Transparent ceramics and magnetic optical devices using the same

Abstract

To provide transparent ceramics that are truly transparent, have few scattering sources from visible to the near-infrared range, and are resistant to laser damage, and to provide a magnetic optical device using the same.SOLUTION: Transparent ceramics are cylindrical in shape having a surface roughness Ra of 1 nm or less on both optical surfaces, with a sample length of 20 mm. When the optical surface of the sample is placed on a black background and an image of the optical surface is acquired using 256-level grayscale, assuming that a grayscale value of the background portion is defined as GB, a highest grayscale value is defined as GH, and a grayscale average value of the central region covering 20*80% of the diameter of the optical surface is defined as GT, the following equation (1) is satisfied: (GT-GB) / (GH-GB)×100≤30%. A magneto-optical device is a light isolator that includes the above transparent ceramic as a Faraday rotator 110.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to transparent ceramics and magneto-optical devices using the same, and more particularly to transparent ceramics having translucency in the visible and infrared regions, and in particular to transparent ceramics used as phosphors, optical lenses, and magneto-optical elements in optical applications, and magneto-optical devices using the same.

Background Art

[0002] Ceramics have been widely manufactured since ancient times for tiles and porcelain, and in recent years for fine ceramics such as piezoelectric elements, superconducting elements, and transparent ceramic bodies, and are important materials in various industries and real life.

[0003] Among various ceramics, the development of transparent ceramics has been accelerated since laser oscillation was reported in the 1990s. The main applications of transparent ceramics are laser materials, optical lenses, phosphors, optical magnetic elements, etc. The characteristics required for transparent ceramics are to have a high transmittance and no optical defects such as visible air bubbles. In particular, in order to achieve the former high transmittance, (1) not containing elements or defects that are absorption factors, (2) not containing micro air bubbles of 2 μm or less, and (3) minimizing the intergranular refractive index difference are required, and strict management in the entire manufacturing process is required.

[0004] Regarding micro air bubbles, as in Patent Document 1 (International Publication No. 2022 / 054595) and Patent Document 2 (International Publication No. 2019 / 187287), it is disclosed that bubbles can be reduced by controlling sintering and the loss can be reduced. However, both patent documents evaluate the insertion loss value in the 1064 nm band. Generally, air bubbles of 2 μm or less, particularly those smaller than the use wavelength, mainly undergo Rayleigh scattering, and the scattering tends to be stronger at shorter wavelengths. Therefore, it is not always a truly transparent ceramic only by the loss coefficient value in the 1064 nm band, and the degree of light scattering at shorter wavelengths must be evaluated.

[0005] Among transparent ceramics, especially regarding magneto-optical elements, there are not only loss coefficients but also important parameters such as laser damage. Laser damage is caused by factors such as the accuracy of the polished surface, defects inside the crystal / transparent ceramics, or absorption. Among these, defects include microbubbles less than 1 μm, and laser damage will occur inside the crystal / transparent ceramics unless these are reduced as much as possible. As described above, a truly transparent transparent ceramic magneto-optical element from visible to near-infrared is required. Regarding transparent ceramics and laser damage, Patent Document 3 (Japanese Patent Application Laid-Open No. 2023-128125), Patent Document 4 (International Publication No. 2022 / 054592), and Patent Document 5 (International Publication No. 2022 / 054596) are already known, but they are respectively about the relationship between polished surface accuracy and laser damage, the relationship between absorption coefficient and laser damage, and the relationship between the loss coefficient in the 1064 nm band and laser damage. There was no known literature that considered scattering from visible to near-infrared and laser damage as in the present invention.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0007] Patent Documents 3, 4, and 5 described above respectively relate to the relationship between polishing surface accuracy and laser damage, the relationship between absorption coefficient and laser damage, and the relationship between loss coefficient in the 1064 nm band and laser damage. These patent documents do not suggest anything about scattering from visible to near-infrared and laser damage.

[0008] Therefore, in view of the above circumstances, an object of the present invention is to provide a truly transparent ceramic with few scattering sources from visible to near-infrared and difficult to cause laser damage, and a magneto-optical device using the same.

