HK1087781B

HK1087781B - Light filter

Info

Publication number: HK1087781B
Application number: HK06107847.4A
Authority: HK
Inventors: 后藤直雪; 片冈球子; D．G．波伦斯奇
Original assignee: 株式会社小原
Priority date: 1999-08-10
Filing date: 2006-07-13
Publication date: 2013-03-28

Description

Light filter

the application is a divisional application with the application number of CN 00118009.6. The application date of the parent case is 6 months and 5 days in 2000; the invention relates to a glass ceramic used as a filter and a filter.

Technical Field

The present invention relates to a glass ceramic used as an optical filter, and also relates to an optical filter using the glass ceramic.

Background

There are filters that cut off or pass a particular wavelength, and there are also filters that reduce the light intensity independent of wavelength. The former includes a band pass filter that passes only one specific wavelength, a notch filter that cuts off the specific wavelength, and high pass and low pass filters that pass only wavelengths shorter or longer than the specific wavelength. The latter includes an ND filter.

The optical filter can also be classified into an absorption type optical filter and an interference type optical filter. A typical absorptive filter is an ND filter, and a typical interference filter is a bandpass filter. A substrate made of a plastic such as those used for photography is applied as an absorption type filter. Since a substrate used as an optical filter subjected to intense laser beams is required to have durability and heat resistance, amorphous glass is exclusively used as such a substrate.

The bandpass filter is made by forming a multilayer dielectric film on a substrate such as glass by alternately laminating an H layer of a dielectric thin film having a high refractive index and an L layer of a dielectric thin film having a low refractive index.

In a bandpass filter applied to a WDM (wavelength division multiplexing) optical communication system, when a narrow pass band width for a pass wavelength is set to apply the bandpass filter to a high density of wavelengths, the temperature stability of the center wavelength of the band poses a problem. More specifically, the band pass filter is a sensitive element in which the center frequency of the band changes even with a slight change in temperature, and therefore, when the band pass filter is applied, the temperature should be adjusted by a temperature controller. However, such a temperature controller cannot be practically applied because of a spatial limitation in the arrangement of the band-pass filter. Since it is necessary to reduce the pass band width as the amount of optical information increases, the temperature stability becomes more important.

In the past, amorphous glass was used as a substrate for bandpass filters. The prior art substrate is insufficient in compressive stress and durability against the film because its thermal expansion property and mechanical strength are not high enough. Further, amorphous glass has low surface hardness, and if high thermal expansion properties are to be provided, a relatively large amount of alkali component must be added, which raises a problem of elution of alkali component during and after the formation of a dielectric film on the substrate. Therefore, amorphous glass is insufficient as a substrate for optical filters, particularly as a substrate for bandpass filters.

Disclosure of Invention

It is therefore an object of the present invention to provide a material suitable for a substrate for an optical filter which will eliminate the above-mentioned disadvantages of the prior art substrates and which has a thermal expansion property sufficient to avoid a change in refractive index at a temperature of an optical filter formed using a single layer or multilayer film (i.e., has a high thermal expansion coefficient and thus gives a compressive stress to the film to improve the temperature stability of the refractive index of the film), and also has a mechanical property which gives the optical filter sufficient durability and also has excellent light transmittance.

It is another object of the invention to provide an optical filter made from such a substrate.

The results of the studies and experiments accumulated by the inventors of the present invention in order to achieve the above objects of the present invention have resulted in the present invention finding that a glass-ceramic having a thermal expansion coefficient, mechanical strength and optical transparency within a specific range is suitable for achieving the objects of the present invention.

According to the present invention, there is provided a glass ceramic for an optical filter, which has a coefficient of thermal expansion of 95X 10 in a temperature range of-20 ℃ to +70 DEG C^-7/℃～140×10^-7In the range/° c.

In one aspect of the invention, the glass-ceramic has a Young's modulus of 85GPa or greater.

In another aspect of the present invention, the glass-ceramic has a flexural strength of 10kg/mm²Or more.

In another aspect of the present invention, the sheet has a transmittance of 60% or more in a wavelength range of 950nm to 1600nm for a sheet having a thickness of 10 mm.

In another aspect of the present invention, the glass-ceramic comprises the following components by weight:

SiO₂ 70～77％

Li₂O 8～12％

K₂O 0.5～3％

MgO+ZnO+SrO+BaO 1～5％

P₂O₅ 1.5～3％

ZrO₂ 2～7％

Al₂O₃ 3～9％

Sb₂O₃+As₂O₃ 0～2％

in another aspect of the present invention, the glass-ceramic contains, as the main crystal phases:

(a) lithium disilicate, and

(b) at least one substance selected from the group consisting of α -quartz, solid solution of α -quartz, α -cristobalite and solid solution of α -cristobalite.

