JPH07112932B2 - Method for producing functional quartz optical fiber base material - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to a method for producing a functional quartz optical fiber preform, more specifically, it can be used for an optical fiber type laser, an optical isolator, etc. The present invention relates to a method for producing a high-performance functional silica optical fiber preform with little contamination.

[Technical background of the invention]

Conventionally, rare earth elements and some transition metal elements (Cr, etc.)
As a method for producing a functionalized silica glass, which contains impurities such as Fe and Cu, and has extremely low contamination with OH groups and other impurities, which is as low as that of a quartz optical fiber, the MCVD method (Ref.SBPool e
t al. Electoron Lett. Vol 21 pp737 ~ (1985), M.WATA
NABE et al. P11-, IOOC-ECOC prce. (1985)) and P
CVD method (H.NAMIKAWA, JJAP, Vo123, ppL409 (198
4)) etc. are known.

For example, the MCVD method has a sub-chamber 4 adjacent to a hollow quartz tube 1 as shown in FIG. 1, and while the main barter 6 is reciprocated, the raw materials SiCl ₄ , O ₂ , GeCl _4, etc. A raw material is introduced from one open end of the quartz tube 1, and a glass thin film is deposited on the inner surface of the quartz tube 1 by a thermal oxidation reaction to form a synthetic clad layer 2. After that, the sub-burner 5 is operated to vaporize the trace component raw material (for example, NdCl ₃ , ErCl ₃ etc.) M, take it into the glass thin film to be deposited, and deposit the synthetic core layer 3 containing the trace component raw material M. , Manufactures optical fiber preforms.

In such a method, since most of the conventional methods for producing an optical fiber preform can be used, there is an advantage that a glass preform having a low OH group concentration and an impurity concentration can be easily produced. There is a drawback that it is difficult to obtain a base material or a rod-shaped base material (a pure quartz glass layer is not included around the base material).

On the other hand, according to the PCVD method shown in FIG. 2, SiCl ₄ (glass raw material) in the saturator 7 held at a predetermined temperature by a temperature controller is introduced into the trace ingredient material holding pipe 19 using Ar gas as a carrier, and the heating furnace Along with the trace amount raw material (for example, NdCl ₃ ) in the trace component raw material boat 12 vaporized by 8, the gas is introduced into the reaction chamber from the nozzle 9. The raw material gas causes an oxidation reaction by the plasma flame 10 generated by the plasma gas inlet 11 and the plasma generating RF coil 17, and synthesizes Nd-doped quartz 14 on the quartz rod 13. The remaining raw material gas is exhausted from the exhaust pipe 15 through the heat exchanger 18.

In such a method, the temperature of the plasma flame 10 (which is said to be in the range of thousands to tens of thousands of degrees) is extremely high. Therefore, in order to avoid sublimation of the synthesized oxide glass and deposit the base material, plasma It is necessary to precisely control the positional relationship between the flame 10 and the target. Further, there is a drawback in that it is necessary to stabilize the flow of the raw material and the working gas in the plasma torch in order to stabilize the plasma flame.

On the other hand, VAD, which is one of the typical manufacturing methods for optical fibers
Consider the synthesis of the base metal in the method. In the VAD method, vapor-phase SiCl ₄ is supplied into an oxyhydrogen flame, and the gas particles generated in the flame are deposited on the end surface of a rotating seed rod, and a porous glass body is grown in the axial direction of the seed rod. Then, the obtained porous preform is subjected to transparent vitrification in an inert atmosphere containing a dehydrating agent to produce an optical fiber preform.

When synthesizing a base material containing rare earth elements or some transition elements using this VAD method, in order to obtain a sufficient vapor pressure because the melting point of the rare earth chloride or transition metal chloride material used is high, 1000 It is necessary to keep the raw material at a high temperature of ℃ or higher (for example, the temperature of NdCl ₃ which gives a saturated vapor pressure of 1 mmHg is 1048 ℃). As a result, in the base metal synthesis by the VAD method, it was necessary to keep all the pipes warm at about 1000 ° C, and it was actually difficult to synthesize the base metal.

As a method to solve this point, after synthesizing a porous base material by the VAD method, a method of adding by vapor phase diffusion by processing in a rare earth chloride raw material atmosphere in an electric furnace during transparent vitrification treatment is known. (Japanese Patent Application No. 54-083505). However, in the present invention, since the rare earth element once added is volatilized again as a chloride in order to perform the dehydration treatment after the diffusion treatment, there is a drawback that it is difficult to add it at a high concentration.

