JPH04115221A - Faraday rotating element - Google Patents

Abstract

PURPOSE:To obtain the rotating element which has high reliability, can be driven with a low electric power and has the high extinction ratio of output light by including a coil for converting the magnetization state of a material (FR) having a magneto-optical effect into the space enclosed with a yoke and the FR and setting the magnetization direction of a permanent magnet having a structure to hold the FR at the direction perpendicular to the propagation direction of light. CONSTITUTION:The circular cylindrical Faraday rotor FR 4 is built in a cylindrical Faraday rotator holder 1. This holder 1 is constituted of the permanent magnet and the magnetization direction thereof is nearly perpendicular to the propagation direction of the light. This magnetization state is substantially not changed by the magnetic field generated by the coil 10. The coil 10 for inverting the magnetization of the FR 4 is wound around the FR 4. The yoke 9 having light pass holes 12a, 12b at the center is so disposed as to include the FR and to include the coil 10 as well. The lead wires 10a, 10b of the coil 10 are taken out to the outside of the yoke 9 and the magnetization state of the FR 4 can be easily changed by passing a current thereto. The easy inversion of the magnetization of the FR 4 to invert the current is possible as well. Since this Faraday rotating element has no mechanically moving parts in such a manner, the element has the high reliability and the driving with the low electric power is possible.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to a Faraday rotation element that has the function of continuously changing the plane of polarization of light using an electric current.

``Prior art'' Coherent optical communication has high sensitivity reception that is more than 10 times that of the current optical intensity modulation method, and has 100
Since it is possible to simultaneously communicate more than one channel with a single fiber, it is expected to be used in long-distance multiplex communication systems that simultaneously transmit audio and video, such as in videophones and video conferences.

The reception method for coherent optical communication uses a heterodyne detection method in which signal light propagating through a fiber interferes with local oscillation light placed on the receiving side. In order to cause two lights to interfere efficiently, it is necessary to match the planes of polarization of the two lights with high accuracy.

However, the fibers actually installed are subject to temperature changes,
Its weak point is that it is susceptible to vibrations and deflections, and the plane of polarization of the signal light on the receiving side fluctuates, reducing the final reception efficiency.

In order to solve this problem, two methods have been considered: a polarization diversity method that corrects the shift in the polarization plane using a circuit, and a method that directly optically makes the polarization plane of the signal light constant using automatic control.

The former polarization diversity method has drawbacks such as a complicated circuit and slow response speed. Regarding the latter optical method, a method using electro-optic effects such as lithium niobate, a method of mechanically compressing a fiber, a method of mechanically rotating a wave plate, etc. have been reported so far. These methods have advantages and disadvantages, and it is not possible at this stage to determine which method is the most efficient. When considering long-term reliability, it is thought that systems with mechanical moving parts will become obsolete and will be gradually replaced by electronic and electromagnetic systems.

In addition to the method for controlling the plane of polarization described above, there is a known method for electromagnetically rotating the plane of polarization that utilizes the Faraday effect, which is one of the magneto-optical effects.

The Faraday effect can be caused by paramagnetic materials or ferromagnetic materials. The angle of rotation of the plane of polarization due to the Faraday effect of the front sound is proportional to the magnetic field applied to the paramagnetic material, so it has the advantage of being easy to control, or if you try to obtain a rotation angle of more than 90 degrees, it will require a large magnetic field (large power). It is not practical as it requires a large sample size.

The rotation angle of the plane of polarization due to the latter Faraday effect is proportional to the net magnetization of the ferromagnetic material in the direction in which the light travels, and even if a rotation angle of 90 degrees or more is to be obtained, a small sample size is sufficient. However, the magnetization state of a ferromagnetic material generally exhibits nonlinear behavior with respect to the magnetic field applied to the ferromagnetic material, making it extremely difficult to control it electromagnetically. In particular, in an unsaturated state where the net magnetization of a ferromagnetic material is close to zero, a multi-domain structure is likely to occur, and when this is used as a polarization control element, the output light becomes elliptically polarized, significantly degrading the extinction ratio. Tend.

