Method for manufacturing optical component

Abstract

To provide an optical component that has a large size, does not include a portion with non-uniform refractive index and is constituted of a YAG single crystal that has less voids at a joint interface, and to provide a method for manufacturing an optical component.SOLUTION: In an optical component, at least a joint surface of an undoped YAG single crystal is a {211} surface, a joint surface of a doped YAG single crystal and the joint surface of an undoped YAG single crystal are mirror polished and joined together, and then joined by diffusion junction by being subjected to pressure of 0.1 kPa or more and 10 kPa or less in a direction perpendicular to the joint surface at a temperature of 1200°C or more and 1600°C or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for manufacturing optical components such as a laser gain medium made of a yttrium aluminum garnet single crystal material and a fluorescent element. Product

Background Art

[0002] In the fields such as laser processing, a small-sized, high-output, and high-beam-quality laser is desired. Recently, solid-state lasers using a laser diode (LD) as an excitation light source have been widely used. Among them, yttrium aluminum garnet (hereinafter referred to as "YAG") crystals doped with neodymium (Nd) or ytterbium (Yb) are the most widely used as solid-state laser materials.

[0003] In order to achieve a high output of the output laser beam in a solid-state laser, heat dissipation of heat generated in the solid, which is a laser gain material, has become a major problem. As a method for effectively dispersing local heat generation, it has been proposed to bond a laser crystal doped with an optically active substance and a crystal not doped with an optically active substance, and a diffusion bonding method and a direct bonding method have been devised (Patent Document 1).

[0004] ​In addition, research on the structure of the laser medium has also been advanced, and a method using a disk-shaped laser gain material is known. By making the laser gain material into a thin disk shape, the light receiving surface of the excitation light from the outside can be increased, and it becomes possible to cool uniformly over the entire disk surface. Among them, the reflective (active mirror type) structure has attracted attention because cooling by a heat sink or fluid becomes possible by applying a reflective film to one surface of the thin film disk. Usually, an active mirror type structure improves mechanical strength and heat dispersibility by joining a thin film disk doped with an optically active substance and a material not doped with an optically active substance (hereinafter, the integrated structure is referred to as a "laser medium bonded body"). Since these materials require a large laser medium body to handle high-power lasers, transparent ceramic materials are used (Patent Document 2).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] As a medium for solid-state lasers, YAG single crystals and YAG ceramics, which are polycrystals, are used. Although YAG ceramics have the advantage of being easily enlarged and manufactured at relatively low cost, there is a problem that the thermal conductivity is lower than that of single crystals because grain boundaries exist, especially the thermal conductivity at low temperatures is smaller than that of single crystals. Therefore, it is desirable to use a YAG single crystal as the laser medium, but large single crystals have problems that crystal growth is difficult and costly, and optical inhomogeneous parts such as refractive index (which tend to occur in the center of the crystal, hereinafter referred to as "core") tend to occur during crystal growth.

[0007] In addition, in a laser medium bonded body obtained by bonding crystals, since smooth polished surfaces are overlapped and bonded, voids are likely to be generated at the interface. Although the voids are considered to be small in size and have little optical influence, it is desirable to have no voids in a high-power laser. Also, in order to improve heat diffusion even slightly, it is desired to have no voids at the bonding interface.

[0008] The present invention has been made in consideration of such problems, and an object thereof is to provide an optical component made of a single crystal that is large-sized, does not include refractive index inhomogeneous portions, and has few voids at the bonding interface, and a method for manufacturing the optical component.

Means for Solving the Problems

[0009] In order to achieve the above object, an optical component according to the present invention is a bonded body of a doped YAG single crystal, which is a yttrium aluminum garnet single crystal doped with an optically active substance, and an undoped YAG single crystal, which is a yttrium aluminum garnet single crystal not doped with an optically active substance, wherein a bonding surface of the undoped YAG single crystal is a {211} plane.

[0010] The optical component according to the present invention is characterized in that a bonding surface of the doped YAG single crystal is a {211} plane.

[0011] A method for manufacturing an optical component according to the present invention is characterized in that after mirror-polishing and bonding a bonding surface of the doped YAG single crystal and a bonding surface of the undoped YAG single crystal, a pressure of 0.1 kPa or more and 10 kPa or less is applied in a direction perpendicular to the bonding surface at a temperature of 1200°C or more and 1600°C or less in the atmosphere, and diffusion bonding is performed.

[0012] The optical component according to the present invention is characterized in that the doped YAG single crystal and the undoped YAG single crystal are grown in the <211> axis direction, and a transmission surface is obtained perpendicular to the crystal growth direction.