Means for Solving the Problems

[0009] To achieve the above object, in one aspect of the present invention, there is provided a cylindrical transparent ceramic having a surface roughness Ra of the optical surfaces at both ends thereof of 1 nm or less, a sample length of 20 mm, and when the optical surface of the sample is placed still on a black background and an image of the optical surface is acquired in 256 gradation gray scale, in the image, the gray scale value of the background portion is G B , the highest gray scale value is G H , and the average gray scale value of the central region of 20 to 80% of the diameter of the optical surface is G T , which satisfies the following formula (1). (G T - G B ) / (G H - G B ) × 100 ≦ 30% ··· (1)

[0010] The crystal structure of the above transparent ceramic is preferably either garnet or C-type rare earth.

[0011] The composition of the above transparent ceramic is preferably represented by the following formula (3). (R 1-x Sc x )3(Al 1-z Sc z )5O 12 ··· (3) (where 0 ≦ x ≦ 0.04, 0 ≦ z ≦ 0.04, 0 ≦ x + z < 0.045, and R contains Y and one or more elements selected from the group consisting of elements having atomic numbers from 65 to 71.)

[0012] In the above transparent ceramics, it is preferable that R in the above formula (3) is Tb.

[0013] The above transparent ceramics preferably have an extinction ratio of 40 dB or more.

[0014] As another aspect, the present invention provides a magneto-optical device comprising the above transparent ceramics.

[0015] 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 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]

[0016] According to the present invention, since there are few scattering sources from visible to near-infrared, it is possible to provide a truly transparent transparent ceramics that can be used as a magneto-optical element for fiber lasers other than the 1064 nm band and a laser material, and a magneto-optical device using the same. [Brief Description of the Drawings]

[0017]

Figure 1

[0018] [1. Transparent Ceramics] First, an embodiment of the transparent ceramics according to the present invention will be described. The transparent ceramics of this embodiment are cylindrical with a surface roughness Ra of 1 nm or less on the optical surfaces at both ends, a sample length of 20 mm, and when the optical surface of the sample is placed still on a black background and an image of the optical surface is acquired in 256 - level gray scale, in the above - mentioned image, the gray - scale value of the background part is G B , the highest gray - scale value is G H , and the average gray - scale value of the central region of 20 - 80% of the diameter of the optical surface is G T , and it satisfies the following formula (1). (G T - G B ) / (G H - G B ) × 100 ≤ 30% ··· (1)

[0019] The transparent ceramics that satisfy the above formula (1) are truly transparent ceramics that are less likely to cause laser damage. As a reason for the occurrence of laser damage, it is considered that during the sintering process of manufacturing the transparent ceramics, bubbles are not completely removed and remain in the transparent ceramics, and damage occurs starting from that part. In the case of such ceramics, when observing the inside from the transmission surface, it is confirmed that it looks white and turbid. Specifically, when a transparent ceramic is placed on a black plate and an image of the optical surface is acquired, the optical surface, that is, the inside of the ceramic may look whiter than the original black background.

[0020] Quantifying this, when the optical surface of a 20 - mm - long sample of transparent ceramics is placed still on a black background, an image of the optical surface is acquired, this image is converted into a 256 - level gray scale, the gray - scale value of the background part in the image is G B , the highest gray - scale value in the image is G H , and the average gray - scale value of the central region of 20 - 80% of the diameter of the optical surface (that is, the average gray - scale value of the background through the transmission surface) is G T , and when the above formula (1) is satisfied, it can be said that the ceramics are truly transparent without bubbles inside and are less likely to cause laser damage.

[0021] The grayscale value represents black as 0 and white as 255. Therefore, in Equation (1), (G H - G B ) represents the grayscale range in the image. The ratio value of (G H - G B ) to (G T - G B ) in Equation (1) (hereinafter referred to as the "grayscale ratio value") represents the degree of light scattering of the transparent ceramics. If this exceeds 30%, it cannot become truly transparent ceramics and will cause laser damage during high-power laser irradiation.

[0022] Note that it is preferable to define the maximum grayscale value in the central region of 20 - 80% of the diameter of the optical surface as G TM and satisfy the following Equation (2). (G TM - G B ) / (G H - G B ) × 100 ≤ 40% ··· (2)

[0023] Equation (2) uses G T instead of G TM in Equation (1). However, if the result of Equation (2) exceeds 40%, it will contain undesirable contamination and heterophase, and the inside of the optical surface will be in a non-uniform state, which is not preferable.

[0024] Note that the method for acquiring the image is not particularly limited as long as an image can be acquired, such as a general digital camera, industrial camera, microscope camera, web camera, etc. The resolution is also not particularly limited, but in order to acquire a more accurate image, it is preferably 1 million pixels or more.

