JP2624985B2 - Manufacturing method of high refractive index difference micro core base material - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for producing an optical fiber preform having a small core diameter and a high refractive index difference between a core and a clad, and relates to a highly efficient fiber Raman laser or the like. A fiber preform having a large nonlinear effect can be produced.

<Prior Art> Conventionally, silica glass fiber, GeO ₂ as the core material
A quartz glass to which glass is added in an amount of 2 to 10 mol% is produced by a MCVD method or a VAD method using a pure silica glass as a cladding material and a quartz tube as a starting tube. Here, the Raman scattering coefficient of GeO ₂ glass added to the core that transmits and traps light is 9.2 times larger than that of SiO ₂ glass (FLGaleener et al., Appl. Phys. Lett., Vol. 32,
P.34,1978), however, in ordinary optical fibers, the Raman characteristics or nonlinear effects that depend almost on the SiO ₂ glass material are dominant, and therefore the nonlinear effects such as the Raman scattering constant are very small. Therefore, in order to generate stimulated Raman scattered light using a normal optical fiber, a large optical power is incident on the fiber, or stimulated Raman generated with pump light using a low-loss optical fiber of 1 to 10 km is used. It was necessary to lengthen the interaction length with the scattered light.

In addition, an optical fiber by the VAD method using a GeO ₂ glass other than the SiO ₂ glass as a core material and a SiO ₂ doped GeO ₂ glass as a cladding material has a stimulated Raman scattered light with a lower pump incident power than a normal optical fiber. (K. Nakamura, et, al., 12thEuropean Conference on
Optical Communication Ecoc'86-11-14). However, since this optical fiber has a large core diameter of 26 μm and a small difference in refractive index of 0.56%, it is difficult to generate stimulated Raman light with a low-power laser such as a semiconductor laser, and other nonlinear efficiency is also small. Was.

Meanwhile, as a method for manufacturing a high refractive index difference small core fiber to pure GeO ₂ glass core, and the P ₂ O ₅ serving as GeO ₂ glass and the cladding material as a core material was added 20ωt% Si
O ₂ glass rods were prepared by the VAD method, and the center of the clad material was perforated to form a tube. The tube was inserted into the core, and a Pyrex (registered trademark of Corning) tube was formed on the outermost periphery as a second clad. (Hosaka et al., Proceedings of the 1987 IEICE National Conference on Semiconductor Materials, P.2-207). However, since this method uses the rod-in-tube method, there is a disadvantage that the fibers are not uniform in the length direction and it is difficult to reduce the loss.

<Problems to be Solved by the Invention> The present invention does not require a large incident power and a long fiber to generate a nonlinear effect such as stimulated Raman scattering, and is particularly small and easy to use like a semiconductor laser. To produce a preform for optical fiber that is uniform in the length direction and low in loss, which can generate stimulated Raman scattered light or generate high-efficiency nonlinear effects (such as the Kerr effect) with a relatively low-power laser. The purpose is to provide a method that can

<Means for Solving the Problems> The configuration of the present invention for achieving the object is to hold and rotate the starting tube horizontally and to introduce a glass raw material gas and oxygen for an optical fiber into the glass tube, An oxidation reaction is caused by heating the glass tube from the outside or the inside, and a layer of a clad glass and a core glass is sequentially deposited on the inner wall of the glass tube, and then the glass tube is solidified.
In the method for producing an optical fiber preform by an MCVD method or a PCVD method, a borosilicate glass tube or an aluminate glass tube is used as the starting tube, and SiO ₂ containing P ₂ O ₅ and B ₂ O ₃ is used as the cladding glass. glass, depositing a glass doped with fluorine in SiO ₂ glass or the glasses containing SiO ₂ glass or B ₂ O ₃ containing P ₂ O _5, pure GeO ₂ glass as the core glass, the SiO ₂ glass GeO ₂ glass added, or fluorine and SiO ₂ glass added
GeO ₂ glass is deposited.

Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows an example of an apparatus for implementing the method of the present invention. As shown in FIG. 1, as an example of borosilicate glass, both ends of a Pyrex tube 3 are rotatably and horizontally held by a chuck 4 as a support, and have an inner diameter of 17 mm, an outer diameter of 20 mm, and a length. The height is 1500mm. A source gas supply unit 1 is connected to one end of the Pyrex pipe 3 via a pipe 2. The source gas supply unit 1 includes SiCl ₄ , GeCl ₄ , PC
Liquids of l ₄ and BBr ₃ are stored in each container, and can be transported by evaporating by supplying Ar gas into the liquid.
In addition, O ₂ gas and SF ₆ gas are stored, and temperature control and flow rate control are possible. These source gases and the like are supplied into the Pyrex pipe 3 via the pipe 2. On the other hand, the other end of the Pyrex tube 3 is rotatably connected to a trap 8 via a rotary connector 7.
The gas that has passed through the trap 8 passes through the solenoid valve 9 and the nozzle
It will be exhausted from 10. Furthermore, Pyrex tube 3
A heating source 5 such as an oxyhydrogen flame burner is disposed below the tube, and a pipe diameter measuring unit 6 is provided adjacent to the heating source 5. The heating source 5 and the tube diameter measuring unit 6 are Pyrex tubes. 3 can be moved. The displacement control unit 11 controls the opening and closing of the solenoid valve 9 based on the signal from the pipe diameter measuring unit 6.

