JPS6135525B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a low-loss, radiation-resistant, polarization-maintaining optical fiber.

An optical fiber for single-mode transmission.The cladding has an elliptical cross-section and anisotropic strain is created in the core, which increases the difference in refractive index in the major and minor axis directions, and therefore in the propagation constant, due to the photoelastic effect. However, a polarization-maintaining optical fiber that preserves the polarization plane is already well known, for example, from Japanese Patent Laid-Open No. 56-99306.

This structure has a cross section as shown in Figure 1.
The cladding 12, which has a layered structure and has an elliptical cross section, is made of SiO ₂ −B ₂ O ₃ glass or SiO ₂ +G ₂ O ₂ +
It has been pointed out that B ₂ O ₃ glass can be used.

The reason why B ₂ O ₃ -containing glass is used for the elliptical cladding 12 is because B ₂ O ₃ is a material that causes anisotropic strain.
This is because it is powerful. 11 is a core, and 13 is an outer jacket.

However, since B ₂ O ₃ suffers from large absorption loss in the long wavelength band of 1.2 μm or more, the use of polarization-maintaining optical fibers has conventionally been limited to short wavelength bands.

Therefore, for the core and the layer directly in contact with the core,
A structure that does not contain B ₂ O ₃ was invented.

It has a four-layer structure as shown in FIG. 2, and because it exhibits extremely excellent characteristics, it has become the mainstream of today's polarization-maintaining optical fibers.

In this four-layer structure, the core 21 is made of SiO ₂ +GeO ₂ glass, and the first cladding 22 in contact with the core 21
is SiO ₂ glass, the second cladding 23 having an elliptical cross section is SiO ₂ +B ₂ O ₃ +P ₂ O ₅ glass, and the outer third cladding 24 is SiO ₂ glass.

In general use, polarization-maintaining optical fibers with this structure do not cause any problems, but
When used in an atmosphere where radiation is present, transmission loss increases significantly due to the effects of radiation. Therefore, a new structure is required that has excellent radiation resistance and does not impair polarization preservation characteristics.

In view of this situation, the present invention has been developed to absorb absorption in a long wavelength band.
It is an object of the present invention to provide a polarization-maintaining optical fiber that has reduced loss, excellent radiation resistance, and is easy to manufacture.

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

In Figure 3, 31 is the core, which has high purity.
Consists of _SiO2 .

32 is a first clad with an elliptical cross section made of SiO ₂ +F glass and does not contain B ₂ O ₃ ;
A second glass with an elliptical cross section made of SiO ₂ + B ₂ O ₃ glass
A third cladding 34 is made of, for example, SiO ₂ glass containing some impurities.

Here, the amount of B ₂ O ₃ in the second cladding 33 is 5 to 15 mol %.

An example of manufacturing a polarization-maintaining optical fiber having such a configuration will be described below.

First, the third cladding has an outer diameter of 18 mm and a thickness of 1.5 mm.
A SiO ₂ +B ₂ O ₃ based glass film is attached to the inner wall surface of a mm transparent quartz glass tube.

It is important in production that the amount of B ₂ O ₃ at this time is 5 to 15 mol %.

A SiO ₂ +F-based glass film is attached to this inner wall surface as the first cladding, and a high-purity glass film is attached to the inner surface.
A film of SiO ₂ is applied.

After that, one end of the glass tube is sealed, the pressure inside the tube is reduced to -5 mmH ₂ O from the other end using a pressure reducer, and the glass tube is heated to approximately 1900℃ with an oxyhydrogen burner, and the burner is moved at a speed of 5 mm/min to solidify the glass tube. . The thus obtained solid base material is heated and drawn to obtain a polarization preserving optical fiber.

What is important here is that the amount of B ₂ O ₃ in the second cladding is 5 to 15 mol %.

That is, in order to form this second clad into an oval shape while keeping the inner core circular, it is necessary to
6 mol% is the critical viscosity, and if the doping amount is less than this, it is difficult to form an elliptical shape.

In addition, 15 to 16 mol% is the critical viscosity at which a high melting point glass layer can be stably formed inside this layer, and if the amount of B ₂ O ₃ is larger than this, it will be difficult to stabilize the core and transmission loss will increase. There is a drawback that it does.

The core is made of high-purity SiO ₂ , so it has high viscosity.
When crushing the second clad into an oval shape, the core is not crushed at the same time.

Although the manufacturing method using only the internal CVD method has been described above, the core rod may be manufactured by the above method up to the first cladding and then integrated by the RT method, or alternatively, the core and the first cladding may be integrated by the RT method.
It is also possible to manufacture a base material by manufacturing a composite rod of the cladding by another manufacturing method and integrating it with the glass tube manufactured by the above method up to the second cladding.

Depending on the amount of dopant in the first clad, the first clad can have either a circular or an elliptical shape, but in the case of a circular shape, the first clad has no particular effect on imparting anisotropy; If it is elliptical, both the second cladding and the second cladding are useful for imparting anisotropy.

Particularly in the case of the present invention, while the core is made of high-purity SiO ₂ and is hard, the first cladding is softer than the core and acts like a cushion. In the short axis direction of the cladding, the thinner the first cladding is, the more effective it is for imparting anisotropy.

The thickness of the first cladding is preferably about 1/2 to twice the core diameter.

The radiation-resistant polarization-maintaining optical fiber of the present invention described above provides the following remarkable effects.

(1) Since the core is made of high-purity SiO ₂ and does not contain dopants such as GeO ₂ or P ₂ O ₅ , there is almost no increase in transmission loss due to the effects of radiation, and it has excellent radiation resistance.

(2) Since the core is made of high-purity SiO ₂ and has a high viscosity, the shape will not collapse even when reduced pressure is applied to form an elliptical layer.

(3) Since the first cladding layer in contact with the core does not contain B ₂ O ₃ , there is almost no absorption loss in the long wavelength band.

(4) The second cladding has an elliptical shape and contains B ₂ O ₃ , which acts as a powerful anisotropic strain imparting material and has excellent polarization preservation properties.

(5) Since the B ₂ O ₃ layer in the second cladding is carefully selected, it can be easily manufactured by a general manufacturing method such as internal CVD method or a combination thereof, and the reproducibility of satisfactory characteristics is high.

[Brief explanation of the drawing]

1 and 2 are cross-sectional explanatory diagrams showing a conventional polarization-maintaining optical fiber, and FIG. 3 is a cross-sectional explanatory diagram showing an embodiment of the present invention. 〓〓〓〓
31: Core, 32: 1st Clad, 33: 2nd
Clad, 34: Third Clad. 〓〓〓〓

Claims (1)

[Claims] 1. A core having a circular cross section, a first cladding having an elliptical cross section outside the core, a second cladding having an elliptical cross section outside the core, and a third cladding having a circular cross section outside the core. And the core is of high purity
The first cladding is made of SiO ₂ +F glass, and the second cladding is made of SiO _{2 +}
A radiation-resistant polarization-maintaining optical fiber made of B ₂ O ₃ glass, characterized in that the amount of B ₂ O ₃ in the second cladding is 5 to 15 mol %.