JPS62270433A - Production of optical fiber preform - Google Patents

Abstract

PURPOSE:To prevent the contamination of core with OH group as far as possible, by preliminarily coating a transparent glass rod with a nitride glass and depositing glass soot on the coated rod. CONSTITUTION:A columnar transparent glass rod 20 is placed on a support 18 in a heating furnace 10, the furnace 10 is supplied with raw material gases 22 consisting of silane gas and ammonia gas and the temperature of the furnace 10 is maintained to 900-1,000 deg.C with a heater 12. A transparent glass rod 20' having nitride glass layer can be produced by this process. The glass rod 20' is taken out of the furnace 10 and glass soot 30 free from dopant is deposited on the glass rod 20' using an oxygen/hydrogen burner 26 as a heat source. The composite preform is heated to obtain an optical fiber preform.

Description

Detailed Description of the Invention 3. Detailed Description of the Invention and Field of Industrial Application 1 This invention relates to a method of manufacturing a low-loss optical fiber preform.

[Prior Art] Methods for manufacturing optical fiber preforms include the MCVD method, the external attachment method, and the VAD method.

In addition to these methods, glass to be the core is prepared in advance as a transparent glass rod, and fine glass powder is deposited on it, resulting in a composite preform of [transparent glass-fine glass powder]. (hereinafter referred to as composite preform) by turning it into transparent glass in a heating furnace.
There is a method for obtaining an optical fiber preform (hereinafter referred to as a composite method).

[Advantages of the composite method] (1) A clear core/cladding boundary can be created. Good controllability of dimensions when converted into optical fiber. In the case of a single mode optical fiber, the cutoff wavelength of the second mode can be easily controlled.

(2) When exposing the two-legged composite preform to an atmosphere containing a doping gas and mixing the dopant into the fine powder, unnecessary dopant does not enter the core.

for example, ・The core part is pure quartz containing Ge, The cladding part is pure quartz containing F, There is an optical fiber consisting of

When making this, if both the core and cladding are made into a preform in the form of fine glass powder and then exposed to an F-containing atmosphere, F will enter both the core and the cladding, and the required specific refractive index will be You can't get a rate difference.

Therefore, in order to obtain the necessary relative refractive index difference, it is necessary to increase the amount of Ge contained in the core. However, it is known that increasing the amount of Ge doping increases so-called Rayleigh scattering loss.

Although there are other dopants other than Ge that increase the refractive index, Ge is currently most commonly used.

[Problems to be Solved by the Invention] Although the present recombination method has the above-mentioned advantages, it also has the following disadvantages.

(1) Normally, fine glass powder is deposited using a burner. When hydrogen, natural gas, etc., which is the easiest to use, is used for combustion, the generated moisture (OH groups) diffuses into the interior from the surface of the transparent glass rod. . As a result, when it is made into a fiber, absorption loss due to OH groups occurs.

(2) Countermeasures against this OH group diffusion are difficult.

For example, even if a high-temperature plasma generated by high-frequency induction is used as a heating source, the device will be much larger than an oxygen/hydrogen burner or the like. Additionally, it becomes necessary to traverse the plasma torch in order to deposit the glass fine powder onto the glass rod.

Furthermore, even if the high-temperature plasma flame itself is anhydrous, the humidity of the atmosphere (usually air) is not zero, so some OH groups are inevitably mixed in.

(3) As mentioned above, in the composite method, O
Contamination of H groups becomes a serious problem.

Incidentally, since the OH groups mixed in the glass fine powder portion of the composite preform can be easily removed by heat treatment in a gas atmosphere containing halogen, there is no problem in this point.

The present invention aims to reduce the incorporation of OH groups into the core as much as possible even when a transparent glass rod is heated with a burner when producing an optical fiber preform by a composite method. .

Means for Solving Problem E] This invention is characterized by: A coating made of nitride glass is applied in advance to a transparent glass rod, and fine glass powder is deposited thereon.

