Transparent ceramic and magneto-optical device using the same

Abstract

To provide a transparent ceramic having a parameter capable of measuring a good extinction ratio in place of observation with a polarizing microscope, and a magneto-optical device using the same.SOLUTION: Δ λ max / L ≤ 0.30 (nm / mm) Expression (1) (Δ λ max is a phase difference maximum value (nm) in the optically effective area, L is an optical path length (mm) of the transparent ceramic, and a phase difference indicates a phase shift between a polarization component along a fast axis of light and a polarization component along a slow axis of light) is satisfied in an optically effective area of 70% or more with respect to an outer shape of the polished surface. The magneto-optical device is constituted by using the transparent ceramic as a magneto-optical material.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to transparent ceramics and magneto-optical devices using the same. [Background technology]

[0002] Ceramics have long been widely used in everyday life, such as tiles and pottery. While most ceramics are opaque, in recent years it has become possible to produce transparent ceramics with transparency comparable to that of single crystals by thoroughly removing bubbles and minimizing the occurrence of heterogeneous phases at grain boundaries and other locations, thereby creating an optically isotropic crystal structure. Transparent ceramics have been applied to laser materials, scintillator materials, phosphors, magneto-optical elements, highly durable windows, and other applications, and it is expected that optical materials with new properties not possible with single crystals will be produced.

[0003] Transparent ceramics must have an optically isotropic crystal structure, and examples of transparent ceramics include C-type rare earth structures, garnet structures, pyrochlore structures, and spinel structures. Among these, transparent ceramics that are expected to be used as magneto-optical elements for fiber lasers are C-type rare earth structures (Patent Document 1), garnet structures (Patent Document 2, Non-Patent Document 1), and pyrochlore structures (Patent Document 3). These structures do not have significant absorption from the visible to near-infrared region and can contain large amounts of terbium, which has a large Verdet constant, and are therefore used as magneto-optical elements.

[0004] For these transparent ceramics to be used as magneto-optical materials, they must satisfy all of the following requirements: low optical absorption, low optical scattering, a high extinction ratio, a sufficient Verdet constant for use in magneto-optical elements, no heterophases, and the transmitted beam must maintain the shape of a Gaussian beam.

[0005] When external light enters the laser light source built into a laser processing machine, the resonance state becomes unstable, causing the oscillation state to become disrupted. In particular, when the emitted light is reflected by the optical system along the way and returns to the light source, the oscillation state is significantly disrupted. To prevent this, an optical isolator is usually installed before the light source, and the above-mentioned magneto-optical element is used in this optical isolator. The extinction ratio is a ratio that indicates the intensity of polarized light. If the extinction ratio value is poor, there will be a lot of returned light from the above-mentioned optical isolator, which will cause unstable laser oscillation. Therefore, when using transparent ceramics as magneto-optical elements, it is common to try to achieve as good an extinction ratio as possible. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 5704097 [Patent Document 2] Patent No. 6438588 [Patent Document 3] Patent No. 6135766 [Patent Document 4] Patent No. 5692127 [Patent Document 5] International Publication No. 2023 / 112508 [Non-patent literature]

[0007] [Non-Patent Document 1] Aung et al. “Development of optical grade (TbxY1-x)3Al5O12ceramics as Faraday rotator material”, Journal of the American Ceramic Society, 100(2017), 4081-4087 [Non-patent document 2] Segawa et al., "Evaluation of homogeneity of molten glass in air using the Schlieren method," New Glass, 28

[0110] , (2013), 30-34 Summary of the Invention [Problem to be solved by the invention]

[0008] It is known that refractive index differences due to foreign matter such as heterogeneous phases affect the extinction ratio. Patent Document 4 discloses a method for selecting an oxide ceramic in which the difference in refractive index between the grain boundaries of the oxide ceramic crystal and the main phase is 0.004 or less as a ceramic magneto-optical material with an extinction ratio of 30 dB or more. Patent Document 5 also clearly states that a garnet composition dramatically reduces material defects caused by differences in refractive index, such as strain and point defects, and the extinction ratio of the element itself can be stably controlled to 35 dB or more. Since these are observed visually or with a metallurgical microscope, it is easy to see that they affect the extinction ratio.

[0009] However, the inventors of the present invention experienced a phenomenon in which the extinction ratio deteriorated even though the refractive index difference was not observed. The difference could not be detected not only by visual inspection or microscopic observation, but also by measurements that visualize the refractive index, such as shadowgraph images and Schlieren images (Non-Patent Document 2). Therefore, they considered the possibility that the extinction ratio deteriorated due to factors other than the refractive index.

[0010] The extinction ratio also influences the photoelastic effect, a phenomenon in which an isotropic, homogeneous, transparent object loses its isotropy and becomes optically anisotropic, exhibiting birefringence, when subjected to external force. Birefringent materials change the polarization state of transmitted light, resulting in increased light leakage. This results in a worsening extinction ratio. This photoelastic effect can be observed with a polarizing microscope. By orthogonally ...

[0011] In view of the above circumstances, the present invention aims to provide a transparent ceramic having a parameter that can be measured to determine whether it has a good extinction ratio, instead of observation with a polarizing microscope, and a magneto-optical device using the same. [Means for solving the problem]

[0012] In order to achieve the above object, one aspect of the present invention is a transparent ceramic having a polished surface, wherein the following formula (1): Δλmax / L≦0.30(nm / mm) Formula (1) (Δλmax is the maximum value (nm) of retardation within the optically effective area, L is the optical path length (mm) of the transparent ceramic, and the retardation indicates the phase shift between the polarized component along the fast axis and the polarized component along the slow axis of the light.)

