JPH09171119A - Dispersion compensation fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a dispersion compensation fiber which eliminates the worry about composite secondary strain. SOLUTION: This dispersion compensation fiber consists of a center core 1 which is uniformly doped with germanium dioxide, an outer core 2 which is disposed on the outer periphery of the center core 1 and is so doped with the germanium dioxide as to decrease the geranium dioxide gradually toward the outer peripheral direction and a clad 3 which is disposed on the outer periphery of the outer core 2 and is uniformly doped with fluorine. This outer core 2 is doped with the fluorine in such a manner that the viscosity on at least its outer periphery side is equaled to the viscosity of the clad 3.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a dispersion compensator, and more particularly to a dispersion compensator.
The present invention relates to a dispersion compensating fiber that can be used for a dispersion compensator in the 1.55 μm wavelength band.

[0002]

2. Description of the Related Art Due to recent technological innovation, a 1.55 μm band optical amplifier using an erbium-doped fiber is approaching a practical stage, and its application to a distribution system such as an optical CATV and a long-distance large-capacity transmission is drawing attention. . With this, existing
Even in the 1.3 μm optical transmission system, there is an increasing demand for high-speed transmission in the 1.55 μm band. However, the standard 1.3 μm single mode fiber has a wavelength of 1.5
Since it has a positive chromatic dispersion of about 17 ps / nm / km at 5 μm, a means for compensating for this is needed.

Currently, as one of the most promising compensation means,
An optical fiber having a negative chromatic dispersion with a sign opposite to that of the positive chromatic dispersion, a so-called dispersion compensating fiber, is used for the existing 1.3 μm.
A method for canceling the dispersion has been proposed by inserting it into a transmission system for business use. The conventional dispersion compensating fiber is provided with a center core in which germanium dioxide (GeO2) is almost uniformly doped (= the relative refractive index difference is substantially uniform), and is provided on the outer circumference of the center core, and GeO2 is directed toward the outer circumference. It is formed by an outer core that is gradually decreased (= the relative refractive index difference gradually decreases in the outer circumferential direction), and a clad that is provided on the outer circumference of the outer core and is doped with fluorine (F). ing.

An example of such a dispersion compensating fiber is
FIG. 4 shows each dopant concentration in the radial direction as a relative refractive index difference based on the refractive index of silica glass (hereinafter, simply referred to as “relative refractive index difference”), and FIG. 5 as a distribution of the relative refractive index difference in the radial direction. Show. In the dispersion compensating fiber, the relative refractive index difference Δ [GeO2] of the center core is + 1.8%, and the relative refractive index difference Δ [F] of the clad is −0.4%. Therefore, the relative refractive index difference Δ between the center core and the clad is 2.2%. The relative refractive index difference Δ [GeO2] of the outer core is almost the same value as the center core at the boundary surface with the center core,
The boundary surface of the clad has almost the same value as the clad.

Here, it is desirable that the dispersion compensating fiber inserted into the existing transmission system for 1.3 μm be as short as possible due to a problem of storage space and the like. Therefore, there is an increasing need for a dispersion compensating fiber having a large negative chromatic dispersion per unit length. In general, the dispersion compensating fiber can increase the negative chromatic dispersion by increasing the relative refractive index difference Δ between the center core and the clad. Further, the dispersion compensating fiber has an outer core between the center core and the clad, in which the relative refractive index distribution gradually decreases toward the outer peripheral direction. Therefore, the dispersion compensating fiber does not have the problem of an increase in structural defect loss that occurs when the relative refractive index difference between the core and the clad is increased in an ordinary optical fiber.

However, in the above-mentioned dispersion compensating fiber, since the refractive index of the outer core is gradually reduced toward the outer peripheral direction, a portion having a viscosity smaller than that of the clad is generated in the outer core corresponding portion in the drawing step. As a result, the above-mentioned dispersion compensating fiber is not suitable for practical use because it has a drawback that the tension is concentrated on that portion and residual stress is generated to cause polarization mode dispersion. This polarization mode dispersion does not adversely affect the amount used in a normal system, but may cause an increase in composite second-order distortion when an analog image signal of optical CATV is transmitted, for example.

