JPH04317432A - Production of elliptic-core type polarization plane maintaining optical fiber - Google Patents

Abstract

PURPOSE:To produce a low-loss and high-extinction-ratio elliptic-core fiber in good yield. CONSTITUTION:A core glass preform 3 consisting of the circular-sectioned core 1 and clad 2 is firstly produced. The preform 3 is heated and softened, a side pressure is exerted to transform the cross section of the clad part into a rectangle, hence the core is transformed to form a rectangular glass preform 5 into an ellipse, and an elliptic core 4 is formed. The clad 6 is then ground so that the periphery of the rectangular preform 5 is made circular to form a ground glass preform 7. An externally applied glass layer 8 is formed on the preform 7 by VAD, etc., and the product is heated, melted and drawn into a fiber.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing an elliptical core type polarization maintaining optical fiber whose core has an elliptical cross-sectional shape.

0002

[Prior Art] Conventionally, one of the methods for manufacturing an elliptical core type polarization maintaining optical fiber (hereinafter referred to as elliptical core fiber) is as follows:
After softening a glass rod with a circular cross section by heating and making the cross section elliptical by applying lateral pressure, this glass rod is inserted into a glass tube, either as is, or a glass filler is filled in the gap between the glass tube and the glass rod. There is a known method in which the wire is heated and then drawn (Japanese Unexamined Patent Publication No. 57-205331).

0003

[Problems to be Solved by the Invention] However, the conventional method described above has the following problems.

(1) When a glass rod with an elliptical cross-section and a cylindrical glass tube are simply fused and integrated, the deformed glass rod returns to its original circular cross-section due to the presence of a gap. As the core returns to its shape, the cross-section of the core also decreases, making it impossible to obtain good polarization preservation characteristics.

(2) If a filler is filled in the gap between the glass rod and the glass tube, the glass rod,
Since air bubbles are trapped in the fused surfaces of the filler and the glass tube, the production yield of the glass base material is poor and the polarization preservation properties are also poor.

(3) In the process of fusing and integrating, the glass base material is heated to a high temperature (approximately 1850° C.) for a long time, so impurities in the quartz tube diffuse near the core, increasing transmission loss.

An object of the present invention is to provide a method for manufacturing an elliptical core fiber that overcomes the above-mentioned drawbacks of the prior art and can produce an elliptical core fiber with low loss and high extinction ratio with good yield.

[0008]

[Means for Solving the Problems] The method of manufacturing an elliptical core fiber of the present invention heats and softens a rod-shaped glass base material for an optical fiber having a circular cross section, consisting of a core and a cladding, and then changes the cross-sectional shape to a rectangular shape by applying lateral pressure. After being crushed into a shape, this rectangular glass base material is heated and drawn at a drawing tension of 50 g or more. Here, the rectangular shape has a shape close to an ellipse because the crushed and flattened portion becomes parallel long sides and the short sides remain rounded.

In this case, a rectangular glass base material formed by crushing is ground so that the cladding has a circular cross section with the core as the central axis, and then the glass base material that has been ground to have a circular cross section as it is or is After attaching the glass layer to the outer periphery again, heating wire drawing may be performed.

0010

[Operation] The present invention improves the ellipticity of the core by heating the glass base material which has been deformed into a rectangular cross-section by heating, or by grinding the cladding part into a circular cross-section with the core as the central axis and then drawing it by heating. It can be made larger and the manufacturing process can be simplified. From this, polarization preservation characteristics, transmission characteristics, and yield can be significantly improved.

When drawing a glass base material that has been deformed into a rectangular shape by heating, it is necessary to control the drawing tension so that the ellipticity of the core does not change. When drawing with a tension of less than 5 g, the flattened portion that is the deformed portion of the base material is almost completely eliminated, and the ellipticity of the fiber core is also almost completely eliminated. At a draw tension of approximately 10 g, the fiber's appearance returns to a fully circular shape, although the core of the fiber remains somewhat oval. At tensions greater than 20 g, the core and fiber appearance become only slightly circular, and increasing to 50 g yields an elliptical core fiber with almost the same shape as the glass preform before drawing.

[0012] In addition, since the elliptical core fiber has a large amount of GeO2 added to the core (relative refractive index difference △n of cladding reference is 1% or more) in order to improve the polarization preservation property, the dependence of Rayleigh scattering loss on the drawing tension is reduced. is large. For this reason, it is desirable to set the drawing tension to 100 g or more in order to achieve particularly low loss.

The glass composition of the cladding may be pure SiO2 or fluorine (F) to lower the processing temperature.
) may be added.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Examples of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 is an explanatory diagram showing the process of manufacturing an elliptical core fiber according to this embodiment.

First, a core glass preform 3 consisting of a core 1 having a circular cross section and a cladding 2 is manufactured (FIG. 1(a)). The core glass preform 3 may be manufactured by either the VAD method or the CVD method.

When this core glass preform 3 is heated and softened and lateral pressure is applied to deform the cladding portion into a rectangular cross section to form a rectangular glass preform, the core becomes oval due to the deformation of the cladding portion. , an elliptical core 4 is formed (FIG. 1(b)).

