Waveguide element manufacturing method

Abstract

To provide a waveguide element manufacturing method with which it is possible to manufacture a waveguide element having superior external appearance and strength.SOLUTION: A waveguide element manufacturing method according to one embodiment of the present invention includes the steps of: forming a plurality of grooves in a first plane of a non-linear optical material substrate with a space therebetween, and constituting at least one of sections of the non-linear optical material substrate located between the grooves adjacent to each other among the plurality of grooves, as a ridge-shaped waveguide path; forming a first low refractive index layer whose refractive index is smaller than that of the non-linear optical material substrate and whose thickness exceeds the groove depth on the first plane of the non-linear optical material substrate so as to cover the ridge-shaped waveguide path; polishing the first low refractive index layer from the side opposite the non-linear optical material substrate and flattening the surface of the first low refractive index layer that is opposite the non-linear optical material substrate; and joining a first support substrate to the flat surface of the first low refractive index layer.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a waveguide element. [Background technology]

[0002] Development of waveguide elements is progressing as one type of nonlinear optical element. Waveguide elements are expected to be applied and developed in a wide range of fields, such as next-generation optical communications and quantum fields. As an example of such a waveguide element, a wavelength conversion element has been proposed, which includes a ferroelectric crystal substrate on which a ridge-type optical waveguide is formed, an overclad formed on a first main surface of the ferroelectric crystal substrate, and an upper substrate bonded to the ferroelectric crystal substrate by an upper adhesive layer via the overclad (see, for example, Patent Document 1). Such a wavelength conversion element is manufactured by forming a pair of ridge grooves parallel to each other on a first main surface of a ferroelectric crystal substrate, forming the portion between the ridge grooves as a ridge-type optical waveguide, depositing an overclad on the first main surface of the ferroelectric crystal substrate, applying an adhesive to the overclad, and adhering an upper substrate to the overclad. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2017-227935 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the manufacturing method of a wavelength conversion element described in Patent Document 1, the overcladding is formed so as to sink into the ridge groove of the dielectric crystal substrate, and the upper adhesive layer on the overcladding is filled into the ridge groove. Therefore, air bubbles may be trapped between the overcladding and the upper adhesive layer. In this case, there is a problem that the appearance of the wavelength conversion element is poor. Furthermore, if air bubbles are mixed between the overclad and the upper adhesive layer, there may be localized areas where the upper adhesive layer and the overclad are not in contact, which may reduce the adhesion between these layers, and the presence of air bubbles may cause unevenness in the upper adhesive layer, which may result in stress being applied to the internal structure of the wavelength conversion element, resulting in insufficient strength of the wavelength conversion element. A primary object of the present invention is to provide a method for manufacturing a waveguide element that can produce a waveguide element having excellent appearance and strength. [Means for solving the problem]

[0005] [1] A method for manufacturing a waveguide element according to one embodiment of the present invention includes the steps of: preparing a nonlinear optical material substrate; forming, at a first surface located on one side of the thickness direction of the nonlinear optical material substrate, a plurality of grooves extending in a direction intersecting the thickness direction at intervals from each other, so as to configure at least one portion of the nonlinear optical material substrate located between adjacent grooves as a ridge waveguide; forming, on the first surface of the nonlinear optical material substrate, a first low refractive index layer having a refractive index smaller than that of the nonlinear optical material substrate and a thickness exceeding the depth of the grooves, so as to cover the ridge waveguide; polishing the first low refractive index layer from the side opposite the nonlinear optical material substrate to flatten the surface of the first low refractive index layer opposite the nonlinear optical material substrate; and bonding a first support substrate to the flat surface of the first low refractive index layer via a first bonding layer. [2] In the method for manufacturing a waveguide element described in [1] above, the step of preparing the nonlinear optical material substrate may include the steps of: forming a second low refractive index layer having a refractive index smaller than that of the nonlinear optical material substrate on a second surface of the nonlinear optical material substrate opposite to the first surface; bonding a second support substrate to the surface of the second low refractive index layer opposite to the nonlinear optical material substrate via a second bonding layer; and polishing the nonlinear optical material substrate from the side opposite to the second low refractive index layer. [3] In the method for manufacturing a waveguide element according to [2] above, the step of preparing the nonlinear optical material substrate may further include a step of forming periodic polarization inversion portions on the second surface of the nonlinear optical material substrate before the step of forming the second low refractive index layer. [4] In the method for manufacturing a waveguide element according to any one of [1] to [3] above, the first bonding layer may be made of a resin material. [5] In the method for manufacturing a waveguide element according to the above [2] or [3], the second bonding layer may be made of a resin material. [6] In the method for manufacturing a waveguide element according to any one of [1] to [5] above, in the step of polishing the first low refractive index layer, the first low refractive index layer may be polished until the dimension between the flat surface of the first low refractive index layer and the ridge waveguide in the thickness direction becomes 1.0 or less relative to the depth of the groove. [7] In the method for manufacturing a waveguide element according to any one of [1] to [6] above, the depth of the groove may be 1.5 μm or more and 3.0 μm or less. [8] In the method for manufacturing a waveguide element according to any one of [1] to [7] above, the nonlinear optical material substrate may be made of lithium niobate and / or lithium tantalate containing magnesium oxide as a dopant. [9] In the method for manufacturing a waveguide element according to any one of the above [1] to [8], the first low refractive index layer may contain silicon dioxide. [Effects of the Invention]

