JPH1130725A - Lower-dispersion optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the low-dispersion optical fiber which has small wavelength dispersion to its overall length and is reduced in 4-light-wave mixture as one of nonlinear effects. SOLUTION: The low-dispersion optical fiber as a single-mode optical fiber is constituted which varies in wavelength dispersion along the length, has a part where the wavelength dispersion in its use wavelength band is positive and a part where the dispersion is negative so that the positive part and negative part cancel each other, and includes a part where the absolute value of the wavelength dispersion is <=0.1 ps/km/nm by <=1% of the overall length.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low dispersion optical fiber, and more particularly to a low dispersion optical fiber having reduced four-wave mixing (hereinafter abbreviated as FWM), which is one of nonlinear effects, and a method of manufacturing the same. It is.

[0002]

2. Description of the Related Art As conventionally known, the low-loss wavelength band of a silica-based optical fiber is 1.4 to 1.7 μm, preferably 1.55 μm. The dispersion-shifted optical fiber (hereinafter, referred to as DSF) is substantially single-mode propagating around a wavelength of 1.55 μm (hereinafter, referred to as a wavelength of 1.55 μm) where the loss of the silica-based optical fiber is minimum, and the chromatic dispersion Is zero or almost zero. Such a DSF changes its structural dispersion by changing its refractive index distribution into various refractive index distribution shapes (hereinafter referred to as a refractive index profile) such as a dual-core type and a ring-core type. Try to make it smaller.

On the other hand, in recent years, high-power density light is generated using a high-output optical booster amplifier and the like, and the high-power density light is transmitted through an optical fiber.
It has been studied to increase the span of relayless transmission.
However, in the conventional DSF, it is known that when the power of normally incident light is several mW or more, a nonlinear effect such as FWM occurs and transmission characteristics deteriorate. Therefore, there is a problem that light with high power density cannot be transmitted effectively, and the transmission distance of an optical signal cannot be extended.

[0004] The mechanism of generation of FWM will be described below (see Document 2). Assume that three light waves i, j, k, whose wavelengths do not always match, are propagating in an optical fiber. At this time, due to the interaction of these three waves, a fourth light wave ijk is newly generated. This fourth lightwave ij
The power of k is represented by the following equation 1.

[0005]

(Equation 1)

The degeneracy coefficient D in the above equation 1 corresponds to the following equation 2 corresponding to the coincidence and non-coincidence of the wavelengths of the three light waves i, j and k.
Indicated by

[0007]

(Equation 2)

[0008] The effective length Leff in the above equation 1 is expressed by the following equation 3 using the transmission loss α and the fiber length L.

[0009]

(Equation 3)

The generation efficiency η in the above equation (1) is expressed by the following equation (4).
It is represented as

[0011]

(Equation 4)

In the above equation (4), Δβ is a propagation constant βijk, βi, β of each of the four light waves represented by the following equation (5).
It is a value obtained from j and βk.

[0013]

(Equation 5)

From the above equation (4), when Δβ is zero, the generation efficiency η becomes maximum. As Δβ increases, the generation efficiency η decreases and approaches zero. Δβ is further calculated by the following equation 6.
Can be rewritten as

[0015]

(Equation 6)

In Equation 6, except for the case where the wavelength dispersion Dc of the waveguide is close to zero, the second term in parentheses can be almost ignored. Therefore, Δβ becomes minimum when Dc is zero, and increases as the absolute value of Dc increases. That is, the generation efficiency η of the FWM depends on the chromatic dispersion of the optical waveguide (optical fiber), and becomes maximum when the chromatic dispersion of the optical waveguide (optical fiber) is zero.

As described above, in order to suppress FWM,
It can be seen that it is effective to shift the chromatic dispersion of the optical fiber from zero to increase its absolute value. However,
Since DSF is an optical fiber whose chromatic dispersion in the wavelength band of 1.55 μm is close to zero, it is inevitable that the wavelength is 1.55 μm.
When used in a band, the generation efficiency η of FWM increases.

