JP2000187193A - Optical attenuator - Google Patents

Abstract

(57) [Summary] [Problem] To reduce variation in characteristics of each device,
The required magnetic field by the electromagnet is reduced, and the wavelength dependence and the temperature dependence are reduced. SOLUTION: A polarizer, a garnet single crystal and an analyzer are arranged in that order, a magnetic field is applied to the garnet single crystal in a direction parallel to an optical axis by an electromagnet, and a magnetic field is applied to a direction perpendicular to the optical axis by a permanent magnet. Is applied, and the Faraday rotation angle is changed by the combined magnetic field to control the amount of transmitted light.
The optical axis is in the <111> direction, and the magnetic field of the permanent magnet is equal to the center (111) plane and the (110) plane on the outermost circle in a stereographic projection centered on the (111) plane perpendicular to the optical axis.
A plane equivalent to the plane, and within 10 degrees closer to the (111) plane located 70 degrees from the center (111) plane from the plane equivalent to the (110) plane, and its direction is the center. From the (111) plane. The displacement path of the resultant magnetic field vector is set within the same range.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical attenuator in which a polarizer, a garnet single crystal having a Faraday effect and an analyzer are combined, and more specifically, a direction parallel to an optical axis with respect to a garnet single crystal by an electromagnet. The present invention relates to an optical attenuator that applies a magnetic field to a magnetic field and applies a magnetic field in a direction perpendicular to the optical axis by a permanent magnet, changes the Faraday rotation angle by the combined magnetic field, and controls the amount of transmitted light.

[0002]

2. Description of the Related Art In an optical communication system or the like, an optical attenuator is used to control the amount of transmitted light. As the optical attenuator, a configuration in which a Faraday rotation angle varying device is incorporated between a polarizer and an analyzer is usually adopted. In the device for varying the Faraday rotation angle, an external magnetic field is applied to the garnet single crystal having the Faraday effect from two or more different directions, and the resultant magnetic field is varied to thereby change the Faraday rotation angle of light transmitted through the garnet single crystal. Is controlling.

For example, Japanese Patent Application Laid-Open No. Hei 6-51255 discloses that a garnet single crystal is saturated by applying a fixed magnetic field equal to or higher than the saturation magnetic field of a garnet single crystal in a direction parallel to the optical axis by a permanent magnet. In that state, a variable magnetic field is applied vertically by an electromagnet to change the resultant magnetic field vector, change the magnetization direction of the garnet single crystal, change the Faraday rotation angle, and control the amount of light coupled to the output side fiber. An attenuator is disclosed. This is because if the garnet single crystal is in an unsaturated state, the extinction ratio is degraded due to the generation of magnetic domains, light is scattered, and the reproducibility of the Faraday rotation angle with respect to the applied magnetic field is poor.

[0004]

However, when the inventors of the present invention made a plurality of optical attenuators by saturating a garnet single crystal in consideration of the above-mentioned situation, the light for obtaining the maximum attenuation was obtained. There has been a problem that the magnetic field of the electromagnet in the direction perpendicular to the axis varies greatly for each manufactured device.

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical attenuator which can reduce variations in characteristics of each device, reduce a magnetic field required by an electromagnet, reduce power consumption of the device, and have a small wavelength dependency and temperature dependency. That is.

[0006]

According to the present invention, a polarizer, a garnet single crystal having a Faraday effect, and an analyzer are arranged in that order, and a magnetic field is applied to the garnet single crystal by an electromagnet in a direction parallel to the optical axis. The optical attenuator has a structure in which a magnetic field is applied in a direction perpendicular to the optical axis by a permanent magnet and a Faraday rotation angle is changed by the combined magnetic field to control the amount of transmitted light. In the present invention, the optical axis is in the <111> direction of the garnet single crystal, and the magnetic field of the electromagnet is also in the <111> direction. The magnetic field of the permanent magnet is centered on the (11) plane of the garnet single crystal perpendicular to the optical axis.
1) plane, a plane equivalent to the (110) plane on the outermost circumference circle, and a plane closer to the (111) plane located 70 degrees from the center (111) plane from the plane equivalent to the (110) plane The direction is given from the center (111) plane toward the outermost circumference circle. The displacement path of the resultant magnetic field vector is represented by a central (111) plane in a stereo projection centered on the (111) garnet single crystal;
A plane equivalent to the (110) plane on the outermost circle and its (11) plane
The plane is set within 10 degrees closer to the (111) plane located 70 degrees from the center (111) plane from the plane equivalent to the (0) plane.