[0006] According to embodiments of the present invention, a waveguide element having excellent appearance and strength can be manufactured. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a schematic diagram illustrating a preparation step included in a method for manufacturing a waveguide element according to an embodiment of the present invention. [Figure 2] FIG. 2 is a schematic explanatory view for explaining a processing step subsequent to the preparation step of FIG. [Figure 3] FIG. 3 is a schematic diagram illustrating the first low refractive index layer forming step subsequent to the processing step of FIG. [Figure 4] FIG. 4 is a schematic explanatory view for explaining the first polishing step subsequent to the first low refractive index layer forming step in FIG. [Figure 5] FIG. 5 is a schematic explanatory view for explaining the first bonding step subsequent to the first polishing step in FIG. [Figure 6] FIG. 6 is a schematic explanatory view for explaining the second low refractive index layer forming step included in the preparation step of FIG. [Figure 7] FIG. 7 is a schematic explanatory view for explaining the second bonding step subsequent to the second low refractive index layer forming step in FIG. [Figure 8] 8 is a schematic perspective view of the waveguide element of FIG. 5 taken along the line VIII-VIII'. [Figure 9] FIG. 9 is a schematic diagram showing the configuration of a periodic polarization inversion portion included in the waveguide element of FIG. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to these embodiments. In addition, in order to clarify the explanation, the width, thickness, shape, etc. of each part may be shown schematically in the drawings compared to the embodiments, but this is merely an example and does not limit the interpretation of the present invention.

[0009] A. Overview of the manufacturing method for waveguide elements Fig. 1 is a schematic diagram illustrating a preparation step included in a method for manufacturing a waveguide element according to an embodiment of the present invention. Fig. 2 is a schematic diagram illustrating a processing step subsequent to the preparation step of Fig. 1. Fig. 3 is a schematic diagram illustrating a first low refractive index layer forming step subsequent to the processing step of Fig. 2. Fig. 4 is a schematic diagram illustrating a first polishing step subsequent to the first low refractive index layer forming step of Fig. 3. Fig. 5 is a schematic diagram illustrating a first bonding step subsequent to the first polishing step of Fig. 4.

[0010] A method for manufacturing a waveguide element according to one embodiment of the present invention includes, in this order, a preparation step, a processing step, a first low refractive index layer forming step, a first polishing step, and a first bonding step. In the preparation step, a nonlinear optical material substrate 1 having a predetermined thickness is prepared. The nonlinear optical material substrate 1 has a first surface 1a located on one side in the thickness direction and a second surface 1b opposite to the first surface 1a. In the processing step, a plurality of grooves 13 are formed on the first surface 1a of the nonlinear optical material substrate 1. Each of the plurality of grooves 13 extends in a direction intersecting the thickness direction of the nonlinear optical material substrate 1. As shown in Fig. 2, in one embodiment, the groove 13 has a bottom surface 13a, a first side surface 13b, and a second side surface 13c. The bottom surface 13a is located between the first surface 1a and the second surface 1b of the nonlinear optical material substrate 1 in the thickness direction of the nonlinear optical material substrate 1. The width direction of the bottom surface 13a typically intersects with the thickness direction of the nonlinear optical material substrate 1 and the direction in which the groove 13 extends. The first side surface 13b connects one end of the bottom surface 13a in the width direction to the first surface 1a. The second side surface 13c connects the other end of the bottom surface 13a in the width direction to the first surface 1a. In another embodiment, groove 13 may have bottom surface 13a and only one of first side surface 13b and second side surface 13c. In other words, groove 13 may be a notch-like shape that is open on one side in the width direction. The plurality of grooves 13 are formed at intervals from one another. At least one of the portions of the nonlinear optical material substrate 1 located between adjacent grooves 13 among the plurality of grooves 13 is configured as a ridge-type waveguide 11. 3, in the first low-refractive-index layer forming step, the first low-refractive-index layer 5 is formed on the first surface 1a of the nonlinear optical material substrate 1 so as to cover the ridge-shaped waveguide 11. The refractive index of the first low-refractive-index layer 5 is smaller than the refractive index of the nonlinear optical material substrate 1. The thickness T of the first low-refractive-index layer 5 exceeds the depth D of the groove 13. In this specification, the "depth of the groove" means the dimension between the first surface 1a of the nonlinear optical material substrate 1 and the bottom surface 13a of the groove 13 in the thickness direction of the nonlinear optical material substrate 1. In the polishing step, the first low refractive index layer 5 is polished from the side opposite to the nonlinear optical material substrate 1. As a result, the surface of the first low refractive index layer 5 opposite to the nonlinear optical material substrate 1 becomes a flat surface 5a, as shown in Fig. 4. In the first bonding step, the first support substrate 2 is bonded to the flat surface 5a of the first low refractive index layer 5 via the first bonding layer 3. According to this method, after forming a plurality of grooves 13 in the nonlinear optical material substrate 1 to provide the ridge-type waveguide 11, the first low-refractive index layer 5 is formed with a thickness T that exceeds the depth D of the grooves 13, and then the surface of the first low-refractive index layer 5 opposite to the nonlinear optical material substrate 1 is polished to form a flat surface 5a. Therefore, when the first support substrate 2 is bonded to the flat surface 5a of the first low-refractive index layer 5 via the first bonding layer 3, it is possible to prevent air bubbles from being mixed in between the first low-refractive index layer 5 and the first bonding layer 3. This makes it possible to smoothly manufacture a waveguide element 100 having excellent appearance and strength.

[0011] 1, in the preparation step, a nonlinear optical material substrate 1 having a predetermined thickness is prepared by any appropriate means. The thickness of the nonlinear optical material substrate 1 prepared in the preparation step is, for example, 0.1 μm to 10 μm, preferably 0.15 μm to 7.0 μm, and more preferably 3.0 μm to 5.0 μm.

