JP2002148578A - Faraday rotation device and optical device which uses the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a Faraday rotation device with improved wavelength characteristics and temperature characteristics and to obtain various kinds of optical devises such as an optical attenuator in which wavelength-dependent and temperature-dependent loss is decreased. SOLUTION: The Faraday rotation device 22 is equipped with permanent magnets 24, 30 magnetized in the axial direction of through holes and Faraday elements 28, 32 showing an identical Faraday rotation direction in the direction of a magnetic field. The permanent magnets are arranged to face the identical magnetization direction along the optical path, and a part of the Faraday elements are disposed in the through hole of the permanent magnet and other Faraday elements are disposed between the permanent magnets so that magnetic fields in the opposite directions are applied on the Faraday elements. When the Faraday rotation angle is to be varied, electromagnets (variable magnetic field applying means) 36 are disposed. Various kinds of optical devices can be constituted by disposing an input fiber collimator 14 and a polarizer 26 in the entrance side of the arranged group of Faraday elements and disposing an analyzer 34 and an output fiber collimator 20 in the exit side.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Faraday rotator in which a Faraday element is provided inside and outside a ring-shaped permanent magnet so that a magnetic field in the opposite direction is applied to the Faraday element, and an optical device using the same. It is about. More specifically, the present invention provides a plurality of Faraday elements in which a plurality of permanent magnets having through holes are arranged along the optical path so as to face the same magnetization direction, and the Faraday rotation direction is the same with respect to the same magnetic field direction. Is accommodated in the through hole of the permanent magnet, and the rest is disposed between the permanent magnets,
The present invention relates to a Faraday rotator in which a reverse magnetic field is applied to some Faraday elements and the remaining Faraday elements. Although this device is not particularly limited, it is useful for an optical attenuator.

[0002]

2. Description of the Related Art In an optical communication system or an optical measurement system, various optical devices such as an optical isolator, an optical circulator, an optical switch, and an optical attenuator are used. These optical devices incorporate a Faraday rotator for rotating the plane of polarization. The Faraday rotation device is configured to apply an external magnetic field to a Faraday element (a magnetic garnet single crystal having a Faraday effect), thereby controlling a Faraday rotation angle of a light beam transmitted through the Faraday element. In this case, a type in which a fixed magnetic field is applied to the Faraday element to keep the Faraday rotation angle constant (Faraday rotator), and a type in which a variable magnetic field is applied to the Faraday element to variably control the Faraday rotation angle (Faraday rotation angle variable device) ).

[0003] A conventional Faraday rotator generally has a structure in which a Faraday element is housed in a cylindrical permanent magnet.
In the case of the Faraday rotation angle variable device, a Faraday element is arranged between two ring-shaped permanent magnets to apply a fixed magnetic field parallel to the light beam direction, and to apply a variable magnetic field orthogonal to the light beam direction by an electromagnet. In general, the Faraday rotation angle is changed by changing the magnetization direction of the Faraday element by changing the resultant magnetic field.

[0004]

In recent years, as wavelength division multiplexing communication has started to be put into practical use, optical devices are required to have small wavelength dependence. Regarding the reduction of the wavelength dependence of the Faraday rotation angle, a configuration has been proposed in which a garnet single crystal base film is combined with a garnet single crystal compensation film in which the Faraday rotation direction is opposite and the Faraday rotation angle is substantially constant.

[0005] In particular, in a Faraday rotation angle variable device incorporated in an optical attenuator or the like, it is necessary to reduce not only the wavelength dependence but also the wavelength dependence loss (the attenuation changes as the wavelength changes). Therefore, in addition to the garnet single crystal base film whose Faraday rotation angle changes due to the variation of the synthetic magnetic field, a garnet single crystal compensation film with a substantially constant Faraday rotation angle is arranged, and the sign of the Faraday rotation angle is placed on the base film and the compensation film. (Japanese Patent Application Laid-Open No. 2000-249) has been proposed in which a compensation film is used to reduce the wavelength change of the Faraday rotation angle of the base film.
997).

In order to realize this, a compensation film corresponding to the basic film is required, but it is not always easy to produce a suitable compensation film corresponding to an arbitrary basic film. It is necessary to cancel out the wavelength characteristics and the temperature characteristics of the basic film and the compensation film as much as possible. However, since the compositions of the two films are completely different, the material design is limited.

