Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth with a subsequent step of heat treating...
Reexamination Certificate
2002-03-27
2004-01-06
Norton, Nadine G. (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth with a subsequent step of heat treating...
C359S280000, C359S281000, C359S282000, C359S283000, C359S284000, C359S324000, C359S484010
Reexamination Certificate
active
06673146
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a Faraday rotator that is formed of a bismuth-substituted rare-earth iron garnet single crystal and does not use a permanent magnet, the bismuth-substituted rare-earth iron garnet single crystal being used as a Faraday rotator for optical isolators and optical circulators. More particularly, the present invention relates to a method of adding a square hysteresis loop to a bismuth-substituted rare-earth iron garnet single crystal having a compensation temperature near room temperature.
2. Description of the Related Art
Recently, optical fiber communications and optical instrumentation have taken a giant leap forward. Optical communications and optical instrumentation commonly use a semiconductor laser that serves as a signal source. A serious problem associated with a semiconductor laser is a so-called reflected light return where the light is reflected back by the end of the optical fiber and returns to the semiconductor laser. If light reflection return occurs, the oscillation of laser becomes unstable. Therefore, an optical isolator is disposed on the output side of the semiconductor laser to block the light reflection return, thereby stabilizing the oscillation of the laser.
Usually, an optical isolator includes a polarizer, an analyzer, a Faraday rotator and a permanent magnet that causes the Faraday rotator to be magnetically saturated. The Faraday rotator plays a critical role in the optical isolator. The Faraday rotator has a thickness of about several tens microns to about 400 &mgr;m and takes the form of a bismuth-substituted rare earth iron garnet single crystal (referred to as BIG hereinafter) grown by liquid phase epitaxy (LPE). Such BIGs include (HoTbBi)
3
Fe
5
O
12
and (TbLuBi)
3
(FeAlGa)
5
O
12
etc.
Intensive research have been carried out on rare earth iron garnets as a recording film for a magneto-optic disk and a variety of papers have been reported (Applied Physics. 2., 1973, pp. 219-228; IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-10, 1974, pp. 480-482; IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-7, 1971, pp. 397-401; and J. Applied. Physics. 53(3), March 1982, pp. 2754-2758, etc.).
These references disclose methods, principles, and theories of recording, storing, and rewriting data, the methods using the temperature dependence of magnetic properties including the square-shaped hysteresis of rare earth iron garnets.
These references describe a nucleation magnetic field (Hn), which is a measure of stability of the square hysteresis loop of magnetization. Nucleation magnetic field (Hn) is a strength of an external magnetic field applied to a garnet at which the direction of magnetization of the garnet is reversed. When a garnet is place in an external magnetic field in an opposite direction to the direction of magnetization of the garnet and the external magnetic field is increased, a tiny area having a magnetization direction opposite to the direction of the external magnetic field. Then, inversion of magnetization direction is triggered from the tiny area and spreads over the entire garnet body in an avalanche fashion. This tiny area is referred to as nucleation and the field strength of the external magnetic field that gives rise to a nucleation is thus referred to as nucleation magnetic field Hn. One of the above references reports that a garnet chip formed to a size of 1 mm square by etching shows a nucleation field, for example, Hn=1200 Oe, and a garnet having formed to a size of 1 mm square by mechanical scribing shows a nucleation field of 26 Oe, for example. The difference in nucleation field implies that a garnet should be free from defects in shape in order to be magnetically stable.
According to the above references, nucleation field Hn is given by the following equation.
Hn=a·Hs+b/Hs (a and b are proportional constants) The nucleation field Hn diverges as the temperature becomes closer to the magnetic compensation temperature of a garnet material at which the saturation magnetization field Hs of the garnet becomes zero. In other words, closer to a compensation point the temperature is, the larger the nucleation field Hn is. A nucleation field Hn of about several thousand can be observed if a garnet material is nearly ideal. Thus, it appears to be preferable to use a BIG having a compensation temperature near room temperature, as a Faraday rotator that does not use a magnet.
It is not until 20 years after the above references that BIGs having a square hysteresis loop were proposed and put into practical use as a magnet-free Faraday rotator. Japanese Patent Laid-open (KOKAI) No. 06-222311 (Laid open on Aug. 12, 1994) discloses that a BIG having a composition of (GdRBi)
3
(FeGaAl)
5
O
12
can be used to manufacture, for example, a magnet-free isolator. EP-0 647 869A1 (Published on Apr. 12, 1995) discloses an isolator having a composition of (GdBi)
3
(FeGaAL)
5
O
12
. The isolator incorporates a Faraday rotator sandwiched between glass polarizers, and shows an insertion loss of 0.4 dB and an extinction ratio of 38.8 dB at a wavelength of 1.31 &mgr;m. The isolator does not require an external magnetic field.
Japanese Patent Laid-open (KOKAI) No. 09-185027 (laid open on Jul. 15, 1997) discloses an embodiment in which a 100 &mgr;m-thick BIG having a composition of Bi
1
Eu
1
Ho
1
Fe
4
Ga
1
O
12
is cut into a slab of 11.5 mm square and a chip of 2 mm square and the slab and chip have a saturation magnetization 4 &pgr;Ms<100 G and shows a square hysteresis loop in the temperature range of −40 to +80° C. Although this reference discloses only magnetic properties of the material, the reference describes that the material can also be applied to optical isolators. Patent Preliminary Publication (KOKAI) No. 09-328398 discloses an embodiment in which a garnet has a composition of (TbBi)
3
(FeAlGa)
5
O
12
, a compensation temperature of zero degrees, a square hysteresis loop, a minimum extinction ratio of 38.8 dB, and a minimum external magnetic field of 164 Oe that can reverse the direction of magnetization of the garnet, and therefore the material can be used as an optical isolator.
Magnet-free optical isolators formed of a BIG can be compact and inexpensive because they do not use a permanent magnet. Optical isolators are subjected to an environmental temperatures higher than 100° C. during manufacture but the garnets are commonly used at room temperature. Thus, it is desirable that the material is stable in a temperature range centered about room temperature. A geometrical defect of a garnet material is a critical factor that affects the stability of the garnet material. A garnet material free from geometrical defects exhibits a strong resistance to an external magnetic field that is applied to the garnet material in such a direction as to destroy the square hysteresis loop of the garnet material.
The inventors manufactured a wafer-shaped BIG having a magnetic compensation temperature near room temperature in the shape of a wafer. Then, the Faraday rotators were manufactured in the following usual manner. The wafer was cut into chips of a size of about 10 mm square. The chips were lapped to such a thickness that the chip has a Faraday rotation angle of about 45 degrees. Then, anti-reflection coating (AR coating) was applied to both surfaces of the chip. Then, the chips were further cut into desired smaller sizes. The Faraday rotators were magnetized at room temperature (24° C.) so that the Faraday rotator acquires a square hysteresis loop. However, most of the magnetized Faraday rotators showed Faraday rotations smaller than they were expected.
This phenomenon is very new and cannot be expected in the usual manner, for example, disclosed in Japanese Patent Preliminary Publication (KOKAI) No. 09-185027, which describes only magnetic properties but suggests applications to isolators. This publication further describes in claim 14 that a BIG preferably has a compensation temperature near room temperature. However, the publication discloses only r
Shirai Kazushi
Takeda Norio
Anderson Matthew
Norton Nadine G.
Photocrystal Inc.
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