On the other hand, 1.3 μm, which is widely used in general,
m-band single mode optical fiber (1.3 μm SM
F is abbreviated to zero near the wavelength of 1.3 μm, but has a wavelength dispersion of about +17 ps / km / nm in the wavelength band of 1.55 μm, and is therefore used in the wavelength band of 1.55 μm. In this case, the generation efficiency η of FWM is low. However, when a DSF having such a large chromatic dispersion is used for transmitting light in the 1.55 μm band, waveform distortion occurs due to group velocity dispersion over the entire length, and transmission characteristics deteriorate. Therefore, in the 1.55 μm wavelength band with low loss, a FWM generation efficiency η is low, the chromatic dispersion over the entire length is small, and transmission characteristics are hardly deteriorated.

[0019]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a low dispersion optical fiber which has a small chromatic dispersion over its entire length and can suppress the occurrence of FWM in a wavelength band to be used.

[0020]

In order to solve the above-mentioned problems, a first invention of the present invention is a single mode optical fiber, wherein chromatic dispersion changes in a length direction, and in the length direction, The wavelength dispersion in the used wavelength band has a positive portion and a negative portion, and these positive and negative portions cancel each other's wavelength dispersion, and A low dispersion optical fiber characterized in that the length of a portion where the absolute value of chromatic dispersion is less than 0.1 ps / km / nm is 1% or less with respect to the entire length. First
In the invention, it is desirable that the average value of the absolute value of the chromatic dispersion over the entire length is 0.3 to 3 ps / km / nm, and preferably 0.5 to 1.5 ps / km / nm. Further, it is desirable that the absolute value of the chromatic dispersion is 5 ps / km / nm or less, preferably 3 ps / km / nm or less. In the first aspect, the chromatic dispersion is canceled out by the portion where the chromatic dispersion becomes positive and the portion where the chromatic dispersion becomes negative, and the chromatic dispersion is averaged over the entire length. Can be reduced. Specifically, the chromatic dispersion over this entire length is preferably 1 ps / km / nm or less, more preferably 0.5 ps / km / nm.
It is preferable that the distance is designed to be s / km / nm or less.

This low dispersion optical fiber can be manufactured by any of the following three methods. The first method is to change the core diameter in the length direction by drawing a fiber preform in which the core diameter and the cladding diameter are constant in the length direction so that the cladding diameter changes in the length direction. Forming an optical fiber whose core diameter changes in the length direction. The second method is to cut a part of the outer peripheral surface of the fiber preform in which the core diameter and the clad diameter are constant in the length direction,
The ratio of the clad diameter / core diameter of the fiber preform is changed in the length direction, and the fiber preform is drawn so that the clad diameter is constant in the length direction, so that the core diameter is increased in the length direction. A method for manufacturing a low dispersion optical fiber, comprising forming an optical fiber that is changing. These first and second methods utilize the fact that chromatic dispersion changes when the core diameter (outer diameter of the core) is changed.
A third method is to form a fiber preform in which the refractive index of the core changes in the length direction by changing the addition amount of the dopant added to the core in the length direction. A method for manufacturing a low dispersion optical fiber, characterized in that an optical fiber whose chromatic dispersion changes in the length direction is formed by drawing the clad diameter so as to be constant in the length direction.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Prior to the present invention, the present inventors disclosed in Japanese Patent Application No. 7-13218 a single-mode optical fiber having a core diameter varying in the longitudinal direction. The equivalent refractive index for the propagation mode changes with the change of the wavelength, and the low dispersion light has a portion where the chromatic dispersion becomes positive and a portion where the chromatic dispersion becomes negative with the change of the equivalent refractive index. A fiber is proposed. In this low dispersion optical fiber, in the portion where the chromatic dispersion is positive and the portion where the chromatic dispersion is negative, the chromatic dispersion is canceled out and averaged in the length direction of the fiber, and the chromatic dispersion over the entire length is zero or almost zero. It becomes. In the present invention, the term “chromatic dispersion” simply refers to the value of the chromatic dispersion in a part of the optical fiber.

