JPH04238303A - Production of monocrystalline optical fiber - Google Patents

Abstract

PURPOSE:To manufacture a monocrystalline optical fiber having good waveguide structure by forming core structure together with clad structure during growing the monocrystalline optical fiber. CONSTITUTION:Lithium niobate crystals 12 having Mg oxide 16 attached or deposited on the surfaces of the crystals 12 are used as the basic material of the monocrystal, and these crystals are heated and molten, a seed crystal 14 comes into contact with the molten basic material 12, and it is pulled upward with its speed being controlled to manufacture the monocrystalline optical fiber 13.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention manufactures nonlinear single-crystal optical fibers used in laser light wavelength conversion elements and photorefractive single-crystal optical fibers used in volumetric multiplexed holograms, which are promising as future high-density optical storage technology. It is about the method.

0002

2. Description of the Related Art FIG. 8 shows an example of a conventional method for manufacturing a single crystal optical fiber. In FIG. 8, the upper part of the base material 102 is heated by irradiation with the heating laser beam 101 to form a melted part 10.
form 5. A single crystal optical fiber 103 is formed by attaching a seed crystal 104 to the melted part 105, moving the seed crystal at a speed V2, and moving the base material 102 at a speed V1. Here, the orientation of the single crystal optical fiber 103 matches the crystal orientation of the seed crystal, and the composition is almost the same as that of the base material. Also, the outer diameter of the single crystal optical fiber is D2 = D1
It is expressed as ×(V1 /V2)1/2. Here, D
1 is the outer diameter of the base material. Therefore, the moving speed of the seed crystal V
2 and the moving speed V1 of the base material, a single crystal optical fiber with a desired outer diameter can be produced. By this method, single crystal fibers of lithium niobate, potassium niobate, sapphire, etc. can be produced.

0003

[Problems to be Solved by the Invention] Generally, lithium niobate and potassium niobate are known as single crystals with large nonlinear optical constants, and their use as wavelength conversion elements is particularly important. While the development of blue-emitting semiconductor lasers is at a standstill, recently, blue laser light generated by the second harmonic of an infrared semiconductor laser and lithium niobate has suddenly attracted attention. However, in the conventional single crystal fiber manufacturing method described above, a core-clad waveguide structure is not formed. In order to create the waveguide structure that is essential for improving the efficiency of nonlinear effects, it was necessary to form a cladding layer by thermally diffusing an element that lowers the refractive index to the outside after growing the single-crystal optical fiber for a long time. Furthermore, fibers in which additive elements have been thermally diffused for long periods of time are extremely brittle, and reducing the diameter of the fiber to create a single mode poses major practical problems, such as breakage during handling and increased scattering loss. It had been pointed out.

Furthermore, the applications of single crystal optical fibers include wavelength conversion, lasers, optical amplification, etc. using nonlinear optical crystals, phase conjugation, holograms, etc. using photorefractive crystals, but in all of these, there are practical problems. In the former, additive elements such as Nd and Er, in the latter, Ce, Fe,
It is necessary to add additional elements such as Cu and V. However, in the conventional single crystal optical fiber manufacturing method described above, it is necessary to prepare a crystal base material to which elements are added in advance as a base material. However, it is not easy to create relatively large crystals by adding desired elements. The element-doped single-crystal optical fibers that can be produced are limited to those such as Nd-doped YAG, which easily produce large crystals even when elements are added. It is also conceivable to use a sintered body as a base material to produce a wide variety of single crystal optical fibers at low cost. However, when forming a single-crystal optical fiber from a pre-purified sintered base material in order to obtain a single crystal with a desired additive element, it is necessary to sinter the additive element because the additive element is in a very small amount. It is difficult to disperse the additives uniformly throughout the body, and it is difficult to prepare a sintered body base material in which the added element concentration is precisely controlled by mixing together with the crystal constituent elements.

In view of the above points, an object of the present invention is to provide a method for manufacturing a single crystal optical fiber having a good waveguide structure by forming the core and cladding structures during the growth of the single crystal optical fiber. It's about doing.

Still another object of the present invention is to provide a method for producing a single crystal optical fiber doped with a desired element without the need to prepare a crystal base material doped with the desired element.

