WO2019230420A1

WO2019230420A1 - Magnetic circuit, faraday rotator, and magneto-optic element

Info

Publication number: WO2019230420A1
Application number: PCT/JP2019/019503
Authority: WO
Inventors: 太志鈴木; 峻也小田原
Original assignee: 日本電気硝子株式会社
Priority date: 2018-05-31
Filing date: 2019-05-16
Publication date: 2019-12-05
Also published as: CN110554522A

Abstract

Provided is a magnetic circuit in which irreversible demagnetization due to an external magnetic field or a temperature increase can be suppressed, and magnetic flux density adequate for a Faraday element can be stably provided. A magnetic circuit 1 having first through third magnets comprising samarium-cobalt-based magnets each provided with a through hole through which light passes, wherein the magnetic circuit 1 is characterized in that: the first through third magnets are disposed in this order on the same axis in a front-rear direction; when the direction in which light passes through the through holes of the magnetic circuit 1 is defined as the optical axis direction, the first magnet 11 is magnetized in the direction perpendicular to the optical axis direction and so that the through-hole side thereof is the N pole thereof, the second magnet 12 is magnetized in the direction parallel to the optical axis direction and so that the first magnet 11 side thereof is the N pole thereof, and the third magnet 13 is magnetized in the direction perpendicular to the optical axis direction and so that the through-hole side thereof is the S pole thereof; and the second magnet 12 has a coercivity equal to or greater than the coercivity of the first and third magnets.

Description

Magnetic circuit, Faraday rotator and magneto-optical element

The present invention relates to a magnetic circuit, a Faraday rotator, and a magneto-optical element.

The Faraday rotator is an element composed of a Faraday element and a magnet that applies a magnetic field to the Faraday element. Since the Faraday rotator has a function of propagating light only in one direction and blocking return light, it has been used as a laser optical oscillator such as an optical communication system or a laser processing system as a magneto-optical element such as an optical isolator.

The wavelength range used in the optical communication system is mainly 1300 nm to 1700 nm, and in the conventional Faraday rotator, rare earth iron garnet has been used for the Faraday element.

On the other hand, the wavelength range used for laser processing and the like in recent years is shorter than the optical communication band, and the vicinity of 1000 nm is mainly used. In this wavelength region, since the rare earth iron garnet cannot be used because of its large light absorption, paramagnetic crystals such as terbium gallium garnet (TGG) have been used for Faraday elements.

Incidentally, in order to use such a Faraday rotator as an optical isolator, the rotation angle (θ) by Faraday rotation needs to be 45 °. This rotation angle is known to have a relationship of the following formula (1) with the length of the Faraday element (L), the Verde constant (V), the magnetic flux density parallel to the optical axis (B).

Θ = V ・ B ・ L (1)

Of these, the Verde constant is a characteristic that depends on the material of the Faraday element. In general, a paramagnetic material such as TGG has a smaller Verde constant compared to rare earth iron garnet, so in order to obtain a Faraday rotation angle of 45 °, the length of the Faraday element and the magnetic flux parallel to the optical axis applied to the Faraday element It was necessary to increase the density. In particular, in recent years, there has been a demand for downsizing of the apparatus. Therefore, an invention has been proposed in which the magnetic flux density applied to the Faraday element is improved by devising the structure of the magnet instead of increasing the size of the Faraday element or the magnet. .

For example, Patent Document 1 discloses a Faraday rotator including a magnetic circuit formed of first to third magnets and a Faraday element. The first magnet is magnetized in a direction perpendicular to the optical axis and toward the optical axis. The second magnet is magnetized in a direction perpendicular to the optical axis and away from the optical axis. A third magnet is disposed between them. The third magnet is magnetized in a direction parallel to the optical axis and in a direction from the second magnet toward the first magnet. In this magnetic circuit, when the length along the length along the optical axis direction of the first magnet and the second magnet L _2, the third optical axis direction is _{_{L 3, L 2/10 ≦}} L 3 ≦ L ₂ is established.

