Magnetic circuit, faraday rotator and magneto-optical device

Abstract

This invention provides a magnetic circuit that, when used in an optical isolator, is less prone to polarizer breakage even when increasing laser output. The magnetic circuit (1) has first to third magnets (11) to (13) respectively provided with through holes for light to pass through. The magnetic circuit (1) is constructed by sequentially arranging the first to third magnets (11) to (13) coaxially in the front-back direction. One of the first magnets (11) and the third magnet (13) is magnetized in a direction (Y) perpendicular to the optical axis (X) such that the through hole (2) side is the N pole. The other magnet of the first magnet (11) and the third magnet (13) is magnetized in a direction (Y) perpendicular to the optical axis (X) such that the through hole (2) side is the N pole. The first magnet (11) is magnetized in a direction (Y) perpendicular to the optical axis (X) so that the through hole (2) side is the (S) pole. The second magnet (12) is magnetized in a direction parallel to the optical axis (X) so that the magnet side of the first magnet (11) and the third magnet (13) with the through hole (2) side as the (N) pole is the (N) pole. The length (L1) of the first magnet (11) along the optical axis (X) is different from the length (L3) of the third magnet (13) along the optical axis (X).

Description

Technical Field

[0001] This invention relates to magnetic circuits, Faraday rotators, and magneto-optical devices. Background Technology

[0002] An optical isolator is a magneto-optical device that directs light in only one direction and prevents reflected light from returning. Optical isolators are used in laser oscillators in optical communication systems and laser processing systems. Generally, an optical isolator includes a Faraday rotator and polarizers positioned at one end and the other end of the Faraday rotator along its optical axis.

[0003] For a long time, the wavelength range used in optical communication systems has been mainly 1300nm to 1700nm, and the Faraday elements of the Faraday rotators in optical isolators use rare earth iron garnet.

[0004] On the other hand, the wavelengths used in laser processing and other applications are shorter than those in optical communication, mainly around 1000 nm. Within this wavelength range, the light absorption of the aforementioned rare-earth iron garnets is high, making them unsuitable. Therefore, Faraday elements made of paramagnetic crystals are generally used, particularly terbium gallium garnet (TGG), which is widely known.

[0005] For use as an optical isolator, the Faraday rotation angle (θ) must be 45°. Regarding this Faraday rotation angle, the length (L), Wilder constant (V), and magnetic flux density (H) parallel to the optical axis are related by the following equation (1).

[0006] θ=V·H·L (1)

[0007] The Wilder constant (V) is material-dependent. Therefore, to adjust the Faraday rotation angle, it is necessary to vary the length (L) of the Faraday element and the magnetic flux density (H) applied to the Faraday element parallel to the optical axis. In particular, recent pursuits in device miniaturization have led to the ability to increase the magnetic flux density applied to the Faraday rotator by altering the magnet's structure, rather than by adjusting the size of the Faraday element or magnet.

[0008] For example, Patent Document 1 disclosed below discloses a Faraday rotator comprising a magnetic circuit composed of first to third magnets and a Faraday element. The first magnet is magnetized in a direction perpendicular to the optical axis and towards 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 towards the first magnet from the second magnet. In this magnetic circuit, when the lengths of the first and second magnets along the optical axis are defined as L2, and the length of the third magnet along the optical axis is defined as L3, the relationship L2 / 10 ≤ L3 ≤ L2 holds.

[0009] Existing technical documents

[0010] Patent documents

[0011] Patent Document 1: Japanese Patent No. 5239431 Summary of the Invention

[0012] The technical problem that the invention aims to solve

[0013] With the increasing output of laser processing in recent years, high laser resistance is required for optical isolators. However, in optical isolators using Faraday rotators such as those in Patent Document 1, polarizer breakage sometimes occurs when the laser output is increased.

[0014] The purpose of this invention is to provide a magnetic circuit, Faraday rotator, and magneto-optical device that, when used in an optical isolator, does not easily cause polarizer damage even when the laser output is increased.

