METHOD FOR PRODUCING AN OPTICAL STRUCTURE

Abstract

Method for fabricating an optical waveguide structure (10, 11), the method comprising: Obtaining a component comprising an optical waveguide (2) with an end face, a reflective film (5) provided on the optical waveguide (2) and reflecting light propagating in the optical waveguide (2), and a metal film (6) provided on the reflective film (5); Subjecting the metal film (6) of the component to a surface oxidation treatment to produce a surface-oxidized (8) film on the metal film (6); and Subsequent polishing of the end surface of the optical waveguide (2) to form a polished end surface.

Description

Technical field

[0001] The present invention relates to a method for manufacturing an optical waveguide structure. STATE OF THE ART

[0002] Recently, intensive research has been conducted on vehicle headlights that utilize a laser light source, one example being a white light source created by combining a blue or ultraviolet laser with a phosphor. The luminous intensity of the excitation light can be increased by focusing the laser beams, and furthermore, the luminous intensity of the excitation light can also be increased by focusing multiple laser beams so that they overlap on the phosphor. As a result, the luminous flux and brightness can be increased simultaneously without altering the light-emitting surface. Therefore, a white light source combining a semiconductor laser and a phosphor has gained attention as a potential replacement for LEDs.For example, the fluorescent glass “Lumiphous™” from Nippon Electric Glass and YAG single-crystal phosphors from the National Institute for Materials Science, Tamura Corporation and Koha Co., Ltd. are proposed as fluorescent glass for use in a vehicle headlight.

[0003] According to patent document 1, a non-reflective film for excitation light and a total internal reflection film for fluorescence are used on an incident surface of an optical waveguide in a flat plate form. The fluorescence, which has been oscillated in the optical waveguide and returned to the incident surface, can be reflected by the total internal reflection film on the incident surface and can be emitted from an emission surface.

[0004] Furthermore, reflective films are formed on the side surfaces of a transparent phosphor of the flat plate type, thereby preventing the emission of fluorescence from the side surfaces of the phosphor of the flat plate type according to patent document 2.

[0005] US 2017 / 0276869 A1 describes a metallized double-clad optical fiber. US 4,418,984 A describes a multi-coated metal-clad optical waveguide. Parsons et al., NASA Technical Memorandum 89822, May 1987, pp. 1-16, describes the effect of an oxygen plasma on uncoated thin aluminum reflective films. US 2016 / 0238784 A1 describes fiber optic illumination systems. JP 2003-35843 A describes a method for fabricating a metal-clad optical fiber, a cladding metal-clad optical fiber, and an optical semiconductor module. US 7,063,779 B2 describes a method for fabricating a metal-clad optical fiber. WO 2017 / 006797 A1 describes an optical element and a light-emitting device. DE 20 2010 017 188 U1 describes an optical waveguide and a semi-finished product for the manufacture of an optical waveguide with bend-optimized properties. DOCUMENT LISTPATENT DOCUMENTS PATENT DOCUMENT 1: WO 2014-203488 A1 PATENT DOCUMENT 2: JP 2014-116081 A SUMMARY OF THE INVENTION (Problem to be solved by the invention)

[0006] However, there are limitations regarding the luminous intensities of the fluorescence and the excitation light in an element, which cause the excitation light to spread through the phosphor in a flat plate shape, thereby producing fluorescence.

[0007] Consequently, the inventors attempted to construct a finned optical waveguide with a phosphor and to fabricate a fluorescence-emitting device. However, in the current fabrication of a phosphor device, it has been observed that fluorescence emanates from one side face of the finned optical waveguide, making it impossible to obtain sufficient fluorescence directed in a desired direction. That is, when excitation light is shone onto phosphor particles in the optical waveguide and fluorescence is emitted by the phosphor particles, the fluorescence is emitted in all directions. As a result, a large portion of the fluorescence propagates in the direction of the side face and the incident face of the optical waveguide, so that a sufficiently high fluorescence output in the desired direction is not achieved.

