JPH0350505A - Single polarization optical fiber - Google Patents

Abstract

PURPOSE:To propagate light in one of two orthogonal polarization modes by equalizing the refractive index of strain inducing parts, which face each other in a 2nd clad across a core at right angles to the axis, to the refractive index of the 2nd clad. CONSTITUTION:The core 11 of the part of a radius (a) from the center has a refractive index n0 and the 1st clad 12 of the outside part of a radius (b) has a refractive index n1 smaller than n0. Further, the 2nd clad 13 and the strain inducing parts 14a and 14b provided in the 2nd clad have the refracting index n2, which is between n0 and n1. Then the single-mode fiber in double-clad structure is provided with the strain inducing parts 14a and 14b to give the core 11 high birefringence, and then light in one of two orthogonal polarization modes is propagated while its propagation constant becomes larger than that of the clad. Consequently, the optical fiber can be manufactured so that the wavelength range where only the single-polarization mode is propagated is as designed.

Description

[Detailed description of the invention] Industrial application field> The present invention is a damaged wave-maintaining fiber that transmits damaged waves required for optical fiber applied measuring instruments, coherent optical transmission systems, etc. while maintaining its characteristics. In particular, the present invention relates to a single-fault optical fiber having a wavelength band in which only a single-fault wavefront is propagated.

2. Description of the Related Art With the advancement of optical communication technology, optical fibers are currently being used in various devices. Among these, optical ICs used in various communication devices assume that the output from the optical fiber is a linear damaged wave in a specified direction, and various measurement devices Then, the light propagating through the optical fiber is required to be a direct I4I damaged wave. Therefore, fix it while preserving the damaged wave surface! Polarization-maintaining optical fibers should be developed to propagate S-scarred waves.

5 and 6 show conventional examples of polarization maintaining optical fibers, and FIGS. 5 and 6 show cross sections of an elliptical core fiber 50 and a non-axisymmetric stress-applying fiber 60, respectively. shows.

As shown in FIG. 5, in the elliptical core fiber 50, the core 5
1 has an elliptical cross section, and the electric field constant differs depending on whether the electric field is parallel to the long axis (X-axis direction) or perpendicular to the long axis (Y-axis direction), and birefringence between these two directions. The index B has the following relationship: Boc (elliptic oblateness)×(relative refractive index Δ)2. However, the relative refractive index Δ is expressed by the following formula using the refractive index n1 of the core 51 and the refractive index n2 of the cladding 52.

Therefore, in the elliptical core fiber 50, by manipulating the refractive indexes n, , n2 of the core 51 and the cladding 52 as well as the elliptic oblateness of the core 51, a high birefringence B can be obtained x
, it is possible to give birefringence to the orthogonal scratch waves in the y2 direction and suppress their energy coupling.

On the other hand, in the non-axisymmetric stress-applying fiber 60 shown in FIG. stress is applied, and this causes strain inside the core 61, causing the core 61, which was originally an isotropic object, to become anisotropic, causing the elliptical core fiber 50 to become anisotropic.
Similar to the core 51 of , a high birefringence is obtained.

Conventional polarization-maintaining fibers, as shown in Part B, have degenerate 2
By imparting birefringence to the two orthogonal flawed wave modes HE,L'' and HE1iF and suppressing the energy coupling between these modes, a single flawed wavefront is preserved, and it is generally called a birefringent fiber. It is.

Problems to be Solved by the Invention> However, in the conventional birefringent fiber as described above, a predetermined polarization is preserved, and if the polarization orthogonal to this polarization differs due to speed, loss, etc. However, they are propagated at the same time. Therefore, in order to apply such a birefringence fiber as a measurement device, for example, there is a problem that an analyzer or the like is required to remove unnecessary orthogonal polarization components and extract only a predetermined polarization component. .

In view of these circumstances, the present invention provides a single individual wave light that propagates only one polarization mode out of two orthogonal polarization modes so that a predetermined polarization extraction means such as an analyzer is not required on the light receiving side. The purpose is to provide fiber.

Means for Solving the Problems> A single polarization optical fiber according to the present invention that achieves the above object includes a core, a first cladding formed around the outer periphery of the core and having a lower refractive index than the core, and a first cladding having a refractive index lower than that of the core. and a second cladding formed outside the first cladding and having a higher refractive index than the first cladding and a lower refractive index than the center of the core, the optical fiber comprising: a second cladding formed outside the first cladding and having a refractive index lower than the center of the core; has a stress applying part which is arranged to sandwich the core in a direction perpendicular to the axis and face each other and gives birefringence to the core, and at least the stress applying part and the second cladding have the same refractive index. It is characterized by

The cross section and refractive index distribution of an example of a single-flaw-maintaining optical fiber having the above configuration are shown in FIGS. 1 and 2. As shown in both figures, the core 11 at the radius a from the center has a refractive index of n0, and the first cladding 12 at the outer radius b has a refractive index of n, which is smaller than n0. , and the second cladding 13 on the outside thereof and the stress applying parts 14a and 14b provided in the second cladding 13 have a refractive index of n2, and n2 is a value between n0 and n1. There is.

In a single mode fiber with a double cladding structure having a refractive index distribution as shown in FIG. Outer diameter ratio a to cladding 12
/b and core 11. At the wavelength determined by the relative refractive index difference between the refractive indexes n, , n2, and n3 of the first cladding 12 and the second cladding 13, the HE mode also becomes a leaky mode and enters a cutoff state.

