JPH0116781B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a preform for a fluoride glass optical fiber capable of transmitting infrared rays in the 2-6 μm band. 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, according to the prior art, Rayleigh scattering is reduced 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 long wavelength side compared to silicon oxide, and then the preform is drawn. Since optical fibers can be produced in such a manner that the loss can be further reduced, there is a demand for a method for forming such optical fiber preforms. 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 cladding - Ag (ClBr) core, and Tl (BrI) core -
Polycrystalline optical fibers such as plastic clad, but in the case of these polycrystalline optical fibers,
The production of ultra-low loss optical fibers is essentially impossible due to the effects of grain boundary scattering loss. C ₂ Cl ₄ liquid core-SiO ₂ clad optical fiber is also known, but
There are major problems in the fabrication of long optical fibers and their connections. 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 ultra-low loss optical fibers in the infrared wavelength region of 2 to 6 μm. At present, nothing is known about optical fiber preforms. The present invention has been made in view of the above-mentioned current situation, and its purpose is to solve the problems of the prior art and to provide a fluoride glass that can transmit infrared rays with a wavelength of 2 to 6 μm and has a high possibility of extremely low loss. The present invention provides an infrared transmitting preform using a material and a dopant for controlling the refractive index. The infrared transmission optical fiber preform of the present invention that achieves the above object contains 28 to 38 mol% BaF ₂ ,
The base glass is a glass having a composition within the range of 2 to 7 mol% of GdF ₃ and 58 to 69 mol% of ZrF ₄ , and the core is composed of 6 mol% or less of PbF ₂ or 16 in the base glass.
Consisting of glass doped with less than mol% _BiF3 ,
Clad contains 24 mol% or less of LiF in the matrix glass,
It is characterized by being made of glass doped with 12 mol% or less of NaF or 8 mol% or less of _AlF3 . Next, to explain the present invention in detail, the base glass of the present invention specifically has a composition shown in the following Table 1, where a to s are compositions within the range of the present invention, and t to w.
indicates a composition outside the scope of the present invention.

【table】

[Table] The refractive index of the matrix glasses a to x in Table-1 is 1.528~
1.530, and there is almost no change depending on the composition. However, compositions from t to w, which are outside the scope of the present invention, are unsuitable as a base glass because microcrystals are generated as shown in the following experimental examples when dopants for the core or cladding are added. In the preform of the present invention, in order to obtain a glass with a refractive index of 1.53 to 1.56 as a core glass, a dopant of 6 mol% or less of PbF ₂ or 16 mol% or less of BiF ₃ is blended with respect to 100 mol% of the base glass. Refractive index 1.53 to 1.51 as glass for cladding
Also less than 24 mol% of LiF, to obtain a glass of
A dopant containing up to 12 mol % of NaF or 8 mol % or less of AlF ₃ is blended. The dopant is added to the base glass at a predetermined ratio.
This is carried out by mixing ZrF ₄ , BaF ₂ , GdF ₃ and a dopant and heating and melting the mixture at about 800 to 850° C. in a crucible. In addition, if NH ₄ F and HF are combined together, 400
The raw material may be completely fluorinated by heating at around 0.degree. C., and then heated and melted. To make a preform from core glass and cladding glass, a rod made of core glass is inserted into a tube made of cladding glass, the cladding glass is melted and brought into close contact with the core rod, and then Manufactured by slow cooling while maintaining the temperature below the glass transition temperature. Next, an experimental example will be shown to clarify the influence of the dopant concentration (amount) in the core glass and the cladding glass. In the following experimental examples, a matrix glass having a composition k was used as the matrix glass. Experimental example 1 (LiF) LiF was added to a mixture of the composition of the matrix glass k at 4, 8,
Glass composition mixture doped with respective proportions of 12, 16, 20, 24, 28 mol% (e.g. LiF4 mol%)
In case of addition, ZrF ₄ 10g, BaF ₂ 5.5g,
A mixture of 0.814 g of GdF ₃ and 0.103 g of LiF) was introduced into a metal crucible, heated and melted at 850°C for 30 minutes using an electric furnace, and placed in a brass mold with a hollow part of 10φ x 130 mm that had been preheated to 250°C. Casting,
After annealing, it was slowly cooled to obtain a glass rod of 10φ×130mm, and its refractive index (η _D ), coefficient of linear expansion (α), glass transition temperature (Tg), and deformation temperature (Td) were measured. The results are shown in Table-2.

