JPH0675195A - Optical control device - Google Patents

Abstract

PURPOSE:To enable stable operation by inserting a layer consisting of a material having the lower ion transfer rate than the ion transfer rate of a buffer layer between a buffer layer and a substrate. CONSTITUTION:The buffer layer 3a is loaded on the substrate 1 of a directional coupler 5 consisting of two pieces of optical waveguides formed on the LiNbO3 or LiTaO3 substrate 1 having an electrooptical effect and the buffer layer 3 is loaded thereon. Electrodes 4a, 4b mainly consisting of a metallic material are formed via the buffer layers 3a, 3 on the optical waveguides 2. Further, a conductive film 6 is loaded thereon. The buffer layer 3a formed between the substrate 1 and the buffer layer 3 is formed by using the material having the low ion transfer rate. Then, the Li ions diffused from the substrate 1 are blocked by the buffer layer 3a having the low ion transfer rate and are prevented from being incorporated into the buffer layer 3 even if the substrate is subjected to a heat treatment at the time of forming the buffer layer 3. The quantity of the ions capable of moving within the buffer layers is, therefore, decreased and the DC drift is decreased.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical control device for modulating a light wave and switching an optical path, and more particularly to a waveguide for performing control using an optical waveguide formed in a LiNbO ₃ or LiTaO ₃ substrate having an electro-optical effect. Type optical switch.

[0002]

2. Description of the Related Art With the practical use of optical communication systems, higher capacity and multifunctional advanced systems are required, and new functions such as generation of higher-speed optical signals, switching of optical transmission lines, and exchanges are required. Is required. In the current practical system, the optical signal is obtained by directly modulating the injection current of the semiconductor laser or the light emitting diode, but in the direct modulation, high-speed modulation of several GHz or more due to the effect of relaxation oscillation and the occurrence of wavelength chirping. However, it is difficult to apply to the coherent optical transmission system because of the wavelength variation. As a means for solving this, there is a method of using an external modulator, and in particular, a waveguide type optical modulator constituted by an optical waveguide formed in an electro-optic crystal substrate has a small size, high efficiency, and high speed. There are advantages.

On the other hand, an optical switch is used as a means for obtaining the function of switching the optical transmission line and switching the network. The optical switch currently in practical use is a prism,
Since the optical path is switched by mechanically moving a mirror, a fiber, etc., there are drawbacks such as low speed, large shape, and unsuitable for matrix formation. As a means for solving this, a waveguide type optical switch using an optical waveguide is being developed, and it has features such as high speed, multi-element integration, and high reliability. In particular, a material using a ferroelectric material such as lithium niobate (LiNbO ₃ ) crystal is characterized by low light absorption and low loss and high efficiency because it has a large electro-optical effect. , Directional coupler type optical modulators or optical switches, total reflection type optical switches, Mach-Zehnder type optical modulators and other various types of optical control devices have been reported.

In recent years, research and development of high-density integration of this waveguide type optical switch have been actively conducted, and according to the literature of Yu Nishimoto et al., IEICE OQE 88-147, L
6 directional coupler type optical switch using iNbO ₃ substrate
We have obtained an 8x8 matrix optical switch with 4 elements integrated. On the other hand, research and development of a device including a single optical switch element such as an external optical modulator has been actively pursued.
The characteristic items of such an optical switch device include stability of operation against environment such as switching voltage (power), crosstalk, extinction ratio, loss, switching speed, temperature and humidity,
In addition, there is stability of operation during continuous application of voltage.

[0005]

