JPH0697315B2 - Loaded directional coupler - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical switch as a component for optical communication and a directional coupler which is promising as an optical modulator.

Configuration of conventional example and its problems In a directional coupler, a pair of optical waveguides are juxtaposed side by side to utilize light coupling, and if a material having an electro-optical effect is used, the degree of light coupling is changed by voltage application. It is possible to perform intensity modulation and switching of light. Usually, such elements are often made of dielectric materials such as LiNbO _3, but when semiconductor materials are used, it becomes possible to integrate with light-emitting elements such as semiconductor lasers, light-receiving elements, and other electrical elements. Very promising from a circuit standpoint.

Usually, as a directional coupler using a semiconductor, a so-called loaded directional coupler that can use a pn junction from the standpoint of modulation power is most advantageous.

FIG. 1 shows a cross-sectional view of a directional coupler using an InP / InGaAsP system, which is a long-wavelength laser material. This is a Te-doped n ⁺ InP buffer layer 2, a non-doped n ^- InGaAsP optical waveguide layer 3, and a Zn-doped P ⁺ InP clad layer 4 which are sequentially epitaxially grown on an S-doped n ⁺ InP substrate 1 and then grown on the P ⁺ InP side. Au-Zn electrode
5, n ⁺ InP After forming Au-Sn electrode 6 on the substrate 1 side, P ⁺ InP
The cladding layer 4 is removed by selectively etching it with an HCl-based etchant leaving the waveguide pattern.
4, 4'are formed, and then the incident end face 11 and the output end face are formed by cleavage. In the n ^- InGaAsP optical waveguide layer 3, the portions directly under the loaded claddings 4, 4'form the three-dimensional waveguides 7, 7 '.

Here, when light is incident on one of the three-dimensional waveguides 7 ', the light waves are coupled between the three-dimensional waveguides 7 and 7', and the light is transferred to the adjacent waveguide 7. Furthermore, one Au / Zn electrode 5, 5'and the Au / Sn on the back surface
When a voltage is applied to the electrode 6 so that a reverse bias is applied to the pn junction, the power ratio of the emitted light 9 and 9'can be changed by the change of the coupling state between the waveguides due to the electro-optical effect, and the modulation and switching of the light can be performed. it can.

By the way, 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 ^- InGaAs
P-type dopant is auto-doped in the P optical waveguide layer 3
The junction surface is formed inside the n ⁻ InGaAsP optical waveguide layer 3. Therefore, in the device structure shown in FIG. 1, the leak current is large and a too large reverse bias cannot be applied. Further, the two waveguides 7 and 7'are not electrically separated from each other, so that a voltage cannot be independently applied to one waveguide, and good optical modulation characteristics cannot be obtained.

In order to achieve complete electrical isolation in the device having such a structure, it is necessary to further etch the n ^-- InGaAsP optical waveguide layer 3 down to the Pn junction surface when etching the loading clad. However, when the optical waveguide layer 3 is etched, the shapes of the waveguides 7 and 7'change, the coupling of the light between the waveguides is weakened, and it takes a very long distance for the light to move to the adjacent waveguide. Further, it is difficult to keep the waveguide parameters constant due to variations in etching. Therefore In
It is a desirable method to stop the etching at the interface between the P clad layers 4 and 4'and the InGaAsP optical waveguide layer 3 and electrically separate the pair of waveguides.

As described above, in the loading type directional coupler having the structure shown in FIG. 1, since the pn junction surface is formed inside the optical waveguide layer 3, it is not possible to apply an electric field to each waveguide independently, which is favorable. The light modulation characteristic could not be obtained.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a directional coupler having good optical modulation characteristics by overcoming the problems of the conventional loaded directional coupler having the above structure.

According to the present invention, in a conventional loaded directional coupler, an n-type and low carrier concentration is provided between an n-type and low carrier concentration optical waveguide layer and a p-type and high carrier concentration second loaded cladding layer. This is a loading type directional coupler having a structure in which a first loading clad layer having a carrier concentration is introduced. By introducing this layer, it is possible to prevent p-type impurities in the second loading clad layer from diffusing into the optical waveguide layer. Further, the individual waveguides can be electrically separated and formed without destroying the shape of the waveguides only by selective etching of the first loading clad layer and the optical waveguide layer, and good optical modulation characteristics can be obtained. .