JP 2009-229802 A

In the above magnetic circuit, a strong magnetic field generated by the interaction between the first magnet and the second magnet is generated in the vicinity of the through hole of the third magnet. This magnetic field is opposite to the magnetization direction of the third magnet. When an external magnetic field opposite to the magnetization direction is generated as described above, it is necessary to consider the influence of the movement of the operating point of the magnet and the demagnetization attributed to it. For explanation, FIG. 7 shows an example of a demagnetization curve (BH curve and JH curve) of a magnet. The BH curve 32 is a curve representing the relationship between the magnetic flux density B and the external magnetic field H. The JH curve 33 is a curve representing the relationship between the magnetization J and the external magnetic field H. The intersections of the curve, the vertical axis, and the horizontal axis mean the residual magnetic flux density Br, the intrinsic coercivity H _cJ and the coercivity H _cB , respectively. The residual magnetic flux density Br is the magnetic flux density remaining in the magnetic material when the external magnetic field is changed from the saturation magnetization state to zero. The coercive force is a value of an external magnetic field when a magnetic field opposite to the magnetization direction of the magnetic material is applied to make the magnetization or magnetic flux density zero, and the former is the intrinsic coercive force H _cJ and the latter is the coercive force. It is expressed as magnetic force _HcB . The magnetic flux density (B), the external magnetic field strength (H), the magnetization strength (J), and the vacuum magnetic permeability (μ ₀ ) of the magnetic material may have a relationship of the following formula (2). Are known.

B = μ ₀ H + J (2)

(Irreversible demagnetization by external magnetic field)
As shown in FIG. 7, when the external magnetic field H1 is applied to the magnet, the operating point a1 of the magnet moves on the BH curve 32 and moves to the operating point b1 close to the H axis. Further, when a larger external magnetic field H2 is applied, the operating point a1 of the magnet further moves to an operating point c1 closer to the H axis. At this time, the operating point c2 obtained by projecting the operating point c1 on the BH curve 32 onto the JH curve 33 is the nick point 34 of the JH curve 33 (the gradient changes and the magnetic flux density rapidly decreases). Change point). Thus, in the case of demagnetization by a larger external magnetic field H2, the operating point c2 obtained by projecting the operating point on the BH curve 32 onto the JH curve 33 exceeds the nick point 34 of the JH curve 33. Thus, when the external magnetic field H2 is removed, the operating point a1 of the magnet moves to the operating point d1. Here, the operating point d1 is an operating point when the external magnetic field H2 is removed from the operating point c1, a straight line parallel to the inclination of the recoil permeability curve passing through the operating point c1, and a straight line passing through the operating point a1 and the origin. Is the intersection of At this time, the difference between the magnetic flux density at the operating point a1 and the magnetic flux density at the operating point d1 is irreversible demagnetization ΔB due to the external magnetic field H2, and is a demagnetization that does not recover unless re-magnetization is performed.

The operating point of the magnet is a point on the BH curve 32 of the magnet as shown in FIG. 7, and is a point indicating the state of the magnetic flux density B and magnetic field H of the magnet in the magnetic circuit. . A straight line drawn from the origin toward this point is called a permeance line 31. It is known that the inclination (B / H) of the permeance line 31 is in the relationship of the vacuum permeability (μ ₀ ), the permeance coefficient (P), and the following equation (3).

B / H = μ ₀ P (3)

(Irreversible demagnetization due to high temperature)
FIG. 8 shows the temperature change of the demagnetization curve of the neodymium magnet. A BH curve 35 at a high temperature, a BH curve 36 at a low temperature, a JH curve 37 at a high temperature, and a JH curve 38 at a low temperature are shown, respectively. When the temperature of the neodymium magnet rises, the residual magnetic flux density Br, the intrinsic coercivity H _cJ and the coercivity H _cB move to _Br ′, H _cJ ′ and H _cB ′, respectively. Then, the BH curve 36 at the low temperature and the JH curve 38 at the low temperature change to the BH curve 35 at the high temperature and the JH curve 37 at the high temperature, respectively. In such a temperature change, when the operating point on the BH curve exceeds the nick point 34 of the BH curve, the magnetic force does not return to the original even if the temperature condition is restored, that is, the magnet is irreversible due to the temperature change. Demagnetization occurs. For example, the operating point a1 moves to the operating point b1 when the external temperature is changed from a low temperature to a high temperature. Since the operating point b1 does not exceed the nick point on the BH curve 35 at a high temperature, when the external temperature is returned from the high temperature to the low temperature, the operating point b1 can return to the operating point a1, that is, reversible demagnetization and Become. On the other hand, the operating point a2 closer to the H-axis moves to the operating point b2 when the external temperature is changed from the low temperature to the high temperature. In this case, since the operating point b2 exceeds the nick point 34 on the BH curve 35 at the time of high temperature, the operating point b2 can return to the operating point a2 even if the external temperature is returned from high temperature to low temperature. Without moving to the operating point c2. At this time, the difference between the magnetic flux density at the operating point a2 and the magnetic flux density at the operating point c2 is irreversible demagnetization ΔB. In particular, as shown in FIG. 8, it is known that the BH curve and the JH curve greatly change to the B axis (magnetic flux density) side of neodymium magnets as the temperature rises. Irreversible demagnetization is likely to occur due to the movement of. The operating point c2 in FIG. 8 can be obtained by the same method as the operating point d1 in FIG.

