JPS636502B2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to polarization maintaining optical fibers.

Various structures have already been proposed as polarization-maintaining optical fibers, but the structure shown in FIG. 1 is currently the most popular.

This is a four-layer optical fiber with a core of
1, a cladding 12, an inner jacket 13, and an outer jacket 14.

The core 11 is made of SiO ₂ glass containing GeO ₂ ,
The cladding ₁₂ is _made of high purity _SiO2 , the inner jacket is made of _SiO2 glass containing _P2O5 and _B2O3 , and the outer jacket is made of _SiO2 glass.

Although this polarization-maintaining optical fiber has extremely excellent properties, it has had some difficulties in terms of radiation resistance.

The polarization preserving optical fiber can be manufactured by a soot deposition method similar to the VAD method, but the internal CVD method is well known in the world, and attempts have also been made to manufacture it using this method.

However, when manufactured using the internal CVD method, the center of the core was missing, resulting in a refractive index distribution as shown in FIG. 2, and satisfactory characteristics were not necessarily obtained.

In view of this situation, the present invention has excellent radiation resistance,
The object of the present invention is to provide a specific configuration of a polarization-maintaining optical fiber that can be easily manufactured by either the VAD method or the internal CVD method and has sufficiently satisfactory characteristics.

In other words, the gist of the present invention is to use a core of high-purity SiO ₂ in order to improve radiation resistance and eliminate the center dropout phenomenon caused by _the internal CVD method. The specific configuration for solving the drawbacks is limited in detail.

The configuration of the present invention will be specifically explained with reference to FIG. 3 showing one embodiment.

In FIG. 3, 31 is a core, 32 is a cladding, 33 is an inner jacket, and 34 is an outer jacket.

The core 31 is made of high-purity SiO ₂ and does not contain any particular dopant, and the cladding 32 is made of
Mainly composed of SiO ₂ and containing P ₂ O ₅ and fluorine, B ₂ O ₃
The inner jacket 33 has a structure that does not include
The outer jacket 34 is composed mainly of SiO ₂ and contains P ₂ O ₅ and B ₂ O ₃ , and the outer jacket 34 is composed mainly of SiO ₂ and does not contain any particular dopant.

Fluorine contained in the cladding 32 is a dopant for controlling the refractive index, and P ₂ O ₅ is a dopant that controls the refractive index and adjusts the viscosity.

Fluorine is a substance that can significantly lower the refractive index with a small amount compared to B ₂ O ₃ and the like.

In other words, even SiO ₂ glass containing 14 to 15 mol % of B ₂ O ₃ can only lower the refractive index of SiO ₂ by about 0.7%, and even if it contains 15 mol % or more of B ₂ O ₃ , the refractive index will decrease further. unable to lower the rate
Increasing the proportion of B ₂ O ₃ will actually increase the refractive index.

On the other hand, 1.5 mol % of fluorine can lower the refractive index by about 0.5%, and 3 mol % can lower the refractive index of SiO ₂ by about 1.0 %.

Therefore, it is extremely easy to form the cladding 32 as a layer with a low refractive index as shown in FIG.

If the refractive index distribution is as shown in Figure 4,
Compared to the case of the shape shown in FIG. 5, the thickness of the cladding 32 and the inner jacket 33 can be made thinner, and manufacturing is easier. That is, the fifth
In the case of the refractive index distribution shape shown in the figure, in order to obtain the same characteristics as the optical fiber having the shape shown in Fig. 4, the total thickness of the cladding 32 and the inner jacket 33 must be increased. First, whether these elements are made by the internal attachment method or the external attachment method, it is troublesome, and furthermore, since a predetermined elliptical shape must be created, the manufacturing conditions are considerably limited.

The amount of fluorine contained in the cladding 32 may be such that the refractive index of the cladding 32 is clearly lower than that of the inner jacket 33, for example, about 2 to 5 mol %. The amount of P ₂ O ₅ contained in the cladding 32 may be minute.

B ₂ O ₃ contained in the inner jacket 33 is a dopant added for the purpose of controlling the refractive index and imparting distortion, and P ₂ O ₅ is a dopant added for the purpose of adjusting the viscosity and controlling the refractive index.

The total amount of B ₂ O ₃ and P ₂ O ₅ is required to be 5 to 20 mol %, and suitably about 10 to 15 mol %.

