JP2913006B2 - Manufacturing method of optical fiber for manufacturing single crystal fiber optical waveguide structure - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing an optical fiber for forming an optical waveguide structure in a single crystal fiber.

[0002]

2. Description of the Related Art As shown in FIG. 4, an optical waveguide has a two-layer structure in which a core called a core 1 is surrounded by a medium called a clad 2 having a lower refractive index. The signal is propagated while being confined (FIG. 4c).
An optical fiber is formed by forming such a refractive index structure on a fiber such as glass or plastic, and as a manufacturing method, a base material called a preform adjusted to a desired refractive index is thinned by utilizing the viscosity of the material. There are known methods such as drawing or linearizing while simultaneously drawing two types of media having different refractive indexes from a nozzle having a double structure.

[0003] In the case of a single crystal medium, a waveguide is generally formed on the surface of a crystal wafer, and an impurity element that mainly increases the refractive index is removed from the crystal surface by thermal diffusion or ion exchange. Are known in the art, and a method of epitaxially growing different materials having different refractive indices is known. There are few examples relating to a fiber-like single crystal medium, and there is only a known case in which a crystal medium serving as a core is grown inside a glass pipe that also serves as a clad portion. In addition, a pull-up method (Czochralski method) is used as a typical example of so-called melt growth in which crystallization is performed from a raw material melt.
There is {Figure 3}.

[0004]

In the pulling method (Czochralski method), one end of a seed crystal 3 is immersed in a raw material melt 6 melted in a bulk crucible 5 to be blended, and a rotary pulling shaft 8 is rotated. In general, in melt growth, the grown crystal 4 and the raw material melt 6 are bounded by a solid-liquid interface 7 corresponding to an isothermal surface equal to the melting temperature (or solidification temperature). It is understood that the size of the solid-liquid interface 7 in contact determines the crystal diameter, and it has been difficult to produce a fiber having a different refractive index between the core portion and the clad portion from one nozzle.

[0005]

The present inventors have observed the solid-liquid interface of the single crystal fiber in detail, and found that the center portion 11 of the single crystal fiber (fiber) 10 as shown in FIG. Although the raw material melt 6 and the fiber 10 are in contact at the solid-liquid interface 7, the fiber 10
Was found not to be in direct contact with the raw material melt 6. Nevertheless, the diameter of the growing fiber 10 is larger than the contact surface of the raw material melt 6, and the raw material melt 6 is supplied along the surface of the central portion 11 at the peripheral portion 12 of the fiber 10. The facts became clear.

[0006] Crystal growth is a phenomenon in which atoms and molecules in the melt are incorporated into the crystal. Therefore, the difference in the supply mechanism of the raw materials necessarily results in the difference in the crystal growth mechanism, and the difference in the growth mechanism results in the generation of inhomogeneity in the grown crystal. In the solid-liquid interface 7 in the above-described state, the central portion 11 is governed by the mechanism in which atoms and molecules are incorporated into the crystal side from the contacting raw material melt 6, that is, the adhesion growth, while the peripheral portion 12 of the fiber 10 is crystallized. The mechanism in which atoms and molecules are supplied from the raw material melt 6 that propagates while wetting the surface, that is, creeping growth occurs. In the creepage growth and the adhesion growth, it is considered that the creepage growth region easily takes in impurity atoms for impurity atoms having a distribution coefficient k> 1, and the adhesion growth region easily takes in impurity atoms for impurity atoms having a distribution coefficient k <1. I have. In a multi-element crystal such as a KLN crystal, the degree of incorporation among the component elements differs, and this is inevitably reflected in the crystal quality. Occurs. When a fiber is grown by adding an impurity element with k> 1 to the raw material and growing the fiber, the refractive index increases when an impurity atom is taken in, and the refractive index at the central portion can be made larger than that at the peripheral portion. The addition of an impurity of a type having a lower refractive index and k <1 can lower the refractive index of the peripheral portion. Using the above principle,
Simultaneously with the growth, a refractive index structure having an optical waveguide effect can be formed in the fiber.

