Glass-ceramics for optical filters and optical filters

Abstract

To provide a crystallized glass for optical filter and an optical filter that have both the transmittance required for a conventional optical filter substrate, and a high thermal expansion coefficient that can respond to stress changes caused by an increase in the number of dielectric films formed on the substrate.SOLUTION: Provided is a crystallized glass for optical filter that is characterized by containing, in terms of oxide conversion mass%, SiO2 component by 35.0% to 55.0%, K2O component by 15.0% to 30.0%, Al2O3 component by 10.0% to 25.0%, ZrO2 component by 0% to 10.0%, TiO2 component by 0% to 15.0%, ZrO2 component and TiO2 component total amount over 0% to 15.0%, and having a thermal expansion coefficient at -30 to 70°C of 125 to 155(×10-7 / °C).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a crystallized glass for an optical filter and an optical filter. [Background technology]

[0002] In recent years, with the ever-increasing popularity of communication devices and the rapid development of the Internet, there has been a demand for even larger scale and higher capacity data communications using optical fiber. Optical fiber data communications use the 1550 nm band of light, which has particularly low optical loss within the fiber. Wavelength division multiplexing (WDM) is also used, which divides this band into multiple smaller bands, transmits multiple light beams simultaneously through the optical fiber, and finally separates and extracts the multiplexed optical signals one by one.

[0003] Furthermore, WDM is classified into two types, Coarse Wavelength Division Multiplexing (CWDM) and Dense Wavelength Division Multiplexing (DWDM), depending on the density of the multiplexed light. CWDM is low-density, with around 16 multiplexed wavelengths. On the other hand, DWDM is high-density, with around 80 to 96 multiplexed wavelengths. For this reason, DWDM is adopted in applications requiring high-capacity communications, such as data communications between operators, countries, and base stations.

[0004] Furthermore, with the full-scale introduction of fifth-generation mobile communication systems (5G) and IoT, there are concerns that the data communication capacity limits of WDM will become apparent.Recently, research has been progressing to overcome the data communication capacity limits by combining the conventional wavelength division multiplexing (WDM) with the new space division multiplexing (SDM).

[0005] Meanwhile, optical filter substrates for optical communications are formed by laminating dielectric layers of low refractive index (e.g., SiO2) and high refractive index (e.g., Ta2O5) layers. To increase data communication capacity, it is preferable to have a narrower bandwidth for multiplexed light, and to accommodate the narrower bandwidth of communication wavelengths, it is necessary to increase the number of layers deposited on the substrate. Therefore, optical filter substrates with a high thermal expansion coefficient are required, from the perspective of adhesion of the laminated layers and to accommodate stress changes that occur as the number of layers increases.

[0006] Patent Document 1 states that the thermal expansion coefficient at -20 to 70°C is 93 × 10 -7 / ℃~130×10 -7 The glass-ceramics for optical filters are characterized by having a Young's modulus of 85 GPa or more at 100°C / °F and containing, as the main crystalline phase, (a) lithium disilicate and (b) at least one selected from α-quartz, α-quartz solid solution, α-cristobalite, and α-cristobalite solid solution. However, the problem of having a maximum average thermal expansion coefficient near 100°C has not been solved.

[0007] Patent Document 2 also describes a ceramic material containing an Al2O3 component and an R2O component (R is at least one selected from Li, Na, and K), in which the main crystalline phase is kalsilite, and having a thermal expansion coefficient of 95 × 10 in the temperature range of -30 °C to 70 °C. -7 / ℃~121×10 -7 / °C. However, an inorganic composition having a thermal expansion coefficient of 125×10 -7 No inorganic compositions exceeding 100°C are disclosed. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-318222 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-031180 Summary of the Invention [Problem to be solved by the invention]

[0009] Therefore, the present invention aims to provide a crystallized glass for optical filters and an optical filter that has a high thermal expansion coefficient that can accommodate stress changes that occur due to the increasing number of films required for substrates for DWDM devices in order to accommodate the recent increase in information communication volume, while maintaining the transmittance required for conventional optical filter substrates. [Means for solving the problem]