[0015] Technical solutions for solving technical problems

[0016] The magnetic circuit of the present invention has first to third magnets respectively provided with through holes for light to pass through. The magnetic circuit is constructed by sequentially arranging the first to third magnets coaxially in the front-back direction. When the direction in which light passes through the through holes of the magnetic circuit is taken as the optical axis direction, one of the first magnets and the third magnet is magnetized in a direction perpendicular to the optical axis direction such that the through hole side is the N pole. The other magnet is magnetized in a direction perpendicular to the optical axis direction such that the through hole side is the S pole. The second magnet is magnetized in a direction parallel to the optical axis direction such that the side of the first magnet and the third magnet whose through hole side is the N pole is the N pole. The length of the first magnet along the optical axis direction is different from the length of the third magnet along the optical axis direction.

[0017] In this invention, preferably the first magnet is magnetized in a direction perpendicular to the optical axis such that the through-hole side is the N pole, the second magnet is magnetized in a direction parallel to the optical axis such that the first magnet side is the N pole, and the third magnet is magnetized in a direction perpendicular to the optical axis such that the through-hole side is the S pole. When the length of the first magnet along the optical axis is set as L1 and the length of the third magnet along the optical axis is set as L3, there exists a relationship L3 < L1.

[0018] In this invention, it is preferable that when the length of the second magnet along the optical axis is set as L2, there exists a relationship of L2 < L3 < L1.

[0019] The Faraday rotator of the present invention includes: a magnetic circuit constructed according to the present invention; and a Faraday element disposed within the through hole of the magnetic circuit and composed of a light-transmitting paramagnetic material.

[0020] In this invention, the paramagnetic material is preferably glass.

[0021] The magneto-optical device of the present invention includes: a Faraday rotator configured according to the present invention; a first optical component disposed at one end of the Faraday rotator in the direction of the optical axis; and a second optical component disposed at the other end of the Faraday rotator in the direction of the optical axis, wherein light passing through the through hole of the magnetic circuit passes through the first optical component and the second optical component.

[0022] In this invention, the first optical component and the second optical component are preferably polarizers.

[0023] The magneto-optical device of the present invention includes: a magnetic circuit having first to third magnets respectively provided with through holes for light to pass through; a Faraday rotator disposed within the through holes of the magnetic circuit and having a Faraday element composed of a light-transmitting paramagnetic material; a first optical component disposed at one end of the Faraday rotator along the optical axis; and a second optical component disposed at the other end of the Faraday rotator along the optical axis. Light passing through the through holes of the magnetic circuit passes through the first optical component and the second optical component. The magnetic circuit is constructed by sequentially arranging the first to third magnets coaxially in the front-back direction. When the direction in which light passes through the through holes of the magnetic circuit is taken as the optical axis... In this configuration, one of the first and third magnets is magnetized in a direction perpendicular to the optical axis such that the through-hole side becomes the N pole; the other of the first and third magnets is magnetized in a direction perpendicular to the optical axis such that the through-hole side becomes the S pole; and the second magnet is magnetized in a direction parallel to the optical axis such that the side of the first and third magnets whose through-hole side is the N pole becomes the N pole. The distance from the center of the Faraday element to the first optical component along the optical axis is different from the distance from the center of the Faraday element to the second optical component along the optical axis.

[0024] In this invention, the second optical component is preferably disposed on the side of the third magnet in the optical axis direction. When the sum of the lengths of the first magnet, the second magnet, and the third magnet along the optical axis direction is set as (L1+L2+L3), and the distance from the center of the Faraday element to the second optical component along the optical axis direction is set as L4, the ratio of L4 / (L1+L2+L3) is in the range of 0.2 or more and less than 0.5.

[0025] In this invention, the Faraday element is preferably disposed at the center of the second magnet.

[0026] Invention Effects

[0027] According to the present invention, a magnetic circuit, a Faraday rotator, and a magneto-optical device can be provided that, when used in an optical isolator, are less prone to polarizer breakage even when the laser output is increased. Attached Figure Description

[0028] Figure 1 This is a schematic cross-sectional view showing the structure of the magnetic circuit according to the first embodiment of the present invention.

