JPS63225543A - Production of base material for optical fiber - Google Patents

Abstract

PURPOSE:To produce the titled base material free from a structural fault, by forming at least one of the dehydrating atmosphere of a gaseous compd. contg. fluorine and the transparent vitrifying atmosphere by means of the reducing atmosphere and introducing a porous glass body thereinto and subjecting it to dehydration and transparent vitrification. CONSTITUTION:A porous glass body 1 for an optical transmission body which consists of a porous glass layer 2 for a core and a porous glass layer 3 for a cladding and has SiO2 as an essential component is inserted into a reactor core pipe 7 provided with a heating oven 4 and an electric heater 8 to the outer circumference. After the inside of the reactor core pipe 7 is regulated to the dehydrating atmosphere incorporating both a gaseous compd. contg. fluorine selected from among SiF4, CF4, SF6, CClF3, CCl3F and reducing gas selected from among CO, SO, NO, H2, and D2 and the porous glass body is heated with the heater 8 and dehydrated, this is subjected to transparent vitrification in the transparent vitrifying atmosphere contg. the above-mentioned reducing gas if necessary.

Description

DETAILED DESCRIPTION OF THE INVENTION Industrial Application Field 1 The present invention relates to a method for manufacturing a preform of an optical fiber used in the field of optical transmission.

“Conventional technology 1: Systems that apply optical fibers are not only used for public communications, but also for
It is used in a wide range of fields including instrumentation of various plants, and its optical fiber base material is widely used in VAD, O
Manufactured by VD method and MCVD method.

Optical fiber preforms manufactured by each of these methods yield optical fibers with low loss close to the theoretical limit, but in order to obtain optical fibers with even lower losses, sufficient control of the process in the manufacturing methods described above is required. , requires close examination.

In particular, when the optical fiber is used at a wavelength of 1.1 Lm, the tail of the absorption peak due to OH groups (wavelength 1.391 Lm) is affected, so it is necessary to reduce loss in the manufacturing process of the optical fiber base material. Dehydration treatment is essential.

Generally, as a means of dehydrating a porous glass body (predecessor of an optical fiber base material) manufactured by the WAD method or OVD method, the porous glass body is often dehydrated in a chlorine gas atmosphere, but fluorine compound gas It has been reported that dehydration of the porous glass body in an atmosphere containing the above is effective in lowering the transparent vitrification temperature of the porous glass body.

That is, when a porous glass body is dehydrated in an atmosphere containing fluorine compound gas, fluorine permeates into the porous glass body, so that the subsequent transparent vitrification temperature is lowered.
The operating cost of a transparent vitrification furnace (electric furnace) can be reduced.

"Problems to be Solved by the Invention" However, when a porous glass body is subjected to dehydration treatment in an atmosphere containing fluorine compound gas, structural defects occur during the treatment process, which causes problems.

In other words, when an optical fiber is made from an optical fiber base material obtained by dehydrating and vitrifying a porous glass body and using this as a core wire to construct an optical cable, even a small amount of H2 molecules will occur within the optical cable. , the H2 molecules diffuse into the optical fiber and react with the structural defects, and an absorption peak (wavelength 152% layer) appears due to increased hydrogen loss.

Therefore, the above optical fiber lacks long-term stability.

In view of the above-mentioned problems, the present invention seeks to provide a method for manufacturing an optical fiber preform that can suppress the occurrence of structural defects that cause increased hydrogen loss.

In order to achieve the intended purpose of the present invention, a porous glass body for an optical transmission body containing S t02 as a main component is dehydrated and made into transparent vitrification under heating conditions. In the method for obtaining an optical fiber preform, the dehydration atmosphere is formed by a compound gas containing at least fluorine, and at least one of the dehydration atmosphere and the transparent vitrification atmosphere is formed by a reducing atmosphere, and It is characterized in that the porous glass body is placed inside and the porous glass body is dehydrated and made into transparent vitrification.

r Effect 1 In the dehydration treatment of the present invention, a dehydration atmosphere is formed using a compound gas containing fluorine. can be lowered.