[0008] Consequently, the inventors conducted investigations to provide a reflective film for covering the side surface of the optical waveguide. However, during the practical fabrication of a phosphor device, it became apparent that the reflective film partially detaches or disappears, and fluorescence emanates from the detached or missing portion of the reflective film, which is problematic.

[0009] An object of the present invention is to provide a method for producing an optical waveguide structure, wherein in the optical waveguide structure with a reflective film on the optical waveguide the local detachment or disappearance of the reflective film can be suppressed and the resulting escape of propagating light to the outside of the optical waveguide can be prevented. (Means of solving the problem)

[0010] This section describes a general optical waveguide structure, including: an optical waveguide with a polished end face; a reflective film that is provided on the optical waveguide and reflects light that propagates in the optical waveguide; a metallic film provided on the reflective film; and a surface-oxidized film that is provided on the metal film and has been formed by surface oxidation of the metal film.

[0011] This generally described optical waveguide structure can be manufactured using the inventive method described later.

[0012] Furthermore, a fluorescence generation device is generally described, which comprises the generally described optical waveguide structure, wherein the optical waveguide comprises a phosphor that transmits excitation light in such a way that fluorescence is generated.

[0013] The present invention provides a method for manufacturing an optical waveguide structure, wherein the method comprises: Obtaining a component comprising an optical waveguide, a reflective film provided on the optical waveguide and reflecting light propagating in the optical waveguide, and a metal film provided on the reflective film; Subjecting the metal film of the component to a surface oxidation treatment to produce a surface-oxidized film on the metal film; and Subsequent polishing of the end surface of the optical waveguide to form a polished end surface. (Effects of the invention)

[0014] The inventors investigated the cause of a local disappearance of the reflective film provided on the optical waveguide and arrived at the following findings.

[0015] This means that manufacturing a chip for an optical fiber device with an integrated optical fiber requires the use of ultrapure water and maintaining a superpure state through separation and purification steps. However, the problem arises that the reflective film on the optical fiber corrodes upon contact with the ultrapure water and partially disappears, rendering it ineffective as a reflective film.

[0016] The inventors then attempted to oxidize the reflective film on the waveguide to prevent its disappearance due to corrosion. This reduces the reflection of light propagating through the optical waveguide. Furthermore, they tried applying an oxide film, composed of aluminum oxide or silicon dioxide, as a protective layer on the reflective film to prevent corrosion. This effectively prevents corrosion of the reflective film during the cleaning process. However, it was shown that the reflective film near the end face was detached due to stress from a machining blade during the polishing of the device's end face, rendering its protective function ineffective.

[0017] Furthermore, the inventors attempted to form a metal film on the reflective film as a protective layer. In this case, it was shown that the protective film, which is composed of oxide, does not detach and that the metal film partially disappears due to corrosion.

[0018] Based on these findings, the inventors attempted to develop a process involving the formation of a metal film on the reflective film, the oxidation of a surface of the metal film by an oxidation process such as plasma ashing, and the formation of a surface-oxidized film composed of an oxide of the metal. It was subsequently found that the disappearance of the reflective film and the metal film formed on it due to corrosion could be prevented, and at the same time, the detachment of the metal film and the reflective film during the polishing step of the end face of the optical waveguide could be suppressed, thereby preventing the leakage of light propagating in the optical waveguide. The present invention was thus made. BRIEF DESCRIPTION OF THE DRAWINGS Fig.1(a) is a cross-sectional view showing the state in which a plating layer 4 and a reflective film 5 are formed on a surface of an optical waveguide 2, and Fig. 1(b) is a cross-sectional view showing the state in which a metal film 6 is formed on the reflective film 5. Fig. Figure 2 is a perspective view showing the state in which the plating layer 4 and the reflection film 5 are formed on the surface of the optical waveguide 2. Fig. Figure 3 is a cross-sectional view showing the state in which the metal film is subjected to surface oxidation, resulting in a surface-oxidized film. Fig. Figure 4 is a perspective view showing the state in which the metal film is subjected to surface oxidation, so that the surface-oxidized film is provided. Fig.Figure 5 is a photograph showing the surroundings of an upper surface of an optical waveguide in an optical waveguide structure according to a comparative example. Fig. Figure 6 is a photograph showing the surroundings of a side surface of the optical waveguide in the optical waveguide structure of the comparison example. Fig. Figure 7 is a photograph showing the surroundings of an upper surface of an optical waveguide structure of the example produced according to the invention. Fig. Figure 8 is a photograph showing the surroundings of a side surface of the optical waveguide in the optical waveguide of the example that was produced according to the invention. MODES FOR EXECUTING THE INVENTION