[0012] 6 and 7, in one embodiment, the preparation step includes a second low refractive index layer forming step, a second bonding step, and a second polishing step, thereby preparing a laminate including a nonlinear optical material substrate 1 having a predetermined thickness. In the second low-refractive-index layer forming step, a second low-refractive-index layer 6 is formed on the second surface 1b of the nonlinear optical material substrate 1. The refractive index of the second low-refractive-index layer 6 is smaller than the refractive index of the nonlinear optical material substrate 1. The thickness of the nonlinear optical material substrate 1 used in the second low-refractive-index layer forming step is typically larger than the thickness of the nonlinear optical material substrate 1 finally prepared in the preparation step. The nonlinear optical material substrate 1 used in the second low-refractive-index layer forming step will be referred to as a thick substrate 10 below. The second surface 1b of the thick substrate 10 (nonlinear optical material substrate 1) is substantially flat. Therefore, in the second low refractive index layer 6 formed on the second surface 1b, the surface opposite to the thick substrate 10 can be substantially flat. In the second bonding step, a second support substrate 7 is bonded to the surface of the second low refractive index layer 6 opposite to the thick substrate 10 via a second bonding layer 4. In the second polishing step, the thick substrate 10 is polished from the side opposite to the second low refractive index layer 6. According to this method, the surface of the second low-refractive index layer 6 opposite to the thick substrate 10 is substantially flat, and the second support substrate 7 is bonded to this surface, so that it is possible to prevent air bubbles from being mixed in between the second low-refractive index layer 6 and the second bonding layer 4. Furthermore, since the thick substrate 10 is polished while being supported by the second support substrate 7 via the second low-refractive index layer 6 and the second bonding layer 4, it is possible to accurately thin the thick substrate 10, and it is possible to stably prepare the nonlinear optical material substrate 1 having the desired thickness. Furthermore, in the manufactured waveguide element 100, the first support substrate 2 is located on one side in the thickness direction, and the second support substrate 7 is located on the other side in the thickness direction, so that it is possible to improve the handleability of the waveguide element 100 (see FIG. 5 ).

[0013] As shown in Fig. 6, in one embodiment, the preparation step further includes a periodic polarization inversion forming step before the second low refractive index layer forming step. In the periodic polarization inversion forming step, periodic polarization inversion portions 12 are formed on the second surface 1b of the thick substrate 10 (see Fig. 8). This allows the manufacture of a waveguide element 100 including periodic polarization inversion portions 12, i.e., a wavelength conversion element 101 (see Fig. 8).

[0014] In this specification, the term "waveguide element" encompasses both a wafer on which at least one waveguide element is formed (waveguide element wafer) and chips obtained by cutting the waveguide element wafer.

[0015] B. Details of the manufacturing method of the waveguide element Next, a method for manufacturing a waveguide element will be described in detail with reference to FIGS.

[0016] B-1. Preparation process B-1-1. Thick board 6, the thick substrate 10 can be described in the same manner as the nonlinear optical material substrate 1 finally prepared in the preparation step, except for the thickness. The thick substrate 10 has any appropriate thickness. The thickness of the thick substrate 10 is, for example, 200 μm to 1000 μm, and preferably 250 μm to 350 μm. Each of the first surface 1a and the second surface 1b of the thick substrate 10 is typically a flat surface extending along a direction perpendicular to the thickness direction of the thick substrate 10. The shape of the thick substrate 10 as viewed in the thickness direction is not particularly limited. Examples of the shape of the thick substrate 10 as viewed in the thickness direction include a circle, an ellipse, and a polygon. The maximum dimension of the thick substrate 10 in a plane direction perpendicular to the thickness direction is, for example, 10 mm to 200 mm, and preferably 100 mm to 200 mm.

[0017] The thick substrate 10 is constructed from any suitable nonlinear optical material, and preferably from a single crystal of the nonlinear optical material. Examples of nonlinear optical materials include lithium niobate (LiNbO3:LN), lithium tantalate (LiTaO3:LT), potassium titanyl phosphate (KTiOPO4:KTP), and potassium lithium niobate (K x Li (1-x) NbO2:KLM), potassium niobate (KNbO3:KN), potassium tantalate niobate (KNb x Ta (1-x) O3:KTN), solid solution of lithium niobate and lithium tantalate, KTP (KTiOPO4), KTN (KTa (1-x) Nb x O3) The nonlinear optical materials may be used alone or in combination. The nonlinear optical material may contain a dopant. Examples of the dopant include magnesium oxide and zinc oxide. The dopants may be used alone or in combination. Among such nonlinear optical materials, lithium niobate (LN) and / or lithium tantalate (LT) containing magnesium oxide as a dopant are preferable. Such a nonlinear optical material has a relatively large nonlinear optical constant d33, and can stably suppress optical damage in the waveguide element 100. The nonlinear optical constant d33 is typically 15 pm / V or more.

[0018] The thick substrate 10 may be an X-cut substrate or a Y-cut substrate, but is preferably a Y-cut substrate, and more preferably a 5° off-axis Y-cut substrate. The refractive index of the thick substrate 10 at 473 Hz is, for example, 2.15 to 2.32, and preferably 2.19 to 2.21.