Furthermore, the third magnet described in Patent Document 1 has a shorter shape in the magnetization direction than the first and second magnets. In such a magnet having a short shape in the magnetization direction, it is necessary to consider a permeance coefficient depending on the shape of the magnet. Usually, the permeance coefficient is small in the magnetizing direction, that is, the magnet having a shape in which the magnetic poles are close to each other. The permeance coefficient is also the slope of the permeance line 31 as shown in equation (3). For example, as shown in FIG. 9, the permeance line 31 passing through the operating point a1 has a larger inclination α than the inclination β of the permeance line 31 passing through the operating point b1, that is, a larger permeance coefficient. . In other words, since a magnet having a small permeance coefficient β has an operating point at a position close to the H-axis, it becomes easier to exceed the nick point 34 on the BH curve 32 and the JH curve 33, and the above-described reverse Irreversible demagnetization is likely to occur due to magnetic fields and high temperatures.

As described above, since the third magnet of the magnetic circuit described in Patent Document 1 is used at a disadvantageous operating point as a magnet, it tends to cause irreversible demagnetization due to a reverse magnetic field or high temperature. In particular, when the magnetic circuit is used as a magneto-optical element such as an optical isolator for a high-power laser, the temperature rise of the magnetic circuit accompanying the temperature rise of the Faraday element due to the high-power light is unavoidable. There was a high possibility of irreversible demagnetization. When irreversible demagnetization occurs in the third magnet in this way, a sufficient magnetic flux density cannot be stably given to the Faraday element, and the Faraday rotator may not perform its original function.

The present invention has been made in view of the above problems, and provides a magnetic circuit that can suppress irreversible demagnetization due to an external magnetic field or a temperature rise, and can stably provide a sufficient magnetic flux density to a Faraday element. To do.

The magnetic circuit of the present invention is a magnetic circuit having first to third magnets made of samarium-cobalt magnets each provided with a through-hole through which light passes. The magnetic circuit includes first to third magnets. The magnets are arranged coaxially in this order in the front-rear direction, and when the direction in which light passes through the through hole of the magnetic circuit is the optical axis direction, the first magnet is in a direction perpendicular to the optical axis direction. And the second magnet is magnetized in a direction parallel to the optical axis direction and the first magnet side is N pole, and the third magnet is magnetized so that the through hole side is N pole. The magnet is magnetized in a direction perpendicular to the optical axis direction so that the through-hole side is an S pole, and the second magnet has a coercive force greater than one or three magnets. In the present invention, the term “coercivity” is used to indicate _HcB .

In the configuration as described above, a strong magnetic field generated by the interaction between the first magnet and the third magnet is generated near the through hole of the second magnet. However, in the magnetic circuit of the present invention, since the coercive force of the second magnet is large, the operating point does not easily exceed the nick point on the BH curve and JH curve, and irreversible demagnetization occurs due to temperature rise and reverse magnetic field. And the magnetic flux density in the through hole portion of the second magnet can be easily maintained. That is, a sufficient magnetic flux density can be stably given to the Faraday element.

Furthermore, the first to third magnets are samarium-cobalt magnets. Samarium-cobalt magnets have the same residual magnetic flux density and coercive force as neodymium magnets, but are characterized by small variations in coercive force due to temperature changes and high Curie temperatures. Therefore, in the magnetic circuit composed of the magnet, the operating point is difficult to exceed the nick point on the BH curve and the JH curve, particularly at high temperatures, and the occurrence of irreversible demagnetization can be suppressed. It becomes easy to maintain a large magnetic flux density in the through hole portion.