[0013] In the present invention, the problem is solved by using the {211} plane of a YAG single crystal not doped with an optically active substance (hereinafter referred to as "undoped YAG") as a bonding surface and joining it to form an optical component such as a laser medium bonded body joined to a YAG single crystal doped with an optically active substance (hereinafter referred to as "doped YAG").

[0014] YAG single crystals are grown by the Czochralski method or the like, and since the normal growth rate is fast and they are easy to grow, they are manufactured by pulling in the <111> axis direction. At that time, a refractive index inhomogeneous portion called a core is likely to form in the center of the grown YAG single crystal. The {211} plane exists on the ingot side surface of the YAG single crystal manufactured by pulling in the <111> axis direction, and it is possible to avoid the core by obtaining a disk material with the {211} plane on the side surface as the main surface.

[0015] In addition, the inventors have developed a technique for growing large YAG single crystals without using an expensive iridium crucible (Japanese Patent Application Laid-Open Nos. 2023-056860 and 2023-056861). Therefore, it has become possible to efficiently obtain a YAG crystal body having a {211} plane that is relatively inexpensive, large-sized, and does not contain inhomogeneous portions as a bonding surface.

[0016] The present invention has found that by using the {211} plane as the bonding surface of the YAG single crystal, it is possible to significantly reduce voids at the bonding interface. Usually, for a YAG single crystal, the crystal is grown in the <111> axis direction and the crystal is obtained by cutting perpendicular to the crystal growth direction, so it is common to use the {111} plane as the bonding surface. However, when the {111} plane is used as the bonding surface, voids are likely to be generated at the interface. In contrast, it has been found that voids are extremely reduced by using the {211} plane as the bonding surface.

[0017] Optical components such as the laser medium bonded body in the present invention can be joined by mirror-polishing the bonding surfaces of the YAG single crystal doped with an optically active substance and the undoped YAG single crystal, and then performing heat treatment at a temperature of 1200°C to 1600°C in the atmosphere under a low pressure of 0.1 kPa or more and 10 kPa or less perpendicular to the bonding surface.

[0018] In the present invention, after growing doped YAG single crystals and undoped YAG single crystals in the <211> axis direction, it is effective to cut them perpendicular to the growth direction (<211> axis direction) to manufacture optical components such as disks. In the method for growing large-sized YAG single crystals developed by the inventors, it is possible to obtain a large-sized crystal body perpendicular to the growth direction while avoiding the core, and the crystal body can be obtained efficiently. Furthermore, by growing a YAG single crystal doped with Nd or Yb in the <211> axis direction, the concentration variation within the crystal is small, and it becomes possible to obtain a crystal body with the {211} plane as the bonding surface while avoiding the core in the central part generated during growth, and it has become possible to efficiently manufacture optical components such as laser medium bonded bodies.

Advantages of the Invention

[0019] According to the present invention, it is possible to provide an optical component and a method for manufacturing an optical component that are single crystal materials without pores (voids) and grain boundaries, without a core, and with extremely few voids at the bonding interface, and that are excellent in optical characteristics and thermal diffusivity.

Brief Description of the Drawings

[0020]

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Embodiments for Carrying Out the Invention

[0021] Hereinafter, embodiments for carrying out the present invention will be described in detail. FIG. 1 (photograph) shows a YAG single crystal grown by the Czochralski method. When the crystal growth method is in the <111> axis direction and the crystal is cut (ring cut) perpendicular to the growth direction, as shown in FIG. 2, there is an optical inhomogeneous portion in the central portion of the crystal. Therefore, it is extremely difficult to obtain a high-quality and large-sized crystal body.

[0022] As shown in FIG. 3, although the core existing in the YAG single crystal may extend in the <110> axis direction, the {211} plane can be obtained so as to be parallel to the crystal growth direction and avoid the core. In addition, the inventors have developed a technique for growing a large-sized YAG single crystal without using expensive iridium or the like. When this method is adopted, it is possible to efficiently obtain a disk-shaped crystal of the {211} plane.

[0023] As a method for producing a laser medium bonded body made of a single crystal material, a crystal doped with an optically active substance and an undoped crystal are bonded by diffusion bonding or direct bonding. Since diffusion bonding requires heat treatment at a high temperature, it is difficult to bond materials with different coefficients of thermal expansion. However, when using the same type of material, strong bonding can be obtained, and there is an advantage that expensive equipment is not required. On the other hand, a method of directly bonding surfaces activated by an Ar beam or the like in a high vacuum at a low temperature has also been put into practical use, but it requires expensive equipment. When bonding the same type of crystals, such as a doped YAG single crystal and an undoped YAG single crystal, diffusion bonding is advantageous because strong bonding can be achieved relatively easily. In the case of diffusion bonding, it is necessary to polish the surface flat. However, if the surface roughness (Ra) is a mirror surface of 0.5 nm or less, in an area of approximately φ50 mm, and the flatness is <λ / 2 (λ: 633 nm), bonding is possible and not much special polishing is required.

