JPS5834405A - Polarization preserving optical fiber - Google Patents

Abstract

PURPOSE:To prevent the increase of the coupling loss and to reduce the scattering loss, by attaching two glass rods having a larger coefficient of thermal expansion than a glass rod at both sides of this glass rod and covering these glass rods with a glass tube. CONSTITUTION:A glass rod 13 having a high coefficient of thermal expansion is attached and melt-stuck at both sides of a glass rod having a core 11 and a cladding layer 12. These glass rods are covered with a glass coat 14. Then these glass rods are heated at a high temperature, and the rod 13 contracts greatly when it is cooled down to the normal temperature from a high temperature. As a result, the core 11 is turned into the double refractive glass. A polarization preserving fiber is obtained owing to the double refractive properties. In this case, the coupling loss does not increase with reduction of the scattering loss since the core 11 is kept circular.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a polarization-maintaining optical fiber used in optical communications, optical sensors, and the like. Optical fibers that guide light while preserving polarization are promising optical transmission lines for use in optical heterodyne communications, fiber optic gyros, pressure sensors, and the like. Conventionally developed polarization-maintaining optical fibers have an elliptical core and an elliptical cladding layer, and the stress applied to the core is distributed in two directions (hereinafter referred to as the X direction and the X direction) within the cross section of the optical fiber. Therefore, they were able to achieve this by making the core glass have birefringence. Making the core circular causes an increase in the coupling loss between the fiber and the laser and the coupling loss between the fibers, and this method is less effective in improving polarization preservation, making it difficult to obtain a polarization preservation optical fiber. It is not a good idea as a method.The Kuzidom 5 is made into a 7-circular shape, and the stress applied to the core is
The method of making the core different in direction does not increase the coupling loss between the fiber and t/-d or the coupling loss between the fibers, and is highly effective in improving polarization preservation, since the core is made circular.
In order to generate a large stress difference between the X direction and the X direction, a large difference in thermal expansion coefficient must be provided between the core and the cladding layer. In order to provide a large difference in thermal expansion coefficient, it is necessary to mix a large amount of dopant such as germanium or phosphorus into the core, which poses the problem of increased scattering loss. Therefore, an object of the present invention is to provide a polarization-maintaining optical fiber that does not increase coupling loss with a laser or fiber-to-fiber coupling and has low scattering loss. In the groove of the present invention, on both sides of the glass rod, two glass rods having a larger coefficient of thermal expansion than the glass rod are in contact with the glass rod in parallel,
The polarization-maintaining optical fiber is characterized in that the three glass rods are covered with a glass tube.

Next, the present invention will be explained by examples using the drawings.

@Figure 1 (al is a diagram explaining one embodiment of the present invention,
This shows the cross-sectional shape of this example at 1.5 in a plane perpendicular to the central axis of the optical fiber, where 11 is a core, 12 is a cladding layer, 13 is a high thermal expansion coefficient glass rod, 14 is a glass coating, and 15 is a The air gap 16 is the axis of symmetry X, which is (1) in Figure 1.
) is the axis of symmetry 16 of the optical fiber shown in FIG.
The refractive index distribution above is shown, 21 is the core, 22 is a high thermal expansion coefficient glass rod, 23 is a glass coating, 24 is a center line, 25
is the vertical axis i11+ indicating the refractive index, and 26 is the horizontal axis indicating the radial direction. In this embodiment, the core 11 was made of quartz glass containing about 1 tem of germanium oxide, the cladding layer 12 was made of fused silica glass, and the high coefficient of thermal expansion glass rod 13 was made of quartz glass containing about 5 oz. of boron oxide. These glass rods are shown in Figure 1 (
As shown in a), the parts where the three wires are in contact with each other are fused and then covered with a glass coating 14. In this case, void 1
The internal pressure of the glass coating 14 is raised by about 10 water column pressure to about 19,000, so that the pressure within the tube of the glass coating 14 is maintained at about 19,000.

Then, the high coefficient of thermal expansion glass rod 13 contracts strongly when it is cooled from a high temperature state to room temperature, and the core glass 11 has an axis of symmetry]6
However, since there is a gap 15 in the X direction perpendicular to the axis of symmetry 16, no tension is applied. Therefore, the refractive index of the core glass 11 in the X direction of the axis of symmetry 116 becomes smaller than the refractive index in the X direction perpendicular to the axis of symmetry 16 due to the photoelastic effect, and the core glass 11 becomes birefringent glass. Due to the above birefringence,
A polarization preserving fiber is obtained. In this example, although the core is kept circular, the germanium oxide concentration contained in the core 11 is as low as about 1,000 nm, so that a polarization-maintaining optical fiber with low scattering loss can be obtained. In the above embodiments, silica-based glass was used as the glass material, but multi-component glass may also be used.

In the above embodiment, both the core glass rod 11 and the high thermal expansion coefficient glass rod have a cylindrical shape, but their cross-sectional shape may be rectangular. As explained above, according to the present invention,
Because the core is kept circular, coupling losses with the laser and between optical fibers do not increase, and despite the large birefringence, the relatively high purity of the core reduces scattering losses. A small polarization-maintaining optical fiber is obtained.

[Brief explanation of the drawing]

FIG. 1 is a diagram illustrating an embodiment of the present invention.
(a) is a diagram showing the cross-sectional shape of the optical fiber, (b) is (
It is a figure which shows the refractive index distribution of the symmetry axis X direction of a). 6. The symbols used in Drawing 1 indicate the following. 11... 2f a, 12... cladding layer, 13... 5- high thermal expansion coefficient glass rod, 14... glass coating,
15...Gap, 16...Axis of symmetry, 21
... Core, 22 ... High thermal expansion fBk coefficient gasis rod, 23 ... Glass coating, 24 ...
...Center line, 25...Vertical axis representing refractive index, 2
6... Horizontal direction representing radial direction++i+1°G-

Claims (1)

[Claims] In optical fibers that guide light while preserving polarization,
Two glass rods having a larger coefficient of thermal expansion than the glass rod are connected in parallel to the glass rod on both sides of the glass rod 1-1, and the three glass rods are covered with a glass tube. polarization preserving optical fiber.