Method for improving laser damage threshold of garnet-type transparent ceramics

Abstract

To provide a garnet-type transparent ceramic, which is a sintered compact of a composite oxide containing at least yttrium and aluminum as major components and which can be used in short-pulse laser systems.SOLUTION: A garnet-type transparent ceramic is a sintered compact of a composite oxide containing yttrium and aluminum. Its optical surface has an average roughness as an arithmetic mean height Sa≤0.70 nm or a root mean square height Sq≤0.89 nm.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to garnet-type transparent ceramics Method for improving laser damage threshold and, more particularly, to paramagnetic garnet-type transparent ceramics containing terbium suitable for forming magneto-optical devices such as isolators Method for improving laser damage threshold relates to 。

Background Art

[0002] In recent years, due to the increasing high power of fiber lasers, the spread of laser processing machines using such fiber lasers has been remarkable. By the way, when external light is incident on the laser light source incorporated in the laser processing machine, a phenomenon occurs in which the resonance state becomes unstable and the oscillation state is disturbed. In particular, when the oscillated light is reflected by the optical system in the middle and returns to the light source, the oscillation state is greatly disturbed. To prevent this, an optical isolator is usually provided on the light emission side of the light source, such as between the laser light source and the optical fiber

[0003] The optical isolator includes a Faraday rotator, a polarizer disposed on the light incident side of the Faraday rotator, and an analyzer disposed on the light emission side of the Faraday rotator. Further, the Faraday rotator is used by applying a magnetic field parallel to the traveling direction of light. At this time, the polarized wave component of light rotates only in a certain direction whether it advances or retreats in the Faraday rotator. Further, the Faraday rotator is adjusted to a length at which the polarized wave component of light is rotated exactly 45 degrees. Here, if the polarization planes of the polarizer and the analyzer are shifted by 45 degrees in the rotation direction of the advancing light, the polarized wave of the advancing light coincides at the polarizer position and the analyzer position and thus transmits. On the other hand, the polarized wave of the returning light rotates 45 degrees in the direction opposite to the rotation direction of the shift angle of the polarization plane of the polarizer shifted by 45 degrees from the analyzer position. Then, the polarization plane of the returning light at the polarizer position is shifted by 45 degrees - (-45 degrees) = 90 degrees with respect to the polarization plane of the polarizer and cannot pass through the polarizer. In this way, it functions as an optical isolator that transmits and emits the advancing light and blocks the returning light

[0004] As the material most widely used as the Faraday rotator constituting the above optical isolator, TGG crystal (Tb3Ga5O 12 ) has been conventionally known (Japanese Patent Application Laid-Open No. 2011-213552 (Patent Document 1)). Although TGG crystal is still widely used for standard fiber laser devices at present, it has recently become clear that it is difficult to use for high-power band fiber laser devices. Specifically, the absorption coefficient inherent in the material is high (absorption coefficient at a wavelength of 1064 nm ~ 0.16% / cm), and the temperature coefficient of the refractive index dn / dt is large (dn / dt = 18×10 -6 K -1 ) at room temperature of 25°C, which is a bottleneck.

[0005] Therefore, there is a strong demand for a new material that has lower absorption than TGG crystal and is applicable to high-power band fiber laser devices. TAG crystal (Tb3Al5O 12 ) is known as one of such candidate materials, but the production of high-quality single crystal TAG has not been invented so far.

[0006] Under such circumstances, recently, it has been disclosed that a dense ceramic sintered body with a composition of (Tb x Y 1-x )3Al5O 12 (x = 0.5 to 1.0) has a higher extinction ratio than the existing TGG crystal (the existing 35 dB is improved to 39.5 dB or more), and the insertion loss can also be reduced (the existing 0.05 dB is improved to 0.01 to 0.05 dB) ("Yan Lin Aung, Akio Ikesue, Development of optical grade (Tb x Y 1-x )3Al5O 12"Ceramics as Faraday Rotator Material," J. Am. Ceram. Soc., (2017), 100(9), 4081 - 4087 (Non-Patent Document 1). The material disclosed in this Non-Patent Document 1 is a material similar to the TAG crystal, but is characterized in that it is made of ceramics, different from the single crystal materials heretofore. As a result, high-quality yttrium-substituted TAG crystals that could not be realized heretofore were obtained.

[0007] Furthermore, Japanese Patent Application Laid-Open No. 2019-199386 (Patent Document 2) discloses paramagnetic garnet-type transparent ceramics having a composition of (Tb 1-x-y Y x Sc y )3(Al 1-z Sc z )5O 12 (where 0.05 ≦ x < 0.45, 0 < y < 0.1, 0.5 < 1 - x - y < 0.95, 0.004 < z < 0.2). This material is a material having a composition similar to that of Non-Patent Document 1, but is different in that Sc is significantly added. When using this material, the extinction ratio is further improved to 40 dB or more compared to Non-Patent Document 1, and even when irradiated with a 100 W laser, the amount of change in the focal position due to the thermal lens is small, and it can be used as an optical isolator at a practically acceptable level, and it is said that a truly practical paramagnetic garnet-type transparent ceramics that is easy to scale up due to being made of ceramics can be provided.

[0008] By the way, recently, in fiber laser systems, not only is there a trend towards higher output, but also a significant trend towards shorter pulses. Generally, when the laser pulse is shortened, the peak value of the energy increases, and material breakdown at the laser output end face of the optical material mainly used in the fiber laser system is likely to occur. For example, "Masahide Ito, Iwafusa Ogura, Destruction of Optical Materials by Laser Light, Production Research of the Institute of Industrial Science, The University of Tokyo, (1984), Vol. 36, No. 6" (Non-Patent Document 2) discloses that generally, the damage threshold of the material surface is lower than that of the interior, and laser damage to the surface is more likely to occur at the output face than at the incident face.

[0009] However, there has been no known example that defines the material surface roughness of optical materials, particularly the control value of the material surface roughness of garnet-type transparent ceramic materials, which have been available for use in short-pulse laser systems up to now.

[0010] Of course, there are prior art examples that attempt to reduce (i.e., smooth) the material surface roughness of garnet-type transparent ceramic materials. For example, "Daniel Ross, Hitomi Yamaguchi, Polishing characteristics of Nd:YAG ceramics with various Nd-dopant concentrations, CIRP Journal of Manufacturing Science and Technology, 27(2019)93-101" (Non-Patent Document 3) discloses a known example in which Nd:YAG ceramics are polished by a latest polishing method called the MAF method using submicron-sized diamond abrasive grains, and the surface roughness of YAG ceramics without Nd doping in particular is reduced (i.e., smoothed) to a range of arithmetic mean height (Sa value) of 0.4 to 0.5 nm in the range of 177 μm × 132 μm. However, Non-Patent Document 3 does not give any examples or suggestions regarding the relationship between surface roughness and laser damage threshold.