[0007]

[Means for Solving the Problems] In order to achieve the above object, the method for manufacturing a single crystal optical fiber according to the present invention is a method for manufacturing a single crystal optical fiber using a seed crystal while heating and melting a base material. The method for manufacturing a crystalline optical fiber is characterized in that a lithium niobate crystal on which a magnesium oxide is previously attached or deposited is used as the base material.

Furthermore, in the present invention, while heating and melting the base material,
A method for manufacturing a single crystal optical fiber in which a single crystal optical fiber is manufactured using a seed crystal, characterized in that a potassium niobate crystal on which a magnesium oxide is previously attached or deposited is used as the base material.

Furthermore, in the present invention, while heating and melting the base material,
In a method for manufacturing a single crystal optical fiber in which a single crystal optical fiber is manufactured using a seed crystal, a sintered body on which a substance containing an element to be added to the single crystal fiber is previously attached or deposited is used as the base material. It is characterized by

0010

[Operation] In the present invention, when producing a single crystal optical fiber by heating and melting the crystal base material and drawing it using a seed crystal, the refractive index of the single crystal is adjusted to realize the produced core-clad structure. A base material is used on which a substance containing the element to be degraded is previously attached or deposited. It is known that adding magnesium (Mg) to lithium niobate and potassium niobate crystals lowers their refractive index (J. Appl. Phys.,
vol. 39, pp. 3033-3038 (1968)
). In the present invention, the base material is a material in which Mg oxide is previously attached or deposited on the crystal surface in order to add Mg. In the molten part of the base material, by adjusting the pulling speed of the seed crystal in consideration of the diffusion rate of the Mg element, the diffusion concentration distribution of the Mg element to the center of the single crystal optical fiber is controlled, and a good core/cladding is achieved. structure can be realized.

In another method of the present invention, a substance containing a desired element to be added is attached or deposited on the surface of the base material in advance. Since the thickness of the deposited or deposited layer can be controlled relatively easily, a desired amount of dopant can be accurately doped into the single crystal optical fiber throughout the single crystal optical fiber fabrication process. Furthermore, by adjusting the pulling speed of the seed crystal in consideration of the diffusion rate of trace elements to be added during single-crystal fiber production, the added elements can be distributed uniformly or concentrated in the radial direction in the molten part of the sintered base material. It can be introduced into a single crystal optical fiber in the form of a gradient. Further, according to this method, since a crystal is not required as a base material, a single crystal optical fiber having a desired additive element can be formed at a relatively low cost.

0012

Embodiments Embodiment 1 Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. An embodiment of the method for manufacturing a single crystal fiber according to the present invention is schematically shown in FIG. In FIG. 1, a layer 16 in which Mg oxide is attached or deposited is previously formed on the surface of a crystal base material 12. Next, the upper part of the base material 12 together with the adhesive layer 16 is heated and melted by the heating laser 11 . At this time, in the melting zone 15, the base material 1
The components No. 2 and the components of the adhesive layer 16 are mixed in a liquid state. This melted part 15 is attached to the seed crystal 14, the seed crystal 14 is moved at a speed V2, and the base material 12 is moved at a speed V1 to draw the single crystal optical fiber 1.
3 is produced. At this time, the components of the base material and the components of the adhesion layer are mixed in a liquid state in the melting zone 15, but since the components of the adhesion layer diffuse at the diffusion rate specific to the element, the seed crystal 1
If the pulling speed in step 4 is made faster than the diffusion speed, it is possible to achieve a state in which the concentration of attached elements is high at the outer circumference of the pulled fiber and low at the inner circumference. Since Mg lowers the refractive index of lithium niobate and potassium niobate,
A waveguide structure having a low refractive index on the outside and a high refractive index on the inner circumference can be realized. In addition, the outer diameter D2 of the single crystal fiber 13
is determined by the above-mentioned equation (1). Here, the crystal orientation of the single crystal fiber 13 is the same as the crystal orientation of the seed crystal 14. The composition of the single crystal fiber 13 is such that Mg element is added to the composition of the base material 12, and a single crystal optical fiber doped with Mg is manufactured.