Among the above-mentioned characteristic items, stable operation is the most important point in practical use. Here, a conventional technique will be described with reference to the drawings. FIG. 4 is a sectional view showing the structure of a conventional optical switch using the directional coupler 5. In FIG. 4, it has an electro-optic effect. LiNb
A buffer layer 3 is loaded on a substrate 1 including a directional coupler 5 composed of two optical waveguides 2 formed on an O ₃ or LiTaO ₃ substrate (hereinafter referred to as a substrate), and the buffer layer 3 is interposed therebetween. Electrodes 4a, 4b mainly made of metal material
Are formed on the optical waveguide 2. And further conductive film 6
Is loaded. According to Sawaki et al., CLEO '86MF-2, page 46, this conductive film 6 is due to the movement of charges generated in the substrate 1 due to the pyroelectric effect of the ferroelectric substance when temperature fluctuation occurs. It has the effect of preventing characteristic instability. That is, it is considered that there is an effect of stabilizing the switch operation with respect to the temperature change, and the Si film is used. The buffer layer 3 is used to prevent the light propagating through the optical waveguide 2 from being absorbed by the electrodes 4a, 4b and the conductive film 6, and is made of an insulator that absorbs very little ordinary light, especially SiO _2. Commonly used. This is because SiO _{2 has} a refractive index of about 1.45, which is smaller than about 2.2 of the refractive index of the LiNbO ₃ or LiTaO ₃ substrate 1 having an electro-optical effect, and has almost no light absorption. If the refractive index is small, the electrode 4
The thickness of the buffer layer 3 required to prevent the absorption of light by the a and 4b and the conductive film 6 can be made thinner than when the refractive index is large. Considering the switching voltage, the electrodes 4a, 4b
When a voltage is applied to the buffer layer 3, since the dielectric constant of the buffer layer 3 is usually smaller than that of the substrate 1, the electric field concentrates on the buffer layer 3. Therefore, the thicker the buffer layer 3, the higher the switching voltage. Therefore, small refractive index as the buffer layer 3, and light absorption SiO ₂ is used extremely small, SiO ₂ in terms of both absorption current refractive index and optical
There is no better material for the buffer layer 3.

However, when SiO ₂ is used as the buffer layer 3 and the electrode 4a is grounded and a DC voltage is continuously applied to the electrode 4b, the ions in the buffer layer are pulled by the electric field and applied to the bottom of each electrode 4a, 4b from the outside. Ions having the opposite sign of the applied voltage gather. Therefore, a counter electric field is generated between the electrodes 4a and 4b with respect to the electric field in the substrate generated by the voltage applied from the outside. The magnitude of this demagnetizing field increases as the total amount of movement of ions increases with time. This is because SiO ₂ generally has high electric insulation, but has high ion conductivity. This phenomenon is generally called DC drift. When the voltage applied from the outside is kept constant, when an anti-electric field is generated, the electric field applied to the optical waveguide is reduced, resulting in characteristic deterioration. That is, when a demagnetizing field that increases with time is generated, a voltage for canceling the demagnetizing field must be applied from the outside in order to restore the characteristics before the demagnetizing field is generated. This means a shift of the operating voltage point of the switch operation, which is a major obstacle to practical use. Further, this ion is mixed in the buffer layer 3 as an impurity in the manufacturing process of the buffer layer 3. In particular, Li, which is an element that constitutes the substrate 1, diffuses from the substrate 1 into the buffer layer 3 due to a thermal process at the time of manufacturing the buffer layer 3, and becomes an ion that causes a DC drift. An object of the present invention is to provide a light control device that can obtain stable operation.

[0007]

A first invention is an optical waveguide formed on a LiNbO ₃ or LiTaO ₃ substrate having an electro-optical effect, a buffer layer formed on the optical waveguide, and a portion near the optical waveguide. A waveguide type optical control device comprising an electrode formed on a buffer layer and a conductive film formed on the electrode or between the electrode and the buffer layer, wherein the buffer layer and the substrate are A layer made of a material having a lower ionic conductivity than the buffer layer is inserted between them.

The second invention is LiN having an electro-optical effect.
an optical waveguide formed on a bO ₃ or LiTaO ₃ substrate,
From a buffer layer formed on the optical waveguide, an electrode composed of a plurality of electrodes formed on the buffer layer near the optical waveguide, and a conductive film formed on the electrode or between the electrode and the buffer layer In the waveguide type optical control device, the material has an effect of trapping ions between the buffer layer and the electrode or the conductive film, and is made of a material having higher ion conductivity than the buffer layer. It is characterized in that layers are inserted.

A third aspect of the present invention is the light control device according to the first aspect, which has an effect of trapping ions between the buffer layer and the electrode or the conductive film, and is more ion-exchanged than the buffer layer. Is characterized in that a layer made of a material having high conductivity is inserted.