Prior Art Documents

Patent Documents

[0011]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0012]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0013] The present invention has been made in view of the above circumstances, and is a sintered body of a composite oxide containing at least yttrium and aluminum as main components, and by controlling the average roughness of the optical surface (precision polished surface) of the sintered body within a specified range, garnet-type transparent ceramics that can also be used in a short-pulse laser system Method for improving laser damage threshold is provided for the purpose of.

Means for Solving the Problems

[0014] The present invention provides the following garnet-type transparent ceramics Method for improving laser damage threshold for the purpose of. 1. A sintered body of a composite oxide containing yttrium and aluminum, In a garnet-type transparent ceramic having two optical surfaces that serve as the incident surface and the exit surface of laser light, with an overall light transmittance of 84% or more and a forward scattering rate of 0.5% or less at a wavelength of 1064 nm and an optical path length of 25 mm , wherein the above two optical surfaces are defined as the average value of the surface roughness of the two optical surfaces with an arithmetic mean height Sa ≤ 0.70 nm or a root mean square height Sq ≤ 0.89 nmaverage roughness having By this, when a short-pulse laser with a wavelength of 1064 nm and a pulse width of 10 ns is incident from one surface of the above two optical surfaces and the laser is emitted from the other surface, the laser damage threshold at the laser light exit surface is 10 J / cm 2 or more, characterized in that garnet-type transparent ceramics Method for improving laser damage threshold . 2. The complex oxide is Furthermore, containing terbium such that The garnet-type transparent ceramics according to 1, which is Method for improving laser damage threshold . 3. In the complex oxide The volume molar concentration of terbium is equal to or higher than the volume molar concentration of yttrium, and of the sintered body The garnet-type transparent ceramics according to 2, wherein the Verdet constant at a wavelength of 1064 nm is 30 rad / (T·m) or higher Method for improving laser damage threshold . 4. For the above sintered body, perform multiple lapping and polishing while gradually replacing the abrasive grains with finer ones, perform the final-stage lapping and polishing using abrasive grains with a particle size of 0.1 μm or less, and then perform optical polishing by CMP finishing polishing in a range where the polishing thickness is less than 1 μm The above optical surface to obtain The garnet-type transparent ceramics according to any one of 1 to 3 Method for improving laser damage threshold . 5. The garnet-type transparent ceramic is a sintered body of a complex oxide represented by the following formula (1), and contains SiO as a sintering aid 2 in an amount of more than 0% by mass and 0.1% by mass or less. Method for improving the laser damage threshold of the garnet-type transparent ceramic according to any one of 1 to 4 (Tb 1-x-y Y x Sc y ) 3 (Al 1-z Sc z ) 5 O 12 (1) (where 0.05 ≦ x ≦ 0.4, 0 ≦ y < 0.08, 0.52 < 1 - x - y < 0.95, 0 ≦ z < 0.15, 0.001 < y + z < 0.2). 6 . Furthermore, having an AR coating layer on the above optical surface such that 1 to 5 The garnet-type transparent ceramics according to any one of Method for improving laser damage threshold . 7. The method for improving the laser damage threshold of the garnet-type transparent ceramic according to any one of 1 to 6, wherein the garnet-type transparent ceramic is used in a short-pulse laser system

Advantages of the Invention

[0015] According to the present invention, when a short-pulse laser with a pulse width of 10 nsec is incident on an optical surface and the laser is emitted from the emission surface, the laser damage threshold of the garnet-type transparent ceramics can be made 10 J / cm 2 or higher.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Figure 3

Figure 4

Modes for Carrying Out the Invention

[0017] Hereinafter, the configuration in an embodiment of the garnet-type transparent ceramics according to the present invention will be described. The garnet-type transparent ceramics according to the present invention is a sintered body of a composite oxide containing yttrium and aluminum, and is characterized by having an optical surface with an average roughness of arithmetic mean height Sa ≦ 0.70 nm or root mean square height Sq ≦ 0.89 nm.

[0018] <Optical Surface of Garnet-Type Transparent Ceramics> Requirements for the optical surface, which is the core part of the garnet-type transparent ceramics (hereinafter sometimes simply referred to as transparent ceramics) of the present invention, and polishing conditions for obtaining the optical surface will be described.

[0019] The garnet-type transparent ceramics of the present invention have an optical surface with an average roughness of arithmetic mean height Sa ≦ 0.70 nm or root mean square height Sq ≦ 0.89 nm. This requirement is based on the following concept.

[0020] That is, the optical surfaces, which are both end surfaces for inputting and outputting the laser of the garnet-type transparent ceramics of the present invention, are precisely polished, and it is preferable to define the average roughness of the polished surface within a certain range. However, all conventional transparent optical devices for inputting and outputting a laser for use are made of single crystals or glass, and the surface roughness of the polished surface of ceramics has never been defined.

[0021] This is partly because sufficiently transparent ceramics that can be used by inputting and outputting high-power laser light have not been obtained so far. Furthermore, the development of high-power laser systems in recent years has been progressing in the direction of shorter pulses, and another reason is that the laser damage threshold has been decreasing more and more, making the problem apparent.

[0022] In addition, transparent ceramics are sintered bodies composed of countless polycrystals (sintered grains). When this transparent ceramics is polished, unlike single crystals or glass, a large number of sintered grains appear on the polished surface. The orientation of these sintered grains is generally in a completely random state. Furthermore, it is also a well-known fact that this sintered grain is a collection of small single crystals (polycrystals), and the hardness and chemical stability of single crystals vary depending on the polishing orientation. Therefore, when this transparent ceramics is polished, sintered grains that are easily polished and sintered grains that are difficult to polish will appear as a large difference in level (grain boundary step) on the surface.

[0023] Note that the transparent ceramics in the present invention are limited to an aggregate (sintered body) of sintered grains having a garnet-type structure. Since the crystallites having a garnet structure have a refractive index independent of the orientation, theoretically, laser light scattering due to crystallite orientation unevenness does not occur. Therefore, when highly densified and internal air bubbles are removed, the cause of laser light scattering can be said to be only the roughness of the incident and exit surfaces of the laser light, that is, the optical surface. If the surface roughness of this optical surface is defined within a range where the incident and exit laser light is not substantially scattered, it is presumed that the laser damage threshold of the transparent ceramics will not decrease.

[0024] Therefore, in the case of transparent ceramics in which laser light is incident and exits on the optical surface, it is necessary to numerically define the surface roughness in a state including this grain boundary step.

