JP7567920B2 - Laser damage resistance evaluation method for paramagnetic garnet-type transparent ceramics - Google Patents

Description

The present invention relates to a method for determining the laser damage resistance of a paramagnetic garnet-type transparent ceramic having transparency in the visible and/or near infrared region, and more particularly to a method for determining the laser damage resistance of a paramagnetic garnet-type transparent ceramic containing terbium that is suitable for forming magneto-optical devices such as optical isolators.

Industrial laser processing machines are equipped with optical isolators to prevent the return of light, such as reflected light, and inside these are mounted terbium-doped glass or terbium gallium garnet crystals (TGG crystals) as Faraday rotators (for example, JP 2011-213552 A (Patent Document 1)). The magnitude of the Faraday effect is quantified by the Verdet constant, and the Verdet constant of TGG crystals is 40 rad/(T·m) (0.13 min/(Oe·cm)), while that of terbium-doped glass is 0.098 min/(Oe·cm). Since the Verdet constant of TGG crystals is relatively large, they are widely used as standard Faraday rotators. Another example is terbium aluminum garnet crystal (TAG crystal). The Verdet constant of the TAG crystal is about 1.3 times that of the TGG crystal, and therefore the length of the Faraday rotator can be shortened, making it a good crystal that can be used in fiber lasers (for example, JP-A-2002-293693 (Patent Document 2) and JP-A-4107292 (Patent Document 3)).

In recent years, a method for producing TAG from transparent ceramics has been disclosed (for example, International Publication No. 2017/033618 (Patent Document 4), "High Verdet constant of Ti-doped terbium aluminum garnet (TAG) ceramics" (Non-Patent Document 1)). In addition, a method for producing transparent ceramics (Tb _x Y _1-x ) ₃ Al ₅ O ₁₂ (0.2≦x≦0.8, or 0.5≦x≦1.0) (YTAG) in which part of the terbium is replaced with yttrium has also been reported (for example, "Fabrication and properties of (Tb _x Y _1-x ) ₃ Al ₅ O ₁₂ transparent ceramics by hot isostatic pressing" (Non-Patent Document 2), "Development of optical grade (Tb _x Y _1-x ) ₃ Al ₅ O ₁₂ ceramics as Faraday rotator material" (Non-Patent Document 3)). Since Tb-containing rare earth aluminum garnet exhibits higher thermal conductivity than TGG, it is expected to become a Faraday element with a smaller thermal lens effect.

As mentioned above, TAG-based transparent ceramics are expected to have a small thermal lens effect, but if absorption occurs at the wavelength used due to the inclusion of impurities or an inappropriate manufacturing process, a thermal lens effect occurs according to the amount of heat generated, and the material's inherent potential cannot be realized. Due to the increasing power of pulsed laser processing machines in recent years, even small absorption of less than 0.1%/cm can lead to heat generation, so managing minute absorption is essential to provide an excellent optical isolator with a small thermal lens effect.

Furthermore, in recent years, the high power and short pulses of pulsed lasers have often caused problems with the Faraday rotator being damaged by the transmission of high power density laser light. "Optical properties and Faraday effect of ceramic terbium gallium garnet for a room temperature Faraday rotator" (Non-Patent Document 4) provides information on the laser damage threshold of TGG single crystals and TGG transparent ceramics due to pulsed laser light with a wavelength of 1,064 nm. If the Faraday rotator is damaged, the transmittance, isolation, and beam quality will deteriorate, and in the worst case, the optical isolator will break down. Possible optical damage caused by pulsed lasers includes ionization due to multiphoton absorption, electron avalanche collapse, and absorption by impurities. In particular, materials with a large absorption coefficient tend to have high laser damage resistance, so absorption management is important in order to provide a Faraday rotator with a high laser damage threshold.

The simplest method to measure the amount of absorption is to measure the transmittance, but the transmittance of transparent ceramics is a value that reflects absorption and scattering (and reflection). Because transparent ceramics contain many scattering sources, it can be difficult to accurately determine the small amount of absorption from the transmittance.

JP 2011-213552 A JP 2002-293693 A Patent No. 4107292 International Publication No. 2017/033618

“High Verdet constant of Ti-doped terbium aluminum garnet (TAG) ceramics”, Optical Materials Express, Vol. 6, No. 1 191-196 (2016) “Fabrication and properties of (TbxY1-x)3Al5O12 transparent ceramics by hot isostatic pressing”, Optical Materials, 72 58-62 (2017) “Development of optical grade (TbxY1-x)3Al5O12 ceramics as Faraday rotator material”, Journal of American Ceramics Society, 100, 4081-4087 (2017) “Optical properties and Faraday effect of ceramic terbium gallium garnet for a room temperature Faraday rotator”, Optical Materials Express, Vol. 19, No. 16 15181-15187 (2011) “Wavelength Dependence of Laser-Induced Damage: Determining the Damage Initiation Mechanisms”, Physical Review Letters, 91, 127402 (2003)

As described above, with the trend toward higher power and shorter pulses in pulsed laser processing machines, there is a demand for Faraday rotators with a high laser damage threshold. However, when testing an optical isolator equipped with a Faraday rotator made of Tb-containing aluminum garnet prototyped as described in the previous literature, a problem occurred in which the Faraday rotator was damaged when irradiated with a certain high-power pulsed laser.

By the way, Tb-containing rare earth aluminum garnet, which has Tb(III) ions as a constituent element, also emits green light due to terbium(III) ions when excited. When Tb-containing rare earth aluminum garnet is used as a luminescent material, it is important that the emission wavelength does not overlap with the absorption bands derived from other elements as a means of increasing its luminescence efficiency. For example, since ions of first transition metals often have d-d transition absorption and charge transfer absorption bands with oxide ions in the visible range, if they are contained as impurities in Tb-containing rare earth aluminum garnet, the emission of Tb ions is reabsorbed by the impurity ions, inhibiting the emission. In addition, when metal oxides are exposed to high temperatures and low oxygen partial pressure, oxygen is removed and defects are generated. Metal oxides with such oxide ion defects often exhibit a gray to black color, which also inhibits the emission of Tb ions. As described above, colored impurity ions and Tb-containing rare earth aluminum garnets with few lattice defects in themselves are excellent as luminescent materials.

Luminescence efficiency (i.e., luminescence quantum yield) is an index that indicates whether a substance emits strong light, and its value is determined by the ratio of the rate constant of the radiative transition from the lowest excited state that accompanies luminescence to the rate constant of the non-radiative transition that does not accompany luminescence. Specifically, it is defined as the probability that the excited state returns to the ground state by radiative transition. If the lifetime of the excited state (the time until the number of excited states decreases to 1/e) is τ, there is a proportional relationship between the luminescence quantum yield and the lifetime of the excited state. In other words, if the Tb-containing rare earth aluminum garnet has the same composition, the amount of impurity ions and defects that cause quenching (non-radiative transition) can be indirectly estimated by measuring the lifetime of the excited state. In addition, since luminescence is a phenomenon that occurs within the crystals of the base material, it is not easily affected by the scattering that is unique to ceramics.

