JP7704050B2 - Method for improving the laser damage threshold of garnet-type transparent ceramics - Google Patents

Description

The present invention relates to a method for improving the laser damage threshold of a garnet-type transparent ceramic, and more particularly to a method for improving the laser damage threshold of a terbium-containing paramagnetic garnet-type transparent ceramic suitable for forming magneto-optical devices such as an isolator .

In recent years, it has become possible to increase the output of fiber lasers, and laser processing machines using these fiber lasers have become increasingly popular. However, when external light enters the laser light source built into the laser processing machine, the resonance state becomes unstable, causing the oscillation state to become disturbed. In particular, when the oscillated light is reflected by the optical system along the way and returns to the light source, the oscillation state is greatly disturbed. To prevent this, an optical isolator is usually installed on the light output side of the light source, such as between the laser light source and the optical fiber.

The optical isolator consists of a Faraday rotator, a polarizer placed on the light input side of the Faraday rotator, and an analyzer placed on the light output side of the Faraday rotator. The Faraday rotator is used by applying a magnetic field parallel to the light's direction of travel. At this time, the polarization line of the light only rotates in a fixed direction whether it moves forward or backward through the Faraday rotator. Furthermore, the Faraday rotator is adjusted to a length that rotates the polarization line of the light exactly 45 degrees. Here, if the polarization planes of the polarizer and analyzer are shifted 45 degrees in the direction of rotation of the forward light, the polarization of the forward light will be transmitted because it matches the polarizer position and the analyzer position. On the other hand, the polarization of the backward light will rotate 45 degrees in the opposite direction to the deviation angle of the polarization plane of the polarizer, which is shifted 45 degrees from the analyzer position. Then, the polarization plane of the returning light at the polarizer position will be shifted 45 degrees - (-45 degrees) = 90 degrees from the polarization plane of the polarizer, and it cannot be transmitted through the polarizer. In this way, it functions as an optical isolator, allowing forward-moving light to pass through and exit, while blocking backward-moving returning light.

TGG crystal (Tb ₃ Ga ₅ O ₁₂ ) has been known as the most widely used material for the Faraday rotator that constitutes the optical isolator (JP Patent Publication 2011-213552 A (Patent Document 1)). TGG crystals are still widely used in standard fiber laser devices, but in recent years it has become clear that they are difficult to use in high-power fiber laser devices. Specifically, the bottlenecks are the high intrinsic absorptance of the material (absorption coefficient at a wavelength of 1064 nm: 0.16%/cm) and the large temperature coefficient dn/dt of the refractive index (dn/dt = 18 × 10 ^-6 K ^-1 at room temperature of 25°C).

Therefore, there is a strong demand for new materials that have lower absorption than TGG crystals and can be used in fiber laser devices with higher power bands. One such candidate material is TAG crystal (Tb ₃ Al ₅ O ₁₂ ), but the production of high-quality single-crystal TAG has not yet been invented.

Under these circumstances, it has been recently disclosed that a dense ceramic sintered body having a composition of (Tb _x Y _1-x ) ₃ Al ₅ O ₁₂ (x = 0.5 to 1.0) has a higher extinction ratio (improved from the existing 35 dB to 39.5 dB or more) and can reduce insertion loss (improved from the existing 0.05 dB to 0.01 to 0.05 dB) compared to the existing TGG crystal (Yan Lin Aung, Akio Ikesue, Development of optical grade (Tb _x Y _1-x ) ₃ Al ₅ O ₁₂ ceramics as Faraday rotor material, J. Am. Ceram. Soc., (2017), 100(9), 4081-4087 (Non-Patent Document 1). The material disclosed in this Non-Patent Document 1 is similar to TAG crystals, but unlike previous single crystal materials, it is characterized by being made of ceramics. This makes it possible to obtain high-quality yttrium-substituted TAG crystals that have not been possible until now.

Furthermore, JP 2019-199386 A (Patent Document 2) discloses a paramagnetic garnet-type transparent ceramic having a composition of (Tb _1-xy Y _x Sc _y ) ₃ (Al _1-z Sc _z ) ₅ O ₁₂ (wherein 0.05≦x<0.45, 0<y<0.1, 0.5<1-x-y<0.95, 0.004<z<0.2). This material has a similar composition to Non-Patent Document 1, but differs in that Sc is significantly added. When this material is used, 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 of a practical level. It is said that a truly practical paramagnetic garnet-type transparent ceramic can be provided that is easy to scale up because it is made of ceramics.

Recently, in fiber laser systems, there has been a remarkable trend not only towards higher output but also towards shorter pulses. Generally, when the laser pulse becomes shorter, the peak energy value increases, and material destruction is more likely to occur at the laser output end face of the optical materials used mainly in the fiber laser system. For example, "Masahide Ito, Iwao Ogura, Destruction of Optical Materials by Laser Light, Tokyo University Institute of Industrial Science, Production Research, (1984), Vol. 36, No. 6" (Non-Patent Document 2) discloses that the damage threshold of the surface of a material is generally lower than that of the interior, and that laser damage to the surface is more likely to occur at the output face than the input face.

However, until now, there have been no publicly known examples that specify control values for the surface roughness of optical materials that can be used in short-pulse laser systems, particularly for garnet-type transparent ceramic materials.

Of course, there are prior art examples that attempt to reduce (i.e., smooth) the surface roughness of garnet-type transparent ceramic materials. For example, see Daniel Ross, Hitomi Yamaguchi, Polishing characteristics of Nd:YAG ceramics with various Nd-dopant concentration, CIRP Journal of Manufacturing Science and Technology, 27 (2019) 93-101" (Non-Patent Document 3) discloses a known example in which the surface roughness of Nd:YAG ceramics, particularly non-Nd-doped YAG ceramics, was reduced (i.e., smoothed) to a range of 0.4 to 0.5 nm in arithmetic mean height (Sa value) in a range of 177 μm x 132 μm by polishing Nd:YAG ceramics using submicron-sized diamond abrasive grains with a state-of-the-art polishing method called the MAF method. However, Non-Patent Document 3 does not provide any examples or suggestions regarding the relationship between surface roughness and laser damage threshold.