[0019] The present invention is described in more detail below with appropriate reference to the drawings.

[0020] The Fig.References 1 to 4 relate to an optical fiber device which can be manufactured according to an embodiment of the present invention.

[0021] As it is in the Fig. 1(a) and Fig. As shown in Figure 2, one or more protrusions 1a are provided on a main surface 1d of a support substrate 1. A ribbed optical waveguide 2 is formed on an upper surface 1b of the protrusion 1a. According to the present example, the ribbed optical waveguide 2 is connected to and fixed to the protrusion 1a. A groove can be formed between a plurality of the adjacent protrusions.

[0022] According to the present example, the ribbed optical waveguide 2 and the support substrate 1 are as a whole covered with a plating layer 4 and a reflective film 5. That is, a reflective film 3 is formed on an upper surface 1b of the protrusion 1a. Furthermore, the plating layer 4 comprises a main-surface covering portion 4d, which covers a main surface 1d of the support substrate 1, a side-surface covering portion 4c, which covers a side surface 1c of the protrusion 1a and a side surface 2c of the optical waveguide 2, and an upper-surface covering portion 4a, which covers an upper surface 2a of the optical waveguide 2. The reflective film 5 also comprises a main-surface covering portion 5d, a side-surface covering portion 5c, and an upper-surface covering portion 5a.According to the present example, the plating layer 4 comprises a lower surface-covering part 4b between a reflective film 3 and the lower surface 2b of the optical waveguide 2.

[0023] Then, as it says in the Fig. As shown in Figure 1(b), a metal film 6 is formed on the reflective film, forming a component 11. According to the present example, the metal film 6 comprises a main surface-covering part 6d, a side surface-covering part 6c, and a top surface-covering part 6a.

[0024] Then, as it says in the Fig. 3 and Fig.As shown in Figure 4, a surface of the metal film 6 is oxidized, forming a surface-oxidized film 8, which consists of an oxide of a metal that forms the metal film 6. An intermediate metal film 7 remains between the surface-oxidized film 8 and the reflection film 5. As a result, in the optical waveguide structure 10 thus obtained, the intermediate metal film 7 comprises a main-surface covering part 7d, a side-surface covering part 7c, and a top-surface covering part 7a, and the surface-oxidized film 8 comprises a main-surface covering part 8d, a side-surface covering part 8c, and a top-surface covering part 8a. Furthermore, according to the Fig. 3 and Fig.4 the plating layer 4, the reflective film 5, the intermediate metal film 7 and the surface-oxidized film 8 the main area 1d of the support substrate 1. Since light does not propagate along the main area 1d of the support substrate 1, the plating layer 4, the reflective film 5, the intermediate metal film 7 and the surface-oxidized film 8 can be omitted on the main area 1d.

[0025] According to such an optical waveguide structure, the disappearance of the reflective film and the metal film on it due to corrosion can be prevented by the effect of the surface oxidized film 8. At the same time, the detachment of the metal film and the reflective film during polishing of the end surface of the optical waveguide can be suppressed, thus preventing the escape of light propagating in the optical waveguide.

[0026] The components of the optical waveguide structure, which can be manufactured according to the inventive method, are described in more detail below.