[0029] Figure 2 This is a schematic cross-sectional view showing the structure of the Faraday rotator according to the first embodiment of the present invention.

[0030] Figure 3 This is a schematic cross-sectional view showing the structure of the magneto-optical device according to the first embodiment of the present invention.

[0031] Figure 4 This is a diagram illustrating an example of the structure of the first magnet of the present invention.

[0032] Figure 5 This is a diagram illustrating an example of the structure of the second magnet of the present invention.

[0033] Figure 6 This is a diagram illustrating an example of the structure of the third magnet of the present invention.

[0034] Figure 7 This is a schematic cross-sectional view showing the structure of the magnetic circuit, Faraday rotator, and magneto-optical device according to the second embodiment of the present invention.

[0035] Figure 8 This is a graph showing the magnetic field strength of the magnetic circuits in Examples 1-2 and Comparative Example 1.

[0036] Figure 9 This is a photograph showing an example of a damaged polarizer. Detailed Implementation

[0037] The preferred embodiments will now be described. However, these embodiments are merely illustrative, and the present invention is not limited to them. Furthermore, in the accompanying drawings, components having substantially the same function are sometimes referred to by the same reference numerals.

[0038] [First Implementation Method]

[0039] Figure 1 This is a schematic cross-sectional view showing the structure of the magnetic circuit according to the first embodiment of the present invention. Figure 2 This is a schematic cross-sectional view showing the structure of the Faraday rotator according to the first embodiment of the present invention. Figure 3 This is a schematic cross-sectional view showing the structure of the magneto-optical device according to the first embodiment of the present invention. In each figure, the letters N and S represent magnetic poles.

[0040] (Magnetic circuit)

[0041] like Figure 1 As shown, the magnetic circuit 1 has a first magnet 11, a second magnet 12, and a third magnet 13, each with a through hole. The magnetic circuit 1 is constructed by sequentially arranging the first magnet 11, the second magnet 12, and the third magnet 13 coaxially in the front-to-back direction. Furthermore, "coaxial arrangement" means that when viewed from the optical axis X, the magnets are arranged so that they overlap near their centers. In this embodiment, the through holes connecting the first magnet 11, the second magnet 12, and the third magnet 13 form the through holes 2 of the magnetic circuit 1.

[0042] The Faraday element 14, described later, can be arranged within the through hole 2 of the magnetic circuit 1. Thus, a Faraday rotator 10 can be constructed for use in magneto-optical devices 20 such as optical isolators and optical circulators.

[0043] The cross-sectional shape of the through hole 2 in the magnetic circuit 1 is not particularly limited and can be rectangular or circular. Rectangular is preferred for ease of assembly, while circular is preferred for imparting a uniform magnetic field.

[0044] Figure 4 This is a diagram showing an example of the structure of the first magnet (viewed from the X-axis). Figure 4 The first magnet 11 shown is constructed by combining four magnetic plates, and has a rectangular (square) cross-sectional shape as a whole. The first magnet 11 can also have a circular cross-sectional shape as a whole. Furthermore, the number of magnetic plates constituting the first magnet 11 is not limited to the above-mentioned number. For example, the first magnet 11 can also be constructed by combining six or eight magnetic plates. By combining multiple magnetic plates to construct the first magnet 11, the magnetic field can be effectively increased. However, the first magnet 11 can also be constructed from a single magnet.

[0045] Figure 5This is a diagram showing an example of the structure of the second magnet (viewed from the X-axis). Figure 5 The second magnet 12 shown is composed of a single magnet. The second magnet 12 has a rectangular (square) cross-sectional shape. The second magnet 12 may also have a circular cross-sectional shape. In addition, the second magnet 12 may also be composed of two or more magnet pieces.

