Transparent ceramic for magneto-optical element and magneto-optical element

Abstract

This invention relates to a transparent ceramic for magneto-optical components, comprising: a paramagnetic garnet-type composite oxide containing terbium, lutetium, and aluminum as shown in formula (1), and 100 ppm to 1000 ppm by mass of Si as a sintering aid. (Tb 1‑x Lu x )3Al5O 12 …(1), where 0.05≤x≤0.45. The magneto-optical element of the present invention is an optical isolator, which includes a transparent ceramic magneto-optical element as a Faraday rotor (110) and a polarizer (120) and an analyzer (130) before and after the Faraday rotor on the optical axis (102), and can be used in the wavelength range of 0.9 to 1.1 μm.

Description

Technical Field

[0001] This invention relates to transparent ceramics for magneto-optical elements and magneto-optical elements, and more particularly to transparent ceramics for magneto-optical elements containing paramagnetic garnet-type composite oxides of terbium suitable for configuration such as optical isolators, and magneto-optical elements using the transparent ceramics for magneto-optical elements. Background Technology

[0002] In recent years, laser processing machines using fiber lasers have become very popular because they can significantly increase output power. However, in laser sources incorporated into laser processing machines, when external light is incident on them, the resonant state becomes unstable and the oscillation state is disturbed. In particular, the oscillation state is significantly disturbed when the oscillating light is reflected by the intermediate optical system and returns to the light source. To avoid this, optical isolators are typically placed, for example, in front of the light source.

[0003] The optical isolator includes a Faraday rotor, a polarizer disposed on the light incident side of the Faraday rotor, and an analyzer disposed on the light emitting side of the Faraday rotor. Furthermore, the Faraday rotor is used by applying a magnetic field parallel to the direction of light travel. In this case, regardless of whether the light moves forward or backward in the Faraday rotor, the polarized wave segment of the light rotates only in one direction. Furthermore, the Faraday rotor is adjusted to have a length such that the polarized wave segment of the light rotates exactly 45 degrees. Here, when the polarization planes of the polarizer and analyzer are offset (misaligned) by 45 degrees in the direction of rotation of the forward-traveling light, the polarized wave of the forward-traveling light is transmitted as the positions of the polarizer and analyzer are aligned. Furthermore, the polarization (wave) of the backward-traveling light rotates 45 degrees in the opposite direction of the offset angle of the polarization plane of the polarizer, which is offset by 45 degrees from the position of the analyzer. In this case, the polarization plane of the returning light at the polarizer position is offset by 45 degrees - (-45 degrees) = 90 degrees relative to the polarization plane of the polarizer, and the polarization of the backward-traveling light cannot be transmitted through the polarizer. In this way, the optical isolator functions to transmit and emit light that is traveling forward and to block light that is traveling backward.

[0004] TGG crystal (Tb3Ga5O) is used as a material for the Faraday rotor in optical isolators. 12 ) and TSAG crystal (Tb (3-x) Sc x )Sc2Al3O 12This is known in related technologies (Patent Documents 1 and 2). TGG crystals are currently widely used in standard fiber laser devices. However, due to the high absorption coefficient of TGG crystals, the maximum usable laser power can be said to be 80W. Furthermore, TSAG crystals have higher rotation angle performance and lower absorption coefficients than TGG crystals, thus enabling the handling of lasers with higher output power than TGG crystals. However, due to the large-scale use of expensive Sc2O3 and poor production stability, TSAG crystals have not yet become a widely used material.

[0005] In addition, as other materials for Faraday rotors, TAG ceramics (Patent Document 3), YTAG ceramics (Non-Patent Document 1), and KTF single crystals (Non-Patent Document 2) have been developed. TAG (Tb3Al5O4) 12 TAG ceramics have a higher Verdet constant than TGG single crystals and are considered suitable materials for high-power applications. YTAG ceramics, in which some terbium is replaced by yttrium, have a lower Verdet constant than TAG ceramics, but a lower absorption coefficient. Therefore, YTAG ceramics can achieve high power ranges not yet achieved with TAG ceramics. The final KTF (KTb3F)... 10 Single crystals have a much lower absorption coefficient than other materials and are considered capable of handling the highest power known to date in Faraday rotors. However, all three types of Faraday rotors suffer from poor manufacturing stability. Furthermore, KTF single crystals have a low laser damage threshold, making them susceptible to damage from short-pulse lasers.

[0006] We have developed C-type rare earth element (Tb,Y)₂O₃ (Patent Document 4) and garnet-type (Tb,Y,Sc)₃(Al,Sc)₅O₃ as novel magneto-optical materials. 12 (Patent Document 5). The former, a C-type rare-earth Faraday rotor, has a high Wilder constant and a high absorption coefficient. Therefore, its use in high-power applications is limited. Furthermore, the latter, a garnet-type rotor, has a Wilder constant that is 0.9 to 1.3 times that of TGG single crystals, and its absorption coefficient is equal to or less than that of TGG single crystals. Therefore, the garnet-type is considered optimal for high-power applications. Moreover, by adding a small amount of Sc, the garnet-type exhibits improved manufacturing stability compared to YTAG.

[0007] However, despite (Tb,Y,Sc)3(Al,Sc)5O 12 It has a low absorption coefficient, but also a low thermal conductivity. Therefore, (Tb,Y,Sc)3(Al,Sc)5O has been determined. 12It is not suitable for high-power applications exceeding 200W. When a Faraday rotor is irradiated with a strong laser, a temperature distribution appears within the rotor, resulting in a thermal lensing effect. The thermal lensing effect depends on the absorption coefficient and thermal conductivity. A lower absorption coefficient is better, and a higher thermal conductivity is better. Materials with low absorption and high thermal conductivity similar to YTAG are needed, but such materials have not yet been found.

