JP3881505B2 - Faraday rotator and optical device using the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a Faraday rotation device in which a reverse magnetic field is applied to a Faraday element by providing Faraday elements inside and outside an annular permanent magnet, and an optical device using the same. More specifically, in the present invention, a plurality of permanent magnets having through holes are arranged so as to face the same magnetization direction along the optical path, and a part of the plurality of Faraday elements having the same Faraday rotation direction with respect to the same magnetic field direction. The Faraday rotation device relates to a Faraday rotation device in which a reverse magnetic field is applied to a part of the Faraday elements and the remaining Faraday elements. is there. This device is not particularly limited, but is useful for an optical attenuator.
[0002]
[Prior art]
In optical communication systems or optical measurement systems, various optical devices such as optical isolators, optical circulators, optical switches, and optical attenuators are used. These optical devices incorporate a Faraday rotator that rotates the plane of polarization. The Faraday rotator 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 Faraday rotation angle is maintained by applying a fixed magnetic field to the Faraday element (Faraday rotator), and a Faraday rotation angle is controlled by applying a variable magnetic field to the Faraday element (Faraday rotation angle variable device). )
[0003]
A conventional Faraday rotator generally has a structure in which a Faraday element is accommodated in a cylindrical permanent magnet. In the case of a 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 a variable magnetic field orthogonal to the light beam direction is applied 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]
[Problems to be solved by the invention]
Incidentally, in recent years, since wavelength multiplexing communication has been put into practical use, optical devices are required to have low wavelength dependency. In order to reduce the wavelength dependence of the Faraday rotation angle, a configuration has been proposed in which a garnet single crystal basic film and a garnet single crystal compensation film having a reverse Faraday rotation direction and a substantially constant Faraday rotation angle are proposed.
[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 dependency, but also the wavelength dependency loss (the attenuation changes as the wavelength changes). Therefore, in addition to the garnet single crystal basic film whose Faraday rotation angle changes due to the change of the synthetic magnetic field, a garnet single crystal compensation film having a substantially constant Faraday rotation angle is arranged, and the sign of the Faraday rotation angle is placed on the basic film and the compensation film. There has been proposed a structure in which the difference in wavelength of the Faraday rotation angle of the basic film is reduced by using a compensation film (see Japanese Patent Application Laid-Open No. 2000-249997).
[0006]
In order to realize this, a compensation film corresponding to the basic film is required, but it is not always easy to produce an appropriate compensation film corresponding to an arbitrary basic film. This is because the wavelength characteristics and temperature characteristics of the basic film and the compensation film need to be offset as much as possible, but the material design is limited because the compositions of the two are completely different.
[0007]
An object of the present invention is to use a plurality of Faraday elements having the same Faraday rotation direction, and to have a structure in which the wavelength coefficient and the temperature coefficient of the Faraday rotation angle work in reverse, so that the wavelength characteristic and the temperature characteristic are improved. An improved Faraday rotator is provided. Another object of the present invention is to employ a structure in which the wavelength coefficient and the temperature coefficient of the Faraday rotation angle work in reverse while using a plurality of Faraday elements having the same Faraday rotation direction. It is to provide various optical devices such as an optical attenuator with reduced temperature dependent loss.
[0009]
[Means for Solving the Problems]
  BookThe invention includes 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, and variable magnetic field applying means Each permanent magnet is arranged to face the same magnetization direction along the optical path, and some Faraday elements are accommodated in the through holes of the permanent magnet, and the remaining Faraday elements are interposed between the permanent magnets. The permanent magnet applies a reverse magnetic field to a part of the Faraday elements and the remaining Faraday elements, and the variable magnetic field applying means applies a magnetic field to the remaining Faraday elements in a direction different from the light beam direction.The maximum rotation angle by the Faraday element located between the permanent magnets is θ _1max , The minimum rotation angle is θ _1min The rotation angle by the Faraday element built in the permanent magnet is θ ₂ When
| Θ _1min | ≦ | θ ₂ | ≦ | θ _1max ｜
Satisfy a relationshipThis is the Faraday rotation device. This Faraday rotator can be used for an optical switch, an optical attenuator, a polarization scrambler, and the like.
