Single crystal fiber manufacturing method

Abstract

The present disclosure provides a method for manufacturing a Nd:YAG single crystal fiber that has such a concentration distribution in the radial direction that the Nd concentration is maximized at the central axis of the single-crystal fiber. The method for manufacturing a Nd:YAG single crystal fiber of the present disclosure comprises: preparing a rod-shaped base metal containing a single crystal or polycrystal of YAG together with Nd and Ca; melting one end of the base metal to form a molten part; and contacting the molten part with a seed crystal and growing the single crystal fiber while pulling up the seed crystal.

Description

[Technical Field]

[0001] The present disclosure relates to a method for manufacturing a Nd:YAG single crystal fiber. [Background technology]

[0002] Yttrium aluminum garnet (hereinafter referred to as YAG) is Y3Al5O 12 It is a composite oxide of yttrium (Y) and aluminum (Al) represented by the chemical formula and is known as a typical laser medium material. Furthermore, solid-state lasers using neodymium (Nd)-containing YAG (hereinafter referred to as Nd:YAG) as a laser medium are widely used in the fields of processing and medicine, and Nd:YAG is also used as an amplifying medium in optical amplifiers.

[0003] A semiconductor laser (hereafter referred to as LD: Laser Diode), which has a high electrical-to-optical conversion efficiency, is often used as the excitation light for oscillating Nd:YAG lasers using Nd:YAG as a laser medium. To improve the efficiency of LD-pumped oscillation in lasers using single crystals as the laser medium, such as Nd:YAG lasers, a method using single crystal fiber as the laser medium has been developed. Generally, high-power LDs have poor focusing ability due to their multi-transverse mode oscillation. However, by using a single crystal fiber with a diameter of 50-500 μm as the laser medium, longitudinal waveguiding can be achieved, confining the excitation light within the cross section of the multimode waveguide. This allows for efficient stimulated emission within the multimode waveguide, improving the oscillation efficiency of lasers using single crystals as the laser medium.

[0004] The main methods for manufacturing single crystal fibers for lasers include the micro pull-down method (hereinafter referred to as the μ-PD method) and the laser heated pedestal growth method (hereinafter referred to as the LHPG method). The μ-PD method has the advantage of distributing the Nd concentration almost uniformly within the fiber cross section, but it also has the problem of only being able to manufacture large diameter fibers with a diameter of approximately 500 μm or more. On the other hand, the LHPG method can manufacture single crystal fibers with a diameter of 50-500 μm, so it is thought that the LHPG method is suitable for manufacturing Nd:YAG single crystal fibers as the laser medium mentioned above.

[0005] FIG. 1 is a diagram conceptually illustrating a process for manufacturing a single crystal fiber using the LHPG method. FIG. 2 is a flowchart illustrating an exemplary method 20 for manufacturing a single crystal fiber using the LHPG method. The exemplary LHPG method for manufacturing a single crystal fiber 20 is performed in an oxygen-containing atmosphere, and as shown in FIGS. 1 and 2 , includes irradiating a tip of a preform 11 with a carbon dioxide laser 16 for heating to form a molten zone 12 (corresponding to step 21 in FIG. 2 ), and bringing a seed crystal 18 into contact with the molten zone 12 and growing a single crystal fiber 14 while pulling up the seed crystal 18 (corresponding to step 22 in FIG. 2 ) (see, for example, Non-Patent Document 4). Conventional manufacturing methods have used a Nd:YAG single crystal or polycrystal as the preform 11 used in the method 20 for manufacturing a Nd:YAG single crystal fiber using the LHPG method.

[0006] In order to use the above-mentioned multimode waveguide as a laser medium and achieve fundamental transverse mode oscillation, it is desirable that the concentration distribution of Nd, which is the luminescence center responsible for gain, in the radial cross section be maximum at the central axis of the fiber. Similarly, for efficient amplification in an optical amplifier, a concentration distribution in which the Nd concentration is maximum at the axial center of the fiber is also desirable. However, according to previous reports, it is known that in Nd:YAG single crystal fibers fabricated by the LHPG method, the Nd concentration distribution has maximums on the circumference equidistant from the central axis of the fiber (see, for example, Non-Patent Document 1).