[0008] Thermal conductivity is a specific value of a material and is affected by crystal structure, composition, defects, grain boundaries, etc. In transparent ceramics, grain boundaries are as thin as less than 1 nm, so their impact on thermal conductivity is small. Furthermore, in uncolored transparent ceramics, very few defects are considered to exist, and thermal conductivity at room temperature is practically equivalent to that of single crystals. Therefore, crystal structure or composition determines thermal conductivity.

[0009] Non-patent documents 3 and 4 illustrate examples of studies on the effect of composition on thermal conductivity. According to these non-patent documents, thermal conductivity decreases rapidly when yttrium aluminum garnet is doped with any other rare earth elements. As described above, a single composition is particularly preferred for improving thermal conductivity; however, using a mixture may result in a decrease in thermal conductivity.

[0010] Non-Patent Document 5 describes a formula relating to changes in thermal conductivity when using a mixture. According to this formula, the thermal conductivity of the mixture is affected by the individual thermal conductivity, composition ratio, and atomic weight difference between the constituent atoms. In the example presented so far, the atomic weight difference is approximately 70 to 80 due to the substitution of rare earth elements with yttrium. Therefore, a rapid decrease in thermal conductivity is conceivable.

[0011] Furthermore, if thermal conductivity is affected by atomic weight difference, it can be assumed that the smaller the atomic weight difference, the smaller the decrease in thermal conductivity. Non-Patent Literature 5 shows the thermal conductivity when ytterbium is added to lutetium aluminum garnet. Since the atomic weight difference between lutetium and ytterbium is as small as 2, it was found that the decrease in thermal conductivity was minimized. Therefore, it can be assumed that in order to increase (Tb,Y,Sc)3(Al,Sc)5O 12 The thermal conductivity of Y is such that changing Y to Lu is effective.

[0012] Non-patent document 6 shows (Tb 0.72 Lu 0.28 )3Al5O 12The Wilder constant and thermal conductivity of the single crystal. The Wilder constant is shown to be slightly lower than that of the TGG single crystal, and the thermal conductivity is shown to be 1.4 times that of the TGG single crystal. To increase the Wilder constant, the proportion of Tb needs to be increased. However, it is known that it is difficult to prepare compositions with different compositions than those described above in the case of single crystals (Non-Patent Document 7). Therefore, to date, no LuTAG-based Faraday rotor has been confirmed to have a Wilder constant and thermal conductivity equal to or greater than those of the TGG single crystal.

[0013] Patent document 6 shows (Tb,Lu,Sc)3(Al,Sc)5O 12 Sc has a larger atomic weight difference than Tb and Lu, which leads to a significant reduction in thermal conductivity. In Patent Document 6, the amount of Sc added is as high as at least about 0.10% by weight (based on metallic Sc), which may result in a decrease in thermal conductivity. Therefore, even with the composition of Patent Document 6, the maximum usable power is only 150W, and further increases in power are required. List of reference documents Patent documents

[0014] Patent Document 1: JP 2011-213552A Patent Document 2: JP 2002-293693A Patent Document 3: JP 6438588B Patent Document 4: JP 5397271B Patent Document 5: JP 6879264B Patent Document 6: JP 6881390B Non-patent literature

[0015] Non-patent literature 1: "Fabrication and Properties of (TbxY1-x)3Al5O12 Transparent Ceramics by Hot Isostatic Pressing", Optical Materials, 72, 58-62 (2017) Non-patent literature 2: “Promising Materials for High Power Laser Isolator”, LaserTech Journal, Vol.13, Issue 3, 18-21, (2016) Non-patent literature 3: “Effects of rare-earth doping on thermal conductivity in Y3Al5O12 crystals”, Optical Materials, Vol.31, Issue 5, 720-724 (2009) Non-patent literature 4: “Crystal growth and properties of (Lu,Y)3Al5O12”, Journal of Crystal Growth, Vol 260, Issue 2, 159-165 (2004) Non-patent literature 5: "Thermal and laser properties of Yb:LuAG for kW thin disklasers", Optical Express, Vol. 18, Issue 20, 20712-20722 (2010) Non-patent literature 6: “Magneto-optical property of terbium-lutetium-aluminumgarnet crystals”, Optical Materials, 66, 207-210 (2017). Summary of the Invention The problem to be solved by the present invention

[0016] The present invention was made in view of the above circumstances, and its object is to provide a transparent ceramic and a magneto-optical element containing a paramagnetic garnet-type composite oxide of terbium and lutetium that can be mounted in a high-power laser of 200W or more. Problem-solving methods

[0017] To achieve the above objectives, one aspect of the present invention provides a transparent ceramic for magnetic optical elements, comprising: a paramagnetic garnet-type composite oxide containing terbium, lutetium, and aluminum as shown in formula (1), and 100 ppm to 1000 ppm by mass of Si as a sintering aid. (Tb 1-x Lu x )3Al5O 12 ……(1) In the formula, 0.05≤x≤0.45 is satisfied.

[0018] The transparent ceramic used for the magneto-optical element may also contain less than 1000 ppm of Sc as a sintering aid.

[0019] In the transparent ceramic for the magneto-optical element, the thermal conductivity at room temperature is preferably 4.2 W / (m·K) or higher.

[0020] In the transparent ceramic used for the magneto-optical element, the extinction ratio is preferably 35 dB or higher.

[0021] In the transparent ceramic used for the magneto-optical element, the loss coefficient at 1064 nm is preferably 0.002 cm⁻¹. -1 the following.

[0022] In the transparent ceramic for the magnetic optical element, the crystal grain size of the ceramic is 1 μm or more and 40 μm or less.

[0023] In the transparent ceramic for the magneto-optical element, when a laser with a wavelength of 1064 nm and an output power of 200 W is applied, the extinction ratio is preferably 35 dB or higher.

