Transparent ceramic for faraday rotator, manufacturing method thereof, and magneto-optical device

Abstract

To provide a transparent ceramic for a Faraday rotator having a high laser damage threshold, its manufacturing method, and a magneto-optical device.SOLUTION: A transparent ceramic for a Faraday rotator according to the present invention comprises a sintered body containing a composite oxide primarily composed of terbium, has a cylindrical or prismatic shape, both end faces of which are optically mirrored, and the total number of voids, heterogeneous phases, bubble clusters, and foreign matter present within the transparent ceramic and having a maximum side length of 10 μm or more is 0.08 or less per 1 mm3. This transparent ceramic for a Faraday rotator can be manufactured by using a raw material powder for sintering from which coarse particles or foreign matter with a maximum side length exceeding 10 μm have been removed. The magneto-optical device is an optical isolator equipped with the transparent ceramic as a Faraday rotator 110.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to transparent ceramics for a Faraday rotator having translucency in the visible and / or near-infrared region, a method for manufacturing the same, and a magneto-optical device. More specifically, the present invention relates to transparent ceramics for a Faraday rotator containing terbium suitable for constituting a magneto-optical device such as an optical isolator, a method for manufacturing the same, and a magneto-optical device.

Background Art

[0002] Since an optical isolator has a function of transmitting forward light and blocking backward light, it is used to prevent damage to a laser oscillation source and stabilize the output by installing it between a light-emitting light source and a workpiece. Inside this optical isolator, terbium-added glass or terbium gallium garnet crystal (TGG crystal) is mounted as a Faraday rotator (for example, Japanese Patent Application Laid-Open No. 2011-213552 (Patent Document 1)). The magnitude of the Faraday effect is quantified by the Verdet constant. The Verdet constant of the TGG crystal is 40 rad / (T·m) (0.13 min / (Oe·cm)), and that of the terbium-added glass is 0.098 min / (Oe·cm). Since the Verdet constant of the TGG crystal is relatively large, it is widely used as a standard Faraday rotator. In addition, there is a terbium aluminum garnet crystal (TAG crystal). Since the Verdet constant of the TAG crystal is about 1.3 times that of the TGG crystal, the length of the Faraday rotator can be shortened, so it is a crystal that can be used for fiber lasers and has good properties (for example, Japanese Patent Application Laid-Open No. 2002-293693 (Patent Document 2), Japanese Patent Application Laid-Open No. 2004-539464 (Patent Document 3)).

[0003] Other than the above crystals, in Japanese Patent Application Laid-Open No. 2010-285299 (Patent Document 4), oxides single crystals and transparent oxide ceramics of (Tb x R 1-x )2O3 (R is an element such as scandium, yttrium, lanthanoid series, 0.4 ≦ x ≦ 1.0) are disclosed as materials having a large Verdet constant. Among these, this (Tb x R1-x ) The Verdet constants of oxide single crystals of 1-x Y x 2O3 and transparent oxide ceramics are 1.4 to 2.5 times those of TGG crystals. As described above, they can be miniaturized more than before and are preferable materials.

[0004] In recent years, methods for fabricating TAG with transparent ceramics have been disclosed (for example, International Publication No. 2017 / 033618 (Patent Document 5), “High Verdet constant of Ti-doped terbium aluminum garnet (TAG) ceramics” (Non-Patent Document 1)). Also, a method for fabricating transparent ceramics of (Tb x Y 1-x )3Al5O 12 (0.2 ≤ x ≤ 0.8, or 0.5 ≤ x ≤ 1.0) (YTAG) has also been reported (for example, “Fabrication and properties of (Tb x Y 1-x )3Al5O 12 transparent ceramics by hot isostatic pressing” (Non-Patent Document 2), “Development of optical grade (Tb x Y 1-x )3Al5O 12 ceramics as Faraday rotator material” (Non-Patent Document 3)). TAG ceramics and YTAG ceramics are said to be materials suitable for high-power applications because they have a smaller thermal lens effect compared to TGG single crystals.

[0005] By the way, due to the recent increase in the output power of laser light sources, the problem of damage to Faraday rotors often occurs when laser light with a high power density passes through. "Optical properties and Faraday effect of ceramic terbium gallium garnet for a room temperature Faraday rotator" (Non-Patent Document 4) shows information on the laser damage threshold of TGG single crystals and TGG transparent ceramics by pulsed laser light with a wavelength of 1,064 nm. When the Faraday rotor is damaged, the transmittance, isolation, and beam quality deteriorate. In the worst case, the optical isolator malfunctions. Optical damage can be considered due to ionization by multi-photon absorption, electron avalanche breakdown, and absorption by impurities. In particular, in transparent ceramics, the presence of scattering sources such as grain boundaries and bubbles is pointed out to lower the laser damage threshold ("Microstructure and Optical Properties of Hot Isostatic Pressed Nd:YAG Ceramics" (Non-Patent Document 5)).

[0006] As methods for increasing the laser damage threshold, the above-mentioned Patent Document 5, International Publication No. 2022 / 054592 (Patent Document 6), and International Publication No. 2022 / 054596 (Patent Document 7) have been published. All of these methods attempt to optimize the production conditions of the sintered body, such as pre-sintering, re-sintering, and annealing, and minimize oxygen defects, bubbles, and heterogeneous phases contained inside the sintered body that cause absorption and scattering, thereby improving the damage resistance.

[0007] However, if there are coarse particles such as uncrushed particles and hard secondary agglomerated particles that were not removed by the raw material powder inside the sintered body, they will cause abnormal grain growth, pores (voids), heterogeneous phases, and the generation of bubble groups. If these exist, scattered light (Hv scattering) with birefringence and optical anisotropy will be generated due to internal stress, but it cannot be removed in subsequent processes such as sintering. Similarly, Hv scattering will occur when foreign substances from the outside world are mixed in. If such scattering sources exist, diffraction, scattering, and refraction of the incident beam will occur when the laser light is irradiated on the transparent ceramics, which will cause laser damage. Therefore, it is most important not to include coarse particles such as uncrushed particles and secondary agglomerated particles and foreign substances from the outside world in the raw material powder. There is no description regarding the conditions of the raw material powder in the above Patent Documents 5, 6, and 7, and there are no known documents that consider the scattering source and laser damage.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Patent Document 8

Non-Patent Documents

[0009]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

Non-Patent Document 5

Non-Patent Document 6

Summary of the Invention

Problems to be Solved by the Invention

[0010] As described above, with the increasing high power of recent laser light sources, the development of transparent ceramics with high laser damage thresholds has been demanded. However, although the inventors of the present application were able to reduce the amount of bubbles, grain boundaries, heterogeneous phases, and foreign substances inside the ceramics to a certain level by optimizing the pre-sintering temperature and the re-sintering temperature as in the above prior art documents, when a test was conducted on an optical isolator equipped with a transparent ceramic fabricated as in the prior art document as a Faraday rotator, a problem occurred in that the Faraday rotator was damaged when irradiated with a pulsed laser of a certain high power.

