Optical waveguide medium, optical modulation element, and method for manufacturing an optical modulation element

Abstract

The present invention provides a hybrid optical waveguide medium, an optical modulation element, and a method for manufacturing an optical modulation element that offer excellent mass-producibility and low cost. [Solution] The optical waveguide medium 1 comprises a substrate 10, an ABX3 type ferroelectric thin film 11 formed on the substrate 10, a first dielectric thin film 12 formed on the ABX3 type ferroelectric thin film 11, a second dielectric thin film 13 formed on the first dielectric thin film 12, and a third dielectric thin film 14 formed on the first dielectric thin film 12 and the second dielectric thin film 13. The first dielectric thin film 12, the second dielectric thin film 13, and the third dielectric thin film 14 constitute an embedded optical waveguide structure in which the second dielectric thin film 13 is embedded by the first dielectric thin film 12 and the third dielectric thin film 14. The refractive indices of the first dielectric thin film 12 and the third dielectric thin film 14 are lower than the refractive indices of the ABX3 type ferroelectric thin film 11 and the second dielectric thin film 13.

Description

Technical Field

[0001] The present invention relates to an optical waveguide medium, an optical modulator, and a method for manufacturing an optical modulator, and particularly relates to an optical waveguide medium, an optical modulator, and a method for manufacturing an optical modulator using a ferroelectric ABX3 crystal.

Background Art

[0002] With the spread of the Internet, the communication volume has increased exponentially, and the importance of optical fiber communication has been extremely enhanced. Optical fiber communication converts an electrical signal into an optical signal and transmits the optical signal through an optical fiber, and has characteristics of wide bandwidth, low loss, and strong noise resistance.

[0003] As a method for converting an electrical signal into an optical signal, a direct modulation method using a semiconductor laser and an external modulation method using an optical modulator are known. Direct modulation does not require an optical modulator and is low-cost, but there is a limit to high-speed modulation, and the external modulation method is used for high-speed and long-distance applications.

[0004] As an optical modulator, a Mach-Zehnder type optical modulator using a ferroelectric ABX3 crystal typified by a lithium niobate film (LiNbO3) has been put into practical use (for example, see Patent Document 1). The Mach-Zehnder type optical modulator uses an optical waveguide (Mach-Zehnder optical waveguide) having a structure of a Mach-Zehnder interferometer that divides light emitted from one light source into two, passes them through different paths, and then overlaps them again to cause interference.

[0005] Patent Document 1 describes a ridge-type optical modulator including a waveguide layer made of a lithium niobate film formed on a substrate, the waveguide layer having a slab portion having a predetermined thickness and a ridge portion protruding from the slab portion.

[0006] Ferroelectric ABX3 crystals are excellent electro-optic materials with low-loss light propagation characteristics, large electro-optic coefficients, linear modulation response, and large modulation bandwidth. However, most conventional optical modulation devices using ferroelectric ABX3 crystals are manufactured using non-standard etching techniques or partially etched ridge waveguides, resulting in poor waveguide shape reproducibility compared to silicon photonic optical modulation devices.

[0007] Therefore, as an alternative to optical modulation elements using ridge waveguides, hybrid devices combining a lithium niobate thin film and a waveguide made of silicon nitride (Si3N4) have been developed (see, for example, Patent Document 2).

[0008] The hybrid device disclosed in Patent Document 2 has a structure in which a substrate with a cladding layer in which a waveguide structure is embedded is bonded to an electro-optic modulation layer containing a lithium niobate thin film. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Patent No. 7538209 [Patent Document 2] U.S. Patent No. 10788689 [Overview of the Initiative] [Problems that the invention aims to solve]

[0010] However, structures such as the one disclosed in Patent Document 2, which involves bonding a cladding layer and an electro-optic modulation layer, are time-consuming to manufacture, making mass production difficult and resulting in high costs.

[0011] The present invention has been made to solve these conventional problems, and aims to provide a hybrid optical waveguide medium, an optical modulation element, and a method for manufacturing an optical modulation element that are highly productive and low-cost. [Means for solving the problem]

[0012] To solve the above problems, the optical waveguide medium according to the present invention comprises a substrate, an ABX3 type ferroelectric thin film formed on the substrate, a first dielectric thin film formed on the ABX3 type ferroelectric thin film, a second dielectric thin film formed on the first dielectric thin film, and a third dielectric thin film formed on the first dielectric thin film and the second dielectric thin film. The first dielectric thin film, the second dielectric thin film, and the third dielectric thin film constitute an embedded optical waveguide structure in which the second dielectric thin film is embedded by the first dielectric thin film and the third dielectric thin film. The refractive indices of the first dielectric thin film and the third dielectric thin film are lower than the refractive indices of the ABX3 type ferroelectric thin film and the second dielectric thin film.

[0013] To solve the above problems, the optical modulation element according to the present invention comprises a substrate, an ABX3 type ferroelectric thin film formed on the substrate, a first dielectric thin film formed on the ABX3 type ferroelectric thin film, a second dielectric thin film formed on the first dielectric thin film, a third dielectric thin film formed on the first dielectric thin film and the second dielectric thin film, and a traveling wave electrode formed on the third dielectric thin film, consisting of a signal electrode and a ground electrode, for applying an electric field to the ABX3 type ferroelectric thin film. The first dielectric thin film, the second dielectric thin film, and the third dielectric thin film embed the second dielectric thin film with the first dielectric thin film and the third dielectric thin film, forming adjacent first and second embedded optical waveguide structures. The refractive indices of the first dielectric thin film and the third dielectric thin film are lower than the refractive indices of the ABX3 type ferroelectric thin film and the second dielectric thin film.