[0010] In order to solve the above problems, the inventors conducted extensive testing and research, and as a result, they discovered a glass composition and blend for crystallized glass for optical filters that has a desired thermal expansion coefficient while maintaining transmittance by improving the expansion characteristics through adjustment of the network former component and alkali metal oxide component and crystallizing the glass, and by controlling the particle size of the precipitated crystals through adjustment of the ZrO2 component or TiO2 component, and thus completed the present invention. Specifically, the present invention provides the following:

[0011] (1) Mass % of oxide equivalent SiO2 content: 35.0% to 55.0% K2O content: 15.0% to 30.0% Al2O3 component 10.0% to 25.0%, ZrO2 content: 0% to 10.0% TiO2 content: 0% to 15.0% The total amount of ZrO2 and TiO2 components is over 0% to 15.0%. Contains Thermal expansion coefficient at -30 to 70°C is 125 to 155 (×10 -7 / °C).

[0012] (2) In terms of oxide, mass % Li2O content: 0% to 7.0% Na2O content: 0% to 5.0% MgO content: 0% to 7.0% The crystallized glass for an optical filter according to (1), characterized by containing:

[0013] (3) As a crystalline phase, KAlSiO4 or KAlSiO4 solid solution The crystallized glass for an optical filter according to (1) or (2), comprising:

[0014] (4) The crystallized glass for an optical filter according to any one of (1) to (3), characterized in that a 1 mm thick sample has a spectral transmittance of 90% or more at 1550 nm.

[0015] (5) An optical filter comprising the crystallized glass according to any one of (1) to (4) and a dielectric film formed on the glass. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide crystallized glass for an optical filter and an optical filter that has an improved thermal expansion coefficient while maintaining the transmittance required for an optical filter. DETAILED DESCRIPTION OF THE INVENTION

[0017] The composition ranges of each component constituting the crystallized glass for optical filters of the present invention are described below. In this specification, unless otherwise specified, the content of each component is expressed as mass% relative to the total mass of the composition converted into oxides. Here, "composition converted into oxides" refers to the composition of each component contained in the glass, assuming that the oxides, composite salts, metal fluorides, etc. used as raw materials for the glass components of the present invention are all decomposed and converted into oxides during melting, with the total mass of the oxides produced being 100 mass%.

[0018] [Glass components] In terms of oxides, it contains 35.0% to 55.0% SiO2, 15.0% to 30.0% K2O, 10.0% to 25.0% Al2O3, 0% to 10.0% ZrO2, 0% to 15.0% TiO2, and the total amount of ZrO2 and TiO2 is over 0% to 15.0%.

[0019] [Required and optional ingredients] The SiO2 component is a glass framework component and is also an essential component for forming the crystals that precipitate when the base glass is heat-treated. In particular, by increasing the SiO2 content to 35.0% or more, base glass can be produced stably. Therefore, the lower limit of the SiO2 component content is preferably 35.0% or more, more preferably 37.0% or more, and even more preferably 40.0% or more. On the other hand, by keeping the content of the SiO2 component at 55.0% or less, excessive increases in viscosity and deterioration of meltability can be suppressed, so the upper limit of the content of the SiO2 component is preferably set at 55.0% or less, more preferably at 53.0% or less, more preferably at 50.0% or less, and even more preferably at 45.0% or less.

[0020] When the LiO content exceeds 0%, it promotes the melting reaction of the raw materials, lowers the melting temperature of the glass, and is an effective component for improving meltability. However, an increased content can cause a deterioration in chemical durability and a change in the precipitated crystalline phase. Therefore, the lower limit of the content of the Li2O component is preferably more than 0%, more preferably 0.5% or more, and even more preferably 1.0% or more. On the other hand, by keeping the content of the Li2O component at 7.0% or less, it is possible to suppress the deterioration of devitrification, changes in the precipitated crystal phase, and deterioration of chemical durability that are caused by an excessive content. Therefore, the upper limit of the content of the Li2O component is preferably 7.0% or less, more preferably 6.0% or less, and even more preferably 5.0% or less.

[0021] When the Na2O content exceeds 0%, it adjusts the meltability of the glass and is also a component that forms crystals that precipitate when the base glass is heat-treated. Therefore, the lower limit of the content of the Na2O component is preferably more than 0%, and more preferably 0.5% or more. On the other hand, by keeping the content of the Na2O component at 5.0% or less, it is possible to suppress deterioration of chemical durability and changes in the precipitated crystal phase. Therefore, the upper limit of the Na2O content is preferably 5.0% or less, more preferably 3.0% or less, and even more preferably 2.0% or less.

