Crystallized glass fiber, optical fiber, wavelength conversion device, light source, and crystallized glass fiber production method

Abstract

This crystallized glass fiber has a first region extending inward in the radial direction from the outer peripheral edge, and the first region includes a crystalline phase having a crystal orientation in the radial direction.

Description

Crystallized glass fiber, optical fiber, wavelength conversion device, light source, and method for manufacturing crystallized glass fiber

[0001] The present invention relates to a crystallized glass fiber, an optical fiber, a light source, a wavelength conversion device, and a method for manufacturing a crystallized glass fiber. This application claims priority to U.S. Patent Application No. 63 / 535,071 filed on August 29, 2023, the contents of which are incorporated herein by reference.

[0002] Optical fibers are generally made of silicon dioxide (SiO ２ Optical fibers are made of a material whose main component is ZnO, and can be easily manufactured using established processes such as fiber drawing, allowing for inexpensive mass production. Optical fibers also have extremely high transparency and extremely low transmission loss, making them used in a variety of fields as a transmission medium for long-distance, high-speed, and large-capacity optical communications. Optical fibers are expected to continue to be used to the fullest extent in applied technologies in the optical field, and are also expected to serve as a transmission medium for quantum communications in the future.

[0003] The low loss and easy formability of optical fibers are known to be due to their isotropic structure. Conventional optical fibers are often used in static devices, such as optical waveguiding, wavelength-specific reflection, and optical amplification, and most of these applications involve enhancing or improving the inherent properties of the base material. In recent years, various approaches have been explored to impart optical nonlinearity to glass materials in order to develop fiber-type dynamic optical functional devices. For example, the crystallization of functional phases in glass precursors, i.e., glass-ceramic (GC) technology, is a promising method for expressing not only permanent optical nonlinearity but also unique optical nonlinearity.

[0004] Conventional crystallized glass includes, for example, transparent crystallized glass having a structure in which nanometer-order crystallized glass particles are dispersed in an amorphous glass matrix (Non-Patent Document 1).

[0005] Y. Takahashi et al. , Appl. Phys. Lett. 86, 091110 (2005)

[0006] However, in the transparent crystallized glass of Non-Patent Document 1, it is difficult to control the crystal orientation, which results in variations in the crystal orientation, making it impossible to obtain a crystallized optical fiber that stably possesses optical nonlinearity, making it difficult to apply it to dynamic devices and put it into practical use.

[0007] The object of the present invention is to provide a crystallized glass fiber, an optical fiber, a wavelength conversion device, a light source, and a method for producing a crystallized glass fiber, which can exhibit dynamic optical functionality by imparting optical nonlinearity while maintaining the transparency, low loss, and easy formability inherent to glass.

[0008] To achieve the above object, the inventors conducted extensive research and found that by providing a crystalline phase oriented radially in a first region extending from the outer peripheral end toward the radially inward direction of a crystallized glass fiber, a radially polarized structure is formed in the radial cross section of the fiber, and the isotropic structure of the glass is macroscopically disrupted, resulting in a first-order electro-optic effect (Pockels effect) and a second-order nonlinear optical effect. As a result, dynamic optical functionality can be exhibited while maintaining transparency, low loss, and easy formability. Furthermore, the inventors found that the radially polarized structure is equivalent to a polarization-reversed structure for intrinsic transverse mode light of any polarization direction, and that quasi-phase matching can be achieved by selecting appropriate cladding and core diameters. Furthermore, the inventors found that by providing a second region containing an amorphous phase in the center of the crystallized glass fiber, radially inside the first region, the crystallized glass fiber can suppress the crystal orientation of the crystalline phase and reduce the refractive index difference between the crystalline and amorphous phases, thereby achieving excellent low loss equivalent to that of an optical single crystal. In addition, it was found that when the crystallized glass fiber having the above-mentioned configuration is used as an optical fiber or a fiber-type optical device, it has high connection affinity with the glass fiber, can achieve low loss at the connection part, and can provide excellent productivity and long-term stability.

[0009] That is, the present invention provides the following configurations: [1] A crystallized glass fiber having a first region extending in a radially inward direction from an outer peripheral end portion, the first region including a crystalline phase oriented along the radial direction.

[0010] [2] The crystallized glass fiber according to the above [1], further comprising a second region located radially inside the first region and at the center of the crystallized glass fiber, the second region containing an amorphous phase.

[0011] [3] The crystallized glass fiber according to the above [1], wherein amorphous phase particles are contained within the domains of the crystalline phase in the first region or between the domains adjacent to each other.

[0012] [4] The crystallized glass fiber according to the above [1], wherein the crystalline phase of the first region has a Fresnoite-type crystalline structure.

[0013] [5] The crystallized glass fiber according to the above [1], wherein the crystalline phase of the first region is composed of an oxide containing strontium (Sr), titanium (Ti) and silicon (Si), an oxide containing barium (Ba), titanium (Ti) and silicon (Si), or an oxide containing barium (Ba), titanium (Ti) and germanium (Ge).

[0014] [6] The crystallized glass fiber according to the above [1], wherein the propagation loss of light in the wavelength range of 400 nm to 1800 nm is 5.0 dB / cm or less.

[0015] [7] An optical fiber comprising at least one core portion made of the crystallized glass fiber described in [1] above, and a cladding portion covering the at least one core portion.

[0016] [8] The optical fiber according to the above [7], wherein the relative refractive index difference Δn of the refractive index of the cladding portion to the refractive index of the crystallized glass in the crystallized glass fiber constituting the core portion is 1% or less.

[0017] [9] An optical phase and intensity modulation device comprising the crystallized glass fiber according to [1] above or the optical fiber according to [7] above.

[0018]

[10] A wavelength conversion device comprising the crystallized glass fiber according to [1] above or the optical fiber according to [7] above.

[0019]

[11] A light source for generating quantum entangled light, comprising the crystallized glass fiber according to [1] above or the optical fiber according to [7] above.

[0020]

[12] An optical switch comprising the crystallized glass fiber according to [1] above or the optical fiber according to [7] above.

[0021]

[13] A phase matching adjustment method for applying an external electric field to the crystallized glass fiber or the optical fiber to adjust the phase matching in a pseudo manner using the optical phase and intensity modulation device described in [9] above, in one or more of the wavelength conversion device described in

[10] above, the light source for generating quantum entangled light described in

[11] above, and the optical switch described in

[12] above.

[0022]

[14] A method for producing a crystallized glass fiber, comprising the steps of: preparing a mixture of a Sr compound, a Ti compound, and a Si compound; a mixture of a Ba compound, a Ti compound, and a Si compound; or a mixture of a Ba compound, a Ti compound, and a Ge compound; heating and melting the mixture to obtain a melt; molding the melt into a glass fiber precursor; and heat-treating the glass fiber precursor at 700°C to 1000°C for 0.5 hours or more to crystallize it.

