JP7704049B2 - Terbium-containing paramagnetic garnet-type transparent ceramics and magneto-optical devices - Google Patents

Description

The present invention relates to terbium-containing paramagnetic garnet-type transparent ceramics, and more specifically to terbium-containing garnet-type transparent ceramics suitable for forming magneto-optical devices such as optical isolators, and magneto-optical devices using the garnet-type transparent ceramics as magneto-optical materials.

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.

As materials used as the Faraday rotator constituting the optical isolator, _TGG crystal ( _Tb3Ga5O12 ) and _TSAG crystal ((Tb _{(3-x) Scx} ) _{Sc2Al3O12 )} have been known (JP Patent Publication 2011-213552 (Patent Document 1), JP Patent Publication 2002-293693 (Patent Document 2)). TGG crystal is currently widely used in standard fiber laser devices. On _the other hand, the Verdet constant of the TSAG crystal is said to be about 1.3 times that of the TGG crystal, and it is also a material that could be used in fiber laser devices. However, because Sc is an extremely expensive raw material, its adoption has not progressed in terms of manufacturing costs.

TGG crystals (Tb ₃ Ga ₅ O ₁₂ ) are relatively easy to manufacture and can be pulled up to a diameter of 4 inches. They are both highly available and cost-effective, and are currently widely used as Faraday rotators for fiber laser devices. However, several problems have become apparent, and it has become clear that their use in fiber laser devices, particularly in the high power band, is difficult. Specifically, the high material-specific absorption coefficient (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) are bottlenecks.

Therefore, there is a strong demand for a new material that is less expensive than TSAG crystal, and has lower absorption and a smaller temperature coefficient of refractive index _dn /dt than TGG crystal. One such candidate material is TAG crystal ( _Tb3Al5O12 ), but the production _of high-quality single crystal TAG has not yet been invented.

In spite of such restrictions, 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 existing TGG crystals (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 a material 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 3) 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.

Incidentally, in the known examples disclosed in Non-Patent Document 1 and Patent Document 3, it is certainly true that the loss can be reduced as expected compared to TGG crystal, and the deterioration of the beam quality can be suppressed even when irradiated with high power. These findings probably indicate an effect based on the fact that the intrinsic absorption rate of the material is lower than that of TGG crystal. In fact, if a material with a low intrinsic absorption rate can be used as a Faraday rotator, it is preferable because it enables the output of the fiber laser device to be further increased. On the other hand, there is no knowledge about the temperature coefficient of the refractive index dn/dt in any of the known examples, and the further problem is that it is completely unclear how the new material is positioned compared to the temperature coefficient of the refractive index dn/dt = 18 × 10 ^-6 (K ^-1 ) of the conventional TGG crystal.

The necessary condition for a Faraday rotator that can be applied to a high-power fiber laser device is a combination of low absorption and a small temperature coefficient of refractive index dn/dt (i.e., both of these requirements are necessary), but there is no knowledge or prior disclosure of this.

JP 2011-213552 A JP 2002-293693 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

The present invention has been made in consideration of the above circumstances, and aims to provide a terbium-containing paramagnetic garnet-type transparent ceramic, which is a sintered body of a complex oxide containing terbium, yttrium, and aluminum, in which the volume molar concentration of the terbium is equal to or greater than the volume molar concentration of the yttrium, has low absorption at a wavelength of 1064 nm, and has a temperature coefficient of refractive index that is half or less than that of existing TGG crystals, and a magneto-optical device using the same.

In order to achieve the above object, the present invention provides the following terbium-containing paramagnetic garnet-type transparent ceramics and magneto-optical device.
1.
A terbium-containing paramagnetic garnet-type transparent ceramic, which is a sintered body of a complex oxide represented by the following formula (1) , characterized in that the Verdet constant at a wavelength of 1064 nm is 30 rad/(T·m) or more, and the average temperature coefficient of refractive index (dn(1064)/dt) _ave at a wavelength of 1064 nm and temperatures of 20, 30 and 40°C is 9.0 x 10 ^-6 (K ^-1 ) or less.
(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, 0.0005≦y<0.08, 0.6<1−x−y<0.95, 0.0004≦z<0.15, and 0.0009≦y+z<0.2.)
2 .
2. The terbium-containing paramagnetic garnet-type transparent ceramic according to 1 , further comprising more than 0 mass % and 0.1 mass % or less of SiO ₂ as a sintering aid.
3 .
3. A magneto-optical device comprising the paramagnetic garnet-type transparent ceramic according to claim 1 or 2 as a magneto-optical material.
4 .
4. The magneto-optical device according to 3 , which is an optical isolator that has the terbium-containing paramagnetic garnet-type transparent ceramic as a Faraday rotator and has polarizing materials in front of and behind the optical axis of the Faraday rotator and can be used in the wavelength range of 0.9 μm to 1.1 μm.