[0014] To solve the above problems, the method for manufacturing an optical modulation element according to the present invention includes the steps of: forming an ABX3 type ferroelectric thin film on a substrate by sputtering; forming a first dielectric thin film on the ABX3 type ferroelectric thin film; forming a second dielectric thin film on the first dielectric thin film; forming a third dielectric thin film on the first dielectric thin film and the second dielectric thin film; and forming a traveling wave electrode consisting of a signal electrode and a ground electrode on the third dielectric thin film for applying an electric field to the ABX3 type ferroelectric thin film. The first dielectric thin film, the second dielectric thin film, and the third dielectric thin film are arranged so that the second dielectric thin film is embedded by the first dielectric thin film and the third dielectric thin film, thereby forming adjacent first and second embedded optical waveguide structures. The refractive indices of the first dielectric thin film and the third dielectric thin film are lower than the refractive indices of the ABX3 type ferroelectric thin film and the second dielectric thin film. [Effects of the Invention]

[0015] The present invention provides a hybrid optical waveguide medium, an optical modulation element, and a method for manufacturing an optical modulation element that offer excellent mass-producibility and low cost. [Brief explanation of the drawing]

[0016] [Figure 1] This is a schematic cross-sectional view of an optical waveguide medium according to the first embodiment of the present invention. [Figure 2] This is a schematic plan view of an optical modulation element according to a second embodiment of the present invention. [Figure 3] This is a cross-sectional view along line AA in Figure 2. [Figure 4] This figure schematically illustrates the manufacturing process of an optical modulation element according to a second embodiment of the present invention. [Figure 5] This is a schematic plan view of an optical modulation element according to a third embodiment of the present invention. [Figure 6] This is a cross-sectional view along line BB in Figure 5. [Figure 7] This figure schematically illustrates the manufacturing process of an optical modulation element according to a third embodiment of the present invention. [Figure 8]FIG. (a) shows a simulation model of the optical waveguide medium of Example 1, and FIG. (b) shows the electric field distribution of light. [Figure 9] FIG. shows a simulation model of the optical modulator of Example 2. [Figure 10] FIG. is a graph showing the full width at half maximum of light intensity and the electric field efficiency when the thickness and width of the second dielectric thin film are changed in the optical modulator of Example 3. [Figure 11] FIG. is a graph showing the propagation loss and the electric field efficiency when the distance between the electrode and the ABX3-type ferroelectric thin film is changed in the optical modulator of Example 4. [Figure 12] FIG. shows a simulation model of the optical modulator of Example 5. [Figure 13] FIG. shows (a) a graph showing the full width at half maximum of light intensity when the width of the second dielectric thin film is changed in the optical modulator of Example 5, and (b) a graph showing the electric field efficiency when the material of the cladding layer and the width of the second dielectric thin film are changed. [Figure 14] FIG. is a graph showing the full width at half maximum of light intensity and the electric field efficiency when the thickness of the first dielectric thin film is changed in the optical modulator of Example 6. [Figure 15] FIG. shows (a) a simulation model of the optical modulator of Example 7, and (b) a graph showing the electric field efficiency when the width of the second dielectric thin film is changed. [Figure 16] FIG. shows (a) a simulation model of the optical modulator of Example 8, and (b) a graph showing the electric field efficiency when the distance between the signal electrode and the ground electrode is changed. [Figure 17] FIG. shows (a) a simulation model of the optical modulator of Example 9, and (b) a graph showing the electric field efficiency when the distance between the signal electrode and the ground electrode is changed.

BEST MODE FOR CARRYING OUT THE INVENTION

[0017] Hereinafter, embodiments of an optical waveguide medium, an optical modulator, and a method for manufacturing an optical modulator according to the present invention will be described with reference to the drawings. Note that the dimensional ratios of the respective components on each drawing do not necessarily match the actual dimensional ratios.

[0018] (First embodiment) The optical waveguide medium of the first embodiment will be described using Figure 1. The optical waveguide medium 1 of the first embodiment comprises a substrate 10, an ABX3 type ferroelectric thin film 11, a first dielectric thin film 12, a second dielectric thin film 13, and a third dielectric thin film 14.

[0019] The ABX3 type ferroelectric thin film 11 is a thin film of ferroelectric ABX3 crystal, which is an electro-optic material, and is, for example, a c-axis oriented lithium niobate film or a lithium tantalate film. The ABX3 type ferroelectric thin film 11 is a sputtered thin film formed on a substrate 10 by the sputtering method.

[0020] By using a lithium niobate film or a lithium tantalate film as the ABX3 type ferroelectric thin film 11, the optical waveguide medium 1 can exhibit excellent electro-optic effects. Furthermore, because the ABX3 type ferroelectric thin film 11 is a sputtered thin film, the optical waveguide medium 1 is excellent in terms of mass production and cost reduction.

[0021] The first dielectric thin film 12 is formed on the ABX3 type ferroelectric thin film 11. The third dielectric thin film 14 is formed on the first dielectric thin film 12 and the second dielectric thin film 13. The first dielectric thin film 12, the second dielectric thin film 13, and the third dielectric thin film 14 constitute an embedded optical waveguide structure in which the second dielectric thin film 13 is embedded by the first dielectric thin film 12 and the third dielectric thin film 14.

[0022] The second dielectric thin film 13 is formed on the first dielectric thin film 12. The cross-sectional shape of the second dielectric thin film 13 can be any shape that allows light to be guided, for example, it may be a rectangle as shown in Figure 1, or it may be a square, trapezoid, or triangle. The second dielectric thin film 13 is made of silicon nitride (Si3N4), which has excellent optical wave guiding properties and processability.

[0023] The first dielectric thin film 12 and the third dielectric thin film 14 function as cladding layers that confine light to the second dielectric thin film 13 and the ABX3 type ferroelectric thin film 11. For this reason, the refractive indices of the first dielectric thin film 12 and the third dielectric thin film 14 are lower than those of the ABX3 type ferroelectric thin film 11 and the second dielectric thin film 13. Hereafter, the first dielectric thin film 12 and the third dielectric thin film 14 will be collectively referred to as the cladding layer 16.

[0024] The first dielectric thin film 12 is preferably made of a highly transparent material, such as aluminum oxide (Al2O3), silicon dioxide (SiO2), LaAlO3, LaYO3, ZnO, HfO2, MgO, or Y2O3. Similarly, the third dielectric thin film 14 can also be made of Al2O3, SiO2, LaAlO3, LaYO3, ZnO, HfO2, MgO, or Y2O3. The first dielectric thin film 12 and the third dielectric thin film 14 may or may not be made of the same material.

