Fluoroborosilicate glass clad article and night vision device

There are disclosed fluoroborosilicate glasses that are particularly adapted to being drawn with lead silicate core glasses to produce clad glass fibers useful in forming fiber optic bundles to be incorporated in night vision equipment. The cladding glass has a refractive index not over about 1.45 and a coefficient of thermal expansion not over about 120.times.10.sup.-7 /.degree.C. The clad fiber has a numerical aperture approximating or equal to one.

INTRODUCTION 
This invention is concerned with fluoroborosilicate glasses having physical 
properties that particularly adapt them to use as cladding glasses for 
clad glass fibers. A specific application is the production of fiber optic 
bundles for use as optical elements in night vision equipment. 
These fiber optic bundles are composed of clad fibers having a high 
numerical aperture (N.A.), wherein the value of N.A. approaches, and 
desirably is equal to, one. Numerical aperture is a function of the 
refractive indices of two glasses employed as core and cladding glasses in 
an optical fiber. It is defined by the equation 
EQU N.A.=(N.sub.1.sup.2 -N.sub.2.sup.2).sup.1/2 
where N.sub.1 and N.sub.2 are the refractive indices of the core and 
cladding glasses, respectively. 
It is immediately apparent that, in order to obtain a high numerical 
aperture, glasses having widely divergent refractive indices must be 
employed. However, consideration must also be given to a number of other 
physical properties as well. Thus, to form a strong clad fiber, free from 
such flaws as cracks, striae, crystals and seeds, the core and cladding 
glasses must be compatible during sealing, redrawing and cooling steps. 
This means that consideration must be given to such properties as 
coefficient of thermal expansion, glass softness point, and phase 
separation tendency. 
It has been the practice, heretofore, to produce fiber optic bundles for 
night vision equipment by a multiple step process. A suitable core glass 
is cast into circular bars, and a cladding glass is drawn as tubing. A bar 
is then placed inside a length of tubing, and the glasses are redrawn as 
clad cane. This clad cane is packed into new bundles with other pieces of 
clad cane and redrawn again. After the redraw step is repeated a number of 
times, the resulting bundles are cut and polished on either end. They are 
then used as either faceplates or inverters (after twisting 180.degree.) 
in night vision goggles. 
Glass requirements are stringent. In addition to a high numerical aperture, 
it is desirable, but not critical, that the glasses have relatively 
similar viscosities, generally about 10.sup.4 -10.sup.6 poises, at an 
appropriate redraw temperature. The relative thermal expansions of each 
glass must be compatible with forming a clad cane. Finally, the glasses 
must not be susceptible to devitrification, or reaction with each other, 
during redraw. 
PURPOSES 
One purpose of the invention is to simplify the rather tedious procedure 
heretofore used in forming fiber optic bundles. 
Another purpose is to provide cladding glasses having properties such that 
clad fibers can be drawn directly from adjacent glass melts. 
A further purpose is to provide a family of fluoroborosilicate cladding 
glasses that are compatible with lead silicate core glasses having high 
refractive indices. 
A still further purpose is to provide fiber optic fibers having a numerical 
aperture equal to or approaching one. 
Yet another purpose is to provide a family of fluoroborosilicate glasses 
having a unique combination of properties including a refractive index not 
over 1.45, a linear coefficient of thermal expansion over the temperature 
range of 25.degree.-300.degree. C. of not over 120.times.10.sup.-7 
/.degree.C., a softening point below 600.degree. C., and a viscosity at 
the liquidus of not under 40,000 poises. 
A further purpose is to provide a clad glass fiber having a numerical 
aperture equal to or approaching one, and being composed of a core glass 
and a cladding glass wherein the cladding glass has a refractive index not 
over about 1.45 and a coefficient of thermal expansion not more than 
30.times.10.sup.-7 /.degree.C., and preferably not more than 
10.times.10.sup.-7 /.degree.C., greater than the core glass. 