[0011] The present invention has been made in view of the above circumstances, and an object thereof is to provide transparent ceramics for a Faraday rotator having a high laser damage threshold, a manufacturing method thereof, and a magneto-optical device.

Means for Solving the Problems

[0012] To achieve the above object, in one aspect of the present invention, there is provided transparent ceramics for a Faraday rotator including a sintered body containing a complex oxide mainly composed of terbium, wherein the transparent ceramics have a cylindrical or prismatic shape, both end faces thereof are optical mirror surfaces, and the total number of voids, heterogeneous phases, bubble groups, and foreign substances having a maximum side length of 10 μm or more existing inside the transparent ceramics is 0.08 or less per 1 mm 3 or less.

[0013] The crystal structure of the transparent ceramics may be a garnet structure or a bixbyite structure.

[0014] The above composite oxide may be a garnet-type composite oxide containing at least terbium and aluminum. In this case, the sintered body contains SiO2 as a sintering aid in an amount of more than 0% by mass and 0.1% by mass or less in terms of Si.

[0015] The laser damage threshold of the above transparent ceramics at a wavelength of 1,064 nm and a pulse width of 5 ns is 10 J / cm 2 or more, which is preferable.

[0016] As another aspect, the present invention is a method for producing the above transparent ceramics for a Faraday rotator, wherein a sintered body containing the above composite oxide is obtained using a sintering raw material powder from which coarse particles or foreign substances having a maximum side length exceeding 10 μm have been removed.

[0017] As still another aspect, the present invention is a magneto-optical device configured using the above transparent ceramics for a Faraday rotator.

[0018] The magneto-optical device of the present invention may be an optical isolator that includes the above transparent ceramics as a Faraday rotator and includes polarization materials before and after on the optical axis of the Faraday rotator and is usable in a wavelength band of 0.9 μm or more and 1.1 μm or less.

Advantages of the Invention

[0019] Thus, according to the present invention, by using a sintering raw material powder from which coarse particles such as unsintered particles or aggregated secondary particles having a maximum side length exceeding 10 μm and foreign substances mixed from the outside have been removed, scattering sources such as voids, heterogeneous phases, bubble groups, and foreign substances remaining inside the transparent ceramics can be reduced to a certain amount, thereby providing a transparent ceramic for a Faraday rotator having a high laser damage threshold and suitable for forming a magneto-optical device such as an optical isolator.

Brief Description of the Drawings

[0020]

Figure 1

Embodiments for Carrying Out the Invention

[0021] [1. Transparent Ceramics for Faraday Rotator] First, an embodiment of the transparent ceramics for a Faraday rotator according to the present invention will be described. The transparent ceramics for a Faraday rotator of this embodiment include a sintered body containing a composite oxide mainly composed of terbium (Tb), and this transparent ceramics has a cylindrical or prismatic shape, and both end faces thereof are optical mirror surfaces. The total number of voids, heterophases, bubble groups, and foreign substances with a maximum side length of 10 μm or more remaining inside the transparent ceramics is 0.08 or less per 1 mm 3 or less.

[0022] This transparent ceramics preferably has a cubic crystal structure. Although it is not particularly limited as long as it is cubic, it is preferably a garnet structure or a perovskite structure.

[0023] In the case of garnet-type transparent ceramics, the composite oxide mainly composed of Tb is preferably a terbium aluminum composite oxide represented by the following formula (1). (Tb 1-x-y Re x Sc y )3(Al 1-z Sc z )5O 12 ···(1) (In the formula, Re is one or more elements selected from the group consisting of Y and rare earth elements having atomic numbers from 65 to 71, 0.05 ≦ x ≦ 0.45, 0 < y < 0.1, 0.5 < 1 - x - y < 0.95, 0.001 < z < 0.15, 0 < y + z < 0.2.)

[0024] In the garnet structure represented by the formula (1), the site mainly occupied by Tb, that is, the front parentheses of the formula (1) is called the A site, and the site mainly occupied by aluminum (Al) is called the B site.

[0025] At the A site of formula (1), Tb is an element having the largest Verdet constant among trivalent rare earth ions and has extremely small absorption in the 1,070 nm region (wavelength band of 0.9 μm or more and 1.1 μm or less) used in fiber lasers. Therefore, it is the most suitable element for use as a material for an optical isolator in this wavelength range. However, Tb(III) ions are easily oxidized to produce Tb(IV) ions. When Tb(IV) ions are generated in a metal oxide, they absorb light at a wide range of wavelengths from ultraviolet to near-infrared. Therefore, it is desirable to eliminate them as much as possible. In addition, since rare earth elements have specific f-f transition absorptions, it is preferable to select an element that has no absorption in the wavelength band used. Specifically, it is particularly preferable to be selected from Y, Tb, and Lu that have no absorption from visible to near-infrared. Further, when the transparent ceramics of the present invention are used as a Faraday rotator, it is more preferable to contain 60 mol% or more of Tb from the viewpoint of having a high Verdet constant.

[0026] At the B site of formula (1), Al is a material having the smallest ionic radius among trivalent ions that can stably exist in an oxide having a garnet structure and is an element that can minimize the lattice constant of a Tb-containing garnet-type composite oxide. If the lattice constant of the garnet structure can be reduced without changing the content of Tb, it is preferable because the Verdet constant per unit length can be increased.

