OPTICAL WAVEGUIDES OF ALUMINUM GARNET

Abstract: 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. When there is sufficient lattice mismatch between garnet substrate and cladding layer, strain is induced which causes stress with resultant birefringence in the waveguiding layer. Such birefringent waveguides maintain polarization of light propagated therein.

Description: OPTICAL WAVEGUIDES OF ALUMINUM GARNET Field of the Invention This invention provides optical waveguide structures of crystalline aluminum garnet of a high refractive index which are epitaxially clad with crystalline aluminum garnet of a lower refractive index. 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 clad with or imbedded in a medium of lower refractive index. High temperature waveguides are commonly made of sapphire, but these have significant optical loss due to the lack of a suitable cladding material. Usually, a metal overcoat is applied to protect the guide from the environment, but the transmission efficiency of this structure is low. P.J. Chandler et al. [P.J. Chandler et al., Electron. Lett. 25, 985 (1989)] disclose an ion-implantation technique to make an aluminum garnet slab waveguide in (Y,Nd) 3 A1 5 0 12 . This technique makes use of the displacement of atoms in the crystal from their usual lattice positions to generate regions of a small refractive index change. This technique is not suitable for making high temperature waveguides, since the crystal structure will relax to its equilibrium state when exposed to high temperature. In an ideal waveguide, linear polarization of guided light would be maintained. However, in practice, polarization in an actual waveguide changes, so that light that has traveled some distance in the waveguide emerges SUBSTITUTESHEET unpolarized. For some fiber optic sensors and advanced communication systems, it is required that the optical waveguides have polarization preserving properties.. Summary of the Invention In the invention waveguides, 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 R 3 (A1,T) 5 0 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+i, Ca+2, Mg + 2 or Sr-2, then T must include one or more charge-compensating ions selected from the group consisting of Fe +4 , Ge+ 4 , Hf+ 4 , Ir+ 4 , Mo 1 "*, N - 4 , Os+ 4 , Pb +4 , Pt +4 , Re+ 4 , Rh +4 , Ru+ 4 , Si +4 , Sn +4 , Ta +4 , Ti +* *, Zr *4 , V+4, W+4, As+s, Mo+5, Nb+s, Re+ 5 , Sb+ 5 , Ta+ 5 , U+ 5 , V+s, Mo* 6 , Re- 6 , W +6 , and Re +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 +2 and Mg +2 , then T should include one or both of Ge^ and Si+4 in equimolar concentration relative to the Ca +2 and/or Mg + 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 3 Al 5 0ι 2 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 sufficient to permit epitaxial deposition. However, for polarization-maintaining waveguides there must be sufficient mismatch between the waveguiding aluminum garnet and the cladding layer to impose a strain on the waveguiding medium, thereby causing a stress with resultant birefringence of the wave propagating in the high refractive index aluminum garnet . In the event YAG is chosen for the epitaxial cladding layer, then the aluminum garnet compositions of the types (Y,Lu) 3 (Al,In) 5 0 12 , (Y,Lu) 3 (Al,Sc) 5 0 12 , (Tb,Lu) 3 A1 5 0 12 and Ho 3 Al 5 0ι 2 have been found to meet the criteria of large difference in refractive indexes and lattice matching particularly well. In the event that polarization maintenance is desired, and YAG is chosen for the epitaxial cladding layer, then the aluminum garnet compositions of the type Tbι. 75 Luι. 25 Al 5 θι 2 and Ho 3 AlsOι have been found to meet the criteria of large difference in refractive indexes and required degree of lattice mismatch for stress-induced birefringence in the wav propagating layer particularly well. Brief Description of the Drawings In the annexed drawings, Fig. 1 illustrates a layer (1) of an aluminum garnet of high refractive index and lattice constant match to YAG epitaxially deposited on a YAG substrate (2) and then epitaxially overcoated with an epitaxial YAG layer (3) to form a "sandwich" structure in which the high refractive index waveguiding layer is clad with the lower refractive index cladding layer in a "slab" waveguide geometry; Fig. 2 illustrates an aluminum garnet fiber (4) epitaxially coated with a YAG layer (5) to form a waveguiding fiber; Fig. 3 illustrates a rib waveguide produced by an epitaxial process as the slab guide of Fig. 1, except that the waveguiding layer is patterned into a "rib" before cladding (reference numerals as in Fig. 1); Fig. 4 illustrates a channel waveguide formed as a variation of a rib waveguide in which the guiding material is deposited in a channel in the substrate layer before cladding (reference numerals as in Fig. 1) ; Fig. 5 shows the polarization extinction ratio vs. input angle in a birefringent planar waveguide of the present invention having a stressed wave propagating (Tb,Lu) 3 Al 5 0ι 2 layer epitaxially deposited on a lower refractive index yttrium aluminum garnet (Y 3 lsOι 2 YAG) layer; Figs. 6 and 7 show transmission intensity vs. wavelength in two different stressed birefringent (Tb,Lu) aluminum garnet waveguides epitaxially deposited on a YAG substrate; and Figs. 8 and 9 show transmission intensity vs. wavelength in two different stressed birefringent holmium aluminum garnet waveguides epitaxially deposited on a YAG substrate. Detailed Description Garnets are oxides of the general composition R 3 Ts0 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 are mechanically strong and highly resistant to chemical attack. They are high temperature materials . For example, yttrium aluminum garnet (YAG) has a melting point of 1947°C, a density of 4.55 g/cc, a hardness o 8.5 moh, a thermal conductivity at 300 K of 0.13 W/cm/K, and a refractive index of 1.84 at 550 nm. Aluminum garnets are optically transparent to long wavelengths. 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 3 Sc 2 Al 3 0_, 2 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 3 Sc 2 Al 3 0ι 2 or "GSAG") , gives a refractive index of 1.97 at visible wavelengths. Fabrication of high temperature aluminum garnet optical waveguides 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, as iullustrated in Figs. 1 through 4. Epitaxial crystal growth process allows deposition of garnet layers on garnet substrates, 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)) . 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°C for a period of about eight hours, to homogenize the components, and then supercooled to about 20°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 μm/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. 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. The refractive index of aluminum garnet can be predicte 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 1 ) 3 (Al,Ti) 5 0 12 and (R2) 3 (A1,T2) 5 0ι 2 is sufficient to give the refractive inde of any intermediate aluminum garnet composition (Ri,R2) 3 (Al,Ti,T2) 5 0ι 2 . 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 %. For birefringent, polarization- maintaining waveguides, the mismatch should be larger than about 0.001%. 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) . SUBSTITUTE S E T An exemplary procedure for X-ray measurement of strain in aluminum garnet epitaxial waveguiding layers on garnet substrate wafers is as follows. Lattice constant measurement of epitaxial waveguiding layers of aluminum garnet on YAG is by standard X-ray diffraction techniques. The (444) and (888) reflections of the garnet diffraction of the (111) layers and wafer substrates are observed with Cu K- alpha-1 radiation collimated by a germanium first crystal in a double-crystal diffractometer. Because of the Poisson distortion of the lattice constants as measured in the direction perpendicular to the plane of the wafer substrates, the following equation is used to calculate the mismatch of lattice constant between the epitaxial layer and the substrate crystal wafer from the mismatch measured in the direction normal to the plane: (Δa/a) = [(l-V)/(l+v)] (Δd/d) where (Δa/a) is the corrected lattice constant mismatch, (Δd/d) is the lattice constant mismatch measured along the direction normal to the plane of the substrate, and v is the Poisson ratio. For aluminum garnets, [ (1-v) / (1+V) ] = 0.6. For layer compositions of the general formula (Y_.