[0022] When the K2O content exceeds 0%, it is one of the components that constitute the crystals that precipitate upon heat treatment of the raw glass, and is also an essential component that contributes to improving the meltability of the glass. The lower limit of the K2O content is preferably 15.0% or more, more preferably 17.0% or more, and even more preferably 18.0% or more. On the other hand, by keeping the content of the K2O component at 30.0% or less, deterioration of chemical durability is suppressed, so the upper limit of the content of the K2O component is preferably set to 30.0% or less, more preferably 28.0% or less, and even more preferably 26.0% or less.

[0023] When the Al2O3 component is contained at more than 0%, it is one of the components that constitute the crystals that precipitate upon heat treatment of the base glass, and is also an effective component for increasing the chemical durability and mechanical strength of the glass and improving the devitrification resistance of the glass melt, and is therefore an essential component of the crystallized glass for light filters of the present invention. Therefore, the lower limit of the content of the Al2O3 component is preferably 10.0% or more, more preferably 12.0% or more, and even more preferably 14.0% or more. On the other hand, by keeping the content of the Al2O3 component at 25.0% or less, it is possible to reduce the deterioration of meltability and devitrification caused by excessive content, so the upper limit of the content of the Al2O3 component is preferably set at 25.0% or less, more preferably at 22.0% or less, and even more preferably at 19.0% or less.

[0024] The P2O5 component is an optional component that, when contained in excess of 0%, serves as a nucleating agent for precipitated crystals, lowers the viscosity of the melt, and contributes to improving the stability of the glass. However, if contained in excess, the precipitated crystal phase may change, the glass may become more susceptible to devitrification, and vitrification may become difficult. Therefore, the lower limit of the P2O5 component is preferably set to more than 0%, and more preferably to 0.5% or more. The upper limit is preferably 5.0% or less, more preferably 3.0% or less, more preferably 2.5% or less, and even more preferably 2.0% or less.

[0025] The B2O3 component is an optional component that, when contained in excess of 0%, contributes to lowering the viscosity of the melt and forming a glass network structure, thereby contributing to the stability of the glass. However, as the content increases, the precipitated crystal phase changes and chemical durability decreases, so the lower limit of the B2O3 component content is preferably set to more than 0%, and more preferably to 0.5% or more. The upper limit is preferably 5.0% or less, more preferably 3.0% or less, more preferably 2.5% or less, and even more preferably 2.0% or less.

[0026] When the MgO content exceeds 0%, it is an optional component that not only improves the meltability of the glass but also prevents the precipitated crystals from becoming coarse, but also contributes to improving the meltability of the glass. Therefore, the lower limit of the content of the MgO component is preferably more than 0%, and more preferably 1.0% or more. On the other hand, by keeping the content of the MgO component at 7.0% or less, it is possible to reduce devitrification caused by an excessive content. Therefore, the upper limit of the content of the MgO component is preferably 7.0% or less, more preferably 5.0% or less, more preferably 4.0% or less, even more preferably 3.8% or less, and even more preferably 3.2% or less.

[0027] ZnO and SrO are optional components that contribute to lowering the viscosity of the melt and improving the stability of the glass when contained in an amount exceeding 0%, respectively. However, excessive content of these components can cause changes in the precipitated crystal phase and make the glass more susceptible to devitrification. Therefore, the ZnO component and the SrO component may or may not be contained within the range that does not impair the physical properties of the material. The content of each of these components can be set to 0% to 3.0%.

[0028] The ZrO2 component acts as a nucleating agent when its content exceeds 0%, and therefore not only facilitates crystal precipitation but also contributes to the refinement of precipitated crystals and the improvement of mechanical strength. In particular, in the present invention, it is a component that can adjust the grain size of the crystals. Therefore, the lower limit of the content of the ZrO2 component is preferably more than 0%, more preferably 1.0% or more, more preferably 2.0% or more, more preferably 3.0% or more, and even more preferably more than 3.0%. On the other hand, by keeping the content of the ZrO2 component at 10.0% or less, it is possible to reduce devitrification and deterioration of meltability caused by an excessive content of the ZrO2 component. Therefore, the upper limit of the content of the ZrO2 component is preferably 10.0% or less, more preferably 8.0% or less, and even more preferably 6.0% or less.