[Outline of Invention]

The present invention has been made in view of the above points, solves the disadvantage of adding a high melting point trace component (rare earth element) element by the VAD method, and has a large size that cannot be synthesized by the conventional MCVD and PCVD methods. It is an object of the present invention to provide a method for producing a functional quartz optical fiber preform that enables the production of a preform.

Therefore, the method for producing a functional quartz optical fiber preform according to the present invention is a method for producing a functional quartz optical fiber preform in which a porous body produced by the VAD method is heat-treated and a rare earth element is added to (a) in advance. A first step of treating the porous body in an inert gas atmosphere containing a dehydration gas in a core tube in which a solid raw material containing a rare earth element is arranged; (b) the dehydration in the core tube A second step of replacing the gas-containing inert gas with a pure inert gas and holding the same for a certain period of time, (c) moving the solid raw material containing the rare earth element along a temperature gradient formed in the core tube It is characterized by comprising the third step of adjusting the temperature of the solid raw material and heating it to make the porous body into a transparent glass.

According to the method for producing a functional quartz optical fiber preform according to the present invention, a porous body is formed in advance by the VAD method, and after the porous body is dehydrated, a trace component raw material (rare earth element) is added. Therefore, there is an advantage that the above-mentioned trace component raw material does not volatilize, and therefore a base material to which a high concentration of trace component is added can be manufactured. Also, VA
Since the porous body is produced by the method D, there is an advantage that the base material can be made large.

[Detailed Description of the Invention] According to the method for producing a functional quartz optical fiber preform according to the present invention, a porous glass body is first produced by the VAD method.

This porous glass body is not basically limited in the present invention, and any porous glass body for a quartz optical fiber preform manufactured by the VAD method may be used.

This porous glass body is treated in an inert gas atmosphere containing a dehydrating gas, and then heat treated in an inert glass atmosphere.

Conventionally, the addition of the trace component raw material has been carried out by the steps of synthesizing a porous body, adding a trace component raw material, (oxidation), dehydration, and vitrification.

In the present invention, since the steps of synthesizing the porous body → dehydration → heat treatment → adding the minor component raw material and vitrification are performed, volatilization of the minor component raw material does not occur.

That is, taking Nd ₂ O ₃ addition as an example, first, NdC
After diffusing into the porous body with l ₃ (gas), Nd ₂ O ₃ added to the porous body by oxidation reacts with Cl ₂ which is generally used as a dehydrating agent in the next dehydration treatment step. ,
Once formed, Nd ₂ O ₃ volatilizes. This is shown by the following reaction formula.

Nd ₂ O ₃ (s) + 3Cl ₂ (g) → 2NdCl ₃ (g) +3/20 ₂ (g) As a result, it becomes impossible to add rare earths to a high concentration, which is what our inventors It was also confirmed by experiments.

On the other hand, according to the method of the present invention, since the dehydration treatment is performed prior to the addition of the rare earth element, the problem of volatilization of the rare earth element due to the above reaction does not occur.

This dehydration treatment can effectively use the dehydration method used in the conventional technique of this kind. For example, it can be performed by heating in an atmosphere containing a dehydrating agent such as a halogen-containing gas.

The temperature of this dehydration treatment and the subsequent heat treatment in an inert gas atmosphere is preferably 1400 ° C. or lower.
When dehydration and heat treatment were performed at temperatures higher than 1400 ° C., as is clear from Example 4 below, the bulk density of the porous body became so high as to be unsuitable for diffusion addition of the trace component raw material, and the trace component raw material was This is because there is a risk that it cannot be added.

Following this dehydration treatment, heat treatment is performed in an inert gas atmosphere in the present invention.

This heat treatment step is performed to replace the dehydrating agent with an inert gas and control the partial pressure of the dehydrating agent in the transparent vitrification step. Therefore, the amount of addition of the trace component can be adjusted by changing the time of this heat treatment. That is, when the partial pressure of the dehydrating agent (for example, chlorine) is large, it easily volatilizes and the addition amount is small, and when the partial pressure of the dehydrating agent is small, the addition amount is large (see Example 2).

The micro component is added to the porous body by exposing the porous body that has been dehydrated and heat-treated as described above to an inert gas atmosphere containing a raw material of the micro component.

The trace element raw material can be a rare earth element such as Nd, Er, Vb, Ce.

