Transparent ceramic for magneto-optical elements and magneto-optical device

Abstract

The present invention provides: a transparent ceramic for magneto-optical elements, the transparent ceramic having practical transparency applicable to high power, such as a high thermal conductivity as compared with conventional ones, even as a composite oxide that contains a rare earth element having a large ionic radius; and a magneto-optical device.　This transparent ceramic for magneto-optical elements contains a sintered body of a composite oxide represented by formula (1), and the average ionic radius of the rare earth element in the composite oxide satisfies formula (2). Formula (1): (Tb1-x-yR1xR2y)3Al5O12 (In the formula, R1 is Y or Lu, R2 is Gd or La, 0 < x ≤ 0.4, 0 < y ≤ 0.4, and 0.05 ≤ x + y ≤ 0.45.)　Formula (2): (average ionic radius) = (1-x-y)DTb + xDR1 + yDR2 < 104.2 (In the formula, DTb is the ionic radius in the 8-coordination site of Tb3+, DR1 is the ionic radius in the 8-coordination site of R13+, and DR2 is the ionic radius in the 8-coordination site of R23+.) This magneto-optical device is an optical isolator which comprises the above-described transparent ceramic as a Faraday rotator 110.

Description

Transparent ceramics for magneto-optical elements and magneto-optical devices

[0001] The present invention relates to transparent ceramics for magneto-optical elements and magneto-optical devices, and more particularly to transparent ceramics for magneto-optical elements suitable for constituting magneto-optical devices such as optical isolators, and magneto-optical devices using the same.

[0002] In recent years, as higher output has become possible, the use of laser processing machines using fiber lasers has become increasingly common. However, when external light enters the laser light source built into the laser processing machine, the resonance state becomes unstable, causing the oscillation state to be disrupted. In particular, when the oscillated light is reflected by the optical system along the way and returns to the light source, the oscillation state is significantly disrupted. To prevent this, an optical isolator is usually installed before the light source.

[0003] An optical isolator is a magneto-optical device consisting of a Faraday rotator, a polarizer placed on the light input side of the Faraday rotator, and an analyzer placed on the light output side of the Faraday rotator. The Faraday rotator is a magneto-optical element that applies a magnetic field parallel to the light's direction of propagation. The polarization of the light rotates in only one direction, whether it is traveling forward or backward through the Faraday rotator. Furthermore, the Faraday rotator is adjusted to a length that rotates the polarization of the light exactly 45 degrees. If the polarization planes of the polarizer and analyzer are offset by 45 degrees from the direction of rotation of the forward light, the polarization of the forward light will be transmitted because it coincides with the polarizer position and the analyzer position. On the other hand, the polarization of the backward light will be rotated by 45 degrees, in the opposite direction to the polarization plane offset of the polarizer, which is offset by 45 degrees from the analyzer position. As a result, the polarization plane of the returning light at the polarizer position is shifted by 45 degrees - (-45 degrees) = 90 degrees relative to the polarization plane of the polarizer, and the returning light cannot pass through the polarizer. In this way, the polarizer functions as an optical isolator, allowing forward light to pass through and exit, while blocking backward returning light.

[0004] As a material used for the Faraday rotator constituting the optical isolator, TGG (Tb ３ Ga ５ O １２ ) crystals or ceramics, TAG(Tb３ Al ５ O １２ ) ceramics are known (Patent Document 1, Patent Document 2). In addition, the inventors of the present application have previously substituted a part of the A site of TAG with Y or Lu (Y ｘ Tb １－ｘ ) ３ Al ５ O １２ (hereinafter referred to as "Y-TAG") or (Lu x Tb １－ｘ ) ３ Al ５ O １２ (hereinafter referred to as "Lu-TAG") has been developed (Patent Documents 3 and 4).

[0005] Japanese Patent Publication No. 2011-213552 Japanese Patent No. 6438588 Japanese Patent No. 6879264 Japanese Patent No. 6881390

[0006] TGG crystals are currently widely used in standard fiber laser devices, but due to their high absorption coefficient, their usable laser power is said to be limited to 80 W. On the other hand, TAG crystals have higher rotation angle performance than TGG crystals and can accommodate lasers with higher power outputs than TGG crystals, but their absorption coefficient is similar to that of TGG crystals, limiting their ability to achieve higher power outputs. Y-TAG ceramics have a lower absorption coefficient than TAG ceramics, allowing for higher power outputs, but their low thermal conductivity means they cannot be used in high-power lasers of 200 W or more. KTF single crystals have an extremely small absorption coefficient, making them ideal for high-power applications, but their low laser damage threshold means they may be damaged by short-pulse lasers.

[0007] Furthermore, the characteristics of Y-TAG and Lu-TAG are: (1) a smaller absorption coefficient than TAG or TGG due to a low Tb concentration; (2) a Verdet constant comparable to that of conventional TGG; and (3) a thermal conductivity higher than that of conventional TGG, making them suitable for use as high-power Faraday rotators. Until now, the rare earth elements substituted into TAG-based Faraday rotators have been limited to Y or atomic numbers 66 to 71. This is because Y or rare earth elements with atomic numbers 66 to 71 have an octacoordinate system and a trivalent ionic radius of 104 pm or less, allowing for stable formation of aluminum garnet. It was generally believed that rare earth elements with atomic numbers 57 to 64 have an octacoordinate system and a trivalent ionic radius greater than 104 pm, preventing the formation of aluminum garnet when substituted into the A site of TAG.

