JPS5816203A - Constant polarization type optical fiber - Google Patents

Abstract

PURPOSE:To obtain an optical fiber with a small loss in a longer wavelength zone by forming the 1st clad of an optical fiber with high purity SiO2, making the refractive index of the 2nd ellipsoidal clad higher than that of the 1st clad, and forming the 3rd clad having a lower refractive index than the 2nd clad. CONSTITUTION:For example, the core 1 is formed with GeO2-SiO2, the 1st clad 2 with high purity SiO2, the 2nd ellipsoidal clad 3 with GeO2+B2O3+SiO2, and the 3rd clad 4 with SiO2. The refractive index difference DELTAn1 between the core 1 and the 2nd clad 3 is about 0.5%, and that DELTAn2 between the 2nd clad 3 and the 1st clad 2 is about 0.1%. The thickness of the clad 2 is made larger than the radius of the core 1, the refractive index of the clad 2 is made lower than that of the core 1, and the refractive index of the clad 3 is made lower than that of the core 1 and higher than that of the clad 2. The 3rd clad 4 having a lower refractive index than the clad 3 is formed at the outside of the clad 3. Thus, a constant polarization type optical fiber with a small loss even in a longer wavelength zone is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical fiber for optical transmission, particularly a low-loss constant polarization type optical fiber.

The cladding is made into an ellipse, the core is anisotropically strained, and the photoelastic effect greatly increases the difference in refractive index and propagation constant in the long- and short-axis directions, thereby preserving the polarization of the long and short axes. Conventionally, many polarized optical fibers for λ=0.63 μm have been used. B2O3 is considered to be a promising cladding material that causes anisotropic strain, but B2O
3 suffers a large absorption loss in the long wavelength band of 1.6 μm or more, which is a major reason why the use of the conventional undefined polarization optical fiber is limited to the short wavelength band.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the above-mentioned drawbacks of the prior art and provide a polarization-constant optical fiber that exhibits low loss in a long wavelength band.

That is, the gist of the present invention is to have a four-layer structure in cross section, the width of the low refractive index layer around the core to be equal to or larger than the core radius, and the refractive index of the elliptical cladding layer to the first cladding consisting of the core and the low refractive index layer. The reason is that a refractive index distribution is added between the two.

FIG. 1 shows a cross-sectional structure of an embodiment of the polarization constant optical fiber of the present invention, and FIG. 2 shows its refractive index distribution.

The core 1, the first clad 2, the elliptical second clad 6, and the outermost third clad 4 each have a composition of G e O2-8.
j, 02, Sin, , GeO2-1-B20B+Si
O2, S]02.

Refractive index difference △n1 between core 1 and elliptical second cladding 30
．． 5%, refractive index difference △n2zO between the elliptical second cladding 6 and the first cladding 2, 1%, core diameter 6 μm, thickness of the first cladding 2 6 μmt, outer diameter 125 μm 5', cutoff wavelength 1.1 μm. A single mode optical fiber was obtained.

In the case of a single-mode optical fiber, the electromagnetic field component leaks out to the shifrad at a normalized frequency different from that of a multi-mode optical fiber, so the first cladding was given a thickness equal to the radius of the core.

Now, the factors that determine polarization stability are the ellipticity of the elliptical cladding and the amount of boron, and generally, increasing the amount of boron improves polarization stability. In a single mode optical fiber, even if the thickness of the first layer is set as described above, the cutoff wavelength changes due to slight changes in the core diameter and refractive index, which changes the normalized frequency at the operating wavelength. Therefore, the electromagnetic field component may sometimes seep into the second elliptical cladding. When such an electromagnetic field component is in a waveguide mode, part of the wave energy is absorbed by the elliptical second cladding portion, resulting in an increase in transmission loss.

In order to prevent this, such a waveguide mode is designated as a leaky mode.
The refractive index of the elliptical second cladding is made larger than that of the first cladding so that only a low-loss mode in which the electromagnetic field components are concentrated at the center of the core can be guided.

In this way, the optical fiber is 1 dB/Km at λ-1, 2 μm.
Even at .lambda.-1.3 .mu.m, it was possible to achieve a loss as low as 1 to 2 aB.

The ellipticity C of the prototype original fiber was ε=60%, and the coupling was about 4 .mu.m (.lambda.-1.6 .mu.m).

The optical fiber of the present invention as described above has the following remarkable effects.

(1) The first cladding was made of high-purity 8102, and the refractive index of the elliptical second cladding was made higher than that of the first cladding.
An optical fiber with low loss and excellent polarization characteristics up to long wavelength bands can be obtained.

(2) Since the increase in loss due to electromagnetic field seepage into the cladding due to changes in the normalized frequency is small, low loss characteristics are stable even if there are longitudinal variations in core diameter or refractive index difference. .

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing an embodiment of the present invention, and FIG. 2 is a diagram showing its refractive index distribution. 1: core, 2: first cladding, 3: second cladding, 4: third cladding. 7-1 Figure 72

Claims (1)

[Claims] 1. The optical fiber has a glass core 1 at its center, and around the outer periphery of the glass core there is a concentric ring made of high-purity 5i02 with a thickness greater than or equal to the core radius and a refractive index smaller than that of the core. 1st
A second clad 6 having an elliptical cross section with a refractive index smaller than that of the core and larger than the first clad is provided on the outer periphery of the first clad 2, and a second clad 6 having a refractive index on the outer periphery of the second clad 2. A sixth clad 4 whose ratio is smaller than the second clad
A polarization-constant optical fiber characterized by having: 2. The second cladding is G e02-B 203-, Si
The constant polarization type optical fiber according to the above item, characterized in that it is made of O2-based glass.