JPS6053285B2 - Constant polarization optical fiber - Google Patents

Description

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

The cladding is made into an ellipse, causing anisotropic strain in the core, and the photoelastic effect increases the difference in refractive index and propagation constant in the major and minor axis directions, preserving polarization in the major and minor axes. Conventionally, the so-called constant polarization type optical fiber has a wavelength of λ■0.63 μm.
Many things are used for this purpose.

The cladding material that causes anisotropic strain is B, 0. is said to be the most likely, but B and O. suffers from a large absorption loss in the long wavelength band of 1.3 μm or more, which is a major reason why the use of polarization-controlled optical fibers has conventionally been limited to short wavelength bands. An object of the present invention is to eliminate the nine colors of the prior art described above and to provide a constant polarization type optical fiber with low loss 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, and the refractive index of the elliptical cladding layer is set between the core and the first cladding consisting of the low refractive index layer. The reason is that the refractive index distribution is added as shown in .

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 composition of the core 1, first cladding 2, elliptical second cladding 3, and outermost third cladding 4 is GeO2+SlO.
2, slO2, GeO2+B3, O3+SiO2, Si
O.

Refractive index difference Δn1 to 0.5% between core 1 and third cladding 4
, the refractive index difference between the elliptical second cladding 3 and the third cladding 4 is ΔN2 to 0.1%, and the refractive index of the first cladding 2 is equal to that of the third cladding 4.

Core diameter: 6 μm, first cladding 2 thickness: 3 μm, outer diameter: 125
A single mode optical fiber with a cutoff wavelength of 1.1 μm was obtained. In the case of a single mode optical 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 equivalent 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, as the amount of boron increases, polarization stability improves.

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, and this causes the normalized frequency at the operating wavelength to change. Because of this change, 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, the waveguide mode is made into a leaky mode, and an elliptical second
The refractive index of the cladding is made larger than that of the first cladding.
Such an optical fiber is 1pB/Km at λ=1.2μm.
, it was possible to achieve a low loss of about 1 to 2 dB/k even when λ=1.3 μm.

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

The major axis and the minor axis, ε=? ×100(%).

! For comparison, a case where the refractive index distribution is as shown in FIG. 3 will be described.

In the case of such a refractive index distribution, the first cladding also operates as if it were part of the core, and a cladding mode is likely to occur.

As a result, the extinction ratio and bending loss value deteriorate, and the influence of B2O contained in the second cladding also appears.If the first cladding and the second cladding are made equal, approximately Although the same effects as the present invention can be expected, it is extremely difficult to produce the same in a strict sense in actual industrial production, and unless the present invention is adopted, there is a possibility that the result will be like the comparative example in Figure 3. The optical fiber of the present invention as described above has the following remarkable effects.

(1) Since the first cladding is made of high purity 102 and the refractive index of the elliptical second cladding is made higher than that of the first cladding, an optical fiber with low loss and excellent polarization characteristics up to a long wavelength band can be obtained.

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

[Brief explanation of drawings]

FIG. 1 is a sectional view showing an example of the present invention, FIG. 2 is a diagram showing its refractive index distribution, and FIG. 3 is a diagram showing the refractive index distribution of a comparative example. 1... Core, 2... First cladding, 3...
...Second cladding, 4...Third cladding.

Claims (1)

[Claims] 1 A glass core 1 is provided at the center of the optical fiber, and a first concentric ring made of high-purity SiO_2 is formed on the outer periphery of the glass core and has a thickness greater than or equal to the core radius and a curvature smaller than that of the core.
A second cladding 3 having a refractive index smaller than the core and larger than the first cladding and having an elliptical cross section is provided on the outer periphery of the first cladding 2. 1. A polarization-constant optical fiber characterized in that it has a third cladding 4 having a smaller index than the second cladding.