[0019] B-1-2. Periodic polarization inversion formation process In one embodiment, the thick substrate 10 is first subjected to a periodic polarization inversion forming step, in which periodic polarization inversions 12 are formed on the second surface 1b of the thick substrate 10. More specifically, a comb-like electrode pattern is formed on the second surface 1b of the thick substrate 10. The period of the comb corresponds to the polarization inversion period Λ in the periodic polarization inversions 12 (see FIG. 9). Next, a voltage is applied to the thick substrate 10 in the c-axis direction via the electrode pattern (see FIG. 9). This forms the periodic polarization inversions 12. Thereafter, the electrode pattern is removed by etching.

[0020] 9, the periodic polarization inversion section 12 has any appropriate configuration capable of exhibiting quasi-phase matching (QPM). The periodic polarization inversion section 12 has first polarization sections 12a and second polarization sections 12b (polarization inversion domains) alternately arranged in a first plane direction perpendicular to the thickness direction of the thick substrate 10. The first polarized portion 12a is polarized in the c-axis direction. The second polarized portion 12b is polarized in the opposite direction to the first polarized portion 12a. In the illustrated example, the first polarized portion 12a is polarized in a second plane direction that is perpendicular to both the thickness direction and the first plane direction of the thick substrate 10. The domain widths of the first polarized portion 12a and the second polarized portion 12b are adjusted arbitrarily and appropriately. The polarization inversion period Λ of the periodic polarization inversion portions 12 is, for example, 1 μm to 50 μm, and preferably 3 μm to 30 μm. The polarization inversion ratio (polarization inversion domain width / polarization inversion period) is, for example, 0.1 to 0.9, and preferably 0.3 to 0.7.

[0021] In the periodic polarization inversion forming step, a plurality of the periodic polarization inversions 12 may be formed. There is no particular limitation on the number of periodic polarization inversions 12. The number of periodic polarization inversions 12 is, for example, 1 to 200,000. When a plurality of periodic polarization inversions 12 are formed on the second surface 1b, the plurality of periodic polarization inversions 12 are typically arranged in parallel at intervals in the direction of the second surface of the thick substrate .

[0022] B-1-3. Second low refractive index layer forming process Next, as shown in FIG. 6, a second low refractive index layer 6 is formed on the second surface 1b of the thick substrate . More specifically, the material of the second low refractive index layer 6 is deposited on the second surface 1b of the thick substrate 10 by any appropriate deposition method. Examples of the film formation method include sputtering, vacuum deposition, ion plating, chemical vapor deposition (CVD), and atomic layer deposition (ALD). Of the film formation methods, sputtering is preferred. The second low refractive index layer 6 may be made of any suitable low refractive index material. The refractive index of the low refractive index material is lower than that of the nonlinear optical material described above. Examples of low refractive index materials include silicon dioxide (SiO), aluminum oxide (AlO), tantalum oxide (TaO), and silicon nitride (SiN). The low refractive index materials may be used alone or in combination. Among low refractive index materials, silicon dioxide (SiO2) is preferable. When the second low refractive index layer 6 contains silicon dioxide, the refractive index of the second low refractive index layer 6 can be made sufficiently small, and excellent thermal stability can be imparted to the second low refractive index layer 6.

[0023] In the illustrated example, the second low refractive index layer 6 is formed over the entire second surface 1b of the thick substrate . The surface of the second low refractive index layer 6 opposite to the thick substrate 10 is typically a flat surface along a direction perpendicular to the thickness direction of the thick substrate 10. The flatness of the surface of the second low refractive index layer 6 opposite to the thick substrate 10 is, for example, 0.0001 μm to 0.1 μm, and preferably 0.0001 μm to 0.01 μm. The flatness is measured in accordance with, for example, JIS B 0621. The thickness of the second low refractive index layer 6 is, for example, 0.01 μm to 10 μm, and preferably 0.1 μm to 1.0 μm. The refractive index of the second low refractive index layer 6 at 473 Hz is, for example, 1.40 to 1.60, and preferably 1.45 to 1.49.

[0024] B-1-4.Second joining process Next, as shown in FIG. 7, a second support substrate 7 is bonded to the second low refractive index layer 6 via a second bonding layer 4. The second bonding layer 4 may be made of an inorganic material or an organic material. Examples of inorganic materials that can be used to form second bonding layer 4 include silicon dioxide, amorphous silicon, and tantalum oxide. When second bonding layer 4 is made of an inorganic material, second bonding layer 4 may be formed by directly bonding second low refractive index layer 6 and second support substrate 7. Examples of organic materials constituting the second bonding layer 4 include curable resins such as ultraviolet (UV) curable resins and thermosetting resins; and thermoplastic resins; preferably curable resins, and more preferably UV curable resins. Specific examples of curable resins include phenolic resins and epoxy resins.

[0025] In one embodiment, the second bonding layer 4 is made of an organic material. When the second bonding layer 4 is made of an organic material, the second bonding layer 4 can relieve stress caused by the difference in thermal expansion coefficient between the second low refractive index layer 6 and the second support substrate 7 when the temperature of the waveguide element 100 changes. As a result, the characteristics of the waveguide element 100 can be sufficiently maintained even when the temperature changes. When the organic material constituting the second bonding layer 4 is a curable resin, a curable adhesive containing a monomer component corresponding to the curable resin is applied to the surface of the second low refractive index layer 6 opposite the thick substrate 10 by any appropriate application method (typically spin coating). As a result, a coating film made of the curable adhesive is formed on the surface of the second low refractive index layer 6. In the illustrated example, the coating film is formed over the entire surface of the second low refractive index layer 6 opposite to the thick substrate 10. Thereafter, the coating film is heated and / or irradiated with UV light while being in contact with the second support substrate 7, thereby curing the curable adhesive, thereby forming the second bonding layer 4 made of the curable resin, which bonds the second low refractive index layer 6 and the second support substrate 7 together. The thickness of the second bonding layer 4 is, for example, 0.001 μm to 10 μm, and preferably 0.3 μm to 3.0 μm.