[0025] Also, the background color is preferably black in order to make the white bubble group stand out, and it is preferably in a state where it is 30 or less in the 256 - gradation grayscale value. For the pedestal, it is advisable to select a material that is difficult to reflect light such as a fluorescent lamp (for example, the light-shielding and anti-reflection black film manufactured by Shibuya Optics Co., Ltd.) or to devise the position of the lighting.

[0026] The lighting is not particularly limited as long as light close to white, such as a fluorescent lamp or a white LED, can be obtained. It is not preferable if it has an orange tint like a halogen lamp because it becomes difficult to see the state of internal scattering. The position where the lighting is applied can be either the upper part or the side, and an image may also be acquired in a general fluorescent lighting environment such as an office. At that time, the lighting intensity needs to be 300 lux or more.

[0027] The method for analyzing the image is not particularly limited, but programming processing using OpenCV or image analysis software such as ImageJ (manufactured by the National Institutes of Health, USA) may also be used. The gray-scale average value G T and the maximum value G TM are the average values of the gray-scale values in the central region from 20% to 80% of the diameter of the optical surface, rather than the entire optical surface. In the central region less than 20% of the diameter of the optical surface, the overall image of the optical surface cannot be grasped. Also, in the central region exceeding 80% of the diameter of the optical surface, information other than the inside is obtained due to the focus of the camera and the influence of the side surface of the ceramics, and the transparency of the optical surface cannot be correctly evaluated.

[0028] In addition, when there is absorption in the entire visible light range due to oxygen defects or other impurities in the transparent ceramics, this measured value may be low. Therefore, the transmittance in the visible light region needs to be 70% or more, and if it is less than 70%, it will not be an effective measurement. When there is a large absorption in the visible light region, removal of oxygen defects by annealing treatment and purification of raw materials are essential. Here, the so-called large absorption refers to the absorption existing over 400 - 700 nm, and for example, the absorption over 450 - 500 nm, which is the characteristic absorption of Tb, can be excluded.

[0029] The transparent ceramics of this embodiment preferably have a cubic crystal structure. If it is cubic, there is no particular limitation, but it is particularly preferable to have a crystal structure selected from either garnet or C-type rare earths.

[0030] In the case of the garnet type, it is more preferable that it has an aluminum garnet structure represented by the following formula (3). (R 1-x Sc x )3(Al 1-z Sc z )5O 12 ··· (3) (In the formula, 0 ≦ x ≦ 0.04, 0 ≦ z ≦ 0.04, 0 ≦ x + z < 0.045)

[0031] In formula (3), R is one or more selected from the group consisting of Y and rare earth elements having atomic numbers from 65 to 71. When aluminum mainly enters the B site of the crystal structure represented by formula (3), it is known that at the 8 - coordinate A site, the ionic radius that can be stably coordinated as garnet is about 104 pm or less, and all the above - mentioned element groups can stably exist as aluminum garnet. Note that for the rare earth element, it is preferable to select an element that has a unique f - f transition absorption and no absorption in the used wavelength band. Specifically, it is particularly preferable to be selected from Y, Tb, and Lu, which have no absorption from visible to near - infrared. Further, when the transparent ceramics of the present embodiment are used in a magneto - optical device, from the viewpoint of having a high Verdet constant, it is more preferable to contain Tb at 60 mol% or more.

[0032] In the case of the garnet type, for the purpose of stabilizing the crystal structure, Sc may be contained. Sc can dissolve in both the A site and the B site, and functions as a buffer that can correct a subtle compositional deviation due to weighing error. The addition amount is within the range of x and z shown in the above formula (3). Since Sc is expensive compared to other elements, when x + z exceeds 0.045, it is not preferable because it is not cost - effective.

[0033] Also, in the case of the garnet type, Si may be added as a sintering aid. For example, it can be added as an Si - based inorganic compound such as SiO2 or an Si - based polymer compound such as tetraethoxysilane (TEOS). 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.

[0034] In the present specification, the "addition amount" refers to the amount of the sintering aid intentionally added. Therefore, when the addition amount is 0 mass ppm, it indicates that there is no intentional addition of the sintering aid, excluding the case where the corresponding element is contained as an impurity in the raw material powder.