In the magnetic circuit of the present invention, it is preferable that the first to third magnets have a coercive force of 650 kA / m or more. By having such a coercive force, occurrence of irreversible demagnetization in the second magnet can be suppressed.

In the magnetic circuit of the present invention, the length along the optical axis direction of the second magnet is preferably equal to or longer than the length along the optical axis direction of the first and third magnets. If it does in this way, generation | occurrence | production of the irreversible demagnetization in a 2nd magnet can be suppressed, and it will become easy to hold | maintain the magnetic flux density in the through-hole part of a 2nd magnet largely.

In the magnetic circuit of the present invention, the length along the optical axis direction of the second magnet is preferably larger than the length along the optical axis direction of the first and third magnets.

In the magnetic circuit of the present invention, it is preferable that the second magnet has a larger coercive force than the first and third magnets.

In the magnetic circuit of the present invention, the cross-sectional area of the through hole is preferably 100 mm ² or less. By setting the cross-sectional area of the through hole to 100 mm ² or less, the magnetic flux density tends to increase.

The magnetic circuit of the present invention is a magnetic circuit having first to third magnets each provided with a through-hole through which light passes, and the first to third magnets are coaxial in the front-rear direction. The first magnet is in a direction perpendicular to the optical axis direction and the through hole side is N poles, where the direction in which light passes through the through hole of the magnetic circuit is the optical axis direction. The second magnet is magnetized in a direction parallel to the optical axis direction and the first magnet side is an N pole, and the third magnet is in the optical axis direction. Magnetized in a vertical direction and with the through-hole side as an S pole, the second magnet has a coercive force greater than that of the first and third magnets, and is along the optical axis direction of the second magnet. The length is not less than the length along the optical axis direction of the first and third magnets.

In the configuration as described above, a strong magnetic field generated by the interaction between the first magnet and the third magnet is generated near the through hole of the second magnet. However, since the coercive force of the second magnet is large, the operating point is unlikely to exceed the nick point on the BH curve and JH curve, and the occurrence of irreversible demagnetization due to temperature rise and reverse magnetic field can be suppressed. The magnetic flux density in the through-hole portion of the second magnet can be easily kept large. Moreover, since the length along the optical axis direction of the second magnet is equal to or longer than the length along the optical axis direction of the first and third magnets, the occurrence of irreversible demagnetization in the second magnet can be suppressed. It is possible to easily maintain a large magnetic flux density in the through-hole portion of the second magnet.

The Faraday rotator of the present invention is characterized by comprising the above magnetic circuit and a Faraday element made of a paramagnetic material that is disposed in a through hole in the magnetic circuit and transmits light.

In the Faraday rotator of the present invention, the paramagnetic material is preferably a glass material.

The magneto-optical element of the present invention includes the Faraday rotator, a first optical component disposed at one end in the optical axis direction of the magnetic circuit of the Faraday rotator, and a second optical component disposed at the other end. The light passing through the through hole of the magnetic circuit passes through the first optical component and the second optical component.

In the magneto-optical element of the present invention, it is preferable that the first optical component and the second optical component are polarizers.

According to the present invention, it is possible to provide a magnetic circuit that can suppress irreversible demagnetization due to an external magnetic field or a temperature rise and can stably give a sufficient magnetic flux density to a Faraday element.

It is typical sectional drawing which shows an example of the structure of the magnetic circuit of this invention. It is a figure which shows an example of the structure of the 1st magnet in this invention. It is a figure which shows an example of the structure of the 2nd magnet in this invention. It is a figure which shows an example of the structure of the 3rd magnet in this invention. It is typical sectional drawing which shows an example of the structure of the Faraday rotator of this invention. It is a typical sectional view showing an example of the structure of the magneto-optical element of the present invention. It is a figure which shows an example of the demagnetization curve (BH curve and JH curve) of a magnet. It is a figure which shows an example of the temperature change (BH curve and JH curve) of the demagnetization curve of a neodymium magnet. It is a figure which shows an example of the demagnetization curve (BH curve and JH curve) of a magnet.

Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments. Moreover, in each drawing, the member which has the substantially the same function may be referred with the same code | symbol.

