Tb-containing rare earth-aluminum garnet-type ceramics and their manufacturing method

Abstract

[Problem] To provide a Tb-containing rare earth-aluminum garnet ceramic which: has a Verdet constant of substantially the same level as that of a TGG single crystal used in an isolator; has an insertion loss and extinction ratio equal to or greater than those of a TGG single crystal; generates less heat when a high-power laser is applied thereto; and is less likely to cause a thermal lens effect or thermal birefringence. [Solution] The present invention relates to: a Tb-containing rare earth-aluminum garnet ceramic characterized by containing a garnet polycrystal represented by the compositional formula (TbxRe1-x)3(AlySc1-y)5O12 (therein, Re represents at least one among Y and Lu, x=1.0-0.5, and y=1.0-0.6), and containing Si and at least one of Ca and Mg; a method for producing the same; and an isolator device using the ceramic.

Description

[Technical Field]

[0001] The present invention relates to novel Tb-containing rare earth-aluminum garnet-type ceramics and a method for producing the same. [Background technology]

[0002] The most promising material capable of polarizing light (Faraday rotation) in the 1 μm to visible region is Tb-containing garnet. Among these, TGG (Tb3Ga5O 12 ) can be used as an element with the Faraday effect because relatively large materials can be produced using the Czochralski method (CZ method).

[0003] In particular, the Verdet constant of the above material in the 1 μm range is 36 radT -1 m -1 The insertion loss is relatively large, at around 0.1 to 0.2 dB when the medium length is 20 mm under a 1 T magnetic field, and it can also be used as an isolator for fiber lasers used in processing, making it a mainstream material in recent years.

[0004] In 1995, it was proven that ceramics (polycrystalline) with grain boundaries could be given performance close to that of single crystals. Regarding TGG ceramics, materials with relatively low optical loss have also been reported (Non-Patent Document 1), but they have the drawback of being inferior to single crystals in the short wavelength range below the visible range, and as a result, they also have the drawback of having a slightly low laser damage threshold.

[0005] TAG (Tb3Al5O 12 ) single crystals (Non-Patent Documents 2, 3) are promising, but this material cannot be produced by the CZ method due to problems with the phase diagram, and relies on the floating zone method (FZ method), which allows only small crystals to be synthesized. Furthermore, the FZ method has problems with crystal size and makes it difficult to produce high-quality materials, making it impossible to apply this single crystal as an industrial material.

[0006] To solve this problem, ceramic technology has been used to synthesize transparent materials using SiO2 (SiO2 obtained by decomposition of tetraethoxysilane (TEOS)) alone or TEOS + MgO as a sintering aid (Non-Patent Documents 4, 6).

[0007] Non-Patent Document 4 reports that transparent TAG ceramics were synthesized by adding TEOS, but the material's structure is very heterogeneous, with many voids or impurity phases precipitated inside the material, and the transmittance of a 2.4 mm thick sample is extremely low at 15 to 70%, making it unsuitable for use as a Faraday element.

[0008] In Non-Patent Document 6, TEOS and MgO are used as sintering aids, but the transmittance of the sample (1.5 mm thick) is about 70 to 80%, and the optical loss described in this paper is several hundred to several thousand times greater than the practical level, making it unsuitable for optical applications.

[0009] On the other hand, by adding a small amount of TEOS (SiO2) as a sintering aid, (Tb x Y 1-x )3AlO 12 For the first time, ceramics with optical properties comparable to those of single-crystal TGG have been obtained from ceramics with x = 0.5 to 1.0 (Non-Patent Document 5). The Verdet constant of this material changes with increasing Tb concentration in the garnet, and is higher at x = 1.00 (i.e., TbAlO 12 The Verdet constant for the wavelength of 1064 nm is 60 radT. -1 m -1 A material with a higher Verdet constant has the advantage of being able to reduce the magnetic capacitance that constitutes the isolator device.

[0010] In Patent Document 1 or Patent Document 2 (hereinafter, both are collectively referred to as "Patent Document 1, etc."), Re3Al5O 12In a ceramic material represented by the formula (Re is an element with atomic number 65 to 71), a Faraday element is synthesized by adding Si and Y as sintering aids, or Si and Lu as sintering aids. In Patent Document 1 and the like, the optical properties required for an isolator are achieved, as in Non-Patent Document 5, but since Si is a sintering aid, heat generation during laser irradiation cannot be guaranteed. Note that Patent Document 1 and the like describe the addition of Si and Y as sintering aids, or Si and Lu as sintering aids, but both are not necessarily sintered in Re3Al5O 12 It cannot be said that Si behaves as a sintering aid, and in fact only Si acts as a sintering aid.

[0011] For Faraday elements, low insertion loss (optical loss when a magnetic field is applied to a material and laser light passing through the medium is polarized) and a large extinction ratio are important, but suppressing heat generation is even more important for practical use. This is because, while the purpose of an isolator device using a Faraday element is to shut down the returning laser wave, the laser light passing through the Faraday element has a certain amount of power. As the laser power increases, heat is generated inside the material, and this heat causes the thermal lens effect or thermal birefringence, which changes the performance of the emitted laser, resulting in problems such as reduced beam quality and fluctuations in focal length, and ultimately causing it to lose its function as a processing laser.

[0012] Here, the thermal lens effect refers to the phenomenon in which heat generation due to the above-mentioned reasons causes a temperature distribution, in other words, a refractive index distribution, within the medium, resulting in a deterioration in beam quality and a change in focal length. Thermal birefringence is the occurrence of birefringence in the medium due to heat generation by the laser (which is basically unlikely to exist in cubic crystalline materials that do not generate heat), which reduces the extinction ratio of the material and reduces the laser's shutdown ability.

[0013] The optical loss of TGG single crystal is about 0.2% / cm, but this value includes not only scattering but also absorption by the host material. If laser light absorption and heat generation occur inside the host material, the refractive index will change with the temperature rise of the material, dn / dT=1.8×10 -5 K, which is large, and the thermal lens effect easily occurs. However, the light source for communication that uses optical fiber as a transmission source has a wavelength of 1.5 μm and a power level of several tens of mW, so the heat generation of the isolator is extremely small and does not pose a practical problem.

[0014] In contrast, fiber lasers used in the visible to 1 μm band or general solid-state lasers that use crystalline materials as their oscillation source have high output, some reaching the kW level. Heat generation in such fiber or solid-state laser isolators reduces the extinction ratio, which is the isolator's lifeline (i.e., thermal birefringence drastically reduces the ability to shut down laser light). Furthermore, the non-uniform refractive index caused by the temperature distribution inside the medium causes the focused spot of the laser light to fluctuate. This results in serious problems in laser applications.

[0015] For example, when the above-mentioned TGG single crystal is irradiated with a laser of about 50 W or more emitted from a fiber laser for processing purposes, a significant thermal lens effect occurs, causing problems that make processing difficult. The ceramic materials described in Patent Document 1 etc. or Non-Patent Document 5 have the advantage of a slightly larger Verdet constant and have optical properties comparable to those of TGG single crystal, but the problem of heat generation has not been fundamentally solved.

[0016] The reason for this is unclear, but it is thought to be due to the following mechanism of action: 4+ ) located at the tetrahedral coordination site of rare earth aluminum garnet during the sintering process. 3+ However, due to the difference in valence, a cation defect is formed. It is believed that this cation defect absorbs the laser light, generating heat and causing the thermal birefringence or thermal lens effect. [Prior art documents] [Patent documents]

[0017]

Patent Document 1

Patent document 2

Non-licensed literature

[0018] [Non-licensed document 1] H. Yoshida, K. Tsubakimoto et. al, "Optical Properties and Faraday , Optics Express, vol. 19, No.16, 15181-87(2011). [Non-licensed document 2] M. Geho, T. Sekijima, T. Fujii, “Growth of Terbium Aluminum Garnet (Tb3Al5O12:TAG) Single Crystals by the Hybrid Laser Floating Zone Method”, J. Cryst. Growth, 267:188-793 (2004). [Non-licensed document 3] Takashi Fujii and Mikio Shita, "Evaluation of photometric properties of Tb3Al5O12 single crystal growth based on the effect of Fiberglass", New Glass, vol. 18, No.4, 32-36 (2003).

Non-licensed Document 4

Non-licensed Document 5

[0019] Thus, there is a new demand for TGG ceramics and the like to reduce problems such as the thermal lens effect and thermal birefringence that arise from the increased output of fiber lasers, but promising materials that can solve these problems have not yet been provided.

[0020] Therefore, the main object of the present invention is to provide a Tb-containing rare earth-aluminum garnet-type ceramic that has a Verdet constant similar to that of the TGG single crystal used in isolators, an insertion loss and extinction ratio equal to or greater than those of the TGG single crystal, and further generates less heat when a high-power laser is used, and is less likely to cause the thermal lens effect or thermal birefringence. [Means for solving the problem]

[0021] As a result of extensive research into the problems of the prior art, the inventors discovered that the above-mentioned object can be achieved because a material obtained by a specific manufacturing method has a unique structure, and thus completed the present invention.