[0007] FIG. 3 is a conceptual diagram illustrating the appearance of a molten zone 12 during the manufacturing process of an Nd:YAG single crystal fiber using the LHPG method according to the prior art. In the figure, the convection 13 of the melt in the molten zone 12 is indicated by a line with an arrow. As described above, in the manufacturing of an Nd:YAG single crystal fiber using the LHPG method, a Nd:YAG single crystal or polycrystal is used as the preform 11. According to existing reports, the segregation coefficient of Nd in YAG crystals is known to be 0.21, so the concentration of Nd in the molten zone 12 is approximately five times higher than that in the preform 11 (see, for example, Non-Patent Document 2). Furthermore, it is known that Nd substitutes for the cation sites of Y in Nd:YAG crystals, but the atomic weight of Nd is approximately 1.6 times larger than that of Y. Therefore, the density of the melt in the initially formed molten zone 12 is relatively high, but the density of the newly melted melt is lower than that of the previously formed melt because the Nd content remains the same. That is, this newly melted melt is subjected to a strong buoyancy from the previously formed surrounding melt, and as shown in Fig. 3, convection 13 occurs around the central axis of the fiber, with the flow from the preform 11 side of the molten zone 12 to the grown single crystal fiber 14 side. As a result, the uptake of Nd atoms is maximized near the apex of the upper outer convection flow, where the Nd concentration is highest, and a region 15 where the Nd concentration is locally maximized is formed on the circumference equidistant from the central axis of the fiber.

[0008] Thus, in an Nd:YAG single crystal fiber fabricated by the LHPG method, the radial concentration distribution is such that Nd has its maximum on the circumference equidistant from the central axis of the single crystal fiber. Therefore, from the perspective of improving the efficiency of laser oscillators and optical amplifiers that use Nd:YAG single crystal fibers, there is a need for a method for manufacturing an Nd:YAG single crystal fiber that has a radial concentration distribution in which the Nd concentration is maximum at the central axis of the single crystal fiber. [Prior art documents] [Non-patent literature]

[0009] [Non-Patent Document 1] S. Bera et al., “Dopant segregation in YAG single crystal fibers grown by the laser heated pedestal growth technique”, Journal of Crystal Growth, 547, P125801, 2020 [Non-patent document 2] Kenichi Shiraki and Yasuhiko Kuwano, "Effective Segregation Coefficient of Neodymium in Nd:Y3Al5O12 Single Crystal Growth by Pulling Method", Journal of the Chemical Society of Japan, 7, p. 940, 1978 [Non-patent document 3] A. Sugimoto et al., “Crystal growth and optical characterization of Cr,Ca: Y3A15012, Journal of Crystal Growth”, 140, P349-354, 1994 [Non-patent document 4] S. Ishibashi et al., “Cr,Ca: Y3A15012 laser crystal grown by the laser-heated pedestal growth method”, Journal of Crystal Growth, 183, P614-621, 1998 [Non-Patent Document 5] Yener Kuru et al., “Enhanced co-solubilities of Ca and Si in YAG (Y3Al5O12)”, Physica Status Solidi (c) 5,P3383-3386, 2008 Summary of the Invention

[0010] The present disclosure has been made in view of the above-mentioned problems, and an object of the present disclosure is to provide a method for manufacturing an Nd:YAG single crystal fiber having a radial concentration distribution in which the Nd concentration is maximized at the central axis of the single crystal fiber.

[0011] In response to the above-described problems, the present disclosure provides a method for manufacturing a Nd-containing YAG single crystal fiber, the method comprising: preparing a rod-shaped base material containing a YAG single crystal or polycrystal, Nd, and calcium (Ca); melting one end of the base material to form a molten portion; bringing a seed crystal into contact with the molten portion; and growing the single crystal fiber while pulling up the seed crystal. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a diagram conceptually showing the process of manufacturing a single crystal fiber by the LHPG method. [Figure 2] 1 is a flowchart illustrating an exemplary method 20 for manufacturing a single crystal fiber using the LHPG method. [Figure 3] 1 is a diagram conceptually showing the appearance of a molten part in a manufacturing process of a Nd:YAG single crystal fiber using the LHPG method according to the prior art. [Figure 4] 1 is a diagram conceptually illustrating the appearance of a fusion zone 12 in the manufacturing process of a Nd:YAG single crystal fiber using the LHPG method according to the present disclosure. [Figure 5] 1A and 1B are diagrams illustrating a base material used in an LHPG method in a first embodiment of the present disclosure, where (a) is an axial cross-sectional view and (b) is a radial cross-sectional view. [Figure 6] 1 is a flow chart illustrating a method for manufacturing a single crystal fiber using an LHPG method according to the present disclosure. [Figure 7] 10A and 10B are diagrams illustrating a base material used in an LHPG method in a second embodiment of the present disclosure, where (a) is an axial cross-sectional view and (b) is a radial cross-sectional view. DETAILED DESCRIPTION OF THE INVENTION