[0046] Figure 6 This is a diagram showing an example of the structure of the third magnet (viewed from the X-axis). Figure 6 The third magnet 13 shown is similar to the first magnet 11, constructed by combining four magnet plates, and has a rectangular (square) cross-sectional shape as a whole. The third magnet 13 itself has a circular cross-sectional shape. By combining multiple magnet plates to construct the third magnet 13, the magnetic field can be effectively increased. Furthermore, the third magnet 13 can be constructed by combining six or eight equal magnet plates, or it can be constructed from a single magnet.

[0047] Furthermore, the first magnet 11, the second magnet 12, and the third magnet 13 are composed of permanent magnets. As such permanent magnets, rare earth magnets are particularly preferred, and magnets with samarium cobalt (Sm-Co) as the main component and magnets with neodymium iron boron (Nd-Fe-B) as the main component are especially preferred.

[0048] In magnetic circuit 1, the first magnet 11 and the third magnet 13 are magnetized in a direction Y perpendicular to the optical axis X, with their magnetization directions opposite each other. Specifically, the first magnet 11 is magnetized in the direction Y perpendicular to the optical axis X such that the side with the through-hole 2 is the N pole. The third magnet 13 is magnetized in the direction Y perpendicular to the optical axis X such that the side with the through-hole 2 is the S pole. The second magnet 12 is magnetized in a direction parallel to the optical axis X such that the side with the first magnet 11 is the N pole. Furthermore, in this specification, the direction in which light passes through the through-hole 2 of magnetic circuit 1 is defined as the optical axis X.

[0049] In magnetic circuit 1, the length L3 of the third magnet 13 is shorter than the length L1 of the first magnet 11. Furthermore, the length L2 of the second magnet 12 is shorter than both the length L1 of the first magnet 11 and the length L3 of the third magnet 13. Therefore, in magnetic circuit 1, there exists a relationship of L2 < L3 < L1. Additionally, the lengths L1 of the first magnet 11, L2 of the second magnet 12, and L3 of the third magnet 13 are all lengths along the optical axis direction X.

[0050] The characteristic of this embodiment is that the length L3 of the third magnet 13 is shorter than the length L1 of the first magnet 11. Therefore, when used in an optical isolator, even if the laser output is increased, the breakage of the polarizer is less likely to occur. This point can be explained as follows.

[0051] For a long time, optical isolators have suffered from the problem of polarizer breakage when laser output is increased. The inventors of this invention have conducted in-depth research into the cause and concluded that it is due to a thermal lensing effect in the Faraday element when laser output is increased. Specifically, when laser output increases, the center of the Faraday element becomes hotter, thus creating a temperature gradient. As a result, due to the temperature dependence of the refractive index, a refractive index gradient is generated. Consequently, the Faraday element acts as a lens, causing the laser light passing through it to be focused. Since the optical isolator uses a structure with a Faraday element placed between two polarizers, it is believed that the light passing through the Faraday element will be focused due to the thermal lensing effect, resulting in a smaller incident beam diameter. This increases the energy density reaching the polarizer on the light-emitting side, leading to its breakage. Furthermore, in... Figure 9 The image shows a photograph as an example of a polarizer damaged due to such a thermal lensing effect. Polarizer damage often occurs due to dielectric breakdown, resulting in breakage (such as cracks) on the exiting surface. When such damage occurs, the polarizer's transmittance decreases sharply.

[0052] In contrast, in this embodiment, since the length L3 of the third magnet 13 is shorter than the length L1 of the first magnet 11, the distance between the Faraday element 14 and the polarizer (second optical component 26) can be shortened. Therefore, even if a thermal lensing effect occurs, the amount of light concentrated is small, thus suppressing the increase in energy density and preventing damage to the polarizer.

[0053] Furthermore, when the length L3 of the third magnet 13 is shorter than the length L1 of the first magnet 11, such as Figure 2 As shown, the Faraday element 14 is offset from the center of the magnetic circuit 1, but it has been found that the same magnetic properties as when the Faraday element 14 is arranged at the center of the magnetic circuit 1 can be obtained in this case.