[0027] The specific material of the support substrate is not particularly limited and includes lithium niobate, lithium tantalate, a glass such as fused silica, and a quartz crystal. Furthermore, to prevent heat conduction from a light source to the optical fiber or to suppress heating of the optical fiber itself due to wavelength conversion or external heat, a support substrate with excellent heat dissipation properties can be used. In this case, aluminum oxide, aluminum nitride, silicon carbide, silicon nitride, tungsten, copper-tungsten, magnesium oxide, and the like can be mentioned.

[0028] A layer to prevent detachment, preferably an oxide film, can be formed between the substrate and the plating layer or the reflective layer to prevent the optical waveguide from detaching. Although the material of the oxide film is not specifically limited, it can preferably be aluminum oxide, tantalum oxide, or titanium oxide. Furthermore, its thermal conductivity can preferably be higher than that of a phosphor, with aluminum oxide being the most preferred in this respect.

[0029] The material of the plating layer can be a material with a lower refractive index than that of the optical waveguide material, and the plating layer can also serve as an adhesive layer. The plating layer material is preferably SiO2, Al2O3, MgF2, CaF2, or MgO. Furthermore, given that the heat generated in the phosphor substrate is radiated through the substrate, the plating layer preferably has a higher thermal conductivity than the phosphor, and Al2O3 and MgO are particularly preferred as such materials.

[0030] The material of the reflective film can be a metal film, such as gold, aluminum, copper, silver, or the like; an alloy containing the metal component; or a dielectric multilayer film. Preferably, the reflective film is composed of one or more metals selected from the group consisting of silver, gold, platinum, aluminum, and alloys thereof. When the metal film is used as the reflective film, a metal layer of chromium, nickel, titanium, or the like can be formed as a buffer layer for the metal film to prevent delamination of the plating layer.

[0031] A bonding layer can be provided between the plating layer and the reflective film. The material of such a bonding layer is not specifically limited, but is preferably aluminum oxide, tantalum oxide, or titanium oxide. However, the bonding layer preferably has a higher thermal conductivity than the phosphor, and aluminum oxide is most preferred in this respect.

[0032] Such a bonding layer can be present between the reflective film and the substrate. In this case, it can be formed by creating a bottom-surface cladding layer, a bottom-surface reflective film and a bonding layer on the optical fiber side, creating a bonding layer on the substrate side, and directly bonding both bonding layers. A layer to prevent delamination can be present between the reflective film and the bonding layer, and between the substrate and the bonding layer.

[0033] The metal film formed on the reflective film may preferably be composed of one or more metal(s) selected from the group consisting of titanium, chromium, tantalum, nickel and alloys thereof.

[0034] The process for film formation of the plating layer, the reflective film and the metal film includes, but is not limited to, a sputtering process, a vapor deposition process, a plating process and a CVD process.

[0035] The metal film is then subjected to a surface oxidation treatment to form the surface-oxidized film and to leave the intermediate metal film between the surface-oxidized film and the reflective film. The thickness of the intermediate metal film can preferably be 1 µm or greater to prevent corrosion of the reflective film, and preferably 5 µm or less, and more preferably 3 µm or less, to reduce costs and film formation time.

[0036] Furthermore, the surface oxidation treatment comprises plasma ashing and tempering at a high temperature, and preferably involves plasma ashing. Plasma ashing is a reaction involving the introduction of oxygen into a vacuum, the subsequent generation of an oxygen plasma using a high-frequency electrical source, and the reaction of an object with oxygen radicals. Plasma ashing has been commonly used in the step of removing photoresist or the like. The electrical power supplied during plasma ashing is preferably 200 to 400 W, and the pressure of the oxygen gas is preferably 100 to 200 Pa. The duration of plasma ashing is preferably 30 to 90 minutes.

[0037] The material forming the surface-oxidized film is an oxide of the metal forming the intermediate metal film, which acts as an underlayer, and is a mixed oxide when the intermediate metal film material is an alloy. The thickness of the surface-oxidized film can be preferably 7 nm or greater, and more preferably 8 nm or greater, to protect the intermediate metal film and the reflective film from corrosion. However, in practice, the thickness of the surface-oxidized film is often 10 nm or less.