SUMMARY OF THE INVENTION 
In furtherance of these and other apparent purposes, an aspect of my 
invention is a family of fluoroborosilicate glasses having compositions 
which, in weight percent, consist essentially of 35-52% SiO.sub.2, 8-23% 
Al.sub.2 O.sub.3, the SiO.sub.2 +Al.sub.2 O.sub.3 being at least 53%, 
10-23% B.sub.2 O.sub.3, 15-19% K.sub.2 O, the B.sub.2 O.sub.3 +Al.sub.2 
O.sub.3 being not over 36%, 0-8% Na.sub.2 O, 0-5% alkaline earth metal 
oxides (RO), and containing by analysis 6-12% F, the glasses having 
refractive indices not over about 1.45 and linear coefficients of thermal 
expansion not over about 120.times.10.sup.-7 /.degree.C. Preferably, the 
cladding glass consists essentially of 35-52% SiO.sub.2, 8-13% Al.sub.2 
O.sub.3, 17-23% B.sub.2 O.sub.3, 15-17% K.sub.2 O, 6-9% F, 0-5% alkaline 
earth metal oxides (RO) and 0-8% Na.sub.2 O. However, Li.sub.2 O, and 
refractory oxides such as ZrO.sub.2, should be avoided. 
In another aspect, my invention is a clad fiber having a numerical aperture 
equal to or approximating one, and wherein the cladding glass is a 
fluoroborosilicate having a composition that, in weight percent, consists 
essentially of 35-52% SiO.sub.2, 8-23% Al.sub.2 O.sub.3, the SiO.sub.2 
+Al.sub.2 O.sub.3 being at least 53%, 10-23% B.sub.2 O.sub.3, 15-19% 
K.sub.2 O, the B.sub.2 O.sub.3 +Al.sub.2 O.sub.3 being not over 36%, 0-8% 
Na.sub.2 O, 0-5% alkaline earth metal oxides (RO) and containing by 
analysis 6-12% F, the glass having a refractive index not over about 1.45 
and coefficient of thermal expansion not over about 120.times.10.sup.-7 
/.degree.C. 
Preferably, the cladding glass consists essentially of 35-52% SiO.sub.2, 
8-13% Al.sub.2 O.sub.3, 17-23% B.sub.2 O.sub.3, 15-17% K.sub.2 O, 6-9% F, 
0-5% alkaline earth metal oxides (RO) and 0-8% Na.sub.2 O, the cladding 
glass having a refractive index not over about 1.45 and a coefficient of 
thermal expansion that is not over 10.times.10.sup.-7 /.degree.C. greater 
than that of the core glass. 
The core glass may be a lead silicate glass having a refractive index of at 
least 1.76. Lead silicate glasses particularly suitable as core glasses 
consist essentially of, in percent by weight, 63-72% PbO, 26-32% 
SiO.sub.2, 0-6% BaO, the PbO+BaO content being 66-72%, 0-2% CaO, the 
BaO+CaO being 0-6%, 0-5% Al.sub.2 O.sub.3, and at least one of the alkali 
metal oxides selected from not over 5% Na.sub.2 O and not over 3% K.sub.2 
O, and 0-2% of As.sub.2 O.sub.3, the glass having a refractive index of 
1.76-1.78. 
The invention further contemplates a night vision device embodying a fiber 
optic bundle composed of clad fibers wherein the cladding glass is a 
composition that, in weight percent, consists essentially of 35-52% 
SiO.sub.2, 8-23% Al.sub.2 O.sub.3, the SiO.sub.2 +Al.sub.2 O.sub.3 being 
at least 53%, 10-23% B.sub.2 O.sub.3, 15-19% K.sub.2 O, the B.sub.2 
O.sub.3 +Al.sub.2 O.sub.3 being not over 36%, 0-8% Na.sub.2 O, 0-5% 
alkaline earth metal oxides (RO) and containing by analysis 6-12% F, the 
glass having a refractive index not over about 1.45 and coefficient of 
thermal expansion not over about 120.times.10.sup.-7 /.degree.C. 