[0024] In the transparent ceramic for the magneto-optical element, the Wilder constant is preferably 30 Rad / (T·m) or higher.

[0025] Furthermore, another aspect of the present invention provides a magneto-optical element configured using transparent ceramic.

[0026] The magneto-optical element can be an optical isolator, which includes a transparent ceramic for the magneto-optical element as a Faraday rotor and polarizing materials before and after the Faraday rotor on the optical axis, and can be used in the wavelength range of 0.9 μm to 1.1 μm. Invention Effects

[0027] Based on the present invention, a truly practical transparent ceramic for magneto-optical elements can be provided, which has a higher thermal conductivity than the prior art Faraday rotor and the highest absorption coefficient due to the inclusion of the paramagnetic garnet-type composite oxide and sintering aid described above. Therefore, it has an extinction ratio of more than 35 dB even under laser irradiation of more than 200 W. In other words, it is a practical transparent ceramic for magneto-optical elements suitable for high-power laser applications, as well as a magneto-optical element using the transparent ceramic for magneto-optical elements. Attached Figure Description

[0028] Figure 1 This is a schematic cross-sectional view showing an example configuration of an optical isolator using transparent ceramic as a Faraday rotor with the magneto-optical element of the present invention. Detailed Implementation

[0029] Transparent ceramics for magneto-optical components First, an embodiment of the transparent ceramic for magneto-optical elements of the present invention will be described. The transparent ceramic for magneto-optical elements comprises a paramagnetic garnet-type composite oxide as shown in formula (1), and 100 to 1000 ppm by mass of Si and 1000 ppm by mass of Sc as sintering aids. (Tb 1-x Lu x )3Al5O 12 ……(1) In the formula, 0.05≤x≤0.45 is satisfied.

[0030] In equation (1), terbium (Tb) is the material with the highest Wilder constant among paramagnetic elements other than iron (Fe). In particular, when terbium is contained in oxides with a garnet structure, terbium is completely transparent at a wavelength of 1064 nm, and is therefore the most suitable element for optical isolators in the aforementioned wavelength range.

[0031] Because lutetium (Lu) forms a more stable garnet phase than the perovskite phase when combined with aluminum to form a complex oxide, lutetium is a preferred element for use in this invention. Furthermore, compared to other rare earth elements, lutetium exhibits no characteristic absorption (ff transition) in the visible-near infrared region and has an atomic weight difference of only 16 with terbium. Therefore, lutetium is an optimal element for addition to produce Faraday rotors with high thermal conductivity.

[0032] At position B in formula (1), aluminum (Al) is the material with the smallest ionic radius among trivalent ions that can stably exist in oxides with a garnet structure, and is the element that can minimize the lattice constant of paramagnetic garnet-type oxides containing Tb. Preferably, the lattice constant of the garnet structure can be reduced without changing the Tb content, because the Wild constant per unit length can be increased. Furthermore, since aluminum is a light metal and its diamagnetic properties are weaker than those of gallium, it is expected that aluminum will have the effect of relatively increasing the magnetic flux density generated inside the Faraday rotor, which also makes it possible to increase the Wild constant per unit length. Therefore, aluminum is preferred. The Wild constant of actual TAG ceramics is increased to 1.25 to 1.5 times that of TGG. Therefore, even when the relative concentration of terbium is reduced by replacing some terbium ions with lutetium ions, the Wild constant per unit length can be maintained at the same level as, higher than, or slightly lower than that of TGG, so aluminum is a suitable constituent element in this invention.

[0033] In equation (1), the range of x is preferably 0.05 ≤ x ≤ 0.45, and more preferably 0.1 ≤ x ≤ 0.4. When x is less than 0.05, the effect of replacing part of the terbium with lutetium cannot be obtained, and the conditions for generating substantial TAG are not achieved. As a result, it is difficult to stably manufacture high-quality ceramic sintered bodies with low scattering and low absorption, which is not preferred. Furthermore, when x is greater than 0.45, the Wilder constant at a wavelength of 1064 nm becomes less than 30 rad / (T·m), which is also not preferred. In addition, when the relative concentration of terbium is too low, the total length required for the laser with a wavelength of 1064 nm to rotate 45 degrees increases to more than 25 mm, which is longer than the total length of the TGG single crystal, and is therefore not preferred.

[0034] The transparent ceramic for magneto-optical elements of the present invention comprises a composite oxide as shown in formula (1) as the main component. Furthermore, as an auxiliary component, Si, acting as a sintering aid, is added at a concentration of 100 to 1000 ppm by mass. When a predetermined amount of Si is added as a sintering aid, the precipitation of perovskite-type heterogeneous phases is suppressed, thus ensuring the transparency of the transparent ceramic for magneto-optical elements. Furthermore, the Si added at the predetermined amount undergoes vitrification during sintering at 1400°C or higher and produces a liquid-phase sintering effect, thereby promoting the densification of the garnet-type ceramic sintered body. However, when more than 1000 ppm by mass of Si is added, the extinction ratio is less than 35 dB when a 200 W laser beam with a wavelength of 1064 nm is applied to a 20 mm long (optical path length) transparent ceramic for magneto-optical elements; therefore, it is necessary to set the amount of Si to be added to 1000 ppm by mass or less.

[0035] In addition, Si can be added as a sintering aid, for example, as a Si-based inorganic compound such as SiO2 or a Si-based polymer compound such as tetraethoxysilane (TEOS). In this case, for metallic Si, it is preferable to adjust the amount to be added to between 100 ppm by mass and 1000 ppm by mass. Preferably, the lower limit of the amount of Si to be added is 200 ppm by mass or more, and more preferably 500 ppm or more. Preferably, the upper limit of the amount of Si to be added is 800 ppm by mass or less.

