Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2002-05-21
2004-06-08
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S282000
Reexamination Certificate
active
06747782
ABSTRACT:
INDUSTRIAL FIELD OF THE INVENTION
This invention relates to a device for adjusting the angle of Faraday rotation (Faraday rotator) and also to an optical attenuator using such a device. The Faraday rotation angle-adjusting device and optical attenuator according to the present invention are specifically used in optical transmission communication systems. The invention, in particular, improves the temperature dependence of Faraday rotation angle and narrows the scatter of variations among products in the angle of Faraday rotation with externally applied variable magnetic fields. Moreover, the invention reduces the variable magnetic field required to achieve a desired low-current characteristic or specific amount of attenuation (proportional to the angle of Faraday rotation).
PRIOR ART
Owing to the striking expansion of transmission capacities, there is a growing demand for high-density-wavelength multiplex transmission systems. This has accordingly increased the need for variable optical attenuators that dynamically adjust the quantity of light required for the systems. Applications for the attenuators include the control of light quantities for individual channels and simultaneous attenuation of multiplex light rays. Among those optical attenuators there is one type that utilizes magneto-optical effect. That type usually involves a layout in which a component capable of changing the angle of Faraday rotation a light beam is disposed between a polarizer and an analyzer. With one such component for changing the Faraday rotation angle, external magnetic fields are applied from two or more different directions to a single crystal of garnet having a Faraday effect so as to make the composite external field variable, whereby the Faraday rotation angle of the light that passes through the single crystal of garnet is controlled (Registered Japanese Patent 2,815,509).
To be more specific, the Registered Japanese Patent 2,815,509 discloses an optical attenuator which, while keeping a fixed magnetic field greater than the saturation magnetic field of a single crystal of garnet applied to the crystal in a direction parallel to the optical axis by means of a permanent magnet, applies a variable magnetic field to the crystal in a direction perpendicular to the optical axis by an electromagnet, thereby changes the composite magnetic field vector, and changes the direction of magnetization and hence the Faraday rotation angle of the single garnet crystal so that the quantity of light coupled to the fiber on the leaving side can be controlled.
Another method is known as a means of decreasing the temperature dependence of optical attenuators, which comprises applying external magnetic fields in the directions where the amount of change of the Faraday rotation angle due to the temperature dependence of the angle between the direction of magnetization of Faraday elements and the direction of the beam and the amount of change of the Faraday rotation angle due to the temperature dependence with the Faraday effect are different in code from each other and the absolute value of either amount is less than twice that of the other amount, whereby the changes of Faraday rotation angle with temperature are restricted. (Japanese Patent Application Kokai No. 11-249095).
PROBLEMS THAT THE INVENTION IS TO SOLVE
With the foregoing in view, a plurality of optical attenuators were experimentally fabricated. The experiments presented a problem of wide scatter among the specimens of the temperature dependence values of the attenuation and the electromagnet field perpendicular to the optical axis required to attain the maximum attenuation. Another problem that arose was the requirement of a large variable magnetic field (and hence a large driving current) to achieve a desired attenuation (proportional to the angle of Faraday rotation).
Therefore, the solution of these problems in accordance with the present invention is to provide an optical attenuator that narrows the scatter of characteristics among devices, reduces the temperature dependence, and possesses good low-current characteristics.
Also, in view of the foregoing, we experimentally fabricated a plurality of optical attenuators and found that they show, in common, that an increase in the quantity of attenuation is accompanied with an increase in the polarization dependence loss (hereinafter abbreviated to “PDL”) up to more than one decibel at peak. The value is by far the greater than those of non-polarization-dependent optical isolators, the optical devices that similarly take the advantage of magneto-optical effect and are already in wide use with optical transmission systems. Typical PDL values of the non-polarization-dependent optical isolators are of the order of 0.1 dB.
A study of the difference revealed that, when birefringent elements are used as a polarizer and an analyzer, ordinary and extraordinary rays pass through a Faraday element along different paths and accordingly the distributions of magnetic fields that are applied to different portions of the Faraday element vary too. Factors that can subtly influence the magnetic field distributions are, for example, the direction of application of variable magnetic field, shape and size of the yoke of an electromagnet that produces the variable field, and the relative positions of the yoke and Faraday element.
FIG. 22
illustrates, by way of example, a conventional member for attaching a Faraday element to an optical attenuator. The member has a cutout
20
for receiving a yoke and an opening
15
formed in the center for the passage of a light ray. A Faraday element recess
21
is formed at the bottom of the cutout
20
in alignment with the opening
15
, and a Faraday element (not shown) consisting of one or more garnet crystal plates is received in the recess. The Faraday element is secured in place with a thermosetting or ultraviolet-curing resin filled in resin-filling ports
16
,
17
open to the cutout
20
. Yokes
10
,
10
of electromagnets are inserted on both sides of the opening
15
into the cutout
20
and are similarly fixed with a hardening resin. There is no means of positioning the inner ends of the yokes
10
of electromagnets, and the yokes simply secured with respect to the Faraday element fail to maintain a constant positional relationship. Moreover, the combined area of the yokes relative to the Faraday element is restricted. These and other factors are deemed responsible for the problem of increased PDL.
Therefore, another problem that the present invention is to solve are settled by the provision of an optical attenuator having favorable PDL characteristics.
MEANS OF SOLVING THE PROBLEMS
The present invention provides a Faraday rotator in which magnetic field is applied to Faraday elements, optical axis of said Faraday elements being in the <111> direction of single crystal of garnet, characterized in that
three single crystals of garnet of substantially the same thickness having a Faraday effect are used to form the Faraday elements and the Faraday elements are arranged in such a manner that a first magnetic field is applied to one of the Faraday elements, in the direction perpendicular to a plane in a range extending 5 deg. each to the left and right of the line connecting the (111) plane in the center of a stereographic projection chart with the (−1−12) plane on the outermost circumference or a plane equivalent thereto in the chart, whereas a second magnetic field is applied to the remaining two elements, in the direction perpendicular to plane in a range extending 5 deg. each to the left and right of the line connecting the (111) plane in the center of the stereographic projection chart with the (−101) plane on the outermost circumference or a plane equivalent thereto in the chart. Each of said first and second magnetic fields may be a composite magnetic field formed by a pair of magnetic fields.
The present invention also provides a Faraday rotator in which both a magnetic field parallel to and a magnetic field perpendicular to the optical axis a
Goto Masanori
Iwatsuka Shinji
Takayama Seiichi
Drinker Biddle & Reath LLP
Hanig Richard
Sugarman Scott J.
TDK Corporation
LandOfFree
Faraday rotator and optical attenuator does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Faraday rotator and optical attenuator, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Faraday rotator and optical attenuator will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3319435