[0024] For the heat treatment for diffusion bonding, an electric furnace or the like is used, and the treatment is carried out at a temperature of 1200°C to 1600°C in the atmosphere. During the heat treatment, positioning is performed with a firing jig, and it is sufficient to apply a load of 0.1 kPa or more and 10 kPa or less from a direction perpendicular to the bonding surface of the doped YAG single crystal or (and) the bonding surface of the undoped YAG single crystal. Bonding is possible without using equipment such as a hot press. It is also possible to bond both crystals by the load due to their own weight. The load corresponding to the weight is about 0.1 kPa. On the other hand, even if the load exceeds 10 kPa, the bonding strength does not change. If the heat treatment temperature is less than 1200°C, the bonding strength may not be sufficient, and if it is 1600°C or more, voids may grow at the interface.

[0025] Since the doped YAG single crystal and the undoped YAG single crystal are usually grown in the <111> axis direction, a disk is obtained by cutting in a ring shape perpendicular to the axis direction. When a {111} plane doped YAG single crystal and a {111} plane undoped YAG single crystal are diffusion bonded, minute voids may be generated at the bonding interface. Since the size of the voids is smaller than 1 μm, it is considered that there is no problem in normal use. However, for a high-intensity laser medium, having fewer voids is optically and thermally preferable.

[0026] By making the bonding surface of the YAG single crystal the {211} plane, it is possible to significantly reduce the voids at the bonding interface. Since the {211} plane is a plane that easily appears as a facet during crystal growth and is considered a stable plane, it is thought that the voids will decrease by using the {211} plane as the bonding surface. When observing the voids at the bonding interface between {111} planes, since the {211} plane may appear in the voids, it is difficult for voids to be generated when the {211} plane is used as the bonding surface. Note that as long as at least the bonding surface of the undoped YAG single crystal is the {211} plane, the bonding surface of the doped YAG single crystal may be the {211} plane or a plane other than {211}.

[0027] YAG single crystals doped with Nd or Yb are used as laser media, and it is common to grow crystals from a melt by the Czochralski method or the like. When growing crystals from a melt, the ratio of the element to be doped incorporated into the crystal varies depending on the element, and the segregation coefficient indicating that ratio is approximately 0.2 for Nd and approximately 1.0 for Yb.

[0028] In the case of Nd-doped YAG single crystals, segregation is likely to occur, so a concentration gradient in the crystal growth direction is likely to be formed. Therefore, when obtaining a crystal parallel to the crystal growth direction, the in-plane concentration distribution becomes large, which is not desirable. Therefore, when growing a crystal in the <111> axis direction, the {111} plane perpendicular to the crystal growth direction is used. Usually, since the thickness is thin, it is hardly affected by optical inhomogeneous parts. Also, in the case of a large crystal, it is possible to obtain the crystal avoiding the core. When bonding a Nd-doped YAG single crystal with a {111} plane and an undoped YAG single crystal with a {211} plane, the voids at the interface are significantly reduced compared to the case of bonding a Nd-doped YAG single crystal with a {111} plane and an undoped YAG single crystal with a {111} plane.

[0029] In contrast, in the case of a Yb-doped YAG single crystal, the segregation coefficient is approximately 1.0, and almost no concentration gradient occurs depending on the growth direction. Therefore, even when crystal growth is performed in the <111> axis direction, it is possible to obtain the {211} plane in the longitudinal direction. When a Yb-doped YAG single crystal with a {211} plane is joined to an undoped YAG single crystal with a {211} plane, a laser medium joined body with fewer voids can be produced. Of course, as in the case of an Nd-doped YAG single crystal, even if it is the {111} plane, if it is joined to an undoped YAG single crystal with a {211} plane, a good joined body with fewer voids at the joined interface can be obtained.

[0030] By using the technique of growing a large YAG single crystal, crystals can be obtained more efficiently by growing the crystal in the <211> axis direction. It is possible to cut out a crystal perpendicular to the growth direction from the crystal grown in the <211> axis direction and obtain a crystal body avoiding the core. Whether it is a crystal not doped with an optically active substance or a crystal doped with an optically active substance, since it is cut perpendicular to the growth direction, there is no concentration unevenness, and a crystal body avoiding the core can be obtained.

[0031] The technology related to the optical component composed of a YAG single crystal as the laser medium described so far can also be applied to lighting using a fluorescence-emitting material doped with cerium (Ce).

