JPH0467105A - Polarization maintaining optical fiber - Google Patents

Abstract

PURPOSE:To obtain the optical fiber which can be easily produced, can be reduced in cost and mass produced and has small crosstalks by constituting the fiber of a core having an elliptic sectional shape and a clad enclosing the core and lowering the softening point temp. of the clad lower than the softening point temp. of the core. CONSTITUTION:This optical fiber consists of the core 11 having the elliptic sectional shape and the clad 12 enclosing the core. The softening point temp. of the clad 12 is set lower than the softening point temp. of the core 11. Such optical fiber is formed by drawing a fiber base material consisting of the elliptic core material and the clad material. Since the softening point temp. of the core material is higher than the softening point temp. of the clad material, the axial stress remains in the core 11. Stress anisotropy is induced in the section of the optical fiber from this axial residual stress, by which the double refractive index of modes is generated. The optical fiber which can be easily produced, can be reduced in cost and mass produced and has the small crosstalks is obtd. in this way.

Description

[Detailed Description of the Invention] <Industrial Application Minute Hand> The present invention is directed to a transmission medium for a coherent optical transmission system, a transmission medium for an ultrahigh-speed transmission system, and a coupling between optical circuit elements having m-wave characteristics. The present invention relates to polarization-maintaining optical fibers required for applications such as polarization maintaining optical fibers.

<Conventional technology> The structure of the conventionally known polarization-maintaining optical fiber is
As shown in the figure. As shown in the figure, this polarization-maintaining optical fiber consists of a core 1 and a cladding 2, and has stress-applying parts 3 on both sides of a cough 11 at positions symmetrical to the core 11, and these stress-applying parts The portion 3 applies stress to the core 1 and the vicinity of the core 1 to impart anisotropy in the refractive index. In the polarization maintaining optical fiber of this structure, Y, 5asaki
rDe+tign and Fabricati by et al.
on of Low-Loss and Low Cr
oggtalk Polarization-Main
The paper entitled “taining 0ptical fibers” (IEEE Lightwave technology
logic, LT-4, No, 4.1097-110
2, 1986).

In order to produce such a polarization-maintaining optical fiber, as shown in FIG. After inserting and integrating the applying member 3A, it is necessary to draw the wire.

<Problems to be Solved by the Invention> As mentioned above, the conventional polarization-maintaining optical fibers shown in FIGS. 4 and 5 require processing such as drilling and polishing into the fiber base material 4. Due to the high cost, it has been difficult to obtain a polarization-maintaining optical fiber with good characteristics at a high yield because it is difficult to cut the price of the optical fiber, and processing such as drilling and polishing is difficult. Furthermore, in order to increase the modal birefringence and reduce crosstalk, it is necessary to use a material with a high coefficient of thermal expansion for the stress applying part, but such a material with a high coefficient of thermal expansion is difficult to obtain. There is also the problem of not being able to do so.

On the other hand, an optical fiber whose core has an elliptical cross-sectional shape and is not provided with the above-mentioned stress applying portion is also known to have polarization-maintaining properties, but it cannot be put to practical use at all.

In view of these circumstances, it is an object of the present invention to provide an inertia wave-maintaining optical fiber that is easy to manufacture, can be manufactured at low cost, can be mass-produced, and has a small four-stoke.

A wave-maintaining optical fiber according to the present invention that achieves the above object is an optical fiber comprising a core having an elliptical cross-sectional shape and a cladding surrounding the core. It is characterized in that its softening point temperature is lower than that of the core.

Effect> When a fiber base material consisting of a core material with an elliptical cross-sectional shape and a clad material surrounding this core material and whose softening point temperature is lower than that of the core material is drawn, the core material is Tension is applied, and axial stress remains in the core after drawing. This emphasizes the anisotropy of the core shape and increases the mode birefringence.

Example of implementation Below, the present invention will be explained based on examples.

FIG. 1 shows a cross section of a polarization maintaining optical fiber according to one embodiment. As shown in the figure, this polarization maintaining optical fiber is
A 111 with an elliptical cross-sectional shape and a cladding 1 surrounding it
2 and Karana 9, cladding 12 from the softening point temperature of core 11
has a lower softening point temperature.

Such a polarization-maintaining optical fiber is made by drawing a fiber base material consisting of an elliptical core material and a cladding material. Directional stress will remain.

That is, as shown in FIG. 2, the cross-sectional area of the core 11 is A.
1. The cross-sectional area of cladding 12 is A2°, and the tension when drawing is T
Then, the optical fiber axial stress σ2 in the cladding 12 is σ,=T/ (A, 10A,) The optical fiber axial stress σ, in the core 11 is c1=
cr, XA2/A.