Preferably, the cladding glass consists essentially of 35-52% SiO.sub.2, 
8-13% Al.sub.2 O.sub.3, 17-23% B.sub.2 O.sub.3, 15-17% K.sub.2 O, 6-9% F, 
0-5% alkaline earth metal oxides (RO) and 0-8% Na.sub.2 O, the cladding 
glass having a refractive index not over about 1.45 and a coefficient of 
thermal expansion that is not over 10.times.10.sup.-7 /.degree.C. greater 
than that of the core glass. 
PRIOR LITERATURE 
An article by Fraser and Upton, in the Journal of the American Ceramic 
Society, 27, 121-128 (1944) and entitled "Optical Fluor-Crown Glasses", 
reports studies on the effect of composition variations on optical 
properties of glasses used in optical elements. The composition components 
varied were silica, alumina, boric oxide, potash and fluorine. 
U.S. Pat. No. 2,407,874 (Fraser) discloses fluor-crown optical glasses 
containing silica, alumina, boric oxide, alkali metal oxide and fluorine. 
U.S. Pat. No. 2,433,882 (Armistead) discloses glasess colored green with 
cobalt and iron halides, containing oxides of silicon, boron, aluminum and 
sodium and/or potassium, and having a minor addition of fluoride. 
U.S. Pat. No. 3,671,380 (Ali et al.) discloses cladding glasses for clad 
fibers. The cladding glasses have relatively low refractive indices and 
are composed of B.sub.2 O.sub.3, SiO.sub.2, Al.sub.2 O.sub.3 and K.sub.2 
O. 
U.S. Pat. No. 3,764,354 (Ritze) discloses fluoroborosilicate glasses having 
low refractive indices and optical paths independent of temperature. The 
glasses consist essentially of SiO.sub.2, B.sub.2 O.sub.3, Al.sub.2 
O.sub.3, alkali oxide plus fluoride, Sb.sub.2 O.sub.3 and additional 
fluorine. 
U.S. Pat. No. 4,102,693 (Owen et al.) discloses photochromic borosilicate 
glasses having dispersed silver halide crystals and composed of SiO.sub.2, 
B.sub.2 O.sub.3, Al.sub.2 O.sub.3 and alkali metal oxide (R.sub.2 O). 
GENERAL DESCRIPTION 
The present invention arose from a search for cladding and core glasses 
having compatible properties that would enable their being drawn directly 
from melts as clad fiber having a numerical aperture equal to or 
approximating one. 
Such a drawing process is sometimes referred to as a double crucible, or 
double orifice, drawing process. It is referred to, for example, in 
connection with resistor cane production in U.S. Pat. No. 3,437,974 
(Spiegler). It is illustrated in U.S. Pat. No. 3,209,641 (Upton), and, in 
somewhat more complex, double clad tubing production, in U.S. Pat. No. 
4,023,953 (Megles, Jr. et al.). 
Briefly, in simple form, a core glass may be melted in, or transferred in 
molten form to, a cylindrical chamber having a drawing orifice. This 
chamber is surrounded by a second, concentric, cylindrical, chamber wall 
spaced from the first chamber. The cladding glass is melted in, or 
transferred to, this second chamber which also has a drawing orifice 
concentric with and surrounding the first orifice. The two glasses are 
allowed to flow out simultaneously and unite to form the desired clad cane 
as drawn. 
The properties required in new cladding glasses for drawing as clad cane 
are: (1) similarity in viscosity to a usable core glass at least within a 
range of forming temperatures, (2) low liquidus temperature and high 
viscosity at the liquidus temperature, (3) low refraction index, (4) 
reaction-free interface when applied as a cladding and (5) thermal 
expansion compatible with typical high lead silicate glasses. 