The present invention has been made in view of the above circumstances, and has an object to provide a method for judging the laser damage resistance of TAG-based, TYAG-based or TYSAG-based paramagnetic garnet-type transparent ceramics having a high laser damage threshold.

As mentioned above, it has been difficult to distinguish between scattering and absorption, which is the cause of optical loss in Faraday rotators using transparent ceramics. In addition, because testing for laser damage involves destruction of the sample, a non-destructive, indirect evaluation method was required. Therefore, originally, the emission intensity of the light emitter and the laser damage of the Faraday rotator are completely unrelated phenomena, but the present inventors have intensively investigated the emission lifetime derived from the ^5D4 → ^7F5 transition of Tb(III) ions and the laser damage threshold as a Faraday rotator for Tb-containing rare earth aluminum garnet (TAG system, TYAG system or TYSAG system), and have found that _{Tb -} containing rare earth aluminum garnet (paramagnetic garnet type transparent ceramics) with a long emission lifetime has a high laser damage threshold, and Tb-containing rare earth aluminum garnet (paramagnetic garnet type transparent ceramics) with a short emission lifetime has a low laser damage threshold, that is, in paramagnetic garnet type transparent ceramics _, there is a correlation between the emission lifetime derived from _the ^5D4 → ^7F5 transition of Tb(III) ions and the laser damage threshold. In addition, since the measurement of the emission lifetime is performed non-destructively, it has been found that the laser damage resistance of the Faraday rotator can be easily evaluated. The present inventors have intensively investigated based on these findings and have made the present invention.

That is, the present invention provides the following method for determining the laser damage resistance of a paramagnetic garnet-type transparent ceramic.
1.
A method for judging the laser damage resistance of a paramagnetic garnet-type transparent ceramic comprising a sintered body of a garnet-type composite oxide represented by the following formula ( 1' ₎ , characterized in that when the lifetime of light emission derived from the ^5D4 → ^7F5 transition of Tb(III) ions contained in the sintered body is 346 _μs or longer, the method judges that the paramagnetic garnet-type transparent ceramic has a laser damage threshold of 10 J/cm2 or more ^at a wavelength of 1,064 nm and a pulse width of 5 ns.
(Tb _1-xy Y _x Sc _y ) ₃ (Al _1-z Sc _z ) ₅ O ₁₂ ( 1' )
(Wherein, 0≦x<0.45, 0≦y<0.08, and 0≦z< 0.16 .)
2 .
2. The method for evaluating laser damage resistance of a paramagnetic garnet-type transparent ceramic according to 1 , wherein the maximum intensity of light emitted by excitation light having a wavelength of 350 nm is between 500 and 600 nm.
3 .
3. The method for evaluating laser damage resistance of a paramagnetic garnet-type transparent ceramic according to 1 or 2, wherein the sintered body further contains a sintering aid.
4 .
The method for determining laser damage resistance of a paramagnetic garnet-type transparent ceramic according to any one of 1 to 3 , wherein the sintered body is subjected to HIP treatment and further annealing treatment in an oxidizing atmosphere.
5 .
5. The method for evaluating laser damage resistance of a paramagnetic garnet-type transparent ceramic according to any one of 1 to 4 , wherein the titanium content in the sintered body is less than 2,000 ppm.

According to the present invention, it is possible to provide a paramagnetic garnet-type transparent ceramic having a laser damage threshold of 10 J/ ^cm2 or more at a wavelength of 1,064 nm and a pulse width of 5 ns, and thus to provide a material suitable for forming magneto-optical devices such as optical isolators.

1 is a schematic cross-sectional view showing an example of the configuration of an optical isolator using the paramagnetic garnet-type transparent ceramic according to the present invention as a Faraday rotator.

[Paramagnetic garnet-type transparent ceramics]
The paramagnetic garnet-type transparent ceramics according to the present invention will now be described.
The paramagnetic garnet-type transparent ceramic of the present invention comprises a sintered body of ^a garnet-type composite oxide represented by the following formula (1), and is characterized in that the lifetime of luminescence derived from the ^5D4 → _7F5 transition of Tb( _III ) ions contained in the sintered body is 346 μs or longer.
(Tb _1-xy Y _x Sc _y ) ₃ (Al _1-z Sc _z ) ₅ O ₁₂ (1)
(In the formula, 0≦x<0.45, 0≦y<0.08, 0≦z<0.16, and 0.001<y+z<0.20.)

In the garnet crystal structure represented by formula (1), the site primarily occupied by Tb, i.e., the first half of the parentheses in formula (1), is referred to as the A site, and the site primarily occupied by Al, i.e., the second half of the parentheses in formula (1), is referred to as the B site.

In the A site of formula (1), terbium (Tb) is an element with the largest Verdet constant among trivalent rare earth ions, and has extremely small absorption in the 1,070 nm region (wavelength band 0.9 μm to 1.1 μm) used in fiber lasers, making it the most suitable element for use as a material for optical isolators in this wavelength range. However, Tb(III) ions are easily oxidized to generate Tb(IV) ions. When Tb(IV) ions are generated in metal oxides, they absorb light over a wide range of wavelengths from ultraviolet to near infrared, so it is desirable to eliminate them as much as possible. One strategy to prevent the generation of Tb(IV) ions is to adopt a crystal structure in which Tb(IV) ions are unstable, i.e., a garnet structure.

Yttrium (Y) has an ionic radius about 2% smaller than that of terbium, and when it combines with aluminum to form a composite oxide, it is a material that can stably form a garnet phase rather than a perovskite phase and reduce residual strain in the crystallites, which makes it possible to prevent scattering due to different phases, deterioration of the extinction ratio due to internal stress, and ff transition absorption of terbium ions, making it an important constituent element in the present invention. Furthermore, by replacing some of the terbium ions with yttrium ions, the sinterability is leveled out, making it possible to limit the amount of residual pores in the ceramic sintered body to 1 ppm or less, making it an element that can be preferably used in the present invention.

In the B site of formula (1), aluminum (Al) is a material with the smallest ionic radius among trivalent ions that can exist stably in an oxide having a garnet structure, and is an element that can minimize the lattice constant of a paramagnetic garnet-type oxide containing Tb. If the lattice constant of the garnet structure can be reduced without changing the Tb content, it is preferable because the Verdet constant per unit length can be increased. Furthermore, since aluminum is a light metal, it has weak diamagnetism compared to gallium, and is expected to have the effect of relatively increasing the magnetic flux density generated inside the Faraday rotator, which is also preferable because it can increase the Verdet constant per unit length. In fact, the Verdet constant of TAG ceramics is improved to 1.25 to 1.5 times that of TGG. Therefore, even if the relative concentration of terbium is reduced by replacing some of the terbium ions with yttrium ions, it is possible to keep the Verdet constant per unit length equal to or slightly lower than that of TGG, making it a suitable constituent element in the present invention.