Therefore, the area A1 of the core 11 is the area A2 of the cladding 12.
Since the stress is very small compared to , most of the stress remains in the core 11 after the wire is drawn. This axial residual stress induces stress anisotropy in the cross section of the optical fiber, resulting in mode birefringence.

In the present invention, the elliptical cross-sectional shape of the core refers to a core other than a circular shape and a non-axisymmetric shape, and is not limited to the above-mentioned elliptical shape.

Preferably, the shape has two axes of symmetry that are perpendicular to each other, such as an ellipse.

(Example 1) S i O2 as the core 11 and F- as the cladding 12
3iO□ was used, and the relative refractive index difference of the core 11 was set to 0.3%. The long axis of the elliptical core 11 is 8 μm, and the short axis is 1 μm.
．． The outer diameter of the optical fiber was 125 μm.

The produced polarization-maintaining optical fiber has a mode birefringence of 5 x 10 when the tension at the time of drawing the optical fiber is 20 g.
-' became. Further, the crosstalk was -30 dB and the loss was -0.3 dB/-, and polarization maintaining characteristics comparable to those of conventional polarization maintaining optical fibers were obtained.

(Example 2) The conditions were the same as in Example 1 except that the long axis of the elliptical core 11 was 8 μm and the short axis was 2.5 μm.

The produced polarization-maintaining optical fiber has a mode birefringence of 4.
X 10-', and the mode refractive index was smaller than that of Example 1 when the drawing tension was the same.

(Example 3) When the conditions were the same as in Example 1 except that the long axis of the elliptical core 11 was 8 μm and the short axis was 4 μm, the mode birefringence was as good as Example 2 and an even smaller value. .

In addition to the embodiments described above, for example, the core 11 is Aj,
O,・S i O2 and cladding 12 is F-Si02° Core 11 is S i O2 and cladding 12 is B2O3・Si
O2゜Core 11 is Aj203, cladding 12 is B2O3
- Even with SiO2, the softening point temperature of the core 11 is higher than the softening point temperature of the cladding 12, and the tension at the time of drawing remains in the core 11 after the optical fiber is drawn, and a similar effect can be expected.

Further, FIG. 3 shows the relationship between the optical fiber axial stress in the core 11 and the mode refractive index in the polarization maintaining optical fibers of Examples 1 to 3 described above. The figure shows stress analysis performed using the finite element method.

Regarding the finite element method, see K. Okamot.

Others (Koyoru, rStress Analysis of
Optiem! Fibers by a Finit
e Element Method J (I
EEE Journal of Quantum El
electronics QE-17, pages 2123-2129, 1981).

In the optical fiber of Example 1, the axial stress was 5 kg/
mm', a mode birefringence of 6X10' was obtained, but on the other hand, when the ellipticity was lowered as in Examples 2 and 3,
Even if the axial stress was the same at 5 kg/mm', the mode birefringence was as small as 4.3 x 10-'3.3 x 10-'.In Examples 1 to 8, when the drawing tension was 20 g, Since the axial stress is about 5 kg/mm'', a high modal birefringence can be obtained due to residual stress due to wire drawing.

As described above, the polarization maintaining optical fiber of the present invention can be made to have a high mode refractive index by appropriately selecting the tension during drawing.

<Effects of the Invention> As explained above, the polarization maintaining optical fiber of the present invention has the following effects:
It consists of an elliptical core and a cladding surrounding it, and the softening point temperature of the cladding is lower than that of the core, so tension remains on the core during drawing, which emphasizes the anisotropy of the core shape. As a result, the mode birefringence increases. In addition, the polarization-maintaining optical fiber of the present invention does not require the step of inserting a stress applying member unlike the conventional method, so it can be mass-produced.
It is easy to reduce the price.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a polarization-maintaining optical fiber according to one embodiment;
Fig. 2 is an explanatory diagram showing the principle of the polarization-maintaining optical fiber of the present invention, Fig. 3 is a graph showing the relationship between axial stress in the core and mode birefringence, and Fig. 4 is a polarization diagram according to the prior art. A cross-sectional view showing the holding optical fiber, and FIG. 5 are explanatory views showing the manufacturing process thereof. In the drawing, 11 is a core, and 12 is a cladding.

Claims (2)

[Claims] (1) A polarization-maintaining optical fiber comprising a core having an elliptical cross-sectional shape and a cladding surrounding the core, wherein the softening point temperature of the cladding is lower than the softening point temperature of the core. . (2) In claim 1, the core is SiO_2, the cladding is F.SiO_2, and the core is Al_2O_3.SiO_2.
The cladding is F.SiO_2, the core is SiO_2 and the cladding is B_2O_3.SiO_2, or the core is Al_2.
A polarization-maintaining optical fiber characterized in that it is O_3 and its cladding is B_2O_3·SiO_2.