[0027] In the case of garnet-type transparent ceramics, it may contain the complex oxide represented by the above formula (1) as the main component, and further Si that serves as a sintering aid may be added as a sub-component. However, if it is less than 100 mass ppm, the above effect cannot be sufficiently obtained, which is not preferable. Also, if the Si content exceeds 1000 mass ppm, there is a possibility of generating minute light absorption due to crystal defects caused by the excessively contained Si, which is not preferable. Therefore, it is preferable to add Si so that it is 100 mass ppm or more and 1000 mass ppm or less. Note that the Si source to be added is not limited to SiO2, and molecular Si such as tetraethoxysilane (TEOS) is also possible. At that time, it is preferable to adjust the addition amount so that it is 100 mass ppm or more and 1000 mass ppm or less in terms of Si conversion.

[0028] In the case of garnet-type transparent ceramics, Sc may be contained in the complex oxide for the purpose of stabilizing the crystal structure. Since Sc is an element that can be solid-solved in both the A site and the B site of garnet, the larger the addition amount, the more the precipitation of perovskite-type heterogeneous phases is suppressed, and the production of transparent ceramics becomes easier. Therefore, there is no problem in that transparent ceramics can be produced even without adding Sc, but in order to stabilize the production, it is better to make y + z > 0 in formula (1). Also, since Sc is expensive in terms of raw materials, it is not preferable from the viewpoint of production cost to dopantly add Sc excessively and unnecessarily. Therefore, it is preferable to make 0 < y + z < 0.2.

[0029] In the case of the pyrochlore-type transparent ceramics, the complex oxide containing Tb as the main component is preferably the complex oxide represented by the following formula (2). (Tb a Re 1-a )2O3···(2) (In the formula, Re is one or more elements selected from the group consisting of Y and rare earth elements having an atomic number of 65 to 71, and 0.5 ≦ a ≦ 0.9.)

[0030] In formula (2), if a is less than 0.5, a high Verdet constant cannot be obtained, so a value of 0.5 or more is preferred. Further, if a is greater than 0.9, the phase transition of terbium cannot be controlled and tetravalent terbium is likely to be generated. Therefore, x is preferably 0.9 or less, more preferably 0.8 or less.

[0031] In the case of the Biogabite type transparent ceramics, since the grain growth rate can be controlled, it is preferable to add a sintering aid as a sub-component. In the case of the Biogabite type transparent ceramics, examples of the sintering aid include oxides of Group 4 elements such as titanium, zirconium, and hafnium, and oxides of Group 2 elements such as magnesium and calcium. Among them, HfO2 and ZrO2 with a small refractive index difference from the oxide in formula (2) are preferable, and HfO2 is most preferable. The sintering aid is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, and particularly preferably 0.5% by mass or more in terms of metal conversion. Further, if the sintering aid is more than 2% by mass, the unsolvated sintering aid precipitates at the grain boundary and becomes a scattering source. Therefore, 2% by mass or less is preferable.

[0032] The transparent ceramics for the Faraday rotator of the present embodiment preferably have a laser damage threshold of 10 J / cm 2 or more at a wavelength of 1,064 nm and a pulse width of 5 ns, regardless of whether they are of the garnet type or the Biogabite type. Since the transparent ceramics of the present embodiment are assumed to be used as a Faraday rotator, it is preferable that they are not damaged by a pulsed laser (have laser damage resistance). The damage threshold is preferably as high as possible. In the case of a wavelength λ = 1,064 nm and a pulse width of 5 ns, 10 J / cm 2 or more is preferable, and 15 J / cm 2 or more is more preferable. The method for determining whether the laser damage threshold at a wavelength of 1,064 nm and a pulse width of 5 ns is 10 J / cm 2 or more is to apply an energy density of 10 J / cm 2Laser light with a wavelength of 1,064 nm and a pulse width of 5 ns was irradiated for 1 minute, and it was assumed that no laser damage occurred when the intensity of the emitted laser beam detected by a power monitor installed in the subsequent stage did not drop by 5% or more. The same measurement was performed at 10 points on the optical surface, and if a decrease in laser intensity was observed even once, it was determined that laser damage had occurred.

[0033] By the way, the laser-induced damage threshold (LIDT) depends on the wavelength, pulse width, and beam spot diameter of the irradiated laser light. Therefore, when a laser damage test at a wavelength of 1,064 nm, a pulse width of 5 ns, and an irradiation beam diameter of 100 μm (Gaussian distribution 1 / e 2 intensity) cannot be carried out, the LIDT scaling can be used as the laser damage threshold at a wavelength of 1064 nm and a pulse width of 5 ns. Here, according to "Wavelength Dependence of Laser-Induced Damage: Determining the Damage Initiation Mechanisms" (Non-Patent Document 6), as a general rule for scaling (converting) from the initial conditions of wavelength (λ1), pulse width (τ1), and irradiation beam diameter (φ1) to a new wavelength (λ2), pulse width (τ2), and irradiation beam diameter (φ2), Equation (S1) can be applied. LIDT(λ2,τ2,φ2)=LIDT(λ1,τ1,φ1)×(λ1 / λ2)×(τ2 / τ1) 1 / 2 ×(φ1 / φ2) 2 ···(S1)

[0034] Therefore, according to the following Equation (S2), the laser damage threshold at a wavelength of 1064 nm, a pulse width of 5 ns (irradiation beam diameter 100 μm) can be converted from the laser damage threshold (LIDT) measured under conditions of wavelength (λ1(nm)), pulse width (τ1(ns)), and irradiation beam diameter (φ1(μm)) different from those of a wavelength of 1,064 nm, a pulse width of 5 ns, and an irradiation beam diameter of 100 μm (Gaussian distribution 1 / e 2 intensity). LIDT(1064,5,100)=LIDT(λ1,τ1,φ1)×(λ1 / 1064)×(5 / τ1) 1 / 2 ×(φ1 / 100) 2···(S2)

[0035] The transparent ceramics for a Faraday rotor according to this embodiment have no more than 0.08 scattering sources (voids, heterophases, bubble groups, foreign substances with a maximum side length of 10 μm or more) per 1 mm 3 inside the transparent ceramics, so that the laser damage threshold can be increased. In particular, the laser damage threshold at a wavelength of 1,064 nm and a pulse width of 5 ns can be made 10 J / cm 2 or higher.