[0007]

When the complex second-order distortion increases, the bit error rate increases and the polarization dispersion loss (PD) increases.
The problem of occurrence of L) arises. Therefore, there has been an urgent need to develop a dispersion-compensating fiber that does not cause complex second-order distortion, that is, has no polarization mode dispersion and has a large negative chromatic dispersion in a short length.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide a dispersion compensating fiber in which polarization mode dispersion does not occur and which has a large negative chromatic dispersion in a short length. The present invention, a center core GeO2 is almost uniformly doped, an outer core provided on the outer periphery of the center core and GeO2 is gradually reduced toward the outer peripheral direction, and fluorine provided on the outer periphery of the outer core In the dispersion compensating fiber made of a doped clad, the outer core is doped with fluorine so that the viscosity of the outer core is at least equal to that of the clad.

In the present invention, in order to avoid the problem of increased structural defect loss by increasing the relative refractive index difference between the center core and the clad, which substantially determines the magnitude of negative wavelength dispersion, the refractive index of the outer core is reduced. The ratio gradually decreases toward the outer circumference. In this case, the dispersion compensating fiber is
In the drawing step, a portion having a lower viscosity than the clad portion does not occur in the outer core corresponding portion, so that the tension is concentrated in that portion and residual stress does not occur. For this reason,
The manufactured dispersion compensating fiber is free from the polarization mode dispersion due to the above-mentioned reasons, and hence, the composite second-order distortion does not occur.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described in detail with reference to the drawings. As shown in FIG. 1, the dispersion compensating fiber includes a center core 1 having a substantially uniform relative refractive index difference, and an outer core 2 provided on the outer periphery of the center core 1 and having a relative refractive index difference gradually decreasing toward the outer peripheral direction. Cladding 3 provided on the outer periphery of the outer core 2 and doped with fluorine
Is formed.

The dispersion compensating fiber has a dopant concentration and a relative refractive index difference shown in FIGS. 2 and 3, respectively, in the radial direction. Here, FIG. 2 shows the relative refractive index difference in which the dopant concentration in the radial direction is based on the refractive index of quartz glass, and FIG. 3 shows the relative refractive index difference in the radial direction. In the figure, reference numeral 5 is a center core corresponding portion, reference numeral 6 is an outer core corresponding portion, and reference numeral 7 is a clad corresponding portion. The same applies to FIGS.

The dispersion compensating fiber described above was manufactured as follows. That is, first, a GeO 2 -doped quartz porous base material to be the center core and the outer core was prepared by the VAD method. Here, it is conventionally known that when sintering a porous silica preform in a SiF4 atmosphere, the doping amount of fluorine changes depending on the soot density. Therefore, in the production of the quartz porous base material, the soot density of the portion corresponding to the outer core was controlled so as to decrease toward the outer side in the radial direction by controlling the burner position, the oxygen gas amount, the hydrogen gas amount, and the like. . The porous base material thus produced was dehydrated and sintered under the conditions shown in Table 1 to obtain a core rod having an outer core formed outside the center core.

Next, the core rod is drawn to have an outer diameter of 12 mm, and the surface is subjected to hydrofluoric acid etching. Then, a clad made of fluorine-doped quartz glass with Δ [F] =-0.4% is attached to the outer periphery by an external attachment method. Then, a porous preform for optical fiber was manufactured. The optical fiber preform was obtained by dehydrating and sintering the porous preform for optical fiber under the conditions shown in Table 2.

[0014]

[Table 1]

[0015]

[Table 2] The optical fiber preform obtained as a result has a relative refractive index difference distribution as shown in FIG. Also, the concentration of each dopant is shown in FIG.
As shown in FIG.

That is, the relative refractive index difference Δ [G of the center core 1
eO2] = + 2.0%, relative refractive index difference Δ [F] = − 0.2% of the cladding 3,
In the outer peripheral portion of the outer core 2, the relative refractive index difference Δ [GeO2] = +0.3
%, And the relative refractive index difference Δ [F] = − 0.3% at the inner peripheral portion of the clad 3. Both GeO2 and F are known to reduce the viscosity of quartz glass. At this time, GeO2 and F are
Doping so that the relative refractive index difference between the outer peripheral portion of the outer core 2 and the inner peripheral portion of the clad 3 satisfies the relationship of Δ [GeO2] / Δ [F] = − 3. The viscosities are equal. That is, if the same amount of GeO2 and F are added to the silica glass in the relative refractive index difference, F will be 3 of GeO2.
Double the viscosity of the quartz glass.