[0022] That is, the present invention relates to the following Tb-containing rare earth-aluminum garnet-type ceramics and a method for producing the same. 1. Composition formula (Tb x Re 1-x )3(Al y Sc 1-y )5O 12 (wherein Re represents at least one of Y and Lu, x=1.0 to 0.5, and y=1.0 to 0.6), and further comprising at least one of Ca and Mg, and Si. 2. The Tb-containing rare earth-aluminum garnet-type ceramic according to item 1, wherein the contents of Si, Ca, and Mg (converted into oxides) are 50 to 500 ppm by weight as SiO2, and the total content of CaO and MgO is 100 to 2000 ppm by weight. 3. The Tb-containing rare earth-aluminum garnet-type ceramic according to item 1, wherein the weight ratio of Si, Ca and Mg (as calculated as oxides) [(CaO+MgO) / SiO2] is 1 or more. 4. The Tb-containing rare earth-aluminum garnet-type ceramics according to item 1, having an average crystal grain size in the range of 1 to 30 μm. 5. The Tb-containing rare earth-aluminum garnet-type ceramic according to item 1, having a relative density of 99.999% or more. 6. The Tb-containing rare earth-aluminum garnet-type ceramic according to item 1, in which substantially no crystalline phase other than the garnet-type polycrystalline body and no amorphous phase are detected. 7. The Tb-containing rare earth-aluminum garnet-type ceramic according to item 1, wherein, at a thickness t at which light of a wavelength λ nm (where 1200≧λ≧550) undergoes 45° Faraday rotation, the insertion loss for said light is 0.2 dB or less and the extinction ratio is 30 dB or more. 8. A method for producing Tb-containing rare earth-aluminum garnet-type ceramics, comprising the steps of: (1) (1-1) A powdered mixed raw material containing (a) terbium oxide having an average primary particle size of 1 μm or less, (b) aluminum oxide having an average primary particle size of 1 μm or less, (c) at least one of a calcium compound having an average primary particle size of 1 μm or less and a magnesium compound having an average primary particle size of 1 μm or less, and (d) a silicon compound having an average primary particle size of 1 μm or less, and having a stoichiometric composition of a garnet composition; or (1-2) (a) The average primary particle size is 1 μm or less (Tb x Re 1-x )3(Al y Sc 1-y )5O 12 (b) an oxide of Re (where Re represents at least one of Y and Lu, x = 1.0 to 0.5, and y = 1.0 to 0.6), (b) at least one of a calcium compound having an average primary particle size of 1 μm or less and a magnesium compound having an average primary particle size of 1 μm or less, and (c) a silicon compound having an average primary particle size of 1 μm or less, and having a stoichiometric composition of a garnet composition. in an alcohol to obtain a mixture; (2) a step of press-molding the mixture to obtain a green compact; (3) pre-sintering the green compact at a temperature of 1450 to 1600°C to obtain a pre-sintered body having a relative density of 95 to 99%; (4) subjecting the pre-sintered body to HIP treatment at a temperature of 1500 to 1680°C and a pressure of 49 to 196 MPa; and (5) Annealing the sintered body obtained by the HIP treatment at 1200 to 1500°C A method for producing Tb-containing rare earth-aluminum garnet-type ceramics, comprising: 9. The manufacturing method according to item 8, wherein Tb2O3 is used as the terbium oxide. 10. A Faraday element comprising the Tb-containing rare earth-aluminum garnet-type ceramic according to any one of items 1 to 7. 11. An optical isolator device including the Faraday element according to item 10. [Effects of the Invention]

[0023] According to the present invention, it is possible to provide a Tb-containing rare earth-aluminum garnet-type ceramic that has a Verdet constant similar to that of the TGG single crystal used in isolators, an insertion loss and an extinction ratio equal to or greater than those of the TGG single crystal, and further generates less heat when a high-power laser is used, and is less likely to cause the thermal lens effect or thermal birefringence.

[0024] The present invention provides a high-performance Faraday element for high-power lasers in the visible to near-infrared region. As a result, it is possible to provide an isolator with excellent protection performance in the high-power region, which cannot be achieved with TGG single crystals. Of course, it goes without saying that the Faraday element made from the ceramic of the present invention can also be applied to lasers with relatively low power.

[0025] The material of the present invention, which has a Tb concentration of 65% or more, has a Verdet constant equal to or greater than that of a TGG single crystal, which allows the medium length to be shortened or the magnets used to be reduced by up to 40%, making it possible to manufacture downsized isolator devices, and miniaturization could trigger market expansion.

[0026] Furthermore, the ceramics of the present invention do not have the absorption of TGG single crystals, so their basic performance is more suitable for high-power applications than TGG. By adding SiO2 as described in Patent Document 1 or Non-Patent Document 5, a low insertion loss of about 0.1 dB and a low extinction ratio are obtained, but this is only measured using a laser with a wavelength of 1 μm and a power of about 10 mW to measure the basic characteristics. When SiO2 is added to Tb-containing rare earth-aluminum garnet-type ceramics, Si 4+ The ions eventually reach the Al 3+This results in the substitution of ions with cation defects, forming cation defects. When laser light is irradiated onto Tb-containing rare earth-aluminum garnet-type ceramics with a defect structure, heat generation occurs, resulting in a thermal lens effect or thermal birefringence, although this is an improvement over TGG. In this invention, based on the finding that the addition of SiO2 alone is insufficient in terms of the overall characteristics of the Faraday element, it has been discovered that heat generation can be effectively suppressed by adding other specific elements in combination, and as a result, a material optimal for low- to high-power lasers has been successfully provided.

[0027] As described above, the transparent ceramics of the present invention have low insertion loss comparable to or superior to that of TGG single crystals obtained by the conventional CZ method, and can exhibit a high extinction ratio that was difficult to achieve with conventional technology. Furthermore, their greatest feature is that they can handle high power, demonstrating properties that are applicable to industry.

[0028] Furthermore, the manufacturing method of the present invention can be carried out basically in a manner similar to that of ceramic manufacturing methods, except for the specific conditions that are set. Therefore, it is possible to produce large-sized media that have been difficult to produce using the FZ method until now, and since it is also suitable for mass production, it is suitable for industrial-scale manufacturing. [Brief explanation of the drawings]

[0029] [Figure 1] Figure 1(a-1) shows a reflection microscope photograph of the ceramic of the present invention (stoichiometric composition). Figure 1(a-2) shows a reflection microscope photograph of a ceramic with an Al2O3-rich composition. Figure 1(b-1) shows a transmission polarizing microscope photograph of the ceramic of the present invention. Figure 1(b-2) shows a transmission polarizing microscope photograph of a ceramic with a Re2O3-rich composition. Figure 1(b-3) shows a transmission polarizing microscope photograph of a ceramic with residual pores having an average diameter of 1 μm. Figure 1(b-4) shows a transmission polarizing microscope photograph of a ceramic with residual pores of several to 10 μm. Figure 1(b-5) shows a transmission polarizing microscope photograph of a ceramic with inclusions remaining due to non-uniformity during manufacturing. [Figure 2]Figure 2(a) shows ceramics with a grain boundary phase. Figure 2(b) shows ceramics with clean grain boundaries. (Both ceramics with the (Tb0.80Y0.20)3Al5O12) composition were observed with a transmission electron microscope at 200,000 magnifications.) [Figure 3] This is a schematic diagram showing the method for evaluating magneto-optical properties. In this measurement, a 20 mm long TGG single crystal and a 13 mm long TAG (Tb3Al5O12) ceramic were used. [Figure 4] The magneto-optical properties of TGG single crystal and (Tb0.65Y0.35)3Al5O12 ceramics are shown. By irradiating the material with laser light under a 1 T magnetic field and measuring the transmission loss versus the rotation angle of the output polarizer, the insertion loss and extinction ratio can be obtained. [Figure 5] A schematic diagram of an isolator device equipped with a Faraday element is shown. A permanent magnet (Nd-FeB) is placed around the Faraday element, and polarizers and apertures are located on the input and output sides of the laser. The device is made of metal. DETAILED DESCRIPTION OF THE INVENTION

[0030] 1. Tb-containing rare earth-aluminum garnet-type ceramics The Tb-containing rare earth-aluminum garnet-type ceramics of the present invention (the ceramics of the present invention) have the composition formula (Tb x Re 1-x )3(Al y Sc 1-y )5O 12 (wherein Re represents at least one of Y and Lu, x=1.0 to 0.5, and y=1.0 to 0.6), and is characterized in that it contains at least one of Ca and Mg, and Si.

[0031] The ceramic of the present invention has a basic composition represented by the formula (Tb x Re 1-x )3(Al y Sc 1-y )5O 12(where Re represents at least one of Y and Lu, x = 1.0 to 0.5, and y = 1.0 to 0.6). The composition substituted with Re and / or Sc has any of the following combinations: a) 0.5≦x<1, 0.6≦y<1, b) 0.5≦x≦1, 0.6≦y<1, c) 0.5≦x<1, 0.6≦y≦1. Note that the above x can be set to, for example, x = 0.99 to 0.5. The above y can be set to, for example, y = 0.99 to 0.6.

