Optical waveguides of aluminum garnet

Optical waveguides suitable for use in high temperature environments are constructed of a waveguiding body composed of a first crystalline aluminum garnet, which is clad with an epitaxially deposited layer of a second crystalline aluminum garnet. The second crystalline aluminum garnet has a lower refractive index than the first crystalline aluminum garnet.

FIELD OF THE INVENTION 
This invention provides optical waveguide/structures of crystalline 
aluminum garnet of a high refractive index which are clad with crystalline 
aluminum garnet of a lower refractive index. These clad waveguides can be 
in the form of fibers, slabs, channels, ribs, or any of the typical 
optical waveguide structures. They are useful at high temperature. 
BACKGROUND OF THE INVENTION 
Waveguides are structures which are used to conduct electromagnetic 
radiation from point to point, much as wire conducts electric current. In 
an optical waveguide, this electromagnetic radiation is light in either a 
narrow or broad range of wavelengths which may be contained in the visible 
spectrum, or the invisible spectra such as ultraviolet or infrared. 
All forms of optical waveguides have as a waveguiding medium a material of 
high refractive index imbedded in a medium of lower refractive index. As 
an example, a glass fiber of refractive index 1.45, suspended in a vacuum 
or air of refractive index 1.0, will act as an optical waveguide. More 
usually, such waveguides are clad with a material, necessarily of lower 
refractive index, to protect them from ambient conditions. Foreign 
material in contact with an unclad waveguide will reduce its transmission 
efficiency by scatter of the waveguided light out of the waveguiding 
medium, and thus the need for cladding. An example of a clad waveguide is 
a germania (GeO.sub.2) - doped silica (SiO.sub.2) glass fiber coated with 
a layer of silica glass (SiO.sub.2) for which there is a 1% difference in 
refractive index between the core and cladding. 
High temperature waveguides are commonly made of sapphire, a crystal form 
of the high melting point oxide Al.sub.2 O.sub.3 (melting point 
2054.degree. C.). Optical waveguides of sapphire have significant optical 
loss due to the lack of a suitable cladding material. A metal overcoat is 
used to protect such waveguides from the environment, but the transmission 
efficiency of this structure is low. A low loss optical waveguide requires 
a higher refractive index core surrounded by a lower refractive index 
cladding, and this is not provided in the metal-clad sapphire core 
waveguides. 
P. J. Chandler et al. [P. J. Chandler et al., Electron. Lett. 25, 985 
(1989)] have used an ion-implantation technique to produce a slab 
waveguide in the aluminum garnet (Y,Nd).sub.3 Al.sub.5 O.sub.12. This 
ion-implantation technique, unlike the technique of the present invention, 
makes use of the displacement of atoms in the crystal from their usual 
positions in the crystal lattice to generate regions of a small refractive 
index change. This ion-implantation technique is not suitable for use in 
high temperature waveguides, since the crystal structure will relax to its 
equilibrium state after exposure to high temperature. 
It is an object of the present invention to provide high temperature 
waveguides having a lower refractive index cladding. 
SUMMARY OF THE INVENTION 
This invention provides optical waveguide structures of crystalline 
aluminum garnet of a high refractive index which are clad with an 
epitaxial layer of an aluminum garnet having a lower refractive index. 
Suitably, the aluminum garnets for the higher refractive index body of the 
waveguide and for the epitaxial cladding layer are selected from aluminum 
garnets of the composition 
EQU R.sub.3 (Al,T).sub.5 O.sub.12 
wherein 
R represents one or more of the elements selected from the group consisting 
of calcium, magnesium, sodium, strontium, yttrium, lanthanum, 
praseodymium, neodymium, samarium, europium, gadolinium, terbium, 
dysprosium, holmium, erbium, thulium, ytterbium and lutetium; and 
T represents one or more of the 3-valent elements selected from the group 
consisting of gallium, indium, and scandium; 
with the provisos that 
(1) the molar ratio of the combined concentration of indium plus scandium 
to aluminum does not exceed 2:3; and that 
(2) if R is one or more of Na.sup.+1, Ca.sup.+2, Mg.sup.+2 or Sr.sup.+2, 
then T must include one or more charge-compensating ions selected from the 
group consisting of Fe.sup.+4, Ge.sup.+4, Hf.sup.+4, Ir.sup.+4, Mo.sup.+4, 
Nb.sup.+4, Os.sup.+4, Pb.sup.+4, Pt.sup.+4, Re.sup.+4, Rh.sup.+4, 
Ru.sup.+4, Si.sup.+4, Sn.sup.+4, Ta.sup.+4, Ti.sup.+4, Zr.sup.+4, 
V.sup.+4, W.sup.+4, As.sup.+5, Mo.sup.+5, Nb.sup.+5, Re.sup.+5, Sb.sup.+5, 
Ta.sup.+5, U.sup.+5, V.sup.+5, Mo.sup.+6, Re.sup.+6, W.sup.+6, and 
Re.sup.+7, in proportions sufficient to achieve an average cation charge 
of three in the crystal. 
