JP3106710B2 - Light control device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to the modulation of light waves, relates to an optical control device that performs such optical path switching, the waveguide-type optical control device in particular performs control using an optical waveguide provided on the substrate
About

[0002]

2. Description of the Related Art As optical communication systems have been put into practical use,
Further, there is a demand for an advanced system having a large capacity and multiple functions, and it is necessary to add new functions such as generation of an advanced optical signal, switching and switching of an optical transmission line.
In the current practical system, the optical signal is obtained by directly modulating the injection current of a semiconductor laser or a light emitting diode.
There are drawbacks such as difficulty in high-level modulation of about GHz or higher, and difficulty in application to a coherent optical transmission system due to 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. Currently used optical switches mechanically move prisms, mirrors, fibers, etc. There are some disadvantages. 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, lithium niobate (LiNb
A material using a ferroelectric material such as O ₃ ) crystal has features such as low light absorption and low loss, and high efficiency due to having a large electro-optic effect. Various types of light control elements such as a sex coupler type optical modulator / switch, a total reflection optical switch, and a Mach-Zehnder type optical modulator have been reported.

FIG. 4 shows a plan view (a) and a sectional view (b) of a directional coupler type optical switch as an example of a conventional light control device. In FIG. 4A, strip-shaped optical waveguides 2 and 3 are formed on a lithium niobate crystal substrate 1 by diffusing titanium (Ti) to make the refractive index larger than that of the substrate. And 3 are close to each other by about a few μm at the center of the substrate to form a directional coupler 4. A control electrode 5 is formed on the optical waveguide constituting the directional coupler 4 via a buffer layer 6 for preventing light absorption by the electrode. FIG. 4B shows the directional coupler 4.
2 is a cross-sectional view perpendicular to the optical waveguides 2 and 3.

In FIG. 4, incident light 7 incident on the optical waveguide 2 gradually transfers light energy to the adjacent optical waveguide 3 as it propagates through the directional coupler 4 and passes through the directional coupler 4. The energy is transferred to the optical waveguide 3 by almost 100% and becomes the emission light 8. On the other hand, when a voltage is applied to the control electrode 5, the refractive index of the optical waveguide under the control electrode changes due to the electro-optic effect, and a phase velocity mismatch occurs between the waveguide modes propagating through the optical waveguides 2 and 3. The state of connection between the two changes. That is, the incident light 7 becomes the outgoing light 9 from the optical waveguide 2 or the outgoing light 8 from the optical waveguide 3 depending on the presence or absence of the control voltage to the control electrode 5.

[0005]

However, in the conventional optical switch as shown in FIG. 4, electric charges in the crystal are locally accumulated at the interface between the crystal and the film by the application of a DC voltage, and the electric field intensity acting on the light wave is increased. Change, that is, DC drift is likely to occur, and there is a problem in device stability.

An object of the present invention, except for the problems of the conventional optical control device described above, a stable light control de characteristics for a long time
To provide vice .

[0007]

A first optical control device according to the present invention is a waveguide type optical device comprising an optical waveguide installed in a lithium niobate crystal substrate and a control electrode installed near the optical waveguide. In the control device, the niobium
An optical control device characterized in that the concentration of hydrogen atoms or hydrogen ions in the lithium oxide crystal substrate is 1 × 10 ¹⁹ cm ⁻³ or less.

A second light control device according to the invention, two
In a waveguide type optical control device comprising an optical waveguide provided in a lithium niobate crystal substrate and a control electrode provided near the optical waveguide, the lithium niobate crystal base
The concentration of hydrogen atoms or hydrogen ions on the plate surface is 1 × 10 ¹⁹
A light control device having a surface layer of not more than cm ⁻³ .

A third light control device according to the invention, two
In a waveguide-type optical control device including an optical waveguide provided in a lithium-obate crystal substrate and a control electrode provided in the vicinity of the optical waveguide, a niobate between the control electrodes is provided.
The light control device is characterized in that the concentration of hydrogen atoms or hydrogen ions in the lithium crystal substrate is 1 × 10 ¹⁹ cm ⁻³ or less.

