Glass for a light filter and light filter

Details Glass for a light filter and light filter Glass for a light filter and light filter

2000-04-17

2002-10-08

Jones, Deborah (Department: 1775)

Stock material or miscellaneous articles

Composite

Of quartz or glass

C501S055000, C501S069000, C501S900000

Reexamination Certificate

active

06461733

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to glass for a light filter and also to a light filter using glass.
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. 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 having a coefficient of thermal expansion, mechanical strength and light transmittance within specific ranges is suitable for achieving these objects of the invention.
According to the invention, there is provided glass for a light filter having a coefficient of thermal expansion within a range from 90×10
−7
/° C. to 120×10
−7
/° C. within a temperature range from −20° C. to +70° C.
In one aspect of the invention, the glass has Young's modulus of 75 GPa or over.
In another aspect of the invention, the glass has Vickers hardness of 550 or over.
In another aspect of the invention, light transmittance for plate thickness of 10 mm is 90% or over within a wavelength range from 950 nm to 1600 nm.
In another aspect of the invention, the glass has a composition which comprises, in weight percent:
one or more ingredients selected from the group consisting of SiO
2
, B
2
O
3
and P
2
O
5
in the total amount of 35-55%;
one or more ingredients selected from the group consisting of TiO
2
, La
2
O
3
, ZrO
2
, Nb
2
O
5
, Ta
2
O
5
, WO
3
and Y
2
O
3
in the total amount of 20-45%;
one or more ingredients selected from the group consisting of MgO, CaO, SrO, BaO and ZnO in the total amount of 3-20%;
one or more ingredients selected from the group consisting of Li
2
O, Na
2
O and K
2
O in the total amount of 5-30%; and
one or both of Sb
2
O
3
and As
2
O
3
in the total amount of 0-1%.
In another aspect of the invention, the glass is substantially free of PbO.
In another aspect of the invention, there is provided a light filter which is made by forming a dielectric film on the above described glass.
In still another aspect of the invention, there is provided a light filter which is made by forming a dielectric film on glass having a larger coefficient of thermal expansion than dielectric which constitutes the dielectric film.
DETAILED DESCRIPTION OF THE INVENTION
Reasons for limiting the thermal expansion property, Young's modulus, Vickers hardness, light transmittance and composition of the glass for a light filter according to the invention will be described below. The composition of the glass is expressed on the oxide basis (in weight percent).
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 is 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 90×10
−7
/° C. or over, sufficient compression stress can be imparted to the film within a temperature range in which the glass is used as a band-pass filter and that, if the coefficient of thermal expansion exceeds 120×10
−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 95×10
−7
/° C. to 115×10
−7
/° C. and a more preferable range thereof is 105±5×10
−7
/° C.
In the band-pass filter, the temperature stability of the center wavelength depends to some extent on a 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

Affiliated with

Also associated with

Rating

Glass for a light filter and light filter does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Glass for a light filter and light filter, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Glass for a light filter and light filter will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2932212