A tubing glass, available from Corning Glass Works under Corning Code 7052 
and heretofore used as cladding, has a refractive index of about 1.48. 
However, that index requires a core glass with a refractive index of at 
least 1.79 to provide a N.A. of one. Glasses having such high indices are 
known, but do not generally lend themselves to drawing as fiber or cane. 
Also, such glasses tend to have expansion and viscosity characteristics 
that negate compatibility in clad articles. 
A number of glass systems have relatively low refractive indices, but are 
subject to other deficiencies. Borate glasses are notorious for poor 
durability, as are fluorophosphates. The latter also have high thermal 
expansion coefficients and low liquidus viscosities. Fused silica is 
difficult to form even by itself. 
The fluoroborosilicate glasses, reported as optical element glasses, looked 
promising. However, initial melts tended to phase separate and/or exhibit 
rather high expansion coefficients. I have now found that the several 
requirements can be met by employing lead silicate glasses as core glasses 
with a narrowly limited range of fluoroborosilicate cladding glasses. 
The lead silicate core glasses generally have refractive indices of at 
least about 1.76 and thermal expansion coefficients on the order of at 
least 75.times.10.sup.-7 /.degree.C. and preferably near 
100.times.10.sup.-7 /.degree.C. Glasses having particular utility for 
present redraw purposes consist essentially of, in weight percent, 63-72% 
PbO, 26-32% SiO.sub.2, 0-6% BaO, the PbO+BaO content being 66-72%, 0-2% 
CaO, the BaO+CaO being 0-6%, 0-5% Al.sub.2 O.sub.3, and at least one of 
the alkali metal oxides selected from not over 5% Na.sub.2 O and not over 
3% K.sub.2 O and 0-2% As.sub.2 O.sub.3, the glass having a refractive 
index of about 1.76-1.78. 
My new cladding glasses must meet several requirements. Initially, as 
previously indicated, a numerical aperture of one requires a large 
difference in the refractive indices of the core and cladding glasses. For 
example, a core glass of about 1.77 requires a cladding glass with an 
index not over about 1.45. 
The liquidus of the cladding glass should be at least below the drawing 
temperature of the core glass. Further, the viscosities of the core and 
cladding glasses should be reasonably close at the forming or drawing 
temperature. Thus, it is generally desirable that the two glasses have 
viscosities of about 10.sup.4 -10.sup.6 poises at the drawing temperature. 
In general, optimum strength in a clad fiber requires that the coefficient 
of thermal expansion of the cladding glass be no more than 
10.times.10.sup.-7 /.degree.C. units above that of the core glass. 
Preferably, the coefficient is equal to or below that of the core glass. 
Accordingly, I prefer cladding glasses having compositions consisting 
essentially of, in percent by weight as calculated on an oxide basis, 
35-52% SiO.sub.2, 8-13% Al.sub.2 O.sub.3, 17-23% B.sub.2 O.sub.3, 15%-17% 
K.sub.2 O, 6-9% F and 0-8% Na.sub.2 O and 0-5% alkaline earth metal oxides 
(RO). 
However, I have found that, if a lesser degree of strength in the clad 
fiber can be tolerated, a surprising degree of mismatch can exist. 
Accordingly, cladding glasses may have a coefficient of thermal expansion 
as high as 120.times.10.sup.-7 /.degree.C. providing that is not over 
30.times.10.sup.-7 /.degree.C. units above the coefficient of the core 
glass and the ratio of the core glass diameter to the thickness of the 
cladding glass is not over about 20:1. In this broader aspect then, my 
glasses may consist, in percent by weight, essentially of 35-52% 
SiO.sub.2, 8-23% Al.sub.2 O.sub.3, the SiO.sub.2 +Al.sub.2 O.sub.3 being 
at least 53%, 10-23% B.sub.2 O.sub.3, 15-19% K.sub.2 O, the B.sub.2 
O.sub.3 +Al.sub.2 O.sub.3 being not over 36%, 0-8% Na.sub.2 O, 0-5% 
alkaline earth metal oxides (RO), and containing by analysis 6-12% F. 