JP 2011-213552 A JP 2019-199386 A

Yan Lin Aung, Akio Ikesue, Development of optical grade (TbxY1-x)3Al5O12 ceramics as Faraday rotator material, J.Am.Ceram.Soc., (2017), 100(9), 4081-4087 Masahide Ito and Iwao Ogura, Destruction of Optical Materials by Laser Beams, Institute of Industrial Science, University of Tokyo, Production Research, (1984), Vol. 36, No. 6 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

The present invention has been made in view of the above circumstances, and aims to provide a method for improving the laser damage threshold of a garnet-type transparent ceramic that is a sintered body of a complex oxide containing at least yttrium and aluminum as main components, and that can be used in a short-pulse laser system by controlling the average roughness of the optical surface (precision-polished surface) of the sintered body within a specified range.

In order to achieve the above object, the present invention provides the following method for improving the laser damage threshold of a garnet-type transparent ceramic.
1.
A method for improving the laser damage threshold of a garnet-type transparent ceramic, comprising: a sintered body of a complex oxide containing yttrium and aluminum; the garnet-type transparent ceramic has two optical surfaces serving as an entrance surface and an exit surface of a laser beam, the two optical surfaces having a total light transmittance of 84% or more at a wavelength of 1064 nm and a forward scattering rate of 0.5% or less when the optical path length is 25 mm; the two optical surfaces have an arithmetic mean height Sa≦0.70 nm or a root mean square height Sq≦0.89 nm as the average value of the surface roughness of the two optical surfaces; and when a short pulse laser having a wavelength of 1064 nm and a pulse width of 10 ns is incident on one of the two optical surfaces and the laser is emitted from the other surface, the laser damage threshold at the laser beam exit surface is set to 10 J/cm2 or more ^.
2.
2. The method for improving the laser damage threshold of a garnet-type transparent ceramic according to 1 , wherein the composite oxide further contains terbium.
3.
3. The method for improving the laser damage threshold of a garnet-type transparent ceramic according to claim 2, wherein the volume molar concentration of terbium in the composite oxide is equal to or greater than the volume molar concentration of yttrium, and the Verdet constant of the sintered body at a wavelength of 1064 nm is 30 rad/(T·m) or greater.
4.
The method for improving the laser damage threshold of a garnet-type transparent ceramic according to any one of 1 to 3, further comprising the steps of performing a plurality of lapping steps on the sintered body while gradually changing the abrasive grain size to smaller grain sizes, performing a final lapping step using abrasive grains having a grain size of 0.1 μm or less, and then performing optical polishing by CMP finish polishing to a polishing thickness of less than 1 μm to obtain the optical surface.
5.
5. The method for improving the laser damage threshold of a garnet-type transparent ceramic according to any one of 1 to 4, wherein the garnet-type transparent ceramic is a sintered body of a complex oxide represented by the following formula (1) and contains more than 0 mass % and 0.1 mass % or less of SiO2 _as a sintering aid.
(Tb _1-xy Y _x Sc _y ) ₃ (Al _1-z Sc _z ) ₅ O 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, and 0.001<y+z<0.2.)
6 .
6. The method for improving the laser damage threshold of a garnet-type transparent ceramic according to any one of 1 to 5 , further comprising an AR coating layer on the optical surface.
7.
7. The method for improving the laser damage threshold of a 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.

According to the present invention, when a short-pulse laser having a pulse width of 10 nsec is incident on an optical surface and the laser is emitted from the emission surface, it is possible to increase the laser damage threshold of the garnet-type transparent ceramic to 10 J/ ^cm2 or more.

FIG. 1 is a schematic cross-sectional view showing an example of the configuration of an optical isolator using the garnet-type transparent ceramic of the present invention as a Faraday rotator. FIG. 2 is a schematic diagram showing an example of a laser damage threshold measurement system used in the examples. FIG. 2 is an optical microscope photograph showing the shape of damage marks generated on the laser emission surface of the garnet-type transparent ceramic of the example. FIG. 2 is a reference graph showing the relationship between the average roughness and the laser damage threshold of a precision-polished surface to be optically utilized according to the present invention.

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

<Optical surface of garnet-type transparent ceramics>
The requirements for the optical surface, which is the core of the garnet-type transparent ceramics of the present invention (hereinafter, sometimes simply referred to as transparent ceramics), and the polishing conditions for obtaining said optical surface will be described below.

The garnet-type transparent ceramic of the present invention has 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 idea.

That is, it is preferable that the optical surfaces of the garnet-type transparent ceramic of the present invention, which are both end faces through which the laser enters and exits, are precisely polished and that the average roughness of the polished surface is regulated within a certain range. However, transparent optical devices that use lasers to enter and exit have all been made of single crystal or glass up until now, and the surface roughness of the polished ceramic surface has never been regulated.

One reason for this is that until now it has not been possible to obtain ceramics that are sufficiently transparent to allow high-power laser light to be incident and emitted. Another reason is that in recent years, the development of high-power laser systems has progressed in the direction of shorter pulses, which has further reduced the laser damage threshold and made the problem more apparent.

Transparent ceramics are sintered bodies made up of countless polycrystals (sintered grains), and when these transparent ceramics are polished, unlike single crystals or glass, numerous sintered grains appear on the polished surface. The orientation of these sintered grains is generally completely random. Furthermore, these sintered grains are a collection of small single crystals (polycrystals), and it is a well-known fact that single crystals vary in hardness and chemical stability depending on the polishing orientation. Therefore, when these transparent ceramics are polished, the sintered grains that are easy to polish and the sintered grains that are difficult to polish appear on the surface as uneven steps (grain boundary steps).

