JP2001255433A - Dispersed flat optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a dispersed flat optical fiber that is suitable for wavelength multiplexed soliton transmission and the like using a light amplifier and that is equipped with a structure for effectively suppressing emergence of nonlinear optical phenomenon. SOLUTION: The dispersed flat optical fiber 100 relating to this invention is equipped with a core area 200 constituted of a first core 601-third core 603 and a clad area 300, and is provided, at least as characteristics at a wavelength of 1,550 nm, with a dispersion whose absolute value is 5 ps/nm/km or below, effective section of 45 μm2 or above, transmission loss of 0.5 dB/turn or less in a state bent at a diameter of 32 mm, dispersed slope of 0.03 ps/nm2/km or below, and a cut-off wavelength of 1.0 μm or above at a length of 2 m.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dispersion flat optical fiber suitable for a communication system using an optical fiber network.

[0002]

2. Description of the Related Art A communication system constituted by an optical fiber network is capable of long-distance and large-capacity communication, and is further increased in capacity by time division multiplex transmission or wavelength multiplex soliton transmission (for example, N.Edagawa, et al., "Long Dista
nce Soliton WDM transmissionusing a dispersion-fla
ttened fiber ", OFC'97, PD19). Such an optical communication system includes a high-performance transceiver for transmitting and receiving signal light, an optical amplifier for optically amplifying the signal light, and an optical fiber for transmitting the signal light. And the like.
In the optical amplifier which is indispensable for obtaining the / N ratio, the wavelength band in which the light can be amplified is 1530 nm to 1560 nm. Therefore, the wavelength of the usable signal light is substantially limited to within this wavelength bandwidth of 30 nm. .

[0003]

In an optical communication system using an optical amplifier, the intensity of signal light in an optical fiber increases, so that nonlinear optical phenomena such as four-wave mixing and self-phase modulation occur in the optical fiber. It happens in. Among them, generation of four-wave mixing can be suppressed by setting the absolute value of dispersion at the wavelength of signal light to about 1 ps / nm / km to 5 ps / nm / km. On the other hand, self-phase modulation and chromatic dispersion (hereinafter simply referred to as dispersion)
Although there is no problem in the case of soliton transmission of signal light of a single wavelength, there is the following problem in the transmission of a plurality of wavelength-multiplexed signal lights. That is,
A dispersion-shifted fiber having a zero dispersion wavelength shifted to a wavelength of about 1550 nm generally has a chromatic dispersion slope of about 0.07 ps / nm ² / km (hereinafter simply referred to as a dispersion slope). The dispersion value differs depending on the wavelength. For this reason, the balance with the self-phase modulation is broken, and the signal light pulse is broken. Therefore, in such a case, a dispersion flat optical fiber having an extremely small dispersion slope is required.

The nonlinear refractive index of the optical fiber is set to N2.
_Assuming that the effective area is A _eff , the power of the signal light is P, and the effective length of the optical fiber is L _eff , the generation amount of the nonlinear optical phenomenon of the optical fiber is given by the following equation. N2 · P · L _eff / A _eff

[0005] In order to increase the signal light power P and obtain a high S / N ratio without increasing the generation amount of the nonlinear optical phenomenon, it is necessary that the effective area A _eff be large.
In the case of time-division multiplex transmission of a single wavelength, in order to suppress the occurrence of nonlinear optical phenomena, not only increase the effective area A _eff , but also set the absolute value of dispersion to 1 ps / nm / km.
In addition, it is necessary to make the dispersion slope extremely small. Furthermore, in order to suppress an increase in loss when an optical fiber is converted into a cable, a small bending loss is required, and for that purpose, the cutoff wavelength must be set to an appropriate value.

The above-described nonlinear refractive index N2 is defined as follows. That is, the refractive index N of the medium under strong light changes depending on the optical power. Therefore, the lowest order effect on the refractive index N is expressed by the following equation. N = N0 + N2 · E ²

Here, N0: refractive index for linear polarization N2: nonlinear refractive index for third-order nonlinear polarization E: photoelectric field amplitude Under strong light, the refractive index N of the medium is a normal value N0 and the photoelectric field amplitude E And the increase in proportion to the square of.
In particular, the proportional constant N2 (unit: m ² / V ² ) of the second term is called a nonlinear refractive index.

Further, the effective area A _eff is described in
As shown in -248251, it is given by the following equation.

[0009]

(Equation 2) Here, E is the electric field associated with the propagating light, and r is the radial distance from the center of the core.

The dispersion slope is defined by the slope of a graph showing dispersion characteristics in a predetermined wavelength band.

As a result of studying a conventional dispersion flat optical fiber, the inventors have found the following problems. That is, the conventional dispersion-flattened optical fiber, although the dispersion slope is small, the effective area of 30μm ² ~40μm ² about only a high optical power density in the core region, nonlinear optical phenomena such as four-wave mixing is strongly generated Easy to do.
Therefore, the conventional dispersion flat optical fiber is not suitable for a wavelength division multiplexing optical communication system using an optical amplifier.

For example, M. Ohashi, et al., "Dispersion-
modified Single-Mode Fiber by VAD Method with Low
Dispersion in the 1.5μm Wavelength Region ", ECOC '
88, pp. 445-448, discloses a dispersion flat optical fiber having a central core (first core) and a second core surrounding the central core.
A core, a core region including a third core surrounding the second core, and a cladding surrounding the core region;
By adding a Ge element to the central core (first core), the relative refractive index difference of the central core with respect to the cladding is reduced by 0.87%.
And adding the element F to the second core to reduce the relative refractive index difference of the second core region with respect to the cladding region.
It has a triple-clad refractive index profile in which the relative refractive index difference of the third core with respect to the cladding is increased to 0.23% by adding Ge element to the third core.

In this conventional dispersion flat optical fiber, the dispersion slope at a wavelength of 1550 nm is 0.0
Although 23 ps / nm ² / km is obtained, its effective area A _eff is only about 37 μm ² .

Also, see Y. Kubo, et al., "Dispersion Flat
tened Single-Mode Fiber for 10,000km Transmission
System ", ECOC'90, pp. 505-508, discloses a dispersion-flat optical fiber having a central core and a second core surrounding the central core.
A core region comprising a core, a cladding surrounding the core region, and a Ge element added to the central core to increase the relative refractive index difference of the central core region with respect to the cladding to 0.9%; It has a W-type refractive index profile in which the relative refractive index difference of the second core with respect to the cladding is reduced to -0.4% by adding the element F to the core region.