[0025] As a result of intensive studies based on the above-described concept, the inventors of the present invention have succeeded in achieving the object of not substantially scattering the incident and exit laser light and not decreasing the laser damage threshold of the transparent ceramics by defining the average roughness of the optical surface of the garnet-type transparent ceramics within the above range. Further, the optical surface is preferably a precision polished surface. Specifically, the average roughness of the optical surface (precision polished surface) of the garnet-type transparent ceramics is such that the arithmetic mean height Sa ≤ 0.70 nm or the root mean square height Sq ≤ 0.89 nm, and it is preferable that the arithmetic mean height Sa ≤ 0.70 nm and the root mean square height Sq ≤ 0.89 nm, and it is more preferable that the arithmetic mean height Sa ≤ 0.65 nm and the root mean square height Sq ≤ 0.80 nm. Thereby, the laser damage threshold when a short pulse laser with a pulse width of 10 nsec is incident and exited is 10 J / cm 2 or more, preferably 10.3 J / cm 2 or more can be ensured. Specifically, when a short pulse laser with a wavelength of 1064 nm and a pulse width of 10 ns is incident from one of the two optical surfaces and the laser is exited from the other surface, the laser damage threshold on the laser light exit surface is preferably 10 J / cm 2 or more.

[0026] Here, the arithmetic mean height Sa and the root mean square height Sq are based on ISO 25178 which defines "surface roughness". That is, the arithmetic mean height Sa is the average value of the absolute values of the heights of each point within the measurement area with respect to the average surface. The root mean square height Sq is a value corresponding to the standard deviation of the distances from the average surface to each point within the measurement area. Also, in the present invention, for an arbitrary area of 90% (area ratio) on the optical axis center side excluding the outer peripheral area of 10% (area ratio) in the optical surface (precision polished surface), it is preferable to define the average roughness including the grain boundary step within the ranges of the arithmetic mean height Sa and the root mean square height Sq for an area of 800 μm square (800 μm × 800 μm).

[0027] Also, it is preferable that the optical surface consists of two surfaces, an incident surface and an exit surface of laser light, and the average roughness of the arithmetic mean height Sa and the root mean square height Sq is the average value of the surface roughnesses of these two optical surfaces.

[0028] In the present invention, as long as an optical surface having the above average roughness can be realized, in the optical polishing process in the manufacturing process of the garnet-type transparent ceramics described later, the conditions for forming the optical surface, particularly the polishing conditions, are not limited. However, generally, when ceramics are excessively subjected to CMP processing (chemical mechanical polishing processing), the grain boundary step increases significantly. Therefore, it is preferable to finish in a state that is as flat and has as small roughness as possible by lap polishing using abrasive grains without chemical action. Specifically, diamond abrasive grains, alumina abrasive grains, ceria abrasive grains, B4C abrasive grains, etc. are preferably selected, and it is preferable to carefully perform lap processing while gradually changing from abrasive grains with a small count (large particle size) to abrasive grains with a large count (small particle size) using these.

[0029] Also, the final abrasive grain size (particle size) at this time should be at the submicron level. Specifically, it is preferable to go through a finishing lapping process using free abrasive grains with a small particle size of 0.3 μm or less, more preferably 0.1 μm or less. However, it is known that generally, only lapping treatment leaves small surface scratches and very shallow abrasions. Therefore, within a range where the polishing thickness is less than 1 μm, more preferably less than 200 nm, very slight CMP finishing polishing may be performed. For CMP polishing abrasive grains such as colloidal silica at this time, it is preferable to use free abrasive grains with a small particle size of 0.3 μm or less, more preferably 0.1 μm or less.

[0030] Moreover, the transparent ceramics of the present invention preferably further have an AR coating layer on the above optical surface. That is, it is preferable to further perform AR coating treatment on both end surfaces (optical surfaces) that are precisely polished as described above and are to be used optically. When an AR coating is applied to the optical surface which is the input / output surface of laser light, Fresnel reflection at the material interface can be suppressed, and therefore it is preferable because a decrease in the laser damage threshold can be prevented.

[0031] <Composition of Garnet-Type Transparent Ceramics> The garnet-type transparent ceramics of the present invention are a sintered body of a composite oxide containing yttrium and aluminum. Since garnet-type ceramics are cubic crystals, there is no azimuthal dependence of the refractive index, and even though they are ceramics, they can be finished into an optical material with little laser scattering similar to a single crystal. In addition, a sintered body of a composite oxide containing yttrium and aluminum can significantly increase the melting point by containing aluminum as a main component, has a Mohs hardness of 8 or more and is hard, and also has a high melting point of 2000 °C or more, making it a very strong material. Therefore, it is preferable because the laser damage threshold can be increased.

[0032] In addition, the garnet-type transparent ceramics of the present invention are preferably a sintered body of a composite oxide further containing terbium. That is, by substituting a part of the yttrium sites in the above composition with terbium, that is, making it a sintered body of a composite oxide mainly containing at least yttrium, terbium, and aluminum, the Faraday effect can be imparted, so it can be used as a magneto-optical material, which is preferable. In this case, it is particularly preferable that the content volume molar concentration of terbium is equal to or higher than the content volume molar concentration of yttrium. When the content volume molar concentration of terbium in the composition constituting the garnet-type transparent ceramics is controlled to be equal to or higher than the content volume molar concentration of yttrium, the Verdet constant at a wavelength of 1064 nm becomes 30 rad / (T·m) or higher.

[0033] Furthermore, as the detailed composition of the transparent ceramics of the present invention, it is preferably represented by the following formula (1). (Tb 1-x-y Y x Sc y )3(Al 1-z Sc z )5O 12 (1) (In the formula, 0.05 ≦ x ≦ 0.4, 0 ≦ y < 0.08, 0.52 < 1 - x - y < 0.95, 0 ≦ z < 0.15, 0.001 < y + z < 0.2.)

[0034] Here, by adding terbium (Tb) in the concentration range represented by formula (1), a sufficient Verdet constant can be ensured, and by adding yttrium (Y) in the concentration range represented by formula (1), suppression of heterogeneous phases and minimization of internal strain can be ensured. In addition, by adding scandium (Sc) in the concentration range represented by formula (1), complete disappearance of heterogeneous phases has been achieved. Furthermore, by adding aluminum (Al) in the concentration range represented by formula (1), a sufficient Verdet constant is ensured, and a relatively high thermal conductivity is imparted.