[0035] The transparent ceramics of the present embodiment are composed of the above main components and sub-components, but may further contain other elements. Examples of other elements typically include rare earth elements such as cerium (Ce), or various impurity groups such as sodium (Na), calcium (Ca), magnesium (Mg), phosphorus (P), tantalum (Ta), molybdenum (Mo), etc. When the total amount of Tb is 100 parts by mass, the content of other elements is preferably 10 parts by mass or less, more preferably 0.1 part by mass or less, and particularly preferably 0.001 part by mass or less.

[0036] The transparent ceramics of the present embodiment exhibit a colorless and transparent appearance, and preferably have an insertion loss at a wavelength of 1064 nm with an optical path length of 20 mm of 0.040 dB or less.

[0037] The transparent ceramics of the present embodiment preferably have a Verdet constant at a wavelength of 1064 nm of 30 rad / (T·m) or more, more preferably 36 rad / (T·m) or more. When the Verdet constant is 36 rad / (T·m) or more, replacement with the existing material TGG single crystal can be performed without changing the component design, which is particularly preferable because it is simple. The upper limit of the Verdet constant under the above conditions is not particularly limited, but may be, for example, 60 rad / (T·m) or less. Note that the Verdet constant has wavelength dependence, and the Verdet constant increases as the wavelength becomes shorter, so the upper limit value changes depending on the wavelength.

[0038] [2. Method for manufacturing transparent ceramics] The method for manufacturing the transparent ceramics of the present embodiment includes, for example, a step of press-molding using raw material powder, a step of degreasing the molded body, and then a step of sintering to obtain a sintered body having a relative density densified to at least 95% or more, a step of performing hot isostatic pressing (HIP) treatment on the sintered body, and then a step of annealing treatment. Hereinafter, the raw material powder and each step will be described in detail.

[0039] [2-1. Raw Material Powder] As starting materials used in this manufacturing method, metal powders of terbium, yttrium, lutetium, aluminum, zirconium, hafnium, or aqueous solutions thereof such as nitric acid, sulfuric acid, and uric acid, or oxide powders of the above elements can be preferably used.

[0040] The preparation of the raw material powder is roughly classified into two types: the breakdown type and the buildup type, but it is not particularly limited as long as it can be made transparent. The buildup type is a method of adjusting the raw material powder for molding by pulverizing various powders. Although it has merits in productivity, there are problems in the uniformity of the composition. On the other hand, the buildup type is a method of obtaining powder by nucleation and grain growth from solutions of various elements. Although it has great merits in the uniformity of the composition, it is a method with difficulties in productivity and reproducibility. In the present embodiment, it is not particularly limited as long as it can be highly made transparent.

[0041] In the breakdown method, it is most preferable to weigh various oxide powders and perform grinding treatment either wet or dry. The purity of the various oxide powders is preferably 99.9% or more, more preferably 99.99% or more. Also, the primary particle size of the various powders is preferably 0.05 μm or more and 100 μm or less. If it is less than 0.05 μm, not only is it difficult to control the uniformity of the ceramics due to high particle aggregability, but rapid densification occurs in the sintering process, making it difficult to control the bubble discharge, so it is not preferable. Also, if it exceeds 100 μm, it is unsuitable because it cannot be ground into fine particles by wet grinding or dry grinding. The grinding treatment can be either wet or dry, and any of ball mill treatment, bead mill treatment, jet mill treatment, and homogenizer treatment can be suitably used. The grinding treatment is preferably carried out until the median value (D50) of the particle size distribution of the primary particles is less than 1 μm.

[0042] In the build-up method, a method of synthesizing powder from a solution containing various elements and firing it at 1300°C or lower is preferable. Examples of the precursors of the various elements include chlorides, nitrates, carbonates, and sulfates, and they are not particularly limited. Also, examples of the powder synthesis methods include coprecipitation method, complex polymerization method, and homogeneous precipitation method, and they are not particularly limited as long as highly transparent ceramics can be manufactured. In any synthesis method, the primary particle size is preferably 0.05 μm or more, and its shape is not particularly limited. Depending on the properties of the obtained powder, after firing, it may be subjected to grinding treatment either wet or dry, and similar to the breakdown method, it is not particularly limited to the grinding method.

[0043] Note that for the subsequent manufacturing yield stability and quality improvement, organic additives such as dispersants, binders, plasticizers, and lubricants may be added. In that case, wet grinding is performed, and the method of adding them to the slurry is the most stable and thus preferable. There is no particular limitation on the addition amount, and it is sufficient if the target characteristics are obtained.