In this conventional dispersion flat optical fiber, the dispersion slope at a wavelength of 1550 nm is 0.0.
Although 23 ps / nm ² / km is obtained, its effective area A _eff is 30 μm ² or less.

On the other hand, a normal dispersion-shifted fiber has a relatively large effective area A _eff of about 50 μm ^2, which is relatively large. However, since the dispersion slope is about 0.07 ps / nm ² / km, the influence of dispersion is large. Are also not suitable for long-distance optical transmission (see, for example, Y. Terasawa, et al., "Des
ign Optimization of Dispersion Shifted Fiber withE
nlarged Mode Field Diameter for WDM Transmission ",
IOOC'95, FA2-2).

Therefore, any of the conventional optical fibers is
It is not suitable for application to time division multiplex transmission or wavelength multiplex soliton transmission using an optical amplifier.

The present invention has been made to solve the above-mentioned problems, and provides a dispersion-flat optical fiber having a structure suitable for time division multiplex transmission and wavelength multiplex soliton transmission in an optical communication system including an optical amplifier. It is intended to be.

[0019]

The dispersion flat optical fiber according to the present invention has a wavelength band of 1.55 μm (1500 nm).
(At 1600 nm) at a center wavelength of 1550 nm, dispersion having an absolute value of 5 ps / nm / km or less, an effective area of 50 μm ² or more, and 0.02 ps / n.
It has a dispersion slope of m ² / km or less and a cutoff wavelength of 1.0 μm or more at a length of 2 m.

Such a dispersion flat optical fiber has a wave
The absolute value of the dispersion at a length of 1550 nm is 5 ps / nm /
km or less and the dispersion slope is 0.02 ps /
nm ^Two/ Km or less, the difference in dispersion value between each signal light
The difference is small. In addition, the effective area is 50 μm^TwoIs over
Therefore, the signal light propagating in the dispersion flat optical fiber is
Power density can be kept low. This allows nonlinear light
The occurrence of chemical phenomena is effectively suppressed and transmitted with a high S / N ratio
Becomes possible. Furthermore, this dispersion flat optical fiber
Has excellent bending properties.

In order to effectively suppress the occurrence of nonlinear optical phenomena, the effective sectional area must be 50 μm ² as described above.
As described above, it is more preferable that the thickness be 70 μm ² or more, but this increase in the effective area causes an increase in bending loss. Such an increase in bending loss is not preferable for making the dispersion flat optical fiber into a cable. On the other hand, suppression of nonlinear optical phenomena can also be realized by intentionally generating dispersion in the dispersion flat optical fiber.

Therefore, in the dispersion flat optical fiber according to the present invention, as the preferable relationship between the effective cross section and the dispersion slope, the effective cross section is 45 μm ² or more and the dispersion slope is 0.03 ps / nm ² / km or less. It is also possible to realize this.

In the dispersion flat optical fiber having the above-described various characteristics, the core region is provided with a first core having a predetermined refractive index and provided on the outer periphery of the first core and higher than the first core. It can be realized by comprising the second core having a refractive index (ring core structure). The core region includes a first core having a predetermined refractive index, a second core provided on an outer periphery of the first core and having a lower refractive index than the first core, and an outer periphery of the second core. And a third core having a higher refractive index than the second core (three-layer core structure). The clad region is a first region provided on an outer periphery of the core region.
A depressed clad structure including a clad (inner clad) and a second clad (outer clad) provided on the outer periphery of the first clad and having a higher refractive index than the first clad may be provided. The dispersion flat optical fiber can be realized by combining any of the above-described ring core structure and three-layer core structure with the depressed clad structure.

Further, the dispersion flat optical fiber having the above-described ring core structure and segment structure and having the above-mentioned various characteristics has a characteristic of 0.15 ps / wavelength at 1550 nm in consideration of a case where it is actually applied as a transmission line.
It is preferable to further have a polarization dispersion of km ^1/2 or less. In consideration of the use of a cable, the dispersion flat optical fiber preferably has a transmission loss of 0.5 dB / turn or less when bent to a diameter of 32 mm, and particularly has the above-described three-layer core structure. Preferably, the cutoff wavelength at a length of 2 m is 1.4 μm or more.

In order to reduce the dispersion slope and to increase the cutoff wavelength to reduce the bending loss in the above-described dispersion-flattened optical fiber having a three-layer core structure, the third core must have a large size. I found that it was contributing. That is, in the dispersion flat optical fiber having the three-layer core structure, the outer diameter of the second core is b, the outer diameter of the third core is c, and the relative refractive index difference of the third core with respect to the reference region of the cladding region is Δn _3. It is preferable that the following relationship be satisfied. Δn ₃ ≧ 0.25% 0.40 ≦ b / c ≦ 0.75 In other words, the profile volume of the third core has a radial distance r from the center in the core region and a distance r from the center. When the relative refractive index difference between the cladding region and the reference region is Δn (r), it is preferable that the following condition is satisfied.

[0026]

(Equation 3) The clad region may have the above-described depressed clad structure. In this case, the reference region for defining the relative refractive index difference of each glass region is the second clad.

In a dispersion flat optical fiber composed of a combination of a three-layer core structure and a depressed clad structure, the outer diameter of the first core is a, the outer diameter of the second core is b, and the outer diameter of the third core is c, when the relative refractive index difference of the first core with respect to the second cladding is Δn ₁ , and the relative refractive index difference of the first cladding with respect to the second cladding is Δn ₄ , the following relationship is preferably satisfied. 0.40% ≦ Δn ₁ ≦ 0.90% Δn ₄ ≦ −0.02% 0.20 ≦ a / c ≦ 0.35 20 μm ≦ c ≦ 30 μm In addition, considering the manufacturing variation, the dispersion according to the present invention is considered. Flat optical fibers must be designed to tolerate such manufacturing variations. Specifically, a manufacturing variation of about ± 2% occurs in the outer diameter of the core region between the manufactured lots (the outer diameter can be controlled only up to about ± 2%). Therefore, in the dispersion-flattened optical fiber according to the present invention, it is necessary to suppress as much as possible the characteristic fluctuation, particularly the fluctuation of the dispersion slope, caused by the fluctuation of the outer diameter of the core region.