FIG. 1 is a stereographic projection centered on the (111) plane of a garnet single crystal. Adjacent concentric circles mean surfaces that differ by 10 degrees from each other, and adjacent radial lines mean surfaces that differ by 10 degrees from each other.
Therefore, any plane of the garnet single crystal can be shown as a point in this stereographic view. A plane equivalent to the (110) plane appears every 60 degrees on the outermost circle, and a plane equivalent to the (111) plane appears every 120 degrees on a circle located 70 degrees from the center. A (111) plane at the center, a plane equivalent to the (110) plane on the outermost circumference circle, and a (111) plane located 70 degrees from the center (111) plane from the plane equivalent to the (110) plane The range within 10 degrees closer to is the six fan-shaped areas with diagonal lines. Here, the planes equivalent to the (110) plane on the outermost circle are the (-101) plane and the (-11) plane.
0) plane, (01-1) plane, (10-1) plane, (1-10) plane,
(0-11) plane. The plane equivalent to the (111) plane on the circle located 70 degrees from the center is (-111)
Plane, (11-1) plane and (1-11) plane. (In the crystal plane notation, a negative index is represented by drawing a horizontal bar over the index, but in this specification, it is not possible to do so because it is indicated by adding a minus sign to the index. .)

An external magnetic field applied to the garnet single crystal is
By fixing the garnet single crystal in consideration of the orientation, the reproducibility of the Faraday rotation angle with respect to the applied magnetic field can be improved. At this time, the magnetization of the garnet single crystal is oriented in the direction perpendicular to the optical axis by the permanent magnet, and the magnet is rotated in the optical axis direction by the electromagnet, so that the magnetic field of the electromagnet can be reduced. Further, in a stereo projection view centering on the (111) plane of the garnet single crystal perpendicular to the optical axis, the magnetic field of the permanent magnet is equivalent to the (111) plane at the center and the (110) plane on the outermost circle. And (70) from the (111) plane at the center from the plane equivalent to the (110) plane.
11) The direction is within 10 degrees closer to the plane, and the direction is given from the center (111) plane to the outermost circle, so that when the magnetic field of the electromagnet is zero, the magnetization of the garnet single crystal Approach in the direction perpendicular to the optical axis with a small magnetic field,
Faraday rotation angle can be reduced to 10 degrees or less and maximum attenuation 20
dB or more can be obtained.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In terms of characteristics, in particular, the magnetic field of a permanent magnet is centered on a (111) plane of a garnet single crystal centered on a (111) plane perpendicular to the optical axis.
The direction of the plane equivalent to the (110) plane on the outermost circle from the plane, and the displacement path of the resultant magnetic field vector is represented by a stereographic projection centered on the (111) plane of the garnet single crystal.
It is desirable to make it on a line connecting the center (111) plane and a plane equivalent to the (110) plane on the outermost circumference circle. However, considering the assembling accuracy and the like, the best condition can be maintained if it is within a narrow range of about 2 degrees from the straight line.

The garnet single crystal used in the present invention is produced by, for example, a liquid phase epitaxial (LPE) method (RB
i) ₃ Fe ₅ O ₁₂ or (RBi) ₃ (FeM) ₅ O
₁₂ (where R is at least one element selected from rare earth elements including yttrium, and M is at least one element that can be substituted for iron). Examples of M include elements such as Ga, In, and Al. In addition, the garnet single crystal is Y ₃ Fe ₅
O ₁₂ may be used. As a method for producing a garnet single crystal, the above-mentioned LPE method is desirable from the viewpoint of mass productivity and the like.

