-InP current blocking layer (buried layer), 16 transparent waveguide core layer

Claims

A semiconductor laser that emits laser light;
An electroabsorption optical modulator that is integrated with the semiconductor laser and modulates the laser light;
The electroabsorption optical modulator includes a first conductive semiconductor layer, an optical waveguide core layer, a second conductive semiconductor layer, and a first electrode formed under the first conductive semiconductor layer, which are sequentially stacked. And a second electrode formed on the second conductivity type semiconductor layer directly above the optical waveguide core layer,
The light absorption coefficient of the optical waveguide core layer is changed by a voltage applied between the first electrode and the second electrode,
An optical modulator integrated semiconductor laser, wherein the width of the optical waveguide core layer gradually decreases toward a light emitting end of the electroabsorption optical modulator.
Embedded layers embedding both sides of the optical waveguide core layer;
A transparent waveguide core layer formed around the optical waveguide core layer and in contact with the optical waveguide core layer;
The second electrode is not formed directly above the transparent waveguide core layer, or is not in contact with the second conductivity type semiconductor layer,
The refractive index of the transparent waveguide core layer is a value between the refractive index of the buried layer and the refractive index of the optical waveguide core layer,
2. The optical modulator integrated semiconductor laser according to claim 1, wherein a band gap energy of the transparent waveguide core layer is larger than a band gap energy of the optical waveguide core layer.
3. The optical modulator integrated semiconductor laser according to claim 2, wherein a total width of the optical waveguide core layer and the transparent waveguide core layer is gradually reduced toward an emission end of the electroabsorption optical modulator.
The optical modulator integrated semiconductor laser according to any one of claims 1 to 3, wherein the optical waveguide core layer extends to a light emitting end.