Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2001-09-10
2003-04-08
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S282000
Reexamination Certificate
active
06545795
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magneto-optical member which is used in a 45 degree Faraday rotator as one constituent member of an optical isolator for use in an optical fiber communication system, an optical measurement system and the like, and also to an optical isolator using the magneto-optical member. Now, the optical isolator includes a polarizer, an analyzer, a 45 degree Faraday rotator which is interposed between the polarizer and the analyzer and which has a magneto-optical effect such as the Faraday effect, and a magnet for applying magnetic field to the 45 degree Faraday rotator, and serves to propagate light emitted from a light source (semiconductor laser) to a transmission path such as an optical fiber or the like without any loss, and to block reflected light from the optical fiber or the like so as to prohibit the reflected light from returning to the light source (semiconductor laser).
2. Description of the Related Art
In an optical fiber communication system having a semiconductor laser as a light source, in particular, an optical system based on a high speed digital transmission or an analog direct modulation mode, if reflected light from optical connector connections, optical circuit components and the like which are used in an optical fiber circuit returns to the semiconductor laser or an optical amplifier, it becomes difficult to maintain high quality transmission due to degradation of frequency characteristics or generation of noises. An optical isolator is used for the purpose of removing the reflected light.
FIG. 7
shows the construction of an optical isolator, and
FIG. 8
shows the principles of operation thereof.
In
FIGS. 7 and 8
, an optical isolator
1
is generally constituted by a polarizer
2
A and an analyzer
2
B which allow only the light component having a fixed plane of polarization to pass through, a 45 degree Faraday rotator
3
which is provided between the polarizer
2
A and the analyzer
2
B and rotates the plane of polarization of light by 45 degrees, and a permanent magnet
4
for applying a magnetic field H to the 45 degree Faraday rotator
3
.
Light
301
which travels in the forward direction shown in a part (I) of FIG.
8
is not polarized, but after having passed through the polarizer
2
A, becomes light
302
which has only a component oriented in the direction of the polarization of the polarizer
2
A. Then, after having passed through the 45 degree Faraday rotator
3
, the light
302
becomes light
303
in which the polarization direction is rotated by 45 degrees. If the polarization direction of the analyzer
2
B is adjusted so as to run parallel to the polarization direction of the light which has been rotated by 45 degrees, the light passes through the analyzer
2
B with a minimum loss.
On the other hand, as shown in a part (II) of
FIG. 8
, as for light
305
which has been reflected and propagates from the optical fiber or the like in the backward direction, only light
306
having a component oriented in the polarization direction of the analyzer
2
B passes through the analyzer
2
B to be made incident on the 45 degree Faraday rotator
3
in the backward direction. This light
306
is further rotated by 45 degrees in the same direction as when traveling in the forward direction on the basis of the non-reciprocity peculiar to the Faraday effect. As a result, after having passed through the 45 degree Faraday rotator
3
, the light
306
becomes light
307
which has a polarization direction perpendicular to the polarization direction of the polarizer
2
A and therefore is blocked at the polarizer
2
A so as not to return to the light source.
Of the constituent members of the optical isolator
1
, the 45 degree Faraday rotator
3
has a large influence on the performance of the optical isolator
1
. The 45 degree Faraday rotator
3
is required to have a small element length necessary for rotating the plane of polarization by 45 degrees and a large light transmittance. Up to now, a magneto-optical member which is employed as the 45 degree Faraday rotator
3
may be made, for example, of a yttrium iron garnet (YIG) bulk single crystal (about 2 mm in thickness), or of a bismuth-substituted rare earth iron garnet (BiYIG) thick film single crystal in which a part of yttrium is substituted with bismuth having a large magneto-optical performance index (several hundred &mgr;m in thickness). Recently, the BIYIG thick film single crystal is employed as the magneto-optical member in many cases because it is advantageous in the miniaturization of the optical isolator.
This BiYIG thick film single crystal is produced by utilizing a liquid phase epitaxial (LPE) growth method. In order to carry out a stable liquid phase epitaxial growth, many production parameters need to be accurately controlled, and therefore it is difficult to grow uniform single crystals over a large area with a high yield. In addition, the fact that it takes 20 hours or more to grow the single crystals and that an expensive non-magnetic GGG (gadolinium gallium garnet) single crystalline wafer needs to be employed for the substrate is an obstacle to the reduction in cost.
Under the situation described above, in order to solve the above-mentioned problems associated with the magneto-optical member which is produced by utilizing the liquid phase epitaxial growth method, the present inventors disclosed in Japanese Patent Application No. Hei 11-283512 the magneto-optical member (45 degree Faraday rotator) made of one-dimensional magnetic photonic crystal which causes the enhancement of the magneto-optical effect (the Faraday effect is one kind of magneto-optical effect) due to the localization of light. Then, though the above-mentioned magneto-optical member made of the one-dimensional magnetic photonic crystal is of polycrystals with a thickness of several &mgr;m, a large Faraday rotation angle can be obtained.
In this connection, the one-dimensional magnetic photonic crystal is also described in JOURNAL OF THE MAGNETICS SOCIETY OF JAPAN, Vol. 23, pp. 1861 to 1866 (1999). The one-dimensional magnetic photonic crystal is structured such that a layer of magnetic substance and a layer of dielectric substance are alternately and multiply laminated with each thickness thereof irregular and formed into a thin film, or that two dielectric films, which are each formed of multiple layers having two kinds of dielectric substances different from each other and alternately laminated with each thickness thereof regular, sandwich a layer of magnetic substance having a thickness different from the layers of dielectric substance.
Of the above-mentioned structures, the latter is identical to the structure known long as the Fabry-Perot resonator structure and is found to be easily manufactured and at the same time to have a large enhancement as well. A magneto-optical member
10
made of one-dimensional magnetic photonic crystal is shown in
FIG. 9
as an example.
The magneto-optical member
10
made of one-dimensional magnetic photonic crystal includes two periodic dielectric multilayer films
13
and
14
in each of which two kinds of dielectric substances (dielectric thin films)
11
and
12
are alternately laminated with each thickness thereof regular, and a magnet-optical thin film (made of magnetic substance)
15
which is provided between the two periodic dielectric multilayer films
13
and
14
.
The periodic dielectric multilayer films
13
and
14
play a part of the reflecting mirror of the Fabry-Perot resonator. While a (Ta
2
O
5
/SiO
2
) system multilayer film is generally employed as the periodic dielectric multilayer films
13
and
14
, a (Si/SiO
2
) system multilayer film has also been proposed in which a large Faraday rotation angle can be obtained with a smaller number of laminations than in the (Ta
2
O
5
/SiO
2
) system multilayer film. The thickness of each of the dielectric substances (dielectric thin films)
11
and
12
needs to be designed in such a way that its optical length (actual thickness&tim
Inoue Mitsuteru
Kato Hideki
Matsushita Takeshi
Takayama Akio
Epps Georgia
Minebea Co. Ltd.
Oliff & Berridg,e PLC
Thompson Timothy
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