[0032] Among the lanthanide rare earth elements, Tb exhibits the greatest Faraday effect in the visible to near-infrared region. For this reason, Dy, Ho, and other elements that exhibit the same Faraday effect, though less so than Tb, have many strong optical absorption bands in the specified wavelength region. Therefore, in the present invention, the element that exhibits the Faraday effect is limited to Tb alone.

[0033] To form a garnet structure, only Y or Lu is selected as other lanthanide elements that can form stable Tb-containing rare earth-aluminum garnet-type ceramics, which do not have an absorption band like Dy, Ho, etc. and have a smaller ionic radius than Tb. In terms of the Verdet constant of the material, Tb3Al5O, where Tb is 100% (x=1), is used. 12 However, the ionic radius of Tb is the largest possible size for forming a rare earth (100% Tb)-aluminum-garnet structure, which can make the ceramics of the present invention somewhat unstable. This is why Y, Lu, and Sc, which have smaller ionic radii than Tb and no absorption bands, are used as some of the rare earth elements for the garnet structure. Furthermore, the addition of Sc results in a more stable TAG than TAG, and there is no absorption in the visible to 1.5 μm wavelength range, which is another reason for its selection.

[0034] The ceramic of the present invention contains at least one of Ca and Mg and Si (hereinafter, these are also collectively referred to as "additives"). By employing such a specific combination of additives, it is possible to provide a ceramic element that generates very little heat during high-power operation.

[0035] Although Si is an element necessary for the transparency of the ceramics of the present invention, simply adding this element promotes grain growth while also improving transparency, resulting in an increase in residual pores, which are a source of scattering. At the same time, the formation of lattice defects also leads to the absorption of irradiated laser energy and the generation of heat. In contrast, the present invention effectively suppresses or prevents the adverse effects of residual pores or heat generation by using Si in combination with at least one of Ca and Mg.

[0036] Regarding these additives, Patent Document 1 and Non-Patent Document 5 achieve the optical properties required for isolators, but essentially only Si is used as an additive, which fails to suppress heat generation. The fundamental difference with the present invention is the discovery that, in addition to Si, which is effective in making materials transparent, it is important to use Mg and / or Ca in combination, which homogenize the material's structure, further improving optical properties, while simultaneously suppressing heat generation. The sintering aids of the present invention are essentially substituted into the lattice of Tb-containing rare earth-aluminum-garnet ceramics during the sintering process, and therefore do not become a source of light scattering, resulting in extremely low insertion loss. Therefore, it is preferable that the present invention does not include, for example, a combination of Si with Y or Lu as an additive.

[0037] The content of the additives is not limited and can be set appropriately depending on, for example, the composition shown in the composition formula above, the desired physical properties, etc., but it is usually desirable to set it as follows in oxide equivalent: Si is 50 to 500 weight ppm as SiO2 content, and particularly 100 to 500 weight ppm. Furthermore, the total content of Ca and Mg as CaO and MgO is 100 to 2000 weight ppm, and particularly 300 to 2000 weight ppm. This makes it possible to make the material transparent and reliably suppress heat generation, making it possible to provide a ceramic element with higher performance.

[0038] The proportion of the additive is not particularly limited, but it is generally preferable that the weight ratio of (CaO+MgO) / SiO2 calculated as oxide is 1 or more, more preferably 1.5 or more, and most preferably 2.0 or more. The upper limit of the proportion is not limited as long as the Si, which is essential for the transparency of the material, is contained in an SiO2 content of 50 weight ppm or more, but can be, for example, about 6. By setting the proportion within the above range, it can be used more reliably even in a high power range of, for example, 200 W or more.

[0039] The ceramic of the present invention is composed of a garnet-type polycrystalline body having the above composition, and its purity, excluding additive components such as sintering aids, is 99.8% by weight or more (preferably 99.9% by weight or more).

[0040] In particular, in the ceramics of the present invention, it is preferable that no crystalline phase or amorphous phase (hereinafter, both of which are collectively referred to as "second phase") other than the garnet-type polycrystalline body is substantially detected. Typical examples of the second crystalline phase include a corundum phase (hexagonal) or a perovskite phase (tetragonal) containing the elements (cations and oxygen ions) that constitute the ceramics of the present invention. Furthermore, examples of phases that appear at the grain boundaries include, in addition to the corundum phase, a crystalline phase or an amorphous phase composed of an oxide of the elements (cations) that constitute the ceramics of the present invention. In the present invention, the above-mentioned "not detected" means that the second phase cannot be confirmed by either 1) analysis using a transmission polarizing microscope or 2) analysis using a high-resolution transmission electron microscope (HR-TEM).

[0041] What is particularly important about the present invention is that the composition of the ceramic of the present invention is nearly identical to the stoichiometric composition, and not only is no phase other than the garnet crystal phase substantially detected inside the ceramic of the present invention, but also that because of the garnet structure, i.e., the cubic crystal structure, even birefringence is substantially not detected.

[0042] The ceramic of the present invention is a polycrystalline body, and its average crystal grain size is preferably 1 to 30 μm. If the average crystal grain size is too small, scattering may tend to increase as the laser wavelength used becomes shorter. If the average crystal grain size is too large, pores, which are the largest source of scattering, may easily remain inside the particles.

[0043] The ceramic of the present invention typically has a porosity of 20 ppm or less, preferably 10 ppm or less, and even more preferably 1 ppm or less. That is, the ceramic of the present invention has high density (particularly a relative density of 99.999% or more), and therefore can exhibit excellent optical properties. While the refractive index of residual pores is 1.00, the refractive index of the ceramic of the present invention is approximately 1.85. Therefore, the high density reduces the number of scatterers, which in turn contributes to reducing insertion loss in magneto-optical properties.

[0044] Furthermore, when the ceramic of the present invention contains residual pores, the average pore diameter is not particularly limited, but is generally desirably 1 μm or less, and more desirably 0.5 μm or less. If the average pore diameter is too large, it may cause damage when a high-power laser, especially a pulsed laser with a high peak power, is inserted into the Faraday element. The average pore diameter can be determined, for example, using a transmission microscope. The number of scatterers (number of residual pores) can be determined by measuring the number of scatterers using laser irradiation, and the pore volume can be calculated from the result of this scatterer number and the average pore diameter.

[0045] The ceramic of the present invention preferably has an insertion loss of 0.2 dB or less and an extinction ratio of 30 dB or more at a thickness t at which light of wavelength λ nm (where 1200 ≧ λ ≧ 550) undergoes 45° Faraday rotation. Because the ceramic of the present invention contains a relatively large amount of Tb, the transmission wavelength band of the Faraday element is 550 to 1400 nm. It can be practically used within this range. However, as the Verdet constant decreases at longer wavelengths, the element length becomes longer, and there are no particular applications for wavelengths of 1200 to 1400 nm, so the wavelength is limited to this range.

[0046] Here, the thickness (distance through which light passes) of the ceramic of the present invention as a measurement sample is defined as the thickness t at which the light undergoes a 45-degree Faraday rotation. For example, at a wavelength of 1064 nm, the Verdet constant is 36 radT. -1 m -1 When a sample of this material is placed in a magnetic field of 1 T, the medium length is approximately 20 mm. (Faraday equation: θ=VHL (where θ is the Faraday rotation angle, V is the Verdet constant, H is the magnetic flux density, and L is the length of the Faraday element).) The ceramic material of the present invention, which has the above-mentioned properties, can exhibit performance equivalent to or superior to that of the TGG single crystal material, which has been the mainstream so far, in terms of insertion loss and extinction ratio, which are basic characteristics of isolators.

[0047] The insertion loss is typically 0.2 dB or less, and preferably 0.1 dB or less. A low insertion loss is important because it allows the laser light energy entering the material to be emitted without loss in a strong magnetic field. To measure basic performance, the insertion loss of a TGG single crystal was measured using a low-power (10 mW) YAG laser (wavelength 1.064 μm) in a strong magnetic field of 1 T, and was found to be approximately 0.1 dB (0.05 dB at the lowest loss point). It is not difficult to achieve an insertion loss of approximately 0.1 dB, equivalent to that of a single crystal, with the ceramic of the present invention. By optimizing the material synthesis, it is possible to achieve an insertion loss of approximately 0.02 dB or less (which is difficult with TGG single crystals). Therefore, the lower limit of the insertion loss can be set to approximately 0.002 dB, but this is not limiting.

[0048] Furthermore, the extinction ratio, which is the ability to shut down the return wave of the laser, is usually 30 dB or more, and preferably 40 dB or more. Since an isolator device can demonstrate the ability to shut down the return wave of the laser as a protection system for a laser oscillator, the high extinction ratio is an important parameter that indicates the degree of completion of the protection performance. Generally, the extinction ratio of a TGG single crystal is about 30 dB, while the ceramic of the present invention can achieve an extinction ratio of 34 to 47 dB (measurement limit). Therefore, the upper limit of the extinction ratio can be, for example, about 47 dB, but is not limited to this.