According to the experiments performed by the inventors, it is found that the value of the dispersion slope tends to decrease as the outer diameter of the core region increases, and further increase as the outer diameter of the core region increases. (The dispersion slope takes a minimum value when the outer diameter of the core region is a predetermined value.) Therefore, in the dispersion flat optical fiber according to the present invention, in order to suppress the characteristic fluctuation due to manufacturing variations, the dispersion slope of the dispersion flat optical fiber is in the range of ± 2% around the outer diameter of the core region where the dispersion slope takes a minimum value. The outside diameter of the core region is set in the inside. Specifically, when the range of the outer diameter variation of the core region is ± 2% with respect to the design value, the variation amount of the dispersion slope is 0.0%.
03 ps / nm ² / km or less.

[0029]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of a dispersion flat optical fiber according to the present invention will be described below with reference to FIGS. In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description.

The dispersion flat optical fiber according to the present invention is applicable to long-haul submarine cables and the like, and
One or two or more signal lights in the μm wavelength band (1500 nm to 1
This is a quartz single-mode optical fiber for transmitting one or two or more signal lights having a center wavelength within a range of 600 nm. Further, the dispersion flat optical fiber includes a core region extending along a predetermined axis, and a cladding region provided on an outer periphery of the core region, in order to reduce dispersion of dispersion between propagating signal lights. The dispersion slope is designed to be small in the signal wavelength band of 1.55 μm.

Specifically, the dispersion flat optical fiber according to the present invention has various characteristics at a wavelength of 1550 nm, a dispersion having an absolute value of 5 ps / nm / km or less, a dispersion of 45 μm ² or more, preferably 50 μm ² or more, and more preferably 70 μm ² or more.
An effective area A _{eff of at} least m ² and 0.03 ps / nm ² /
It has a dispersion slope of not more than km, preferably not more than 0.02 ps / nm ² / km, and a cutoff wavelength of not less than 1.0 μm at a length of 2 m.

Further, the dispersion flat optical fiber according to the present invention has various characteristics at a wavelength of 1550 nm,
It preferably has a polarization dispersion of 0.15 ps / km ^1/2 or less, and a transmission loss when bent to a diameter of 32 mm is preferably 0.5 dB / turn or less.
The transmission loss was measured using a mandre having a diameter of 32 mm.
This is a value obtained by converting the transmission loss measured in a state where the optical fiber to be measured is wound several times in l) per turn, and is hereinafter referred to as bending loss.

As a configuration for realizing the above-mentioned various characteristics, a core region extending along a predetermined axis is provided with a first core having a predetermined refractive index and an outer periphery of the first core, and a core region extending from the first core. May be a ring core structure having a second core having a high refractive index, a first core having a predetermined refractive index, and a lower refractive index provided on the outer periphery of the first core and lower than the first core. A second core having a ratio;
A three-layer core structure including a third core provided on the outer periphery of the core and having a higher refractive index than the second core may be used. In the case of a three-layer core structure, the cutoff wavelength at a length of 2 m needs to be 1.4 μm or more. Also,
The clad region provided on the outer periphery of the core region includes a first clad (inner clad) provided on the outer periphery of the core region, and a refractive index higher than the first clad provided on the outer periphery of the first clad. And a second clad (outer clad) having a depressed clad structure.

In particular, in the case of a dispersion flat optical fiber having a three-layer core structure, the outer diameter of the second core is b, the outer diameter of the third core is c, the reference region of the cladding region (when the cladding region is a single layer, When the relative refractive index difference of the third core with respect to the second core (when the cladding itself has a depressed cladding structure and the second cladding) is Δn ₃ , the following relationship is preferably satisfied. Δn ₃ ≧ 0.25% 0.40 ≦ b / c ≦ 0.75 The profile volume of the third core is represented by r at a radial distance from the center in the core region and at a position at a distance r from the center. When the relative refractive index difference between the cladding region and the reference region is Δn (r), it is preferable that the following condition is satisfied.

[0035]

(Equation 4) Further, in a dispersion-shifted fiber composed of a combination of a three-layer core structure and a depressed clad structure, the first
The outer diameter of the core is a, the outer diameter of the second core is b, the outer diameter of the third core is c, and the relative refractive index difference of the first core with respect to the second cladding is Δ
Assuming that n _{1 is} the relative refractive index difference of the first cladding relative to the second cladding Δn ₄ , the following relationship is preferably satisfied. 0.40% ≦ Δn ₁ ≦ 0.90% Δn ₄ ≦ −0.02% 0.20 ≦ a / c ≦ 0.35 20 μm ≦ c ≦ 30 μm The relative refractive index of each region relative to the reference region of the cladding region The difference is given by the following equation (note that each parameter does not change in order). _{(N T -n R) / n} R

Here, n _T is the refractive index of the target glass region, and n _R is the refractive index of the reference cladding region (where,
If the cladding region has a depressed cladding structure, the second
(Refractive index of the cladding). Further, in this specification, the relative refractive index difference of each glass region is expressed in percentage. Therefore, the relative refractive index difference displayed as a negative value is
It means a region having a lower refractive index than the reference region.

The outer diameter of the core region is preferably set within a fluctuation range of ± 2% around the value at which the dispersion slope becomes a minimum value. Specifically, the inventors set the outer diameter of the core region ±
The variation amount of the dispersion slope with respect to the variation of 2% is 0.003.
It has been found that it is preferable to design to be ps / nm ² / km or less.

Hereinafter, embodiments of the dispersion flat optical fiber according to the present invention will be described in order. The dispersion flat optical fibers according to the first and second embodiments described below each have a core region having a ring core structure and a cladding region having a depressed cladding structure. Further, the dispersion flat optical fiber according to the third embodiment has a core region having a three-layer core structure and a cladding region having a depressed cladding structure. Further, the dispersion flat optical fiber according to the fourth embodiment has a core region having a three-layer core structure and a single-layer cladding region.

(First Embodiment) First, FIG. 1A is a diagram showing a cross-sectional structure of a dispersion flat optical fiber according to a first embodiment, and FIG. 3 is a refractive index profile shown along the line L1 of FIG. Note that the line L
Reference numeral 1 denotes a line intersecting at a point O ₁ indicating the central axis of the dispersion flat optical fiber 100. The dispersion flat optical fiber 100 according to the first embodiment includes a core region 200 having a ring core structure and a cladding region 300 having a depressed cladding structure. The core region 200 is provided on a first core 201 having an outer diameter a1 having a refractive index n ₁ and an outer periphery of the first core 201 and having a refractive index n ₂ (>).
n ₁ ) having a second core 202 having an outer diameter b1. In addition, the cladding region 300 includes the second core 202.
And a first cladding 301 having an outer diameter c1 having a refractive index n ₃ (= n ₁ ) and an outer diameter c1.
And a second clad 302 having a refractive index of n ₄ (> n ₃ ).