Further, in this low dispersion optical fiber, stimulated Brillouin scattering is less likely to occur due to a change in core diameter along the length direction (see Document 1).
Stimulated Brillouin scattering relies on Brillouin frequency shift. The Brillouin frequency shift changes with a change in the core diameter and a change in the amount of dopant added to the core. If the Brillouin frequency shift changes in the length direction of the optical fiber, the occurrence of stimulated Brillouin scattering can be suppressed. Further, in a portion where the chromatic dispersion changes from a positive portion to a negative portion (or a negative portion to a positive portion), there is a portion where the absolute value of the chromatic dispersion decreases and approaches zero. ,
In this portion, FWM is likely to occur. However, conversely, in the portion where the absolute value of chromatic dispersion is not near zero, FWM
Can be suppressed. As described above, although the low dispersion optical fiber can suppress the generation of FWM in a part thereof, no study has been made to suppress the generation of FMW over its entire length. Therefore, the present invention improves and develops the low-dispersion optical fiber so that F
This is to suppress generation of WM.

The wavelength band used by the low dispersion optical fiber of the present invention is usually the 1.55 μm wavelength band,
It refers to a wavelength range of 1575 nm. Low dispersion optical fiber of the present invention (hereinafter sometimes simply referred to as optical fiber)
The total length is not particularly limited, but is usually several km to several tens km.
It is said. Also, a single mode optical fiber,
The refractive index shape is a step-index type, a dual-core type having a stepped core portion, or the like. The dual-core type is preferable because bending loss can be suppressed low.

In the low dispersion optical fiber of the present invention, the equivalent refractive index for the propagation mode changes in the length direction, and the chromatic dispersion changes with the change of the equivalent refractive index.
The equivalent refractive index for the propagation mode can be changed by changing the core diameter. Another method for changing the equivalent refractive index for the propagation mode is as follows.
For example, it is possible to adopt a method in which the core diameter is constant and the amount of a dopant such as germanium added to the core is adjusted by changing the amount in the length direction.

In this low dispersion optical fiber, the chromatic dispersion changes in the longitudinal direction from a positive portion to a negative portion or from a negative portion to a positive portion as the equivalent refractive index changes with respect to the propagation mode. It is changing. The chromatic dispersion becomes positive and negative, and the chromatic dispersion cancels out in the longitudinal direction and is averaged, so that the chromatic dispersion over the entire length can be reduced. Specifically, the chromatic dispersion over the entire length can be reduced to 1 ps / km / nm or less, and more preferably, it is suitably designed to be 0.5 ps / km / nm or less.

At this time, the portion where the chromatic dispersion becomes positive and the portion where the chromatic dispersion becomes negative only have to be adjacent to each other at least one by one in the longitudinal direction of the optical fiber. The length of the portion where the wavelength dispersion becomes positive or the portion where the wavelength dispersion becomes negative is not particularly limited, and can be adjusted so as to obtain desired optical characteristics, but each is usually about several hundred m to several km. .

In this low-dispersion optical fiber, a portion where the chromatic dispersion shifts from a positive portion to a negative portion and a portion where the chromatic dispersion shifts from a negative portion to a positive portion along the length direction. In some portions, there is a portion where the absolute value of the chromatic dispersion is less than 0.1 ps / km / nm, and the length of this portion is set to 1% or less of the entire length. That is, FWM
Since the ratio of the portion where the absolute value of the chromatic dispersion in which is easily generated is small, the generation of FWM can be suppressed over the entire length. If this length exceeds 1%, it is difficult to sufficiently suppress the generation of FWM. In addition, in this low dispersion optical fiber, since the Brillouin frequency shift that changes with the change in the core diameter and the amount of the dopant added to the core changes in the length direction, the occurrence of stimulated Brillouin scattering is reduced. Can be suppressed.

Preferably, the value obtained by averaging the absolute values of the chromatic dispersion over the entire length is 0.3 to 3 ps / km / nm.
Preferably, it is 0.5 to 1.5 ps / km / nm.
By doing so, the FW of 0.3 ps / km / nm or more can be obtained.
Since the chromatic dispersion is averagely distributed in a range where M does not easily occur, FWM can be further suppressed. Also, 3p
By not exceeding s / km / nm, it is possible to suppress wavelength distortion over the entire length and to suppress deterioration of transmission characteristics.