These composition limits on the several glass components should be observed 
with reasonable care. Thus, at least 6% fluorine, by analysis, is required 
to maintain a softening point below 600.degree. C. and a low refractive 
index below 1.45. In general, not more than about 12% can be maintained in 
the glass, any excess being lost by volatilization. In order to provide 
analyzed contents of 6-12% F, about 10-15%, or more, should be 
incorporated in the glass batch. It will be appreciated, of course, that 
the amount lost by volatilization will depend somewhat on other 
constituents, and on melting conditions. 
Al.sub.2 O.sub.3 apparently serves to stabilize the fluorine and maintain 
it in the glass during melting. However, increasing Al.sub.2 O.sub.3 
hardens the glass and increases the liquidus. Hence, Al.sub.2 O.sub.3 
should not exceed 23%. The total content of SiO.sub.2 +Al.sub.2 O.sub.3 
should exceed 53% to restrain the expansion coefficient. 
B.sub.2 O.sub.3 tends to lower the expansion coefficient, as well as soften 
the glass, but may cause excessive F volatilization. However, with 
contents below about 10%, the glasses tend to phase separate and become 
opals. Above 23% B.sub.2 O.sub.3, F volatilization precludes analyzed F 
contents above 6%. 
K.sub.2 O softens the glass, but amounts over about 19% unduly increase the 
refractive index and the expansion coefficient. Amounts below 15% tend to 
make the glass too hard. 
In general, low coefficients of thermal expansion are favored by employing 
low Al.sub.2 O.sub.3, K.sub.2 O and F contents and high B.sub.2 O.sub.3 
content. Accordingly, my preferred compositions contain 8-13% Al.sub.2 
O.sub.3, 15-17% K.sub.2 O, 6-9% F and 17-23% B.sub.2 O.sub.3, 0-8% 
Na.sub.2 O and 0-5% alkaline earth metal oxides (RO).

SPECIFIC EXAMPLES 
The invention is further illustrated with reference to compositions for 
several cladding glasses as set forth in Table I. The compositions are 
given on an oxide basis, with fluorine being shown separately on an 
elemental basis. However, it will be appreciated that the fluorine is 
incorporated in a glass batch as fluorides, e.g., aluminum and/or 
potassium fluorides, and enters the glass structure in place of oxygen. 
The compositions are given in parts by weight. Since each composition 
totals approximately 100, the individual amounts may be taken as 
percentages by weight. 
A glass batch was mixed corresponding to each composition. The batches are 
melted in covered platinum crucibles for four hours at temperatures 
varying from 1250.degree.-1400.degree. C. as required for successful 
melting. The melts were poured into molds to form 6".times.6" (15 
cm.times.15 cm) patties which were annealed. The patties were divided into 
sections and prepared for measurement of various physical properties. The 
observed data are also recorded in Table I. R.I. represents refractive 
index. C.T.E. represents coefficient of thermal expansion expressed in 
terms of .times.10.sup.-7 /.degree.C. S.P. represents softening point in 
terms of .degree.C. The visual appearance of each sample is also recorded 
illustrating the absence of devitrification or opalization. 
TABLE I 
______________________________________ 
1 2 3 4 5 6 
______________________________________ 
SiO.sub.2 
46.6 46.7 42.8 36.8 49.1 39.6 
Al.sub.2 O.sub.3 
9.0 15.0 19.6 21.7 12.5 15.0 
B.sub.2 O.sub.3 
21.0 15.0 11.3 15.5 12.5 21.0 
K.sub.2 O 
15.3 15.3 15.8 15.8 17.8 15.3 
F (anal.) 