[0011]

(Embodiment 1) In order to reduce the power consumption of the optical attenuator, it is necessary to rotate the magnetization of the garnet single crystal with a small magnetic field and change the Faraday rotation angle. Therefore, the present inventors fabricated an optical attenuator having the configuration shown in FIG. 2A and examined the relationship between the Faraday rotation angle, the amount of attenuation, and the magnetic field of the electromagnet. The light emitted from the optical fiber 10 passes through the polarizer 14 via the lens 12, the garnet single crystal 16 having the Faraday effect, and the analyzer 18, and is coupled to the optical fiber 22 via the lens 20.
A magnetic field is applied to the garnet single crystal 16 in the optical axis direction by an electromagnet 24, and a magnetic field is applied in a direction perpendicular to the optical axis by a pair of permanent magnets 26 and 28. The magnetic field generated by the permanent magnets 26 and 28 at the crystal position is 150 Oe. Here, the polarizer 14 and the analyzer 18 are a composite polarizing prism in which two right-angled triangular prisms are joined via a polarization separation film, and are arranged so that the angle between the polarization planes passing therethrough is 90 degrees. .

A garnet single crystal was prepared as follows.
Including _{PbO-B 2 O 3 -Bi 2} O 3 as a flux in,
Lattice constant is 1 by liquid phase epitaxy (LPE) method.
2.496 °, composition of (CaGd) ₃ (MgZrGa)
₅ O ₁₂ , a Bi-substituted rare earth iron garnet single crystal (LPE film, composition Tb _1.00 Y _0.65 Bi) was placed on the (111) plane of a nonmagnetic garnet substrate having a diameter of 1 inch and a thickness of 500 μm.
_1.35 Fe _4.05 Ga _0.95 O ₁₂ , a film thickness of 450 μm) were grown. The obtained LPE film was cut into a 3 mm square, the substrate was removed by polishing, and then heat-treated at 1100 ° C. for 8 hours in the air. The heat treatment is performed to reduce uniaxial magnetic anisotropy due to growth induction. And it is polished again, 3 × 3 ×
Mirror polishing was performed to a shape of 0.31 mm, and an antireflection film was deposited. Finally, it was cut to 1 × 1 × 0.31 mm. VSM
When the saturation magnetization 4πMs of the garnet single crystal was measured by a (vibrating sample magnetometer) and found to be 120 Gauss, the magnetic field by the permanent magnet was set to 150 Oersted to saturate the garnet single crystal. Then, the light is deposited on the surface of the garnet single crystal antireflection film, that is, (111)
The measurement was performed so as to be perpendicular to the surface.

First, the orientation of the garnet single crystal was examined by X-ray diffraction. As a result, the A-plane to E-plane of the garnet single crystal shown in FIG.
1) It was found that they corresponded to the planes A 'to E' in the stereographic projection centering on the plane. Therefore, as shown in FIG.
The garnet single crystals 16 are aligned in the same direction, light is incident in the direction of the arrow, and the displacement path of the synthetic magnetic field vector by the permanent magnet and the electromagnet is 0 to 110 degrees (i.e., al) in FIG.
At 10 degrees. FIG. 6 is basically the same as FIG. 1 except that the displacement path and the angle of the resultant magnetic field vector are added. FIG. 7 shows the measurement results of the Faraday rotation angle and the attenuation amount for the paths a to f, and FIG. 8 shows the measurement results of the Faraday rotation angle and the attenuation amount for the paths g to l. Table 1 shows the attenuation when the magnetic field of the electromagnet in the paths a to l is zero. In addition, 120-230 degrees, 240-35
The measurement was also performed at 0 °, but the description was omitted because the measurement results were the same as those from 0 to 110 °. In the measurement, a 1.55 μm semiconductor laser was used as a light source.