[0038] The optical waveguide device can be used as a passive optical component. Alternatively, the optical waveguide device can be used as a wavelength conversion device or as a waveguide-type fluorescence generation device.

[0039] The optical waveguide device can be a non-grid type device that does not include a grating (diffraction grating) in the optical waveguide, or it can be a grating device.

[0040] The material that forms the optical waveguide can be a chemically stable material with a high refractive index, and includes lithium niobate, lithium tantalate, a solid solution of lithium niobate-lithium tantalate, tantalum pentoxide, and silicon nitride.

[0041] In the case of a phosphor-type waveguide device, the optical waveguide can be composed of a phosphor. The phosphor can be a fluorescent glass, a single crystal, or a polycrystal. The fluorescent glass is formed by distributing rare-earth ions in a glass base.

[0042] Examples of glass used as a base include oxide glass containing silicon oxide, boron oxide, calcium oxide, lanthanum oxide, barium oxide, zinc oxide, phosphorus oxide, aluminum fluoride, magnesium fluoride, calcium fluoride, strontium fluoride or barium chloride, and YAG (yttrium aluminum garnet).

[0043] Although the rare earth ions distributed in the glass are preferably Tb, Eu, Ce and Nd, the rare earth ions may be La, Pr, Sc, Sm, Er, Tm, Dy, Gd or Lu.

[0044] Y3Al5O is used as a phosphor single crystal. 12 , Ba5Si 11 Al7N 25 and Tb3Al5O 12Preferably, the dopant material used in the phosphor consists of rare earth ions, such as Tb, Eu, Ce, and Nd. To suppress thermal decomposition, the phosphor is preferably a single crystal; however, a polycrystal can also reduce heat resistance at grain boundaries and increase transmittance if the polycrystal is a high-density material, and can therefore act as an optical waveguide.

[0045] A semiconductor laser made of a GaN material with high reliability for exciting the phosphor for illumination is preferred as the light source. Alternatively, a light source such as a laser array arranged in a one-dimensional form can be implemented. The light source can be a super-light-emitting diode or a semiconductor optical amplifier (SOA).

[0046] Although a method for generating white light from the semiconductor laser and the phosphor is not specifically limited, the following methods are possible.

[0047] A method of generating yellow fluorescence using a blue laser and a phosphor, thereby obtaining white light.

[0048] A method of generating red and green fluorescence using a blue laser and a phosphor, thereby obtaining white light.

[0049] A method of generating red, blue and green fluorescence by a blue laser or an ultraviolet laser through a phosphor, thereby obtaining white light.

[0050] A method of generating blue and yellow fluorescence by a blue laser or an ultraviolet laser through a phosphor, thereby obtaining white light. EXAMPLES

[0051] A fiber optic device was used with the one in the Fig. 3 and Fig. 4 forms shown.

[0052] In particular, an optical waveguide 2 was fabricated on a substrate 1, consisting of a 4-inch wafer with a thickness of 1 mm and made of aluminum nitride, by sputtering using a reflective film 3 composed of an Ag-based alloy and a plating layer 4b composed of Al₂O₃. The optical waveguide 2 material was YAG. An aluminum oxide plating layer 4 was then formed as a film at 8000 angstroms, and an Ag reflective film 5 was formed as a film at 1000 angstroms on the surfaces of the optical waveguide 2 and the substrate 1. A Ti metal film 6 was then formed as a film on the reflective film 5 with a thickness of 2.56 µm. The formation of all films was carried out by sputtering.

