JPH01207705A - Wide wavelength band/low dispersion single mode optical fiber - Google Patents

Abstract

PURPOSE:To obtain strong bending characteristics in a wide mode field diameter without losing wide wavelength and low dispersion characteristics by specifying the diameter of a 1st core and a specific refractive index difference between the 1st core and an outermost clad. CONSTITUTION:The title optical fiber has the distribution of refractive indexes so that the refractive index of the 1st core 1 is larger than that of the outermost layer clad 5, the refractive index of a 2nd core is almost equal to that of the clad 5, the refractive index of a 1st clad 3 is smaller than that of the clad 5, and the refractive index of a 2nd clad 4 is larger than that of the clad 5 and smaller than that of the 1st core 1. The diameter of the 1st core 1 is 4-6mum and the specific refractive index difference between the 1st core 1 and the outermost layer clad 5 is 0.45-0.7%. Consequently, the single mode optical fiber provided with high bending loss characteristics in a large mode field diameter can be obtained without losing the wide wavelength band and low dispersion characteristics.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention is directed to optical fibers, more specifically, for optical communications that transmit light with a wavelength of 1.5 μm to 1.6 μm, where the loss of the optical fiber is minimal. The present invention relates to a wide wavelength range, low dispersion single mode optical fiber that achieves a wide wavelength range and low dispersion, and is less affected by increased loss due to microbending during cable production.

[Prior Art] It is generally known that the transmission band of a single mode optical fiber is limited by chromatic dispersion. Chromatic dispersion is given by the sum of material dispersion that depends on the fiber material and structural dispersion that depends on the refractive index distribution of the fiber.

The material dispersion of a silica-based optical fiber is; around a wavelength of 1.3 μm. Conventional single mode optical fibers are designed in a large range of normalized frequency v values where the structural dispersion is almost identical, so the wavelength at which the dispersion becomes zero is approximately 1.3 μm.
, and large-capacity transmission was possible at this wavelength.

In addition, in recent years, the wavelength 1 where the loss of silica fiber is the minimum
．． A dispersion-shifted optical fiber in which the τ dispersion wavelength is shifted to the 1.5 μm band has been put into practical use by realizing a large structural dispersion in the vicinity of 5 μm.

Since it achieves dispersion at wavelengths of For this reason, research is being actively conducted aimed at creating optical fibers with a wide wavelength range and low dispersion that can achieve low dispersion over a wide wavelength range.

FIGS. 2 and 3 respectively show a cross-sectional view of a W-shaped layered index-gradient optical fiber and its refractive index distribution as an example of a conventional wide-wavelength low-dispersion optical fiber.

Here, 6 is a core, 3 is a first cladding around the core 6, and 5 is an outermost cladding disposed around the first cladding 3. Note that the structural parameters of this W-shaped optical fiber are selected such that the bending loss is substantially minimized at a mode field diameter of 8 μm.

In FIG. 3, Δ1 and Δ− are respectively core 6
and the relative refractive index difference of the first cladding 3, 2a and 2ac
are the diameters of the core 6 and the first cladding 3, respectively. Here, these relative refractive index differences are defined by the following equation.

Δ+-(n2-nc2)/2n2 Δ-(n(,'-ne')/2no' n, nl and nc are the refractive index differences between the core 6, first cladding 3 and outermost cladding 5, respectively. .

An example of the dispersion wavelength characteristic of the W-shaped optical fiber shown in FIG. 4 obtained by theoretical calculation is shown. The theoretical calculations performed here were performed using the method disclosed in Japanese Patent Application Laid-Open No. 62-52508, in which the refractive index distribution of the optical fiber is divided into multiple layers in the radial direction, and the electric field distribution in each layer is calculated. We used a method to numerically determine the propagation constant from the boundary conditions of the electromagnetic field components in each layer.

The W-type optical fiber is a fiber that operates in a single mode while achieving low dispersion in a wide wavelength range due to the mode cut filter effect of the outermost cladding 5.

From FIG. 4, it can be seen that in the wavelength range of about 1.35 to 1.6 μm, the wide wavelength range low dispersion of tik (iiaO=±3 ps/km/nm or less) is satisfied.

FIG. 5 shows the relationship between the mode field diameter and allowable bending radius of a W-shaped optical fiber at a wavelength of 1255 μm.

Here, the allowable bending radius refers to the bending radius at which the increase in loss due to uniform bending at the used wavelength is 0.01 dB/km, and the smaller this value is, the stronger the fiber is against bending.