[0032] <Example> Hereinafter, the comparative examples and examples will be described (see Table 1 below). First, a method for producing a crystal bonded body of a comparative example will be described. An undoped YAG single crystal grown in the <111> axis direction was cut perpendicular to the growth axis, ground, and the bonding surface was mirror-polished to produce a disk with a diameter of φ54 mm and a thickness of 6 mm. Similarly, a YAG single crystal doped with 1 at% Nd was grown in the <111> axis direction, cut perpendicular to the growth direction, ground, and the bonding surface was mirror-polished to produce a disk with a diameter of φ54 mm and a thickness of 1.6 mm. These two disks were stacked, a load of 1.3 kPa was applied with a weight, and heat treatment was performed at 1500 °C for 10 h in the atmosphere to bond them. The bonded crystal was subjected to outer circumference grinding and double-sided polishing to obtain a bonded body with a diameter of φ50 mm and a thickness of 5.2 mm (the thickness of the Nd:YAG single crystal was 0.2 mm). The appearance thereof is shown in Fig. 4. In addition, the measurement results of the transmitted wavefront accuracy by a laser interferometer (Verifire manufactured by ZYGO) are shown in Fig. 5. The transmitted wavefront accuracy was 0.89λ (λ = 633 nm) in a φ42.5 mm area due to the influence of the core in the crystal.

[0033] The crystal bonded body of the example will be described. From an undoped YAG single crystal grown in the <111> axis direction, a crystal was cut out so that the {211} plane became the bonding surface parallel to the growth direction, ground, and the bonding surface was mirror-polished to produce a disk with a diameter of φ54 × 6 mm. For the Nd-doped YAG single crystal, a sample prepared in the same manner as in the comparative example was used. The appearance of a crystal bonded body with a diameter of φ50 mm and a thickness of 5.2 mm (the thickness of the Nd:YAG single crystal was 0.2 mm) prepared by the same procedure as in the comparative example is shown in Fig. 6, and the transmitted wavefront accuracy is shown in Fig. 7. Since the core in the crystal was avoided, the transmitted wavefront accuracy was small and a good result of 0.12λ was obtained (Example 1).

[0034] The crystal bonded body of Example 2 was produced in the same manner as in Example 1. A disk with a diameter of φ30 × 3 mm was produced from an undoped YAG crystal grown in the <111> axis direction so that the {211} plane became the bonding surface. Also, a YAG single crystal doped with 5 at% Yb was grown in the <111> axis direction, and a disk with a diameter of φ30 × 1 mm was produced by cutting out the crystal so that the {211} plane became the bonding surface parallel to the growth direction. These two disks were stacked, a load of 4.3 kPa was applied, and heat treatment was performed at 1500 °C for 10 h in the atmosphere to bond them.

[0035]

Table 1

[0036] The observation of the bonding interface will be described. Fig. 8 is a scanning electron microscope (SEM) photograph of the interface obtained by bonding the {111} plane of a doped YAG single crystal and the {111} plane of an undoped YAG single crystal by heat treatment at 1500°C for 10 h. Small voids of about 0.1 μm in size are scattered here and there.

[0037] In contrast, Fig. 9 is an SEM photograph of the interface obtained by bonding the {111} plane of an Nd-doped YAG single crystal and the {211} plane of an undoped YAG single crystal by heat treatment at 1500°C for 10 h (Example 1). Fig. 10 is an SEM photograph of the interface obtained by bonding the {211} plane of a Yb-doped YAG single crystal and the {211} plane of an undoped YAG single crystal by heat treatment at 1500°C for 10 h (Example 2). It can be seen that in both Example 1 and Example 2, there are almost no voids at the bonding interface and the bonding is good.

[0038] As described above, the embodiments of the present invention have been described in detail, but the present invention is not limited to the above embodiments. And various design changes can be made to the present invention without departing from the matters described in the claims.

Claims

1. A method for manufacturing an optical component, which is a bonded body of a doped YAG single crystal that is a yttrium aluminum garnet single crystal doped with an optically active substance and an undoped YAG single crystal that is a yttrium aluminum garnet single crystal not doped with an optically active substance, comprising: growing the doped YAG single crystal and the undoped YAG single crystal in the <211> axis direction, and obtaining a transmission plane perpendicular to the crystal growth direction, manufacturing the optical component in which the bonding surface of the doped YAG single crystal and the bonding surface of the undoped YAG single crystal are {211} planes. A method for manufacturing an optical component, characterized by the above.

2. After mirror-polishing and bonding the bonding surface of the doped YAG single crystal and the bonding surface of the undoped YAG single crystal, in the atmosphere, at a temperature of 1200 °C or higher and 1600 °C or lower, applying a pressure of 0.1 kPa or higher and 10 kPa or lower in a direction perpendicular to the bonding surface, and performing diffusion bonding. The method for manufacturing an optical component according to claim 1, characterized by the above.

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