Compositions – Light transmission modifying compositions
Reexamination Certificate
2002-12-13
2004-08-03
Thibodeau, Paul (Department: 1773)
Compositions
Light transmission modifying compositions
C252S584000, C359S280000, C359S324000, C428S692100, C428S702000
Reexamination Certificate
active
06770223
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to articles and systems (collectively “articles”) that comprise a Faraday rotator that does not require a bias magnet.
BACKGROUND OF THE INVENTION
Faraday rotator materials are a necessary component in non-reciprocal devices such as magnetooptic isolators, circulators and switches. These devices have found extensive application in telecommunications and other fields. The preferred materials for these applications at telecommunications wavelengths are bismuth-doped rare-earth iron gamets. At the principal near infrared telecommunications wavelengths of about 1310 nm and about 1550 nm, these magnetic gamets have a high degree of transparency and reasonably high specific Faraday rotations (Faraday rotation per unit thickness).
Because these bismuth-doped rare-earth iron garnet materials are not congruently melting, nearly uniform crystals can only be grown by flux techniques, typically by liquid phase epitaxy on substrates of non-magnetic garnet. This technology is well reviewed by V. J. Fratello and R. Wolfe (in Magnetic Film Devices, edited by M. H. Francombe and J. D. Adam, Volume 4 of
Handbook of Thin Film Devices: Frontiers of Research, Technology
and
Applications
, Academic Press, 2000). As is detailed therein, a close room temperature lattice match of the film to the substrate is required to prevent cracking of the film, the substrate or both. Films are generally grown only on one side of the substrate to allow stress relief by bending. To grow a thick film >300 &mgr;m as is required for telecommunications device applications, the film and substrate lattice parameters should match to within ±0.1%~±0.012 A, preferably within ±0.05%~±0.006 Å. The range of film compositions possible is therefore constrained by the available substrate materials.
Bismuth-doped garnet film compositions have an enhanced Faraday rotation over the pure rare-earth iron garnets. Doping with bismuth strongly effects the electric dipole term in the magnetooptic coefficients through superexchange and spin-orbit interactions (see, for example, P. Hansen and J.-P. Krumme,
Thin Solid Films
114, 69 (1984) and H. LeGall, M. Guillot, A. Marchand, Y. Nomi, M. Artinian and J. M. Desvignes,
J. Mapn. Soc
. Jpn. 11, Supplement S1, 235 (1987)). The total Faraday rotation of the garnet, &THgr;
F
, may be characterized as the sum of the following:
(1) The iron lattice contribution, which can be determined from the Faraday rotation of yttrium iron garnet (YIG-Y
3
Fe
5
O
12
), &THgr;
F
(YIG), since yttrium is a non-magnetic ion and does not contribute to the magnetic or magnetooptic properties. This iron lattice contribution is a small positive Faraday rotation.
(2) The rare earth (designated as R) contribution, which can be determined from the difference between the Faraday rotations of YIG and a pure-rare earth iron garnet (RIG-R
3
Fe
5
O
12
), &THgr;
F
(RIG)-&THgr;
F
(YIG). Since bismuth substitutes for rare earths in the gamets, this contribution will be diluted by bismuth substitution. (For data on the pure rare-earth iron gamets at telecommunications wavelengths see J. F. Dillon, Jr., S. D. Albiston and V. J. Fratello,
J. Magn. Soc. Jpn
. 11, Supplement S1, 241 (1987)). These rare earth contributions are typically smaller than the overall iron lattice Faraday rotation and may be positive or negative.
(3) The bismuth contribution, &THgr;
F
(Bi), which is well characterized by a single line shape (see G. B. Scott and D. E. Lacklison,
IEEE Trans. Maqn
. 12, 292 (1976)). This contribution is linearly dependent on the bismuth contribution at least up to 2 atoms per formula unit of bismuth (see T. Tamaki, H. Kaneda, T. Watanabe and K. Tsushima,
J. Magn. Soc
. Jpn. 11, Supplement S1, 391 (1987) and T. Okuda, T. Katayama, K. Satoh, T. Oikawa, H. Yamamoto and N. Koshizuka,
Proc. of the Fifth Symposium on Magnetism and Magnetic Materials
, ed. by H. L. Huang and P. C. Kuo (World Scientific, Singapore, 1989)). This contribution is large and negative.