[0010]

[0011]

[0012]

The DC described in the section of the problem to be solved by the invention
One cause of the drift phenomenon is that impurity ions existing in the lithium niobate crystal substrate move due to an external electric field and are localized on the surface of the crystal substrate to form an electric field. Normally, lithium ion (H ⁺ )
Alternatively, a hydroxyl ion (OH ⁻ ) is contained in an amount of several hundred ppm to several%. These ions are generated when moisture in the atmosphere diffuses into the crystal during crystal growth, poling, a waveguide forming process, and the like. The authors have experimentally confirmed that these hydrogen ions or hydroxyl ions are impurity ions that cause the DC drift phenomenon. Therefore,
The DC drift phenomenon can be reduced by reducing the concentration of hydrogen ions or hydrogen atoms.

The concentration of hydrogen ions or hydrogen atoms in the lithium niobate crystal can be reduced by heating the crystal in an atmosphere free of hydrogen ions or hydrogen elements. Therefore, by heating the crystal in a vacuum,
Crystals having a low concentration of hydrogen ions or hydrogen atoms can be obtained. In addition, when oxygen is contained in the constituent elements of the lithium niobate crystal, heating the crystal in high-purity oxygen gas having a low concentration of hydrogen atoms causes hydrogen ions or water ions.
At the same time as the effect of reducing the DC drift by reducing the concentration of elemental atoms, an oxygen control in the lithium niobate crystal is supplemented, and an optical control device with small light scattering loss can be obtained.

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.

[0015]

1 is a plan view (a) and a sectional view (b) of a directional coupler type optical switch which is an embodiment of an optical control device according to claim 1 of the present invention. As in the example of FIG. 4, optical waveguides 2 and 3 of about 3 to 10 μm formed by thermally diffusing titanium (Ti) on a lithium niobate (LiNbO ₃ ) crystal substrate 1 at about 900 to 1100 ° C. for several hours. Are formed, and both optical waveguides are several μm apart from each other at the center of the substrate.
The directional coupler 4 is configured as close as possible. The control electrode 5 is formed thereon via the buffer layer 6. The buffer layer 6 may not always be necessary depending on the polarization direction of light or the electrode material. The concentration of hydrogen atoms or hydrogen ions in the lithium niobate crystal substrate 1 is 1 × 10 ¹⁹
cm ^-3 or less. The conventional lithium niobate substrate 12 contains hydrogen atoms or hydrogen ions of 3 × 10 ¹⁹ cm ⁻³ or more, and these hydrogen ions are polarized by an electric field applied from the outside, and DC drift occurs. Therefore, by reducing the hydrogen ion concentration, DC drift can be reduced, and an optical control device with stable characteristics can be obtained.

FIG. 2 is a plan view (a) and a sectional view (b) of a directional coupler type optical switch which is an embodiment of the optical control device according to the second aspect of the present invention. The configurations of the optical waveguides 2 and 3, the buffer layer 6, and the control electrode 5 are the same as those in FIG. On the surface of the lithium niobate substrate 12, a hydrogen ion concentration of 1 ×
A surface layer 10 of 10 ¹⁹ cm ^-3 or less is provided;
The lower portion of the surface layer 10 has a hydrogen ion concentration of 3 × 10 ¹⁹ cm ⁻³ or more, similarly to the conventional substrate. The effect of reducing the DC drift caused by the polarization of the hydrogen ions is the same as that of the device of the first aspect. As the hydrogen ion concentration decreases, the conductivity due to hydrogen ion conduction decreases. Therefore, when the layer 10 having a low hydrogen ion concentration is provided on the surface of the lithium niobate substrate 12, the conductivity of the surface layer 10 decreases. Usually, on the surface of the lithium niobate substrate 12, there is a layer having a higher conductivity than the bulk crystal due to processing strain and the like. Therefore, by reducing the hydrogen ion concentration on the surface of the substrate 12, Offset the layers,
A substrate having a uniform conductivity can be obtained. As a result, in addition to the DC drift caused by the polarization of the hydrogen ions, a transient DC drift caused by a difference in electric constant between the layer structures in the cross section of the device can be reduced, and the characteristics can be more stable. A light control device is obtained.

FIG. 3 is a plan view (a) and a sectional view (b) of a directional coupler type optical switch which is an embodiment of the optical control device according to claim 3 of the present invention. The configurations of the optical waveguides 2 and 3, the buffer layer 6, and the control electrode 5 are the same as those in FIG. A region 11 having a hydrogen ion concentration of 1 × 10 ¹⁹ cm ⁻³ or less is provided between the control electrodes 5 on the surface of the lithium niobate substrate 1, and the other regions are the same as the conventional substrate 12. The concentration is 3 × 10 ¹⁹ cm ⁻³ or more. When a voltage is applied to the control electrode 5 from the outside, a maximum electric field is applied between the control electrodes 5. Therefore, even if the hydrogen ion concentration between the control electrodes 5 is simply reduced, the DC drift reduction effect due to the polarization of the hydrogen ions is reduced. Is obtained. Further, in this structure, it is not necessary to reduce the hydrogen ions in the optical waveguides 2 and 3, so that the effect of an increase in light loss which is likely to be generated when the hydrogen ions are reduced can be reduced. Therefore, a low-loss light control device with stable characteristics can be provided.