In this example, MgO was added as an oxide of Mg on the surface of a lithium niobate crystal matrix having an outer diameter of 200 μm.
A material that had been formed in advance to a thickness of approximately 1 μm by electron beam evaporation was used as a base material. CO as a heating laser
2 A laser was used. A lithium niobate single crystal whose axial direction was the a-axis direction was used as a seed crystal. The pulling speed of the seed crystal is 2 mm/min, and the feeding speed of the base material is 0.32 mm.
FIG. 2 shows the distribution of Mg element concentration and refractive index change in the fiber radial direction of the grown single crystal optical fiber when drawn at a speed of /min. It can be seen that the Mg element concentration is high at the outer circumference and becomes lower toward the inner circumference. On the other hand, the refractive index change increases in proportion to the Mg element concentration (the absolute value of the refractive index decreases), showing a so-called graded refractive index distribution. As a result, an optical fiber with a fiber outer diameter of approximately 80 μm and an effective core diameter of approximately 30 μm was obtained. here,
The crystal orientation of single crystal fiber is the same as the seed crystal orientation (a-axis)
Met. In addition, this fiber was equipped with Y having a wavelength of 1.06 μm.
When an AG laser was applied, it was confirmed that a second harmonic with a wavelength of 0.53 μm was generated.

<Example 2> M was added as an adhesion layer to the crystal base material.
A potassium niobate single crystal fiber was manufactured using potassium niobate having an outer diameter of 200 μm and having a thickness of 1 μm of gO deposited by electron beam evaporation. Potassium niobate seed crystal at speed 2.5
Pull up the base material at a speed of 0.4 mm/min.
FIG. 3 shows the Mg element concentration distribution and refractive index change when the wire is drawn by moving by n. As in the case of lithium niobate described above, it can be seen that the refractive index change increases in proportion to the Mg element concentration, and a waveguide structure is formed. Experiments using a YAG laser confirmed that this potassium niobate single crystal optical fiber can also be used as a highly efficient wavelength conversion element.

<Example 3> Further, using a lithium niobate crystal with a thickness of 150 μm and MgO deposited on the outside to a thickness of 2 μm as a base material, the seed crystal was pulled up at a speed of 3 mm/min, and the base material was 0.33 mm thick. A single crystal fiber with an outer diameter of 50 μm was formed by drawing the fiber while moving at a speed of 50 μm. FIG. 4 shows the change in refractive index in the fiber radial direction. It was also found that the present invention can realize a single mode single crystal fiber with an effective core diameter of 10 μm or less. The wavelength conversion efficiency of this single mode single crystal fiber was more than 10 times higher than that of the conventional fiber.

By the way, the conventional method of forming a waveguide structure using long-term thermal diffusion has the drawback that the fiber becomes brittle and the end face or surface is easily damaged, resulting in poor light coupling efficiency. Since the single crystal fiber produced by this method basically does not require long-term annealing, it has the following advantages: (1) high mechanical strength and low scattering loss; and (2) high incidence efficiency. In addition, after fabrication by this method, the temperature at about 1000°C
It has been found that by annealing for 5 hours or less, the strain generated during fabrication is released, which may be effective in reducing the loss of guided light. That is, the method for producing a single crystal fiber according to the present invention essentially provides a single crystal optical fiber with excellent waveguide properties, but these properties may be further improved by short-time annealing.

<Embodiment 4> During wire drawing, the pulling speed V2 of the seed crystal 14 is made sufficiently slower than the diffusion speed of the added element in the adhesion layer, so that the added element in the fusion zone 15 is uniform with the base material element. becomes a liquid mixture. Therefore, the composition of the single crystal optical fiber 13 obtained is as follows:
The composition has a composition in which the additive elements of the adhesion layer 16 containing additive elements are added to the composition of No. 2. That is, a single crystal optical fiber doped with elements is manufactured.