[0054] Furthermore, this embodiment describes the case where light is incident from the side of the first magnet 11, but light can also be incident from the side of the third magnet 13. In this case, by making the length L1 of the first magnet 11 shorter than the length L3 of the third magnet 13, the breakage of the polarizer can be minimized even when the laser output is increased. Therefore, in this invention, the length L1 of the first magnet 11 and the length L3 of the third magnet 13 can be different.

[0055] Alternatively, in this case, the first magnet 11 may be magnetized with the through hole 2 side as the S pole, the third magnet 13 may be magnetized with the through hole 2 side as the N pole, and the second magnet 12 may be magnetized with the first magnet 11 side as the S pole.

[0056] Furthermore, in this embodiment, the length L2 of the second magnet 12 is shorter than the length L1 of the first magnet 11 and the length L3 of the third magnet 13. Therefore, the magnetic properties can be further improved, and a larger magnetic field can be applied to the Faraday element 14. By applying a large magnetic field to the Faraday element 14, the isolation characteristics as an optical isolator can be further improved.

[0057] In this embodiment, the ratio L1 / L3 of the length L1 of the first magnet 11 to the length L3 of the third magnet 13 is preferably 1.01 or more, more preferably 1.03 or more, even more preferably 1.05 or more, even more preferably 1.07 or more, particularly preferably 1.10 or more, most preferably 1.11 or more, preferably 3.00 or less, more preferably 2.90 or less, even more preferably 2.80 or less, even more preferably 2.70 or less, particularly preferably 2.60 or less, and most preferably 2.50 or less.

[0058] When L1 / L3 is above the aforementioned lower limit, even increasing the laser output can reduce the likelihood of polarizer breakage. Furthermore, when L1 / L3 is below the aforementioned upper limit, a larger magnetic field can be applied to the Faraday element 14.

[0059] Furthermore, the ratio of the length L2 of the second magnet 12 to the length L3 of the third magnet 13, L2 / L3, is preferably 0.05 or more, more preferably 0.10 or more, even more preferably 0.15 or more, particularly preferably 0.20 or more, preferably 0.95 or less, more preferably 0.85 or less, even more preferably 0.80 or less, and particularly preferably 0.75 or less.

[0060] When L2 / L3 is within the aforementioned range, a larger magnetic field can be applied to the Faraday element 14.

[0061] (Faraday rotator)

[0062] Figure 2 The Faraday rotator 10 shown is a device used in the magneto-optical device 20, which will be described later, such as an optical isolator and an optical circulator. The Faraday rotator 10 includes a magnetic circuit 1 and a Faraday element 14 disposed within a through-hole 2 of the magnetic circuit 1. The Faraday element 14 is made of a light-transmitting paramagnetic material.

[0063] Faraday rotator 10 has Figure 1 The magnetic circuit 1 of the first embodiment shown can therefore make it less likely for the polarizer to break when used in an optical isolator, even if the laser output is increased.

[0064] Furthermore, in the Faraday rotator 10, light can be incident from either the first magnet 11 side or the third magnet 13 side. When incident from the third magnet 13 side, by making the length L1 of the first magnet 11 shorter than the length L3 of the third magnet 13, even if the laser output is increased, the breakage of the polarizer can be minimized.

[0065] Furthermore, the cross-sectional shape of the Faraday element 14 does not necessarily have to be the same as the cross-sectional shape of the through hole 2 of the magnetic circuit 1, but it is preferable to be the same from the viewpoint of applying a uniform magnetic field.

[0066] The Faraday element 14 can be made of a paramagnetic material. Glass is preferred. A Faraday element 14 made of glass exhibits less variation in the Wilder constant and less reduction in the extinction ratio due to defects such as those found in single-crystal materials, and is less affected by stress from the binder, thus ensuring a stable Wilder constant and a high extinction ratio. Furthermore, when the Faraday element 14 is made of a glass material with low thermal conductivity, the aforementioned thermal lensing effect is more easily generated, thus more effectively achieving the effects of the present invention.

