JP2606552B2 - Light control device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light control device for modulating a light wave, switching an optical path, and the like, and more particularly, to a light control device of a waveguide type for controlling using an optical waveguide provided in a substrate.

[0002]

2. Description of the Related Art As optical communication systems have been put into practical use,
Further, advanced systems having a large capacity and multiple functions are required, and new functions such as generation of higher-level optical signals, switching of optical transmission lines, and exchange are required.
In the current practical system, an optical signal is obtained by directly modulating the injection current of a semiconductor laser or a light emitting diode.
There are drawbacks in that high-speed modulation of about 0 GHz or more is difficult, and it is difficult to apply to a coherent optical transmission system because of wavelength fluctuation. As a means for solving this problem, there is a method using an external modulator. In particular, a waveguide type optical modulator constituted by an optical waveguide formed in a substrate has features of small size, high efficiency, and high speed. On the other hand, an optical switch is used as a means for obtaining an optical transmission line switching or network switching function. Optical switches currently in practical use are:
It is a mechanism for mechanically moving a prism, a mirror, a fiber, and the like, and has disadvantages such as low speed, insufficient reliability, and a large size alone, which is unsuitable for matrix formation.

What is being developed as a means for solving this problem is a waveguide type optical switch using an optical waveguide, which has features such as high speed, integration of many elements, and high reliability. . In particular, those using a ferroelectric material such as lithium niobate (LiNbO ₃ ) have characteristics such as low light absorption, low loss, and high efficiency due to a large electro-optic effect. Conventionally, various types of light 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 are a plan view and a sectional view of a Mach-Zehnder type optical modulator as an example of a conventional light control device. FIG. 5B is a dashed line b− in FIG.
FIG. 4 shows a cross-sectional view of a portion b ′. In FIG. 5A,
Strip-shaped optical waveguides 21, 22, 23, and 2 formed by diffusing titanium (Ti) on the lithium niobate crystal substrate 1 cut out perpendicular to the Z axis to have a larger refractive index than the substrate.
4 are formed. The optical waveguide 21 branches off into optical waveguides 22 and 23 near the center of the substrate and then joins again to form an optical waveguide 24, which constitutes a Mach-Zehnder interference system. A control electrode 61 is formed on the Mach-Zehnder two branch optical waveguides 22 and 23 via a buffer layer 62 for preventing light absorption by the electrodes.

[0005] The incident light 5 incident on the optical waveguide 21 is divided into two optical waveguides 22 and 23, and after propagating through the respective branches, merges into the optical waveguide 24 to become the output 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 arranged 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 formed by the electro-optic effect.
The refractive indexes of the optical waveguides 2 and 23 change, the light propagating through the optical waveguides 22 and 23 undergoes phase modulation in opposite directions, and the power of the waveguide mode of the optical waveguide 24 after the merge 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 acting on the light wave is generated. A phenomenon in which the intensity changes, that is, a DC drift easily occurs, and there is a problem in device stability.

An object of the present invention is to provide a light control device whose characteristics are stable for a long period of time, excluding the above-mentioned problems of the conventional light control device.

[0008]

According to the present invention, there is provided a Mach-Zehnder interference-type light control device comprising an optical waveguide formed on a dielectric crystal substrate having an electro-optic effect and a control electrode provided in the vicinity thereof. As a 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 in the same Mach-Zehnder interferometer, and propagation of light in the thickness direction of the substrate and light to the optical waveguide. and a second control electrode for applying a direction perpendicular to the direction of the electric _{field, Γ d r 13 V d L} d / g d = αΓ t r 22 V t L t / g t ( although, alpha is a vertical electric field Large DC drift of lateral electric field
比 is the electric field correction coefficient, and r ₁₃ and r ₂₂ are the electro-optic
Number, V is applied voltage, L is electrode length, g is electrode spacing,
Where d and t are electrodes for applying a vertical electric field.
And an electrode to which a horizontal electric field is applied)
Voltage, electrode length, electrode spacing
Is set .

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 of the optical waveguide. A first control electrode for applying an electric field in the thickness direction of the substrate to the optical waveguide, and a thickness direction of the substrate and a propagation direction of light to the optical waveguide in the same directional coupler. and a second control electrode for applying a vertical direction of the electric _{field, Γ d r 13 V d L} d / g d = αΓ t r 22 V t L t / g t ( although, alpha vertical field and horizontal field Of DC drift of
比 is the electric field correction coefficient, and r ₁₃ and r ₂₂ are the electro-optic
Number, V is applied voltage, L is electrode length, g is electrode spacing,
Where d and t are electrodes for applying a vertical electric field.
And an electrode to which a horizontal electric field is applied)
Voltage, electrode length, electrode spacing
Is set .

