JPS6245182B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a preform for a fluoride glass optical fiber capable of transmitting infrared rays in the wavelength band of 2 to 6 μm.

Conventional optical fiber preforms are mainly composed of silicon oxide (SiO ₂ ) glass, but this glass material has infrared absorption caused by the vibration of Si-O bonds, so Rayleigh scattering loss and infrared absorption The low-loss wavelength range that exists between the loss and the loss is limited to the visible range to the near-infrared range (wavelength 0.6 to 1.7 μm).
It has not been possible to obtain an optical fiber with low loss in a wavelength range longer than that. On the other hand, Rayleigh scattering decreases in inverse proportion to the fourth power of the wavelength, so a preform is formed from a glass material whose infrared absorption edge is located on the longer wavelength side compared to silicon oxide, and the preform is drawn to form an optical fiber. It is thought that the loss can be further reduced by manufacturing the optical fiber, and there is a demand for a method for forming such an optical fiber preform.

Optical fibers for communications have a waveguide structure in which a core with a high refractive index is covered with a cladding with a lower refractive index, but currently, optical fibers for infrared transmission that have a waveguide structure are known as , AgCl clad - Ag (ClBr) core, Tl (BrI) core - plastic clad, etc. However, in the case of these polycrystalline optical fibers, due to the effect of grain boundary scattering loss, It is essentially impossible to create ultra-low loss optical fibers. C ₂ Cl ₄ liquid core-SiO ₂ clad optical fibers are also known, but there are major problems in producing long optical fibers and connecting them.

In addition, fluoride glass is attracting attention as a material that eliminates the drawbacks of the various fiber materials mentioned above and has a high possibility of realizing an ultra-low loss optical fiber in the infrared wavelength region of 2 to 6 μm. There have been no specific examples introduced so far regarding the manufacturing method for optical fiber preforms having the same structure, and it is also true that the internal attachment method or vapor phase axis attachment method, which is a manufacturing method for silicon oxide optical fiber preforms, cannot be applied as is. it is obvious.

The present invention was made in view of the above-mentioned current situation, and its purpose is to solve the drawbacks of the prior art and to create a fluoride glass that can transmit infrared rays with a wavelength of 2 to 6 μm and has an extremely low loss. An object of the present invention is to provide a method for easily manufacturing an optical fiber preform made of.

In order to achieve this object, in the method for manufacturing an infrared transmission optical fiber preform of the present invention, a fluoride glass rod of an appropriate shape such as a cylinder is heated in a melt of an aluminum compound at a temperature below the deformation temperature of the fluoride glass rod. The fluoride glass rod is heated at a temperature below the deformation temperature of the fluoride glass rod in a solution containing one type of compound selected from aluminum compounds, lithium compounds, and sodium compounds as a solute. Aluminum ion Al ³⁺ in the melt or aluminum ion Al ³⁺ in the solution, lithium ion
Ion exchange is performed between Li ⁺ and sodium ions Na ⁺ to lower the refractive index of the peripheral portion of the fluoride glass rod, thereby forming a clad structure to produce a preform.

The fluoride glasses used in the present invention include conventionally known ZrF ₄ -ThF ₄ -BaF ₂ and ZrF ₄ -
BaF ₂ −LnF ₃ (L is a lanthanide element), ZrF ₄ −
ZrF _4- based fluoride glass such as BaF ₂ −YF ₃ , or
The refractive index of HfF ₄ -based fluoride glass is similar to that of AlF ₃ and
Fluoride glass with higher fluoride content than LiF and NaF is preferable.

The method for manufacturing a preform according to the present invention basically involves placing a prefabricated fluoride glass rod of an appropriate shape such as a cylindrical shape for the core in a melt of various Al compounds, or in a mixture of an Al compound, a Li compound, or a Na compound. The glass rod is immersed in a heated solution as a solute, and the relationship between cations such as Zr ⁴⁺ , Gd ³⁺ , Ba ²⁺ etc. around the glass rod and cations such as Al ³⁺ , Li ⁺ , Na ⁺ etc. in the melt or solution is detected. Ion exchange is accelerated by heating at a high temperature below the deformation temperature of the glass rod, thereby forming a clad waveguide structure.

The present invention will be explained in detail below with reference to the drawings.