[Explanation] (1) It is known that the diffusion of OH groups in quartz glass does not simply diffuse as water or OH, but behaves quite differently between 0 and H groups.

For example, "Glass Handbook" (published by Breakfast Shoten)
According to the authors, in experiments using polarized elements, it is stated that 0 is more than two orders of magnitude more difficult to diffuse than H.

That is, it can be said that the diffusion of OH groups in quartz glass is dominated by the diffusion of H produced by dissociation of OH groups, especially at high temperatures.

(2) Quartz glass has extremely high permeability for small molecules such as hydrogen, and has high permeability compared to other glasses for helium gas, which has small molecules similar to hydrogen. It is known to have.

(3) From this, when attaching fine glass powder generated by heating onto a transparent glass rod, in order to prevent OH groups from diffusing into the transparent glass rod, it is necessary to add hydrogen to the surface of the transparent glass rod. It turns out that it is sufficient to create a barrier against this.

(4) As a result of studies, the present inventors have found that it is effective to thinly coat the surface of a transparent glass rod with a nitride glass film.

[Nitride glass film coating method] In FIG. 1, 10 is the entire heating furnace, 12 is a heater, 14 is a furnace core tube (made of quartz glass), 16 is a thermometer, 17
is an exhaust port.

18 is a support, on which a transparent glass rod 20 is placed.

Maintain the furnace temperature at 800-100°C.

As source gases 22, silane gas (SiHo), ammonia gas (NHj), and nitrogen gas are introduced.

The composition ratio is NH3:S i H4=1:100NH3:N
2 = I:1000

Under the above conditions, 0.1 to 0.3
1 angstrom/sec, nitride glass (si3Na)
A film can be deposited.

The final thickness of this film is suitably about 500 angstroms.

Note that 02 may be added to the raw material gas 22 to change the nitride glass film to a mixed composition of s i and N4-5io.

Further, it is also possible to produce a nitride glass film having a composition other than St, for example, containing Ge or the like. In that case, germane (GeHn) or the like is used as the source gas.

[Subsequent process] The following is the same as before.

(1) Since a nitride glass film serving as a barrier against H is formed on the transparent glass rod 20, fine glass powder can be deposited using an oxygen/hydrogen burner.

An outline of the pattern is shown in Figure 2.

Reference numeral 20 is a transparent glass rod on which a nitride glass film is formed, and a dummy quartz glass rod 24 is connected to it and pulled up while rotating.

An oxygen/hydrogen burner 26 is used as a heat source.

For example, 5iC14 and Ar are used as the raw material gas 28 to deposit glass fine powder 30 made of 5i02 without a dopant.

(2) Thereafter, the powder is transparently vitrified in a heating furnace, for example, in an atmosphere containing F, and F is doped into the fine powder at that time.

[Example] (1) A pure quartz transparent glass rod 20 with an outer diameter of 81 pairs and a length of 400 mm was prepared and placed in a heating furnace IO of 8500 G (see Fig. 1).

The conditions in the furnace were maintained as follows: NH3:S iHa :1:10O NH3:N2 =l:1000. Heating time is approximately 40 minutes. As a result, a 513N4 film of about 500 angstroms was deposited on the surface.

(2) Next, glass fine powder 30 containing no dopant was deposited to a thickness of 80 mm using the oxygen/hydrogen burner 26 as a heat source (see FIG. 2).

The conditions at that time are: ・02 20 pairs/pair n * H22041/win @ Si Cl 4 250ec/sin pot Ar3
00cc/sin - Rotation speed 10-30 rpm - Pulling speed 1.2-1/■1n (3) Furthermore, this composite preform was heated at 16800C.
It was made into transparent glass in a heating furnace.

At that time, there were 5 moles of CF4 and 95 moles of CF4 in the furnace.
of helium gas was flowed. The CF4 gas is decomposed in the furnace to liberate active phase F. This F was doped into the quartz glass fine powder.