[0010]

The light control device of the present invention includes a control electrode for applying an electric field in a thickness direction of a substrate (hereinafter, this direction is referred to as a vertical direction) to an optical waveguide as an electric field application means for light control;
The optical waveguide includes both a control electrode for applying an electric field in a direction perpendicular to a thickness direction of the substrate and a direction of light propagation (hereinafter, this direction is referred to as a lateral direction). According to our experiments,
When a vertical electric field is applied, the DC drift occurs in a direction that weakens the applied electric field (hereinafter, such a DC drift is referred to as a positive DC drift). Conversely, when a horizontal electric field is applied, the DC drift occurs. Occurs in a direction to increase the applied electric field (hereinafter, such a DC drift is caused by a negative DC
Drift). Therefore, by combining the vertical electric field and the horizontal electric field, the positive DC drift and the negative DC drift can be offset, and the DC drift can be reduced.

As described above, the light control device of the present invention can provide a light control device that is more stable 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. The broken line b in FIG.
A sectional view of the -b 'portion is shown in FIG. FIG. 1C is a cross-sectional view taken along a broken line cc ′ in FIG.

On a Z-plate lithium niobate crystal substrate 1,
Optical waveguides 21, 22, 23 of about 3 to 10 μm formed by thermally diffusing titanium at about 900 to 1100 ° C. for several hours.
And 24 are formed. The optical waveguide 21 branches into two optical waveguides 22 and 23, and then merges to form an optical waveguide 24, thereby forming 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 of the same magnitude and opposite directions are applied to the waveguides 22 and 23, respectively. The control electrode 25 is the optical waveguide 22,
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 provided so as to sandwich the optical waveguides 22 and 23,
A lateral electric field 8 is applied to 2 and 23. As described in the operation section, a positive DC drift is generated for a vertical electric field, and a negative DC drift is generated for a horizontal electric field. In addition, the time constants of the two DC drifts are substantially equal. Therefore, T due to the control electrodes 25 and 26
Refractive index variation of the E-mode or transverse direction of polarization [Delta] n _d and Δ
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 ³ V _t L _t / (2g _t ) (2) Here, D (t) is the DC drift amount 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 the electro-optic coefficients, n _o is the refractive index of ordinary light, V is the applied voltage, L
Represents an electrode length, g represents an electrode interval, and suffixes d and t respectively represent an electrode 25 for applying a vertical electric field and an electrode 26 for applying a horizontal electric field. Therefore, as shown in FIG. 1A, the vertical electric field electrode 25 and the horizontal electric field electrode 2
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, electrode length, electrode spacing, etc. of each electrode so as to satisfy the relationship of g _t (3), the DC at each branch is adjusted.
Phase change due to drift can be reduced to 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 amount of phase change in each branch can be made zero, the DC drift of the output light intensity of the Mach-Zehnder optical modulator is also made zero And a stable optical modulator can be obtained.

FIGS. 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 taken along the broken line bb 'of FIG. 2A. Optical waveguides 21, 22, 23 and 24 are placed on Z-plate lithium niobate crystal substrate 1 in the same manner as in the example of FIG.
Are formed to constitute a Mach-Zehnder interference system. A control electrode 31 is formed on the optical waveguides 22 and 23. The control electrode 31 is placed so as to overlap the optical waveguide 22 and sandwich the optical waveguide 23. The control electrode 31 applies a vertical electric field 7 to the photoelectrode 22 and a horizontal electric field 8 to the optical waveguide 23. An electric field is applied to the optical waveguides 22 and 23 of the two branches of the Mach-Zehnder so that their 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, by adjusting the applied voltage of the electrodes, the electrode length, the electrode interval, and the like so that the control electrode 31 satisfies the relationship of Expression (3), DC of each phase change in branch
The drift is canceled at the time of merging, and the change in output light intensity due to the DC drift can be made zero, so that 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.

