JPH0427905A - Polarization keeping optical fiber - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a polarization-maintaining optical fiber, and particularly to a polarization-maintaining optical fiber suitable for use in optical fiber sensors and optical components.

[Conventional technology]

An example of a conventional polarization maintaining optical fiber is a PANDA type optical fiber having a cross section shown in FIG. In this fiber, a cladding 2 covers a core 1 and a stress applying section 3 having a circular cross section provided on both sides of the core 1.

When a base material corresponding to such a cross-sectional structure is manufactured and this base material is drawn into a fiber, tensile stress is generated from the stress applying portion 3 to the core 1. Due to the photoelastic effect caused by this stress, two orthogonal polarized waves (X polarized wave and Y polarized wave)
A difference occurs in the relative refractive index difference with respect to light (birefringence).

For this reason, there is a difference in the propagation constants of the X and Y polarized lights, making it difficult for coupling between the polarized waves to occur, so that the light propagates through the fiber while the polarized waves are maintained.

Furthermore, since the refraction of light in a PANDA polarization maintaining optical fiber is birefringence, the loss that occurs when the fiber is bent is greater for Y polarization.

Therefore, by selecting an appropriate fiber structure, fiber bending diameter, and fiber length to be bent, polarization-maintaining optical fiber can be used as a polarizer that reduces bending loss only for Y polarization (and transmits only X polarization). It is also possible to use

FIG. 8 shows the refractive index distribution of the PANDA type polarization-maintaining optical fiber shown in FIG. FIG. 8(A) shows the refractive index (n) distribution in the X-axis direction, and FIG. 8(B) shows the refractive index (n) distribution in the Y-axis direction. The refractive index of the stress applying part 3 is the same as that of the cladding 2.
As shown in FIG. 8, the refractive index distribution is different in the X-axis direction and the Y-axis direction. but,
Since the refractive index distribution near the core 1 through which light actually propagates is the same in the X-axis direction and the Y-axis direction, the anisotropy of the refractive index distribution has little effect on birefringence in this optical fiber.

[Problem to be solved by the invention]

It is expected that the polarization maintaining characteristic of a polarization maintaining optical fiber will improve as the refractive index difference (birefringence) between orthogonal x and Y polarizations increases. Further, even when using this optical fiber for a polarizer, a polarizer having better characteristics can be obtained by using an optical fiber with a large birefringence index. Therefore, it is desirable that the polarization-maintaining optical fiber has a large birefringence index.

However, in conventional stress-applied polarization-maintaining optical fibers such as the PANDA type, the stress generated is determined by the structure of the stress-applying part and its material, which limits the birefringence that can be obtained. .

Furthermore, if the structure and material of the stress applying section are set in order to obtain as large a stress as possible, excessive stress will be concentrated near the core. This causes core deformation and irregularities at the core-clad interface, resulting in a problem in that good polarization maintaining characteristics cannot be obtained.

An object of the present invention is to solve the above-mentioned problems and provide an optical fiber having good polarization maintaining characteristics.

[Means for Solving the Problems] In order to achieve such an object, the present invention comprises a core,
two intermediate members sandwiching the core and in contact with the core and disposed in the diametrical direction of the fiber; and two stress applying parts disposed in the diametrical direction in contact with each of the intermediate members. , a cladding that covers the core, the intermediate member, and the stress applying section, each having a refractive index of (
core)〉(cladding)〉(intermediate member) and (cladding)
〉 (stress applying part).

Further, the present invention includes a core, two stress applying parts disposed on both sides of the core and in contact with the core in the diametrical direction of the fiber, and a cladding covering the core and the stress applying part. , the magnitude of each refractive index is (core)〉(
cladding)> (stress applying part).

[For production]

In the optical fiber according to the present invention, side bits are provided on both sides of the core, so the refractive index on the outside of the core is smaller than the cladding in the X-axis direction, and the refractive index distribution is anisotropic even near the core. The sex is great. Therefore, the optical fiber of the present invention can also obtain birefringence due to the anisotropy of the refractive index portion, and due to the synergistic effect with the birefringence due to stress from the stress-applying portion, it has a birefringence larger than that of conventional polarization-maintaining optical fibers. It becomes possible to obtain.

Therefore, in the optical fiber of the present invention, a large birefringence can be obtained without concentrating an excessive stress near the core that would cause core deformation or irregularity at the core-clad interface, so that good polarization can be achieved. An optical fiber with retention properties is obtained. Furthermore, if the optical fiber of the present invention is used for a polarizer, better polarizer characteristics than conventional polarization-maintaining optical fibers can be obtained.

〔Example〕

Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 is a cross-sectional view of an example of a polarization-maintaining optical fiber according to the present invention. (2) is a core 1 in which GeO* is added to SiO□ glass. 2 is a cladding made of Si0g glass. 3 is 820. on 5ins glass. This is a stress applying part with added . 4 is a side bit made of SiO* glass doped with fluorine.

Figure 2 shows the refractive index distribution of the optical fiber shown in Figure 1. Figure 2 (A) shows the refractive index distribution in the X-axis direction, and Figure 2 (B) shows the refractive index distribution in the Y-axis direction. shows. In this example, the relative refractive index difference between the core cladding was 0.3%, and the relative refractive index difference between the stress applying part and the cladding and between the side bit and the cladding was 0.5%.

In addition, in this example, the stress applying part 3 and the side bit 4
Although the stress applying part 3 is made to be separate from the core 1, it is also possible to make the stress applying part 3 itself have a role as the side bit 4 by making it contact with the core 1 or by overlapping a part of these parts. It is possible.