In this embodiment, the base material 12 is made of strontium barium niobate single crystal (Sr0.4 Ba0.6 Nb2, which is one of the photorefractive single crystals).
Raw material carbonate and oxide B corresponding to the composition of O6)
A sintered base material with an outer diameter of approximately 1 mm, which was prepared by weighing and sintering aCO3, SrCO3, and Nb2O5, was added with C as an additive element.
e. Using CeO as the adhesion layer 16 containing additive elements and a CO2 laser as the heating laser, a Ce-doped strontium barium niobate single crystal optical fiber with an outer diameter of about 400 μm was fabricated. Note that the adhesion layer 16 containing the additive element was formed by electron beam evaporation of CeO on the surface of the sintered body base material 12, and its film thickness was controlled by a crystal oscillator film thickness monitor. A cross section perpendicular to the central axis of the base material and a cross section along the central axis are shown in FIGS. 5(a) and 5(b), respectively. As shown in FIGS. 5(a) and 5(b), the adhesion layer 16 was formed on the surface of the sintered phase base material 12 to a substantially uniform thickness, and the thickness of the adhesion layer was approximately 0.5 μm. . By the way, as described above, the additive element concentration distribution of the manufactured single crystal fiber strongly depends on the pulling speed of the seed crystal 14 and the diffusion rate of the additive element. Here, the pulling speed V2 of the strontium/barium niobate seed crystal is set to 0.06 mm/min.
FIG. 6 shows the Ce concentration distribution in the radial direction of the single crystal optical fiber when drawn at a feeding speed of 0.6 mm/min and 0.6 mm/min (the feeding speed of the base material is 0.01 mm/min and 0.1 mm/min, respectively). Pulling speed is 0.6mm/min
In the case of 0.06 mm/
It can be seen that in the case of min, a substantially uniform distribution is shown at a concentration of about 0.05%. The Ce uniformly doped single crystal optical fiber manufactured at the latter pulling speed has a wavelength of 514
．． It was found that the 5 nm Ar+ laser exhibits photorefractive behavior and is applicable as a medium for volumetric multiplex holography. In addition, V-doped strontium barium niobate single crystal optical fiber, Cr
Doped strontium/barium niobate single crystal optical fibers were also fabricated using a similar method.

<Example 5> Next, as the base material 12, lithium niobate single crystal (L), which is one of the nonlinear optical single crystals, was used.
Raw material oxide LiO,N corresponding to the composition of iNbO3)
A sintered body base material with an outer diameter of approximately 500 μm obtained by weighing and sintering b2 O5 was prepared using Mg as an additive element, MgO as an adhesion layer 16 containing the additive element, and CO2 as a heating laser beam 11.
A Mg-doped lithium niobate single crystal optical fiber with an outer diameter of about 100 μm was fabricated using a laser. Note that the adhesion layer 16 containing the additive element was formed by electron beam evaporation of MgO on the surface of the sintered body base material 12. The adhesive layer 16 was formed on the surface of the sintered body base material 12 to have a substantially uniform thickness, and the thickness of the adhesive layer was approximately 1 μm. The doped element concentration distribution of the produced single crystal fiber is determined by the seed crystal 14 as described above.
depends strongly on the pulling rate and the diffusion rate of the added element. Here, the pulling rate V2 of the lithium niobate seed crystal is
is 0.7mm/min, and the moving speed of the base material is 0.028m.
FIG. 7 shows the distribution of Mg concentration in the radial direction of a single-crystal optical fiber in m/min. It can be seen that a concentration gradient is formed where the concentration is high on the outer circumference side of the fiber and low on the inner circumference side. Due to the Mg distribution as shown in Figure 7, the refractive index distribution in the radial direction of the single crystal optical fiber is lower at the outer circumference and higher at the inner circumference, making it possible to form a waveguide structure with a so-called graded refractive index distribution. I understand. This waveguide structure allows the guided laser light to propagate efficiently, so we confirmed a high rate of wavelength conversion in a second harmonic generation experiment using a YAG laser (wavelength: 1.06 μm).

[0020] Also, using this method, an outer diameter of about 30 μm
By producing the following lithium niobate single crystal optical fiber, a single mode waveguide can be formed, and a significant improvement in wavelength conversion efficiency can be expected. In addition, Zn-doped lithium niobate single-crystal optical fibers, Cu-doped lithium niobate single-crystal optical fibers, and Ni-doped lithium niobate single-crystal optical fibers were also produced using the same method.

