JPH06347737A - Optical control device - Google Patents

Abstract

PURPOSE:To provide the optical control device which is stable in characteristics over a long period of time by diminishing the DC drift which has been a problem in the conventional optical control device. CONSTITUTION:Optical waveguides 21, 22, 23 and 24 formed by diffusing titanium are formed on a lithium niobate crystal substrate 1 of a Z-plate to constitute a Mach-Zehunder interference system. Control electrodes 25 and 26 are formed on the optical waveguides 22 and 23. These control electrodes 25, 26 are so formed respectively of the same size and are so constituted that electric fields of opposite directions are impressed to the optical waveguides 22, 23. The control electrode 25 is so installed as to overlap on the optical waveguides 22, 23. The electric field 7 of a vertical direction is impressed to the optical waveguides 22, 23. On the other hand, the control electrode 26 is so installed as to hold the optical waveguides 22, 23 and the electric field 8 of a transverse direction is impressed to the optical waveguides 22, 23.

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 type optical control device for controlling using an optical waveguide provided in a substrate.

[0002]

2. Description of the Related Art As the practical use of optical communication systems progresses,
There is a demand for advanced systems with large capacity and multiple functions, and new functions such as generation of higher-level optical signals, switching of optical transmission lines, and replacement are 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. However, direct modulation has an effect such as relaxation oscillation.
There are drawbacks such as difficulty in high-speed modulation of around 0 GHz or more, and difficulty in application to the coherent optical transmission system due to 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 a substrate is characterized by small size, high efficiency and high speed. On the other hand, an optical switch is used as a means for obtaining the function of switching the optical transmission line and the switching function of the network. Currently used optical switches are
It mechanically moves a prism, a mirror, a fiber, and the like, and has drawbacks such as low speed, insufficient reliability, large size of a single unit, and unsuitable for matrix formation.

What is being developed as a means for solving this is a waveguide type optical switch which also uses an optical waveguide, and is characterized by 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. Conventionally, various types of optical control elements such as a directional coupler type optical modulator / switch, a total reflection type optical switch, and a Mach-Zehnder type optical modulator have been reported.

FIGS. 5A and 5B show a plan view and a sectional view of a Mach-Zehnder type optical modulator as an example of a conventional optical control device. FIG. 5B is a broken line b- of FIG.
The sectional view of b'part is shown. In FIG. 5 (a),
Band-shaped optical waveguides 21, 22, 23, and 2 formed by diffusing titanium (Ti) on the lithium niobate crystal substrate 1 cut out perpendicularly to the Z-axis to make the refractive index larger than that of the substrate.
4 are formed. The optical waveguide 21 branches into optical waveguides 22 and 23 near the center of the substrate, and then merges again to become an optical waveguide 24, which constitutes a Mach-Zehnder type interference system. A control electrode 61 is formed on the two optical waveguides 22 and 23 of the Mach-Zehnder via a buffer layer 62 for preventing light absorption by the electrodes.

Incident light 5 that has entered the optical waveguide 21 is split into two optical waveguides 22 and 23, propagates through the respective branches, and then joins the optical waveguide 24 to become emitted light 6. Usually 2
The optical waveguides 22 and 23 of the two branches have the same optical path length, and the control electrode 61 is installed so that the two branches have the same size and the electric field 7 in the opposite direction is applied. When a voltage is applied to the control electrode 61, the optical waveguide 2 below the control electrode 6 is caused by the electro-optic effect.
The refractive indexes of the optical waveguides 2 and 23 change, the lights propagating through the optical waveguides 22 and 23 undergo phase modulation in opposite directions, and the power of the waveguide mode of the optical waveguides 24 after merging changes.

[0006]

However, in the conventional light control device as shown in FIG. 5, the electric charge in the crystal is locally accumulated at the interface between the crystal and the film by the application of the DC voltage, and the electric field acts on the light wave. A phenomenon in which the intensity changes, that is, a DC drift is likely to occur, and there is a problem in device stability.