[0013] The wavelength of light used in measuring the phase difference may be 520 nm.

[0014] The transparent ceramic may have a cylindrical shape. The crystal structure of the transparent ceramic may be a C-type rare earth structure, a garnet structure, or a pyrochlore structure. The transparent ceramic preferably has a composition containing Tb.

[0015] The transparent ceramic according to the present invention preferably has a Verdet constant of 35 Rad / T·m or more, and an extinction ratio of 35 dB or more when irradiated with a laser having a wavelength of 1070 nm.

[0016] Another aspect of the present invention is a magneto-optical device, which is constructed using the transparent ceramics described above as a magneto-optical material. [Effects of the Invention]

[0017] The inventors measured the phase difference to quantify birefringence. The phase difference indicates the phase shift between the polarized component along the fast axis (fast component) of light and the polarized component along the slow axis (slow component), and is a parameter correlated with birefringence. By measuring the phase difference, it was possible to quantify birefringence, which cannot be observed using the polarizing microscope, and further confirmed its correlation with the extinction ratio. Therefore, by satisfying the above formula (1), it is possible to provide transparent ceramics with a good extinction ratio and magneto-optical devices using such ceramics. In particular, by suppressing internal stress, i.e., birefringence, it is possible to provide transparent ceramics with a high extinction ratio of 35 dB or more. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a cross-sectional view schematically showing an example of an optical isolator using the transparent ceramic according to the present invention as a Faraday rotator. [Figure 2] 10(a) is an image showing the phase difference distribution of Example 7, and FIG. 10(b) is an image showing the phase difference distribution of Comparative Example 8. FIG. [Figure 3] 10 is a graph showing phase difference histograms within the optically effective area in Example 7 and Comparative Example 8. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, one embodiment of the transparent ceramic according to the present invention will be described. The transparent ceramic of this embodiment satisfies the following formula (1) within an optically effective area of ​​70% or more of the outer shape of its polished surface. Δλmax / L≦0.30(nm / mm) Formula (1) Here, Δλmax is the maximum value of retardation (nm) within the optically effective area, L is the optical path length (mm) of the transparent ceramic, and the retardation indicates the phase shift between the polarized component along the fast axis and the polarized component along the slow axis of the light.

[0020] The crystal structure of the transparent ceramic of this embodiment is preferably a cubic system such as a garnet type, C-type rare earth type, pyrochlore type, or spinel type. Although it is possible to make transparent ceramics with tetragonal or hexagonal systems other than cubic systems by making the crystal grains finer, the transparency of tetragonal and hexagonal transparent ceramics is poor due to the refractive index difference at the crystal plane grain boundaries, making them undesirable. In the case of a cubic system, there is no difference in the refractive index in each direction, that is, the refractive index difference at the crystal plane grain boundaries is small, making it possible to produce highly transparent ceramics.

[0021] The transparent ceramic of this embodiment preferably contains a total of 99% or more of the main component elements, and the total of sintering aids and metal impurities other than the main components must be kept to less than 1%, more preferably less than 0.5%. In particular, metal impurities such as iron, chromium, and nickel act as absorbents and sometimes cause coloration, so it is most preferable to keep the amount as small as possible, i.e., less than 10 ppm in total. There are no particular restrictions on the amount of sintering aids, as long as they are kept to less than 20,000 ppm and can achieve high transparency.

[0022] When the crystal structure of the transparent ceramic is garnet type, A3B5O 12 (A is an element containing at least one selected from scandium, yttrium, and lanthanides, and B is an element containing at least one selected from aluminum, gallium, and scandium). In the case of a garnet structure, if the composition formula deviates from the above formula by 1% or more, a different phase such as a perovskite phase or an alumina phase will precipitate, reducing the transparency of the ceramic. An example of the composition is yttrium-aluminum garnet (Y3Al5O 12 ), terbium-gallium garnet (Tb3Ga5O 12 ), yttrium-terbium-aluminum garnet ((Y 0.4 Tb 0.6 )3AlO 12 ) are listed.

[0023] When the crystal structure is garnet type, Si can be included as a sintering aid element. Sintering aids not only have the effect of lowering the sintering temperature, but also have the effect of widening the narrow transparent region of garnet. The amount of sintering aid added is calculated in terms of metal (e.g., Y3Al5O 12 The Si content (weight of Si relative to the total weight of the ceramic) is preferably 1 ppm or more and 1000 ppm or less, more preferably 10 ppm or more and 1000 ppm or less, and even more preferably 50 ppm or more and 500 ppm or less. If the sintering aid content exceeds 1000 ppm, a heterogeneous phase originating from the sintering aid will be generated at the grain boundaries of the ceramic, which is not preferable.

[0024] When the crystal structure of the transparent ceramic is a C-type rare earth structure, it has a composition formula of C2O3 (C is an element containing at least one selected from scandium, yttrium, and lanthanoids). Examples of the composition include Y2O3, (Y 0.4 Tb 0.6 )2O3. Meanwhile, the composition formula C2O3 also has A-type rare earth structure and B-type rare earth structure, but because the A-type rare earth structure is trigonal and the B-type rare earth structure is monoclinic, they cannot be made into transparent ceramics. Each rare earth structure is determined by the type of rare earth oxide and the manufacturing temperature, so the manufacturing temperature is controlled to obtain a C-type rare earth structure.