[0053] The metal film was then subjected to ashing in a plasma ashing system at an oxygen pressure of 133 Pa and an electrical power input of 250 W. The ashing time was also varied, as shown in Table 1. Following this, the film was machined with a saw to expose the end faces, and then ultrasonic cleaning was performed to create a rib-type optical waveguide structure. Table 1 Example according to the invention 1 Example 2 according to the invention Example according to the invention 3 Example according to the invention 4 Comparative example 1 Film thickness of the metal film before ashing 2,56 µm 2,56 µm 2,56 µm 2,56 µm 2,56 µm Duration of ashling 10 min 30 min 60 min 90 min 0 Film thickness of the metal oxide film 7.6 nm 8.5 nm 9.5 nm 9.3 nm No

[0054] Furthermore, the film thickness of the surface-oxidized film (titanium oxide film) was measured using an X-ray reflection (XRR) technique. Additionally, photographs of the structures of each sample were taken after separation to examine the layer structures and the presence or absence of defects.

[0055] As shown in Table 1, the thickness of the surface-oxidized film, which is composed of titanium oxide, increases with a longer ashing duration, reaching its maximum value after an ashing treatment of 60 minutes. Furthermore, a three-layer structure consisting of a surface-oxidized film, an intermediate metal film, and a reflective film was confirmed according to all examples of the invention. However, the ashing treatment was not carried out according to Comparative Example 1.

[0056] As it is in the Fig. As shown in Figure 5, in accordance with comparative example 1, the side of the upper surface of the optical waveguide was corroded with pure water, so that the disappearance of the reflective film was observed in an area indicated by an arrow. Furthermore, as shown in the Fig.Figure 6 shows that, according to the structure of the comparative example 1, the side of the surface of the optical waveguide is corroded by pure water, so that the disappearance of the reflection film was observed.

[0057] On the other hand, according to the structure of the example according to the invention (Ti / Ag-based alloy / Al2O3), as it is described in the Fig. Figure 7 shows that corrosion or the disappearance of the reflective film on the upper surface of the optical waveguide was not observed, and as shown in the Fig. As shown in Figure 8, corrosion or the disappearance of the reflective film was not observed on the side of the side surface either.

[0058] As further examples according to the invention, Ta, Ni or Cr was used as the metal component of the surface-oxidized film instead of Ti, an Al-based alloy was used for the reflective film and SiO2 was used for the plating layer, which led to similar results.

[0059] Furthermore, according to the respective optical waveguide structures of the inventive examples 1, 2, 3 and 4, the detachment of the metal film and the reflective film was not observed even after polishing the end surfaces.

Claims

[1] Method for fabricating an optical waveguide structure (10, 11) comprising: Obtaining a component comprising an optical waveguide (2) with an end face, a reflective film (5) provided on the optical waveguide (2) and reflecting light propagating in the optical waveguide (2), and a metal film (6) provided on the reflective film (5); Subjecting the metal film (6) of the component to a surface oxidation treatment to produce a surface-oxidized (8) film on the metal film (6); and Subsequent polishing of the end surface of the optical waveguide (2) to form a polished end surface. [2] Method according to claim 1, wherein the surface oxidation treatment comprises plasma ashing. [3] Method according to claim 1 or 2, wherein the optical waveguide structure (10, 11) is treated with ultrapure water. [4] Method according to any one of claims 1 to 3, wherein the reflective film (5) comprises one or more metal(s) selected from the group consisting of silver, gold, platinum, aluminium and alloys thereof. [5] Method according to any one of claims 1 to 4, wherein the metal film (6) comprises one or more metal(s) selected from the group consisting of titanium, chromium, tantalum, nickel and alloys thereof. [6] Method according to any one of claims 1 to 5, wherein the optical waveguide (2) has a main surface, side surfaces (2c) and a top surface (2a), wherein the reflective film (5) comprises a main surface-covering part (5d), side surface-covering parts (5c) and a top surface-covering part (5a), and wherein the metal film (6) comprises a main surface covering part (6d), side surface covering parts (6c) and a top surface covering part (6a). [7] Method according to any one of claims 1 to 6, further comprising a support substrate (1) wherein the optical waveguide (2) is provided on a main surface (1d) of the support substrate (1). [8] Method according to any one of claims 1 to 7, wherein the optical waveguide (2) comprises lithium niobate, lithium tantalate, a solid solution of lithium niobate-lithium tantalate, tantalum pentoxide or a phosphor.

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