In another aspect of the present invention, the glass-ceramic is substantially Na-free₂O and PbO.

In another aspect of the present invention, there is provided an optical filter made by forming a dielectric film on the above glass-ceramic.

In still another aspect of the present invention, there is provided an optical filter made by forming a dielectric film on a glass ceramic having a thermal expansion coefficient larger than that of a dielectric constituting the dielectric film.

The reason why the glass ceramic used for the optical filter according to the present invention is limited in its thermal expansion coefficient, Young's modulus, bending strength, transmittance, composition of the glass ceramic, main crystal phase, etc. will be described below. As in their base glasses, the compositions of glass-ceramics are expressed on an oxide basis (percent by weight).

First, explanation will be made regarding the thermal expansion performance. As mentioned earlier, the thermal stability of the central wavelength of the band (band) is important and the glass-ceramic has a larger thermal expansion coefficient than the material constituting the desired film. As a result of studies and experiments conducted by the present inventors, it was found that, in the temperature range of-20 ℃ to +70 ℃, if the thermal expansion coefficient is 95X 10^-7at/DEG C or above, the film can be given sufficient compressive stress in a temperature range where the glass ceramic is used as a kind of band-pass filter, and if the coefficient of thermal expansion exceeds 140X 10^-7/° c, problems such as separation of the film from the substrate, for example, will occur where the difference in the coefficient of thermal expansion between the substrate and the filter becomes so large. The preferred range of thermal expansion coefficient is 110X 10^-7/℃～130×10^-7More preferably, it is within a range of 120. + -. 5X 10/. degree.C^-7/℃。

In a bandpass filter, the temperature stability at the center wavelength is related to some extent to the temperature coefficient of refractive index of the dielectric constituting the thin film, and, to a greater extent, to the thermal expansion coefficient of the substrate. This is because the refractive index is determined by the film atomic density of the thin film. That is, the higher the membrane atom density of the thin film, the smaller the change due to the temperature of the center frequency. The film atom density of the thin film is greatly affected by the thermal expansion coefficient of the substrate used as the optical filter on which the thin film is formed. More specifically, during film formation, the temperature of the substrate becomes about 200 ℃, and the substrate is thereby greatly expanded. A thin film is formed on such an expanded substrate, and the thin film is susceptible to compressive stress due to the difference in the thermal expansion coefficient between them when the substrate is cooled. As a result, the film atom density of the thin film increases, and thus the refractive index also increases. As the thermal expansion coefficient of the substrate increases, the compressive stress applied to the dielectric thin film formed on the substrate increases as a result of a change in refractive index due to a decrease in temperature when the optical filter is used. Therefore, it is necessary to set the thermal expansion coefficient of the glass ceramic at a value larger than that of the dielectric thin film.

In view of the severe conditions applied in the band-pass filter, in addition to the above properties, the strength of the glass-ceramic against mechanical deformation, i.e., the bending strength and Young's modulus, cannot be ignored.

Regarding the Young's modulus, since the substrate is processed into small pieces (2 mm. times.2 mm or less) after the thin film is formed, a high Young's modulus and a high bending strength are required for the substrate. The glass-ceramic should preferably have a Young's modulus of 85GPa or more and a bending strength of 10kg/mm in view of the next process²Or a combination thereof.

Regarding the transmittance, if the transmittance is low, troubles such as reduction of the signal-to-noise ratio will occur in generating a signal. Therefore, it is required that the transmittance should be as large as possible, and it is found that the transmittance must be at least 60%. The wavelength range of 95nm to 1600nm applied to the bandpass filter is required for such wavelength range, and the transmittance of a sheet having a thickness of 10mm is 60% or more. In terms of transmittance in such a wavelength range, the transmittance should preferably be 75% or more, more preferably 80% or more.

With respect to the crystal phase growth in the glass ceramic, the glass ceramic should preferably include, as their main crystal phases: (a) lithium disilicate, and (b) at least one selected from the group consisting of α -quartz, α -quartz solid solution, α -cristobalite and α -cristobalite solid solution. As these crystal phases grow in the glass ceramic, the bending strength and Young's modulus can be remarkably increased, and the thermal expansion coefficient in the temperature range of-20 ℃ to +70 ℃ can be extended to 95X 10^-7/℃～140×10^-7Range/° c. Further, in applying the band pass filter, transmittance in a 10mm thick sheet can be obtained at 60% or more in a wavelength range of 950nm to 1600 nm.

The reason for limiting the above-mentioned composition range of the base glass will now be described.