[0023] According to the present invention, it is possible to provide a crystallized glass fiber, an optical fiber, a wavelength conversion device, a light source, and a method for producing a crystallized glass fiber, which are capable of exhibiting dynamic optical functionality by imparting optical nonlinearity while maintaining the transparency, low loss, and easy formability inherent to glass.

[0024] Fig. 1 is a perspective view schematically showing the configuration of a crystallized glass fiber according to an embodiment of the present invention. Fig. 2 is an axial cross-sectional view of the crystallized glass fiber of Fig. 1. Fig. 3 is a radial cross-sectional view of the crystallized glass fiber of Fig. 1. Fig. 4 is a partially enlarged cross-sectional view of the crystallized glass fiber of Fig. 2. Fig. 5A is a diagram showing the crystal structure of a domain of a crystalline phase in the first region of Fig. 4. Fig. 5B is a diagram showing a TiO 2 having a pyramidal structure in the crystal structure of Fig. 5A. ５1. FIG. 6 is a diagram for explaining the Pockels effect in the crystallized glass fiber of FIG. 1. FIG. 7 is a diagram for explaining quasi-phase matching in the crystallized glass fiber of FIG. 1. FIG. 8 is a diagram for explaining asymmetry due to structural imperfection in the crystallized glass fiber of FIG. 1. FIG. 9 is a diagram for explaining a quasi-phase matching adjustment technique in the crystallized glass fiber of FIG. 8 using the Pockels effect shown in FIG. 6. FIG. 10 is an axial cross-sectional view showing a modified example of the crystallized glass fiber of FIG. 1. FIG. 11 is a radial cross-sectional view of the crystallized glass fiber of FIG. 10. FIG. 12 is a partially enlarged cross-sectional view of the crystallized glass fiber of FIG. 10. FIG. 13 is a diagram for explaining a manufacturing method of a crystallized glass fiber according to an embodiment of the present invention. FIG. 14 is a radial cross-sectional view showing an example of the configuration of an optical fiber comprising a crystallized glass fiber according to an embodiment of the present invention. FIG. 15 is a radial cross-sectional view showing a modified example of the optical fiber of FIG. 14. FIG. 16 is a radial cross-sectional view showing another modified example of the optical fiber of FIG. 14. Fig. 17A is a side transmission image showing the results of observation of the crystallized glass fiber obtained in Example 1 by the crossed Nicols method. Fig. 17B is a side transmission image showing the results of observation of the crystallized glass fiber obtained in Example 1 by the parallel Nicols method. Fig. 18A is a side transmission image showing the results of observation of the crystallized glass fiber obtained in Example 2 by the crossed Nicols method. Fig. 18B is a side transmission image showing the results of observation of the crystallized glass fiber obtained in Example 2 by the parallel Nicols method. Fig. 19A is a side transmission image showing the results of observation of the crystallized glass fiber obtained in Example 3 by the crossed Nicols method. Fig. 19B is a side transmission image showing the results of observation of the crystallized glass fiber obtained in Example 3 by the parallel Nicols method. Fig. 20 is a radial cross-sectional transmission image showing the results of observation of the crystallized glass fiber obtained in Example 5 by the crossed Nicols method. Fig. 21A is a side transmission image showing the results of observation of the crystallized glass fiber obtained in Example 6 by the crossed Nicols method. Fig. 21B is a side transmission image showing the results of observation by the parallel Nicol method of the crystallized glass fiber obtained in Example 6. Fig. 22 is an image showing the results of analysis of the texture structure of the crystallized glass fiber obtained in Example 1 by electron backscatter diffraction (EBSD).Fig. 23 is a transmission electron microscope (TEM) image showing the results of observing the microstructure of the crystallized glass fiber obtained in Example 1. Fig. 24 is an image showing the results of EBSD analysis of the microstructure of the crystallized glass fiber obtained in Example 3. Fig. 25 is a TEM image showing the results of EBSD analysis of the microstructure of the crystallized glass fiber obtained in Example 3. Fig. 26 is an image showing the results of EBSD analysis of the microstructure of the crystallized glass fiber obtained in Example 4. Fig. 27 is a graph showing the results of plotting light transmission intensity against each length of the crystallized glass fibers obtained in Examples 1 to 3. Fig. 28 is a diagram explaining the apparatus used when measuring changes in interference light intensity of the crystallized glass fiber obtained in Example 3. Fig. 29A is a graph showing changes in interference light intensity with applied voltage when y-polarized light is incident on the crystallized glass fiber obtained in Example 3. Fig. 29B is a graph showing changes in interference light intensity with applied voltage when z-polarized light is incident on the crystallized glass fiber obtained in Example 3. Fig. 30A is a graph showing the change in interference light intensity with applied voltage when y-polarized light is incident on the crystallized glass fiber obtained in Example 6. Fig. 30B is a graph showing the change in interference light intensity with applied voltage when z-polarized light is incident on the crystallized glass fiber obtained in Example 6. Fig. 31 is a graph showing the change over time in transmitted light when laser light is continuously introduced into the crystallized glass fiber obtained in Example 3.

[0025] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0026] [Configuration of Crystallized Glass Fiber] Fig. 1 is a perspective view schematically showing the configuration of a crystallized glass fiber according to a first embodiment of the present invention, Fig. 2 is an axial cross-sectional view, and Fig. 3 is a radial cross-sectional view. As shown in Figs. 1 to 3, the crystallized glass fiber 1A has a first region 10A extending radially inward from the outer peripheral end 10a, and the first region 10A contains a crystalline phase with crystals oriented along the radial direction. The crystallized glass fiber 1A of this embodiment also has a second region 20A containing an amorphous phase, which is located radially inward of the first region 10A and in the center of the crystallized glass fiber 1A.

[0027] The outer diameter of the crystallized glass fiber 1A is not particularly limited, but may be, for example, 3 μm or more, 5 μm or more, or 10 μm or more. The outer diameter of the crystallized glass fiber 1A is also not particularly limited, but may be, for example, 1000 μm or less, 700 μm or less, 400 μm or less, or 200 μm or less.

[0028] The first regions 10A form a radial polarization structure in which the polarization axes Z are radially oriented in a radial cross section. Specifically, the first regions 10A are arranged radially so as to have infinite rotational symmetry with respect to the radial center of the crystallized glass fiber 1A. In the first regions 10A, the crystal phase is radially oriented in a radial cross section at any position in the axial direction of the crystallized glass fiber 1A. The configuration of the first regions 10A will be described in detail later.

