Optical waveguide device

Abstract

To enable an optical waveguide element, including an optical waveguide of a single-crystal ferroelectric thin film having electrooptic effect, to have polarization nondependence with simple constitution.SOLUTION: An optical waveguide element has a channel optical waveguide which is provided to an epitaxial ferroelectric thin film arranged on a (110) plane in parallel with a single crystal substrate surface and has electrooptic effect, and electrodes which are arranged in parallel with a [111] direction of the epitaxial ferroelectric thin film and provided on both sides of the channel optical waveguide, wherein an electric field by the electrodes is applied to light entering the channel optical waveguide perpendicularly to the traveling direction.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an optical waveguide element including an optical waveguide layer of a single-crystal ferroelectric thin film having an electro-optic effect. [Background technology]

[0002] In optical waveguide devices such as optical modulators and optical switches, polarization independence is important in addition to high speeds ranging from several tens of megahertz to several tens of gigahertz. For example, in radio-on-fiber systems, randomly polarized light supplied from a central-station light source to an antenna is used for upstream transmission by a polarization-independent optical modulator, which is important for reducing the size and cost of the antenna. Furthermore, in optical communication networks, polarization-independent optical switches are essential because optical switches are required to connect polarization-nonmaintaining optical fibers. For this reason, technologies have been proposed to modulate TE and TM light in a polarization-independent manner, such as by making the input Y-branch asymmetric (Patent Document 1), by providing a folded optical waveguide so that an electric field is applied to both polarizations (Patent Document 2), or by rotating one polarization and combining it with the other polarization component to achieve polarization independence (Patent Document 3).

[0003] In optical waveguide devices, LiNbO3 is widely used as a single-crystal ferroelectric material for the optical waveguide. 1-x La x (Zr y Ti 1-y ) 1-x / 4 With the establishment of a method for producing a single-crystal epitaxial thin film of O3 (PLZT) (Patent Document 4), the application of PLZT ferroelectric thin films to optical waveguide elements is progressing. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 5-196903 [Patent Document 2] Publication 2005-275210 [Patent Document 3] Japanese Translation of PCT International Publication No. 2018-520375 [Patent Document 4] Japanese Unexamined Patent Application Publication No. 2000-329959 [Patent Document 5] Japanese Unexamined Patent Application Publication No. 2006-58837 [Summary of the Invention] [Problems to be Solved by the Invention]

[0005] An object of the present invention is to achieve polarization independence with a simple configuration in an optical waveguide element including an optical waveguide layer of a single-crystalline ferroelectric thin film having an electro-optic effect. [Means for Solving the Problems]

[0006] To achieve the above object, the present invention provides the following configuration. [1] An aspect of the present invention includes a channel optical waveguide provided in an epitaxial ferroelectric thin film having an electro-optic effect and arranged in a (110) plane parallel to the surface of a single-crystalline substrate, and electrodes provided on both sides of the channel optical waveguide and arranged parallel to the

[0111] direction of the epitaxial ferroelectric thin film. The electric field by the electrodes is applied in a direction perpendicular to the traveling direction with respect to the light incident on the channel optical waveguide. [2] In the above aspect, the optical waveguide element is a Mach-Zehnder interferometer type optical modulator. [3] In the above aspect, the optical waveguide element is a Mach-Zehnder interferometer type optical switch. [4] In the above aspect, the epitaxial ferroelectric thin film is a Pb 1-x La x (Zr y Ti 1-y ) 1-x / 4 O3 thin film in the range of 0 < x < 0.3 and 0.2 < y < 1.0. [5] In the above aspect, the epitaxial ferroelectric thin film is a BaTiO3 thin film. [6] In the above aspect, the epitaxial ferroelectric thin film has a thickness of 100 nm to 3000 nm. [7] In the above aspect, the single crystal substrate is sapphire (Al2O3), magnesium oxide (MgO), or strontium titanate (SrTiO3). [8] In the above aspect, the channel optical waveguide is characterized in that it can couple an optical fiber having a mode field diameter of 3.0 μm to 7.0 μm. [Effects of the Invention]