[0044] [2-2. Forming Process] In this manufacturing method, a normal pressing process can be preferably used. That is, a very common pressing process of filling a mold and applying pressure from a certain direction, a CIP (Cold Isostatic Pressing) process or a WIP (Warm Isostatic Pressing) process 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 molded body, and there is no particular limitation. Alternatively, a hot pressing process, a spark plasma sintering process, a microwave heating process, etc., which not only perform the forming process but also sinter in one go during forming, can also be preferably used. Furthermore, it is also possible to produce a molded body by a casting molding method instead of the pressing molding method. Molding methods such as pressure casting molding, centrifugal casting molding, and extrusion molding can also be adopted by optimizing the shape and size of the oxide powder as the starting material and the combination with various organic additives.

[0045] [2-3. Debinding Process] In this manufacturing method, a normal debinding process can be preferably used. That is, it is possible to go through a temperature-raising debinding 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 debinding temperature is preferably 270°C or higher and 1000°C or lower. If it is lower than 270°C, it is difficult to completely remove the organic additives. On the other hand, at a temperature higher than 1000°C, densification progresses before the sintering process, making it difficult to obtain a transparent sintered body with low scattering.

[0046] [2-4. Sintering Process] In this manufacturing method, a general sintering process can be preferably used. That is, a heating 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 an inert gas, an oxygen gas, a hydrogen gas, a helium gas, etc., or sintering under reduced pressure (in a vacuum) is also possible, but sintering in a vacuum where high transparency can be achieved is most preferable.

[0047] The sintering temperature in the sintering process is preferably 1400 - 1780°C, particularly preferably 1450 - 1750°C. When the sintering temperature is within this range, it is preferable because densification is promoted while suppressing heterogeneous precipitation.

[0048] The sintering holding time in the sintering process is about several hours, but the relative density of the sintered body must be densified to at least 93% or more. If it is less than 93%, it is unsuitable because a transparent body cannot be obtained in the subsequent HIP treatment process. It is necessary to control the sintering holding time so that the relative density of the sintered body is 93% or more.

[0049] The crystal grain size in the sintering process is preferably 1 μm or more and 40 μm or less, more preferably 5 μm or more and 35 μm or less. If the crystal grain size is less than 1 μm, it is unsuitable because the transparency deteriorates due to minute compositional deviations between crystal grains. Also, if it is 40 μm or more, there is a risk of desulfurization occurring in the subsequent polishing process, so it is not preferable. It is preferable to set the sintering temperature and sintering holding time so that the crystal grain size is within the appropriate range.

[0050] [2 - 5. Hot Isostatic Pressing (HIP) Treatment Process] In this manufacturing method, after the sintering process, a hot isostatic pressing (HIP) treatment is further performed. As the type of pressurized gas medium used in the HIP treatment process, 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 - 300 MPa, more preferably 100 - 300 MPa. If the pressure is less than 50 MPa, the transparency improvement effect may not be obtained, and if it exceeds 300 MPa, no further transparency improvement can be obtained even if the pressure is increased. It is convenient and preferable that the applied pressure is 196 MPa or less, which can be processed by a commercially available HIP device.

[0051] In addition, the processing temperature in the HIP processing step is set in the range of 1000 to 1780 °C, preferably 1100 to 1730 °C. If the processing temperature exceeds 1780 °C, the risk of oxygen deficiency generation increases, which is not preferable. Also, if the processing temperature is less than 1000 °C, almost no effect of improving the transparency of the sintered body can be obtained. Regarding the holding time at the above processing temperature, there is no particular limitation, 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.

[0052] Note that the heater material, heat insulating material, and processing container for HIP processing are not particularly limited, but graphite, or molybdenum (Mo), tungsten (W), platinum (Pt) can be preferably used, and yttrium oxide and gadolinium oxide can also be preferably used as the processing container. Particularly when the processing temperature is 1500 °C or lower, platinum (Pt) can be used as the heater material, heat insulating material, and processing container, and the pressurized gas medium can be Ar - O2, so it is preferable because the generation of oxygen deficiency during HIP processing can be prevented.