[0010]
In this way, by accommodating a part of the Faraday elements in the through holes of the permanent magnets and arranging the remaining Faraday elements between the permanent magnets, a reverse magnetic field is applied to the Faraday elements, thereby the same While using the Faraday element having the Faraday rotation direction, it is possible to provide a function of canceling the wavelength and temperature coefficient of the Faraday rotation angle. This is a feature of the present invention. For example, in the case of a configuration having variable magnetic field applying means, as compared with the prior art, a Faraday element that is located between permanent magnets and to which a variable magnetic field is applied functions as a basic film, and a Faraday element built in the permanent magnet functions as a compensation film. It will be.
[0011]
As a typical example, there are two permanent magnets and a plurality of Faraday elements, some Faraday elements are accommodated in the through holes of one permanent magnet, and the rest of the Faraday elements are between both permanent magnets. Arranged configuration, having two permanent magnets and three or more Faraday elements, some Faraday elements are housed in respective through holes of both permanent magnets, and the remaining Faraday elements are both permanent magnets There is a configuration arranged between. These are usually used in a state where all the Faraday elements are magnetically saturated by a magnetic field generated by a permanent magnet.
[0012]
The present invention is an optical device using the Faraday rotator as described above, 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. A wedge-shaped birefringent plate is used as a polarizer and an analyzer, an input fiber collimator having an input fiber and a collimator lens is arranged on the input side of the polarizer, and an output fiber collimator having an output fiber and a collimator lens is arranged on the output side of the analyzer. There is a configuration.
[0013]
  The present invention also includes two permanent magnets having through holes and magnetized in the axial direction of the through holes, a plurality of Faraday elements having the same Faraday rotation direction with respect to the same magnetic field direction, and variable It has a magnetic field applying means, a polarizer and an analyzer, and both permanent magnets are arranged to face the same magnetization direction along the optical path, and some Faraday elements are accommodated in the through holes of the permanent magnets. The remaining Faraday elements are disposed between the permanent magnets, the permanent magnets apply a fixed magnetic field that reversely saturates some of the Faraday elements and the remaining Faraday elements, and the variable magnetic field applying means applies the remaining Faraday elements. A variable magnetic field is applied to the element in a direction different from the light beam direction, and a polarizer is placed on the input side and an analyzer on the output side of all Faraday elements.The maximum rotation angle by the Faraday element located between the permanent magnets is θ _1max , The minimum rotation angle is θ _1min The rotation angle by the Faraday element built in the permanent magnet is θ ₂ When
| Θ _1min | ≦ | θ ₂ | ≦ | θ _1max ｜
To satisfy the relationshipOptical attenuator.
[0014]
Such an optical attenuator is most effective as an example of use of the Faraday rotator. In that case, it is preferable that all the Faraday elements are composed of magnetic garnet single crystals having the same composition and the same characteristics. A fiber type optical attenuator can be configured by arranging an input fiber collimator having an input fiber and a collimator lens on the input side of the polarizer and an output fiber collimator having an output fiber and a collimator lens on the output side of the analyzer.
[0015]
As a concrete example, there are two permanent magnets and two Faraday elements. From the input side to the output side, the permanent magnet, the polarizer, the Faraday element, the Faraday element built in the permanent magnet, the detector There is a configuration in which the photons are arranged along the optical path in order. In this case, the rotation angle by the Faraday element located between the permanent magnets in the crossed Nicols state is θ_1kThe polarizer optical axis angle θ from the direction of the electromagnet magnetic field by the variable magnetic field applying means_fThe
θ_f≒ -θ_1k/ 2 + nπ / 2 (where n = 0, 1)
Is preferable.
[0016]
  As another specific example, there are two permanent magnets and two Faraday elements, and from the input side to the output side, a polarizer, a Faraday element built in the permanent magnet, a Faraday element, an analyzer The arrangement of the permanent magnets in the order of the optical path, the two permanent magnets and the three Faraday elements, and the polarizer and the Faraday element built in the permanent magnet from the input side to the output side The Faraday element, the Faraday element built in the permanent magnet, and the analyzer are arranged in this order along the optical path. In these cases, the rotation angle by the Faraday element located between the permanent magnets in the crossed Nicol state is θ_1k,Located between the polarizer and the Faraday elementThe rotation angle by the Faraday element built in the permanent magnet is θ₂The polarizer optical axis angle θ from the direction of the electromagnet magnetic field by the variable magnetic field applying means_fThe
θ_f≒ -θ_1k/ 2-θ₂+ Nπ / 2 (where n = 0, 1)
Is preferable.