An object of the present invention is to provide an optical control device having stable characteristics over a long period of time, except for the above-mentioned problems of the conventional optical control device.

[0008]

The present invention provides a Mach-Zehnder interference type optical control device comprising an optical waveguide formed on a dielectric crystal substrate having an electro-optical effect and a control electrode installed in the vicinity thereof, As means for applying an electric field for light control, a first control electrode for applying an electric field in the thickness direction of the substrate to the optical waveguide and a propagation direction of light in the substrate to the optical waveguide are provided in the same Mach-Zehnder interferometer. And a second control electrode for applying an electric field in a direction perpendicular to the direction.

Further, the present invention provides a directional coupler type optical control device comprising an optical waveguide formed on a dielectric crystal substrate having an electro-optical effect and a control electrode provided in the vicinity thereof for optical control. As means for applying the electric field, the first control electrode for applying an electric field in the thickness direction of the substrate to the optical waveguide and the optical waveguide in the thickness direction of the substrate and the propagation direction of light are provided in the same directional coupler. And a second control electrode for applying an electric field in a vertical direction.

[0010]

The light control device of the present invention comprises, as means for applying an electric field for light control, a control electrode for applying an electric field in the thickness direction of the substrate (hereinafter, this direction is referred to as a vertical direction) to the optical waveguide.
The optical waveguide is provided with both a thickness direction of the substrate and a control electrode for applying an electric field in a direction perpendicular to the light propagation direction (hereinafter, this direction is referred to as a lateral direction). According to the experiments by the inventors,
When a vertical electric field is applied, DC drift occurs in a direction that weakens the applied electric field (hereinafter, such DC drift is referred to as positive DC drift), and conversely, when a horizontal electric field is applied, DC drift occurs. Occurs in the direction in which the applied electric field is strengthened (hereinafter, such a DC drift will be a negative DC
Called drift). Therefore, by combining the vertical electric field and the horizontal electric field, the positive DC drift and the negative DC drift can be canceled and the DC drift can be reduced.

From the above, the light control device of the present invention can provide a more stable light control device than the conventional one.

[0012]

1 (a), 1 (b) and 1 (c) are a plan view and a sectional view of a Mach-Zehnder type optical modulator which is an embodiment of an optical control device according to the present invention. Broken line b in FIG.
A sectional view of the portion -b 'is shown in FIG. A cross-sectional view taken along the broken line cc 'of FIG. 1A is shown in FIG.