[0035] Note that the numerical ranges for x, y, and z in formula (1) are as follows in detail. In formula (1), the range of x is 0.05 ≦ x ≦ 0.4, preferably 0.1 ≦ x ≦ 0.4, and more preferably 0.2 ≦ x ≦ 0.4. When x is within this range, the perovskite-type heterophase can be reduced to a level where it cannot be detected by X-ray diffraction (XRD) analysis.

[0036] When x is less than 0.05, the effect of substituting part of terbium with yttrium cannot be obtained and it becomes substantially the same as the condition for producing TAG. Therefore, it is not preferable because it becomes difficult to stably manufacture a high-quality ceramic sintered body with low scattering and low absorption. Also, when x is greater than 0.4, the Verdet constant at a wavelength of 1064 nm becomes less than 32 rad / (T·m), which is not preferable. Furthermore, when the relative concentration of terbium becomes excessively low, the total length required to rotate the laser light with a wavelength of 1064 nm by 45 degrees becomes longer than 25 mm, making the manufacturing difficult, so it is not preferable.

[0037] In formula (1), the range of y is 0 ≦ y < 0.08, preferably 0.001 < y < 0.004, and more preferably 0.002 < y < 0.004. When y is within this range, it is preferable because the perovskite-type heterophase can be reduced to a level where it cannot be detected by X-ray diffraction (XRD) analysis. Furthermore, it is preferable because it can prevent an excessive decrease in the thermal conductivity due to the homogeneity of the sintered body and grain boundary scattering.

[0038] When y is 0.08 or more, while the effect of suppressing the precipitation of the perovskite-type heterophase or the alumina heterophase is saturated and unchanged, uneven sintering, residual sintering strain, or residual grain boundary scattering occurs due to the excessive effect of the sintering suppression effect of scandium. As a result, a local decrease in the extinction ratio and a decrease in the average value of the thermal conductivity occur, which is not preferable.

[0039] In formula (1), the range of 1 - x - y is 0.52 < 1 - x - y < 0.95, and more preferably 0.6 ≦ 1 - x - y < 0.8. When 1 - x - y is within this range, a large Verdet constant can be ensured and high transparency can be obtained at a wavelength of 1064 nm.

[0040] (1) In the formula, the range of z is 0 ≦ z < 0.15, preferably 0.001 < z < 0.004, and more preferably 0.02 ≦ z < 0.004. When z is within this range, it is preferable because the perovskite-type heterogeneous phase can be reduced to a level where it cannot be detected by X-ray diffraction (XRD) analysis. Furthermore, it is preferable because excessive decrease in the thermal conductivity due to the homogeneity of the sintered body and grain boundary scattering can be prevented.

[0041] When z is 0.15 or more, while the effect of suppressing the precipitation of the perovskite-type heterogeneous phase or the alumina heterogeneous phase saturates and does not change, uneven sintering, residual sintering distortion, or residual grain boundary scattering occurs due to the excessive effect of the sintering suppression effect of scandium, and as a result, local decrease in the extinction ratio and decrease in the average value of the thermal conductivity occur, which is not preferable.

[0042] In the transparent ceramics of the present invention, scandium (Sc) can be added within the ranges of y and z in the above formula (1). The addition amounts y and z of scandium each include 0 when considered alone. However, when considered as the composition of the entire paramagnetic garnet-type transparent ceramics, it is preferable that y + z exceeds 0.001 and is added within the range of the formula (1) because a highly transparent sintered body can be stably manufactured.

[0043] That is, in the formula (1), the range of y + z is 0.001 < y + z < 0.2, more preferably 0.002 < y + z < 0.005, and even more preferably 0.003 < z < 0.005. When y + z is within this range, it is preferable because the perovskite-type heterogeneous phase can be reduced to a level where it cannot be detected by X-ray diffraction (XRD) analysis. Furthermore, it is preferable because excessive decrease in the thermal conductivity due to the homogeneity of the sintered body and grain boundary scattering can be prevented.

[0044] When y + z is 0.001 or less, the risk of precipitation of perovskite-type heterogeneous phases or alumina heterogeneous phases increases, which is not preferable. Further, when y + z is 0.2 or more, while the effect of suppressing the precipitation of perovskite-type heterogeneous phases or alumina heterogeneous phases saturates and does not change, uneven sintering, residual sintering strain, or residual grain boundary scattering due to the excessive effect of the sintering suppression effect of scandium occurs. As a result, local reduction of the extinction ratio and reduction of the average value of the thermal conductivity occur, which is not preferable.

[0045] In the transparent ceramics of the present invention, it is preferable that the main components of the 6-coordinate site and the 4-coordinate site in the garnet structure are aluminum (Al). When the main components of these sites can be composed of aluminum (Al), the bondability of the crystal is improved.

[0046] Further, in the transparent ceramics of the present invention, terbium (Tb) and yttrium (Y) are selected as the main components of the 8-coordinate site, and the concentration of terbium is 1 - x - y (0.52 < 1 - x - y < 0.95), and the concentration of yttrium is x (0.05 ≤ x ≤ 0.4). It is preferably controlled within the range. When the concentration of terbium is controlled within this range, the Verdet constant at a wavelength of 1064 nm can be ensured to be 32 rad / (T·m) or more.

[0047] Incidentally, the paramagnetic garnet-type transparent ceramics obtained in the present invention contain, as the main components, components having a composition within the range represented by the formula (1), and as the sub-components, SiO2 that serves as a sintering aid is contained in a range of 0.1 mass% or less (that is, the content is more than 0 mass% and 0.1 mass% or less). When SiO2 is contained as the sintering aid within this range, the transparency of the obtained paramagnetic garnet-type ceramics is improved to a level that can withstand practical use and is stabilized, which is preferable.

[0048] Here, "contained as a main component" means containing 90% by mass or more of the composite oxide represented by the above formula (1). The content of the composite oxide represented by the formula (1) is preferably 99% by mass or more, more preferably 99.9% by mass or more, still more preferably 99.99% by mass or more, and particularly preferably 99.999% by mass or more.

[0049] In addition, the transparent ceramics of the present invention are composed of the above main component and sub-components, but may further contain other elements. Examples of other elements include rare earth elements such as lutetium (Lu) and cerium (Ce), or various impurity groups such as sodium (Na), calcium (Ca), magnesium (Mg), phosphorus (P), tungsten (W), and molybdenum (Mo).

[0050] When the total amount of Y, or Tb and Y 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 (substantially zero).

[0051] By manufacturing the transparent ceramics of the present invention under appropriate conditions with the composition of the above-described conditions, a truly transparent garnet-type transparent ceramic sintered body having a total light transmittance of 84.0% or more at a wavelength of 1064 nm with an optical path length of 25 mm and a forward scattering rate of 0.5% or less can be stably obtained with good reproducibility. Further, its thermal conductivity can be finished to 4.8 W / m·K or more. The measurement of the thermal conductivity can be evaluated by the laser flash method in accordance with JIS R1611.