An object of the present invention is to use a structure in which the wavelength coefficient and the temperature coefficient of the Faraday rotation angle act in reverse, while using a plurality of Faraday elements having the same Faraday rotation direction, thereby achieving wavelength characteristics and An object of the present invention is to provide a Faraday rotating device having improved temperature characteristics. Another object of the present invention is to use a structure in which the wavelength coefficient and the temperature coefficient of the Faraday rotation angle act in reverse, while using a plurality of Faraday elements having the same Faraday rotation direction, thereby achieving wavelength dependency and An object of the present invention is to provide various optical devices such as an optical attenuator with reduced temperature-dependent loss.

[0008]

According to the present invention, a plurality of permanent magnets having a through hole and magnetized in the axial direction of the through hole have the same Faraday rotation direction with respect to the same magnetic field direction. It comprises a plurality of Faraday elements, each permanent magnet is arranged so as to face the same magnetization direction along the optical path, some Faraday elements are accommodated in through holes of the permanent magnet,
The remaining Faraday element is a Faraday rotator arranged between the permanent magnets so that opposite magnetic fields are applied to some of the Faraday elements and the remaining Faraday elements. This Faraday rotation device can be used for an optical isolator, an optical circulator, and the like.

Further, the present invention provides a plurality of permanent magnets having a through hole and magnetized in the axial direction of the through hole, and a plurality of Faraday elements having the same Faraday rotation direction with respect to the same magnetic field direction. , A variable magnetic field applying means, each permanent magnet is arranged so as to face the same magnetization direction along the optical path, a part of the Faraday element is accommodated in a through hole of the permanent magnet, and the remaining Faraday elements are The permanent magnet is arranged between the permanent magnets, and the permanent magnet applies a magnetic field in a direction opposite to some of the Faraday elements and the remaining Faraday elements, and the variable magnetic field applying means applies a direction different from the light beam direction to the remaining Faraday elements. Is a Faraday rotator adapted to apply a magnetic field to the Faraday rotator. This Faraday rotation device can be used for an optical switch, an optical attenuator, a polarization scrambler, and the like.

As described above, by storing a part of the Faraday element in the through hole of the permanent magnet and disposing the remaining Faraday elements between the permanent magnets, a magnetic field in the opposite direction is applied to the Faraday element. Thereby, while using the Faraday element having the same Faraday rotation direction,
A function to cancel the wavelength and the temperature coefficient of the Faraday rotation angle can be provided. This is a feature of the present invention.
For example, in the case of a configuration having a variable magnetic field applying unit, the Faraday element positioned between the permanent magnets and to which a variable magnetic field is applied functions as a base film, and the Faraday element built in the permanent magnet functions as a compensation film, as compared with the related art. Will be.

A typical example has two permanent magnets and a plurality of Faraday elements, some Faraday elements are accommodated in through holes of one permanent magnet, and the other Faraday elements are both permanent magnets. Having two permanent magnets and three or more Faraday elements, some of the Faraday elements are housed in respective through holes of both permanent magnets, and the remaining Faraday elements are both Are arranged between the permanent magnets. In these, usually, all the Faraday elements are used in a state where they are magnetically saturated by a magnetic field generated by a permanent magnet.

The present invention is an optical device using the Faraday rotator as described above, in which a polarizer is arranged on the input side and an analyzer is arranged on the output side of the array of a plurality of Faraday elements. A wedge-shaped birefringent plate is used as a polarizer and an analyzer. An input fiber collimator having an input fiber and a collimating lens is arranged on the input side of the polarizer, and an output fiber collimator having an output fiber and a collimating lens is arranged on the output side of the analyzer. There is a configuration.

The present invention also provides two permanent magnets having a through hole and magnetized in the axial direction of the through hole, and a plurality of Faraday elements having the same Faraday rotation direction with respect to the same magnetic field direction. , A variable magnetic field applying means, a polarizer and an analyzer, both permanent magnets are arranged so as to face the same magnetization direction along the optical path, and some Faraday elements are located in the through holes of the permanent magnets. And the remaining Faraday element is disposed between the permanent magnets, and the permanent magnet applies a fixed magnetic field that causes a part of the Faraday element and the remaining Faraday element to be magnetically saturated in the opposite direction, and the variable magnetic field applying means includes: An optical attenuator in which a variable magnetic field is applied to the remaining Faraday elements in a direction different from the light beam direction, and a polarizer is arranged on the input side and an analyzer is arranged on the output side of the array of all Faraday elements. .

As an example of the use of the Faraday rotation device, such an optical attenuator is most effective. In that case, it is preferable that all the Faraday elements are formed of a magnetic garnet single crystal having the same composition and the same characteristics. A fiber-type optical attenuator can be configured by disposing an input fiber collimator having an input fiber and a collimator lens on the input side of a polarizer and an output fiber collimator having an output fiber and a collimator lens on the output side of the analyzer.