The case where the present invention is implemented using the accelerated heating ion exchange tank shown in FIG. 1 will be described. A glass flask 1 contains a solution containing an Al compound, an Al compound, a Li compound, or a Na compound as a solute, which becomes liquid at high temperatures, and a cylindrical shape is placed in the ion exchange melt or ion exchange solution 2. A fluoride glass rod 3 is suspended by a platinum wire 4, and an electric motor 5 is connected to the rod.
While stirring the ion-exchange melt or solution 2 with a stirrer 6 driven by a mantle heater 7, the ion-exchange melt or ion-exchange solution 2 is heated to a temperature below the deformation temperature of the fluoride glass rod 3, e.g. Heat to 250-300°C. A cooler 8 is attached to the flask 1. Cooling water is introduced into the cooler 8 from a cooling water introduction pipe 9, and the vapor generated when the solution 2 in the flask 1 boils is liquefied by the cooler 8. and keep the amount of solution in flask 1 constant. 10 is a cooling water discharge pipe. In this way, the formation cations of the fluoride glass rods 3
Ion exchange is performed between Zr ⁴⁺ , Gd ³⁺ , Ba ²⁺ and Al ³⁺ , Li ⁺ , Na ⁺ , thereby forming a clad structure having a lower refractive index than the center of the rod around the glass rod 3 .

The preform obtained in this way has a waveguide structure that was impossible with conventional technology, and the optical fiber obtained by drawing this is an infrared transmission optical fiber with low loss in the wavelength range of 2 to 4 μm. It is valid as

Next, examples of the present invention will be described, but the present invention is not limited thereto.

Example 1 ZrF ₄ 21g (63 mol%), BaF ₂ 11.55g (33 mol%), GdF ₃ 17g (4 mol%) and NH ₄ F・HF5
g was weighed, ground and mixed in a mortar. This was introduced into a metal crucible and heated at 400℃ for 30 minutes using an electric furnace to completely fluorinate the raw material, and then heated at 850℃ for 30 minutes.
Melt by heating for minutes. This is casted into a brass mold shown in FIGS. 2A to 2D.

Figure 2A shows the assembled state of the mold used here, 11 is a brass mold outer frame, and as shown in Figure 2B, sheath members 11A and 11 are divided into three parts.
Consisting of B and 11C, these sheath members 11A~
After fitting the lower end of 11C to the brass bottom cover 12, attach the brass ring 13 as shown in FIG. 14C to fix the sheath members 11A to 11C to form a mold. The above-mentioned molten material was poured into such a mold and casting was performed to obtain a glass rod of 9φ×100 mm. The outer periphery of this rod was optically polished and used as a starting material for ion exchange.

Next, this base material 3 was introduced into the ion exchange tank shown in Fig. 1, and after being fixed with a platinum wire 4, aluminum benzylate Al(C ₇ H ₇ O) ₃ was introduced into the flask 1 and heated at a temperature of 70°C. Aluminum benzylate was introduced in such a way that it melted and the starting matrix 3 was completely submerged in the solution. Next, the solution 2 is heated to a temperature of 283 to 284°C by the mantle heater 7 and refluxed through the cooling pipe 8.
After 24 hours, the reflux was stopped, and the base material 3 was taken out from the flask 1, and then annealed in another electric furnace for annealing, and cooled from 250° C. to room temperature over 24 hours. The cross section of the resulting preform 15 as shown in FIG. 3A was observed with an interference microscope,
When calculating the refractive index distribution along the dotted line 16, the third
It was found that a waveguide structure having a continuous refractive index change as shown in Figure B was formed. Here, the relative refractive index difference was 0.6% between the highest point at the center of the core and the lowest point at the periphery.

Example 2 Same as Example 1, ZrF ₄ (63 mol%)-BaF ₂ (33
mol %) - GdF ₃ (4 mol %) glass rod 3
(9φ×100 mm), an aluminum bromide melt was introduced into the ion exchange tank and refluxed at 264° C. After 36 hours, it was taken out from the flask 1 and annealed. Similar to Example 1, the refractive index distribution shown in FIG. 3B was obtained.

Example 3 Same as Example 1, ZrF ₄ (63 mol%)-BaF ₂ (33
mol %) - GdF ₃ (4 mol %) glass rod 3
(9φ x 100mm), introduced the melt of aluminum diethylmalonate Al(C ₇ H ₁₁ O ₄ ) ₃ into the ion exchange tank, heated it to 300°C with stirring, and after 24 hours took it out from inside flask 1 and annealed it. did. the result,
Line 1 of the preform 17 as shown in FIG. 4A
A characteristic as shown in FIG. 4B was obtained as a refractive index distribution along the angle 8.

Example 4 Same as Example 1, ZrF ₄ (63 mol%)-BaF ₂ (33
Glass rod (9 mol %) - GdF ₃ (4 mol %)
_{φ _ _}
After heating and stirring at 300° C. for 12 hours, the mixture was taken out from flask 1 and annealed. As a result, Figure 3B
A preform with a refractive index distribution was obtained.

