Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2000-05-19
2002-07-09
Lester, Evelyn A (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S283000, C359S324000, C359S484010
Reexamination Certificate
active
06417952
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a device for controlling Faraday rotational angle of a light which transmits through a garnet single crystal by applying external magnetic fields from more than one (or, at least two) directions to a garnet single crystal having a Faraday effect and varying the synthesized magnetic field of the more than one directionally applied external magnetic fields. More particularly, the present invention relates to and provides a Faraday rotation angle varying device, which comprises a base film of a garnet single crystal which varies Faraday rotation angle by varying the synthesized magnetic field and, in combination, a compensating film of a garnet single crystal which has a substantially fixed Faraday rotation angle, so that a wavelength variation component of the Faraday rotation angle of the base film is reduced by means of the compensating film. The Faraday rotation angle varying device of the present invention is useful particularly for optical devices such as an optical attenuator, etc.
In the field of an optical communication system, it has been required to provide an optical attenuator for controlling light transmission (that is, a transmitted light quantity), or a polarizing scrambler for continuously and cyclically varying the direction of polarization. In these devices such as the optical attenuator and polarizing scrambler is mounted a Faraday rotation angle varying device. The Faraday rotation angle varying device is constructed to control Faraday rotation angle of a light which transmits through the garnet single crystal by applying an external magnetic field to the garnet single crystal having a Faraday effect from at least two directions and varying the synthesized or composite magnetic field.
FIGS. 1A and 1B
show a typical example of an optical device employing a Faraday rotation angle varying device.
FIG. 1A
designates an entire structure of an optical attenuator and
FIG. 1B
a structural feature of the Faraday rotation angle varying device. As illustrated, a polarizer
10
, a Faraday rotation angle varying device
12
and an analyzer
14
are disposed in turn. An incident light from an input fiber
16
is made into parallel lights by a collimator lens
18
and passes through, in turn, the polarizer
10
, a garnet single crystal
20
of the Faraday rotation angle varying device, and the analyzer
14
, and then converged by a collimator lens
22
and collected by an output fiber
24
. A fixed magnetic field which is parallel with the direction of light is applied to the garnet single crystal
20
by permanent magnets
26
,
28
, and a variable magnetic field which is perpendicular to the direction of light by means of an electromagnet
30
. By changing the synthesized magnetic field to change the magnetization direction of the garnet single crystal, the Faraday rotation angle is varied so that a quantity of light transmitted to the analyzer
14
is controlled.
More specifically, the polarizer and the analyzer are aligned such that an angle of polarized surface of the light transmitting therethrough is set to be 105 degrees and, when the magnetic field of the electromagnet is zero (0), Faraday rotation angle of the garnet single crystal becomes maximum, that is, 96 degrees. Since the angle of polarizing directions of the polarizer and the analyzer is 105 degrees, a quantity of light (a quantity of outgoing light) passing through the analyzer is reduced due to an angle of deviation, the reduction being extremely small. By contrast, as the Faraday rotation angle is reduced by applying a magnetic field to the electromagnet, the angle of deviation becomes increased to thereby increase a quantity of attenuation (that is, a quantity of outgoing light is reduced). When Faraday rotation angle is 15 degrees, the analyzer is of a so-called crossed Nicol state relative to the analyzer, so that the attenuation quantity becomes maximum.
Recently, by a new a practical application of a wavelength multiplex communication system, there has been an industrial requirement that the optical device, has less wavelength dependency. Thus, it is also required that a Faraday rotation angle varying device, as well, has less wavelength dependency.
In the example of the optical attenuator described above, when a magnetic field of the electromagnet is zero (0, there is no substantial change in a quantity of light (outgoing light quantity) passing through the analyzer even when there is more or less change in Faraday rotation angle due to a wavelength variation. When a magnetic field is applied to the electromagnet to increase a deviation of angle, it causes an increase of attenuation (that is, reduction of quantity of outgoing light), and a quantity of attenuation (dB) at that time is represented by the following formula (equation):
D
=−10*log(10
(−ko/10)
+sin
2
(&Dgr;&thgr;))
provided that:
ko: extinction ratio of garnet single crystal; and
&Dgr;&thgr;: deviation of angle from a crossed Nicol state.
From the formula described above, it is understood that a quantity of attenuation is dependent upon the square of a sine function of the deviation angle at the position near the crossed Nicol state at which a large attenuation is obtained and extremely sensitive to the angle. In other words, in this region there is a problem that an attenuation is extensively varied by a change of Faraday rotation angle due to a wavelength variation.
With reference to Faraday rotator which uses a constant state of Faraday rotation angle, there have been many attempts and suggestions to reduce a wavelength dependency as disclosed in, for example, Japanese Patent Publication (Unexamined) No. 2-256,018 and No. 10-273,397. However, with respect to a Faraday rotation angle varying device which permits variation or adjustment of Faraday rotation angle, there is no attempt for reducing a wavelength dependency and, therefore, with reference to an optical attenuator for wavelength multiplex communication, it has been strongly required that a wavelength dependency is reduced at a region adjacent to the attenuation quantity which is particularly required in accordance with the state and condition of use.
SUMMARY OF THE INVENTION
It is, therefore, a general object of the present invention to provide a new Faraday rotation angle varying device which can reduce a wavelength dependency.
Another object of the present invention is to provide a Faraday rotation angle varying device which has less wavelength dependency and less temperature dependency.
According to the present invention, there is provided a Faraday rotation angle varying device in which an external magnetic field is applied from at least two directions to a garnet single crystal having a Faraday effect and varying a synthesized magnetic field so that Faraday rotation angle of light transmitting through the garnet single crystal is controlled, comprising:
a base film of garnet single crystal having a rotation angle varied in accordance with variation of a synthesized magnetic field, and
a compensating film of a garnet single crystal having a constant Faraday rotation angle,
wherein the base film has a wavelength coefficient sign and the compensating film has a wavelength coefficient sign different from the sign of a wavelength coefficient of the base film, so that a wavelength variation component of the Faraday rotation angle of the base film is reduced by the compensating film.
The outer magnetic fields are, in general, applied from two directions, that is, a parallel direction and a perpendicular direction relative to a light direction and, in that case, it is preferred that the magnetic field which is parallel to the light direction is a fixed magnetic field formed by the permanent magnet which has a magnetic strength for permitting the base film to be magnetically saturated, whereas the perpendicular magnetic field is a variable magnetic field applied by the electromagnet.
The outer magnetic fields are always applied to the base film from the two directions or more, whereas an adjustmen
Kawai Hirotaka
Umezawa Hiromitsu
FDK Corporation
Lester Evelyn A
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