-_ . - b ,R i a,R2b) 3 AI 3 (Ali-c,Sc c ) 2 O i2 this strain of the epitaxial layer on Y 3 AlsO-_ 2 can be expressed as Strain(%) = (a) (k_.) + (b) (k 2 ) + (c) (2.25) where k_ , ,k 2 are selected from Table 1, below: Table 1 Lu -0.73 Yb -0.59 Tm -0.36 Er -0.16 Y 0.0 Ho 0.01 Dy 0.35 Tb 0.62 Gd 0.94 Eu 1.02 Sm 1.40 Nd 1.95 Pr 2.33 La 3.07 Table 2 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 2 Ho 3 Al 5 O 12 Tb_.. 63 LUi. 37 Al 5 O 12 Tbι. 4 7 ι.53 Al 5 O12 T i.io Tmi.go Al 5 0 12 Tbo.βi Er 2 . 39 Al 5 Oχ 2 Dy 2 .o9 L 0 .9i Al 5 0 12 Dyι.95 ϊ ι.05 Al 5 O12 Dyι.59 T l. 4 ι Al 5 O12 Dyi.oo Er 2 .oo Al 5 0 12 Gd 1#33 Lu x . 67 Al 5 0 12 Gdi.ι-7 Ybι. 83 Al 5 O i2 Gdo. 8 Tm 2 .ι 6 Al 5 0 12 Gd 0<44 Er 2>56 Al 5 0_. 2 Yo.7β Ll - l . 22 SCo.5 2 Al 4 . 48 O 12 Yθ.66 Y *t>2.3 SCo. 4 Al 4φ56 Oχ 2 Yθ.4 T 2.56 SCo. 2 9 Al 4 . 71 0_, 2 Ϊ0.21 E-C2.79 SCo.14 AI4.86 χ 2 Dyo.67 Lu 2 .33 SCo. 5 Al 4 . 55 Oχ 2 Dyo.56 Yb 2 . 44 Sc 0 ,38 AI4.6 2 0 1 Dyo.37 Tm 2 . 63 Sc 0 . 25 Al 4<75 O12 Dyo.is Er 2 .β2 Sc 0 .i2 Al 4 . 88 0_. 2 Tb 0 .6i Lu 2#39 Sc 0 . 4 ι Al 4 . 59 O x2 Tbo.Sl Yb2.49 SCo.34 l .66 O12 Tbo.33 Tm 2 . 67 Sc 0 . 22 Al 4 . 78 O12 Tbo.ie Er 2#84 Sco.io Al 4 . 90 0_, 2 Gdo.55 Lu 2 . 45 Sc 0 .36 Al . 64 0 12 o. 6 Yb 2 .54 Sco.30 Al 4φ 0 Oχ 2 Gdo.29 Tm 2 . 71 Sco.20 Al 4 . 80 012 Gdo.i Er 2 .86 Sco.o9 Al _ 9 _. O12 Cai.oo Tb 2 .oo Sii.oo Al .oo O12 Ca 0 . 6 i Dy 2 . 39 Si 0 .6i l 4 , 39 0 12 Ca_,. 28 Gd 1#72 Siχ. 28 Al 3ι72 0 12 Caι.27 Luι.73 Gβι.27 Al 3 . 73 Oι 2 Caι.12 ϊbi.ββ G β ι.ι 2 Al 3 . 88 0 12 Ca 0 .79 T 2 .2i Ge 0 .79 AI4.21 O12 Ca 0 . 4 ι Er . 5 9 Ge 0 . 4 ι Al 4<59 O12 CS l . 8 8 Y 1 . 12 Sii. 8 8 Sc 0# 5 Al 2 . 3 0 12 Ca2.02 D o.98 Si 2 .02 SCQ.66 Al 2 . 32 Oι 2 SUBSTITUTE SHEET Ca 2 , 10 Tbo.go Si 2 .χo Sc 0 .60 Al 2 . 3 o Oχ 2 Ca 2 _ 18 Gdo.8 2 Si .ι 8 SC0.55 Al 2>27 0_. 2 Table 3, below, list exemplary aluminum garnet compositions suitable for polarization-maintaining waveguides of the present invention, which acquire a strain of about 0.01 % on yttrium aluminum garnet (YAG) : Table 3 Lu 0 . 94 Dy 2 .o6Al 5 Oι 2 LUl.36T ι. 64 Al 5 0 12 LUi. 67 Gd 1 .33Al 5 Oi2 LU1.73EUL27A15012 1-Uι .96Sm_..0 lsOι 2 Lu 2 . 17 Nd 0 . 8 3Al 5 O 12 Lu 2 . 27 Pr 0 .73AlsOi 2 Lu 2 . 42 La 0 .58AI5O12 ϊbl.09*Dyi.9lAl5θl2 Ω>1.51-n1. Al 5 0i2 ϊbι. 82 Gdι.ιβAl 5 0i2 Ybι.8βEUι.i2 l 5 0i2 ϊb 2 . ιoSm 0 .90AI5O12 Yb 2 .29Nd 0 . lAl5θι 2 Yb 2 . 38 Pr 0 .62Al5θι 2 Yb 2 . 51 La 0 . 49 Al 5 O 12 -Iπiι. 4*Oyι.5βAl5θi2 Tm 1 . 87 Tbι.i 3 Al 5 0ι 2 Tm 2 .ι 5 Gdo. 8 sAl 5 Oι 2 Tm 2 . 20 Euo. 8 oAl 5 Oι 2 Tm 2 . 37 Sm 0 .63 l5θι 2 Tm 2 .5 2 Ndo. 8Al 5 0 12 Tm 2 . 59 Pr 0 .4iAl5θι 2 Tm 2 . sβLao .32Als0 12 Er 2 .ooDyι.oo l5θι 2 Er 2 . 35 Tbo. 65 Al 5 Oι 2 Er 2 . 54 Gd 0 .46 l5θi2 Er 2 . 5 7Eu 0 . 43 Al 5 Oι 2 Er 2 . 67 S o .33 l5θ i2 Ho 3 . 00 Al 5 Oι 2 LU 3 SC 0 .66Al 4 .34θι 2 Yb 3 Sco.53Al 4 . 47 Oι 2 Tm 3 SC 0 .33Al 4 .6 7 θι 2 Er3SC0.15Al4.85O1;> Y3SC0. oιAl .99O12 Er 2 . 76 Ndo. 24 Al 5 Oι 2 Er 2 . 8 oPro.2θAl 5 Oι 2 r 2 . 84 Lao.i6Al5θι 2 Y2.9lDyo.09Al 5 Oi2 Ϊ2.9sTbo.o5Al 5 Oi2 Y 2 .97Gd 0 . 03 Al 5 O X2 Y 2 .97EU0. 0 3AlsOι 2 Y 2 .98Sm 0 .02Al5 i2 Y 2 .9 8 Nd 0 .02Al5θi2 Y2.99 Pr 0.0l lsOi2 Y 2 . 99 La 0 . oι lsOι 2 Table 4, below, list exemplary aluminum garnet compositions suitable for polarization-maintaining waveguides of the present invention, which acquire a strain of about 0.025 % on yttrium aluminum garnet (YAG) : Table 4 Lu 0 . 9 oDy 2 .ιoAl 5 Oι 2 u 1 .32Tbι.6βAlsOi2 Lu 1 . 64 Gd 1 . 36 Al 5 Oi 2 Luι. 71 Eu 1 . 29 Al 5 0ι 2 Lu_.. 94 Smi . 06 lsOι 2 LU 2 . ι 5 Nd 0 .8 5 l 5 θι 2 IiU 2 .26Pro.74Al5θi2 Lu 2 . 4 oLao .60 I5O12 Y ι. 04 Dy 1 . 9 6Al 5 Oi2 Yb 1 . 48 Tb x .52Al 5 0 1 2 Ω1.79Gd 1 .2iAl 5 0 X 2 •Ω>1.85E ι.ιsAl 5 0i2 Η>2. cπSπio . 93 Al5θι 2 Yb 2 . 