Here, the refractive index profile 150 of FIG. 1B shows the refractive index of each region along the line L1 in FIG. 1A, and the region 151 is the line L1 of the first core 201.
The upper portions and the regions 152 correspond to the respective portions on the line L1 of the second core 202, the regions 153 correspond to the respective portions on the line L1 of the first clad 301, and the regions 154 correspond to the respective portions on the line L1 of the second clad 302. Is equivalent.

In the first embodiment, the first cladding 302 (the reference region of the cladding region 300) is not
Relative refractive index difference Δn _{1 of} core 201 and first cladding 301
The relative refractive index difference [Delta] n ₃ of, are both -0.6%, second
The relative refractive index difference Δn ₂ of the second core with respect to the cladding 302 is 0.7%. The outer diameter a1 of the first core 201 is 3.04 μm, and the outer diameter b1 of the second core 202 is 7.26 μm.
m, the outer diameter c1 of the first cladding 301 is 13.2 μm, and the outer diameter of the dispersion flat optical fiber 100 (the outer diameter of the second cladding 302) is 125 μm. The relative refractive index difference of each region is given as follows. Δn ₁ = (n ₁ −n ₄ ) / n ₄ Δn ₂ = (n ₂ −n ₄ ) / n ₄ Δn ₃ = (n ₃ −n ₄ ) / n ₄

The present inventors have designed the first embodiment configured as described above.
Wavelength 1 of dispersion flat optical fiber 100 according to the embodiment
Various characteristics at 550 nm were measured. As a result, the dispersion value at a wavelength of 1550 nm is 0.17 ps / nm / k.
m (<| 5 | ps / nm / km), the effective area at the wavelength of 1550 nm A _eff is 58μm ² (> 45μm ^2),
The cut-off wavelength at a length of 2 m is 1.153 μm (> 1.
0 μm). Further, the dispersion slope has a wavelength of 153.
0.018 ps / nm ² / km at 0 nm, wavelength 1550
0.007 ps / nm ² / km in nm (<0.03 ps
/ Nm ² / km), 0.000 ps at 1560 nm wavelength
/ Nm ² / km. Further, the polarization dispersion value at a wavelength of 1550 nm is 0.11 ps / km ^1/2 (<0.1
5 ps / km ^1/2 ). FIG. 2 shows the first
4 is a graph illustrating dispersion characteristics of the dispersion flat optical fiber 100 according to the embodiment.

(Second Embodiment) FIG. 3 shows a second embodiment of the present invention.
4 is a refractive index profile of the dispersion flat optical fiber according to the embodiment. The cross-sectional structure of the dispersion flat optical fiber according to the second embodiment is the same as the structure shown in FIG. Therefore, the refractive index profile 16
0 corresponds to the refractive index of each region along the line L1 in FIG. 1A, the region 161 is each part on the line L1 of the first core 201, and the region 162 is the line L1 of the second core 202. The upper portions and the regions 163 correspond to the respective portions on the line L1 of the first clad 301, and the regions 154 correspond to the respective portions on the line L1 of the second clad 302.

In the second embodiment, the outer diameter a1 of the first core (refractive index n ₁ ) corresponding to the region 161 is 3.75.
μm, a second core corresponding to the region 162 (refractive index n ₂ >
n ₁ ) has an outer diameter b of 8.25 μm, and the outer diameter c _{1 of} the first cladding (refractive index n ₃ = n ₁ ) corresponding to the region 163 is 15.
0 μm, the second clad (refractive index n
₄ > n ₃ ) has an outer diameter of 125 μm. Further, the relative refractive index difference Δn ₁ (= (n ₁ −
n ₄ ) / n ₄ ) and the relative refractive index difference Δn ₃ (=
(N ₃ −n ₄ ) / n ₄ ) are both -0.60%,
The relative refractive index difference Δn ₂ of the second core with respect to the second clad (=
(N ₂ −n ₄ ) / n ₄ ) is 6.3%.

The present inventors have proposed the second embodiment constructed as described above.
Wavelength 1550 of dispersion flat optical fiber according to embodiment
Various properties in nm were measured. As a result, the wavelength 155
The dispersion value at 0 nm is 0.12 ps / nm / km (<
| 5 | ps / nm / km ), the effective area A _eff is 72 .mu.m ² at a wavelength of 1550nm (> 45μm ^2), length 2
The cutoff wavelength at m is 1.187 μm (> 1.0 μm).
m). The dispersion slope is 1530n wavelength.
0.0096ps / nm ² / km in m, wavelength 1550n
-0.0120 ps / nm ² / km (<0.003
ps / nm ² / km), −0.02 at a wavelength of 1560 nm
It was 65 ps / nm ² / km. Further, the wavelength 155
The polarization dispersion value at 0 nm is 0.10 ps / km
^1/2 (<0.15 ps / km ^1/2 ). FIG.
Is a graph showing the dispersion characteristics of the dispersion flat optical fiber according to the second embodiment.

(Third Embodiment) FIG. 5A is a diagram showing a cross-sectional structure of a dispersion flat optical fiber according to a third embodiment, and FIG. 5B is a diagram showing a line in FIG. 6 is a refractive index profile shown along L2. Note that a line L2 indicates a point O ₂ indicating the central axis of the dispersion flat optical fiber 500.
Is the line that intersects. The dispersion flat optical fiber 500 according to the third embodiment has a core region 60 having a three-layer core structure.
0, cladding region 700 of depressed cladding structure
And The core region 600 includes a first core 601 having an outer diameter a2 having a refractive index n ₁ and the first core 601.
And a second core 602 having an outer diameter b2 having a refractive index n ₂ (<n ₁ ) and an outer diameter b2 provided on the outer circumference of the second core 602 and having a refractive index n ₃ (> n ₂ ). A third core 603 having a diameter c2. The clad region 700 is provided on the outer periphery of the third core 603 and provided on the outer periphery of the first clad 701 having an outer diameter d having a refractive index n ₄ (<n ₃ ). A second clad 702 having a refractive index n ₅ (> n ₄ ) is provided.