[0052] <Manufacturing Method of Garnet-Type Transparent Ceramics> [Raw Materials] As raw materials used in the present invention, at least oxide powders of terbium, yttrium, scandium, and aluminum are used as starting materials. At this time, the raw material purity is preferably 99.9% by mass or more, and particularly preferably 99.99% by mass or more. Weigh a predetermined amount of those elements, and further contain silicon oxide (SiO2) in an amount exceeding 0 mass% and not exceeding 0.1 mass%, and process it appropriately by a wet ball mill or a bead mill.

[0053] In the garnet-type oxide powder raw material used in the present invention, various organic additives may be added for the purpose of improving quality stability and yield in the subsequent ceramics manufacturing process. In the present invention, these are not particularly limited. That is, various dispersants, binders, lubricants, plasticizers, etc. can be preferably used. However, as these organic additives, it is preferable to select a high-purity type that does not contain unnecessary metal ions. In addition, the addition order of each organic additive needs to be appropriately designed so as not to inhibit the control of the properties (particle size distribution, etc.) of the raw material to be manufactured.

[0054] [Manufacturing process] In the present invention, the above raw material powder can be used to press-mold into a predetermined shape, or the wet slurry can be directly cast and molded to produce a molded body. The obtained molded body is sufficiently degreased, and then sintered to produce a sintered body with a relative density densified to at least 94% or more. As a subsequent process, it is preferable to perform a hot isostatic pressing (HIP) treatment. If the hot isostatic pressing (HIP) treatment is performed as it is, the paramagnetic garnet-type transparent ceramics will be reduced and a slight oxygen deficiency will occur. Therefore, it is preferable to perform a mild oxidation HIP treatment or an annealing treatment (oxidation annealing treatment) in an oxidizing atmosphere after the HIP treatment to recover the oxygen deficiency. Thereby, a transparent garnet-type oxide ceramic without defect absorption can be obtained.

[0055] (Molding) In the present invention, a normal pressing process can be preferably used. That is, a uniaxial pressing process in which a mold is filled and pressed from a certain direction, a cold isostatic pressing (CIP) process in which it is hermetically stored in a deformable waterproof container and pressed by hydrostatic pressure, or a warm isostatic pressing (WIP) process can be preferably used. The applied pressure may be appropriately adjusted while checking the relative density of the obtained molded body, and is not particularly limited. However, for example, if it is managed within a pressure range of about 300 MPa or less that can be handled by a commercially available CIP device or WIP device, the manufacturing cost may be reduced. Alternatively, a hot pressing process, a spark plasma sintering process, a microwave heating process, etc. in which not only the forming process but also sintering is carried out all at once 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, centrifugal casting, 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.

[0056] (Debinding) In the present invention, a normal debinding process can be preferably used. That is, it is possible to go through a temperature-rising 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 not particularly limited, but when using a raw material mixed with an organic additive, it is preferable to raise the temperature to a temperature at which the organic component can be decomposed and removed.

[0057] (Sintering) In the present invention, 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, or sintering under reduced pressure (in a vacuum) is also possible. However, since it is preferable to finally prevent the generation of oxygen deficiency, more preferable atmospheres include an oxygen gas and a reduced-pressure oxygen gas atmosphere.

[0058] The sintering temperature in the sintering process of the present invention is preferably 1400 to 1780 °C, particularly preferably 1480 to 1750 °C. When the sintering temperature is within this range, it is preferable because densification is promoted while suppressing heterogeneous precipitation.

[0059] In the sintering process of the present invention, a sintering holding time of about several hours is sufficient, but the relative density of the sintered body must be densified to at least 94% or more. Further, if it is held for 10 hours or longer to densify the relative density of the sintered body to 99% or more, it is more preferable because the final transparency is improved.

[0060] (Hot Isostatic Pressing (HIP)) In the manufacturing method of the present invention, a step of further performing a hot isostatic pressing (HIP) treatment can be provided after the sintering step.

[0061] At this time, as the type of pressurized gas medium, an inert gas such as argon or nitrogen, or Ar - O2 can be preferably used. The pressure applied by the pressurized gas medium is preferably 50 to 300 MPa, more preferably 100 to 300 MPa. If the pressure is less than 50 MPa, the transparency improvement effect may not be obtained. If it exceeds 300 MPa, further transparency improvement cannot be obtained even if the pressure is increased, and there is a risk of excessive load on the apparatus and damage to the apparatus. It is convenient and preferable that the applied pressure is 196 MPa or less, which can be processed by a commercially available HIP apparatus.

[0062] Also, the treatment temperature (predetermined holding temperature) at that time is set in the range of 1100 to 1780 °C, preferably 1200 to 1730 °C. If the heat treatment temperature exceeds 1780 °C, the risk of oxygen deficiency generation increases, which is not preferable. Also, if the heat treatment temperature is less than 1100 °C, almost no transparency improvement effect of the sintered body can be obtained. Regarding the holding time of the heat treatment 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.

[0063] Note that the heater material, heat insulating material, and processing container for HIP treatment are not particularly limited, but graphite, or molybdenum (Mo), tungsten (W), or platinum (Pt) can be preferably used. As the processing container, yttrium oxide or gadolinium oxide can also be preferably used. Particularly when the processing temperature is 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, which is preferable because it can prevent the generation of oxygen deficiency during HIP treatment. When the processing temperature exceeds 1500°C, graphite is preferable as the heater material and heat insulating material. In this case, any one of graphite, molybdenum (Mo), or tungsten (W) is selected as the processing container, and then any one of yttrium oxide or gadolinium oxide is selected as a double container inside it. If an oxygen releasing material is filled in the container, it is preferable because the amount of oxygen deficiency generated during HIP treatment can be suppressed to be as small as possible.

[0064] (Annealing) In the manufacturing method of the present invention, after the HIP treatment, oxygen deficiency may occur in the obtained transparent ceramic sintered body, and it may exhibit a slightly light gray appearance. In that case, it is preferable to perform an oxidation annealing treatment (oxygen deficiency recovery treatment) in an oxygen atmosphere or in air at a temperature equal to or lower than the HIP treatment temperature, typically 1000 to 1500°C, preferably 1400°C or higher, more preferably 1450°C or higher and 1500°C or lower. The holding time in this case is not particularly limited, but it is preferably selected to be a time sufficient for recovering oxygen deficiency and within a time that does not wastefully consume electricity by performing a long - time treatment. By this oxygen annealing treatment, even a transparent ceramic sintered body that exhibits a slightly light gray appearance in the HIP treatment process can be made into a colorless and transparent paramagnetic garnet - type transparent ceramic body without defect absorption.