27 do. 73 Al5θι 2 Yb 2 . 37 Pro. 63 Al 5 Oι 2 Yb 2 . 50 La 0 .5 θ Al5θι 2 Tmι.37Dy 1 . 6 3Al 5 Oi2 Tmι. 82 Tbι.ι 8 l 5 0i2 Tm 2 . 11 Gdo. 89 Al 5 Oι 2 Tm 2<16 Euo. 84 Al 5 Oι 2 Tm 2 . 34 Sm 0 .66Al 5 Oι 2 Tm 2 . 50 Ndo.5oAl 5 0 12 Tm 2 . 57 Pro. 43 Al 5 Oι 2 Tm 2 . 66 La 0 . 34 Al 5 Oι 2 Erx.gxDyx.og lsOi ∑ Er 2 , 2 gTbo .7lAlsOι 2 Er 2 , 5oGd 0 .50 I5O12 Er 2 .53Euo. 4 7Al 5 Oι 2 Er 2. 6 4Sπi θ .36Al5θι 2 Er 2 . 74 Nd 0 . 26 Al 5 O X2 Er 2 . 78 Pro. 22 Al 5 Oι 2 r 2 . 83 Lao.i 7 AlsOι 2 Y2.79Dy 0 .2iAl 5 Oι 2 Y 2 . ββTbo . i2 AlsOι 2 γ 2 _ g 2 Gd 0 . oβAlsOi*. Y 2 . 9 3EU 0 .0 7 AlsOι 2 Y 2 . 95 Smo.o5 l 5 Oι 2 Y2.96 d 0 .0 Al 5 Oι 2 Y 2.97-?-t " 0.03Al5θi2 * Y 2 .g 8 Lao.o2AlsOi2 Ho 2 . 87 Dy 0 .ι 3 Al 5 Oι 2 Ho 2 .g 3 Tbo.o 7 Al 5 Oι 2 Ho 2 . sGdo .05AI5O12 Hθ 2 . 96 Eu 0 . 04 Al5θι 2 Ho 2 . 7Smo .03AI5O12 Hθ 2 . 98 Nd 0 .θ2Al5θι 2 Hθ 2 .g 8 P o.θ2Al5θi2 Hθ # g Lao . O1AI5O 1 2 Lu 3 SCo.6 7 Al 4 .3 3 Oι 2 Yb 3 SCo .5δAl 4 .45θi2 Tm 3 SC 0 .3 4 Al 4 .6 6 θι 2 Er 3 SCo.i6Al 4 . 84 θι*_ Y3SC0. o 2 Al 4 . βOi2 Ho 3 SCo . QIA1 4 . 99 0ι 2 Table 5, below, list exemplary aluminum garnet compositions suitable for the polarization-maintaining waveguides of the present invention, which acquire a strain of about 0.05 % on yttrium aluminum garnet (YAG) : SUBSTITUTE S Table 5 L 0 . 83 Dy 2 .i7 l 5 Oι 2 LUι. 2 7Tbι.73Al 5 Oi2 Lui . 6θ dι . 4 o lsOι 2 LUj_.6t.EUi..34 AI5O12 Lui . g 0 Smι . 10 l 5 Oi 2 Lu 2 .i 3 Nd 0 . 8 7Al 5 Oi2 Lu 2 .24P o. 7e l 5 Oi2 LU2. 38 Lao .62AI5O12 Ybo. 6 Dy2.0 Al 5 Oi2 Ybi.ήx bi.sg lsOiz Yb 1 .75Gd 1 .25 l 5 0 1 2 ϊbl.BlEUi.igAlsOi∑ Yb 2 .o 4 Sm 0 . 9 6Al 5 Oι 2 Yb 2 .24Ndo. 7 6Al 5 Oι 2 ϊ 2 . 34 Pro.66Al 5 Oi2 Yb 2 . 8 La 0 .5 2 Al 5 Oι 2 Tmι. 2 Dyι.73Al 5 Oi2 Tmι.74Tbι.2βAl 5 0i2 Tm 2 .o5Gd 0 .g 5 Al 5 Oι 2 Tm 2 .ι 1 Euo. 8 9Al 5 Oi2 Tm 2 .3oSmo .70AI5O12 Tm 2 . 47 Nd 0 .53Al 5 O 12 m 2 .5 4 ro. 6Al 5 Oi 2 Tm 2 . 64 La 0 .3 6 Al 5 O 12 Erι.7 6 Dyι.2 4 Al 5 0i2 Er 2 .ι 9 Tb 0 . 8 χAl 5 Oi 2 Er 2 . 3 Gd 0 .5 7 Al 5 Oι 2 Er 2 . 47 Eu 0 .53Al 5 O 12 Er 2 .6oSmo .40AI5O12 Er 2 . 70 Ndo. 3 oAl 5 0-_ 2 Er 2 . 75 Pro. 2 5Al 5 Oι 2 Er 2 . 80 La 0 . 2 oAlsOι 2 Y2.5 7 Dy 0 . 43 Al 5 O 12 *2.76Tb 0 .24Al 5 O 12 Y2. 8 Gd 0 .ι 6 Al 5 Oι 2 Y2.85E 0.15AI5O12 Y 2 .S9Sm 0 .il l50i2 Y 2 . 92 Nd 0 .0 8 Al5θι 2 Y2.9 Pr 0 .06Al 5 Oi2 Y 2 . gsLa 0 .05 AI5O12 Hθ 2 . 65 Dy 0 .35Al5θι 2 Hθ 2 . 80 bo.2θAl5θi2 Ho 2 . 87 Gd 0 . ι 3 Al 5 0i2 Ho 2 . 88 Eu 0 .ι 2 Al 5 Oι 2 Ho 2.9iSm 0 .09 I5O12 Hθ2.g Nd 0 .o6AlsOi2 Ho 2 . 9 5Pro.o5Al 5 Oι 2 Ho 2 .961-a-o .0 AI5O12 Lu 3 Sc 0 .6θAl 4 .31O12 Yb 3 SCo. 5 7Al 4 .43θl 2 Tm 3 SC 0 . 3 6Al 4 .64θl2 Er 3 SCo.i9Al 4 . 81 Oi2 3SCo. θ 4Al 4 . 9 6θι 2 Hθ 3 SCo .04A1 4 .96 i-2 Table 6, below, list exemplary aluminum garnet compositions suitable for polarization-maintaining waveguides of the present invention, which acquire a strain of about 0.1 % on yttrium aluminum garnet (YAG) : Table 6 Lu 0 .69Dy 2 .3iAl 5 Oi2 LUι.ι 6 Tbι. 8 4Al 5 0i2 Luι.5iGd x . 49 Al 5 0 12 Lu 1 . 58 Euι. 2Al 5 Oi2 IiU 1.8 3 Sm-_ . ι 7 Al 5 0i2 Lu 2 . cπNdo . g 3 Al 5 0i2 Lu 2 .i9Pr 0 .8iAl 5 Oi2 Lu 2 .34Lao. 6 6Al 5 Oi2 Yb 0 . 8θ Dy 2 .2 θ Al 5 0 12 bi . gεSmi .0 4 AI5O12 Yb2.l9Ndo.8lAl 5 Oi2 Yb 2 . 29 Pr 0 .7iAl 5 θι 2 Yb 2 .43Lao.57Al 5 Oi2 Tm 1 .o6Dyι.9 4 Al 5 Oi2 Tmι.59T ι. 4 ιAl 5 C:2 SUBSTITUTE SHEET Tm 1 .94Gd 1 . 06 l5θi2 Tm 2 .ooE ι.ooAl5θi2 Tm2.22Sm 0 .78 l5θi2 Tm 2 . oNdo.6o lsOi2 Tm 2 . 4 9Pro.5lAl5θi2 Tm 2 .6oLao. θ Al5θi 2 Er 1 . 47 Dy 1 .53Al 5 Oi2 E 2. ooTbχ .00 I5O12 Er 2 .29 d 0 .7iAl5θi2 Er 2 . 34 E 0 .66 l5θi2 Er 2 .5oSm 0 .50AI5O12 Er 2 .6 3 Ndo.37Al 5 0 1 2 Er 2 . 69 Pr 0 .3iAl 5 Oi2 r2.7βLao .24AI5O12 Y2.14Dyo.86Al 5 Oi2 Y2.52 0 .48 l 5 Oi2 Y2.68 do.32Al5θi2 Y2.71EU0.29A15O12 Y2.7 9 Smo.2iAl 5 Oj.2 Y2.85 do.ι 5 Al5θ 12 Y2.8 7 pro.13A15.D 12 Y 2 . 9 oLa c .ιo l5θi2 Hθ 2 .2lDyo.79Al 5 Oi2 Hθ2.56Tbo. 44 Al 5 0: 2 Ho 2 . 7 iGd 0 . 29 Al 5 O 12 HO2.73EU0.27A15O12 H02.8iSπto .19AI5O12 Ho 2 . 86 Nd 0 . 14 Al 5 O 12 Hθ2. 88 Pro.ι 2 Al 5 0 1 2 H02 , 9 ιLao . 09 AI5O 2 Lu 3 SCo.7 4 Al 4 .26θl2 Yb 3 SC 0 .6lAl 4 .3 9 Oi2 Tm 3 Sco. 4 ιAl 4 . 5 gOi2 Er 3 SC 0 .23Al 4 .77θi2 Y3SC 0 .09 l 4 . 