[0036] In addition to Si as a sintering aid, Sc can be added in amounts up to 1000 ppm by mass. With the addition of Sc, similar to Si, the precipitation of perovskite-type heterogeneous phases is suppressed, thus improving the transparency of the transparent ceramic for magneto-optical components. Furthermore, since Sc is an element capable of forming solid solutions at both the A and B sites of a garnet-type structure, the larger the amount added, the easier it is to manufacture the transparent ceramic. However, Sc has an atomic weight of 44.96, which is significantly different from the atomic weights of Tb and Lu, which are the main components. Therefore, a large addition leads to a deterioration in thermal conductivity. Therefore, adding Sc in amounts greater than 1000 ppm by mass is not preferable.

[0037] Sc can be added as, for example, an inorganic compound based on Sc such as Sc₂O₃. Preferably, the lower limit of the amount of Sc to be added is 100 ppm by mass or more. Preferably, the upper limit of the amount of Sc to be added is 800 ppm or less. Furthermore, it is preferred that the total amount of Si and Sc is 1000 ppm by mass or less.

[0038] In this specification, the terms "amount to be added" or "amount added" refer to the amount of sintering aid that is intentionally added. Therefore, a case where the amount to be added is 0 ppm by mass indicates that the sintering aid is not intended to be added, and excludes the possibility that the corresponding element is contained as an impurity in the raw material powder.

[0039] Furthermore, the phrase "containing as a main component" means that the transparent ceramic for magneto-optical elements contains 90% by mass or more of the composite oxide shown in formula (1). The content of the composite oxide shown in formula (1) is preferably 99% by mass or more, more preferably 99.9% by mass or more, and even more preferably 99.99% by mass or more.

[0040] The transparent ceramic for the magneto-optical element of the present invention comprises the main and auxiliary components described above, but may also contain other elements. Examples of other elements include rare earth elements such as yttrium (Y) and cerium (Ce), and typical examples of various impurities include sodium (Na), calcium (Ca), magnesium (Mg), phosphorus (P), tungsten (Ta), and molybdenum (Mo).

[0041] When the total amount of Tb is 100 parts by mass, the content of other elements is preferably 10 parts by mass or less, more preferably 0.1 parts by mass or less, and even more preferably 0.001 parts by mass or less.

[0042] The transparent ceramic used in the magneto-optical element of the present invention has a colorless and transparent appearance, and exhibits a wavelength of 0.002 cm⁻¹ at a wavelength of 1064 nm with an optical path length of 20 mm. -1 The following are the loss coefficients. There is no particular restriction on the lower limit of the loss amount, but it can be, for example, 0.0001 cm. -1The above. In this invention, the term "loss coefficient" is a coefficient representing the performance of transparent ceramics, and is expressed by the following formula. Loss coefficient [cm] -1 ]=10×log(I / I0) / (sample length[cm]) In the expression, I represents the intensity of transmitted light (the intensity of light that is linearly transmitted through a sample with a length of 20 mm), and I0 represents the intensity of incident light.

[0043] The thermal conductivity of the paramagnetic garnet-type ceramic of the present invention is 4.2 W / (m·K) or higher. Methods for measuring thermal conductivity are broadly classified into steady-state methods and unsteady-state methods. Examples of steady-state methods include the heat flow meter method, and examples of unsteady-state methods include the laser flash method, the cyclic heating method, and the thermal ray method. In the present invention, any measurement method can be used. Among these methods, the laser flash method is the most preferred method from the viewpoint that the sample size can be smaller than that of other measurement methods and that completely transparent ceramics can be easily manufactured. There is no particular upper limit to the thermal conductivity, but it can be, for example, 8.0 W / (m·K) or lower.

[0044] In the transparent ceramic for magneto-optical elements of the present invention, the Wilder constant at a wavelength of 1064 nm is preferably 30 rad / (T·m) or higher, and more preferably 36 rad / (T·m) or higher. A Wilder constant of 36 rad / (T·m) or higher is particularly preferred because it allows for easy replacement of existing materials with TGG single crystals without changes to the component design. There is no particular upper limit to the Wilder constant, but it can be, for example, 60 rad / (T·m) or lower.

[0045] The magneto-optical element of the present invention uses transparent ceramic as a single ceramic element and has an extinction ratio of 35 dB or more. Within the garnet composition range of the present invention, material defects such as strain and point defects are significantly reduced, thus the extinction ratio of a single material element is stably controlled to 35 dB or more. There is no particular upper limit to the extinction ratio, but it can be, for example, below 50 dB.

[0046] Furthermore, in the transparent ceramic for the magneto-optical element of the present invention, when a laser with a wavelength of 1064 nm is incident with an incident power of 200 W and an optical path length of 20 mm, the extinction ratio is 35 dB or higher. When a high-power laser is applied, heat generation (heating) corresponding to the absorption of the transparent ceramic is observed, and a temperature distribution corresponding to the thermal conductivity is caused by the heat generation, resulting in thermal birefringence. In this case, even if the extinction ratio is 35 dB or higher at low power (e.g., 1 W), the extinction ratio will decrease with high-power irradiation. If the extinction ratio is less than 35 dB at high power, there is a high possibility of damage to the laser source, which is therefore not preferred. There is no particular limit to the upper limit of the extinction ratio at high power, and it can be, for example, 50 dB or less.

[0047] The changes in the focal position and diameter of the emitted beam are affected by the same heat generation and thermal conductivity as during high-power irradiation with respect to the extinction ratio. These are known as thermal lensing effects, which occur due to the refractive index distribution resulting from the temperature distribution caused by heat generation. Regarding the change in the focal position of the emitted beam, the rate of change between the focal position when no sample is placed and the focal position when the beam passes through the sample is preferably less than 10%. Regarding the change in the diameter of the emitted beam, the rate of change between the initial beam diameter and the beam diameter when the beam passes through the sample is preferably less than 10%. In this invention, during high-power radiation testing, acceptance decisions can be made based on any one of the changes in extinction ratio, focal position, and beam diameter.