As a specific example, two permanent magnets and two permanent magnets
There is a configuration in which a permanent magnet, a polarizer, a Faraday element, a Faraday element built in the permanent magnet, and an analyzer are arranged along the optical path in order from the input side to the output side. . In this case, _assuming that the rotation angle of the Faraday element positioned between the permanent magnets in the crossed Nicols state is θ _1k , the polarizer optical axis angle θ _f from the direction of the electromagnet magnetic field by the variable magnetic field applying means is θ _f ≒ −θ _1k / 2 + nπ / 2 (where n = 0, 1).

As another specific example, there are two permanent magnets and two Faraday elements, from the input side to the output side, a polarizer, a Faraday element built in the permanent magnet, and a Faraday element. , An analyzer, and a permanent magnet are arranged in this order along the optical path. The analyzer has two permanent magnets and three Faraday elements. From the input side to the output side, a polarizer,
There is a configuration in which a Faraday element incorporated in a permanent magnet, a Faraday element, a Faraday element incorporated in a permanent magnet, and an analyzer are arranged along an optical path in this order. In these cases,
When the rotation angle of the Faraday element located between the permanent magnets in the crossed Nicols state is θ _1k and the rotation angle of the Faraday element incorporated in the permanent magnet is θ ₂ , the polarizer optics from the direction of the electromagnet magnetic field by the variable magnetic field applying means the axial angle _{θ f, θ f ≒ -θ 1k} / 2-θ 2 + nπ / 2 ( where, n = 0,
1) is preferable.

Further, in these optical attenuators,
When the maximum rotation angle of the Faraday element located between the permanent magnets is θ _1max , the minimum rotation angle is θ _1min , and the rotation angle of the Faraday element built in the permanent magnet is θ ₂ , | θ _1min | ≦ | θ ₂ | It is preferable to satisfy the relationship of | ≦ | θ _1max |. Furthermore, a wedge-shaped birefringent plate is used for the polarizer and the analyzer, and the optical axis of the analyzer is 0 degrees or more and 90 degrees when viewed from the direction of the optical axis of the polarizer in the rotation direction of the Faraday element located between the permanent magnets. It is better to do the following.

In the optical attenuator, it is also effective to provide an optical filter function unit having a wavelength-dependent loss whose sign is opposite to that of the wavelength-dependent loss caused by the Faraday rotator. In this case, the maximum wavelength-dependent loss slope of the optical attenuator without the optical filter
1. When the minimum wavelength-dependent loss slope is WDL2, the wavelength-dependent loss slope X of the optical filter function unit is X 部 − (WDL1 + WDL2) / 2, or an arbitrary value in the optical attenuator without the optical filter function unit. Assuming that the maximum wavelength-dependent loss slope in the use attenuation region is WDL3 and the minimum wavelength-dependent loss slope is WDL4, the wavelength-dependent loss slope X of the optical filter function unit is X ≒ − (WDL3 + WDL4) / 2. Is good.

[0019]

FIG. 1 is an explanatory view showing an embodiment of an optical attenuator according to the present invention. A Faraday rotation device 22 is provided between an input fiber collimator 14 having an input fiber 11 and a collimating lens 11 mounted on a ferrule 10 and an output fiber collimator 20 having an output fiber 17 and a collimating lens 18 mounted on a ferrule 16. Deploy. The Faraday rotation device 22 has two permanent magnets and two Faraday elements, and, from the input side to the output side, a permanent magnet 24, a polarizer 26 formed of a wedge-shaped birefringent plate (for example, rutile crystal), One Faraday element 28, a second Faraday element 32 incorporated in the permanent magnet 30, and an analyzer 34 composed of a wedge-shaped birefringent plate (for example, rutile crystal) are arranged along the optical path in this order.
This is a structure in which an electromagnet (variable magnetic field applying means) 36 for applying a magnetic field to the Faraday element 28 located between 4, 4 and 30 in a direction different from the light beam direction is provided. The two permanent magnets 24 and 30 each have an annular shape, are two-pole magnetized in the axial direction of the through hole, and are arranged so as to face the same magnetizing direction. The two Faraday elements 28 and 32 are
The Faraday rotation direction is the same in the same magnetic field direction, and here, the one having the same characteristics and the same composition is used.