[0013] Various embodiments of the present disclosure will be described in detail below with reference to the drawings. The same or similar reference numerals indicate the same or similar elements, and redundant description may be omitted. The following description is an example, and some configurations may be omitted or modified, or additional configurations may be added, as long as they do not deviate from the gist of one embodiment of the present disclosure.

[0014] The method for manufacturing an Nd:YAG single crystal fiber according to the present disclosure is similar to the prior art in that it uses the LHPG method, but differs from the prior art in that an additional element other than Nd is added to the YAG base material, and the fluid behavior of the melt is changed in the molten zone 12 as shown in Figure 2, thereby forming a region 15 with a maximum Nd concentration at the center axis of the fiber.

[0015] As described above, in the conventional method 20 for manufacturing Nd:YAG single crystal fiber 14 using the LHPG method, a density gradient in the molten zone 12 according to the segregation coefficient of Nd in the YAG crystal results in a radial concentration distribution in which the Nd concentration is maximized in regions away from the central axis of the single crystal fiber 14. However, if an element with a small segregation coefficient and a small atomic weight is added, the molten zone is formed while maintaining the density of the melt equivalent to that of the base material, thereby suppressing the density gradient in the melt. In other words, it is possible to manufacture an Nd:YAG single crystal fiber 14 using the LHPG method, which has a radial concentration distribution in which the Nd concentration is maximized at the central axis of the single crystal fiber 14.

[0016] Calcium (Ca) is an additional element that satisfies these conditions. The conditions for adding Ca to Nd:YAG are described in detail below.

[0017] In Nd:YAG with Ca added, if the ratio of the number of Nd atoms to the number of Y atoms that make up the YAG is x and the ratio of the number of Ca atoms is y, the chemical formula of the substance is Nd 3x Ca 3y Y 3(1-x-y)AlO 12 The atomic weights of the constituent atoms of this Ca-doped Nd:YAG are Y: 88.91, Al: 26.98, O: 16.00, Nd: 144.24, and Ca: 40.08, so the molecular weight S(x, y) of Ca-doped Nd:YAG can be expressed by (Equation 1).

[0018]

number

[0019] Since the volume change of the YAG melt due to the presence of added elements (Nd and Ca) to the YAG is considered to be insignificant here, S(x, y) in (Equation 1) is essentially a parameter corresponding to the density.

[0020] On the other hand, the segregation coefficient k eff For Nd, k eff,Nd =0.21, and k for Ca eff,Ca = 0.1 (see, for example, Non-Patent Documents 2 and 3). Therefore, the concentration of each added element in the melt is eff,Nd =x / 0.21, Ca is y / k eff,Ca =y / 0.1.

[0021] Based on these findings, if we assume that a Ca-doped Nd:YAG single crystal fiber is being grown (stable growth) using the LHPG method, the conditions under which the density of the newly melted liquid becomes equal to or greater than that of the previously formed melt can be expressed by (Equation 2).

[0022]

number

[0023] Therefore, by substituting the values ​​into (Equation 1) and (Equation 2), the condition for generating convection similar to that of the melt containing no additives can be expressed by (Equation 3).

[0024]

number

[0025] On the other hand, when adding other elements (e.g., Ca) to YAG, if the Ca is added in excess, the free energy of the YAG crystal increases, making it impossible to maintain the crystal structure. Furthermore, the amount of Ca intercalating between the lattices rather than at the Y cation sites increases, which can result in the YAG becoming amorphous, or the added Ca being phase-separated into calcium oxide (CaO). In other words, in the method for manufacturing a Nd:YAG single crystal fiber according to the present disclosure, there is an upper limit to the amount of Ca that can be added to the YAG.