Here, the refractive index profile 510 of FIG. 5B shows the refractive index of each region along the line L2 in FIG. 5A, and the region 511 is the line L2 of the first core 601.
Each region above, region 512 is each region on line L2 of second core 602, region 513 is each region on line L2 of third core 603, region 514 is each region on line L2 of first clad 701, The region 515 corresponds to each portion on the line L2 of the second clad 702.

In the third embodiment, the first clad 702 (the reference region of the clad region 700) is not
The relative refractive index difference Δn ₁ of the core 601 is 0.58%, and the relative refractive index difference Δn ₂ of the second core 602 with respect to the second clad 702.
Is −0.10%, the relative refractive index difference Δn ₃ of the third core 603 with respect to the second cladding 702 is 0.40%, and the relative refractive index difference Δn ₄ of the first cladding 701 with respect to the second cladding 702.
Is -0.27%. Also, the outer diameter a of the first core 601
2 is 5.8 μm, and the outer diameter b2 of the second core 602 is 16.2
μm, the outer diameter c2 of the third core 603 is 23.2 μm,
The outer diameter d of the cladding 701 is 46.4 μm, and the outer diameter of the dispersion flat optical fiber 500 (the outer diameter of the second cladding 702) is 125 μm. The relative refractive index difference of each region is given as follows. Δn ₁ = (n ₁ −n ₅ ) / n ₅ Δn ₂ = (n ₂ −n ₅ ) / n ₅ Δn ₃ = (n ₃ −n ₅ ) / n ₅ Δn ₄ = (n ₄ −n ₅ ) / n ₅

[0049] The present inventors have developed the third embodiment constructed as described above.
Wavelength 1 of dispersion flat optical fiber 500 according to the embodiment
Various characteristics at 550 nm were measured. As a result, the dispersion value at a wavelength of 1550 nm is -2.2 ps / nm / k.
m (<| 5 | ps / nm / km), the effective area A _{eff at} a wavelength of 1550 nm is 50 μm ² (> 45 μm ² ),
The cutoff wavelength at a length of 2 m is 1.920 μm (> 1.
0 μm). Further, the dispersion slope has a wavelength of 153.
0.0129 ps / nm ² / km at 0 nm, wavelength 155
0.0172 ps / nm ² / km at 0 nm (<0.03
ps / nm ² / km), 0.019 at a wavelength of 1560 nm
It was 8 ps / nm ² / km. Further, the wavelength 1550
The polarization dispersion value in nm is 0.06 ps / km ^1/2 (<
0.15 ps / km ^1/2 ). FIG. 6 is a graph showing the dispersion characteristics of the dispersion flat optical fiber according to the third embodiment.

(Fourth Embodiment) FIG. 7 shows a fourth embodiment of the present invention.
4 is a refractive index profile of the dispersion flat optical fiber according to the embodiment. The dispersion flat optical fiber according to the fourth embodiment has a core region having a three-layer core structure similar to the core region 600 in FIG. 5A and a cladding region having a single layer, and the refractive index profile 170 is triple. It is also called a clad type refractive index profile.

The refractive index profile 170 basically corresponds to the refractive index of each region along the line L2 in FIG. 5A, and the region 171 corresponds to each region on the line L2 of the first core 601. , The region 172 is each part on the line L2 of the second core 602, the region 173 is each part on the line L2 of the third core 603,
The region 174 corresponds to each portion on the line L2 of the second clad 702. However, in the dispersion flat optical fiber of the fourth embodiment, there is no region corresponding to the first clad in FIG.

In the fourth embodiment, the outer diameter a2 of the first core (refractive index n ₁ ) corresponding to the region 171 is 7.3 μm.
m, the second core corresponding to the region 172 (refractive index n ₂ <n ₁ )
Has an outer diameter b1 of 15.0 μm and a third diameter corresponding to the region 173.
The outer diameter c2 of the core (refractive index n ₃ > n ₂ ) is 22.0 μm, and the outer diameter of the cladding (refractive index n ₄ <n ₃ ) corresponding to the region 174 is 125 μm. Also, the relative refractive index difference Δn ₁ (= (n ₁ −n ₄ ) / of the first core with respect to the cladding (reference region) /
n ₄₎ 0.58%, the relative refractive index of the second core difference with respect to the cladding _{Δn 2 (= (n 2 -n} 4) / n 4) is -0.18%,
The relative refractive index difference Δn ₃ (= (n ₃
-N ₄₎ / n ₄₎ is 0.27%.

The present inventors have designed the fourth embodiment constructed as described above.
Wavelength 1550 of dispersion flat optical fiber according to embodiment
Various properties in nm were measured. As a result, the wavelength 155
The dispersion value at 0 nm is -0.37 ps / nm / km
(<| 5 | ps / nm / km), the effective area A _{eff at} a wavelength of 1550 nm is 52.8 μm ² (> 45 μm).
m ² ) and the cutoff wavelength at a length of 2 m is 1.713 μm
(> 1.0 μm). Also, dispersion slope, 0.0005ps / nm ² / km at a wavelength of 1530 nm, a wavelength ^{1550nm 0.0005ps / nm 2 / km (} <
0.003ps / nm ² / km), it was 0.0010ps / nm ² / km at a wavelength of 1560 nm. 20m diameter
The transmission loss (bending loss) when bent at m is 3.2d
B / m. Further, the polarization dispersion at a wavelength of 1550 nm is 0.08 ps / km ^1/2 (<0.15 ps / km
km ^1/2 ). FIG. 8 is a graph showing the dispersion characteristics of the dispersion flat optical fiber according to the fourth embodiment.

FIG. 9 shows the first to fourth embodiments described above.
The wavelength 15 of each of the dispersion flat optical fibers according to the embodiment
5 is a table summarizing various characteristics at 50 nm. Each of these
Any of the dispersion flat optical fibers according to the embodiments has a dispersion flat optical fiber.
Is less than 5 ps / nm / km and the effective area A_eff
Is 45 μm^TwoAs described above, the dispersion slope is 0.03 ps / nm. ^Two
/ Km or less, and the cutoff wavelength at a length of 2 m is 1.
0 μm or more, polarization dispersion is 0.15 ps / km^1/2Below
is there.

