Birefringent glass waveplate containing copper halide crystals

A birefringent waveplate that is composed of an integral, transparent, glass body, the glass body consisting of non-absorbing crystalline particles dispersed in a glassy matrix, the dispersed crystalline particles being selected from the group consisting of copper chloride, copper bromide and mixtures thereof, the dispersed crystalline particles having a high aspect ratio and being oriented and aligned along a common axis, whereby the waveplate is rendered birefringent so that polarized components of light transmitted through the waveplate have a phase shift introduced.

RELATED APPLICATIONS 
U.S. application Ser. No. 07/959,988, now U.S. Pat. No. 5,375,012, was 
filed Oct. 13, 1992 by N. F. Borrelli and T. P. Seward, III under the 
title BIREFRINGENT GLASS WAVEPLATE and assigned to the same assignee as 
the present application. It is directed to a birefringent waveplate 
composed of an integral, transparent, glass body having a thermally 
developed, dispersed phase composed of particles having a high aspect 
ratio and being oriented and aligned in one direction. 
U.S. application Ser. No. 08/270,052, now U.S. Pat. No. 5,517,356, was 
filed Jul. 1, 1994 by R. J. Araujo et al. under the title GLASS POLARIZER 
FOR VISIBLE LIGHT and assigned to the same assignee as the present 
application. It is directed to a glass polarizer that exhibits permanent 
dichroic behavior and that is effective across the entire visible spectrum 
of 400-700 nm. The glass has an R.sub.2 O--Al.sub.2 O.sub.3 --B.sub.2 
O.sub.3 --SiO.sub.2 base glass composition and a precipitated cuprous 
and/or cadmium halide crystal phase. The crystals are elongated, and at 
least a portion of the crystals are partially reduced to colloidal copper. 
FIELD OF THE INVENTION 
The field is birefringent waveplates and optical systems embodying such 
waveplates. 
BACKGROUND OF THE INVENTION 
A waveplate is also referred to as a linear phase retarder, or as a 
retarder plate. A waveplate introduces a phase shift between components of 
polarized light transmitted through the plate. It functions in an optical 
system to modify and control the relative phase of constituent beams. 
A waveplate is a body of material in which the refractive index differs 
along two unique orthogonal directions. As a result, light rays travel at 
different velocities in the two directions. Consequently, a ray 
transmitted in one direction is retarded relative to a ray transmitted in 
the other direction. In crystals, these two transmission directions are 
often referred to as ordinary and extraordinary ray directions. The path 
difference k.lambda. between the two rays, expressed in wavelengths, is 
given by 
EQU k.lambda.=.+-.l(n.sub.e -n.sub.o) 
where 
n.sub.e =refractive index of the extraordinary ray, 
n.sub.o =refractive index of the ordinary ray, 
l=physical thickness of the plate, and 
.lambda.=wavelength of the light ray. 
"k" can be considered the retardation expressed in integrals or fractions 
of a wavelength. The phase difference between two rays traveling through a 
birefringent material is 2.pi./.lambda. times the path difference. 
Therefore, the phase difference, called the plate retardation .delta., may 
be expressed as, 
##EQU1## 
Thus, if a phase difference of .pi./2 is introduced between the ordinary 
and extraordinary rays, the plate is termed a quarter-wave plate. The same 
characterization is true for any condition expressed by (2.pi.)m+.delta. 
when "m" is an integer. When "m" is zero, the term zero-order waveplate is 
used; when "m" is other than zero, the plate is termed a multiple order 
waveplate. 
The simplest retardation plate is a slice cut out of a uniaxial crystal, 
the slice being cut so that the optic axis lies in a plane parallel to the 
face of the plate. Heretofore, the principal materials used in waveplate 
production were crystalline materials such as quartz, calcite and mica. 