[0007] According to the present invention, a polarization-independent optical waveguide element can be realized with a simple configuration. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic plan view showing an example of a configuration in which the optical waveguide element is an optical modulator. [Figure 2] FIG. 2 is a diagram for explaining the arrangement of the epitaxial ferroelectric thin film. [Figure 3] FIG. 3 is a graph showing the intensities of the TE polarized wave and the TM polarized wave versus the applied voltage. [Figure 4] FIG. 4 is a plan view showing a schematic diagram of the relationship between the (110) plane of the epitaxial ferroelectric thin film, the electrodes, and the input light. [Figure 5] FIG. 5 is a schematic plan view showing an example of a configuration in which the optical waveguide element is an optical switch. [Figure 6] FIG. 6 is a schematic cross-sectional view taken along line A in FIG. 1, showing an embodiment of the optical waveguide element. [Figure 7] FIG. 7 is a schematic cross-sectional view taken along line A in FIG. 1, showing an embodiment of the optical waveguide element. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the optical waveguide device according to the present invention will be described in detail while referring to the drawings. In different configuration examples, the same or similar components are denoted by the same reference numerals.

[0010] FIG. 1 is a schematic plan view showing an optical modulator as a configuration example of the optical waveguide device of the present invention. The optical modulator 1 includes a substrate 10, an optical waveguide layer 20 disposed on the substrate 10, and an electrode 30. The optical waveguide layer 20 is formed of an epitaxial ferroelectric thin film. The epitaxial ferroelectric thin film is preferably a Pb 1-x La x (Zr y Ti 1-y ) 1-x / 4 O3 (PLZT) thin film, and the range of 0 <x <0.3 and 0.2 <y <1.0, particularly the range of 0.08 <x <0.1 and 0.6 <y <0.7 is preferable.

[0011] In the illustrated example, the optical waveguide layer 20 has a pair of channel optical waveguides 21 and 22 that branch from an input optical waveguide where input light is incident at a first Y-branch portion 20a. The channel optical waveguides 21 and 22 are arranged in parallel at a predetermined interval. The channel optical waveguides 21 and 22 merge at a second Y-branch portion 20b, and output light is output from an output optical waveguide. It is desirable to miniaturize the Y-branch portion by using an MMI (Multi Mode Interference) 3 dB coupler.

[0012] In the illustrated example, the electrodes 30 include one signal electrode 31 disposed between the channel optical waveguides 21 and 22, a first ground electrode 32 extending parallel to the signal electrode 31 across the channel optical waveguide 21, and a second ground electrode 33 extending parallel to the signal electrode 31 across the channel optical waveguide 22. Each of the electrodes 31, 32, and 33 is a traveling-wave electrode with a length of 1 to 10 mm, and both ends of each electrode extend to the input and output terminals at the edges of the substrate and are connected to an external voltage control unit. As another example, each of the electrodes 31, 32, and 33 can be a lumped-element electrode, which, although its response speed is inferior to that of a traveling-wave electrode, is useful for constructing a large-scale optical switch.

[0013] The substrate 10 is a single crystal substrate, and is preferably a (0112) (r-plane) sapphire (Al2O3). As another example, the substrate may be a (110) plane of magnesium oxide (MgO) or a (110) plane of strontium titanate (SrTiO3).

[0014] The optical modulator 1 is configured as a Mach-Zehnder interferometer. An example of optical modulation will be described. An electric field E1 generated by a voltage between the signal electrode 31 and the ground electrode 32 is applied to the channel optical waveguide 21, and an electric field E2 generated by a voltage between the signal electrode 31 and the ground electrode 33 is applied to the channel optical waveguide 22. A voltage of several tens of volts is applied between the ground electrodes 32 and 33 in advance to perform poling, thereby aligning the polarization axes in a specific direction. This inverts the phase of the light passing through the channel optical waveguides 21 and 22. When an electric field is applied, the amplitude of the output light is zero, and when no electric field is applied, the amplitude of the output light is the same as that of the input light. This results in optical modulation. It is also effective to create a voltage difference between the ground electrodes 32 and 33 while superimposing a bias voltage on them.

[0015] 2(a) is a schematic diagram illustrating the orientation of an epitaxial ferroelectric thin film in an optical waveguide element of the present invention. (b) is a further simplified view of (a). In the illustrated example, the epitaxial ferroelectric thin film is PLZT. The crystal structure of PLZT is a perovskite structure, with Pb or La at each vertex of a cubic lattice, Zr or Ti at the body center, and O at each face center.