[0065] (Optical polishing) In the present invention, for the garnet-type transparent ceramics that have undergone the above series of manufacturing processes, both end faces on the axis for optical utilization are optically polished to form optical surfaces. The polishing conditions are as described above. At this time, when the measurement wavelength λ = 633 nm, the optical surface accuracy is preferably λ / 2 or less, and particularly preferably λ / 8 or less. In addition, it is also possible to further reduce the optical loss by appropriately forming an AR coating layer (antireflection film) on the optically polished surface.

[0066] In the above manner, the garnet-type transparent ceramics of the present invention can be provided. The transparent ceramics containing terbium can be used as a Faraday rotator operable in the wavelength band of 0.9 μm or more and 1.1 μm or less.

[0067] [Magneto-optical device] Furthermore, since the garnet-type transparent ceramics of the present invention are also assumed to be used as a magneto-optical material when containing terbium, after applying a magnetic field parallel to the optical axis to the garnet-type transparent ceramics, it is preferable to configure and utilize a magneto-optical device by setting the polarizer and analyzer such that their optical axes are shifted by 45 degrees from each other. That is, the garnet-type transparent ceramics of the present invention are suitable for magneto-optical device applications, and are particularly preferably used as a Faraday rotator of an optical isolator having a wavelength of 0.9 to 1.1 μm.

[0068] FIG. 1 is a schematic cross-sectional view showing an example of an optical isolator, which is an optical device having a Faraday rotator made of the garnet-type transparent ceramics of the present invention as an optical element. In FIG. 1, the optical isolator 100 includes a Faraday rotator 110 made of the garnet-type transparent ceramics of the present invention, and a polarizer 120 and an analyzer 130, which are polarization materials, are provided before and after the Faraday rotator 110. Further, the optical isolator 100 is preferably arranged in the order of the polarizer 120, the Faraday rotator 110, and the analyzer 130, and a magnet 140 is placed on at least one of their side surfaces.

[0069] In addition, the above optical isolator 100 can be suitably used in an industrial fiber laser device. That is, it is suitable for preventing the reflected light of the laser light emitted from the laser light source from returning to the light source and causing unstable oscillation.

Examples

[0070] 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.

[0071] [Examples 1 to 3, Comparative Examples 1 to 3] Terbium oxide powder, yttrium oxide powder, scandium oxide powder manufactured by Shin-Etsu Chemical Co., Ltd., and aluminum oxide powder manufactured by Dainippon Chemical Co., Ltd. were obtained. Further, a liquid of tetraethyl orthosilicate (TEOS) manufactured by Kishida Chemical Co., Ltd. was obtained. The purity of all the powder raw materials was 99.95 mass% or more, and the purity of the liquid raw material was 99.999 mass% or more. Using the above raw materials, the mixing ratio was adjusted to obtain a composite oxide raw material with a final composition of (Tb 0.6 Y 0.398 Sc 0.002 )3(Al 0.998 Sc 0.002 )5O 12 . At this time, TEOS was also weighed so that the added amount was 0.05 mass% in terms of SiO2 and added simultaneously. Subsequently, the composite oxide raw material was dispersed and mixed in ethanol using an alumina ball mill apparatus. The treatment time was 15 hours. Thereafter, spray drying treatment was performed to produce granular raw materials with an average particle size of 20 μm. The obtained oxide raw materials were subjected to uniaxial press molding and hydrostatic press molding at a pressure of 198 MPa to obtain 6 CIP compacts. All of the compacts were degreased in a muffle furnace under the conditions of 1000 °C for 3 hours to obtain degreased compacts. Subsequently, the degreased compacts were charged into a vacuum sintering furnace and treated at 1550 °C for 3 hours to obtain sintered bodies. At this time, the sintered relative density of the samples was all in the range of 94 to 98%. Each of the obtained sintered compacts was charged into a HIP furnace made of a carbon heater and subjected to HIP treatment under the conditions of 200 MPa, 1600 °C, and 2 hours in Ar. Almost no graying (oxygen deficiency absorption) was confirmed visually for any of the obtained sintered compacts. However, as a precaution, all of the obtained ceramic sintered compacts were annealed at 1450 °C for 30 hours in an air heating furnace to sufficiently recover the oxygen deficiency. Thus, first, samples of garnet-type transparent ceramics were prepared.

[0072] Subsequently, the obtained samples of garnet-type transparent ceramics were ground into a rod shape with a diameter of 5 mm and a length of 25 mm. Next, both end faces of the obtained rod-shaped samples of transparent ceramics were subjected to optical polishing under the respective conditions shown in Table 1 (Examples 1 to 3, Comparative Examples 1 to 3). At this time, all abrasive grains were free abrasive grains, and polishing was performed by changing the type and size of the abrasive grains. Regarding the abrasive grain size, it is known that when precision polishing is performed with sub-micron-sized abrasive grains by gradually decreasing the size from the micron size at which general rough polishing can be performed, the average roughness of the polished surface becomes smaller. Therefore, polishing was carried out by changing the number of stages of polishing while changing this abrasive grain size. The type of surface plate and CMP polishing using colloidal silica were made standard. The optical surface accuracy of each of the obtained polished samples was λ / 8 (measurement wavelength λ = 633 nm) or more.

[0073] Subsequently, the surface roughness of each of the obtained polished rod-shaped samples was measured as follows. (Method for measuring surface roughness) The surface roughness of the polished surface was measured by a method conforming to ISO 25178. Specifically, using a surface roughness measuring device ZeGage Plus manufactured by ZYGO Corporation, the surface roughness was measured by the vertical scanning type low coherence interferometry (CSI method). The light source was a white light source, the objective lens was a 10-fold Mirau type, the measurement area was 830 μm × 830 μm, and the measurement was performed under the condition that the lateral resolution was 0.81 μm. However, when obtaining the surface roughness value, the outer peripheral values of the measurement area of the measurement data were not counted, and only the data in the 800 μm × 800 μm region at the center of the measurement area was referred to. The surface roughness was calculated based on two criteria. One is Sa (arithmetic mean height), which is the average value of the absolute values of the heights of each point within the measurement area with respect to the average surface. The other is Sq (root mean square height), which is a value corresponding to the standard deviation of the distances from the average surface to each point within the measurement area. The average roughness calculated in this way is summarized in Table 1.