[0010]

In the light control device of the first invention, since the layer having a lower ionic conductivity than the buffer layer is inserted between the substrate and the buffer layer, Li ions diffused from the substrate and mixed in the buffer layer are The amount can be lowered.

In the light control device of the second invention,
By inserting a layer that can capture ions but has a higher ionic conductivity than the buffer layer above the buffer layer, it diffuses out of the substrate compared to when this layer is used instead of the buffer layer. The amount of Li ions mixed in the buffer layer can be reduced, and ions can be effectively trapped in this layer.

In the light control device of the third invention,
According to the first invention, the amount of Li ions diffused from the substrate and mixed in the buffer layer can be reduced, and further, the mixed Li ions can be effectively captured by the second invention. Therefore, it is possible to suppress the increase in switching voltage that may occur in the first invention by optimizing the film thickness of these layers.

According to these inventions, the amount of ions that can move in the buffer layer can be reduced. As a result, D
C drift can be reduced and the light control device can be operated stably.

[0014]

The present invention will be described below with reference to the drawings. FIG. 1 is a sectional view showing the structure of a light control device according to an embodiment of the first invention.

FIG. 1 is a sectional view showing the structure of an optical switch according to the first invention using a directional coupler 5. In FIG. 1, LiNbO ₃ or Li having an electro-optic effect is used.
The buffer layer 3a is loaded on the substrate 1 including the directional coupler 5 composed of the two optical waveguides 2 formed on the TaO ₃ substrate 1, and the buffer layer 3 is loaded on the buffer layer 3a.
Electrodes 4a and 4b mainly made of a metal material are formed on the optical waveguide 2 via the. Further, the conductive film 6 is further loaded. A material having a low ionic conductivity is used for the buffer layer 3a formed between the substrate 1 and the buffer layer 3. With such a structure, even if a heat treatment is performed when forming the buffer layer 3, Li ions diffused from the substrate 1 are blocked by the buffer layer 3a having a low ionic conductivity and do not mix into the buffer layer 3. Therefore, the amount of ions that can move in the buffer layer can be reduced, and DC drift can be reduced. Further, the buffer layer 3 just below the electrodes 4a and 4b is made of a conventional material, and the buffer layer 3a for preventing the mixture of ions is made thin so that the voltage in the substrate 1 is applied to the electrodes 4a and 4b. Since it has almost no effect on the electric field strength of, the rise of the switching voltage can be suppressed. As a material having a low ionic conductivity for forming the buffer layer 3a, MgF ₂ , Si ₃ N
There are ₄ etc.

FIG. 2 is a sectional view showing the structure of a light control device according to an embodiment of the second invention. In the structure shown in this embodiment, the buffer layer 3 is loaded on the substrate 1 including the directional coupler 5, and the buffer layer 3b is loaded thereon, and the electrode 4 mainly made of a metal material is interposed via the buffer layers 3 and 3b.
a and 4 b are formed on the optical waveguide 2. Further, the conductive film 6 is further loaded. A material capable of capturing ions is used for the buffer layer 3b. When the ionic conductivity of the buffer layer 3b is higher than that of the buffer layer 3, by using this structure, the amount of Li ions diffused from the substrate 1 is smaller than that when only the buffer layer 3 is used, which is effective. Capable of capturing ions, D
C drift can be reduced. As a material for forming the buffer layer 3b, SiO ₂ doped with P (phosphorus) is used.
and so on.

FIG. 3 is a sectional view showing the structure of an optical control device according to an embodiment of the third invention. In the structure shown in this embodiment, the buffer layer 3 is formed on the substrate 1 including the directional coupler 5.
a, a buffer layer 3 thereon, and a buffer layer 3b thereon, and electrodes 4a and 4b mainly made of a metal material are formed on the optical waveguide 2 through the buffer layers 3a, 3 and 3b. . Further, the conductive film 6 is further loaded. The buffer layer 3a is made of a material having a lower ionic conductivity than the buffer layer 3, and the buffer layer 3b is made of a material having a higher ionic conductivity than the buffer layer 3 that can trap ions. By using this structure, the amount of Li ions diffused from the substrate 1 and mixed in the buffer layer can be reduced, and the ions can be captured more effectively. Further, in the case of only the first invention, by increasing the film thickness of the buffer layer 3a, it is possible to suppress the mixture of Li, while the high dielectric constant also increases the switching voltage, but the buffer layer 3b is used. It is possible to suppress the amount of Li ions and suppress an increase in the switching voltage by optimizing the film thickness of.

