JPH08201745A - Faraday effect element, optical device using same and production thereof - Google Patents

Abstract

PURPOSE: To provide a Faraday effect element capable of shifting Faraday rotation angle by 90 deg. with a small-sized low power consuming electromagnet and to produce an optical device using this element at high yield. CONSTITUTION: A bismuth-substd. iron garnet single crystal grown by a liquid phase epitaxial method is segmented into a thin plate of a specified dimension. The thin plate is polished to have a mirror surface and then heat treated at temp. higher than the Curie temp. for demagnetization. Thus, the residual magnetization is eliminated and Faraday rotation angle is inverted with small magnetic power. Then the optical device is assembled and subjected to various inspections in the succeeding processes avoiding application of any larger magnetic field than a specified intensity of magnetic field. Thus, even when residual magnetization generates, its value is small so that the faraday effect can be maintained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Faraday effect element which is a main element of a Faraday rotator used for an optical device such as an optical switch and an optical isolator, an optical device using the Faraday effect element, and a method for manufacturing them. The present invention relates to a Faraday effect element composed of a thin plate of bismuth-substituted iron garnet single crystal grown by a liquid phase epitaxial method.

[0002]

2. Description of the Related Art As is well known, a Faraday rotator is constructed by combining a Faraday effect element made of a thin plate of bismuth-substituted iron garnet single crystal grown by a liquid phase epitaxial method and a permanent magnet or an electromagnet. To be done. An example thereof is shown in FIG. Coil 1
The Faraday effect element 3 is arranged in the magnetic gap portion of the C-shaped yoke 2 wound with, and the direction of the magnetic field applied to the Faraday effect element 3 is reversed by reversing the exciting current flowing in the coil 1. When a positive magnetic field is applied, the Faraday rotation angle is −45 °, and when a negative magnetic field is applied, the Faraday rotation angle is + 45 °. That is,
The Faraday rotation angle is set to 9 depending on the direction of the current flowing through the coil 1.
You can switch 0 °. In addition, the yoke 2 of the electromagnet
As a semi-hard magnetic material such as chrome / stainless steel SUS420J2, a magnetic field of a predetermined value or more is continuously applied to the Faraday effect element 3 by the residual magnetism of the yoke 2 even after supplying a predetermined exciting current pulse to the coil 1. To configure. That is, it is sufficient to supply the exciting current pulse only when switching the Faraday rotation angle.

An example of the structure of an optical switch using the Faraday rotator configured as described above is shown in FIG. Polarization beam splitters 4 and 5 are arranged on both sides of the Faraday effect element 3, two optical fibers 6a and 6b are arranged on one side on the optical axis connecting these, and optical fibers 7a and 7b are arranged on the opposite side. To do. By switching the Faraday rotation angle of the Faraday effect element 3 as described above, for example, the light beam from the optical fiber 3a is guided to the optical fiber 7a or the optical fiber 7b.
Switch the light transmission path.

[0004]

The operating characteristics of the Faraday rotator shown in FIG. 1 are shown in FIG. In the figure, the horizontal axis H is the magnetic field applied to the Faraday effect element 3, and the vertical axis θ is the Faraday rotation angle. ± h1 on the horizontal axis is the effective magnetic field generated when a predetermined exciting current is supplied to the coil 1 of the electromagnet, and ± h2 is the residual of the yoke 2 when the supply of the exciting current is stopped. It is the effective magnetic field generated by magnetism.

FIG. 1A shows one of the ideal characteristics,
The magnetic field is symmetrical with respect to the magnetic field H = 0 and has no hysteresis. If the magnetic field of + h2 is applied to the Faraday effect element 3, the Faraday rotation angle is kept stable at -45 °, and the magnetic field of -h2 is If applied, the Faraday rotation angle is kept stable at + 45 °. In this case, the Faraday rotation angle is + 45 ° or − by supplying a positive or negative exciting current pulse having a predetermined amplitude to the coil 1 as described above.
The angle can be switched to 45 °, and after the switching, the Faraday rotation angle is kept at + 45 ° or −45 ° without flowing an exciting current.

Moreover, since there is no hysteresis, the Faraday rotation angle is also instantly reversed with the reversal of the applied magnetic field, so that the switching speed becomes high. Here, the reversal of the Faraday rotation angle means a change from + 45 ° to −45 °, or vice versa.