[0014]

[Table 1]

Focusing on the point where the magnetic field of the electromagnet is zero, the Faraday rotation angle becomes zero and the attenuation becomes maximum when the displacement path of the composite magnetic field vector due to the externally applied magnetic field is d and j. In the case of the paths a and g, the Faraday rotation angle became the maximum value of-or +, and the amount of attenuation became the minimum. This is as shown in FIG. 9A for the path a and as shown in FIG. 9B for the path g, and the magnetization of the garnet single crystal is influenced by the external magnetic field and the anisotropic magnetic field. This is probably because they are oriented in the direction of the synthesized magnetic field vector. From these results, it is necessary to fix the displacement path of the composite magnetic field vector due to the externally applied magnetic field in order to suppress the variation in the characteristics, and to further increase the maximum attenuation, the displacement path of the composite magnetic field vector due to the externally applied magnetic field is required. It can be seen that the displacement path needs to be limited. According to Table 1, within 10 degrees closer to the (111) plane located at 70 degrees from the center (111) plane from the path d or j (the shaded area in FIG. 1), a maximum attenuation of 20 dB or more is obtained. It can be seen that an optical attenuator having good characteristics can be obtained in this range. Further, in the optical attenuator having this configuration, the maximum attenuation is obtained when the Faraday rotation angle is the minimum, so that the wavelength dependence and the temperature dependence are small.

In the path a and the like, when the magnetic field of the electromagnet is applied, the attenuation becomes large at first, and the maximum attenuation is obtained when the Faraday rotation angle becomes zero. Becomes difficult, which is not preferable.
Even outside the hatched sector in FIG. 1, if the magnetic field of the permanent magnet is increased, the magnetization of the garnet single crystal approaches perpendicular to the optical axis when the magnetic field of the electromagnet is zero, and the Faraday rotation angle becomes zero. Get closer. When a large magnetic field is applied by an electromagnet, the same result as that obtained by the path d or j can be obtained.
However, doing so increases the magnetic field of the electromagnet required to rotate the magnetization, so the device becomes larger,
It is not preferable because the power consumption increases.

Example 2 An optical attenuator having the structure shown in FIG. 2B was manufactured. This has substantially the same configuration as that shown in FIG. 2A, except that a permanent magnet is provided on only one side. Therefore, corresponding members are denoted by the same reference numerals, and description thereof will be omitted. Here, the permanent magnet 26 has an N pole on the garnet single crystal 16 side, and the magnetic field of the permanent magnet at the crystal position is 150 Oersted. A wedge-shaped birefringent crystal (made of rutile) was used for the polarizer 14 and the analyzer 18. Faraday rotation angle is 7
At 5 degrees, the polarizer 14 and the analyzer 18 were arranged so as not to be most coupled to the optical fiber 22 on the emission side. FIG. 10 shows the relationship between the Faraday rotation angle and the attenuation with respect to the magnetic field of the electromagnet when the displacement path of the synthetic magnetic field vector applied from the outside is the path d in FIG. Maximum attenuation was obtained when the magnetic field of the electromagnet was about 250 Oe.

(Comparative Example) For comparison, an optical attenuator having the same configuration as that of the related art shown in FIG. 3 was manufactured. The optical attenuator shown in FIG. 3 is obtained by reversing the relationship between the electromagnet and the permanent magnet of the optical attenuator shown in FIG.
A magnetic field is applied in the optical axis direction by 6, 28, and a magnetic field is applied by the electromagnet 24 in a direction perpendicular to the optical axis. When the Faraday rotation angle was 15 degrees, the polarizer 14 and the analyzer 18 were arranged so as not to couple most to the optical fiber 22 on the emission side. The magnetic field generated by the permanent magnets 26 and 28 at the crystal position is 150 Oe. FIG. 11 shows the relationship between the Faraday rotation angle and the attenuation with respect to the magnetic field of the electromagnet when the displacement path of the synthetic magnetic field vector applied from the outside is the path d in FIG. The maximum attenuation was obtained when the magnetic field of the electromagnet was about 800 Oe. This value was three times or more the value of the optical attenuator shown in Example 2.

[0019]

As described above, the present invention applies a magnetic field to a garnet single crystal in a direction parallel to the optical axis by an electromagnet and applies a magnetic field in a direction perpendicular to the optical axis by a permanent magnet. When the Faraday rotation angle is changed by the synthetic magnetic field, the direction of the magnetic field of the permanent magnet and the displacement path of the synthetic magnetic field vector are specified, so the reproducibility of the Faraday rotation angle with respect to the applied magnetic field is improved, and the characteristics of each device vary. Can be suppressed, the magnetic field of the electromagnet is small, the power consumption can be reduced, and the device can be downsized. In addition, since the maximum attenuation is obtained when the Faraday rotation angle is minimum, the wavelength dependence and the temperature dependence are reduced.