[0025] When the crystal structure is a C-type rare earth structure, at least one element selected from Si, Mg, Zr, Hf, and Al can be included as a sintering aid. The amount of sintering aid added is preferably 1 ppm to 20,000 ppm, more preferably 100 ppm to 10,000 ppm, calculated as metal. In the case of a C-type rare earth structure, the amount of sintering aid does not need to be reduced as much as in the case of a garnet structure, but if it exceeds 20,000 ppm, a different phase will occur, which is not preferable.

[0026] When transparent ceramics are used as Faraday rotators for optical isolators, the rare earth elements (i.e., A in the above composition formula for garnet structures, and C in the above composition formula for C-type rare earth structures) must contain 60 mol% or more of terbium. The required characteristics of a Faraday rotator are transparency (no light absorption) in the wavelength range of use and a large Verdet constant. Terbium has the largest Verdet constant of any paramagnetic material other than iron and has no characteristic absorption from the visible to near-infrared range, making it an ideal element for Faraday rotators. If the amount of terbium relative to the total rare earth elements is less than 60 mol%, the Verdet constant of the Faraday rotator will be small, which is undesirable. Terbium can be trivalent or tetravalent, with tetravalent being the most stable valence, but it must remain trivalent. Tetravalent terbium is undesirable because it absorbs light in the visible range and has a brown appearance. If the crystal structure is a garnet structure, terbium is more stable in the trivalent state, so there is no need to worry about it. However, if the crystal structure is a C-type rare earth structure, terbium tends to become stable tetravalent terbium, so careful attention must be paid to changes in valence during production.

[0027] <<Phase difference measurement>> The retardation of the transparent ceramic of this embodiment is measured as follows, and satisfies the above formula 1. When measuring low retardation, the waveplate rotation method, the waveplate fixed method, and the circularly polarized light incidence method are commonly used, but any of these measurement methods is effective in the present invention. The measurement wavelength can be any wavelength, including the visible light region (e.g., 530 nm), near-infrared (e.g., 960 nm or 1100 nm), and ultraviolet, and is not particularly limited as long as it avoids the absorption wavelength of the transparent ceramic sample itself. A commercially available retardation measurement device can be used as the measurement device, or a homemade one can be used. It is preferable that the phase difference resolution be 0.1 nm or less, as this allows for the observation of fine phase differences.

[0028] The phase difference must be measured while minimizing local temperature distribution, stress, and other factors that cause the photoelastic effect. Temperature distribution, in particular, has a significant impact on the phase difference, so careful attention must be paid. Placing the sample on the stage using your hands is undesirable because the temperature of your hands can affect the measurement. It is preferable to use tweezers or similar. Furthermore, since a temperature difference between the sample and the stage can cause a phase difference, it is preferable to wait a certain amount of time after placing the sample on the stage before starting measurement to ensure a uniform temperature between the sample and the stage. While there are no particular limitations, leaving the sample on the stage for at least 30 seconds is preferable, with at least 1 minute being preferable, and at least 5 minutes being even more preferable. Note that for large samples (e.g., a 30 mm diameter × 40 mm long cylinder), leaving the sample on the stage for at least 1 hour is preferable, as 10 minutes is insufficient. The optimal leaving time varies depending on the sample shape, so it is best to determine the time based on the sample.

[0029] When measuring the phase difference of elongated transparent ceramics such as magneto-optical elements, it is preferable to adjust the optical system so that measurements can be made with parallel light. When there is a focal point, as with a normal lens, the measurement results are significantly affected by the periphery, which can lead to results that differ from the actual situation, and this is not desirable. Methods for measuring with parallel light include the use of telecentric lenses or homemade lenses made by combining convex and concave lenses. Although there is some influence from the periphery even when measuring with parallel light, this is not a problem as long as measurements can be made within an optically effective area of ​​at least 70% of the outer shape of the polished surface.

[0030] The term "optically effective area" as used herein refers to the optically effective region of the polished surface of the transparent ceramic, i.e., the region that functions effectively as a magneto-optical material when incident light passes through and exits the transparent ceramic. For example, in the case of a cylindrical transparent ceramic, it refers to the region of the circular polished surface on the optically useful axis, excluding the optically ineffective outer edge, i.e., the region toward the center of the polished surface. An optically effective area of ​​70% or more of the outer shape of the polished surface refers to an area that accounts for 70% or more of the area of ​​the polished surface. The upper limit of the outer shape of the polished surface is the region excluding the outer edge, as described above, and varies depending on the shape of the polished surface, but may be, for example, 95%.

[0031] The shape of the sample to be measured is not particularly limited, but a columnar structure such as a cylinder or square prism is preferable. Furthermore, the polished surface is preferably a plane perpendicular to the sample's optical path, i.e., vertical polishing, and the surface roughness should preferably be less than 5 nm in arithmetic mean roughness Ra. If the polished surface is not perpendicular, the influence of the outer periphery may be picked up, potentially resulting in inaccurate values. Furthermore, if the surface roughness is more than 5 nm in arithmetic mean roughness Ra, the roughness of the polished surface will have an effect, making it impossible to obtain accurate data. An anti-reflection coating (AR coating) may be applied to both end faces of transparent ceramics, but it must be confirmed that this does not pose a problem for the measurement wavelength.