[0026] The second support substrate 7 is made of any suitable inorganic material. Examples of inorganic materials that can be used for the second support substrate 7 include lithium niobate (LiNbO3:LN), lithium tantalate (LiTaO3:LT), silicon (Si), glass, sialon (Si3N4-Al2O3), mullite (3Al2O3·2SiO2, 2Al2O3·SiO2), aluminum nitride (AlN), silicon nitride (Si3N4), magnesium oxide (MgO), sapphire, quartz, crystal, gallium nitride (GaN), silicon carbide (SiC), and gallium oxide (Ga2O3). The inorganic materials can be used alone or in combination. Of the inorganic materials that can be used for the second support substrate 7, LN is preferred. The second support substrate 7 has any appropriate thickness. The thickness of the second support substrate 7 is, for example, 300 μm to 1000 μm.

[0027] B-1-5.Second polishing process Next, the thick substrate 10 is thinned by polishing it from the side opposite to the second low refractive index layer 6 by any appropriate polishing method. More specifically, the thick substrate 10 having a thickness of 200 μm or more is thinned to a nonlinear optical material substrate 1 having a thickness of 10 μm or less (see FIG. 1). An example of the polishing method is chemical mechanical polishing (CMP polishing). This prepares a nonlinear optical material substrate 1 having a thickness of 10 μm or less, as shown in Fig. 1. In the illustrated example, a laminate including, in this order, the nonlinear optical material substrate 1 having a thickness of 10 μm or less, a second low refractive index layer 6, a second bonding layer 4, and a second support substrate 7 is prepared.

[0028] B-2. Processing process As shown in FIG. 2, in the processing step, a plurality of grooves 13 are formed on the first surface 1a of the nonlinear optical material substrate 1 by any appropriate processing method. Examples of the processing method include laser processing and reactive ion etching (RIE), and preferably laser processing.

[0029] In one embodiment, each of the plurality of grooves 13 extends in a first surface direction perpendicular to the thickness direction of the nonlinear optical material substrate 1. The grooves 13 have any appropriate shape in a cross section cut in a direction perpendicular to the extending direction of the grooves 13. The cross-sectional shape of the grooves 13 is typically a substantially U-shape that opens to the first surface 1a side, or a substantially L-shape that opens to the first surface 1a side and one side in the width direction. Groove 13 having a generally U-shaped cross section has bottom surface 13a, first side surface 13b, and second side surface 13c. Groove 13 having a generally L-shaped cross section has bottom surface 13a, and first side surface 13b or second side surface 13c. In one embodiment, the bottom surface 13a is substantially parallel to the first surface 1a of the nonlinear optical material substrate 1. Each of the first side surface 13b and the second side surface 13c may be substantially parallel to the thickness direction of the nonlinear optical material substrate 1, or may intersect with the thickness direction of the nonlinear optical material substrate 1. In the illustrated example, each of the first side surface 13b and the second side surface 13c intersects with the thickness direction of the nonlinear optical material substrate 1.

[0030] The dimension (depth D) of the groove 13 in the thickness direction of the nonlinear optical material substrate 1 is, for example, 30% to 100%, and preferably 40% to 60%, when the thickness of the nonlinear optical material substrate 1 is taken as 100%. The depth D of the groove 13 is, for example, 0.5 μm to 5.0 μm, and preferably 1.5 μm to 3.0 μm. When the depth D of the groove 13 is in this range, a ridge-type waveguide 11 having a sufficient height can be formed.

[0031] The plurality of grooves 13 are arranged in parallel at intervals in a second surface direction perpendicular to the first surface direction of the nonlinear optical material substrate 1. The plurality of grooves 13 are substantially parallel to each other. All of the plurality of grooves 13 may have a generally U-shaped cross section, or some of the plurality of grooves 13 may have a generally L-shaped cross section and the remaining may have a generally U-shaped cross section. Groove 13 having a generally L-shaped cross section is typically located at an end of the plurality of grooves 13 in the second surface direction. There is no particular limitation on the number of the plurality of grooves 13. The number of the plurality of grooves 13 is, for example, 2 to 54.

[0032] In the nonlinear optical material substrate 1, the portions located between adjacent grooves 13 among the plurality of grooves 13 are configured as ridge waveguides 11. Therefore, the number of ridge waveguides 11 formed in the processing step is typically the number of grooves 13 minus one. The ridge waveguide 11 extends in the same direction as the grooves 13. One end face in the extending direction of the ridge waveguide 11 is configured as an input end face where a light wave is incident (see FIG. 8). The other end face in the extending direction of the ridge waveguide 11 is configured as an output end face where a light wave is emitted (see FIG. 8).

[0033] The ridge waveguide 11 has any appropriate shape in a cross section cut in a direction perpendicular to the extension direction of the ridge waveguide 11. Examples of the cross-sectional shape of the ridge waveguide 11 include quadrilaterals such as a square, a rectangle, a trapezoid, and a parallelogram. In the illustrated example, the cross-sectional shape of the ridge waveguide 11 is a trapezoid that tapers away from the second surface 1b.

[0034] The dimension (height) of the ridge waveguide 11 in the thickness direction of the nonlinear optical material substrate 1 is typically substantially the same as the depth D of the grooves 13. The width of the ridge waveguide 11 is substantially the same as the distance between adjacent grooves 13, and is, for example, 1 μm to 10 μm.