In putting optical fibers into practical use, the mode field diameter and the allowable bending radius are important factors. Regarding the mode field diameter, in order to enable low-loss connection, it is currently considered that a value of 8 μm or more is required in the used wavelength range. Further, regarding the allowable bending radius, in order to suppress an increase in loss after cable formation and installation, it is necessary to set the allowable bending radius to a value of 6 cm or less at the wavelength used.

[Problems to be Solved by the Invention] However, as can be seen from FIG. 5, in a W-type optical fiber, when the mode field diameter is set to 8 μm or more, the allowable bending radius becomes about 7c+n or more.
There were major drawbacks to its practical application.

In conventional W-shaped optical fibers, since the first cladding exists around the core, the influence of this on the waveguide mode has been ignored. In the conventional W-shaped optical fiber, as the depth Δ- and width (ae-a) of the first cladding are narrowed,
It has characteristics that make it easy to achieve low dispersion over a wide wavelength range.

However, in this case, due to the influence of the first cladding, the confinement of the optical power in the core becomes strong and the mode field diameter becomes small. moreover,
The first cladding also has the effect of reducing the normalized propagation constant of the propagation mode, which also has the disadvantage of increasing the allowable bending radius.

SUMMARY OF THE INVENTION Therefore, the object of the present invention is to provide a wide wavelength range low dispersion optical fiber which solves the drawback of conventional wide wavelength range low dispersion optical fibers, that is, the bending characteristics become weak when the mode field diameter is increased. Our goal is to provide the following.

[Means for Solving the Problems] In order to achieve such an object, the present invention sequentially forms a first core, a second core, a first clad, a second clad, and an outermost clad in concentric circles from the inside. The refractive index of the first core is greater than the refractive index of the outermost cladding, the refractive index of the second core is approximately equal to the refractive index of the outermost cladding, and the refractive index of the first cladding is , the refractive index of the second cladding is smaller than the refractive index of the outermost cladding, and the refractive index of the second cladding is larger than the refractive index of the outermost cladding, and
A single mode optical fiber having a refractive index distribution smaller than the refractive index of the core, the diameter of the first core is 4 to 6 μm, and the relative refractive index difference of the first core with respect to the outermost cladding is 0.45 ~0.7%.

[Function] According to the present invention, without impairing wide wavelength range low dispersion,
We aim to realize a single mode optical fiber with a large mode field diameter and good bending loss characteristics.

In order to improve the above-mentioned drawbacks, the optical fiber of the present invention has the following features:
By providing the second core between the core and the first cladding, it is possible to increase the mode field diameter without impairing low dispersion in a wide wavelength range. Furthermore, by arranging a second cladding having a larger refractive index than the outermost cladding outside the first cladding,
The normalized propagation constant is increased to improve bending characteristics.

[Example] Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows a cross-sectional structure of an embodiment of the optical fiber of the present invention, where 1 is a first core, 2 is a second core, 3 is a first cladding, 4 is a second cladding, and 5 is an outermost layer. It is clad.

1st core 1. Second core 2. First cladding 3. The second cladding 4 and the outermost cladding 5 are sequentially arranged concentrically in multiple layers from the inside, and the refractive index of the first core l is larger than the refractive index of the outermost cladding 5, and the refractive index of the second core 2 is The refractive index of the first cladding 3 is approximately equal to the refractive index of the outermost cladding 5, the refractive index of the first cladding 3 is smaller than the refractive index of the outermost cladding 5, and the refractive index of the second cladding 4 is the refractive index of the outermost cladding 5. , and smaller than the refractive index of the first core 1. The diameter of the first core 1 is 4 to 6 μm, and the relative refractive index difference between the first core 1 and the outermost cladding 5 is 0.45 to 0.7.
%.

FIG. 6 is a diagram showing an example of the refractive index distribution of the optical fiber of the present invention shown in FIG.
．． Let the relative refractive index difference and diameter of the first cladding 3 and the second cladding 4 be Δ", Δ0.Δ-9Δ, and 2a, 2a, 2a2, 2ae, respectively.

Note that here, the relative refractive index difference is defined by the following equation using the refractive index nC of the outermost layer cladding 5 as a reference.

Δ"= (n2-ne')/2n2 Δ.-(n,'-ne')/2no'Δ-=(n,'
-nc')/2nl'Δr''(n2'-n%)/2n2' Here, n, no, nl, n2 and nc are respectively the first core 1. the second core 2. the first cladding 3. This is the refractive index of the second cladding 4 and the outermost cladding 5.
The above theoretical calculations were also performed using the method disclosed in Japanese Patent Application Laid-Open No. 62-52508.