By the way, the Faraday effect element 3 has a process in which a thin plate having a predetermined size is cut out from a bismuth-substituted iron garnet single crystal grown by a liquid phase epitaxial method, and the surface is mirror-finished and an antireflection coating is applied to the surface. After that, the Faraday rotator is constructed by combining this with an electromagnet as shown in FIG. 1, but even if the Faraday rotator is mass-produced by the same process, it is shown in FIGS. 2 (B) and 2 (C). As described above, many elements having a predetermined hysteresis in the characteristics of the applied magnetic field and the Faraday rotation angle are generated.

When the hysteresis is roughly divided, as shown in FIG. 1B, when the generated magnetic field ± h1 when a predetermined exciting current is supplied to the coil 1 of the electromagnet is applied, the Faraday rotation angle Also, as shown in FIG. 6C, there is a relatively small hysteresis that causes the Faraday rotation angle to be reversed even when the magnetic fields ± h1 are applied. The latter case is a defective product.

Even if the Faraday effect element is manufactured by the same process as described above, the variation in the characteristics of the individual elements is very large, and the Faraday rotation having the characteristics in the normal range as shown in FIGS. 3A and 3B is performed. Only a limited number of elements that can form a child were obtained. Conventionally, when constructing a Faraday rotator, the characteristics of each Faraday effect element were measured (whether or not the Faraday rotation angle is actually reversed by applying a predetermined magnetic field, or the magnetic field required to reverse the Faraday rotation angle). Was measured) to select good products and defective products. As a result, the incidence of defective products reached about 50%.

Even in the Faraday effect element having the characteristics shown in FIG. 3C, the Faraday rotation angle can be changed from + 45 ° to -4 by increasing the output of the electromagnet to be combined.
It is possible to switch up to 5 °, but such measures are totally impractical in designing optical devices. That is, it is important to switch the Faraday rotation angle by 90 ° with a compact and low power consumption electromagnet.

The present invention has been made in view of the above background, and an object thereof is to solve the above problems and to switch the Faraday rotation angle by 90 ° with an electromagnet of small size and low power consumption. The purpose is to produce a Faraday rotator with good yield.

[0012]

DISCLOSURE OF THE INVENTION The inventors of the present invention sought the cause of the above-mentioned variation in characteristics. As a result, it was found that the Faraday effect element composed of a thin plate of bismuth-substituted iron garnet single crystal grown by the liquid phase epitaxial method is a kind of magnetic material and exhibits characteristics similar to a semi-hard magnetic material. That is, the bismuth-substituted iron garnet single crystal produces a large amount of magnetic flux inside when a strong magnetic field is applied, and a large amount of magnetic flux remains even if the magnetic field is removed. Then, it was found that this residual magnetism is the cause of the variation.

If a strong magnetic field is applied to the material crystal or the crystal thin plate somewhere in the manufacturing process of the Faraday effect element described above, the Faraday effect element formed is magnetized (has residual magnetism). Some Faraday effect elements are almost non-magnetized, while others are considerably strongly magnetized. This results in variations in the above characteristics. The Faraday effect element having the characteristics shown in FIG. 2A has almost no residual magnetism. Faraday effect elements having other characteristics have residual magnetism.
Then, a magnetic field is applied when measuring the magnetic characteristics of the manufactured Faraday rotator for inspection of good / defective products, etc., but if the Faraday rotation angle does not rotate at that time, a larger magnetic field will be used to confirm the operation. It has been found that the residual magnetism is further increased and the characteristics are further deteriorated.

As described above, it was clarified that the conventional problem is caused by the hysteresis as the semi-hard magnetic material of the Faraday effect element. And as a means to solve this problem,
A thin plate having a predetermined size was cut out from a bismuth-substituted iron garnet single crystal grown by a liquid phase epitaxial method, the surface of the thin plate was mirror-finished, and then heat treatment was performed at a Curie temperature or higher to demagnetize. After the heat demagnetization, various optical devices were manufactured and assembled without applying a certain magnetic field strength or more.

[0015]

[Operation] The arrangement of magnetic moments possessed by the atoms of a magnetic material suddenly changes when the ambient temperature exceeds a certain value, and the directions of the respective magnetic moments become irregular,
The magnetic domains disappear and the magnetism as a whole disappears. The temperature at this time is called the Curie temperature, and heat demagnetizing the magnetic material at the Curie temperature or higher to demagnetize it is called thermal demagnetization or magnetic annealing. The Faraday effect element after thermal demagnetization has no remanence, and if a Faraday rotator is constructed by combining it with a required electromagnet or permanent magnet so that a large magnetic field is not applied to this, variations in characteristics due to remanence (defective product) Can be almost completely eliminated.

