CA 02307708 2000-OS-OS GLASS SUBSTRATE FOR NARROW BANDWIDTH FILTfiRS This invention generally relates to a glass substrate that may be used for an interference filter. $~;br~mnci nYTha invPnrinn Substrates that demand high expansion with good chzmical durability are often manufactured from optical glasses. Optical glasses may be rmploycd in various applications, such as substrates for interference filters used in fiber optic systems. Generally, these irnerferrnce filters are fabricated from multiple layers of conducting and 1 U insulating materials or films that Together result in a filter that passes only a narrow bandwidth of incident radiation. Such filters are described in Oprical Filter hesigrr and Ancrlys~s-A Signal Processing Approach by Christie K. Madsen and liars H. Zhao published by John Wiley & Sons, 1999. The thermal expansion value of a glass substrate can have important implications 1 S on device performance. As an example, a mismatch in thermal expansion between the glass substrate and a coating film can impose undue stress on the film. This stress can be calculated by the following formula: o = En""~ocOT where En,m is the Young's tnodulus ofthe film, E1a is the mismatch in thermal 20 expansion coefficient between the film and substrate, and DT is the shift in temperature from the preparation temperature ofthe film to the temperature ofuse, which is usually room temperature. One solution is to prepare and maintain the film at the t~mperatur~ of its intrndcd use. Howcvrr, transient stresses develop even for slight departures from the ?5 film creation temperature_ Therefore, it is highly desirablz to minimize the rmsmatch in thermal expansion between the film and the glass substrate. -1- CA 02307708 2000-OS-OS In one particular application, there is a strong demand for a glass substrate capable ofbeing incorporated into an interferCncc fihcr for dense wave division (DWD) or dense wave division multiplexing (DWDM) applications. Such interference filters have high requirements in narrowing the bandwidth of light transmission. These filters require bandwidths of less than 200GHz pass frequency in Lla2 1.~ ~m wavelength region. Desirably, these filters have a substrate That has a thermal expansion matching the coatings applied to the substrate. Generally, a substrate having a coefficient of thezmal expansion of about 90 - about 1?0 x lU-'/'C (-30.degree.C to +70.degree.C) would satisfy this requirement. Desirably, the substrate would also have a refractive index na = of about 1.50 - about 1.b0, a digital uansmtitance exceeding about 90% at a wavelength at about 1.5 ~cm, and good durability. Ahhough there are glasses such as F7 and PSK S~ provided by Schott Glass Technologies, Inc. having a coefficient of thermal expansion of 98 x 10-'/.degree.C and 119 x 10''/.degree.C> respectively, there are no current commercial glasses available having a combination of all these properties for use in various applications, such as interference filters. It is also desired that such substrate glasses not contain primary coloranu such as M~ Co. Ni. V, Fe. Cr> and Cu, e.g., because they can impede transmission or not contain rare earth additives such as Er oxide or Nd oxide, which can have absorption bands in the visible or near-infrared regions of the electromagnetic spectrum, or not include large amounts ofPbO, which can degrade chemical durability, or not include CoO, Ni02, Fe0 and Cu0 which can impede transmission in the near infrared region of the electromagnetic spectrum. Furthermore, it would be desirable to manufacture sn interference filter substrate eeonomically_ One such desirable process for making interference filter 2~ subsuates from optical glasses is tank melting. In this case, refining agents such as arsenic oxide and antimony oxide are ofren used during the tank melting process to produce glass with high optical quality and low hubble content. However, ofren optical glass substrates contain cerium oxide. Cerium can form a solarization couple with either of these compounds, resulting in browrimg of the glass when it is exposed to short wavelength radiation. The glass browning can cause the loss tn optical transmission. Consequently, glasses having cerium oxide would generally be undesirable