[0067] The glass material used in the Faraday element 14 preferably contains more than 20% Tb₂O₃ (calculated as a molar percentage of oxides), more preferably 25% or more, more preferably 29% or more, more preferably 30% or more, more preferably 31% or more, more preferably 35% or more, more preferably 40% or more, more preferably 45% or more, more preferably 48% or more, and particularly preferably 51% or more. By increasing the Tb₂O₃ content in this way, a good Faraday effect is easily obtained. Furthermore, since Tb exists in the glass material in a trivalent or tetravalent state, all of these values ​​are expressed in this specification as values ​​converted to Tb₂O₃.

[0068] In the glass material used for Faraday element 14, Tb 3+ The proportion of Tb relative to the total Tb is preferably 55% or more, more preferably 60% or more, further preferably 80% or more, and particularly preferably 90% or more. If Tb 3+ If the proportion of Tb relative to the total Tb is too small, the light transmittance in the wavelength range of 300nm to 1100nm is likely to decrease.

[0069] (Magneto-optical devices)

[0070] Figure 3 The magneto-optical device 20 shown is an optical isolator. The magneto-optical device 20 includes... Figure 2The diagram shows a Faraday rotator 10, a first optical component 25 disposed at one end of the optical axis direction X of the magnetic circuit 1, and a second optical component 26 disposed at the other end. In this embodiment, the first optical component 25 and the second optical component 26 are polarizers. The light transmission axis of the second optical component 26 is tilted at 45° relative to the light transmission axis of the first optical component 25.

[0071] Light incident on the magneto-optical device 20 passes through the first optical component 25, becoming linearly polarized light, and then enters the Faraday element 14. The incident light is rotated by 45° by the Faraday element 14 and passes through the second optical component 26. A portion of the light passing through the second optical component 26 becomes reflected light, passing through the second optical component 26 at a polarization plane angle of 45°. The reflected light passing through the second optical component 26 is then rotated by the Faraday element 14 by another 45°, becoming an orthogonal polarization plane at 90° relative to the light transmission axis of the first optical component 25. Therefore, the reflected light cannot penetrate the first optical component 25 and is blocked.

[0072] The magneto-optical device 20 of the present invention has Figure 1 The magnetic circuit 1 of the first embodiment is shown. In other words, the distance from the center of the Faraday element 14 to the first optical component 25 along the optical axis X is different from the distance from the center of the Faraday element 14 to the second optical component 26 along the optical axis X. Therefore, even if the laser output is increased, the breakage of the polarizer can be made less likely to occur.

[0073] Furthermore, in the magneto-optical device 20, when the sum of the lengths of the first magnet 11, the second magnet 12, and the third magnet 13 along the optical axis direction X is set to (L1+L2+L3), and the distance from the center of the Faraday element 14 to the second optical component 26 (the end face of the Faraday rotator 10 side of the second optical component 26) along the optical axis direction X is set to L4, the ratio of L4 / (L1+L2+L3) is preferably 0.2 or more, more preferably 0.25 or more, even more preferably 0.27 or more, particularly preferably 0.3 or more, preferably less than 0.5, more preferably 0.49 or less, even more preferably 0.48 or less, and particularly preferably 0.47 or less.

[0074] When L4 / (L1+L2+L3) is above the lower limit mentioned above, a larger magnetic field can be applied to the Faraday element 14. Furthermore, when L4 / (L1+L2+L3) is below the upper limit mentioned above, even if the laser output is increased, the polarizer is less likely to break.

[0075] In this embodiment, the position of the end face of the Faraday rotator 10 on the third magnet 13 side along the optical axis X is the same as the position of the end face of the second optical component 26 on the Faraday rotator 10 side along the optical axis X. Therefore, in this embodiment, L4 is the same distance along the optical axis X from the center of the Faraday element 14 to the end face of the Faraday rotator 10 on the third magnet 13 side.

[0076] Furthermore, in this embodiment, the Faraday element 14 is positioned at the center of the second magnet 12. In other words, the center of the Faraday element 14 coincides with the center of the second magnet 12 along the optical axis X. In this case, a larger magnetic field can be applied to the Faraday element 14.