More desirably, R represents one or more of the elements selected from the 
group consisting of calcium, magnesium, yttrium, lanthanum, praseodymium, 
neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, 
erbium, thulium, ytterbium and lutetium, and if R is one or both of 
Ca.sup.+2 and Mg.sup.+2, then T should include one or both of Ge.sup.+4 
and Si.sup.+4 in equimolar concentration relative to the Ca.sup.+2 and/or 
Mg.sup.+2 to achieve an average cation charge of three in the crystal. 
An especially suitable aluminum garnet for the cladding layer is yttrium 
aluminum garnet, Y.sub.3 Al.sub.5 O.sub.12 or "YAG" 
The R and T elements in the aluminum garnet for the waveguide body and for 
the cladding layer, and their proportions, are selected to give as large 
as possible a refractive index difference while still maintaining lattice 
constant matching. In the event YAG is chosen for the epitaxial cladding 
layer, then the aluminum garnet compositions of the types (Y,Lu).sub.3 
(Al,In).sub.5 O.sub.12, (Y,Lu).sub.3 (Al, Sc).sub.5 O.sub.12, 
(Tb,Lu).sub.3 Al.sub.5 O.sub.12 and Ho.sub.3 Al.sub.5 O.sub.12 have been 
found to meet the criteria of large difference in refractive indexes and 
lattice matching particularly well.

DETAILED DESCRIPTION OF THE INVENTION, OF THE PREFERRED EMBODIMENTS, AND OF 
THE BEST MODE PRESENTLY CONTEMPLATED FOR ITS PRACTICE 
Garnets are oxides of the general composition R.sub.3 T.sub.5 O.sub.12, 
wherein R and T respectively represent elements which form large and small 
ions of positive charge (cations). Garnets are resistant to chemical 
attack and high temperatures. There is much diversity in garnet 
composition, since R and T can be combinations of one or several elements 
cohabiting a crystal sublattice, and R and T range over much of the 
Periodic Table. 
Aluminum garnets, R.sub.3 Al.sub.5 O.sub.12, are mechanically strong and 
highly resistant to chemical attack. They are high temperature materials. 
As mentioned above, yttrium aluminum garnet (YAG) has a melting point of 
1947.degree. C. Other properties of YAG, which is a representative 
aluminum garnet, are a density of 4.55 g/cc, a hardness of 8.5 moh, a 
thermal conductivity at 300K of 0.13 W/cm/K, and a refractive index of 
1.84 at 550 nm. 
The aluminum garnets are optically transparent to long wavelengths. YAG is 
used as a host crystal for lasing ions, Nd:YAG being the prime example 
with laser emission at the infrared wavelength of 1.06 .mu.m. The melting 
point of the aluminum garnets is nearly the same as that of sapphire. One 
advantage that the aluminum garnets have over sapphire in high temperature 
waveguide applications is that the infrared absorption edge of the former 
extends to longer wavelengths, allowing efficient use of these waveguides 
at longer wavelengths of light. Another advantage of the aluminum garnets 
is the mature state of the art of their epitaxial growth. The epitaxial 
crystal growth process allows deposition of garnet layers on garnet 
substrates. Waveguiding and cladding layers can be deposited by the 
epitaxial crystal growth process, allowing fabrication of clad aluminum 
garnet optical waveguides. 
The common technique for the epitaxial crystal growth of garnet is the 
liquid phase epitaxy technique, more specifically the horizontal dipping 
technique with rotation, as developed by H. J. Levinstein et al., (Appl. 