[0008] The present invention has been made in view of the above circumstances, and aims to provide a transparent ceramic for magneto-optical elements, and a magneto-optical device using the same, which, as a composite oxide containing a rare earth element with a large ionic radius, has high thermal conductivity and is in the best group of absorption coefficients compared to conventional Faraday rotators, and even when irradiated with laser light of 100 W or more, the rate of change in beam diameter due to thermal lensing is 10% or less, so that it has practical transparency that can be applied up to high powers, and is easy to scale up because it is a ceramic sintered body.

[0009] In order to achieve the above object, one aspect of the present invention is a transparent ceramic for a magneto-optical element, which comprises a sintered body of a complex oxide represented by the following formula (1), wherein (Tb １－ｘ－ｙ R1 ｘ R2 ｙ ) ３ Al ５ O １２ ... Formula (1) (In the formula, R1 is an element selected from the group consisting of Y and Lu, R2 is an element selected from the group consisting of Gd and La, and 0<x≦0.4, 0<y≦0.4, 0.05≦x+y≦0.45.) The average ionic radius of the rare earth element in the composite oxide satisfies the following formula (2): Average ionic radius=(1−x−y)D Ｔｂ +xD Ｒ１ +yD Ｒ２<104.2...Formula (2) (wherein, D Ｔｂ is Tb ３＋ Ionic radius at the 8-coordinated site, D Ｒ１ is R1 ３＋ Ionic radius at the 8-coordinated site, D Ｒ２ is R2 ３＋ is the ionic radius at the 8-coordinated site.)

[0010] The transparent ceramic for magneto-optical elements of the present invention may further contain 3 mass % or less of Sc as a sintering aid. The transparent ceramic for magneto-optical elements of the present invention may further contain 100 mass ppm or more and 1000 mass ppm or less of Si as a sintering aid. R1 may be Lu, and R2 may be Gd.

[0011] The transparent ceramic for magneto-optical elements of the present invention preferably has a thermal conductivity of 4.5 W / m K or more. The transparent ceramic for magneto-optical elements of the present invention preferably has an in-line transmittance at 1060 nm of 78% or more. The transparent ceramic for magneto-optical elements of the present invention preferably has a Verdet constant of 30 Rad / T m or more.

[0012] Another aspect of the present invention is a magneto-optical device that is constructed using the above-mentioned transparent ceramics for magneto-optical elements.

[0013] The magneto-optical device of the present invention may be an optical isolator that includes the above-mentioned transparent ceramic for magneto-optical elements as a Faraday rotator and has polarizing materials in front of and behind the optical axis of the Faraday rotator and can be used in the wavelength range of 0.9 μm or more and 1.1 μm or less.

[0014] According to the present invention, a composite oxide containing terbium, yttrium or lutetium as a rare earth element with a small ionic radius, and lanthanum or gadolinium as a rare earth element with a large ionic radius is provided, and the average ionic radius of the rare earth elements in this composite oxide is set to less than 104.2 pm. This makes it possible to provide a transparent ceramic for magneto-optical elements and a magneto-optical device using the same, which have high thermal conductivity compared to conventional Faraday rotators, and also have an absorption coefficient that is in the best group. Even when irradiated with laser light of 100 W or more, the rate of change in beam diameter due to thermal lensing is 10% or less, and therefore have practical transparency that allows application up to high powers, and which can be easily scaled up because it is a ceramic sintered body.

[0015] 1 is a cross-sectional view showing a configuration example of an optical isolator using the transparent ceramic for a magneto-optical element according to the present invention as a Faraday rotator.

[0016] [1. Transparent ceramics for magneto-optical devices] First, one embodiment of the transparent ceramics for magneto-optical devices according to the present invention will be described. The transparent ceramics for magneto-optical devices according to this embodiment includes a sintered body of a complex oxide represented by the following formula (1): (Tb １－ｘ－ｙ R1 ｘ R2 ｙ ) ３ Al ５ O １２ ...(1)

[0017] In formula (1), terbium (Tb) is a material with the largest Verdet constant among paramagnetic elements except for iron (Fe), and is completely transparent at a wavelength of 1064 nm, especially when contained in an oxide having a garnet structure, making it the most suitable element for use in optical isolators in this wavelength range.

[0018] R1 is Tb ３＋R1 is an element having an ionic radius smaller than that of Y or a group consisting of rare earth elements having atomic numbers 66 to 71. Although these elements can be produced as aluminum garnets by themselves, this embodiment provides an unprecedented aluminum garnet for Faraday rotators by combining them with a rare earth element having a large ionic radius, which will be described later. When the wavelength band to be used is from visible to near infrared, yttrium (Y) or lutetium (Lu), which are colorless in the corresponding wavelength region, is used as R1.

[0019] Among these, Lu has the smallest ionic radius among the lanthanoid elements, and when combined with aluminum to form a composite oxide, it forms a garnet phase more stably than a perovskite phase, making it an element that can be preferably used in this embodiment. Furthermore, compared to other rare earth elements, Lu does not have characteristic absorption (ff transition) in the visible to near-infrared region, making it an optimal element to add.