The transparent ceramics in the present invention are limited to aggregates (sintered bodies) of sintered grains having a garnet structure. The refractive index of crystallites having a garnet structure is independent of orientation, so in theory, laser light is not scattered due to uneven orientation of the crystallites. Therefore, when the material is highly densified and internal air bubbles are removed, the only cause of scattering of laser light is the roughness of the incident and exit surfaces of the laser light, i.e., the optical surface. It is presumed that the laser damage threshold of the transparent ceramics will not decrease if the surface roughness of this optical surface is set within a range in which the incident and exiting laser light is not substantially scattered.

Therefore, for transparent ceramics that have optical surfaces through which laser light enters and leaves, it becomes necessary to numerically specify the surface roughness, including the grain boundary steps.

As a result of intensive research based on the above-mentioned concept, the inventors have succeeded in achieving the object of preventing the incident and outgoing laser light from being substantially scattered and preventing a decrease in the laser damage threshold of the transparent ceramic by defining the average roughness of the optical surface of the garnet-type transparent ceramic within the above-mentioned range. Moreover, 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 ceramic is arithmetic mean height Sa≦0.70 nm or root mean square height Sq≦0.89 nm, preferably arithmetic mean height Sa≦0.70 nm and root mean square height Sq≦0.89 nm, more preferably arithmetic mean height Sa≦0.65 nm and root mean square height Sq≦0.80 nm. This ensures a laser damage threshold of 10 J/cm ² or more, preferably 10.3 J/cm 2 or more, when a short pulse laser with a pulse width of 10 nsec is incident and emitted. In detail, it is preferable that the laser damage threshold at the laser light emission surface is 10 J/cm ² or more 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 emitted from the other surface.

Here, the arithmetic mean height Sa and the root mean square height Sq are based on ISO 25178, which specifies "surface roughness". In other words, the arithmetic mean height Sa is the average of the absolute values of the heights of each point in the measurement area relative to the average surface. The root mean square height Sq is a value equivalent to the standard deviation of the distance from the average surface to each point in the measurement area. In addition, in the present invention, it is preferable to specify the average roughness, including grain boundary steps, within the range of the arithmetic mean height Sa and the root mean square height Sq for a measurement range of 800 μm square (800 μm x 800 μm) in any area of 90% (area ratio) of the optical axis center side excluding the outer peripheral area 10% (area ratio) of the optical surface (precision polished surface).

It is also preferable that the optical surface is composed of two surfaces, an entrance surface and an exit surface for laser light, and that the average roughness of the arithmetic mean height Sa and the root mean square height Sq is the average value of the surface roughness of the two optical surfaces.

In the present invention, as long as an optical surface having the above average roughness can be realized, the conditions for forming the optical surface, particularly the polishing conditions, are not limited in the optical polishing step in the manufacturing process of garnet-type transparent ceramics described later. However, in general, when ceramics are subjected to excessive CMP (chemical mechanical polishing) treatment, the grain boundary step becomes larger and larger, so it is preferable to finish the surface as flat and as small in roughness as possible by lapping using abrasive grains that have no chemical action. Specifically, diamond abrasive grains, alumina abrasive grains, ceria abrasive grains, B ₄ C abrasive grains, etc. are suitably selected, and it is preferable to use these to carefully perform lapping treatment while gradually replacing abrasive grains with small grit (large grain size) with abrasive grains with large grit (small grain size). In addition, the surface plate used at this time may be made of either metal or cloth.

The final abrasive size (grain size) should be submicron level. Specifically, it is preferable to go through a finish lapping process using free abrasive grains with a small grain size of 0.3 μm or less, more preferably 0.1 μm or less. However, it is known that lapping alone generally leaves small surface scratches or very shallow scratches. Therefore, CMP finish polishing may be performed very slightly to a polishing thickness of less than 1 μm, more preferably less than 200 nm. In this case, it is preferable to use free abrasive grains with a small grain size of 0.3 μm or less, more preferably 0.1 μm or less, such as colloidal silica.

The transparent ceramic of the present invention preferably further has an AR coating layer on the optical surface. That is, it is preferable to further apply an AR coating to both end faces (optical surfaces) to be optically utilized that have been precisely polished as described above. If an AR coating is applied to the optical surfaces that are the entrance and exit surfaces of the laser light, Fresnel reflection at the material interface can be suppressed, and therefore a decrease in the laser damage threshold can be prevented, which is preferable.

<Composition of garnet-type transparent ceramics>
The garnet-type transparent ceramic of the present invention is a sintered body of a complex oxide containing yttrium and aluminum. Garnet-type ceramics are cubic crystals, so there is no orientation dependency of the refractive index, and even though they are ceramics, they can be finished into an optical material with small laser scattering like single crystals. In addition, the sintered body of a complex oxide containing yttrium and aluminum contains aluminum as a main component, so that the melting point can be significantly increased, and the Mohs hardness is as hard as 8 or more, and the melting point is as high as 2000°C or more, making it a very strong material, which is preferable because it can increase the laser damage threshold.

The garnet-type transparent ceramic of the present invention is preferably a sintered body of a complex oxide further containing terbium. That is, if a part of the yttrium sites in the above composition is replaced with terbium, that is, if a sintered body of a complex oxide containing at least yttrium, terbium, and aluminum as main components is used, the Faraday effect can be imparted, and it is preferable that it can be used as a magneto-optical material. In this case, it is particularly preferable that the volume molar concentration of terbium is equal to or higher than the volume molar concentration of yttrium. If the volume molar concentration of terbium in the composition constituting the garnet-type transparent ceramic is controlled to be equal to or higher than the volume molar concentration of yttrium, the Verdet constant at a wavelength of 1064 nm will be 30 rad/(T·m) or more.