[0029] When the TiO2 content exceeds 0%, it acts as a nucleating agent, which not only facilitates crystal precipitation but also contributes to the refinement of precipitated crystals and the improvement of mechanical strength. In particular, in the present invention, it is a component that can adjust the grain size of the crystals. Therefore, the lower limit of the content of the TiO2 component is preferably more than 0%, more preferably 1.0% or more, more preferably 2.0% or more, more preferably 3.0% or more, more preferably 4.0% or more, and even more preferably more than 4.0%. On the other hand, by keeping the content of the TiO2 component at 15.0% or less, the decrease in transmittance can be suppressed. Therefore, the upper limit of the content of the TiO2 component is preferably 15.0% or less, more preferably 12.0% or less, more preferably 9.0% or less, more preferably 8.0% or less, more preferably 7.0% or less, and even more preferably 6.0% or less.

[0030] The Sb2O3 component is an optional component that can degas the glass melt when its content exceeds 0%. Therefore, the lower limit of the content of the Sb2O3 component may be preferably set to more than 0%, and more preferably to 0.03% or more. On the other hand, by keeping the content of the Sb2O3 component at 1.0% or less, the decrease in transmittance can be suppressed. Therefore, the upper limit of the content of the Sb2O3 component may be preferably set to 1.0% or less, more preferably 0.5% or less, and even more preferably 0.2% or less.

[0031] The Gd2O3 component is a component that reduces the formability and devitrification of the glass melt when contained in an amount of 3.0% or more. Therefore, the upper limit of the Gd2O3 content is preferably set to less than 3.0%, and it may not be contained at all.

[0032] In addition, La2O3 component, Y2O3 component, Yb2O3 component, Eu2O3 component, Dy2O3 component, Er2O3 component, Tb2O3 component, Pr6O 11 The composition may or may not contain any of the following components: Nd2O3 component, Tm2O3 component, Sm2O3 component, Ho2O3 component, CeO2 component, CaO component, BaO component, Ta2O5 component, Nb2O5 component, WO3 component, TeO2 component, Bi2O3 component, GeO2 component, Ga2O3 component, and SnO2 component. The content of each of these components may be 0% to less than 3.0%.

[0033] When the total content of the ZrO2 component and the TiO2 component exceeds 0%, they function as nucleating agents for precipitated crystals and also contribute to the refinement of precipitated crystals and the improvement of the mechanical strength of the material. In particular, in the present invention, they are components that can adjust the grain size of the crystals. Therefore, the lower limit of the total content of the ZrO2 component and the TiO2 component is preferably more than 0%, more preferably 5.0% or more, more preferably 7.0% or more, more preferably 9.0% or more, more preferably 9.2% or more, more preferably 9.5% or more, and even more preferably 9.7% or more. On the other hand, by keeping the total content of the ZrO2 component and the TiO2 component at 15.0% or less, it is possible to suppress deterioration in transmittance and sudden changes in physical properties during crystallization. Therefore, the upper limit of the total content of the ZrO2 component and the TiO2 component is preferably 15.0% or less, and more preferably 12.0% or less.

[0034] By setting the ratio of the total content of the Na2O component, the K2O component, and the MgO component to the content of the SiO2 component [(Na2O+K2O+MgO) / SiO2] to be 0.40 or more, the thermal expansion characteristics of the crystallized glass can be improved. Therefore, the upper limit of the ratio of the total content of Na2O, K2O, and MgO to the content of SiO2 [(Na2O + K2O + MgO) / SiO2] is preferably 1.00 or less, more preferably 0.90 or less, more preferably 0.80 or less, and even more preferably 0.70 or less, while the lower limit is preferably 0.40 or more, more preferably 0.45 or more, and even more preferably 0.50 or more.

[0035] By setting the ratio of the total content of the Na2O component and the K2O component to the content of the SiO2 component [(Na2O+K2O) / SiO2] to be 0.30 or more, the thermal expansion characteristics of the crystallized glass can be improved. Therefore, the upper limit of the ratio of the total content of Na2O and K2O to the content of SiO2 [(Na2O + K2O) / SiO2] is preferably 0.80 or less, more preferably 0.75 or less, and even more preferably 0.70 or less, while the lower limit is preferably 0.30 or more, more preferably 0.35 or more, and even more preferably 0.40 or more.