Description of Embodiments FIG. 2 is a sectional view showing the structure of a loading type directional coupler of the present invention. Here, 1 to 6 are as shown in FIG.
Reference numerals 0 and 10 'are the loaded n ^-- InP cladding layers according to the present invention. The carrier density of each layer is 3 for S-doped n ⁺ -InP substrate 1.
× 10 ¹⁸ / cm ³ , Te-doped n ⁺ -InP buffer layer 2 is 3 × 10 ¹⁸
/ cm ^3, an undoped n ^- -InGaAsP layer 3 is 1 × 10 ¹⁶ / cm ^3, an undoped n ^- -InP cladding layer 10, 10 '1 × 10 ¹⁶ / cm ^3, Zn-doped P ⁺ -InP layer 4 and 4 ′ Is 8 × 10 ¹⁷ / cm ³ .

This device was grown epitaxially and then Au / Z was formed on the surface.
n / electrodes 5 and 5 ', Au / Sn electrode 6 is vapor-deposited on the back surface, a stripe pattern is formed on the front surface by photolithography, and parts other than the Au / Zn electrodes 4 and 4'are etched. Subsequently, the HCl-based etchant is used to remove the InP layer other than the loaded InP cladding layers 4, 4 ', 10, 10' by selective etching, and the incident end 11 and the outgoing end 12 are formed by cleavage. Actually, when a reverse bias was applied between the Au / Zn electrode 5 and the 5'Au / Sn electrode 6 of the device thus manufactured, the leak current was 1 μA or less when the breakdown voltage was 20 V or more and 10 V was applied. . 2 more
The two Au / Zn electrodes 5 and 5 ′ also showed a withstand voltage of 20 V or more and were electrically independent.

The optical modulation characteristics of the loaded directional coupler of this structure are shown in FIG. Here, the horizontal axis is the reverse voltage applied between the Au / Zn electrode Au / Sn electrode 6, and the vertical axis is the emitted light at each waveguide end 9,
9'relative light output P ₁ , P ₂ .

As a structural parameter of this device, T = 1.0 μmH ₁ = 0.3
μm H ₂ = 1.0 μm W = 3.0 μm S = 3.0 μm L = 2 mm.
It can be seen from FIG. 3 that an extinction ratio of about 20 dB is obtained with an applied voltage of 24 V and good optical modulation characteristics are obtained.

As described above, according to this embodiment, the loading type directional coupler
n ^- n between -InGaAsP optical waveguide layer and the P ⁺ InP cladding layer ^- -InP
By introducing the layer, a pair of loaded waveguides that can be electrically separated can be formed only by performing selective etching of InP, and an excellent optical modulation characteristic can be obtained by the element of this structure. In this embodiment, the material is limited to the InP-based material, but the material is not limited to these as long as the selective etching between the optical waveguide layer and the cladding layer can be used.

EFFECTS OF THE INVENTION The loaded directional coupler according to the present invention is provided between the n-type optical waveguide layer having a low carrier concentration and the p-type second loading clad layer having a high carrier concentration, and the first n-type carrier having a low carrier concentration. By providing the loading clad layer, it is possible to prevent p-type impurities in the second loading clad layer from diffusing into the optical waveguide layer, and to electrically perform only by selective etching of the first loading clad layer and the optical waveguide layer. A pair of loaded waveguides separated from each other can be formed, and good optical modulation characteristics can be obtained by using this element, and its practical effect is great.

[Brief description of drawings]

FIG. 1 is a structural diagram of a conventional loading type directional coupler, FIG. 2 is a structural diagram of a loading type directional coupler in an embodiment of the present invention, and FIG. 3 is a loading type directional coupler in an embodiment. It is a light modulation characteristic view of. 1 …… n ⁺ −InP substrate, 2 …… InP buffer layer, 3 …… n ⁻
InGaAsP layer, 4,4 '... P ⁺ InP layer, 10,10' ... n ^- InP clad layer.

Claims (1)

[Claims] 1. An n-type semiconductor substrate, an n-type optical waveguide layer formed on the substrate and having a low carrier concentration, and a composition capable of being selectively etched on the optical waveguide layer with the optical waveguide layer. A pair of striped first loading clad layers having a low carrier concentration and a predetermined width and being parallel to each other; and a p layer formed on the first loading clad layer with a width substantially equal to that of the first loading clad layer. A pair of striped second loading clad layers having a high carrier concentration and an electrode formed on the second loading clad layer, wherein the first loading clad layer comprises the second loading clad layer and the second loading clad layer. The p-type impurities are prevented from diffusing into the optical waveguide layer, and the first and second loading clad layers form a pair of stripes. The first and second loading clad layers directly under the stripes. Of the optical waveguide An optical waveguide is formed on each of the optical waveguides, and the pair of stripes are located close to each other so that light guided through the optical waveguides can be coupled to each other. Then, by applying a reverse bias, the refractive index of each of the optical waveguides is independently changed, and the light emitted from the optical waveguides is modulated.