JP2002174802A - Optical attenuator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical attenuator which shows a nearly constant change of attenuation for the change of control current and is large in the maximum attenuation. SOLUTION: This optical attenuator is provided with a polarizing element, Faraday rotators 11, 12 and a magnetic field applying means which controls so that the direction of magnetization of the Faraday rotators is varied corresponding to the direction of optical path. Therein, the Faraday rotators 11, 12 are constituted by using approx. square planar Bi-substituted rare earth iron garnet crystal and the ratio a/b of thickness (a) in the direction of optical path to length (b) of the side of the approximate square in one sheet of Bi- substituted rare earth iron garnet crystal is a value >=0.5 and, preferably, is >=0.6.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical attenuator for attenuating and adjusting light intensity in an optical communication system or the like, and more particularly, to a variable type having a Faraday rotator suitable for use in a wavelength division multiplexing optical communication device. Optical attenuator.

[0002]

2. Description of the Related Art An optical attenuator that controls a Faraday rotation angle by an electric current to attenuate light operates as a highly reliable optical attenuator because it has no mechanically movable part. In particular, an optical attenuator using a Faraday rotator that controls the Faraday rotation angle by controlling the Faraday rotation angle by changing the angle between the direction of magnetization and the direction of light transmission while the magnetization of the Faraday rotator is saturated. Does not occur, and operates as an attenuator with little polarization dependence and excess loss.

FIG. 4 is a diagram showing the arrangement of optical components used in a conventional optical attenuator. As shown in FIG. 4, light emitted from the optical fiber 41a is converted into a parallel light beam by a lens 42a, and is incident on a wedge-shaped birefringent crystal 43a. When passing through this crystal, the ordinary light component and the extraordinary light component of light undergo refraction according to their respective refractive indexes. Next, each light is applied to the variable Faraday rotator 4.
After the rotation of the polarization plane of 0 to 90 ° by 4, the light passes through the wedge-shaped birefringent crystal 43 b, is converted into a convergent light beam by the lens 42 b, and a part of the light is coupled to the optical fiber 41 b.

Here, wedge-shaped birefringent crystals 43a and 43
The direction of the c-axis (optical axis) of b is substantially orthogonal. Therefore, when the Faraday rotation angle by the variable Faraday rotator 44 is approximately 90 °, light that has passed through the wedge-shaped birefringent crystal 43a as ordinary light also passes through the wedge-shaped birefringent crystal 43b as ordinary light. As a result, changes in the traveling direction of light received by the wedge-shaped birefringent crystals 43a and 43b cancel each other, and all ordinary light components are coupled to the optical fiber 41b. The situation is the same for light passing through the wedge-shaped birefringent crystal 43a as extraordinary light.

On the other hand, when the Faraday rotation angle is 0 °, light passing through one of the wedge-shaped birefringent crystals as ordinary light passes through the two wedge-shaped birefringent crystals to pass the other as extraordinary light. The change in the direction of travel does not cancel out. As a result, light does not couple to the optical fiber 41b.

Accordingly, when the Faraday rotation angle is 90 °, the attenuation is zero, and the attenuation increases as the Faraday rotation angle approaches 0 °.

FIG. 5 is a perspective view showing a Faraday rotator and a magnetic circuit used in a conventional optical attenuator. As shown in FIG. 5, an electromagnet is formed by the coil 54 and the yoke 53, a magnetic field of variable intensity is generated in a direction perpendicular to the optical path, and a gap between the yokes 53 is provided with three Faraday rotators. ing. On the other hand, a fixed magnetic field is applied in a direction parallel to the optical path.

When the current flowing through the coil 54 is zero,
The direction of the magnetic field applied to the Faraday rotator coincides with the direction of the optical path, and about 90 ° Faraday rotation occurs. When the current flowing through the coil 54 increases, the direction of the magnetic field approaches the direction of the magnetic field generated by the electromagnet, and the Faraday rotation angle decreases.

Therefore, in order to increase the dynamic range of the attenuation, the Faraday rotation angle must be changed from 0 to 90 °. Further, if there is a macro defect inside the crystal, the direction of magnetization cannot be smoothly changed inside the crystal. Therefore, three Faraday rotators are used for crystal growth requirements.

A conventional optical attenuator provided with a variable Faraday rotator is configured as described above.

[0011]

However, in the conventional optical attenuator, the change in the amount of attenuation with respect to the change in the current flowing through the coil is not constant, and the maximum amount of attenuation may not be sufficient. FIG. 3B shows an attenuation characteristic curve of the conventional optical attenuator. The horizontal axis indicates the current flowing through the electromagnet, and the vertical axis indicates the amount of light attenuation. Current is 5
At ４０40 mA, the rise of the attenuation is large, but the maximum attenuation does not tend to increase even if the current is increased.

Accordingly, it is an object of the present invention to provide an optical attenuator that exhibits a substantially constant change in attenuation with respect to a change in control current and has a large maximum attenuation.