The method of the present invention is implemented as follows using the apparatus having the above configuration.

First, the Pyrex tube 3 supported by the chuck 4 is
Rotate at about rpm and feed gas 1
And only O ₂ gas is flowed at a rate of 500 cc / min. In this state, the heating source 5 is reciprocated several times (about 5 to 10 cm / min).
Then, the Pyrex tube 3 is heated to a temperature of about 800 ° C. and baked. The baking is performed to make the inner wall of the pipe very smooth. When a Pyrex tube is heated above its softening point, it shrinks concentrically and becomes thinner. This pipe diameter is measured by the pipe diameter measuring unit 6 and compared with the lower limit pipe diameter set in the displacement control unit 11 in advance. When the pipe diameter of the Pyrex 3 is larger than the lower limit pipe diameter, the displacement control unit 11 opens the solenoid valve 9 and discharges the gas flowing through the pipe without resistance.

On the other hand, when the pipe diameter is smaller than the lower limit pipe diameter, the exhaust amount control unit 11 closes the solenoid valve 9, so that the gas outlet in the pipe is only a small gap of the nozzle 10. Since the O ₂ gas is constantly supplied into the Pyrex tube 3 from the raw material gas supply unit 1, the pressure in the tube increases, and the heated portion of the Pyrex tube 3 expands to overcome the surface tension to be stored. When the expanded pipe diameter matches or becomes larger than the set value, the displacement control unit 11 opens the solenoid valve 9 and
Reduce the pressure in the tube to atmospheric pressure. By performing such an operation in the longitudinal direction of the Pyrex tube 3 together with the movement of the heating raw material 5, the shape is adjusted to match the set value.

Next, from the raw material gas supply unit 1 together with 500 cc / min of O ₂ gas
12 ℃, 200cc / min SiCl ₄ bubbled with Ar gas, 12 ℃,
PCl _{3 at} 200 cc / min, BBr ₃ at 12 ° C. and 200 cc / min are flowed into the Pyrex tube 3,
Heat to ℃ to cause an oxidation reaction, containing P ₂ O ₅ and B ₂ O ₃ as glass to be clad, that is, clad glass
A glass layer of SiO ₂ (hereinafter abbreviated as P ₂ O ₄ / B ₂ O ₃ / SiO ₂ ) is deposited on the inner wall (about 17 mm) of the Pyrex tube. By reciprocating the heating source 5 about 20 times, the above glass layer becomes approximately
The thickness is 36 μm.

Subsequently, GeCl ₄ at 12 ° C. and 50 cc / min is flowed into the Pyrex tube together with O ₂ gas, heated to about 1000 ° C. by the heating source 5 to cause an oxidation reaction, and a glass serving as a core, that is, GeO ₂ glass as a core glass Further layers are deposited. By reciprocating the heating source 5 about five times, the thickness of the GeO ₂ glass layer becomes about 0.4 μm.

Thereafter, the heating source 5 is slowly moved, the inflow from the displacement control unit 11 is stopped, the heating source 5 is heated to about 1000 to 1100 ° C., and the heating source 5 is reciprocated about three times to be solidified, and the cylindrical base material ( The diameter was about 11 mm and the length was 500 mm). In the present invention, a Pyrex tube is used as a starting tube instead of a conventional quartz tube, and the Pyrex glass has a softening point close to that of the Pyrex glass.
P ₂ O ₅ / B ₂ O ₂ / SiO ₂ glass and pure GeO ₂ glass are used as a cladding material and a core material, respectively. For this reason, solidification could be achieved at a low temperature. If a conventional quartz tube is used as a starting tube, pure GeO ₂ glass deposited in the quartz tube at the time of solidification (about 1900 ° C.) sublimates, and a core cannot be formed.

As both ends of the manufactured base material cool from high temperature to room temperature,
Pyrex glass, P ₂ O ₅ / B ₂ O ₂ / SiO ₂ glass and pure GeO ₂
This is a portion where cracks are likely to occur due to thermal stress depending on the difference in linear expansion coefficient of glass. Therefore, both ends of the base material were stretched as shown in FIG. As shown in FIG. 3, the diameter of the core 12 is about 0.5 mm, and the diameter of the clad 13 is about 5 m.
m. In addition, as shown in FIG.
The refractive indices n of 2, clad 13, and Pyrex glass were 1.61, 1.458, and 1.474, respectively. Therefore, the refractive index difference Δ =
(N ₁ −n ₂ ) / n ₁ (n ₁ : refractive index of the core, n ₂ : refractive index of the clad) was as high as 9.44%.