[0029] The second region 20A is formed continuously along the axial direction of the crystallized glass fiber 1A. In this embodiment, the second region 20A has a constant radial dimension of the crystallized glass fiber 1A, but this is not limited thereto, and the radial dimension may be different at any two positions along the axial direction of the crystallized glass fiber 1A. Furthermore, although the second region 20A is formed continuously along the axial direction of the crystallized glass fiber 1A, this is not limited thereto, and the second region 20A may be formed intermittently.

[0030] 4 is a partially enlarged cross-sectional view of the crystallized glass fiber 1A of FIG. 2. As shown in FIG. 4, the crystalline phase of the first region 10A includes multiple domains 11A, 11A, ... formed along the radial direction of the crystallized glass fiber 1A. The multiple domains 11A, 11A are formed by crystal growth from the outer peripheral end 10a toward the axis X located at the radial center of the crystallized glass fiber 1A, and are formed radially around the axis X. The crystallized glass fiber 1A includes amorphous phase particles 12A within the crystalline phase domains 11A in the first region 10A. The particles 12A are approximately spherical, approximately polyhedral, or irregular, and multiple particles are scattered within the domains 11A. Hereinafter, the particles 12A will also be referred to as "particulate amorphous phase." The crystallized glass fiber 1A also includes amorphous phase particles 13A between adjacent domains 11A, 11A. The particles 13A are strip-shaped and extend in a direction from the outer peripheral edge 10a toward the inside in the radial direction, that is, along the crystal growth direction Y of the domain 11A. Hereinafter, the particles 13A are also referred to as "strip-shaped amorphous phase."

[0031] In this embodiment, the crystalline phase of the first region 10A has a Fresnoite-type crystalline structure. Specifically, a domain 11A of the crystalline phase is configured with a Fresnoite-type crystalline structure. The crystalline phase is not particularly limited, but may be, for example, ２ TiB ２ O ８ (A is any element of Sr or Ba, and B is any element of Si or Ge). The crystalline phase of the first region 10A is made of, for example, an oxide containing strontium (Sr), titanium (Ti) and silicon (Si), an oxide containing barium (Ba), titanium (Ti) and silicon (Si), or an oxide containing barium (Ba), titanium (Ti) and germanium (Ge). Specific examples of oxides containing strontium (Sr), titanium (Ti) and silicon (Si) include Sr ２ TiSi ２ O ８ Specific examples of oxides containing barium (Ba), titanium (Ti) and silicon (Si) include Ba ２ TiSi ２ O ８Specific examples of oxides containing barium (Ba), titanium (Ti) and germanium (Ge) include Ba ２ TiGe ２ O ８ When the domain 11A of the crystalline phase is configured with the fresnoite type crystal structure as described above, TiO having a pyramidal structure can be used as the domain 11A. ５ The alignment of the apical oxygen atoms induces a macroscopic spontaneous polarization, which is the origin of the Pockels effect. As a result, spontaneous polarization is formed radially in the radial cross section of the crystallized glass fiber 1A, and the Pockels effect and second-order nonlinear optical properties can be exhibited, independent of the polarization direction of the incident light.

[0032] As shown in Figure 6, in the crystal phase of the first region 10A, due to its radial polarization structure, a different refractive index change occurs for each region 10A when a uniform electric field is applied in the radial direction. The Pockels coefficient (described later) for each region 10A is the same, but the angle between the applied electric field direction and the polarization axis Z is different, so the refractive index n ０ Specifically, the refractive index n ０ The refractive index of the region 10Aa above the region where the refractive index is equal to n is higher, and the refractive index of the region 10Ab below the region where the refractive index is equal to n is lower. ０ The Pockels coefficient of the first region 10A is the same for light of any polarization direction, and the Pockels effect does not vary depending on the incidence conditions.

[0033] 7, due to the radial polarization structure in the first region 10A, light of any polarization direction and intrinsic transverse mode propagating through the crystallized glass fiber 1A passes alternately through the upper region 10Aa and the lower region 10Ab at a constant period. Because the crystalline phases of the regions 10Aa and 10Ab have second-order nonlinear optical coefficients with different signs, the crystallized glass fiber 1A is equivalent to a periodically polarization-inverted structure, and quasi-phase matching is established for light of a specific transverse mode, resulting in highly efficient wavelength conversion and quantum light generation.

[0034] FIG. 6 shows a case where the radial cross-sectional shape of the crystallized glass fiber 1A is a perfect circle or an approximately perfect circle, but the present invention is not limited to this. The present invention can also be applied to a case where the radial cross-sectional shape of the crystallized glass fiber is a distorted circle, for example, where the peripheral curvature in the radial cross-section is not constant.

[0035] As shown in FIG. 8, the crystallized glass fiber 1A' has a refractive index n ０ Assume that the region 10Aa' above the portion where the circumferential curvature is equal to the region 10Ab' has a smaller circumferential curvature than the region 10Aa', and the region 10Ab' below the region 10Ab' has a larger circumferential curvature than the region 10Ab'. If asymmetry occurs in the radially polarized structure of the first region 10A', the polarization inversion structure becomes non-periodic, and quasi-phase matching is no longer possible. When the crystallized glass fiber 1A' has the above-described configuration, a refractive index distribution under the Pockels effect similar to that shown in FIG. 6 is used, and an external electric field is applied along the radial direction from the lower region 10Ab' with a larger circumferential curvature to the upper region 10Aa' with a smaller circumferential curvature, as shown in FIG. 9. In this case, light in the intrinsic transverse mode propagates slowly in the upper region 10Aa' with a higher refractive index and propagates quickly in the lower region 10Ab' with a lower refractive index, resulting in the crystallized glass fiber 1A' being equivalent to a periodically polarized structure. This compensates for the non-periodicity of the glass fiber 1A' and enables quasi-phase matching. Furthermore, by adjusting, for example, the Pockels coefficient and / or the magnitude of the applied external electric field depending on the degree of difference in the peripheral curvature between the upper region 10Aa' and the lower region 10Ab' in the crystallized glass fiber 1A', it is possible to compensate for the non-periodicity and thereby realize quasi-phase matching.