Furthermore, the specific composition of the transparent ceramic of the present invention is preferably represented by the following formula (1).
(Tb _1-xy Y _x Sc _y ) ₃ (Al _1-z Sc _z ) ₅ O 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, and 0.001<y+z<0.2.)

Here, a sufficient Verdet constant can be ensured by adding terbium (Tb) in the concentration range expressed by formula (1), and suppression of heterogeneous phases and minimization of internal strain can be ensured by adding yttrium (Y) in the concentration range expressed by formula (1). Also, complete annihilation of heterogeneous phases is achieved by adding scandium (Sc) in the concentration range expressed by formula (1).
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.

The detailed numerical ranges for x, y, and z in formula (1) are as follows:
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 in this range, the perovskite type heterophase can be reduced to a level that is not detected by X-ray diffraction (XRD) analysis.

If x is less than 0.05, the effect of substituting part of the terbium with yttrium is not obtained, and the conditions are essentially the same as those for producing TAG, which is undesirable because it is difficult to stably produce a high-quality ceramic sintered body with low scattering and low absorption. Also, if x is greater than 0.4, it is undesirable because the Verdet constant at a wavelength of 1064 nm is less than 32 rad/(T·m). Furthermore, if 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 exceeds 25 mm, which is undesirable because it makes production difficult.

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 in this range, it is preferable because the perovskite-type heterophase can be reduced to a level that is not detectable by X-ray diffraction (XRD) analysis. Furthermore, it is preferable because it is possible to prevent an excessive decrease in thermal conductivity due to the homogeneity of the sintered body and grain boundary scattering.

When y is 0.08 or more, the effect of inhibiting the precipitation of the perovskite-type heterophase or alumina heterophase saturates and remains unchanged, while the sintering inhibition effect of scandium is excessively effective, resulting in uneven sintering, residual sintering distortion, or residual grain boundary scattering, which is undesirable as it results in a local decrease in the extinction ratio and a decrease in the average value of the thermal conductivity.

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 in this range, a large Verdet constant can be ensured and high transparency can be obtained at a wavelength of 1064 nm.

In formula (1), 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 in this range, it is preferable because the perovskite-type heterophase can be reduced to a level that is not detectable by X-ray diffraction (XRD) analysis. Furthermore, it is preferable because it can prevent an excessive decrease in thermal conductivity due to the homogeneity of the sintered body and grain boundary scattering.

When z is 0.15 or more, the effect of inhibiting the precipitation of the perovskite-type heterophase or alumina heterophase saturates and remains unchanged, while the sintering inhibition effect of scandium is excessively effective, resulting in uneven sintering, residual sintering distortion, or residual grain boundary scattering, which results in a local decrease in the extinction ratio and a decrease in the average value of the thermal conductivity, which is not preferable.

In the transparent ceramics of the present invention, scandium (Sc) can be added within the range of y and z in the above formula (1). The amount of scandium added, y and z, includes 0 when viewed alone. However, when viewed as the composition of the entire paramagnetic garnet-type transparent ceramics, it is preferable to add y+z in excess of 0.001 within the range of formula (1), since this makes it possible to stably produce a highly transparent sintered body.

That is, in 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 in this range, it is preferable because the perovskite-type heterophase can be reduced to a level that is not detectable by X-ray diffraction (XRD) analysis. Furthermore, it is preferable because it is possible to prevent an excessive decrease in thermal conductivity due to the homogeneity of the sintered body and grain boundary scattering.

When y + z is 0.001 or less, the risk of precipitation of perovskite-type heterophases or alumina heterophases increases, which is not preferable. Also, when y + z is 0.2 or more, while the effect of inhibiting the precipitation of perovskite-type heterophases or alumina heterophases saturates and remains unchanged, the sintering inhibition effect of scandium is excessively effective, resulting in uneven sintering, residual sintering distortion, or residual grain boundary scattering, which is not preferable as a result of localized decreases in the extinction ratio and a decrease in the average value of thermal conductivity.

In the transparent ceramics of the present invention, it is preferable that the main component of the 6-coordinated sites and 4-coordinated sites in the garnet structure is aluminum (Al). If the main component of these sites can be aluminum (Al), the bonding strength of the crystal is improved.

In addition, in the transparent ceramics of the present invention, it is preferable to select terbium (Tb) and yttrium (Y) as the main components of the 8-coordination site, and to control the terbium concentration in the range of 1-x-y (0.52<1-x-y<0.95) and the yttrium concentration in the range of x (0.05≦x≦0.4). When the terbium concentration is controlled in this range, it is possible to ensure a Verdet constant of 32 rad/(T・m) or more at a wavelength of 1064 nm.

The paramagnetic garnet-type transparent ceramic obtained by the present invention preferably contains as a main component the components represented by the formula (1) and as a secondary component _SiO2 acting as a sintering aid in an amount of up to 0.1 mass% (i.e., the content is more than 0 mass% and 0.1 mass% or less). When _SiO2 is contained as a sintering aid in this range, the transparency of the obtained paramagnetic garnet-type ceramic is improved to a practical level and is stable, which is preferable.

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

The transparent ceramics of the present invention are composed of the above-mentioned main components and sub-components, but may further contain other elements. Typical examples of other elements include rare earth elements such as lutetium (Lu) and cerium (Ce), and various impurities such as sodium (Na), calcium (Ca), magnesium (Mg), phosphorus (P), tungsten (W), and molybdenum (Mo).

The content of other elements is preferably 10 parts by mass or less, more preferably 0.1 parts by mass or less, and particularly preferably 0.001 parts by mass or less (substantially zero), when the total amount of Y, or Tb and Y, is 100 parts by mass.

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

<Method of manufacturing garnet-type transparent ceramics>
[Raw materials]
The raw materials used in the present invention are at least terbium, yttrium, scandium, and aluminum oxide powders, and the raw material purity is preferably 99.9% by mass or more, and more preferably 99.99% by mass or more.
These elements are weighed in predetermined amounts, and silicon oxide (SiO ₂ ) is further added in an amount of more than 0 mass % and 0.1 mass % or less, and the mixture is appropriately treated using a wet ball mill or a bead mill.