Then, in order to provide stress applying parts 14a and 14b to the single mode fiber with such a double clad structure, and to give the core 11 high birefringence, two orthogonal single wave modes, ie, H E, mode and HE mode, were proposed. Among Y mode, −
The propagation constant of the 4 damaged wave mode is larger than the propagation constant of the cladding and can be propagated, but the propagation constant of the other polarization mode becomes smaller than or equal to the propagation constant of the cladding and is in a cut-off state. There will now be such a wavelength range. In other words, this wavelength range becomes a single sweep wave range.

In addition, the cutoff wavelength is greatly influenced by the refractive index distribution of the cladding, and the cutoff wavelength depends on the refractive index distribution of the cladding. If there is a difference in ratio, etc., the cutoff wavelength of the actual fiber may differ from the design, but in the present invention, the second cladding 13 and the stress applying part 14a
, 14b, the refractive index is matched with that of the refractive index, so that manufacturing as designed is possible.

In the present invention, it is desirable that the refractive index n0 of the core 11 and the refractive index n2 of the stress-applying parts 14a and 14b be as close as possible. Manufacturing becomes easier by adding a lower refractive index.

Example: A single wave optical fiber having the same configuration as shown in FIG. 1 was manufactured.

In this example, the core 11 is a pure SI0 with a refractive index of 1.458.
2 glass, the first cladding 12 included in this core 11 is made of SiO2-F glass with a refractive index of 1.447, and the second cladding 13 containing the first cladding 12 is made of SiO2-F glass with a refractive index of 1.4.
51 SiO■-F glass, stress applying portions 14a, 14b provided on both sides of the core 11 in the direction orthogonal to the axis.
It was constructed from jtSin2-B, O, glass.

To manufacture such a single polarization optical fiber, the method shown in FIG.
As shown in l, first, the refractive index is 1,458 and the diameter is 1.6.
In between is pure Sin2 glass N31, a SiO2-F glass layer 32 with a refractive index of 1.447 and a diameter of 4.8 mm, and a SiO glass layer with a refractive index of 1.451 and a diameter of 40+w. 1st floor glass I? !
A glass rod 30 consisting of 33 is formed by a VAD method.

Then, as shown in FIG. 3 (bl), the glass rod 30
In the cross section, a hole 35 with a diameter of 11 mm is centered at two points A and B at a distance of 9.4 m from the center on the diameter.
a, forming 35b. Next, as shown in FIG. 3 (cl), both sides of these holes 35a and 35b were flame polished, and after cleaning by immersing them in a 10% sulfuric acid solution for 30 minutes, these holes 35a and 35b were polished with 15 layers of %B2
A glass rod 34a, 34b with a diameter of 10 mm including S.sub.iO2-B20 and glass rods 34a and 34b are respectively inserted to form an optical fiber base material. Then, this base material was drawn using a carbon resistance furnace at a temperature of about 2,000°C and a drawing speed of about 30 m/min.
A polarization-maintaining fiber with a core diameter of 5 μm was obtained by drawing to i25 μm.

The polarization maintaining fiber created in this way has a wavelength of 1
．． The birefringence B for 3 μm is 5×10−′, and the leakage loss of HE,, x%− for a wavelength of 1.55 μm is 1. 0 dB/km pH E, y-mode leakage loss was 25.0 dB/km.

This polarization maintaining fiber has a loss wavelength characteristic as shown in FIG.

In this way, the damaged wave-maintaining fiber of this example is HE,...
It can be seen that the leakage loss of the Y mode is clearly larger than that of the HE mode, indicating that it is a single-damaged optical fiber.

Effects of the Invention> As explained above, the single polarization optical fiber according to the present invention has a wavelength range in which either the HE'' mode or the HE' mode is in a cut-off state, Since only one polarization mode can be propagated in this wavelength range, when applied to a fiber sensor, for example, the sensitivity is greatly increased, and furthermore, a predetermined means for extracting flawed waves such as an analyzer on the light receiving side is unnecessary, and measurement is possible. , very useful in fields such as optical communications.

In addition, in the single-flaw optical fiber of the present invention, by matching the refractive index of the stress applying part provided in the second cladding with the second cladding, the wavelength range in which only the single-swept wave mode propagates can be adjusted as designed. It becomes possible to manufacture.

[Brief explanation of drawings]

No.lrI! J is a cross-sectional view showing an example of a single wave optical fiber according to the present invention, FIG. 2 is its refractive index distribution diagram, and FIG.
al to {e) are explanatory diagrams showing the manufacturing process of the single-polarized optical fiber according to the example, FIG. 4 is a graph showing the loss wavelength characteristics of the single-polarization optical fiber according to the example, and FIGS. 5 and 6 Each figure is a sectional view showing a polarization maintaining fiber according to the prior art. In the drawings, 11 is a core, 12 is a first cladding, 13 is a second cladding, 14m and 14b are stress applying parts, 31 is a pure Si2 glass layer, 32 is a SiO2-F glass layer, and 33 is a SiO2-F glass. The layers 34a and 34b are SiO2-B20, and the glass rods 35a and 35b are holes. Do,

Claims (2)

[Claims] (1) a core, a first cladding formed on the outer periphery of the core and having a lower refractive index than the core, and a first cladding formed outside the first cladding and having a higher refractive index than the first cladding; An optical fiber comprising a second cladding having a lower refractive index than a central portion of the core, the core being disposed in the second cladding so as to sandwich the core in a direction perpendicular to the axis and facing each other, and having birefringence in the core. What is claimed is: 1. A single-polarized optical fiber comprising a stress-applying portion that provides a stress-applying portion, and at least the stress-applying portion and the second cladding have the same refractive index. (2) The single polarization optical fiber according to claim 1, wherein the material of the core is pure quartz containing no additives or quartz added with an additive having the effect of reducing the refractive index.