The absolute value of the chromatic dispersion is desirably 5 ps / km / nm or less, preferably 3 ps / km / nm or less. Even if there is a portion where the absolute value of the chromatic dispersion exceeds 5 ps / km / nm, if the chromatic dispersion can be canceled by the positive portion and the negative portion, the chromatic dispersion as the entire length can be reduced. . However, if the length of the portion where the chromatic dispersion is larger than necessary is long, large wavelength distortion occurs in this portion, and it may be difficult to cancel the wavelength distortion.

As described above, in the low dispersion optical fiber described above, the chromatic dispersion is canceled out and averaged in the longitudinal direction by the portion where the chromatic dispersion becomes positive and the portion where the chromatic dispersion becomes negative. Therefore, chromatic dispersion over the entire length can be reduced, and wavelength distortion due to group velocity dispersion can be suppressed. At the same time, since the length of the portion where the chromatic dispersion is 0.1 ps / km / nm or less is 1% or less of the entire length, the occurrence of FWM can be suppressed. Further, since the Brillouin frequency shift changes along the length direction, the occurrence of stimulated Brillouin scattering can be suppressed at the same time.

(Reference 1) Y. Miyajima, M. Ohashi and K. Na
kajima, "Novel dispersion-managedfiber for suppress
ing FWM and an evaluation of its dispersiondistrib
ution ", OFC'96.PD7-1,1996 (Reference 2) N.Shibata, RPBraun, and RGWaarts," Phas
e-mismacth dependenceof efficiency of wave generat
ion through four-wave mixing in a single-mode opti
calfiber ", J. Quantum electron., QE-23, 1205-1210, 1987

[0033]

【Example】

(Examples) Examples will be described below with reference to manufacturing methods. First, a fiber preform (hereinafter abbreviated as a preform) for a dual-core dispersion-shifted optical fiber having a refractive index profile shown in FIG. 1 was prepared. In the figure, reference numeral 1 denotes a central core, and the central core 1
And a step core portion 2 provided on the outer periphery thereof, a core 3 is formed. A clad 4 is provided on the outer periphery of the core 3. Central core 1 and stair core 2
Is made of SiO ₂ to which germanium is added, and the central core 1 has a higher amount of germanium added than the stepped core 2 and has a higher refractive index. Cladding 4 is pure Si
It is made of O ₂ and has a lower refractive index than the step core portion 2.

Δ1 is the relative refractive index difference between the cladding 4 and the central core 1, Δ2 is the relative refractive index difference between the step core 2 and the cladding 4, 2a is the outer diameter of the central core 1, 2b is the step core. 2 shows the outer diameter (outer diameter of the core 3). In this embodiment, b
/ A is 3.1, Δ1 is 0.85%, Δ2 is 0.085%
Met. The outer diameter of the clad 4 is 40.0 mm, the outer diameter of the core 3 is 5.8 mm, and the ratio of (the outer diameter of the clad 4) / (the outer diameter of the core 3) is 6.9. It was constant in the length direction.

Further, the relationship between the outer diameter of the cladding 4 (cladding diameter) and the outer diameter of the core 3 (core diameter) and the relationship between the outer diameter of the core 3 and chromatic dispersion when the base material is drawn in advance are described. In advance, the base material was drawn according to these relationships, and an optical fiber was manufactured such that the outer diameter of the clad 4 changed in the length direction. That is, if the outer diameter of the clad 4 is changed in the length direction, the outer diameter of the core 3 provided in contact with the clad 4 can be changed accordingly. Then, the equivalent refractive index changes with the change of the outer diameter of the core 3, and as a result, an optical fiber whose chromatic dispersion changes in the length direction can be obtained.