These crystalline materials are well recognized as being highly 
birefringent. Because of their large birefringent values, the thickness of 
a zeroth order waveplate would necessarily be impractically thin. For 
example, the thickness of such a plate would be on the order of 25 
microns. Therefore, a practical waveplate, produced from such crystalline 
materials, must be of a higher order, that is, a multiple of 2.pi. plus 
the phase difference. 
A recent publication by P. D. Hale and G. W. Day, "Stability of 
Birefringent Linear Retarders (Waveplates)", Applied Optics, 27 (24), 
5146-53 (1988), discusses various types of waveplates and their features. 
In particular, the publication discusses how retardance in the various 
types varies with temperature, angle of light ray incidence and 
wavelength. For example, the effect of a slight deviation in angle of 
incidence is magnified by the multiple order of retardation inherent in an 
integral, crystalline waveplate. The term "integral" indicates a unitary, 
crystalline waveplate composed of a single material. 
The authors conclude that, for a waveplate application requiring high 
stability, a low order, and ideally zero order, waveplate should be 
chosen. Since a zero-order, integral plate is impractically thin, it is 
common practice to resort to compound waveplates. Thus, to obtain a 
90.degree. retardation (quarter-wave), a positive plate of 
360.degree.+90.degree. is sealed to a negative plate of 360.degree.. This 
provides the desired 90.degree. retardation required with the multiple 
orders cancelling out. 
The related Borrelli-Seward application discloses a birefringent glass 
waveplate in which non-absorbing, oriented and aligned particles having a 
high aspect ratio are dispersed in the glass. The particles derive their 
birefringence through form birefringence. The glass is a phase-separable 
glass selected from the group consisting of lead borate and bivalent metal 
oxide silicate glasses and alkali metal oxide aluminosilicate glasses from 
which silver halide crystals are separated. 
These prior waveplates are highly advantageous. However, it would be 
desirable to provide an increase in the degree of phase shift per unit 
thickness. The present invention accomplishes this desired end by 
employing a glass having a different system of dispersed particles. 
SUMMARY OF THE INVENTION 
Our invention resides in a birefringent waveplate that is composed of an 
integral, transparent, glass body. The glass body comprises non-absorbing, 
crystalline particles dispersed in a glassy matrix, the dispersed, 
crystalline particles being selected from the group consisting of copper 
chloride, copper bromide and mixtures thereof, the dispersed crystalline 
particles having a high aspect ratio, and being oriented and aligned along 
a common axis, whereby the waveplate is rendered birefringent so that 
polarized components of light transmitted through the waveplate have a 
phase shift introduced.

PRIOR ART 
In addition to the art already noted, additional art is described in a 
separate document. 
DESCRIPTION OF THE INVENTION 
The related Borrelli-Seward application teaches that a birefringent glass 
can be produced by applying stress to, at a temperature near the glass 
softening temperature, a glass containing silver halide particles. The 
stress elongates the particles, and orients and aligns them in the 
direction of the stress. Initially, the birefringent property went 
unnoticed because the silver halide was subsequently reduced to a metallic 
state to render the glass polarizing. 
The present application is predicated on discovery that a glass containing 
copper bromide and/or chloride (CuBrCl) crystals may also be rendered 
birefringent by applying stress to elongate the crystals. As in the case 
of the glass containing silver halide crystals, the CuBrCl crystals must 
not be reduced to the metal. Quite surprisingly, it has been found that 
the degree of birefringence obtainable in a glass containing CuBrCl 
particles is substantially greater than that obtained in a silver halide 
glass. 
The present discovery has two significant effects. The thickness of a 
waveplate for a given degree of birefringence can be reduced. This is 
important where miniaturization and compactness are essential. Conversely, 
for a waveplate of given thickness, the degree of birefringence can be 
increased. This has the potential of meeting requirements for applications 
in telecommunication equipment. 