(Magnetic circuit 1)
FIG. 1 is a schematic cross-sectional view showing the structure of the magnetic circuit of the present invention. The magnetic circuit 1 includes a first magnet 11, a second magnet 12, and a third magnet 13 each having a through hole. The magnetic circuit 1 includes a first magnet 11, a second magnet 12, and a third magnet 13 that are coaxially arranged in this order in the front-rear direction. In addition, arrange | positioning coaxially means arrange | positioning so that the center vicinity of each magnet may overlap seeing from an optical axis direction. In the present embodiment, the through holes 2 of the magnetic circuit are configured by connecting the through holes of the first magnet 11, the second magnet 12, and the third magnet 13. The letters N and S in FIG. 1 indicate magnetic poles, and the same applies to other drawings described later.

In the magnetic circuit 1, the first magnet 11 and the third magnet 13 are magnetized in the direction perpendicular to the optical axis, and the magnetization directions are opposed to each other. Specifically, the first magnet 11 is magnetized in a direction perpendicular to the optical axis so that the through hole side is an N pole. The third magnet 13 is magnetized in a direction perpendicular to the optical axis so that the through hole side is an S pole. The second magnet 12 is magnetized in a direction parallel to the optical axis so that the first magnet 11 side has an N pole.

The first to third magnets constituting the magnetic circuit 1 are preferably composed of magnets mainly composed of samarium-cobalt (Sm—Co). Since the samarium-cobalt magnet has a Curie temperature of 600 ° C. or higher, irreversible demagnetization at high temperatures can be suppressed. Further, the temperature dependence of the residual magnetic flux density of the samarium-cobalt magnet is generally about -0.03% / ° C, and the neodymium magnet is about -0.1% / ° C. The temperature dependence of the coercive force is about −0.5% / K for neodymium magnets and about −0.2% / K for samarium-cobalt magnets. Therefore, when the samarium-cobalt magnet is used, it is possible to more effectively suppress the decrease in the residual magnetic flux density and the coercive force of the magnet due to the temperature rise of the magnetic circuit 1. A magnet other than samarium-cobalt (Sm—Co) as a main component may be used.

The first to third magnets constituting the magnetic circuit 1 preferably have a coercive force of 650 kA / m or more, more preferably 660 kA / m, further preferably 700 kA / m, and particularly preferably 750 kA / m. If the coercive force is low, the second magnet 12 tends to irreversibly demagnetize because the operating point approaches the H-axis due to the strong magnetic field created by the interaction between the first magnet 11 and the third magnet 13. Become. Further, the larger the coercive force, the more stable the magnetic circuit 1 can be obtained at higher temperatures. However, the upper limit of the coercive force obtained with the samarium-cobalt magnet is practically 1000 kA / m.

Moreover, it is preferable that the coercive force of the first magnet 11 and the third magnet 13 is equal. In this way, a uniform magnetic field can be applied to the second magnet 12. But the coercive force of the 1st magnet 11 and the coercive force of the 3rd magnet 13 do not need to be equal.

The second magnet 12 has a coercive force greater than that of the first and third magnets. Specifically, the coercive force of the second magnet 12 is one or more times that of the first and third magnets, preferably 1.05 times or more, and particularly preferably 1.1 times or more. In this way, even if a strong magnetic field generated by the interaction between the first magnet 11 and the third magnet 13 is generated near the through hole of the second magnet 12, the operation of the second magnet 12 is performed. The point becomes difficult to exceed the nicks on the BH curve and JH curve, and the occurrence of irreversible demagnetization due to temperature rise and reverse magnetic field can be suppressed, and the magnetic flux density in the through hole portion of the second magnet 12 can be reduced. Large and easy to hold. The coercive force obtained with a samarium-cobalt magnet is actually about 400 to 1000 kA / m. Therefore, the coercive force of the second magnet 12 is the maximum, preferably 2.5 times or less of the coercive force of the first and third magnets, more preferably 2 times or less, and 1.8 times or less. It is particularly preferred. When the coercive force of the first magnet 11 and the coercive force of the third magnet 13 are not equal, the first and third magnets have the above values for higher coercive force. And

Further, the residual magnetic flux density (Br) of the first to third magnets constituting the magnetic circuit 1 is preferably 0.7T or more, more preferably 0.8T or more, and 0.9T or more. It is particularly preferred. In this way, a region having a high magnetic flux density can be formed in the vicinity of the through hole of the second magnet 12, and a 45 ° rotation angle can be given to the Faraday element 14 described later.