Each of the dispersion flat optical fibers has a very small dispersion slope and a flat dispersion characteristic, and the effective area _Aeff is as large as 45 μm ² or more, so that the power density of the signal light in the dispersion flat optical fiber is small. The generation of nonlinear optical phenomena is suppressed effectively by being kept low, and transmission with a high S / N ratio is possible. Further, since the cutoff wavelength at a length of 2 m is 1.0 μm or more, these dispersion flat optical fibers have excellent bending characteristics (small bending loss). Therefore, these dispersion flat optical fibers are suitable for use in time division multiplex transmission or wavelength multiplex soliton transmission using an optical amplifier. In particular, the dispersion flat optical fiber (FIG. 3) according to the second embodiment has an effective area A _eff of 70 μm ² or more, so that the power density of signal light in the dispersion flat optical fiber can be suppressed lower (non-linear optical fiber). The occurrence of the phenomenon is further suppressed). In the dispersion flat optical fiber according to the third embodiment (FIGS. 5A and 5B), the cutoff wavelength at a length of 2 m is longer than the signal light wavelength. However, considering that the actual transmission distance of the signal light is several hundred km to several thousand km as described later, the higher-order mode is attenuated, so that there is no problem.

The present invention is not limited to the above-described embodiments, and various modifications are possible. For example, the refractive index profiles shown in the first to fourth embodiments are examples, and the refractive index profile for realizing the dispersion flat optical fiber according to the present invention may have various other modes.

As an application example of the dispersion flat optical fiber having the refractive index profile shown in FIG. 5B, the inventors sampled a sample 1 in which the profile volume of the third core was changed as shown in FIG. , And the relationship between the outer diameter c2 of the core region (outer diameter of the third core) c2 and the dispersion slope (ps / nm ² / km), and the outer diameter c2 of the third core. And the cutoff wavelength (μm) at a length of 2 m was examined. Specifically, in the prepared samples 1 to 5, the ratio of the outer diameter of the second core to the outer diameter of the third core (b2 / c2) and the relative refractive index difference Δn ₃ of the third core to the second clad are changed. Have been. On the other hand, in the prepared samples 1 to 5, other parameters are common, and the ratio (a2 / c2) of the outer diameter of the first core to the outer diameter of the third core is 0.25, and the outer diameter of the third core is The ratio (d / c2) of the outer diameter of the first clad to the first cladding is 2.0, the relative refractive index difference Δn ₁ of the first core to the second cladding is 0.6%,
The relative refractive index difference Δn ₂ of the second core with respect to the second cladding and the relative refractive index difference Δn ₄ of the first cladding are both −0.05.
%.

FIGS. 11A and 11B show samples 1 to
5 is a graph showing a change in dispersion slope (ps / nm ² / km) with respect to the outer diameter of the core region (outer diameter c2 of the third core) for FIGS. 12 (a) and 12 (b). 5 is a graph showing a change in cutoff wavelength (μm) with respect to the outer diameter c2 of the third core for No. 5. In each of FIGS. 11A, 11B, 12A and 12B, S1 is a graph of sample 1, S2 is a graph of sample 2, S3 is a graph of sample 3, and S4 is a sample 4 S5 shows the graph of Sample 5.

As can be seen from these graphs, the second
Relative refractive index difference Δn of the third core with respect to the cladding_ThreeIs too low
Or the outer diameter of the second core relative to the outer diameter of the third core
If the ratio (b2 / c2) is too large, the dispersion slope
0.03ps / nm^Two/ Km or less
No. In addition, the cut-off wavelength at a length of 2 m becomes shorter,
Loss increases. Therefore, the dispersion slope
Bending loss by increasing the cut-off wavelength at 2m length
In order to improve the loss, the relative refractive index difference Δn of the third core _ThreeTo
It is necessary to set a large value and increase the width of the third core
There is. Incidentally, the core region of the above-described three-layer core structure (the
(With three cores) according to the third and fourth embodiments.
In a dispersion flat optical fiber, the
The 3 core profile volumes are 13.8 each
% ・ Μm^Two, 8.7% · μm^Two(See FIG. 9).

[0060]

(Equation 5)

Next, the inventors of the present invention have a third embodiment (FIG. 5).
As application examples of the dispersion-shifted fiber according to (a) and (b)), samples 6 to 9 having refractive index profiles shown in FIGS. 13A, 13B, and 14 are prepared, respectively. Wavelength 1550n for samples 6 to 9
Various properties at m were measured. The structure of the prepared samples 6 to 9 is the same as the cross-sectional structure shown in FIG. 5A, and includes a core region including a first core, a second core and a third core, and a first clad. And a cladding region composed of a second cladding.

Sample 6 has a refractive index profile 520 shown in FIG.
20 corresponds to the refractive index of each region along the line L2 in FIG. Also, the refractive index profile 520
, The region 521 is each part of the first core 601 on the line L2, the region 522 is each part of the second core 602 on the line L2, the region 523 is each part on the line L2 of the third core 603,
The region 524 is a portion on the line L2 of the first cladding 701,
The region 525 corresponds to each portion of the second clad 702 on the line L2.

In sample 6, this corresponds to region 521
First core (refractive index n₁) Has an outer diameter a of 5.7 μm,
The second core (refractive index n_Two<N₁) Outside diameter
b2 is 14.7 μm, the third core corresponding to region 523
(Refractive index n_Three> N_Two) Has an outer diameter c2 of 22.6 μm and a region 5
The first clad (refractive index n_Four= N_Two) Outside diameter
d is 45.2 μm, the second clad corresponding to region 525
(Refractive index n_Five> N_Four) Has an outer diameter of 125 μm. Also,
Relative refractive index difference Δn of the first core with respect to the second cladding ₁(=
(N₁-N_Five) / N_Five) Is 0.60% against the second clad.
Relative refractive index difference Δn of the second core_Two(= (N_Two-N_Five) /
n_Five) And the relative refractive index difference Δn of the first cladding_Four(= (N_Four−
n_Five) / N_Five) Is -0.05% for both, the second clad
Relative refractive index difference Δn of the third core_Three(= (N_Three-N_Five) /
n_Five) Is 0.30%. Furthermore, the outer diameter of the third core
The ratio of the outer diameter of the first core to the first core (a2 / c2) is 0.25,
The ratio of the outer diameter of the second core to the outer diameter of the third core (b2 / c)
2) is 0.65.