[0036] The amorphous phase of the particles 12A and the particles 13A is not particularly limited. For example, the crystalline phase of the first region 10A is xSr ２ TiSi ２ O ８ When x is mol %, the amorphous phase of the particles 12A and 13A can be represented by, for example, the following formula (1): (α-2x)SrO—(β-x)TiO ２ -(γ-2x)SiO ２ (α+β+γ=100, 30≦α≦40, 15≦β≦25, 40≦γ≦50, 15≦x≦25) (α, β, and γ are mole percents) (1) The crystal phase of the first region 10A is xBa ２ TiSi ２ O ８ When x is mol %, the amorphous phase of the particles 12A and 13A can be represented by, for example, the following formula (2): (α-2x)BaO-(β-x)TiO ２ -(γ-2x)SiO ２ (α+β+γ=100, 25≦α≦35, 15≦β≦25, 45≦γ≦55, 15≦x≦25) (α, β, and γ are mole percents) (2) Furthermore, the crystal phase of the first region 10A is xBa ２ TiGe ２ O ８ When x is mol %, the amorphous phase of the particles 12A and 13A can be expressed by, for example, the following formula (3): (α-2x)BaO-(β-x)TiO ２ -(γ-2x)GeO ２ (α+β+γ=100, 30≦α≦40, 10≦β≦20, 45≦γ≦55, 10≦x≦20) (α, β, γ are mol%)...(3)

[0037] The amorphous phase of the second region 20A is not particularly limited. For example, the crystalline phase of the second region 20A may be xSr ２ TiSi ２ O ８ In this case, the amorphous phase of the second region 20A can be expressed by the above formula (1), similarly to the amorphous phases of the particles 12A and 13A. ２ TiSi ２ O ８In this case, the amorphous phase of the second region 20A can be expressed by the above formula (2), similarly to the amorphous phases of the particles 12A and 13A. ２ TiGe ２ O ８ In this case, the amorphous phase of the second region 20A can be expressed by the above formula (3), similarly to the amorphous phases of the particles 12A and 13A.

[0038] The crystallized glass fiber of this embodiment is obtained via a glass fiber precursor, which will be described later, and in actual use, can constitute the core of the optical fiber shown in Figures 14 to 16, which will be described later. In particular, when the cladding outside the core is made of glass, the cross section of the optical fiber is dominated by glass, regardless of whether it is before or after the crystallization heat treatment, and this allows for low-loss and easy splicing, similar to the splicing of all-glass optical fibers, which is an established precision technology.

[0039] The crystalline phase of the first region 10A has a Fresnoite crystal structure, but is not limited to this, and may have a crystal structure other than the Fresnoite crystal structure.

[0040] 10 is an axial cross-sectional view showing a modified example of the crystallized glass fiber of FIG. 1, FIG. 11 is a radial cross-sectional view of the crystallized glass fiber of FIG. 10, and FIG. 12 is a partially enlarged cross-sectional view of the crystallized glass fiber of FIG. 10.

[0041] 10 to 12, the crystallized glass fiber 1B has a first region 10B extending radially inward from the outer peripheral end 10b, and the first region 10B contains a crystalline phase oriented radially. As described above, the crystallized glass fiber 1B does not have a second region containing an amorphous phase in its central portion.

[0042] The crystalline phase of the first region 10B includes a plurality of domains 11B, 11B, ... formed along the radial direction of the crystallized glass fiber 1B. The domains 11B, 11B are formed by crystal growth from the outer peripheral end 10b toward the axis X located at the radial center of the crystallized glass fiber 1B, and are formed radially around the axis X. The tips of the radially formed domains 11B, 11B may be in contact with each other at the radial center. Furthermore, the crystallized glass fiber 1B includes amorphous phase particles 12B within the crystalline phase domains 11B in the first region 10B. The particles 12B are approximately spherical, approximately polyhedral, or irregular, and are scattered in plurality within the domains 11B.

[0043] The crystallized glass fiber 1B also contains amorphous phase particles 13A between adjacent domains 11B, 11B. The particles 13A are columnar with a substantially circular or polygonal cross section and extend radially inward from the outer peripheral end 10b, i.e., along the crystal growth direction Y of the domains 11B.

[0044] In this modification, the crystalline phase of the first region 10B has a Fresnoite-type crystalline structure. The structure and composition of the crystalline phase of the first region 10B are the same as those of the first region 10A of the crystallized glass fiber 1A, and therefore, a description thereof will be omitted.

[0045] [Method for producing crystallized glass fiber] The method for producing a crystallized glass fiber according to this embodiment includes a preparation step, a melt obtaining step, a molding step, and a crystallization step ( FIG. 13 ). The method for producing a crystallized glass fiber may include other steps before the preparation step, after the crystallization step, or between the steps, without departing from the spirit of the present invention.

[0046] (Preparation Step) In the preparation step, a mixture of Sr compound, Ti compound and Si compound, a mixture of Ba compound, Ti compound and Si compound, or a mixture of Ba compound, Ti compound and Ge compound is prepared (Step S1). When a compound containing strontium (Sr), titanium (Ti) and silicon (Si) is to be obtained as a product, a mixture of Sr compound, Ti compound and Si compound is prepared. The Sr compound, Ti compound and Si compound are not particularly limited, but may be, for example, SrCO ３ , TiO ２ and SiO ２ When a compound containing barium (Ba), titanium (Ti) and silicon (Si) is to be obtained as a product, a mixture of a Ba compound, a Ti compound and a Si compound is prepared. The Ba compound, the Ti compound and the silicon compound are not particularly limited, but examples thereof include BaCO ３ , TiO ２ and SiO ２ When a compound containing barium (Ba), titanium (Ti), and germanium (Ge) is to be obtained as a product, a mixture of a Ba compound, a Ti compound, and a Ge compound is prepared. The Ba compound, the Ti compound, and the Ge compound are not particularly limited, but examples thereof include BaCO ３ , TiO ２ and GeO ２ Examples include:

[0047] The Sr compound content of the mixture of Sr compound, Ti compound, and Si compound is preferably 30 mol% to 40 mol%, more preferably 32 mol% to 38 mol%, and even more preferably 32 mol% to 35 mol%. The Ti compound content of the mixture is preferably 15 mol% to 25 mol%, more preferably 17 mol% to 23 mol%, and even more preferably 17 mol% to 21 mol%. The Si compound content of the mixture is preferably 40 mol% to 50 mol%, more preferably 42 mol% to 48 mol%, and even more preferably 45 mol% to 48 mol%.

[0048] The content of the Ba compound in the mixture of the Ba compound, the Ti compound, and the Si compound is preferably 25 mol% to 35 mol%, more preferably 26 mol% to 31 mol%, and even more preferably 27 mol% to 30 mol%. The content of the Ti compound in the mixture is preferably 15 mol% to 25 mol%, more preferably 16 mol% to 21 mol%, and even more preferably 17 mol% to 20 mol%. The content of the Si compound in the mixture is preferably 45 mol% to 55 mol%, more preferably 48 mol% to 55 mol%, and even more preferably 50 mol% to 54 mol%.

[0049] The content of the Ba compound in the mixture of the Ba compound, the Ti compound, and the Ge compound is preferably 25 mol% to 40 mol%, more preferably 30 mol% to 37 mol%, and even more preferably 33 mol% to 37 mol%. The content of the Ti compound in the mixture is preferably 10 mol% to 20 mol%, more preferably 13 mol% to 17 mol%, and even more preferably 15 mol% to 17 mol%. The content of the Ge compound in the mixture is preferably 45 mol% to 55 mol%, more preferably 48 mol% to 54 mol%, and even more preferably 48 mol% to 52 mol%.

