Faraday rotator and magneto-optical element using the same

Details Faraday rotator and magneto-optical element using the same Faraday rotator and magneto-optical element using the same

2000-05-26

2002-02-26

Epps, Georgia (Department: 2873)

Optical: systems and elements

Optical modulator

Having particular chemical composition or structure

C359S280000, C423S263000, C117S942000

Reexamination Certificate

active

06351331

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a Faraday rotator and a method for preparing the same as well as a magneto-optical element in which the Faraday rotator is incorporated and an optical isolator as one of the magneto-optical elements.
In an optical circuit, which constitutes an optical communication system, there have been used a variety of magneto-optical elements such as an optical isolator, an optical circulator, an optical switch, a magnetic field sensor and an optical attenuator. Such a magneto-optical element comprises a Faraday rotator as an optical component and therefore, the operating ability of the magneto-optical element is changed due to the temperature of the environment in which the element is operated (hereunder referred to as “use environment”). For this reason, a temperature-control device is fitted to the magneto-optical element in the use environment whose temperature is severely changed, but the use of the control device is not necessary if the temperature-dependency of the Faraday's rotational angle of the Faraday rotator is low.
As to the improvement of the Faraday rotator in the temperature-dependency, Japanese Patent No. 2,679,157 discloses the use of a terbium-bismuth-gallium iron garnet crystal. This crystalline body is grown by the flux method and therefore, it suffers from a problem such as low reproducibility and productivity and difficulty in processing.
On the other hand, it has been tried to prepare a garnet-epitaxial film according to the liquid phase epitaxy, but the usually available garnet crystals are those obtained by replacing or substituting Gd
3
Ga
5
O
12
with Ca, Mg, Zr (an NOG substrate manufactured by Shin-Etsu Chemical Co., Ltd. and having a lattice constant of 12.496±0.003 Å) and Sm
3
Ga
5
O
12
(an SGG substrate having a lattice constant of 12.439 Å).
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to, in a broad sense, solve the foregoing problems associated with the conventional techniques and to, in a narrow sense, provide a Faraday rotator whose temperature-dependency in the Faraday's rotational angle is low, a method for efficiently preparing the same, a magneto-optical element, which makes use of the rotator and whose quality and characteristic properties are less sensitive to a temperature change in the use environment and an optical isolator, at a low price.
The inventors of this invention have variously investigated any change in the Faraday's rotational angle of the Faraday rotator due to temperature changes in the use environment, have found that the temperature-dependency of the Faraday's rotational angle is adversely affected by the chemical species of ions, which occupy the positions
24
c
in the garnet crystal structure constituting the Faraday rotator and have thus completed the present invention.
According to a first aspect of the present invention, there is provided a Faraday rotator, which consists of a garnet crystal represented by the following compositional formula and having a lattice constant of 12.470±0.013 Å:
(Tb
1−(a+b+c)
Ln
a
Bi
b
M1
c
)
3
(Fe
1−d
M2
d
)
5
O
12
  I
In Formula I, Ln is an element selected from the group consisting of rare earth elements other than Tb; M1 represents an element selected from the group consisting of Ca, Mg and Sr; M2 is an element selected from the group consisting of Al, Ti, Si and Ge; and a to d are numerals satisfying the following relations: 0≦a≦0.5, 0<b≦0.2, 0≦c≦0.02 and 0≦d≦0.1.
Among the foregoing Ln, preferred are, for instance, La, Pr, Nd, Gd, Dy, Ho, Yb, Lu and Tm. These elements have ionic radii different from one another and these elements can be incorporated into the crystal in an appropriate amount which falls within the range: 0≦a≦0.5 so that the lattice constant thereof falls within the range: 12.470±0.013 Å. If the rate b of Bi present exceeds 0.2, the lattice constant of the resulting crystal is beyond the desired range defined above. The elements Ca, Mg and Sr represented by M1 have a light absorption-inhibitory effect to thus improve the optical transmission and it is sufficient to use these elements in a trace amount (rate c) on the order of not more than 0.02. The elements Al, Ti, Si and Ge represented by M2 are replaced with Fe atoms in the garnet crystal. The element Al is involved in the lattice constant and saturated magnetic flux density of the crystal. The elements Ti, Si and Ge serve to prevent light absorption like the elements Ca, Mg and Sr, when the coexisting Fe ions are in the divalent state. If the rate d of these elements exceeds 0.1, the lattice constant of the resulting crystal is beyond the desired range defined above and the saturated magnetic flux density would exceed 1000 gauss (Gs).
As specific examples of the garnet crystals represented by Formula I, preferred be one represented by the following compositional formula: Tb
2.48
Bi
0.52
Fe
5
O
12
.
According to a second aspect of the present invention, there is provided a method for preparing a Faraday rotator, which comprises the step of growing a garnet crystal represented by Formula I and having a lattice constant of 12.470±0.013 Å on a substituted or unsubstituted gadolinium-gallium-garnet crystalline substrate having a lattice constant of 12.472±0.013 Å according to the liquid phase epitaxy.
More specifically, the method for preparing a Faraday rotator is the liquid phase epitaxy, which comprises the steps of, for instance, mixing garnet components such as a combination of Tb
4
O
7
, Bi
2
O
3
and Fe
2
O
3
with flux components such as a combination of B
2
O
3
and PbO, melting the resulting mixture in a platinum crucible, and immersing a paramagnetic garnet crystalline substrate in the molten mixture while maintaining the temperature of the melt constant and rotating the substrate to thus grow the foregoing desired crystal on the substrate. The lattice constant of the paramagnetic garnet substrate is 12.472±0.013 Å and therefore, it is possible to use a substituted crystalline substrate obtained by the substitution of a gadolinium-gallium-garnet crystal with Ca and/or Zr through addition thereof.
According to a third aspect of the present invention, there is provided a Faraday rotator, which consists of a garnet crystal represented by the following compositional formula II and having a lattice constant of 12.470±0.013 Å:
(Tb
1−(a+b+c+e)
Ln
a
Bi
b
M1
c
Eu
e
)
3
(Fe
1−d
M2
d
)
5
O
12
  II
In Formula II, Ln represents an element selected from the group consisting of rare earth elements other than Tb and Eu; M1 represents an element selected from the group consisting of Ca, Mg and Sr; M2 is an element selected from the group consisting of Al, Ti, Si and Ge; and a to e are numerals satisfying the following relations: 0≦a≦0.5, 0<b≦0.2, 0≦c≦0.02, 0≦d≦0.1 and 0<e≦0.3.
The Faraday rotator according to the third aspect of the present invention is identical to the rotator according to the first aspect of the present invention except that a part of Tb present in the garnet crystal is substituted with Eu. In these Faraday rotators, the Faraday's rotational angle varies as a quadratic curve with respect to the temperature, but Eu serves to control the temperature at which the quadratic curve has a peak. Thus, the peak temperature of the Faraday's rotational angle can be controlled to a desired value (such as room temperature) by adjusting the e value to a level of not more than 0.3. In addition, Eu has an effect of inhibiting light absorption to thus improve the optical transmission of the rotator, like Ca, Mg and Sr. Therefore, if Eu is incorporated into the crystal to control the peak temperature of the Faraday's rotational angle, the rate c of Ca, Mg and Sr represented by M1 can be reduced in proportion thereto.
As specific examples of garnet crystals represented by Formul

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