Example 5 Same as Example 1, ZrF ₄ (63 mol%)-BaF ₂ (33
mol %) - GdF ₃ (4 mol %) glass rod 3
(9 φ x 100 mm), a solution of 30 g of aluminum propoxide Al(C ₃ H ₇ O) ₃ dissolved in 500 c.c. of triethylene glycol was introduced into the ion exchange tank, and heated under reflux at 288°C. flask 1 after 36 hours
Glass rod 3 was taken out from inside and annealed. As a result, a preform having a refractive index distribution as shown in FIG. 3B was obtained.

Example 6 Same as Example 1, ZrF ₄ (63 mol%)-BaF ₂ (33
mol %) - GdF ₃ (4 mol %) glass rod 3
(9 φ x 100 mm), a solution of 50 g of aluminum benzoate Al (C ₇ H ₅ O ₂ ) ₃ dissolved in 500 c.c. of dodecylbenzene was introduced into the ion exchange tank.
After 24 hours of heating and stirring at °C, glass rod 3 was taken out from flask 1 and annealed. As a result, a preform having a refractive index distribution as shown in FIG. 3B was obtained.

Example 7 Same as Example 1, ZrF ₄ (63 mol%)-BaF ₂ (33
mol %) - GdF ₃ (4 mol %) glass rod 3
(9 φ x 100 mm), a solution of 30 g of lithium salicylate LiC ₇ H ₅ O ₂ dissolved in 500 c.c. of tetraethylene glycol was introduced into the ion exchange tank.
After 24 hours of heating and stirring at 300°C, glass rod 3 was taken out from flask 1 and annealed. As a result, a preform having a refractive index distribution as shown in FIG. 3B was obtained.

Example 8 Same as Example 1, ZrF ₄ (63 mol%)-BaF ₂ (33
mol %) - GdF ₃ (4 mol %) glass rod 3
(9φ x 100mm), and the ion exchange tank was a solution of 10g of sodium acetate NaC ₂ H ₃ O ₂ dissolved in dibutyl sebacate H ₉ C ₄ OOC (CH ₂ ) ₈ COOC ₄ H ₉ 500c.c. was introduced, heated and stirred at 300°C, and after 24 hours, glass rod 3 was taken out from inside flask 1 and annealed. As a result, a preform having a refractive index distribution as shown in FIG. 4B was obtained.

As explained above, according to the present invention, it is possible to easily form a cladding portion having a lower refractive index than the center of the rod at the outer peripheral portion of the fluoride glass rod. Further, by changing the temperature and conditions of the melt or solution for ion exchange, a preform having a refractive index distribution in each part can be manufactured. Fluoride optical fibers made from such preforms are
Wavelength range of 2 to 6 μm, which was impossible with conventional technology
It has the advantage of being able to realize transmission in

Although the fluoride glass rod has been described above as an example of a cylindrical shape, it goes without saying that the rod can have any desired shape.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing an example of an ion exchange device used to carry out the present invention, FIG. B is a sectional view taken along the line B-B, FIG. 2C is a plan view of the bottom cover, FIGS. 3A and B are a sectional view and a refractive index distribution diagram of the preform formed according to the present invention, and FIG. 4A and B are a cross-sectional view and a refractive index distribution diagram, respectively, of a preform formed according to another example of the present invention. 1... Glass flask, 2... Ion exchange melt or ion exchange solution, 3... Fluoride glass rod, 4... Platinum wire, 5... Electric motor, 6
... Stirrer, 7 ... Mantle heater, 8 ... Cooler, 9 ... Cooling water introduction pipe, 10 ... Cooling water discharge pipe, 11 ... Brass mold outer frame, 11A-1
1C... Tripart sheath member, 12... Brass bottom cover, 13... Brass ring, 14A to 14C...
Screw, 15, 17... Preform cross section, 16, 1
8...Position on the preform corresponding to the refractive index distribution.

Claims (1)

[Claims] 1. Heating a fluoride glass rod in a melt of an aluminum compound at a temperature below the deformation temperature of the fluoride glass rod, or in a solution containing one compound among aluminum compounds, lithium compounds, and sodium compounds as a solute. The fluoride glass rod is heated to a temperature below the deformation temperature, and the constituent cations of the fluoride glass rod are combined with the aluminum ions Al ³⁺ in the melt or the aluminum ions Al ³⁺ , Li ⁺ , Na ⁺ in the solution. A method for manufacturing a preform for an infrared transmission optical fiber, characterized in that the preform is manufactured by forming a clad structure by lowering the refractive index of the peripheral portion of the fluoride glass rod by performing ion exchange between the rods.