[0035] When the manufacturing method of the waveguide element includes the periodic polarization inversion forming step described above, in the processing step, a plurality of grooves 13 are formed so that at least one of the ridge-type waveguides 11 includes a periodic polarization inversion portion 12 (see FIG. 8).

[0036] B-3. ​​First low refractive index layer forming process Next, as shown in FIG. 3, a first low refractive index layer 5 is formed on the first surface 1a of the nonlinear optical material substrate 1 on which the plurality of grooves 13 have been formed. More specifically, the material of the first low refractive index layer 5 is deposited on the first surface 1a of the nonlinear optical material substrate 1 by any appropriate deposition method. Examples of the film formation method in the first low refractive index layer formation step include sputtering, vacuum deposition, ion plating, chemical vapor deposition (CVD), and atomic layer deposition (ALD), and preferably sputtering. Examples of materials for the first low refractive index layer 5 include the same low refractive index materials as those for the second low refractive index layer 6, and preferably silicon dioxide (SiO2). When the first low refractive index layer 5 contains silicon dioxide, the refractive index of the first low refractive index layer 5 can be made sufficiently small, and excellent thermal stability can be imparted to the first low refractive index layer 5.

[0037] In the illustrated example, a first low-refractive index layer 5 is formed over the entire first surface 1a of the nonlinear optical material substrate 1. The first low-refractive index layer 5 is formed so as to follow the grooves 13 and ridge-type waveguide 11 provided in the nonlinear optical material substrate 1. Therefore, each of the multiple grooves 13 is filled with the first low-refractive index layer 5. As a result, unevenness corresponding to the grooves 13 and ridge-type waveguide 11 is formed on the surface of the first low-refractive index layer 5 opposite to the nonlinear optical material substrate 1. The thickness T of the first low-refractive index layer 5 before the first polishing step is greater than the depth D of the grooves 13. The thickness T is, for example, more than 1 time, and preferably 2 times or more, the depth D of the grooves 13. If the ratio of the thickness T of the first low-refractive index layer 5 to the depth D of the grooves 13 is equal to or greater than this lower limit, even if the surface of the first low-refractive index layer 5 opposite to the nonlinear optical material substrate 1 is polished until it becomes substantially flat in the first polishing step, it is possible to prevent the ridge-type waveguide 11 from being exposed from the first low-refractive index layer 5. On the other hand, the thickness T of the first low refractive index layer 5 before the first polishing step is, for example, 10 times or less, or, for example, 2 times or less, the depth D of the groove portion 13. The thickness T of the first low refractive index layer 5 before the first polishing step is, for example, 1.5 μm to 6.0 μm. The refractive index of the first low refractive index layer 5 at 473 Hz is, for example, 1.40 to 1.60, and preferably 1.45 to 1.49.

[0038] B-4.First polishing process 4, the first low refractive index layer 5 is polished from the side opposite to the nonlinear optical material substrate 1. More specifically, the first low refractive index layer 5 is polished by any appropriate polishing method so as to remove irregularities from the surface of the first low refractive index layer 5 opposite to the nonlinear optical material substrate 1. The polishing method may be, for example, chemical mechanical polishing (CMP polishing). The amount of polishing in the first polishing step is typically equal to or greater than the depth D of the groove 13.

[0039] As a result, the surface of the first low refractive index layer 5 opposite to the nonlinear optical material substrate 1 becomes a flat surface 5a. The flat surface 5a of the first low refractive index layer 5 typically extends along a direction perpendicular to the thickness direction of the nonlinear optical material substrate 1. The flatness of the flat surface 5a of the first low refractive index layer 5 is, for example, 0.0001 μm to 0.1 μm, and preferably 0.0001 μm to 0.01 μm.

[0040] The distance between the flat surface 5a of the first low-refractive-index layer 5 and the ridge-type waveguide 11 is, for example, 1.0 or less, preferably 0.7 or less, and more preferably 0.5 or less, relative to the depth D of the groove 13. On the other hand, the distance between the flat surface 5a of the first low-refractive-index layer 5 and the ridge-type waveguide 11 is, for example, 0.1 or more, or for example, 0.2 or more, relative to the depth D of the groove 13. When the dimension between the flat surface 5a and the ridge waveguide 11 relative to the depth D of the groove portion 13 is within this range, the first low refractive index layer 5 can stably cover the ridge waveguide 11, and the waveguide element 100 can be made thinner. The distance between the flat surface 5a of the first low refractive index layer 5 and the ridge-type waveguide 11 is, for example, 0.1 μm to 3.0 μm, and preferably 0.5 μm to 1.5 μm.

[0041] B-5.First joining process Next, as shown in FIG. 5, the first support substrate 2 is bonded to the first low refractive index layer 5 via the first bonding layer 3. The first bonding layer 3 may be made of an inorganic material or an organic material. Examples of inorganic materials constituting the first bonding layer 3 include the same inorganic materials as those of the second bonding layer 4 described above. Examples of the organic material constituting the first bonding layer 3 include the same organic material as that of the second bonding layer 4 described above.