The concentration of hydrogen ions or hydrogen atoms in the lithium niobate crystal can be reduced by heating the crystal in an atmosphere free of hydrogen ions or hydrogen elements. Therefore, by heating the crystal in a vacuum,
Crystals having a low concentration of hydrogen ions or hydrogen atoms can be obtained. For example, when a lithium niobate substrate having a thickness of about 1 mm is heated at 700 ° C. for about 2 hours in a vacuum at a pressure of about 1 × 10 ⁻⁵ Torr, the hydrogen ion concentration of the entire substrate is 5 × 10 ¹⁷ cm ^{−. 3} or less. Also this Nio
When oxygen is contained in the constituent elements of a crystal, such as a lithium butyrate crystal, heating the crystal in high-purity oxygen gas with a low concentration of hydrogen atoms results in hydrogen ions or hydrogen atoms
Reduction by DC drift-reducing effect of the concentration at the same time, two
Oxygen vacancies in the lithium obate crystal are compensated for, and a light control device with small light scattering loss can be obtained. To reduce the hydrogen ion concentration of only the vicinity of the surface of the substrate as shown in FIG. 2,
The substrate may be heated at a lower temperature or for a shorter time than in the case where the hydrogen ion concentration of the entire substrate is reduced. As shown in FIG. 3, in order to reduce the hydrogen ion concentration only between the control electrodes 5, silicon dioxide (SiO ₂ ) or silicon nitride (Si ₃ N ₄ ) After masking with, for example, heating may be performed similarly.

[0019]

As described above, in the light control device of the present invention, a light control device having more stable characteristics than conventional light control devices can be obtained. Therefore, the characteristics are long
Can provide a stable light control device
You.

[Brief description of the drawings]

FIG. 1 is a plan view (a) and a sectional view (b) showing a first embodiment of a light control device according to the present invention.

FIG. 2 is a plan view (a) and a sectional view (b) showing a second embodiment of the light control device according to the present invention.

FIG. 3 is a plan view (a) and a sectional view (b) showing a third embodiment of the light control device according to the present invention.

FIG. 4 is a plan view (a) and a cross-sectional view (b) showing an example of a conventional light control device.

[Explanation of symbols]

1 Lithium niobate crystal substrate having a hydrogen ion concentration of 1 × 10 ¹⁹ cm ⁻³ or less 2,3 Optical waveguide 4 Directional coupler 5 Control electrode 6 Buffer layer 7 Incident light 8,9 Outgoing light 10 Hydrogen ion concentration 1 × 10 ¹⁹ cm ^-3 or less of lithium niobate surface layer 11 of hydrogen ion concentration of 1 × 10 ¹⁹ cm ^-3 or less of lithium niobate region 12 the hydrogen ion concentration of 1 × 10 ¹⁹ cm ^-3 or less of lithium niobate crystal substrate

Continuation of the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) G02F 1/035 G02B 6/12 G02F 1/313 JICST file (JOIS)

Claims (3)

(57) [Claims] 1. A waveguide-type optical control device comprising an optical waveguide provided in a lithium niobate crystal substrate and a control electrode provided near the optical waveguide.
A light control device, wherein the concentration of hydrogen atoms or hydrogen ions in the lithium oxide crystal substrate is 1 × 10 ¹⁹ cm ⁻³ or less. 2. A waveguide-type light control device comprising an optical waveguide provided in a lithium niobate crystal substrate and a control electrode provided near the optical waveguide, wherein
An optical control device comprising a lithium oxide crystal substrate surface having a surface layer having a hydrogen atom or hydrogen ion concentration of 1 × 10 ¹⁹ cm ⁻³ or less. 3. A waveguide type optical control device comprising an optical waveguide provided in a lithium niobate crystal substrate and a control electrode provided in the vicinity of the optical waveguide, wherein the lithium niobate crystal substrate is provided between the control electrodes. An optical control device, wherein the concentration of hydrogen atoms or hydrogen ions therein is 1 × 10 ¹⁹ cm ⁻³ or less.