(4) As a result, the refractive index distribution of the finally obtained optical fiber preform had a step-like shape, as shown in FIG.

(5) In this case, the deposition density of the glass fine powder is approximately 1
It was set as low as /8. The reason is.

This is to ensure rapid diffusion of F into the glass fine powder.

(8) The outer diameter of the preform obtained as a result of the above operations was approximately 45 mm. From this number, the cladding/core diameter ratio is approximately 6.

Therefore, this preform was heated and stretched again.
The outer diameter was 15 mm.

Then, on the transparent glass rod, in the same way as above, ■
A series of steps of depositing pure quartz glass fine powder and heating to transparent vitrification in an F-containing atmosphere were repeated.

The outer diameter of the final preform was approximately 30 mm.

(7) Figure 4 shows the characteristics of the optical fiber obtained by drawing this preform.

This optical fiber becomes a single mode optical fiber at a wavelength of 1.3 g due to the core and relative refractive index difference settings.

The absorption loss peak at a wavelength of 1.39 g in the figure is due to OH groups remaining in the fiber. The absorption loss due to OH groups is about 1 dB/km, which means that the remaining OH groups in the core portion through which light effectively passes is on the order of 20 ppb.

(8) Normally, if the OII group loss peak value at 1.38 μ- is 2 dB/km or less, there is no effect of the tail of OH group absorption on the 1 and 3 Iho wavelengths, which are most used in optical communications. Therefore, the results of this example are at a level that is sufficiently practical.

[Comparative Experiment] For comparison, an optical fiber was produced using exactly the same process without applying nitride coating to the transparent glass rod that was the starting core member.

The results are shown in FIG.

The loss due to OH group is about 5 at a wavelength of 1.38 μm.
It was as large as 0 dB/1u+, and was originally unsatisfactory as a long-distance optical fiber. In other words, wavelength!
, 3 pm, the loss was about 2 dB/ka+.

E. Alternative Embodiments] (1) The method of the present invention is not necessarily limited to optical fibers with a pure quartz core and an F-doped quartz cladding.

The optical fiber may contain a dopant in its core, or may contain no dopant in its cladding.

(2) Alternatively, the optical fiber may include a dopant such as Ge in a part of the cladding.

For example, FIG. 6 shows the refractive index profile of a 1.55 p dispersion flat optical fiber.

In this example, in the portion other than the central core, layers of F-doped quartz glass and layers of quartz glass doped with both F and Ge are arranged alternately in a concentric cylindrical shape.

[Effects of the Invention] Since the -1- of the transparent glass rod is coated with nitride glass in advance and the fine glass powder is deposited on the -1-, OH groups are not absorbed into the transparent glass rod that is to become the core. contamination can be greatly reduced.

Therefore, using the most convenient oxygen/hydrogen burner, the combined process can be carried out to obtain high performance optical fibers.

[Brief explanation of drawings]

Figure 1 shows the transparent glass rod 1, which is the key point of the present invention.
Figure 2 is an explanatory diagram of the process of forming a nitride glass film on -, Figure 2 is an explanatory diagram of the process of crucible IIL of glass fine powder, and Figure 3 is a refractive index distribution diagram of the optical fiber obtained by the present invention. 'S4 is a wavelength characteristic diagram of the transmission loss of the optical fiber of the present invention, Figure 5 is a wavelength characteristic diagram of the transmission loss of the optical fiber according to the conventional method, and Figure 6 is the dispersion flat for 1.55 μm obtained by the present invention. Refractive index distribution diagram of type optical fiber. lO: Heating furnace 12: Heater 14: Furnace tube 16: Thermometer 20: Transparent glass rod

Claims (1)

[Claims] A method for producing an optical fiber preform by heating a composite preform obtained by depositing fine glass powder on the outside of a cylindrical transparent glass rod, comprising: A method for producing an optical fiber preform, comprising applying a coating made of nitride glass in advance and depositing fine glass powder thereon.