Further, it is preferable that the residual magnetic flux densities of the first magnet 11 and the third magnet 13 are equal. In this way, a uniform magnetic field can be applied to the second magnet 12. However, the coercive force of the first magnet 11 and the residual magnetic flux density of the third magnet 13 may not be equal.

In the magnetic circuit 1 of the present invention, the length along the optical axis direction of the second magnet 12 is preferably equal to or longer than the length along the optical axis direction of the first magnet 11 or the third magnet 13. Specifically, the length of the second magnet 12 is preferably at least 1 time, more preferably at least 1.01 times the length of the first magnet 11 and the third magnet 13. 1.05 times or more is particularly preferable. In this way, the length in the magnetization direction of the second magnet 12 is relatively improved, and the permeance coefficient of the second magnet 12 is increased, so that the operating point of the second magnet 12 is moved to the B-axis side. The effect of suppressing irreversible demagnetization increases. In addition, if the length of the second magnet 12 along the optical axis direction is too large, the interaction between the first magnet 11 and the third magnet 13 is weakened, so that the magnetic flux density is near the through hole of the second magnet 12. It becomes impossible to form a large area. Therefore, the length of the second magnet 12 along the optical axis direction is preferably 2 times or less, more preferably 1.5 times or less, and particularly preferably 1.4 times or less. When the length along the optical axis direction of the first magnet 11 and the length along the optical axis direction of the third magnet 13 are not equal, the light of the longer magnet of the first and third magnets. It shall have said value with respect to the length along an axial direction.

In the magnetic circuit 1 of the present invention, it is preferable that the length along the optical axis direction of the first magnet 11 is equal to the length along the optical axis direction of the third magnet 13. In this way, a uniform magnetic field can be applied to the second magnet 12. But the length along the optical axis direction of the 1st magnet 11 and the length along the optical axis direction of the 3rd magnet 13 do not need to be equal.

In the magnetic circuit 1 of the present invention, the cross-sectional shape of the through hole 2 of the magnetic circuit is not particularly limited, and may be a rectangle or a circle. A rectangle is preferable in terms of facilitating assembly, and a circle is preferable in terms of providing a uniform magnetic field.

Preferably the cross-sectional area of the through-hole 2 of the magnetic circuit is 100 mm ² or ^less, more preferably ^{^{3mm 2 ~ 80mm 2, 5mm 2}} ~ 60mm more preferably ^{^2,} particularly preferably 7 mm 2 ~ 50 mm ^2. If the cross-sectional area becomes too large, a sufficient magnetic flux density cannot be obtained, and if it is too small, it becomes difficult to arrange the Faraday element 14 in the through hole 2 of the magnetic circuit.

FIG. 2 is a diagram showing an example of the structure of the first magnet. The first magnet 11 shown in FIG. 2 is configured by combining four magnet pieces. The number of magnet pieces constituting the first magnet 11 is not limited to the above. For example, the first magnet 11 may be configured by combining six or eight magnet pieces. By configuring the first magnet 11 by combining a plurality of magnet pieces, the magnetic field can be effectively increased. But the 1st magnet 11 may consist of a single magnet.

FIG. 3 is a diagram showing an example of the structure of the second magnet. The 2nd magnet 12 shown in FIG. 3 consists of one single-piece magnet. The second magnet 12 may be configured by combining two or more magnet pieces.

FIG. 4 is a diagram showing an example of the structure of the third magnet. The third magnet 13 shown in FIG. 4 is configured by combining four magnet pieces in the same manner as the first magnet 11. By configuring the third magnet 13 by combining a plurality of magnet pieces, the magnetic field can be effectively increased. The third magnet 13 may be configured by combining six or eight magnet pieces, or may be a single magnet.

(Faraday rotator 10)
FIG. 5 is a schematic cross-sectional view showing an example of the structure of the Faraday rotator of the present invention. The Faraday rotator 10 is an apparatus used for a magneto-optical element 20 described later, such as an optical isolator or an optical circulator. The Faraday rotator 10 includes a magnetic circuit 1 and a Faraday element 14 disposed in the through hole 2 of the magnetic circuit. The Faraday element 14 is made of a paramagnetic material that transmits light.

Since the Faraday rotator 10 has the magnetic circuit 1 of the present invention shown in FIG. 1, irreversible demagnetization due to an external magnetic field or temperature rise is suppressed, and a sufficient magnetic flux density can be stably given to the Faraday element 14. Therefore, it can be used stably.