[0007] Such a fact has not been discovered by the pulling method until now, but the reason is that the solid-liquid interface is extremely thin with respect to the crystal diameter, the appearance changes depending on the temperature conditions, and especially the pulling method. According to the method, the solid-liquid interface is located directly above a high-temperature crucible, so that the temperature gradient cannot be increased, and the crystal is rotating during growth, making observation difficult. However, upon careful observation, although the scale was different, it was also observed by the pulling method, and it was confirmed that this fact was a phenomenon commonly occurring in melt growth.

The present invention is based on the above-mentioned new fact. In the process of growing a fibrous single crystal for continuously crystallizing a raw material melt whose supply amount is controlled via a fine nozzle, the temperature near the solid-liquid boundary is increased. By using a difference in crystal growth mechanism between the central part and the peripheral part of the crystal by steepening the gradient, it is possible to induce inhomogeneities with different refractive indexes in the fiber. It is.

[0009]

In the case of lowering the raw material melt by the lowering method or the like, the diameter of the lowering nozzle is made smaller, the temperature gradient near the solid-liquid interface is made steep, and the single crystal fiber is grown while continuously lowering in this state. . As a result, the crystal growth mechanism differs between the central part and the peripheral part of the solid-liquid interface, and a fiber having an optical waveguide effect can be obtained.

[0010]

[Example 1] The basic principle of growing a crystal having a small diameter and a long length like a single crystal fiber is to crystallize while continuously supplying a small amount of melt, and we use the simplest method. A certain reduction method was adopted. This method draws the raw material melt from a fine-diameter nozzle inserted into the bottom of the crucible, and is characterized in that the temperature conditions near the solid-liquid interface can be selected in a relatively wide range (FIG. 2). Resistance heating used
An example of the mold crucible 13 has a rough size of length 8 × width 4
× height 5 × plate thickness 0.1 mm, the nozzle 9 of the melt outlet is 100 to 800 μm in diameter, and both are made of platinum. As a heat source, a resistance heating method in which current is directly applied to the resistance heating type crucible 13 was adopted, and the heat source was installed in a small vertical electric furnace (FIG. 2A) having a five-stage heating zone structure. Resistance in the middle of the electric furnace
The anti-heating type crucible 13 was used for temperature control around the crucible with its upper and lower stages sandwiched between coiled sub-heaters 14, and for heat retention and annealing with its upper and lower portions further sandwiched between two cylindrical electric furnaces 15. The electric furnace main body is installed in the pull-down driving device, and the raw material melt 6 passing through the nozzle 9 of the crucible 13 at the center stage is attached to the tip of the seed crystal 3 attached to the drive shaft 16.
Was lowered while growing crystals. The pull-down speed is 0.5 to 30 millimeters per hour and the crystal is not rotating. In addition, 17 is a heating wire.

In the case of lithium niobate (LiNbO3, abbreviated as LN), lithium carbonate and niobium pentoxide are used as starting materials, and a congruent composition ratio Li / LN = 0.48.
6 and then mixed, pressed and fired (900 to 960
(7 ° C., 7 to 10 hours) cycle is repeated three times, and titanium dioxide (TiO 2) is
Four kinds of 1 mol%, 3 mol%, 5 mol% addition and no addition were prepared, and the growth of a single crystal fiber was attempted for each. The shape of the solid-liquid interface 7 during growth shown in FIG. 1 is affected by the temperature of the melt, the temperature gradient near the solid-liquid interface, the pull-down speed, and the like. When the melt temperature was increased and the temperature gradient was increased and the temperature gradient was increased, the central portion 10 of the single crystal fiber became thinner. On the other hand, when the temperature gradient was reduced, the fiber diameter decreased, and the ratio of the core to the diameter increased accordingly. By selecting optimal temperature conditions such as melt temperature, nozzle diameter, sub-heater, pull-down speed, etc., a single crystal fiber having a diameter of 50 to 800 microns and a crystal orientation c-axis could be grown. Diameter of grown fiber is 50
For fibers near 0 microns, perpendicular to the growth direction
Cut out a piece of crystal and use a polarizing microscope and two-beam interference microscopy
Using a mirror, a concentric optical defect
Confirm that homogeneity is formed, and
A near-field pattern cut to 10 mm and polished at both ends was observed. As a result, a clear optical waveguide effect was observed in all four types of fibers including those without additives.