[0036] [2. Method for manufacturing transparent ceramics for a Faraday rotor] Next, an embodiment of a method for manufacturing transparent ceramics for a Faraday rotor according to the present invention will be described. The manufacturing method of this embodiment includes a step of producing a sintering raw material powder containing a complex oxide mainly composed of Tb such as a garnet-type complex oxide represented by the above-described formula (1) or a perovskite-type complex oxide represented by the formula (2), a step of molding using this sintering raw material powder, a step of degreasing the molded body, a step of pre-sintering the molded body, a step of subjecting the pre-sintered body to hot isostatic pressing (HIP) treatment, an optional step of re-sintering the HIP-treated body, a step of subjecting the HIP-treated body or the re-sintered body to oxidation annealing treatment, and a step of optically polishing the obtained transparent ceramic body. Hereinafter, the sintering raw material powder and each step will be described in detail.

[0037] (2-1. Sintering raw material powder) The method for producing the sintering raw material powder is not particularly limited, and a coprecipitation method, a pulverization method, a spray pyrolysis method, a sol-gel method, an alkoxide hydrolysis method, or any other synthesis method may be used. In some cases, the obtained ceramic raw material of rare earth complex oxide may be appropriately treated with a wet ball mill, a bead mill, a jet mill, a dry jet mill, a hammer mill, etc. to obtain a desired particle size. For example, it is preferable to use a solid-phase reaction method in which a plurality of types of oxide particles are mixed and fired to produce uniformity by thermal diffusion of ions, or a coprecipitation method in which hydroxides, carbonates, etc. are precipitated from an ion-containing solution in which oxide particles are dissolved and then fired to form oxides to produce uniformity, to obtain the sintering raw material powder.

[0038] In the case of a solid-phase reaction method in which multiple types of oxide particles are mixed and fired to produce uniformity by thermal diffusion of ions, as starting materials, metal powders of terbium, yttrium, lutetium, scandium, aluminum, zirconium, hafnium, or those obtained by dissolving the metal powders in an aqueous solution such as nitric acid, sulfuric acid, or uric acid, or oxide powders of the above elements can be preferably used. Further, the purity of the above raw materials is preferably 99.9% by mass or more, and particularly preferably 99.99% by mass or more. Weigh a predetermined amount of these starting materials so that they have a composition corresponding to formula (1) or formula (2), mix them, and then fire them to obtain a calcined raw material of the desired metal oxide. Crush this to obtain a raw material powder for sintering.

[0039] However, the calcination temperature at this time is preferably less than 1100 °C, and more preferably 1050 °C or less. When the calcination temperature is higher than 1100 °C, necking of the sintered raw material powder starts, and it becomes difficult to control the bubble discharge in the sintering process, which is not preferable. The lower limit of the calcination temperature is not particularly limited as long as high transparency can be achieved, and calcination may not be performed. The firing time may be 1 hour or more, and the heating rate at that time is preferably 100 °C / h or more and 500 °C / h or less. The firing atmosphere is not particularly limited, and an air atmosphere, an oxygen atmosphere, an oxygen-containing atmosphere, or an Ar atmosphere can be preferably used. Examples of the firing apparatus include a vertical muffle furnace, a horizontal tubular furnace, a rotary kiln, etc., and it is not particularly limited as long as the target temperature can be reached and oxygen flow is possible.

[0040] Further, the raw material powder for sintering preferably contains a sintering aid. For example, in the case of garnet-type transparent ceramics, together with the above starting materials, tetraethoxysilane (TEOS) or SiO2 powder is added as a sintering aid in an amount of 100 ppm by mass or more and 1,000 ppm by mass or less in terms of Si based on the total raw material powder (the composite oxide + sintering aid constituting the above formula (1)), mixed, and fired to obtain a fired raw material. Note that the purity of the sintering aid is preferably 99.9% by mass or more, and particularly preferably 99.99% by mass or more. The sintering aid may be added during the preparation of slurrying the raw material powder for sintering described later. Further, when no sintering aid is added, it is preferable to select the raw material powder for sintering (that is, the above composite oxide powder) having a nano-sized primary particle diameter and extremely high sintering activity. Such a selection may be made as appropriate.

[0041] Similarly, in the case of perovskite-type transparent ceramics, together with the above starting materials, HfO2, ZrO2, etc. are added as a sintering aid in an amount of 0.1% by mass or more and 2.0% by mass or less in terms of metal based on the total raw material powder (the composite oxide + sintering aid constituting the above formula (2)). Note that the purity of the sintering aid is preferably 99.9% by mass or more, and particularly preferably 99.99% by mass or more.

[0042] Next, the obtained fired raw material is pulverized to obtain a raw material powder for sintering. If molding is performed using a raw material powder containing coarse particles, it will cause the formation of large voids in the molded body, abnormal grain growth that causes intragranular bubbles, and further heterogeneous precipitation, and it will be very difficult to discharge them outside the system even if the sintering process is devised. The pulverization method can be selected from either dry or wet methods, but in any pulverization method, it is preferable to remove coarse particles such as aggregated secondary particles or uncrushed particles with a filter or the like.

[0043] For example, in the case of wet grinding, the fired raw material is slurried by various grinding (dispersion) methods such as ball mills, bead mills, homogenizers, jet mills, ultrasonic irradiation, etc. and ground (dispersed) to primary particles. The dispersion medium of this wet slurry is not particularly limited as long as it enables high transparency of the finally obtained ceramics. Examples include alcohols such as lower alcohols having 1 to 4 carbon atoms and pure water. Also, various organic additives may be added to this wet slurry for the purpose of improving quality stability and yield in the subsequent ceramic manufacturing process. In the present invention, these are not particularly limited either. That is, various dispersants, binders, lubricants, plasticizers, etc. can be suitably used. However, as these organic additives, it is preferable to select a high-purity type that does not contain unnecessary metal ions.