The birefringent mechanism is termed "form birefringence". The anisotropic 
behavior stems from the asymmetric depolarization fields of the elongated 
particle. That is, the dipole moment, .mu., is proportional to the local 
field, through the polarizability, .alpha., 
EQU .mu.=.alpha.E.sub.loc 
and the local field is made up of the external field and the depolarization 
field, which is taken to be proportional to the polarization, P, 
EQU E.sub.loc =E.sub.0 +4.pi.LP 
The depolarization factor, L, is a function of the shape, or form, of the 
imbedded particle. The values of L are defined for the convenient symmetry 
directions of the particle. The sum of the three values of L must equal 
unity. For a sphere, L.sub.a =L.sub.b =L.sub.c =1/3; for a long cylinder, 
L.sub.c =0, L.sub.a =L.sub.b =1/2, where the c direction is taken along 
the axial direction of the cylinder. 
The form birefringent property derives from the anisotropic nature of the 
polarizability. Eliminating E.sub.0 between the previous two equations 
yields the expression for the polarizability. This, in turn, can be 
expressed as refractive index difference .DELTA.n, as follows, for the two 
polarizing directions in the material, 
EQU .DELTA.n=(V.sub.f /2n.sub.0)(N.sup.2 -1){1/[L.sub.1 (N.sup.2 
-1)+1]-1/[L.sub.2 (N.sup.2 -1)+1]} 
In this equation, V.sub.f is the volume fraction of the separated phase, 
n.sub.0 is the average refractive index of the glass, N is the ratio of 
the refractive index of the separated phase to the surrounding phase, and 
L.sub.1 and L.sub.2 are the respective depolarization factors for the 1 
and 2 directions of the embedded particles. One should note that the 
larger the value of N is, the larger the resulting n for the same aspect 
ratio of the particle. 
The birefringent characteristics of a glass containing elongated, CuBrCl 
particles, taken with the inherent thermal and environmental stability of 
such a glass, renders it particularly useful for waveplate purposes. Thus, 
the degree of birefringence permits producing a zero order waveplate in an 
integral body having a practical thickness because the value of N is of 
the order of 2.1/1.5=1.4. In addition, the magnitude of the birefringence 
permits producing a zero order waveplate in an integral body having a 
practical thickness. With wavelengths in the visible range, plate 
thicknesses of 0.5 to 1.5 mm are possible. Somewhat greater thicknesses 
may be required in the infra-red range, but are still practical. 
For waveplate purposes, a glass containing CuBrCl particles is subjected to 
only the step of elongating the particles under stress, at an elevated 
temperature. Glass composition families and elongation conditions are 
described in considerable detail in the related applications. Both the 
composition and treatment conditions are equally applicable for present 
purposes. Accordingly, the disclosures of the applications are 
incorporated by reference herein in their entirety. 
In general, any glass containing CuBrCl particles, whether photochromic or 
simply phase separated, may be employed. Also, stress may be applied by 
any known means at a temperature generally in the range between the 
annealing temperature and the softening point of the glass. There are, 
however, other significant factors that must be considered in producing an 
acceptable waveplate. 
One of these factors is the amount of light scattering that can be 
tolerated. Light scattering is related to the size and aspect ratio of the 
particle involved. Therefore, the particle size of the CuBrCl produced 
during an initial glass heat treatment must be balanced against the degree 
of light scattering permitted. 
The ideal waveplate particle would have a high aspect ratio, preferably 
greater than 5:1. At the same time, it should be of minimal size in both 
dimensions to minimize light scattering. 
In the case of photochromic glasses, the photochromic behavior might need 
to be eliminated. Alternatively, a closed optical system, or a suitable UV 
blocking filter, may be employed. Otherwise, ambient UV radiation tends to 
activate the crystals and darken the glass. Thus, non-photochromic glasses 
are generally preferred. However, where the wavelength of use of the 
waveplate is beyond the wavelength of the photochromic absorption band, 
e.g. beyond 800 nm, no appreciable absorption is observed. 