[0053] When the processing temperature is 1500 °C or higher, graphite is preferable as the heater material and heat insulating material. In this case, any one of graphite, molybdenum (Mo), and tungsten (W) is selected as the processing container, and further, any one of yttrium oxide and gadolinium oxide is selected as the double container inside it, and then the container is filled with an oxygen releasing material. This is preferable because the amount of oxygen deficiency generated during HIP processing can be suppressed to be as small as possible.

[0054] Note that after the HIP processing step, in order to achieve further low scattering, a sintering step may be performed again, and then the HIP processing step may be performed again. Regarding the number of times of the sintering step and the HIP processing step, there is no particular limitation, and it may be repeated until low scattering is achieved.

[0055] [2 - 6. Annealing Process] In this manufacturing method, after the HIP treatment process, oxygen deficiency may occur in the obtained transparent sintered body, and it may exhibit a slightly light gray appearance. In that case, it is preferable to perform annealing treatment (oxygen deficiency recovery treatment) in an oxygen atmosphere or an air atmosphere at a temperature below the treatment temperature in the HIP treatment process, typically 1000 to 1500 °C. The holding time in this case is not particularly limited, but it may be carried out for a time sufficient for recovering the oxygen deficiency, preferably 10 hours or more, more preferably 20 hours or more.

[0056] By such annealing treatment, even if the ceramic sintered body exhibits a slightly light gray appearance in the HIP treatment process, it is possible to obtain a completely colorless and transparent transparent ceramic with less defect absorption.

[0057] [2-7. Optical Polishing] In this manufacturing method, for the transparent ceramic obtained through the above series of processes, both end faces on the axis for its optical use are optically polished to form optical surfaces. When the accuracy of the optical surface 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 Ra of the optical surface obtained by polishing is set to 1 nm or less. Note that it is also possible to further reduce the optical loss by appropriately forming an antireflection film on the polished optical surface. Note that the surface roughness Ra represents the arithmetic mean roughness of the contour curve as specified in JIS B 0601.

[0058] As described above, it is possible to manufacture a transparent ceramic that satisfies the above formula (1), and to provide a truly transparent ceramic that is less likely to cause laser damage.

[0059] [3. Magneto-Optical Device] When the above-described transparent ceramics are used as a magneto-optical element, the magneto-optical device of the present embodiment is configured using the above transparent ceramics. Specifically, it is preferable to configure and use a magneto-optical device by applying a magnetic field parallel to the optical axis to the above transparent ceramics and then setting a polarizer and an analyzer so that their optical axes are shifted by 45 degrees from each other. In particular, the above transparent ceramics are preferably used 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, and includes a magneto-optical element made of the above-described paramagnetic garnet-type transparent ceramics as a Faraday rotator. As shown in FIG. 1, the optical isolator 100 includes, inside its housing 102, a Faraday rotator 110 made of the above-described paramagnetic garnet-type transparent ceramics, a polarizer 120 made of a polarizing 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 plane of the polarizer 120 and the polarization vibration plane of the analyzer 130 are arranged so 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 surface of the side surface of the Faraday rotator 110 in the housing 102.

[0061] Such an optical isolator 100 can be suitably used for 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 with reference to examples and comparative examples, but the present invention is not limited to the examples.

[0063] [Examples 1 to 5, Comparative Examples 1 to 3] Terbium oxide powder (Tb4O7, manufactured by Shin-Etsu Chemical Co., Ltd.) with a purity of 99.999% or more, yttrium oxide powder (Y2O3, manufactured by Shin-Etsu Chemical Co., Ltd.), and aluminum oxide powder (Al2O3, manufactured by Daming Chemical Co., Ltd.) were weighed in predetermined amounts so as to obtain the compositions of the composite oxides of Examples 1 to 5 and Comparative Examples 1 to 4 shown in Table 1, and mixed. Further, tetraethoxysilane (Si(OEt)4, manufactured by Kishida Chemical Co., Ltd., hereinafter referred to as "TEOS") was added as a sintering aid so as to be 1000 ppm in terms of Si in the obtained transparent ceramics.