Here, a composite oxide whose constituent elements are only Tb, Y, and Al may not have a garnet structure due to a subtle weighing error, making it difficult to stably manufacture transparent ceramics that can be used for optical applications. Therefore, in the present invention, scandium (Sc) is added as a constituent element to eliminate the composition deviation due to a subtle weighing error. Sc is a material with an intermediate ionic radius that can be dissolved in both the A site and the B site in an oxide having a garnet structure, and is a buffer material that can adjust the distribution ratio to the A site (rare earth site consisting of Tb and Y) and the B site (aluminum site) to exactly match the stoichiometric ratio and thereby minimize the energy of crystallite formation when the compounding ratio of the rare earth elements consisting of Tb and Y and Al deviates from the stoichiometric ratio due to variations in weighing. In addition, it is an element that can limit the presence ratio of the alumina heterophase in the garnet parent phase to 1 ppm or less, and can limit the presence ratio of the perovskite type heterophase in the garnet parent phase to 1 ppm or less, and can be added to improve product yields.

In formula (1), the range of x is 0≦x<0.45, preferably 0.1≦x≦0.4, and more preferably 0.2≦x≦0.4. When x is in this range, the Verdet constant at room temperature (23±15°C) and wavelength of 1,064 nm is 30 rad/(T·m) or more, and it can be used as a Faraday rotator. In addition, the larger x is in this range, the smaller the thermal lens effect tends to be, which is preferable. In addition, when x is 0.45 or more, the Verdet constant at wavelength of 1,064 nm is less than 30 rad/(T·m), which is not preferable. In other words, if the relative concentration of Tb is excessively diluted, when a general magnet is used, the total length required to rotate the laser light of wavelength 1,064 nm by 45 degrees exceeds 30 mm, which is not preferable because it becomes difficult to manufacture.

In formula (1), the range of y is 0≦y<0.08, preferably 0.002≦y≦0.07, and more preferably 0.003≦y≦0.06. When y is in this range, the perovskite-type heterophase can be reduced to a level that is not detectable by X-ray diffraction (XRD) analysis. Furthermore, this is preferable because the number of perovskite-type heterophases (typically 1-1.5 μm in diameter and granular, light brown in color) present in a 150 μm x 150 μm field of view observed under an optical microscope is one or less. In this case, the ratio of the perovskite-type heterophase to the garnet parent phase is 1 ppm or less.

When y is 0.08 or more, the risk of antisite defect absorption, in which Tb and Y enter the B site and Al enters the A site, increases, which is not preferable. Also, when y is 0.1 or more, in addition to substituting part of Tb with Y, part of Tb is also substituted with Sc, which results in an unnecessary decrease in the solid solution concentration of Tb, which is undesirable as it reduces the Verdet constant. Also, since the raw material cost of Sc is high, unnecessary excessive doping of Sc is not preferable in terms of manufacturing costs.

In formula (1), the range of z is 0≦z<0.16, preferably 0.01≦z≦0.15, and more preferably 0.03≦z≦0.15. When z is in this range, the perovskite-type heterophase is not detected by XRD analysis. Furthermore, this is preferable because the number of perovskite-type heterophases (typically 1-1.5 μm in diameter and granular, light brown in color) present in a 150 μm x 150 μm field of view observed under an optical microscope is one or less. In this case, the ratio of the perovskite-type heterophase to the garnet parent phase is 1 ppm or less.

When z is 0.16 or more, the risk of antisite defect absorption, in which Tb and Y occupy the B site and Al occupies the A site, increases, which is undesirable. In addition, the value of y, i.e., the substitution ratio of Tb by Sc, increases in conjunction with an increase in the value of z, which results in an unnecessarily lowered solid solution concentration of Tb, which undesirably reduces the Verdet constant. Furthermore, since the raw material cost of Sc is high, unnecessarily excessive doping of Sc is also undesirable in terms of manufacturing costs.

In formula (1), the range of y+z is 0.001<y+z<0.20. When y+z is in this range, the perovskite-type heterophase is not detected by XRD analysis. Furthermore, when observed by optical microscope, the number of perovskite-type heterophases (typically 1-1.5 μm in diameter and granular, light brown in color) in a field of view of 150 μm x 150 μm is preferably one or less. In this case, the ratio of the perovskite-type heterophase to the garnet parent phase is 1 ppm or less. Note that the effect of the present invention can be obtained even if the range of y+z is 0≦y+z≦0.001, but this is not preferable because the heterophase is more likely to occur due to weighing errors in the raw materials, resulting in a decrease in yield.

In addition, in the paramagnetic garnet-type transparent ceramics of the present invention, the sintered body preferably further contains a sintering aid. Specifically, it is preferable that the sintering aid contains more than 0 mass% and 0.1 mass% or less (more than 0 ppm and 1,000 ppm or less) of _SiO2 . If the content exceeds 0.1 mass% (1,000 ppm), there is a risk of a slight amount of light absorption occurring due to crystal defects caused by the excessive Si contained.
The titanium content in the sintered body is preferably less than 2,000 ppm.

The paramagnetic garnet-type transparent ceramic of the present invention exhibits Tb emission. Specifically, the lifetime of the emission derived from the ^5D4 → ^7F5 transition of the Tb(III) ^ion contained in the sintered body is 346 μs or more, preferably 500 μs or more. The method for measuring the "lifetime of the emission derived from the _5D4 → _7F5 transition of the Tb(III ₎ ion contained in the sintered body _" is to irradiate the target paramagnetic garnet-type transparent ceramic with excitation light of 350 nm wavelength, and measure the ^emission lifetime of the component of the emission with a wavelength of 545 nm with a fluorescence spectrophotometer. Specifically, the time dependence of the emission decay of the emission component with a wavelength of 545 nm is measured, and assuming that this is a single emission lifetime, the emission lifetime is the time at which the emission intensity becomes 1/e (about 37%) of the emission intensity at the start of emission.
It is also preferable that the maximum intensity of the emission emitted by excitation light with a wavelength of 350 nm is between 500 and 600 nm.

The paramagnetic garnet-type transparent ceramic of the present invention preferably has a laser damage threshold of 10 J/ ^cm2 or more at a wavelength of 1,064 nm and a pulse width of 5 ns. Since the paramagnetic garnet-type transparent ceramic of the present invention is intended to be used as a Faraday rotator, it is preferable that it is not damaged by a pulse laser (has laser damage resistance). The damage threshold is preferably as high as possible, and in the case of a wavelength λ=1,064 nm and a pulse width of 5 ns, it is preferably 10 J/cm2 or ^more , and more preferably 15 J/cm2 or ^more . The method for determining whether or not "the laser damage threshold of a wavelength of 1,064 nm and a pulse width of 5 ns is 10 J/ ^cm2 or more" is to irradiate a laser beam having an energy density of 10 J/ ^cm2 , a wavelength of 1,064 nm, and a pulse width of 5 ns to any multiple locations (five or more, preferably ten or more) of the target paramagnetic garnet-type transparent ceramic, and observe the target area with an optical (polarized) microscope or the like. If the entire area is not damaged, it is determined that the "laser damage threshold is 10 J/ ^cm2 or more," and if even one location is damaged, it is determined that the "laser damage threshold is not 10 J/ ^cm2 or more."