[0050] (Melt Obtaining Step) In the melt obtaining step, the mixture is heated and melted to obtain a melt (step S2). When the mixture is a mixture of an Sr compound, a Ti compound, and an Si compound, the melting temperature is preferably 1200°C to 1500°C, more preferably 1300°C to 1500°C, and even more preferably 1400°C to 1500°C. In this case, the melting time is preferably 0.5 hours or more, and more preferably 1 hour or more.

[0051] When the mixture is a mixture of a Ba compound, a Ti compound, and a Si compound, the melting temperature is preferably 1200° C. to 1600° C., more preferably 1300° C. to 1600° C., and even more preferably 1400° C. to 1600° C. In this case, the melting time is preferably 0.5 hours or more, and more preferably 1 hour or more.

[0052] When the mixture is a mixture of a Ba compound, a Ti compound, and a Ge compound, the melting temperature is preferably 1100° C. to 1400° C., more preferably 1200° C. to 1400° C., and even more preferably 1300° C. to 1400° C. In this case, the melting time is preferably 0.5 hours or more, and more preferably 1 hour or more.

[0053] (Molding Step) In the molding step, a glass fiber precursor is molded from the melt (step S3). The molding method is not particularly limited, and examples thereof include a pulling method and an extrusion method. The pulling method is not particularly limited, and can be performed using known equipment and manufacturing conditions. For example, the longitudinal end of a glass rod made of quartz or the like is immersed in a melt held in a crucible made of platinum or the like under predetermined temperature conditions, and the glass rod is pulled up at a predetermined pulling speed. After the glass rod is pulled up a predetermined distance, the linear glass body connected to the glass rod is cut to obtain a glass fiber precursor.

[0054] When a glass fiber precursor is obtained from a mixture of a Sr compound, a Ti compound, and a Si compound, the composition of the glass fiber precursor can be expressed, for example, by the following formula (1): αSrO-βTiO ２ -γSiO ２ (α+β+γ=100, 30≦α≦40, 15≦β≦25, 40≦γ≦50) (α, β, and γ are mole percents) (1) When a glass fiber precursor is obtained from a mixture of a Ba compound, a Ti compound, and a Si compound, the composition of the glass fiber precursor can be expressed, for example, by the following formula (2): αBaO-βTiO ２ -γSiO ２ (α+β+γ=100, 25≦α≦35, 15≦β≦25, 45≦γ≦55) (α, β, and γ are mole percents) (2) Furthermore, when a glass fiber precursor is obtained from a mixture of a Ba compound, a Ti compound, and a Ge compound, the composition of the glass fiber precursor can be expressed, for example, by the following formula (3): αBaO-βTiO ２ -γGeO ２ (α+β+γ=100, 30≦α≦40, 10≦β≦20, 45≦γ≦55) (α, β, γ are mol%)...(3)

[0055] The molding method is not limited to the pulling method, but may also be, for example, a method in which a melt is cooled to form a preform, and the preform is drawn to form a glass fiber precursor (drawing method).

[0056] (Crystallization Step) In the crystallization step, the glass fiber precursor is heat-treated at 700°C to 1000°C for 0.5 hours or more to crystallize it (Step S4). When an oxide containing strontium (Sr), titanium (Ti), and silicon (Si) is to be obtained as a product, the heat treatment temperature is preferably 740°C to 980°C, more preferably 770°C to 850°C, and even more preferably 790°C to 820°C. In this case, the heat treatment time is preferably 0.5 hours or more, more preferably 1 hour or more, and even more preferably 12 hours or more.

[0057] When an oxide containing barium (Ba), titanium (Ti), and silicon (Si) is obtained as a product, the heat treatment temperature is preferably 740° C. to 980° C., more preferably 770° C. to 880° C., and even more preferably 770° C. to 820° C. In this case, the heat treatment time is preferably 0.5 hours or more, more preferably 1 hour or more, and even more preferably 12 hours or more.

[0058] When an oxide containing barium (Ba), titanium (Ti), and germanium (Ge) is obtained as a product, the heat treatment temperature is preferably 700° C. to 900° C., more preferably 750° C. to 850° C., and even more preferably 770° C. to 820° C. In this case, the heat treatment time is preferably 0.5 hours or more, more preferably 1 hour or more, and even more preferably 12 hours or more.

[0059] By setting the heat treatment conditions for crystallizing the glass fiber precursor to 700°C to 1000°C and 0.5 hours or more, during crystal nucleation, crystal nucleation at the outer peripheral edge of the glass fiber precursor is promoted, and a dense crystal nucleus structure is formed at the outer peripheral edge. Furthermore, during crystal growth, crystals selectively grow from the crystal nuclei toward the radial center, thereby forming a first region of a crystalline phase. Furthermore, by setting the heat treatment conditions during crystal growth as described above, the crystal growth rate is reduced, and excess components including silicon (Si) or germanium (Ge) migrate to the radial center as the domains grow, resulting in the formation of a second region of an amorphous phase at the radial center. At this time, excess components including silicon (Si) or germanium (Ge) are less likely to be incorporated into the domains, thereby reducing the size of the amorphous phase particles formed within the domains. Furthermore, the driving force for the supercooled liquid to flow between adjacent domains is reduced, thereby reducing the size of the amorphous phase particles formed between the domains. This crystallization step makes it possible to easily form a periodic polarization inversion structure or an equivalent polarization inversion structure in the crystallized glass fiber. The crystallized glass fiber of this embodiment is manufactured through the above steps.

[0060] In the crystallized glass fiber obtained by the manufacturing method of this embodiment, the propagation loss of light in the wavelength range of 400 nm to 1800 nm is, for example, 5.0 dB / cm or less. Furthermore, the propagation loss of light in the same wavelength range is preferably 4.0 dB / cm or less, more preferably 2.0 dB / cm or less, and even more preferably 0.5 dB / cm or less. Thus, the crystallized glass fiber of this embodiment can achieve low propagation loss.

[0061] Fig. 14 is a radial cross-sectional view showing an example of the configuration of an optical fiber including a crystallized glass fiber 1A according to an embodiment of the present invention. As shown in Fig. 14, the optical fiber 100A has the crystallized glass fiber 1A constituting the core and a cladding 101A covering the crystallized glass fiber 1A. The optical fiber 100A may also have a protective layer (not shown) covering the cladding 101A.

