JPS5831303A - Constant polarization type optical fiber - Google Patents

Abstract

PURPOSE:To obtain an optical fiber with a small loss up to a longer wavelength zone by adopting a 4-layered sectional structure, using P2O5-B2O3-SiO2 glass as the material of the 2nd elliptical clad, and making the refractive index of the clad highter than that of the 1st clad made of high purity SiO2. CONSTITUTION:The 1st concentric clad 2 made of high purity SiO2 is formed aroud a glass core 1. The thickness of the clad 2 is larger than the radius of the core 1, and the refractive index is lower than that of the core 1. The 2nd clad 3 made of P2O5-B2O3-SiO2 glass and having an elliptical section is formed around the 1st clad 2. The refractive index of the clad 3 is lower than that of the core 1 and higher than that of the 1st coad 2. The 3rd clad 4 having a lower refractive index than the 2nd clad 3 is further formed around the clad 3. Thus, the desired constant polarization type optical fiber 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 subjected to anisotropic strain, and the photoelastic effect increases the difference in refractive index and propagation constant in the major and minor axes, thereby preserving the polarization of the major and minor 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 from a large absorption loss in the long wavelength band of 1.2 μm or more, which is a major reason why the use of polarization-constant optical fibers has conventionally been limited to short wavelength bands.

Previously, we proposed providing a first cladding of high purity 5i02 as a polarization-constant optical fiber J and provided one solution, but the present invention takes into account the manufacturing conditions of the solid CvD method, etc. ,
It is an object of the present invention to provide a constant polarization optical fiber that can achieve an even lower loss ratio in a long wavelength band.

That is, the gist of the present invention is that the cross section has a four-layer structure, the width of the low refractive index layer around the core is equal to or larger than the core radius, the material of the elliptical cladding layer is P2O5-B2O3 to SioJ glass, and the refractive index is equal to that of the core. The refractive index distribution is such that it lies between the first cladding and the first cladding consisting of the low refractive index layer.

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

The core 1, first clad 2, elliptical second clad 3, and outermost third clad 4 each have a composition of GeO2-BiO.
9, sio, 2. p2o, +B, 203+Si○2
,810!2.

The refractive index difference between the core 1 and the elliptical second cladding 3 = 0.
5%, refractive index difference Δn,k between the elliptical second cladding 3 and the first cladding 2 is 0.05%, the core diameter is 6μJ, the thickness of the first cladding 2 is 4μmt, the outer diameter is 125μmg6, and the cutoff wavelength is 1.2μm. obtained a single mode optical fiber.

In the case of a single mode original fiber, unlike a multimode optical fiber, the electromagnetic field components seep into the cladding due to the normalized frequency, 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 cladding 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.

As a result of trial production, such an original fiber was 0 at λ = 1.2 μm.
．． 7dB/Km, 0.8dB/ even at λ=1.3p m
It was Km.

The ellipticity e of the prototype optical fiber was ε=60%, and the coupling length was about 4 wn (λ=1.3 μm).

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

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

(2) Since the thickness of the first cladding made of high-purity Sin is large, there is little increase in loss due to electromagnetic field seepage into the cladding due to changes in the normalized frequency, and low-loss characteristics are stable.

(3) P, Ol, 10B20B+S i in the second cladding
Since it uses 02 series gas, it has not been prototyped in the past.
c, Ge090B, o8+s i Compared to O□-based glasses, low-temperature reactions are possible, and there is no OH group diffusion from the reeds and third cladding toward the core, resulting in low loss.

[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, f 1 mouth 2E Δ

Claims (1)

[Claims] A glass core 1 is provided at the center of the original fiber, and a concentric first clad 2 having a thickness greater than or equal to the core radius and a refractive index smaller than that of the core is provided on the outer periphery of the glass core. has a refractive index smaller than that of the core and the first
Second cladding 3'f with an elliptical cross section larger than the cladding:
1, a third clad 4 having a smaller refractive index than the second clad on the outer periphery of the second clad A, wherein the first clad is made of high purity S]02, and the second clad is made of P20y, - B203-8 i A polarization-constant optical fiber characterized by being made of O2-based glass.