[0025] The second dielectric thin film 13 and the ABX3 type ferroelectric thin film 11 are optically coupled via the first dielectric thin film 12. The second dielectric thin film 13, the first dielectric thin film 12, and the ABX3 type ferroelectric thin film 11 function as a three-dimensional optical waveguide. Since the second dielectric thin film 13 has a higher refractive index than the first dielectric thin film 12, it suppresses the wide horizontal spread of light in the ABX3 type ferroelectric thin film 11. Therefore, the optical waveguide of the optical waveguide medium 1 can be fabricated without processing the ABX3 type ferroelectric thin film 11.

[0026] The substrate 10 is not particularly limited as long as its refractive index is lower than that of the ABX3 type ferroelectric thin film 11, but a substrate that can form the ABX3 type ferroelectric thin film 11 as an epitaxial film with excellent crystallinity is preferred, and a sapphire single crystal substrate or a silicon single crystal substrate is preferred. The crystal orientation of the substrate 10 is not particularly limited.

[0027] Furthermore, lithium niobate films and lithium tantalate films have the property of being easily formed as c-axis oriented epitaxial films on single crystal substrates of various crystal orientations. Since the crystals constituting the c-axis oriented lithium niobate film or lithium tantalate film have triple symmetry, it is desirable that the underlying substrate 10 also has the same symmetry. In the case of a sapphire single crystal substrate, a c-plane substrate is preferred, and in the case of a silicon single crystal substrate, a (111)-plane substrate is preferred.

[0028] Here, an epitaxial film is a film whose crystal orientation is aligned with that of the underlying substrate or film. When the film plane is considered the XY plane and the film thickness direction is the Z axis, the crystals are aligned in the X, Y, and Z axis directions. For example, an epitaxial film can be proven by firstly confirming the peak intensity at the orientation position using 2θ-θ X-ray diffraction, and secondly confirming the poles.

[0029] Specifically, firstly, when measurements are performed by 2θ-θ X-ray diffraction, the peak intensities of all planes other than the target plane must be 10% or less, preferably 5% or less, of the maximum peak intensity of the target plane. For example, in a c-axis oriented epitaxial film of lithium niobate, the peak intensities of planes other than the (00L) plane are 10% or less, preferably 5% or less, of the maximum peak intensity of the (00L) plane. (00L) is a general term for equivalent planes such as (001) and (002).

[0030] Secondly, in pole measurement, it is necessary for the poles to be visible. The conditions for confirming the peak intensity at the first orientation position mentioned above only show orientation in one direction, and even if the first condition mentioned above is obtained, if the crystal orientation is not aligned in the plane, the X-ray intensity will not increase at a specific angular position, and the poles will not be visible. Since LiNbO3 has a trigonal crystal structure, there are three poles in LiNbO3(014) in a single crystal.

[0031] In the case of lithium niobate films, it is known that epitaxial growth occurs in a so-called twinned state, where crystals rotated 180° around the c-axis are symmetrically bonded. In this case, since two of the three poles are symmetrically bonded, there are six poles. Furthermore, when a lithium niobate film is formed on a (100)-plane silicon single crystal substrate, since the substrate is symmetrical four times, 4 × 3 = 12 poles are observed. In this invention, lithium niobate films grown epitaxially in a twinned state are also included as epitaxial films.

[0032] The composition of the lithium niobate film is Li x NbA y O z The formula is as follows: A represents an element other than Li, Nb, and O. x is between 0.5 and 1.2, preferably between 0.9 and 1.05. y is between 0 and 0.5. z is between 1.5 and 4, preferably between 2.5 and 3.5. The elements of A include K, Na, Rb, Cs, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Cr, Mo, W, Fe, Co, Ni, Zn, Sc, Ce, etc., and combinations of two or more types are also acceptable.

[0033] The thickness of the ABX3 type ferroelectric thin film 11 is preferably 2 μm or less. This is because if the thickness exceeds 2 μm, it becomes difficult to form a high-quality film. On the other hand, if the thickness of the ABX3 type ferroelectric thin film 11 is too thin, the light confinement in the ABX3 type ferroelectric thin film 11 weakens, and light leaks into the substrate 10 and cladding layer 16. Also, when an electric field is applied to the ABX3 type ferroelectric thin film 11, the change in the effective refractive index of the optical waveguide may become small. For this reason, the thickness of the ABX3 type ferroelectric thin film 11 is preferably at least 1 / 10 of the wavelength of the light used.

[0034] For forming the ABX3 type ferroelectric thin film 11, it is desirable to use film formation methods such as sputtering, CVD (Chemical Vapor Deposition), or sol-gel method. The c-axis of the ABX3 type ferroelectric thin film 11 is oriented perpendicular to the main surface of the substrate 10, and by applying an electric field parallel to the c-axis, the optical refractive index changes in proportion to the electric field.

[0035] When a sapphire single crystal substrate is used as the substrate 10, the ABX3 type ferroelectric thin film 11 can be epitaxially grown directly on the sapphire single crystal substrate. When a silicon single crystal substrate is used as the substrate 10, the ABX3 type ferroelectric thin film 11 is formed by epitaxial growth via a cladding layer (not shown). The cladding layer (not shown) should have a lower refractive index than the ABX3 type ferroelectric thin film 11 and be suitable for epitaxial growth. For example, using Y2O3 as the cladding layer (not shown) allows for the formation of a high-quality ABX3 type ferroelectric thin film 11.

[0036] Furthermore, a method for forming the ABX3 type ferroelectric thin film 11 is also known, which involves thinly polishing or slicing a ferroelectric ABX3 crystal substrate. This method has the advantage of obtaining the same properties as a single crystal and can be applied to the present invention.

[0037] The optical waveguide medium 1 can be manufactured at high mass productivity and low cost without using bonding techniques by stacking an ABX3 type ferroelectric thin film 11, a first dielectric thin film 12, a second dielectric thin film 13, a third dielectric thin film 14, and an electrode layer 15 on a substrate 10 in this order. The hybrid optical waveguide medium 1 manufactured in this way has a low-defect film structure with precisely controlled film thickness, enabling the realization of a low-loss optical waveguide.

[0038] (Second embodiment) Next, an optical modulation element according to a second embodiment of the present invention will be described with reference to the drawings. Components similar to those in the first embodiment will be denoted by the same reference numerals, and their descriptions will be omitted as appropriate. Similarly, descriptions of operations similar to those in the first embodiment will also be omitted as appropriate.