According to the terbium-containing paramagnetic garnet-type transparent ceramic of the present invention, the average value (dn(1064)/dt) _ave of the temperature coefficients of refractive index dn(1064,20)/dt, dn(1064,30)/dt, and dn(1064,40)/dt is 9.0×10 ^-6 (K ^-1 ) or less, which is roughly half that of conventional TGG crystals, so that when the terbium-containing paramagnetic garnet-type transparent ceramic of the present invention is applied to a laser device, the amount of thermal lens generation caused by changes in the refractive index of the material is also halved. In other words, it can be applied to high-output laser devices that are at least twice as high as those of existing TGG crystals.

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

<Terbium-containing paramagnetic garnet-type transparent ceramics>
The terbium-containing paramagnetic garnet-type transparent ceramics (sometimes simply referred to as transparent ceramics) according to the present invention will be described below.
The transparent ceramic of the present invention is a sintered body of a complex oxide containing terbium, yttrium, and aluminum, in which the volume molar concentration of the terbium is equal to or greater than the volume molar concentration of the yttrium, and is characterized in that the Verdet constant at a wavelength of 1064 nm is 30 rad/(T·m) or more, and the average temperature coefficient of refractive index (dn(1064)/dt) _ave at a wavelength of 1064 nm and temperatures of 20, 30, and 40°C is 9.0 x 10 ^-6 (K ^-1 ) or less.

The Verdet constant of the transparent ceramic of the present invention at a wavelength of 1064 nm is 30 rad/(T·m) or more, and preferably 32 rad/(T·m) or more. The above Verdet constant can be obtained by controlling the volume molar concentration of terbium in the composition constituting the garnet-type oxide sintered body to be equal to or greater than the volume molar concentration of yttrium, and by manufacturing the transparent ceramic under appropriate conditions in each process as described below.

Furthermore, the average value (dn(1064)/dt) ave of the temperature coefficients of refractive index dn(1064,20)/dt, dn(1064,30)/dt, and dn(1064,40)/dt of the transparent ceramic of the present invention at a wavelength of 1064 nm _{and temperatures} of 20, 30, and 40° C. is 9.0×10 ^-6 (K ^-1 ) or less, and preferably 8.9×10 ^-6 (K ^-1 ) or less. The average value of the temperature coefficient of refractive index (dn(1064)/dt) ave can be obtained by setting the composition of the transparent ceramic of the present _invention within the composition range represented by formula (1) described later and producing the transparent ceramic under appropriate conditions in each step as described later in the method for producing the transparent ceramic.

The temperature coefficient of refractive index dn(λ, T)/dt referred to here is the temperature coefficient of refractive index of the target transparent ceramic at wavelength λ (nm) and temperature T (°C), and is determined by a method conforming to JIS B7071-2 (Method of measuring refractive index of optical glass - Part 2: V-block method). The average value of the temperature coefficient of refractive index (dn(1064)/dt) _ave at a wavelength of 1064 nm and temperatures of 20, 30, and 40°C specified in the present invention is determined as follows.
First, a triangular prism-shaped block sample made of transparent ceramic to be measured, with the two bottom faces forming right-angled isosceles triangles, is placed on a V-block prism having a predetermined refractive index N, and four types of standard laser light with different wavelengths (e.g., laser light with wavelengths of 785.4 nm, 830.5 nm, 1310 nm, and 1550 nm) are incident on the normal to the wall of the prism, and the refractive index n of the block sample to be measured for each wavelength is calculated from the angle (deflection angle i ^* ) of the light that is refracted by the V-block prism and the block sample and emitted from the wall on the opposite side of the prism, using the following formula: At this time, the temperatures (sample temperatures) of the V-block prism and the block sample are set to 20° C., 30° C., and 40° C., and the refractive indexes for each of the four wavelengths at each sample temperature are measured.

n={N ² +sini ^*・(N ² -sin ² i ^* ) ^1/2 } ^1/2

Next, from the refractive index values calculated at the four wavelengths for sample temperatures of 20°C, 30°C, and 40°C, the refractive index at a wavelength of 1064 nm for sample temperatures of 20°C, 30°C, and 40°C is derived based on Sellmeier's dispersion formula, and the temperature coefficients of the refractive index at a wavelength of 1064 nm and temperatures of 20°C, 30°C, and 40°C are respectively defined as dn(1064,20)/dt, dn(1064,30)/dt, and dn(1064,40)/dt. The average of these temperature coefficients is defined as the average value (dn(1064)/dt) ave of the temperature coefficients of the refractive index dn(1064,20)/dt, dn(1064,30)/dt, and dn(1064,40)/dt of the transparent ceramic to be measured at a wavelength of 1064 nm and _temperatures of 20, 30, and 40°C.
Such measurements may be carried out using a precision refractometer (for example, Kalnew Precision Refractometer KPR-3000, manufactured by Shimadzu Corporation) in a fully automatic measurement mode of the device.