[0020] R2 is Tb ３＋ The element has an ionic radius larger than that of Tb and can be selected from the group consisting of rare earth elements with atomic numbers of 60 to 64. ３＋ Elements with a larger ionic radius than R2 are elements that would not normally be produced as aluminum garnet, but by combining them with the aforementioned elements with a smaller ionic radius, it is possible to provide a novel aluminum garnet for Faraday rotators. When the wavelength range used is from visible to near infrared, gadolinium (Gd) or lanthanum (La), which are colorless in the corresponding wavelength range, are used as R2.

[0021] Among them, Gd is ３＋ Its ionic radius is close to that of rare earth elements, making it advantageous for compounding with elements with smaller ionic radii to produce aluminum garnet. In addition, since it does not have characteristic absorption (ff transition) in the visible to near-infrared region, it is an ideal element to add to Faraday rotators.

[0022] In the B site of formula (1), aluminum (Al) is the material with the smallest ionic radius among trivalent ions that can stably exist in oxides having a garnet structure, and is the element that can minimize the lattice constant of Tb-containing paramagnetic garnet-type oxides. Reducing the lattice constant of the garnet structure without changing the Tb content is preferable because it increases the Verdet constant per unit length. Furthermore, since aluminum is a light metal, its diamagnetic properties are weaker than those of gallium, and it is expected to have the effect of relatively increasing the magnetic flux density generated inside the Faraday rotator, which is also preferable because it increases the Verdet constant per unit length. In fact, the Verdet constant of TAG ceramics is 1.25 to 1.5 times that of TGG. Therefore, even if the relative concentration of terbium is reduced by substituting some of the terbium ions with other rare earth elements, it is possible to keep the Verdet constant per unit length equivalent to or slightly lower than that of TGG, making it a preferred constituent element in this embodiment.

[0023] In formula (1), the ranges of x and y are 0<x≦0.4, 0<y≦0.4, and 0.05≦x+y≦0.45. If x+y is less than 0.05, the effect of partially substituting terbium with other rare earth elements is not obtained, and the conditions are essentially the same as those for producing TAG. This makes it difficult to stably produce high-quality ceramic sintered bodies with low scattering and low absorption, which is undesirable. Furthermore, if x+y exceeds 0.45, the Verdet constant at a wavelength of 1064 nm is less than 30 rad / (T·m), which is undesirable. Furthermore, if the relative concentration of terbium is excessively diluted, the total length required to rotate a 1064 nm laser beam by 45 degrees exceeds 25 mm, which is longer than that of a TGG single crystal, which is undesirable. More preferably, 0.1≦x+y≦0.4.

[0024] Here, the average ionic radius of the rare earth element in the composite oxide of formula (1) can be calculated by the following formula (2), and as shown in formula (2), the average ionic radius is less than 104.2 pm. Average ionic radius = (1 - x - y) D Ｔｂ +xD Ｒ１ +yD Ｒ２ <104.2...Formula (2) (wherein, DＴｂ is Tb ３＋ Ionic radius at the 8-coordinated site, D Ｒ１ is R1 ３＋ Ionic radius at the 8-coordinated site, D Ｒ２ is R2 ３＋ is the ionic radius at the 8-coordinated site.)

[0025] An average ionic radius exceeding 104.2 pm is inappropriate because it is impossible to form a garnet. The average ionic radius is preferably 104.0 pm or less, more preferably 103.0 pm or less, and even more preferably 102.0 pm or less. The lower limit of the average ionic radius is not particularly limited as long as aluminum garnet can be formed, but may be, for example, 97.0 pm. For the ionic radius of each element, please refer to literature information such as Shannon et al., Acta A 32 (1976) 751.

[0026] The transparent ceramic for magneto-optical elements of this embodiment contains a composite oxide represented by formula (1) as a main component. Here, "containing as a main component" means that the composite oxide represented by formula (1) is contained in an amount of 90 mass% or more. The content of the composite oxide represented by formula (1) is preferably 99 mass% or more, more preferably 99.9 mass% or more, even more preferably 99.99 mass% or more, and particularly preferably 99.999 mass% or more.

[0027] The transparent ceramic for magneto-optical devices of this embodiment may contain silicon (Si) as a sintering aid in an amount of 100 ppm by mass or more and 1000 ppm by mass or less. Adding Si as a sintering aid in this amount suppresses perovskite-type heterophase precipitation, thereby ensuring the transparency of the transparent ceramic for magneto-optical devices. Furthermore, Si added in this amount vitrifies during sintering at 1400°C or higher, resulting in a liquid-phase sintering effect and promoting the densification of the sintered composite oxide. However, adding Si in an amount exceeding 1000 ppm by mass results in a beam diameter change rate of 10% or more when a 100-W laser beam with a wavelength of 1064 nm is irradiated onto a transparent ceramic for magneto-optical devices having a length (optical path length) of 20 mm. Therefore, the amount of Si added must be 1000 ppm by mass or less.

[0028] The sintering aid Si can be, for example, SiO ２ or a Si-based polymer compound such as tetraethoxysilane (TEOS). In this case, the amount added is adjusted so that the Si content is 100 ppm by mass or more and 1000 ppm by mass or less.

[0029] Scandium (Sc) may also be added as a sintering aid in addition to or instead of Si. Adding Sc, like Si, suppresses perovskite-type heterophase precipitation and improves the transparency of transparent ceramics for magneto-optical devices. Furthermore, since Sc is an element that can form a solid solution in both the A site and the B site of a garnet-type structure, the greater the amount added, the easier it is to manufacture transparent ceramics. However, Sc is expensive, and adding too much increases material costs. Therefore, adding more than 3.5% by mass is not preferable, and adding 3.0% by mass or less is more preferable. When adding Sc, the lower limit is preferably set to, for example, 0.5% by mass or more in order to fully exert its effects.