On the Z plate lithium niobate crystal substrate 1,
Optical waveguides 21, 22 and 23 of about 3 to 10 μm formed by thermally diffusing titanium at 900 to 1100 ° C. for several hours.
And 24 are formed. The optical waveguide 21 branches into two to become optical waveguides 22 and 23, and then merges to form an optical waveguide 24, which constitutes a Mach-Zehnder interference system.
Control electrodes 25 and 26 are formed on the optical waveguides 22 and 23. The control electrodes 25 and 26 are configured such that electric fields having the same magnitude and opposite directions are applied to the waveguides 22 and 23, respectively. The control electrode 25 is the optical waveguide 22,
It is installed so as to overlap with 23, and a vertical electric field 7 is applied to the optical waveguides 22 and 23. On the other hand, the control electrode 26 is installed so as to sandwich the optical waveguides 22 and 23, and
A horizontal electric field 8 is applied to the electrodes 2 and 23. As described in the action section, a positive DC drift occurs with respect to the vertical electric field, and a negative DC drift occurs with respect to the lateral electric field. Moreover, the time constants of both DC drifts are almost equal. Therefore, T due to the control electrodes 25 and 26
E mode, that is, the refractive index changes Δn _d and Δ of the polarized light in the lateral direction
n _{t respectively, Δn d = [1-D} (t)] Γ d r 13 n o 3 V d L d / (2g d) (1) Δn t = [1 + αD (t)] Γ t r 22 n o It can be expressed as ³ V _t L _t / (2g _t ) (2). Here, D (t) is the amount of DC drift due to the vertical electric field, α is the ratio of the magnitude of the DC drift amount between the vertical electric field and the horizontal electric field, Γ is the electric field correction coefficient, r ₁₃ and r ₂₂ are electro-optical coefficients, and n _o is the refractive index of ordinary light, V is the applied voltage, L
Is the electrode length, g is the electrode spacing, and suffixes d and t are the electrode 25 for applying a vertical electric field and the electrode 26 for applying a horizontal electric field, respectively. Therefore, as shown in FIG. 1A, the vertical electric field electrode 25 and the horizontal electric field electrode 2 are formed.
If the 6 cascaded, since the phase variation of the Mach-Zehnder each branch is equal to the sum of the amount of phase change at each control _{electrode, Γ d r 13 V d L} d / g d = αΓ t r 22 V t L t / By adjusting the applied voltage of each electrode, the electrode length, the electrode interval, etc. so as to satisfy the relationship of g _t (3), DC in each branch
The phase change due to drift can be made zero. Since the output light intensity of the Mach-Zehnder is determined by the phase difference between the two branches, if the DC drift of the phase change amount at each branch can be made zero, the DC drift of the output light intensity of the Mach-Zehnder optical modulator will also be made zero. Therefore, a stable optical modulator can be obtained.

2A and 2B are a plan view and a sectional view of a Mach-Zehnder type optical modulator according to another embodiment of the present invention.

FIG. 2B is a sectional view of the broken line bb 'in FIG. 2A. Similar to the example of FIG. 1, the optical waveguides 21, 22, 23 and 24 are formed on the Z-plate lithium niobate crystal substrate 1.
Are formed to form a Mach-Zehnder interference system. A control electrode 31 is formed on the optical waveguides 22 and 23. The control electrode 31 is installed so as to overlap the optical waveguide 22 and sandwich the optical waveguide 23. A vertical electric field 7 is applied to the photoelectrode 22 and a horizontal electric field 8 is applied to the optical waveguide 23 by the control electrode 31. An electric field is applied to the optical waveguides 22 and 23 of the two branches of the Mach-Zehnder so that the respective phase changes are in opposite directions. Since the output light intensity of the Mach-Zehnder is determined by the phase difference between the two branches, the control electrode 31 adjusts the voltage applied to the electrodes, the electrode length, the electrode interval, etc. so that the control electrode 31 satisfies the two conditions. DC of the amount of phase change in each branch
The drifts are canceled at the time of merging, the change in the output light intensity due to the DC drift can be made zero, and a stable light intensity modulator can be obtained.

FIGS. 3A, 3B and 3C are a plan view and a sectional view of a directional coupler type optical switch according to an embodiment of the present invention.

A sectional view taken along the broken line bb 'in FIG. 3A is shown in FIG. 3B, and a sectional view taken along the broken line cc' is shown in FIG. 3C. Z
Plate 900 titanium on the lithium niobate crystal substrate 1
〜1 to 100 formed by thermal diffusion at about 1100 ° C for several hours
Optical waveguides 41 and 42 of about 0 μm are formed,
In the central part of the substrate, both optical waveguides are close to each other by several μm to form a directional coupler. Control electrodes 43 and 44 are installed on it. The control electrode 43 is the optical waveguide 4
The optical waveguides 41 and 42 are installed so as to overlap with the optical waveguides 41 and 42.
A vertical electric field 7 is applied to. On the other hand, the control electrode 44
Are installed so as to sandwich the optical waveguides 41 and 42, and a lateral electric field 8 is applied to the optical waveguides 41 and 42. With the above configuration, the DC drift of the directional coupler type optical switch can be reduced by the same principle as that of the Mach-Zehnder type modulator of FIG.

4A and 4B are a plan view and a sectional view of a directional coupler type optical switch according to another embodiment of the present invention.

