JPH08201639A - Low dispersion optical fiber - Google Patents

Abstract

(57) [Summary] [Object] To obtain a low-dispersion optical fiber having a chromatic dispersion characteristic of zero or near zero at the transmission wavelength, for example, 1.55 μm, and having a different operation principle from the conventional dispersion shift wavelength. [Structure] The core diameter of a single-mode optical fiber is changed in the length direction of the fiber. When the core diameter is changed, the chromatic dispersion changes accordingly. The core diameter is changed in a range that spans the portion of the core diameter where the wavelength dispersion is positive and the portion of the core diameter where the wavelength dispersion is negative.

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 whose chromatic dispersion at the transmission wavelength of light is close to zero, and a method for producing the same.

[0002]

2. Description of the Related Art A single mode optical fiber for 1.3 μm band which is widely used at present has a chromatic dispersion of almost zero at 1.3 μm, but has a chromatic dispersion of +10 ps // at a wavelength of 1.55 μm, which causes little loss due to Rayleigh scattering. When the signal transmission speed is high or the transmission distance is long, it becomes a great disadvantage. Therefore, 1.55μ
A dispersion-shifted fiber whose wavelength dispersion at m is close to zero and a dispersion flat fiber whose wavelength dispersion is close to zero from 1.3 μm to 1.55 μm have been developed.

Such a dispersion-shifted fiber or a dispersion flat fiber has a refractive index profile of a dual core type, a ring core type or the like to change the structural dispersion so as to reduce the wavelength dispersion in these wavelength regions. Is.
However, these dispersion-shifted fibers and dispersion flat fibers require precise adjustment of the core diameter in order to control the dispersion value. Particularly in a dispersion flat fiber, it is necessary to control the core diameter very precisely, and the refractive index distribution is complicated, so that there is a drawback that it is not easy to manufacture.

[0004]

Therefore, an object of the present invention is to obtain a new type of optical fiber having zero or near zero chromatic dispersion, which is different from the conventional dispersion shift fiber and dispersion flat fiber.

[0005]

The object of the present invention is to change the core diameter of a single-mode fiber in its length direction and to obtain a positive chromatic dispersion at a transmission wavelength that changes in accordance with the change of the core diameter. This is achieved by changing the core diameter so as to change in a range that spans the portion of and the portion of the core diameter that becomes negative.

[0006]

[Function] When the core diameter is changed so that the chromatic dispersion changes in the range where the chromatic dispersion at the transmission wavelength becomes positive and the core diameter becomes negative, the chromatic dispersion changes in the fiber length direction. The chromatic dispersions of A and B are canceled out and averaged, and the value can be zero or a value close to zero.

The present invention will be described in detail below. FIG.
[Fig. 2] schematically shows an example of the low dispersion optical fiber of the present invention. The low-dispersion optical fiber 1 is a single-mode optical fiber, and the core diameter of the core 2 is uniformly reduced from the starting end 1A to the terminating end 1B of the fiber 1, and the fiber diameter is from the starting end 1A to the terminating end 1B. Is constant until. The single-mode optical fiber here is
It is a fiber that can perform substantially single mode transmission,
Even if the secondary mode can theoretically be transmitted, it will be attenuated at a relatively short distance, for example, at a distance of 1 km or less,
Refers to a fiber that may be considered as substantially single mode transmission.

Although the length of the fiber 1 is not particularly limited,
It is usually several kilometers to several tens of kilometers. The refractive index distribution shape of the fiber 1 is a step index type, a dual core type having a staircase core portion, or the like, and the shape at the starting end 1A and the shape at the ending end 1B are substantially similar. Then, the core diameter r _A in the starting end 1A of the fiber 1, the core diameter r _B at the end 1B are defined as follows.

FIG. 2 shows a certain transmission wavelength (for example, 1.55 μm) corresponding to a change in core diameter of a single mode optical fiber.
It is a graph which shows the example of a change of chromatic dispersion in m) typically. As shown in this graph, when the core diameter of the single-mode fiber changes, the chromatic dispersion also changes in accordance with the change, and exhibits a change represented by a curve such as curve (a) or curve (d). Four kinds of curves (a) shown in the graph
(D) is for a single-mode fiber having different structural parameters, and the relationship between the core diameter and chromatic dispersion is generally expressed by a quadratic curve. In the curve (d), the minimum point is the core diameter. In the small region, the curve (C) has the minimum point in the region where the core diameter is large.

By appropriately determining the structural parameters of the single mode fiber, as shown in the curve (a) and the curve (c) in the graph of FIG. There will be a range of core diameters where the negative core diameter portion continues. Then, the core diameter r _{A at} the starting end and the core diameter r _B at the terminating end of the fiber are determined within the range of the core diameter satisfying such conditions.