It is difficult to prepare a base material having the same composition as the base material of the present invention by the VAD method. The reason is that cracks due to thermal stress based on the difference in linear expansion between pure GeO ₂ glass and P ₂ O ₅ / B ₂ O ₂ / SiO ₂ glass are less likely to occur as the core diameter is smaller. V that makes it difficult to reduce the core diameter to 1 mm or less
This is because in the AD method, after being heated to be transparent vitrified, a crack is generated by thermal stress at the time of cooling. Also, it is difficult to match the softening temperature and the linear expansion coefficient of pure GeO ₂ glass and P ₂ O ₅ / B ₂ O ₂ / SiO ₂ glass.

In the above embodiment, P ₂ O ₅ / B ₂ O ₂ /
SiO ₂ glass was used, but instead of P ₂ O ₅ / SiO
₂ , B ₂ O ₂ / SiO ₂ or glass obtained by adding a small amount of fluorine thereto can also be used. Further, as the core glass, instead of the pure GeO ₂ glass of the above embodiment, only SiO ₂ or Si
GeO ₂ glass to which trace amounts of O ₂ and fluorine are added can also be used. The manufacturing method is the same as the above procedure. Further, in the above embodiment, the MC using the oxyhydrogen burner as the heating source 5 was used.
Although the VD method has been used, a PCVD (plasma CVD) method can be used instead. For example, the pressure inside the Pyrex tube is reduced to 10 ^-7 Torr, the microwave generated at a power of 200 to 1500 watts is used to heat the Pyrex tube using a microwave cabidi as a heating source 5 and a microwave generator of 2.45 GHz to generate plasma. The core layer and the clad layer may be deposited while the inner surface of the tube is heated by the method. In this way, the outside temperature of the Pyrex tube does not rise, thus reducing tube shrinkage (Fredy Weli
ng J. Appl, Phys Vol. 57, no. 99, PP. 4441-4446, 1985).

<Effect of the Invention> As described above in detail with reference to the examples, the method of the present invention can reduce the core diameter of the base material to about 0.5 mm, which is caused by the difference in linear expansion coefficient between the core glass and the clad glass. Low thermal stress and no cracks. Also, MCVD
Since the method is a closed tube system, there is an advantage that the loss can be reduced.

Furthermore, when an optical fiber is manufactured from the preform according to the method of the present invention, when the refractive index difference is 8 to 9% and the wavelength used is 0.5 to 1.2 μm, single mode operation becomes possible with a core diameter of about 0.6 to 1 μm. A power density about 100 times higher than that of a conventional silica single mode optical fiber can be achieved from the same light source. Therefore, in addition to the increase in nonlinear effects represented by Raman scattering,
There is an advantage in improving the resolution of laser processing, a laser scalpel, an image guide, and a thermal image guide.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing one example of an apparatus for carrying out the method of the present invention, FIG. 2 and FIG. 3 are longitudinal sectional views of the prepared base materials,
FIG. 4 is a graph showing a refractive index distribution. In the drawing, 1 is a raw material gas supply section, 2 is a pipe, 3 is a Pyrex pipe, 4 is a chuck, 5 is a heating source, 6 is a pipe diameter measuring section, 7 is a rotary connector, 8 is a trap, 9 is a solenoid valve, 10 Is a nozzle, 11 is a displacement control unit, 12 is a core, 13 is a clad, and 14 is Pyrex glass.

Claims (1)

(57) [Claims] An oxidizing reaction is generated by rotating a starting tube while keeping the starting tube horizontal, introducing a glass raw material gas for optical fiber and oxygen into the glass tube, and heating the glass tube from the outside or the inside. After sequentially depositing a layer of a clad glass and a core glass on the inner wall of the glass tube, and solidifying the glass tube, in a method of producing an optical fiber preform by an MCVD method or a PCVD method, a borosilicate glass tube is used as the starting tube. Or using an alumina silicate glass tube, containing P ₂ O ₅ and B ₂ O ₃ as the cladding glass
SiO ₂ glass, depositing a glass doped with fluorine in SiO ₂ glass or the glasses containing SiO ₂ glass or B ₂ O ₃ containing P ₂ O _5, pure GeO ₂ glass as the core glass, SiO ₂ A method for producing a small core material with a small difference in refractive index, comprising depositing GeO ₂ glass to which glass is added or GeO ₂ glass to which fluorine and SiO ₂ glass are added.