After the thermal demagnetization, for example, the magnetic field applied to measure the characteristics and the magnetic field generated when the device is actually used as an optical device are suppressed to a certain level or less to perform various treatments. It does not occur, or if it does occur, it is not so big. Therefore, the Faraday rotation angle can be reversed with a small magnetic force. Further, since the magnetic field is once demagnetized and the strength of the magnetic field applied thereafter is small, the residual magnetism generated in the Faraday rotator is also small and the variation is small.

[0017]

The preferred embodiments of the Faraday effect element, the optical device using the same and the manufacturing method thereof according to the present invention will be described below in detail with reference to the accompanying drawings. An example of a method of manufacturing a Faraday effect element will be described below. First, in this example, (GdBi) ₃ (FeAlGa) ₅ O ₂ was adopted as a specific example of the bismuth-substituted iron garnet single crystal grown by the liquid phase epitaxial method. The Curie temperature thereof is 260 ° C., the saturation magnetic flux density at 25 ° C. is 80 gauss, and the compensation temperature is −5 ° C. The size of the thin plate cut out from this was 2 × 7 × 0.33 mm. Both the front and back surfaces of this crystal thin plate are mirror-finished. After that, the required number of the crystal thin plates are placed on a platinum plate, placed in an electric furnace, heat-treated in the atmosphere, and thermally demagnetized. In this heat treatment, the temperature raising rate and the temperature lowering rate were 120 ° C./h, the maximum temperature was 350 ° C., and the holding time was 30 minutes.

In this way, the Faraday effect element according to the present invention is manufactured, and the heat treatment in this way completely demagnetizes the crystal thin plate (Faraday effect element), and the residual magnetic flux density becomes zero.

An example of a method for manufacturing an optical device will be described. The Faraday effect element manufactured as described above is used, and after that, the Faraday effect element is carefully exposed to a large magnetic field. Assemble with each component that makes up the device. And in this embodiment,
A magnetic field exceeding 400 Oe at maximum was not applied. In addition, each of the above-mentioned components is an excitation means for applying a predetermined magnetic field to the Faraday effect element, or a ferrule or an optical fiber or other optical component for causing light to enter or exit the device in a predetermined polarization state. Etc.

An example of the optical device manufactured in this way is the optical switch shown in FIG. 1 cited in the description of the conventional example. That is, the Faraday effect element 3 heat-treated as described above is arranged in the magnetic gap portion of the C-shaped yoke 2 around which the coil 1 is wound.
The direction of the magnetic field applied to the Faraday effect element 3 is reversed by reversing the exciting current applied to the Faraday effect element 3. The coil 1, the yoke 2, and the exciting current supplying means (not shown) constitute an exciting means.

In this example, a semi-hard magnetic material such as chrome / stainless steel SUS420J2 is used as the yoke 2. As a result, even after the predetermined exciting current pulse is supplied to the coil 1, the magnetic field of the predetermined value or more is continuously applied to the Faraday effect element 3 due to the residual magnetism of the yoke 2.
The self-holding function is demonstrated. That is, it is sufficient to supply the exciting current pulse only when switching the Faraday rotation angle. A magnetic field of 200 Oe (h1 in FIG. 2) is generated when an exciting current pulse having a predetermined amplitude is supplied, and a magnetic field of 100 Oe (h2 in FIG. 2) is generated after the pulse is stopped. As a result, the operating characteristics of all manufactured Faraday rotators became usable characteristics as shown in FIGS. 2 (A) and 2 (B). In other words, there is no need to select Faraday effect elements, and the number of defective products for the above-mentioned reasons is zero.

FIG. 3 is a characteristic diagram in which positive and negative magnetic fields are alternately applied to the Faraday effect element manufactured as described above, and the magnetic field required to reverse the Faraday rotation angle at that time is measured. First, there is no hysteresis up to 200 Oe as shown in the center of the figure, and the Faraday rotation angle returns to 0 degrees when the applied magnetic field is set to 0 Oe. And -30
When a magnetic field of 0 Oe is applied, the applied magnetic field remains +45 degrees even if the applied magnetic field is 0 Oe, resulting in hysteresis.
Then, in order to reverse the Faraday rotation angle, it was necessary to apply a magnetic field of about 100 Oe or more in the opposite direction. As is clear from the figure, the stronger the applied magnetic field, the larger the magnetic field required to reverse the Faraday rotation angle.