As indicated earlier, the phase shift of a waveplate may be expressed as, 
##EQU2## 
where l is the thickness of the waveplate. The desire for a thinner glass 
waveplate has been recognized; also, the desirability of increasing the 
phase shift obtainable per unit thickness. 
The form birefringence, .DELTA.n, is determined by the aspect ratio of the 
elongated particle, the number of particles per unit volume, and the ratio 
of the refractive indices between the elongated phase and the matrix 
glass. The increase in the effect from the aspect ratio is controllable by 
the applied stress. It is essentially constant after an aspect ratio of 
about 5/1 is achieved. Further, the index ratio of elongated to matrix 
phase is difficult to change in a significant way. 
This leaves the particle number density as the most efficacious route to 
increasing the phase shift per millimeter. To a large extent, this is 
determined by solubility of the halide constituents that can be achieved 
in the glass; also, to the ability to bring those constituents out of 
solution upon reheating. 
The present invention then stems in large measure from our discovery that 
the CuBrCl system enables us to increase the amount of halide phase that 
can be precipitated from a glass. In particular, the amount of CuBrCl 
particles that can be precipitated per unit volume of glass is 
substantially greater than the amount of silver halide particles obtained 
in practicing the invention of the related application. 
Certain conditions are essential, in either a photochromic or 
non-photochromic type glass, to produce a CuBrCl crystal phase. The base 
glass must be an R.sub.2 O--Al.sub.2 O.sub.3 --B.sub.2 O.sub.3 --SiO.sub.2 
glass. The glass batch must contain a source of copper, as well as a 
source of halogen selected from chlorine and bromine. To produce the 
present birefringent glass, the following additives, in weight percent 
based on the glass, are considered desirable: 0.4-1.0% CuO, 0.5-1.0% SnO, 
and a halogen selected from the group consisting of 0.25-1.0% Cl, 
0.25-1.0% Br and 0.25-1.5% Cl+Br. 
The crystal phase may be precipitated in the glass as a formed article is 
cooled. However, it is generally desirable to cool the glass rapidly, 
thereby avoiding crystal development. Then, the glass may be reheated to 
precipitate the CuBrCl crystal phase. To this end, the glass is heated 
above its strain point, but below about 900.degree. C. Generally a 
temperature in the range of 650.degree.-850.degree. C. is preferred for 
this purpose, although temperatures in the range of 
500.degree.-900.degree. C. are contemplated. 
To provide CuBrCl crystals in the glass, the glass composition requires at 
least 0.2 weight percent cuprous oxide (Cu.sub.2 O), preferably at least 
0.4%. Up to about 2% Cu.sub.2 O may be employed, but cuprous ions tend to 
disproportionate into cupric ions and neutral atoms at such higher levels. 
Therefore, the preferred maximum Cu.sub.2 O content is about 1.0% by 
weight. The cuprous ion imparts no visible color to the glass, whereas the 
cupric ion generally provides a blue-green color. 
The oxidation state of the copper is influenced by the temperature at which 
the glass batch is melted, by the partial pressure of oxygen to which the 
molten batch is exposed, by the concentration of polyvalent ions in the 
glass, and by the basicity (the R-value) of the glass. The oxides of 
arsenic, antimony and tin are illustrative of polyvalent metal oxides that 
are especially useful since they do not directly impart color to the 
glass. 
Chlorine or bromine must be present to combine with the copper to form the 
necessary crystal phase. Iodine is also effective, but is not normally 
employed. The inclusion of fluorine may be useful, but it does not produce 
cuprous halide crystals in the absence of chlorine or bromine. 
A particularly significant control factor is the R-value, a measure of the 
basicity of a glass. This value is expressed in cation % on an oxide basis 
as calculated from the formula: 
##EQU3## 
M.sub.2 O designates alkali metal oxides, and MO represents alkaline earth 
metal oxides. Cuprous halide crystals can be developed in glasses with 
R-values below 0.15. Nevertheless, the development is slow, there is no 
substantial advantage in these glasses, they tend to be difficult to melt 
and they have poor chemical durability. Glasses with an R-value greater 
than 0.30 do not provide the desired crystal phase, except under certain 
compositional conditions. Glasses with a value over 0.45 are not suitable 
under any condition. Glasses with an R-value of about 0.25 are generally 
optimal for the development of a cuprous halide crystal phase. 