[0039] Figure 2 is a plan view showing the configuration of the optical modulation element 2 according to a second embodiment of the present invention, illustrating the entire optical modulation element including the traveling wave electrode. Figure 3 is a cross-sectional view of the optical modulation element 2, taken along line AA in Figure 2.

[0040] As shown in Figure 2, the optical modulation element 2 comprises a Mach-Zehnder optical waveguide 20 formed above the substrate 10 and having first and second optical waveguides 20a and 20b arranged parallel to each other, a signal electrode 30 provided along the first optical waveguide 20a, and a ground electrode 31 provided along the second optical waveguide 20b.

[0041] The Mach-Zehnder optical waveguide 20 is an optical waveguide having the structure of a Mach-Zehnder interferometer, and has first and second optical waveguides 20a and 20b branched from a single input optical waveguide 20i by a demultiplexer 20c, and the first and second optical waveguides 20a and 20b are combined into a single output optical waveguide 20o via a combiner 20d. The input light to the input optical waveguide 20i is demultiplexed by the demultiplexer 20c, travels through the first and second optical waveguides 20a and 20b respectively, is then combined by the combiner 20d, and output as modulated light from the output optical waveguide 20o.

[0042] One end 30i of the signal electrode 30 is a signal input terminal to which an electrical signal (modulated signal) is input. The other end 30o of the signal electrode 30 is connected to the ground electrode 31 via a termination resistor (not shown). As a result, the signal electrode 30 and the ground electrode 31 function as traveling wave electrodes.

[0043] As shown in Figure 3, the optical modulation element 2 has a multilayer structure in which an ABX3 type ferroelectric thin film 11, a first dielectric thin film 12, a second dielectric thin film 13 (13a, 13b), a third dielectric thin film 14, and an electrode layer 15 are stacked on the substrate 10 in this order. The first dielectric thin film 12, the second dielectric thin films 13a, 13b, and the third dielectric thin film 14 embed the second dielectric thin films 13a, 13b with the first dielectric thin film 12 and the third dielectric thin film 14, forming adjacent first and second embedded optical waveguide structures.

[0044] In other words, the optical modulation element 2 of this embodiment has the structure of the optical waveguide medium 1 of the first embodiment, with two first and second optical waveguides 20a and 20b composed of a second dielectric thin film 13a and 13b, a first dielectric thin film 12, and an ABX3 type ferroelectric thin film 11, and an electrode layer 15 provided on the upper surface of the third dielectric thin film 14. The electrode layer 15 includes a traveling wave electrode consisting of a signal electrode 30 and a ground electrode 31 for applying an electric field to the ABX3 type ferroelectric thin film 11.

[0045] The first and second optical waveguides 20a and 20b are formed by the optical coupling of the second dielectric thin films 13a and 13b with the Z-cut ABX3 type ferroelectric thin film 11 which has an electro-optic effect. Therefore, the refractive indices of the first and second optical waveguides 20a and 20b change to +Δn and -Δn, respectively, depending on the electric field applied to the ABX3 type ferroelectric thin film 11 from the signal electrode 30 and the ground electrode 31, and the phase difference between the pair of optical waveguides changes. A signal light modulated by this change in phase difference is output from the output optical waveguide 20o.

[0046] The signal electrode 30 is provided superimposed on the second dielectric thin film 13a constituting the first optical waveguide 20a in order to modulate the light traveling through the first optical waveguide 20a. The ground electrode 31 is provided superimposed on the second dielectric thin film 13b constituting the second optical waveguide 20b in order to modulate the light traveling through the second optical waveguide 20b.

[0047] The materials for the signal electrode 30 and the ground electrode 31 can be any material with high electrical conductivity, but it is desirable to use highly electrically conductive metallic materials such as Au, Cu, Ag, and Pt in order to reduce signal propagation loss at high frequencies.

[0048] The cladding layer 16 also functions as a buffer layer to prevent light propagating through the first and second optical waveguides 20a and 20b from being absorbed by the signal electrode 30 and the ground electrode 31.

[0049] The thickness of the cladding layer 16 is preferable when it is thick to reduce the light absorption of the signal electrode 30 and the ground electrode 31 (hereinafter collectively referred to simply as "electrodes"), and preferable when it is thin to apply a high electric field to the first and second optical waveguides 20a and 20b. In other words, there is a trade-off relationship between the light absorption of the electrodes and the applied voltage to the electrodes, so it is necessary to set an appropriate thickness depending on the purpose. A higher dielectric constant of the cladding layer 16 is preferable because it can reduce VπL (an index representing electric field efficiency), and a lower refractive index of the cladding layer 16 is preferable because it can make the cladding layer 16 thinner.

[0050] Here, Vπ is the half-wavelength voltage, defined as the difference between the voltage V1 at which the optical output of the optical modulator is maximum and the voltage V2 at which it is minimum, and represents the drive voltage. VπL is the product of the drive voltage, which is the voltage applied to the signal electrode 30, and the electrode length L of the signal electrode 30, and is one indicator of the performance of the optical modulator. The smaller VπL, the smaller the size and the lower the drive voltage.

[0051] Generally, materials with a high dielectric constant also have a high refractive index. Therefore, it is important to consider the balance between the two and select a material with a high dielectric constant and a relatively low refractive index. As an example, Al2O3 has a relative dielectric constant of approximately 9 and a refractive index of approximately 1.6, making it a suitable material for the cladding layer 16.

[0052] Figure 4 is a schematic diagram illustrating the manufacturing process of the optical modulation element 2 according to this embodiment.

[0053] As shown in Figure 4, in the manufacturing of the optical modulation element 2, a sapphire single crystal substrate is prepared as the substrate 10, and an ABX3 type ferroelectric thin film 11, such as a lithium niobate film, is formed on the entire main surface of the substrate 10 by sputtering. Furthermore, the upper surface of the ABX3 type ferroelectric thin film 11 is planarized by chemical mechanical polishing (CMP) (step S1).

[0054] Next, a first dielectric thin film 12 made of SiO2 or Al2O3, etc., with a thickness of 100 to 600 nm is formed on the upper surface of the ABX3 type ferroelectric thin film 11 by CVD (step S2).

[0055] Next, a second dielectric thin film 13 made of Si3N4 with a thickness of 100 to 1000 nm is formed on the upper surface of the first dielectric thin film 12 by CVD (step S3).