Furthermore, in the present invention, at least one of the dehydration atmosphere and the transparent vitrification atmosphere is formed in a reducing atmosphere, so that the optical fiber preform obtained by processing the porous glass body in the atmosphere is An optical fiber with no structural defects and no absorption peak at a wavelength of 1.52 #L, that is, an optical fiber with long-term stability, can be obtained from the optical fiber preform.

The reasons for this will be explained below.

When a porous glass body is treated in a compound gas containing fluorine, peroxy linkage (P
s+xoxy Linkage) occurs, and the conoperoxy linkage remains even at the stage of the optical fiber preform.

(1) When the optical fiber base material is spun (heated and drawn), the peroxy linkage causes the reaction of formula (2) and breaks.

1si-0-0-! 3i Mi +5i-0-0°+st=
+'e...= (2) If a hydrogen molecule exists here, the equation (3) will be obtained.

=st+ 1/2 H2-+=Sil(a*aas**
＊＊＊＊＊＊＊＊++ (3) =siH in the above has an extremely large absorption peak near the wavelength 4.4#L intestine, and the third harmonic of its stretching vibration is at the wavelength 1.527Ll.
giving rise to an absorption peak of l.

In the method of the present invention, the porous glass body is further exposed to an atmosphere containing He and a reducing agent (eg, CO).

In such a case, the peroxy linkage in the porous glass body after being dehydrated by the above formula (1) is reduced and changed by the reaction of formula (4).

=st-o-o-si=+co→=si-0-9i=+
CO2...Φ...(0 or peroxy linkage changes to 1si-0-8imi).

Such Mi5i-0-8ii bond is extremely strong;
During the above-mentioned spinning, the bonds are hardly broken.

This is the case in electron spin resonance (ESR) measurements.
1si-0'! This can be estimated from the fact that Jiii is hardly detected.

In this way, since the =Si-0-9iEi bond is not broken during the above-described spinning, =St is not generated in the optical fiber preform, even if hydrogen enters the preform.

=SiH does not occur.

Therefore, when producing an optical fiber from the optical fiber preform produced by the method of the present invention, it is possible to suppress the occurrence of the intestinal absorption peak of wavelength 1.52#L, which is thought to be caused by SiH, and to maintain long-term stability. An optical fiber is obtained.

Example 1 An example of the method of the present invention will be described below with reference to the drawings.

FIG. 1 illustrates a means for dehydrating a porous glass body and converting it into transparent vitrification.

In FIG. 1, a quartz-based porous glass body 1 consists of a porous glass layer 2 for a core and a porous glass layer 3 for a cladding.

As an example, the core porous glass layer 2 is made of 5i02-Ge.
The porous glass layer 3 for cladding is made of pure Si02.

In FIG. 1, a heating furnace 4 is comprised of a quartz core tube 7 having a gas inlet 5 and a gas outlet 6, and an electric heater 8 provided around the outer periphery of the furnace core tube 7.

The optical fiber preform 11 shown in FIG. 2 is obtained by processing the above-mentioned porous glass body 1 by the method of the present invention, and the optical fiber preform 11 includes a core glass 12 and a cladding glass 13. ing.

FIG. 3 illustrates a method (WAD method) for producing the porous glass body 1 described above.

In FIG. 3, the burner 21 for core synthesis and the burners 22, 23, and 24 for cladding synthesis each have a multi-tube structure (quadruple tube structure), and a predetermined gas supply system is installed at their base end. is connected.

In FIG. 3, the exhaust pipe 25 is made of porous glass! , are arranged at predetermined positions corresponding to the burners 21 to 24, etc., and the burners 21 to 24 and the exhaust pipe 25 are assembled into a reaction vessel (not shown) as is well known.

In the WAD method shown in FIG.
When making 5iC14 in the burner 21 for core synthesis.
, GeCl4,12 e02 , Ar, etc. are supplied,
5il for each burner 22, 23, 24 for cladding synthesis
141H2102.