The magnetic field from both permanent magnets 24, 30 is
The size is such that both the Faraday elements 28 and 32 are magnetically saturated in opposite directions. Only the magnetic field of the permanent magnet 30 is applied to the second Faraday element 32 incorporated in the permanent magnet 30, and the first Faraday element 28 located between the two permanent magnets 24 and 30 receives both magnetic fields. Permanent magnet 24,3
A fixed magnetic field in the light beam direction by 0 and a variable magnetic field in a direction different from the light beam direction by the electromagnet 36 are combined and applied.
Since the Faraday rotation angle of the first Faraday element 28 changes and the Faraday rotation angle of the second Faraday element 32 is substantially constant due to the variation of the synthetic magnetic field, the total Faraday rotation angle thereof changes, and The wavelength and the temperature coefficient are canceled by the two Faraday elements 28 and 32.

The operating principle of this optical attenuator is basically the same as that of a conventional optical attenuator using a birefringent crystal plate as a polarizer and an analyzer. For example, the polarizer 26 and the analyzer 3
In the case where 4 is arranged so that the optical axes of both birefringent crystal plates are parallel to each other, the following operation is performed. Light emitted from the input fiber 11 and converted into a parallel beam by the lens 12 is separated by the polarizer 24 into ordinary light and extraordinary light. The polarization directions of the ordinary light and the extraordinary light are orthogonal to each other. Then, when each light passes through the Faraday rotation angle varying device 22, the polarization direction is rotated according to the magnitude of the magnetization parallel to the optical path, and is further separated into ordinary light and extraordinary light by the analyzer 34, respectively. . Some ordinary light and extraordinary light emitted from the analyzer 34 are parallel to each other, and are coupled to the output fiber 17 by the lens 18. However, the remaining extraordinary light and ordinary light emitted from the analyzer 34 are not parallel to each other, but spread. , Lens 1
It does not couple to the output fiber 17 even though it passes through 8.

When the magnetic field applied by the electromagnet 36 is zero, the Faraday rotation angle is 90 degrees (the magnetization is parallel to the direction of the light beam), and the ordinary light emitted from the polarizer 26 is emitted from the analyzer 34 as extraordinary light, Since the extraordinary light emitted from 26 is emitted from the analyzer 34 as ordinary light, it does not couple to the output fiber 17 even though it passes through the lens 18. On the other hand, if the magnetic field applied by the electromagnet 36 is sufficiently large, the Faraday rotation angle approaches 0 degrees, and the ordinary light emitted from the polarizer 26 is almost directly emitted from the analyzer 34 as ordinary light, and the extraordinary light emitted from the polarizer 26 is Since the light is emitted from the analyzer 34 almost as it is as extraordinary light, both lights are parallel and all are coupled to the output fiber 17 by the lens 180. In this manner, depending on the strength of the magnetic field applied by the electromagnet 36, the first
Of the Faraday element 28 rotates, and the Faraday rotation angle changes in a range from about 90 degrees to about 0 degrees, and accordingly, the amount of light coupled to the output fiber 17 changes, and as a variable optical attenuator, Will work.

However, actually, due to the power applied to the electromagnet, the Faraday rotation angle is limited to the first Faraday element 28.
Is made to be 90 degrees or more when facing the light beam direction, and is changed in an angle range smaller than 90 degrees. For example, when the magnetization is oriented in the light beam direction, the Faraday rotation angle becomes 96 degrees, and the magnetic field of the electromagnet is applied to reduce the Faraday rotation angle to 15 degrees. In that case, if the angle between the optical axes of the birefringent crystals, which are the polarizer and the analyzer, is set to 105 degrees, when the Faraday rotation angle is 15 degrees, a crossed Nicols state occurs,
A large attenuation can be obtained. Therefore, even in such a case, the operation principle is the same as described above.

FIG. 2 is an explanatory view showing another embodiment of the optical attenuator according to the present invention. The basic configuration is shown in FIG.
Therefore, corresponding members are denoted by the same reference numerals for simplification of description, and descriptions thereof are omitted. In this embodiment, an optical filter function unit 38 having a wavelength-dependent loss whose sign is opposite to that of the wavelength-dependent loss caused by the Faraday rotator is provided. The optical filter function 38 is, for example, an optical filter in which an optical filter film having a desired characteristic made of a dielectric multilayer film is formed on a glass substrate.

The characteristics of the optical filter function section are, for example, that the maximum wavelength-dependent loss slope in the optical attenuator without the optical filter function section is WDL1, and the minimum wavelength-dependent loss slope is WDL2.
In this case, the wavelength-dependent loss slope X of the optical filter function unit is expressed as X （− (WDL1 + WDL2) / 2, or the maximum wavelength dependence in an arbitrary attenuated region in the optical attenuator without the optical filter function unit. When the loss slope is WDL3 and the minimum wavelength-dependent loss slope is WDL4, the wavelength-dependent loss slope X of the optical filter function unit is set to be X ≒ − (WDL3 + WDL4) / 2. Thereby, the wavelength-dependent loss can be reduced.