[0048] Manufacturing method of transparent ceramic for magneto-optical components Next, one embodiment of the manufacturing method of the transparent ceramic for magneto-optical elements of the present invention will be described. In this embodiment, the raw material powder is first pressed into a predetermined shape and then degreased. Then, it is sintered to produce a dense sintered body having a relative density of at least 95%. As a subsequent step, hot isostatic pressing (HIP) is preferably performed. When HIP is performed as is, the transparent ceramic for magneto-optical elements is reduced, resulting in slight oxygen defects. Therefore, a slight oxidation HIP treatment is preferably performed, or an annealing treatment in an oxidizing atmosphere is performed after HIP treatment to restore oxygen defects. Thus, a transparent ceramic for magneto-optical elements that is transparent, has no defect absorption, and contains paramagnetic garnet-type composite oxides can be obtained. The raw materials and processes will be described below.

[0049] 1. Raw materials The raw materials used in this embodiment may appropriately include: metal powders containing terbium, lutetium, scandium and aluminum; aqueous solutions of nitric acid, sulfuric acid, uric acid, etc.; or oxide powders of the elements described above; etc.

[0050] The preparation of oxide powders for transparent ceramics can be broadly categorized into two types: breakdown and build-up. There are no particular limitations as long as transparency can be achieved. The build-up method involves pulverizing various powders to prepare oxide powders for transparent ceramics. It has advantages in production efficiency but suffers from problems with compositional uniformity. Conversely, the build-up method obtains powders through nucleation and grain growth from solutions of various elements. It offers significant advantages in compositional uniformity but is less efficient and reproducible. In this invention, there are no particular limitations regarding the preparation of oxide powders for transparent ceramics, as long as high transparency can be achieved.

[0051] In the decomposition type, it is most preferable to weigh the various oxide powders and sintering aids and pulverize them by wet or dry methods. The purity of each of the various oxide powders is preferably 99.9% or higher, more preferably 99.99% or higher. Furthermore, the primary particle size of each of the various powders is preferably 0.05 to 100 μm. When the primary particle size is less than 0.05 μm, it is difficult to control the uniformity of the ceramic due to the high agglomeration of the grains, and densification occurs rapidly during the sintering process, making it difficult to control the removal of bubbles, therefore this is not preferred. Furthermore, when the diameter is greater than 100 μm, it is impossible to pulverize it into fine particles by wet or dry pulverization, therefore a diameter greater than 100 μm is unsuitable. Pulverization can be carried out by wet or dry methods, and any one of ball milling, bead milling, jet milling, and homogenization can be appropriately used. Pulverization is preferably carried out until the center value (D50) of the primary particle size distribution is less than 1 μm.

[0052] In the stacking type, a preferred method includes the following steps: synthesizing powder from a solution containing various elements (which may contain not only the main components but also sintering aids), and baking at a temperature below 1300°C. Examples of precursors for the various elements include chlorides, nitrates, carbonates, and sulfates, and there are no particular limitations on the precursors. Furthermore, examples of powder synthesis methods include co-precipitation, polymerization complexation, and homogeneous precipitation, and there are no particular limitations on the powder synthesis method, as long as it can produce highly transparent ceramics. In any synthesis method, the primary particle size is preferably 0.05 μm or larger, and there are no particular limitations on the shape of the primary grains. Depending on the properties of the obtained powder, it can be pulverized by wet or dry methods after baking, and as in the decomposition type, there are no particular limitations on the pulverization method.

[0053] In addition, organic additives, such as dispersants, binders, plasticizers, and lubricants, can be added to improve the stability of subsequent manufacturing yields and quality. In this case, the method of wet-milling the additives and adding them to the slurry is preferred because it is the most stable method. There are no particular limitations on the amount added, as long as the desired properties are achieved.

[0054] 2. Forming In the manufacturing method of this embodiment, conventional compression molding processes can be appropriately used. That is, very common compression molding processes, in which a mold is filled and pressure is applied in a certain direction, or cold isostatic pressing (CIP) or warm isostatic pressing (WIP), in which hydrostatic pressure is applied after airtight storage in a deformable waterproof container. The pressure to be applied can be appropriately adjusted while confirming the relative density of the molded body to be obtained, and there are no particular limitations. Alternatively, hot pressing, spark plasma sintering, microwave heating, etc., can also be appropriately used, in which not only the molding process is performed during molding but also the sintering treatment is performed. In addition, the molded body can be manufactured by slip casting instead of compression molding. By optimizing the combination of the shape and size of the oxide powders used as starting materials and various organic additives, molding methods such as pressure slip casting, centrifugal slip casting, and extrusion molding can also be used.

[0055] 3. Degreasing In the manufacturing method of this embodiment, a conventional degreasing process can be appropriately used. That is, the heating and degreasing process can be carried out in a heating furnace. Furthermore, there are no particular restrictions on the type of atmosphere gas, and air, oxygen, hydrogen, etc., can be appropriately used. The degreasing temperature is also preferably 270°C or higher and 1000°C or lower. At temperatures below 270°C, it is difficult to completely remove organic additives. In addition, at temperatures above 1000°C, densification occurs before the sintering process, making it difficult to obtain a transparent sintered body with low scattering.

[0056] 4. Sintering In the manufacturing method of this embodiment, conventional sintering processes can be appropriately used. That is, heating and sintering processes, such as resistance heating or induction heating, can be appropriately used. In this case, there are no particular limitations on the atmosphere, and various atmospheres such as inert gases, oxygen, hydrogen, and helium can be used, or sintering can be performed under reduced pressure (in a vacuum). However, sintering in a vacuum, where high transparency can be achieved, is most preferred.