Phys. Lett. 19, 486 (1971)). This liquid phase epitaxy technique was 
developed to a high state of the art in research on magnetic bubble memory 
materials. Magnetic bubble memory devices utilize epitaxial layers of rare 
earth iron garnet on gadolinium gallium garnet (GGG) substrates. Such 
layers must be nearly defect-free for proper device operation. 
The growth of an epitaxial garnet layer by liquid phase epitaxy proceeds as 
follows. A garnet substrate is carefully cleaned and mounted in a 
substrate holder which allows horizontal rotation and vertical 
translation. The substrate is then "dipped" by vertical translation into a 
tube furnace containing a platinum crucible holding the molten constituent 
oxides of the garnet which is to be epitaxially deposited. These oxides 
are dissolved in a suitable melt solvent, usually a lead oxide based 
solvent first heated to 1000.degree. C. and then supercooled to about 
20.degree. C. below the temperature at which garnet crystals will grow 
(the saturation temperature). 
The substrate submerged in the growth solution is rotated at about 100-250 
rev/min, and a garnet layer is epitaxially grown on the substrate at a 
rate of about 0.5-1.0 .mu.m/min. After time sufficient for growth of the 
desired layer thickness, the substrate is pulled vertically from the 
growth solution, and the clinging solution is "spun-off" at high speed. 
The substrate, now with an epitaxial layer, is removed from the furnace, 
and remaining traces of solidified growth solution are removed by 
treatment with a suitable solvent, usually hot nitric acid. 
Substitutions of elements in YAG can greatly increase the refractive index. 
Substitution of some of the aluminum by scandium to form yttrium scandium 
aluminum garnet (Y.sub.3 Sc.sub.2 Al.sub.3 O.sub.12 or "YSAG") increases 
the refractive index from 1.84 to 1.88 at visible wavelengths. A 
simultaneous replacement of yttrium by gadolinium and aluminum by scandium 
to form another aluminum garnet, gadolinium scandium aluminum garnet 
(Gd.sub.3 Sc.sub.2 Al.sub.3 O.sub.12 or "GSAG"), gives a refractive index 
of 1.97 at visible wavelengths. 
Fabrication of high temperature optical waveguides in accordance with the 
present invention involves cladding an aluminum garnet substrate of 
waveguiding structure with an epitaxial aluminum garnet layer having a 
lower refractive index than the substrate. The body of the optical 
waveguide within which the light is transmitted is always formed of a 
single crystal. Optical waveguiding structures of aluminum garnet can be 
fabricated in a variety of forms, such as fibers, slabs, channels, or 
ribs. For example, with reference to FIG. 1, an epitaxial layer 1 of an 
aluminum garnet of high refractive index and lattice constant match to YAG 
can be epitaxially deposited on a YAG substrate 2 and then epitaxially 
overcoated with a further epitaxial YAG layer 3 to form a "sandwich" 
structure in which the high refractive index waveguiding layer 1 is clad 
with the lower refractive index cladding layers 2 and 3 in a "slab" 
waveguide geometry. 
Similarly, as illustrated in FIG. 2, a fiber 21 of a single crystal higher 
refractive index aluminum garnet can be epitaxially coated with a layer 22 
of a lower refractive index aluminum garnet composition, such as YAG, to 
form a waveguiding fiber. 
A rib waveguide as illustrated by FIG. 3 can be produced by the same 
epitaxial process as for a slab, as illustrated by FIG. 1, except that the 
higher refractive index waveguiding layer 31 (which has been epitaxially 
deposited on lower refractive index substrate 32) is patterned into a 
"rib" before it is epitaxially clad with lower refractive index layer 33. 
Such "rib" waveguides have been produced in iron garnets by Pross et al. 
[E. Pross et al., Appl. Phys. Lett. 52, 682 (1988)]. Similar waveguides in 
iron garnet have been reported by R. Wolfe et al. [R. Wolfe et al., J. 
Appl. Phys. 56, 426 (1990); J. Appl. Phys. 57, 960 (1990)]. The difference 
in refractive index between waveguiding and cladding layers in the 
reported iron garnet waveguides is about 0.3%. 
A channel waveguide as illustrated by FIG. 4 is a variation of a rib 
waveguide in which the guiding aluminum garnet crystal 41 is deposited in 
a channel in the substrate 42, and then clad with a further layer 43. 