These contributions yield the approximate formula:
&THgr;
F
(Bi
X
R
3-X
Fe
5
O
12
)=(
X
/3)×[&THgr;
F
(Bi)+&THgr;
F
(YIG)]+[(1-(
X
/3)]×
F
(RIG)
If a partial substitution of gallium, aluminum or any other diamagnetic ion is made for any of the iron in the garnet, e.g. Bi
X
R
3-X
Fe
5-Y
Ga
Y
O
12
, all these Faraday rotations are reduced by dilution, though not necessarily in a linear manner.
As the bismuth doping is increased, its contribution to the Faraday rotation first cancels the small positive contribution of the rare earth iron garnet (for Bi
X
Y
3-X
Fe
5
O
12
this occurs at X~0.10-0.15 atoms per formula unit at 1310-1550 nm), then the Faraday rotation increases in magnitude in the negative direction. The upper limit of specific Faraday rotation, &THgr;
F
/t, results from the maximum allowable bismuth doping in the film. This occurs because of the onset of misfit dislocations in the film, which degrade the optical quality unacceptably (see V. J. Fratello, S. J. Licht, C. D. Brandle, H. M. O'Bryan and F. A. Baiocchi,
J. Cryst. Growth
142, 93 (1994)). This maximum bismuth concentration is a complex function of growth conditions and gallium or aluminum substitution but is typically in the range 1.2-1.5 atoms per formula unit.
The range of film compositions that can be grown is constrained by the lattice match to available substrates. Because bismuth has a large ionic size, substrates with higher lattice parameters are required.
Most commonly used and commercially available in large diameters is calcium-magnesium-zirconium substituted gadolinium gallium garnet (CMZ:GGG-{Gd
2.68
Ca
0.32
}[Ga
1.04
Mg
0.32
Zr
0.64
](Ga
3
)O
122
). The literature value of the lattice parameter of this material is 12.498 Å (D. Mateika, R. Laurien and Ch. Rusche,
J. Cryst. Growth
56, 677 (1982)), but slightly lower values are sometimes quoted as well.
Some thick film crystal growth has been performed on neodymium gallium garnet (NdGG-Nd
3
Ga
5
O
12
) substrates of lattice parameter 12.504 Å. However this material has a significantly worse match of coefficient of thermal expansion to the thick film materials than CMZ:GGG or GSGG. Therefore it is more prone to breakage or catastrophic formation of misfit dislocations (V. J. Fratello, S. J. Licht, C. D. Brandle, H. M. O'Bryan and F. A. Baiocchi,
J. Cryst. Growth
142, 93 (1994). For these reasons NdGG is not suitable as a substrate for thick film gamets with high bismuth concentrations.
The next higher lattice parameter commercially available material is gadolinium scandium gallium garnet (GSGG-Gd
2.957
Sc
1.905
Ga
3.138
O
12
lattice parameter 12.560 Å (V. J. Fratello, C. D. Brandle and A. J. Valentino,
J. Cryst. Growth
80, 26 (1987)).
Since, as is stated above, good thick film growth requires a room temperature lattice match of ±0.012 Å, preferably ±0.006 Å, this leaves a considerable lattice parameter range not attainable on commercially available substrates.
To operate most non-reciprocal devices, the magnetic garnet must be maintained in a single domain state. Most magnetic gamets spontaneously demagnetize into multiple domains to minimize their free energy. Device designs have traditionally used a bias magnet to maintain the magnetic garnet in the single domain state required for device operation. However Pulliam et al. (
J. Appl. Phys
. 53, 2754 (1982)) Identified large stable magnetic domains in garnet films and Brandle et al. (U.S. Pat. Nos. 5,608,570 and 5,801,875) identified the necessary magnetic conditions to maintain such domains over the temperature range of device operation. The specific teachings of the Brandle et al. patents are as follows:
(1) To maintain a saturated magnetic state without a bias magnet, a saturation magnetization 4&pgr;M
s
<100 G must be maintained over the device operating range, e. g. −40° to 85° C.
(2) To this end, the material composition must avoid or minimize rare earth ions wi
Abbott Robert R.
Fratello Vincent J.
Licht Steve J.
Mnushkina Irina
Bernatz Kevin M.
Integrated Photonics, Inc.
Locke Liddell & Sapp LLP
Thibodeau Paul
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