[0032] In this way, the phase difference within an optically effective area of ​​70% or more of the outer shape of the polished surface of the transparent ceramic can be measured, and the mode and maximum value (Δλmax) of that area can be obtained. max The comparative phase difference value Δλmax / L is calculated by dividing the extinction ratio (nm) by the optical path length L of the sample (or the sample length in the case of a columnar structure) (mm). The transparent ceramic of this embodiment has Δλmax / L that satisfies the above formula (1), i.e., 0.30 (nm / mm) or less. Δλmax / L is preferably 0.25 (nm / mm) or less, and more preferably 0.20 (nm / mm) or less. The transparent ceramic of this embodiment that satisfies the above formula (1) can have a good extinction ratio.

[0033] <<Extinction ratio measurement>> The extinction ratio is measured by using an optical system that has at least a laser light source, polarizer, analyzer, and photodetector, passing a beam with a diameter that covers 70% of the effective diameter through the sample, measuring the light intensity I0' when the polarization plane of the analyzer is aligned with that of the polarizer, and then rotating the polarization plane of the analyzer by 90 degrees to make it perpendicular to the polarization plane of the polarizer, and measuring the received light intensity I' again, and then calculating the extinction ratio using the following formula. Extinction ratio (dB)=-10×log 10 (I' / I0')

[0034] The optical system is not particularly limited as long as it satisfies the above principle, and a commercially available extinction ratio measuring device can be used, or a homemade one can be used. The wavelength is preferably 1060±10 nm, which is used for fiber lasers, but is not particularly limited.

[0035] The transparent ceramic of this embodiment can have a good extinction ratio of, for example, 35 dB or more. Such transparent ceramic is suitable for use in magneto-optical devices or laser media, and can keep the beam quality difference between the incident beam and the transmitted beam to less than 1.1 when irradiated with a high power of 150 W or more.

[0036] FIG. 1 is a cross-sectional view schematically illustrating an example of an optical isolator, which is a magneto-optical device. The optical isolator 100 includes a Faraday rotator 110 made of the transparent ceramic material, a polarizer 120 made of a polarizing material, and an analyzer 130 inside a housing 102. These components are arranged in the order of polarizer 120, Faraday rotator 110, and analyzer 130 along the optical axis 112 of the Faraday rotator. The polarization plane of the polarizer 120 and the polarization plane of the analyzer 130 are arranged so that the relative angle is 45°. The optical isolator 100 also includes a magnet 140 attached to at least one side of the Faraday rotator 110 inside a housing 150 to apply a magnetic field to the Faraday rotator 110.

[0037] 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, causing the oscillation to become unstable.

[0038] <<Method for manufacturing transparent ceramics>> The transparent ceramic of this embodiment can be obtained by a manufacturing method including, for example, a step of forming a raw material to obtain a green body, a step of degreasing the green body to obtain a degreasing body, a step of sintering the degreasing body to obtain a sintered body, a step of annealing the sintered body, a step of processing the sintered body after the annealing treatment, and a step of measuring the phase difference of the obtained transparent ceramic. The raw materials used and each step will be described below.

[0039] <Preparation of raw powder for sintering> The first step in manufacturing transparent ceramics is to prepare the raw material powder for sintering. Raw material powder for sintering can be either a powder (mixed raw material) made by grinding and mixing various oxides using a wet or dry method, or a powder (co-precipitated raw material) made by precipitating particles from a solution in which various elements are dissolved.

[0040] (In the case of mixed raw materials) The raw oxide and sintering aid are weighed to achieve the desired composition. The purity of the raw oxide is preferably 99.9% or higher, more preferably 99.99% or higher, and even more preferably 99.999% or higher. Purity less than 99.9% is undesirable because it may contain elements other than the main components, such as heavy metals such as iron, chromium, and nickel, as well as rare earth elements other than the main components. The primary particle size of the raw material is not particularly limited as long as it can be made transparent and the half-width of the Schlieren image taken and quantified falls within a specified range, but a size of 0.05 μm or more and less than 5 μm is preferred. Note that particle sizes that are too large are unsuitable because they tend to cause compositional inconsistencies within the transparent ceramic. The sintering aid can be either an oxide or a compound. Examples of oxides include SiO2, MgO, ZrO2, and HfO2, and examples of compounds include tetraethoxysilane (TEOS), hafnium chloride, zirconium chloride, and aluminum chloride.

[0041] The raw material oxides and sintering aids, weighed to achieve the desired composition as described above, are mixed and ground by either wet or dry milling. Examples of wet mixing include ball mill mixing, bead mill mixing, jet mill mixing, and ultrasonic irradiation. Examples of dry mixing include dry ball milling and agate treatment. The mixing method is not particularly limited as long as it results in transparency. However, from the standpoints of productivity and contamination prevention, wet ball mill mixing, bead mill mixing, and jet mill mixing are preferred. When using a ball mill or bead mill, the type of balls or beads must be carefully selected, and materials containing the same elements as the starting raw material powder and sintering aids should be selected. For example, alumina balls are preferred for aluminum garnet, and zirconia balls are preferred for C-type rare earth structures and sintering aids, respectively. The processing conditions for each process are not particularly limited, as long as they are optimized to result in transparency.

[0042] Examples of dispersion media for wet mixing include primary to tertiary lower alcohols and pure water, and are not particularly limited as long as they can be made transparent. Furthermore, organic additives called dispersants may be added to improve dispersibility, and examples include oxyethylene alkyl ether-based dispersants, dodecylbenzene sulfonic acid-based dispersants, and polyethylene glycol-based dispersants. Furthermore, to improve subsequent moldability, organic binders may be added, and examples include polyvinyl acetate-based binders, polyvinyl alcohol-based binders, polyvinyl butyral-based binders, and polyacrylic acid-based binders. However, the type and amount of binder are not particularly limited as long as they can maintain shape retention during molding and the half-width when a Schlieren image is captured and digitized falls within a predetermined range.