[0030] Sc is, for example, Sc ２ O ３Scandium can be added as an oxide such as scandium fluoride, scandium nitrate, scandium isoproxide, or any other organic or inorganic scandium compound. The amount of each is adjusted to achieve the above-mentioned concentration in terms of Sc.

[0031] In this specification, the term "added amount" refers to the amount of sintering aid intentionally added. Therefore, an added amount of 0 ppm by mass indicates that no sintering aid is intentionally added, and excludes cases where the corresponding element is contained as an impurity in the raw material powder.

[0032] The transparent ceramic for magneto-optical devices of the present invention is composed of the above-mentioned main component and subcomponents, but may further contain other elements. Typical examples of other elements include rare earth elements such as cerium (Ce), or various impurities such as sodium (Na), calcium (Ca), magnesium (Mg), phosphorus (P), tungsten (Ta), and molybdenum (Mo). The content of the other elements is preferably 10 parts by mass or less, more preferably 0.1 parts by mass or less, and particularly preferably 0.001 parts by mass or less, based on 100 parts by mass of the total amount of Tb.

[0033] The transparent ceramic for magneto-optical elements of this embodiment has a colorless and transparent appearance, and its linear transmittance at a wavelength of 1064 nm with an optical path length of 20 mm is 78% or more. In this specification, "linear transmittance" refers to the linear transmittance when the transmission spectrum measured in a blank (space) state without placing a sample in the measurement optical path is taken as 100%. It is more preferable that the linear transmittance under the above conditions is 80% or more. The upper limit of the linear transmittance is not particularly limited, but it may be, for example, 86% or less.

[0034] The transparent ceramic for magneto-optical devices of this embodiment preferably has a Verdet constant at a wavelength of 1064 nm of 30 rad / (T m) or more, more preferably 36 rad / (T m) or more. A Verdet constant of 36 rad / (T m) or more is particularly preferable because it allows for easy replacement with TGG single crystals, which are existing materials, without requiring any changes to the component design. The upper limit of the Verdet constant under the above conditions is not particularly limited, but may be, for example, 60 rad / (T m) or less.

[0035] The transparent ceramic for magneto-optical elements of this embodiment preferably has a thermal conductivity of 4.5 W / m K or more, and more preferably 4.8 W / m K or more. A thermal conductivity of 4.5 W / m K or more is preferable because it can reduce the thermal lens effect. The upper limit of the thermal conductivity is not particularly limited, but may be, for example, 8.0 W / m K or less.

[0036] Furthermore, the transparent ceramic for magneto-optical elements of this embodiment exhibits a beam diameter change rate of 10% or less due to thermal lensing when laser light having a wavelength of 1064 nm is incident with an optical path length of 20 mm, a beam diameter of 1.6 mm, and an incident power of 200 W. If the beam diameter change rate due to thermal lensing at a certain incident power is 10% or less, the material can be installed in a system with that incident power, i.e., the thermal lensing characteristics are satisfactory. Since the transparent ceramic for magneto-optical elements of this embodiment can maintain a beam diameter change rate of 10% or less at a high power incident of 100 W, this material can essentially be used in a high-power laser system for 100 W. The lower limit of the beam diameter change rate under the above conditions is not particularly limited, but may be, for example, 0.1% or more.

[0037] The transparent ceramic for magneto-optical elements of this embodiment has an extinction ratio of 40 dB or more as a ceramic element alone. Within the composition range shown in formula (1), material defects such as distortion and point defects are dramatically reduced, so the extinction ratio of the material element alone is stably controlled to 40 dB or more. The upper limit of the extinction ratio is not particularly limited, but may be, for example, 50 dB or less.

[0038] [2. Manufacturing Method of Transparent Ceramics for Magneto-Optical Devices] The manufacturing method of the transparent ceramics for magneto-optical devices of this embodiment includes, for example, a step of press-molding a raw material powder, a step of degreasing the molded body, a step of sintering the molded body to obtain a densified sintered body having a relative density of at least 95% or more, a step of hot isostatic pressing (HIP) the sintered body, and a step of annealing the sintered body. The raw material powder and each step will be described in detail below.

[0039] [2-1. Raw Material Powder] As the starting material used in the present production method, metal powders of terbium, yttrium, lutetium, gadolinium, lanthanum, and aluminum, or aqueous solutions thereof in nitric acid, sulfuric acid, uric acid, or the like, or oxide powders of the above elements can be suitably used.

[0040] The preparation of raw material powders is broadly divided into two types: breakdown type and build-up type, but there are no particular limitations as long as it can be made transparent. The build-up type is a method of preparing raw material powder for molding by pulverizing various powders, which has the advantage of productivity but has issues with composition uniformity. On the other hand, the build-up type is a method of obtaining powder by nucleation and grain growth from a solution of various elements, which has the great advantage of composition uniformity but has problems with productivity and reproducibility. In the present embodiment, there are no particular limitations as long as it can be made highly transparent.

