JPH08220362A - Dispersion shift optical fiber - Google Patents

Abstract

PURPOSE: To obtain a dispersion shift optical fiber which is large in a mode field diameter(MFD) and has a small nonlinear effect by adopting the constitution having a core diameter of a small diameter which is small in the value among the core diameters at which wavelength dispersion is made zero at a specific wavelength. CONSTITUTION: The outer side of a central core part 1 is provided with a staircase core part 2 and a core 3 is composed of such central core part 1 and the staircase core part 2. The outer side of the core 3 is provided with a clad 4. The core diameter (b) of the small diameter having the small value among the core diameters of the core 3 which has a staircase type refractive index distribution and at which the wavelength dispersion is made zero at the wavelength 1.55μm is selected. The core is otherwise so formed that the correction factor (k) in the relation Aeff=k.π/4.(MFD)<2> between the effective sectional area (Aeff) and MFD of the fiber attains >=0.95. The small core diameter among these core diameters (b) is adopted in such a manner, by which the correction factor (k) and the MFD are increased and further, (k) is specified to >=0.95, by which Aeff is increased and the nonlinear effect is lowered.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dispersion-shifted optical fiber (hereinafter abbreviated as DSF) having a stepwise refractive index distribution and having almost zero chromatic dispersion at a wavelength of 1.55 μm. The effect is reduced.

[0002]

2. Description of the Related Art A DSF is an optical fiber having a chromatic dispersion value of zero or almost zero in a wavelength band of 1.55 μm where the loss of a silica optical fiber is minimum. The one having a stepwise refractive index distribution as shown in 1 is general. In FIG. 1, reference numeral 1 is a central core portion, and a staircase core portion 2 is provided outside the central core portion 1.
The core 3 is composed of the central core portion 1 and the staircase core portion 2, and the clad 4 is provided outside the core 3. Further, in FIG. 1, a indicates the diameter of the central core portion 1, b indicates the outer diameter of the staircase core portion 2, Δ ₁ is the relative refractive index difference between the central core portion 1 and the cladding 4, and Δ ₂ is the staircase core. Part 2
And the relative refractive index difference between the clad 4 and the clad 4.

The DSF having this stepwise refractive index distribution is
Compared to other types of DSFs, such as step type and triangular type, which have a refractive index distribution, the bending loss is small and the mode field diameter (hereinafter abbreviated as MFD) is a little large. There is a normal 1.
MFD compared to single mode optical fiber for 3μm band
Is small, about 8 μm.

When the MFD of the optical fiber is small, it is not only disadvantageous in terms of connection loss, but also when the power density of the light propagated in the optical fiber is large, for example, in the case of an optical amplifier, it is nonlinear. The effect is increased, and inconveniences such as deterioration of transmission characteristics occur.

[0005]

Therefore, an object of the present invention is to provide a DS with a large MFD and a small nonlinear effect.
To get F.

[0006]

The problem is to select a small core diameter having a small value among core diameters having a stepwise refractive index distribution and having zero wavelength dispersion at a wavelength of 1.55 μm. Alternatively, it can be solved by setting the correction coefficient (k) in the relational expression between the effective area of the fiber and the MFD to 0.95 or more.

The present invention will be described in detail below. In general,
The magnitude of the nonlinear effect in an optical fiber is n ₂ / Aeff
It is represented by. Here, n ₂ is the nonlinear refractive index of the optical fiber, and Aeff is the effective area of the optical fiber. Therefore, the magnitude of the nonlinear effect is inversely proportional to Aeff. On the other hand, it is known that there is a relationship represented by the following relational expression Aeff = k · π / 4 · (MFD) ² between Aeff and MFD in the DSF, and the value of the correction coefficient k is approximately 0.944. Has been reported to be constant (Y.NAMI
HIRA, ELECTRONICS LETTERS. Volume 30, Issue 3, Issue 2
62, February 1994).

From the above relational expression, Aeff of DSF is simply determined only by MFD, and if MFD becomes large, Aeff
It can be seen that the eff increases and the nonlinear effect decreases. However, as described above, D having a stepwise refractive index profile
In SF, since MDF is about 8 μm, Aeff is almost constant and the nonlinear effect cannot be reduced. Therefore, if the correction coefficient k in the above relational expression can be increased, the Aeff can be increased and the non-linear effect can be reduced. Therefore, the correction coefficient k, which has been considered to be constant until now, is corrected. It was examined whether the coefficient k could be increased.

The DS having the stepwise refractive index distribution shown in FIG.
With respect to F, when the outer diameter b of the core 3 (hereinafter referred to as the core diameter) is changed while keeping the refractive index distribution in a similar shape,
It is known that there are up to four core diameters b at which the wavelength dispersion value becomes zero at a wavelength of 1.55 μm (Nishide et al., IEICE Tech. OQE 87.4, 1987). It is also known that changing the parameters such as b / a, Δ ₁ and Δ ₂ in the refractive index distribution results in two core diameters at which the chromatic dispersion value becomes zero. In such a range, there are often two core diameters, and conventionally, a large core diameter that can reduce bending loss is exclusively used.