FIG. 9 shows the structure of a Faraday rotary element having a core type magnetic circuit, which has already been filed by the present inventors. In this structure, a Faraday rotator (FR) '4 made of YIG single crystal or the like is arranged in the center of the hollow yoke 9, and the wire ring 10 is FR4.
wrapped around.

In this magnetic circuit, a closed magnetic path is formed by the yoke 9 and the FR4, and it is possible to magnetically saturate the FR4 with an extremely small amount of electric power.

FIG. 10 shows the operating characteristics of the Faraday rotation element having the conventional structure shown in FIG. As you can see from this figure, about 6
It is possible to saturate the Faraday rotation angle to 45 degrees with a minute current of 0mA (power approximately 0.3mW), but in the region where the Faraday rotation angle changes rapidly near zero current, the operation is not only unstable. , the extinction ratio is significantly degraded to 17 dB, which is not preferable when used as a polarization plane control element for coherent optical communication or the like.

"Problems to be Solved by the Invention" As stated above, conventional Faraday rotary elements using the Faraday effect require a large amount of electric power and are extremely large in size in the case of paramagnetic materials. In the case of ferromagnetic materials, there were problems such as deterioration of extinction ratio.

An object of the present invention is to provide a Faraday rotation element having a magnetic circuit with a new structure in order to solve these problems.

"Means for solving the l\Ut point" In order to achieve the above object, the Faraday rotation element of the present invention is provided with a light passage hole penetrating the center of the hollow yoke. A substance (FR) having a magneto-optical effect is provided at the center of the interior, and the FR and the yoke form a magnetic circuit close to a closed magnetic circuit, and the magnetization state of the FR is changed in the space surrounded by the yoke and the FR. The structure containing the wire ring for causing the FR and holding the FR in close proximity is constituted by a permanent magnet, and the direction of magnetization of the permanent magnet is perpendicular to the propagation direction of the light. It is a feature.

"Effects" By using the configuration of the present invention, it is possible to provide a Faraday rotation element that is extremely reliable, can be driven with low power, and has a high extinction ratio of output light. This will be explained in detail using an example.

FIG. 1 is an embodiment showing the principle structure of the Faraday rotation element of the present invention.

As shown in the perspective view of FIG. 2, the cylindrical Faraday rotator FR4 is housed in the cylindrical Faraday rotator holder 1. As shown in the perspective view of FIG. In this embodiment, FR4 is made of YIG single crystal. Further, the holder 1, which is a feature of the present invention, is composed of a permanent magnet, and its magnetization direction is approximately perpendicular to the propagation direction of light. This magnetization state is wire ring 10
It hardly changes depending on the magnetic field generated by the magnetic field.

A wire ring 10 is wrapped around the FR4 to reverse the magnetization of the FR4. Around this, there is a light passage hole 12 in the center.
FR4 is included in the center of the yoke 9 having a and 12b,
It is arranged so that it also includes O.

In this embodiment, the yoke 9 is made of a soft magnetic material such as Hermalloy, and is therefore dirty. The second yoke 9 and the second yoke 1 form a 4F configuration of a magnetic circuit that is almost a closed magnetic path.

Lead wire 10 a + of wire ring lO] Ob is yoke 9
F
The magnetization state of R,4 can be easily changed. Therefore, when the current is reversed, the magnetization of FR4 can also be easily reversed.

As is clear from comparing the structure of the present invention shown in Fig. 1 with the structure of the prior art shown in Fig. 9, the difference between the two is the material or the F of the permanent magnet.
This is the addition of R holder 1.

In the conventional structure shown in FIG. 9, there is also a second method in which a non-magnetic material is used as the FR holder. In the magnetic circuit of the present invention, the magnetization direction of the permanent magnet of the holder 1 or the propagation direction of light (optical axis)
It is characterized by being perpendicular to.

FIG. 3 shows another embodiment of the Faraday rotation element of the present invention. In this case, a cylindrical permanent magnet 2 is arranged outside the wire ring 10. FIG. 4 is a sectional view taken along line AA' in FIG. 3. The direction of magnetization of the permanent magnet 2 is along one radial direction of the cylinder.

In this case, since the permanent magnet 2 is further away from the magnet FR4 than in the above embodiment, the magnetic field applied to the magnet FR4 becomes weaker. However, as shown in the cross-sectional view of FIG. 4, the magnetic charge on the outside of the cylindrical magnet 2 is effectively prevented by the outer circumferential yoke 9, so that the magnetic field applied to the FR 4 becomes stronger accordingly.

This is because if there is no yoke 9, the magnetic field due to the magnetic charges on the outside of the cylindrical magnet will be in the opposite direction to the magnetic field due to the magnetic charges on the inside.
This is because it acts to weaken the magnetic field applied to FR4.

FIG. 5 shows the operating characteristics of the Faraday rotation element having the structure of the invention shown in FIG. When the applied current is +200 mA, the Faraday rotation angle reaches the saturation value of +50 degrees, and when the applied current is returned to zero, the Faraday rotation angle becomes almost 0 degrees,
During that time, it changes almost linearly. Reverse the applied current and
At 00 mA, the Faraday rotation angle similarly reaches the saturation value of 150 degrees.

On the other hand, the extinction ratio of the output horn light was more than 30 dB at ±100 mA, but it did not become less than 28 dB even when the current was near zero. As is clear from comparison with the operating characteristics of the prior art shown in FIG. 1.0, the structure of the present invention has an effect of improving the extinction ratio by about 10 dB near zero current. This is considered to be because the generation of a multi-domain structure near zero current was suppressed due to the bias magnetic field in the orthogonal direction by the FR holder 1.

FIGS. 6 and 7 are diagrams showing other embodiments of the present invention.

These are cases in which a protrusion 11 is provided at the center of the hollow yoke 9 in order to increase the volume of the wire ring 10. The yoke 9 can be divided and assembled using a dividing portion 13 for easy assembly. FIG. 8 shows a case where a notch 14 is provided at the periphery in order to release the heat generated by the coil to the outside.

"Effects of the Invention" As described above, the Faraday rotary element of the present invention has high reliability because it has no mechanically moving parts, and can be driven with low power, so it can be used as a unique and important optical-related component. It meets the various demands of industry.

[Brief explanation of the drawing]

1 to 4 are diagrams showing the structure of an embodiment of the invention, FIG. 5 is a characteristic diagram showing the operating characteristics of the embodiment of the invention, and FIGS. 6 to 8 are diagrams showing other embodiments of the invention. FIG. 9 is a diagram showing an example, and FIG. 9 is a diagram for explaining the structure of the prior art, and FIG. 10 is a diagram for explaining the operating characteristics of the prior art. 1; Faraday rotator (FR,) holder (permanent magnet)
, 2; Cylindrical magnet 4: Faraday rotator (FR), 10. Wire ring, 9: Yoke, 10a, 10b; Current terminal 11; Protrusion, 12; Passing hole, 13; Divided portion, 14; Peripheral notch

Claims (1)

[Claims] 1) A light passage hole is provided so as to pass through the center of a hollow yoke, a material (FR) having a magneto-optical effect is provided at the center of the inside of the hollow yoke, and the FR and the In a Faraday rotation element that forms a magnetic circuit close to a closed magnetic path by a yoke and includes a wire ring for changing the magnetization state of the FR in a space surrounded by the yoke and the FR, the direction of propagation of light (optical axis) The bias magnetic field almost perpendicular to the FR
A Faraday rotator element characterized in that a voltage is applied to the Faraday rotator. 2) In the Faraday rotation element according to item 1, the method of applying the bias magnetic field is realized by a structure that is close to the FR and that holds it constituted by a permanent magnet. Faraday rotating element. 3) The Faraday rotator element according to item 1, wherein the method of applying the bias magnetic field is realized by a permanent magnet placed outside the wire ring.