[0016] In the present invention, the epitaxial ferroelectric thin film constituting the optical waveguide layer is arranged so that the (110) plane of the crystal structure of the epitaxial ferroelectric thin film is parallel to the surface of the substrate 10. A method for fabricating the PLZT (110) plane parallel to the surface of the single crystal substrate is described in, for example, Patent Document 4.

[0017] An example of the manufacturing method is outlined below. A precursor solution containing Pb:La:Ti dissolved in a predetermined ratio is spin-coated onto a sapphire Al2O3 (1102) single crystal substrate. Next, the coating layer is thermally decomposed in a temperature range where crystallization does not occur, to form an amorphous thin film. Next, a ferroelectric thin film is grown on the single crystal substrate by solid phase epitaxial growth at a predetermined temperature (400-1200°C) in an oxygen-containing atmosphere. By adjusting the temperature at this time, an epitaxial ferroelectric thin film with a (110) plane parallel to the surface of the single crystal substrate is obtained. The temperature is adjusted, for example, by increasing the temperature at 10°C / sec in a humidified O2 atmosphere, holding it at 300°C for 2 minutes, then increasing the temperature at 10°C / sec to 650°C and holding it for 10 minutes, and then cooling. Another example of temperature adjustment is by increasing the temperature at 10°C / sec in a humidified O2 atmosphere, holding it at 300°C for 2 minutes, then increasing the temperature at 80°C / sec to 750°C and holding it for 10 minutes, and then cooling. This allows a thin film with a thickness of, for example, 90 nm to grow. By repeating this process, an epitaxial ferroelectric thin film with a desired thickness can be obtained. The thickness of the epitaxial ferroelectric thin film can be set to 100 nm to 3000 nm.

[0018] Similar epitaxial ferroelectric thin films can be obtained using the same method with ABO3-type ferroelectrics other than PLZT, such as BaTiO3 thin films.

[0019] Furthermore, in the present invention, the channel optical waveguides 22 and 23 are formed so that the direction of propagation of the input light is the

[0111] direction of the crystal structure of the epitaxial ferroelectric thin film. 1 1] (second upper bar), [ 1 11] (first upper bar), [ 1 1 1 ](1st, 3rd upper bar), [1 11 This is equivalent to the notation

[0111] (upper bar on the second and third characters), and is typically written as

[0111] .

[0020] In the present invention, the signal electrode 31 and ground electrodes 32, 33 are formed to extend in the

[0111] direction of the epitaxial ferroelectric thin film. As a result, the direction of the electric field formed between the signal electrode 31 and each ground electrode 32, 33 is perpendicular to the

[0111] direction of the epitaxial ferroelectric thin film. In other words, an electric field is applied in a direction perpendicular to the propagation direction of light incident on the channel optical waveguides 21, 22. The angle of the signal electrode 31 and ground electrodes 32, 33 with respect to the

[0111] direction is within ±10 degrees, preferably within ±5 degrees, and more preferably within ±1 degree. As a result, the electro-optical effects for TE polarization and TM polarization become approximately equal.

[0021] Figure 3 is a graph showing the intensities of the TE polarized wave and the TM polarized wave versus the applied voltage. The intensities of the TE polarized wave and the TM polarized wave are almost the same, and the electro-optic effects are almost equal.

[0022] 4 shows the relationship between the (110) plane of the epitaxial ferroelectric thin film forming the channel optical waveguides 22 and 23, the electrodes 31, 32, and 33, and the input light. The polarization directions TE and TM of the input light are indicated in the diagram. The thickness of the epitaxial ferroelectric thin film is set in the range of 100 nm to 3000 nm, which satisfies the single-mode condition according to the refractive index difference between the ferroelectric thin film and the single-crystal substrate. Furthermore, optical fibers with a mode field diameter of 3.0 μm to 7.0 μm, preferably 3.0 μm to 5.0 μm, are coupled to both ends of the optical modulator 1. As a result, it is possible to reduce coupling loss as well as polarization-dependent loss. This configuration allows the present invention to achieve polarization independence with low drive voltage and small electrodes. Also, an IQ modulator can be configured by connecting four optical modulators shown in Figure 1 in parallel.