9 ιOi2 Hθ 3 SC 0 .08Al 4 .92θi2 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. The thickness of the cladding layer 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 are no limits to the thickness, other than those dictated by practical considerations of construction, expense of application, etc. Desirably, the thickness of the cladding layer is 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 SUBSTITUTE SHEET 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 μ , more desirably at least about lOμm; preferably at least about 25 μm, and more preferably yet at least about 100 μm. Example 1 A melt was prepared for the epitaxial crystal growth of an aluminum garnet layer of composition (Y,Lu) 3 (Al, In) 5 0 12 , by melting together the oxides in the following proportions: PbO 952.87 g B 2 0 3 24.77 g A1 2 0 3 8.04 g ln 2 0 3 8.75 g Y 2 0 3 2.81 g Lu 2 0 3 3.30 g An epitaxial layer of the approximate composition Y 2 Lu 1 A i . 7 ln 0 . 3 Oι 2 was grown by the liquid phase epitaxial crystal growth process detailed above on a substrate wafer of YAG to produce a slab waveguide. Growth conditions and product properties were as follows : growth temperature: 926.5 °C growth rate: 1.17 μm/min thickness: 2.34 μm lattice constant (Angs.) : 12.0150 refractive index (at 633nm) 1.8424. 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) 3 Al 5 0ι 2 , as detailed in Table 7, below: SUB TITUTE S Table 7 Composition of Melt in Grams for the Growth of Optical Waveguides of Compositi Epitaxial layers of the approximate composition Tbι.7 5 L ι. 25 Al 5 Oi 2 were grown by the liquid phase epitaxial crystal growth process detailed above on substrate wafers of YAG. Growth conditions and product properties were as shown in Table 8, 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 8 Properties of Slab Waveguides of (Lu,Tb) 3 AI5O 12 Epitaxially Grown on YAG Substrates. Example No. : Growth Temp (°C) : Growth Rate (μm/min) : Thickness (μm) : Ref. Ind. (at 633 nm) Lattice Const. (Angs.) 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 9, below. Also shown in Table 9 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. SUBSTITUTE SHEET Optical transmission loss measurements were made on the (Lu,Tb) 3 Al 5 0ι 2 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 10) gave an optical loss of lil ± 1.2 dB/cm. An optical loss of the order of 1 dB/cm is considered adequate for most applications. SUBSTITUTESHEET Example 6 A melt was prepared for the epitaxial crystal growth of aluminum garnet layers of composition (Tb,Lu) 3 Al 5 0ι 2 , by melting together the oxides in the following proportions: PbO 765.12 g B 2 0 3 19.89 g A1 2 0 3 7.61 g Tb 2 0 3 4.88 g Lu 2 0 3 2.50 g An epitaxial layer of the approximate composition Tb1.75Lu 1 .-_5Al.5O12 was grown at 958°C at a growth rate of 1.97 μm/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 μm. 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 rutile prism, and then extracted from the waveguide by another rutile prism. The distance between prisms fixes the optical path length, and the optical loss is 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) 3 Al 5 0ι , by melting together the oxides in the following proportions: PbO 760.10 g B203 19.76 g A1203 10.24 g Tb203 6.96 g Lu203 2.94 g This melt has a saturation temperature of about 1070°C and a growth temperature of about 1055°C. Epitaxial layers of the approximate composition Tbι. 75 Luι. 25 Al 5 Oι 2 were grown from this melt on YAG substrate wafers to be overcoated with a cladding SUBSTITUTE SHEET layer of YAG in order to fabricate clad waveguides of aluminum garnet. A melt for the liquid phase epitaxy of YAG was formulated of composition shown below to overcoat the optical waveguiding layer of (Tb,Lu) 3 Al 5 0ι 2 with a cladding layer of YAG: PbO 765 .24 g B 2 0 3 19 . 89 g A1 2 0 3 9 . 40 g Y 2 0 3 5 . 