Here, the contributions of GeO2 and F to the relative refractive index difference in each core, Δ [GeO2] and Δ [F], are the porous preforms for optical fibers prepared under the same conditions as described above. SiF4
It was determined by comparison with the refractive index distribution of the optical fiber preform obtained by sintering in an atmosphere not containing. A dispersion compensating fiber was drawn from the optical fiber preform manufactured as described above, and immediately a coating layer of an ultraviolet curable resin was applied. In this way, the outer core diameter is 2.5 μm and the clad diameter is 125 μm.
m, a coating diameter of 240 μm and a strip length of about 10 km were obtained.

When the polarization mode dispersion of the manufactured dispersion compensating fiber was measured by the Jones matrix method, the three values were 0.089 ps / Km ^-1/2 , 0.123 ps / Km ^-1/2 and 0.
Also Izure and 108ps / Km ^-1/2 0.1ps / Km ^-1/2 was around. It is equivalent to the polarization mode dispersion value (0.099ps / Km ^-1/2 ) of the standard 1.3 μm single-mode fiber. As a comparative example, as shown in FIG. 4, a center core corresponding portion 5 in which GeO2 is substantially uniformly doped in the radial direction and a center core corresponding portion 5 are provided on the outer periphery and GeO2 is doped so that the GeO2 gradually decreases toward the outer peripheral direction. Three conventional dispersion compensating fibers including the outer core corresponding portion 6 and the clad corresponding portion 7 provided on the outer periphery of the outer core corresponding portion 6 and doped with fluorine were manufactured.

Here, the relative refractive index difference Δ [GeO2] of the center core corresponding portion 5 is + 1.8%, and the relative refractive index difference Δ of the clad corresponding portion 7 is Δ.
[F] was −0.4%. Therefore, the relative refractive index difference Δ between the two is 2.2%. The refractive index distribution is as shown in FIG. 5, and the shape is almost the same as that of FIG. 3 showing the refractive index distribution of the dispersion compensating fiber manufactured in this embodiment.

When the polarization mode dispersion of the dispersion compensating fiber thus obtained was measured by the Jones matrix method, the three values were 0.790 ps / Km ^-1/2 and 0.420 ps / Km, respectively.
^-1/2 and 0.334ps / Km ^-1/2 are standard polarization mode dispersion values of standard single-mode fiber for 1.3 μm (0.099ps / Km ^-1/2 )
It was a much larger value.

[0021]

According to the present invention, since a portion having a lower viscosity than the clad equivalent portion does not occur in the outer core equivalent portion in the drawing step, the tension is concentrated in that portion and residual stress does not occur. Therefore, polarization mode dispersion does not occur due to the above-mentioned reasons, and there is no concern about composite second-order distortion.

[Brief description of the drawings]

FIG. 1 is a perspective view of a dispersion compensating fiber according to the present invention.

FIG. 2 is a dopant concentration distribution diagram showing each dopant concentration in the radial direction of the dispersion compensating fiber according to the present invention as a relative refractive index difference based on the refractive index of silica glass.

FIG. 3 is a relative refractive index difference distribution diagram showing the relative refractive index difference in the radial direction of the dispersion compensating fiber according to the present invention.

FIG. 4 is a dopant concentration distribution diagram showing each dopant concentration in the radial direction of the dispersion compensating fiber according to the comparative example as a relative refractive index difference based on the refractive index of silica glass.

FIG. 5 is a relative refractive index difference distribution diagram showing the relative refractive index difference in the radial direction of the dispersion compensating fiber according to the comparative example.

[Explanation of symbols]

 1 ... Center core 2 ... Outer core 3 ... Clad

Claims (1)

[Claims] 1. A center core substantially uniformly doped with germanium dioxide, an outer core provided on the outer periphery of the center core, and doped with germanium dioxide so that the amount of germanium dioxide gradually decreases in the outer peripheral direction, and an outer periphery of the outer core. Dispersion compensating fiber provided with a clad doped with fluorine, wherein the outer core is doped with fluorine so that at least the viscosity on the outer peripheral side is equal to that of the clad. .