for use as an interference filter substrate. -2- CA 02307708 2000-OS-OS Generally, the glass substrate of the prasent invention can provide a thermal expansion coefficient. a. of about 90 - about 130 x 10-'/.degree.C, preferably about 100 - less than about 119 or about 120 ?: 10-'I.degree.C, or optimally about 105 - about 115 x 10- '!.degree.C, over a temperature range of -30.degree.C to +70.degree.C_ hforcover, the glass substrate of the present invention preferably provides a refractive index value na from about 1.SU about 1.60 and a digital transmittancc exceeding about 90% at a wavelen~ h of 1.5 hem for a 1 millimeter thickness. Dzsirably, the substrate of the present in~r~ntion also has ?ood chemical durability. As a result, the glass substrates of the present invention are suited for various appllcatlons. Oxte exen?plary ~pplieation is a fiber optic system having an interference filter that includes a glass substrate ofthe present invention and, thrrcon, at least one interference film Iayer. Other possible uses of substrates of the present invention can include any sort of (narrow) bandwidth filter applicauor~s, especially when temperature variations occur after production. One desired embodiment of the invention is a glass substrate including in weight percent based on oxide; about 44 - about ~4 Si02; about 13 - about 28 of at least one alkali metal oxide; 0 - about 6 B:O j, and/or YZU3; about 2 - about 15 of at least one alkaline earth metal oxide and/or ZnO; and about 11 - about 16 of at least one colorless rare earth metal oxide. Preferably, the glass substrate includes a plurality of aJl;.ali metal oxides, such as about 7 - about 15 weight percent based on oxidz Na,O and about 6 - about 13 weight percent based on oxide K20. In addition, thr glass substrate can include 0 - about 2 weight percent based on oxide A12O3 and 0 - about 3 weight percent based on oxide ZrO2. Also, the glass substrate can include a refining agent, such as Asz03 or Sb=O~. Desirably, the glass substrate includes about 0.1 - about 1.0 weight percent based on oxide thereof, e.g., As2O,. What is more, the glass substrate can include at least one alkaline earth metal oxide selected from CaO, SrO, or HaO, andior ainc oxide, desirably about 2 - about ~ w~i~ht pzrcent baszd on oxide EaO. Furthermore, the glass substrate can include at least one colorless rare earth metal oxide selected _3_ CA 02307708 2000-OS-OS frozzl La=O3 or Ho~03. desirably about 11 to about 16 weight pcrccnt based on oxide, e.g., LazO;. In addition, the glasses of this invention can indude nnall amounts of other additives, e.g_, NbzO~, TiOa, ZrO~ at levels of up to about 3 weight percent on an oxide basis. As used herein, the term "colorless rare earth metal oxide" means a rare earth metal oxide substantially not having an absorption band between 140 - 150 manometers. Generally, glass substrates of the present invention can include in approximate weight percent based on oxide the following components and ranges: Table 1 Omd~ (.C'/o) Gcn~ral Preferred Optunal Rangy Raage Ratxge 510, 44-54 44-47 4~.~-46.5 8=~~ 0-6 2-6 4.5-S.S w1,0, 0-2 0-? 0-1 5 Y=O.~ D-6 0-4 0.3 S B20j, A1;0" Y:O, 0-6 2-6 4-~.5 Li~O 0-4 0-2 0-1.5 Na.O 7-IS 13-IS 14-1~ Kz0 6-13 6-12.~ 8-70 S" R-O where R = Li, 13-28 ?0-?7 Z2-z6 Na, 1: Ba0 0-5 2-5 3-.4.~ Mg0 0-5 0-4 D-3 Ca0 U-~ 0-4 0-3 SIO 0-5 0-4 0-3 Zn0 0-10 2-10 4-10 ~ R'G where R' - Ba, 2-15 5-15 8-15 Mg, Ca, Sr, Zn Ls=O, lI-I6 13-16 I4-16 TiO; D-3 1-3 1. ~-3 Nb,05 0-I 0-1 0-D.5 3 0 ZrO; 0-3 0-Z 5 0-2 ~t~.,0~ 0.1-1 0.1-1 0.1-0.5 -4- CA 02307708 2000-OS-OS Thesz glasses see prepared by selecting conventional starting materials corresponding to the end weight oxide percentages of the cooled glass, and using conventional melting techniques. Without fiu~her elaboration, it is believed that one skilled in the art can, using the preceding description, utiliza the present invention to its fullest extent. The following preferred specific ctnbodimatts are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever. In the foregoing and in the following examples, all temperatures are set forth uncorrected in 4egrees Celsius; and, unless otherwise indicated, a.ll parts arid percentages arc by weight. The entire disclosures of all