[0026] Here, as the Faraday element 28 and 32, composition _{Tb 1.00 Y 0.65 Bi 1.35 Fe 4.05} Ga 0.95 O
A magnetic garnet single crystal film represented by No. ₁₂ and grown on a nonmagnetic garnet single crystal substrate by an LPE (liquid phase epitaxial) method was used. Note that the magnetic garnet single crystal film is obtained by removing the non-magnetic garnet single crystal substrate and polishing both surfaces so as to have a predetermined thickness.

The maximum rotation angle of the first Faraday element 28 located between the two permanent magnets ₂₄ and ₃₀ is θ _1max ,
When the rotation angle of the second Faraday element 32 built in the permanent magnet 30 is θ ₂ and θ _1max = −θ ₂ , the rotation angle of the first Faraday element 28 when the electromagnet magnetic field is zero is 96 degrees, The rotation angle of the second Faraday element 32 is -96.
Degree, the angle between the polarizer 26 and the analyzer 34 is 90 degrees, and the optical filter characteristic is X を-(WDL1 + WDL2) / 2 or X ≒-
When (WDL3 + WDL4) / 2, the first Faraday element 2
If the sign of the Faraday rotation angle wavelength coefficient of the second Faraday element 32 is opposite to the sign of the temperature coefficient of the second Faraday element 32, an optical attenuator having zero temperature / wavelength dependence in a high attenuation region at zero electromagnet magnetic field (current 0 mA) is realized. It becomes possible, and furthermore, the wavelength dependency can be reduced in an arbitrary use attenuation range by the optical filter characteristics.

FIG. 3 shows the temperature characteristics, and FIG. 4 shows the wavelength dependence. These have a wavelength-dependent loss slope of -0.4 in an optical attenuator without an optical filter function unit.
This is the case of dB. The wavelength-dependent loss between the attenuation of 2 dB and 30 dB is ± 0.1 dB.

-Θ ₂ = θ ₁ (θ ₁ : the rotation angle of the first Faraday element 28 located between the permanent magnets;
θ ₂ : the rotation angle of the second Faraday element 32 built in the permanent magnet 30), the signs of the wavelength coefficients (degrees / nm) are opposite to each other, so that the wavelength dependence of the Faraday rotation angle cancels out. Thus, a point where the wavelength dependence becomes zero is obtained. The point at which this becomes zero is determined by setting θ ₂ within a range satisfying the relationship of | θ _1min | ≦ | θ ₂ | ≦ | θ _1max |, and adjusting the angle between the polarizer and the optical axis of the analyzer. Can be determined by the amount of attenuation.

For example, when the electromagnet magnetic field is zero, the rotation angle of the first Faraday element 28 is 96 degrees, the rotation angle of the second Faraday element 32 is -15 degrees, and the angle between the polarizer and the optical axis of the analyzer is 90 degrees. In the case of degrees, when the rotation angle of the first Faraday element is 15 degrees, the state becomes a crossed Nicols state, the attenuation becomes maximum (attenuation 25 dB), and the wavelength dependence is canceled by the second Faraday element (FIG. 5). A). Under the same conditions, if the angle between the polarizer and the optical axis of the analyzer is 80 degrees, the point where the wavelength dependence is canceled out is when the rotation angle of the second Faraday element is 15 degrees (in the crossed Nicol state, (Not reached) Attenuation amount is around 15 dB (see FIG. 5B). By adjusting the Faraday rotation angle θ2 of the _second Faraday element and the angle between the polarizer and the optical axis of the analyzer, a region where the wavelength-dependent loss is flat can be provided, and the optical filter characteristics can be used.

Further, the azimuth not affected by the Cotton Mouton effect, that is, the azimuth of the plane of polarization incident on the first Faraday element (the optical axis angle of the polarizer from the direction of the electromagnet magnetic field) θ
By setting _f as follows, high attenuation characteristics can be obtained. θ _f ≒ −θ _1k / 2 + nπ / 2 (where n = 0,1) (θ _1k is the rotation angle by the first Faraday element in the crossed Nicols state) For example, the Faraday rotation angle by the first Faraday element is 96 degrees When the Faraday rotation angle by the second Faraday element is -15 degrees and the angle between the polarizer and the optical axis of the analyzer is 90 degrees, θ _1k = 15 degrees and θ _f = −7.5 degrees + nπ /
It becomes 2. Attenuation characteristics in this configuration (1545 nm, 25 ° C.
6 is shown in FIG. 6, and the temperature characteristic (example of 1545 nm) is shown in FIG.

