JPS60212727A - Loading type directional coupler - Google Patents

Abstract

PURPOSE:To form a pair of loading type waveguides separated electrically by only selective etching of a clad layer, and to obtain a good optical modulation characteristic by providing a loading clad layer of high specific resistance and the first or the second conductive type, between an optical waveguide of high specific resistance and the first conductive type and a loading clad layer of low specific resistance and the second conductive type. CONSTITUTION:As for an element, after a multi-layer epitaxial growth, Au/Zn electrodes 5, 5' and an Au/Sn electrode 6 are vapor-deposited on the surface and the rear side, respectively, a stripe-like pattern which becomes a waveguide is formed on the surface by photolighography, and other part than Au/Zn electrodes 4, 4' is etched. Subsequently, an InP layer of other part than loading InP clad layers 4, 4' and 10, 10' is removed by selecting etching by using an etchant of an HCl compound, and an incident end 11 and an emitting end 12 are formed by cleavage, by which the element can be manufactured. In this way, by leading in an n<->-InP layer between an n<-> InGaAsP optical waveguide layer of a loading type directional coupler and a P<+> InP clad layer, a pair of loading type waveguides separated electrically by only executing the selective etching can be formed, and a good optical modulation characteristic is obtained by the element of this structure.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an optical switch as an optical communication component and a directional coupler that is promising as an optical modulator.

Conventional configuration and its problems A directional coupler utilizes optical coupling by arranging a pair of optical waveguides in close proximity, and if a material with an electro-optic effect is used, the degree of optical coupling can be changed by applying a voltage. It is possible to perform intensity modulation and switching of light. Usually, such elements are often made of dielectric materials such as LiNbO2, but when semiconductor materials are used, they can be integrated with light-emitting elements such as semiconductor lasers, light-receiving elements, and other electrical elements, making it possible to create optical integrated circuits. This is very promising from this standpoint.

As a directional coupler using a semiconductor, a so-called loaded directional coupler that can utilize a pn junction is most advantageous from the standpoint of modulation power and the like.

Figure 1 shows InP/InGa, which is a long wavelength laser material.
A cross-sectional view of a directional coupler using an AsP system is shown.

This is a To-doped n'' on an S-doped n+InP substrate 1.
InP buffer single layer 21 non-dorg n-InGaAsP
Optical waveguide layer 3, Zn-doped PJnP cladding layer 4
Sequentially epitaxy '/+# and grow A on the P+InP side.
After forming an Au-8n electrode on the n+InP side of the u-Zn electrode 6°, the P''InP cladding layer 6 is selectively etched with an HCl-based etchant to remove the loaded cladding layer 4, 4', leaving a waveguide pattern. is formed, and an incident end face 11 and an outgoing end face are further formed by cleavage.

The portion of the n-I nGaAs P optical waveguide layer 3 directly below the loaded cladding 4.4' is the three-dimensional waveguide 7, 7'.
form.

Here, when light enters one three-dimensional waveguide 7', the light waves are coupled between the three-dimensional waveguides 7 and 7', and the adjacent waveguide 7
to move to. Furthermore, one Au/Zn electrode 6, 6'
When a voltage is applied to the Au/Sn electrode 6 on the back surface so as to apply a reverse bias to the pn junction, the power ratio of the output lights 9 and 9' changes due to the change in the coupling state between the waveguides due to the electro-optic effect, resulting in light modulation. and switching can be performed.

However, in order to effectively modulate this light, the pair of waveguides 7 and 7' must be electrically separated. However, in reality, during epitaxial growth n-I
A P-type dopant is autodoped into the nGaAsP optical waveguide layer 3, and a pn junction is formed inside the n-I nGaAsP optical waveguide layer 3. Therefore, in the element structure shown in FIG. 1, leakage current is large, and a very large reverse bias cannot be applied. Furthermore, the two waveguides 7, 7' are not electrically isolated, and voltage cannot be applied independently to one waveguide, making it impossible to obtain good optical modulation characteristics.

In order to achieve complete electrical isolation in a device with such a structure, it is necessary to use n-InGa when etching the loaded cladding.
The AaP optical waveguide layer 3 must be further etched below the pn junction. However, when the optical waveguide layer 3 is etched, the shape of the waveguides 7, 7' changes, and the coupling of light between the waveguides becomes weaker, so that it takes a very long distance for light to migrate to an adjacent waveguide. Furthermore, it is difficult to keep the waveguide parameters constant due to variations in etching.

Therefore, it is desirable to stop the etching at the interface between the InP cladding layers 4, 4' and the InGaAaP optical waveguide layer 3, and to electrically separate the pair of waveguides.

In this way, in the loaded directional coupler having the structure shown in FIG. It was not possible to obtain optical modulation characteristics.