Incidentally, the laser damage threshold (LIDT) depends on the wavelength, pulse width and beam spot diameter of the irradiated laser light. Therefore, if it is not possible to perform a laser damage test with a wavelength of 1,064 nm, a wavelength of 5 ns and an irradiation beam diameter of 100 μm (Gaussian distribution 1/ ^e2 intensity), the laser damage threshold may be set to a wavelength of 1,064 nm and a pulse width of 5 ns by using LIDT scaling. Here, according to "Wavelength Dependence of Laser-Induced Damage: Determining the Damage Initiation Mechanisms" (Non-Patent Document 5), the formula (S1) can be applied as a general rule for scaling (converting) the initial conditions of wavelength (λ1), pulse width (τ1) and irradiation beam diameter (φ1) to new wavelength (λ2), pulse width (τ2) and irradiation beam diameter (φ2).
LIDT (λ2, τ2, φ2) = LIDT (λ1, τ1, φ1) × (λ1/λ2) × (τ2/τ1) ^1/2 × (φ1/φ2) ² (S1)
Therefore, using the following formula (S2), it is possible to convert the laser damage threshold (LIDT) measured under conditions of wavelength (λ1 (nm)), pulse width (τ1 (ns)), and irradiation beam diameter (φ1 (μm)) different from those of wavelength 1,064 nm, wavelength 5 ns, and irradiation beam diameter 100 μm ( ^Gaussian distribution 1/e2 intensity) to the laser damage threshold of a wavelength of 1,064 nm and a pulse width of 5 ns (irradiation beam diameter 100 μm).
LIDT (1064, 5, 100) = LIDT (λ1, τ1, φ1) × (λ1/1064) × (5/τ1) ^1/2 × (φ1/100) ² (S2)

In addition, in a paramagnetic garnet-type transparent ceramic, which is a sintered body of the garnet-type composite oxide represented by the above formula (1), the lifetime of the luminescence originating from _the ^5D4 → ^7F5 transition of the Tb(III ₎ ions contained in the sintered body is measured, and if the luminescence lifetime is 346 μs or more, it can be determined that the laser damage threshold at a wavelength of 1,064 nm is 10 J/ ^cm2 or more.

In addition, as described below, it is preferable that the paramagnetic garnet-type transparent ceramic of the present invention is obtained by subjecting the above-mentioned sintered body to HIP treatment and then annealing treatment in an oxidizing atmosphere.

Here, the paramagnetic garnet-type transparent ceramics of the present invention may be produced by the following procedure.
(raw powder for sintering)
First, a raw powder for sintering containing the garnet-type composite oxide powder (ceramic powder) represented by the above formula (1) is prepared.

The method for producing the raw powder for sintering of the above-mentioned garnet-type complex oxide used in the present invention is not particularly limited, but may be a coprecipitation method, a grinding method, a spray pyrolysis method, a sol-gel method, an alkoxide hydrolysis method, or any other synthesis method. In some cases, the obtained ceramic raw material of rare earth complex oxide may be appropriately processed by a wet ball mill, a bead mill, a jet mill, a dry jet mill, a hammer mill, etc. to obtain a desired particle size. For example, a solid-phase reaction method in which multiple types of oxide particles are mixed and fired to produce uniformity by thermal diffusion of ions, or a coprecipitation method in which hydroxides, carbonates, etc. are precipitated from an ion-containing solution in which oxide particles are dissolved, and then fired to produce oxides, thereby producing uniformity, may be used to produce the raw powder for sintering.

In the case of the solid-phase reaction method in which multiple types of oxide particles are mixed and fired to produce uniformity by thermal diffusion of ions, metal powders consisting of terbium, yttrium, scandium, and aluminum, or the metal powders dissolved in an aqueous solution of nitric acid, sulfuric acid, uric acid, etc., or oxide powders of the above elements can be suitably used as starting materials. The purity of the above raw materials is preferably 99.9% by mass or more, and particularly preferably 99.99% by mass or more. The starting materials are weighed in a predetermined amount so as to have a composition corresponding to formula (1), mixed, and then fired to obtain a fired raw material mainly composed of a cubic garnet-type oxide of the desired composition. The firing temperature at this time is preferably 950°C or more and lower than the sintering temperature to form a garnet structure, and more preferably 1,100°C or more and lower than the sintering temperature to be performed thereafter. The firing time may be 1 hour or more, and the heating rate at this time is preferably 100°C/h or more and 500°C/h or less. The firing atmosphere is preferably air or an oxygen-containing atmosphere, and is not suitable for nitrogen, argon, or hydrogen. The firing apparatus is not particularly limited as long as it can reach the target temperature and oxygen flow. The term "main component" as used herein means that the main peak obtained from the powder X-ray diffraction results of the firing raw material is a diffraction peak derived from the garnet structure. When the ratio of the perovskite-type heterophase to the garnet parent phase is 1 ppm or less, the powder X-ray diffraction pattern will essentially only detect a single garnet phase pattern.

In addition, the raw powder for sintering preferably contains a sintering aid. For example, tetraethoxysilane (TEOS) is added as a sintering aid together with the above starting raw materials in an amount of more than 0 ppm and not more than 1,000 ppm (more than 0 mass% and not more than 0.1 mass%) in terms of _SiO2 in the entire raw powder (garnet-type complex oxide powder + sintering aid), or _SiO2 powder is added in an amount of more than 0 ppm and not more than 1,000 ppm (more than 0 mass% and not more than 0.1 mass%) in the entire raw powder (garnet-type complex oxide powder + sintering aid), mixed and fired to obtain a fired raw material. If the amount added exceeds 1,000 ppm, there is a risk of slight light absorption occurring due to crystal defects caused by the excess Si. The purity is preferably 99.9 mass% or more. The sintering aid may be added when preparing the raw powder slurry described later. In addition, when no sintering aid is added, it is advisable to select the raw material powder for sintering (i.e., the above-mentioned composite oxide powder) to be used, which has a nano-sized primary particle size and has extremely high sintering activity. Such a selection may be made appropriately.