[0049] The extinction ratio is an important factor in protection systems. Its value easily varies depending on the laser's incident power. In industrial applications, most isolators used in the visible to near-infrared wavelength range operate in the 1 μm band. When a transparent material is irradiated with laser light, heat is generated within the material as the power increases. This heat generates a temperature distribution within the material, which means a refractive index distribution (thermal lens effect) occurs within the material. The Faraday element itself exhibits a lens effect, causing the focal length of the laser light to fluctuate. If the focal length changes, laser processing becomes difficult and the element's functionality is completely lost. Furthermore, heat generation causes birefringence within the Faraday element (thermal birefringence), which reduces the extinction ratio with laser power. If the extinction ratio becomes extremely low, the material will no longer be able to protect against returning laser waves, potentially damaging the laser oscillator.

[0050] In this regard, the TGG single crystal has a slight optical absorption (a trace of Tb 4+ Because of this, the aforementioned thermal lens effect and thermal birefringence become more pronounced as the laser power increases, making it unsuitable for high-power applications without cooling. If fiber laser light with a wavelength of 1 μm is focused to 300 μm and inserted into an uncooled TGG single crystal, the extinction ratio drops even at around 50 W, making it difficult to protect the laser device. In contrast, the ceramics of the present invention generate very little heat, making the thermal lens effect and thermal birefringence less likely to occur, and therefore can maintain and achieve the extinction ratio described above.

[0051] In the ceramic of the present invention, the Verdet constant, which indicates the strength of the Faraday effect, is usually, but not limited to, 80 to 25 radT. -1 m -1 This value can be controlled by the Tb concentration. For example, 12 The composition that is the lower limit of the Tb concentration (Tb 0.5 Re 0.5 )3AlO 12 So, about 60 to 29 radT -1 m -1 This is because some of the Al sites are replaced by Sc (Tb x Re 1-x )3(Al y Sc 1-y )5O 12 However, although there are some differences, similar trends are observed (i.e., 60-29radT -1 m -1 If the Verdet constant is too small, it is necessary to make the medium length longer in a 1T magnetic field (and to use a long magnet house that matches the medium length), and if you do not want to make the medium length longer, you will need to use a powerful magnet calculated from the Verdet constant.

[0052] The ceramic of the present invention may be either colored and transparent or colorless and transparent, but colorless and transparent is particularly preferred. 4+ Since no coloring due to residual Tb was observed, it is believed that the material is essentially free of Tb 4+ Therefore, the present invention can provide a colorless and transparent ceramic.

[0053] 2. Manufacturing method for Tb-containing rare earth-aluminum garnet-type ceramics The ceramics of the present invention can be suitably produced by the following production method: That is, a method for producing Tb-containing rare earth-aluminum garnet-type ceramics, comprising the steps of: (1) (1-1) A powdered mixed raw material containing (a) terbium oxide having an average primary particle size of 1 μm or less, (b) aluminum oxide having an average primary particle size of 1 μm or less, (c) at least one of a calcium compound having an average primary particle size of 1 μm or less and a magnesium compound having an average primary particle size of 1 μm or less, and (d) a silicon compound having an average primary particle size of 1 μm or less, and having a stoichiometric composition of a garnet composition; or (1-2) (a) The average primary particle size is 1 μm or less (Tb x Re 1-x )3(Al y Sc 1-y )5O 12 (b) an oxide of Re (where Re represents at least one of Y and Lu, x = 1.0 to 0.5, and y = 1.0 to 0.6), (b) at least one of a calcium compound having an average primary particle size of 1 μm or less and a magnesium compound having an average primary particle size of 1 μm or less, and (c) a silicon compound having an average primary particle size of 1 μm or less, and having a stoichiometric composition of a garnet composition. in an alcohol to obtain a mixture (mixing step); (2) a step of obtaining a green compact by press-molding the mixture (molding step); (3) a step of pre-sintering the green compact at a temperature of 1450 to 1600°C to obtain a pre-sintered body having a relative density of 95 to 99% (pre-sintering step); (4) a step of HIP-treating the pre-sintered body at a temperature of 1500 to 1680°C and a pressure of 49 to 196 MPa (HIP treatment step); (5) Annealing the sintered body obtained by the HIP treatment at 1200 to 1500°C (annealing step) The ceramics of the present invention can be suitably produced by the method for producing Tb-containing rare earth-aluminum garnet-type ceramics, which is characterized by comprising the steps of:

[0054] Mixing process First, as the mixing step, either (1-1) or (1-2) above can be adopted depending on the raw materials used to form the basic composition of the ceramic of the present invention.

[0055] In the above (1-1), a powder mixed raw material is used that contains (a) terbium oxide having an average primary particle size of 1 μm or less, (b) aluminum oxide having an average primary particle size of 1 μm or less, (c) at least one of a calcium compound having an average primary particle size of 1 μm or less and a magnesium compound having an average primary particle size of 1 μm or less, and (d) a silicon compound having an average primary particle size of 1 μm or less, and that has a stoichiometric composition of a garnet composition.

[0056] In the above (1-2), instead of the above (a) and (b), (a) the average primary particle diameter is 1 μm or less (Tb x Re 1-x )3(Al y Sc 1-y )5O 12 (Here, Re represents at least one of Y and Lu, x = 1.0 to 0.5, and y = 1.0 to 0.6.) Except for using oxides, it is the same as (1-1) above. The composition substituted with Re and / or Sc has any of the following combinations: a) 0.5≦x<1, 0.6≦y<1; b) 0.5≦x≦1, 0.6≦y<1; c) 0.5≦x<1, 0.6≦y≦1. Note that the above x can be set to, for example, x = 0.99 to 0.5. The above y can be set to, for example, y = 0.99 to 0.6.

[0057] In the above (1-1), oxides of elements constituting the basic composition of the ceramic of the present invention can be suitably used as starting materials for preparing the powder mixed raw material. Therefore, in addition to terbium oxide (Tb4O7, Tb2O3), aluminum oxide (α-Al2O3, γ-Al2O3, etc.), when substituting a portion of the Tb site or Al site, yttrium oxide (YO3), lutetium oxide (Lu2O3), scandium oxide (Sc2O3), etc. are preferably used.

[0058] Although the valence of the elements in these oxides is not particularly limited, it is particularly preferable to use Tb2O3 as the terbium oxide in the present invention. Tb4O7 is a commonly available commercially available Tb source material. While it is possible to use this to produce the ceramics of the present invention that can be used in Faraday elements, it is preferable to use Tb2O3 because it can be produced more uniformly and stably.

[0059] Although commercially available Tb4O7 theoretically has a molar ratio of Tb:O = 4:7, in practice, this ratio is not accurate. Therefore, even when very small amounts are added, variations in the rare earth (Tb) portion of the garnet structure during the preparation of the ceramics of the present invention can occur. This not only hinders the attainment of completely consistent properties, but also increases the risk of partial incorporation of TbO2 clusters. For these reasons, adjusting the composition is difficult when using Tb4O7 as the starting material. However, this can be achieved by (1) determining the exact Tb:O ratio in advance and then fine-tuning it during weighing, or (2) weighing the raw materials at a Tb:O ratio of 4.00:7.00 and then re-adjusting the raw material composition based on feedback from microstructural observations of the fired samples (determining whether they are Al-rich or rare earth-rich). However, this process is complex and requires significant adjustments. In contrast, in the present invention, extensive research and investigation was carried out to achieve more perfect uniformity of the material, including composition adjustment, and it was confirmed that Tb2O3 is the most preferable Tb source, as described above.

[0060] Currently, Tb2O3 is not generally produced by raw material manufacturers. Therefore, it can be used as a starting material by synthesizing Tb2O3. There are no particular limitations on the method for producing Tb2O3, but one method includes a step of heat treating Tb4O7 (black to brown) at 1200 to 1300°C in a reducing atmosphere such as hydrogen or CO gas, which can suitably obtain white Tb2O3 powder. Tb2O3 is a pure Tb 3+ and O 2-Since this powder is composed of Tb2O3, it is possible to produce a sintered body with a more optically uniform structure when producing ceramics with a garnet structure. However, since the Tb2O3 powder obtained by the above heat treatment sinters with other powder particles during the heat treatment, causing strong agglomeration, it is preferable to pulverize it using a ball mill or the like.

[0061] These raw materials are usually used in powder form, and in this case, the average primary particle size is usually 1 μm or less, as described above, and preferably 0.01 to 0.5 μm.

[0062] On the other hand, in the above (1-2), as the Tb supply source and the Al supply source, (Tb x Re 1-x )3(Al y Sc 1-y )5O 12 (wherein Re represents at least one of Y and Lu, x = 1.0 to 0.5, and y = 1.0 to 0.6) oxide (hereinafter referred to as "TbAlO 12 The other points are the same as above (1-1). 12 The b3Al5O system oxide can be prepared by a known method such as coprecipitation. 12 The average primary particle size of the base oxide is also preferably 1 μm or less, and particularly preferably 0.01 to 0.5 μm.