[0023] 5 is a schematic plan view showing an example of a configuration in which the optical waveguide element is an optical switch. The main part of the optical switch 2 having a pair of channel optical waveguides 21 and 22 has substantially the same configuration as the optical modulator 1 shown in FIG. 1. The main part of the optical switch 2 is also configured as a Mach-Zehnder interferometer type. Although not specifically shown, the Y branching sections 20c and 20d where the pair of channel optical waveguides 21 and 22 branch and merge have an optical multiplexing / branching structure such as an MMI, and the electric field formed between the signal electrode 31 and each of the ground electrodes 32 and 33 switches which of the two output waveguides to output to. Such an optical switch configuration is publicly known (for example, see Patent Document 5).

[0024] By combining multiple optical switches in Figure 5, it is possible to configure a 1xN tree-type optical switch such as 1x2, 1x4, 1x8, 1x16, or 1x32, or an NxN matrix-type optical switch such as 2x2, 4x4, 8x8, or 16x16.

[0025] Fig. 6 is a schematic cross-sectional view showing an example of an optical waveguide element taken along line A in Fig. 1. In the example shown, an optical waveguide layer 20 is formed on a substrate 10. The substrate 10 may be made of MgO or SrTiO3 in addition to sapphire (Al2O3). The optical waveguide layer 20 has a ridge-shaped channel optical waveguide 21 that has been processed by etching to a width of 1.0 μm to 3.0 μm. The channel optical waveguide 21 is formed to protrude from the slab optical waveguide 23 by 100 nm to 1000 nm. A signal electrode 31 and a ground electrode 32 are formed on the slab optical waveguide 23 at a predetermined interval on both sides of the channel optical waveguide 21. The electrodes are preferably made of gold (Au).

[0026] Fig. 7 is a schematic cross-sectional view showing another embodiment of the optical waveguide element taken along line A in Fig. 1. The configuration different from Fig. 5 is that an insulating SiO2 layer 40 is formed between electrodes 31 and 32, and this layer 40 covers the channel optical waveguide 21 and the slab optical waveguide 23.

[0027] Although the present invention has been described above with reference to exemplary configurations, various modifications are possible in keeping with the spirit of the present invention, and these modifications are also encompassed by the present invention. [Explanation of symbols]

[0028] 1 Optical modulator 2. Optical switch 10 Substrate 20 Optical waveguide layer 20a Y-branch 20b Y-branch 20c Y-junction 20d Y-branch 21, 22: Channel optical waveguide 23 Slab Optical Waveguide 31 signal electrode 32, 33 Ground electrode

Claims

1. a channel optical waveguide provided in an epitaxial ferroelectric thin film having an electro-optic effect and a (110) plane arranged parallel to the surface of the single crystal substrate; electrodes arranged parallel to the [111] direction of the epitaxial ferroelectric thin film and provided on both sides of the channel optical waveguide; 2. The optical waveguide element according to claim 1, wherein an electric field generated by the electrodes is applied to the light incident on the channel optical waveguide in a direction perpendicular to the traveling direction of the light.

2. 2. The optical waveguide element according to claim 1, wherein the optical waveguide element is a Mach-Zehnder interferometer type optical modulator.

3. 2. The optical waveguide element according to claim 1, wherein the optical waveguide element is a Mach-Zehnder interferometer type optical switch.

4. The epitaxial ferroelectric thin film is Pb in the range of 0<x<0.3 and 0.2<y<1.

0. １－ｘ La ｘ (Zr ｙ Ti １－ｙ ) １－ｘ／４ O ３ 2. The optical waveguide element according to claim 1, wherein the optical waveguide element is a thin film.

5. The epitaxial ferroelectric thin film is BaTiO ３ 2. The optical waveguide element according to claim 1, wherein the optical waveguide element is a thin film.

6. 2. The optical waveguide element according to claim 1, wherein the epitaxial ferroelectric thin film has a thickness of 100 nm to 3000 nm.

7. The single crystal substrate is sapphire (Al ２ O ３ ), magnesium oxide (MgO), or strontium titanate (SrTiO ３ 6. The optical waveguide element according to claim 1, wherein

8. 2. The optical waveguide element according to claim 1, wherein the channel optical waveguide is capable of coupling an optical fiber having a mode field diameter of 3.0 μm to 7.0 μm.

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