Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Reexamination Certificate
2001-08-14
2004-04-06
Sample, David (Department: 1755)
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
C501S069000, C501S070000, C501S073000
Reexamination Certificate
active
06716779
ABSTRACT:
BACKGROUND OF THE INVENTION
A. Field of the Invention
The present invention relates to glasses suitable for being shaped into small, flat windows used as substrates onto which thin film coatings of a suitable material are applied to form optical interference filters. More particularly, the invention relates to a glass composition in which relative proportions of components thereof may be varied to adjust the thermal expansion coefficient of the glass to relatively high values which are tailored to suit characteristics of particular thin film coatings, enabling fabrication of optical interference filters having transmission bands which shift minimally in wavelength in response to varying temperatures and other environmental stresses.
B. Description of Background Art
Optical interference filters fabricated by applying onto a thin, flat transparent glass substrate one or more coatings of dielectric or conductive films each having a thickness which is a multiple of one-quarter wavelength of a light of a particular wavelength are well known and widely used. Optical thin film interference filters are of various types, including low pass or high pass filters which transmit light having wavelengths longer or shorter than a particular cut-off wavelength, band pass filters which transmit only light having wavelengths within a particular band of wavelengths, and notch filters which transmit light over a range of wavelengths comprising a pass band while reflecting or absorbing light in a smaller range of wavelengths centered about a notch wavelength contained within the pass band. One common example of a thin film optical interference filter is the optical anti-reflection coating on the lenses of binoculars, which forms therewith a relatively wide band pass filter for light within the visible spectrum, minimizing reflections from the surface of objective lenses of light received from an object viewed and maximizing transmission of light through the eyepieces to the eyes of a viewer.
Performance requirements for band pass filters used in certain optical communications applications, such as Dense Wavelength Division Multiplexing (DWDM) are more demanding than those imposed on other applications for optical interference filters, as will now be explained.
In Dense Wavelength Division Multiplexing (DWDM) light energy in discrete wavelength bands is modulated with radio frequency signals containing audio, video or digital information including telephone conversations, television transmissions and digital computer data signals. In a typical DWDM system, an optical signal generated by a laser and having a center wavelength in the near infrared portion of the electromagnetic spectrum, e.g., 1.5 microns, (1500 nanometers) is subdivided into a plurality of wavelength bands which comprise separate optical carrier channels. Each optical carrier channel may have a bandwidth of about 0.2 nanometers, and be separated from one another by about 8 nanometers. The amplitude or phase of each of the optical carrier channel signals is modulated by a plurality of radio frequency sub-carrier channel signals, e.g., having a bandwidth of about 25 GHz and a channel separation of about 100 GHz. Each RF channel is in turn modulated at a lower frequency with information such as digitized telephone conversation signals, television signals or other digital data.
In a simplified example, a DWDM system may employ separate optical carrier channel signals each having a band width of 0.2 nanometer and center wavelengths of 1500, 1499.2, 1498.4, 1500.8, and 1501.6, nanometers, respectively. The plurality of optical carrier channel signals is optically combined or “multiplexed” onto a single optical beam, which may then be transmitted on a single optical fiber. Optical multiplexing may be performed using a resonant cavity filter, such as the one depicted in FIG. 3 of U.S. Pat. No. 5,953,134, the entire specification of which is incorporated herein by reference. The resonant cavity filter described therein employs a plurality of individual interference filters, each one being highly transmissive to light in a particular wavelength band, and highly reflective to all other wavelengths of light.
At the receiving end of an optical fiber or other transmission media through which combined or multiplexed optical signals are transmitted, a resonant optical cavity provided with a separate interference filter for each optical channel may be used to divide or “de-multiplex” the combined signal into separate optical beams which are arranged to impinge on a plurality of separate photo-detectors, one for each optical channel, thus allowing signal information contained on each optical carrier channel to be directed to appropriate destinations for the information signals on each channel, where the information may be recovered by demodulating the optical carrier signal.
Because of the very narrow bandwidth and close center wavelength spacing required of interference filters used for DWDM applications as described above, both the center wavelength and bandwidth must remain precisely fixed in spite of variations in ambient temperature, humidity, and other environmental conditions encountered by DWDM systems. Otherwise, data transmitted over adjacent optical channels could intermix, be reduced substantially in signal-to-noise ratio, or be lost entirely. Thus, the glass which is used for substrates onto which dielectric coatings are applied to form interference filters for use in DWDM applications must have properties which differ from those of existing glass compositions, for the following reasons.
Conventional glasses may be broadly categorized as “soft” or “hard.” Soft glasses typically have a linear coefficient of thermal expansion (&agr; or “CTE”) of greater than 60×10
−7
, while hard glasses usually have a CTE of less than 60×10
−7
. The softer glasses generally have a lower Young's modulus and are generally more subject to surface degradations by environmental conditions such as high temperatures, humidity and/or corrosive atmospheres. On the other hand, it has been determined that glass used as a substrate for receiving dielectric coatings to form highly wavelength-stable interference filters of the type required for DWDM applications should have a relatively high Young's modulus, to provide required dimensional stability, but must also have a thermal coefficient of expansion which is substantially larger than that typical of hard glasses. Moreover, it has been found that to maintain the center wavelength and bandwidth of optical interference filters stable enough for use in DWDM applications, the thermal expansion coefficient of the glass filter substrate must be rather precisely tailored to suit properties of the particular dielectric coatings applied to the substrate. Typical dielectric coating materials include oxides of titanium, tantalum, niobium, silicon and aluminum, and other substances. It is believed that better wavelength stability is obtained using glass substrates with higher coefficients of expansion, because the high CTE's tend to produce compressive stresses in metal oxide coatings deposited on the glass, when the glass cools down to ambient temperature after being heated to a temperature typically exceeding 200° C. during the coating process, which is typically done in a low pressure chamber.
In apparent recognition of the desirability of providing a glass with special properties for use as substrates for DWDM interference filters, NA0YURI, in Patent Publication Number EP 1081512 disclosed a glass for a light filter stated to be capable of preventing variations of refractive index in a band pass filter, to have a coefficient of thermal expansion within a range from 90×10
−7
/° C. to 120×10
−7
/° C. within a temperature range of −20° C. to +70° C., and, preferably a Young's modulus of 75GPa or over, a Vickers hardness of 550 or over, and light transmittance for plate thickness of 10 mm of 90% or over within a wavelength range of 950 nm to 1600 nm
Chapin William L.
OptoElectronics International, Inc.
Sample David
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