Various organic additives may be added to the garnet-type oxide powder raw material used in the present invention for the purpose of improving the quality stability and yield in the subsequent ceramic manufacturing process. In the present invention, these are not particularly limited. In other words, various dispersants, binders, lubricants, plasticizers, etc. can be suitably used. However, it is preferable to select high-purity types of organic additives that do not contain unnecessary metal ions. In addition, the order of addition of each organic additive must be appropriately designed so as not to interfere with the management of the properties (particle size distribution, etc.) of the raw material to be manufactured.

[Manufacturing process]
In the present invention, the raw material powder is pressed into a predetermined shape, or the wet slurry is cast in place to produce a molded body. The obtained molded body is thoroughly degreased and then sintered to produce a sintered body with a relative density of at least 94%. It is preferable to carry out a hot isostatic pressing (HIP) process as a subsequent process. If the hot isostatic pressing (HIP) process is carried out as is, the paramagnetic garnet-type transparent ceramics will be reduced and some oxygen deficiency will occur. Therefore, it is preferable to carry out a slight oxidation HIP process or an annealing process in an oxidizing atmosphere (oxidation annealing process) after the HIP process to recover the oxygen deficiency. This makes it possible to obtain a transparent garnet-type oxide ceramics without defect absorption.

(molding)
In the present invention, a normal press molding process can be suitably used. That is, a very common uniaxial pressing process in which a mold is filled and pressurized from a certain direction, a cold isostatic pressing (CIP) process in which a mold is sealed and stored in a deformable waterproof container and pressurized with hydrostatic pressure, and a warm isostatic pressing (WIP) process can be suitably used. The applied pressure can be appropriately adjusted while checking the relative density of the obtained molded body, and is not particularly limited. For example, the manufacturing cost can be reduced by managing the pressure within a pressure range of about 300 MPa or less that can be handled by a commercially available CIP device or WIP device. Alternatively, a hot press process, a discharge plasma sintering process, a microwave heating process, etc. in which not only the molding process but also sintering is performed at once during molding can also be suitably used. Furthermore, it is also possible to produce a molded body by a casting molding method instead of a press molding method. Molding methods such as pressure casting, centrifugal casting, and extrusion molding can also be employed by optimizing the shape and size of the oxide powder starting material and the combination of various organic additives.

(Degreasing)
In the present invention, a normal degreasing process can be suitably used. That is, a temperature-raising degreasing process using a heating furnace can be used. The type of atmospheric gas used is not particularly limited, and air, oxygen, hydrogen, etc. can be suitably used. There is no particular limit to the degreasing temperature, but if a raw material containing an organic additive is used, it is preferable to raise the temperature to a temperature at which the organic components can be decomposed and removed.

(Sintering)
In the present invention, a general sintering process can be preferably used. That is, a heat sintering process such as a resistance heating method or an induction heating method can be preferably used. The atmosphere at this time is not particularly limited, and sintering can be performed in various atmospheres such as an inert gas, oxygen gas, hydrogen gas, helium gas, or under reduced pressure (vacuum). However, since it is preferable to prevent the occurrence of oxygen deficiency in the end, more preferable atmospheres include an oxygen gas atmosphere and a reduced pressure oxygen gas atmosphere.

The sintering temperature in the sintering process of the present invention is preferably 1400 to 1780°C, and more preferably 1480 to 1750°C. A sintering temperature in this range is preferable because it promotes densification while suppressing the precipitation of different phases.

In the sintering process of the present invention, a sintering holding time of several hours is sufficient, but the relative density of the sintered body must be densified to at least 94%. It is even more preferable to hold the sintering for 10 hours or more to densify the relative density of the sintered body to 99% or more, as this improves the final transparency.

(Hot Isostatic Pressing (HIP))
In the manufacturing method of the present invention, an additional step of hot isostatic pressing (HIP) can be provided after the sintering step.

In this case, the type of pressurized gas medium is preferably an inert gas such as argon or nitrogen, or Ar- _O2 . 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, and if the pressure exceeds 300 MPa, no further improvement in transparency can be obtained even if the pressure is increased, and the load on the device becomes too heavy, which may damage the device. It is convenient and preferable that the applied pressure is 196 MPa or less, which can be processed by a commercially available HIP device.

The treatment temperature (predetermined holding temperature) is set in the range of 1100 to 1780°C, preferably 1200 to 1730°C. Heat treatment temperatures above 1780°C are undesirable because they increase the risk of oxygen vacancies. Heat treatment temperatures below 1100°C are undesirable because they do not improve the transparency of the sintered body. There are no particular restrictions on the holding time of the heat treatment temperature, but holding for too long is undesirable because it increases the risk of oxygen vacancies. Typically, it is preferably set in the range of 1 to 3 hours.

The heater material, heat insulating material, and processing vessel for HIP processing are not particularly limited, but graphite, molybdenum (Mo), tungsten (W), and platinum (Pt) can be preferably used, and yttrium oxide and gadolinium oxide can also be preferably used as the processing vessel. In particular, when the processing temperature is 1500°C or lower, platinum (Pt) can be used as the heater material, heat insulating material, and processing vessel, and the pressurized gas medium can be Ar- _O2 , which is preferable because it can prevent the occurrence of oxygen deficiency during HIP processing. When the processing temperature exceeds 1500°C, graphite is preferable as the heater material and heat insulating material, but in this case, it is preferable to select graphite, molybdenum (Mo), or tungsten (W) as the processing vessel, and further select yttrium oxide or gadolinium oxide as a double vessel inside it, and then fill the vessel with an oxygen release material, since the amount of oxygen deficiency generated during HIP processing can be suppressed to a minimum.