SiO₂The component is a very heavyThe desired composition is formed by heating a base glass to form lithium disilicate, alpha-quartz solid solution, alpha-cristobalite and alpha-cristobalite solid solution, which constitute the main crystalline phases of the glass-ceramic. If the content of this component is below 70%, the crystal phase growth in the glass ceramic will be unstable and their crystal structure tends to become too coarse, while if the content of this component exceeds 77%, difficulties may arise in melting and shaping the base glass.

Li₂The O component is a very important component, and lithium disilicate is formed as one of the main crystal phases by heating the base glass. If the amount of this component is less than 8%, difficulty arises in the growth of this crystal phase and the melting of the base glass, whereas if the amount of this component exceeds 12%, the crystal phase growth will be unstable, and its crystal structure tends to become too coarse and chemical durability deteriorates.

K₂The O component is effective for improving the melting property of the glass and preventing the crystal phase structure from growing too coarse, etc. The amount of such a component is preferably 0.5% or more. However, an excessive amount of such a component causes the crystal phase structure to grow too coarsely, causes a change in the crystal phase and lowers the chemical durability, and therefore, the amount of such a component should preferably be limited to 3% or less.

The MgO, ZnO, SrO and BaO components are effective in improving the melting property of the glass, preventing the crystal structure from growing too coarse, and enabling the adjustment of the transmittance by adjusting the refractive index of the glass phase constituting the basic phase (matrix phase). If the total amount of these components is below 1%, these effects cannot be obtained, whereas if the total amount of these components exceeds 5%, the resulting crystal phase is unstable, and thus the crystal structure tends to become too coarse.

P₂O₅The component is necessary as a nucleating agent. In order to obtain such an effect, the amount of such a component is preferably 1.5% or more. In order to prevent devitrification and maintain stability of the base glass during mass production, the composition ofThe amount should be limited to 3% or less.

ZrO₂Component is a very important component like P₂O₅The components, as well, function as a nucleating agent for glass and are effective for producing finer grown grains and improving the mechanical strength and chemical durability of the material, etc. In order to obtain these effects, it is preferable that the amount of such a component is 2% or more. If such a component is added in excess, difficulty arises in melting the base glass, and one such as ZrSiO₄Will remain unmelted. For this reason, the amount of such components should be limited to 9% or less.

A1₂O₃The component is a component which improves the chemical durability and mechanical strength, particularly the bending strength, of the glass-ceramic. To achieve this object of the invention, it is necessary to add 3% or more of this component, preferably 4% or more. If the component is added in excess, the melting property and resistance to devitrification of the glass are lowered, and in addition, β -spodumene and β -cristobalite grow into a crystal phase. Beta-spodumene and beta-cristobalite are crystals having a very small thermal expansion coefficient, and the growth of these crystals greatly lowers the thermal expansion coefficient of the resulting glass ceramic. Therefore, the amount of such a component is preferably 9% or less, and most preferably 8% or less.

Sb₂O₃And As₂O₃The components may be added as a refining agent. It is sufficient to add one or both of these components in a total amount of 2% or less. Preferably 1% or less in total.

The substantial exclusion of Na in this glass-ceramic will be described below₂The reasons for O and PbO. In a multilayer film formed on a substrate, Na contained in the material₂O causes a problem in that Na ions are eluted into the multilayer film, deteriorating the film properties. As for PbO, such a component is not required from the viewpoint of environmental protection, and the use of such a component should be avoided as much as possible.

In order to manufacture the glass-ceramic for an optical filter according to the present invention, a base glass having the above composition is melted, subjected to hot forming and/or cold forming, and heat-treated at a temperature in the range of 500 to 600 ℃ for about 1 to 7 hours to generate a kind of crystal nuclei, and then further heat-treated at a temperature in the range of 700 to 780 ℃ for about 1 to 7 hours to be crystallized.

The glass ceramic obtained by the heat treatment thus contains, as its main crystal phases: (a) lithium disilicate, and (b) at least one selected from the group consisting of α -quartz, α -quartz solid solution, α -cristobalite solid solution.

The glass-ceramic thus obtained is ground and polished by a conventional method to provide a glass-ceramic substrate for use as an optical filter with a surface roughness (Ra) (arithmetic average roughness) in the range of 1.0A to 5.0A. The glass ceramic according to the present invention is suitable for an interference type optical filter in which a multilayer dielectric film formed by alternately laminating a dielectric film having a high refractive index (H layer) and a dielectric film having a low refractive index (L layer) is formed on a glass ceramic substrate, and is particularly suitable for a bandpass filter having a multilayer dielectric film.