[0043] (In the case of co-precipitated raw materials) Particles of the desired composition can also be precipitated from an aqueous solution containing various elements (main components and sintering aids). This type of precipitation from solution has the advantage of increasing the elemental uniformity of the particles and reducing compositional variations in the transparent ceramics. Particle precipitation methods include coprecipitation, in which a basic solution is dropped into an acidic solution containing various dissolved elements to obtain particles, and complex polymerization, but are not particularly limited as long as they can be made transparent. The particle shape is preferably spherical or card-house shaped, ranging from 50 nm to 500 nm. Coarse particles larger than 1 μm, which expose crystal faces, are not preferred because they deteriorate sinterability. The resulting powder must be fired at 600°C to 1200°C. Temperatures below 600°C are inadequate because of insufficient crystallization, while temperatures above 1200°C are undesirable due to poor sinterability. The fired powder is then processed in the same manner as the mixed raw materials to form a slurry.

[0044] <Forming process> The molding is carried out to obtain the desired shape. The molding method is broadly divided into dry molding and wet molding, but is not particularly limited. Examples of dry molding include uniaxial press molding and cold isostatic pressing (CIP), and examples of wet molding include slip casting and centrifugal molding. Regardless of whether the molding is dry or wet, a molded body that has achieved a certain desired shape may be further subjected to CIP treatment. The molding relative density to the true density is preferably 50% or more and less than 65%, and more preferably 55% or more and less than 62%. If the molding relative density is less than 50%, there will be many voids inside the molded body, making it difficult to produce a dense sintered body. On the other hand, if it is 65% or more, the molded body will be too densely packed, making it difficult for the internal gas to escape, which is undesirable as it will deteriorate the transparency of the transparent ceramic.

[0045] Regardless of the molding method, the molded body must be molded without bending. If the molded body is bent, stress will be applied in the direction of the bend, which may ultimately create a photoelastic effect and worsen the extinction ratio. In particular, with wet slip molding, if the inside and outside are not as wet or dry as they are when drying, stress will be generated due to the density difference between the inside and outside, and the photoelastic effect may worsen the extinction ratio. The molding process can be one of the factors that worsen the extinction ratio, so it is important to mold the body as evenly and as smoothly as possible.

[0046] <Degreasing process> If the resulting molded body contains organic matter such as a binder, it must be degreased. The molded body obtained above must be degreased in a heating furnace to remove the organic matter. There are no particular restrictions on the type of heating furnace; a muffle furnace or an oxygen furnace can be used. The degreasing atmosphere is not particularly limited as long as it can remove the organic matter, but it is preferable to perform the degreasing in air or an oxygen atmosphere to promote the combustion of the organic matter. A degreasing temperature of at least 270°C is required. Temperatures below 270°C are unsuitable because organic matter may remain. On the other hand, the upper limit of the degreasing temperature is 1100°C; temperatures higher than 1100°C are not preferred because they result in poor sinterability. The upper limit of the degreasing temperature may vary depending on the material, and it is best to determine the optimal degreasing temperature for each material.

[0047] <Sintering process> The degreased body obtained above is subjected to a sintering process to make it transparent. There are two main types of sintering processes: a method in which transparency is achieved by a single sintering process, and a method in which transparency is achieved by sintering followed by hot isostatic pressing (HIP). However, there are no particular limitations on the method used as long as a high degree of transparency is achieved. To further increase transparency, a second sintering or HIP process may be performed after transparency has been achieved. The number of times that the second sintering or HIP process is performed is not particularly limited, and it may be repeated multiple times until the desired properties are achieved.

[0048] The sintering process is selected from vacuum sintering, oxygen sintering, air sintering, spark plasma sintering, hot press sintering, and microwave sintering, and is not particularly limited as long as it ultimately leads to high transparency. In the case of vacuum sintering, the degree of vacuum is 10 -3 Pa or less is preferable. The sintering temperature is not particularly limited as long as transparency is possible, and it is most preferable to determine the optimum sintering temperature depending on the material. The optimum sintering temperature is the temperature at which transparency is achieved when transparency is achieved by a single sintering process, while it is the temperature at which the relative density after sintering is 94% or more but less than 99% when transparency is achieved by subsequent HIP treatment. In the latter case, a relative density of less than 94% will not achieve transparency in the subsequent HIP treatment, while a relative density of 99% or more is not preferable because it will be difficult to crush the internal bubbles by HIP.

[0049] The temperature rise profile for the sintering process is preferably between 20°C / h and 300°C / h, and it is also possible to change the temperature rise rate during the sintering process. A fast temperature rise rate has the effect of making it easier to increase the sintered density, but once the temperature reaches the point where crystal grains grow, bubbles are trapped within the crystal grains. These intragranular bubbles cannot be eliminated by any subsequent treatment, so they must be avoided when producing advanced transparent ceramics. On the other hand, a slow temperature rise rate will prevent intragranular bubbles from forming even if the crystal grains grow, but it will make it difficult to increase the sintered density. Therefore, it is effective to switch the temperature rise rate, such as using a fast temperature until the crystal grains grow and then a slower rate once the crystal grains have grown.

[0050] Furthermore, if moisture adsorbed on the particle surface or oxygen contained in tetravalent Tb is desorbed and remains inside the sintered body, it will remain inside the ceramic as a source of bubbles. To avoid this, the sintering temperature can be maintained between 200°C and 1000°C for 8 hours or more during the temperature rise to promote the desorption of moisture and oxygen. Below 200°C, the amount of moisture desorbed is small, so the effect is weak. At temperatures above 1000°C, densification begins to progress, so the effect is also weak, making this unsuitable.