(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 the body may have a faint light gray appearance. In that case, it is preferable to perform an oxidation annealing treatment (oxygen deficiency recovery treatment) in an oxygen atmosphere or air at a temperature below the HIP treatment temperature, typically 1000 to 1500°C, preferably 1400°C or higher, more preferably 1,450°C to 1,500°C. In this case, the holding time is not particularly limited, but it is preferable to select a time that is sufficient to recover the oxygen deficiency and is within a time that does not consume electricity by performing the treatment for a long time unnecessarily. By the oxygen annealing treatment, even if the transparent ceramic sintered body has a faint light gray appearance in the HIP treatment process, it can be made into a colorless, transparent, paramagnetic garnet-type transparent ceramic body without defect absorption.

(optical polishing)
In the present invention, the garnet-type transparent ceramics that has undergone the above-mentioned series of manufacturing steps is optically polished at both end faces on the axis to be optically utilized to form optical surfaces. The polishing conditions are as described above. In this case, the optical surface accuracy is preferably λ/2 or less, particularly preferably λ/8 or less, when the measurement wavelength λ=633 nm. It is also possible to further reduce optical loss by appropriately forming an AR coating layer (anti-reflection film) on the optically polished surface.

In this manner, the garnet-type transparent ceramic of the present invention can be provided. Transparent ceramics containing terbium can be used as a Faraday rotator that can operate in the wavelength range of 0.9 μm or more and 1.1 μm or less.

[Magneto-optical devices]
Furthermore, since the garnet-type transparent ceramic of the present invention is also expected to be used as a magneto-optical material when it contains terbium, it is preferable to apply a magnetic field parallel to the optical axis of the garnet-type transparent ceramic, and then set a polarizer and an analyzer so that their optical axes are shifted by 45 degrees from each other to form a magneto-optical device. That is, the garnet-type transparent ceramic of the present invention is suitable for use in magneto-optical devices, and is particularly suitable for use as a Faraday rotator in an optical isolator with a wavelength of 0.9 to 1.1 μm.

Figure 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 ceramic of the present invention as an optical element. In Figure 1, the optical isolator 100 includes a Faraday rotator 110 made of the garnet-type transparent ceramic of the present invention, and a polarizer 120 and an analyzer 130, which are polarizing materials, are provided in front of and behind the Faraday rotator 110. In addition, 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 preferably placed on at least one of the side surfaces of the polarizer 120, the Faraday rotator 110, and the analyzer 130.

The optical isolator 100 can also be used effectively in industrial fiber laser devices. In other words, it is effective in preventing reflected light from the laser light emitted from the laser light source from returning to the light source, causing the oscillation to become unstable.

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

[Examples 1 to 3, Comparative Examples 1 to 3]
We obtained terbium oxide powder, yttrium oxide powder, and scandium oxide powder manufactured by Shin-Etsu Chemical Co., Ltd., and aluminum oxide powder manufactured by Taimei Chemical Co., Ltd. Furthermore, we obtained liquid tetraethyl orthosilicate (TEOS) manufactured by Kishida Chemical Co., Ltd. The purity of the powder raw materials was 99.95% by mass or more, and the purity of the liquid raw materials was 99.999% by mass or more.
Using the above raw materials, _a composite oxide raw material was prepared by adjusting _the mixing ratio to have a final composition _of ( _{Tb0.6Y0.398Sc0.002} ) _{3 ( Al0.998Sc0.002} ) _5O12 . At this time, TEOS was also weighed and added at the same time so that the amount added was 0.05 mass % in terms of _SiO2 .
The composite oxide raw material was then dispersed and mixed in ethanol using an alumina ball mill for 15 hours.
Then, a spray-drying process was carried out to produce granular raw materials each having an average particle size of 20 μm. The obtained oxide raw materials were subjected to uniaxial pressing and isostatic pressing at a pressure of 198 MPa to obtain six CIP molded bodies. All of the molded bodies were degreased in a muffle furnace at 1000° C. for 3 hours to obtain degreased molded bodies.
The degreased green body was then placed in a vacuum sintering furnace and sintered at 1550° C. for 3 hours to obtain a sintered body. At this time, the sintered relative density of each sample was within the range of 94 to 98%.
The obtained sintered bodies were placed in a carbon heater HIP furnace 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 observed in the appearance of any of the obtained sintered bodies. However, to be on the safe side, all of the obtained ceramic sintered bodies were annealed in an atmospheric heating furnace at 1450°C for 30 hours to sufficiently recover the oxygen deficiency. In this way, first, samples of garnet-type transparent ceramics were prepared.

Subsequently, the obtained sample of garnet-type transparent ceramics was ground into a rod shape having a diameter of 5 mm and a length of 25 mm.
Next, both end faces of the obtained rod-shaped transparent ceramic samples were optically polished under the conditions shown in Table 1 (Examples 1 to 3, Comparative Examples 1 to 3). In this case, all abrasive grains were free abrasive grains, and the type and size of the abrasive grains were changed for polishing. It is known that the average roughness of the polished surface is reduced by gradually decreasing the abrasive grain size from a micron size that can generally be used for rough polishing, and then using submicron-sized abrasive grains for precision polishing. Therefore, polishing was performed by changing the abrasive grain size and the number of polishing stages. The type of surface plate and the CMP polishing using colloidal silica were standard. The optical surface precision of each of the obtained polished samples was λ/8 (measurement wavelength λ=633 nm) or higher.

Next, the surface roughness of each of the polished rod-shaped samples obtained was measured in the following manner.
(Method of measuring surface roughness)
The surface roughness of the polished surface was measured according to ISO 25178. Specifically, the surface roughness was measured by vertical scanning low coherence interferometry (CSI) using a surface roughness measuring device ZeGage Plus manufactured by ZYGO Corporation. Measurements were performed under the following conditions: a white light source, a 10x Mirau type objective lens, a measurement area of 830 μm×830 μm, and a lateral resolution of 0.81 μm. However, when determining the surface roughness value, the outer periphery value of the measurement area of the measurement data was not counted, and only the data of the 800 μm×800 μm area in the center of the measurement area was referenced.
The surface roughness was calculated based on two criteria: Sa (arithmetic mean height), which is the average of the absolute values of the heights of each point in the measurement area relative to the average surface, and Sq (root mean square height), which is the standard deviation of the distances from the average surface to each point in the measurement area.
The average roughnesses thus calculated are shown in Table 1.