FIG. 2 schematically shows this optical fiber as a sectional view taken along the axis. In this optical fiber, a large diameter portion A having an outer diameter of about 132 μm of the cladding 4 and a small diameter portion B having an outer diameter of 122 μm are alternately arranged approximately every 10 km along the length direction. While the outer diameter of the cladding 4 is increased or decreased in the length direction, the cladding 4 is drawn so as to have a total length of about 40 km. In the figure, reference symbol C indicates a transition portion from the large diameter portion A to the small diameter portion B or from the small diameter portion B to the large diameter portion A. The end of the optical fiber having an outer diameter of about 132 μm is referred to as an E end, and the other end having an outer diameter of about 122 μm is referred to as an S end. In this optical fiber, as the outer diameter of the clad 4 increases and decreases, the core 3 extends in the longitudinal direction.
The outer diameter of the core 3 in the large diameter portion A is about 19.2 μm, and the outer diameter of the core 3 in the small diameter portion B is 1
It was 7.7 μm.

FIG. 3 is a graph showing the relationship between the outer diameter of the cladding 4 and the position in the longitudinal direction of the optical fiber.
3 is an enlarged view showing a change in the outer diameter of the clad 4 at a transition portion C from the large diameter portion A to the small diameter portion B shown in FIG. FIG. 5 is a diagram showing a measured value obtained by measuring chromatic dispersion by cutting 1 km from each of the E end and the S end of the optical fiber and a change in the outer diameter of the cladding 4 shown in FIG. 6 is a graph showing a change in chromatic dispersion in a direction. Looking at the graph of FIG. 5 along with the change of the outer diameter of the cladding 4 of FIG.
(Negative chromatic dispersion) in the small-diameter portion B, and these are alternately located in the length direction via the transition portion C.

In the transition portion C, there is a portion where the chromatic dispersion approaches zero and its absolute value is less than 0.1 ps / km / nm. One condition in the low dispersion optical fiber of the present invention is that the chromatic dispersion is 0.1 ps / k.
The length of the portion that is less than m / nm is less than 1% of the whole. Therefore, it can be seen that it is preferable to increase the amount of change in chromatic dispersion per length at the transition portion C as much as possible.

In this embodiment, specifically, the length of the portion where the absolute value of chromatic dispersion is less than 0.1 ps / km / nm is 0.08% of the total length, and the absolute value of chromatic dispersion is 1.05 p
s / km / nm or less.
The average value over 0.9 km is 0.92 ps / km /
nm.

Comparative Example A material similar to the base material used in the examples was drawn so that chromatic dispersion was almost zero in the wavelength band of 1.55 μm and constant in the length direction.

Table 1 shows design conditions and optical characteristics of the example and the comparative example. In Table 1, the mode field diameter, the transmission loss, the bending loss, the chromatic dispersion at the entire length, and the chromatic dispersion at the S end and the E end are measured values at a wavelength of 1.55 μm.

[0042]

[Table 1]

Further, the generation efficiency η of these FWM is expressed by F
FIG. 6 is a graph showing the relationship between the frequency difference between the probe light and the pump light for generating the WM light and the generation efficiency η.
In the figure, ▲ indicates the measured value of the example, and ● indicates the measured value of the comparative example. Curve A is the calculated value (theoretical value) of the example, and curve B is the calculated value (theoretical value) of the comparative example. From the results shown in Table 1 and FIG. 6, the optical fiber of the example according to the present invention has a small chromatic dispersion over the entire length, has optical characteristics equivalent to those of the comparative example, and has a low dispersion with a small FWM generation efficiency η. It is clear that the optical fiber is used.

The method of manufacturing an optical fiber having a chromatic dispersion that changes in the length direction is the same as the method described in this embodiment.
For example, the following method can be adopted. As another example of changing the outer diameter of the core, first, as shown in FIG. 7, a part of the outer peripheral surface of the base material is cut, and the ratio of (the outer diameter of the clad 4A) / (the outer diameter of the core 3A) is reduced. A small portion A 'is formed, and a portion B' having a large (outer diameter of the clad 4A) / (outer diameter of the core 3A) and a portion A 'having a smaller size are alternately provided. Prepare Next, when the base material thus formed is drawn so that the outer diameter of the clad 4A becomes constant in the length direction, the outer diameter of the core 3A becomes relatively large at the portion indicated by A ', and the B The outer diameter of the core 3A becomes smaller in the portion indicated by '. As a result, the outer diameter of the core 3A changes in the length direction, and with this change, the equivalent refractive index for the propagation mode changes in the length direction, and the chromatic dispersion changes in the length direction. Can be formed.