[0026] When Ca is added to YAG crystals, it substitutes for the Y cation sites in YAG, just like Nd, and it is known that when added as CaO, the crystal structure of YAG is maintained up to approximately 8 at.% (see, for example, Non-Patent Document 5). When adding CaO, silicon (Si) is sometimes added to maintain electrical neutrality (see, for example, Non-Patent Document 5). In this case, since Si substitutes for the Al sites, Al with the same atomic number as the added Si is subtracted from the raw material. Because the effective ionic radius of Si is smaller than that of Al, the segregation coefficient of Si relative to YAG is approximately 1, and it does not affect convection in the melt.

[0027] From the above, the upper limit of the number of Ca atoms to be added to YAG, expressed as a ratio to the number of Y atoms (corresponding to the above-mentioned y), is 0.08. Taking this into account in (Equation 3), (Equation 4) is derived, and this (Equation 4) represents the condition range for the amount of Ca added in the present disclosure.

[0028]

number

[0029] From the above, if an Nd:YAG single crystal fiber is produced by the LHPG method under conditions that satisfy (Equation 4), the Nd:YAG single crystal fiber produced will have a concentration distribution in which the Nd concentration is maximized at the central axis of the fiber.

[0030] FIG. 4 is a conceptual diagram illustrating the appearance of a molten zone 12 during the manufacturing process of an Nd:YAG single crystal fiber using the LHPG method according to the present disclosure. As in FIG. 3, in the figure, convection 13 of the melt in the molten zone 12 is indicated by a line with an arrow. Furthermore, unlike the preform 11 in the prior art, the preform 17 in the present disclosure is a YAG single crystal or polycrystal containing Nd and Ca so as to satisfy Equation 4. Based on the above-described principle, in the manufacturing of an Nd:YAG single crystal fiber using the LHPG method according to the present disclosure, no density gradient of the melt occurs in the molten zone 12 due to differences in segregation coefficients and atomic weights. Therefore, unlike the prior art, convection 13 does not exhibit fluid behavior such as flowing from the preform 17 side of the molten zone 12 toward the grown single crystal fiber 14 around the central axis of the single crystal fiber 14. As a result, the Nd concentration reaches a maximum at the central axis of the single crystal fiber 14, which is the axis of symmetry for convection 13.

[0031] Thus, according to the present disclosure, it is possible to manufacture, by the LHPG method, an Nd:YAG single crystal fiber having a radial concentration distribution in which the Nd concentration is maximized at the center axis of the fiber. Therefore, compared with fibers manufactured by conventional techniques, Nd:YAG single crystal fibers manufactured by the manufacturing method of the present disclosure can easily achieve fundamental transverse mode oscillation, thereby enabling the realization of highly efficient laser oscillators and optical amplifiers.

[0032] In addition, the method for manufacturing Nd:YAG single crystal fiber according to the present disclosure changes only the composition of the preform, and the LHPG method itself remains unchanged from the conventional method. Therefore, there is no need to significantly change the existing manufacturing process, and there is also the advantage that the existing production line can be maintained.

[0033] (First embodiment) A first embodiment of the present disclosure will be described in detail below with reference to the drawings. In this embodiment, the base material used in the LHPG method is a YAG single crystal or polycrystal containing Nd and Ca as additive elements.

[0034] FIG. 5 illustrates a preform 17 used in the LHPG method in this embodiment, with FIG. 5(a) being an axial cross-sectional view and FIG. 5(b) being a radial cross-sectional view. The preform 17 in this embodiment is a rod material having a YAG single crystal or polycrystal as a matrix, with Nd and Ca contained within the matrix as additive elements. As described above, the preform 17 is configured so that the Nd and Ca contents satisfy Equation 4. An Nd:YAG single crystal fiber 14 is manufactured from the preform 17 having such a shape by the LHPG method. While the preform 17 is preferably a round bar (cylinder) as shown in the figure, the shape is not limited thereto as long as it is rod-like.

[0035] The base material 17 can be produced by techniques such as melting, sintering, element diffusion, and ion implantation using an ion beam, but the production method is not limited to these.

[0036] 6 is a flowchart illustrating a method 60 for manufacturing a single crystal fiber using the LHPG method according to an embodiment of the present disclosure. The method 60 for manufacturing a single crystal fiber using the LHPG method according to an embodiment of the present disclosure further includes, prior to the conventional manufacturing method shown in FIG. 2, preparing a preform containing Nd and Ca in a YAG single crystal or polycrystal so as to satisfy Equation 4 (corresponding to step 61 in FIG. 6). Note that, although Nd:YAG single crystal fibers are typically grown in an atmosphere containing oxygen, the manufacturing method according to the present disclosure may be performed in an atmosphere not containing oxygen.