[0064] Then, using ethanol (manufactured by Kanto Chemical Co., Ltd.) as a dispersion medium, this mixed raw material was subjected to wet ball milling treatment. The media of the ball mill used were 2 mm alumina balls (manufactured by Nikkato Corporation). To the slurry obtained by the ball milling treatment, 1 wt% of polyvinyl alcohol (manufactured by Kanto Chemical Co., Ltd.) was added as a binder, and granulated by spray drying. Uniaxial press molding and CIP treatment were performed on the obtained granules so as to have a predetermined shape, and degreased in air at 500 °C in a muffle furnace. Subsequently, vacuum sintering treatment (10 -3 Pa, 1500 to 1700 °C) and subsequent HIP treatment (198 MPa, 1500 to 1700 °C) were performed, and air annealing treatment was carried out at 1450 °C for 10 hours. The obtained transparent body was optically polished and processed so as to have a diameter of 5 mm × length of 20 mm, an optical surface accuracy of λ / 8, and a surface roughness Ra of the optical surface of 1 nm or less to obtain a ceramic sample.

[0065] Regarding the ceramic sample thus obtained, the insertion loss, extinction ratio, Verdet constant, and grayscale value by image analysis were measured. Note that an antireflection film was formed during the measurement of the insertion loss, but the measurement was performed without forming an antireflection film during other measurements. The antireflection film was adjusted according to the measurement wavelength of the insertion loss. Also, the laser damage of the ceramic sample was observed.

[0066] (Measurement of insertion loss) The insertion loss was measured by using a light source manufactured by NKT Photonics, and an in-house optical system using a power meter and a Ge-amplified photodetector manufactured by Gentec. The light intensity was measured when light with a wavelength of 1064 nm was transmitted through a beam diameter of 1 to 3 mmφ, and was obtained based on the following formula. During the measurement, the sample was rotated, and the maximum value obtained at that time was taken as the measured value. Insertion loss [dB] = 10 × log(I / I0) (sample length 20 mm) (In the formula, I represents the transmitted light intensity (the intensity of light that has linearly passed through a 20-mm-long sample), and I0 represents the incident light intensity.)

[0067] (Measurement of extinction ratio) The extinction ratio was measured by using a light source manufactured by NKT Photonics and an in-house optical system using a collimator lens, a polarizer, a work stage, an analyzer, a power meter manufactured by Gentec, and a Ge photodetector. With the light with a wavelength of 1064 nm set to a beam diameter of 3 mmφ, the light was transmitted through the sample. In this state, the light intensity I0' was measured when the polarization plane of the analyzer was made to coincide with the polarization plane of the polarizer. Subsequently, after rotating the polarization plane of the analyzer by 90 degrees to make it orthogonal to the polarization plane of the polarizer, the received light intensity I' was measured again, and then it was calculated based on the following formula. Extinction ratio (dB) = -10 × log 10 (I’ / I0’)

[0068] (Measurement of Verdet constant) First, a ceramic sample was mounted on the optical isolator shown in Fig. 1. That is, each obtained 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. After inserting polarizers at both ends thereof, a high-power laser (beam diameter: 1.6 mm) manufactured by IPG Photonics Japan Corporation was used to irradiate high-power laser light with a wavelength of 1064 nm from both end faces to determine the Faraday rotation angle θ. The Faraday rotation angle θ was defined as the angle that exhibits the maximum transmittance when the polarizer on the output side is rotated. Then, the Verdet constant was calculated based on the following formula. Note that the magnitude (H) of the magnetic field applied to the sample was a value calculated by simulation from the dimensions of the above measurement system, the residual magnetic flux density (Br), and the coercive force (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 (in this case, 0.020 m).)

[0069] (Measurement of grayscale value) In a 400 lux fluorescent lamp illumination environment, using a microscope camera and lens manufactured by Thorlabs, the background was made of glass coated with an ultra-low reflection paint (S-MUSOU, manufactured by Shibuya Optics Co., Ltd.). The optical surface of the sample was placed on this black paint-coated glass, and a camera image was acquired from above. Note that in order to accurately obtain the internal information of the sample, the distance and magnification were adjusted so that the number of pixels related to the sample was 5000 or more. The obtained image was grayscaled using image analysis software Image-J to acquire 256-level grayscale information. Then, using Image-J, the grayscale value G B of the background part, the maximum grayscale value G H in the image, and the grayscale average value G T of the part of the sample optical surface were calculated, and the grayscale ratio value represented by the above formula (1) was obtained. Note that the calculation of the grayscale average value G T was performed for the central region of 80% of the diameter of the optical surface.