Next, the obtained sintered raw material is pulverized to obtain raw material powder for sintering. Either dry or wet pulverization can be used, but pulverization is required to obtain a highly transparent ceramic. For example, in the case of wet pulverization, the sintered raw material is slurried by various pulverization (dispersion) methods such as ball mills, bead mills, homogenizers, jet mills, ultrasonic irradiation, etc., and pulverized (dispersed) to primary particles. The dispersion medium of this wet slurry is not particularly limited as long as it can make the final ceramic highly transparent, and examples of such dispersion medium include alcohols such as lower alcohols with 1 to 4 carbon atoms, and pure water. In addition, various organic additives may be added to this wet slurry for the purpose of improving the quality stability and yield in the subsequent ceramic manufacturing process. In the present invention, these are also 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 the case of wet pulverization, the dispersion medium of the slurry is finally removed to obtain raw material powder for sintering.

[Manufacturing process]
In the present invention, the above-mentioned raw powder for sintering is used to press-mold into a predetermined shape, followed by degreasing and then sintering to produce a sintered body with a relative density of at least 94%. It is preferable to carry out hot isostatic pressing (HIP) as a post-process. If hot isostatic pressing (HIP) is carried out directly, 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 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 manufacturing method of the present invention, a normal press molding process can be suitably used. That is, a very common press process in which the material is filled into a mold and pressed from a certain direction, or a cold isostatic pressing (CIP) process or a warm isostatic pressing (WIP) process in which the material is sealed and stored in a deformable waterproof container and pressed with hydrostatic pressure can be suitably used. The applied pressure can be appropriately adjusted while checking the relative density of the resulting 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. 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 possible to produce a molded body by a casting molding method instead of a press molding method. Molding methods such as pressurized casting molding, centrifugal casting molding, and extrusion molding can also be adopted by optimizing the combination of the shape and size of the oxide powder, which is the starting material, and various organic additives.

(Degreasing)
In the manufacturing method of the present invention, a normal debinding process can be suitably used. That is, a heating debinding process using a heating furnace can be used. The type of atmospheric gas used at this time is not particularly limited, and air, oxygen, hydrogen, etc. can be suitably used. There is no particular limit to the debinding temperature, but if a raw material containing an organic additive is used, it is preferable to heat the raw material to a temperature at which the organic component can be decomposed and eliminated.

(Sintering)
In the manufacturing method of 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 1,500 to 1,780°C, and particularly preferably 1,550 to 1,750°C. Sintering temperatures in this range are preferable because they promote densification while suppressing heterogeneous phase precipitation. A sintering holding time of several hours in the sintering process of the present invention is sufficient, but the relative density of the sintered body must be densified to at least 94%.

(Hot Isostatic Pressing (HIP))
In the manufacturing method of the present invention, a step of performing hot isostatic pressing (HIP) can be additionally provided after the sintering step. In addition, the type of pressurized gas medium at this time can be 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, even if the pressure is increased, no further transparency improvement can be obtained, and the load on the device may be excessive, which may damage the device. It is convenient and preferable that the applied pressure is 196 MPa or less, which can be processed with a commercially available HIP device.

The treatment temperature (predetermined holding temperature) is set in the range of 1,000 to 1,780°C, preferably 1,100 to 1,730°C. A treatment temperature of more than 1,780°C is not preferable because the risk of oxygen deficiency increases. A treatment temperature of less than 1,000°C hardly improves the transparency of the sintered body. The holding time of the treatment temperature is not particularly limited, but holding for too long a period of time is not preferable because the risk of oxygen deficiency increases. Typically, it is preferably set in the range of 1 to 3 hours. The heater material, heat insulating material, and treatment container for the HIP treatment 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 treatment container. In particular, when the treatment temperature is 1,500° C. or less, platinum (Pt) can be used as the heater material, heat insulating material, and treatment vessel, and Ar—O ₂ can be used as the pressurized gas medium, which is preferable since it prevents the occurrence of oxygen deficiency during HIP treatment. When the treatment temperature exceeds 1,500° 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 treatment vessel, and further select either yttrium oxide or gadolinium oxide as a double vessel inside it, and fill the vessel with an oxygen release material, since this minimizes the amount of oxygen deficiency during HIP treatment.

(Annealing)
In the manufacturing method of the present invention, after the HIP treatment, oxygen deficiency may occur in the obtained transparent ceramic sintered body, resulting in a gray appearance. In that case, it is preferable to perform oxygen annealing treatment (oxygen deficiency recovery treatment) in an oxygen atmosphere at a temperature equal to or lower than the HIP treatment temperature, typically 1,000 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 is possible to turn it into a colorless, transparent paramagnetic garnet-type transparent ceramic with little defect absorption.

(Optical Polishing)
In the manufacturing method of the present invention, it is preferable to optically polish both end faces on the optically utilized axis of the paramagnetic garnet-type transparent ceramics that has undergone the above-mentioned series of manufacturing steps. In this case, the optical surface accuracy is preferably λ/2 or less, and particularly preferably λ/8 or less, when the measurement wavelength λ is 633 nm. It is also possible to further reduce optical loss by appropriately forming an anti-reflection film on the optically polished surface.

In this manner, it is possible to provide a paramagnetic garnet-type transparent ceramic which is a sintered body of a paramagnetic garnet-type composite oxide containing terbium represented by the above formula ₍ 1), and in which the lifetime of the emission derived from the ^5D4 → ^7F5 transition of the Tb(III ₎ ion contained in the sintered body is 346 μs or more. In addition, it is possible to provide a laser damage threshold of 10 J/ ^cm2 or more at a wavelength of 1,064 nm and a pulse width of 5 ns.

[Magneto-optical devices]
Furthermore, since the paramagnetic garnet-type transparent ceramic of the present invention is intended to be used as a magneto-optical material, it is preferable to apply a magnetic field parallel to the optical axis of the paramagnetic 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 magneto-optical material 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.

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 magneto-optical material of the present invention as an optical element.
1, an optical isolator 100 includes a Faraday rotator 110 made of the magneto-optical material 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 the optical isolator 100, the polarizer 120, the Faraday rotator 110, and the analyzer 130 are arranged in this order on an optical axis 112, and it is preferable that a magnet 140 is placed on at least one of the side surfaces of these elements.

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

[Method for assessing laser damage resistance of paramagnetic garnet-type transparent ceramics]
The method for determining the laser damage resistance of a paramagnetic garnet-type transparent ceramic of the present invention is characterized in that the laser damage resistance of a paramagnetic garnet-type transparent ceramic made of a sintered body of a garnet-type composite oxide represented by the following formula (1' ₎ is determined based on the lifetime of emission derived from _the ^5D4 → ^7F5 transition of Tb(III) ions contained in the sintered body.
(Tb _1-xy Y _x Sc _y ) ₃ (Al _1-z Sc _z ) ₅ O ₁₂ (1')
(In the formula, 0≦x<0.45, 0≦y<0.08, and 0≦z<0.16.)

Here, when the lifetime of the emission derived from _the ^5D4 → ^7F5 transition of the Tb(III) ion is 346 _μs or more, it is preferable to determine that the laser damage threshold at a wavelength of 1,064 nm and a pulse width of 5 ns is 10 J/cm2 or ^more .
The details of the sintered body of the garnet-type composite oxide represented by formula (1') and the method for producing the same are the same as those of the paramagnetic garnet-type transparent ceramics and the method for producing the same of the present invention described above, except for the condition "0.001<y+z<0.20".