[0077] in addition, Figure 3 The magneto-optical device 20 shown is an optical isolator, but it can also be an optical circulator. In this case, the first optical component 25 and the second optical component 26 can be a wavelength plate or a beam splitter. However, the magneto-optical device 20 is not limited to optical isolators and optical circulators.

[0078] [Second Implementation]

[0079] Figure 7 This is a schematic cross-sectional view showing the structure of the magnetic circuit, Faraday rotator, and magneto-optical device according to the second embodiment of the present invention.

[0080] like Figure 7 As shown, in magnetic circuit 31, the lengths L1 of the first magnet 11 and L3 of the third magnet 13 are shorter than the length L2 of the second magnet 12. Furthermore, the length L3 of the third magnet 13 is also shorter than the length L1 of the first magnet 11 in magnetic circuit 31. Therefore, in magnetic circuit 31, there exists a relationship of L3 < L1 < L2.

[0081] The Faraday rotator 40 includes the magnetic circuit 31 and a Faraday element 14 disposed within the through hole 2 of the magnetic circuit 31. Furthermore, the magneto-optical device 50 includes the Faraday rotator 40, a first optical component 25 disposed at one end of the magnetic circuit 31 along the optical axis X, and a second optical component 26 disposed at the other end. Other aspects are the same as in the first embodiment.

[0082] Alternatively, as in the second embodiment, the lengths L1 of the first magnet 11 and L3 of the third magnet 13 can be shorter than the length L2 of the second magnet 12. In this case, by making the lengths L1 of the first magnet 11 and L3 of the third magnet 13 different, even if the laser output is increased, the breakage of the polarizer can be made less likely.

[0083] However, in this invention, it is preferable that the lengths L1 of the first magnet 11, L2 of the second magnet 12, and L3 of the third magnet 13 satisfy the relationship L2 < L3 < L1. In this case, the magnetic properties can be further improved, and a larger magnetic field can be applied to the Faraday element 14.

[0084] Furthermore, in this embodiment, the Faraday element 14 is also positioned at the center of the second magnet 12. In other words, the center of the Faraday element 14 coincides with the center of the second magnet 12 along the optical axis X. In this case, a larger magnetic field can be applied to the Faraday element 14.

[0085] Example

[0086] The present invention will now be described in more detail based on specific embodiments. The present invention is not limited to any of the following embodiments and can be implemented with appropriate modifications without altering its spirit.

[0087] (Examples 1-5 and Comparative Example 1)

[0088] Table 1 shows Examples 1 to 5 and Comparative Example 1 of the present invention.

[0089] Table 1

[0090]

[0091] The magnetic circuits of Examples 1-5 and Comparative Example 1 have a structure in which the entire structure has a square cross-sectional shape of 40mm × 40mm and includes a through hole 2 with a square cross-sectional shape of 4mm × 4mm. Furthermore, the lengths L1, L2, and L3 of the first magnet 11, the second magnet 12, and the third magnet 13 along the optical axis direction X are shown in Table 1. Therefore, in Examples 1-4, L1, L2, and L3 are... Figure 1 The magnetic circuit shown is the same as 1. Furthermore, in embodiment 5, L1, L2, and L3 are the same as... Figure 7 The magnetic circuit 31 shown is the same. In addition, the lengths of each magnet along the optical axis X in Comparative Example 1 are L1 = L3 > L2.

[0092] All magnets used are Nd-Fe-B based magnets. The remanent magnetic flux density of the magnet is 1.25T, and the coercivity is 940kA / m.

[0093] Figure 8 The magnetic field strength of the magnetic circuits in Examples 1-2 and Comparative Example 1 is shown. The horizontal axis represents the distance in the optical axis direction X. Furthermore, regarding the distance in the optical axis direction X, the first optical component 25 side is set to 0 mm. When the lengths of L1 and L3 are the same as in Comparative Example 1, the location with the strongest magnetic field strength is the portion 15 mm from the center of the magnetic circuit.