The material of each part has a refractive index of (core)>(cladding)
> (side bit), and the coefficient of linear expansion of the stress applying portion 3 may be larger than that of the cladding, and the materials are not limited to those specifically shown.

In this embodiment, a PANDA type optical fiber having a circular stress applying portion as a stress applying structure will be described, but the present invention is not limited to this (any structure may be used as long as birefringence is caused by stress).

The following two types of polarization-maintaining optical fibers were produced based on the present invention.

1.1.3 μm Polarization-maintaining optical fiber for 3 μm cross-sectional shapes at each step are shown.

■Manufacturing a single mode fiber preform for the 1.3 μm band using the VAD method.

■Fluorine added 5 inches for side bits on both sides of core 11
2.Drill hole 4A for inserting the glass rod.

(2) Insert the fluorine-doped 5-ins glass rod for the side bit into the hole 4A, and then collapse it to form the side bit 4B.

■ B20 for stress applying part so as to be in contact with side bit 4B
．． Open hole 3A for inserting the added Si0g glass rod.

■ Stress applying part B20 to hole 3A. After inserting the added Sing glass rod, it is drawn and integrated, resulting in a core of 1° and a cladding of 2. An optical fiber consisting of the stress applying part 3 and the side bit 4 is obtained.

The refractive index distribution in the X and Y axis directions of the fabricated fiber is shown in FIG. 4, and the specifications are summarized in Table 1.

FIG. 4(A) shows the refractive index distribution in the X direction, with the symbol n++
The refractive index indicated by nz+ns and n4 is core 1,
Cladding 2. The refractive index of the stress applying part 3 and the side bit 4 is shown. FIG. 4(B) shows the refractive index distribution in the Y direction,
The refractive indices indicated by symbols n6 and n6 indicate the refractive indices of the core 1 and the cladding 2.

Table 1 Polarization-maintaining optical fiber for 1.3 μm μm For comparison of original characteristics, a polarization-maintaining optical fiber similar to the optical fiber according to this example except that it does not have the side bit 4 was fabricated. When its characteristics were evaluated, the birefringence was 3.3
l0-', the crosstalk was -35 dB. As a result, it was confirmed that the present invention allows a larger birefringence than the conventional one to be obtained, and as a result, crosstalk indicating polarization maintaining characteristics is improved.

2.0.85 μm Band Polarization Maintaining Optical Fiber for Polarimetry Figure 5 shows the cross-sectional shape at each step.

■Manufacturing a single mode fiber preform for the 0.85 μm band using the VAD method.

■A side bit and stress applying part B20. Hole 3A for inserting doped 5iOa glass rod
open it.

■B20 for side bit and stress applying part in hole 3A. Addition 5
After inserting the ins glass rod, draw a line and integrate it.
An optical fiber consisting of a core 1, a cladding 2 and a stress applying section 3 is obtained.

The refractive index distribution in the X and Y axis directions of the fabricated fiber is shown in FIG. 6, and the specifications are summarized in Table 2. In Figure 6, the fourth
The same reference numerals are given to the same parts as in the figure. As can be seen from FIGS. 6 and 4, the polarization-maintaining optical fiber for 1.3 μm and 85 μm-band polarization measurement shows similar refractive index distributions.

Table 2 Specifications of polarization-maintaining optical fiber for polarimetry in the 0.85 μm band 1 m of the main fiber 311 has a diameter of 30 I! A fiber polarizer was made by winding it on a 1ffl reel, and the wavelength was 0.85.
When the characteristics were evaluated in μm, the extinction ratio was 47 dB, and the excess loss was 0.08 dB, which showed better polarizer characteristics than conventional ones, indicating that the optical fiber of this example can also be used for polarizers. was confirmed.

〔Effect of the invention〕

As explained above, in the present invention, since the optical fiber has a large birefringence, there is an effect that good polarization maintaining characteristics can be obtained, and when the optical fiber of the present invention is used as a polarizer, has the effect that good polarizer characteristics can be obtained.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view of a polarization-maintaining optical fiber according to the present invention, Figure 2 is a diagram showing the refractive index distribution in the X and Y axis directions of the polarization-maintaining optical fiber shown in Figure 1, and Figure 3 is a diagram showing the polarization-maintaining optical fiber shown in Figure 1. Cross-sectional views of each manufacturing process of a polarization-maintaining optical fiber for measuring 1 and 3 μm μm according to the invention; FIG. 4 is a diagram showing the refractive index distribution in the X and Y axis directions of the optical fiber shown in FIG. 3; FIG. Cross-sectional views of each manufacturing process of the polarization-maintaining optical fiber for polarimetry in the 0 and 85 μm band according to the invention; FIG. 6 is a diagram showing the refractive index distribution in the X and Y axis directions of the optical fiber shown in FIG. 5; FIG. is a cross-sectional view of a conventional PANDA type polarization-maintaining optical fiber, and FIG. 8 is a diagram showing the refractive index distribution in the X and Y-axis directions of the conventional polarization-maintaining optical fiber. DESCRIPTION OF SYMBOLS 1... Core, 2... Clad, 3... Stress applying part, 4... Side bit.

Claims (1)

[Scope of Claims] 1) A core, two intermediate members disposed in the diametrical direction of the fiber in contact with the core, and two intermediate members disposed in the diametrical direction in contact with each of the intermediate members. established 2
a cladding that covers the core, the intermediate member, and the stress applying portion, each of which has a refractive index such that (core)>(cladding)>(intermediate member) and (cladding) ＞(stress applying part). 2) having a core, two stress applying parts disposed in the diametrical direction of the fiber, sandwiching the core and in contact with the core, and a cladding covering the core and the stress applying part,
The size of each refractive index is (core) > (cladding) >
(stress imparting part).