TABLE I, below, sets forth, in terms of oxides and halogens, the 
approximate ranges, in weight percent, for compositions for glasses in 
which CuBrCl crystals can be precipitated in the glass. The first column 
identifies the composition components; the second column, the ranges for 
non-photochromic (Non-PC) glasses; the third column, ranges for all 
glasses within the scope of the present invention (Cons.). 
TABLE I 
______________________________________ 
Comp. Non-PC Cons. 
______________________________________ 
SiO.sub.2 48-80 40-80 
B.sub.2 O.sub.3 10-35 4-35 
Al.sub.2 O.sub.3 0-12 0-26 
Li.sub.2 O 0-4 0-8 
Na.sub.2 O 0-14 0-15 
K.sub.2 O 0-12 0-20 
Li.sub.2 O+Na.sub.2 O+K.sub.2 O 
4-15 2-20 
CaO+BaO+SrO 0-10 0-10 
Cu.sub.2 O 0.2-1.6 0.2-2 
CdO 0-2 0-2 
ZrO.sub.2 0-12 0-12 
SnO.sub.2 0-2.5 0-2.5 
As.sub.2 O.sub.3 +Sb.sub.2 O.sub.3 
0-2 0-2 
Cl 0-1.75 0-2 
Br 0-1.0 0-2 
Cl+Br 0.25-2.0 0.25-2.0 
F 0-2 0-2 
R-value 0.15-0.45 
0.15-0.45 
______________________________________ 
TABLE II sets forth some typical photochromic glass compositions in terms 
of oxides and halogens. These compositions are calculated from the glass 
batch in parts by weight approximating 100. It will be appreciated that up 
to 25% of the copper, and up to as much as 60% of the halogen content, may 
be lost during melting of the batch. 
TABLE II 
______________________________________ 
1 2 3 4 5 6 
______________________________________ 
SiO.sub.2 
58.3 55.2 58.4 57.7 59.2 59.5 
Al.sub.2 O.sub.3 
9.0 12.0 9.0 9.0 9.5 11.4 
B.sub.2 O.sub.3 
20.0 20.0 20.0 20.0 20.1 17.4 
Na.sub.2 O 
10.1 10.0 10.0 10.0 4.4 5.7 
F 1.4 0.7 1.2 1.5 -- -- 
Cl 0.9 -- 0.9 0.9 0.5 0.5 
Br -- 1.5 -- -- 0.5 0.5 
Cu 0.5 0.3 0.5 0.9 0.4 0.58 
Cd -- 0.3 -- -- -- -- 
Li.sub.2 O 
-- -- -- -- 1.9 2.0 
K.sub.2 O 
-- -- -- -- 2.9 1.5 
SnO.sub.2 
-- -- -- -- 0.5 0.66 
______________________________________ 
TABLE III sets forth several typical compositions for non-photochromic 
glasses. The compositions are presented in terms of oxides and halogen 
contents as calculated from the batch in parts by weight approximating 
100. Again, analyses will show substantially lower copper and halogen 
contents. 
All of the compositions shown in TABLES II and III represent glasses 
suitable for producing articles in accordance within the present 
invention. 