Then, an experiment similar to the above was carried out by replacing the Faraday effect element to be measured, and the phase of the magnetic field required to reverse the applied magnetic field was obtained. As a result,
The characteristics shown in are obtained. As is clear from the figure, it was confirmed that the magnetic fields required for reversal also increased as the applied magnetic field increased, although there were variations among the elements. The applied magnetic field in which hysteresis starts to be generated varies with each element, but the applied magnetic field when used as an optical device is 200 Oe (200
In view of the fact that the Faraday rotation angle is inverted by Oe), it is understood that the magnetic field strength that can be applied in the manufacturing process is preferably 400 Oe or less. Then, this magnetic field strength directly corresponds to a predetermined magnetic field strength in the exciting means. Further, when the applied magnetic field required for use as an optical device is 100 Oe, the magnetic field strength that can be applied in the above manufacturing process is 200 Oe or less.

Further, although the characteristics vary among the respective elements as described above, the degree of the variation is smaller as the applied magnetic field is smaller. Therefore, as in the present invention, by controlling the magnetic field applied after thermal demagnetization so as to be equal to or less than a certain value, there is a secondary effect that variations in the characteristics of the manufactured elements are reduced.

The optical device using the Faraday rotator is not limited to the 4-port optical switch shown in FIG. 1B, but can be applied to various optical switches and optical isolators. In the case of the optical isolator, a permanent magnet is used as the exciting means. In this case, as described above, in the present invention, the residual magnetism of the Faraday effect element is basically absent, and even if the residual magnetism is generated due to a magnetic field applied at the time of various inspections thereafter, the residual magnetism is small, so that the variation is small. Since a small permanent magnet can be used, the optical device can be downsized.

[0026]

According to the present invention, a thin plate of bismuth-substituted iron garnet single crystal, which is a material of a Faraday effect element, is subjected to a simple heat treatment to make it magnetically virgin (demagnetized to remain. Therefore, when a Faraday rotator is constructed by combining a Faraday effect element with an electromagnet or a permanent magnet that is an exciting means, the characteristics of the applied magnetic field and the Faraday rotation angle can be made uniform. it can. Therefore, as in the past, it is possible to eliminate the waste of disposing a product that has a characteristic defect due to the influence of residual magnetism due to the magnetic history in the manufacturing process as a defective product, and to provide a high-performance optical device with uniform characteristics. Mass production is efficient and cheap.

[Brief description of drawings]

FIG. 1A is a configuration diagram of a Faraday rotator common to an embodiment of the present invention and a conventional example. (B) is a block diagram of an optical switch common to the embodiment of the present invention and the conventional example.

FIG. 2 is a graph showing operating characteristics of a Faraday rotator that changes due to residual magnetism of a Faraday effect element.

FIG. 3 is a diagram showing a relationship between a magnetic field applied to a Faraday effect element and a magnetic field necessary for reversing a Faraday rotation angle.

FIG. 4 is a diagram showing a relationship between a magnetic field applied to a Faraday effect element and a magnetic field required to reverse the Faraday rotation angle.

[Explanation of symbols]

 1 Coil 2 Yoke 3 Faraday Effect Element 4,5 Polarization Beam Splitter 6a, 6b, 7a, 7b Optical Fiber

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Teruo Kiyomiya 5-36-11 Shinbashi, Minato-ku, Tokyo Fuji Electric Chemical Co., Ltd.

Claims (5)

[Claims] 1. A Faraday effect element comprising a thin plate of a bismuth-substituted iron garnet single crystal grown by a liquid phase epitaxial method, which is heat-treated at a temperature not lower than the Curie temperature to be demagnetized. 2. A Faraday characterized in that a thin plate of a predetermined size is cut out from a bismuth-substituted iron garnet single crystal grown by a liquid phase epitaxial method, the surface of the thin plate is mirror-finished, and then heat-treated at a Curie temperature or higher to demagnetize it. Method for manufacturing effect element. 3. An optical device comprising a Faraday rotator in which the Faraday effect element according to claim 1 is combined with an exciting means for applying a predetermined magnetic field. 4. The exciting means includes one of an electromagnet and a permanent magnet.
The optical device described in. 5. A Faraday effect element manufactured by the method according to claim 2 is used, and the Faraday effect element is incorporated into a component forming an optical device including a predetermined exciting means, and after demagnetization, a constant value is obtained. A method for manufacturing an optical device, wherein each treatment is performed without applying a magnetic field exceeding the magnetic field strength.