[0056] Next, a photoresist is spin-coated onto the upper surface of the second dielectric thin film 13 and cured. Furthermore, the photoresist is exposed and developed using a photomask to form a resist pattern 35p corresponding to the second dielectric thin films 13a and 13b (step S4).

[0057] Next, the second dielectric thin film 13 is etched using the resist pattern 35p as a mask to form the second dielectric thin films 13a and 13b. Reactive ion etching (RIE) or ion milling can be used as the etching method. Then, the resist pattern 35p is removed (step S5).

[0058] Next, a third dielectric thin film 14 made of SiO2 or Al2O3, etc., is formed on the first dielectric thin film 12 and the second dielectric thin films 13a, 13b by CVD. Furthermore, the upper surface of the third dielectric thin film 14 is planarized by CMP (step S6). In this way, the second dielectric thin films 13a, 13b are embedded by the first dielectric thin film 12 and the third dielectric thin film 14, forming adjacent first and second embedded optical waveguide structures.

[0059] Subsequently, electrode layers 15 including signal electrodes 30 and ground electrodes 31, which cover the upper surfaces of the second dielectric thin films 13a and 13b respectively, are sequentially formed on the upper surface of the third dielectric thin film 14 (step S7). This completes the optical modulation element 2.

[0060] (Third embodiment) Next, an optical modulation element according to a third embodiment of the present invention will be described with reference to the drawings. Components similar to those in the first or second embodiment will be denoted by the same reference numerals, and their descriptions will be omitted as appropriate. Similarly, descriptions of operations similar to those in the first or second embodiment will also be omitted as appropriate.

[0061] Figure 5 is a plan view showing the configuration of an optical modulation element 3 according to a third embodiment of the present invention, illustrating the entire optical modulation element 3 including the traveling wave electrode. Figure 6 is a cross-sectional view of the optical modulation element 3, taken along the line BB in Figure 5.

[0062] As shown in Figure 5, the optical modulation element 3 comprises a Mach-Zehnder optical waveguide 20 formed above the substrate 10 and having first and second optical waveguides 20a and 20b arranged parallel to each other, a signal electrode 30 provided between the first and second optical waveguides 20a and 20b in a plan view, a ground electrode 31a provided at a position opposite the signal electrode 30 with the first optical waveguide 20a in between in a plan view, and a ground electrode 31b provided at a position opposite the signal electrode 30 with the second optical waveguide 20b in between in a plan view.

[0063] The first and second optical waveguides 20a and 20b are composed of a second dielectric thin film 13a and 13b and an X-cut ABX3 type ferroelectric thin film 11 having an electro-optic effect. Therefore, the refractive indices of the first and second optical waveguides 20a and 20b change to +Δn and -Δn, respectively, due to the electric field applied to the ABX3 type ferroelectric thin film 11 from the signal electrode 30 and the ground electrodes 31a and 31b, and the phase difference between the pair of optical waveguides changes. A signal light modulated by this change in phase difference is output from the output optical waveguide 20o.

[0064] As shown in Figure 6, the optical modulation element 3 has a multilayer structure in which an ABX3 type ferroelectric thin film 11, a first dielectric thin film 12, second dielectric thin films 13a, 13b, a third dielectric thin film 14, and an electrode layer 15 are stacked on the substrate 10 in this order. The electrode layer 15 includes a signal electrode 30 and ground electrodes 31a, 31b for applying an electric field to the ABX3 type ferroelectric thin film 11.

[0065] The signal electrode 30 is provided between the second dielectric thin films 13a and 13b that constitute the first and second optical waveguides 20a and 20b in a plan view, in order to modulate the light traveling through the first and second optical waveguides 20a and 20b. The ground electrodes 31a and 31b are arranged to sandwich the signal electrode 30 in order to modulate the light traveling through the first and second optical waveguides 20a and 20b.

[0066] In other words, the optical modulation element 3 of this embodiment differs from the optical modulation element 2 of the second embodiment in the arrangement of the signal electrode 30 and the ground electrodes 31a and 31b because the ABX3 type ferroelectric thin film 11 is X-cut. The other configurations and operation of the optical modulation element 3 of this embodiment are the same as those of the optical modulation element 2 of the second embodiment.

[0067] Figure 7 is a schematic diagram illustrating the manufacturing process of the optical modulation element 3 according to this embodiment.

[0068] Steps S11 to S16 are the same as steps S1 to S6 in the manufacturing process of the optical modulation element 2 of the second embodiment.

[0069] After step S16, an electrode layer 15 is sequentially formed on the upper surface of the third dielectric thin film 14, including a signal electrode 30 provided between the second dielectric thin films 13a and 13b in a plan view, a ground electrode 31a provided at a position opposite the signal electrode 30 with the second dielectric thin film 13a in between in a plan view, and a ground electrode 31b provided at a position opposite the signal electrode 30 with the second dielectric thin film 13b in between in a plan view (step S17). With the above steps, the optical modulation element 3 is completed. [Examples]

[0070] (Example 1) The electric field distribution (light intensity distribution) of light propagating through the optical waveguide formed by the second dielectric thin film 13 and the ABX3 type ferroelectric thin film 11 of the optical waveguide medium 1 of the first embodiment was determined by simulation.

[0071] Figure 8(a) shows the simulation model of the optical waveguide medium 1 used in this simulation. The information of each layer that makes up this simulation model is as follows.

[0072] The substrate 10 is a sapphire single crystal (Al2O3) substrate. The ABX3 type ferroelectric thin film 11 is a lithium niobate (LN) film with a thickness of 0.3 μm. The first dielectric thin film 12 is SiO2 with a thickness of 0.1 μm. The second dielectric thin film 13 is a rectangular parallelepiped-shaped Si3N4 with a thickness of 0.1 μm and a width of 0.45 μm. The third dielectric thin film 14 is SiO2 with a thickness of 0.6 μm (however, the thickness in the area where the second dielectric thin film 13 is formed is 0.5 μm).

[0073] Figure 8(b) shows the simulation results. In Figure 8(b), areas with a strong electric field are shown in white, and areas with a weak electric field are shown in black. The two white horizontal lines indicate the location of the ABX3 type ferroelectric thin film 11. The white rectangle indicates the location of the second dielectric thin film 13.