Figure 7 shows the calculation results showing the relationship between 2ac and Δ0 of the optical fiber of the present invention when the dispersion σ is within zps/km/nm at a wavelength of 1.5 to 1.6 μm. be. In the example of FIG. 7, the shape parameters of the optical fiber of the present invention are a/ac, at/ac, a2/ae.
and Δ0/Δ2. Δ−/Δ3. The values of Δ, /Δ0 are 0.3 in fiber groups l and 2, respectively.
5゜0.6.0.8.0. -0.7.0.5 and 0.
4.0.55.0.8°0, -0.7.0.6.

In FIG. 7, in the region where the value of Δ1 is smaller than the black dot, the wavelength slope of the waveguide dispersion becomes small, so the wavelength slope of the material dispersion cannot be canceled, and the dispersion condition (1
(σ1≦2 ps/km/nm) cannot be satisfied.

In addition, in FIG. 7, when other shape parameters are kept constant, for a certain value of Δ0, a change in dispersion conditions in the range of ±2 ps/km/nm, a change in 2ac is approximately 2o, iμm.
Therefore, wide wavelength range low dispersion optical fiber has 2ae and Δ
It is considered that the relationship 3 is almost uniquely determined.

FIG. 8 shows Δ0 and wavelength 1.5 for fiber groups 1 and 2 of the optical fiber of the present invention shown in FIG.
This is a calculation result of the relationship between the mode field diameter and the allowable bending radius at 5 μm.

It can be seen that the mode field diameter and the allowable bending radius increase as Δ" decreases. Furthermore, from FIG. 8, it is clear that the relationship between the mode field diameter and the allowable bending radius is almost uniquely expressed. Therefore, from this relationship when the shape parameters of the optical fiber are changed, the characteristics of the optical fiber of the present invention can be optimized.

FIG. 9 shows the relationship between the allowable bending radius R and the mode field diameter 2W, which are uniquely determined as described above. FIG. 9 shows the calculation results using ξ1 (-Δr/Δ゛), which represents the magnitude of the refractive index of the second cladding 4, as a parameter at a wavelength of 1.55 μm, and shows the bending property improvement effect of the second cladding 4. represents.

In Figure 9, a/ac, a17'ac, a2/
ae and Δ0/Δ1. The values of Δ−/Δ” are, respectively,
0.4.0.6°0.8 and O, -0.7, and ae and Δ0 were determined to satisfy the above dispersion conditions.

From FIG. 9, it can be seen that when the mode field diameter 2W is constant, the bending characteristics are improved by increasing ξ1, that is, by increasing the refractive index of the second cladding 4.

However, the maximum value of the mode field diameter 2W that satisfies the dispersion condition (black circle in the figure; there is no solution that satisfies the dispersion condition with a mode field diameter larger than the black circle) is
It can be seen that the refractive index of the second cladding 4 decreases as the refractive index becomes steeper.

FIG. 1O is a calculation result showing the mode field diameter expansion effect of the second core 2, and ξ2 representing the size of the first core 1.
(= a/ae) as a parameter, wavelength 1.55μ
The relationship between the allowable bending radius R and the mode field diameter 2W at m is shown.

In Figure 1θ, at/ac, a2/aC and Δ
. /Δ3゜Δ−/Δ9. The values of Δ and /Δ4 are, respectively,
0.6, 0.8 and 0. -0.7.0.4. From FIG. 10, it can be seen that ξ2 is small, that is, the width of the second cladding 4 (-
By increasing (a, -a)/ac), the maximum value of the mode field diameter can be increased. In this case, the bending properties are also slightly degraded, but this can be improved by increasing ξ by a small amount.

In this way, by reducing the refractive index and ξ of the second cladding 4 and appropriately adjusting the width of the second core 2,
The fiber of the present invention has a large mode field diameter and can be resistant to bending. Further, the characteristics of dispersion and mode field diameter are largely dependent on the structural parameters of the first core 1.

Figure 11 shows the above-mentioned dispersion conditions and mode field diameter 7.
This is the calculation result of the relationship between 28 and Δゝ of the optical fiber of the present invention that satisfies the requirement of μm or more and the allowable bending radius of 6cn+ or less, and the shape parameter is 2a.

Combinations for which a solution of Δ1 exists are selected. The black circles are plots of calculation results obtained when the above conditions are satisfied by changing the shape parameters as appropriate; in reality, solutions also exist in this vicinity. From this, 2a and Δ9 are approximately 4 to 6 μm and 0.45 to 0.7%, respectively.
It can be seen that the range is within the range of .