In addition, if the average value (dn(1064)/dt) _ave of the temperature coefficients of refractive index dn(1064,20)/dt, dn(1064,30)/dt, and dn(1064,40)/dt at a wavelength of 1064 nm and temperatures of 20, 30, and 40° C. can be controlled to 9.0×10 ⁻⁶ (K ⁻¹ ) or less, the average value (18×10 ⁻⁶ (K −1 ) of the temperature coefficients of refractive index dn(1064,20)/dt, dn(1064,30)/dt, and dn(1064,40)/dt for the TGG crystal at a wavelength of 1064 nm and temperatures of 20, 30, and 40° ^C. ) is less than half the value of the TGG crystal, and the amount of thermal lens generation caused by the change in refractive index when the transparent ceramic of the present invention is irradiated with a high-power laser having a wavelength of 1064 nm is less than half that in the case of the TGG crystal, and the high-power application resistance is essentially doubled or more, which is preferable.

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, and preferably 0.001<z<0.004. When z is in this range, the perovskite-type heterophase can be reduced to a level that is not detectable by X-ray diffraction (XRD) analysis, which is preferable. 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 the 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, and the average value (dn(1064)/dt) _ave of the temperature coefficient of the refractive index at a wavelength of 1064 nm and temperatures of 20, 30, and 40° C. can be reduced. In particular, as in the above formula (1), if the ratio of the 6-coordinated sites and the 4-coordinated sites of aluminum (Al) is 1-z (0≦z<0.15), the average value (dn(1064)/dt) _ave of the temperature coefficient of the refractive index at a wavelength of 1064 nm and temperatures of 20, 30, and 40° C. can be controlled to 9.0×10 ⁻⁶ (K ⁻¹ ) or less.

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-coordinated 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, the Verdet constant at a wavelength of 1064 nm can be ensured to be 32 rad/(T·m) or more. When the terbium concentration is in this range and the yttrium concentration is controlled in the above range, the average value (dn(1064)/dt) _ave of the temperature coefficient of refractive index at a wavelength of 1064 nm and temperatures of 20, 30, and 40° C. can be controlled to 9.0×10 ⁻⁶ (K ⁻¹ ) or less.
When these two components Tb and Y simultaneously satisfy the above-mentioned ranges, it becomes possible for the two characteristics (Verdet constant, average value of the temperature coefficient of the refractive index (dn(1064)/dt) _ave ) to be satisfied simultaneously.

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 Tb and Y is 100 parts by mass.

By manufacturing the transparent ceramics of the present invention with the composition described above, it is possible to achieve a thermal conductivity of 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 terbium-containing paramagnetic garnet-type transparent ceramics>
[Raw materials]
The raw materials used in the present invention are terbium, yttrium, scandium, and aluminum oxide powders, and the purity of the raw materials 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 (insertion?) 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 these organic additives that do not contain unnecessary metal ions. In addition, the order of addition of each organic additive must be properly 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 1440 to 1780°C, and particularly preferably 1470 to 1730°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 present invention, a step of performing hot isostatic pressing (HIP) can be additionally 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 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 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, the terbium-containing paramagnetic garnet-type transparent ceramic of the present invention can be provided. The transparent ceramic 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 terbium-containing 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 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 transparent ceramic material of the present invention is suitable for magneto-optical device applications, and is particularly suitable for use as a Faraday rotator for 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 terbium-containing paramagnetic 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 terbium-containing paramagnetic 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 6, Comparative Examples 1 to 6]
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, 12 types of composite oxide raw materials having the final compositions shown in Table 1 were prepared by adjusting the mixing ratio.
The mixing ratio was adjusted by weighing and mixing the oxide powders of terbium, yttrium, aluminum, and scandium so that the mole numbers of each were the molar ratios of the compositions in Table 1. Then, TEOS was weighed and added to each raw material in an amount equivalent to _SiO2 of the mass % (wt %) in Table 1.