In the step of adding the minor component, the amount of the minor component added can be adjusted by adjusting the temperature of the above-mentioned minor component raw material (Example 1).

Example 1 FIG. 3 is a schematic view of an apparatus for carrying out the method of the present invention. As is clear from this figure, this device has a structure including an electric heater 20 and a quartz furnace core tube (having a gas inlet and a gas outlet) 22 equipped with a rotatable and vertically movable seed rod 21. Has become.

A porous body 23 is deposited on the seed rod 21 of the quartz core tube 22, and a boat 24 containing NdCl ₃ is installed at the bottom of the core tube 22. The temperature distribution in the quartz furnace tube 22 is shown on the right side of FIG.

First, a pure quartz porous base material was produced by the VAD method. The conditions for producing this porous matrix were SiCl ₄ (40 ° C, carrier Ar gas 100cc) with an oxyhydrogen burner (H ₂ 2.9 min, O ₂ 6.0 / min)
And the growth rate was 42 mm / hour.

This porous base material was attached to the soaking part of the quartz furnace tube 22 in the electric furnace. NdCl ₃ raw material is a quartz boat placed at the bottom of the quartz core tube 2
The temperature of the NdCl ₃ contained in the boat 4 was adjusted by changing the vertical position of the boat 24. That is, according to the temperature distribution shown on the right side of FIG. 3, there is a portion where the temperature changes vertically in the bottom portion of the core tube 22. Therefore, the temperature of NdCl ₃ can be changed by moving the boat 24 up and down corresponding to this temperature change portion.

Next, while flowing He gas at 5 / min into the core tube 22,
The furnace temperature was raised to 00 ° C. Next, He (5 / min),
After keeping it at 1000 ℃ in a mixed atmosphere of Cl ₂ (70 cc / min) and dehydrating it for 1 hour, it was further 48 in a He (5 / min) atmosphere.
Held for hours.

Finally, the temperature was raised to 1500 ° C. in an atmosphere of He (5 / min). The heating rate is 500 ℃ / hour from 0 to 1000 ℃, 1000
Up to 1500 ° C / min was 200 ° C / hour.

FIG. 4 shows the relationship between the NdCl ₃ temperature and the amount of Nd added. As is clear from FIG. 4, it was found that the addition concentration of Nd ³⁺ ions can be uniquely controlled by adjusting the NdCl ₃ temperature.

Example 2 Using the same apparatus as in Example 1, the temperature of the electric furnace was controlled by the program shown below.

The temperature of the NdCl ₃ raw material was constant at 1400 ° C. Fifth
The figure shows the relationship between heat treatment time and the amount of Nd added. As is clear from FIG. 5, after the Cl ₂ dehydration treatment, 1000
Nd after vitrification can be adjusted by adjusting the holding time at ℃
It was found that the concentration of added ³⁺ ions could be changed. on the other hand,
The residual OH group concentration in the present invention was about 50 ppb.

Example 3 First, a pure quartz porous base material having an outer diameter of 20 mmφ and a length of 30 cm was synthesized by the VAD method. Next, Nd-added vitrification treatment was carried out by holding for 2 hours by the method as shown in Example 2.
The base material after vitrification had a diameter of 10 mm, a length of 17 cm, and an Nd ³⁺ ion addition concentration of about 100 ppm. After forming a porous layer (pure quartz) serving as a clad on the base material by an external attachment method, F-containing transparent vitrification was performed in an electric furnace in a mixed atmosphere of He and SF ₆ . The relative refractive index difference between the core and the clad was 0.3%. Next, using a graphite resistance furnace, the cutoff wavelength was 0.
The wire was drawn so as to have a thickness of 85 μm.

FIG. 6 shows the loss wavelength characteristic of the manufactured optical fiber. 1
At a wavelength shorter than μm, the loss is 1000 dB / Km, but at 1.0 to 1.3 μm, a sufficiently low loss of about 4 dB / Km is achieved, and by the method according to the present invention, high quality is achieved.
It was found that a Nd-added base material can be manufactured.

Example 4 Using the porous base material produced under the same conditions as in Example 1, dehydration, Nd addition and transparent vitrification treatment were performed using the same electric furnace. However, the following method was used as the dehydration treatment.

First, place the porous base material in the soaking part of the electric furnace and
Dehydration was performed under a mixed atmosphere of He (5 / min) and Cl ₂ (70 cc / min) up to ℃. The temperature raising condition is 50 at 0 to 1000 ° C.
0 ° C./hour, and 1000 to 1400 ° C., 200 ° C./hour.