[0057] In this embodiment, the sintering temperature in the sintering process is preferably from 1400°C to 1780°C, and particularly preferably from 1450°C to 1750°C. The sintering temperature is preferably within this range because it promotes densification while suppressing heterogeneous phase precipitation.

[0058] For the sintering process of this embodiment, a sintering holding time of several hours is sufficient, but the sintered body must be densified to a relative density of at least 93%. If the relative density is less than 93%, a transparent body cannot be obtained in the subsequent HIP process, and this is undesirable. The sintering holding time must be controlled to ensure that the relative density of the sintered body reaches 93% or more.

[0059] The grain size during the sintering process is preferably 1 to 40 μm, and more preferably 5 to 35 μm. When the grain size is less than 1 μm, the transparency deteriorates due to minute compositional variations between the grains, which is undesirable. Similarly, a grain size greater than 40 μm is also undesirable because there is a risk of desulfurization during the subsequent polishing process. It is preferable to set the sintering temperature and sintering holding time to ensure the grain size is within this range.

[0060] Furthermore, the average particle size (average sintered particle size) of the sintered particles in the target sintered body is obtained by measuring the particle size of the sintered particles using a metal microscope, and the specific description of the average particle size is as follows. That is, for the pre-sintered body, a reflective image of the sintered body surface is captured using a metal microscope with a 50x objective lens and a reflection mode. Specifically, an image of the entire optically effective area of ​​the target sintered body is captured, taking into account the effective image size of the objective lens, and the captured images are analyzed. In this case, firstly, a diagonal line is drawn on each captured image, the total number of sintered particles crossed by the diagonal line is counted, and then the value obtained by dividing the length of the diagonal line by the total number of counts is defined as the average particle size of the sintered particles in the image. The average particle sizes in the captured images read during the analysis process are summed together and then divided by the number of captured images. The resulting value is defined as the average sintered particle size of the target sintered body.

[0061] 5. Hot isostatic pressing (HIP) In the manufacturing method of this embodiment, a hot isostatic pressing (HIP) process can be provided additionally after the sintering process.

[0062] In this case, an inert gas, such as argon, nitrogen, or Ar-O2, can be appropriately used as the type of pressurized gas medium. The pressure applied by the pressurized gas medium is preferably 50 to 300 MPa, and more preferably 100 to 300 MPa. At pressures less than 50 MPa, the improvement in transparency may not be achieved. Furthermore, at pressures greater than 300 MPa, even with increased pressure, further improvement in transparency cannot be obtained. The applied pressure is preferably below 196 MPa, which can be easily handled using commercially available HIP (High-Intensity Permeable) devices.

[0063] Furthermore, the processing temperature (specified holding temperature) in this case is set between 1000°C and 1780°C, preferably between 1100°C and 1730°C. A heat treatment temperature higher than 1780°C is not preferred because it increases the risk of oxygen defects. Additionally, at heat treatment temperatures below 1000°C, it is difficult to obtain the improved transparency of the sintered body. There are no particular restrictions on the holding time at the heat treatment temperature, but holding for too long is not preferred because it increases the risk of oxygen defects. Typically, the holding time is preferably set between 1 and 3 hours.

[0064] There are no particular limitations on the heater materials, insulation materials, and processing containers used for HIP treatment. However, graphite, molybdenum (Mo), tungsten (W), and platinum (Pt) can be suitably used, and yttrium oxide and gadolinium oxide can also be suitably used as processing containers. In particular, the processing temperature is preferably below 1500°C because platinum (Pt) can be used as the heater material, insulation material, and processing container, and Ar-O2 can be used as the pressurized gas medium, which can prevent oxygen vacancies during HIP treatment.

[0065] When the processing temperature is above 1500°C, graphite is preferred as both the heater material and the insulation material. In this case, it is preferable to select any one of graphite, molybdenum (Mo), and tungsten (W) as the processing container, select yttrium oxide or gadolinium oxide as the double container within the processing container, and then fill the container with an oxygen-releasing material, because oxygen defects that occur during HIP processing can be suppressed as much as possible.

[0066] Furthermore, after the HIP treatment, a sintering process can be performed again to further reduce scattering, and then a further HIP treatment can be performed. There is no specific limit to the number of sintering and HIP treatments, and the process can be repeated until low scattering is achieved.

[0067] 6. Annealing treatment In the manufacturing method of this embodiment, oxygen defects may occur in the obtained transparent ceramic sintered body after HIP treatment, and the appearance may be slightly light gray. In this case, the annealing treatment (oxygen defect recovery treatment) is preferably carried out in an oxygen atmosphere or an air atmosphere at a temperature not higher than the HIP treatment temperature, typically 1000°C to 1500°C. In this case, there is no particular limitation on the holding time. However, the holding time can be long enough to recover from oxygen deficiency, and is preferably 10 hours or more, more preferably 20 hours or more.

[0068] By using oxygen annealing, even transparent ceramic sintered bodies that have a slightly light gray appearance obtained in the HIP process can be entirely made into transparent ceramic bodies for magneto-optical components. These ceramic bodies are transparent and free from defect absorption.

[0069] 7. Optical polishing In the manufacturing method of this embodiment, for the transparent ceramic for magneto-optical elements that has undergone the above-described series of manufacturing processes, it is preferable to optically polish the two end surfaces on the optical axis. In this case, at a measurement wavelength λ of 633 nm, the optical surface accuracy is preferably λ / 2 or less, and particularly preferably λ / 8 or less. Light loss (optical depletion) can also be further reduced by appropriately forming an anti-reflective film on the optically polished surface.