[Brief description of the drawings]

FIG. 1 is a stereographic projection centered on a (111) plane showing a range of the present invention.

FIG. 2 is a basic configuration diagram of an optical attenuator according to the present invention.

FIG. 3 is a configuration diagram of a conventional optical attenuator.

FIG. 4 is an explanatory diagram showing a stereo projection diagram centered on an actual plane and a (111) plane of a garnet single crystal.

FIG. 5 is an explanatory view showing an arrangement state of a garnet single crystal.

FIG. 6 is a stereo projection showing displacement paths and angles of an external combined magnetic field vector.

FIG. 7 is a graph showing a relationship between a Faraday rotation angle and an attenuation amount with respect to a magnetic field of an electromagnet in paths a to f.

FIG. 8 is a graph showing a relationship between a Faraday rotation angle and an attenuation amount with respect to a magnetic field of an electromagnet in paths g to l.

FIG. 9 is an explanatory view showing the influence of the crystal magnetic anisotropy of a garnet single crystal on paths a and g.

FIG. 10 is a graph showing the relationship between the Faraday rotation angle and the attenuation with respect to the magnetic field of the electromagnet in one of the best modes of the present invention.

FIG. 11 is a graph showing a relationship between a Faraday rotation angle and an attenuation amount with respect to a magnetic field of an electromagnet according to a conventional technique.

[Explanation of symbols]

 10,22 Optical fiber 12,20 Lens 14 Polarizer 16 Garnet single crystal 18 Analyzer 24 Electromagnet 26,28 Permanent magnet

Claims (6)

[Claims] 1. A polarizer, a garnet single crystal having a Faraday effect, and an analyzer are arranged in that order. A magnetic field is applied to the garnet single crystal in a direction parallel to an optical axis by an electromagnet, and light is applied by a permanent magnet. An optical attenuator that applies a magnetic field in a direction perpendicular to the axis and changes the Faraday rotation angle by the combined magnetic field to control the amount of transmitted light. The optical axis is the <111> direction of garnet single crystal, and the permanent magnet Is the garnet single crystal perpendicular to the optical axis (11
1) In a stereographic projection centered on a plane, a (111) plane at the center, a plane equivalent to the (110) plane on the outermost circumference circle, and a plane equivalent to the (110) plane are located at the center (111) plane.
10 nearer the (111) plane located 70 degrees from the plane
And the direction is given from the center (111) plane to the outermost circle, and the displacement path of the resultant magnetic field vector is a stereo projection of the garnet single crystal centered at (111). , The plane equivalent to the (110) plane on the center, the (110) plane on the outermost circumference circle, and the center (11) from the plane equivalent to the (110) plane.
1) An optical attenuator characterized by being within 10 degrees within a range closer to the (111) plane located 70 degrees from the plane. 2. In a stereographic projection centered on the (111) plane of a garnet single crystal perpendicular to the optical axis, the magnetic field of the permanent magnet is (11) on the outermost circle from the center (111) plane.
The direction of the plane equivalent to the (0) plane, and the displacement path of the resultant magnetic field vector is represented by the (111) plane at the center and the ( The optical attenuator according to claim 1, wherein the optical attenuator is on a line connecting surfaces equivalent to the (110) surface. 3. The optical attenuator according to claim 1, wherein the garnet single crystal is Y ₃ Fe ₅ O ₁₂ . 4. The garnet single crystal is (RBi)_ThreeFe_Five
O₁₂Or (RBi) _Three(FeM)_FiveO₁₂(However, R is a
At least one element selected from rare earth elements including thorium
Element, M is one or more elements that can be substituted for iron).
3. The optical attenuator according to 1 or 2. 5. The optical attenuator according to claim 1, wherein the polarizer and the analyzer are composite polarizing prisms. 6. The optical attenuator according to claim 1, wherein the polarizer and the analyzer are birefringent crystals.