OBJECTS OF THE INVENTION It is an object of the present invention to overcome the problems of the conventional loaded directional couplers described above and to provide a directional coupler having good optical modulation characteristics.

Composition of the Invention The present invention provides a loaded directional coupler having a conventional structure between an optical waveguide layer of a first conductivity type and high resistivity and a loaded InP cladding layer of a second conductivity type and low resistivity. This is a loaded directional coupler with a structure in which a loaded InP cladding layer of the first or second conductivity type and high resistivity is introduced.By introducing this layer, the shape of the waveguide can be changed only by selective etching of the cladding layer. Individual waveguides can be electrically separated and formed without breaking, and good optical modulation characteristics can be obtained.

DESCRIPTION OF EMBODIMENTS FIG. 2 is a sectional view showing the structure of a loaded directional coupler of the present invention. Here, 1 to 6 are as shown in Figure 1, and 1
0 and 10' are loaded n"'-InP cladding layers according to the present invention. The carrier density of each layer is S-doped n"-
InP substrate 1 is 3 x 1018/d, Te-doped ♂-InP buffer 1 and layer 2 is 3 x 1 o18. z7
, non-dorg n"'-I nGaAs P layer 3 is lX
10/-, non-doped n-InP cladding layer 10, 1
0' is I X 1016/ctA, ZnDawg P+-
InP layers 4, 4' are 8×1010111.

This device is made of Au on the surface after multilayer epitaxial growth.
/Zn electrode 5.5', Au/s n electrode 6 is vapor-deposited on the back surface, and a striped pattern that will become a waveguide is formed on the surface by photophosphorography to form Au/Zn electrode 4.
, 4', and then the loaded InP cladding layer 4.4', 4' is etched using an HC1-based etchant.
The InP layer at the portion other than 10.10' is removed by selective etching, and the input end 11.10' is removed by selective etching. It can be manufactured by forming the output end 12 by cleavage. Au/Zn electrode of the device actually made by pressing in this way 6. or 5'A
When a reverse bias is applied between the u/Sn electrode 6, the breakdown voltage is 20
The leakage current was less than 1μ8 when 10V was applied.Furthermore, the leakage current between the two Au/Zn electrodes 5 and 5' was 20V or less.
They exhibited a breakdown voltage of V or higher and were electrically independent.

FIG. 3 shows the optical modulation characteristics of a loaded directional coupler with this structure. Here, the horizontal axis is the reverse voltage applied between the Au/Zn electrode and the Au/Sn electrode 6, and the vertical axis is the relative optical output P1゜P2 of the emitted light 9, 9' at each waveguide end. .

The structural parameters of this element are T = 1. c>μ
mH1=0.3 prn H2==1. Opm W=
3.0 prn S = 3.0. canL=2m. It can be seen from FIG. 3 that an extinction ratio of about 20 dB was obtained with an applied voltage of 24 V, and good optical modulation characteristics were obtained.

As described above, according to this embodiment, by introducing the n7-InP layer between the n''-InGaAsP optical waveguide layer and the P''InP cladding layer of the loaded directional coupler, selective etching of InP is achieved. By simply carrying out this process, a pair of electrically isolated loaded waveguides can be formed, and an element with this structure can obtain good optical modulation characteristics.

Note that the n InP layer in this embodiment may be a P--InP layer. Further, in this embodiment, the material is limited to InP, but it is not limited to this as long as selective etching between the optical waveguide layer and the cladding layer can be utilized.Advantages of the InventionThe loaded directional coupler of the present invention is , a high resistivity loaded cladding layer of the first or second conductivity type is applied between the high resistivity first conductivity type leading wave layer and the low resistivity loaded landing layer of the low resistivity second conductivity type; Therefore, a pair of electrically separated loaded waveguides can be formed only by selective etching of the cladding layer, and by using this device, good optical modulation characteristics can be obtained, and its practical effects are great.

[Brief explanation of the drawing]

Fig. 1 is a structural diagram of a conventional loaded directional coupler, Fig. 2 is a structural diagram of a loaded directional coupler in an embodiment of the present invention, and Fig. 3 is a structural diagram of a loaded directional coupler in an embodiment. FIG.

Claims (1)

[Claims] On a semiconductor substrate of a first conductivity type and a low specific resistance, or on a buffer layer of a first conductivity type and the same material as the semiconductor substrate on the semiconductor substrate, the refractive index is lower than that of the semiconductor substrate with a first conductivity type. and a high resistivity optical waveguide layer, and a second layer on top of the optical waveguide layer.
a loaded cladding layer of conductivity type and low resistivity, electrodes are provided on a lower surface of the semiconductor substrate and an upper surface of the loaded cladding layer, and a first or second electrode is provided between the optical waveguide layer and the loaded cladding layer. A loaded directional coupler characterized in that a second loaded cladding layer of conductivity type and high specific resistance is provided.