[0070] In this way, transparent ceramics for magneto-optical elements can be provided, comprising a paramagnetic garnet-type composite oxide containing terbium and lutetium as main components, and exhibiting a thermal conductivity of 4.2 W / mK or higher at room temperature. Furthermore, transparent ceramics for magneto-optical elements can be provided, exhibiting a Wilder constant of 30 rad / (T·m) or higher at a wavelength of 1064 nm, and an extinction ratio of 35 dB or higher when a laser beam with a wavelength of 1064 nm is incident with an incident power of 200 W, an optical path length of 20 mm, and a beam diameter of 1.6 mm.

[0071] Magnetic optical elements Furthermore, one embodiment of the magneto-optical element of the present invention will be described. The magneto-optical element of the present invention is configured using the transparent ceramic described above. The transparent ceramic described above can be used as the magneto-optical material. Specifically, it is preferable to configure and use the magneto-optical element by applying a magnetic field parallel to the optical axis of the transparent ceramic, and to provide a polarizer and an analyzer such that their optical axes are offset from each other by 45 degrees. In particular, the transparent ceramic of the present invention is suitable for use as a Faraday rotor of an optical isolator having a wavelength of 0.9 to 1.1 μm.

[0072] Figure 1 This is a schematic cross-sectional view illustrating an example of an optical isolator as a magneto-optical element, which includes a Faraday rotor formed of transparent ceramic using the magneto-optical element of the present invention as an optical element. Figure 1 As shown, the optical isolator 100 includes: a Faraday rotor 110 formed of transparent ceramic from the magneto-optical elements described above, housed in a housing 102; and a polarizer 120 and an analyzer 130 formed of polarizing material. These are arranged along the optical axis 104 of the Faraday rotor in the order of polarizer 120, Faraday rotor 110, and analyzer 130. The polarization vibration surfaces of the polarizer 120 and the analyzer 130 are arranged such that the relative angle is 45°. Furthermore, the optical isolator 100 includes a magnet 140 for applying a magnetic field to the Faraday rotor 110 around it in the housing 102.

[0073] Optical isolator 100 can be appropriately used in industrial fiber laser devices (not shown). The optical isolator prevents laser light emitted from the laser source from reflecting back to the light source and causing oscillation instability. Example

[0074] The present invention will now be described in detail with reference to embodiments. Examples 1 to 4 and Comparative Examples 1 to 4 Manufacturing of transparent ceramics Terbium oxide powder (Tb4O7, manufactured by Shin-Etsu Chemical Co., Ltd.), lutetium oxide powder (Lu2O3, manufactured by Shin-Etsu Chemical Co., Ltd.), alumina powder (Al2O3, manufactured by Daimei Chemical Co., Ltd., grade TM-DAR), tetraethoxysilane [Si(OC2H5)4, manufactured by Kishida Chemical Co., Ltd., hereinafter referred to as TEOS] as a sintering aid, and scandium oxide powder (Sc2O3, manufactured by Shin-Etsu Chemical Co., Ltd.) were weighed according to specified amounts, and wet ball milling was performed using ethanol (manufactured by Kanto Chemical Co., Ltd.) as the dispersion medium. 2mm alumina balls (manufactured by Nikkato Corporation) were used as the ball mill media. 1% by weight of polyvinyl alcohol (manufactured by Kanto Chemical Co., Ltd.) was added as a binder to the slurry obtained by ball milling, and granulation was performed by spray drying. The obtained particles were uniaxially pressed and CIP treated to obtain a specified shape, and then degreased in air in a muffle furnace at 500°C. Next, vacuum sintering was performed (10... -3The sample underwent a HIP treatment (198 MPa, 1600 °C) followed by atmospheric annealing at 1450 °C for 10 hours. The resulting transparent body was polished and processed to obtain a φ5 mm × 20 mm diameter with an optical surface finish of λ / 8. Comparative Examples 1 to 4 were made transparent in the same manner, but without adding the lutetium oxide powder of the examples or by replacing it with yttrium oxide powder (Y₂O₃, manufactured by Shin-Etsu Chemical Co., Ltd.).

[0075] Loss coefficient measurement An optical system was constructed using a light source manufactured by NKT Photonics, a power meter manufactured by Gentec, and a Ge (germanium) photodetector. When light... The loss coefficient is measured using the intensity of light with a wavelength of 1064 nm when the beam diameter is transmitted, and is obtained based on the following formula. Loss coefficient [cm] -1 ]=10×log(I / I0) / (sample length[cm]) In the expression, I represents the intensity of transmitted light (the intensity of light that is linearly transmitted through a sample with a length of 20 mm), and I0 represents the intensity of incident light.

[0076] Thermal conductivity measurement Thermal conductivity was measured according to JIS R 1611-1997 (Test method for measuring 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 one surface of it was irradiated with a laser. The temperature difference between the laser-irradiated surface and the opposite surface was measured, and the thermal diffusivity α was determined using the half-time method. The density ρ was measured by the Archimedes method, and the specific heat C was measured by differential scanning gravimetric analysis. Thermal conductivity was determined by the product of thermal diffusivity α, density ρ, and specific heat C.