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

[Example 1]
Example 1 shows a case where y in formula (1) is fixed at 0.004, z is fixed at 0.03 (y+z=0.034), and the value of x is set to 0≦x≦0.396.
Yttrium oxide powder, terbium oxide powder, and scandium oxide powder manufactured by Shin-Etsu Chemical Co., Ltd., and aluminum oxide powder manufactured by Taimei Chemical Co., Ltd. were obtained. In addition, tetraethyl orthosilicate (TEOS) manufactured by Kishida Chemical Co., Ltd. and polyethylene glycol 200 liquid manufactured by Kanto Chemical Co., Ltd. were obtained. The purity of the powder raw material was 99.9% by mass or more, and the purity of the liquid raw material was 99.999% by mass or more. Using the above raw materials, a total of four types of raw material powders with the final composition shown in Table 1 were prepared by adjusting the mixing ratio as follows. The titanium content in each raw material powder was less than 1 ppm.

(Raw materials for Example 1-1 and Comparative Example 1-1)
Terbium, yttrium, aluminum and scandium were weighed out so that the mole numbers were Tb:Y:Sc:Al=1.794: _1.194 : _0.162 :4.850, respectively _{, to prepare a mixed powder for (Tb0.598Y0.398Sc0.004)3(Al0.97Sc0.03)5O12 . Next , TEOS was} added as a sintering aid, weighed out so that the amount added was 100 ppm in terms of _SiO2 , to prepare a raw material.

(Raw materials for Examples 1-2 and Comparative Examples 1-2)
Terbium, yttrium, aluminum and scandium were weighed out so that the mole numbers were Tb:Y:Sc:Al=2.091: _0.897 : _0.162 :4.850, respectively _{, to prepare a mixed powder for (Tb0.697Y0.299Sc0.004)3(Al0.97Sc0.03)5O12 . Next , TEOS was} added as a sintering aid, weighed out so that the amount added was 100 ppm in terms of _SiO2 , to prepare a raw material.

(Raw materials for Examples 1-3 and Comparative Examples 1-3)
Terbium, yttrium, aluminum and scandium were weighed out so that the mole numbers were Tb:Y:Sc:Al=2.391: _0.597 : _0.162 :4.850, respectively _{, to prepare a mixed powder for (Tb0.797Y0.199Sc0.004)3(Al0.97Sc0.03)5O12 . Next , TEOS was} added as a sintering aid, weighed out so that the amount added was 100 ppm in terms of _SiO2 , to prepare a raw material.

(Raw materials for Examples 1-4 and Comparative Examples 1-4)
The mole numbers of terbium, aluminum, and scandium were weighed out to give Tb:Sc:Al=2.988: _0.162 :4.850, respectively, to prepare a mixed powder for ( _{Tb0.996Sc0.004} ) ₃ ( _Al0.97Sc0.03 ) _5O12 . Next, _TEOS was weighed out as a sintering aid so that the amount added was 100 ppm in terms of _SiO2 , and _this was used as a raw material.

Next, each was placed in a polyethylene pot, taking care to prevent mixing, and polyethylene glycol 200 was added as a dispersant to the oxide powder at 0.5% by mass. Each was dispersed and mixed in ethanol using a ball mill. The processing time was 24 hours. After that, a spray drying process was carried out to produce granular raw materials with an average particle size of 20 μm.

These granular raw materials were then placed in an yttria crucible and sintered in a high-temperature muffle furnace at 1,100°C for three hours to obtain sintered raw material powders of the respective compositions. The four types of powder raw materials obtained were each subjected to uniaxial press molding and isostatic pressing at a pressure of 198 MPa to obtain CIP compacts. The resulting compacts were then degreased in a muffle furnace at 1,000°C for two hours.

As an example, the degreased molded body was placed in an oxygen atmosphere furnace and treated at 1,730°C for 3 hours to obtain a total of four types of sintered bodies. At this time, the sintered relative density of all samples was 94% or more. Each of the obtained sintered bodies was placed in a carbon heater HIP furnace and HIP-treated under conditions of 200 MPa, 1,700°C, and 2 hours in Ar. The HIP-treated sintered bodies were then placed again in the oxygen atmosphere furnace and subjected to oxidation annealing treatment at 1,450°C for 6 hours.

As a comparative example, the degreased molded body was placed in a vacuum furnace and treated at 1,600°C for three hours to obtain a total of four types of sintered bodies. At this time, the sintered relative density of all samples was 94% or more. Each of the obtained sintered bodies was placed in a HIP furnace made of a carbon heater and HIP-treated under conditions of 200 MPa, 1,600°C, and 3 hours in Ar. In accordance with prior art documents, the HIP-treated sintered bodies were not subjected to oxidation annealing treatment.

Each transparent ceramic thus obtained was cylindrically ground to a diameter of 10 mm, and then ground and polished to a length of 10 mm. Furthermore, both optical end faces of each sample were subjected to final optical polishing with an optical surface precision of λ/8 (when the measurement wavelength λ = 633 nm). Next, the optically polished samples were coated with an anti-reflection film designed to have a central wavelength of 1,064 nm.

Each sample obtained in the above manner was evaluated by measuring the luminescence lifetime and performing laser damage tests as follows.

(Luminescence lifetime measurement)
The luminescence lifetime was measured using a modular fluorescence spectrophotometer Fluorolog-3 (manufactured by HORIBA Seisakusho). The measurement location was a cylindrically ground rough surface. The excitation light was 350 nm, and the lifetime was measured for the luminescence component with a wavelength of 545 _nm . When the excitation light was applied, the measured sample became excited and returned to the ground state while releasing energy via the lowest excited state ^5D4 . At this time, luminescence occurred. Note that luminescence with maximum intensity was observed between wavelengths of 500 and 600 nm in all samples.
In this case, the change in emission intensity over time F(t) is given by the following formula.
F(t)=F ₀ ×exp(-t/τ)
(In the formula, F ₀ is the emission intensity at t = 0, and τ is the emission lifetime (μs).)
In other words, the time required for F(t) to become 1/eF ₀ (approximately 37%) is the emission lifetime.
The luminescence lifetime τ was determined by measuring the time dependence of luminescence decay and by the least squares method using the above formula, assuming a single luminescence lifetime.

(Laser damage test)
The laser damage threshold was measured using a Nd:YAG laser with a wavelength of 1,064 nm and a pulse width of 5 ns. The irradiation angle was perpendicular to the polished surface (optical end surface of the sample), the irradiation size was a beam diameter of 100 μm (Gaussian distribution 1/e ² intensity), and the energy density was 10 to 11 J/cm ^2. 10 shots were irradiated while changing the position within the optical effective area of the optical end surface of the sample, and the number of damages was counted with a polarizing microscope. Specifically, a cross-Nicol image of the entire optical effective area was observed from the optical end surface direction using a polarizing microscope with a magnification of 12.5 times, and when Hv (Horizontal-vertical) light scattering was confirmed, it was determined that this was caused by laser damage distortion and that laser damage occurred at that position. Note that the position of the laser damage (inside the material, the exit end surface, etc.) was not important.
The above results are shown in Table 1.