Then, the materials were dispersed and mixed in ethanol using an alumina ball mill, while taking care to prevent each material from being mixed with the other material. The treatment time was 15 hours.
Then, spray drying was performed to produce granular raw materials with an average particle size of 20 μm. Each of the 12 types of oxide raw materials obtained was subjected to uniaxial press molding and isostatic pressing at a pressure of 198 MPa to obtain CIP molded bodies. In addition, to confirm the manufacturing stability, three rod-shaped samples were prepared under each condition, and one block-shaped sample was also molded separately. All the obtained molded bodies were degreased in a muffle furnace at 1000°C for 3 hours to obtain degreased molded bodies.

The degreased green bodies were then placed in a vacuum sintering furnace and sintered at 1500-1600°C for 3 hours under reduced pressure of less than 1.0 x ^10-3 Pa to obtain 12 types of sintered bodies (3 rod-shaped bodies and 1 block-shaped body each). The sintering temperature was adjusted so that the relative sintered density of each sample was in the range of 94-98%.

The resulting sintered bodies were placed in a carbon heater HIP furnace and HIP-treated in Ar at 200 MPa, 1600°C, and 2 hours. The HIP bodies (pressure sintered bodies) obtained showed almost no graying (oxygen deficiency absorption) in appearance. However, to be on the safe side, each HIP body (pressure sintered body) obtained was annealed in an atmospheric heating furnace at 1450°C for 30 hours while managing each lot, to fully recover from oxygen deficiencies. In this way, a total of 12 types of ceramic samples were prepared for the examples and comparative examples.

Next, of the resulting ceramic samples, the rod-shaped samples were first ground and polished to give them a uniform rod shape with a diameter of 5 mm and a length of 25 mm, and then 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).

For each sample 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).
Measurements were performed at a wavelength of 1064 nm using a spectrophotometer V-670 manufactured by JASCO Corporation. First, the total light transmittance was 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 in advance, and receiving the collected light with a detector. The obtained illuminance was set as I ₀ , and then the workpiece was set in the device, and this time the dispersed light was incident on the workpiece, and the transmitted light was again collected with an integrating sphere and received with a detector. The obtained illuminance was determined as I 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 order to take into account the effects of reproducibility and variation, measurements were taken for three samples under each condition, and the average values were calculated to determine the total light transmittance and forward scattering rate of each sample. In addition, if the beam diameter is made thicker 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.

For each of the rod-shaped samples (three sets each) of the Examples and Comparative Examples prepared by the final optical polishing in the above manner, both optical end faces were coated with an anti-reflection film (AR coat) designed to have a central wavelength of 1064 nm. The Verdet constant of each of the obtained 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).)
The average value of the Verdet constants of three samples produced under each set of conditions was taken as the Verdet constant for each set of conditions.

Next, the block-shaped samples prepared above for each condition of the Examples and Comparative Examples were cut and ground to produce triangular prism samples with a side length of 20 mm, a thickness of 10 mm, and two bottom faces of a right-angled isosceles triangle. At this time, the angle of the right corner of the triangular prism was controlled to be 90 degrees ± 0.1 degrees. After that, the two faces constituting the right corner of the triangular prism were lapped and polished to make them flat.
Using the right-angled isosceles triangular prism block samples thus obtained under each condition of the Examples and Comparative Examples, the average values of the temperature coefficients of refractive index (dn(1064)/dt) _ave at a wavelength of 1064 nm and temperatures of 20, 30 and 40° C. were measured in the following manner.
(Method of measuring (dn(1064)/dt) _ave )
The average temperature coefficient of refractive index (dn(1064)/dt) _ave of the terbium-containing paramagnetic garnet transparent ceramic at a wavelength of 1064 nm and temperatures of 20, 30, and 40° C. was measured in accordance with JIS B7071-2 (Method of measuring refractive index of optical glass - Part 2: V-block method).

A precision refractometer, KALNEW KPR-3000, manufactured by Shimadzu Corporation, was selected as the measuring device, and measurements were performed in the fully automatic measurement mode of the device. Specifically, a glass material V-block prism having a known refractive index N for high refractive index samples was selected and set in the device, and each block-shaped sample prepared as described above was placed on it, and four types of standard laser light with different wavelengths (laser light with wavelengths of 785.4 nm, 830.5 nm, 1310 nm, and 1550 nm (standard equipment of the device)) was incident from the normal line of the wall surface of the prism, and the refractive index n of the block-shaped sample to be measured for each wavelength was calculated using the following formula from the angle (deviation angle i ^* ) of the light refracted by the V-block prism and the block-shaped sample that transmitted at this time and emitted from the wall surface on the opposite side of the prism. At this time, the temperatures (sample temperatures) of the V-block prism and the block sample were set to 20° C., 30° C., and 40° C., and the refractive indexes for each of the four wavelengths at each sample temperature were measured. The sample temperatures were controlled at 20°C, 30°C, and 40°C by placing the prism and block-shaped samples on a sample stage. The samples were left stationary in this state while the temperature of the samples themselves was monitored. After the temperature had stabilized and 5 minutes had passed, the sample surface temperatures were counted as 20°C, 30°C, and 40°C, respectively.