[0017]
  In these optical attenuators,sideA wedge-shaped birefringent plate is used for the photon and analyzer, and the optical axis of the analyzer is set to 0 degrees or more and 90 degrees or less 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 good.
[0018]
In the optical attenuator, it is also effective to add 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 that case, when the maximum wavelength-dependent loss gradient in the optical attenuator without the optical filter function unit is WDL1, and the minimum wavelength-dependent loss gradient is WDL2, the wavelength-dependent loss gradient X of the optical filter function unit is
X ≒-(WDL1 + WDL2) / 2
Or the wavelength dependence of the optical filter function section when the maximum wavelength-dependent loss slope is WDL3 and the minimum wavelength-dependent loss slope is WDL4 in any use attenuation region in the optical attenuator without the optical filter function section. Loss slope X is
X ≒-(WDL3 + WDL4) / 2
It is good to do.
[0019]
【Example】
FIG. 1 is an explanatory view showing an embodiment of an optical attenuator according to the present invention. A Faraday rotator 22 is provided between an input fiber collimator 14 having an input fiber 11 and a collimator lens 11 attached to the ferrule 10, and an output fiber collimator 20 having an output fiber 17 and a collimator lens 18 attached to the ferrule 16. Deploy. The Faraday rotator 22 has two permanent magnets and two Faraday elements. From the input side to the output side, a permanent magnet 24, a polarizer 26 made of a wedge-shaped birefringent plate (for example, a rutile crystal), a first One Faraday element 28, a second Faraday element 32 built in a permanent magnet 30, and an analyzer 34 made of a wedge-shaped birefringent plate (for example, a rutile crystal) are disposed along the optical path in this order. The Faraday element 28 located between 30 is provided with an electromagnet (variable magnetic field applying means) 36 that applies a magnetic field in a direction different from the light beam direction. The two permanent magnets 24 and 30 are both in an annular shape, are two-pole magnetized in the axial direction of the through hole, and are arranged to face the same magnetizing direction. The two Faraday elements 28 and 32 have the same Faraday rotation direction with respect to the same magnetic field direction, and have the same characteristics and the same composition here.
[0020]
The magnetic field generated by both permanent magnets 24 and 30 has a magnitude that magnetically saturates both Faraday elements 28 and 32 in the opposite direction. Only the magnetic field generated by the permanent magnet 30 is applied to the second Faraday element 32 built in the permanent magnet 30, and both of the first Faraday element 28 positioned between the permanent magnets 24 and 30 are A fixed magnetic field in the light beam direction by the permanent magnets 24 and 30 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 is changed by changing the composite magnetic field, and the Faraday rotation angle of the second Faraday element 32 is substantially constant, the total Faraday rotation angle thereof will change, and Both Faraday elements 28, 32 will cancel the wavelength and temperature coefficient.
[0021]
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, when the polarizer 26 and the analyzer 34 are arranged so that the optical axes of the birefringent crystal plates are parallel to each other, the operation is as follows. The light emitted from the input fiber 11 and converted into a parallel beam by the lens 12 is separated into ordinary light and abnormal light by the polarizer 24. The polarization directions of ordinary light and extraordinary light are orthogonal to each other. 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 abnormal 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, but the remaining extraordinary light and ordinary light exiting from the analyzer 34 are not parallel to each other but spread. Even if it passes through the lens 18, it is not coupled to the output fiber 17.
[0022]
When the magnetic field applied by the electromagnet 36 is zero, the Faraday rotation angle is 90 degrees (magnetization is parallel to the light beam direction), and ordinary light emitted from the polarizer 26 is emitted as abnormal light from the analyzer 34 and emitted from the polarizer 26. The extraordinary light emitted from the analyzer 34 as ordinary light is not coupled to the output fiber 17 even if it passes through the lens 18. On the other hand, when 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 emitted as it is from the analyzer 34 as ordinary light, and the extraordinary light emitted from the polarizer 26 is Since the light exits from the analyzer 34 as abnormal light almost as it is, both lights are parallel and all are coupled to the output fiber 17 by the lens 180. In this manner, the magnetization of the first Faraday element 28 rotates according to the strength of the magnetic field applied by the electromagnet 36, and the Faraday rotation angle changes in the range from about 90 degrees to about 0 degrees, and the output is accordingly generated. The amount of light coupled to the fiber 17 will be different and will function as a variable optical attenuator.