FIG. 8 is an explanatory view showing another embodiment of the optical attenuator according to the present invention. Input fiber collimator 14 having input fiber 11 and collimating lens 12
And a Faraday rotation device 42 between the output fiber 17 and the output fiber collimator 20 having the collimator lens 18. Here, the Faraday rotation device is 2
A polarizer 44 made of a wedge-shaped birefringent plate (for example, rutile crystal), a Faraday element 48 built in a permanent magnet 46, and a Faraday element, each of which has two permanent magnets and two Faraday elements from the input side to the output side. Element 50, analyzer 52 made of wedge-shaped birefringent plate (for example, rutile crystal), permanent magnet 5
4, the permanent magnets 46 and 54 are arranged along the optical path.
In this structure, an electromagnet (variable magnetic field applying means) 56 for applying a magnetic field to the Faraday element 50 located between the two in a direction different from the light beam direction is provided. Two permanent magnets 46,5
Reference numeral 4 denotes an annular shape, which is magnetized in two poles in the axial direction of the through hole and is arranged so as to face the same magnetizing direction.

FIG. 9 is an explanatory view showing still another embodiment of the optical attenuator according to the present invention. A Faraday rotation device 62 is arranged between an input fiber collimator 14 having an input fiber 11 and a collimator lens 12 and an output fiber collimator 20 having an output fiber 17 and a collimator lens 18. Here, the Faraday rotator has two permanent magnets and three Faraday elements, and a polarizer 64 and a permanent magnet 66 made of a wedge-shaped birefringent plate (for example, rutile crystal) are arranged from the input side to the output side. The Faraday element 68, the Faraday element 70, the Faraday element 74, which is embedded in the permanent magnet 72, and the analyzer 76 composed of a wedge-shaped birefringent plate (for example, rutile crystal) are arranged along the optical path in this order. An electromagnet (variable magnetic field applying means) 78 for applying a magnetic field to the Faraday element 70 located between 66 and 72 in a direction different from the light beam direction is provided. The two permanent magnets form an annular shape,
The magnets are magnetized in the axial direction of the through holes, and are arranged so as to face the same magnetizing direction. In this embodiment, the Faraday element is divided into two parts in consideration of the thickness of the Faraday element incorporated in the permanent magnet.

The Faraday element built in the permanent magnet is
Although it performs the function of the compensation film in the prior art, if it is located on the input side as in the embodiments shown in FIGS. 8 and 9, it is necessary to change the optical axis of the polarizer by a corresponding amount. is there. _Assuming that the rotation angle of the Faraday element located between the permanent magnets in the crossed Nicols state is θ _1k , the optical axis angle θ _f of the polarizer from the direction of the electromagnet magnetic field is θ _f ≒ −θ _1k / 2−θ ₂ + nπ / 2. (However, n = 0,
1) is preferable. The polarization plane incident on the Faraday element located between the permanent magnets is different between the case where there is a Faraday element built in the permanent magnet and the case where there is no Faraday element built in the permanent magnet between the Faraday element and the polarizer located between the permanent magnets. is there. For example, the Faraday rotation angle of the Faraday element located between the permanent magnets is 96 degrees, and the Faraday rotation angle of the Faraday element built in the permanent magnet located on the input side is-.
When the angle between the polarizer and the optical axis of the analyzer is 90 degrees, θ _1k = 15 degrees, and θ _f = 7.5 degrees + nπ / 2.
Becomes

Although the optical filter function section is not provided in the embodiment shown in FIGS. 8 and 9, similar to the embodiment shown in FIG.
An optical filter function unit having a wavelength-dependent loss whose sign is opposite to that of the wavelength-dependent loss caused by the Faraday rotator can be provided at an arbitrary position. Thereby, the wavelength-dependent loss can be reduced.

[0036]

As described above, according to the present invention, a plurality of permanent magnets are arranged so as to face the same magnetization direction along the optical path, and a part of the Faraday element is accommodated in a through hole of the permanent magnet. Since the remaining Faraday elements are arranged between the permanent magnets, it is possible to apply a magnetic field in the opposite direction to some of the Faraday elements and the remaining Faraday elements, and the Faraday rotation direction is the same with respect to the same magnetic field direction. While using a certain Faraday element, a function of canceling the wavelength and the temperature coefficient of the Faraday rotation angle can be provided. And
In addition to the effect of reducing the wavelength-dependent loss, there is also the effect of reducing the temperature-dependent loss. In other words, each Faraday element has the same composition
The advantage is that there is no need to develop a garnet single crystal having a new composition as a compensation film having the same characteristics and a characteristic matching the characteristics of the main Faraday element, and the economic effect is extremely large.