[0036] By setting the ratio of the total content of the Na2O component and the K2O component to the content of the Al2O3 component [(Na2O+K2O) / Al2O3] to be 1.00 or more, the thermal expansion characteristics of the crystallized glass can be improved. Therefore, the upper limit of the ratio of the total content of Na2O and K2O to the content of Al2O3 [(Na2O + K2O) / Al2O3] is preferably 1.80 or less, more preferably 1.70 or less, more preferably 1.60 or less, more preferably 1.55 or less, and even more preferably 1.50 or less. On the other hand, the lower limit is preferably 1.00 or more, more preferably 1.10 or more, more preferably 1.20 or more, and even more preferably 1.30 or more.

[0037] The ratio of the content of the ZrO2 component to the total content of the ZrO2 component and the TiO2 component [ZrO2 / (ZrO2+TiO2)] is a parameter that contributes to improving the transparency of the crystallized glass by making the precipitated crystals finer, when set to 0.10 or more and 0.70 or less. Therefore, the upper limit of the ratio of the content of the ZrO2 component to the total content of the ZrO2 component and the TiO2 component [ZrO2 / (ZrO2+TiO2)] is preferably 0.70 or less, more preferably 0.65 or less, more preferably 0.60 or less, more preferably 0.58 or less, and even more preferably 0.55 or less. On the other hand, the lower limit is preferably 0.10 or more, more preferably 0.15 or more, more preferably 0.20 or more, and even more preferably 0.25 or more.

[0038] [Manufacturing method] The crystallized glass for optical filters of the present invention is produced, for example, as follows: Raw materials such as oxides, hydroxides, carbonates, nitrates, fluorides, chlorides, and metaphosphate compounds are mixed uniformly so that each component falls within a predetermined content range, the mixture is poured into a platinum crucible, and melted in an electric furnace at a temperature range of 1300 to 1600°C depending on the melting difficulty of the glass composition, stirred and homogenized, then cooled to an appropriate temperature, poured into a mold, and slowly cooled to produce a glass base material.

[0039] The crystallization process is not particularly limited, but for example, the glass base material was heated at a temperature range of 500°C to 560°C for 5 hours to form crystal nuclei, and then heated at a temperature range of 570°C to 800°C for 3 hours to crystallize. The precipitated crystalline phase was analyzed and identified using a Bruker D8 DISCOVER X-ray diffractometer.

[0040] The precipitated crystalline phase of the crystallized glass for an optical filter of the present invention is preferably KAlSiO4 or a KAlSiO4 solid solution in order to improve the thermal expansion coefficient.Other precipitated crystalline phases may or may not include Rn2MgSiO4 (where Rn is one or more selected from Li, Na, and K), Rn2SiO3 (where Rn is one or more selected from Li, Na, and K), Li2TiO3, K2ZrSi2O7, etc.

[0041] This crystallized glass was lapped and then polished to obtain a crystallized glass substrate for optical filters. By forming a dielectric multilayer film on the surface of this crystallized glass substrate, it can be used as an optical filter for SDM or DWDM.

[0042] [Coefficient of thermal expansion] The linear expansion coefficient was measured in the temperature range of -30°C to 70°C using a Mac Science TD5000S, based on the Japan Optical Glass Industry Association standard JOGIS-16 (2019), "Method for measuring the average linear expansion coefficient of optical glass near room temperature." The sample was cut into a cylindrical shape with a diameter of 4 mm and a length of 20 mm, and the linear expansion coefficient was calculated from the slope of the expansion curve, which shows the relationship between temperature and material elongation, in the temperature range of -30°C to 70°C. The optical filter of the present invention preferably has a lower limit of the thermal expansion coefficient at -30°C to 70°C of 125 (×10 -7 / ℃), more preferably 126(×10 -7 / ℃), and most preferably 127(×10 -7 / °C) or more. The upper limit is preferably 155 (×10 -7 / ℃), more preferably 154(×10 -7 / °C), and more preferably 153 (× 10 -7 / °C) or less.