[0094] By constructing the magnetic circuit 1 with L1, L2, and L3 having different lengths as in Examples 1 and 2, the position of maximum magnetic field strength can be shifted outward from the center. In Example 1, the position of maximum magnetic field strength is 19.7 mm, and in Example 2, it is 16 mm. Furthermore, the maximum magnetic field strength is approximately the same in Examples 1, 2, and Comparative Example 1.

[0095] Optical isolators were assembled using these magnetic circuits, and laser resistance tests were conducted.

[0096] The Faraday element uses a cylindrical Faraday rotation glass material element with a diameter of 3 mm, a length of 6 mm, and a Wilder constant of 0.21 min / Oe·cm.

[0097] The polarizer is a polarization beam splitter made of two prisms of BDA glass (manufactured by Nippon Electric Glass Co., Ltd.) joined together by optical contact through a polarization separation film. The laser used has a pulse width of 10 ns and a repetition frequency of 200 kHz.

[0098] In the laser resistance test, the laser output was gradually increased while monitoring the light output of the transmissive isolator. The output at which the transmittance decreased sharply was measured as the threshold for laser damage. The results are shown in Table 1 above.

[0099] As shown in Table 1, the results of the laser resistance tests showed that the laser damage threshold for the optical isolators in Examples 1-5 was 40-100 W. In contrast, the laser damage threshold for the optical isolator in Comparative Example 1 was 30 W. Furthermore, the sharp decrease in transmittance in all samples was caused by damage to the polarizer on the exit side.

[0100] Explanation of reference numerals in the attached figures

[0101] 1, 31... Magnetic circuit

[0102] 2... Through hole

[0103] 10, 40... Faraday rotator

[0104] 11... First Magnet

[0105] 12……Second Magnet

[0106] 13……The Third Magnet

[0107] 14...Faraday elements

[0108] 20, 50... Magneto-optical devices

[0109] 25……First optical component

[0110] 26……Second optical component.

Claims

1. A magneto-optical device, characterized by, include: A Faraday rotator includes: a magnetic circuit having first to third magnets respectively provided with through holes for light to pass through; and a Faraday element disposed in the through holes of the magnetic circuit and having a paramagnetic material that is transmissible to light. A first optical component, which is disposed at one end along the optical axis of the Faraday rotator; and A second optical component is disposed at the other end of the optical axis of the Faraday rotator. Light passing through the through-hole of the magnetic circuit, and then through the first optical component and the second optical component, The magnetic circuit is constructed by sequentially arranging the first to third magnets coaxially in the front-to-back direction. When the direction of light passing through the through hole in the magnetic circuit is taken as the optical axis direction. One of the first magnet and the third magnet is magnetized in a direction perpendicular to the optical axis, such that the through-hole side is the N pole. The first magnet and another of the third magnets are magnetized in a direction perpendicular to the optical axis, such that the through-hole side is the S pole. The second magnet is magnetized in a direction parallel to the optical axis, such that the through-hole side of the first magnet and the third magnet are both N-pole magnets. When light is incident from the side of the first optical component along the optical axis, the distance from the center of the Faraday element to the second optical component along the optical axis is shorter than the distance from the center of the Faraday element to the first optical component along the optical axis. The second optical component is disposed on the side of the third magnet in the optical axis direction. When the sum of the lengths of the first magnet, the second magnet, and the third magnet along the optical axis is set as (L1+L2+L3), and the distance from the center of the Faraday element to the second optical component along the optical axis is set as L4, L4 / (L1+L2+L3) is in the range of 0.2 or more and less than 0.

5.

2. The magneto-optical device as described in claim 1, characterized in that: The Faraday element is positioned at the center of the second magnet.

3. The magneto-optical device as described in claim 1 or 2, characterized in that: The paramagnetic material is glass.

4. The magneto-optical device as described in claim 1 or 2, characterized in that: The first optical component and the second optical component are polarizers.

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