In the present invention, the effect of canceling the wavelength and the temperature coefficient of the Faraday rotation angle is effective especially in a region where a large attenuation is obtained, and therefore, the effect of improving characteristics when applied as an optical attenuator is large. Further, by providing the optical filter function unit, it is possible to greatly reduce the wavelength-dependent loss in an arbitrary light attenuation region.

From the above, the optical attenuator of the present invention is particularly effective for a WDM (wavelength multiplexing) transmission system using EDFAs (erbium-doped optical fiber amplifiers) in multiple stages.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing one embodiment of an optical attenuator according to the present invention.

FIG. 2 is an explanatory diagram showing another embodiment of an optical attenuator incorporating an optical filter function unit.

FIG. 3 is a graph showing temperature characteristics.

FIG. 4 is a graph showing wavelength dependence.

FIG. 5 is a graph showing wavelength dependence.

FIG. 6 is a graph showing attenuation characteristics.

FIG. 7 is a graph showing temperature characteristics.

FIG. 8 is an explanatory view showing another embodiment of the optical attenuator according to the present invention.

FIG. 9 is an explanatory view showing still another embodiment of the optical attenuator according to the present invention.

DESCRIPTION OF SYMBOLS 11 Input fiber 12 Collimating lens 14 Input fiber collimator 17 Output fiber 18 Collimating lens 20 Output fiber collimator 22 Faraday rotator 24, 30 Permanent magnet 26 Polarizer 28, 32 Faraday element 34 Analyzer

Continuing from the front page (72) Inventor Hiroki Kawai 5-36-11 Shimbashi, Minato-ku, Tokyo Inside Fuji Electric Chemical Co., Ltd. (72) Inventor Hidenori Nakata 5-36-11 Shimbashi, Minato-ku, Tokyo Fuji Electric Chemical Co., Ltd. In-house F term (reference) 2H049 BA08 BA42 BB03 BC25 2H079 BA01 BA02 CA04 DA13 EB18 HA11 JA08 KA01 KA05 KA11 2H099 AA01 CA06 DA09

Claims (20)