n={N ² +sini ^*・(N ² -sin ² i ^* ) ^1/2 } ^1/2

Next, from the refractive index values calculated at the four wavelengths for sample temperatures of 20°C, 30°C, and 40°C, the refractive index at a wavelength of 1064 nm for sample temperatures of 20°C, 30°C, and 40°C was derived based on Sellmeyer's dispersion formula, and the temperature coefficients of the refractive index at a wavelength of 1064 nm and temperatures of 20°C, 30°C, and 40°C were determined as dn(1064,20)/dt, dn(1064,30)/dt, and dn(1064,40)/dt, respectively. The average of these temperature coefficients was designated as the average value (dn(1064)/dt) ave of the temperature coefficients of the refractive index dn(1064,20)/dt, dn(1064,30)/dt, and dn(1064,40)/dt of the transparent ceramic to be measured at a wavelength of 1064 nm and _temperatures of 20, 30, and 40°C.
The above results are summarized in Table 2.

（Ｎ／Ａ：測定不能）

(N/A: Unmeasurable)

From the results in Table 2, in producing the transparent ceramics of the present invention, in Comparative Examples 1 and 4, which do not contain any yttrium, the total light transmittance was below 84% and the forward scattering coefficient exceeded 1.0%. This means that many scattering sources were generated in the transparent ceramics, and when a high-power laser was irradiated, the thermal lens effect was intensified, indicating that it would be difficult to use them for industrial purposes.
Conversely, in the samples of Comparative Examples 2, 3, 5, and 6 in which the ratio of yttrium (x in formula (1)) was greater than 0.59 (i.e., the ratio of terbium 1-x-y was 0.4 or less), the Verdet constant was less than 32 rad/(T·m). In this case, the size needs to be larger than that of an optical isolator using TGG crystal, which is a conventional Faraday rotator material, and this poses a problem in that it cannot be installed in future high-power laser systems that are being miniaturized.
For all of Examples 1 to 6, a total light transmittance of 84% or more, a forward scattering coefficient of 1.0% or less, and a Verdet constant of 32 rad/(T·m) or more were ensured, making them suitable for installation in high-power laser systems. Furthermore, for all of the samples of Examples 1 to 6, the average value of the temperature coefficient of refractive index (dn(1064)/dt) _ave at a wavelength of 1064 nm and temperatures of 20, 30, and 40° C. was 9.0×10 ^-6 (K ^-1 ) or less, which was confirmed to be approximately half the (dn(1064)/dt) _ave of the TGG crystal, which is a conventional material.
When the terbium-containing paramagnetic garnet-type transparent ceramics of the present invention, in which the average value of the temperature coefficient of refractive index (dn(1064)/dt) _ave at a wavelength of 1064 nm and temperatures of 20, 30 and 40° C. is less than half that of existing materials, is installed in a high-power laser system, the high-power application resistance is substantially improved by more than two times, making it possible to provide a higher-density high-power laser system that was previously unrealizable.

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.

100 Optical isolator 110 Faraday rotator 120 Polarizer 130 Analyzer 140 Magnet

Claims (4)

A terbium-containing paramagnetic garnet-type transparent ceramic, which is a sintered body of a complex oxide represented by the following formula (1) , characterized in that the Verdet constant at a wavelength of 1064 nm is 30 rad/(T·m) or more, and the average temperature coefficient of refractive index (dn(1064)/dt) _ave at a wavelength of 1064 nm and temperatures of 20, 30 and 40°C is 9.0 x 10 ^-6 (K ^-1 ) or less.
(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, 0.0005≦y<0.08, 0.6<1−x−y<0.95, 0.0004≦z<0.15, and 0.0009≦y+z<0.2.) The terbium-containing paramagnetic garnet-type transparent ceramic according to claim 1 , further comprising more than 0 mass % and 0.1 mass % or less of _SiO2 as a sintering aid. 3. A magneto-optical device comprising the paramagnetic garnet-type transparent ceramic according to claim 1 or 2 as a magneto-optical material. 4. The magneto-optical device according to claim 3 , which is an optical isolator having the terbium-containing paramagnetic garnet-type transparent ceramic as a Faraday rotator and having polarizing materials in front of and behind the optical axis of the Faraday rotator and usable in the wavelength range of 0.9 μm to 1.1 μm.