[0062] In the configuration of the optical fiber 100A, the relative refractive index difference Δn between the refractive index of the cladding portion 101A and the refractive index of the crystallized glass in the crystallized glass fiber 1A constituting the core portion is not particularly limited, but is preferably 1% or less.

[0063] As a modification of the optical fiber 100A shown in Fig. 14, an optical fiber 100B may have a crystallized glass fiber 1A constituting a core, a first cladding 101B covering the crystallized glass fiber 1A, and a second cladding 102B covering the first cladding 101B, as shown in Fig. 15. The first cladding 101B may be composed of a crystalline phase or an amorphous phase. The optical fiber 100B may also have a protective layer (not shown) covering the second cladding 102B.

[0064] 16, as another modification of the optical fiber 100A of Fig. 14, an optical fiber 100C may have a plurality of crystallized glass fibers 1A, 1A, ... constituting a plurality of cores, and a cladding 101C covering the plurality of crystallized glass fibers 1 A. The optical fiber 100C may have another cladding covering the cladding 101C, or may have a protective layer covering the cladding 101C or the other cladding.

[0065] In the case of a single-mode optical fiber, the outer diameter (core diameter) of the crystallized glass fiber 1A is preferably, for example, 3 μm to 10 μm. In the case of a multi-mode optical fiber, the outer diameter (core diameter) of the crystallized glass fiber 1A is preferably, for example, 10 μm to 60 μm.

[0066] The optical fiber of this embodiment can provide an optical fiber with low propagation loss and large transmission capacity that is compatible with single mode, which is mainly used for long-distance communications, or multimode, which is mainly used for short- to medium-distance communications. It can also provide an optical fiber cable in which a plurality of the optical fibers are bundled together.

[0067] The crystallized glass fiber or optical fiber of this embodiment can be used for, but is not limited to, an optical phase and intensity modulation device, a wavelength conversion device, a light source for generating quantum entangled light, and / or an optical switch. For example, an optical phase and intensity modulation device may include the crystallized glass fiber or the optical fiber, and any one of the wavelength conversion device, the light source for generating quantum entangled light, and the optical switch may include the optical phase and intensity modulation device. Furthermore, any one of the wavelength conversion device, the light source for generating quantum entangled light, and the optical switch may include the crystallized glass fiber or the optical fiber.

[0068] In this embodiment, for example, the optical phase and intensity modulation device can be used to apply an external electric field to the crystallized glass fiber or the optical fiber in one or more of a wavelength conversion device, a light source for generating quantum entangled light, and an optical switch, thereby enabling pseudo-phase matching adjustment. This allows for easy phase matching adjustment using a crystallized glass fiber that exhibits dynamic optical functionality, and also enables excellent productivity and long-term stability to be achieved.

[0069] Examples of the present invention will be described below, but the present invention is not limited to the examples shown below.

[0070] Example 1 32SrO-20TiO ２ -48SiO ２ (mol%) (hereinafter also referred to as STS48) ３ , TiO ２ and SiO ２ The above were weighed and mixed, and the resulting mixture was heated and melted in an air atmosphere at 1500°C for 1 hour to obtain a melt. Thereafter, the end of a glass rod was immersed in the melt placed in a platinum crucible, and the glass rod was pulled up a predetermined distance. The linear glass body connected to the glass rod was then cut to obtain a glass fiber precursor made of STS48. The obtained glass fiber precursor was then heat-treated at 940°C for 1 hour to crystallize it, and a glass fiber precursor with an outer diameter of about 100 μm and a crystalline phase of Sr ２ TiSi ２ O ８ A crystallized glass fiber consisting of the above was prepared.

[0071] Example 2 A crystallized glass fiber was produced in the same manner as in Example 1, except that the heat treatment conditions for crystallization were changed to 830° C. and 12 hours.

[0072] Example 3 A crystallized glass fiber was produced in the same manner as in Example 1, except that the heat treatment conditions for crystallization were changed to 800° C. for 36 hours and the outer diameter was set to about 90 μm.

[0073] Example 4 35SrO-20TiO ２ -45SiO ２ (mol%) (hereinafter also referred to as STS45) ３ , TiO ２ and SiO ２ A crystallized glass fiber was produced in the same manner as in Example 1, except that a glass fiber precursor made of STS45 was obtained using a mixture prepared by weighing and mixing the above ingredients, and that the glass fiber precursor was heat-treated at 960°C for 1 hour to perform crystallization.

[0074] Example 5: 28BaO-18TiO ２ -54SiO ２　 BaCO ３ , TiO ２ and SiO ２ The resulting mixture was heated and melted in an air atmosphere at 1400°C for 1 hour to obtain a melt. Then, the end of a glass rod was immersed in the melt placed in a platinum crucible, and the glass rod was pulled up a predetermined distance. The linear glass body connected to the glass rod was cut, and strain was removed at 730°C for 1 hour to obtain 28BaO-18TiO ２ -54SiO ２ The glass fiber precursor thus obtained was then heat-treated at 885°C for 3 hours to crystallize it, resulting in a glass fiber precursor having an outer diameter of about 200 µm and a crystalline phase of Ba. ２ TiSi ２ O ８ A crystallized glass fiber consisting of the above was prepared.

[0075] Example 6 37BaO-15TiO ２ -48GeO ２BaCO ３ , TiO ２ and GeO ２ The resulting mixture was heated and melted in an air atmosphere at 1200°C for 1 hour to obtain a melt. Then, the end of a glass rod was immersed in the melt placed in a platinum crucible, and the glass rod was pulled up a predetermined distance. The linear glass body connected to the glass rod was cut, and strain was removed at 630°C for 1 hour to obtain 37BaO-15TiO ２ -48GeO ２ The glass fiber precursor thus obtained was then heat-treated at 825°C for 1 hour to crystallize it, resulting in a glass fiber precursor having an outer diameter of about 250 µm and a crystalline phase of Ba. ２ TiGe ２ O ８ A crystallized glass fiber consisting of the above was prepared.

[0076] The crystallized glass fibers obtained in Examples 1 to 6 were measured and evaluated by the following methods.

[0077] [Polarizing Microscope Observation] Using a polarizing microscope (Nikon Corporation, product name "ECLIPSE LV100POL"), the crystallized glass fibers obtained in Examples 1 to 3, 5, and 6 were observed by either the crossed Nicols method, the parallel Nicols method, or both. The results are shown in Figures 17A to 20B. In Example 1, it was confirmed that a crystalline phase with crystals oriented along the radial direction was formed at any position in the longitudinal direction of the crystallized glass fiber (Figure 17A). It was also confirmed that linear voids were formed along the longitudinal direction at the radial center of the crystallized glass fiber (Figure 17B, arrow portion in the figure).