[0070] (Observation of laser damage) Using the parameters of the power E (W), pulse width τ (ns), repetition frequency H (Hz), and incident beam diameter D (μm) of the light source laser that outputs at a wavelength of 1064 nm, the full fluence F converted to a pulse width of 10 ns is 20 J / cm² according to the following formula. 2 Adjust the optical system so that this condition is met. F = (2 × E / H) ÷ (π × ((D × 10 ―4 ⁻³) / 2) 2 )² × √(10 / τ) (J / cm²) 2 )

[0071] Then, a laser of 10 J / cm² 2 was incident on one sample optical surface, transmitted through it, and the laser beam emerging from the other optical surface was monitored with a power meter installed downstream of the sample. If the laser was continuously applied to the sample for 1 minute and 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. If a decrease in laser intensity was observed even once, it was determined that laser damage had occurred.

[0072] The above measurement results and observation results are shown in Table 1. As in Examples 1 to 4, when the grayscale ratio value was 30% or less, it was found that no laser damage occurred and it functioned well as an isolator material. On the other hand, as in Comparative Examples 1 to 3, when the grayscale ratio value exceeded 30%, not only laser damage but also insertion loss and extinction ratio deteriorated, making it difficult to use as an isolator. In the claims shown in Table 1 and the comparative examples, the Verdet constant was 52 rad / T·m for all of them.

[0073]

Table 1

[0074] [Examples 5 to 10] Transparent ceramics of Examples 5 to 10 were produced in the same manner as in Example 1, except that the composition of the transparent ceramics was changed to the composition shown in Table 2. The sintering temperature and HIP temperature were finely adjusted according to the composition. Similar to Examples 1 to 4, when the grayscale ratio value was 30% or less in Examples 5 to 10, good stability against laser damage was obtained. That is, it was found that truly transparent ceramics could be obtained even with the compositions shown in Examples 5 to 10.

[0075]

Table 2

[0076] [Comparative Example 4] Transparent ceramics were produced in the same procedure as in Example 1, but a sample with a surface roughness Ra of 6 nm on the optical surface was used as Comparative Example 4. The grayscale ratio value of this sample was 37%. There was a tendency for the grayscale ratio value to deteriorate due to the influence of polishing of the optical surface. Furthermore, when a high-power laser with a wavelength of 1064 nm and 20 J / cm 2 was incident on this sample, laser damage occurred on the optical surface, and it was found that it was difficult to use it as an isolator.

[0077] Although the present invention has been described with the above embodiments, the present invention is not limited to the above embodiments, and can be changed within the scope that those skilled in the art can conceive, such as other embodiments, addition, modification, deletion, etc. 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 Signs

[0078] 100 Optical isolator 102 Housing 104 Optical axis 110 Faraday rotator 120 Polarizer 130 Analyzer 140 Magnet

Claims

1. A cylindrical transparent ceramic having a surface roughness Ra of 1 nm or less on the optical surfaces at both ends thereof, having a sample length of 20 mm, with the optical surface of the sample placed stationary on a black background, and when an image of the optical surface is acquired in 256 - gradation gray scale, in the said image, the gray scale value of the background portion is G Ｂ , the highest gray scale value is G Ｈ , and the average gray scale value of the central region of 20 - 80% of the diameter of the said optical surface is G Ｔ is defined, and a transparent ceramic satisfying the following formula (1). (G Ｔ - G Ｂ ) / (G Ｈ - G Ｂ ) × 100 ≦ 30% ··· (1)

2. The transparent ceramic according to Claim 1, wherein the crystal structure of the said transparent ceramic is either garnet or C - type rare earth.

3. The transparent ceramic according to Claim 1 or 2, wherein the composition of the said transparent ceramic is represented by the following formula (3). (R １－ｘ Sc ｘ ) ３ (Al １－ｚ Sc ｚ ) ５ O １２ ··· (3) (In the formula, 0 ≦ x ≦ 0.04, 0 ≦ z ≦ 0.04, 0 ≦ x + z < 0.045, and R contains any one or more elements of Y and elements with atomic numbers 65 - 71.)

4. The transparent ceramic according to Claim 3, wherein R in the said formula (3) is Tb.

5. The transparent ceramic according to Claim 1 or 2, having an extinction ratio of 40 dB or more.

6. A magneto - optical device configured by using the transparent ceramic according to Claim 1 or 2 as a magneto - optical material.

7. The magneto-optical device according to claim 6, which comprises the above transparent ceramics as a Faraday rotator, and has polarization materials provided before and after on the optical axis of the Faraday rotator, and is an optical isolator that can be used at a wavelength of 0.9 μm or more and 1.1 μm or less.

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