The present invention is effective not only for the optical control device composed of the directional coupler but also for all the optical control devices of the Mach-Zehnder type, the total reflection type using the crossed waveguides and the like. It's clear.

[0019]

In the light control device of the present invention, the amount of Li ions mixed from the substrate into the buffer layer and moving in the buffer layer can be reduced, so that DC drift can be reduced and stable operation can be always obtained. .

[Brief description of drawings]

FIG. 1 is a sectional view of a structure of a light control device according to a first invention.

FIG. 2 is a sectional view of a structure of a light control device according to a second invention.

FIG. 3 is a sectional view of a structure of a light control device according to a third invention.

FIG. 4 is a sectional view of the structure of a conventional light control device.

[Explanation of symbols]

1 LiNbO ₃ or LiTa having electro-optic effect
Substrate of O ₃ 2 Optical waveguide 3, 3a, 3b Buffer layer 4a, 4b Electrode 5 Directional coupler 6 Conductive film

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[Procedure amendment]

[Submission date] November 16, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] 0015

[Correction method] Change

[Correction content]

FIG. 1 is a sectional view showing the structure of an optical switch according to the first invention using a directional coupler 5. In FIG. 1, LiNbO ₃ or Li having an electro-optic effect is used.
The buffer layer 3a is loaded on the substrate 1 including the directional coupler 5 composed of the two optical waveguides 2 formed on the TaO ₃ substrate 1, and the buffer layer 3 is loaded on the buffer layer 3a.
Electrodes 4a and 4b mainly made of a metal material are formed on the optical waveguide 2 via the. Further, the conductive film 6 is further loaded. A material having a low ionic conductivity is used for the buffer layer 3a formed between the substrate 1 and the buffer layer 3. With such a structure, even if a heat treatment is performed when forming the buffer layer 3, Li ions diffused from the substrate 1 are blocked by the buffer layer 3a having a low ionic conductivity and do not mix into the buffer layer 3. Therefore, the amount of ions that can move in the buffer layer can be reduced, and DC drift can be reduced. Further, the buffer layer 3 just below the electrodes 4a and 4b is made of a conventional material, and the buffer layer 3a for preventing the mixture of ions is made thin so that the voltage in the substrate 1 is applied to the electrodes 4a and 4b. Since it has almost no effect on the electric field strength of, the rise of the switching voltage can be suppressed. As a material having a low ionic conductivity for forming the buffer layer 3a, MgF ₂ , Si ₃ N
₄ , Si, etc.

Claims (3)

[Claims] 1. An optical waveguide formed on a LiNbO ₃ or LiTaO ₃ substrate having an electro-optical effect, a buffer layer formed on the optical waveguide, and a plurality of layers formed on the buffer layer near the optical waveguide. In a waveguide type optical control device comprising an electrode made of and a conductive film formed on the electrode or between the electrode and the buffer layer, the ionic conductivity of the buffer layer between the buffer layer and the substrate is higher than that of the buffer layer. An optical control device characterized in that a layer made of a material having a low dielectric constant is inserted. 2. An optical waveguide formed on a LiNbO ₃ or LiTaO ₃ substrate having an electro-optical effect, a buffer layer formed on the optical waveguide, and a plurality of layers formed on the buffer layer near the optical waveguide. In a waveguide type optical control device comprising an electrode made of and a conductive film formed on the electrode or between the electrode and the buffer layer, between the buffer layer and the electrode or the conductive film. A light control device, wherein a layer made of a material capable of trapping ions of the buffer layer is inserted. 3. The light control device according to claim 1, wherein a layer made of a material capable of trapping ions of the buffer layer is inserted between the buffer layer and the electrode or the conductive film. An optical control device characterized by.