[0023]
However, in reality, for the convenience of the power applied to the electromagnet, the Faraday rotation angle is set to 90 degrees or more when the magnetization of the first Faraday element 28 is directed in the light beam direction, and an angle range smaller than 90 degrees. Change with. For example, when the magnetization is directed in the direction of the light beam, the Faraday rotation angle is 96 degrees, and the magnetic field of the electromagnet is applied to reduce it to 15 degrees. In this case, if the angle formed by the optical axes of the birefringent crystals that are the polarizer and the analyzer is set to 105 degrees, when the Faraday rotation angle is 15 degrees, a crossed Nicols state is obtained, and a large amount of attenuation is obtained. Accordingly, even in such a case, the operating principle is the same as described above.
[0024]
FIG. 2 is an explanatory view showing another embodiment of the optical attenuator according to the present invention. Since the basic configuration is the same as that in FIG. 1, the same reference numerals are given to corresponding members for the sake of simplicity, 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 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, for example.
[0025]
The characteristics of the optical filter function unit are, for example, the wavelength-dependent loss slope X of the optical filter function unit when the maximum wavelength-dependent loss gradient is WDL1 and the minimum wavelength-dependent loss gradient is WDL2 in an optical attenuator without the optical filter function unit. But,
X ≒-(WDL1 + WDL2) / 2
If the maximum wavelength-dependent loss slope is WDL3 and the minimum wavelength-dependent loss slope is WDL4 in an arbitrary attenuation range in an optical attenuator without an optical filter function section, the wavelength dependence of the optical filter function section Loss slope X is
X ≒-(WDL3 + WDL4) / 2
Set to be. Thereby, the wavelength dependent loss can be reduced.
[0026]
Here, as the Faraday elements 28 and 32, the composition is Tb._1.00Y_0.65Bi_1.35Fe_4.05Ga_0.95O₁₂A magnetic garnet single crystal film grown on a nonmagnetic garnet single crystal substrate by the LPE (liquid phase epitaxial) method was used. The magnetic garnet single crystal film is polished on both sides so as to have a predetermined thickness after removing the non-magnetic garnet single crystal substrate.
[0027]
The maximum rotation angle by the first Faraday element 28 located between both permanent magnets 24 and 30 is θ_1maxThe rotation angle of the second Faraday element 32 built in the permanent magnet 30 is θ₂And θ_1max= −θ₂In this case, when the electromagnetic 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 -96 degrees, and the angle formed by the polarizer 26 and the analyzer 34 is 90 degrees. When the optical filter characteristic is X≈− (WDL1 + WDL2) / 2 or X≈− (WDL3 + WDL4) / 2, the Faraday rotation angle wavelength coefficient and temperature coefficient of the first Faraday element 28 and the second Faraday element 32 are calculated. If the sign is reversed, it is possible to realize an optical attenuator with zero temperature and wavelength dependence in an electromagnet magnetic field zero (current 0 mA), and further, wavelength dependence due to optical filter characteristics in an arbitrary attenuation range. It becomes possible to decrease.
[0028]
FIG. 3 shows the temperature characteristics, and FIG. 4 shows the wavelength dependence. These are cases where the wavelength dependent loss slope in the optical attenuator without the optical filter function part is −0.4 dB. The wavelength dependent loss between the attenuation 2 dB and 30 dB is ± 0.1 dB.
[0029]
-Θ₂= Θ₁(Θ₁: Rotation angle by first Faraday element 28 located between permanent magnets, θ₂: In the case of the rotation angle by the second Faraday element 32 built in the permanent magnet 30), the signs of the mutual wavelength coefficients (degrees / nm) are opposite, so that the Faraday rotation angle wavelength dependence is offset, A point where the wavelength dependence becomes zero is obtained. This zero point is θ₂The
| Θ_1min| ≦ | θ₂| ≦ | θ_1max｜
By adjusting the angle between the polarizer and the analyzer optical axis, it can be determined with an arbitrary amount of attenuation.
