JP2005208516A - Faraday rotator device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To control variation in attenuation quantity of a variable optical attenuator against variation in surrounding temperature to a low level by controlling a temperature characteristic of a Faraday rotation angle versus a supplied current against surrounding temperature to a low level. <P>SOLUTION: A permanent magnet is disposed so as to apply a fixed magnetic field vertical to an advancing direction of light passing through a magnetooptic crystal to the magnetooptic crystal, and an electromagnet is disposed so as to apply a variable magnetic field parallel to the advancing direction of the light to the magnetooptic crystal. Under a condition in which a current supplied to the electromagnet is constant, the ratio of residual field intensity of the permanent magnet to generated field intensity of the electromagnet is made to get smaller according as the temperature gets higher, and temperature deviation of the Faraday rotation angle against the current supplied to the electromagnet is canceled by temperature characteristics of the fixed magnetic field by the permanent magnet and the variable magnetic field by the electromagnet and a temperature characteristic of the magnetooptic crystal. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a Faraday rotation device capable of changing the Faraday rotation angle of incident light. More specifically, the composite magnetic field obtained by a permanent magnet and an electromagnet has a temperature characteristic so as to cancel the deviation of the Faraday rotation angle with respect to a change in environmental temperature. The present invention relates to a Faraday rotation device having This technique is useful, for example, for a variable optical attenuator that adjusts the light intensity in the field of optical communication.

  In an optical communication system or an optical measurement system, a Faraday rotation device that rotates a polarization plane is incorporated. As is well known, a Faraday rotation device is configured to apply an external magnetic field to a magneto-optic crystal (a magnetic garnet single crystal having a Faraday effect), thereby controlling the Faraday rotation angle of a light beam transmitted through the magneto-optic crystal. ing. As an example, there is a type in which a Faraday rotation angle is variably controlled by applying a variable magnetic field to the magneto-optical crystal. In that case, a combined magnetic field of a fixed magnetic field by a permanent magnet and a variable magnetic field by an electromagnet is usually applied.

  Incidentally, the magneto-optical crystal has temperature characteristics at its Faraday rotation angle. Generally, the direction of the temperature characteristic is such that the Faraday rotation angle decreases as the temperature increases, and the Faraday rotation angle increases as the temperature decreases. No material with the opposite characteristics has been found so far. Therefore, in a variable optical attenuator using a Faraday rotation device, the temperature characteristic of the magneto-optic crystal affects the drive current versus attenuation characteristic. For example, even when a magneto-optical crystal having a temperature coefficient of 0.001 [deg / ° C./deg] per 1 Faraday rotation angle is used, a deviation of about 10 dB occurs in the vicinity of the maximum attenuation.

  Patent Document 1 proposes a variable optical attenuator that takes the maximum amount of attenuation when the angle between the traveling direction of light and the synthesized magnetic field applied to the magneto-optical crystal coincides. However, even in this case, a temperature fluctuation of a little less than 0.7 dB occurs in principle.

  In Patent Document 2, a configuration in which a fixed magnetic field is applied parallel to the traveling direction of light passing through a magneto-optic crystal and a variable magnetic field is applied vertically is applied to an electromagnet yoke with a Ni—Fe alloy or a Fe—Co—Ni alloy. A technique for correcting the temperature by using the above is disclosed. However, such a yoke material is expensive and difficult to put into practical use.

Non-Patent Document 1 also discloses a technique for reducing temperature fluctuations by a constant voltage driving method using temperature characteristics of electrical resistance.
JP-A-9-236784 JP 2002-341302 A “Reduction of temperature variation in attenuation of magneto-optic VOA” Nagae, Kawamata (2002 IEICE General Conference, C-3-132)

  The problem to be solved by the present invention is that the temperature characteristic of the supply current versus the Faraday rotation angle is large with respect to the environmental temperature.

  The present invention relates to a magneto-optical crystal, a permanent magnet arranged so that a fixed magnetic field is applied substantially perpendicular to the direction of travel of light passing through the magneto-optical crystal, and the travel of light through the magneto-optical crystal. In a Faraday rotating device having an electromagnet arranged so that a variable magnetic field is applied substantially parallel to the direction, the generated magnetic field strength of the electromagnet is controlled under the condition that the current supplied to the electromagnet is constant. The ratio of the permanent magnet's residual magnetic field strength decreases as the temperature increases, and the Faraday rotation angle relative to the supply current to the electromagnet depends on the temperature characteristics of the fixed magnetic field by the permanent magnet and the variable magnetic field by the electromagnet and the temperature characteristics of the magneto-optic crystal. This is a Faraday rotation device in which the angle deviation due to the temperature of the lens is canceled.