[0063] As the additives in (1-1) and (1-2) above, (c) at least one of a calcium compound having an average primary particle size of 1 μm or less and a magnesium compound having an average primary particle size of 1 μm or less, and (d) at least one of a silicon compound having an average primary particle size of 1 μm or less and a liquid silicon compound are used.

[0064] Any of these compounds can be used as long as it does not impair the effects of the present invention. Calcium compounds include, for example, inorganic compounds such as CaCO3, Ca(OH2), CaO, and CaF2, and organic compounds such as calcium alkoxides. These compounds can be used alone or in combination of two or more. Magnesium compounds include, for example, inorganic compounds such as MgCO3, MgO, Mg(OH)2, and MgF2, and organic compounds such as magnesium alkoxides. These compounds can be used alone or in combination of two or more. Silicon compounds include, for example, inorganic compounds such as SiO2 powder for fillers and colloidal silica, and organic compounds such as alkoxysilanes (e.g., tetraethoxysilane (TEOS) and tetrapropoxysilane (TPOS)). These compounds can be used alone or in combination of two or more.

[0065] Among these, in the present invention, in addition to oxides, compounds that decompose upon heating and ultimately become oxides (particularly hydroxides, carbonates, alkoxides, etc.) can also be suitably used.

[0066] These inorganic and organic compounds are preferably of high purity. For example, the purity of the inorganic compounds is usually 99.8% by weight or more, preferably 99.9% by weight (3N) or more, and more preferably 99.99% by weight (4N) or more. These raw materials themselves can also be commercially available. The purity of the organic compounds can be 99% by weight or more, preferably 99.9% by weight or more.

[0067] These calcium compounds, magnesium compounds and silicon compounds are usually used in powder form, with an average primary particle size of usually 1 μm or less, preferably 0.01 to 0.5 μm, as described above.

[0068] In the present invention, these components are weighed out so as to obtain a stoichiometric composition that matches the garnet composition, and then wet-mixed in alcohol. As the alcohol, industrial alcohol containing ethanol as the main component (e.g., product name "Solmix" (registered trademark, Japan Alcohol Sales Co., Ltd.) is economically effective, but there is no particular limitation, and alcoholic solvents containing primary alcohols such as ethanol and isopropyl alcohol can be suitably used.

[0069] Furthermore, when using a raw material powder (garnet powder) that is not hygroscopic or hydratable, or when using a combination of raw materials that hardly react with water, even in the case of reaction sintering, water may be contained in the alcohol. In other words, in the present invention, not only 100% alcohol but also alcohol diluted with water is included in the "alcohol" of the present invention.

[0070] The amount of alcohol used is not limited, and can usually be about 100 to 300 parts by weight per 100 parts by weight of the powder mixed raw material.

[0071] Furthermore, grinding media (balls) can be used during wet mixing as needed, which effectively prevents particles from agglomerating during mixing, allowing a mixture of fine particles to be prepared.

[0072] As the grinding media, for example, alumina balls with a purity of 99.6% by weight or more, garnet balls composed of the constituent elements of the present invention such as YAG, etc. can be used to minimize contamination from the balls. Furthermore, when grinding media are used, it is preferable to use a container made of synthetic resin for the same reasons as above. Therefore, both the container and grinding media are preferably made of synthetic resins such as fluororesins (e.g., Teflon (registered trademark)) and various engineering plastics, or YAG (Y3Al5O4) with a purity of 99.6% or more composed of the constituent elements of the present invention. 12 A grinding / mixing device using a garnet pot such as a pulverizer or a mixer can also be suitably used. These devices can also be publicly known or commercially available devices.

[0073] When grinding media are used, the amount used can be, for example, 500 to 2000 parts by weight per 100 parts by weight of the powder mixed raw material, but is not limited to this.

[0074] The mixing time may be any time that allows the powder mixed raw material to have a uniform composition, and is usually about 10 to 40 hours, but is not limited to this.

[0075] After the mixing step is completed, the resulting mixture can be collected. The mixture is usually obtained in the form of a slurry. For example, the slurry can be stored in a heat-resistant plastic container, a Teflon (registered trademark) container, or the like. The slurry can be subjected to the molding step as is, but if necessary, the slurry can also be subjected to drying treatment, granulation treatment, or the like to obtain a slurry that can be subjected to the molding step.

[0076] The drying process can include, for example, 1) a step of evaporating the solvent from the slurry at a temperature of 70 to 90°C to obtain a dry powder, and 2) a step of crushing the dried powder and then passing it through a sieve of about 100 mesh to obtain a powder.

[0077] The granulation process can be, for example, a method including the steps of granulating and drying the slurry. Granulation and drying can be carried out by known methods such as stirring granulation, rolling granulation, spray drying, etc. In this case, the average particle size of the granulated product (granules) is not limited, but it is usually desirable to set it to about 10 to 50 μm.

[0078] Molding process In the molding step, the mixture is press-molded to obtain a green compact. The term "the mixture" as used herein refers to the mixture obtained in the above step (without adjusting the moisture content) as well as the mixture with the moisture content adjusted. The solid content (or moisture content) of the mixture can be appropriately adjusted, for example, by the drying process described above.

[0079] The press molding method is not particularly limited, and one or a combination of two or more known methods such as uniaxial press molding, cold isostatic pressing (CIP molding), etc. can be used. For example, after uniaxial press molding as the primary molding, a green compact can be obtained by CIP as the secondary molding.

[0080] The pressure applied during compaction can be set appropriately depending on the particle size and composition of the raw material powder used, but it is generally preferable to apply pressure so that the density of the resulting compact is approximately 40 to 65% of the theoretical density.

[0081] For example, an isolator element for laser emission using a fiber laser or rod can be molded into a round rod shape (e.g., diameter: 8 mm x length: any length), and for slab laser emission applications, it can be molded into a slab shape with a rectangular cross section. For isolators for pumping fiber lasers or solid-state lasers, the shape must be controlled according to the required shape. When using CIP molding, the pressure can be appropriately set within a range of approximately 98 to 198 MPa, but is not limited to this. When molding in two stages as described above, uniaxial press molding can be performed using a mold within a range of approximately 5 to 20 MPa, followed by CIP molding within a range of approximately 98 to 198 MPa.

[0082] The obtained green compact is subjected to a preliminary sintering process. However, if an organic binder or grinding media (especially synthetic resin media) is used in the molding process, it is preferable to remove the organic components by firing the obtained green compact. In this case, the firing conditions (removal of organic substances) are not limited, and can be, for example, about 600 to 1100°C in an oxidizing atmosphere.

[0083] Pre-sintering process In the pre-sintering step, the green compact is pre-sintered at a temperature of 1450 to 1600°C to obtain a pre-sintered body having a relative density of 95 to 99%.

[0084] The pre-sintering temperature is typically 1450 to 1600°C, with 1500 to 1600°C being particularly preferred, but can be adjusted appropriately depending on the sintering characteristics of the raw materials used. Therefore, for example, it can be set to 1500 to 1580°C. If the temperature is too low, even if an insufficiently densified pre-sintered body (a sintered body with many open pores) is HIPed, the pressure cannot be transmitted to the interior, resulting in insufficient densification and insufficient transparency. If the temperature is too high, the constituent particles are too large relative to the residual pore size, and the residual pores cannot be efficiently removed, resulting in insufficient densification and transparency. In the worst case, abnormal grain growth (non-uniform microstructure) can form, further inhibiting densification, non-uniformity, and transparency. The pre-sintering time can be appropriately set depending on the density and particle size of the pre-sintered body, the size of the green compact, etc.

[0085] The heating rate during pre-sintering can be set appropriately depending on, for example, the sinterability and size of the green compact, but is preferably set to 200°C / hr or less, particularly in the temperature range of 1000°C or higher. This makes it possible to more effectively reduce the amount of residual porosity.

[0086] The pre-sintering atmosphere is preferably a hydrogen atmosphere, an oxygen atmosphere, or a vacuum, but is more preferably a vacuum. -1 ~10 -5 It is preferable to set the pressure to about Pa.

[0087] The pre-sintered body thus obtained is subjected to a HIP treatment process. In this case, the pre-sintered body has a relative density of 95 to 99%, preferably 98 to 99.9%. Furthermore, it is desirable that the average crystal grain size of the pre-sintered body be 5 μm or less. These characteristics can be more reliably obtained by pre-sintering within the above-mentioned range of conditions.

[0088] HIP treatment process In the HIP treatment step, the pre-sintered body is subjected to HIP treatment at a temperature of 1500 to 1680° C. and a pressure of 49 to 196 MPa.

[0089] The temperature in the HIP treatment is usually 1500 to 1680° C., and preferably 1550 to 1650° C. By setting the temperature within this range, the amount and size of pores remaining in the resulting sintered body can be effectively reduced.