applications, patents and publications, cited above or below are hereby incorporated by reference. The above-described glasses of the pmsent invention can be prepared as follows. Selected chemical compounds are weighed in desired amounts and melted within a crucible at tcmperaTUres in cxcc5s of 1200.degree.C to produce vitrified msterial. Once the melting is completed, the glass melt is stirred and refitted at temperatures up to 1450 .degree. C for three hours prior to casting the molten glass in a steel mold. The csst glass is held st X00.degree.C for tv~~o hours before being cooled io room temperature at a cooling rare e.g., of SO.degree.C/hr. Table 2 below depicts three glasses made according to this procedure. -S- CA 02307708 2000-OS-OS Table 2 Oxide (wr'~o)EXAMPLE E?CAIbIPLE EXAMPLE 3 1 2 SiO., 53.80 ~i.98 46 61 B_Os 1.97 S 10 5 32 $ Al,~, ~ 02 _.. 1.11 Li20 3.8~ 1.02 Na=O 7.27 14.28 14. 87 1C-O 9.32 12.3a 6 42 Ba0 2.16 4 08 2 93 I O 'ISO, 2 ~5 1.79 Zn0 9.49 4.32 La=O, 11.57 15.43 14.72 Nbs05 0.50 As..O, 0.20 0.20 0.20 1S F~:~,pI.F 4 Several measurrmcnts arc made for the glass composirions depicted in Table 2. These measurements include re&active index, cvzfficient of thermal expansion, digital percent transmission, Vd, Young's Modulus, Poisson's Ratio, thctznal conductivity, Tb, softening point, and density. The coefficient of thermal expansion is measured using 20 push-rod dilatomctric methods over -30.degree.C to =70.degree.C with a rate of change of 1.5 degrees C/min. The digital percent transmittance is measured at 1.5 microns (thiclrness 1 mm) usinb a Pcrkin-Elmer Model Lambda 9 device. The refractive index and Vd arc measured using a v-block refractometer in accordance with lournal of Scientific Instruments 1$ 23~4 (194~1). The Young's Modules and Poisson's Ratio are determined 25 by utilizixlg a Matec Pulse Echo Overlap Sysiem model 6600. The thermal conductivity is measured using a Dynatech C-Matic Thermal Conductance Tester model TCHM-DV. The Ts can be determined by using either a Harrop Labofatories Dilatometer Model .A.T- 710 or the Theta Industries Dilatomater Model 1200C. The softening paint can be determined using conventional instruments except the so$enirlg point temperature is 30 reported at the glass substrate's fixed viscosity of I O' 6 poise. These measurements are -6- CA 02307708 2000-OS-OS undertaken in substantial accordance with standard procedure. Tlle results of thcsc mcasurcments are depicted in Table 3. Table 3 Property EX4MPLE EXAMPLE EX.~.VIPLE 1 2 3 n 1.56367 1.56516 1.56842 Cocff:cicm of Tltcrmal93 113 101 expnnsion fz om -30' C I4 ~:-70 C ( 10''/K] Dzgxtal Percent Transmztiancc91.3% 91.4/" 91.2"/ at l.S~em (thzclazcss 1 mm) Vd 55.34 3212 S2.6a YounS's MoCUlus [Gpa]79 74 77 Poisson's Rxtxo 0.248 0.258 0 249 Thermal Conductivity (W/mC) 0.885 0.790 0.845 25 C 0.937 0.834 0.894 90C Tg ( C) 444 477 X72 Safrzning Point (C) 591 608 604 10' poise Density, p (gm/cm'] 2.87 2.88 2.91 As d~ypicted in Table 3, glasses of the present invzntion exhibit a refractive index within the range, 1.50 - 1.60, a coefficient of the~lal expansion from -30.degree.C to -r 70.degree.C u2thin the range, 90 - 130 x 10-~/.degree.C, and digital transmittance at 1.5 ~m and a thiclrness of 1 mm greater than 90.degree.l.degree.. Accozdirlgly, th,e glasses of the presem invention are desirable as substrates for interference filters especially for applications of less than ?00 GHz bandwidth fa~clge_ 2~ A glass of the present invention having a coefficient of thermal expansion (-30.degree.C to +70.degree.C) of about 107 x 10-'/.degree.C would have a composition as depicted below in Table 4: CA 02307708 2000-OS-OS Table 4 Orlde N~21~'hT PercenT Based on Oxldc SiO~ 4.96 B.,O, 5 .22 S Al=O, 1.07 LlaO 0.51 NaZO 14.63 K,O 9..~ 1 Ba0 3 54 2n0 2.17 TtO, 3.17 La_O, 1.10 As,03 0.21 The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants andlor operating conditions of this invention for those used in the preceding examples. From the foregoing description, one racilled in the art can easily ascertain the essential characteristics ofthis invention and, without departing from the spirit and scope thereof, can make various changes and rnodificattons of the invention to adapt it to various usages and conditions. _g_