  In the present invention, in particular, a combination in which the temperature fluctuation of the generated magnetic field strength of the electromagnet under a condition where the current supplied to the electromagnet is constant is substantially negligible compared to the temperature fluctuation of the residual magnetic field strength of the permanent magnet. It is preferable to do. In that case, the permanent magnet is selected from those having a temperature characteristic of the residual magnetic field of about -0.05 to -0.25% / ° C.

  Using such a Faraday rotation device, placing an analyzer on the exit side of the magneto-optic crystal, and setting the analyzer orientation so that the attenuation is maximized when no current is supplied to the electromagnet, it is possible to achieve a good temperature A variable optical attenuator exhibiting characteristics can be obtained. Also, a polarizer is placed on the entrance side of the magneto-optic crystal and an analyzer is placed on the exit side, so that the crystal axis orientation of the polarizer and analyzer is maximized so that the amount of attenuation is maximized when no current is supplied to the electromagnet. You may set so that it may be substantially orthogonal.

  The Faraday rotation device according to the present invention cancels the angular deviation due to the temperature of the Faraday rotation angle with respect to the current supplied to the electromagnet, by the temperature characteristics of the fixed magnetic field by the permanent magnet and the variable magnetic field by the electromagnet and the temperature characteristics of the magneto-optic crystal. Specifically, under a condition where the current supplied to the electromagnet is constant, it is possible to suppress the rotation angle shift with respect to the change in the environmental temperature to 1 degree or less when the rotation angle is around 0 degrees. In addition, by applying to a variable optical attenuator, it is possible to suppress the temperature variation of the attenuation amount to 0.2 dB or less. Furthermore, there is an advantage that it can be made of an inexpensive material and can be easily put into practical use.

The Faraday rotation angle including the temperature characteristics and the traveling direction of light in the magneto-optical crystal is
θ _F (t) = θ (t ₀ ) × (1−αΔt) × cos β = θ (t) × cos β
It can be expressed as However,
β: angle formed by the traveling direction of light and the synthetic magnetic field applied to the magneto-optic crystal θ (t ₀ ): Faraday rotation angle of the magneto-optic crystal at temperature t ₀ α: temperature coefficient per deg of the magneto-optic crystal [deg / ° C. / deg]
Δt = t−t ₀
It is. Therefore, unless F = 90 degrees, the temperature characteristic of the Faraday rotation angle always occurs. θ (t) is the Faraday rotation angle (how many rotors) of the magneto-optical crystal at the temperature t. Faraday rotation is difficult to rotate on the high temperature side and easy to rotate on the low temperature side with respect to the room temperature. From the above equation, it can be seen that cos β should have a characteristic opposite to θ (t) in order to cancel the temperature characteristic of the Faraday rotation angle. However, since β generally takes a wide range of about 0 to 90 °, it is impossible to cancel the entire range, and it is only necessary to cancel within the necessary limited range. Conceptually, as shown in FIG. 1, the temperature characteristics of the synthesized magnetic field are such that cos β is larger (the synthesized magnetic field vector is tilted) at higher temperatures and cos β is smaller (the synthesized magnetic field vector is standing) at lower temperatures. is there.

  By the way, the temperature characteristic of the residual magnetization of the permanent magnet is generally negative. That is, the higher the temperature, the lower the residual magnetic field strength. On the other hand, the electromagnet is used in a region that does not reach the magnetization saturation point, and the generated magnetic field depends on the drive current. Therefore, the temperature characteristics of the magnetic field generated by the electromagnet (temperature characteristics of the yoke material) can be almost ignored. Therefore, the Faraday rotation device passes through the magneto-optical crystal, the permanent magnet arranged so that a fixed magnetic field is applied perpendicular to the traveling direction of the light passing through the magneto-optical crystal, and the magneto-optical crystal. A configuration including an electromagnet arranged so that a variable magnetic field is applied in parallel to the traveling direction of light is suitable. In this configuration, since the fixed magnetic field may be relatively weak, a ferrite-based magnet and a bonded magnet can be used, and the range of material selection is widened.