Optical waveguides of aluminum garnet, as provided by this invention, are 
most conveniently prepared by the liquid phase epitaxy crystal growth 
technique. The growth of an aluminum garnet crystal layer by liquid phase 
epitaxy on an aluminum garnet substrate, for example, a wafer of YAG, 
proceeds as follows: A substrate crystal of YAG, or YAG with a previous 
overgrowth of a garnet crystal, is carefully cleaned and mounted in a 
substrate holder which allows horizontal rotation and vertical 
translation. The substrate is then "dipped" by vertical translation into a 
tube furnace containing a platinum crucible holding the molten constituent 
oxides of the aluminum garnet which is to be grown dissolved in a lead 
oxide based solvent, or other suitable solvent as is known in common 
crystal growth practice. In the case of a lead oxide based solvent, this 
mixture (termed a "melt") is first heated to about 1150.degree. C. for a 
period of about eight hours, to homogenize the components, and then 
supercooled to about 20.degree. C. below the temperature at which garnet 
crystals will grow (the saturation temperature). 
After the substrate is dipped into the growth solution, it is rotated at 
about 100-250 rev/min, and an aluminum garnet layer is epitaxially grown 
on the substrate at a rate of about 0.5-2.5 .mu.m/min. After time 
sufficient for growth of the desired layer thickness, the substrate is 
pulled vertically from the growth solution, and the clinging solution is 
"spun-off" by rotation at high speed. The substrate, now with an epitaxial 
layer, is removed from the furnace, and remaining traces of solidified 
growth solution are removed in hot nitric acid. As this epitaxial crystal 
growth technique is in common and widespread use, the distinguishing 
features of this invention are the compositions of the epitaxial crystals 
and the compositions of the melts from which they are grown. 
Purity of starting materials is important, since many impurity components 
will cause optical absorption in the waveguides and reduce the 
transmission efficiency. For example, holmium, a rare-earth impurity, 
absorbs strongly at the wavelength of a red helium-neon gas laser, 632.8 
nm. The rare earths are chemically similar and difficult to separate, so 
that such impurity absorption is a common problem. In general, the purity 
of the rare earth components of a melt should be at least 99.9%, and the 
purity of the lead oxide solvent should be at least 99.999%. 
The difference in refractive index between the higher refractive index 
aluminum garnet single crystal waveguiding body and the lower refractive 
index epitaxial aluminum garnet coating should be at least least about 
0.02%, preferably at least about 0.1%, more preferably at least about 
0.5%. There is no upper limit on the difference in refractive indices. Any 
aluminum garnet combination having sufficiently different refractive 
indices is suitable for present purposes, so long as the lattice constants 
of these garnets are sufficiently close to permit epitaxial deposition of 
one on the other. To permit such epitaxial deposition, the lattice 
mismatch should not be larger than about 1.4%, desirably not larger than 
about 0.15%. Preferably, it is less than about 0.05%. 
The refractive index of aluminum garnet can be predicted to serve as a 
guide to composition selection for use in the waveguides of the present 
invention, as described by K. Nassau, Physics Today, September 1984, p. 42 
in an article entitled Dispersion--our current understanding. Briefly, the 
refractive index of aluminum garnets is a function of wavelength, and the 
ultraviolet and infrared absorption bands of the crystal. Knowledge of the 
absorption parameters (which can be readily determined using conventional 
procedures) allows calculation of the refractive index for a particular 
composition at any wavelength by the "Sellmeier" equation. For example, a 
linear combination of the refractive indices of the terminal aluminum 
garnet compositions (R.sup.1).sub.3 (Al,T.sup.1).sub.5 O.sub.12 and 
(R.sup.2).sub.3 (Al,T.sup.2).sub.5 O.sub.12 is sufficient to give the 
refractive index of any intermediate aluminum garnet composition 
(R.sup.1,R.sup.2).sub.3 (Al,T.sup.1,T.sup.2).sub.5 O.sub.12. 
The lattice constants of the aluminum garnets useful for making the present 
waveguides are determined using conventional X-ray diffraction procedures, 
as for example described in W. L. Bond, Precision Lattice Constant 
Determination, Acta Cryst. 13, 814-818 (1960); W. L. Bond, Precision 
Lattice Constant Determination: Erratum, Acta Cryst. A31, 698 (1975); and 
R. L. Barnes, A Survey of Precision Lattice Parameter Measurements as a 
Tool for the Characterization of Single-Crystal Materials, Mat. Res. Bull. 