FIG. 3A is a sectional view taken along a broken line bb 'of FIG. 3A, and FIG. 3C is a sectional view taken along a broken line cc' of FIG. Z
On a lithium niobate crystal substrate 1, 900 titanium was added.
3-1 formed by thermal diffusion at about 1100 ° C for several hours
Optical waveguides 41 and 42 of about 0 μm are formed,
At the center of the substrate, the two optical waveguides are close to each other up to several μm to form a directional coupler. Control electrodes 43 and 44 are provided thereon. The control electrode 43 is an optical waveguide 4
The optical waveguides 41 and 42 are installed so as to overlap with the optical waveguides 41 and 42.
Is applied with a vertical electric field 7. On the other hand, the control electrode 44
Are disposed 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 on the same principle as that of the Mach-Zehnder type modulator of FIG.

FIGS. 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.

FIG. 1B is a sectional view taken along a broken line bb 'of FIG. 1A. As in the case of FIG. 3, optical waveguides 41 and 42 are formed on a Z-plate lithium niobate crystal substrate 1, and both optical waveguides are close to each other up to several μm at the center of the substrate, and are directional couplers. Is composed. A control electrode 51 is provided thereon. The control electrode 51 is connected to the optical waveguide 4
1 and so as to sandwich the optical waveguide 22. 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 on the same principle as that of the Mach-Zehnder type modulator of FIG.

[0020]

As described above, in the light control device of the present invention, since the DC drift can be reduced, a light control device having more stable characteristics than the conventional light control device can be obtained.

[Brief description of the drawings]

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

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

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

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

FIG. 5 is a plan view and a cross-sectional view showing an example of a conventional light control device.

[Explanation of symbols]

 DESCRIPTION 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 Outgoing light 7 Vertical electric field 8 Horizontal electric field 62 Buffer layer

Claims (6)

(57) [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 provided in the vicinity thereof, means for applying an electric field for light 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. Second to apply
And a control _{electrode, Γ d r 13 V d L} d / g d = αΓ t r 22 V t L t / g t ( although, alpha is the DC drift amount of the vertical electric field and a lateral field size
比 is the electric field correction coefficient, and r ₁₃ and r ₂₂ are the electro-optic
Number, V is applied voltage, L is electrode length, g is electrode spacing,
Where d and t are electrodes for applying a vertical electric field.
And lateral direction of the applied voltage of each of the electrodes so as to satisfy the relation of representing the electrodes for applying an electric field), the electrode
A light control device characterized by setting a length and an electrode interval . 2. The optical waveguide according to claim 1, wherein said optical waveguide is a first branch optical waveguide.
And a Mach-Zehnder interferometer having a second branch optical waveguide
Wherein the first control electrode is a part of a first branch optical waveguide.
And a part of the second branch optical waveguide.
And the second control electrode is provided on a part of the first branch optical waveguide.
And a part of the second branch optical waveguide.
The light control device according to claim 1, wherein: 3. The optical waveguide comprises a Mach-Zehnder interference system having a first branch optical waveguide and a second branch optical waveguide, and the first control electrode overlaps the first branch optical waveguide. Installed, wherein the second control electrode is a second control electrode.
The light control device according to claim 1, wherein the light control device is provided so as to sandwich the branch optical waveguide. 4. A directional coupler type optical control device comprising an optical waveguide formed on a dielectric crystal substrate having an electro-optic effect and a control electrode provided in the vicinity thereof.
As a means for applying an electric field for light control, a first method for applying an electric field in the thickness direction of the substrate to the optical waveguide 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.
_{Includes, Γ d r 13 V d L} d / g d = αΓ t r 22 V t L t / g t ( although, alpha is the DC drift amount of the vertical electric field and a lateral field size
比 is the electric field correction coefficient, and r ₁₃ and r ₂₂ are the electro-optic
Number, V is applied voltage, L is electrode length, g is electrode spacing,
Where d and t are electrodes for applying a vertical electric field.
And lateral direction of the applied voltage of each of the electrodes so as to satisfy the relation of representing the electrodes for applying an electric field), the electrode
A light control device characterized by setting a length and an electrode interval . 5. The optical waveguide according to claim 1, wherein said optical waveguide is an adjacent first optical waveguide.
And a directional coupler having a second optical waveguide,
The first control electrode includes a portion of a first optical waveguide and a second control electrode.
The second waveguide is installed so as to overlap a part of the optical waveguide.
The control electrode is a part of the first optical waveguide and a part of the second optical waveguide.
Claims characterized by being installed so as to sandwich the part
5. The light control device according to 4. 6. The optical waveguide forms a directional coupler having a first optical waveguide and a second optical waveguide that are close to each other, and the first control electrode overlaps a first branch optical waveguide. 5. The light control device according to claim 4, wherein the second control electrode is disposed so as to sandwich a second branch optical waveguide. 6.