[0078] In Example 2, it was confirmed that a crystalline phase with crystal orientation along the radial direction was formed at any position along the longitudinal direction of the crystallized glass fiber ( FIG. 18A ).It was also confirmed that a plurality of point-like voids were formed along the longitudinal direction at the radial center of the crystallized glass fiber ( FIG. 18B , arrows in the figure).

[0079] In Example 3, it was confirmed that a crystalline phase with crystal orientation along the radial direction was formed at any position in the longitudinal direction of the crystallized glass fiber ( FIG. 19A ). It was also confirmed that no voids were formed in the radial center of the crystallized glass fiber, and that a uniform phase difference was generated along the longitudinal direction ( FIG. 19B ). Therefore, it was found that the size of the voids in the radial center of the crystallized glass fiber could be reduced with a decrease in the heat treatment temperature in the crystallization step, and that a crystallized glass fiber without voids in the radial center could be produced.

[0080] In Example 5, the radial cross section of the crystallized glass fiber was observed by the crossed Nicols method, and it was confirmed that a crystalline phase with crystal orientation along the radial direction was formed at any position along the longitudinal direction of the crystallized glass fiber ( FIG. 20 ).

[0081] In Example 6, it was confirmed that a crystalline phase with crystal orientation along the radial direction was formed at any position along the longitudinal direction of the crystallized glass fiber (FIG. 21A).It was also confirmed that linear voids were formed along the longitudinal direction at the radial center of the crystallized glass fiber (FIG. 21B, arrow portion in the figure).

[0082] [Observation of texture and microtexture] Fig. 22 is an image showing the results of analyzing the texture of the crystallized glass fiber obtained in Example 1 by EBSD (manufactured by JEOL, apparatus name "JSM-7100F"), and Fig. 23 is a TEM image (manufactured by JEOL, apparatus name "JEM-2100F") showing the results of observing the microtexture of the crystallized glass fiber obtained in Example 1.

[0083] In Fig. 22, it was confirmed that the first region of the crystallized glass fiber contained a crystalline phase with crystals oriented along the radial direction. It was also confirmed that multiple domains contained in the crystalline phase of the first region grew from the outer peripheral end toward the radial center of the crystallized glass fiber, and were formed radially with the radial center as the reference. In Fig. 23, it was confirmed that a granular amorphous phase was present within the crystalline phase domains in the first region. In addition, it was confirmed that a band-like amorphous phase was present between adjacent domains in the first region.

[0084] FIG. 24 is an image showing the results of EBSD analysis of the texture structure of the crystallized glass fiber obtained in Example 3, and FIG. 25 is a TEM image showing the results of observation of the microtexture structure of the crystallized glass fiber obtained in Example 3.

[0085] In Figure 24, it was confirmed that the first region of the crystallized glass fiber contained a crystalline phase with crystals oriented along the radial direction. It was also confirmed that multiple domains contained in the crystalline phase of the first region grew from the outer peripheral end toward the radial center of the crystallized glass fiber, and were formed radially with the radial center as the reference. Furthermore, it was confirmed that a second region was formed in the center (black portion) of the crystallized glass fiber, radially inside the first region. Unlike the colored first region, this second region was black, and its analysis was poor, suggesting that it was composed of an amorphous phase. In Figure 25, it was confirmed that a granular amorphous phase was present within the crystalline phase domains in the first region. In addition, it was confirmed that a band-like amorphous phase was present between adjacent domains in the first region.

[0086] Fig. 26 is an image showing the results of EBSD analysis of the texture of the crystallized glass fiber obtained in Example 4. In Example 4, the glass fiber precursor was STS45, and it was found that the crystal nucleation rate was faster and the domain size was smaller as a result, compared to Example 1 in which the glass fiber precursor was STS48 (see Fig. 22).

[0087] [Propagation Loss Measurement] The propagation loss of the crystallized glass fiber was measured using the cutback method. A cured resin was prepared by embedding the crystallized glass fiber obtained in Example 1, and the cured resin was cut out to prepare a sample so that the crystallized glass fiber had a predetermined length. One end of the crystallized glass fiber in the sample was connected to a light source using a commercially available single-mode optical fiber, and a slit and a photodiode were arranged in this order on the other end side of the crystallized glass fiber in the sample. Light with a wavelength of 633 nm or 1550 nm was introduced from the light source into one end of the crystallized glass fiber, and the transmitted light emitted from the other end of the crystallized glass fiber was detected by the photodiode, and the intensity of the transmitted light was measured using a power meter connected to the photodiode. The transmitted light intensity was measured in the same manner for Examples 2 and 3. The results of plotting the light transmission intensity against each length of the crystallized glass fiber obtained in Examples 1 to 3 are shown in Figure 27.

[0088] 27 shows that in Example 1, the propagation loss of light with a wavelength of 633 nm was 13.2±0.6 dB / cm. In Example 2, the propagation loss of light with a wavelength of 633 nm was 2.8±0.7 dB / cm, which was significantly reduced compared to Example 1. Furthermore, in Example 3, the propagation loss of light with a wavelength of 633 nm was 1.61±0.08 dB / cm, which was further reduced compared to Examples 1 and 2. Furthermore, in Example 3, the propagation loss of light with a wavelength of 1550 nm, which is in the communication wavelength band, was 0.37±0.04 dB / cm, which was further reduced compared to when light with a wavelength of 633 nm was used.

[0089] [Measurement of Changes in Interference Light Intensity] The change in interference light intensity was measured and evaluated using the device shown in Figure 28. A pair of electrodes was positioned facing each other in contact with the outer periphery of the crystallized glass fiber obtained in Example 3, and the fiber was embedded in resin and cured. The cured resin had a refractive index of 1.46, and the crystallized glass fiber had an outer diameter (distance between the electrodes) of 93.3 µm, a length of 10.65 mm, and a refractive index of 1.7067. While changing the voltage applied between the pair of electrodes, polarized light parallel to the y-axis (wavelength 633 nm, also referred to as y-polarized light) was introduced from a light source into one end of the crystallized glass fiber. The transmitted light emitted from the other end of the crystallized glass fiber was focused by a lens and detected by a photodetector, and the intensity of the transmitted light (interference light intensity) was measured. The intensity of transmitted light was also measured for polarized light parallel to the z-axis (wavelength 633 nm, also referred to as z-polarized light) in the same manner. Figures 29A and 29B show the results of the change in interference light intensity depending on the applied voltage when y-polarized light and z-polarized light were incident on the crystallized glass fiber. In the figure, the upper triangular waveform is the applied voltage, and the lower waveform is the optical interference intensity.