[0013]

In order to solve the above-mentioned problems, a method of smoothly changing the internal magnetic field in the Faraday rotator can be considered. From this viewpoint, it is desirable that the shape of the crystal is a spheroid whose demagnetizing field is constant inside the crystal. However, in actuality, in this shape, the Faraday rotator has a refractive power for light, and there is a surface that is difficult to use, and the Faraday rotator having a rectangular parallelepiped shape is easier to use.

Therefore, in the present invention, the conditions for crystal growth of the Faraday rotator are improved, the thickness of the Faraday rotator in the optical path direction is increased, and the degree of freedom of selecting the dimensional ratio in the rectangular parallelepiped shape is increased. Then, the shape of the Faraday rotator is determined so as to smoothly change the direction of the resultant magnetic field vector inside the crystal.

That is, the optical attenuator of the present invention is an optical attenuator comprising a polarizing element, a Faraday rotator, and a magnetic field applying means for controlling the direction of magnetization of the Faraday rotator to change in the direction of the optical path. The Faraday rotator is formed using a substantially square plate-shaped Bi-substituted rare earth iron garnet crystal, and has a thickness a in the optical path direction and a length b of a substantially square side of one Bi-substituted rare earth iron garnet crystal. Is an optical attenuator having a ratio a / b of more than 0.5, preferably 0.6 or more.

Further, according to the present invention, the Bi-substituted rare earth iron garnet crystal is produced by a liquid phase epitaxial method, and the magnetic anisotropy caused by the selective arrangement of the constituent elements generated during the crystal growth process is obtained. This is an optical attenuator removed by heat treatment after growth.

The present invention is also an optical attenuator for reducing the coercive force Hc of the Faraday rotator to 20 Oe or less.

Further, according to the present invention, the magnetic field applying means includes a fixed magnetic field in a light path direction by a permanent magnet, an electromagnet for generating a magnetic field substantially perpendicular to the light path direction, and a variable current power supply capable of adjusting a current flowing through the electromagnet. An optical attenuator to be configured.

The present invention also relates to the above Faraday rotator.
To Gd_3-xBi_xFe_5-yzAl _yGa_zO
₁₂(However, 0 <x <1.5, 0 ≦ y <0.5, 0 ≦ z
An optical attenuator using a crystal that is <0.5).

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An optical attenuator according to an embodiment of the present invention will be described below.

FIG. 1 is a perspective view showing an optical attenuator according to an embodiment of the present invention.

FIG. 2 is a perspective view showing an arrangement of optical components used in the optical attenuator according to the embodiment of the present invention. 21a and 21b are optical fibers; 22a and 22b are lenses; 23a and 23b are wedge-shaped birefringent crystals;
Reference numeral 4 denotes a set of Faraday rotators, which includes two Faraday rotators 11 and 12.

The Faraday rotator will be described in more detail with reference to FIG. A fixed magnetic field parallel to the optical path and a magnetic field of an electromagnet constituted by the yoke 13 and the coil 14 are applied to the Faraday rotators 11 and 12. The yoke 13 was made of silicon steel. The current flowing through the coil 14 was changed by a variable current power supply.

The material crystal of the Faraday rotators 11 and _{12 is a} single crystal of (GdBi) ₃ (FeAlGa) ₅ O _12, which is formed on a nonmagnetic garnet substrate by a liquid phase epitaxy method, and is configured for crystal growth. In order to remove magnetic anisotropy caused by the selective arrangement of elements, heat treatment was performed at 1100 ° C. or more for 10 hours or more.

Further, the obtained (GdBi) ₃ (FeAlGa) ₅
The saturation magnetization 4πMs of the O ₁₂ single crystal was 150 Gauss, and the coercive force Hc was about 10 Oe.

Further, the shapes of the Faraday rotators 11 and 12 are square plates, and the dimensions are as follows: the thickness a is 0.55 mm;
The length b of the side of the optical surface was 0.8 mm, and the optical surface was coated with an anti-reflection coating.

FIG. 3 is a diagram showing attenuation characteristics in the embodiment of the present invention and the conventional optical attenuator. FIG.
FIG. 3A is a diagram illustrating an attenuation characteristic of the optical attenuator according to the embodiment of the present invention, and FIG.
FIG. 11 is a diagram illustrating attenuation characteristics of the optical attenuator already described as a conventional example.

In FIG. 3A, the horizontal axis represents the control current flowing through the coil, and the vertical axis represents the amount of light attenuation. When the current ranges from 5 mA to 80 mA, the amount of attenuation is 1.2.
It changes almost linearly from 3 dB to 28.0 dB. Separately from this figure, the polarization dependent loss (PD
L) gradually increases with the current value, but the attenuation amount is 1
The polarization dependent loss (PDL) at 9.5 dB is 0.5d
B.

On the other hand, in FIG. 3B showing a conventional example, the attenuation tends to saturate as the current increases, and the maximum value remains at 23.5 dB at a current value of 85 mA. At a current of about 40 mA, the polarization dependent loss (PDL) increased to exceed 0.8 dB.
The Faraday rotator used at this time had a square plate shape, the sides of the square were 0.8 mm in length, and the thickness was 0.8 mm.
The 37 mm one consists of three pieces.