[0041] In the breakdown method, it is most preferable to weigh the various oxide powders and perform wet or dry pulverization. The purity of the various oxide powders is preferably 99.9% or higher, more preferably 99.99% or higher. Furthermore, the primary particle size of the various powders is preferably 0.05 μm or more and 100 μm or less. A particle size of less than 0.05 μm is not preferable because it is difficult to control the uniformity of the ceramic due to high particle cohesion, and rapid densification occurs during the sintering process, making it difficult to control the release of air bubbles. Furthermore, a particle size exceeding 100 μm is unsuitable because it cannot be pulverized to fine particles by wet or dry pulverization. The pulverization process can be either wet or dry, and any of ball milling, bead milling, jet milling, and homogenizer processing can be suitably used. The pulverization process is preferably performed until the median value (D50) of the particle size distribution of the primary particles is less than 1 μm.

[0042] In the build-up method, a powder is preferably synthesized from a solution containing various elements and fired at 1300°C or less. Examples of precursors of the various elements include chlorides, nitrates, carbonates, and sulfates, but are not particularly limited. Examples of the powder synthesis method include coprecipitation, complex polymerization, and homogeneous precipitation, and are not particularly limited as long as highly transparent ceramics can be produced. Regardless of the synthesis method, the primary particle diameter is preferably 0.05 μm or more, and the shape is not particularly limited. Depending on the properties of the obtained powder, it may be subjected to a wet or dry pulverization process after firing, and, as with the breakdown method, the pulverization method is not particularly limited.

[0043] In order to improve the stability of the production yield and the quality of the resulting product, organic additives such as dispersants, binders, plasticizers, lubricants, etc. may be added. In this case, the most stable method is to perform wet grinding and add the additives to the slurry. There are no particular restrictions on the amount of additives added, as long as the desired properties are obtained.

[0044] [2-2. Molding Process] In this manufacturing method, a typical press molding process can be suitably used. Specifically, a typical press process in which the material is filled into a mold and pressurized from a certain direction, or a cold isostatic pressing (CIP) process or warm isostatic pressing (WIP) process in which the material is sealed in a deformable, waterproof container and pressurized with hydrostatic pressure can be suitably used. The applied pressure can be adjusted appropriately while checking the relative density of the resulting molded body, and is not particularly limited. Alternatively, a hot press process, a spark plasma sintering process, or a microwave heating process, which not only performs the molding process but also sintering at once, can also be suitably used. Furthermore, molded bodies can also be produced by slip casting instead of press molding. Other molding methods, such as pressure casting, centrifugal casting, and extrusion molding, can also be employed by optimizing the shape and size of the starting oxide powder and the various organic additives.

[0045] [2-3. Debinding Step] In this manufacturing method, a normal debinding step can be suitably used. That is, a temperature-raising debinding step using a heating furnace can be performed. There are also no particular restrictions on the type of atmospheric gas used, and air, oxygen, hydrogen, etc. can be suitably used. The debinding temperature is also preferably 270°C or higher and 1000°C or lower. If the temperature is lower than 270°C, it is difficult to completely remove the organic additives. On the other hand, if the temperature is higher than 1000°C, densification will progress before the sintering step, making it difficult to obtain a transparent sintered body with low scattering.

[0046] [2-4. Sintering Process] In this manufacturing method, a general sintering process can be suitably used. That is, a heat sintering process such as a resistance heating method or an induction heating method can be suitably used. The atmosphere at this time is not particularly limited, and sintering can be performed in various atmospheres such as an inert gas, oxygen gas, hydrogen gas, or helium gas, or under reduced pressure (vacuum). However, sintering in a vacuum is most preferable, as it allows for a high degree of transparency.

[0047] The sintering temperature in the sintering step is preferably 1400 to 1780° C., particularly preferably 1450 to 1750° C. If the sintering temperature is in this range, densification is promoted while suppressing the precipitation of heterophases, which is preferable.

[0048] A sintering holding time of a few hours is sufficient in the sintering process, but the relative density of the sintered body must be densified to at least 93%. A density of less than 93% is unsuitable because it is not possible to obtain a transparent body in the subsequent HIP treatment process. The sintering holding time must be controlled so that the relative density of the sintered body is 93% or more.

[0049] The crystal grain size in the sintering process is preferably 1 μm or more and 40 μm or less, and more preferably 5 μm or more and 35 μm or less. A crystal grain size below 1 μm is unsuitable because the transparency deteriorates due to minute compositional differences between the crystal grains. Furthermore, a crystal grain size of 40 μm or more is not preferable because there is a risk of desulfurization occurring during the subsequent polishing process. It is preferable to set the sintering temperature and sintering holding time so that the crystal grain size falls within the applicable range.

[0050] [2-5. Hot Isostatic Pressing (HIP) Treatment Step] In this manufacturing method, after the sintering step, an additional hot isostatic pressing (HIP) treatment is performed. The type of pressurized gas medium used in the HIP treatment step is an inert gas such as argon or nitrogen, or Ar—O ２ can be suitably used. The pressure applied by the pressurized gas medium is preferably 50 to 300 MPa, more preferably 100 to 300 MPa. If the pressure is less than 50 MPa, the transparency improvement effect may not be obtained, and if the pressure exceeds 300 MPa, further improvement in transparency cannot be obtained even if the pressure is increased. For convenience, it is preferable that the applied pressure be 196 MPa or less, which can be processed using a commercially available HIP device.

[0051] The treatment temperature in the HIP treatment step is set in the range of 1000 to 1780°C, preferably 1100 to 1730°C. Treatment temperatures above 1780°C are undesirable because they increase the risk of oxygen vacancies. Treatment temperatures below 1000°C are undesirable because they do not provide much of an effect of improving the transparency of the sintered body. There are no particular restrictions on the holding time at the treatment temperature, but holding for too long a time is undesirable because it increases the risk of oxygen vacancies. Typically, the temperature is preferably set in the range of 1 to 3 hours.