[0090] The pressure for the HIP treatment is usually 49 to 196 MPa, and particularly preferably 98 to 196 MPa.

[0091] The atmosphere for the HIP treatment is not particularly limited, and can be, for example, an oxygen-containing atmosphere, an inert gas atmosphere, or the like. Oxygen-containing atmospheres can be suitably used, for example, O2-Ar mixed gas or O2-N2 mixed gas with an oxygen concentration of 20% by volume or less. In the present invention, an inert gas atmosphere, such as argon gas or nitrogen gas, is particularly preferred.

[0092] The HIP treatment time can be changed as appropriate depending on the HIP treatment temperature and the like, but it is usually sufficient to set it within the range of 1 to 10 hours.

[0093] In the HIP treatment, the temperature increase rate and the temperature decrease rate are not particularly limited, but are usually about 300 to 600° C. / hr.

[0094] Annealing process In the annealing step, the sintered body obtained by the HIP treatment is annealed at 1200 to 1500° C. By carrying out the annealing step, lattice defects, distortion, etc. that may exist inside the sintered body can be more reliably removed.

[0095] The annealing temperature is usually 1200 to 1500°C, and preferably 1250 to 1400°C. The treatment atmosphere may be an oxygen-containing atmosphere, such as atmospheric air, O2-Ar mixed gas, O2-N2 mixed gas, or the like. The pressure is not particularly limited, but the treatment is usually carried out under atmospheric pressure (normal pressure). The annealing time can be changed as appropriate depending on, for example, the size of the sintered body, the treatment temperature, etc., but is usually within the range of about 3 to 10 hours.

[0096] The annealed sintered body can be used as it is or after being processed into a predetermined shape as necessary as an optical material for an optical isolator, etc. The processing method is not particularly limited, and can be carried out according to a known cutting method, polishing method, etc.

[0097] 3. Use of the ceramic of the present invention The ceramics of the present invention can be used as various optical materials. In particular, they are effective as Faraday rotators for optical isolator devices. When the ceramics of the present invention are used as optical isolators, the same configuration as that of known isolator devices can be adopted, except that the ceramics of the present invention are used as Faraday rotators.

[0098] Figure 4 shows an example of the configuration of an optical isolator using the ceramics of the present invention. A permanent magnet such as Nd-Fe-B is placed around the material, and polarizers are placed on both the input and output sides of the laser light. What makes this optical isolator different from a typical optical isolator is that the ceramics of the present invention are used in the Faraday rotator. While TGG single crystals are used in the Faraday elements of typical isolators used in the visible to near-infrared range, the polycrystalline ceramics of the present invention can be used in place of these TGG single crystals.

[0099] <Embodiments of the Invention> In the present invention, the oxides that constitute the material (one or more rare earth oxides, namely TbO, LuO, and YO, AlO, and ScO) and each additive are weighed, and then the material is synthesized by the following steps: wet mixing, drying, uniaxial molding, cold isostatic pressing, degreasing, pre-sintering, and hot isostatic pressing. The resulting transparent sintered body is annealed in an oxygen-containing atmosphere at 1200-1500°C to remove lattice defects and distortions within the material, and then machined into a desired shape (e.g., a rod shape with a diameter of 5 mm and a length of 20 mm). Next, the input / output surfaces (both 5 mm diameter surfaces) through which the laser light passes are finished to a flatness of λ / 10 (λ=633 nm), a parallelism of less than 10 sec, and a surface roughness of approximately Ra 0.2 nm. The polished surfaces are then subjected to an AR (anti-reflection) coating treatment in the wavelength range to be used, resulting in a Faraday element (ceramic of the present invention).

[0100] Figure 1 shows the (Tb 0.8 Y 0.2 )3AlO 12These are micrographs of transparent ceramics. Figure 1(a-1) is a reflection microscope photograph of a polished and thermally etched sample of the just-stoichiometric composition. Figure 1(a-2) is a reflection microscope photograph of a ceramic with a 0.02% Al2O3 enrichment from the just-stoichiometric composition, showing abnormal particles. Figure 1(b-1) is a transmission polarizing microscope photograph of a polished sample of the just-stoichiometric composition, showing uniform coloring (substantially no birefringence detected) and virtually no residual porosity. Figure 1(b-2) is a transmission polarizing microscope photograph of a ceramic with a 0.02% rare earth enrichment from the just-stoichiometric composition, showing birefringence. Figure 1(b-3) is a transmission polarizing microscope photograph of the just-stoichiometric composition ceramic. Birefringence is not detectable, but 3 ppm of pores with an average depth of 1 μm are present. Figure 1(b-4) is a transmission polarizing microscope photograph of the ceramic of the just stoichiometric composition, and although there is virtually no birefringence, there are 200 ppm of pores of several to tens of μm in size. Figure 1(b-5) is a transmission polarizing microscope photograph of the just stoichiometric composition ceramic, and there is virtually no birefringence, but there are inclusions [(TbY)Al2O3] of several to tens of μm in size due to poor mixing.

[0101] To evaluate the material, the measurement device shown in Figure 3 is used to measure the Verdet constant of the material and determine the required medium length for the capacity of the magnet to be used. As shown in the figure, a magnet with a magnetic flux density of 1T is placed around the material for evaluation purposes, and polarizers are placed on both ends of the material. When a 1.064μm, 10mW Nd:YAG laser is irradiated, polarization of the laser light occurs within the Faraday element in contact with the 1T magnet, and by rotating the polarizer on the output side, the laser light output versus polarization angle can be measured.

[0102] Here too, the Verdet constant can be found from Faraday's theoretical formula θ=VHL (where θ is the Faraday rotation angle, V is the Verdet constant, H is the magnetic flux density, and L is the medium length).

[0103] Figure 4 shows the 20mm long (Tb 0.65 Y0.35 )3AlO 12 The polarization characteristics of ceramics placed in a 1T magnetic field are shown. (For reference, the polarization characteristics of a TGG single crystal with nearly the same Verdet constant are also shown.) The laser light is shut off at positions of -45 and +135 degrees relative to the rotation angle of the output polarizer. As the output polarizer is rotated, it transmits light sequentially, and the maximum transmittance (minimum transmission loss) is achieved when the input polarizer and output polarizer are at a position of +45 degrees. The less loss there is in the material, the smaller this value is, and this value is the insertion loss (dB). When the output polarizer is rotated further, the laser light is shut off, and returns to its original value at θ = +135 degrees. When the output polarizer is rotated at -45 → +315 (-45) deg. (i.e., one rotation from 0 to 360 (0) deg. results in two cycles: extinction (maximum transmission loss) → transmission (minimal transmission loss) → extinction → transmission), which indicates that the design of the medium length (calculated from the Verdet constant) against the magnetic field is matched. The value in the shutdown state when the polarizer is parallel is called the extinction ratio, and this value indicates the superiority of the laser light shutdown ability. As is clear from Figure 4, the ceramics (polycrystalline body) of the present invention, which have a Verdet constant similar to that of TGG single crystal, have magneto-optical properties that are comparable to or superior to those of TGG single crystal.

[0104] The fundamental magneto-optical properties of materials are measured at low laser power levels, but practical laser powers are high, and the heat generated inside the material at this time causes the thermal lens effect or thermal birefringence, so a Faraday element and isolator with stable properties even at high power are essential.

[0105] The composition of the ceramic of the present invention, additives, etc. can be detected or measured, for example, by the following methods.

[0106] (a) Additives The additives added to the material can be detected by detecting the Si, Mg, and Ca contents in the material in ppm units using, for example, a general analytical instrument, ICP (Inductive Coupled Plasma)-MASS.

[0107] (b) Composition analysis of ceramic materials The ceramics of the present invention are assumed to be a single garnet phase by XRD, but the material composition can be identified by X-ray fluorescence analysis (XRF) to determine the composition of Tb, Y, Lu, and Al. Composition analysis can be performed by a general method using a calibration curve.

[0108] (c) Stoichiometric composition Generally, whether or not a material is a single phase is determined by X-ray diffraction analysis (XRD), but the determination of the just stoichiometric composition of the present invention is impossible using conventional methods. Therefore, in the present invention, the determination can be made by observing the material structure with a reflecting microscope and a transmission polarizing microscope. The minimum requirement is that the transparent ceramics be composed of uniform particles, as shown in Figure 1(a-1). The image shown in Figure 1(b-1) is a transmission + polarized microscope photograph, and the fact that the image is uniform even under polarized light means that there is no birefringence (there are no particles other than cubic crystals = garnet), and the state that has been reached is called a just stoichiometric composition. FIG. 1(b-2) shows a composition slightly shifted toward the rare earth side, and although no impurity phase can be detected, birefringence (color unevenness) is present, resulting in large optical loss, and therefore falling outside the scope of the present invention. Figure 1(a-2) shows a sample with a slight alumina composition deviation, with abnormal particles observed locally. Ceramics with such a microstructure exhibit birefringence around the abnormal particles and a large amount of residual porosity, making them outside the scope of this invention. [Example]

[0109] The features of the present invention will be described in more detail below with reference to examples and comparative examples, but the scope of the present invention is not limited to these examples.