[0074] The simulation results shown in Figure 8(b) indicate that the second dielectric thin film 13 and the ABX3 type ferroelectric thin film 11 are optically coupled, and the position where the light intensity is maximum is located within the ABX3 type ferroelectric thin film 11.

[0075] (Example 2) The electric field efficiency VπL was determined by simulation when light with a wavelength of 637 nm was propagated through the first and second optical waveguides 20a and 20b of the optical modulation element 2 of the second embodiment.

[0076] Figure 9 shows the simulation model of the optical modulation element 2 used in this simulation. Figure 9 is an enlarged view of the area around the first optical waveguide 20a, and the second optical waveguide 20b is not shown.

[0077] In the simulation model of this embodiment, the ground electrode 31 is assumed to consist of a pair of electrodes 31a and 31b that sandwich the signal electrode 30 from both sides. The information of each layer constituting this simulation model is as follows.

[0078] The substrate 10 is a sapphire single crystal (Al2O3) substrate. The ABX3 type ferroelectric thin film 11 is a lithium niobate (LN) film with a thickness of 0.3 μm. The first dielectric thin film 12 is SiO2 with a thickness of 0.1 μm. The second dielectric thin films 13a and 13b are Si3N4 in a rectangular parallelepiped shape with a thickness of 0.1 μm and a width of 0.45 μm. The third dielectric thin film 14 is SiO2 with a thickness of 0.6 μm (however, the thickness in the area where the second dielectric thin film 13 is formed is 0.5 μm).

[0079] The distance between the second dielectric thin films 13a and 13b that constitute the first and second optical waveguides 20a and 20b, respectively, is 60 μm. The signal electrode 30 and the ground electrodes 31a and 31b are made of Au with a thickness of 2 μm. The width of the signal electrode 30 is 3 μm. The distance between the signal electrode 30 and the ground electrodes 31a and 31b is 2 μm.

[0080] The above simulation revealed that the electric field efficiency VπL of the simulation model in this embodiment is 9.9 Vcm for light with a wavelength of 637 nm.

[0081] (Example 3) The appropriate width and thickness of the second dielectric thin film 13 were determined by simulation when light with a wavelength of 637 nm was propagated through the first and second optical waveguides 20a and 20b of the optical modulation element 2 of the second embodiment. The information of each layer constituting the simulation model of the optical modulation element 2 used in this simulation is the same as that of the simulation model of Example 2, except for the width and thickness of the second dielectric thin film 13.

[0082] The graph in the upper left of Figure 10 shows the simulation results of the full width at half maximum (FWHM) of the light intensity in the X direction (horizontal direction) when the width W (SiN width) of the second dielectric thin film 13, made of Si3N4, is varied while the thickness T (SiN thickness) of the second dielectric thin film 13 is fixed at 0.1 μm. Here, the FWHM of the light intensity in the X direction is the distance between two positions in the X direction where the electric field strength decreases by 3 dB from the maximum value in the electric field distribution when light with a wavelength of 637 nm is propagated through the first and second optical waveguides 20a and 20b.

[0083] From these simulation results, it was found that the wider the width W of the second dielectric thin film 13, the smaller the FWHM becomes, and the stronger the confinement of light in the X direction in the ABX3 type ferroelectric thin film 11.

[0084] The graph in the upper right of Figure 10 shows the simulation results of the electric field efficiency VπL when the width W of the second dielectric thin film 13 is varied while the thickness T of the second dielectric thin film 13 is fixed at 0.1 μm. From these simulation results, it was found that VπL is approximately minimized when the width W of the second dielectric thin film 13 is 1.2 μm.

[0085] The graph in the lower left of Figure 10 shows the simulation results of the FWHM in the X direction of light intensity when the thickness T of the second dielectric thin film 13 is varied while the width W of the second dielectric thin film 13 is fixed at 1.2 μm. From these simulation results, it was found that the wider the width W of the second dielectric thin film 13, the smaller the FWHM becomes, and the stronger the confinement of light in the X direction in the ABX3 type ferroelectric thin film 11.

[0086] The graph in the lower right of Figure 10 shows the simulation results of the electric field efficiency VπL when the thickness T of the second dielectric thin film 13 is varied while the width W of the second dielectric thin film 13 is fixed at 1.2 μm. From these simulation results, it was found that when the thickness T of the second dielectric thin film 13 is 0.15 μm, VπL becomes 4.6 Vcm, and VπL is nearly minimized. In other words, it was found that the preferred size of the second dielectric thin film 13 for which VπL is nearly minimized for light with a wavelength of 637 nm is a thickness T of 0.15 μm and a width W of 1.2 μm.

[0087] (Example 4) The appropriate distance between the lower surfaces of the signal electrode 30 and the ground electrodes 31a and 31b (hereinafter collectively referred to simply as "electrodes") and the upper surface of the ABX3 type ferroelectric thin film 11 was determined by simulation when light with a wavelength of 637 nm was propagated through the first and second optical waveguides 20a and 20b of the optical modulation element 2 of the second embodiment. The information of each layer constituting the simulation model of the optical modulation element 2 used in this simulation is the same as that of the simulation model of Example 2, except for the distance D between the lower surface of the electrode and the upper surface of the ABX3 type ferroelectric thin film 11, i.e., the thickness of the cladding layer 16.

[0088] The upper graph in Figure 11 shows the simulation results of the propagation loss (PL) due to light absorption at the electrode when the distance D (LN-Au distance) between the lower surface of the electrode and the upper surface of the ABX3 type ferroelectric thin film 11 is varied. From these simulation results, it was found that when the distance D is 0.6 μm or more, the propagation loss PL becomes 0 dB / cm.

[0089] The graph at the bottom of Figure 11 shows the simulation results of the electric field efficiency VπL when the distance D between the bottom surface of the electrode and the top surface of the ABX3 type ferroelectric thin film 11 is varied. From these simulation results, it was found that as the distance D decreases, VπL also decreases.

[0090] In other words, for light with a wavelength of 637 nm, the preferred distance D at which the propagation loss PL becomes 0 dB / cm and VπL is nearly minimized is 0.6 μm, and at that distance VπL is 4.2 V cm.