[0018] The heating temperature applied at this time is such that the viscosity is 107.6.
The optimum temperature is near Poise's softening point. To apply lateral pressure, it is sufficient to do as shown in FIG. 2 or 3. In the method shown in FIG. 2, the entire core glass base material 3 is heated uniformly, sandwiched between graphite plates or molybdenum plates 10 from above and below, and lateral pressure is applied to deform it. In the method shown in FIG. 3, the core glass preform 3 is heated sequentially from the end and rolled and deformed using a pressure roller 11 made of graphite or molybdenum.

Next, the rectangular glass preform 5 which has been deformed into a rectangular cross-section is heated and melted as it is to form a drawn fiber, or
Alternatively, the ground glass base material 7 is formed by grinding the cladding 6 so that the outer periphery of the rectangular glass base material 5 has a circular cross section (
After forming an external glass layer 8 on the outer periphery of the ground glass base material 7 by a VAD method or the like (FIG. 1(d)), if necessary, the glass layer 8 is heated and melted to form a drawn fiber. do.

As mentioned above, a glass tube can be made by heating a glass base material that has been deformed into a rectangular cross-section by heating, or by grinding the cladding part into a circular cross-section with the core as the central axis and then drawing it by heating. We decided to directly manufacture elliptical core fiber without using glass tubes,
As in the case where a filler is used to fill the gap between the core and the core, the deformed glass rod returns to its original circular cross-sectional shape during welding, resulting in a reduction in the cross-sectional area of the core. No air bubbles are trapped in the fused surface, good polarization preservation characteristics are obtained, and the manufacturing yield of the glass base material is improved. In addition, there is no need to heat the glass base material at high temperatures for a long time during the process of fusion and integration.
It is also possible to prevent impurities in the quartz tube from diffusing near the core and increasing transmission loss. Therefore, it can be widely used in optical fiber application systems such as optical gyros that require polarization preserving characteristics.

Next, a concrete example will be shown.

Example 1 First, GeO2-Si with Δn=1.5% was prepared by VAD method.
A glass base material 3 having a diameter of 20 mm and consisting of an O2 glass core and a SiO2 glass cladding was manufactured. The ratio D/a between the core diameter a and the cladding diameter D was set to 40.

Next, this glass base material 3 is heated and softened, and lateral pressure is applied, so that the short axis side diameter/long axis side diameter = 0.4 (FIG. 1(b))
A rectangular glass base material 5 was obtained. This rectangular glass base material 5 is drawn in a resistance heating furnace with a drawing tension of 100 g, and the outer diameter of the long axis side of the fiber is 125 μm, the outer diameter of the short axis side is 50 μm, and the ellipticity of the core (1 - core short axis diameter An optical fiber having a core of 0.65 was obtained. The loss of this fiber at a wavelength of 0.85 μm is 3 dB/km
The extinction ratio was -30 dB (converted to 1 km of fiber), which was good.

Example 2 By VAD method △n=1.5%, D/a=40, diameter 2
A 3 mm core glass preform 3 was produced, and lateral pressure was applied to deform it to a ratio of minor axis side diameter/long axis side diameter = 0.25.

Next, the cladding portion 6 was ground around the core so that the outer periphery of the rectangular glass preform 5 was completely round, thereby obtaining a ground glass preform 7 having a diameter of 10 mmφ. Furthermore, the diameter of this ground glass base material 7 is tripled by V
After externally attaching the clad glass layer 8 again by the AD method, this externally attached glass preform 9 was drawn to have an outer diameter of 125 μm to obtain an optical fiber with a core ellipticity of 0.76. The transmission characteristics of the optical fiber were the same as in Example 1.

[0026]

[Effects of the Invention] As described above, according to the present invention, the following effects can be obtained.

(1) Since the core can be formed into an ellipse by simple heating processing, it is possible to manufacture high performance elliptical core fibers with excellent processing accuracy and high yield.

(2) Since the core ellipticity of the fiber can be increased, the extinction ratio characteristic is good.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing an example of a process for manufacturing an elliptical core fiber according to the present invention.

FIG. 2 is an explanatory diagram showing the pressurization method of this embodiment.

FIG. 3 is an explanatory diagram showing another pressurizing method of this embodiment.

[Explanation of symbols]

1 Core 2 Clad 3 Core glass base material 4 Oval core 5 Rectangular glass base material 6　Grinding removal section 7　Grinded glass base material 8 External glass layer 9 External glass base material 10 Graphite plate (or molybdenum plate) 11 Pressure roller

Claims (2)

[Claims] Claim 1: A rod-shaped glass base material for an optical fiber with a circular cross section consisting of a core and a cladding is heated and softened, the cross-sectional shape is crushed into a rectangular shape by applying lateral pressure, and then this rectangular glass base material is wire-drawn. A method for manufacturing an elliptical core type polarization maintaining optical fiber, characterized by heating and drawing with a force of 50 g or more. [Claim 2] After grinding the rectangular glass base material so that the cladding has a circular cross section with the core as the central axis, the glass base material is ground as it is or on the outer periphery of the glass base material that has been ground so as to have a circular cross section. 2. The method for manufacturing an elliptical core type polarization maintaining optical fiber according to claim 1, wherein the elliptical core type polarization maintaining optical fiber is heated and drawn after the layer is externally attached.