Ar, etc. are supplied, and each of these burners 21% 22.23
Glass particles generated by the flame hydrolysis reaction of each gas supplied to the 5i02-Ge
Porous glass W for core made of O2 (Δ・Soda 0.3K)
! 3. A porous glass layer for cladding consisting of 2 and pure 5if2. That is, a porous glass body l is produced.

When the porous glass body l is for a single mode, the porous glass layer 2 for the core and the porous glass for the cladding are arranged so that the ratio of the glass 12 for the core and the glass 13 for the cladding after transparent vitrification is 1 nia. The outer diameter ratio with layer 3 is set.

When the porous glass body 1 produced by the method described above is dehydrated and made into transparent vitrification by the method shown in FIG. At the same time, this atmosphere, that is, the inside of the furnace tube 7 is maintained at a high temperature by an electric heater 8.

The porous glass body 1 is inserted into such an atmosphere, dehydrated and made into transparent glass, and becomes an optical fiber preform 11.

Next, specific examples of the present invention and comparative examples thereof will be explained.

Specific Example When a porous glass body 1 was dehydrated and made into transparent vitrification by the means shown in FIG. 1, it was carried out in the following manner.

During the dehydration process, the electric heater 8 maintains the highest temperature part in the furnace core tube 7 at 800°C, and the dehydration atmosphere in the furnace core tube 7 is adjusted to He: 241/■in and CO:
1 mjl/sin and 5iFs: 1101/sin, and the porous glass body l was drawn down at a speed of 150 m/h.
The lubricant l was dehydrated by moving the lubricant in a vacuum cleaner.

At the stage of transparent vitrification, the temperature of the above-mentioned highest temperature part of the furnace tube 7 is set to 1.
While maintaining the temperature at 400° C., a transparent vitrification atmosphere in the furnace tube 7 was formed with He: 24 mm/cm, and the base material 1 after dehydration was transformed into transparent vitrification at the above-mentioned drawing speed.

After the optical fiber preform 11 obtained through these dehydration steps and transparent vitrification steps was reduced in diameter by heating and drawing, it was jacketed with an anhydrous synthetic quartz tube to obtain a preform rod for spinning.

When spinning an optical fiber from such a preform rod, the spinning temperature is approximately 2000°C and the drawing speed is 80 l.
(2) An optical fiber was spun with a tensile force of 15 g for 1 lil, and the optical fiber immediately after spinning was coated with an ultraviolet curable resin.

The outer diameter of the single mode optical fiber thus obtained was 125μ.
The outer diameter of the coating was 400, the final layer was φ.

As described above, single mode coated optical fibers were made from three preform rods, and the cutoff wavelengths of these optical fibers were measured.
It was within the range of 1.2 to 1.3μ.

In the above, from each optical fiber obtained by spinning,
Take a 1000cm length each and heat them at 100°C.
The sample was placed in a hydrogen atmosphere, treated for 30 minutes, taken out, and left in the air (room temperature) for 24 hours.

After that, the wavelength-loss characteristics of each of the above optical fibers were measured using a monochromator, and the wavelength 1.52
When it was confirmed whether or not an absorption peak of 1 Lm appeared, no absorption peak was observed in any of the optical fibers.

As explained in the above formulas, this indicates that there is no structural defect in the optical fiber that reacts with hydrogen to generate the absorption peak.

Comparative Example When Miki's single mode coated optical fiber is made through these processes, such as dehydration and transparent vitrification of a porous glass body, preparation of a preform rod, and optical fiber spinning and coating, transparent vitrification is performed. The same procedure as in the specific example was carried out except that the atmosphere was not a reducing atmosphere.

The cutoff wavelength of these optical fibers is 1
．． It was within the range of 2 to 1.3 #L■.