[0030]
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 formed by the polarizer and the analyzer optical axis is 90 degrees. In this case, when the rotation angle of the first Faraday element is 15 degrees, the crossed Nicol state is reached and the attenuation is maximized (attenuation 25 dB), and the wavelength dependence is canceled by the second Faraday element (see A in FIG. 5). . Under the same conditions, when the angle between the polarizer and the analyzer optical axis is 80 degrees, the point at which the wavelength dependence is canceled is when the rotation angle of the second Faraday element is 15 degrees (in the crossed Nicols state) (Not reached) near attenuation of 15 dB (see B in FIG. 5). Faraday rotation angle θ of the second Faraday element₂By adjusting the angle between the polarizer and the analyzer optical axis, it is possible to provide a region having a flat wavelength-dependent loss, and the optical filter characteristics can be used.
[0031]
Further, the direction not affected by the Cotton Mouton effect, that is, the direction of the polarization plane incident on the first Faraday element (polarizer optical axis angle from the direction of the electromagnetic field) θ_fBy setting as shown in the following equation, high attenuation characteristics can be obtained.
θ_f≒ -θ_1k/ 2 + nπ / 2 (where n = 0, 1)
(Θ_1kIs the rotation angle by the first Faraday element in the crossed Nicol state)
For example, when the Faraday rotation angle by the first Faraday element is 96 degrees, the Faraday rotation angle by the second Faraday element is -15 degrees, and the angle between the polarizer and the analyzer optical axis is 90 degrees, θ_1k= 15 degrees and θ_f= −7.5 degrees + nπ / 2. FIG. 6 shows the attenuation characteristics (example of 1545 nm, 25 ° C.) in this configuration, and FIG. 7 shows the temperature characteristics (example of 1545 nm).
[0032]
FIG. 8 is an explanatory view showing another embodiment of the optical attenuator according to the present invention. A Faraday rotator 42 is disposed between the input fiber collimator 14 having the input fiber 11 and the collimator lens 12 and the output fiber collimator 20 having the output fiber 17 and the collimator lens 18. Here, the Faraday rotator has two permanent magnets and two Faraday elements. From the input side to the output side, a polarizer 44 made of a wedge-shaped birefringent plate (for example, a rutile crystal) and a permanent magnet 46 are provided. The built-in Faraday element 48, Faraday element 50, analyzer 52 made of a wedge-shaped birefringent plate (for example, rutile crystal), and permanent magnet 54 are arranged in this order along the optical path, and are positioned between the permanent magnets 46 and 54. The Faraday element 50 is provided with an electromagnet (variable magnetic field applying means) 56 for applying a magnetic field in a direction different from the light beam direction. The two permanent magnets 46 and 54 have an annular shape, and are two-pole magnetized in the axial direction of the through hole, and are arranged so as to face the same magnetization direction.
[0033]
FIG. 9 is an explanatory view showing still another embodiment of the optical attenuator according to the present invention. A Faraday rotator 62 is disposed between the input fiber collimator 14 having the input fiber 11 and the collimator lens 12 and the output fiber collimator 20 having the output fiber 17 and the collimator lens 18. Here, the Faraday rotator has two permanent magnets and three Faraday elements. From the input side to the output side, a polarizer 64 made of a wedge-shaped birefringent plate (for example, a rutile crystal) and a permanent magnet 66 are provided. A built-in Faraday element 68, a Faraday element 70, a Faraday element 74 built in a permanent magnet 72, and an analyzer 76 made of a wedge-shaped birefringent plate (for example, rutile crystal) are arranged along the optical path in this order. In this structure, an electromagnet (variable magnetic field applying means) 78 for applying a magnetic field in a direction different from the light beam direction is provided for the Faraday element 70 positioned between 66 and 72. The two permanent magnets are both in an annular shape, are magnetized in the axial direction of the through hole, and are arranged so as to face the same magnetization direction. In this embodiment, the thickness of the Faraday element incorporated in the permanent magnet is taken into consideration and the two are installed in two.