[Table] As is clear from Table 2, the refractive index can be arbitrarily adjusted from 1.526 to 1.510 by doping with LiF, and it can be used as a low refractive index glass for cladding. Also, LiF28 mol%
When added, the glass begins to devitrify, and its wide range of vitrification makes it a promising dopant for fluoride optical fibers. When using a matrix glass other than glass number K, the refractive index change is exactly the same, 1.526
It can be adjusted arbitrarily from ~1.510. However, for glass numbers t, u, v, and w, LiF is 4 to 24.
It was found that microcrystals were generated inside the glass rod when it was doped in a mole % range, making it unsuitable as a base glass. Experimental example 2 (NaF) 2, 4,
Glass composition mixtures doped with respective proportions of 6, 8, 10, 12, 14, 16, and 18 mol% (e.g.
In case of adding 6 mol% of NaF, 10g _{of ZrF4} ,
A mixture of 5.5 g of BaF ₂ , 0.814 g of GdF ₃ , and 0.255 g of NaF) was introduced into a metal crucible and heated to 850°C using an electric furnace.
Heat melt for 30 minutes and preheat to 250℃ 10φ
It was casted in a brass mold with a hollow part of 130 mm, annealed and then slowly cooled to obtain a glass rod of _10φ x 130 mm. The deformation temperature (Td) was measured.
The results are shown in Table-3.

[Table] As is clear from Table 3, the refractive index can be arbitrarily adjusted from 1.526 to 1.514 by doping with NaF, and it can be used as a low refractive index glass for cladding. In addition, crystals begin to form inside the glass when 16 mol% of NaF is added, and its wide range of vitrification makes it a promising dopant for fluoride optical fibers. Even when a matrix glass other than glass number k was used, exactly the same change in refractive index was exhibited, and the refractive index could be arbitrarily adjusted from 1.526 to 1.514. However, for glass numbers t, u, v, w
It was found that when doping NaF in a range of 2 to 14 mol%, microcrystals were generated inside the glass rod, making it unsuitable as a base glass. Experimental example 3 (AlF ₃ ) 2, ₄ ,
A glass composition mixture doped at respective proportions of 6, 8, and 10 mol % (for example, in the case of adding 4 mol % of AlF ₃ , 20 g of ZrF ₄ , 11 g of BaF ₂ , 1.628 g of GdF ₃ ,
A mixture of 0.515 g of AlF ₃ ) was introduced into a metal crucible, heated and melted at 850°C for 30 minutes using an electric furnace, and the
Casting is done in a brass mold with a hollow part of 10φ x 130mm heated to
A glass rod of ×130 mm was obtained, and its refractive index (η _D ), coefficient of linear expansion (α), glass transition temperature (Tg), and deformation temperature (Td) were measured. The results are shown in Table 4.

[Table] As is clear from Table 4, the refractive index can be arbitrarily adjusted within the range of 1.519 to 1.513 by doping with AlF ₃ and it can be used as a low refractive index glass for cladding. Also,
Crystals begin to form inside the glass when 10 mol% of AlF ₃ is added. When a matrix glass other than glass number k is used, the refractive index changes are exactly the same, ranging from 1.519 to
It can be adjusted to any refractive index of 1.513.
However, in glass numbers t, u, v, and w, microcrystals were generated inside the glass rod when AlF ₃ was doped in the range of 2 to 8 mol %, and it was found that these glasses were unsuitable as base glasses. Experimental example 4 (PbF ₂ ) _2, 4,
A glass composition mixture doped at respective proportions of 6 and 8 mol % (for example, in the case of adding 4 mol % of PbF _{2 ,} 10 g of ZrF ₄ , 5.5 g of BaF ₂ , 0.814 g of GdF ₃ ,
A mixture of 0.971 g of PbF ₂ ) was introduced into a metal crucible, heated and melted at 850°C for 30 minutes using an electric furnace, and
It was casted into a brass mold with a hollow part of 10φ x 130mm heated to
A 130 mm glass rod was obtained, and its refractive index (η _D ), coefficient of linear expansion (α), glass transition temperature (Tg), and deformation temperature (Td) were measured. The results are shown in Table-5.