Note that the maximum value (0.7%) and minimum value (0.4%) of Δ0
5%) are due to the mode field diameter and allowable bending radius conditions, respectively, and the first core 1
The range of change in the core diameter 2a is caused by a change in dispersion conditions within a range of ±2 ps/km/nm.

The relative refractive index difference Δ of the second core 2. As for the calculation result,
It was found that when Δ0/Δ1 is within the range of approximately 0.1 to −0.1, the effects on the bending and mode field diameter characteristics are small.

Next, Table 1 shows that the dispersion value is within ±2 ps/km/nm at a wavelength of 1.5 μm to 1.6 μm, and the dispersion value is within ±2 ps/km/nm at a wavelength of 1.5 μm to 1.6 μm.
The mode field diameter at 55 μm is approximately 8 μm,
A design example of an optical fiber of the present invention having an allowable bending radius of 6 cm or less will be shown.

TABLE 1 Here, fibers A and B have the structural parameters indicated by the filled circles in fiber groups 1 and 2 in FIG. 7, respectively. The shape parameters of these fibers are designed by the method shown in FIG. 9 and FIG. 1O.

Note that by using FIGS. 7 and 8, it is possible to determine 2ac and Δ0 of the fiber.

FIG. 12 shows the relationship between the bending characteristics and the mode field diameter of the optical fiber of the present invention obtained from FIG. 8, and FIG. 13 shows the calculation results of its dispersion wavelength characteristics.

As is clear from FIG. 12, the optical fiber of the present invention is capable of reducing the allowable bending radius at the operating wavelength to 6 cm or less when the mode field diameter is 8 μm, and as is clear from FIG. It can be seen that low dispersion (within ±2 ps/km/nm) can be achieved in the range (approximately 1.5 μm to 1.67 μm for fiber B).

[Effects of the Invention] As described above, the present invention has the advantage that strong bending characteristics can be obtained in a wide mode field diameter without impairing low dispersion in a wide wavelength range. Therefore, by using the fiber of the present invention, there is an advantage that a wavelength multiplexed capacity transmission system can be easily realized.

[Brief explanation of the drawing]

Fig. 1 is a cross-sectional view showing an example of the cross section of an optical fiber of the present invention, Fig. 2 is a cross-sectional view showing an example of a cross section of a conventional W-layer graded index optical fiber, and Fig. 3 is a cross-sectional view of a conventional W-layer optical fiber. A characteristic diagram showing the refractive index distribution of a refractive index distribution optical fiber. Figure 4 is a characteristic diagram showing the dispersion wavelength characteristics of a conventional W-shaped optical fiber. Figure 5 is a characteristic diagram showing the dispersion wavelength characteristics of a conventional W-shaped optical fiber at a wavelength of 1.55 μm. A characteristic diagram showing the relationship between mode field diameter and allowable bending radius. Figure 6 is a characteristic diagram showing the refractive index distribution of the optical fiber of the present invention. Figure 7 is a characteristic diagram of 2a and Δ0 that satisfy the dispersion condition in the optical fiber of the present invention. FIG. 8 is a characteristic diagram showing the relationship between Δ9, mode field diameter, and allowable bending radius in the optical fiber of the present invention. FIG. 9 is a characteristic diagram showing the bending characteristic improvement effect of the second cladding in the optical fiber of the present invention. FIG. 10 is a characteristic diagram showing calculation results regarding the relationship between the allowable bending radius and the mode field diameter shown in FIG. FIG. 11 is a characteristic diagram showing the relationship between 28 and Δ3 when the shape parameters are changed in the optical fiber of the present invention when the mode field diameter is 7 μm or more; FIG. 13 is a characteristic diagram showing the relationship between the mode field diameter and allowable bending radius at a wavelength of 1.55 μm of the optical fiber of the present invention, and FIG. 13 is a characteristic diagram showing the dispersion wavelength characteristic of the optical fiber of the present invention. l...First core, 2...Second core, 3...First cladding, 4...Second cladding, 5...Outermost layer cladding, 6...Core.

Claims (1)

[Scope of Claims] 1) A first core, a second core, a first clad, a second clad, and an outermost clad are arranged concentrically in multiple layers in order from the inside, and The refractive index of one core is
greater than the refractive index of the outermost cladding, the refractive index of the second core is approximately equal to the refractive index of the outermost cladding, and the refractive index of the first cladding is smaller than the refractive index of the outermost cladding, The refractive index of the second cladding is greater than the refractive index of the outermost cladding, and
A single mode optical fiber having a refractive index distribution smaller than the refractive index of the core, wherein the first core has a diameter of 4 to 6 μm, and the relative refractive index difference of the first core with respect to the outermost cladding is 0.45
~0.7%.