[0037] In the Nd:YAG single crystal fiber manufactured by this method, as described above, the density gradient of the melt is suppressed by the addition of Ca, and therefore the molten part 12 has the appearance shown in Fig. 4. Therefore, the manufactured Nd:YAG single crystal fiber 14 has a concentration distribution in which the Nd concentration is maximum at the central axis.

[0038] (Second embodiment) A second embodiment of the present disclosure will be described in detail below with reference to the drawings. In this embodiment, a base material used in the LHPG method has an oxide layer on its outer surface.

[0039] 7A and 7B are diagrams illustrating a base material 70 used in the LHPG method according to one embodiment of the present disclosure, with FIG. 7A being an axial cross-sectional view and FIG. 7B being a radial cross-sectional view. The base material 70 in this embodiment further includes a substrate 71 and an oxide layer 72 formed on the outer surface of the substrate 71. Here, as an example, the substrate 71 is described as a single crystal or polycrystalline Nd:YAG, and the oxide layer 72 is described as CaO, but as described below, other forms are also possible. However, the amounts of Nd and Ca in the entire base material 70, including the substrate 71 and the oxide layer 72, must satisfy Equation 4.

[0040] The oxide layer 72 can be formed on the outer surface of the substrate 71 by techniques such as, for example, physical vapor deposition methods such as vacuum deposition and magnetron sputtering, chemical vapor deposition methods such as plasma CVD and photo-CVD, liquid phase deposition methods such as plating and sol-gel methods, thermal spraying methods such as flame spraying and plasma spraying, and sintering methods such as hot isostatic pressing and spark plasma sintering, but the film formation techniques are not limited to these.

[0041] If the Nd:YAG single crystal fiber 14 is manufactured by the method shown in FIG. 6 using the preform 70 of this embodiment configured as described above, the Nd:YAG single crystal fiber 14 having a concentration distribution in which the Nd concentration is maximized at the central axis can be obtained, as in the first embodiment.

[0042] In this embodiment, the base material 71 of the base material 70 is made of Nd:YAG and the oxide layer 72 is made of CaO, but the combination of materials is not limited to this. For example, the base material 71 may be made of YAG containing Ca as an additive element (Ca:YAG), and the oxide layer 72 may be made of neodymium oxide (Nd2O3). The additive elements of YAG (Nd and Ca) may be contained in either the base material 71 or the oxide layer 72.

[0043] Additionally, the base material 70 may be the base material 17 described in the first embodiment, with the oxide layer 72 formed on the outer surface thereof.

[0044] However, in either case, as described above, the amounts of Nd and Ca in the base material 70 as a whole must be configured to satisfy (Equation 4). [Industrial Applicability]

[0045] The single crystal fiber manufacturing method according to the present disclosure, unlike conventional techniques, makes it possible to manufacture a Nd:YAG single crystal fiber having a concentration distribution in which the Nd concentration is maximized at the central axis. Nd:YAG single crystal fibers having such a concentration distribution can easily achieve fundamental transverse mode oscillation, and are therefore expected to be applicable to laser oscillators and optical amplifiers.

Claims

1. 1. A method for producing a neodymium (Nd)-containing yttrium aluminum garnet (YAG) single crystal fiber, comprising: A rod-shaped base material is prepared, the base material including a single crystal or polycrystal of YAG, Nd, calcium (Ca), and silicon; melting one end of the base material to form a molten portion; bringing a seed crystal into contact with the molten portion and growing the single crystal fiber while pulling up the seed crystal; A method for manufacturing a single crystal fiber, comprising:

2. a ratio of the number of Nd atoms contained in the base material to the number of yttrium (Y) atoms contained in the YAG single crystal or polycrystal is defined as a first ratio; When the ratio of the number of Ca atoms contained in the base material to the number of Y atoms contained in the YAG single crystal or polycrystal is defined as a second ratio, 2. The method for producing a single crystal fiber according to claim 1, wherein the second ratio is not less than 0.47 times the first ratio and not more than 0.

08.

3. 3. The method for producing a single crystal fiber according to claim 1, wherein the base material has a parent phase of a YAG single crystal or polycrystal, and Nd and Ca are contained within the parent phase.

4. 4. The method for producing a single crystal fiber according to claim 1, wherein the preform further comprises a substrate and an oxide layer formed on an outer surface of the substrate.

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