Next, the sample 7 has the refractive index profile 530 shown in FIG. 13B, and the refractive index profile 530 is the refractive index of each region along the line L2 in FIG. Is equivalent to In the refractive index profile 530, the region 531 corresponds to the line L2 of the first core 601.
Each region above, the region 532 is each region on the line L2 of the second core 602, the region 533 is each region on the line L2 of the third core 603, the region 534 is each region on the line L2 of the first clad 701, The region 535 corresponds to each portion of the second clad 702 on the line L2. In the prepared sample 7, the shape of the refractive index profile in the region 533 (corresponding to the radial refractive index profile of the third core region) is shown in FIG.
This is different from the shape of the region 513 in the refractive index profile 510 shown in FIG.

In sample 7, this corresponds to region 531
First core (refractive index n₁) Has an outer diameter of 5.6 μm,
The second core (refractive index n_Two<N₁) Outside diameter
b2 is 12.6 μm, the third core corresponding to region 533
(Refractive index n_Three> N_Two) Has an outer diameter c2 of 24.2 μm and a region 5
The first clad (refractive index n_Four= N_Two) Outside diameter
d is 48.4 μm, the second clad corresponding to the region 535
(Refractive index n_Five> N_Four) Has an outer diameter of 125 μm. Also,
Relative refractive index difference Δn of the first core with respect to the second cladding ₁(=
(N₁-N_Five) / N_Five) Is 0.60% against the second clad.
Relative refractive index difference Δn of the second core_Two(= (N_Two-N_Five) /
n_Five) And the relative refractive index difference Δn of the first cladding_Four(= (N_Four−
n_Five) / N_Five) Is -0.05% for both, the second clad
Relative refractive index difference Δn of the third core_Three(= (N_Three-N_Five) /
n_Five) Is 0.41%. Furthermore, the outer diameter of the third core
The ratio of the outer diameter of the first core to the first core (a2 / c2) is 0.23,
The ratio of the outer diameter of the second core to the outer diameter of the third core (b2 / c)
2) is 0.52.

Sample 8 has the refractive index profile 540 shown in FIG.
Corresponds to the refractive index of each region along the line L2 in FIG. In the refractive index profile 540, the region 541 is each part on the line L2 of the first core 601; the region 542 is each part on the line L2 of the second core 602;
The region 543 corresponds to each portion on the line L2 of the third core 603, the region 544 corresponds to each portion on the line L2 of the first clad 701, and the region 545 corresponds to each portion on the line L2 of the second clad 702. Note that the prepared sample 8 is located in the region 541.
The shape of the refractive index profile (corresponding to the radial refractive index profile of the first core region) is different from the shape of the region 511 in the refractive index profile 510 shown in FIG.

In sample 8, this corresponds to region 541.
First core (refractive index n₁) Has an outer diameter a of 8.6 μm,
A second core (refractive index n_Two<N₁) Outside diameter
b2 is 17.6 μm, the third core corresponding to region 543
(Refractive index n_Three> N_Two) Has an outer diameter c2 of 25.2 μm and a region 5
The first cladding (refractive index n_Four= N_Two) Outside diameter
d is 50.4 μm, the second clad corresponding to the region 545
(Refractive index n_Five> N_Four) Has an outer diameter of 125 μm. Also,
Relative refractive index difference Δn of the first core with respect to the second cladding ₁(=
(N₁-N_Five) / N_Five) Is 0.85% against the second clad
Relative refractive index difference Δn of the second core_Two(= (N_Two-N_Five) /
n_Five) And the relative refractive index difference Δn of the first cladding_Four(= (N_Four−
n_Five) / N_Five) Is -0.05% for both, the second clad
Relative refractive index difference Δn of the third core_Three(= (N_Three-N_Five) /
n_Five) Is 0.29%. Furthermore, the outer diameter of the third core
The ratio of the outer diameter of the first core to the first core (a2 / c2) is 0.34,
The ratio of the outer diameter of the second core to the outer diameter of the third core (b2 / c)
2) is 0.74.

On the other hand, Sample 9 has a refractive index profile similar to that of Sample 6 described above (see FIG. 13A). Therefore, in the sample 9, the outer diameter a2 of the first core (refractive index n ₁ ) corresponding to the region 521 is 6.6 μm.
m, the second core corresponding to the region 522 (refractive index n ₂ <n ₁ )
Has an outer diameter b of 18.9 μm and a third diameter corresponding to the region 523.
The outer diameter c2 of the core (refractive index n ₃ > n ₂ ) is 25.5 μm, and the outer diameter d of the first cladding (refractive index n ₄ = n ₂ ) corresponding to the region 524 is 41.0 μm, corresponding to the region 525. The outer diameter of the second cladding (refractive index n ₅ > n ₄ ) is 125 μm. Further, the relative refractive index difference Δn ₁ of the first core with respect to the second clad.
(= (N ₁ −n ₅ ) / n ₅ ) is 0.50%, and the relative refractive index difference Δn ₂ of the second core with respect to the second cladding (= (n ₂ −n ₅ ))
/ N ₅ ) and the relative refractive index difference Δn ₄ (= (n ₄
−n ₅ ) / n ₅ ) are both -0.15%, and the relative refractive index difference Δn ₃ (= (n ₃ −n ₅ ) of the third core with respect to the second cladding.
/ N ₅ ) is 0.43%. Further, the ratio of the outer diameter of the first core to the outer diameter of the third core (a2 / c2) is 0.2.
6, the ratio of the outer diameter of the second core to the outer diameter of the third core (b2
/ C2) is 0.74. With the above configuration, a dispersion flat optical fiber having a positive dispersion value and an extremely small dispersion slope can be obtained.

Samples 6 to 9 designed as described above
FIG. 15 shows various characteristics at a wavelength of 1550 nm.
Note that the profile volume of the third core given as described above is preferably equal to or greater than 7.0% · μm ^{2 in} consideration of the values of the above samples 3 to 9.