48 g This melt produced epitaxial layers of YAG at a growth temperature of about 1095*- * C at a growth rate of about 1.5 μ /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: Gu din Laye : layer thickness (μm) growth rate (μm/min) refractive index at 632.8 nm Cl d Layer layer thickness (μm) growth rate (μm/min) refractive index at 632.8 nm Polarization is a consequence of the nature of electromagnetic waves. An electromagnetic wave contains two fields, one electric and one magnetic, oscillating perpendicular to each other and propagating in a direction perpendicular to both, ϋnpolarized light is made up of many waves with their electric and magnetic fields oriented randomly. If all the electric fields (and, hence, the magnetic fields as well) were aligned parallel to one another, the light would be linearly polarized. In an ideal waveguide the linear polarization will be maintained. However, it is known that the linearly polarized light in an actual waveguide is coupled into modes of different polarization, so that light that has traveled some distance in the waveguide emerges unpolarized. This change in the polarization state can be attributed to deformations of the film from its symmetry, or to anisotropy of the waveguide material . In single mode waveguides, the loss of linear polarization can be described as a coupling process between orthogonally polarized modes . There are always two modes of the same kind, but with transverse electric field components that are polarized perpendicular to each other. Film deformations or anisotropies of the waveguide material couple these orthogonally polarized modes to each other, causing the polarization of the superposition field of all the modes to change. In multimode waveguides the picture is further complicated by the fact that all the modes couple to their perpendicularly polarized counterparts, so that the polarization of the total wave field becomes random, resembling the polarization state of incoherent light. For some fiber optic sensors and advanced communication systems, it is required that the optical waveguides have * polarization preserving properties. Two types of polarization-sensitive single-mode waveguides are available commercially. One type is a true single-polarization waveguide that can transmit light in one linear polarization but not in the other. The other type is polarization- maintaining; that is, it maintains the polarization of the light that originally entered the waveguide by isolating the two orthogonal polarizations from each other while they travel down the same single-mode guide. Although both single-polarization and polarization- maintaining fiber waveguides are available today, polarization-preserving planar waveguides are still desired for many applications, and high temperature polarization preserving planar waveguides are preferred for special applications. It is known that stress will induce birefringence in a material and that lattice constant mismatch between crystal layers will induce stress . SUBSTITUT Epitaxial waveguiding layers of aluminum garnet on aluminum garnet substrates were prepared to obtain planar birefringent waveguides. When linearly polarized light enters such waveguides in certain orientations, the linear polarization will be perserved by the stress-induced birefringence. Usually, light is neither totally polarized nor unpolarized, but a mixture of the two types. Thus, the two orthogonal plane polarized components representing the wave will have unequal amplitudes. In such a case, it is said that the light is partially polarized. A measure of this condition is the degree of polarization V defined as V = I p /(I p +I U ) I p and I u are the constituent flux densities of polarized and unpolarized light, respectively. I p + I u is the total irradiance and V is the fractional polarized componen . The examples below illustrate birefringent polarization- maintaining aluminum garnet waveguides. Example 8 Polarization preservation in aluminum garnet waveguides: fTh.Lu 3A15012 epitaxial waveσuidinσ layers on YAG To measure the polarization preserving properties of epitaxial waveguides of aluminum garnet, a 50 μm thick epitaxial layer of (Tb,Lu) 3 Al 5 0i 2 was prepared on