This range is mainly limited by viscosity, and if the total amount of B ₂ O ₃ and P ₂ O ₅ is less than 5 mol%,
This is because the viscosity is high and it is extremely difficult to form the inner jacket 33 into an elliptical shape as shown in FIG.

In addition, if the total amount of B ₂ O ₃ and P ₂ O ₅ exceeds 20 mol%, when manufacturing by internal CVD method,
It is unsuitable because the viscosity is low and a layer with a higher viscosity inside cannot be deposited, and at the same time, 20 mol%
This is because if the value exceeds 100%, it becomes difficult to control the refractive index.

Furthermore, in the inner jacket 33
The ratio of B ₂ O ₃ is preferably in the range of 1.0 to 2.0 times that of P ₂ O ₅ . i.e. 1.0 for P ₂ O ₅
If it is less than twice that, it will not be possible to keep the refractive index clearly lower than that of the core 31 and the outer jacket 34, so it needs to be 1.0 times or more.

If the proportion of B ₂ O ₃ is significantly higher than that of P ₂ O ₅ and is 2.0 times or more, the difference in coefficient of thermal expansion will be large compared to the outer jacket 34, etc., and the viscosity will not be low enough to cause cracks. This is an undesirable condition in which it is easy to enter.

For the outer jacket 34, a generally commercially available industrial silica glass tube can be used, but it goes without saying that synthetic quartz or high-purity SiO ₂ may also be used.

The polarization-maintaining optical fiber of the present invention can be manufactured not only by an internal CVD method including depressurization, but also by an external CVD method, as disclosed in, for example, Japanese Patent Laid-Open No. 125233/1983. , V.A.D.
It can also be produced by the Rod Inch-Yub method, which is a mixture of these methods.

Furthermore, the polarization preserving optical fiber obtained in this manner is of course used for practical use by being coated with a predetermined resin composition, and the coating layer of the resin composition may be one or more layers. .

The polarization maintaining optical fiber as described above has the following remarkable effects.

(1) Since the core is made of high-purity SiO ₂ , it has good radiation resistance.

(2) Since the core is made of high-purity SiO ₂ that does not contain dopants, there is no center hole even when manufactured using the internal CVD method, and the characteristics are stable.

(3) Since the core is made of high-purity SiO ₂ , it has high viscosity and does not deform during the process of forming the inner jacket into an ellipse.

(4) Since the cladding does not contain B ₂ O ₃ , there is almost no absorption loss in the long wavelength band, and practical use in the long wavelength band is possible.

(5) Since the amounts of B ₂ O ₃ and P ₂ O ₅ contained in the inner jacket are appropriately selected, manufacturing is easy and reproducible whether by the internal method, external method, or other methods. Highly sexual.

(6) Since the refractive index of the cladding is lower than that of the inner jacket, the thickness of the cladding and the inner jacket can be made thinner even with the same characteristics, and manufacturing is easy and reproducibility is high.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing a conventional polarization-maintaining optical fiber, FIG. 2 is a diagram showing the refractive index distribution of a conventional polarization-maintaining optical fiber, and FIG. 3 is a cross-sectional view showing an embodiment of the present invention. , FIG. 4 is a diagram showing the refractive index distribution of an example of the present invention, and FIG. 5 is a diagram showing the refractive index distribution of a comparative example for comparison with the present invention. 31: core, 32: cladding, 33: inner jacket, 34: outer jacket.

Claims (1)

[Claims] 1. In a polarization-maintaining optical fiber having a core with a circular cross section, a cladding provided on the outer periphery, an inner jacket with an elliptical cross section provided on the outer periphery, and an outer jacket provided on the outer periphery. , the core is made of high purity _SiO2 ,
The cladding is made of glass containing SiO ₂ as a main component and P ₂ O ₅ and fluorine; the inner jacket is made of glass containing SiO ₂ as a main component and P ₂ O ₅ and B ₂ O ₃ ; The cladding contains a larger proportion of SiO ₂ than the inner jacket, and the refractive index of the cladding _{is lower} than that of the inner jacket.
The total amount with B ₂ O ₃ is 5 to 20 mol%, and
A polarization-maintaining optical fiber characterized in that the ratio of B ₂ O ₃ to P ₂ O ₅ is 1.0 to 2.0 times in mol%.