For example, regarding the curve (a), two regions (N,
The two core diameters r _A and r _B are determined so that the area of the region (L) having a negative value and the sum of the areas of M) are equal or nearly equal. Similarly, regarding the curve (c), the area of the region where the chromatic dispersion value takes a positive value and the area of the region where the chromatic dispersion value takes a negative value are equal or almost equal to each other.
To determine the core diameters r _A and r _B. By doing so, it is possible to suppress chromatic dispersion to such an extent that there is no practical problem. Furthermore, when lowering the variance value, the following circumstances need to be considered. When the base material processed into a taper shape is spun, a portion having a large outer diameter is spun longer than a portion having a small outer diameter. Therefore, in the average value of the chromatic dispersion over the entire length of the fiber, the weight of the total of the chromatic dispersion on the side where the outer diameter of the base material is large, that is, on the side of r _{B in} FIG. 2 is large. If the core diameters at both ends of the fiber are determined in consideration of the above circumstances, it is possible to suppress the chromatic dispersion to a lower value.

The two core diameters r thus determined
Let _A and r _{B be} the core diameters of the starting end 1A and the ending end 1B of the fiber 1. As a result, the positive chromatic dispersion and the negative chromatic dispersion generated while the light having the transmission wavelength incident from the starting end 1A of the fiber 1 reaches the terminating end 1B are canceled, and as a result, the light is emitted from the terminating end 1B. Light has a chromatic dispersion that approaches zero, for example, chromatic dispersion of ± 2 ps / k
It is possible to keep the value within m · nm.

[0013] In principle, the core diameter r _B of the core diameter r _A and the end 1B of the fiber 1 of the starting end 1A as described above is determined, in fact, between the core diameter r _A and r _B Make sure that the chromatic dispersion at any core diameter is within ± 5 ps / km · nm, that the core diameter where the cutoff wavelength exceeds the transmission wavelength is not too long, and that the bending loss and Rayleigh loss do not become excessive. First, it is necessary to determine the core diameter and other structural parameters. From these points,
As the fiber 1, it is preferable that the refractive index distribution shape is a dual core type having a core composed of a central core portion and a step core portion.

Next, a method of manufacturing the low dispersion optical fiber of the present invention will be described. First, a glass base material whose outer diameter and core diameter are constant in the length direction is prepared, and the refractive index distribution of this glass base material is measured. From this measurement result, various characteristics of the fiber obtained by spinning the glass base material with various core diameters are predicted and calculated. From these characteristics, the transmission wavelength, for example, 1.5
It is possible to obtain how the wavelength distribution at 5 μm changes corresponding to the change in the core diameter, that is, the curve in the graph of FIG. When it is assumed as an example curve this curve (b) was obtained, as described above defines the core diameter r _A in the starting end 1A of the fiber, the core diameter r _B at the end 1B.

Next, the core diameter r _B of the core diameter r _A and the end 1B of the outer diameter and the starting end 1A of the fiber 1 that is set in advance, will define the outer diameter of the length direction of the fiber preform. For that purpose, first, the diameter ratio (core clad ratio) of the core part and the clad part at the starting end of the fiber preform matches the core clad ratio at the terminating end 1B of the fiber 1 after spinning,
The core-clad ratio at the terminal end of the fiber preform must match the core-clad ratio at the starting end 1A of the fiber 1 after spinning. Since the outer diameter of the fiber base material and the diameter of the core part are constant, in order to satisfy the above conditions, the outer diameter of the fiber base material is cut into a taper shape so that the outer diameter at the start end is small and the outer diameter at the end is small. The outer diameter may be increased.

At this time, depending on the initial size of the fiber preform, the thickness of the clad portion may be insufficient and the core-clad ratio at the terminal end may not be satisfied. In this case, a new additional clad portion may be formed by an external attachment method so as to satisfy the core clad ratio, and then ground into a tapered shape. Then, the taper-shaped fiber preform thus obtained is spun to have a constant outer diameter, whereby the low dispersion optical fiber shown in FIG. 1 can be obtained. The outer diameter of this spinning can be made constant by gradually changing the take-up speed.

In such a low dispersion fiber 1,
As described above, since the average value of the chromatic dispersion over the entire length of the fiber 1 is zero or approaches zero, the chromatic dispersion in the entire fiber 1 is within ± 2 ps / km · nm, and in some cases within ± 1 ps / km · nm. Can be suppressed.

Further, since the core diameter changes in the length direction of the fiber 1, stimulated Brillouin scattering is less likely to occur. Stimulated Brillouin scattering occurs when the intensity of light incident on an optical fiber exceeds a certain threshold value.
This is a phenomenon in which the scattered light returns to the incident end of light. Therefore, there is a disadvantage that a high level of light cannot be substantially incident on or transmitted to the optical fiber.

The fact that the stimulated Brillouin scattering is suppressed by changing the core diameter of the single-mode optical fiber in the longitudinal direction has already been found by the present applicant to be in Japanese Patent Application No. 3-16640.
It is disclosed in No. 3. Therefore, this low-dispersion optical fiber has a chromatic dispersion of zero or close to zero, and is capable of inputting and transmitting high-output signal light.

Further, in the above description, one fiber 1
Although the core diameter of the core varies uniformly from the starting end 1A to the terminating end 1B, the low dispersion optical fiber of the present invention is not limited to this, and in the length direction of one fiber, The uniform change in the core diameter may be repeated twice or more, and the chromatic dispersion is averaged to zero in each range of the uniform change in the core, so that the chromatic dispersion is zero as a whole. Or it will be close to zero.

