JPS623402B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an optical fiber having a function of preserving the plane of polarization among optical transmission glass fibers.

[Prior art] As an optical fiber that preserves the plane of polarization, a four-layer structure as seen in Japanese Unexamined Patent Publication No. 57-37305 is already known, and it is superior to the three-layer structure that was originally developed. It exhibits the following characteristics.

In other words, the core contains dopants such as SiO ₂ and GeO ₂ to increase the refractive index, allowing a large difference in refractive index.

Since the first cladding is made of high-purity SiO ₂ , deformation of the core can be prevented and the effects of absorption loss due to B and OH groups can be prevented.

The second elliptical cladding has a low reaction temperature.
Since P ₂ O ₅ +B ₂ O ₃ +SiO ₂ glass is used, the influence of absorption loss due to OH groups can be prevented.

That is the characteristic.

However, with conventional known techniques, it has not been possible to obtain products with industrial stability and excellent properties, and there are still many unknown points in order for general third parties to manufacture them.

[Object of the Invention] In view of the above situation, the present invention has been developed based on the generally known methods (for example, Japanese Patent Application Laid-Open No. 50-120352, Japanese Patent Application Laid-Open No. 56-125233).
The purpose of the present invention is to provide a polarization-maintaining optical fiber that can be stably produced industrially and has even better characteristics using the internal CVD method of the present invention.

[Example] The configuration of the present invention will be specifically described with reference to the drawings showing one example.

FIG. 1 is a cross-sectional explanatory diagram, and 1 is a core;
The composition includes SiO ₂ and a dopant for increasing the refractive index, such as GeO ₂ .

2 is the first clad made of high-purity SiO ₂ , which allows the cladding contained in the second clad 3 to
This blocks the influence of B ₂ O ₃ and prevents the core 1 from breaking.

3 is a second cladding having an elliptical cross section;
Consists of P ₂ O ₅ + B ₂ O ₃ + SiO ₂ glass.

The proportion of P ₂ O ₅ in this second cladding 3 is 3
~10 mol%, and the proportion _{of B2O3} is 2.5-8 mol%
It is.

4 is a third cladding having a circular cross section, and is made of quartz glass containing no particular dopant.

As a result, the refractive index distribution becomes as shown in Figure 2, with core 1 having the highest refractive index and first cladding 2 having the highest refractive index.
The refractive index of the second cladding 3 and the third cladding 4 are approximately equal, and the refractive index of the second cladding 3 is adjusted to be lower than the core 1 and higher than the first cladding 2.

Due to the need to satisfy the above refractive index relationship, the ratio of P ₂ O ₅ and B ₂ O ₃ contained in the elliptical second cladding 3 is such that P ₂ O ₅ is slightly larger than B 2 O 3, and the ratio is significantly higher than that of B ₂ O ₃ . There can be no variation. That is, when B ₂ O ₃ exceeds P ₂ O ₅ , the refractive index of the second cladding 3 becomes lower than that of the first cladding 2, and the above relationship is no longer satisfied.

And the total amount _{of P2O5} and _{B2O3 is} 5.5-18 mol%
This is an important manufacturing requirement.

In other words, if the total amount is less than 5 mol%, the viscosity of the second cladding 3 will be high, making it difficult to form this layer into a predetermined elliptical shape, and even the core will be damaged if the layer is forcibly shaped into an elliptical shape by reducing pressure or the like. Since it deforms, approximately 5.5 mol% or more of P ₂ O ₅ and B ₂ O ₃ are required.

Furthermore, if the total amount of P ₂ O ₅ and B ₂ O ₃ exceeds 18 mol%, the viscosity decreases, making it difficult to deposit the first clad 2 and core 1 inside the second clad 3 using the internal deposition method. . Especially when the total amount of P ₂ O ₅ and B ₂ O ₃ is 20
It becomes extremely difficult to exceed this amount by mol%.
It is desirable that the content be 18 mol% or less.

If the total amount of P ₂ O ₅ and B ₂ O ₃ is 18 mol% or less, the first
It is also easy to deposit the cladding 2 and core 1.

[Comparative Example] Comparative Example 1 2 mol% of P ₂ O ₅ contained in the second cladding,
A polarization-maintaining optical fiber with a four-layer structure was prototyped with the B ₂ O ₃ content being 2 mol % and the other conditions being the same as in the example.

In this case, the viscosity of the second clad is high, so it is difficult to form it into an elliptical shape, and if the amount of pressure reduction is intentionally increased, even the core deforms irregularly, so the ellipticity that can be formed is about 20%. .

Therefore, the bond length is also 16mm (measured at a wavelength of 0.63μm)
It was poor quality.

Comparative Example 2 2 mol% of P ₂ O ₅ contained in the second cladding,
The content of B ₂ O ₃ was 9 mol %, and the rest was the same as in the example.

In this case, since the refractive index of the second cladding is extremely low, the propagating light propagates through the first cladding as if it were part of the core, resulting in a very poor transmission loss of 15 dB/Km (measured at a wavelength of 1.3 μm). It was hot.

Comparative Example 3 P ₂ O ₅ contained in the second cladding was 11 mol%,
The B ₂ O ₃ content was 2 mol %, and the rest was the same as in the example.

In this case, the refractive index of the second cladding became too large, and the cladding mode propagated to the second cladding, making it impossible to function as a polarization-maintaining optical fiber.

Comparative Example 4 P ₂ O ₅ contained in the second cladding was 11 mol%,
When we tried to manufacture the glass with 9 mol% B ₂ O ₃ , the glass membrane flowed as the burner moved during the process of internally attaching the membrane that would become the second cladding.
A uniform glass film could not be formed.

[Effects of the Invention] The polarization-maintaining optical fiber of the present invention described above provides the following remarkable effects.

(1) Since the composition of the first clad and the second clad is appropriate, the deformation of the core, B,
It has excellent characteristics without problems such as absorption loss due to OH groups, etc.

(2) Since the composition of the second cladding is quantitatively appropriate, it can be manufactured easily and industrially stably using the well-known internal CVD method, and has high reproducibility and reliability. .

[Brief explanation of the drawing]

FIG. 1 is an explanatory cross-sectional view showing one embodiment of the present invention, and FIG. 2 is a diagram showing a refractive index distribution. 1: core, 2: first cladding, 3: second cladding, 4: third cladding.

Claims (1)

[Claims] 1 It has a core containing SiO ₂ and a dopant for increasing the refractive index, and high-purity SiO ₂ is placed around the outer periphery of the core.
and has a first cladding having a circular cross section;
A second cladding made of P ₂ O ₅ +B ₂ O ₃ +SiO ₂ glass and having an elliptical cross section is provided on the outer periphery of the cladding.
In a polarization-maintaining optical fiber having a third cladding having a circular cross section on the outer periphery of the cladding, the proportion of P ₂ O ₅ in the second cladding is 3 to 10 mol %, and the proportion of B ₂ O ₃ is 3 to 10 mol %.
The proportion of is 2.5 to 8 mol%, and the total amount of P ₂ O ₅ and B ₂ O ₃ is
5.5 to 18 mol %, and the refractive index of the second cladding is lower than the refractive index of the core and higher than the refractive index of the first cladding.