[0034]
The Faraday element incorporated in the permanent magnet functions as a compensation film in the prior art. However, if it is located on the input side as in the embodiments shown in FIGS. Only the optical axis of the polarizer needs to be changed. The rotation angle by the Faraday element located between the permanent magnets in the crossed Nicol state is θ_1kThe polarizer optical axis angle θ from the direction of the electromagnetic field is_fThe
θ_f≒ -θ_1k/ 2-θ₂+ Nπ / 2 (where n = 0, 1)
Is preferable. The polarization plane incident on the Faraday element located between the permanent magnets differs depending on whether or not there is a 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 positioned between the permanent magnets is 96 degrees, the Faraday rotation angle of the Faraday element built in the permanent magnet positioned on the input side is -15 degrees, and the polarizer and analyzer optical axes When the angle formed is 90 degrees, θ_1k= 15 degrees and θ_f= 7.5 degrees + nπ / 2.
[0035]
8 and 9, the optical filter function unit is not provided. However, as shown in FIG. 2, the wavelength-dependent loss whose sign is opposite to the wavelength-dependent loss caused by the Faraday rotator is shown. It is also possible to attach an optical filter function unit having an arbitrary position. Thereby, the wavelength dependent loss can be reduced.
[0036]
【The invention's effect】
In the present invention, as described above, a plurality of permanent magnets are arranged so as to face the same magnetization direction along the optical path, some Faraday elements are accommodated in the through holes of the permanent magnets, and the remaining Faraday elements are Since it is arranged between permanent magnets, it is possible to apply a reverse magnetic field to some Faraday elements and the remaining Faraday elements, and use Faraday elements that have the same Faraday rotation direction with respect to the same magnetic field direction. However, the function of canceling the wavelength and temperature coefficient of the Faraday rotation angle can be provided. In addition to the wavelength-dependent loss reduction effect, there is also a temperature-dependent loss reduction effect. In other words, each Faraday element may have the same composition and the same characteristics, and there is an advantage that it is not necessary to develop a garnet single crystal having a new composition as a compensation film having characteristics suitable for the characteristics of the main Faraday element. The effect is extremely large.
[0037]
In the present invention, since the effect of canceling the wavelength and temperature coefficient of the Faraday rotation angle is effective particularly in a region where the attenuation is large, the effect of improving characteristics in application as an optical attenuator is great. Furthermore, by providing the optical filter function unit, it is possible to significantly reduce the wavelength-dependent loss in an arbitrary light attenuation region.
[0038]
For these reasons, the optical attenuator of the present invention is particularly effective for a WDM (wavelength multiplexing) transmission system using EDFA (erbium-doped optical fiber amplifier) in multiple stages.
[Brief description of the drawings]
FIG. 1 is an explanatory view showing an 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.
[Explanation 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

Claims (18)

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, and a variable magnetic field applying means, Each permanent magnet is arranged so as to face the same magnetization direction along the optical path. Some Faraday elements are accommodated in the through holes of the permanent magnet, and the remaining Faraday elements are arranged between the permanent magnets. The magnet applies a reverse magnetic field to some Faraday elements and the remaining Faraday elements, and the variable magnetic field applying means is configured to apply a magnetic field in a direction different from the light beam direction to the remaining Faraday elements. When the maximum rotation angle by the Faraday element located between the permanent magnets is θ _1max , the minimum rotation angle is θ _1min , and the rotation angle by the Faraday element built in the permanent magnet is θ ₂ ,
| Θ _1min | ≦ | θ ₂ | ≦ | θ _1max |
A Faraday rotator characterized by satisfying the following relationship . Has two permanent magnets and a plurality of Faraday element, a part of the Faraday element is housed in the through hole of one of the permanent magnet, according to claim is disposed between both the remaining Faraday element of the permanent magnet 1 serial mounting Faraday rotation device. It has two permanent magnets and three or more Faraday elements, some of the Faraday elements are accommodated in the respective through holes of both permanent magnets, and the remaining Faraday elements are arranged between both permanent magnets. Faraday rotation device of claim 1 Symbol placement are. The Faraday rotator according to any one of claims 1 to 3 , wherein each Faraday element is magnetically saturated by a magnetic