Therefore, a plurality of fibers whose core diameter changes uniformly from the start end to the end are spliced in such a direction that the core diameters of the ends are equal,
Similarly, the chromatic dispersion can be maintained at or near zero over the length of the connected fibers.

Specific examples are shown below, but the present invention is not limited thereto. (Example) A fiber preform for a dual core type dispersion shift fiber having a refractive index distribution shown in FIG. 3 was prepared. The outer diameter of this fiber base material is 30 mm, and the diameter of the core part is 3.6 mm.
The core-clad ratio was 8.3, and this dimension was constant in the length direction. The outer diameter of this fiber preform is 1
If the fiber is spun with a constant outer diameter of 25 μm, the core diameter of the fiber is 15 μm.

As a result of the analysis of the refractive index distribution of this fiber preform, it was calculated that the chromatic dispersion at the transmission wavelength of 1.55 μm changes as the core diameter changes as shown by the curve (e) in FIG. Was done. From the curve (e) in FIG. 4, when the core diameter at the start and the core diameter at the end of the fiber are determined so that the areas of both the positive wavelength region and the negative wavelength region are substantially equal, r _A is Values of 25.0 μm and r _B 12.5 μm were obtained.

If the diameter of the fiber 1 after spinning is constant at 125 μm, the core-clad ratio at the starting end 1A of the fiber 1 after spinning is 125/25 = 5, and the core-cladding ratio at the end 1B is 125/12. 5 = 10. On the other hand, the above-mentioned prepared fiber preform has a uniform core-clad ratio of 8.3, so that the glass having the same refractive index as that of the clad portion is adjusted so that the core-clad ratio on the terminal side of the fiber preform becomes 10 or more. Was attached externally, and its outer diameter was set to 36 mm. Then, as shown in FIG. 5, the fiber preform 1 having an outer diameter of 36 mm
1 on its end side 11B to an outer diameter of 36 mm, and its start end side 1
It was ground in a taper shape with an outer diameter of 18 mm at 1A. Reference numeral 12 in FIG. 5 indicates a core portion.

The tapered fiber preform 11 was spun with a constant outer diameter of 125 μm to obtain an optical fiber having a total length of 14 km. When the characteristics of the obtained fiber were measured, they were as follows. Starting end 1A Starting end 1B Clad outer diameter (μm) 125 125 Core diameter (μm) 25 12.5 Mode field diameter (μm) at 1.55 μm 6.43 8.63 1.55 μm wavelength dispersion over the entire length +0. 43 (ps / km / nm)

The threshold value of the amount of incident light that causes stimulated Brillouin scattering of this fiber was about 11 dBm. This value is larger than about 7 dBm of a normal 1.55 μm dispersion-shifted fiber, and it was found that stimulated Brillouin scattering is suppressed.

[0027]

As described above, in the low dispersion optical fiber of the present invention, the chromatic dispersion at the transmission wavelength can be averaged in the length direction of the fiber to be zero or close to zero. Therefore, the spread of the spectrum width of the light from the light source can be made extremely small, and the transmission speed and the transmission distance of the optical signal can be increased. In addition, since stimulated Brillouin scattering can be suppressed at the same time, a high level of light can be incident and transmitted, which also extends the transmission distance.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram schematically showing an example of a low dispersion optical fiber of the present invention.

FIG. 2 is a graph showing the relationship between core diameter and chromatic dispersion in the present invention.

FIG. 3 is a diagram showing a refractive index distribution of a fiber preform used in Examples.

FIG. 4 is a graph showing the relationship between core diameter and chromatic dispersion in an example.

FIG. 5 is a schematic cross-sectional view of a tapered glass preform that has been ground in an example.

[Explanation of symbols]

1 ... Low dispersion optical fiber, 2 ... Core, 1A ... Start end, 1B ...
Termination, 11 ... Fiber preform

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akira Wada 1440 Rokuzaki, Sakura City, Chiba Prefecture, Fujikura Co., Ltd.Sakura Plant (72) Inventor Kazuhiko Aikawa 1440, Rokuzaki, Sakura City, Chiba Prefecture, Fujikura Sakura Plant, Ltd. ( 72) Inventor Shoji Ohashi 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation (72) Inventor Mitsuhiro Tachida 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (3)

[Claims] 1. A single-mode optical fiber, in which the core diameter changes in the length direction of the fiber, and the chromatic dispersion that changes with the change of the core diameter has a positive chromatic dispersion. A low-dispersion optical fiber characterized in that the core diameter changes in a range that spans the negative and negative core diameter portions. 2. The low dispersion optical fiber according to claim 1, wherein the refractive index distribution is a dual core type having a central core portion and a step core portion. 3. A fiber preform having a ratio of the diameter of the core portion to the diameter of the cladding portion which is constant in the longitudinal direction is processed so that its outer peripheral shape is tapered, and each core cladding at both ends thereof is processed. A method for producing a low-dispersion optical fiber, characterized in that the ratio is made to match each core-clad ratio at both ends of the fiber after spinning, and then the fiber is spun at a constant outer diameter.