Polarized optical waveguide and method for manufacturing the same

Abstract: An optical waveguide, a dielectric device and method for fabricating such devices in which a substrate (1) polarized in a first direction and a dielectric thin film (2) formed on the surface of the substrate and polarized in the opposite direction to the substrate. Parallel grooves (1a) may be formed in the substrate (1). In one embodiment, a first dielectric thin film (2) is formed on the substrate (1) and is formed with grooves (2a) and is polarized in the same direction as the substrate (1). Then, a second dielectric thin film (3) is formed over the first dielectric thin film (2) and it is polarized in a direction which is opposite to the substrate (1) and the first thin film (2).

Description: CROSS-REFERENCES TO RELATED APPLICATIONS This application is related to Application Serial No. 553,395, filed July 17, 1990 in which the inventors are Masahiro Yamada, Koichiro Kishima, assigned to the assignee of the present invention and application entitled "Optical Device Having Domain Structure And Method For Manufacturing The Same" in which the inventors are Masahiro Yamada, Koichiro Kishima and Alum Taguchi assigned to the assignee of the present invention. BACKGROUND OF THE INVENTION Field of the invention The present invention relates to optical waveguides and methods of fabricating optical waveguides in accordance with the preamble of independent claims 1, 3, 4 and 6, respectively. Such an optical waveguide and a method of fabricating the same are known from an article by R.S. Madoyan and O.A. Khachaturyan, published in Sov. Phys. Solid State, Vol. 29 (1989), pages 1578-1580. Description of Related Art Application of a LiNbO₃ optical waveguide having a periodic inverted domain structure formed by periodically controlling its domains to optical SHG, for example, by quasi phase matching (abbreviated to "QPM") is described in E.J. Lim et al., Electric Letter. 25 (1989) 731 and J. Webjorn et al., IEEE Photon. Techn. Lett. 1 (1989) 316. Such a periodic inverted domain structure is formed by a method that diffuses Ti into a LiNbO₃ substrate or a method that diffuses out Li₂O from a LiNbO₃ substrate. Periodic inverted domain structures formed by those methods have a construction as shown in FIG. 4. As shown in FIG. 4, the periodic inverted domain structure has inverted domains 102 having a triangular section with depths of d, arranged with a periodic of Λ, and have a direction of spontaneous polarization deviating at an angle of 180° from the direction of spontaneous polarization Ps of a substrate 101 of a z-plate of LiNbO₃ single crystal (a plate of single crystal having major surfaces perpendicular to the c-axis of the crystal), and are formed by selectively diffusing Ti into the +C-surface of the substrate 101 or by selectively diffusing out Li₂O from the +C-surface of the substrate 101. In this periodic inverted domain structure, d is approximately Λ/4. Patent Abstracts of Japan Vol. 9, No. 225 (E-342), 11 September 1985; & JP - A - 60083499 (Hitachi Seisakusho K.K.) 11.05.1985 discloses a piezoelectric device comprising a dielectric crystal substrate (in this case strontium titanate) and a dielectric crystal thin film formed over major surface of the dielectric crystal substrate by an epitaxial growth process (in this case a layer of lead titanate is epitaxially grown on the strontium titanate substrate). SUMMARY OF THE INVENTION The present invention relates to high performance optical waveguide constructed by forming by means of a liquid-phase epitaxial process over one major surface of a dielectric crystal substrate polarized in one direction, a dielectric crystal thin film polarized in a direction opposite to the direction of polarization of the dielectric crystal substrate and which has a Curie temperature higher than the temperature for the liquid-phase epitaxial growth process. The present invention solves the following problem. The depths d of the domains 102 of the conventional LiNbO₃ optical waveguide shown in FIG. 4 decreases with a decrease of the period Λ which reduces the SHG efficiency. A Ti mask and a SiO₂ mask for the diffusion of Ti and the out diffusion of Li₂O cannot completely be removed by etching after the diffusion of Ti and the out diffusion of Li₂O and the Ti mask and the SiO₂ mask tend to remain partially on the surface so as to deteriorate the characteristics of the optical waveguide. A proton exchange annealing method as well as methods using the diffusion of Ti and the out diffusion of Li₂O have been employed for forming optical waveguides. However, the non-linear characteristics of an optical waveguide formed by the proton exchange annealing method is lower than that of LiNbO₃ crystal and hence the SHG efficiency of the optical waveguide is low. It is thus an object of the present invention to provide a high performance optical waveguide such as an optical SHG device capable of functioning with a high SHG efficiency. It is another object of the present invention to provide a method for fabricating a high performance optical waveguide such as an optical SHG device which is capable of operating at a high SHG efficiency. The above objects are respectively solved by the features of independent device claims 1 and 4 as well as by the features of independent method claims 3 and 6 . Other objects, features an advantages of the invention will be readily apparent from the following description of certain preferred embodiments thereof taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1A and 1B are sectional views of an optical waveguide disclosing a first embodiment of the present invention; FIG. 2 is a sectional view of an optical waveguide of a second embodiment of the present invention; FIGS. 3A-3D are sectional views of an optical waveguide of a third embodiment according to the present invention; and FIG. 4 is a sectional view of the prior art optical waveguide. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The optical waveguide according to a first aspect of the present invention comprises a dielectric crystal substrate 1 polarized in one direction and provided with parallel grooves in a predetermined arrangement in one major surface thereof, and a dielectric crystal thin film 2 formed over the major surface of the dielectric crystal substrate 1 has a periodic inverted domain structure in which the depth can be determined regardless of the period, and has the same nonlinear characteristics as the intrinsic nonlinear characteristics of the dielectric crystal. Thus, the optical waveguide of the first aspect of the present invention is a high-performance optical waveguide, such as an optical SHG device capable of operating at a high SHG efficiency. The method of fabricating an optical waveguide according to a second aspect of the present invention results in the fabrication of an optical waveguide comprising a dielectric crystal substrate 1 polarized in one direction and provided in one major surface thereof with parallel grooves in a predetermined arrangement. A dielectric crystal thin film 2 is formed on the major surface of the dielectric crystal substrate 1 and polarized in a direction opposite to the direction of polarization of the dielectric crystal substrate 1. The optical waveguide thus fabricated is a high-performance optical waveguide, such as an optical SHG device capable of operating at a high SHG efficiency. The optical waveguide according to a third aspect of the present invention comprises a first dielectric crystal thin film 2 which is polarized in one direction and which is provided in its major surface with parallel grooves in a predetermined arrangement, and a second dielectric crystal thin film 3 is formed over the major surface of the first dielectric crystal thin film 2 to completely fill the grooves and is polarized in a direction which is opposite to the direction of polarization of the first dielectric crystal thin film 2. Thus the optical waveguide has a periodically inverted domain structure in which the depth can be determined regardless of the period, and which has the same nonlinear characteristics as the intrinsic nonlinear characteristics of the dielectric crystal and is a high-performance optical waveguide, such as an optical SHG device capable of operating a high SHG efficiency. The method of fabricating an optical waveguide according to a fourth aspect of the present invention results in the fabrication of an optical waveguide comprising a first dielectric crystal thin film which is polarized in one direction and having the parallel grooves in predetermined arrangement, and a second dielectric crystal thin film 3 formed over the first dielectric crystal thin film 2 and polarized in a direction opposite to the direction of polarization of the first dielectric crystal thin film 2. The optical waveguide thus fabricated has a periodic inverted domain structure in which the depth can be determined regardless of the period. The optical waveguide has the same nonlinear characteristics as the intrinsic nonlinear characteristics of the dielectric crystal, and is a high-performance optical waveguide, such as an optical SHG device capable of operating at a high SHG efficiency. Preferred embodiments of the present invention will be described hereinafter with reference to the accompanying drawings, in which like or corresponding parts are denoted by the same reference characters. The First Embodiment FIG. 1A illustrates, grooves 1a of a depth t, for example, 0.9 µm, arranged at a period Λ, for example, 9.6 µm, formed in the +c-surface of a substrate 1 which comprises a LiNbO₃ single crystal z-plate (a single crystal plate having major surfaces perpendicular to the c-axis of the crystal) and which is polarized in a direction which is perpendicular to its major surfaces. The grooves 1a can be formed by forming a resist film having a pattern with openings corresponding to the grooves 1a by a lithographic process, and by etching the surface of the substrate 1 to a depth 5 by a dry etching process, such as a reactive ion etching (RIE) process. Then, a LibO₃ thin film 2 of several micrometers in thickness is formed at a temperature of 905°C in a fused mixture of 52% by mol Li₂O, 8% by mol. Nb₂O₅, and 40% by mol V₂O₅, over the +c-surface of the substrate 1 which is provided with