2, 273-282 (1967). 
The Table I below lists illustrative sets of aluminum garnet compositions 
suitable for making the waveguide structure of the present invention with 
YAG as a substrate and YAG as an overcoating layer: 
TABLE I 
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Ho.sub.3 Al.sub.5 O.sub.12 
Tb.sub.1.63 Lu.sub.1.37 Al.sub.5 O.sub.12 
Tb.sub.1.47 Yb.sub.1.53 Al.sub.5 O.sub.12 
Tb.sub.1.10 Tm.sub.1.90 Al.sub.5 O.sub.12 
Tb.sub.0.61 Er.sub.2.39 Al.sub.5 O.sub.12 
Dy.sub.2.09 Lu.sub.0.91 Al.sub.5 O.sub.12 
Dy.sub.1.95 Yb.sub.1.05 Al.sub.5 O.sub.12 
Dy.sub.1.59 Tm.sub.1.41 Al.sub.5 O.sub.12 
Dy.sub.1.00 Er.sub.2.00 Al.sub.5 O.sub.12 
Gd.sub.1.33 Lu.sub.1.67 Al.sub.5 O.sub.12 
Gd.sub.1.17 Yb.sub.1.83 Al.sub.5 O.sub.12 
Gd.sub.0.84 Tm.sub.2.16 Al.sub.5 O.sub.12 
Gd.sub.0.44 Er.sub.2.56 Al.sub.5 O.sub.12 
Y.sub.0.78 Lu.sub.2.22 Sc.sub.0.52 Al.sub.4.48 O.sub.12 
Y.sub.0.66 Yb.sub.2.34 Sc.sub.0.44 Al.sub.4.56 O.sub.12 
Y.sub.0.44 Tm.sub.2.56 Sc.sub.0.29 Al.sub.4.71 O.sub.12 
Y.sub.0.21 Er.sub.2.79 Sc.sub.0.14 Al.sub.4.86 O.sub.12 
Dy.sub.0.67 Lu.sub.2.33 Sc.sub.0.45 Al.sub.4.55 O.sub.12 
Dy.sub.0.56 Yb.sub.2.44 Sc.sub.0.38 Al.sub.4.62 O.sub.12 
Dy.sub.0.37 Tm.sub.2.63 Sc.sub.0.25 Al.sub.4.75 O.sub.12 
Dy.sub.0.18 Er.sub.2.82 Sc.sub.0.12 Al.sub.4.88 O.sub.12 
Tb.sub.0.61 Lu.sub.2.39 Sc.sub.0.41 Al.sub.4.59 O.sub.12 
Tb.sub.0.51 Tb.sub.2.49 Sc.sub.0.34 Al.sub.4.66 O.sub.12 
Tb.sub.0.33 Tm.sub.2.67 Sc.sub.0.22 Al.sub.4.78 O.sub.12 
Tb.sub.0.16 Er.sub.2.84 Sc.sub.0.10 Al.sub.4.90 O.sub.12 
Gd.sub.0.55 Lu.sub.2.45 Sc.sub.0.36 Al.sub.4.64 O.sub.12 
Gd.sub.0.46 Yb.sub.2.54 Sc.sub.0.30 Al.sub.4.70 O.sub.12 
Gd.sub.0.29 Tm.sub.2.71 Sc.sub.0.20 Al.sub.4.80 O.sub.12 
Gd.sub.0.14 Er.sub.2.86 Sc.sub.0.09 Al.sub.4.91 O.sub.12 
Ca.sub.1.00 Tb.sub.2.00 Si.sub.1.00 Al.sub.4.00 O.sub.12 
Ca.sub.0.61 Dy.sub.2.39 Si.sub.0.61 Al.sub.4.39 O.sub.12 
Ca.sub.1.28 Gd.sub. 1.72 Si.sub.1.28 Al.sub.3.72 O.sub.12 
Ca.sub.1.88 Y.sub.1.12 Si.sub.1.88 Sc.sub.0.75 Al.sub.2.37 
O.sub.12 
Ca.sub.2.02 Dy.sub.0.98 Si.sub.2.02 Sc.sub.0.66 Al.sub.2.32 
O.sub.12 
Ca.sub.2.10 Tb.sub.0.90 Si.sub.2.10 Sc.sub.0.60 Al.sub.2.30 
O.sub.12 
Ca.sub.2.18 Gd.sub.0.82 Si.sub.2.18 Sc.sub.0.55 Al.sub.2.27 
O.sub.12 
Ca.sub.1.27 Lu.sub.1.73 Ge.sub.1.27 Al.sub.3.73 O.sub.12 
Ca.sub.1.12 Yb.sub.1.88 Ge.sub.1.12 Al.sub.3.88 O.sub.12 
Ca.sub.0.79 Tm.sub.2.21 Ge.sub.0.79 Al.sub.4.21 O.sub.12 
Ca.sub.0.41 Er.sub.2.59 Ge.sub.0.41 Al.sub.4.59 O.sub.12 
______________________________________ 
The required thickness of the guiding layer (the layer in which the light 
is being propagated) is a function of the relative refractive indices of 
the guiding layer, the cladding layers, the wavelength of the light to be 
guided, and the number of modes which are to be transmitted. Procedures 
for calculating the thickness of the guiding layer based on these 
parameters are well known to those skilled in the art of optical 
waveguiding. 