[0044] In the case of wet grinding, if a plasticizer is added, there is a risk that the particles will aggregate. Therefore, it is preferable to remove coarse particles such as secondary particles or unground particles that have aggregated with a nylon filter or the like and foreign substances mixed in from the outside before adding the plasticizer. In any grinding method, it is preferable to remove coarse particles having a maximum side length of 10 μm or more. If coarse particles having a size of 10 μm or more are present, there is a high risk that various raw material powders will not diffuse sufficiently in the subsequent sintering process or the like, resulting in the occurrence of heterogeneous phases, which is not preferable. Furthermore, coarse particles are likely to undergo abnormal grain growth with a high densification rate during sintering, and will densify before the bubbles are discharged out of the system, becoming internal bubbles in the grains. Once they become internal bubbles in the grains, it is difficult in principle to remove them in the sintering process. Therefore, it is preferable to remove coarse particles from the raw material powder in order not to cause abnormal grain growth. The aperture of the filter for removing the above-mentioned coarse particles and foreign substances is preferably 20 μm or less, more preferably 10 μm or less, and particularly preferably 7 μm or less. Also, suction filtration may be performed to increase the filtration speed. In the case of wet grinding, the raw material powder for sintering is obtained by removing the dispersion medium of the filtered slurry.

[0045] Although the production of the raw material powder for sintering by the solid-phase reaction method has been described in detail as above, the present invention is not limited to the solid-phase reaction method. By removing coarse particles and foreign substances having a maximum side length of 10 μm or more from the raw material powder for sintering with a filter or the like, a raw material powder for sintering used in the method for manufacturing the transparent ceramics for a Faraday rotor of the present invention can be obtained by other methods as well.

[0046] (2-2. Forming step) In the manufacturing method of the present embodiment, a normal pressing step can be preferably used. For example, the raw material powder for sintering may be slurried, granules may be made from the slurry using spray drying, and after filling the granules into a jig, pressing may be performed. As the pressing, a pressing step of filling a very general mold and applying pressure from a certain direction, a CIP (Cold Isostatic Pressing) step or a WIP (Warm Isostatic Pressing) step of hermetically storing in a deformable waterproof container and applying pressure with hydrostatic pressure can be preferably used. The applied pressure may be appropriately adjusted while checking the relative density of the obtained formed body, and is not particularly limited. For example, if it is managed within a pressure range of about 300 MPa or less that can be handled by a commercially available CIP device, the manufacturing cost may be suppressed.

[0047] Alternatively, a hot pressing step, a spark plasma sintering step, a microwave heating step, etc., which perform sintering all at once not only in the forming step but also at the time of forming, can be preferably used. Further, it is also possible to produce a formed body by a casting method instead of the pressing method. Forming methods such as pressure casting, centrifugal casting, and extrusion molding can also be adopted by optimizing the combination of the shape and size of the oxide powder as the starting material and various organic additives.

[0048] (2-3. Debinding step) In the manufacturing method of the present embodiment, a normal degreasing process can be preferably used. That is, it is possible to go through a temperature-raising degreasing process using a heating furnace. Also, the type of the atmosphere gas at this time is not particularly limited, and air, oxygen, hydrogen, etc. can be preferably used. The degreasing temperature is not particularly limited either, but a temperature of 270°C or higher and 1000°C or lower is preferable. It is difficult to completely remove the organic additive. On the other hand, at a temperature higher than 1000°C, densification proceeds before the sintering process, making it difficult to obtain a transparent sintered body with low scattering.

[0049] (2-4. Preliminary sintering process) In the manufacturing method of the present embodiment, as the sintered body before heat sintering, a preliminary sintered body with a relative density of preferably 93% or more and an average sintering particle size of preferably 5 μm or less is produced. At this time, it is necessary to adjust the conditions of temperature and holding time so that the sintering particle size falls within the desired range.

[0050] Here, a general sintering process can be preferably used. That is, a heat sintering process such as a resistance heating method or an induction heating method can be preferably used. The atmosphere at this time is not particularly limited, and various atmospheres such as air, inert gas, oxygen gas, hydrogen gas, and helium gas can be preferably used, but more preferably, sintering under reduced pressure (in vacuum) can be used. The degree of vacuum for preliminary sintering is preferably less than 1×10 -1 Pa, and more preferably less than 1×10 -2 Pa.

[0051] In the case of garnet-type transparent ceramics, the sintering temperature in the preliminary sintering process is preferably 1450 - 1650°C, particularly preferably 1500 - 1600°C. When the sintering temperature is within this range, it is preferable because densification is promoted while suppressing heterogeneous precipitation and grain growth. The sintering holding time in the preliminary sintering process of the garnet-type ceramics of this embodiment is about several hours, but it is preferable to densify the relative density of the preliminary sintered body to 93% or more. Note that when the relative density of the preliminary sintered body becomes higher than 99%, plastic deformation of the internal particles of the sintered body hardly occurs during subsequent hot isostatic pressing (HIP treatment), and it becomes difficult to remove the bubbles remaining in the sintered body. Therefore, the relative density of the preliminary sintered body is preferably 99% or less at most, more preferably 98% or less.

[0052] In the case of perovskite-type transparent ceramics, the sintering temperature in the preliminary sintering process is preferably 1300 - 1650°C, more preferably 1350°C - 1600°C. At 1300°C or lower, almost no sintering occurs, and at 1650°C or higher, oxygen defects in the material occur and become an absorption source, which is not preferable. The sintering holding time may be optimized to reach the target relative density and is not particularly limited, but when the sintering holding time is longer than 20 hours, oxygen defects are likely to occur, which is not preferable. At this time, it is preferable to densify the relative density of the preliminary sintered body to 94% or more.

[0053] For both garnet-type and perovskite-type, the average sintered grain size of the grains in the preliminary sintered body is preferably 5 μm or less, more preferably 3 μm or less, and even more preferably 2.5 μm or less. The average sintered grain size of the sintered grains can be adjusted in consideration of the raw material type, atmosphere, sintering temperature, and holding time. If the sintered grain size is larger than 5 μm, plastic deformation is less likely to occur during subsequent hot isostatic pressing (HIP treatment), and there is a possibility that it becomes difficult to remove the bubbles remaining in the preliminary sintered body.