one side of a 2.54 cm diameter 0.5 mm thick substrate wafer of Y 3 AI 5 O 12 . Polarized light with a polarization extinction ratio [defined as (I ma χ-Imin)/(Imaχ+Imin) ] of 26.2 dB, was coupled into this waveguide, and the angle of polarization with respect to the plane of the waveguide was rotated in 10° increments . At each orientation the polarization extinction ratio of the light coupled out of the waveguide was measured. The results are shown in Fig. 5 as the polarization extinction ratio of the (Tb,Lu) 3 l 5 θi 2 waveguide as a function of the angle of polarization of the incident light. For light polarized along the normal of the waveguide (parallel to the optic axis) , it was found that the polarization extinction ratio cf the exiting wave was 24.6 dB, or a reduction of 1.6 dB. For light polarized in the plane of the waveguide (at 90° to the optic axis), the polarization extinction ratio was 22.4 dB, or a reduction of 3.8 dB. As is to be expected, for light polarized at angles between these extremes the exiting light will be elliptically polarized, as can be seen from Fig. 5. Example 9 Birefringence of (Tb.Lu^Al^O^ epitaxial waveguiding layers on YAG When linearly polarized light impinges on a birefringent crystal with its optical axis 90° to the propagation direction, the light will decompose into two orthogonally polarized components, one polarized parallel to the optic axis and the other polarized at 90° to the optic axis. As both beams pass through the birefringent crystal, they see different polarizations of the electrons and, therefore, experience different refractive indices. As a result, the waves will develop a phase difference as they pass through the crystal. This phase difference depends on the difference in refractive indices, the path length through the crystal, and the wavelength of the light. When a broadband LED or other broadband source is chosen as the light source, the birefringence of the crystal will induce an intensity modulation, or fringes, on the light spectrum. The number of fringes in a given wavelength interval is where B is the birefringence, d is the pathlength through the crystal, and λi and λ 2 are the lower and upper wavelength limit of the spectrum, respectively. In theory, linear strain is anticipated with lattice constant mismatch, so that linear elasticity may be used to compute the linear stress at the interface between an epitaxial layer of Hθ3AlsOι 2 and a Y 3 AI 5 O 12 substrate. H0 3 AI5O 12 may be prepared on Y 3 AI5O 12 with SUBSTITUTE SHEET at least a 0 . 01% lattice mismatch . From one-dimensional linear elasticity, the strain is e = dl/1 = 0 . 01 % = 0 . 0001 . Using Hooke's law, the stress is s = E e = 4.1615 x IO "7 x 0.0001 = 4161.5 psi, where E = 4.1615 x IO 7 is the Young's Modulus of YAG. The photoelastic constant dB/ds of YAG at room temperature is 7.973 x IO-**/psi. using this value of dB/ds for HoAG. the stress-induced birefringence is B = dB/ds x s = 3.3 x 10-!. A SPEX .22 Spectrometer was used to measure the birefringence of (Tb,Lu) 3 Al 5 0i 2 waveguiding layers on Y 3 AlsOι 2 . Light from a wide band source was collimated through a microscope lens and coupled into the waveguides. These waveguides were placed between a pair of crossed polarizers . The birefringence of these planar waveguides was determined by using a white light source (450 nm to 750 nm) and measuring the number of fringes, or using a broad-band light- emitting diode (LED) (760 nm to 840 nm) as the light source to measure the number of fringes . For 16.86 μ (Tb,Lu) 3 l 5 0i 2 waveguiding layers prepared on each side of a Y 3 AI 5 O 12 wafer substrate, it was found that the birefringence was 0.668 x IO- 3 (see Fig. 6) . For a 50.25 μm (Tb,Lu) 3 Al 5 θi 2 waveguiding layer prepared on one side of a Y 3 Al 5 Oi 2 wafer substrate, it was found that the birefringence was 4.08 x 10-3 (see Fig. 7) . Examples 10 - 11 Birefringence of Ho^AlcO-^ epitaxial waveσuidinσ layers on YAG The birefringence of two HoAlsOi 2 waveguiding layers prepared on Y 3 lsOι 2 wafer substrates was evaluated with a white light source (560 nm - 620 nm) and a broad-band LED (750 nm - 850 nm) . It was found that the birefringence was 3.05 x IO -3 for the first specimen (see Fig. 8), and that the birefringence was 2.55 x 10-3 for the second specimen (see Fig. 9) . SUBSTITUTE SHEET The waveguides of the present invention are particularl suited for controlled transmission of light in high temperature environments, as, for example, for optical engine controls for turbine engines, and the like. SUBSTITUTE SHEET