[Claims] A plurality of permanent magnets having a through hole and magnetized in the axial direction of the through hole; and a plurality of Faraday elements having the same Faraday rotation direction with respect to the same magnetic field direction. The permanent magnets are arranged so as to face the same magnetization direction along the optical path, some Faraday elements are housed in the through holes of the permanent magnet, and the other Faraday elements are arranged between the permanent magnets, whereby A Faraday rotator wherein a magnetic field in an opposite direction is applied to some Faraday elements and the remaining Faraday elements. 2. A plurality of permanent magnets having a through hole and magnetized in the axial direction of the through hole, a plurality of Faraday elements having the same Faraday rotation direction with respect to the same magnetic field direction,
A variable magnetic field applying means is provided, each permanent magnet is arranged so as to face the same magnetization direction along the optical path, some Faraday elements are accommodated in through holes of the permanent magnet, and the remaining Faraday elements are permanent. The permanent magnet is arranged between the magnets, the permanent magnet applies a magnetic field of a reverse direction to some Faraday elements and the remaining Faraday elements, and the variable magnetic field applying means applies a direction different from the light beam direction to the remaining Faraday elements. A Faraday rotator, wherein a magnetic field is applied. 3. There are two permanent magnets and a plurality of Faraday elements, a part of the Faraday elements is accommodated in a through hole of one of the permanent magnets, and the other Faraday element is disposed between the two permanent magnets. The Faraday rotation device according to claim 1 or 2, wherein 4. It has two permanent magnets and three or more Faraday elements, some Faraday elements are housed in respective through holes of both permanent magnets, and the other Faraday elements are of both permanent magnets. 3. The method according to claim 1, which is arranged between the two.
The described Faraday rotation device. 5. The Faraday rotation device according to claim 1, wherein all of the Faraday elements are magnetically saturated by a magnetic field generated by a permanent magnet. 6. An optical device using the Faraday rotator according to claim 1, wherein a polarizer is arranged on the input side and an analyzer is arranged on the output side of the array group of the plurality of Faraday elements. 7. An output fiber having a wedge-shaped birefringent plate as a polarizer and an analyzer, an input fiber collimator having an input fiber and a collimating lens on an input side of the polarizer, and an output fiber and a collimating lens on an output side of the analyzer. The optical device according to claim 6, wherein a fiber collimator is arranged. 8. A permanent magnet having a through hole and magnetized in the axial direction of the through hole, a plurality of Faraday elements having the same Faraday rotation direction with respect to the same magnetic field direction,
It has a variable magnetic field applying means, a polarizer and an analyzer, and both permanent magnets are arranged so as to face the same magnetization direction along the optical path, and some Faraday elements are housed in through holes of the permanent magnets The remaining Faraday element is disposed between the permanent magnets, the permanent magnet applies a fixed magnetic field that causes magnetic saturation in a reverse direction to a part of the Faraday element and the remaining Faraday elements, and the variable magnetic field applying unit applies the remaining magnetic field to the remaining Faraday element. An optical attenuator characterized in that a variable magnetic field is applied to the Faraday element in a direction different from the light beam direction, and a polarizer is arranged on the input side and an analyzer is arranged on the output side of an array of all Faraday elements. . 9. The optical attenuator according to claim 8, wherein all the Faraday elements are made of a magnetic garnet single crystal having the same composition and the same characteristics. 10. The light according to claim 8, wherein an input fiber collimator having an input fiber and a collimating lens is arranged on an input side of the polarizer, and an output fiber collimator having an output fiber and a collimating lens is arranged on an output side of the analyzer. Attenuator. 11. A permanent magnet, a polarizer, a Faraday element, a Faraday element built in the permanent magnet, and an analyzer, which have two permanent magnets and two Faraday elements from an input side to an output side. 9. The light emitting device according to claim 8, wherein the light emitting devices are sequentially arranged along the optical path.
11. The optical attenuator according to any one of claims 10 to 10. 12. A permanent magnet, comprising two permanent magnets and two Faraday elements, wherein, from the input side to the output side, a polarizer, a Faraday element built in the permanent magnet, a Faraday element, an analyzer, and a permanent magnet. 9. The light emitting device according to claim 8, wherein the light emitting devices are sequentially arranged along the optical path.
11. The optical attenuator according to any one of claims 10 to 10. 13. A polarizer, a Faraday element built in a permanent magnet, a Faraday element, and a built-in permanent magnet, which have two permanent magnets and three Faraday elements from an input side to an output side. The optical attenuator according to any one of claims 8 to 10, wherein the Faraday element and the analyzer are arranged along the optical path in this order. 14. When the rotation angle of the Faraday element positioned between the permanent magnets in the crossed Nicols state is θ _1k , the polarizer optical axis angle θ _f from the direction of the electromagnet magnetic field by the variable magnetic field applying means is θ _f ≒ − _{θ 1k / 2 + nπ / 2} ( where, n = 0, 1) and claims 11 optical attenuator according. 15. A rotation angle of a Faraday element located between permanent magnets in a crossed Nicols state is θ _1k ,
The rotation angle of the Faraday element built in the permanent magnet is θ
When a _2, a polarizer optical axis angle theta _f from the electromagnet the magnetic field direction by the variable magnetic field applying _{means, θ f ≒ -θ 1k / 2} -θ 2 + nπ / 2 ( where, n = 0,
14. The optical attenuator according to claim 12, wherein 16. When the maximum rotation angle of the Faraday element located between the permanent magnets is θ _1max , the minimum rotation angle is θ _1min , and the rotation angle of the Faraday element built in the permanent magnet is θ ₂ , | θ _1min | The optical attenuator according to any one of claims 8 to 15, which satisfies the following relationship: ≦ | θ ₂ | ≦ | θ _1max | 17. A wedge-shaped birefringent plate is used for a polarizer and an analyzer, and the optical axis of the analyzer is 0 degrees when viewed from the direction of the optical axis of the polarizer in the rotation direction of the Faraday element located between the permanent magnets. The optical attenuator according to any one of claims 8 to 16, wherein the angle is not less than 90 degrees. 18. The optical attenuator according to claim 8, further comprising an optical filter function unit having a wavelength-dependent loss whose sign is opposite to that of the wavelength-dependent loss caused by the Faraday rotator. 19. When the maximum wavelength-dependent loss slope and the minimum wavelength-dependent loss slope of the optical attenuator without the optical filter function unit are WDL1 and WDL2, respectively, the wavelength-dependent loss slope X of the optical filter function unit is X ≒ 19. The optical attenuator according to claim 18, wherein-(WDL1 + WDL2) / 2. 20. In the optical attenuator without an optical filter function unit, when the maximum wavelength-dependent loss slope and the minimum wavelength-dependent loss slope in any use attenuation range are WDL3 and WDL4, the wavelength dependence of the optical filter function unit. The optical attenuator according to claim 18, wherein the loss slope X is X あ る-(WDL3 + WDL4) / 2.