For each of the polished rod-shaped samples obtained as described above, the total light transmittance and the forward scattering rate were measured as follows.
(Method of measuring 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, the measurements were performed at a wavelength of 1064 nm using a spectrophotometer V-670 manufactured by JASCO Corporation.
First, the total light transmittance is measured by irradiating the spectrophotometer V-670 with light dispersed by a spectroscope without setting a workpiece (sample), receiving the light with an integrating sphere set in the device beforehand, and receiving the collected light with a detector. The obtained illuminance is _I0 , and then the workpiece is set in the device, this time the dispersed light is incident on the workpiece, and the transmitted light is again collected with an integrating sphere and received with a detector. The obtained illuminance is I and is calculated by the following formula.

Total light transmittance (%/25mm) = I/I ₀ ×100

Next, the forward scattering rate was measured using the same measurement system except that the reflector on the back of the integrating sphere was removed from the state in which the workpiece was set, and the dispersed light was again incident on the workpiece, and the transmitted light was again collected by the integrating sphere and received by the detector. The obtained illuminance represents the scattered component other than the linear transmitted component, and is defined as I _S and calculated by the following formula.

Forward scattering rate (%/25mm) = I _S /I ₀ ×100

In addition, in order to take into account the effects of reproducibility and variation, three samples were measured for each condition, and the average values were calculated to determine the total light transmittance and forward scattering rate of each sample. In addition, when the beam diameter is made larger than 3 mmφ, the beam begins to be kicked off at the outer periphery of the sample with a diameter of 5 mmφ, so this beam diameter of 3 mmφ was defined as the state in which light is actually incident on the entire surface of the workpiece.

Each of the polished rod-shaped samples of the Examples and Comparative Examples prepared in the above manner was coated on both optical end faces with an anti-reflection film (AR coat) designed to have a central wavelength of 1064 nm. The Verdet constant of each of the obtained AR-coated rod-shaped samples was measured in the following manner.
(Method of measuring the Verdet constant)
The Verdet constant V was calculated based on the following formula: The magnitude (H) of the magnetic field applied to the sample was calculated by simulation from the dimensions of the 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 (0.025 m in this case).)

Finally, the laser damage threshold of each of the AR-coated rod-shaped samples of the Examples and Comparative Examples was evaluated in the following manner.
(Method of measuring laser damage threshold)
The laser damage threshold was measured with reference to ISO 21254. Specifically, the laser damage threshold converted into a pulse width of 10 ns was calculated based on the following formula: The incident energy to the sample was calculated as a fluence F converted into a pulse width of 10 ns using the following formula, using the parameters of the power E (W), pulse width τ (ns), repetition frequency H (Hz), and incident beam diameter D (μm) of the light source laser outputting a wavelength of 1064 nm.

F=(2×E/H)÷(π×((D×10 ^-4 )/2) ² )×√(10/τ)(J/cm ² )
Furthermore, the laser beam incident on one optical surface of the sample at a fluence F, transmitted through it, and emitted from the other optical surface (opposite surface) was monitored by a power meter installed downstream of the sample. In this state, the sample incident fluence F was gradually increased. Then, at a certain point, a phenomenon occurred in which the input value of the power meter suddenly decreased. The incident laser fluence Fmax at this time was read as the laser damage threshold at a wavelength of 1064 nm for each sample. Taking into consideration the variation in the measurement data, the laser damage threshold was measured at 10 points while shifting the incident position on the optical surface, and the average value was calculated as the laser damage threshold.

For reference, an example of a measurement system is shown in Figure 2. In Figure 2, the output from a pulsed laser 11, which is the light source, is kept constant, and the amount of laser light incident on a transparent ceramic sample 17 is controlled by rotating a half-wave plate (HWP) 13 set in the middle of the optical path. Note that the location where laser damage occurred was on the laser exit surface in all samples.
For reference, FIG. 3 shows the appearance (optical microscope photograph) of damage marks caused by laser damage on the emission surface of the sample.

The above results are summarized in Table 2. For further reference, a graph showing the relationship between the average roughness Sa and Sq in Table 1 and the laser damage threshold _Fmax in Table 2 is shown in FIG.

As a result, all of the samples produced in this embodiment, whether they were examples or comparative examples, were highly transparent sintered bodies. That is, all of the samples in 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. In addition, all of the Verdet constants exceeded 30 rad/(T·m).
Nevertheless, the laser damage thresholds of Comparative Examples 1 to 3, in which 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, in which the arithmetic mean height Sa was 0.7 nm or less or the root mean square height Sq was 0.89 nm or less, were all above 10 J/cm ² .
In this way, if the laser damage threshold at a wavelength of 1064 nm converted into a pulse width of 10 ns exceeds 10 J/ ^cm , the material can be used stably without suffering laser damage, even when it is installed in the latest high-output, short-pulse laser system, for example, as a Faraday rotator for an optical isolator unit.