[0042] In one embodiment, the first bonding layer 3 is made of an organic material. When the first bonding layer 3 is made of an organic material, when the temperature of the waveguide element 100 changes, the first bonding layer 3 can relieve stress caused by the difference in thermal expansion coefficient between the first low refractive index layer 5 and the first support substrate 2. As a result, the characteristics of the waveguide element 100 can be sufficiently maintained even when the temperature changes. When the organic material constituting the first bonding layer 3 is a curable resin, a curable adhesive containing a monomer component corresponding to the curable resin is applied to the flat surface 5a of the first low refractive index layer 5 by any appropriate application method (typically, spin coating). As a result, a coating film made of the curable adhesive is formed on the flat surface 5a of the first low refractive index layer 5. In the illustrated example, the coating film is formed on the entire flat surface 5a of the first low refractive index layer 5. Thereafter, the coating film is heated and / or irradiated with UV light to cure the curable adhesive while the first support substrate 2 is in contact with the coating film, thereby forming a first bonding layer 3 made of a curable resin, which bonds the first low refractive index layer 5 and the first support substrate 2 together. The thickness of the first bonding layer 3 is, for example, 0.001 μm to 10 μm, and preferably 0.3 μm to 3.0 μm.

[0043] The first support substrate 2 is made of any appropriate inorganic material. Examples of inorganic materials that can be used to form the first support substrate 2 include the same inorganic materials as those used to form the second support substrate 7. Of the inorganic materials that can be used to form the first support substrate 2, LN is preferred. The first support substrate 2 has any appropriate thickness. The range of the thickness of the first support substrate 2 is, for example, the same as the range of the thickness of the second support substrate 7 described above.

[0044] This completes the manufacturing process of the waveguide element 100. The thickness of the waveguide element 100 is, for example, 300 μm to 2000 μm, and preferably 500 μm to 1000 μm. In one embodiment, the waveguide element 100 comprises, in this order, a first support substrate 2, a first bonding layer 3, a first low refractive index layer 5, a nonlinear optical material substrate 1, a second low refractive index layer 6, a second bonding layer 4, and a second support substrate 7. As described above, the nonlinear optical material substrate 1 is provided with the ridge waveguide 11. The ridge waveguide 11 is surrounded by the first low refractive index layer 5 and the second low refractive index layer 6. Therefore, the ridge waveguide 11 functions as a core, and the first low refractive index layer 5 and the second low refractive index layer 6 function as cladding. Light waves with wavelengths of 0.5 μm to 1.6 μm are typically incident on the ridge waveguide 11. At this time, the first low refractive index layer 5 and the second low refractive index layer 6 can prevent the light waves from leaking out of the ridge waveguide 11. Therefore, the ridge waveguide 11 can propagate the light waves with low loss.

[0045] 8, when the ridge waveguide 11 includes periodic polarization inversions 12, the waveguide element 100 can function as a wavelength conversion element 101. The wavelength conversion element 101 can convert the wavelength of the light wave passing through the ridge waveguide 11 by quasi-phase matching (QPM) of the periodic polarization inversions 12. Wavelength conversion includes, for example, parametric down-conversion (PDC), optical parametric amplification (OPA), second harmonic generation (SHG) or sum frequency generation (SFG). [Example]

[0046] The present invention will be specifically explained below with reference to examples, but the present invention is not limited to these examples. Example 1 (1) Preparation process Magnesium oxide-doped lithium niobate (MgO:LN) substrates were prepared as nonlinear optical crystal substrates (thick substrates). The MgO:LN substrates had an orientation flat (OF) and were oriented at a 5° YZ orientation. The diameter of the MgO:LN substrates was 4 inches, and the thickness of the MgO:LN substrates was 300 μm.

[0047] A comb-shaped electrode pattern was formed on one surface (second surface) in the thickness direction of the MgO:LN substrate, and a voltage was applied in the c-axis direction. This resulted in the formation of periodic polarization inversions on the MgO:LN substrate. The periodic polarization inversions consisted of first polarization inversions and second polarization inversions, which had polarizations in opposite directions, arranged alternately in the first surface direction, which was perpendicular to the thickness direction of the MgO:LN substrate. Three periodic polarization inversions were arranged in parallel, spaced apart from each other, in the second surface direction, which was perpendicular to the first surface direction. After polarization inversion, the comb-shaped electrode was removed by etching.

[0048] Next, SiO2 was deposited by sputtering on the surface of the MgO:LN substrate where the polarization inversion had been formed, to form an SiO2 film (thickness: 0.4 μm) as a second low refractive index layer.

[0049] Next, a curable adhesive was uniformly applied by spin coating to the surface of the SiO2 film opposite the MgO:LN substrate to form a coating film with a thickness of 0.4 μm. The curable adhesive was then cured while a lithium niobate (LN) substrate (thickness: 500 μm) serving as a second support substrate was in contact with the coating film. This formed a cured adhesive layer as a second bonding layer, and the LN substrate was bonded to the SiO2 film (second low refractive index layer) via the cured adhesive layer.

[0050] Next, the MgO:LN substrate was ground and polished from the side opposite to the SiO2 film until the thickness of the MgO:LN substrate became 3.7 μm.

[0051] (2) Processing process Next, four grooves were formed by laser processing on the surface (first surface) of the MgO:LN substrate opposite the SiO2 film. Each of the four grooves had a depth of 2.0 μm. Each of the four grooves extended linearly in the first surface direction, which was perpendicular to the thickness direction of the MgO:LN substrate. The four grooves were arranged in parallel at intervals in the second surface direction, which was perpendicular to the first surface direction. In the MgO:LN substrate, the portions between adjacent grooves among the four grooves were configured as ridge-type waveguides. In other words, three ridge-type waveguides were formed in the MgO:LN substrate. Each of the three ridge-type waveguides included periodic polarization inversion sections.