[0051] HIP treatment is performed to make the aforementioned sintered body with a density of 94% to 99% transparent, or to further enhance the transparency of already transparent ceramics. The HIP pressure medium is selected from argon, nitrogen, or argon plus 20% or less oxygen, as long as it achieves a high degree of transparency. However, when HIP treatment is performed under conditions containing oxygen, a platinum-based heater material is required. While carbon heaters are also available as HIP heater materials, their use in the presence of oxygen is not recommended because it accelerates thinning. On the other hand, when using argon or nitrogen, carbon heaters are preferred. The HIP treatment temperature is not particularly limited, but it is most preferable to keep it within ±100°C of the sintering temperature of the previous process. The HIP treatment pressure is not particularly limited, but is preferably 50 MPa or more and 200 MPa or less.

[0052] <Annealing process> The transparent ceramics obtained by the above process may have oxygen vacancies due to oxygen desorption at high temperatures or reduction by a carbon heater, resulting in a black color. An annealing step may be performed to replenish the lost oxygen. The annealing process may be performed in an atmospheric furnace, an oxygen furnace, or in an oxidizing atmosphere using HIP, but is not particularly limited. The annealing temperature may be any temperature sufficient to recover the oxygen vacancies, preferably between 1300°C and 1600°C.

[0053] In the previous processes involving heating, there is a risk that the photoelastic effect will occur partially and the extinction ratio will deteriorate due to uneven heating caused by the temperature distribution inside the furnace due to the sintering furnace structure, the appearance of anisotropy due to contact with jigs, the influence of nearby samples, dissolution, adhesion of filter media, etc. It is preferable to use a sintering furnace with a temperature distribution that is as uniform as possible, and to consider a layout that avoids contact with sintering jigs as much as possible.

[0054] <Processing process> It is preferable to optically polish both end faces of the transparent ceramic obtained by the above series of steps, which are located on the optically utilized axis. The optical surface precision at this time is preferably λ / 8 or less, and particularly preferably λ / 10 or less, when the measurement wavelength λ is 633 nm. To obtain appropriate phase difference information, the roughness of the polished surface is preferably 5 nm or less in arithmetic mean roughness Ra, more preferably 3 nm or less, and even more preferably 1 nm or less. If the arithmetic mean roughness Ra exceeds 5 nm, the phase difference measurement device will pick up information about the unevenness of the polished surface, making it impossible to observe appropriate phase difference information. An AR coating may be formed on the polished surface of the processed transparent ceramic for anti-reflection purposes.

[0055] <Phase difference measurement process> Then, the phase difference is measured for the transparent ceramic having the polished surface obtained by the processing step. The method for measuring the phase difference has already been explained, so it will be omitted here. The maximum phase difference Δλ obtained max The comparative retardation value Δλmax / L is calculated by dividing Δλmax / L (nm) by the optical path length L (mm) of the transparent ceramic. Products with Δλmax / L that satisfy the above formula (1), i.e., 0.30 (nm / mm) or less, are selected as non-defective products.

[0056] By measuring and evaluating the phase difference in this way, it is possible to quantify birefringence, which cannot be observed using a polarizing microscope, and to provide transparent ceramics with small birefringence inside the ceramics and a good extinction ratio, for example, an extinction ratio of 35 dB or more. [Example]

[0057] The present invention will be explained in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0058] [Examples 1 to 4] (Manufacturing transparent ceramics) Terbium oxide powder (Tb4O7, particle size 50 nm) manufactured by Shin-Etsu Chemical Co., Ltd., yttrium oxide powder (YO3, particle size 50 nm) manufactured by Shin-Etsu Chemical Co., Ltd., and aluminum oxide powder (TM-DAR, particle size 300 nm) manufactured by Taimei Chemical Co., Ltd. were weighed out in predetermined amounts to achieve the compositions shown in Table 1 (50 g total for each composition). In addition, 130 g of ethanol manufactured by Kanto Chemical Co., Ltd. was prepared as a dispersion medium, 200 ppm of tetraethoxysilane (TEOS) manufactured by Kishida Chemical Co., Ltd. was prepared as a sintering aid, and 700 g of alumina balls (φ2 mm, purity 3.5N) manufactured by Nikkato Corporation were prepared as grinding media. These were then placed in a 500 mL resin pot and subjected to ball milling.

[0059] After 20 hours of ball milling, 1 wt% of polyvinyl alcohol (Kanto Chemical Co., Ltd.) was added as a binder to the powder, and the powder was ball milled again for 4 hours. The resulting slurry was granulated by spray drying. The resulting granules were uniaxially pressed and then subjected to CIP treatment (200 MPa) to obtain a molded body with a molding density of 60%.

[0060] The molded body was degreased under the condition of 400°C in air for 3 hours, and then heated at a rate of 300°C / h. -3 The sample was vacuum sintered at 1600°C for 2 hours at a vacuum of 196 MPa. The relative density of the resulting sintered body was 96%. To achieve transparency, HIP treatment was performed using argon as the pressure medium at 196 MPa and 1600°C for 2 hours. Since the sample was slightly dark after HIP treatment, it was annealed in air at 1450°C for 20 hours. After annealing, the transparent ceramic was machined to a diameter of 5 mm and length of 17 mm, and both end faces were optically polished to a surface precision of λ / 8 or less. The outer diameter was first machined to 5 mm, and then both end faces were polished. After cylindrical grinding, the sample was annealed at 1300°C for 6 hours.

