JPH0242202B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a silica core single mode optical fiber.

Optical fibers generally use doped silica containing SiO ₂ as a core material and GeO ₂ or P ₂ O ₅ to increase the refractive index.

This is because using silica as the cladding on the outside of the core is easiest from a manufacturing standpoint, and optical fibers with such a structure exhibit relatively excellent transmission characteristics.

However, as the uses of optical fibers have expanded, cases have arisen where optical fibers are used in places where radiation is present, and demands for radiation resistance have begun to emerge.

According to the results of experiments conducted by the present inventors, it is desirable that the core of the optical fiber be made of high-purity SiO ₂ in order to achieve a certain degree _of radiation resistance. In reality, this was a major problem.

That is, from a manufacturing standpoint, it is convenient for the outermost layer to be made of high-purity silica, so if this is applied to a single mode optical fiber, it will exhibit a refractive index distribution as shown in FIG.

Of course, the high-purity silica in the outermost layer does not need to be as pure as the core, but since it does not contain any particular dopants, its refractive index is approximately equal to that of the core.

In FIG. 1, 11 is a core, 12 is a first cladding, and 13 is a second cladding.

The waves excited here are structurally leaky waves.

Therefore, in order to achieve low loss, the thickness of the first cladding must be sufficiently large relative to the core radius T.
Specifically, if the normalized frequency is 2.2, it must be more than 6 times the core radius. For this reason, when attempting to manufacture an optical fiber with a large core diameter, there may be limitations in terms of the outer diameter. Furthermore, longitudinal variations in the core diameter have a large effect on the attenuation constant of leaky waves, making it difficult to produce a uniform and long fiber.
Because the core diameter cannot be increased for the reasons mentioned above, there is a problem in that it is difficult to connect the fibers to each other and to connect them to a light source.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the drawbacks of the prior art described above and provide a low loss silica core single mode optical fiber.

That is, the gist of the present invention is to provide a single mode optical fiber having a core, a first cladding, a second cladding, and a third cladding, in which the core and the third cladding are made of high-purity silica, the refractive index of the core is n ₁ , the core is The refractive index n ₂ of the first cladding and the refractive index n ₃ of the second cladding adjacent to the core are n ₁ > n ₃ > n ₂ (n ₁ − n ₂ )2 (n ₁ − n ₃ ), and the core When the radius is T, the thickness of the first cladding is δ ₁ T, and the total thickness of the first cladding and the second cladding is δ ₂ T, 0<δ ₁ 1 3.5 _{δ 2} <6.

The configuration of the present invention will be specifically explained with reference to FIG. 2 showing one embodiment.

In FIG. 2, 21 is the core, 22 is the first cladding, 23 is the second cladding, and 24 is the third cladding. Letting their refractive indexes be n ₁ , n ₂ , n ₃ , n ₄ , n ₁ ~n ₄ > n ₃ > n ₂ (n ₁ − n ₂ ) 2 (n ₁ − n ₃ ).

Here, the difference in specific refractive index between the core and the first cladding is Δ ₁ ,
The difference in specific refractive index between the core and the second cladding is Δ ₂ ,
When the relationship (Δ ₁ /Δ ₂ )=2 is established, the single mode operating range becomes 2.25<v′<4.3 as shown in FIG. However, v' is the normalized frequency defined by v'=n ₁ (2π/λ)T2√ ₁ , λ is the wavelength used, and T is the core radius. When the operating point is set to v' = 3.5 in consideration of manufacturing ease and fiber diameter variation, the thickness of the first cladding is δ ₁ T, the distance between the core and the support tube, or the distance between the first cladding and the first cladding. If the total thickness of the second cladding is δ ₂ T, the attenuation constant due to leakage of the LP ₀₁ mode and the LP ₁₁ mode is as shown in FIG. Therefore,
Loss due to leakage in LP ₁₁ mode 10 ⁶ dB/Km or more,
We obtain 0<δ ₁ 1, δ ₂ 3.5 as conditions where the loss due to leakage of the LP ₀₁ mode can be ignored. A fiber that satisfies the above conditions has a single mode, whether long or short (several centimeters). If δ ₁ is made larger than 1, there will be cases where there is a single mode for long lengths (several hundred meters) and two modes for short lengths. This is clear from the relationship between δ ₂ and the attenuation constant due to leakage of the LP ₁₁ mode in FIG. 3. If δ ₂ is made smaller than 3.5, the LP ₀₁ mode will also be attenuated due to leakage.

In FIG. 3, u is the normalized lateral phase number within the core.

Note that by setting (Δ ₁ /Δ ₂ )>2, the operating range of v' shifts to 2.5/4.7, 3.0 to 5.5,
The effective wavelength range becomes significantly wider.

If δ ₂ is larger than 3.5, there will be no transmission problem, but
Considering the relationship between the core diameter and the optical fiber outer diameter,
Too large is not realistic, 3.5δ ₂ <6
is within an appropriate range.

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

(1) Since the core diameter can be made larger than in the conventional structure, it is easier to connect single mode optical fibers to each other and to couple with a light source.

(2) Due to good field distribution confinement, the influence of longitudinal diameter fluctuations is small.

(3) Single-mode operation has a wide wavelength range and is easy to manufacture.

(4) The core is made of high-purity silica and has good radiation resistance.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing the refractive index distribution and cross section of a conventional silica core single mode optical fiber, FIG. 2 is an explanatory diagram showing the refractive index distribution and cross section of the silica core single mode optical fiber of the present invention, and FIG. is a diagram showing the single mode operating range of the silica core single mode optical fiber of the present invention, and FIG. 4 is a diagram showing the relationship between loss due to leakage between δ ₁ and LP ₁₁ mode and between δ ₂ and LP ₀₁ mode. It is. 21: Core, 22: 1st Clad, 23: 2nd
Clad, 24: Third Clad.

Claims (1)

[Claims] 1 core, first clad, second clad, and third clad
In a single mode optical fiber having a cladding, the core and the third cladding are high-purity silica, and the core has a refractive index n ₁ , a first cladding adjacent to the core has a refractive index n ₂ , and a second cladding has a refractive index n ₃ . is in the relationship n ₁ > n ₃ > n ₂ (n ₁ − n ₂ ) 2 (n ₁ − n ₃ ), where T is the core radius, δ ₁ T is the thickness of the first clad, and δ 1 T is the thickness of the first clad and the second clad. A silica core single mode optical fiber characterized in that, where the total thickness with the cladding is δ ₂ T, 0<δ ₁ 1 3.5δ ₂ <6.