[0052] The heater material, heat insulating material, and treatment vessel used in the HIP treatment are not particularly limited, but graphite, molybdenum (Mo), tungsten (W), and platinum (Pt) can be suitably used, and yttrium oxide and gadolinium oxide can also be suitably used for the treatment vessel. In particular, when the treatment temperature is 1500°C or less, platinum (Pt) can be used for the heater material, heat insulating material, and treatment vessel, and Ar-O can be used as the pressurized gas medium. ２ This is preferable because it is possible to prevent the occurrence of oxygen deficiency during the HIP treatment.

[0053] When the processing temperature is 1500°C or higher, graphite is preferred as the heater material and heat insulating material. In this case, it is preferable to select either graphite, molybdenum (Mo), or tungsten (W) as the processing container, and then select either yttrium oxide or gadolinium oxide as a double container inside, and then fill the container with an oxygen release material, as this will minimize the amount of oxygen deficiency that occurs during HIP processing.

[0054] After the HIP treatment, a sintering step may be performed again to further reduce scattering, and then a HIP treatment step may be performed again. The number of times for the sintering step and the HIP treatment step is not particularly limited, and they may be repeated until low scattering is achieved.

[0055] [2-6. Annealing Treatment Step] In this manufacturing method, oxygen deficiency may occur in the obtained transparent sintered body after the HIP treatment step, resulting in a faint light gray appearance. In such cases, it is preferable to perform an annealing treatment (oxygen deficiency recovery treatment) in an oxygen atmosphere or an air atmosphere at a temperature equal to or lower than the treatment temperature in the HIP treatment step, typically 1000 to 1500°C. In this case, the holding time is not particularly limited, but it should be long enough to recover the oxygen deficiency, preferably 10 hours or more, and more preferably 20 hours or more.

[0056] By such annealing, even if the ceramic sintered body has a faintly light gray appearance due to the HIP treatment process, it is possible to obtain a transparent ceramic for magneto-optical elements that is completely colorless and transparent and has little defect absorption.

[0057] [2-7. Optical Polishing] In this manufacturing method, it is preferable to optically polish both end faces of the paramagnetic garnet-type transparent ceramic obtained through the above series of steps, which are located on the optically utilized axis. In this case, the optical surface precision is preferably λ / 2 or less, and particularly preferably λ / 8 or less, when the measurement wavelength λ is 633 nm. It is also possible to further reduce optical loss by appropriately forming an anti-reflection film on the optically polished surface.

[0058] In this manner, a transparent ceramic for magneto-optical devices containing a sintered body of the composite oxide represented by formula (1) can be produced. By producing the transparent ceramic for magneto-optical devices under these production conditions, a linear transmittance of 78% or more at a wavelength of 1064 nm over a length (optical path length) of 20 mm can be provided. Furthermore, a magneto-optical material can be provided that has a thermal conductivity of 4.5 W / m·K or more, a Verdet constant at a wavelength of 1064 nm of 30 rad / (T·m) or more, and a rate of change in beam diameter due to thermal lensing of 10% or less when irradiated with a 1064 nm wavelength laser beam having a beam diameter of 1.6 mm and an incident power of 100 W over an optical path length of 20 mm.

[0059] [3. Magneto-Optical Device] The paramagnetic garnet-type transparent ceramics described above are used as magneto-optical elements, and the magneto-optical device of this embodiment is constructed using the paramagnetic garnet-type transparent ceramics described above. Specifically, it is preferable to construct and use a magneto-optical device by applying a magnetic field parallel to the optical axis of the paramagnetic garnet-type transparent ceramics described above, and then setting a polarizer and an analyzer so that their optical axes are shifted by 45 degrees from each other. In particular, the transparent ceramics described above are suitable for use as Faraday rotators in optical isolators, particularly for wavelengths of 0.9 to 1.1 μm.

[0060] FIG. 1 is a cross-sectional view schematically illustrating an example of an optical isolator, which is a magneto-optical device. The optical isolator 100 includes a Faraday rotator 110 made of the paramagnetic garnet-type transparent ceramics described above, and a polarizer 120 and an analyzer 130 made of a polarizing material, both of which are arranged within a housing 102. These components are arranged in the order of polarizer 120, Faraday rotator 110, and analyzer 130 along the optical axis 104 of the Faraday rotator. The polarization plane of the polarizer 120 and the polarization plane of the analyzer 130 are arranged so that the relative angle is 45°. The optical isolator 100 also includes a magnet 140 attached to at least one side of the Faraday rotator 110 within the housing 102, for applying a magnetic field to the Faraday rotator 110.

[0061] Such an optical isolator 100 can be suitably used in an industrial fiber laser device (not shown). The optical isolator can prevent the laser light emitted from the laser light source from being reflected back to the light source, causing oscillation instability.