[0091] (Example 5) In the second embodiment, when the cladding layer 16 of the optical modulation element 2 is made of aluminum oxide (Al2O3), the appropriate width of the second dielectric thin film 13 for light with a wavelength of 637 nm was determined by simulation. Here, the thickness T of the second dielectric thin film 13 is 0.15 μm.

[0092] Figure 12 shows the simulation model of the optical modulation element 2 used in this simulation. The simulation model in this embodiment is the same as the simulation model in Embodiment 2, except for the material of the cladding layer 16 and the thickness T and width W of the second dielectric thin film 13.

[0093] Figure 13(a) shows the simulation results of the FWHM in the X direction of the light intensity when the width W of the second dielectric thin film 13 is varied. From these simulation results, it was found that the FWHM is almost minimized when the width W of the second dielectric thin film 13 is 2.0 μm.

[0094] Figure 13(b) shows the simulation results of the electric field efficiency VπL when the material of the cladding layer 16 and the width W of the second dielectric thin film 13 are varied.

[0095] When the cladding layer 16 is made of SiO2 and the width W of the second dielectric thin film 13 is 1.2 μm, VπL was 4.2 Vcm. When the cladding layer 16 is made of Al2O3 and the width W of the second dielectric thin film 13 is 1.2 μm, VπL was 2.6 Vcm. When the cladding layer 16 is made of Al2O3 and the width W of the second dielectric thin film 13 is 2.0 μm, VπL was 2.5 Vcm.

[0096] In other words, when the width W of the second dielectric thin film 13 is 1.2 μm, it was found that VπL could be reduced to approximately 60% of the original value by changing the material of the cladding layer 16 from SiO2 to Al2O3. This is thought to be because the dielectric constant of the cladding layer 16 increased, as Al2O3 has a dielectric constant of 9 and SiO2 has a dielectric constant of 4.

[0097] (Example 6) The appropriate thickness of the first dielectric thin film 12 was determined by simulation when light with a wavelength of 637 nm was propagated through the first and second optical waveguides 20a and 20b of the optical modulation element 2 of the second embodiment. Here, the second dielectric thin film 13 has a thickness T of 0.15 μm and a width W of 2.0 μm.

[0098] The information for each layer constituting the simulation model of the optical modulation element 2 used in this simulation is the same as that of the simulation model in Example 5, except for the thickness of the first dielectric thin film 12 and the width W of the second dielectric thin film 13.

[0099] The upper graph in Figure 14 shows the simulation results of the FWHM in the X direction of the light intensity when the thickness of the first dielectric thin film 12 (cover layer thickness) is changed. From these simulation results, it was found that the thinner the first dielectric thin film 12, the smaller the FWHM becomes, and the stronger the confinement of light in the X direction in the ABX3 type ferroelectric thin film 11.

[0100] The graph at the bottom of Figure 14 shows the simulation results of the electric field efficiency VπL when the thickness of the first dielectric thin film 12 (cover layer thickness) is varied. From these simulation results, VπL is found to be almost at its minimum when the thickness of the first dielectric thin film 12 is 50 nm, and its value is 2.36 Vcm.

[0101] These simulation results showed that if the thickness of the first dielectric thin film 12 is 80 nm or less, the VπL for light with a wavelength of 637 nm becomes 2.4 V cm or less. In other words, a good VπL can be obtained even if the thickness of the first dielectric thin film 12 is 0.

[0102] (Example 7) The electric field efficiency VπL for light with a wavelength of 637 nm was determined by simulation when the material of the first dielectric thin film 12 of the optical modulation element 2 of the second embodiment was changed to SiO2.

[0103] Figure 15(a) shows the simulation model of the optical modulation element 2 used in this simulation. The simulation model in this embodiment is the same as the simulation model in Embodiment 6, except for the material and thickness of the first dielectric thin film 12 and the width W of the second dielectric thin film 13.

[0104] Figure 15(b) shows the simulation results of the electric field efficiency VπL (circles) when the width W of the second dielectric thin film 13 is varied. The graph in Figure 15(b) also shows VπL (squares) when the first dielectric thin film 12 is Al2O3 and the width W of the second dielectric thin film 13 is 2.0 μm, as a comparative example.

[0105] From these simulation results, VπL was found to be approximately minimum when the thickness of the first dielectric thin film 12 made of SiO2 was 2.0 μm, with a value of 2.45 Vcm. In other words, VπL when the first dielectric thin film 12 is made of SiO2 is increased by approximately 0.1 Vcm compared to when the first dielectric thin film 12 is made of Al2O3. This is thought to be because the dielectric constant of SiO2 is smaller than that of Al2O3.

[0106] (Example 8) The electric field efficiency VπL was determined by simulation when light with a wavelength of 637 nm was propagated through the first and second optical waveguides 20a and 20b of the optical modulation element 3 of the third embodiment. Figure 16(a) shows the simulation model of the optical modulation element 3 used in this simulation. The information of each layer constituting the simulation model of this embodiment is as follows.

[0107] The substrate 10 is a sapphire single crystal (Al2O3) substrate. The ABX3 type ferroelectric thin film 11 is an X-cut lithium niobate (LN) film with a thickness of 0.3 μm. The first dielectric thin film 12 is SiO2 with a thickness of 100 nm. The second dielectric thin films 13a and 13b are Si3N4 in a rectangular parallelepiped shape with a thickness of 0.15 μm and a width of 2.0 μm. The third dielectric thin film 14 is SiO2 with a thickness of 0.5 μm (however, the thickness in the area where the second dielectric thin film 13 is formed is 0.35 μm).

[0108] The distance between the second dielectric thin films 13a and 13b that constitute the first and second optical waveguides 20a and 20b, respectively, is 60 μm. The signal electrode 30 and the ground electrodes 31a and 31b are made of Au with a thickness of 2 μm. The width of the signal electrode 30 varies depending on the spacing between the electrodes and is 50 to 56 μm. The distance D between the bottom surface of the electrode and the top surface of the ABX3 type ferroelectric thin film 11 is 0.6 μm.

[0109] Figure 16(b) shows the simulation results of the electric field efficiency VπL when the gap G between the signal electrode 30 and the ground electrodes 31a and 31b is varied. From these simulation results, it was found that as the gap G decreases, the VπL also decreases, and when the gap G is 4 μm, the VπL is 0.61 V cm.

