Faraday rotator

Details Faraday rotator Faraday rotator

1999-02-26

2001-09-11

Lester, Evelyn A (Department: 2873)

Optical: systems and elements

Optical modulator

Light wave temporal modulation

C359S280000, C359S281000, C359S282000, C359S484010

Reexamination Certificate

active

06288827

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to a Faraday rotator in which a Faraday element and an external magnetic rield applying means are arranged such that the magnetization direction of the Faraday element is tilted with respect to a light ray direction, and particularly to a Faraday rotator capable of reducing the amount of the temperature-dependent change in Faraday rotation angle by making use of the temperature dependence on an angle &agr; between the magnetization direction of the Faraday element and the light ray direction. Such a Faraday rotator is useful for various optical devices utilizing the Faraday effect, such as an optical attenuator, and an optical isolator.
2. Related Art
Optical communication systems require an optical isolator for allowing light rays to pass therethrough only in one direction, an optical attenuator for controlling the quantity of light rays passing therethrough, etc., and a Faraday rotator for rotating the polarization plane of light rays passing therethrough is incorporated in the optical isolator, optical annenuator, etc. The Faraday rotator is also used for other optical devices such as an optical switch, optical circulator, optical filter, and optical equalizer.
An optical isolator has a configuration, for example, shown in
FIGS. 21A and 21B
in which a 45° Faraday rotator
3
is inserted between a polarizer
1
and an analyzer
2
which are arranged such that the polarization planes of light rays passing through the polarizer
1
and analyzer
2
intersect one another at 45°. The Faraday rotator
3
includes a Faraday element composed of a magnetooptic crystal in combination with a permanent magnet as an external magnetic field applying means. An external magnetic field is applied to the Faraday element by the permanent magnet in such a manner as to correspond to a light ray direction, to realize a magnetic saturation state of the magnetooptic crystal. The magnetooptic crystal is designed to have a thickness allowing the polarization plane of light rays passing therethrough to be rotated 45° in the above magnetic saturation state. When light rays are allowed to pass through the optical isolator in the forward direction, the light rays having passed through the polarizer
1
pass through the analyzer
2
almost with no loss (see FIG.
21
A). On the contrary, when light rays are allowed to pass through the optical isolator in the reverse direction, the light rays having passed through the analyzer
2
cannot pass through the polarizer
1
because the polarization plane of the light rays having passed through the Faraday rotator
3
are rendered perpendicular to the polarizer
1
(see FIG.
21
B). This optical isolator is of a polarization-dependent type; however, there is also known a polarization-independent type (see Japanese Patent Application No. Sho 56-148290).
One example of a prior art optical attenuator is shown in
FIGS. 2A and 2B
. As shown in
FIG. 2A
, a polarizer
14
composed of a wedge-shaped birefringent crystal (for example, rutile), a Faraday rotator
15
, and an analyzer
16
composed of a wedge-shaped birefringent crystal
16
are arranged on the optical axis in this order between an input fiber
12
having a collimate lens
10
and an output fiber
13
having a collimate lens
11
(see Japanese Patent Application No. Hei 4-205044). The Faraday rotator
15
includes, as shown in
FIG. 2B
, a Faraday element (magnetooptic crystal)
17
in combination with a permanent magnet
18
and an electromagnet
19
for applying magnetic fields to the Faraday element
17
in two directions which are 90° offset from each other. The magnetization direction of the Faraday element
17
is matched with the direction of a synthetic magnetic field of a specific magnetic field applied by the permanent magnet
18
and a variable magnetic field applied by the electromagnet
19
. Therefore, the Faraday rotation angle is variable.
For example, when the polarizer
14
and the analyzer
16
are arranged such that the optical axes of both the birefringent crystals thereof are rendered parallel to each other, the optical attenuator operates as follows. Light rays having gone out of the input fiber
12
are converted into a collimated light beam through the first lens
10
and are separated into an ordinary light ray o and an extraordinary light ray e through the polarizer
14
. The polarization direction of the ordinary light ray o is perpendicular to that of the extraordinary light ray e. When the light rays o and e pass through the Faraday rotator
15
, the polarization direction of each of the light rays o and e is rotated depending on the magnitude of a component of the magnetization of the Faraday element
17
in the direction parallel to the optical axis. The light rays o and e are then separated, through the analyzer
16
, into an ordinary light ray o
1
and an extraordinary light ray e
1
, and an ordinary light ray o
2
and an extraordinary light ray e
2
, respectively. As shown by solid lines in
FIG. 2A
, the ordinary light ray o
1
and extraordinary light ray e
2
outgoing from the analyzer
16
are parallel to each other, and are coupled to the output fiber
13
through the second lens
11
. Meanwhile, as shown by broken lines in
FIG. 2A
, the extraordinary light ray e
1
and ordinary light ray o
2
outgoing from the analyzer
16
are not parallel to each other but spread outwardly, and are not coupled to the output fiber
13
through the second lens
11
.
When the magnetic field applied to the Faraday element
17
by the electromagnet
19
comes into zero, that is, when the magnetization direction of the Faraday element
17
is rendered parallel to the optical axis, the Faraday rotation angle of the Faraday element
17
becomes 90°. At this time, the ordinary light ray o having gone out of the polarizer
14
goes out of the analyzer
16
as the extraordinary light ray e
1
. The extraordinary light ray e having gone out of the polarizer
14
goes out of the analyzer
16
as the ordinary light ray o
2
. The light rays e
1
and o
2
are spread outwardly, and are not coupled to the output fiber
13
through the second lens
11
. On the contrary, when the magnetic field applied to the Faraday element
17
by the electromagnet
19
becomes sufficiently large, the Faraday rotation angle of the Faraday element
17
comes closer to 0°. At this time, almost all of the ordinary light ray o having gone out of the polarizer
14
goes out of the analyzer
16
as the ordinary light ray o
1
, and almost all of the extraordinary light ray e having gone out of the polarizer
14
goes out of the analyzer
16
as the extraordinary light ray e
2
. The light rays o
1
and e
2
are parallel to each other, and are all coupled to the output fiber
13
through the second lens
11
. The magnetization of the Faraday element
17
is thus rotated depending on the strength of the magnetic field applied to the Faraday element
17
by the electromagnet
19
, to change the Faraday rotation angle of the Faraday element
17
in a range of about 90 to about 0°, thereby making variable the quantity of the light rays coupled to the output fiber
13
in accordance with the amount of the change in Faraday rotation angle. In this way, the above configuration including the Faraday rotator
15
functions as an optical attenuator.
It should be noted that if the polarizer
14
and the analyzer
16
are arranged such that the optical axes of both the birefringent crystals thereof are perpendicular to each other, the optical attenuator operates in accordance with the manner reversed to that described above. That is to say, when the Faraday rotation angle of the Faraday element
17
becomes 90°, the quantity of light rays passing through the optical attenuator is maximized, while when the Faraday rotation angle of the Faraday element
17
becomes zero, the quantity of light rays passing through the optical attenuator is minimized.
As the Faraday element to be incorporated in the Faraday rotator, there has been, in recent yea

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