From the above results, the emission lifetime of the oxidatively annealed transparent ceramics (Examples 1-1 to 1-4) was 346 μs to 751 μs. The emission lifetime of the non-oxidatively annealed transparent ceramics (Comparative Examples 1-1 to 1-4) was 232 μs to 280 μs. That is, it was confirmed that the emission lifetime of the oxidatively annealed transparent ceramics was longer than that of the non-oxidatively annealed transparent ceramics, at 346 μs or more. It is considered that the tendency of the emission lifetime to become shorter as x becomes smaller for the oxidatively annealed transparent ceramics (Examples 1-1 to 1-4) is due to the concentration quenching of Tb ions. In addition, when irradiated with a laser beam having an energy density of 10 to 11 J/cm ² , the number of laser damages of the oxidatively annealed transparent ceramics (Examples 1-1 to 1-4) was 0, whereas the number of damages of the non-oxidatively annealed transparent ceramics (Comparative Examples 1-1 to 1-4) was 10. In other words, it was confirmed that by performing oxidation annealing treatment after HIP treatment, transparent ceramics with a long emission lifetime of 346 μs or more and a laser damage threshold of higher than 10 J/cm ² can be obtained.

[Example 2]
Example 2 shows the case where x in formula (1) is fixed at 0.4, the value of y is 0≦y≦0.07, the value of z is 0≦z≦0.15, and the value of y+z is 0.001<y+z<0.20.
As in Example 1, yttrium oxide powder, terbium oxide powder, and scandium oxide powder manufactured by Shin-Etsu Chemical Co., Ltd., and aluminum oxide powder manufactured by Taimei Chemical Co., Ltd. were obtained. In addition, tetraethyl orthosilicate (TEOS) manufactured by Kishida Chemical Co., Ltd. and polyethylene glycol 200 liquid manufactured by Kanto Chemical Co., Ltd. were obtained. The purity of the powder raw materials was 99.9% 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 total of three types of raw material powders with the final compositions shown in Table 2 were prepared by adjusting the mixing ratio as follows.

(Raw material for Example 2-1)
Terbium, yttrium, scandium and aluminum were weighed out so that the mole numbers were Tb:Y:Sc:Al=1.797: _1.200 : _0.008 :4.995, respectively, to prepare a mixed powder for ( _{Tb0.599Y0.4Sc0.001} ) ₃ ( _{Al0.999Sc0.001 ) 5O12} . Next, TEOS was added as a sintering aid, weighed out so that the amount added was ₁₀₀ ppm in terms of _SiO2 , to prepare a raw material.

(Raw material for Example 2-2)
Terbium, yttrium, scandium and aluminum were weighed out so that the mole numbers were Tb:Y: _Sc :Al=1.68: _1.20 : _0.52 :4.60, respectively, to prepare a mixed powder for ( _{Tb0.56Y0.4Sc0.04} ) ₃ ( _{Al0.92Sc0.08 ) 5O12} . Next, TEOS was added as a sintering aid, weighed out so that the amount added was 100 ppm in terms of _SiO2 , to prepare a raw material.

(Raw materials for Examples 2-3)
Terbium, yttrium, scandium and aluminum were weighed out so that the mole numbers were Tb:Y: _Sc :Al=1.59: _1.20 : _0.86 :4.35, respectively, to prepare a mixed powder for ( _{Tb0.53Y0.4Sc0.07} ) ₃ ( _{Al0.87Sc0.13 ) 5O12} . Next, TEOS was added as a sintering aid, weighed out so that the amount added was 100 ppm in terms of _SiO2 , to prepare a raw material.

(Raw material for Reference Example 2-1)
The mole numbers of terbium, yttrium and aluminum were weighed out so that Tb:Y:Al=1.8:1.2:5.0, respectively, to prepare a mixed powder for (Tb _0.6 Y _0.4 ) ₃ Al ₅ O ₁₂ . Then, TEOS was added as a sintering aid so that the amount added was 100 ppm in terms of SiO ₂ to prepare a raw material.

Next, each was placed in a polyethylene pot, taking care to prevent mixing, and polyethylene glycol 200 was added as a dispersant to the oxide powder at 0.5% by mass. Each was dispersed and mixed in ethanol using a ball mill. The processing time was 24 hours. After that, a spray drying process was carried out to produce granular raw materials with an average particle size of 20 μm.

These granular raw materials were then placed in an yttria crucible and sintered in a high-temperature muffle furnace at 1,100°C for three hours to obtain sintered raw material powders of the respective compositions. The four types of powder raw materials obtained were each subjected to uniaxial press molding and isostatic pressing at a pressure of 198 MPa to obtain CIP compacts. The resulting compacts were then degreased in a muffle furnace at 1,000°C for two hours.

The degreased molded body was placed in an oxygen atmosphere furnace and treated at 1,730°C for 3 hours to obtain a total of four types of sintered bodies. At this time, the sintered relative density of all samples was 94% or more. Each of the obtained sintered bodies was placed in a carbon heater HIP furnace and HIP-treated under conditions of 200 MPa, 1,700°C, and 2 hours in Ar. The HIP-treated sintered bodies were then placed again in the oxygen atmosphere furnace and subjected to oxidation annealing treatment at 1,450°C for 6 hours.

Each transparent ceramic thus obtained was cylindrically ground to a diameter of 10 mm, and then ground and polished to a length of 10 mm. Furthermore, both optical end faces of each sample were subjected to final optical polishing with an optical surface precision of λ/8 (when the measurement wavelength λ = 633 nm). Next, the optically polished samples were coated with an anti-reflection film designed to have a central wavelength of 1,064 nm.

For each sample thus obtained, the emission lifetime was measured and a laser damage test was performed in the same manner as in Example 1 to evaluate the sample.
The above results are summarized in Table 2.

From the above results, the luminescence lifetime of the transparent ceramics of Examples 2-1 to 2-3 was 705 μs to 729 μs. In addition, the number of damages in all of the transparent ceramics of Examples 2-1 to 2-3 was 0. In other words, it was confirmed that transparent ceramics with luminescence lifetimes of 705 μs to 729 μs and laser damage thresholds of higher than 10 J/ ^cm2 were obtained.