Further, from each optical fiber obtained by spinning, 1,000 cm lengths were taken out, treated with hydrogen in the same manner as in the specific example, and left in air (room temperature) for 24 hours. The loss characteristics were measured using a monochromator to confirm whether or not the absorption peak of the IL layer at a wavelength of 1.52 appeared in these optical fibers, and it was found that the absorption peak appeared in all of the optical fibers.

From the appearance of the above absorption peak, it can be presumed that these optical fibers have a structural defect.

In the case of the method of the present invention, the porous glass body to be subjected to the dehydration treatment and the transparent vitrification treatment may be produced by an OVD method or a sol-gel method.

In the case of the method of the present invention, the dehydration treatment may be performed first and the transparent vitrification treatment may follow, or the dehydration treatment and the transparent vitrification treatment may be performed simultaneously.

In addition, in the above embodiment, the dehydration atmosphere was formed using a compound gas containing fluorine, and a reducing gas such as CO gas was added to the dehydration atmosphere to create a reducing atmosphere. The vitrification step may be performed in a reducing atmosphere.

In addition to 5iFn, the dehydration atmosphere of the method of the present invention contains CF4
.

It is sufficient if it contains one or more compound gases containing fluorine, such as SF6, CG12F2, CGIh, and CChF.

The transparent vitrification atmosphere of the method of the present invention also includes CO gas,
It is sufficient if it contains one or more arbitrary reducing agents such as SO gas and NO gas.

In the case of the present invention, a carbon tube may be used as the furnace core tube of the heating furnace.

“Effect of the Invention 1 As explained above, the method for manufacturing an optical fiber preform according to the present invention forms a dehydration atmosphere using a compound gas containing at least fluorine, and at least - and are formed in a reducing atmosphere, and the porous glass body is placed in this atmosphere to dehydrate and vitrify the predetermined porous glass body, so the transparent vitrification temperature is low, of course. It is possible to suppress the occurrence of structural defects that cause increased hydrogen loss.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram schematically showing an embodiment of the method of the present invention, and FIG.
The figure is a sectional view of an optical fiber preform manufactured by the method of the present invention, and FIG. 3 is an explanatory diagram schematically showing an example of manufacturing a porous glass body used in the present invention. l... Porous glass body 2... Porous glass layer for core 3... Porous glass layer for cladding 4...
・Heating furnace 5...Heating furnace gas inlet 6...Heating furnace gas outlet a...Heating furnace core tube 8...Heating furnace electricity Heater 11...Optical fiber base material 12...Glass for core 13...Glass for cladding Agent Patent attorney Yoshio Saifuji Figure 3 Procedure correction teeth 1988 1 29th of the month

Claims (5)

[Claims] (1) In a method for obtaining an optical fiber base material by dehydrating and transparent vitrifying a porous glass body for an optical transmission body mainly composed of SiO_2 under heating conditions, the dehydration atmosphere is replaced with a compound gas containing at least fluorine. and at least one of the dehydration atmosphere and the transparent vitrification atmosphere is formed by a reducing atmosphere,
A method for producing an optical fiber preform, which comprises placing the porous glass body in such an atmosphere to dehydrate and turn the porous glass body into transparent vitrification. (2) After placing a porous glass body in a dehydration atmosphere and dehydrating the porous glass body, the porous glass body is placed in a transparent vitrification atmosphere to vitrify the porous glass body into transparent glass. A method for manufacturing an optical fiber preform according to claim 1. (3) The light according to claim 1, wherein a porous glass body is placed in an atmosphere that shares a dehydration atmosphere and a transparent vitrification atmosphere, and dehydration and transparent vitrification of the porous glass body are simultaneously performed. Method for manufacturing fiber base material. (4) Dehydration atmosphere is SiF_4, CF_4, SF_6,
The method for manufacturing an optical fiber preform according to any one of claims 1 to 3, which contains one or more of CCl_2F_2, CCl_F_3, and CCl_3F. (5) Claims 1 to 3, wherein the reducing atmosphere contains one or more of CO gas, SO gas, and NO gas.
A method for manufacturing an optical fiber preform according to any one of Items 1 to 3.