Then, hold at 1400 ° C in He (5 / min) atmosphere for 48 hours, then 1500 ° C in NdCl ₃ and He (5 min) mixed atmosphere.
Up to 200 ° C./hour. The temperature of NdCl ₃ was 1400 ° C.

As a result of quantitative analysis of the amount of Nd added after the treatment by chemical analysis,
Nd was found to be the detection limit (1 ppm or less). This indicates that the bulk density of the base material increases to a value unsuitable for the diffusion addition of Nd when subjected to dehydration and heat treatment at temperatures above 1400 ° C.

〔The invention's effect〕

As described above, according to the method for producing a functional quartz optical fiber preform according to the present invention, the porous preform synthesis in the conventional VAD method, the dehydration step of the porous preform and the dopant material for imparting the functionality By separating the addition, there is an advantage that the rare earth element can be easily added. Further, there is an advantage that the dehydration can be sufficiently performed because the porous base material is used.

[Brief description of drawings]

FIG. 1 is a schematic view showing a main part of a conventional base metal synthesizing apparatus by the MCVAD method, FIG. 2 is a schematic view of a conventional base material synthesizing apparatus by the PCVAD method, and FIG. 3 is for carrying out the method of the present invention. FIG. 4 is a schematic view of an example of the apparatus of FIG.
A diagram showing Nd ³⁺ ion addition concentration and NdCl ₃ raw material temperature dependence,
FIG. 5 is a diagram showing the relationship between Nd ³⁺ ion addition concentration and treatment time in Example 2, and FIG. 6 is a diagram showing loss wavelength characteristics of the optical fiber in Example 3. 1 ... Support quartz, 2 ... Synthetic clad layer, 3 ...
Synthetic core layer, 4 ... Trace component raw material chamber, 5 ... Secondary burner, 6 ... Main burner, 7 ... Saturator, 8 ... Trace component raw material heating furnace, 9 ... Raw material blowing nozzle, 10 ... Plasma flame , 11 ...... Plasma glass inlet, 12 ...... Minor component raw material port, 13 ...... Quartz rod, 14 ...... Synthetic quartz (minor component added), 15 ...... Exhaust pipe, 16 ...... Temperature controller,
17 ... RF coil for plasma generation, 18 ... Heat exchanger, 19 ...
… Micro component feed tube, 21 …… Electric furnace heater, 21 …… Seed rod, 22 …… Quartz core tube, 23 …… Porous base material, 24 …… NdCl
₃ raw material boats.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hiroyuki Suda Inventor Hiroyuki Suda 162 Shirahane, Shikatakata, Tokai-mura, Naka-gun, Ibaraki Prefecture, Nippon Telegraph and Telephone Corporation, Ibaraki Telecommunications Research Laboratories (72) Masaharu Horiguchi, Naka-gun, Ibaraki Prefecture No. 162, Shirane, Shirokata, Tokai-mura (56) References, Ibaraki Telecommunications Research Institute, Nippon Telegraph and Telephone Corporation JP-A-61-40831 (JP, A) JP-A-57-170831 (JP, A) JP 56-155036 (JP, A) JP-A-60-231432 (JP, A)

Claims (4)

[Claims] 1. A method for producing a functional quartz optical fiber preform in which a porous body produced by the VAD method is heat-treated to add a rare earth element, wherein (a) a core tube in which a solid raw material containing a rare earth element is arranged in advance. In the first step of treating the porous body under an inert gas atmosphere containing a dehydrating gas, (b) a pure inert gas containing the dehydrating gas in the core tube A second step of substituting with a gas and holding for a certain period of time, (c) moving the solid raw material containing the rare earth element along a temperature gradient formed in the furnace core tube, adjusting the temperature of the solid raw material, and heating the solid raw material; A method for producing a functional quartz optical fiber, which comprises a third step of converting a porous body into a transparent glass. 2. The temperature of the first step and the second step is 1400 ° C.
The method for producing a functional quartz optical fiber preform according to claim 1, wherein: 3. The functional silica optical fiber preform according to claim 1, wherein the treatment time of the second step is changed in order to control the added concentration of the rare earth element. Manufacturing method. 4. The temperature of the solid raw material containing the rare earth element is changed in the third step in order to control the additive concentration of the rare earth element. A method for producing a functional quartz optical fiber preform according to any one of the above.