[0074]

Table 1

[0075] For each polished rod-shaped sample obtained as described above, the total light transmittance and the forward scattering rate were measured as follows. (Measurement method of total light transmittance and forward scattering rate) The total light transmittance and the forward scattering rate were measured with reference to JIS K7105 (ISO 13468-2:1999) and JIS K7136 (ISO 14782:1999). Specifically, measurements were made at a wavelength of 1064 nm using a spectrophotometer V-670 manufactured by JASCO Corporation. First, for the measurement of the total light transmittance, the light dispersed by the spectroscope without setting the work (sample) in the spectrophotometer V-670 was irradiated, and this light was received by an integrating sphere pre-set in the apparatus, and the collected light was received by a detector. The obtained illuminance was designated as I0. Subsequently, the work was set in the apparatus, and this time the dispersed light was incident on the work, and the transmitted light was collected again by the integrating sphere and received by the detector. The obtained illuminance was designated as I, and it was obtained by the following formula. Total light transmittance (% / 25 mm) = I / I0 × 100 Next, for the measurement of the forward scattering rate, in the same measurement system as all except that the reflector on the back surface of the integrating sphere was removed from the state where the work was set, the dispersed light was incident on the work again, and the transmitted light was collected again by the integrating sphere and received by the detector. The obtained illuminance represents the scattering component other than the direct transmission component, and this was designated as I S and it was obtained by the following formula. Forward scatter rate (% / 25 mm) = I S / I0 × 100 In consideration of the influence of reproducibility and variation, three measurements were made for each condition, and the average value was calculated to obtain the total light transmittance and the forward scatter rate value of each sample. Also, when the beam diameter was made larger than 3 mmφ, the beam skirt began to be kicked at the outer circumference of the 5 mmφ diameter sample, so this beam diameter of 3 mmφ was defined as a state where light was incident on almost the entire surface of the work.

[0076] For each polished rod-shaped sample of the examples and comparative examples prepared in the above manner, an antireflection film (AR coat) designed so that the central wavelength was 1064 nm was coated on both optical end faces. The Verdet constant was measured for each obtained rod-shaped sample with an AR coat in the following manner. (Method for measuring Verdet constant) The Verdet constant V was determined based on the following formula. The magnitude (H) of the magnetic field applied to the sample was the 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, 0.025 m).)

[0077] Finally, for each rod-shaped sample with an AR coat of these examples and comparative examples, the laser damage threshold was evaluated in the following manner. (Method for measuring laser damage threshold) The measurement of the laser damage threshold was performed with reference to ISO 21254. Specifically, the laser damage threshold in terms of a pulse width of 10 ns was determined based on the following calculation formula. The incident energy on the sample was calculated as the full fluence F in terms of a pulse width of 10 ns using the following formula with the parameters of the power E (W) of the light source laser output at a wavelength of 1064 nm, the pulse width τ (ns), the repetition frequency H (Hz), and the incident beam diameter D (μm). F = (2×E / H) ÷ (π×((D×10 -4 ) / 2) 2 ) × √(10 / τ) (J / cm 2 ) Furthermore, a laser beam that is incident on one optical surface of the sample with a fluence F, transmitted through the sample, and then exits from the other optical surface (opposite surface) is monitored with a power meter installed downstream of the sample. In this state, the sample-incident fluence F is gradually increased. Then, a phenomenon occurs in which the input value of the power meter suddenly decreases at a certain point. The incident laser fluence Fmax at this time was read as the laser damage threshold at a wavelength of 1064 nm for each sample. In addition, taking into account the variation in the measurement data, this laser damage threshold was measured 10 times while shifting the incident position within the optical surface, and the average value was determined as the laser damage threshold.

[0078] For reference, an example of the measurement system is shown in Fig. 2. In Fig. 2, the output from the pulsed laser 11, which is the light source, is assumed to be constant, and the amount of incident laser light on the transparent ceramic sample 17 is controlled by rotating the HWP (half-wave plate) 13 set in the optical path. Note that the locations where laser damage occurred were all on the surface of the laser exit surface for all samples. For reference, Fig. 3 shows the appearance (optical microscope photograph) of the damage marks due to laser damage on the exit surface of the sample.

[0079] The above results are summarized and shown in Table 2. For further reference, Fig. 4 shows a graph of the relationship between the average roughness Sa and Sq in Table 1 and the laser damage threshold F max in Table 2.

Table 2

[0080] As a result, the samples fabricated in this example were all highly transparent sintered compacts regardless of whether they were examples or comparative examples. That is, all the samples of Examples 1 to 3 and Comparative Examples 1 to 3 had a total light transmittance of 84.0% or more and a forward scattering rate of 0.5% or less. Also, all of their Verdet constants exceeded 30 rad / (T·m). Nevertheless, the laser damage thresholds of Comparative Examples 1 to 3, where the arithmetic mean height Sa of the optical surface (polished surface) exceeded 0.7 nm or the root mean square height Sq calculated by another calculation method exceeded 0.89 nm, were all below 10 J / cm 2 On the other hand, the laser damage thresholds of Examples 1 to 3, where the arithmetic mean height Sa was 0.7 nm or less or the root mean square height Sq was 0.89 nm or less, all exceeded 10 J / cm 2 Thus, when the laser damage threshold at a pulse width of 10 ns in terms of wavelength 1064 nm exceeds 10 J / cm it becomes possible to stably use the material in the latest high-power and short-pulse laser systems, for example, when mounted as a Faraday rotator for an optical isolator unit, without the material suffering laser damage. 2

[0081] [Examples 4 to 6, Comparative Examples 4 to 6] Yttrium oxide powder manufactured by Shin-Etsu Chemical Co., Ltd. and aluminum oxide powder manufactured by Daming Chemical Co., Ltd. were obtained. Furthermore, a liquid of tetraethyl orthosilicate (TEOS) manufactured by Kishida Chemical Co., Ltd. was obtained. The purity of all the powder raw materials was 99.95 mass% or more, and the purity of the liquid raw material was 99.999 mass% or more. Using the above raw materials, the mixing ratio was adjusted to produce a composite oxide raw material with a final composition of Y3Al5O 12 (commonly known as YAG). At this time, TEOS was also weighed so that its added amount would be 0.05 mass% in terms of SiO2 and added simultaneously. Subsequently, the composite oxide raw material was subjected to dispersion and mixing treatment in an alumina ball mill apparatus in ethanol. The treatment time was 15 hours. ​Subsequently, spray drying treatment was carried out to produce granular raw materials with an average particle size of 20 μm in each case. The obtained oxide raw materials were subjected to uniaxial press molding and isostatic pressing treatment at a pressure of 198 MPa to obtain 6 CIP compacts. All of these compacts were degreased in a muffle furnace under the conditions of 1000 °C for 3 hours to obtain degreased compacts. Subsequently, the degreased compacts were charged into a vacuum sintering furnace and treated at 1600 °C for 3 hours to obtain sintered compacts. At this time, the sintered relative density of the samples was all within the range of 94 - 98%. Each of the obtained sintered compacts was charged into a HIP furnace made of a carbon heater and subjected to HIP treatment under the conditions of 200 MPa, 1600 °C, and 2 hours in Ar. Almost no graying (oxygen deficiency absorption) was confirmed in appearance for any of the obtained sintered compacts. However, as a precaution, all of the obtained ceramic sintered compacts were annealed in an air heating furnace at 1450 °C for 30 hours to carry out a treatment to sufficiently recover the oxygen deficiency. Thus, samples of garnet-type transparent ceramics were first prepared.