[0090] From the results in Fig. 29A, it was confirmed that in the crystallized glass fiber of Example 3, when y-polarized light is incident, the interference light intensity increases as the applied voltage increases, and decreases as the applied voltage decreases. Furthermore, from the results in Fig. 29B, it was confirmed that when z-polarized light is incident, the interference light intensity decreases as the applied voltage increases, and increases as the applied voltage decreases. Furthermore, the applied voltages at which the interference light intensity reaches its maximum and minimum were read, and the voltage value Vπ that gives the phase difference π was obtained. Using Vπ, the effective Pockels coefficient was calculated from the following formula:

[0091] (λ: light source wavelength 633 nm, d: distance between electrodes 93.3 μm, n ０ : refractive index when no voltage is applied 1.7067, l: crystallized glass fiber length 10.65 mm)

[0092] As a result, the effective Pockels coefficient when y-polarized light was incident was 0.9 pm / V, and the effective Pockels coefficient when z-polarized light was incident was 1.6 pm / V, confirming that the optical modulation operation in the crystallized glass fiber of Example 3 was generally independent of the polarization direction of the incident light.

[0093] Furthermore, for the crystallized glass fiber obtained in Example 6, the change in the intensity of interference light due to the applied voltage when y-polarized light and z-polarized light were incident was measured in the same manner as for the crystallized glass fiber obtained in Example 3 (d: 228.7 μm, n ０ : 1.8221, ℓ: 6.94 mm). The results are shown in Figures 30A and 30B. From the result in Figure 30A, it was confirmed that in the crystallized glass fiber of Example 6, when y-polarized light was incident, the interference light intensity decreased as the applied voltage increased, and increased as the applied voltage decreased. Furthermore, from the result in Figure 30B, it was confirmed that when z-polarized light was incident, the interference light intensity increased as the applied voltage increased, and decreased as the applied voltage decreased.

[0094] Furthermore, the effective Pockels coefficient when y-polarized light was incident was 4.3 pm / V, and the effective Pockels coefficient when z-polarized light was incident was 3.3 pm / V, confirming the optical modulation operation in the crystallized glass fiber of Example 6.

[0095] [Light damage resistance] 35SrO-20TiO ２ -45SiO ２ (mol %). ３ , TiO ２ and SiO ２ A glass precursor was prepared by the melt quenching method using a mixture obtained by weighing and mixing the above ingredients. In the melt quenching method, the obtained mixture was heated and melted in an air atmosphere at 1500°C for 1 hour, and the resulting melt was poured onto an iron plate heated to 200°C, and then immediately pressed with another iron plate heated to 200°C to obtain a flat glass precursor made of STS45. The obtained glass precursor was polished and then heat-treated at 960°C for 1 hour to crystallize it, obtaining a plate-shaped crystallized glass with a thickness of approximately 1 mm. A laser beam with a wavelength of 1064 nm, an incident light power of 4.4 W, a pulse width of 30 nm, and a repetition frequency of 30 kHz was continuously introduced into the obtained plate-shaped crystallized glass, and the change in transmitted light over time was measured. For comparison, LN (lithium niobate, LiNbO ３ The change in transmitted light over time was measured when laser light was continuously introduced into the edge face of a wafer made of a silicon wafer.

[0096] As a result, as shown in Figure 31, in LN wafer, the ratio of the power of transmitted light to the power of incident light is more than 11 until about 120 minutes have passed since the introduction of laser light, but immediately after about 120 minutes have passed since the introduction of laser light, this ratio drops sharply, and the damage caused by laser light occurs.On the other hand, in the plate-shaped crystallized glass of embodiment 3, the ratio of the power of transmitted light to the power of incident light remains more than 9 until about 300 minutes have passed since the introduction of laser light, and it is confirmed that it does not get damaged even under the long-term laser light, and has high reliability.

[0097] 1A Crystallized glass fiber 1B Crystallized glass fiber 10a Outer peripheral end portion 10A First region 10Aa Region 10Ab Region 10b Outer peripheral end portion 10B First region 11A Domain 11B Domain 12A Particle 12B Particle 13A Particle 20A Second region 100A Optical fiber 100B Optical fiber 100C Optical fiber 101A Cladding portion 101B First cladding portion 101C Cladding portion 102B Second cladding portion

Claims

1. A crystallized glass fiber having a first region extending in a radially inward direction from an outer peripheral end portion, the first region including a crystalline phase oriented along the radial direction.

2. The crystallized glass fiber according to claim 1, further comprising a second region, the second region being located radially inside the first region and at the center of the crystallized glass fiber, the second region including an amorphous phase.

3. The crystallized glass fiber according to claim 1, comprising amorphous phase particles within the domains of the crystalline phase in the first region or between adjacent domains.

4. The crystallized glass fiber according to claim 1, wherein said crystalline phase of said first region has a Fresnoite type crystal structure.

5. The crystallized glass fiber according to claim 1, wherein the crystalline phase of the first region is composed of an oxide containing strontium (Sr), titanium (Ti) and silicon (Si), an oxide containing barium (Ba), titanium (Ti) and silicon (Si), or an oxide containing barium (Ba), titanium (Ti) and germanium (Ge).

6. The crystallized glass fiber according to claim 1, which has a propagation loss of 5.0 dB / cm or less for light in the wavelength range of 400 nm to 1800 nm.

7. An optical fiber comprising at least one core portion made of the crystallized glass fiber according to claim 1, and a cladding portion covering said at least one core portion.

8. The optical fiber according to claim 7, wherein the relative refractive index difference Δn of the refractive index of said cladding portion to the refractive index of the crystallized glass in the crystallized glass fiber constituting said core portion is 1% or less.

9. An optical phase and intensity modulation device comprising the crystallized glass fiber of claim 1 or the optical fiber of claim 7.

10. A wavelength conversion device comprising the crystallized glass fiber according to claim 1 or the optical fiber according to claim 7.

11. A light source for generating quantum entangled light, comprising the crystallized glass fiber according to claim 1 or the optical fiber according to claim 7.

12. An optical switch comprising the crystallized glass fiber according to claim 1 or the optical fiber according to claim 7.

13. A phase matching adjustment method using the optical phase and intensity modulation device as claimed in claim 9, in the wavelength conversion device as claimed in claim 10, the light source for generating quantum entangled light as claimed in claim 11, and the optical switch as claimed in claim 12, by applying an external electric field to the crystallized glass fiber or the optical fiber to perform pseudo-phase matching adjustment.

14. A method for producing a crystallized glass fiber, comprising the steps of: preparing a mixture of a Sr compound, a Ti compound, and a Si compound, a mixture of a Ba compound, a Ti compound, and a Si compound, or a mixture of a Ba compound, a Ti compound, and a Ge compound; heating and melting the mixture to obtain a molten liquid; forming a glass fiber precursor from the molten liquid; and heat treating the glass fiber precursor at 700°C to 1000°C for 0.5 hours or more to crystallize it.

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