[0052] (3) First low refractive index layer forming step Next, SiO2 was sputtered onto the surface (first surface) of the MgO:LN substrate opposite the SiO2 film so as to cover the ridge-shaped waveguide, forming an SiO2 film (thickness 4.0 μm) as a first low refractive index layer.

[0053] (4) First polishing process Next, the SiO2 film (first low refractive index layer) was polished from the side opposite the MgO:LN substrate by CMP (polishing method) until the distance between the surface of the SiO2 film (first low refractive index layer) opposite the MgO:LN substrate and the ridge-shaped waveguide was 0.5 μm. This made the surface of the SiO2 film opposite the MgO:LN substrate flat in the direction perpendicular to the thickness direction of the MgO:LN substrate.

[0054] (5) First joining process Next, a cardo resin (curable adhesive) was uniformly applied to the flat surface of the SiO2 film by spin coating to form a coating film with a thickness of 1.5 μm. After that, a lithium niobate (LN) substrate (thickness: 500 μm) serving as a first support substrate was brought into contact with the coating film, and the cardo resin (curable adhesive) was cured. This formed a cured adhesive layer as a first bonding layer, and the LN substrate was bonded to the SiO2 film (first low refractive index layer) via the cured adhesive layer.

[0055] A wavelength conversion element was manufactured in this way. The wavelength conversion element had a layered structure of an LN substrate (first support substrate) / cured adhesive layer (first bonding layer) / SiO2 film (first low refractive index layer) / MgO:LN substrate (nonlinear optical crystal substrate) / SiO2 film (second low refractive index layer) / cured adhesive layer (second bonding layer) / LN substrate (second support substrate).

[0056] <Comparative Example 1> A wavelength conversion element was manufactured in the same manner as in Example 1, except that the thickness of the SiO2 film as the first low refractive index layer formed in the above-mentioned (3) first low refractive index layer forming process was changed to 0.5 μm, and the above-mentioned (4) first polishing process was not performed.

[0057] <Evaluation> The wavelength conversion elements manufactured in the examples and comparative examples were observed with an optical microscope to confirm the presence of air bubbles. In the wavelength conversion element of comparative example 1, air bubbles were confirmed between the SiO2 film (first low refractive index layer) and the adhesive cured layer (first bonding layer). On the other hand, no air bubbles were confirmed in the laminated structure of the wavelength conversion element of example 1. Therefore, it can be seen that the wavelength conversion element of example 1 is superior in appearance and strength to the wavelength conversion element of comparative example 1. [Industrial Applicability]

[0058] The waveguide elements according to the embodiments of the present invention can be used in a wide range of fields such as next-generation high-speed communications and quantum fields, and can be particularly suitably used as wavelength conversion elements, optical amplifiers, and optical modulators. [Explanation of symbols]

[0059] 1. Nonlinear optical crystal substrate 1a 1st page 1b 2nd side 11 Ridge waveguide 12 Periodic polarization inversion section 13 Groove 2 First support board 3 First bonding layer 4 Second bonding layer 5. First low refractive index layer 5a flat surface 6 Second low refractive index layer 7 Second support board

Claims

1. providing a nonlinear optical material substrate; forming a plurality of grooves at intervals from one another on a first surface located on one side in a thickness direction of the nonlinear optical material substrate, the grooves extending in a direction intersecting the thickness direction, and configuring at least one of the portions of the nonlinear optical material substrate located between adjacent grooves as a ridge-type waveguide; forming a first low refractive index layer having a refractive index lower than that of the nonlinear optical material substrate, the first low refractive index layer having a thickness exceeding the depth of the groove portion, on the first surface of the nonlinear optical material substrate so as to cover the ridge-type waveguide; polishing the first low refractive index layer from the side opposite to the nonlinear optical material substrate to make the surface of the first low refractive index layer opposite to the nonlinear optical material substrate substantially flat; and bonding a first support substrate to the flat surface of the first low refractive index layer via a first bonding layer.

2. The step of preparing a nonlinear optical material substrate includes: forming a second low refractive index layer having a refractive index lower than that of the nonlinear optical material substrate on a second surface of the nonlinear optical material substrate opposite to the first surface; a step of bonding a second support substrate to a surface of the second low refractive index layer opposite to the nonlinear optical material substrate via a second bonding layer; 2. The method for manufacturing a waveguide element according to claim 1, further comprising the step of polishing the nonlinear optical material substrate from the side opposite to the second low refractive index layer.

3. 3. The method for manufacturing a waveguide element according to claim 2, wherein the step of preparing the nonlinear optical material substrate further comprises the step of forming periodic polarization inversion portions on the second surface of the nonlinear optical material substrate before the step of forming the second low refractive index layer.

4. The method for manufacturing a waveguide element according to claim 1 , wherein the first bonding layer is made of a resin material.

5. The method for manufacturing a waveguide element according to claim 2 , wherein the second bonding layer is made of a resin material.

6. 6. The method for manufacturing a waveguide element according to claim 1, wherein in the step of polishing the first low-refractive index layer, the first low-refractive index layer is polished until a dimension between a flat surface of the first low-refractive index layer and the ridge-type waveguide in the thickness direction becomes 1.0 or less with respect to a depth of the groove.

7. The method for manufacturing a waveguide element according to claim 6, wherein the depth of the groove is 1.5 μm or more and 3.0 μm or less.

8. 6. The method for manufacturing a waveguide element according to claim 1, wherein the nonlinear optical material substrate is made of lithium niobate and / or lithium tantalate containing magnesium oxide as a dopant.

9. The method for manufacturing a waveguide element according to claim 1 , wherein the first low refractive index layer contains silicon dioxide.

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