TABLE III 
______________________________________ 
7 8 9 10 11 12 
______________________________________ 
SiO.sub.2 
56.6 52.6 77.5 56.5 55.8 72.8 
Al.sub.2 O.sub.3 
8.5 4.4 1.9 9.4 8.4 0.3 
B.sub.2 O.sub.3 
25.3 17.7 13.6 19.0 24.9 21.2 
Li.sub.2 O 
4.0 1.0 -- 1.1 2.6 0.4 
Na.sub.2 O 
3.6 7.8 3.9 8.5 6.3 1.2 
K.sub.2 O 
-- 1.3 -- 1.5 -- 1.4 
ZrO.sub.2 
-- 10.6 -- -- -- -- 
SnO.sub.2 
0.6 1.0 0.6 0.9 0.6 0.6 
CuO 0.4 0.8 0.5 0.8 0.4 0.5 
Cl 0.6 0.6 0.9 0.3 0.6 1.0 
Br 0.4 0.4 0.5 0.3 0.4 0.7 
F -- 1.8 0.8 1.8 -- -- 
______________________________________ 
Glass batches were formulated on the basis of compositions 5 and 6 using 
standard glass-making materials including sand, alumina, oxides, 
carbonates and halides. The batches were ballmilled to ensure homogeneity, 
and melted in covered crucibles. A 22 Kg (10 lb.) batch for each glass was 
formulated, mixed and melted for 6 hours at 1450.degree. C. For test 
purposes, bars having dimensions of 6.25.times.1.25.times.70 cms. 
(2.5".times.0.5".times.28") were poured from the melts into molds. 
Bars cast from the melts were selected for further treatment. The bars were 
heat treated for 75 minutes at the glass softening point to form the 
requisite crystal phase. The bars were then heat treated at different 
temperatures and stretched under different stresses to provide samples for 
birefringent measurements. 
TABLE IV sets forth the temperatures to which the bars were heated for 
stretching, the pulling stress at these temperatures and the time period 
during which stress was applied. Temperatures are in .degree.C.; pulling 
stresses are in Mp.multidot.as (psi); time in minutes; birefringence in 
degrees/mm. Birefringence was measured at a wavelength of 633 nanometers 
in radial degrees per mm of glass thickness. 
TABLE IV 
______________________________________ 
Glass Temp. Time Stress Birefringence 
______________________________________ 
5 725 75 19.3 (2800) 
128 
5 750 75 19.3 (2800) 
180 
6 700 60 20.7 (3000) 
116 
6 700 60 34.5 (5000) 
191 
6 725 75 20.7 (3000) 
186 
6 725 75 34.5 (5000) 
230 
______________________________________ 
Glass 5 contained 0.82.times.10.sup.20 Cu ions per cm.sup.3 ; glass 6 
contained 1.17 Cu ions per cm.sup.3. 
It is apparent from TABLE IV that birefringence is proportional to the Cu 
concentration. Thus, under identical conditions, the birefringence induced 
in glass 6 is 1.5 times that induced in glass 5. This correlates with Cu 
concentrations in the glasses. 
For comparison, a commercial photochromic glass containing 0.215 wt. % Ag 
(0.3.times.10.sup.20 Ag ions/cm.sup.3) as silver chloride was heated, 
stretched and measured. One sample was stressed for 60 minutes at 
700.degree. C. under a pulling stress of 40 Mp.multidot.as (5800 psi); a 
second sample was stressed for 60 minutes at 725.degree. C. under a 
pulling stress of 40 Mp.multidot.as (5800 psi). 
When measured at 633 nm, the first sample showed a birefringence of 125 
degrees/mm, the second sample measure 170 degrees/mm. The comparison shows 
that copper-containing glass 6 yields a 30-50% higher birefringence than 
the silver-containing glass when treated under similar conditions. 
The invention is further described with reference to the accompanying 
drawing wherein the single FIGURE is a schematic, perspective view of the 
components and operation of a circular polarizer 10. Thus, a beam of light 
12, from a laser 14, for example, passes through a polarizer 16 where it 
is linearly polarized. The beam then proceeds to, and passes through, a 
waveplate 18. If the crystalline optic axis 20 of waveplate 18 is set at 
an angle of 45.degree. to the input polarization plane 22, as shown, the 
emergent light is circularly polarized, that is, its electric field vector 
traces out a helical path 24 as it propagates.