[0061] (Phase difference measurement) The phase difference was measured using a two-dimensional birefringence evaluation system (PA-300) manufactured by Photonic Lattice Co., Ltd. The measurement wavelength was 520 nm, and measurements were performed using a telecentric lens with a magnification of 3x. After placing the sample on the stage, a waiting period of 30 seconds was allowed, and the phase difference within the sample was observed by taking the difference from the background. A dedicated program (PA-View) manufactured by Photonic Lattice Co., Ltd. was used for analysis. A histogram of phase difference was created in an optically effective area of ​​70% or more of the outer shape of the polished surface of the sample, and the maximum phase difference obtained, Δλ max The comparative retardation value Δλmax / L was obtained by dividing the wavelength (nm) by the sample length L (mm).

[0062] (Loss factor measurement) The loss coefficient was measured by measuring the light intensity when light with a wavelength of 1064 nm was transmitted through a beam diameter of 1 mm using an in-house optical system that included a light source manufactured by NKT Photonics, a power meter manufactured by Gentec, and a Ge photodetector, and was calculated using the following formula. Loss coefficient (cm -1 ) = 10 × log(I / I0) / (sample length (cm)) (In the formula, I represents the transmitted light intensity (the intensity of light transmitted in a straight line through a 20 mm long sample), and I0 represents the incident light intensity.)

[0063] (Method for measuring extinction ratio) The extinction ratio was measured by transmitting light with a wavelength of 1064 nm through the sample with a large beam diameter of 3 mm using an optical system manufactured in-house using a light source manufactured by NKT Photonics, a collimator lens, a polarizer, a work stage, an analyzer, a power meter manufactured by Gentec, and a Ge photodetector. In this state, the light intensity I0' was measured when the polarization plane of the analyzer was aligned with that of the polarizer, and then the received light intensity I' was measured again with the polarization plane of the analyzer rotated 90 degrees to make it perpendicular to the polarization plane of the polarizer, and the extinction ratio was then calculated using the following formula. Extinction ratio (dB)=-10×log 10 (I' / I0')

[0064] (Isolator included) As shown in Figure 1, each transparent ceramic sample was inserted into the center of a neodymium-iron-boron magnet with an outer diameter of 32 mm, an inner diameter of 6 mm, and a length of 40 mm. Polarizers were then inserted at both ends, and a high-power laser beam with a wavelength of 1064 nm was incident on both ends using a high-power laser (beam diameter: 1.6 mm) manufactured by IPG Photonics Japan, Inc., to determine the Faraday rotation angle θ. The Faraday rotation angle θ was defined as the angle at which maximum transmittance was achieved when the polarizer on the output side was rotated. The Verdet constant was calculated based on the following equation. The magnitude of the magnetic field (H) applied to the sample was calculated by simulation using the dimensions of the measurement system, the residual magnetic flux density (Br), and the coercivity (Hc) described above. θ=V×H×L (In the formula, θ is the Faraday rotation angle (Rad), V is the Verdet constant (Rad / T m), H is the magnitude of the magnetic field (T), and L is the length of the Faraday rotator (0.017 m in this case).)

[0065] Table 1 shows the measurement results of the loss coefficient, the mode and maximum value of the retardation, and the extinction ratio of each of the samples of Examples 1 to 4 obtained as described above.

[0066] [Comparative Examples 1 to 4] The transparent ceramics of Comparative Examples 1 to 4 were produced in the same manner as in Examples 1 to 4, except that the molding process was performed by slip molding and a partially warped shape was obtained. The measurement results of the loss coefficient, the mode and maximum value of the retardation, and the extinction ratio are also shown in Table 1.

[0067] [Table 1]

[0068] As shown in Table 1, in the samples of Examples 1 to 4 that could be molded without bending during molding, the phase difference mode and phase difference maximum were small, Δλmax / L was less than 0.5, and the extinction ratio was good at 35 dB or more. On the other hand, in the samples of Comparative Examples 1 to 4 that were bent during molding, the phase difference mode and phase difference maximum, and Δλmax / L were all large. This is thought to be because the stress generated during molding remains even after the transparent body is formed, causing a photoelastic effect. As a result, polarization is thought to be eliminated, and the extinction ratio deteriorates.

[0069] Furthermore, when the Verdet constants of the transparent ceramics of Examples 1 to 4 and Comparative Examples 1 to 4 were measured, they were 46 Rad / T·m for Example 1 and Comparative Example 1, 49 Rad / T·m for Example 2 and Comparative Example 2, 53 Rad / T·m for Example 3 and Comparative Example 3, and 60 Rad / T·m for Example 4 and Comparative Example 4. Furthermore, when the extinction ratio of the entire isolator was measured, it was consistently 35 dB or higher for Examples 1 to 4, while it was in the 20 dB range for Comparative Examples 1 to 4. A good extinction ratio of the magneto-optical element also improves the extinction ratio of the entire isolator, which is particularly desirable for industrial lasers used in high-power applications.

[0070] [Examples 5 to 8, Comparative Examples 5 to 8] Transparent ceramics were produced in the same manner as in Examples 1 to 4 and Comparative Examples 1 to 4, except that the composition and crystal structure were changed as shown in Table 2. In the case of the C-type rare earth structure in Examples 5 and 6 and Comparative Examples 5 and 6, the sintering aid was ZrO2, and the amount was 1 mass %. The length of the transparent ceramics with the C-type rare earth structure was 9 mm.