the grooves 1a by a liquid-phase epitaxial growth process so that the grooves 1a are completely filled with LiNbO₃. The Curie temperature of about 1210°C is higher than the temperature for the liquid-phase epitaxial growth process by about 300°C. The direction of spontaneous polarization of the LiNbO₃ thin film 2 formed over the +c-surface of the substrate 1 is opposite to the direction of spontaneous polarization of the substrate 1. It is to be noted that the upper surface of the LiNbO₃ thin film 2 is perfectly flat despite the fact that the surface of the substrate 1 over which the LNbO₃ thin film 2 is formed is provided with grooves 1a. Thus, the optical waveguide in the first embodiment has a LiNbO₃ thin film formed on the +c-surface of the substrate 1 which is provided with the grooves 1a in a periodic arrangement and is polarized in a direction which is opposite to the direction of polarization of the substrate 1 so as to form a substantially ideal periodic inverted domain structure, and the optical waveguide has the same non-linear characteristics as the intrinsic nonlinear characteristics of the LibO₃ crystal. Accordingly, the optical waveguide is a high-performance optical waveguide, such as a QPM optical SHG device capable of operating at a high SHG efficiency. The optical waveguide in the first embodiment is applicable, in addition to the optical SHG device, to optical waveguides utilizing nonlinear optical effect,optical waveguides utilizing acousto-optic effect or electro-optic effect, such as optical switches, modulators and deflectors. Second Embodiment FIG. 2 shows an optical waveguide in a second embodiment according to the present invention which employs a substrate 1 which is similar to that employed in the first embodiment. A LiNbO₃ thin film 2 is formed over the +c-surface of the substrate 1 by a liquid-phase epitaxial growth process similar to that employed in fabricating the optical waveguide in the first embodiment. The thickness of the UNbO₃ film 2 is approximately equal to the depth of the grooves 1a which are completely filled with UNbO₃ and the upper surface of the LiNbO₃ thin film is flat. The polarization of film 2 is opposite to that of the substrate 1. The film 2 in FIG. 2 is much thinner than in FIG. 1B. The optical waveguide in the second embodiment, like the optical waveguide in the first embodiment, is a high-performance optical waveguide, such as a QPM optical SHG device capable of operating at a high SHG efficiency. Third Embodiment A third embodiment is shown in FIG. 3A wherein a LNbO₃ thin film 2 is formed over the +c-surface of a substrate 1 comprising a LNbO₃ crystal z-plate doped with MgO or the like by a liquid-phase epitaxial growth process similar to that employed in fabricating the optical waveguide in the first embodiment. The LiNbO₃ thin film 2 is polarized in a direction which is opposite to the direction of polarization of the substrate 1. Then, the substrate 1 is heated to a temperature near the Curie temperature and an electric field is applied to the heated substrate 1 to invert the polarity of the LiNbO₃ thin film 2 so that the substrate 1 and the LiNbO₃ thin film are polarized in the same direction as shown in FIG. 3B. Then, as shown in FIG. 3C, grooves 2a are formed in the major surface of the LiNbO₃ thin film in a predetermined arrangement similar to the grooves of the first embodiment. Then, as shown in FIG. 3D, a LiNbO₃ thin film 3 is formed on the grooved major surface of the LiNbO₃ thin film 2 by a liquid-phase epitaxial growth process. The LiNbO₃ thin film 3 thus formed by epitaxial growth is polarized in a direction which is opposite to the direction of polarization of the underlying LiNbO₃ thin film 2. The optical device thus fabricated has a substantially ideal periodic inverted domain structure consisting of the LiNbO₃, and thin films 2 and 3 and is a high-performance optical waveguide, such as a QPM optical SHG device capable of operating at a high SHG efficiency. For example, the substrate 1 may comprise a LiNbO₃ single crystal plate, a Li(NbTa)O₃ single crystal z-plate, plates other than the LiNbO₃ z-plate and the MgO-doped LiNbO₃ single crystal z-plate employed in the foregoing described four embodiments. It is possible to employ dielectric crystal substrates other than the LiNbO₃ substrates. Similarly, the LiNbO₃ thin films 2 and 3 may be replaced by dielectric crystal thin films other than the LiNbO₃ thin films. A substrate having an orientation which is different from those of the substrate 1 employed in the foregoing embodiments may be used. Furthermore, the present invention is applicable to forming a dielectric crystal thin film locally having inverted domains on a dielectric crystal substrate. A locally inverted domain structure thus formed is applicable to various optical waveguides respectively utilizing electro-optic effect, acousto-optic effect and nonlinear optical effect. Effect of the invention. As is apparent from the foregoing description, the optical waveguide and the method of fabricating the same realizes a high-performance optical waveguide, such as an optical SHG device capable of operating at a high SHG efficiency.