[0110] Example 1 Commercially available Tb4O7 powder (purity >99.9%, average primary particle size 0.1 μm) and α-Al2O3 powder (purity >99.9%, uniform primary particle size 0.3 μm) were used as starting materials. Additives of 300 ppm colloidal silica, 100 ppm CaO (average primary particle size 0.1 μm, decomposition product of CaCO3), and 900 ppm MgO (average primary particle size 0.05 μm) were added to these powders. These powders were weighed to form a garnet composition. An organic binder (a 20% solution of low-molecular-weight polyvinyl alcohol (PVA)) was added at 3 wt% of the starting powders, and the mixture was placed in a synthetic resin container. A total of 100 g of powder was mixed with 1 kg of high-purity Al2O3 balls (particle size approximately 3 mm) for grinding and 250 ml of ethanol. The mixture was wet-mixed for 15 hours to obtain a slurry. The collected slurry was dried at 90°C to evaporate the ethanol, and the powder was then sieved through a nylon sieve (100 mesh). The powder that passed the sieve was then press-molded under a pressure of approximately 10 MPa, followed by CIP molding under a pressure of 98 MPa. Next, the obtained compact was subjected to 1×10 -3 Pre-sintering was carried out in a vacuum of 100 Pa at a temperature increase rate of 150°C / hr up to a maximum temperature of 1500°C. The relative density of the pre-sintered body was measured by the Archimedes method and was found to be 98%. Next, the pre-sintered body was subjected to HIP treatment. The treatment conditions were: treatment atmosphere: Ar gas, pressure: 196 MPa, temperature: 1550°C, treatment time: 3 hours. Since the HIP-treated sintered body was transparent but gray, it was further annealed in air at 1300°C for 10 hours to remove internal lattice defects. In this way, a sintered body was produced.

[0111] Examples 2 to 27 Sintered bodies were produced in the same manner as in Example 1, except that the compositions and synthesis conditions shown in Tables 1 to 6 were used.

[0112] Examples 28 to 36 Sintered bodies were produced in the same manner as in Example 1, except for the compositions and synthesis conditions shown in Tables 7 and 8. The average primary particle diameter of the raw material powders listed in the tables was in the range of 100 to 300 nm, and the additives used were TEOS (Si source), MgCO3 (Mg source), and CaCO3 (Ca source).

[0113] Examples 37 to 45 The starting material was Tb2O3 powder obtained by hydrogen reduction, which was pulverized to an average primary particle size of 1 μm. Sintered bodies were prepared in the same manner as in Example 1, except that the compositions and synthesis conditions were as shown in Tables 9 and 10.

[0114] Examples 46 to 54 Tables 11-12 show (Tb x Re 1-x )3(Al y Sc 1-y )5O 12 Sintered bodies were prepared in the same manner as in Example 1, except that the ceramic compositions were used and the synthesis conditions shown in Tables 11 and 12 were used. Sc2O3 powder with an average primary particle size of 500 nm was used as the Sc supply source in all Examples. Sc essentially substitutes for Al at hexacoordination sites in the garnet structure, and the compounding ratio of each raw material was calculated taking this into consideration.

[0115] Comparative Examples 1 to 9 The performance of a TGG single crystal produced by the CZ method is shown as Comparative Example 1, and the performance of a TAG single crystal produced by the FZ method is shown as Comparative Example 2. In Comparative Examples 3 to 9, sintered bodies were produced in the same manner as in Example 1, except that the compositions and synthesis conditions shown in Tables 13 to 14 were used.

[0116] Comparative Examples 10 to 13 The synthesis conditions shown in Table 15 were the same as in Example 1, except that the reaction sintering of Tb4O7, Y2O3, Lu2O3, Al2O3 and additives was carried out and the (a) (Tb 0.2 Lu 0.8 )3AlO 12 and (b)(Tb0.4 Y 0.6 )3AlO 12 Ceramics were produced.

[0117] Test Example 1 The following physical properties were measured for the sintered bodies finally obtained in each Example and Comparative Example. The results are shown in Tables 1 to 15. In each table, 1) shows the measured value when a 10 mW Nd:YAG laser was used, and 2) shows the increase in beam diameter (thermal lensing) and extinction ratio (thermal birefringence) when a 200 W laser was used. In each table, the units are "volume ppm" for pore volume, "μm" for average pore diameter, "μm" for average crystal grain diameter, "dB" for insertion loss, and "dB" for extinction ratio.

[0118] (1) Relative density of pre-sintered body The porosity in the pre-sintered body was measured by the Archimedes method (calculated from the dry weight, wet weight, and underwater weight), and the relative density was calculated based on the measurement results.

[0119] (2) Average crystal grain size of sintered body The sintered body was mirror-polished and thermally etched at temperatures ranging from 1500 to 1600°C, and the surface was then measured using a reflecting microscope. More specifically, surface photographs were taken at five random locations (observed at magnifications ranging from 200 to 1000x), and the area per crystal grain was calculated from the value of (area of ​​the observation field) / (number of crystal grains in the observation field). The diameter of the crystal grains, assuming they were spherical, was taken as the average crystal grain size of the sintered body. For example, Figure 1(a-1) shows the ceramic of the present invention, with an average grain size of 10 μm.

[0120] (3) Porosity Measurements were made using a combination of a visible light (transmission) microscope and a scanning electron microscope (SEM). With the visible light (transmission) microscope, the pores present inside the material were observed at a magnification of 100x or more, and the pore diameter was observed, and the porosity was calculated based on this. The number of pores inside the material must be small enough to be counted in order for the insertion loss to be 0.2 dB or less. While ordinary sintered bodies have a relatively large number of pores, which can be measured using the Archimedes method, the ceramics of the present invention are extremely dense, making pore detection extremely difficult. Therefore, the special measurement method described above is used. Small voids are always visible when observed using infrared (transmission) microscopes and visible (transmission) microscopes due to scattering, but pore diameters cannot be determined when the pore size is submicron. Therefore, by using SEM in combination, the diameter of small residual pores of 1 μm or less can be determined, thereby determining the average pore diameter. In other words, in this invention, the number of residual pores in the depth direction (interior) of the material is determined using the above-mentioned transmission polarizing microscope, and then the pore diameters are determined using the high-resolution SEM. The average pore diameter can then be calculated by dividing the total diameter of each pore by the total number of residual pores. An example of measuring the residual pore volume using a transmission microscope is described in "A. Ikesue, T. Kinoshita, K. Kamata, K. Yoshida, "Fabrication and Optical Properties of High-Performance Polycrystalline Nd:YAG Ceramics for Solid-State Lasers," J. Am. Ceram. Soc., 78 [4] 1033 (1995)." Measurement of the residual pore volume using a transmission microscope can also be carried out according to the method described therein. The porosity was calculated by assuming each pore to be spherical and dividing the volume (or area) of each pore and its number (i.e., the total pore volume) by the measured volume (or area) to determine the porosity (volume ppm). For reference, Figure 1(b-1) shows a transmission polarizing microscope photograph of the present invention in which the porosity is <1 ppm (essentially zero). Not a single void can be detected in the depth direction of the material. Figure 1(b-2) shows a ceramic with an average pore diameter of 1 μm and 3 ppm of residual pores. Figure 1(b-3) shows a ceramic with 200 ppm of pores ranging from several to several tens of μm, and the pore diameter and quantity can be counted.

[0121] (4) Presence or absence of a second phase (4-1) Presence or absence of impurity phases Ten randomly selected grain boundaries were observed under a conventional transmission polarizing microscope, and the presence or absence of birefringence (other than the cubic phase) was examined using a polarizer. If any such regions were present, they were determined to be impurity phases. Although the ceramic in Figure 1(b-4) has a just stoichiometric composition, inclusions of several to tens of micrometers in size were detected due to insufficient mixing. These inclusions exhibit significant birefringence under observation with a transmission polarizing microscope, and are therefore outside the scope of this invention. (4-2) Presence or absence of grain boundary phase The presence or absence of grain boundary phases was confirmed by observing the structure (lattice image) of the grain boundary area using HR-TEM (magnifications of several tens to one million), and further observations were made using a transmission polarizing microscope (100x) to check for areas where there was a difference in brightness (birefringence). If such areas were present, they were determined to be grain boundary phases (impurity phases). The ceramic in Figure 2(b) does not contain any grain boundary phases, so the grain boundaries have a clean structure. Figure 2(a) shows ceramics where an impurity phase of about 10 nm exists in the grain boundary area. When the grain boundary phase and the interior of the grains are analyzed using TEM-EDS, their compositions are completely different, so it can easily be determined to be grain boundary phases.