[0077] Methods for measuring extinction ratio Regarding the extinction ratio, an optical system was constructed using a light source, collimating lens, polarizer, stage, analyzer, power meter manufactured by NKT Photonics, and a Ge photodetector manufactured by Gentec. Light with a wavelength of 1064 nm was transmitted through the sample with a beam diameter as high as 3 mmφ. Under these conditions, the intensity I0' of the light was measured when the polarization surfaces of the analyzer and polarizer coincided. The intensity I' of the received light was then measured again when the polarization surface of the analyzer was rotated 90 degrees to be perpendicular to the polarization surface of the polarizer. The extinction ratio was calculated using the following formula. Extinction ratio (dB) = -10 × log10 (I' / I0')

[0078] Evaluation of extinction ratio during high-power irradiation The extinction ratio during high-power irradiation was measured according to JIS C 5877-2:2012. Measurements were performed using a collimated CW laser with linearly polarized light having a wavelength of 1064 nm, an emission power of 200 W, and a diameter of 1.6 mm. The sample, PBS, and power meter were positioned on the optical axis of the laser. First, the PBS was set parallel to the laser's polarization (wavelength), and the transmitted light intensity P was read. / / Next, the PBS was set perpendicular to the polarization (wave), and the transmitted light intensity P was read. ⊥ The extinction ratio (dB) at an incident intensity of 200 W is calculated using the following formula. Extinction ratio (dB) = -10 × log 10 (P ⊥ / P / / )

[0079] Installation of isolators like Figure 1 As shown, each of the obtained ceramic samples 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, and polarizers were inserted at both ends of the magnet. Then, a high-power laser (beam diameter 1.6 mm) manufactured by IPG Photonics Japan, Ltd. was used to incident a high-power laser beam with a wavelength of 1064 nm from both end faces to determine the Faraday rotation angle θ. The Faraday rotation angle θ is the angle representing the maximum transmittance when the polarizer on the emission side is rotated. The Wilder constant was calculated based on the following formula. The magnitude (H) of the magnetic field applied to the sample was used, a value obtained through simulation based on the dimensions of the measurement system, the remanent magnetic flux density (Br), and the holding force (Hc). θ=V×H×L In the expression, θ is the Faraday rotation angle (Rad), V is the Wilder constant (Rad / T·m), H is the magnitude of the magnetic field (T), and L is the length of the Faraday rotor (0.020m in this case).

[0080] Table 1

[0081] Table 1 shows the performance evaluation results of various transparent ceramics. It can be seen that when Lu is substituted at the A site, the thermal conductivity is above 4.2 W / mK, and the loss coefficient is reduced. Therefore, the extinction ratio during 200 W irradiation is above 35 dB. Furthermore, when Y is substituted at the A site, the loss coefficient is approximately the same, but the thermal conductivity is significantly reduced. Therefore, the extinction ratio during 200 W irradiation is less than 35 dB. Based on these results, it has been found that Lu is the preferred substitution element at the A site rather than Y, and it can be used as a high-power Faraday rotor exceeding 200 W.

[0082] Examples 5 to 11 and Comparative Examples 5 to 8 Transparent ceramics were prepared in the same manner as in Example 2, but the amounts of TEOS and Sc2O3 to be added were varied. Table 2 shows the results. When the amount of Sc to be added was less than 1000 ppm by mass, the thermal conductivity did not show a rapid decrease, and the extinction ratio during 200 W irradiation was greater than 35 dB. However, when more than 1000 ppm by mass of Sc was added, although the stability of transparency was improved, the thermal conductivity decreased and the extinction ratio during 200 W irradiation was less than 35 dB. Therefore, it has been determined that the amount of Sc added is preferably less than 1000 ppm by mass. When the amount of Sc is reduced, it is difficult to achieve transparency, but transparency can be achieved by adjusting the amount of Si. It was found that when the amount of Si was also between 100 ppm by mass and 1000 ppm by mass, there was no particular effect on the extinction ratio during high-power irradiation, and when the amount of Si was 0 ppm by mass, transparency could not be achieved due to the generation of heterogeneous phases.

[0083] Table 2

[0084] Although the present invention has been described using the above embodiments, the present invention is not limited to the above embodiments. Various modifications can be made within the scope that those skilled in the art can conceive of, such as using other embodiments, adding, changing, and deleting, as long as the effects and functions of the present invention are demonstrated in any way, they all fall within the scope of the present invention. Brief Explanation of Figure Labels

[0085] 100: Optical isolator 102: Shell 104: Optical axis 110: Faraday rotor 120: Polarizer 130: Analyzer 140: Magnet

Claims

1. A transparent ceramic for a magneto-optical element, comprising: The following formula (1) shows a paramagnetic garnet-type composite oxide containing terbium, lutetium, and aluminum, and 100 ppm to 1000 ppm of Si as a sintering aid. (Tb 1-x Lu x 3Al5O 12 …… (1) In the formula, 0.05≤x≤0.

45.

2. The transparent ceramic for magneto-optical elements as claimed in claim 1, further comprising Sc at 1000 ppm by mass as a sintering aid.

3. The transparent ceramic for magneto-optical elements as described in claim 1 or 2, wherein the thermal conductivity at room temperature is 4.2 W / (m·K) or higher.

4. The transparent ceramic for magneto-optical elements as described in claim 1 or 2, wherein the extinction ratio is 35 dB or higher.

5. The transparent ceramic for magneto-optical elements as described in claim 1 or 2, wherein the loss coefficient at 1064 nm is 0.002 cm⁻¹. -1 the following.

6. The transparent ceramic for magneto-optical elements as described in claim 1 or 2, wherein the crystal grain size of the ceramic is 1. μ m to 40 μ m.

7. The transparent ceramic for magneto-optical elements as described in claim 1 or 2, wherein, The extinction ratio is above 35 dB when a laser with a wavelength of 1064 nm and an output power of 200 W is applied.

8. The transparent ceramic for magneto-optical elements as described in claim 1 or 2, wherein the Wilder constant is 30 Rad / (T·m) or higher.

9. A magneto-optical element configured with transparent ceramic using the magneto-optical element according to any one of claims 1 to 8.

10. The magneto-optical element of claim 9, wherein the optical isolator comprises a transparent ceramic element serving as a Faraday rotor and polarizing materials before and after the Faraday rotor on the optical axis, and the optical isolator is capable of operating at 0.9... μ m to 1.1 μ Used within the wavelength range of m.

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