Magnetic garnet single crystal and faraday rotator using the...

Compositions – Magnetic – Iron-oxygen compound containing

Reexamination Certificate

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C252S062590, C252S062580

Reexamination Certificate

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06641751

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic garnet single crystal and a Faraday rotator utilizing a magneto-optical effect which uses the magnetic garnet single crystal. The Faraday rotator using the magnetic garnet single crystal is used in a magneto-optical element such as, for example, an optical isolator, an optical circulator, an optical attenuator or the like.
2. Description of the Related Art
In an optical communication or an optical application device using semiconductor laser, an optical isolator, an optical circulator or an optical attenuator is widely used. The Faraday rotator can be cited as one of the essential elements for these devices.
Although a YIG (yttrium iron garnet) single crystal and a bismuth (Bi)-substituted rare earth iron garnet single crystal are known for the Faraday rotator, at present, the Faraday rotator using a bismuth-substituted rare earth iron garnet single crystal film formed by a liquid-phase epitaxial (LPE) method is the mainstream.
For example, in the Japanese Patent Publication No. 6-46604, a bismuth-substituted rare earth iron garnet is described. The bismuth-substituted rare earth iron garnet which is grown by a liquid-phase epitaxial growth method has a composition expressed by a general formula R
3−(a+b)
Pb
a
Bi
b
Fe
5−c
M
c
O
12−d
(R is at least one kind of element selected from rare earth elements and elements substitutable with the rare earth elements, M is at least one kind of element selected from elements substitutable with iron elements, a is a number between 0.01 and 0.2, b is a number between 0.5 and 2.0, c is a number between 0.01 and 2.0 and d is a number between 0 and 1). At the same time, a part of M in the aforementioned formula contains a tetravalent element which is belonging to the Group IVA and the Group IVB in the periodic table excluding Pb in which tetravalent element is more than 0.01 as an atomic ratio (c) in the above general chemical formula.
As disclosed in the above publication, Pb
4+
can be eliminated when the Bi-substituted rare earth iron garnet single crystal is grown by the liquid-phase epitaxial method by adding an IV Group element. Therefore, an absorption loss at the time of transmitting light through the Bi-substituted rare earth iron garnet single crystal can be reduced.
By the way, as shown in the example disclosed in the Japanese Patent Publication No. 6-46604, if for example TiO
2
is added as the IV Group element and a single crystal epitaxial film is grown by the liquid-phase epitaxial method, an effect of reducing the light absorption loss of the obtained Bi-substituted rare earth iron garnet single crystal epitaxial film is recognized. However, when the film thickness of the obtained epitaxial film is more than approximately 200 &mgr;m, many crystal defects are confirmed on a film surface. When the Faraday rotator for an optical isolator is fabricated by polishing the surface of such a crystal and forming a non-reflective film, many defects are confirmed in the interior of the Faraday rotator according to an observation using the infrared ray and a reduction of the extinction ratio is also identified.
The invention described in the Japanese Patent Publication No. 6-46604 cites a reduction of the light absorption loss of the Bi-substituted rare earth iron garnet single crystal epitaxial film as a technical subject and none is disclosed with respect to the subject to suppress the generation of crystal defects and to improve the extinction ratio. If the generation of crystal defects in the Bi-substituted rare earth iron garnet single crystal epitaxial film can be suppressed, the extinction ratio of the Faraday rotator can be improved and furthermore, the performance of optical communication parts including the optical isolator can be improved by improving the extinction ratio of the Faraday rotator.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a magnetic garnet single crystal which reduces an absorption loss by adding a tetravalent element and then suppresses a generation of crystal defects.
Another object of the present invention is to provide a Faraday rotator which reduces an insertion loss and improves an extinction ratio.
Therefore, the inventors of the present invention investigate to obtain a single crystal more than approximately 200 &mgr;m which is required to fabricate the Faraday rotator without generating many crystal defects as well as additives for achieving a reduction in light absorption.
As a result, it is found that a remarkable effect is achieved if Pt which can steadily take a tetravalent structure similar to the structure of the IV Group elements is used as an additional element. In other words, when PtO
2
or Pt is melted in the flux and a Bi-substituted rare earth iron garnet single crystal which is more than 200 &mgr;m in film thickness is grown, the number of crystal defects on the epitaxial film surface is considerably reduced, no crystal defect is confirmed even if the single crystal is observed by the polarization microscope using the infrared ray, and the light absorption loss can be substantially naught (zero).
Further, it is found that if Ge is used as an additional element, a remarkable effect is achieved. In other words, when GeO
2
is added and the Bi-substituted rare earth iron garnet single crystal which is more than 200 &mgr;m in film thickness is grown, the number of crystal defects on the epitaxial film surface is considerably reduced, no crystal defect is confirmed either according to the observation of the interior of the single crystal by the polarization microscope using the infrared ray, and the light absorption loss can be substantially naught (zero).
The above objects are achieved by a magnetic garnet single crystal which is grown by the liquid-phase epitaxial growth method and expressed by a general formula Bi
a
Pb
b
A
3−a−b
Fe
5−c−d
B
c
Pt
d
O
12
(A in the formula is at least one kind of element selected from Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, B is at least one kind of element selected from Ga, Al, Sc, Ge and Si and a, b, c and d are 0.8<a<1.4, 0<b≦2.0, 0≦c≦0.9 and 0<d≦2.0 respectively).
In the above magnetic garnet single crystal according to the present invention, the film thickness is more than 200 &mgr;m. Further, in the above magnetic garnet single crystal according to the present invention, 0.5≦b/d≦2.0 is realized.
Furthermore, the above objects are achieved by a magnetic garnet single crystal which is grown by the liquid-phase epitaxial growth method and expressed by the general formula Bi
a
Pb
b
A
3−a−b
Fe
5−c−d
B
c
Ge
d
O
12
(A in the formula is at least one kind of element selected from Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, B is at least one kind of element selected from Ga, Al, Sc, Pt and Si and a, b, c and d are 0<a<3.0, 0<b≦2.0, 0≦c≦2.0 and 0<d≦2.0 respectively).
In the above magnetic garnet single crystal according to the present invention, the film thickness is more than 200 &mgr;m.
Further, the above objects are achieved by a Faraday rotator which is formed from the above magnetic garnet single crystal according to the present invention. Furthermore, an insertion loss of the Faraday rotator according to the present invention is less than 0.1 dB.
An operation of the present invention is described below. Ti
4+
, Pt
4+
or Ge
4+
is mainly substituted to an Fe site of a 6-coordination in a lattice of the Bi-substituted rare earth iron garnet. However, since an ion radius of Ti
4+
is larger than that of Fe
3+
in the 6-coordination, a distortion is generated in the lattice of the Bi-substituted rare earth iron garnet. Therefore, when the epitaxial growth progresses and the film thickness increases, the distortion in the lattice is accumulated and many crystal defects generate. Since the ion radius of Pt
4+
or Ge
4+
is smaller than t

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