[0012]

Embodiment 2 Lithium Potassium Niobate (K3 + x
Li2 + y Nb5-xy O15, KL for short
The single crystal fiber of N) uses lithium carbonate, potassium carbonate, and niobium pentoxide as starting materials, and 0 ≦ x ≦ 0.3,
A single crystal fiber was grown in the same procedure as in Example 1 by synthesizing in a composition range of 0 ≦ y ≦ 0.2. Since KLN performs so-called incongruent melting in which the raw material composition and the crystal composition are different, the crystal itself and the composition of the raw material melt essentially change as the crystal grows. However, it is possible to crystallize a part of the filling raw material, and a single crystal fiber was grown in search of an optimal temperature condition. The growth orientation is always the a-axis. A crystal habit appeared strongly, and a fiber having a rectangular cross section characteristic of a-axis growth was grown. The solid-liquid interface always forms a curved surface whose center is convex toward the melt with respect to the four corners, and crystal non-uniformity is likely to occur without adding impurities. A single crystal fiber having a diameter of 100 to 800 microns and a length of 50 to 70 millimeters is grown, and a fiber having a diameter of about 500 microns is grown as in the case of LN.
A piece of crystal perpendicular to the growth direction was cut out, and as in Example 1,
Concentric optical inhomogeneity forms in fiber cross section
And 5 to 10 millimeters at both ends are polished.
By observing the pattern, the optical waveguide effect was confirmed.

[0013]

As described above, according to the present invention, the method of pulling down the raw material melt from the fine nozzle can set a sharp temperature distribution particularly in the vicinity of the solid-liquid interface, and the difference in the crystal growth mechanism between the central portion and the peripheral portion. Can be emphasized. By utilizing this, regardless of the case of adding or not adding impurities, crystal growth can be performed so that the central portion and the peripheral portion within one single crystal fiber have non-uniform crystallinity. By utilizing the difference in refractive index caused by this, a single crystal fiber having an optical waveguide effect could be realized at the same time as growing.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the principle of the present invention.

FIG. 2 is a schematic diagram specifically illustrating an embodiment of the present invention. (A) is an overall view, (b) is a view showing a main part.

FIG. 3 is an explanatory diagram for explaining a pulling method (Czochralski method), which is a typical example of melt crystal growth.

FIG. 4 is an explanatory diagram illustrating the concept of an optical waveguide. In the drawing, (a) is a cross-sectional structure, (b) is a refractive index distribution, and (c) is a diagram showing a state of light propagation in the waveguide.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Core 2 Clad 3 Seed crystal 4 Growth crystal 5 Crucible for bulk 6 Raw material melt 7 Solid-liquid interface 8 Rotating pull-up shaft 9 Fine nozzle 10 Single crystal fiber 11 Central part of single crystal fiber 12 Peripheral part of single crystal fiber 13 Resistance heating Mold crucible 14 Secondary heater 15 Cylindrical electric furnace 16 Pull down drive shaft 17 Heating wire

Claims (2)

(57) [Claims] 1. A raw material melt whose supply amount is controlled is continuously pulled down and supplied via a fine nozzle or the like ,
To grow single-crystal single crystals and create single-crystal fiber optical waveguide structures
A method for producing an optical fiber to be produced, wherein , in a process of growing the fibrous single crystal , the raw material melt is pulled down from the fine nozzle or the like and supplied while a steep temperature distribution is set near a solid-liquid interface. Thus, in the central portion of the optical fiber, the raw material
Although in contact with the melt at the solid-liquid interface, the optical fiber
At the periphery of the bar, it is in direct contact with the raw material melt.
And so no crystal between said central portion and said peripheral portion
Causing the difference in growth mechanisms, utilizing the difference in the crystal growth mechanism to produce a single crystal fiber optical waveguide structure, characterized in that to produce the optical waveguide structure of the core and the cladding in the single crystal fiber optical fiber Manufacturing method. 2. The method for producing an optical fiber according to claim 1, wherein an impurity which causes a change in the refractive index is added to the raw material.