[0043] [specific gravity] The specific gravity of the glass-ceramics was measured in accordance with Japanese Industrial Standard JIS Z 8807 (2012) "Method for measuring density and specific gravity of solids." The specific gravity of the optical filter of the present invention is preferably 4.0 or less, more preferably 3.5 or less, and even more preferably 3.0 or less.

[0044] [Internal transmittance] The spectral transmittance, including reflection loss, of 10 mm and 1 mm thick, parallel-polished, face-to-face samples was measured with reference to the Japan Optical Glass Industry Association standard JOGIS-17 (2019), "Method for Measuring Internal Transmittance of Optical Glass," and the internal transmittance of the 1 mm thickness (spectral transmittance not including reflection loss) was calculated from these spectral transmittances. The optical filter of the present invention preferably has an internal transmittance at 1550 nm of 90.0% or more, more preferably 92.0% or more, even more preferably 95.0% or more, and most preferably 98.0% or more.

[0045] [Transmittance] Based on the Japan Optical Glass Industry Association standard JOGIS-02 (2019) "Method for measuring color intensity of optical glass," the spectral transmittance of a 1 mm thick, parallel-polished specimen was measured using a Hitachi Measurement was carried out using a spectrophotometer U-4100. The optical filter of the present invention preferably has a spectral transmittance at 1550 nm of 90.0% or more, more preferably 90.2% or more, even more preferably 90.4% or more, and most preferably 90.5% or more. [Example]

[0046] The following examples are provided to illustrate the present invention in detail for illustrative purposes, but it should be noted that these examples are for illustrative purposes only and that many modifications may be made by those skilled in the art without departing from the spirit and scope of the present invention.

[0047] Glass-ceramics of various compositions, as listed in Tables 1 and 2, were prepared as Examples (Nos. 1 to 21) and Comparative Examples 1 and 2. High-purity raw materials typically used in glass-ceramics, such as oxides, hydroxides, carbonates, nitrates, fluorides, chlorides, and metaphosphate compounds, were selected as the raw materials for each component. These were weighed and mixed to achieve the compositional ratios shown in Tables 1 and 2 for each example. The mixture was then placed in a platinum crucible and melted in an electric furnace at a temperature ranging from 1300 to 1600°C, depending on the melting difficulty of the glass composition. The mixture was then stirred and homogenized, cooled to an appropriate temperature, poured into a mold, and slowly cooled. The resulting glass was then crystallized at a predetermined temperature. The thermal expansion coefficient, transmittance, internal transmittance, and specific gravity of each glass-ceramic were measured, and the results are shown in Tables 1 and 2.

[0048] [Table 1]

[0049] [Table 2]

[0050] The crystallized glass for optical filters according to the examples of the present invention has a high thermal expansion coefficient while maintaining transmittance, and is therefore suitable as an optical filter for SDM or DWDM devices.

Claims

1. In terms of oxide, mass % SiO ２ The content of the ingredients is 35.0% to 55.0%. K ２ The content of O component is 15.0% to 30.0%. Al ２ O ３ The content of the ingredients is 10.0% to 25.0%, ZrO ２ The content of the ingredients is 0% to 10.0%, TiO ２ The content of the ingredients is 0% to 15.0%, ZrO ２ Ingredients and TiO ２ The total amount of ingredients is more than 0% to 15.0%; the ratio of the total content of the Na 2 O component and the K 2 O component to the content of the SiO 2 component [(Na 2 O + K 2 O) / SiO 2 ] is 0.58 or more; Thermal expansion coefficient at -30 to 70°C is 139 to 155 (x 10 －７ / °C).

2. In terms of oxide, mass % Li ２ The content of O component is 0% to 7.0%, Na ２ The content of O component is 0% to 5.0%.

2. The crystallized glass for optical filters according to claim 1, wherein the content of MgO component is 0% to 7.0%.

3. As a crystalline phase, KAlSiO ４ or KAlSiO ４ 3. The crystallized glass for an optical filter according to claim 1, which contains a solid solution.

4. 4. The crystallized glass for optical filters according to claim 1, wherein a sample having a thickness of 1 mm has a spectral transmittance of 90% or more at 1550 nm.

5. An optical filter comprising the crystallized glass according to any one of claims 1 to 4 and a dielectric film formed on the glass.

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