As to the thickness of the cladding layer, it can be zero, since 
waveguiding will still occur under conditions in which the waveguiding 
layer is exposed to air (refractive index 1), but the thickness of the 
cladding layer for usual operation is desirably large. Of course, there is 
no limits to the thickness, other than those dictated by practical 
considerations of relating to construction, expense of application, etc. 
In practice the thickness of the cladding layer should be large with 
respect to the ratio of the wavelength to the refractive index difference 
between waveguide and cladding layer. It can be made thinner if greater 
optical loss is tolerable under usual operating conditions at which the 
waveguide will be exposed to an environment of arbitrary refractive index. 
In general, the ratio of the thickness of the cladding layer to the ratio 
of the wavelength to the refractive index difference between waveguide and 
cladding layer should preferably be greater than 0.01, more preferably 
greater than 0.1. In practical operation, the thickness of the cladding 
layer will ordinarily be at least about 3 .mu.m, more desirably at least 
about 10 .mu.m; preferably at least about 25 .mu.m, and more preferably 
yet at least about 100 .mu.m. 
The following examples, which should be interpreted as illustrative rather 
than in a limiting sense, will further explain the present invention. 
EXAMPLE 1 
A melt was prepared for the epitaxial crystal growth of an aluminum garnet 
layer of composition (Y,Lu).sub.3 (Al,In).sub.5 O.sub.12, by melting 
together the oxides in the following proportions: 
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PbO 952.87 g; 
B.sub.2 O.sub.3 
24.77 g; 
Al.sub.2 O.sub.3 
8.04 g 
In.sub.2 O.sub.3 
8.75 g 
Y.sub.2 O.sub.3 
2.81 g 
Lu.sub.2 O.sub.3 
3.30 g 
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An epitaxial layer of the approximate composition Y.sub.2 Lu.sub.1 
Al.sub.4.7 In.sub.0.3 O.sub.12 was grown by the liquid phase epitaxial 
crystal growth process detailed above on a substrate wafer of YAG, thereby 
producing a slab waveguide. Growth conditions and product properties were 
as follows: 
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growth temperature: 926.5.degree. C. 
growth rate: 1.17 .mu.m/min 
thickness: 2.34 .mu.m 
lattice constant (Angs.): 
12.0150 
refractive index (at 633 nm) 
1.8424. 
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Light from a helium-neon gas laser was guided in this slab waveguide using 
the conventional prism coupling technique, employing a rutile prism. This 
waveguiding allowed measurement of the refractive index of the epitaxial 
layer, as shown above. 
EXAMPLES 2-5 
Melts were prepared for the epitaxial crystal growth of aluminum garnet 
layers of composition (Tb,Lu).sub.3 Al.sub.5 O.sub.12, as detailed in 
Table II, below: 
TABLE II 
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Composition of Melt in Grams for the Growth of Optical 
Waveguides of Composition (Tb,Lu).sub.3 Al.sub.5 O.sub.12 
on YAG Substrates. 