field generated by a permanent magnet. Using the Faraday rotation device according to any one of claims 1 to 4, a polarizer on the input side of the sequence group of the plurality of Faraday elements, the optical device placing the analyzer at the output side. A wedge-shaped birefringent plate is used as a polarizer and an analyzer, an input fiber collimator having an input fiber and a collimator lens is arranged on the input side of the polarizer, and an output fiber collimator having an output fiber and a collimator lens is arranged on the output side of the analyzer. The optical device according to claim 5 . Two 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, variable magnetic field applying means, polarization And both permanent magnets are arranged so as to face the same magnetization direction along the optical path, some Faraday elements are accommodated in the through holes of the permanent magnets, and the remaining Faraday elements are Located between the permanent magnets, the permanent magnet applies a fixed magnetic field that reversely saturates some Faraday elements and the rest of the Faraday elements, and the variable magnetic field applying means applies the light beam direction to the remaining Faraday elements. Faraday element so as to apply a variable magnetic field, a polarizer on the input side of the sequence group of all the Faraday element, and configured to position an analyzer on the output side, located between the permanent magnets in a direction different from the Maximum rotation angle theta _1max by the minimum rotation angle theta _1min, when the rotation angle is theta ₂ by Faraday element incorporated in the permanent magnet,
| Θ _1min | ≦ | θ ₂ | ≦ | θ _1max |
An optical attenuator characterized by satisfying the following relationship . The optical attenuator according to claim 7, wherein all Faraday elements are made of magnetic garnet single crystals having the same composition and the same characteristics. An input fiber collimator having an input fiber and collimating lens to the input side of the polarizer, optical attenuator according to claim 7 or 8, wherein placing the output fiber collimator with an output fiber and collimating lens on the output side of the analyzer. It has two permanent magnets and two Faraday elements. From the input side to the output side, the permanent magnet, the polarizer, the Faraday element, the Faraday element built in the permanent magnet, and the analyzer are arranged along the optical path in this order. optical attenuator according to any one of claims 7 to 9 is disposed Te. It has two permanent magnets and two Faraday elements. From the input side to the output side, the polarizer, the Faraday element built in the permanent magnet, the Faraday element, the analyzer, and the permanent magnet follow the optical path in this order. optical attenuator according to any one of claims 7 to 9 is disposed Te. It has two permanent magnets and three Faraday elements. From the input side to the output side, polarizer, Faraday element built in permanent magnet, Faraday element, Faraday element built in permanent magnet, analyzer The optical attenuator according to any one of claims 7 to 9 , wherein the optical attenuator is disposed along an optical path in the order of. When the rotation angle by the Faraday element located between the permanent magnets in the crossed Nicol state is θ _1k , the polarizer optical axis angle θ _f from the electromagnet magnetic field direction by the variable magnetic field applying means is
θ _f ≈ −θ _1k / 2 + nπ / 2 (where n = 0, 1)
And claims 1 0 optical attenuator according. When the rotation angle by the Faraday element located between the permanent magnets in the crossed Nicol state is θ _1k, and the rotation angle by the Faraday element built in the permanent magnet arranged between the polarizer and the Faraday element is θ ₂ , The polarizer optical axis angle θ _f from the electromagnetic field direction by the variable magnetic field applying means,
θ _f ≈−θ _1k / 2−θ ₂ + nπ / 2 (where n = 0, 1)
And claims 1 1 or 1 2 optical attenuator according. A wedge-type 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 or less 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. optical attenuator according to any one of claims 7 to 1 4 and. The optical attenuator according to any one of claims 7 to 15, further comprising an optical filter function unit having a wavelength dependent loss having a sign opposite to that of the wavelength dependent loss caused by the Faraday rotator. In an optical attenuator without an optical filter function unit, when the maximum wavelength-dependent loss gradient is WDL1, and the minimum wavelength-dependent loss gradient is WDL2, the wavelength-dependent loss gradient X of the optical filter function unit is
X ≒-(WDL1 + WDL2) / 2
The optical attenuator according to claim 16, wherein In an optical attenuator without an optical filter function unit, when the maximum wavelength dependent loss slope in an arbitrary 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
The optical attenuator according to claim 16, wherein

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