[0054] (2-5. Hot Isostatic Pressing (HIP) Treatment Process) In the manufacturing method of the present embodiment, after the pre-sintering process, a hot isostatic pressing (HIP) treatment is performed. At this time, as the type of pressurized gas medium, an inert gas such as argon or nitrogen, or Ar-O2 can be preferably 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. If it exceeds 300 MPa, further transparency improvement cannot be obtained even if the pressure is increased, and the load on the device becomes excessive, which may damage the device. It is convenient and preferable that the applied pressure is 196 MPa or less, which can be processed by a commercially available HIP device. The heater material, heat insulating material, and processing container for HIP treatment are not particularly limited, but graphite, molybdenum (Mo), tungsten (W), or platinum (Pt) can be preferably used. As the processing container, yttrium oxide or gadolinium oxide can also be preferably used. Particularly when the processing temperature is 1,500 °C or lower, Pt can be used as the heater material, heat insulating material, and processing container, and the pressurized gas medium can be Ar-O2, which is preferable because it can prevent the occurrence of oxygen deficiency during HIP treatment. When the processing temperature exceeds 1,500 °C, graphite is preferable as the heater material and heat insulating material. In this case, any one of graphite, Mo, and W is selected as the processing container, and then either yttrium oxide or gadolinium oxide is selected as the double container inside. If an oxygen releasing material is filled in the container, it is preferable because the amount of oxygen deficiency generated during HIP treatment can be suppressed to the minimum possible extent.

[0055] (2-6. Re-sintering process) In the manufacturing method of this embodiment, after the HIP treatment, in the case of garnet-type transparent ceramics, recrystallization may be performed for the purpose of grain growth. The recrystallization temperature is preferably 1,650 °C or higher, more preferably 1,700 °C or higher. If it is less than 1,650 °C, grain growth does not occur, which is not preferable. The average grain size of the crystal grains due to recrystallization is preferably 10 μm or more, more preferably 15 μm or more, and particularly preferably 20 μm or more. The holding time of the recrystallization process is not particularly limited, but is preferably 5 hours or more, and more preferably 10 hours or more. The temperature and holding time of the recrystallization process may be appropriately adjusted by checking the average grain size. However, generally, if the sintering temperature is raised too much, unexpected abnormal grain growth will occur, making it difficult to obtain a homogeneous sintered body. Therefore, it is preferable to have a certain margin for the recrystallization temperature, and the size adjustment of the average grain size of the recrystallized body is preferably adjusted by extending the holding time. In addition, the processing temperature (predetermined holding temperature) at that time is set in the range of 1,000 to 1,780 °C, preferably 1,100 to 1,730 °C. If the processing temperature exceeds 1,780 °C, the risk of oxygen deficiency generation increases, which is not preferable. Also, if the processing temperature is less than 1,000 °C, almost no effect of improving the transparency of the sintered body can be obtained. Note that the holding time of the processing temperature is not particularly limited, but if it is held for too long, the risk of oxygen deficiency generation increases, which is not preferable. Typically, it is preferably set in the range of 1 to 3 hours.

[0056] (2-7. Oxidation annealing treatment step) In the manufacturing method of this embodiment, the sintered compact or HIP-treated body that has undergone the above series of processes is reduced particularly in the HIP treatment step or the like, resulting in some oxygen deficiencies and possibly presenting a gray to dark blue appearance. Therefore, oxidation annealing treatment (oxygen deficiency recovery treatment) is performed under an oxidizing atmosphere (oxygen-containing atmosphere) such as in the air. The annealing treatment temperature is 1,400 °C or higher, preferably 1,450 °C or higher. Also, it is preferably 1,500 °C or lower. The holding time in this case is not particularly limited, but it may be performed for a time sufficient for recovering the oxygen deficiency, preferably 10 hours or more, more preferably 20 hours or more. Also, slightly oxidized HIP treatment may be performed. By these treatments, even if the sintered compact is colored, the oxygen deficiency can be recovered, so the size and quantity of the scattering source (scattering contrast source) can be controlled within the specified range, and a transparent ceramic body with less absorption derived from oxygen defects can be obtained. Of course, the essential coloring (absorption) of the material due to the addition of colored elements such as dopants and impurities for imparting functions cannot be removed.

[0057] (2-8. Optical Polishing) In the manufacturing method of this embodiment, for the transparent ceramic body that has undergone the above series of manufacturing processes, both end faces on the axis optically utilized are optically polished. When the optical surface accuracy at this time is measured at a wavelength λ = 633 nm, it is preferably λ / 2 or less, particularly preferably λ / 8 or less. Also, the surface roughness Sa (arithmetic mean height) of the polished surface is preferably 1 nm or less, particularly preferably 0.7 nm or less. The surface roughness Sq (root mean square height) is preferably 1.5 nm or less, particularly preferably 0.89 nm or less. It is also possible to further reduce the optical loss by appropriately forming an antireflection film on the optically polished surface.

[0058] As described above, a transparent ceramic containing a sintered compact containing a composite oxide mainly composed of terbium such as the composite oxide represented by the above formula (1) or (2) can be obtained. In this transparent ceramic, the scattering source remaining inside is 1 mm 3Since it is 0.08 or less per unit area, the laser damage threshold can be increased, and in particular, the laser damage threshold at a wavelength of 1,064 nm and a pulse width of 5 ns can be made 10 J / cm 2 or higher, and thus it is suitable for use as a Faraday rotator.

[0059] [3. Magneto - optical device] Furthermore, an embodiment of the magneto - optical device according to this embodiment will be described. The magneto - optical device of this embodiment is configured using the above - mentioned transparent ceramics for a Faraday rotator. The above - mentioned transparent ceramics for a Faraday rotator can be used as a magneto - optical material. Specifically, after applying a magnetic field parallel to its optical axis to this transparent ceramics for a Faraday rotator, it is preferable to configure and use a magneto - optical device by setting a polarizer and an analyzer such that their optical axes are offset by 45 degrees from each other. The transparent ceramics of this embodiment is preferably used as a magneto - optical device, particularly as a Faraday rotator of an optical isolator having a wavelength of 0.9 to 1.1 μm.

[0060] FIG. 1 is a cross - sectional view schematically showing an example of an optical isolator which is a magneto - optical device including a Faraday rotator made of the transparent ceramics of this embodiment as an optical element. As shown in FIG. 1, the optical isolator 100 includes, inside its housing 102, a Faraday rotator 110 made of the above - mentioned transparent ceramics, a polarizer 120 made of a polarization material, and an analyzer 130. These are arranged in the order of the polarizer 120, the Faraday rotator 110, and the analyzer 130 along the optical axis 104 of the Faraday rotator. The polarization vibration planes of the polarizer 120 and the analyzer 130 are arranged such that the relative angle is 45°. Further, the optical isolator 100 includes a magnet 140 for applying a magnetic field to the Faraday rotator 110 on at least one side surface of the Faraday rotator 110 inside the housing 102.