PbO B.sub.2 O.sub.3 
Al.sub.2 O.sub.3 
Tb.sub.2 O.sub.3 
Lu.sub.2 O.sub.3 
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Example 2 
602.71 15.67 5.30 3.25 1.90 
Example 3 
602.71 15.67 5.99 3.85 1.97 
Example 4 
602.71 15.67 5.99 3.85 1.97 
Example 5 
477.14 12.40 5.30 3.25 1.90 
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Epitaxial layers of the approximate composition Tb.sub.1.75 Lu.sub.1.25 
Al.sub.5 O.sub.12 were grown by the liquid phase epitaxial crystal growth 
process detailed above on substrate wafers of YAG. Growth conditions and 
product properties were as described in Table III, below. Light from a 
helium-neon gas laser was guided in these slab waveguides by the prism 
coupling technique, using a rutile prism. This waveguiding technique 
allowed measurement of the refractive index of the epitaxial layers also. 
TABLE III 
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Properties of Slab Waveguides of (Lu,Tb).sub.3 Al.sub.5 O.sub.12 
Epitaxially Grown on YAG Substrates. 
Example No.: 2 3 4 5 
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Growth Temp (.degree.C.): 
928.0 950.5 951.5 952.0 
Growth Rate 0.78 1.48 0.90 3.53 
(.mu.m/min): 
Thickness (.mu.m): 
3.91 7.42 4.51 4.97 
Ref. Ind. (at 633 nm): 
1.8535 1.8544 1.8540 1.8547 
Lattice Const. (Angs.): 
11.9972 12.0073 -- 12.0126 
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To further illustrate the waveguiding nature of these epitaxial layers, the 
effective refractive index of several of the guiding modes was measured at 
the 632.8 nm wavelength of a helium-neon laser. The results are 
illustrated in Table IV, below. Also shown in Table IV are the calculated 
refractive indices for these modes, based on an ideal model of a step 
change of refractive index between the YAG substrate and the waveguiding 
layer. 
TABLE IV 
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Guided Modes in the Slab Waveguide of 
(Tb,Lu).sub.3 Al.sub.5 O.sub.12 on YAG, of Example 3, at 632.8 nm. 
Mode Number 
Calc. Ref. Index 
Measured Ref. Index 
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1 1.8539 -- 
2 1.8524 1.8525 
3 1.8500 1.8501 
4 1.8466 1.8466 
5 1.8422 1.8423 
6 1.8369 1.8370 
7 1.8308 1.8313 
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Optical transmission loss measurements were made on the (Lu,Tb).sub.3 
Al.sub.5 O.sub.12 slab waveguide of Example 4. Light was guided into the 
epitaxial layer by prism coupling using a rutile prism, and the intensity 
of scattered light along the waveguiding track was probed with a 
fiberoptic cable. Measurement of the light intensity along the track as a 
function of position gives the optical loss directly if reflected light 
from the edge of the wafer does not follow along the same track. Loss 
measurements along five different waveguiding tracks in the layer (Table 
V) gave an optical loss of 1.1.+-.1.2 dB/cm. An optical loss of the order 
of 1 dB/cm is considered adequate for most applications. 
TABLE V 
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Optical Loss for a Guided Mode in 
(Tb,Lu).sub.3 Al.sub.5 O.sub.12 Layer (Example 4) at 632.8 nm 
Measurement No. 
Loss (dB) Path (cm) dB/cm 
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1 0.6 1.0 0.6 
2 1.4 1.2 1.2 
3 1.1 1.2 0.9 
4 1.1 0.6 1.8 
5 0.5 0.5 1.0 
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EXAMPLE 6 
A melt was prepared for the epitaxial crystal growth of aluminum garnet 
layers of composition (Tb,Lu).sub.3 Al.sub.5 O.sub.12, by melting together 
the oxides in the following proportions: 
______________________________________ 
PbO 765.12 g; 
B.sub.2 O.sub.3 
19.89 g; 
Al.sub.2 O.sub.3 
7.61 g 
Tb.sub.2 O.sub.3 
4.88 g 
Lu.sub.2 O.sub.3 
2.50 g 
______________________________________ 
An epitaxial layer of the approximate composition Tb.sub.1.75 Lu.sub.1.25 
Al.sub.5 O.sub.12 was grown at 958.degree. C. at a growth rate of 1.97 
.mu.m/min. by the liquid phase epitaxial crystal growth process detailed 
above on a substrate wafer of YAG. The thickness of the epitaxial layer 
was measured to be 9.8 .mu.m. An optical loss measurement at 632.8 nm was 
performed on this slab waveguide using the dual prism method, wherein 
light is coupled into the waveguide by a prism made of the high refractive 
index material rutile, and then extracted from the waveguide by another 
rutile prism. The distance between prisms fixes the optical path length, 
and the optical loss is readily calculated from measurement of the 
intensity of the incident and the recovered light. The optical loss for 
this waveguide was found to be 1.22 dB/cm. 