[0082] Subsequently, the obtained samples of transparent ceramics were ground into a rod shape with a diameter of 5 mm and a length of 25 mm. Both end faces of the rod-shaped samples of transparent ceramics thus prepared were subjected to optical polishing treatment under the respective conditions (Examples 4 - 6, Comparative Examples 4 - 6) shown in Table 3. The polishing conditions at this time were the same as those in the cases of Examples 1 - 3 and Comparative Examples 1 - 3. Furthermore, the surface roughness of each of the obtained polished rod-shaped samples was measured in the same manner as in the cases of Examples 1 - 3 and Comparative Examples 1 - 3. The average roughness measured and calculated in this way is summarized in Table 3.

[0083]

Table 3

[0084] For each of the above polished rod-shaped samples, the total light transmittance and the forward scattering rate were measured in the same manner as in Examples 1 to 3 and Comparative Examples 1 to 3. Note that since the sample of the transparent ceramics in this example does not contain terbium and the Faraday rotation effect cannot be expected, the Verdet constant of the sample was not evaluated. Finally, for each of the polished rod-shaped samples of Examples 4 to 6 and Comparative Examples 4 to 6, an antireflection film (AR coat) was coated in the same manner as in Examples 1 to 3 and Comparative Examples 1 to 3, and then the laser damage threshold was measured in the same manner as described above. The obtained results are summarized in Table 4.

[0085]

Table 4

[0086] As a result of the above, all the samples prepared in this example, regardless of whether they are examples or comparative examples, were finished as highly transparent sintered bodies. That is, all the samples of Examples 4 to 6 and Comparative Examples 4 to 6 had a total light transmittance of 84.0% or more and a forward scattering rate of 0.5% or less. Nevertheless, when the arithmetic mean height Sa of the optical surface (polished surface) exceeded 0.7 nm, or when the root mean square height Sq calculated by another calculation method exceeded 0.89 nm, the laser damage thresholds of Comparative Examples 4 to 6 were all below 10 J / cm 2 On the other hand, when the arithmetic mean height Sa was 0.7 nm or less, or when the root mean square height Sq was 0.89 nm or less, the laser damage thresholds of Examples 4 to 6 were all above 10 J / cm 2 That is, they were all above. Incidentally, although not shown in this example, the YAG-type transparent ceramics of this example can be used as a laser oscillation medium itself by doping about 1 to 3 mass% of neodymium (Nd).

[0087] Although the present invention has been described above with reference to the embodiments, the present invention is not limited to these embodiments and can be modified within the scope that those skilled in the art can conceive, such as other embodiments, additions, changes, deletions, etc. As long as the functions and effects of the present invention can be achieved in any aspect, it is included in the scope of the present invention.

Description of Reference Numerals

[0088] 11 Pulse laser 12 Amplifier 13 Mirror 14 HWP (Half-Wavelength Plate) 15 PBS 16 Lens 17 Sample 18 Power meter 19 PC 100 Optical isolator 110 Faraday rotator 120 Polarizer 130 Analyzer 140 Magnet

Claims

1. A sintered body of a composite oxide containing yttrium and aluminum, which is a garnet-type transparent ceramic having two optical surfaces serving as an incident surface and an exit surface of laser light, with a total light transmittance at a wavelength of 1064 nm and an optical path length of 25 mm of 84% or more and a forward scattering rate of 0.5% or less. The two optical surfaces have an average roughness of arithmetic mean height Sa ≤ 0.70 nm or root mean square height Sq ≤ 0.89 nm as the average value of the surface roughness of the two optical surfaces. By this, when a short-pulse laser with a wavelength of 1064 nm and a pulse width of 10 ns is incident from one surface of the two optical surfaces and the laser is emitted from the other surface, the laser damage threshold at the laser light exit surface is 10 J / cm² or more. A method for improving the laser damage threshold of garnet-type transparent ceramics.

2. The method for improving the laser damage threshold of garnet-type transparent ceramics according to Claim 1, wherein the composite oxide further contains terbium.

3. The method for improving the laser damage threshold of garnet-type transparent ceramics according to Claim 2, wherein the volume molar concentration of terbium in the composite oxide is equal to or higher than the volume molar concentration of yttrium, and the Verdet constant at a wavelength of 1064 nm of the sintered body is 30 rad / (T·m) or more.

4. The method for improving the laser damage threshold of garnet-type transparent ceramics according to any one of Claims 1 to 3, wherein the sintered body is subjected to a plurality of lapping and polishing operations while gradually replacing the abrasive grains with smaller particle sizes, and the final lapping and polishing operation is performed using abrasive grains with a particle size of 0.1 μm or less. Then, chemical mechanical polishing (CMP) finish polishing is performed in a range where the polishing thickness is less than 1 μm to form the optical surfaces.

5. The method for improving the laser damage threshold of garnet-type transparent ceramics according to any one of Claims 1 to 4, wherein the garnet-type transparent ceramic is a sintered body of a composite oxide represented by the following formula (1) and contains SiO₂ as a sintering aid in an amount exceeding 0 mass% and not exceeding 0.1 mass%. (Tb₁₋ₓ₋ᵧYₓScᵧ)₃(Al₁₋zScₓ)₅O₁₂ (1) (In the formula, 0.05 ≤ x ≤ 0.4, 0 ≤ y < 0.08, 0.52 < 1 - x - y < 0.95, 0 ≤ z < 0.15, 0.001 < y + z < 0.2.)

6. The method for improving the laser damage threshold of garnet-type transparent ceramics according to any one of claims 1 to 5, which further has an AR coating layer on the optical surface.

7. The method for improving the laser damage threshold of garnet-type transparent ceramics according to any one of claims 1 to 6, wherein the garnet-type transparent ceramics are used in a short-pulse laser system.

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