As dielectrics, like TiO₂，Ta₂O₂，Nb₂O₅And SiO₂The inorganic oxide of (2) can be preferably used. In a bandpass filter for use in a wavelength range from 950nm to 1600nm, TiO₂/SiO₂，Ta₂O₂/SiO₂And Nb₂O₅/SiO₂The combination of (b) may be preferably used as a composite of the H layer and the L layer.

The optical filter of the present invention may be provided by forming a dielectric thin film on the surface of a glass-ceramic substrate. For forming such a thin film, a deposition method, a radio frequency ion plating method, a magnetron sputtering method, a plasma plating method, or the like can be applied. Among these methods, the deposition method is particularly preferable.

Detailed Description

Examples of the invention

Examples of the present invention will now be described. Tables 1, 2 and 3 show the compositions, growth crystal phases, thermal expansion coefficients, Young's moduli, bending strengths and transmittances of the glass ceramics used as the optical filters in examples 1 to 8 of the present invention and the glass substrates used as the optical filters in comparative examples of the prior art. In the table, lithium disilicate is abbreviated as "LD".

TABLE 1

TABLE 2

TABLE 3

To make the glass-ceramics of examples 1-8 above, materials including oxides, carbonates, and nitrates were weighed and mixed and melted in a conventional melting apparatus at a temperature range of about 1350 deg.C to 1450 deg.C. The molten glass is stirred uniformly and then formed into a predetermined shape and annealed to provide a formed glass. The formed glass is then heat treated at a temperature in the range of 500 ℃ to 600 ℃ for about 1 to 7 hours to produce crystal nuclei, and then further heat treated at a temperature in the range of 700 ℃ to 780 ℃ for about 1 to 7 hours for crystallization to produce the desired glass-ceramic. Then, the glass ceramics are ground with 800-2000 # diamond particles for 5-30 minutes, and finally, are ground with cerium oxide abrasive grains having an average diameter of 0.02-3 μm for 30-60 minutes. The surface roughness (Ra) (roughness at the center line) is 5A or less.

The grown crystal phase was measured by X-ray diffraction Analyzer (XRD) and energy dispersive X-ray Spectroscopy (EDS).

Comparing examples 1 to 8 with comparative example 1, the thermal expansion coefficient of the glass substrate for optical filter of the prior art was 93X 10^-7/° c, it is not sufficient to impart compressive stress to the formed film. The prior art glass substrate also exhibited a low Young's modulus of 75GPa and a bending strength of 5kg/mm². In contrast, the glass ceramic of the present invention has a sufficient coefficient of thermal expansion to impart compressive stress to the formed film, and also has sufficient Young's modulus and bending strength, indicating that the glass ceramic of the present invention is suitable for use as a substrate for an optical filter.

Forming TiO from the glass-ceramic substrate in the above example₂/SiO₂，Ta₂O₂/SiO₂And Nb₂O₅/SiO₂The multilayer film method of (1), provides an interference type filter having an excellent temperature stability of the center wavelength thereof, and finds an optimum use as a band-pass filter in optical communications.

As described above, according to the present invention, the disadvantages of the prior art substrate are overcome, and a glass-ceramic substrate for use as an optical filter having an excellent temperature stability of the center wavelength is provided. These characteristics, i.e., high transmittance, high thermal expansion property, high young's modulus and high bending strength, are suitable for use as an interference filter, particularly a band-pass filter, and are most suitable for use as WDM (wavelength division multiplexing) and DWDM (dense wavelength division multiplexing) in optical communication systems. In addition, the glass ceramic substrate of the present invention is formed of TiO₂/SiO₂，Ta₂O₂/SiO₂And Nb₂O₅/SiO₂The multilayer dielectric film provides a bandpass filter element having excellent temperature stability of the center wavelength and not only canThe method is used for a ground optical communication system, and can also be used for a space station satellite optical communication system.

Claims

1. An optical filter is made by forming a dielectric film on a glass ceramic having a thermal expansion coefficient of 95 x 10 in a temperature range of-20 ℃ to +70 ℃ which is larger than that of a dielectric constituting the dielectric film^-7/℃～140×10^-7A/° c range and a young's modulus of the glass-ceramic is 85GPa or more, wherein the glass-ceramic contains, as main crystalline phases:

(a) lithium disilicate, and

(b) at least one substance selected from the group consisting of alpha-quartz, solid solutions of alpha-quartz, alpha-cristobalite and solid solutions of alpha-cristobalite,

and the transmittance of the glass ceramic to a 10mm thick sheet is 60% or more in the wavelength range of 950 nm-1600 nm, wherein the glass ceramic comprises the following components in percentage by weight: