Process for manufacturing GRIN lenses by melting a series of layers of frits

A method of manufacturing an optical device having a profile of refractive indices along its optical axis. A desired volume of each of a plurality of types of optical material are dispensed into a mold of known plan area in the form of a frit or a melt and the mold is heated to cause the materials to melt and fuse together to define a contiguous body of optical material having a desired profile. As the optical materials melt, the different types of material separate out so that they are arranged with the most dense material closest to the bottom of the mold and the least dense material closest to the top of the mold. To enhance this effect, the optical materials are layered in the mold in decreasing order of density of the materials from the bottom to the top so that the layers of optical material with the greater densities are closer to the bottom of the mold than the layers of materials with lesser densities.

TECHNICAL FIELD 
This invention relates to methods for making an optical device having a 
gradient of refractive index across its thickness which is useful in a 
process of manufacturing an optical lens having a predetermined gradient 
of refractive index along its optical axis. 
BACKGROUND 
Lenses which have a gradient of refractive indices (GRIN lenses) have been 
known for some time. These lenses have numerous uses in the optics, 
optical fiber and solar technology industries and are useful in designing 
compound lens systems using a single, integral lens or a reduced number of 
lenses. A GRIN lens can exhibit a change of refractive index along its 
optical axis or bi-directionally (both orthogonally radial to and along 
the optical axis). More complex GRIN lenses, which have changes in 
refractive index in three dimensions, are also known. 
An example of a lens which has a chosen gradient in its index of refraction 
both orthogonal to and longitudinally along an optical axis is illustrated 
in U.S. Pat. No. 4,883,522 to Hagerty. Other examples of GRIN lenses can 
also be found in U.S. Pat. No. 4,929,065 to Hagerty and in the documents 
referred to therein. In particular, the '065 Hagerty patent, the 
disclosure of which is incorporated herein by reference, discloses a 
method of manufacturing a GRIN lens which has a large change in index of 
refraction over a significant dimension along only its optical axis. The 
method disclosed in this patent requires that a number of glass wafers 
each having a different refractive index, be stacked on top of one 
another. The stack is then heated to above the fusion temperature of the 
individual wafers which fuse together to define a contiguous unit of 
optical material. The fused stack can then be ground to form the defined 
GRIN lens. 
This method, when used correctly, can produce a good quality GRIN lens with 
an accurately defined gradient in refractive index. The method does, 
however, have the disadvantage that it is a very expensive and time 
consuming to prepare the individual wafers required. This is partly 
because the wafers must be accurately ground to the desired thickness and 
must have very smooth surfaces to reduce the amount of air bubbles which 
would otherwise be trapped between the wafers as they are stacked to form 
a block. The required cutting, grinding and polishing can, in extreme 
cases, result in the loss of as much as 30% to 50% of the glass. 
In an alternative prior method of manufacturing GRIN lenses, two glasses 
are selected, the indices of which represent the end members of the 
gradient profile desired. Each is ground to a powder and then mixed 
together in suitable proportions calculated to create a series of 
mixtures, each of which when fused has the index required for the gradient 
profile. Each mixture is then carefully and successively layered into a 
platinum alloy mold. The mold with glass powder is placed into a furnace 
and slowly heated to fusion temperature, then held at a predetermined 
temperature for diffusion and slowly cooled. 
This method has the disadvantage in that the two glass types in the powder 
usually have different densities, with the higher density glass having a 
lower melting temperature. As the glass particles start to melt, the 
higher density glass melts first and tends to sink to the bottom of the 
mold and the less dense glass particles tend to move up toward the surface 
of the molten denser glass before they melt. As a result, separation of 
glass types within layers occur and mixing between layers occurs. In 
extreme cases the denser and less dense glasses separate into 
substantially two different layers, resulting in a body made up of 
essentially two layers having different refractive indices, as opposed to 
the continuous gradient in refractive index required. Because of this 
effect, it is very difficult to accurately control the gradient of 
refractive index using this process. 
Furthermore, it is very difficult to remove small bubbles of air that are 
trapped in the powder as it melts. This means that the final piece of 
optical material could have unacceptably large defects as a result of 
trapped air. 
Accordingly, a need has arisen for a process of manufacturing a GRIN lens, 
which has a gradient of refractive indices along its optical axis, which 
does not have the above disadvantages. This process should be able to 
produce a GRIN lens with a similar refractive index profile to the "wafer 
process" in the Hagerty patent above, produce a lens of high optical 
quality, and must be reproducible. 
SUMMARY OF THE INVENTION 
Briefly therefore, this invention provides a method for manufacturing an 
optical device from a plurality of layers of different optical materials 
such as different glasses, each of which materials having a different 
refractive index from the others, and in which the densities of the 
material varies with increasing refractive index. According to the 
invention, the layers are formed by successively dispensing in order of 
decreasing density a measured amount of each optical material into a mold. 
The materials are preferably dispensed in the form of a frit. 
Alternatively, they can be dispensed in molten form. The dispensing of the 
material results in a series of layers of such optical materials, each 
layer having a substantially uniform composition, in which the density of 
the materials of layers decreases from the bottom towards the top of the 
mold. By "uniform composition" is meant that, in the case of a frit, all 
the frit particles forming a layer have the same composition. The optical 
materials in the mold are then heated to fuse them together to define a 
contiguous body of optical material. 
The invention may be applied for the manufacture of an optical element 
having a desired profile of refractive index along its optical axis. In 
this case, the thickness of each layer of optical material is determined, 
for instance, by the technique described in the U.S. Pat. No. 4,929,065 in 
order that the fused optical element have the desired characteristic. 
Typically, the optical materials are glasses having different refractive 
indices. 
The products of the method of the present invention are also suitable for 
other uses such as optical limitors, optical intensity detectors, and 
optical switches. Because of the gradient of the mechanical properties of 
the products of the invention, sound will travel at different speeds at 
different locations within the products. Thus it is possible to make use 
of the invention in tunable electronic signal delayed devices and similar 
products. The products of the invention may also have decorative uses. 
When the material is dispensed into a mold of known plan area, the required 
weight of optical material of a given type needed to yield a layer of a 
given thickness may be calculated based on the volume of the layer (based 
on the plan area and the thickness of the layer) and the density of the 
optical material. This has the important advantage of allowing much more 
precise control over the thickness of each layer than prior art methods 
using stacks of plates of glass. The calculated weight of material can 
then be weighed out and dispensed into the mold. 
Preferably the material is layered into the mold in either solid 
particulate or molten form. Generally, the particulate form of the 
material should have particles in a size range of 0.1 to 30 mm, and 
preferably in the range of 0.1 to 6 mm in diameter. Most preferably, the 
particles are in the range of 0.5 to 4 mm. 
The particulate material can be produced by heating a larger piece of 
material and, thereafter, rapidly cooling it. This can be done by 
quenching or by melting the material and pouring it into a bath of cold 
water. If required, the particulate can also be formed by mechanical 
crushing. 
Other details of the method of the invention will become apparent from the 
following detailed description of the invention which is illustrated in 
the several figures of the drawing.

DETAILED DESCRIPTION OF THE INVENTION 
The method of this invention is described with reference to FIGS. 1(a) to 
(c) of the drawings in connection with an embodiment involving the 
manufacture of an optical element having a substantially linear gradient 
of refractive index across its thickness. 
This embodiment of the process of the invention assumes that the desired 
profile of refractive indices of a GRIN lens is known. It also assumes 
that the different glasses and the thickness of each of the different 
layers of these different glasses which will be used to make up the GRIN 
lens are known. Typically this information can be determined using the 
methods used to calculate such parameters in the "wafer process" disclosed 
in the Hagerty '065 patent above or in U.S. Pat. No. 5,262,896 to 
Blankenbecker, which is also incorporated herein by reference. 
Generally speaking, the glasses selected have different densities 
associated with their different refractive indices, with the density of 
the glass within a given family of glasses increasing with increasing 
index. As described below, the different glass types are fused together by 
melting to form a GRIN optical material having the desired profile. This 
fusing takes place in a mold, usually made of an inert material such as 
platinum, in which the glasses making up the GRIN lens are placed in a 
furnace. 
In terms of this invention and given that the plan area of the diffusion 
mold to be used can easily be determined, once the thickness of each layer 
of glass required is known, it is a simple process to calculate the volume 
of each type of glass that is needed for each layer. Furthermore, as the 
density of the glass in each layer is known, these calculated volumes can 
easily be translated into weights of glass required. 
Once the weight of the glass required in each layer has been determined and 
the glass has been cleaned to remove any surface impurities or dirt, 
accurately weighed batches of each glass type are made up. Preferably, the 
glass in these batches is in the form of glass chunks or a glass frit 
which had previously been prepared by methods described below. 
As shown in FIG. 1(a), the batches of glass frit are then successively 
layered into a mold 40, preferably made of platinum, with the particles 42 
of the batch containing glass of the highest density being layered into 
the mold 40 first. This is followed by successive layers of particles 44, 
46, 48 which are layered into the mold in order of descending glass 
density towards the top of the mold 40. This means that the glass with the 
highest density is at the bottom of the mold and that of the lowest 
density is at the top. 
Once this has been done, and as illustrated in FIG. 1(b), the mold is 
placed in a heated furnace to melt the glass in the different layers. This 
figure also shows that, if the different glasses have been selected from 
one "family" of glasses, for instance lead silica glasses, the glass with 
the lowest melting point is normally that of the highest density and 
highest index of refraction. 
This means that the layers of glass toward the bottom of the mold 40 melt 
first to form layers of molten glass 42', 44', 46' with the glass 
particles in the unmolten layer 48 "floating" on top. 
When the temperature is increased above the fusion temperature of the 
highest melting point glass, as illustrated in FIG. 1(c), the glass layer 
48' melts as well so that all the glass is in the molten state and the 
molten layers are fused with one another, by diffusion across their 
interfaces, to form one contiguous body of glass 50 which has the desired 
profile of refractive indices. 
As is evident from the above description, the process of the invention 
relies on the different densities of the glasses to maintain the layers of 
glass in their appropriate positions in the body of optical material. For 
best results, it is preferred that the difference in density between 
adjacent layers be at least 0.1 g/cc. Although the above description 
indicates that the glass of the highest density should be placed in the 
bottom of the mold, this is only a preferred way of arranging the glass. 
The glass could, for instance, be arranged in any convenient manner as the 
glass will in any event separate out into correctly ordered layers when it 
melts. In order for this to happen though, the glass may have to be kept 
at a given melting point or series of melting points for a longer period 
of time than in the layering method described. 
One method of producing a frit of glass particles suitable for weighing is 
to heat a piece of the selected glass to a temperature of between 
400.degree. and 500.degree. C. and then rapidly quench it in distilled 
water. The resulting thermal shock cracks the glass into small chunks with 
sizes ranging from about 0.1 mm to 20 mm in diameter. These glass chunks 
are then dried and may be stored for later weighing. If required, these 
chunks can be crushed using standard mechanical means. 
Another way of preparing the glass chunks is to pour molten glass directly 
into cold distilled water. This results in a fine glass frit with particle 
size range of about 0.1 to 10 mm in diameter. 
The following examples illustrate the process of the invention in the 
context of a GRIN lens: 
EXAMPLE 1 
("Wafer" Process): 
In this Example the prior art "wafer process" was used to produce a GRIN 
optical material which is used as a reference for the GRIN optical 
materials produced in Examples 2 to 5. For this Example and Examples 2 to 
5, seven lead silicate glasses, with properties as listed in the table 
below, were chosen: 
______________________________________ 
Thermal 
Expansion 
Density Coefficient (10.sup.-7 
Glass # nd (g/cm.sup.3) 
1.degree. C.) 
______________________________________ 
1 1.563 3.02 92 
2 1.594 3.28 92 
3 1.594 3.66 90 
4 1.672 4.05 89 
5 1.720 4.29 89 
6 1.765 4.77 91 
7 1.807 5.12 91 
______________________________________ 
For each glass type, cylinders 54 mm in diameter were core-drilled out of a 
30 mm thick block. These cylinders were then sliced into wafers about 2 mm 
in thickness using a diamond saw and fine-ground to a specified thickness 
to a tolerance of .+-.0.05 mm. 
The thicknesses of these glass wafers are: 
______________________________________ 
Glass 1 5.00 mm 
Glass 2 1.65 mm 
Glass 3 1.65 mm 
Glass 4 1.63 mm 
Glass 5 2.10 mm 
Glass 6 1.00 mm 
Glass 7 5.00 mm 
______________________________________ 
The top and bottom layers were deliberately made thicker than required to 
compensate for the removal of inhomogeneous or distorted regions which may 
occur at the ends of the glass block to be produced. Once cut and ground, 
the wafers were cleaned in an ultrasonic cleaner with alcohol as a 
cleaning solvent and thereafter stacked into a 55 mm diameter platinum 
mold with the wafers of higher density glass below those of lower density 
glass. 
This stack was then placed in a high temperature box furnace and treated 
by: 
(i) Raising the furnace temperature from room temperature to 1100.degree. 
C. at a rate of 20.degree. C./min and maintaining that temperature for a 
period of 2 hours; 
(ii) Lowering the furnace temperature to 1000.degree. C. at a rate of 
10.degree. C./min rate and maintaining that temperature for a period of 98 
hours; 
(iii) Then lowering the furnace temperature to 300.degree. C. at a rate of 
1.degree. C./min; and 
(iv) Thereafter shutting down the furnace power and allowing the stack to 
cool with the furnace. 
Samples of gradient index glass were then removed from the mold using 
core-drilling, and prepared for evaluation by means of a refractive index 
profile measurement and a wavefront quality test. The results of these 
tests are plotted on the graph in FIG. 2 and show that lenses produced by 
this method exhibit a nearly linear profile of refractive indices with 
only a 0.25 RMS wavefront distortion. 
EXAMPLE 2 
Using the same seven glasses as in Example 1, chunks of each glass type 
approximately 10.times.10.times.20 mm.sup.3 in size were cut using a 
diamond saw. These chunks were ultrasonically cleaned, and then the 
specific weight (with a tolerance of .+-.0.02 g) of chunks of each type of 
glass required to produce a lens of the same GRIN profile as that produced 
in Example 1 were weighed out. The diffusion was carried out in a 55 mm 
diameter circular cylindrical platinum mold. The weight of each glass 
layer was calculated from the mold size, density and desired thickness for 
each glass layer. The desired layer thickness are the same as those in 
Example 1. The weights of the 7 different layers are: 
______________________________________ 
Glass 1 35.87 g 
Glass 2 12.86 g 
Glass 3 14.35 g 
Glass 4 15.68 g 
Glass 5 21.40 g 
Glass 6 11.33 g 
Glass 7 60.82 g 
______________________________________ 
These glass chunks were then layered into the 55 mm diameter platinum mold 
with the layers of the more dense glass below those of the less dense 
glass, and treated to the same heating conditions as in Example 1. As a 
result of this heating, the different chunks of glass melted and fused 
together to form a solid, contiguous unit which was then tested using the 
same profile testing methods as before. The results of these tests, also 
plotted on the graph in FIG. 2, showed that this method produced a GRIN 
lens material having a better linear profile of refractive indices with 
the same slope as before and exhibiting the same 0.25 RMS wavefront 
distortion. 
A reason for the better profile in this Example is that the wafers used in 
the process described in Example 1 had a deviation in thickness of about 
0.05 mm. Considering that the average wafer thickness is about 1.5 mm, 
this means that the wafer thickness deviated by about 3.3%. For the 
process used in this Example 2, however, the deviation in weight is about 
0.02 g. Considering the average weight of each chunk is about 15 g, this 
translates into a deviation in weight of only about 0.13%. This means that 
the thickness of the molten glass layers during diffusion also varies by 
about .+-.0.13%. In other words, because the control of the weight of the 
chunks is much more precise and accurate than that of the wafer thickness, 
the resulting layer thicknesses in the final GRIN lens are much more 
precisely defined using this process of this invention. 
EXAMPLE 3 
Using the same seven lead silicate glasses as in Example 2, pieces of the 
different glasses were heated to 450.degree. C. and then quenched in cold 
water to form a frit with a particle size ranging from 0.5 to 20 mm. The 
different frits were then weighed to the same specified weights as in 
Example 2. These frits were then layered in the same platinum mold with 
the layers placed in order of decreasing density such that the layer of 
frit of the most dense glass is at the bottom of the mold and the layer of 
frit of the least dense glass is at the top. The layers of frit were then 
subjected to the same heating and subsequent testing processes as before. 
The results, which are also plotted on the graph of FIG. 2, show that using 
the frit of this type produced a GRIN lens material having a linear 
profile of refractive indices with the same slope as in Example 2 and 
exhibiting the same 0.25 RMS wavefront distortion. 
EXAMPLE 4 
In this Example the seven lead silicate glass frits made in Example 3 were 
crushed into particles of 0.1 to 10 mm in size. These finer frits were 
then used to produce a block of GRIN lens material in the same way as 
described above with reference to Example 3. This material was also tested 
in the same way as before. 
The results, which are once again plotted on the graph of FIG. 2, show that 
using this smaller frit also produced a GRIN lens material having a the 
same linear profile of refractive indices as in Example 2 and exhibiting 
the same 0.25 RMS wavefront distortion. 
EXAMPLE 5 
In this Example, samples of the same seven lead silicate glasses used 
before were remelted at 1100.degree. C. for 1 hr and then poured directly 
into cold water. The resulting thermal shock caused the glass to form a 
fine frit with a particle size ranging from 0.1 to 10 mm. These frits were 
used to produce samples of GRIN material as before. 
The results of subsequent testing, which are also plotted on the graph of 
FIG. 2 for comparison, show that using this very small frit also produced 
a GRIN lens material having a the same linear profile of refractive 
indices as before with the same 0.25 RMS wavefront distortion. 
EXAMPLE 6 
In Example 6, an alternative group of six different lead silicate glasses 
were chosen. As can be seen from their properties listed below, their 
refractive indices ranged from 1.734 to 1.964: 
______________________________________ 
THERMAL 
EXPANSION 
GLASS DENSITY COEFFICIENT 
TYPE INDEX (g/cm.sup.3) 
(10.sup.-7 1/C.) 
______________________________________ 
a 1.734 4.548 97 
b 1.779 4.902 96 
c 1.825 5.256 98 
d 1.858 5.663 97 
e 1.917 5.979 99 
f 1.964 6.279 98 
______________________________________ 
Using the same methods as before, the thickness of the different glass 
layers necessary for the desired GRIN profile were determined. As with the 
design in Example 1, the top and bottom layers were deliberately made 
thicker than required. This was to compensate for the removal of 
inhomogeneous or distorted ends which may be result from the manufacturing 
of the GRIN material. Once this was done, glass frits were made, in the 
same way as in Example 3, having a particle size ranging from 0.5 to 10 
mm. These frits were then weighed and arranged in a platinum mold in the 
same way as before. 
The diffusion was carried out in a 55 mm diameter circular cylindrical 
platinum mold. The weight of each glass layer was calculated from the mold 
size, density and required thickness for each glass layer. The weights of 
the 7 different layers are: 
______________________________________ 
Thickness (mm) 
Weight (g) 
______________________________________ 
Glass a 5.00 54.03 
Glass b 2.30 26.79 
Glass c 2.50 31.22 
Glass d 2.60 34.98 
Glass e 1.07 15.20 
Glass f 5.00 74.59 
______________________________________ 
Once again, the frits in the mold were subjected to heating in a high 
temperature box furnace. The heating procedure followed the following 
steps: 
(i) Heating from room temperature to 1000.degree. C. at a rate of 
20.degree. C./min and maintaining that temperature for a period of 2 
hours; 
(ii) Cooling to a temperature of 900.degree. C. at a rate of 10.degree. 
C./min rate and maintaining that temperature for a period of 40 hours; 
(iii) Further cooling to a temperature of 250.degree. C. at a rate of 
2.degree. C./min; and 
(iv) Shutting down the furnace power and allowing the stack to cool with 
the furnace. 
Samples of gradient index glass were then removed from the mold using 
core-drilling and prepared for evaluation by means of a refractive index 
profile measurement and a wavefront quality test as before. The results of 
these tests are plotted on the graph in FIG. 3 and show that the GRIN 
optical material produced exhibits a nearly linear profile of refractive 
indices with only a 0.25 RMS wavefront distortion. 
The results of Examples 2 to 6 show that the process of this invention can 
be used to make high quality optical axis GRIN glass with good 
repeatability. It is believed that the reason for the success of this 
process is because of the difference in densities among the lead-silicate 
glasses used. It is estimated that when the density difference between two 
neighboring glasses is larger than 0.1 g/cm.sup.3, the process of this 
invention will be suitable. 
Also, it should be noted that the Examples show that this process does not 
have a strict requirement as to the size of frit used. Frits with particle 
size ranges from 0.1 mm to 30 mm were used and all yielded approximately 
the same results. Nonetheless, a frit with particles ranging from 0.1 to 5 
mm is preferable as these particles are easier to handle and weigh. It has 
been found that by directly pouring molten glass into cold water (as 
described in Example 5) or some other wet or dry cold bath, will result in 
a frit with most particles being in this size range. Most preferably, the 
frit particles are in the range of about 0.5 to 4 mm. 
An alternative embodiment of this invention is illustrated in FIG. 4. In 
this embodiment, the required profile and thicknesses of the different 
layers of glass are known as before. From the layer thickness and mold 
plan dimension, the volume and/or weight of glass in each layer can be 
determined as before. 
A plurality of glass melters 60 are set up so that each can dispense a 
predetermined volume (or weight) of a specific glass into a mold 62. The 
melters 60 are set up so that the most dense glass is dispensed first 
followed by the less dense glasses in order of decreasing density. This 
process can be automated with molds 62 carried by a conveyor 64. Many 
molds 42 can be filled and processed together in a suitable furnace 66. 
Control of the dispensing of the molten glass can be achieved by a computer 
activated cutter, schematically shown as 68, so that the correct amounts 
of glass, in the correct order are dispensed to the mold 62. The process 
of this embodiment is particularly suited to making large diameter 
optical-axis GRIN lenses and to automation. 
One of the significant advantages of this invention is that the process of 
this invention gives much better control over the thicknesses of the 
different glass layers that make up the GRIN material. This is 
particularly so where the required lenses have larger diameters (greater 
than say, 100 mm in diameter). 
For example, it is very difficult and costly to make a glass wafer which is 
200 mm in diameter and only 1.5 mm thick with its front and back faces 
exactly parallel to each other. Even a 0.1 mm change in thickness can 
cause as much as a 7% change in the slope of the gradient of the profile 
of refractive indices. Using the method of the invention, however, the 
weight of the wafer, typically 200 to 300 grams, can be controlled to as 
accurately as .+-.0.05 grams; a deviation which will cause only a 0.03% 
variation in slope. 
Another advantage, which flows directly from the above advantage, is that 
this quality control is achieved using a process which is much less 
expensive that the prior art "wafer" process. 
Yet another advantage of the invention is that the original pieces if glass 
used to make up the chunks, frits or glass melts, do not have to be of the 
same high optical quality as in the "wafer" process of the prior art. This 
is because at least some stria and bubbles will be eliminated from the 
molten glass as it is kept under the prolonged high temperatures during 
diffusion. 
Still another advantage is that, as the glass in each layer melts, it 
assumes a flat and level upper surface that interfaces with the lower 
surface of the layer directly above it. In addition, if the mold is level, 
each layer will have a uniform thickness which is produced automatically 
without any expensive grinding or cutting. 
Although the present invention has been described above in terms of 
specific embodiments relating to optical material with a linear profile in 
refractive indices, it is anticipated that its principals can be applied 
in to other forms of profiles of refractive indices. Other alterations and 
modifications of the process of the invention will no doubt become 
apparent to those skills in the art. It is therefore intended that the 
following claims be interpreted as covering all such other applications, 
alterations and modifications as fall within the true spirit and scope of 
this invention.