EXAMPLE 7 
A melt was prepared for the epitaxial crystal growth of aluminum garnet 
layers of composition (Tb,Lu).sub.3 Al.sub.5 O.sub.12, by melting together 
the oxides in the following proportions: 
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PbO 760.10 g; 
B.sub.2 O.sub.3 
19.76 g; 
Al.sub.2 O.sub.3 
10.24 g 
Tb.sub.2 O.sub.3 
6.96 g 
Lu.sub.2 O.sub.3 
2.94 g 
______________________________________ 
This melt composition can be described more generally by the following 
molar ratios of the components: 
EQU Al.sub.2 O.sub.3 /(Tb.sub.2 O.sub.3 +Lu.sub.2 O.sub.3)=3.8 
EQU PbO/2B.sub.2 O.sub.3 =6 
EQU (Al.sub.2 O.sub.3 +Tb.sub.2 O.sub.3 +Lu.sub.2 O.sub.3)/(Al.sub.2 O.sub.3 
+Tb.sub.2 O.sub.3 +Lu.sub.2 O.sub.3 +B.sub.2 O.sub.3 +1/2PbO)=0.06 
EQU Tb.sub.2 O.sub.3 /(Tb.sub.2 O.sub.3+Lu.sub.2 O.sub.3)=0.72 
EQU Lu.sub.2 O.sub.3 /(Tb.sub.2 O.sub.3+Lu.sub.2 O.sub.3)=0.28 
This melt has a saturation temperature of about 1070.degree. C. and a 
growth temperature of about 1055.degree. C. Epitaxial layers of the 
approximate composition Tb.sub.1.75 Lu.sub.1.25 Al.sub.5 O.sub.12 were 
grown from this melt on YAG substrate wafers to be overcoated with a 
cladding layer of YAG in order to fabricate clad waveguides of aluminum 
garnet. 
A melt for the liquid phase epitaxy of YAG was formulated with the 
composition listed below to overcoat the optical waveguiding layer of 
(Tb,Lu).sub.3 Al.sub.5 O.sub.12 with a cladding layer of YAG. 
______________________________________ 
PbO 765.24 g; 
B.sub.2 O.sub.3 
19.89 g; 
Al.sub.2 O.sub.3 
9.40 g 
Y.sub.2 O.sub.3 
5.48 g 
______________________________________ 
This melt composition can be described more generally by the following 
molar ratios of the components: 
EQU (Al.sub.2 O.sub.3 /Y.sub.2 O.sub.3)=3.8 
EQU (PbO/2B.sub.2 O.sub.3)=6.0 
EQU (Al.sub.2 O.sub.3 +Y.sub.2 O.sub.3)/(Al.sub.2 O.sub.3 +Y.sub.2 O.sub.3 
+B.sub.2 O.sub.3 +(1/2)PbO)=0.055 
This melt produced epitaxial layers of YAG at a growth temperature of about 
1095.degree. C. at a growth rate of about 1.5 .mu.m/min. Two 
(Tb,Lu)-aluminum garnet optical waveguides prepared previously on YAG 
wafers were epitaxially clad with YAG by this melt. The properties of the 
finished clad waveguides (Guide A and Guide B) were as follows: 
______________________________________ 
Guide A 
Guide B 
______________________________________ 
Guiding Layer: 
layer thickness (.mu.m) 
25.6 11.8 
growth rate (.mu.m/min) 
1.71 2.36 
refractive index at 632.8 nm 
1.8545 1.8545 
Clad Layer 
layer thickness (.mu.m) 
3.26 2.83 
growth rate (.mu.m/min) 
1.63 1.41 
refractive index at 632.8 nm 
1.8284 1.8288 
______________________________________ 
The waveguides of the present invention are particularly suited for 
controlled transmission of light in high temperature environments, as, for 
example, for optical engine controls for turbine engines, and the like. 
Since various changes may be made in the invention without departing from 
its spirit and essential characteristics, it is intended that all matter 
contained in the description shall be interpreted as illustrative only and 
not in a limiting sense, the scope of the invention being defined by the 
appended claims.