  By the way, the larger the central value of the Faraday rotation angle, the greater the angular deviation due to temperature. When the rotation angle deviation is attempted to be zero when the central value of the rotation angle is large (for example, around 90 degrees), it is necessary to combine the permanent magnet materials with values that completely cancel the temperature deviation of the Faraday rotation angle. However, this method is disadvantageous in view of the fact that there are individual variations in the characteristics of magneto-optical crystals and permanent magnets, and that the rotational angle deviation is proportional to the center value. On the other hand, in the vicinity of the rotation angle of zero, this angular deviation is completely zero in principle, and the rotation center value itself is small, so it is easier to take a value close to zero in this vicinity. Therefore, the deviation when the rotation angle is around 0 degrees is suppressed to a small value.

  The temperature coefficient of the Faraday rotation angle of the magneto-optical crystal is generally 0.04 to 0.08 deg / ° C. (value at 45 degrees rotator). The temperature characteristic of the magnetic field generated by the electromagnet (temperature characteristic of the yoke material) is almost zero in silicon steel, which is inexpensive and has a high saturation magnetic flux density. Therefore, the temperature characteristic of the residual magnetic field of the permanent magnet is −0. Select from about 05 to -0.25% / ° C.

  In addition, the magnetic anisotropy of a magneto-optic crystal has a temperature characteristic, which causes a temperature characteristic with respect to the drive current. It is desirable to arrange.

  When configuring a variable optical attenuator, a polarizer is placed on the entrance side of the magneto-optic crystal and an analyzer is placed on the exit side, so that the amount of attenuation is maximized when no current is supplied to the electromagnet. And the crystal axis orientation of the analyzer are set so as to be substantially orthogonal.

  FIG. 2 is an explanatory view showing an embodiment of a Faraday rotation device according to the present invention. In this Faraday rotation device, the electromagnet 12 is arranged so as to apply a variable magnetic field in parallel to the traveling direction of light passing through the magneto-optical crystal 10, and a fixed magnetic field is applied perpendicularly to the traveling direction of light. The permanent magnet 14 is arranged. The electromagnet 12 has a structure in which a coil 18 is wound around a U-shaped yoke 16, and a through hole 20 is formed in the yoke 16 so as not to hinder the passage of light.

  In the present invention, under the condition that the current supplied to the coil 16 of the electromagnet 12 is constant, the ratio of the residual magnetic field strength of the permanent magnet 14 to the generated magnetic field strength of the electromagnet becomes smaller as the temperature becomes higher. The temperature deviation of the Faraday rotation angle with respect to the supply current to the electromagnet 12 is canceled out by the temperature characteristics of the fixed magnetic field by the electromagnet 12, the variable magnetic field by the electromagnet 12, and the temperature characteristics of the magneto-optical crystal 10.

An example of the calculation result is shown in FIG. The temperature characteristics of the permanent magnet were selected so that the rotation angle deviation Δθ was small when the rotation angle was around 0 degrees. The conditions are as follows. A is a reference example and B is the present invention. Although the values at temperatures of −5 ° C. (low temperature), 25 ° C. (room temperature), and + 70 ° C. (high temperature) were obtained, the curves overlap in the figure and are difficult to see, so −5 ° C. (low temperature) and + 70 ° C. Only the characteristics at (high temperature) are shown. The same applies to the following graphs.
-Temperature coefficient of Faraday rotation angle of magneto-optic crystal: 0.045deg / ° C (value at 45 degree rotor)
・ Temperature characteristics of yoke magnetic field generation: 0% / ℃
-Temperature characteristics of the residual magnetic field of the permanent magnet Example A: -0.02% / ° C (for example, Sm-Co magnet)
Example of B: -0.098% / ° C (for example, ferrite magnet)
From these results, it can be seen that the rotation angle deviation Δθ with respect to the change of the environmental temperature can be suppressed to 1 degree or less when the rotation angle is around 0 degree under the constant coil drive current condition. In particular, by selecting an appropriate value for the temperature characteristic of the residual magnetic field of the permanent magnet, as shown in FIG. 3B, the rotation angle with respect to changes in the environmental temperature over a wide range from 0 degrees to 50 degrees. The shift Δθ can be suppressed to 1 degree or less.