[0062] EXAMPLES The present invention will be explained in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0063] [Examples 1 to 5, Comparative Examples 1 to 4, Reference Examples 1 and 2] Terbium oxide powder (Tb ４ O ７ , manufactured by Shin-Etsu Chemical Co., Ltd.), lutetium oxide powder (Lu ２ O ３ , manufactured by Shin-Etsu Chemical Co., Ltd.), gadolinium oxide powder (Gd ２ O ３ , manufactured by Shin-Etsu Chemical Co., Ltd.), and aluminum oxide powder (manufactured by Taimei Chemical Co., Ltd.) were weighed out and mixed in predetermined amounts so as to obtain the composite oxide compositions of Examples 1 to 5, Comparative Examples 1 to 4, and Reference Examples 1 and 2 shown in Table 1. In addition, tetraethoxysilane (Si(OEt) ４ , manufactured by Kishida Chemical Co., Ltd., hereinafter referred to as TEOS) was added so that the content of the resulting transparent ceramic was 1000 ppm in terms of Si.

[0064] Then, this mixed raw material was subjected to a wet ball mill treatment using ethanol (manufactured by Kanto Chemical Co., Ltd.) as a dispersion medium. 2 mm alumina balls (manufactured by Nikkato Corporation) were used as the ball mill media for TEOS. 1 wt% of polyvinyl alcohol (manufactured by Kanto Chemical Co., Ltd.) was added as a binder to the slurry obtained by the ball mill treatment, and the mixture was granulated by spray drying. The obtained granules were subjected to uniaxial press molding and CIP treatment to obtain a predetermined shape, and then air degreasing was performed in a muffle furnace at 500°C. Subsequently, a vacuum sintering treatment (10 ―３ The transparent body was subjected to a pressure of 198 MPa and 1700°C, followed by HIP treatment (198 MPa and 1600°C), followed by atmospheric annealing at 1450°C for 10 hours. The resulting transparent body was polished and processed to a diameter of 5 mm, a length of 20 mm, and an optical surface precision of λ / 8 to obtain a ceramic sample. The linear transmittance, extinction ratio, thermal conductivity, beam diameter change rate, and Verdet constant of this ceramic sample were then measured.

[0065] (Measurement of Linear Transmittance) Linear transmittance was measured based on the light intensity when light with a wavelength of 1064 nm was transmitted through a beam diameter of 1 to 3 mmφ using an optical system manufactured in-house using a light source manufactured by NKT Photonics and a power meter and Ge photodetector manufactured by Gentec, and was calculated based on the following formula: Linear transmittance (%) = I / I ０ × 100 (wherein I is the transmitted light intensity (the intensity of light transmitted in a straight line through a sample with a length of 20 mm), I ０ indicates the incident light intensity.)

[0066] (Measurement of extinction ratio) The extinction ratio was measured by using an optical system manufactured in-house using a light source manufactured by NKT Photonics, a collimator lens, a polarizer, a work stage, an analyzer, a power meter manufactured by Gentec, and a Ge photodetector. The extinction ratio was measured by transmitting light with a wavelength of 1064 nm through a sample with a large beam diameter of 3 mm, and then aligning the polarization plane of the analyzer with the polarization plane of the polarizer. ０The extinction ratio (dB) was then measured, and the polarization plane of the analyzer was rotated by 90 degrees to be perpendicular to the polarization plane of the polarizer, and the received light intensity I' was measured again. The extinction ratio (dB) was then calculated based on the following formula: １０ (I' / I ０ ')

[0067] (Measurement of Thermal Conductivity) Thermal conductivity was measured in accordance with JIS R 1611-1997 (Test method for thermal diffusivity, specific heat, and thermal conductivity of fine ceramics by laser flash method). A disc-shaped transparent ceramic sintered body with a diameter of 10 mm and a thickness of 2 mm was prepared, and laser irradiation was performed on one side. The difference in temperature rise between the laser-irradiated surface and the opposite surface was measured, and the thermal diffusivity α was determined by the half-time method. The density ρ was measured by the Archimedes method, and the specific heat C was measured by differential scanning thermogravimetry. The thermal conductivity was determined as the product of the thermal diffusivity α, density ρ, and specific heat C.

[0068] (Measurement of Beam Diameter Change Rate) A 100 W output parallel beam was emitted from a high-power laser (beam diameter 1.6 mm) manufactured by IPG Photonics Japan, Inc., and the beam diameter Ra was measured using a beam profiler. A sample was then set on the line of the emitted beam, and the beam diameter Rb at the time the sample was set was remeasured using the beam profiler. The beam diameter change rate was calculated from the measurement results of Ra and Rb using the following formula: Beam diameter change rate (%) = |Ra - Rb| / R1 × 100

[0069] (Measurement of Verdet Constant) First, a ceramic sample was mounted on the optical isolator shown in Figure 1. Specifically, each ceramic sample was inserted into the center of a neodymium-iron-boron magnet with an outer diameter of 32 mm, an inner diameter of 6 mm, and a length of 40 mm. Polarizers were then inserted at both ends of the sample. A high-power laser beam with a wavelength of 1064 nm was then incident from both end faces using a high-power laser (beam diameter 1.6 mm) manufactured by IPG Photonics Japan Inc., to determine the Faraday rotation angle θ. The Faraday rotation angle θ was defined as the angle exhibiting the maximum transmittance when the polarizer on the output side was rotated. The Verdet constant was then calculated based on the following equation. The magnitude (H) of the magnetic field applied to the sample was calculated by simulation from the dimensions of the measurement system, the residual magnetic flux density (Br), and the coercivity (Hc). θ=V×H×L (where θ is the Faraday rotation angle (Rad), V is the Verdet constant (Rad / T·m), H is the magnitude of the magnetic field (T), and L is the length of the Faraday rotator (0.020 m in this case).)