[0122] (5) Insertion loss and extinction ratio "(Tb x Y 1-x )3AlO 12 The method used was almost the same as that described in “Faraday ceramics as a rotator material”, J. Am. Ceram. Soc., DOI: 10.1111 / jace.14961 (2017),” except that a 10mW Nd:YAG laser with a wavelength of 1064nm was used as the light source. First, we used an Nd-Fe-B magnet to determine the Verdet constant from the applied magnetic field and Faraday rotation angle, and then determined the thickness of the material that would allow for 45-degree polarization. After optically polishing both surfaces of the material of the specified thickness to a λ / 10 flatness, we applied an AR (anti-reflection) coating for the measurement wavelength band. Following the setup example for magneto-optical characterization shown in Figure 3, we performed general optical measurements and evaluation as an isolator. As shown in Figure 3, we used a 1064 nm, 10 mW Nd:YAG laser. Polarizers (both crossed at 45 degrees) were installed on the laser input and output sides of the isolator, and polarization characteristics were measured under a 1 Tesla magnetic field. In this case, we used a 5 mm diameter x 20 mm long TGG single crystal grown by the CZ method as a reference. Figure 4 shows the TGG single crystal and (Tb 0.65 Y 0.35 )3AlO 12 The measured values ​​of ceramics are shown. Both materials are adjusted to a diameter of 5 mm x length of 20 mm, and both 5 mm diameter surfaces are optically polished. When a magnetic field of 1 T is applied to this material and the Verdet constant is measured, both are 36 radT. -1 m -1 The Faraday element was placed in a 1T magnetic field, and the output polarizer was set at -45° relative to the input side, which was taken as the starting point. The output polarizer was rotated while irradiating a constant output Nd:YAG laser, and the amount of laser light transmitted (transmission loss) was measured. Since the input and output polarizers are orthogonal at the starting point, the extinction ratio is calculated from this point, and the point at which the transmittance is maximum (transmission loss is minimum) when the output polarizer is rotated 90° from the starting point (45° in the figure) is the insertion loss. If the Verdet constant is measured and the medium length is adjusted, the TGG single crystal and polycrystalline ceramics always exhibit similar behavior, but the difference between each material is the absolute values ​​of the insertion loss and extinction ratio.

[0123] (6) Transmitted wavefront distortion The measurement method was as follows: the sample was optically polished, and then AR coated, and then measured with an interferometer.

[0124] [Table 1]

[0125]

Table 2

[0126]

Table 3

[0127]

Table 4

[0128]

Table 5

[0129]

Table 6

[0130]

Table 7

[0131]

Table 8

[0132]

Table 9

[0133]

Table 10

[0134]

Table 11

[0135]

Table 12

[0136] [Table 13]

[0137] [Table 14]

[0138] [Table 15]

[0139] As is clear from these results, the ceramics of the present invention containing the specified additives are composed of fine crystal grains and a single crystal phase (garnet-type polycrystalline body) with almost no residual pores, and therefore have low insertion loss and a high extinction ratio. In addition, the increase rate of the beam diameter is suppressed to +4%, which means that heat generation is effectively suppressed.

[0140] In contrast, the TGG single crystal of Comparative Example 1 has relatively good insertion loss and extinction ratio, but the beam diameter increases at a high rate, and its high-power characteristics are poor.Furthermore, the TAG single crystal of Comparative Example 2 has inferior basic characteristics to TGG, and its high-power characteristics are also poor.

[0141] It can be seen that, although some characteristics are good in Comparative Examples 3 to 9, the beam diameter increase rate is particularly high or is immeasurable (measurement is not possible due to too low transmittance or optical characteristics). In particular, Comparative Examples 3 and 4, which contain only Si as an additive, show a beam diameter increase rate of +12% to +25%, indicating that characteristics deteriorate in the high power range.

[0142] Comparative Examples 10 and 11 have compositions with a low Tb content, and the insertion loss and extinction ratio at a wavelength of 1.064 μm are 0.05 dB and 0.04 dB, respectively, and 39 dB and 38 dB, respectively, which are superior to TGG single crystals. Furthermore, the reduction in the extinction ratio and the increase in the beam diameter when irradiated with a 200 W laser were also greater than those described in the examples. However, the Verdet constants of these ceramics are 11 radT, respectively. -1 m -1 (Comparative Example 10) and 23radT -1 m -1 The magnetic field lengths of 1 T calculated from the Verdet constant were 65 mm and 32 mm, respectively, making the device extremely large. Thus, although the element function itself was good, it was outside the scope of the present invention in terms of device volume.

[0143] Comparative Examples 12 and 13 were synthesized using 99.9% pure Tb4O7 and YO3, and 99.5% pure alumina (main impurities were Na and Fe), and the magneto-optical properties and optical uniformity were comparable to those of the Examples. However, heat was generated when irradiated with a 200 W laser, and it was confirmed that the thermal lens and thermal birefringence results were inferior.

[0144] Furthermore, among the examples, for example, the transmitted wavefront distortion of the samples in the examples (Examples 1 to 9) using Tb4O7 as the starting material was λ / 8 to λ / 12, whereas the transmitted wavefront distortion of the samples in the examples (Examples 37 to 45) using Tb2O3 as the starting material was λ / 10 to λ / 15, indicating that using Tb2O3 as the starting material ensures higher uniformity. Improved transmitted wavefront distortion means improved beam quality after passing through the Faraday element, and maintaining beam quality enables the laser function to be maximized. [Industrial Applicability]

[0145] The present invention relates to a Tb-containing aluminum garnet transparent ceramic that is transparent and has magneto-optical properties, and is applicable to Faraday rotators in the visible to near-infrared region, and can exhibit excellent performance particularly as a high-power optical isolator.

Claims

1. Composition formula (Tb ｘ Re １－ｘ ) ３ (Al ｙ Sc １－ｙ ) ５ O １２ (wherein Re represents at least one of Y and Lu, x=1.0 to 0.5, and y=1.0 to 0.6), and further comprising at least one of Ca and Mg, and Si.

2. The content of Si, Ca and Mg (converted to oxides) is SiO ２ 2. The Tb-containing rare earth-aluminum garnet-type ceramic according to claim 1, wherein the total content of CaO and MgO is 50 to 500 ppm by weight.

3. Weight ratio of Si, Ca and Mg (oxide equivalent) [(CaO + MgO) / SiO ２ 2. The Tb-containing rare earth-aluminum garnet-type ceramic according to claim 1, wherein the value of [R] is 1 or more.

4. 2. The Tb-containing rare earth-aluminum garnet-type ceramic according to claim 1, wherein the average crystal grain size is in the range of 1 to 30 μm.

5. 2. The Tb-containing rare earth-aluminum garnet-type ceramic according to claim 1, which has a relative density of 99.999% or more.

6. 2. The Tb-containing rare earth-aluminum garnet-type ceramic according to claim 1, wherein substantially no crystalline phase or amorphous phase other than said garnet-type polycrystal is detected.

7. 2. The Tb-containing rare earth-aluminum garnet-type ceramic according to claim 1, wherein, at a thickness t at which light of a wavelength λ nm (where 1200≧λ≧550) undergoes 45° Faraday rotation, the Tb-containing rare earth-aluminum garnet-type ceramic has an insertion loss of 0.2 dB or less and an extinction ratio of 30 dB or more for said light.

8. A method for producing Tb-containing rare earth-aluminum garnet-type ceramics, comprising the steps of: (1) (1-1) A powder mixed raw material containing (a) terbium oxide having an average primary particle size of 1 μm or less, (b) aluminum oxide having an average primary particle size of 1 μm or less, (c) at least one of a calcium compound having an average primary particle size of 1 μm or less and a magnesium compound having an average primary particle size of 1 μm or less, and (d) a silicon compound having an average primary particle size of 1 μm or less, and having a stoichiometric composition of a garnet composition; or (1-2) (a) The average primary particle size is 1 μm or less (Tb ｘ Re １－ｘ ) ３ (Al ｙ Sc １－ｙ ) ５ O １２ (b) an oxide of Re (where Re represents at least one of Y and Lu, x = 1.0 to 0.5, and y = 1.0 to 0.6), (b) at least one of a calcium compound having an average primary particle size of 1 μm or less and a magnesium compound having an average primary particle size of 1 μm or less, and (c) a silicon compound having an average primary particle size of 1 μm or less, and having a stoichiometric composition of a garnet composition. in an alcohol to obtain a mixture; (2) a step of press-molding the mixture to obtain a green compact; (3) pre-sintering the green compact at a temperature of 1450 to 1600°C to obtain a pre-sintered body having a relative density of 95 to 99%; (4) subjecting the pre-sintered body to HIP treatment at a temperature of 1500 to 1680°C and a pressure of 49 to 196 MPa; and (5) Annealing the sintered body obtained by the HIP treatment at 1200 to 1500°C A method for producing Tb-containing rare earth-aluminum garnet-type ceramics, comprising:

9. Terbium oxide as Tb ２ O ３ The method of claim 8, wherein

10. A Faraday element comprising the Tb-containing rare earth-aluminum garnet-type ceramic according to any one of claims 1 to 7.

11. An optical isolator device comprising the Faraday element of claim 10.

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