6.8 8.0 10.3 10.1 8.0 7.8 
R.I. 1.45 1.45 1.45 1.45 1.45 1.45 
C.T.E. 99 104 112 111 107 112 
S.P. 571 592 585 539 591 537 
Appearance 
clear clear clear 
clear clear 
clear 
______________________________________ 
By way of illustrating how small deviations in composition outside the 
prescribed limits can alter properties, compositions and properties for 
six additional glasses are shown in Table II below. 
TABLE II 
______________________________________ 
7 8 9 10 11 12 
______________________________________ 
SiO.sub.2 
41.6 39.7 32.7 40.6 36.4 44.0 
Al.sub.2 O.sub.3 
15.0 16.3 25.8 18.8 18.7 16.3 
B.sub.2 O.sub.3 
13.0 14.1 15.5 18.8 18.7 9.8 
K.sub.2 O 
20.3 16.6 15.8 13.5 17.7 16.6 
F (anal.) 
10.3 13.2 10.1 8.3 8.5 13.2 
R.I. 1.45 1.43 -- 1.46 1.45 -- 
C.T.E. 132 145 -- 90 123 -- 
S.P. 554 517 -- 612 528 -- 
Appearance 
clear clear opal clear clear 
opal 
______________________________________ 
Table III sets forth compositions for four glasses (Examples 13-16) that 
are particularly suitable for core glasses, as well as for two glasses 
(Examples 17 and 18) that are not suitable. Also included are physical 
properties as in Tables I and II plus annealing point (Ann.) and strain 
point (Str.) in .degree.C. and Internal Liquidus (Int. Liq.) also in 
.degree.C. 
TABLE III 
______________________________________ 
13 14 15 16 17 18 
______________________________________ 
SiO.sub.2 
30.5 29.4 27.5 28.3 27.0 27.5 
PbO 63.7 64.6 63.6 64.3 62.6 63.7 
BaO 5.6 1.8 5.6 3.1 5.5 5.6 
CaO -- 0 0 0.9 0 2.8 
Na.sub.2 O 
-- 1.6 3.1 1.6 0 0 
K.sub.2 O 
-- 2.4 0 1.6 4.6 0 
As.sub.2 O.sub.3 
0.2 0.2 0.2 0.2 0.2 0.2 
Soft (.degree.C.) 
605 535 531 546 548 620 
Ann. (.degree.C.) 
473 396 401 412 416 489 
Str. (.degree.C.) 
437 363 370 379 384 456 
CTE 75 99 100 94 104 82 
(.times. 10.sup.-7 /.degree.C.) 
R.I. 1.77 1.770 1.770 
1.765 
1.770 
1.785 
Int. Liq. 
-- 720 703 707 819 744 
(.degree.C.) 
______________________________________ 
The high K.sub.2 O content in Example 17, and the high CaO content in 
Example 18, appear to sharply increase the liquidus temperature. This 
interfered with drawing clad fiber which was produced by melting core and 
cladding glasses in a double crucible apparatus generally similar to that 
illustrated in the Upton patent mentioned earlier. 
To illustrate the mismatch in thermal expansion coefficients that can be 
tolerated under some circumstances, a lead silicate glass having the 
composition of Example 13 in Table III was melted in the inner chamber. 
The cladding glass was melted in the outer chamber, and had the 
composition shown in Table I as Example 2. The drawing temperature for the 
clad fiber was about 820.degree. C. 
The higher expansion coefficient of the cladding glass presented the 
potential problem of cracking. Accordingly, the ratio of core diameter to 
cladding thickness was varied during the run. Since a ratio of 16:1 was 
specified for product purposes, the ratio was set initially somewhat 
higher at 20:1. When no cracking or other defect was observed, the ratio 
was increased to 25:1. At this ratio some cracking was observed. This 
confirms that an expansion mismatch can be tolerated, but that it is 
desirable to minimize it if other requirements permit. No reaction at the 
interface between the glasses were observed.