[0070] The measurement results for Examples 1 to 5 and Comparative Examples 1 to 4 are shown in Table 1. The average ionic radius was calculated using the above-mentioned formula (2). These results are also shown in Table 1.

[0071]

[0072] As shown in Table 1, the ceramic samples of Examples 1 to 5, which had an average ionic radius of less than 104.2 pm, all achieved high transparency, with a beam diameter change rate of 10% or less. On the other hand, Comparative Examples 1 to 3, which had an average ionic radius of 104.2 pm or greater, melted during sintering and did not achieve transparency. This is presumably because aluminum garnet was unstable at high temperatures and became liquid at average ionic radii of 104.2 pm or greater. Comparative Example 4 achieved transparency, but its Verdet constant was small, requiring a longer length for use as a Faraday rotator, which is undesirable. Measurement results for LuTAG and TAG are also shown in Reference Example 1 and Reference Example 2, respectively. Examples 1 to 5 are comparable to these and are considered usable for high-power applications.

[0073] [Examples 6 to 10] Ceramic samples were prepared and tested in the same manner as in Example 3, except that Sc was further added as a sintering aid and the amount of Si added was changed as shown in Table 2. The results are shown in Table 2.

[0074]

[0075] As shown in Table 2, the addition of Sc and Si to any of the ceramic samples of Examples 6 to 10 does not cause any particular problems in the characteristics and can be used for high-power applications exceeding 100 W. The addition of Sc and Si broadens the range of composition for transparency, making it possible to stably provide highly transparent ceramics even if there are factors of variation, such as weighing errors or differences in the amount of moisture adsorbed by the raw materials.

[0076] [Examples 11 to 13, Comparative Examples 5 to 7] Ceramic samples were prepared in the same manner as in Example 1, except that Y was used instead of Lu and La was used instead of Gd, so as to obtain the composite oxide compositions of Examples 11 to 13 and Comparative Examples 5 to 7 shown in Table 3, and the respective tests were carried out. The results are shown in Table 3. The raw material was yttrium oxide powder (Y ２ O ３ , manufactured by Shin-Etsu Chemical Co., Ltd.), lanthanum oxide powder (La ２ O ３ , manufactured by Shin-Etsu Chemical Co., Ltd.) was used.

[0077]

[0078] As shown in Table 3, even when Y was used instead of Lu and La was used instead of Gd, the ceramic samples of Examples 11 to 13, which had an average ion radius of less than 104.2 pm, were highly transparent, demonstrating their potential for use in high-power applications. On the other hand, the samples of Comparative Examples 5 to 7, which had an average ion radius of 104.2 pm or more, melted during sintering.

[0079] Although the present invention has been described using the above-mentioned embodiments, the present invention is not limited to the above-mentioned embodiments, and can be modified within the scope of what a person skilled in the art can conceive, such as other embodiments, additions, changes, deletions, etc., and any aspect is included in the scope of the present invention as long as it achieves the effects of the present invention.

[0080] 100 Optical isolator 102 Housing 110 Faraday rotator 120 Polarizer 130 Analyzer 140 Magnet

Claims

1. A transparent ceramic for magneto-optical elements comprising a sintered body of a complex oxide represented by the following formula (1): (Tb １－ｘ－ｙ R1 ｘ R2 ｙ ) ３ A ５ O １２ ...Formula (1) (wherein R1 is an element selected from the group consisting of Y and Lu, R2 is an element selected from the group consisting of Gd and La, and 0<x≦0.4, 0<y≦0.4, 0.05≦x+y≦0.45.) A transparent ceramic for a magneto-optical element, in which the average ionic radius of the rare earth element in the composite oxide satisfies the following formula (2): Average ionic radius=(1-x-y)D Ｔｂ +xD Ｒ１ +yD Ｒ２ <104.2...Formula (2) (wherein, D Ｔｂ is Tb ３＋ Ionic radius at the 8-coordinate site of D Ｒ１ is R1 ３＋ Ionic radius at the 8-coordinate site of D Ｒ２ is R2 ３＋ is the ionic radius at the 8-coordinate site.) 2. The transparent ceramic material for magneto-optical elements according to claim 1, further comprising 3 mass % or less of Sc as a sintering aid.

3. The transparent ceramics for magneto-optical elements according to claim 1 or 2, further comprising Si as a sintering aid in an amount of 100 ppm by mass or more and 1000 ppm by mass or less.

4. The transparent ceramics for magneto-optical elements according to claim 1 or 2, wherein R1 is Lu and R2 is Gd.

5. The transparent ceramic material for magneto-optical devices according to claim 1 or 2, which has a thermal conductivity of 4.5 W / m·K or more.

6. The transparent ceramics for magneto-optical devices according to claim 1 or 2, which has an in-line transmittance at 1060 nm of 78% or more.

7. The transparent ceramic material for magneto-optical elements according to claim 1 or 2, which has a Verdet constant of 30 Rad / T·m or more.

8. A magneto-optical device constructed using the transparent ceramic for magneto-optical elements according to any one of claims 1 to 7.

9. The magneto-optical device according to claim 8, which is an optical isolator that comprises the above-mentioned transparent ceramics for magneto-optical elements as a Faraday rotator and has polarizing materials in front of and behind the optical axis of the Faraday rotator and can be used in the wavelength range of 0.9 μm or more and 1.1 μm or less.

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