Glass-ceramics for a light filter

Details Glass-ceramics for a light filter Glass-ceramics for a light filter

2002-02-04

2004-01-13

Group, Karl (Department: 1755)

Compositions: ceramic

Ceramic compositions

Devitrified glass-ceramics

C501S005000

Reexamination Certificate

active

06677259

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to glass-ceramics for a light filter.
There are light filters which cut or pass light of a specific wavelength and there are also light filters which reduce intensity of light without depending upon wavelength. The former includes a band-pass filter which passes only a specific wavelength, a notch pass filter which cuts a specific wavelength and high-pass and low-pass filters which pass only wavelengths shorter or longer than a specific wavelength. The latter includes an ND filter.
Light filters can be classified also into an absorption type filter and an interference type filter. A representative absorption type filter is the ND filter and a representative interference type filter is the band-pass filter. A substrate made of plastic is used for absorption type filters such as those for photography. Since a substrate for light filters which are subject to a strong laser beam requires durability and heat resistance property, amorphous glass is exclusively employed for such substrate.
The band-pass filters are made by forming, on a substrate made of, e.g., glass, a multi-layer film of dielectric 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 band-pass filter which is used for the WDM(wavelength division multiplexing) optical communication system, temperature stability of the center wavelength of the band poses a problem when a narrow band width for passing wavelengths is set for applying the band-pass filter to a wavelength of a higher density. More specifically, the band-pass filter is a sensitive element in which the center frequency of the band varies even with a slight variation in temperature and, therefore, temperature compensation should be made by a temperature controller when the band-pass filter is used. Such temperature controller, however, cannot actually be employed because of limitation in the space where the band-pass filter is located. The temperature stability has become a matter of increasing importance since it is necessary to reduce the band width as the amount of light information increases.
In the past, amorphous glass has been used as a substrate for the band-pass filter. This prior art substrate is not sufficient in its compressive stress to the film and its durability since its thermal expansion property and mechanical strength are not sufficiently high. Further, amorphous glass has a low surface hardness and, moreover, a relatively large amount of alkali ingredient must be added if a high thermal expansion property is to be provided and this poses a problem of elution of alkali ingredient during and after forming of the dielectric film on the substrate. Thus, amorphous glass cannot sufficiently satisfy the demands for a substrate for a light filter, particularly a substrate for a band-pass filter.
It is, therefore, an object of the invention to provide a material suitable for a substrate for a light filter which has eliminated the above described disadvantages of the prior art substrate and has a thermal expansion property which is sufficient for avoiding variation in the refractive index at a temperature at which a filter formed with a mono-layer or multi-layer film is used (i.e., having a high coefficient of thermal expansion and thereby imparting compressive stress to the film to improve temperature stability of the refractive index of the film) and also has a mechanical property which imparts sufficient durability to the filter and further has excellent light transmittance.
It is, another object of the invention to provide a light filter made of such substrate.
SUMMARY OF THE INVENTION
Accumulated studies and experiments made by the inventors of the present invention for achieving the above described objects of the invention have resulted in the finding, which has led to the present invention, that glass-ceramics having a coefficient of thermal expansion, mechanical strength and light transmittance within specific ranges are suitable for achieving these objects of the invention.
According to the invention, there are provided glass-ceramics for a light filter having a coefficient of thermal expansion within a range from 95×10
−7
/°C. to 140×10
−7
/°C. within a temperature range from −20° C. to +70° C., having light transmittance for plate thickness of 10 mm which is 60% or over within a wavelength range from 950 nm to 1600 nm, containing, as predominant crystal phases,
(a) lithium disilicate, and
(b) at least one selected from the group consisting of &agr;-quartz, &agr;-quartz solid solution, &agr;-cristobalite and &agr;-cristobalite solid solution, and having a composition which comprises 2-7 weight percent of ZrO
2
and is substantially free of Nb2O
5
, SnO
2
, MoO
3
, TiO
2
, B
2
O
3
, Cr
2
O
3
and MnO.
In one aspect of the invention, the glass-ceramics have Young's modulus of 85 GPa or over.
In another aspect of the invention, the glass-ceramics have bending strength of 10 kg/mm
2
or over.
In another aspect of the invention, the glass-ceramics are substantially free of Na
2
O and PbO.
In another aspect of the invention, the glass-ceramics have a composition which further comprises, in weight percent:
SiO
2
70-77% 
Li
2
O
8-12% 
K
2
O
0.5-3%  
MgO + ZnO + SrO + BaO
1-5%
P
2
O
5
1.5-3%  
Al
2
O
3
3-9%
Sb
2
O
3
+ As
2
O
3
 0-2%.
DETAILED DESCRIPTION OF THE INVENTION
Reasons for limiting the thermal expansion property, Young's modulus, bending strength, light transmittance, composition and predominant crystal phases of the glass-ceramics for a light filter according to the invention will be described below. The composition of the glass-ceramics is expressed on the oxide basis (in weight percent) as in their base glasses.
Description will be made first about thermal expansion property. As described previously, the temperature stability of the center wavelength of the band is very important and glass-ceramics having a larger coefficient of thermal expansion than a material which constitutes the film are required. The studies and experiments conducted by the inventors of the present invention have resulted in the finding that, if the coefficient of thermal expansion within the temperature range from −20° C. to +70° C. is 95×10
−7
/°C. or over, sufficient compression stress can be imparted to the film within a temperature range in which the glass-ceramics are used as a band-pass filter and that, if the coefficient of thermal expansion exceeds 140×10
−7
/°C., difference in the coefficient of thermal expansion between the substrate and the filter becomes so large that problems such as separation of the film from the substrate take place. A preferable range of the coefficient of thermal expansion is 110×10
−7
/°C. to 130×10
−7
/°C. and a more preferable range thereof is 120±5×10
−7
/°C.
In the band-pass filter, the temperature stability of the center wavelength depends to some extent on refractive index temperature coefficient of a dielectric which constitutes the thin film and, to a larger extent than that, on a coefficient of thermal expansion of the substrate. This is because refractive index is determined by a film atomic density of the thin film. That is, the higher the film atomic density of the thin film is, the smaller becomes variation caused by the temperature of the center frequency. The film atomic density of the thin film is greatly influenced by the coefficient of thermal expansion of the substrate for the light filter on which the thin film is formed. More specifically, the temperature of the substrate during the film forming process becomes about 200° C. and the substrate thereby is considerably expanded. The thin film is formed on this expanded substrate and, as the substrate is cooled, the thin film is subjected to compressive stress due to difference in the coefficient of thermal ex

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