Next, a description will be given of an experiment conducted using a variable Faraday rotator similar to the Faraday rotator shown in FIG. 1 to find a condition under which the Faraday rotation angle smoothly changes with respect to a change in current. I do.

A = 0.45-0.6 mm, b = 0.6-1.
Using a combination of two square plate-shaped Faraday rotators within a range of 0 mm, the change in the Faraday rotation angle was examined by changing the current of the electromagnet.

As a result, when a / b was less than 0.5, an irreversible behavior with respect to the change in the Faraday rotation angle was observed when the Faraday rotation angle per sheet was around 10 °.
On the other hand, when a / b is 0.5 or more, the change in the Faraday rotation angle becomes smooth, and when an optical attenuator is manufactured, the attenuated amount and the polarization dependent loss (PDL) are practically reduced. There is something usable.

In particular, when a / b was 0.6 or more, excellent characteristics were obtained in the linearity of the optical attenuation with respect to the current value, the maximum attenuation, and the polarization dependent loss (PDL).

Even if a / b is 0.6 or more, a Faraday rotator of a practically usable optical attenuator can be used as long as it is not a crystal having excellent uniformity obtained by improving crystal growth conditions. Cannot be used as

The garnet single crystal has a coercive force Hc of 2
If it exceeds 0 Oe, the Faraday rotation angle cannot be smoothly changed due to a change in the current of the electromagnet.

As described above, in the Faraday rotator using the garnet thick film, when the dimensional ratio was selected so as to increase the thickness, an optical attenuator having excellent attenuation characteristics could be manufactured.

By the way, the garnet thick film Gd _3-x Bi _x
The composition of _{Fe 5-y-z Al y} Ga z O 12 is chosen as follows. As for the substitution amount x of Bi, the larger one,
Although it is preferable because the Faraday rotation ability is increased, in practice, less than 1.5 is selected due to the limitation on crystal growth.

[0038] Also, the substitution amount z of substitution amount y and Ga of Al, as both increased, so lowering the saturation magnetization 4PaiM _S, the larger is, it is possible to reduce the current of the electromagnet. However, a value of less than 0.5 is selected due to the limitation of the temperature dependence of the Faraday rotation ability.

[0039]

According to the present invention, it is possible to provide an optical attenuator that exhibits a substantially constant change in the amount of attenuation with respect to a change in the current for controlling the amount of light attenuation, and can increase the maximum amount of attenuation. it can.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a Faraday rotator and a magnetic circuit used in an optical attenuator according to an embodiment of the present invention.

FIG. 2 is a perspective view showing an arrangement of an optical component used in the optical attenuator according to the embodiment of the present invention.

FIG. 3 is a diagram showing attenuation characteristics in the embodiment of the present invention and a conventional optical attenuator. FIG. 3A is a diagram illustrating an attenuation characteristic in an optical attenuator according to an embodiment of the present invention, and FIG. 3B is a diagram illustrating an attenuation characteristic in a conventional optical attenuator.

FIG. 4 is a diagram showing an arrangement of optical components used in a conventional optical attenuator.

FIG. 5 is a perspective view showing a Faraday rotator and a magnetic circuit used in a conventional optical attenuator.

[Explanation of symbols]

11, 12, 51 Faraday rotator 13, 53 Yoke 14, 54 Coil 21a, 21b, 41a, 41b Optical fiber 22a, 22b, 42a, 42b Lens 23a, 23b, 43a, 43b Wedge-shaped birefringent crystal 24 Faraday rotator Pair 44 Variable Faraday rotator

Claims (5)

[Claims] 1. An optical attenuator comprising: a polarizing element; a Faraday rotator; and magnetic field applying means for controlling a direction of magnetization of the Faraday rotator to change in a direction of an optical path, wherein the Faraday rotator is substantially square. A plate-shaped Bi-substituted rare earth iron garnet crystal,
The ratio a / b of the thickness a of the Bi-substituted rare earth iron garnet crystal in the optical path direction to the length b of the substantially square side is 0.5.
An optical attenuator characterized by the above. 2. The Bi-substituted rare earth iron garnet crystal is manufactured by a liquid phase epitaxial method, and a magnetic anisotropy caused by a selective arrangement of constituent elements generated during the crystal growth process is subjected to a heat treatment after the crystal growth. The optical attenuator according to claim 1, wherein the optical attenuator has been removed. 3. The optical attenuator according to claim 1, wherein the Bi-substituted rare earth iron garnet crystal has a holding power Hc of 20 Oe or less. 4. The magnetic field applying means includes a fixed magnetic field in a direction of an optical path by a permanent magnet, an electromagnet for generating a magnetic field substantially perpendicular to the direction of the optical path, and a variable current power supply capable of adjusting a current flowing through the electromagnet. The optical attenuator according to claim 1, wherein Wherein said Bi-substituted rare earth iron garnet _{crystal, Gd 3-x Bi x Fe} 5-y-z Al y Ga z O
₁₂ (where 0 <x <1.5, 0 ≦ y <0.5, 0 ≦ z
The optical attenuator according to any one of claims 1 to 4, wherein <0.5).