In the present invention, a core diameter smaller than the conventional one is adopted. FIG. 2 shows the correction coefficient k and the MFD of the DSF having the stepwise refractive index distribution shown in FIG. 1, which are calculated by changing b / a, Δ ₁ and Δ _2.
This is for a = 4.5, Δ ₁ = 0.96%, and Δ ₂ = 0.15%. The points marked with x and ● on the curve in this graph are the wavelength of 1.55 at the value of this parameter.
The point of the core diameter at which the wavelength dispersion in μm becomes zero is shown, and the x mark shows a small core diameter (small diameter solution), and the ● mark shows a large core diameter (large diameter solution). It should be noted that in the MFD range of the graph of FIG. 2, the MFD increases as the core diameter b decreases.

From the graph of FIG. 2, the correction coefficient k is 0.945 when a large core diameter is adopted as the core diameter b at which the wavelength dispersion becomes zero at the wavelength of 1.55 μm.
Is 6.6 μm, and when a small core diameter is adopted, the correction coefficient k is 0.967 and the MFD is 8.5 μm. As is clear from this result, by adopting a core having a small core diameter, the correction coefficient k becomes larger than 0.944 and the MFD also becomes large. Therefore, by adopting the small diameter solution, the correction coefficient k and the MFD are increased,
It can be seen that Aeff increases and the nonlinear effect decreases.

The graph of FIG. 3 shows that when b / a, Δ ₁ and Δ ₂ in the stepwise refractive index distribution of FIG.
For many combinations of core diameters b at which the chromatic dispersion value becomes zero at 55 μm (combinations of small diameter solution and large diameter solution),
Similar to FIG. 2, the correction coefficient and the MFD at that time are calculated and plotted, where x indicates a small core diameter, and ● indicates a large core diameter. As is clear from this graph, D at which the chromatic dispersion value is zero at 1.55 μm
As for SF, when a large core diameter b is adopted, it is found that the correction coefficient k is concentrated and distributed in a narrow range of 0.941 to 0.948. It seems that it was understood as variation.

On the other hand, when a small core diameter b is adopted as in the present invention, the correction coefficient k is distributed over a wide range from 0.945 to 0.97 and the MFD becomes large. It can be seen that the correction coefficient k rapidly increases as the MFD value increases, and the MFD values are distributed in the larger range of 7.8 μm to 8.6 μm.

As described above, by adopting the smaller core diameter b among the core diameters b at which the wavelength dispersion is zero at the wavelength of 1.55 μm, the correction coefficient k and MFD are increased, and the correction coefficient k is set to 0. By setting 95 or more, A
The non-linear effect can be greatly reduced by increasing eff. As a concretely preferable value, the correction coefficient k is 0.95.
To 0.97 and the MFD is 8.0 to 8.5 μm.
Range.

Although there is a concern that the bending loss may be increased by adopting a small core diameter, the degree of freedom in design is slightly reduced by appropriately changing other parameters, for example, increasing Δ _1. However, it can be suppressed to such an extent that there is no practical problem.

[0016]

As described above, in the DSF of the present invention, the MFD becomes large, the connection loss becomes small, and the effective area Aeff becomes large, so that the nonlinear effect can be suppressed to a low level. it can. Therefore, it can be suitably used for applications with high power density such as optical amplifiers.

[Brief description of drawings]

FIG. 1 is a diagram showing a stepwise refractive index distribution.

FIG. 2 is a graph showing the relationship between the correction coefficient k and MFD for a DSF having the refractive index distribution shown in FIG.

FIG. 3 is a correction coefficient k and MF for many combinations of core diameters having a wavelength dispersion value of zero at a wavelength of 1.55 μm.
It is a graph which shows the relationship with D.

[Explanation of symbols]

 b ... Core diameter

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kuniharu Himeno 1440 Rokuzaki, Sakura City, Chiba Prefecture Fujikura Co., Ltd.Sakura Plant (72) Ryozo Yamauchi 1440, Rokuzaki, Sakura City, Chiba Prefecture Fujikura Sakura Plant, Co., Ltd.

Claims (3)

[Claims] 1. A dispersion-shifted optical fiber having a stepwise refractive index distribution, which has a chromatic dispersion of almost zero at a wavelength of 1.55 μm, and has a chromatic dispersion of zero at a wavelength of 1.55 μm. A dispersion-shifted optical fiber having a small core diameter with a small value. 2. A dispersion-shifted optical fiber having a staircase type refractive index distribution, which has a wavelength dispersion of almost zero at a wavelength of 1.55 μm, and has a relational expression between an effective cross-sectional area and a mode field diameter of the optical fiber. Correction coefficient (k) is 0.9
A dispersion-shifted optical fiber of 5 or more. 3. A dispersion-shifted optical fiber having a chromatic dispersion at a wavelength of 1.55 μm which is substantially zero and having a stepwise refractive index distribution, wherein a core diameter of which the chromatic dispersion is zero at a wavelength of 1.55 μm A dispersion-shifted optical fiber having a small core diameter with a small value and having a correction coefficient (k) in the relational expression between the effective cross-sectional area of this optical fiber and the mode field diameter of 0.95 or more.