The seed crystal for pulling a single-crystal optical fiber is preferably a single crystal having the same composition as the intended single-crystal optical fiber, but it is not necessary to contain additive elements. Further, even if the composition is slightly different, it can be used as long as the difference in lattice constant is within about 3% from that of a single crystal optical fiber.

[0022]

As explained above, according to the method for manufacturing lithium niobate and potassium niobate single crystal optical fibers according to the present invention, an adhesion layer containing Mg element that lowers the refractive index is used as a base material. adhering or depositing,
The base material is melted together with the attached layer, and the addition of additive elements and the production of single-crystal optical fiber are performed in the same process.
It has the following effects.

(1) A waveguide structure with a good refractive index distribution can be realized without performing long-term thermal diffusion.

(2) Since long-term thermal diffusion is not performed, a single crystal optical fiber with excellent mechanical strength and low waveguide loss can be realized.

(3) Since crystal growth and formation of the waveguide structure are performed in the same process, manufacturing time and costs can be reduced.

Furthermore, according to the method of manufacturing a single crystal optical fiber of the present invention, a sintered body is used as a base material, an adhesion layer or deposit containing an additive element is formed on the base material, and the base material is removed together with the adhesion layer. Since melting, mixing of additive elements, and production of a single crystal optical fiber are performed in the same process, the following effects are achieved.

(4) A single crystal optical fiber containing an additive element can be formed without using a crystal base material containing an additive element that is difficult to produce.

(5) After forming the sintered body base material, an adhesion layer containing the additive element is attached or deposited, so that trace elements can be added accurately. Further, by adjusting the pulling speed of the seed crystal, a desired additive element concentration distribution can be achieved.

(6) Since a sintered base material is used, a desired doped single crystal optical fiber can be formed at a relatively low cost without using an expensive crystal base material.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing an embodiment of a method for manufacturing a single crystal optical fiber according to the present invention.

FIG. 2 is a diagram showing the Mg concentration and the radial distribution of refractive index change in the Mg-doped lithium niobate single crystal optical fiber produced in this example.

FIG. 3 is a diagram showing the Mg concentration and the radial distribution of refractive index change in the Mg-doped potassium niobate single crystal optical fiber produced in this example.

FIG. 4 is a diagram showing the radial distribution of refractive index change in the single-mode Mg-doped lithium niobate single-crystal optical fiber manufactured in this example.

FIG. 5 is a cross-sectional view and a vertical cross-sectional view showing the structure of a base material on which an adhesion layer containing an additive element is attached or deposited.

FIG. 6 is a diagram showing the radial distribution of Ce concentration in the Ce-doped strintium barium niobate single crystal optical fiber manufactured in this example.

FIG. 7 is a diagram showing the radial distribution of Mg concentration and refractive index change in the Mg-doped lithium niobate single crystal optical fiber produced in this example.

FIG. 8 is a schematic diagram showing an example of a conventional method for manufacturing a single crystal optical fiber.

[Explanation of symbols]

11,101 Laser light for heating 12,102 Base material 13,103 Single crystal optical fiber 14,104 Seed crystal 15,105 Melting part 16 Adhesive layer containing additive elements

Claims (3)

[Claims] Claim 1: A method for manufacturing a single crystal optical fiber in which a single crystal optical fiber is manufactured by using a seed crystal while heating and melting a base material, in which lithium niobate on which magnesium oxide is preliminarily attached or deposited is used. A method for manufacturing a single crystal optical fiber, characterized in that a crystal is used as the base material. 2. A method for manufacturing a single crystal optical fiber in which a single crystal optical fiber is manufactured using a seed crystal while heating and melting a base material, in which potassium niobate on which magnesium oxide is preliminarily attached or deposited. A method for manufacturing a single crystal optical fiber, characterized in that a crystal is used as the base material. 3. A method for producing a single-crystal optical fiber in which a single-crystal optical fiber is produced using a seed crystal while heating and melting a base material, in which a substance containing an element to be added to the single-crystal fiber is coated on the surface in advance. A method for manufacturing a single-crystal optical fiber, characterized in that an attached or deposited sintered body is used as the base material.