[0110] (Example 9) The electric field efficiency VπL was determined by simulation when light with a wavelength of 637 nm was propagated through the first and second optical waveguides 20a and 20b of the optical modulation element 3 of the third embodiment. Figure 17(a) shows the simulation model of the optical modulation element 3 used in this simulation. The information of each layer constituting the simulation model of this embodiment is as follows.

[0111] The substrate 10 is a sapphire single crystal (Al2O3) substrate. The ABX3 type ferroelectric thin film 11 is an X-cut lithium niobate (LN) film with a thickness of 0.3 μm. The first dielectric thin film 12 is aluminum oxide (Al2O3) with a thickness of 100 nm. The second dielectric thin films 13a and 13b are Si3N4 in a rectangular parallelepiped shape with a thickness of 0.15 μm and a width of 2.0 μm. The third dielectric thin film 14 is Al2O3 with a thickness of 0.5 μm (however, the thickness in the area where the second dielectric thin film 13 is formed is 0.35 μm).

[0112] The distance between the second dielectric thin films 13a and 13b that constitute the first and second optical waveguides 20a and 20b, respectively, is 60 μm. The signal electrode 30 and the ground electrodes 31a and 31b are made of Au with a thickness of 2 μm. The width of the signal electrode 30 varies depending on the spacing between the electrodes and is 50 to 56 μm. The distance D between the bottom surface of the electrode and the top surface of the ABX3 type ferroelectric thin film 11 is 0.6 μm.

[0113] Figure 17(b) shows the simulation results of the electric field efficiency VπL when the gap G between the signal electrode 30 and the ground electrodes 31a and 31b is varied. These simulation results are almost the same as those of Example 8, in which the cladding layer 16 is SiO2, and the smaller the gap G, the smaller the VπL, with VπL being 0.53 Vcm when the gap G was 4 μm. [Explanation of Symbols]

[0114] 1 Optical waveguide medium 2,3 Optical Modulators 10 circuit boards 11 ABX3 type ferroelectric thin film 12 First Dielectric Thin Film 13,13a,13b Second dielectric thin film 14. Third Dielectric Thin Film 15 Electrode layer 16. Cladding layer 20 Mach-Zehnder Optical Waveguide 20a First optical waveguide 20b Second optical waveguide 20c branch 20d Multiplexing section 20i input optical waveguide 20o output optical waveguide 30 Signal electrodes 31,31a,31b Ground electrode

Claims

1. circuit board and ABX formed on the substrate ３ A type of ferroelectric thin film and The aforementioned ABX ３ A first dielectric thin film formed on a ferroelectric thin film, A second dielectric thin film formed on the first dielectric thin film, The invention comprises a first dielectric thin film and a third dielectric thin film formed on the second dielectric thin film, The first dielectric thin film, the second dielectric thin film, and the third dielectric thin film constitute an embedded optical waveguide structure in which the second dielectric thin film is embedded by the first dielectric thin film and the third dielectric thin film. The refractive indices of the first dielectric thin film and the third dielectric thin film are the ABX ３ An optical waveguide medium characterized by having a refractive index lower than that of a ferroelectric thin film and the second dielectric thin film.

2. The aforementioned ABX ３ The optical wave guide medium according to claim 1, wherein the ferroelectric thin film is a lithium niobate film or a lithium tantalate film.

3. The aforementioned ABX ３ The optical waveguide medium according to claim 2, wherein the ferroelectric thin film is a sputtered thin film.

4. The optical waveguide medium according to claim 2 or 3, wherein the substrate is a sapphire single crystal substrate.

5. The optical waveguide medium according to claim 1, wherein the first dielectric thin film and / or the third dielectric thin film is made of silicon dioxide or aluminum oxide.

6. The optical wave guide medium according to claim 1, wherein the second dielectric thin film is made of silicon nitride.

7. circuit board and ABX formed on the substrate ３ A type of ferroelectric thin film and The aforementioned ABX ３ A first dielectric thin film formed on a ferroelectric thin film, A second dielectric thin film formed on the first dielectric thin film, A third dielectric thin film formed on the first dielectric thin film and the second dielectric thin film, Formed on the third dielectric thin film, the ABX ３ A traveling wave electrode consisting of a signal electrode and a ground electrode is provided for applying an electric field to a ferroelectric thin film. The first dielectric thin film, the second dielectric thin film, and the third dielectric thin film are arranged such that the second dielectric thin film is embedded by the first dielectric thin film and the third dielectric thin film, forming adjacent first and second embedded optical waveguide structures. The refractive indices of the first dielectric thin film and the third dielectric thin film are lower than those of the ３ ３ type ferroelectric thin film and the second dielectric thin film, respectively, which is characterized by an optical modulation element.

8. The aforementioned ABX ３ The optical modulation element according to claim 7, wherein the ferroelectric thin film is a lithium niobate film or a lithium tantalate film.

9. The aforementioned ABX ３ The optical modulation element according to claim 8, wherein the ferroelectric thin film is a sputtered thin film.

10. The optical modulation element according to claim 8 or 9, wherein the substrate is a sapphire single crystal substrate.

11. The optical modulation element according to claim 7, wherein the first dielectric thin film and / or the third dielectric thin film is made of silicon dioxide or aluminum oxide.

12. The optical modulation element according to claim 7, wherein the second dielectric thin film is made of silicon nitride.

13. ABX on the circuit board ３ A process for forming a type ferroelectric thin film by sputtering, The aforementioned ABX ３ A step of forming a first dielectric thin film on a type ferroelectric thin film, A step of forming a second dielectric thin film on the first dielectric thin film, A step of forming a third dielectric thin film on the first dielectric thin film and the second dielectric thin film, The aforementioned ABX ３ The process includes the step of forming a traveling wave electrode, consisting of a signal electrode and a ground electrode, on the third dielectric thin film for applying an electric field to the ferroelectric thin film, The first dielectric thin film, the second dielectric thin film, and the third dielectric thin film are arranged such that the second dielectric thin film is embedded by the first dielectric thin film and the third dielectric thin film, forming adjacent first and second embedded optical waveguide structures. The refractive indices of the first dielectric thin film and the third dielectric thin film are the ABX ３ A method for manufacturing an optical modulation element, characterized in that the refractive index is lower than that of the type ferroelectric thin film and the second dielectric thin film.

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