Claims: We claim: 1. An optical waveguide comprising, in combination, a waveguiding body composed of a first crystalline aluminum garnet, and a cladding comprising an epitaxially deposited layer of a second crystalline aluminum garnet, wherein said second crystalline aluminum garnet has a lower refractive index than said first crystalline aluminum garnet. 2. The optical waveguide of claim 1 wherein 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 R 3 (Al,T) 5 0i 2 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+i, Ca+2, Mg+2 or Sr+2, then T must include one or more charge-compensating ions selected from the group consisting of Fe+ 4 , Ge +4 , Hf +4 , Ir +4 , Mo +4 , b+4, Os* 4 , Pb+ 4 , Pt +4 , Re +4 , Rh +4 , Ru +4 , Si- 1 - 4 , Sn +4 ., Ta +4 , Ti+4, Zr+ 4 , V+ 4 , W +4 , As +5 , Mo+5, Nb+s, Re + s, Sb + 5, Ta+5, U +5 , V+s, Mo +6 , Re + e, W + 6, and Re +7 , in proportions sufficient to achieve an average cation charge of three in the crystal. SUBSTITUTE SHEET 3. The optical waveguide of claim 2 wherein the aluminu garnets for the higher refractive index body of the waveguide and for the epitaxial cladding layer are selected from aluminum garnets of the composition • R 3 (Al,T) 5 0i 2 wherein 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 an 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 indu plus scandium to aluminum does not exceed 2 : 3; and that (2) if R is one or both of Ca+2 and Mg+2, then T includes one or both of Ge +4 and Si + *> in equimolar concentration relative to the Ca +2 and/or Mg τ 2 to achieve an average cation charge of three in the crystal. . The optical waveguide of claim 1 wherein the body of the waveguide is an aluminum garnet selected from the composition (Y,Lu) 3 (Al, In) 5 0ι 2 , (Y,Lu) 3 (Al,Sc) 5 0ι 2 , (Tb,Lu) 3 Al5θ 12 and Ho 3 Al 5 0ι 2 . 5. The optical waveguide of claim 4 wherein the epitaxial cladding layer is yttrium aluminum garnet . SUBSTITUTE SHEET 6. The optical waveguide of claim 3 comprising an yttrium aluminum garnet base, having epitaxially deposited thereon an aluminum garnet waveguiding body of the composition R 3 (A1, T) 5 O 12 having a higher refractive index than said yttrium aluminum garnet, and having an epitaxial cladding layer of yttrium aluminum garnet on said aluminum garnet of the composition R 3 (Al,T) 5 O 12 . 7. The optical waveguide according to any one of claims 1 through 6, wherein the lattice dimensions of between said first and said second crystalline aluminum garnet are sufficiently different to impose a strain of at least 0.001% on said crystalline aluminum garnet waveguiding body, thereby causing a stress with resultant birefringence in said crystalline aluminum garnet waveguiding body. 8. The method for forming an optical waveguide according to claim 1 which comprises providing a base of a first aluminum garnet, epitaxially depositing thereon a second aluminum garnet having a higher refractive index than said first aluminum garnet, and epitaxially depositing on said second aluminum garnet a third aluminum garnet cladding layer having a ( lower refractive index than said second aluminum garnet. 9. The method of claim 8 wherein said first and said third aluminum garnet are both yttrium aluminum garnet. SUBSTITUTE SHEET

Inventors: GUALTIERI DEVLIN M, HOU JANPU, MORRIS ROBERT C, VAN DE VAART HERMAN, GUALTIERI, DEVLIN, M., HOU, JANPU, MORRIS, ROBERT, C., VAN DE VAART, HERMAN
Applicants: ALLIED SIGNAL INC, ALLIED-SIGNAL INC.
Application year: 1992
Main classification: G02B6
IPC codes: G02B6