Further, light may be incident on the Faraday rotator 10 from the first magnet 11 side or may be incident from the third magnet 13 side.

Further, the cross-sectional shape of the Faraday element 14 and the cross-sectional shape of the through-hole 2 of the magnetic circuit are not necessarily matched, but are preferably matched from the viewpoint of providing a uniform magnetic field.

A paramagnetic material can be used for the Faraday element 14. Among these, it is preferable to use a glass material. The Faraday element 14 made of a glass material has a stable Verde constant and a high extinction ratio because there is little fluctuation of the Verde constant due to defects such as a single crystal material and there is little influence of the stress from the adhesive. Can keep.

The glass material used for the Faraday element 14 preferably has a Tb ₂ O ₃ content of more than 40%, more preferably 45% or more, even more preferably 48% or more, and 51% in terms of mol% of oxide. The above is particularly preferable. Thus, it becomes easy to acquire a favorable Faraday effect by increasing the content of Tb ₂ O ₃ . In addition, although Tb exists in a trivalent or tetravalent state in glass, all of these are expressed as values converted to Tb ₂ O ₃ in this specification.

In the glass material used for the Faraday element 14, the ratio of Tb ^{3+ to} the total Tb is preferably 55% or more in terms of mol%, more preferably 60% or more, further preferably 80% or more, and 90% or more. It is particularly preferred. If the ratio of Tb ^{3+ to} the total Tb is too small, the light transmittance at wavelengths of 300 nm to 1100 nm tends to decrease.

(Magneto-optical element 20)
FIG. 6 is a schematic cross-sectional view showing an example of the structure of the magneto-optical element of the present invention. The magneto-optical element 20 shown in FIG. 6 is an optical isolator. The magneto-optical element 20 includes the Faraday rotator 10 illustrated in FIG. 5, the first optical component 25 disposed at one end in the optical axis direction of the magnetic circuit 1, and the second optical disposed at the other end. And a component 26. The first optical component 25 and the second optical component 26 are polarizers in this embodiment. The light transmission axis of the second optical component 26 is inclined 45 ° with respect to the light transmission axis of the first optical component 25.

The light incident on the magneto-optical element 20 passes through the first optical component 25, becomes linearly polarized light, and enters the Faraday element 14. The incident light is rotated 45 ° by the Faraday element 14 and passes through the second optical component 26. Part of the light that has passed through the second optical component 26 becomes reflected return light, and the polarization plane passes through the second optical component 26 at an angle of 45 °. The reflected return light that has passed through the second optical component 26 is further rotated by 45 ° by the Faraday element 14 and becomes an orthogonal polarization plane of 90 ° with respect to the light transmission axis of the first optical component 25. Therefore, the reflected return light cannot be transmitted through the first optical component 25 and is blocked.

Since the magneto-optical element 20 of the present invention has the magnetic circuit 1 of the present invention shown in FIG. 1, irreversible demagnetization due to an external magnetic field or temperature rise is suppressed, and a sufficient magnetic flux density for the Faraday element 14 can be stabilized. Therefore, it can be used stably.

Although the magneto-optical element 20 shown in FIG. 6 is an optical isolator, the magneto-optical element 20 may be an optical circulator. In this case, the first optical component 25 and the second optical component 26 may be a wave plate or a beam splitter. However, the magneto-optical element 20 is not limited to an optical isolator and an optical circulator.

Hereinafter, the present invention will be described based on examples, but the present invention is not limited to these examples.

Table 1 shows Examples 1 to 7 and Comparative Example 1 of the present invention.

The average values of the magnetic flux densities of the magnetic circuits of Examples 1 to 7 and Comparative Example 1 have the coercive force H _cB , the residual magnetic flux density Br, and the length L so as to satisfy the conditions in Table 1 above. This was measured by simulating a case where a magnet structure as shown in FIG. 1 is configured with the first to third magnets. The average value of the magnetic flux density is based on the assumption that a Faraday rotating glass element having a diameter of 3 mm, a length of 10 mm, and a Verde constant of 0.21 min / Oe · cm is used. It represents a simulated value of the average value of magnetic flux density in the length of ± 5 mm from the center to the optical axis direction. The “length” is a length along the optical axis direction, and is simply expressed as a length or L in this embodiment.