[0071] Table 2 shows the measurement results of the loss coefficient, the mode and maximum values ​​of the phase difference, and the extinction ratio for Examples 5 to 8 and Comparative Examples 5 to 8. Furthermore, as the measurement results of the phase difference, Fig. 2(a) shows the phase difference distribution on the polished surface of the sample of Example 7, and Fig. 2(b) shows the phase difference distribution on the polished surface of the sample of Comparative Example 8. In the figures, the darker the area, the larger the phase difference, with areas of 10 nm or more being the darkest. Fig. 3 also shows phase difference histograms within the optically effective area of ​​Example 7 and Comparative Example 8.

[0072] [Table 2]

[0073] As shown in Table 2, in Examples 5 to 8, in which the molded bodies were not bent even though the composition and crystal structure were changed as shown in Table 2, the extinction ratio was good at 35 dB or more and Δλmax / L was small. On the other hand, in Comparative Examples 5 to 8, which had the same composition and crystal structure but were bent during molding, the Δλmax / L was large and the extinction ratio was poor.

[0074] 2(a) and (b), in Example 7, the entire optically effective area on the center side of the polished surface of the sample was shown lightly, and the phase difference was very small, while in Comparative Example 8, many areas were shown darkly even within the optically effective area, and birefringence within the ceramic was observed. As shown in the phase difference histogram in Figure 3, in Example 7, the phase difference was 2 nm or less in almost all regions within the optically effective area, whereas in Comparative Example 8, the frequency peak was a phase difference of about 2 nm, and a wide range of phase differences from about 1 nm to about 4 nm was observed with relatively high frequency.

[0075] [Comparative Examples 9 to 11] Comparative Example 9 was prepared in the same manner as Example 7 except that foreign matter was intentionally introduced inside, Comparative Example 10 was prepared in the same manner as Example 7 except that sintering was carried out in a location outside the hot zone of the vacuum sintering furnace, Comparative Example 11 was prepared in the same manner as Example 7 except that the sample was pressed firmly against a peripheral jig during heating, and Comparative Example 12 was prepared in the same manner as Example 7 except that annealing was not carried out after cylindrical grinding. The results are shown in Table 3. As shown in Table 3, the extinction ratio of all of the samples in Comparative Examples 9 to 12 was poor, making them difficult to use as magneto-optical elements.

[0076] [Table 3]

[0077] (Isolator included) Transparent ceramics containing Tb exhibit magneto-optical properties and can be used as isolator materials. To investigate whether the resulting transparent ceramics could be used as isolators, we measured their Verdet constants. As shown in Figure 1, each ceramic sample was inserted into the center of a neodymium-iron-boron magnet with an outer diameter of 32 mm, an inner diameter of 6 mm, and a length of 40 mm. Polarizers were then inserted at both ends of the magnet. A high-power laser beam with a wavelength of 1064 nm was then incident on both ends using a high-power laser (beam diameter: 1.6 mm) manufactured by IPG Photonics Japan, Inc., to measure the Faraday rotation angle θ. The Faraday rotation angle θ was defined as the angle at which maximum transmittance was achieved when the polarizer on the output side was rotated. The Verdet constant was calculated using the following equation: The magnitude of the magnetic field (H) applied to the sample was calculated by simulation using the dimensions of the measurement system, the residual magnetic flux density (Br), and the coercivity (Hc). θ=V×H×L (In the formula, θ is the Faraday rotation angle (Rad), V is the Verdet constant (Rad / T m), H is the magnitude of the magnetic field (T), and L is the length of the Faraday rotator (0.017 m in this case).)

[0078] The Verdet constants of the transparent ceramics of Example 7 and Comparative Example 7 were measured and found to be 45 Rad / T·m for both. Furthermore, the extinction ratio of the entire isolator was measured and found to be 36 dB for Example 7 and 27 dB for Comparative Example 7. A good extinction ratio for the magneto-optical element also improves the extinction ratio of the entire isolator, which is particularly desirable for industrial lasers used in high-power applications.

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

[0080] 100 Optical isolator 102 Case 104 Optical axis 110 Faraday rotator 120 Polarizer 130 Analyzer 140 Magnet

Claims

1. A transparent ceramic having a polished surface, wherein the following formula (1) is satisfied within an optically effective area of ​​70% or more of the outer shape of the polished surface: Δλmax / L≦0.30 (nm / mm) ...Formula (1) (Δλmax is the maximum value (nm) of phase difference within the optically effective area, L is the optical path length (mm) of the transparent ceramic, and the phase difference indicates the phase shift between the polarization component along the fast axis and the polarization component along the slow axis of the light.)

2. 2. The transparent ceramic according to claim 1, wherein the wavelength of light used in measuring the phase difference is 520 nm.

3. The transparent ceramic according to claim 1, wherein the transparent ceramic has a cylindrical shape.

4. 2. The transparent ceramic according to claim 1, wherein the crystal structure of the transparent ceramic is a C-type rare earth structure, a garnet structure, or a pyrochlore structure.

5. The transparent ceramic according to claim 1, wherein the transparent ceramic has a composition containing Tb.

6. 2. The transparent ceramic according to claim 1, wherein the Verdet constant is 35 Rad / T·m or more.

7. 2. The transparent ceramic according to claim 1, wherein the extinction ratio is 35 dB or more when the transparent ceramic is irradiated with a laser having a wavelength of 1070 nm.

8. A magneto-optical device constructed using the transparent ceramic according to claim 1 as a magneto-optical material.

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