[Table] As is clear from Table 5, the refractive index can be arbitrarily adjusted within the range of 1.532 to 1.540 by doping with PbF ₂ , and it can be used as a glass rod for a high refractive index core. Also,
When 8 mol% of PbF ₂ is added, crystals begin to form inside the glass. When using a matrix glass other than glass number k, the refractive index changes are exactly the same, ranging from 1.532 to 1.540.
can be adjusted to any desired refractive index. However, for glass numbers t, u, v, and w, PbF ₂ is
It was found that when doped in a range of 6 mol %, microcrystals were generated inside the glass rod, making it unsuitable as a base glass. Experimental example 5 (BiF ₃ ) _2, 4,
A glass composition mixture doped at respective proportions of 6, 8, 12, 16, and 20 mol% (for example, in the case of adding 10 mol% of BiF ₃ , 20 g of ZrF ₄ , 11 g of BaF ₂ ,
A mixture of 1.628 g of GdF ₃ and 5.62 g of BiF ₃ ) was introduced into a metal crucible, heated and melted at 850°C for 30 minutes using an electric furnace, and a brass mold with a hollow part of 10φ x 130 mm was heated to 250°C in advance. After annealing and cooling, a glass rod of 10φ x 130mm was obtained, and its refractive index (η _D ), coefficient of linear expansion (α), glass transition temperature (Tg), and deformation temperature (Td) were measured. The results are shown in Table-6.

[Table] As is clear from Table 6, the refractive index can be arbitrarily adjusted from 1.528 to 1.562 by doping with BiF ₃ , and it can be used as a high refractive index core glass. Furthermore, when 20 mol% of BiF ₃ is added, crystals begin to form inside the glass, and its wide range of vitrification makes it a promising dopant for fluoride optical fibers. Even when a host glass other than glass number k is used, the refractive index changes in exactly the same way, and the refractive index can be adjusted to any desired value within an extremely wide range of 1.528 to 1.562. However, in glass numbers t, u, v, and w, microcrystals were generated inside the glass rod when BiF ₃ was doped in a range of 2 to 16 mol %, and it was found that these glasses were unsuitable as base glasses. Next, the present invention will be described with reference to Examples, but the present invention is not limited thereto in any way. Example ZrF ₄ (63 mol%)-BaF ₂ (33 mol%)-GdF ₃ (4
A glass tube for cladding (outer diameter 12φ x 130 mm, hollow part 7φ x 130 mm) was prepared by doping the matrix glass _{of ZrF 4 (} 63 mol%) -
A glass rod for the core (6.8φ x 100mm) was prepared by doping 6 mol% of BiF ₃ into a base glass of BaF ₂ (33 mol%) - GdF ₃ (4 mol%), and the glass rod for the core was inserted into the glass tube for the cladding. and by band melting
Draw the preform at 400-500℃, 2-6μ
refractive index difference 1.2%, transmission loss 0.2 to 0.5 dB/m (wavelength 3 to 4 μm), outer diameter 240 μm, with low loss window at m
We were able to fabricate an optical fiber for infrared transmission with a core diameter of 70 μm. Since the optical fiber of the present invention uses a fluoride raw material that is poorly soluble in water and has low toxicity, it is
It has extremely low hygroscopicity and toxicity compared to glass and fiber such as Tl (BrI) and ZnCl ₂ . As is clear from the above description, the infrared transmitting optical fiber preome of the present invention is an optical fiber preome capable of transmitting infrared rays using fluoride glass, which was completely unknown in the past. An optical fiber for infrared transmission that has a low-loss window in the band and is non-hygroscopic and non-toxic can be manufactured as a practical communication line. If the raw material is highly purified, there will be less influence of Rayleigh scattering in the 3-4 μm band, and there is an advantage that an optical fiber with extremely low loss can be produced. Furthermore, according to the present invention, the relative refractive index difference between the core and the cladding can be set arbitrarily within the range of around 1%, making it easy to manufacture single mode optical fibers, and since it transmits infrared rays, the core diameter can be set large, and the connection There are advantages that are advantageous to

Claims (1)

[Claims] 1 BaF ₂ 28-38 mol%, GdF ₃ 2-7 mol% and
The base glass is a glass having a composition within the range of 58 to 69 mol% ZrF ₄ , and the core is made of glass doped with 6 mol% or less of PbF ₂ or 16 mol% or less of BiF ₃ to the matrix glass. 24 mol% or less of LiF, 12 mol% or less of NaF in the matrix glass
Or, a preform for an infrared transmission optical fiber, characterized in that it is made of glass doped with 8 mol% or less of AlF ₃ .