[Examples 4 to 6, Comparative Examples 4 to 6]
We obtained yttrium oxide powder manufactured by Shin-Etsu Chemical Co., Ltd. and aluminum oxide powder manufactured by Taimei Chemical Co., Ltd. Furthermore, we obtained liquid tetraethyl orthosilicate (TEOS) manufactured by Kishida Chemical Co., Ltd. The purity of the powder raw materials was 99.95% by mass or more, and the purity of the liquid raw materials was 99.999% by mass or more.
Using the above raw materials, a composite oxide raw material with _a final composition _{of Y3Al5O12} (commonly known as YAG) was prepared by adjusting the mixing ratio. At this time, TEOS was also weighed and added at the same time so that the amount added was 0.05 mass% in terms of _SiO2 .
The composite oxide raw material was then dispersed and mixed in ethanol using an alumina ball mill for 15 hours.
Then, a spray-drying process was carried out to produce granular raw materials each having an average particle size of 20 μm. The obtained oxide raw materials were subjected to uniaxial pressing and isostatic pressing at a pressure of 198 MPa to obtain six CIP molded bodies. All of the molded bodies were degreased in a muffle furnace at 1000° C. for 3 hours to obtain degreased molded bodies.
The degreased green body was then placed in a vacuum sintering furnace and sintered at 1600° C. for 3 hours to obtain a sintered body. At this time, the relative sintered density of each sample was within the range of 94 to 98%.
The obtained sintered bodies were placed in a carbon heater HIP furnace 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 observed in the appearance of any of the obtained sintered bodies. However, to be on the safe side, all of the obtained ceramic sintered bodies were annealed in an atmospheric heating furnace at 1450°C for 30 hours to sufficiently recover the oxygen deficiency. In this way, first, samples of garnet-type transparent ceramics were prepared.

Subsequently, the obtained transparent ceramic sample was subjected to a grinding process so as to have a rod shape with a diameter of 5 mm and a length of 25 mm.
Both end faces of the thus prepared rod-shaped transparent ceramic samples were subjected to optical polishing under the conditions (Examples 4 to 6, Comparative Examples 4 to 6) shown in Table 3. The polishing conditions were the same as those in Examples 1 to 3 and Comparative Examples 1 to 3.
Furthermore, the surface roughness of each of the polished rod-shaped samples obtained was measured in the same manner as in Examples 1-3 and Comparative Examples 1-3.
The average roughnesses thus measured and calculated are shown in Table 3.

The total light transmittance and forward scattering rate of each of the above polished rod-shaped samples were measured in the same manner as in Examples 1 to 3 and Comparative Examples 1 to 3. Note that since the transparent ceramic samples of this example did not contain terbium and therefore the Faraday rotation effect could not be expected, the Verdet constant of the samples was not evaluated.
Finally, each of the polished rod-shaped samples of Examples 4 to 6 and Comparative Examples 4 to 6 was coated with an anti-reflection film (AR coat) in the same manner as in Examples 1 to 3 and Comparative Examples 1 to 3, and the laser damage threshold was measured in the same manner as described above.
The results are summarized in Table 4.

As a result, all of the samples prepared in this embodiment, regardless of whether they were examples or comparative examples, were finished as highly transparent sintered bodies. That is, all of the samples in 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, the laser damage thresholds of Comparative Examples 4 to 6, in which 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 4 to 6, in which the arithmetic mean height Sa was 0.7 nm or less or the root mean square height Sq was 0.89 nm or less, were all above 10 J/cm ² .
Although not shown in this embodiment, the YAG type transparent ceramics of this embodiment can be used as a laser oscillation medium itself by doping it with about 1 to 3 mass % of neodymium (Nd).

Although the present invention has been described above using the above-mentioned embodiment, the present invention is not limited to this embodiment, and can be modified within the scope of what a person skilled in the art can imagine, such as other embodiments, additions, modifications, deletions, etc., and any aspect is within the scope of the present invention as long as it provides the effects of the present invention.

11 Pulse laser 12 Amplifier 13 Mirror 14 HWP (half wave 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 (7)

A method for improving the laser damage threshold of a garnet-type transparent ceramic, comprising: a sintered body of a complex oxide containing yttrium and aluminum; the garnet-type transparent ceramic has two optical surfaces serving as an entrance surface and an exit surface of a laser beam, the two optical surfaces having a total light transmittance of 84% or more at a wavelength of 1064 nm and a forward scattering rate of 0.5% or less when the optical path length is 25 mm; the two optical surfaces have an arithmetic mean height Sa≦0.70 nm or a root mean square height Sq≦0.89 nm as the average value of the surface roughness of the two optical surfaces; and when a short pulse laser having a wavelength of 1064 nm and a pulse width of 10 ns is incident on one of the two optical surfaces and the laser is emitted from the other surface, the laser damage threshold at the laser beam exit surface is set to 10 J/cm2 or more ^. 2. The method for improving the laser damage threshold of a garnet-type transparent ceramic according to claim 1 , wherein the composite oxide further contains terbium. 3. A method for improving the laser damage threshold of a garnet-type transparent ceramic as described in claim 2, wherein the volume molar concentration of terbium in the composite oxide is equal to or greater than the volume molar concentration of yttrium, and the Verdet constant of the sintered body at a wavelength of 1064 nm is 30 rad/(T·m) or greater. The method for improving the laser damage threshold of a garnet-type transparent ceramic according to any one of claims 1 to 3, wherein the sintered body is subjected to multiple lapping processes while gradually changing the abrasive grain size to smaller grain sizes, followed by a final lapping process using abrasive grains having a grain size of 0.1 µm or less, and then optical polishing is performed by CMP finish polishing to a polishing thickness of less than 1 µm to obtain the optical surface. 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 5. The method for improving the laser damage threshold of a garnet-type transparent ceramic according to claim 1, wherein the garnet-type transparent ceramic contains more than 0 mass % and 0.1 mass % or less of
(Tb _1-x-y Y _x S.C. _y ) ₃ (Al _1-z S.C. _z ) ₅ O 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, and 0.001<y+z<0.2.) 6. The method for improving a laser damage threshold of a garnet-type transparent ceramic according to claim 1 , further comprising the step of providing an AR coating layer on said optical surface. 7. The method for improving the laser damage threshold of a garnet-type transparent ceramic according to claim 1, wherein the garnet-type transparent ceramic is used in a short pulse laser system.