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

Example

[0062] Hereinafter, the present invention will be described more specifically by way of examples, reference examples, and comparative examples, but the present invention is not limited to these examples.

[0063] [Example 1] The powder raw materials of Example 1-1 to Example 1-9 and Comparative Example 1-1 to Comparative Example 1-6 were prepared as follows. Yttrium oxide powder, lutetium oxide powder, terbium oxide powder, scandium oxide powder manufactured by Shin-Etsu Chemical Co., Ltd., hafnium oxychloride octahydrate (HfClO·8H2O) powder manufactured by Daiichi Rare Element Chemical Industry Co., Ltd., and aluminum oxide powder manufactured by Dainippon Chemical Co., Ltd. were obtained. Further, tetraethyl orthosilicate (TEOS) manufactured by Kishida Chemical Co., Ltd. and polyethylene glycol 200 liquid manufactured by Kanto Chemical Co., Ltd. were obtained. The purity of all the powder raw materials was 99.9 mass% or more, and the purity of the liquid raw materials was 99.999 mass% or more. Using the above starting materials, the mixing ratio was adjusted to obtain the composition formulas shown in Table 1, and sintering raw material powders having a total of 15 chemical compositions were prepared.

[0064]

Table 1

[0065] As an example, for the obtained nine types of raw powder for sintering, while taking care to prevent mutual mixing, they were placed in polyethylene pots, and each was subjected to dispersion and mixing treatment in a ball mill apparatus in ethanol. The treatment time was 24 hours. Polyethylene glycol 200 was added to the obtained slurry as a binder so as to be 1.0% by mass based on the raw powder for sintering. Thereafter, the slurry was filtered through a nylon filter with an opening of 20 μm to remove uncrushed particles and secondarily agglomerated particles. The obtained slurry was subjected to spray drying treatment to produce granular raw materials with an average particle size of 20 μm for all of them.

[0066] For the six types of raw powder for sintering in the comparative example, granular raw materials with an average particle size of 20 μm were produced in the same procedure as in the example except that filtration was not performed.

[0067] Subsequently, these granular raw materials were each subjected to uniaxial press molding and isostatic pressing treatment at a pressure of 198 MPa to obtain CIP compacts. The obtained compacts were degreased in a muffle furnace under the conditions of 1,000 °C for 2 hours.

[0068] For the degreased bodies obtained in Example 1-1 to Example 1-7 and Comparative Example 1-1 to Comparative Example 1-5, they were charged into a vacuum furnace, and pre-sintering treatment was performed at 1,600 °C for 2 hours under a reduced pressure of less than 1.0×10 -2 Pa to obtain a total of 11 pre-sintered compacts. At this time, the sintered relative density of the samples was all 93% or more. Each of the obtained pre-sintered compacts was charged into a HIP furnace made of a carbon heater, and hot isostatic pressing (HIP) treatment was performed under the conditions of 196 MPa, 1,600 °C, and 3 hours in Ar. Subsequently, the hot isostatically pressed compact was charged into a vacuum furnace again, and re-sintering treatment was performed at 1,700 °C for 20 hours under a reduced pressure of less than 1.0×10 -2 Pa to obtain a re-sintered compact. Finally, the re-sintered compact was subjected to oxidation annealing treatment at 1,450 °C for 30 hours in the atmosphere.

[0069] For the degreased bodies obtained in Example 1-8, Example 1-9 and Comparative Example 1-6, they were charged into a vacuum furnace, and 1.0×10 -2Three kinds of pre-sintered compacts were obtained by pre-sintering at a temperature of 1,500 °C for 2 hours under a reduced pressure of less than Pa. At this time, the sintered relative density of each sample was 94% or more. Each of the obtained pre-sintered compacts was charged into a HIP furnace made of a carbon heater and subjected to hot isostatic pressing (HIP) treatment under the conditions of 196 MPa, 1,600 °C, and 3 hours in Ar.

[0070] As Reference Example 1-1, a TGG single crystal manufactured by Northrop Grumman Corporation was prepared.

[0071] Each of the thus obtained transparent ceramics and TGG single crystal was cylindrically ground to a diameter of 4 mm and ground and polished to a length of 17 mm. Further, the optical both end faces of each sample were finally optically polished so that the optical surface accuracy was λ / 8 (when the measurement wavelength λ = 633 nm), the surface roughness Sa was 0.7 nm or less, and the surface roughness Sq was 0.89 nm or less.

[0072] For each of the samples obtained as described above, the observation of the scattering source and the laser damage test were performed as follows.

[0073] (Observation method of scattering source) Using a polarizing microscope manufactured by Zeiss with an objective lens magnification of 1.25 times, the cross-nicol image of the entire optically effective region was observed from the optical end face direction, and all voids, different phases, bubble groups, and foreign matters with different contrasts from the base material were photographed as scattering sources. Also, when the shape of the transparent ceramic in the optically effective region is a cylindrical shape with a radius of r mm and a length of l mm, the cylindrical region from the center to r × 0.95 mm is the range. As scattering sources, all those with a size of 10 μm or more in the longitudinal direction were counted. For the observation of the scattering source, when the scattering source was small, an objective lens with a magnification of 5 times or 10 times was used to measure its size. Then, the number of scattering sources was divided by the volume of the product shape, and the number of scattering sources per 1 mm 3 was calculated. The third decimal place was rounded off. The results are shown in Table 2.

[0074] (Laser damage test) The measurement of the laser damage threshold was performed using a Nd:YAG laser with a wavelength of 1,064 nm and a pulse width of 5 ns. The irradiation angle was perpendicular to the polished surface (the optical end face of the sample), and the irradiation size was a beam diameter of 100 μm (Gaussian distribution 1 / e 2 intensity), and irradiation was performed so that the energy density was 10 J / cm 2 . The laser beam emerging from the other optical surface was monitored with a power meter installed downstream of the sample. The laser was continuously applied for 1 minute, and if the laser intensity did not drop by 5% or more during that time, it was considered that no laser damage occurred. The same measurement was performed at 10 points on the optical surface, and if a decrease in laser intensity was observed even once, it was determined that laser damage had occurred. The results are shown in Table 2.