Claims: An optical waveguide comprising: a dielectric crystal substrate (1) polarized in one direction, and a dielectric crystal thin film (2) formed over one major surface (+C) of the dielectric crystal substrate (1) by a liquid-phase epitaxial growth process and polarized in a direction opposite to the direction of polarization of the dielectric crystal substrate (1) and having a Curie temperature which is higher than the temperature used for the epitaxial growth process, characterized in that said dielectric crystal substrate is provided with parallel grooves (1a) arranged with predetermined distances to each other in said major surface (+C) and said dielectric thin film (2) completely fills said grooves (1a). An optical waveguide according to claim 1, wherein the period (Λ) of said parallel and periodic grooves (1a) is constant. A method of fabricating an optical waveguide, comprising following steps: Providing a dielectric crystal substrate (1) which is polarized in one direction, forming by a liquid-phase epitaxial growth process over one major surface of said dielectric crystal substrate (1), a dielectric crystal thin film (2) polarized in a direction opposite to the direction of polarization of the dielectric crystal substrate (1) and having a Curie temperature higher than the temperature used for the liquid-phase epitaxial growth process, characterized in that the method further comprises a step of forming parallel grooves (la) in a predetermined arrangement in said one major surface of said dielectric crystal substrate (1), said grooves subsequently being completely filled with said dielectric crystal thin film (2) on said major surface. An optical waveguide comprising: a dielectric crystal substrate (1) polarized in one direction; a first dielectric crystal thin film (2) formed by a liquid-phase epitaxial growth process over one major surface of the dielectric crystal substrate (1) and having a Curie temperature higher than the temperature for the liquid-phase epitaxial growth process, characterized in that said first dielectric crystal thin film is provided in its upper surface with parallel grooves (2a) arranged with predetermined distances to each other, and is polarized in the direction of polarization of the dielectric crystal substrate; and that a second dielectric crystal thin film (3) formed by a liquid-phase epitaxial growth process is provided over said major surface of the first dielectric crystal thin film (2) and polarized in a direction opposite to the direction of polarization of said dielectric crystal substrate and having a Curie temperature which is higher than the temperature for the liquid-phase epitaxial growth process. An optical waveguide according to claim 4, wherein the period (Λ) of said parallel and periodic grooves (2a) is constant. A method of fabricating an optical waveguide comprising following steps: providing a dielectric crystal substrate (1) polarized in one direction, forming by a liquid-phase epitaxial growth process over one major surface of said dielectric crystal substrate (1) a first dielectric crystal thin film (2) which is polarized in a direction which is opposite to the polarization of the dielectric crystal substrate (1) and which has a Curie temperature which is higher than the temperature for the liquid-phase epitaxial growth process; characterized by further steps of: inverting the polarity of the first dielectric crystal thin film (2); forming grooves (2a) in a predetermined parallel arrangement in the major surface of the first dielectric crystal thin film (2); and forming a second dielectric crystal thin film (3) polarized in a direction opposite to the direction of polarization of the dielectric crystal substrate over the major surface of the first dielectric crystal thin film (2) by a liquid-phase epitaxial growth process.

Inventors: SAITOH MASAKI, TAMADA HITOSHI, TANEYAMA YUKIE, YAMADA ATSUO, SAITOH, MASAKI, TAMADA, HITOSHI, TANEYAMA, YUKIE, YAMADA, ATSUO
Applicants: SONY CORP, SONY CORPORATION
Application year: 1991
Main classification: G02F1
IPC codes: G02F1, G02B6