In addition to the method of structurally changing the equivalent refractive index for the propagation mode as described above, a method of adjusting the amount of dopant added to the core can also be adopted. For example, in the dual-core type refractive index profile shown in FIG. 1, the core 3 is made of Si doped with a dopant such as germanium, which has a function of increasing the refractive index.
Consists of O ₂ . Then, the addition amount of the dopant to the core 3 is adjusted by changing the addition amount in the length direction, the addition amount of the dopant changes in the length direction, and the outer diameter of the clad 4 and the outer diameter of the core 3 are constant in the length direction. Form the base material. If this is drawn so that the outer diameter of the cladding 4 is constant in the length direction, the outer diameter of the core 3 is constant, and the equivalent refractive index for the propagation mode is changed in the length direction by the change in the amount of the dopant added. And an optical fiber whose chromatic dispersion changes in the length direction can be obtained.

[0046]

As described above, in the low dispersion optical fiber of the present invention, the chromatic dispersion is canceled out and averaged by the portion where the chromatic dispersion is positive and the portion where the chromatic dispersion is negative, and the chromatic dispersion over the entire length is reduced. As a result, wavelength distortion due to group velocity dispersion can be suppressed, and the length of the portion where the chromatic dispersion is 0.1 ps / km / nm or less is 1% or less of the entire length. , FWM can be suppressed. Further, since the Brillouin frequency shift changes in the length direction, the occurrence of stimulated Brillouin scattering can be suppressed. As described above, even when the wavelength band used is 1.55 μm band, the chromatic dispersion over the entire length is small, and the nonlinear effects of stimulated Brillouin scattering and F
Since WM can be suppressed, deterioration of transmission characteristics can be suppressed.

[Brief description of the drawings]

FIG. 1 is a graph showing a refractive index profile of a fiber preform for a dual-core dispersion-shifted optical fiber used in Examples.

FIG. 2 is a sectional side view of the optical fiber according to the embodiment, taken along an axis, schematically showing the optical fiber;

FIG. 3 is a graph showing the relationship between the outer diameter of the cladding of the optical fiber of the example and the position in the length direction of the optical fiber.

4 is an enlarged graph of a main part of FIG.

FIG. 5 is a graph predicting and showing a change in chromatic dispersion in the length direction of the optical fiber of the example.

FIG. 6 is a graph showing measurement results of the generation efficiency of the FWM of the example and the comparative example.

FIG. 7 is a cross-sectional view of a fiber preform used in another example of the method of manufacturing a low dispersion optical fiber according to the present invention, taken along the axis.

[Explanation of symbols]

 3,3A ... core, 4,4A ... cladding

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Kuniharu Himeno, 1440, Mukurosaki, Sakura-shi, Chiba Prefecture, Fujikura Sakura Factory Co., Ltd. 72) Inventor Ryozo Yamauchi 1440, Murosaki, Sakura-shi, Chiba Prefecture Inside Fujikura Sakura Factory (72) Inventor Kenji Kurokawa 3-2-1, Nishishinjuku, Shinjuku-ku, Tokyo Nippon Telegraph and Telephone Corporation (72) Inventor Kazuhide Nakajima Nippon Telegraph and Telephone Corporation 3-1-2, Nishi-Shinjuku, Shinjuku-ku, Tokyo (72) Inventor Yoshiaki Miyajima 3-2-1, Nishi-Shinjuku, Shinjuku-ku, Tokyo, Japan

Claims (1)

[Claims] 1. A single-mode optical fiber having a portion in which chromatic dispersion changes in a length direction and a portion in which chromatic dispersion in a used wavelength band becomes positive and a portion which becomes negative in the length direction. The positive part and the negative part cancel each other's chromatic dispersion, and the absolute value of the chromatic dispersion is 0.1 ps with respect to the entire length.
A low-dispersion optical fiber, wherein the length of a portion less than / km / nm is 1% or less.