[Example 3]
In Example 3, x=0, y=0, z=0.002 (y+z=0.002) in formula (1) are used, and the effect of including paramagnetic impurities that absorb the wavelength to be used is shown. As an example of paramagnetic impurities, the effect of doping with Ti is shown with reference to Non-Patent Document 1.
Terbium oxide powder manufactured by Shin-Etsu Chemical Co., Ltd., aluminum oxide powder manufactured by Taimei Chemical Co., Ltd., and titanium oxide powder manufactured by Kojundo Chemical Laboratory Co., Ltd. were obtained. In addition, tetraethyl orthosilicate (TEOS) manufactured by Kishida Chemical Co., Ltd. and polyethylene glycol 200 liquid manufactured by Kanto Chemical Co., Ltd. were obtained. The purity of the powder raw materials was 99.9% 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 total of two types of raw material powders with the final composition shown in Table 3 were prepared by adjusting the mixing ratio as follows.

(Raw material for Example 3-1)
A mixed powder for _Tb3 ( _{Al0.998Sc0.002} ) _5O12 was prepared by weighing out the moles of terbium, scandium, and aluminum to be Tb:Sc:Al=3:0.01: _4.99 , respectively. Next, TEOS was added as a sintering aid in an amount of 100 _ppm calculated as _SiO2 , to prepare a raw material.

(Raw materials for Comparative Example 3-1 and Comparative Example 3-2)
With reference to Non-Patent Document ₁ , a mixed powder for _Tb3 ( _{Al0.990Sc0.002Ti0.008} )5O12 was prepared by weighing out the moles of terbium, scandium, aluminum and titanium in the ratio Tb:Sc:Al:Ti=3:0.01:4.95: _0.04 . Then, TEOS was added as a sintering aid in an amount of ₁₀₀ ppm calculated as _SiO2 , to prepare a raw material. The titanium content at this time was 2.4 x ¹⁰³ _ppm .

Next, each was placed in a polyethylene pot, taking care to prevent mixing, and polyethylene glycol 200 was added as a dispersant to the oxide powder at 0.5% by mass. Each was dispersed and mixed in ethanol using a ball mill. The processing time was 24 hours. After that, a spray drying process was carried out to produce granular raw materials with an average particle size of 20 μm.

These granular raw materials were then placed in an yttria crucible and sintered in a high-temperature muffle furnace at 1,100°C for three hours to obtain sintered raw material powders with the respective compositions. The two types of powder raw materials obtained were each subjected to uniaxial press molding and isostatic pressing at a pressure of 198 MPa to obtain CIP compacts. The obtained compacts were degreased in a muffle furnace at 1,000°C for two hours.

As an example, _{a degreased molded body of Tb3(Al0.998Sc0.002)5O12 was placed in} an oxygen atmosphere furnace and treated at 1,650°C for 3 hours to obtain a sintered body. At this time, the sintered relative density of the sample was ₉₈ %. The obtained sintered body was placed in a carbon heater HIP furnace and HIP-treated under conditions of 200 MPa, 1,650°C, and 2 hours in Ar. The HIP-treated sintered body was then placed again in an oxygen atmosphere furnace and subjected to oxidation annealing treatment at 1,450°C for 6 hours, which was designated as Example 3-1.

Referring to Non-Patent Document ₁ , _{a degreased molded body of Tb3(Al0.990Sc0.002Ti0.008)5O12 was placed in} a vacuum furnace and sintered at 1,600°C for ₈ hours to obtain a sintered body as a comparative example. The sintered relative density of each sample was 99% or more. The external appearance was translucent and brown in color. Comparative Example 3-1 was a sample that was not subjected to oxidation annealing treatment after vacuum sintering.

The vacuum sintered body was then placed in an oxygen atmosphere furnace and subjected to oxidation annealing treatment at 1,450°C for 6 hours, resulting in Comparative Example 3-2. The appearance was translucent and the color was brown.

Each transparent ceramic thus obtained was cylindrically ground to a diameter of 10 mm, and then ground and polished to a length of 10 mm. Furthermore, both optical end faces of each sample were subjected to final optical polishing with an optical surface precision of λ/8 (when the measurement wavelength λ = 633 nm). Next, the optically polished samples were coated with an anti-reflection film designed to have a central wavelength of 1,064 nm.

For each sample thus obtained, the emission lifetime was measured and a laser damage test was performed in the same manner as in Example 1 to evaluate the sample.
The above results are summarized in Table 3.

From the above results, the emission lifetime of the transparent ceramics not doped with titanium (Example 3-1) was 374 μs. The emission lifetimes of the transparent ceramics doped with titanium (Comparative Examples 3-1 and 3-2) were 198 μs and 234 μs. That is, it was confirmed that the emission lifetime of the titanium-doped transparent ceramics was shorter than that of the titanium-doped transparent ceramics regardless of the presence or absence of oxidation annealing treatment. In addition, when irradiated with a laser beam having an energy density of 10 to 11 J/cm ² , the number of laser damages of the titanium-doped transparent ceramics (Example 3-1) was 0, whereas the number of laser damages of the titanium-doped transparent ceramics (Comparative Examples 3-1 and 3-2) was 10. In other words, it was confirmed that the transparent ceramics with an emission lifetime of less than 234 μs had a laser damage threshold of less than 10 J/cm ² regardless of the presence or absence of oxidation annealing treatment.

Although the present invention has been described above using the above embodiments, the present invention is not limited to these embodiments and can be modified, such as by other embodiments, additions, changes, deletions, etc., within the scope of what a person skilled in the art can conceive. As long as the effects of the present invention are achieved in any form, it is within the scope of the present invention.

100 Optical isolator 110 Faraday rotator 112 Optical axis 120 Polarizer 130 Analyzer 140 Magnet 150 Housing

Claims (5)

A method for judging the laser damage resistance of a paramagnetic garnet-type transparent ceramic comprising a sintered body of a garnet-type composite oxide represented by the following formula ( 1' ₎ , characterized in that when the lifetime of light emission derived from the ^5D4 → ^7F5 transition of Tb(III) ions contained in the sintered body is 346 _μs or longer, the method judges that the paramagnetic garnet-type transparent ceramic has a laser damage threshold of 10 J/cm2 or more ^at a wavelength of 1,064 nm and a pulse width of 5 ns.
(Tb _1-xy Y _x Sc _y ) ₃ (Al _1-z Sc _z ) ₅ O ₁₂ ( 1' )
(Wherein, 0≦x<0.45, 0≦y<0.08, and 0≦z< 0.16 .) 2. The method for judging laser damage resistance of a paramagnetic garnet-type transparent ceramic according to claim 1 , wherein the maximum intensity of light emitted by excitation light having a wavelength of 350 nm is between 500 and 600 nm. 3. The method for evaluating laser damage resistance of a paramagnetic garnet-type transparent ceramic according to claim 1, wherein the sintered body further contains a sintering aid. 4. The method for evaluating laser damage resistance of a paramagnetic garnet-type transparent ceramic according to claim 1, wherein the sintered body is subjected to HIP treatment and further annealing treatment in an oxidizing atmosphere. 5. The method for determining laser damage resistance of a paramagnetic garnet-type transparent ceramic according to claim 1, wherein the titanium content in the sintered body is less than 2,000 ppm.

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