  The most preferred structure of the Faraday rotation device is shown in FIG. Electromagnets 32a and 32b are arranged before and after the optical path of the magneto-optic crystal 30, and permanent magnets 34a and 34b are arranged on the left and right (or top and bottom) of the optical path. Here, both the electromagnets 32a and 32b have a structure in which a coil 38 is wound around the outer periphery of the cylindrical yoke 36, and the central hole 40 of the cylindrical yoke 36 serves as an optical path. The permanent magnets 34a and 34b are plate magnets magnetized in the thickness direction. In this structure, since the U-shaped yoke is not used, there is an advantage that there is no protrusion of the member in the direction perpendicular to the light traveling direction, and the size and diameter can be reduced.

  When the coil 38 is energized, the magnetic flux passes through the cylindrical yoke 36 and a magnetic field is generated outside. By energizing the coils 18 of both the electromagnets 32a and 32b in the same direction, a magnetic field is applied to the magneto-optic crystal 30 positioned therebetween in a direction parallel to the traveling direction of light, and the current value is controlled. Thus, the strength of the magnetic field can be varied. The two permanent magnets 34a and 34b are magnetized in the same direction, whereby a fixed magnetic field is applied to the magneto-optical crystal 30 in a direction perpendicular to the light traveling direction. Therefore, a fixed magnetic field perpendicular to the light traveling direction and a variable magnetic field parallel to the light traveling direction are simultaneously applied to the magneto-optic crystal 30, and the resultant magnetic field direction and the light traveling direction are Faraday rotation occurs in the incident light according to the cosine component.

FIG. 5 shows the measurement results for the Faraday rotation device having this structure. The materials used are as follows.
(1) Magneto-optical crystal: Bi-substituted rare earth iron garnet LPE film (crystal length corresponding to a 110 degree rotator for a wavelength of 1550 nm)
(2) Electromagnet and yoke material: Silicon steel with high saturation magnetic flux density (temperature characteristics are almost 0% / ° C)
Coil: 800 turns each (3) Permanent magnet: Ferrite magnet (temperature characteristic of residual magnetization Br is -0.12% / ° C)
As can be seen from FIG. 5, the rotation angle deviation Δθ can be suppressed to 1 degree or less in a wide range of the rotation angle from 0 degree to about 60 degrees, and a very good result was obtained. In particular, in the range of 0 degrees to 4 degrees, the angular deviation due to temperature was 0.1 degrees or less.

  To construct a variable optical attenuator using such a Faraday rotation device, in principle, a polarizer is arranged on the incident side of the magneto-optical crystal of the Faraday rotation device shown in FIG. 2, and an analyzer is arranged on the emission side. The crystal axis orientations of the polarizer and the analyzer may be set so as to be orthogonal. A magneto-optical crystal having a crystal length that causes a Faraday rotation of 90 degrees or more is used.

An example of the calculation result is shown in FIG. The conditions are as follows.
-Temperature coefficient of Faraday rotation angle of magneto-optic crystal: 0.045deg / ° C (value at 45 degree rotor)
・ Temperature characteristics of yoke magnetic field generation: 0% / ℃
Polarizer and analyzer are orthogonally arranged A in FIG. 6 is a case where the temperature characteristics of the residual magnetic field of the permanent magnet is 0% / ° C. (reference example). ΔATT, which is the temperature variation (maximum-minimum) of attenuation during constant current drive, is a little less than 0.7 dB. This ΔATT curve changes as shown in FIG. 6B depending on the temperature characteristics of the residual magnetic flux of the permanent magnet used. That is, the temperature variation ΔATT of the attenuation amount can be controlled by selecting a permanent magnet having an appropriate value for the temperature characteristic of the residual magnetic flux. In the present invention, the temperature characteristic of the residual magnetic field of the permanent magnet is selected from about -0.05 to -0.25% / ° C. In particular, from the results shown in FIG. 6B, it can be seen that it is preferable to set to −0.08 to −0.12% / ° C. As a result, the temperature variation ΔATT of the attenuation amount can be reduced to 0.2 dB or less.