A sectional view taken along the broken line bb 'of FIG. 1A is shown in FIG. Similar to the case of FIG. 3, the optical waveguides 41 and 42 are formed on the Z-plate lithium niobate crystal substrate 1, and both optical waveguides are close to each other up to several μm in the central portion of the substrate and the directional coupler is used. Are configured. The control electrode 51 is installed on it. The control electrode 51 is the optical waveguide 4
The optical waveguide 22 is placed so as to overlap with the optical waveguide 1. A vertical electric field 7 is applied to the optical waveguide 41 by the control electrode 51, and a horizontal electric field 8 is applied to the optical waveguide 42.
Is applied. With the above configuration, the DC drift of the directional coupler type optical switch can be reduced by the same principle as in the case of the Mach-Zehnder type modulator of FIG.

[0020]

As described above, in the light control device of the present invention, the DC drift can be reduced, so that the light control device having stable characteristics can be obtained as compared with the conventional light control device.

[Brief description of drawings]

FIG. 1 is a plan view and a cross-sectional view showing an example of a light control device according to the present invention.

FIG. 2 is a plan view and a sectional view showing an example of a light control device according to the present invention.

FIG. 3 is a plan view and a cross-sectional view showing an example of a light control device according to the present invention.

FIG. 4 is a plan view and a cross-sectional view showing an example of a light control device according to the present invention.

5A and 5B are a plan view and a cross-sectional view showing an example of a conventional light control device.

[Explanation of symbols]

 1 Lithium niobate crystal substrate 21, 22, 23, 24, 41, 42 Optical waveguide 25, 26, 31, 43, 44, 51, 61 Control electrode 5 Incident light 6 Emission light 7 Vertical electric field 8 Horizontal electric field 62 Buffer layer

Claims (6)

[Claims] 1. A Mach-Zehnder interference type optical control device comprising an optical waveguide formed on a dielectric crystal substrate having an electro-optical effect and a control electrode installed in the vicinity thereof, and means for applying an electric field for optical control. In the same Mach-Zehnder interferometer, a first control electrode for applying an electric field in the thickness direction of the substrate to the optical waveguide and an electric field in the direction perpendicular to the thickness direction of the substrate and the light propagation direction are applied to the optical waveguide. A light control device comprising: a second control electrode for applying. 2. The optical waveguide constitutes a Mach-Zehnder interference system having a branch portion, the first control electrode is installed so as to overlap the branch portion, and the second control electrode is provided with the branch portion. The light control device according to claim 1, wherein the light control device is installed so as to sandwich it. 3. The optical waveguide constitutes a Mach-Zehnder interference system having a first branched optical waveguide and a second branched optical waveguide, and the first control electrode is overlapped with the first branched optical waveguide. The light control device according to claim 1, wherein the light control device is installed, and the second control electrode is installed so as to sandwich the second branched optical waveguide. 4. A directional coupler type optical control device comprising an optical waveguide formed on a dielectric crystal substrate having an electro-optical effect and a control electrode installed in the vicinity thereof, wherein an electric field for optical control is provided. As an applying means, a first electric field is applied to the optical waveguide in the thickness direction of the substrate in the same directional coupler.
And a second control electrode for applying an electric field to the optical waveguide in a direction perpendicular to the thickness direction of the substrate and the light propagation direction. 5. The optical waveguide constitutes a directional coupler having a proximity portion, the first control electrode is installed so as to overlap the proximity portion, and the second control electrode is disposed in the proximity portion. 5. The light control device according to claim 4, wherein the light control device is installed so as to sandwich it. 6. The optical waveguide constitutes a directional coupler having a first optical waveguide and a second optical waveguide which are adjacent to each other, and the first control electrode is arranged to overlap the first optical waveguide. The light control device according to claim 4, wherein the light control device is installed, and the second control electrode is installed so as to sandwich the second optical waveguide.