As is apparent from Table 1, in Examples 1 to 7, the average value of the magnetic flux density in the length of ± 5 mm from the center of the through hole of the second magnet in the optical axis direction is 1.13 to 1.34 T. Even under the influence of a strong magnetic field created by the interaction between the first magnet and the third magnet, irreversible demagnetization hardly occurs in the vicinity of the through hole of the second magnet. Obtained.

Comparative Example 1, a second _{magnet, H cB} is 413kA / m, but except that Br is small and 0.95T is a magnetic circuit manufactured in the same manner as in Example 3, the through-hole of the second magnet The average value of the magnetic flux density in the length of ± 5 mm from the center to the optical axis direction was as small as 1.01 T.

DESCRIPTION OF SYMBOLS 1 Magnetic circuit 2 Through-hole 10 of magnetic circuit Faraday rotator 11 1st magnet 12 2nd magnet 13 3rd magnet 14 Faraday element 20 Magneto-optical element 25 1st optical component 26 2nd optical component 31 Permeance line 32 BH curve 33 JH curve 34 Knick point 35 BH curve at high temperature 36 BH curve at low temperature 37 JH curve at high temperature 38 JH curves at low temperature a1, b1, c1 , D1, a2, b2, c2 Operating points H, H1, H2 External magnetic field ΔB Irreversible demagnetization HcJ, HcJ ′ Intrinsic coercive force HcB, HcB ′ coercive force Br, Br ′ residual magnetic flux density α, β permeance coefficient

Claims

A magnetic circuit having first to third magnets made of samarium-cobalt magnets each provided with a through-hole through which light passes,
The magnetic circuit comprises the first to third magnets arranged coaxially in this order in the front-rear direction,
When the direction in which light passes through the through hole of the magnetic circuit is the optical axis direction, the first magnet is in a direction perpendicular to the optical axis direction and the through hole side is an N pole. Magnetized,
The second magnet is magnetized in a direction parallel to the optical axis direction so that the first magnet side is an N pole,
The third magnet is magnetized in a direction perpendicular to the optical axis direction and so that the through hole side is an S pole,
The magnetic circuit, wherein the second magnet has a coercive force greater than that of the first and third magnets.
2. The magnetic circuit according to claim 1, wherein the first to third magnets have a coercive force of 650 kA / m or more.
3. The magnetic circuit according to claim 1, wherein a length along the optical axis direction of the second magnet is equal to or longer than a length along the optical axis direction of the first and third magnets.
The length along the optical axis direction of the second magnet is larger than the length along the optical axis direction of the first and third magnets, according to any one of claims 1 to 3. Magnetic circuit.
The magnetic circuit according to any one of claims 1 to 4, wherein the second magnet has a coercive force larger than that of the first and third magnets.
The magnetic circuit according to any one of claims 1 to 5, wherein a cross-sectional area of the through hole is 100 mm 2 or less.
A magnetic circuit having first to third magnets each provided with a through-hole through which light passes,
The magnetic circuit comprises the first to third magnets arranged coaxially in this order in the front-rear direction,
When the direction in which light passes through the through hole of the magnetic circuit is the optical axis direction, the first magnet is in a direction perpendicular to the optical axis direction and the through hole side is an N pole. Magnetized,
The second magnet is magnetized in a direction parallel to the optical axis direction so that the first magnet side is an N pole,
The third magnet is magnetized in a direction perpendicular to the optical axis direction and so that the through hole side is an S pole,
The second magnet has a coercive force greater than that of the first and third magnets;
The magnetic circuit characterized in that a length along the optical axis direction of the second magnet is equal to or longer than a length along the optical axis direction of the first and third magnets.
A Faraday comprising: the magnetic circuit according to any one of claims 1 to 7; and a Faraday element made of a paramagnetic material that is disposed in the through hole of the magnetic circuit and transmits light. Rotor.
The Faraday rotator according to claim 8, wherein the paramagnetic material is a glass material.
The Faraday rotator according to claim 8 or 9,
A first optical component disposed at one end in the optical axis direction of the magnetic circuit of the Faraday rotator and a second optical component disposed at the other end;
A magneto-optical element in which light passing through the through hole of the magnetic circuit passes through the first optical component and the second optical component.
The magneto-optical element according to claim 10, wherein the first optical component and the second optical component are polarizers.