  The most preferred structure of the variable optical attenuator is shown in FIG. Here, the Faraday rotation device having the structure shown in FIG. 4 is used as it is. Therefore, the corresponding members are denoted by the same reference numerals, and description thereof is omitted. In this embodiment, a polarizer 42 is installed on the incident side of the magneto-optic crystal 30 and an analyzer 44 is installed on the exit side. The types and installation positions of the polarizer 42 and the analyzer 44 are arbitrary, but are preferably provided between the magneto-optical crystal 30 and the electromagnets 32a and 32b as shown in the figure. This is because a certain distance is required between the electromagnets 32a and 32b, and the polarizer 42 and the analyzer 44 can be inserted using the space. Here, wedge-shaped birefringent plates (for example, rutile) are used, and their crystal axis orientations are set to be orthogonal to each other.

  Typically, a Faraday rotation device of 90 to 100 degrees is used, and the analyzer 44 has a substantially orthogonal orientation with respect to the polarizer 42. The incident light is separated into ordinary light and extraordinary light by the polarizer 42, and their polarization planes are rotated by the magneto-optical crystal 30 according to the direction of the combined magnetic field, and further pass through the analyzer 44 to further separate the optical paths, thereby producing parallel light components. Will be coupled to the exit side. When the drive currents of both the electromagnets 32a and 32b are zero, the polarization plane does not rotate, so that it can hardly be coupled to the emission side, and the attenuation is maximized. On the other hand, when the drive current of the electromagnet is sufficiently large, the plane of polarization rotates approximately 90 degrees, so that most of the plane is coupled to the emission side, and the attenuation is minimized.

The measurement results for the variable optical attenuator having this structure are shown in FIG. The materials used are as follows.
(1) Magneto-optical crystal: Bi-substituted rare earth iron garnet LPE film (crystal length corresponding to a 110 degree rotator for a wavelength of 1550 nm)
(2) Electromagnet and yoke material: Silicon steel with high saturation magnetic flux density (temperature characteristics are almost 0% / ° C)
Coil: 800 turns each (3) Permanent magnet: Ferrite magnet (temperature characteristic of residual magnetization Br is -0.12% / ° C)
As can be seen from FIG. 8, the change in attenuation with respect to the change in environmental temperature can be suppressed to 0.1 dB or less under a constant coil drive current condition.

The image figure of the temperature characteristic of the synthetic magnetic field which should be. Explanatory drawing which shows one Example of the Faraday rotation device which concerns on this invention. The graph which shows an example of the calculation result. The perspective view which shows the most preferable structural example of a Faraday rotation device. The graph which shows an example of the measurement result. The graph which shows an example of the calculation result as a variable optical attenuator. The perspective view which shows the most preferable structural example of a variable optical attenuator. The graph which shows an example of the measurement result.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Magneto-optical crystal 12 Electromagnet 14 Permanent magnet 16 Yoke 18 Coil 20 Through-hole

Claims (4)

A magneto-optical crystal, a permanent magnet arranged so that a fixed magnetic field is applied substantially perpendicular to the traveling direction of light passing through the magneto-optical crystal, and the traveling direction of light passing through the magneto-optical crystal In a Faraday rotation device comprising an electromagnet arranged such that a variable magnetic field is applied substantially in parallel,
Under a condition where the current supplied to the electromagnet is constant, the ratio of the residual magnetic field strength of the permanent magnet to the generated magnetic field strength of the electromagnet becomes smaller as the temperature rises, and the fixed magnetic field of the permanent magnet and the variable magnetic field of the electromagnet are reduced. A Faraday rotation device characterized in that an angular shift due to a temperature of a Faraday rotation angle with respect to a current supplied to an electromagnet is canceled out by a temperature characteristic and a temperature characteristic of a magneto-optical crystal.   2. The Faraday rotation according to claim 1, wherein the temperature fluctuation of the generated magnetic field strength of the electromagnet under the condition that the current supplied to the electromagnet is constant can be substantially ignored as compared with the temperature fluctuation of the residual magnetic field strength of the permanent magnet. device.   Using the Faraday rotation device according to claim 1 or 2, the analyzer is arranged on the exit side of the magneto-optical crystal, and the analyzer orientation is set so that the attenuation is maximized when no current is supplied to the electromagnet. A variable optical attenuator. The Faraday rotation device according to claim 1 or 2, wherein a polarizer is arranged on the incident side of the magneto-optic crystal and an analyzer is arranged on the emission side, so that the attenuation is maximized when no current is supplied to the electromagnet. The variable optical attenuator is set so that the crystal axis orientations of the polarizer and the analyzer are substantially orthogonal to each other.

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