[0075]

Table 2

[0076] From the above results, in Examples 1-1 to 1-9, the number of scattering sources was 0.08 or less per 1 mm 3 , and no laser damage was observed. In contrast, in Comparative Examples 1-1 to 1-6, the number of scattering sources was 0.11 or more per 1 mm 3 , and laser damage was observed. That is, by removing coarse particles, foreign substances, etc. with a filter, the number of scattering sources became 0.08 or less per 1 mm 3 , and it was confirmed that the laser damage threshold was 10 J / cm 2 or more. Also, it was confirmed that Examples 1-1 to 1-9 all had a higher laser damage threshold than the TGG crystal of Reference Example 1-1.

[0077] [Example 2] As Example 2, the case where the composition of the raw material powder for sintering was fixed under the conditions of Example 1-2, Example 1-7, and Example 1-8, and the filter opening size was changed is shown. For each composition of the raw material powder for sintering, the filter opening was set to 20 μm (Example 2-1, Example 2-5, Example 2-9), 10 μm (Example 2-2, Example 2-6, Example 2-10), 7 μm (Example 2-3, Example 2-7, Example 2-11), 1 μm (Example 2-4, Example 2-8, Example 2-12), 75 μm (Comparative Example 2-1, Comparative Example 2-4, Comparative Example 2-7), 50 μm (Comparative Example 2-2, Comparative Example 2-5, Comparative Example 2-8), and 30 μm (Comparative Example 2-3, Comparative Example 2-6, Comparative Example 2-9), and each sample was prepared. For each of the obtained samples, the number of scattering sources and the number of laser damages were evaluated in the same manner as in Example 1. The results are summarized in Table 3.

[0078]

Table 3

[0079] From the above results, in Examples 2-1 to 2-12, the number of scattering sources was 0.06 or less per 1 mm 3 and no laser damage was observed. On the other hand, in Comparative Examples 2-1 to 2-9, the number of scattering sources was 0.09 or more per 1 mm 3 and laser damage was observed. That is, by performing filtration with a filter having an opening of 20 μm or less, the number of scattering sources becomes 0.7 or less per 1 mm 3 and it was confirmed that the laser damage threshold is 10 J / cm 2 or more.

[0080] [Example 3] As an example of a magneto-optical device, an example of constructing an optical isolator using a transparent ceramic in which no laser damage was observed (Example 1-6) as Example 3 and a transparent ceramic in which the number of laser damages was observed (Comparative Example 1-4) as Comparative Example 3 is shown. An optical isolator having the same configuration as in Patent Document 8 was fabricated using each transparent ceramic as a Faraday rotator.

[0081] (Durability Test of Optical Isolator) The durability test of the optical isolator was evaluated by transmitting pulsed laser light with a wavelength of 1,030 nm, a pulse width of 14 ps, an average power of 150 W, and a repetition frequency of 600 kHz through the optical isolator. The beam diameter was set to approximately parallel light with a diameter of 1.0 mmφ (Gaussian distribution 1 / e 2 intensity). The transmitted light was expanded by an expander, and the durability of the optical isolator was evaluated by observing the time dependence of the transmitted light intensity with a power meter.

[0082] For the optical isolator (Example 3) equipped with a transparent ceramic (Examples 1-6) in which no laser damage was observed, even after performing a durability test for 100 hours or more, the intensity of the transmitted light had a change rate of less than 2% with respect to the initial value. On the other hand, for the optical isolator (Comparative Example 3) equipped with a transparent ceramic (Comparative Examples 1-4) in which laser damage was observed, the intensity of the transmitted light decreased to a value of 50% or less with respect to the intensity of the incident light almost simultaneously with the start of the test, so the test was aborted. As described above, it was confirmed that when no laser damage was observed, in other words, when the laser damage threshold was 10 J / cm 2 or more, a highly durable optical isolator with a transmission rate that does not decrease even after continuous operation for 100 hours or more can be obtained.

[0083] Although the present invention has been described with the above embodiments, the present invention is not limited to these embodiments, and can be changed within the scope that those skilled in the art can conceive, such as other embodiments, additions, changes, deletions, etc., and as long as the effects of the present invention are achieved in any aspect, it is included in the scope of the present invention.

Explanation of Reference Numerals

[0084] 100 Optical isolator 102 Housing 104 Optical axis 110 Faraday rotator 120 Polarizer 130 Analyzer 140 Magnet

Claims

1. A transparent ceramic for a Faraday rotor including a sintered body containing a complex oxide containing terbium as a main component, wherein the transparent ceramic has a cylindrical or prismatic shape, both end faces thereof are optical mirror surfaces, and the total number of voids, heterogeneous phases, bubble groups, and foreign substances having a maximum side length of 10 μm or more present inside the transparent ceramic is 0.08 or less per 1 mm ３ or less for the transparent ceramic for a Faraday rotor.

2. The transparent ceramic for a Faraday rotator according to Claim 1, wherein the crystal structure of the transparent ceramic is a garnet structure or a perovskite structure.

3. The above complex oxide is a garnet-type complex oxide containing at least terbium and aluminum, and the above sintered body contains SiO ２ in an amount of more than 0% by mass and 0.1% by mass or less in terms of Si, which is the transparent ceramics for a Faraday rotator according to claim 1.

4. The laser damage threshold of the above-mentioned transparent ceramics at a wavelength of 1,064 nm and a pulse width of 5 ns is 10 J / cm ２ The transparent ceramics for a Faraday rotator according to claim 1 or 2, wherein the above is satisfied.

5. A method for manufacturing a transparent ceramic for a Faraday rotator according to Claim 1 or 2, comprising obtaining a sintered body containing the above complex oxide using a sintering raw material powder from which coarse particles or foreign substances having a maximum side length exceeding 10 μm have been removed.

6. A magneto-optical device configured using the transparent ceramic for a Faraday rotator according to Claim 1 or 2.

7. The magneto-optical device according to Claim 6, which includes the above transparent ceramic as a Faraday rotator and has polarizing materials provided before and after the Faraday rotator on the optical axis thereof, and is an optical isolator usable in a wavelength band of 0.9 μm or more and 1.1 μm or less.

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