Further, the inventors of the present invention have prepared a dispersion-flat optical fiber (sample 7) having a refractive index profile shown in FIG. A plurality of samples prepared for the relationship with various properties were measured. Note that the prepared sample has a relative refractive index difference Δ of the first core with respect to the second clad.
n ₁ (= (n ₁ −n ₅ ) / n ₅ ) is 0.61%, and the relative refractive index difference Δn ₂ (= (n ₂ −
n ₅ ) / n ₅ ) and the relative refractive index difference Δn ₄ (=
(N ₄ −n ₅ ) / n ₅ ) are both -0.05%, and the relative refractive index difference Δn ₃ of the third core with respect to the second cladding (= (n ₃ −
n ₅₎ / n ₅₎ is 0.35%. The second clad was made of pure quartz. Further, the first core with respect to the outer diameter of the third core is
The ratio of the outer diameter of the core (a2 / c2) is 0.23, and the ratio of the outer diameter of the second core to the outer diameter of the third core (b2 / c2) is 0.
52, the ratio (d / c2) of the outer diameter of the first clad to the outer diameter of the third core is 1.8.

FIGS. 16A and 16B show the measurement results of the prepared samples. FIG. 16A shows the outer diameter c2 (μm) of the third core.
And the relationship between the effective area ^{_{A eff (μm 2), (}} b) bending the relationship between the loss (dB / m) when bent at the outer diameter c2 ([mu] m) and 20mm diameter of the third core, (c ) Is the relationship between the outer diameter c2 (μm) of the third core and the dispersion slope (ps / nm).
² / km), and (d) shows the outer diameter c2 (μm) of the third core.
m) and dispersion (ps / nm / km). In each graph, curves L3, L4, L5,
L6 is a theoretical curve obtained by calculation, and the plotted points (dots) represent measured values.

As can be seen from these graphs, the obtained dispersion flat optical fiber has a dispersion value of -4 to +4 ps / nm / km (<| 5 | p) at a wavelength of 1550 nm.
s / nm / km) and 0.026 to 0.028 ps / n
m ² / km dispersion slope (<0.03 ps / nm ² / k
m) and an effective area A _eff (> 45 μm) of 47 to 52 μm ^2.
m ² ).

In particular, the point to be noted in FIG. 16C is that in the region indicated by A in the figure, the fluctuation of the dispersion slope is small even if the outer diameter of the third core changes. . In general, when an optical fiber is manufactured, a manufacturing variation of about ± 2% occurs in the outer diameter of the core region between the manufactured lots (the outer diameter can be controlled only to about ± 2%). Therefore, if the outer diameter of the core region can be controlled within a range in which the dispersion slope dispersion is suppressed, it is possible to avoid the occurrence of the characteristic dispersion of each product (dispersion flat optical fiber) due to the production dispersion.

FIG. 17 is a graph showing the relationship between the variation of the outer diameter of the core region and the variation of various characteristics for the sample 8. In particular, (a) shows the outer diameter C2 of the third core and the dispersion (p
s / nm / km) and dispersion slope (ps / nm ² / k)
m), (b) is the variation rate (%) of the outer diameter of the core region.
Of the dispersion slope (ps / nm ² / km /
%). In FIG. 17A, D indicates the dispersion of Sample 8, and DS indicates the dispersion slope of Sample 8.

As can be seen from FIG. 17A, as the diameter of the region (mainly the core region) where the signal light propagates increases, the dispersion value increases, but the dispersion slope is the outer diameter of the third core. When c2 is a certain value, it takes a minimum value. In particular, in the vicinity of the minimum value, the fluctuation of the dispersion slope becomes small. Specifically, as can be seen from the graph shown in FIG. 17B, if the dispersion flat optical fiber is designed by setting the outer diameter of the core region to an appropriate value, the outer diameter can be controlled ±
It is possible to suppress the variation of the dispersion slope to 0.003 ps / nm ² / km /% or less with respect to the variation rate of the outer diameter of the core region of 2%.

Next, the bending loss (dB / m) and the cutoff wavelength in the dispersion flat optical fiber according to the present invention will be described.

FIG. 18A shows that the outer diameter c2 of the third core is 2
FIG. 7 is a graph in which the measured value of the bending loss (dB / m) with respect to the change in the bending diameter (mm) is plotted while being fixed at 2.4 μm, and L7 in the figure represents a theoretical value obtained by calculation. As can be seen from this graph, the bending loss at a bending diameter of 32 mm is 0.04 dB / m (= 0.004 d / m).
B / turn), which is a good value.

On the other hand, FIG. 18B shows the outer diameter c of the third core.
2 is a graph in which the measured value of the cutoff wavelength (μm) with respect to the change in the fiber length (km) is plotted with 2 fixed to 22.8 μm, and L8 in the figure represents a theoretical value obtained by calculation. Note that the slope of the line L8 is −252 nm /
Decade. As you can see from this graph,
Even if the cutoff wavelength at a length of 2 m is 2.080 μm, a single mode operation is guaranteed if the length is about 200 m. Therefore, when the cutoff wavelength at a length of 2 m is longer than the signal light wavelength. However, there is no practical problem (usually, in the case of a submarine cable or the like, an optical transmission line is formed by fusing a plurality of optical fibers of about 5 km).

FIG. 19A shows the profile volume (% · μm ² ) and the dispersion slope (ps / nm ² / km) of the third core for the samples 1 to 5 described above.
FIG. 19B is a graph showing the relationship between the profile volume (% · μm ² ) of the third core and the cutoff wavelength (μm) for Samples 1 to 5. . As described above, in the dispersion flat optical fiber having the core region of the three-layer core structure (the third and fourth embodiments having the third core), the cutoff wavelength at a length of 2 m is 1.4 μm or less. Need to be. Also,
The dispersion slope is preferably not more than 0.03 ps / nm ² / km. Therefore, in order to satisfy these restrictions, as shown in FIGS. 19A and 19B, the profile volume of the third core given as described above is 7.0% ·
μm ² or more. In FIGS. 19A and 19B, S1 to S5 are samples 1 to 5 described above, respectively.
5 shows the dispersion slope and cutoff wavelength for the profile volume.

[0080]

As described above in detail, the dispersion flat optical fiber according to the present invention has an absolute value of dispersion at a wavelength of 1550 nm of 5 ps / nm / km or less.
In addition, since the dispersion slope is equal to or less than 0.03 ps / nm ² / km, it is possible to reduce the difference in dispersion value between the signal lights over the entire used wavelength band. Further, the effective area is preferably 45 μm ² or more. By realizing an appropriate relationship between these effective areas and the dispersion slope, the power density of the signal light in the dispersion flat optical fiber is suppressed to be low, and the nonlinear optical phenomenon Is effectively suppressed, thereby enabling transmission at a high S / N ratio. Furthermore, since the cutoff wavelength at a length of 2 m is 1.0 μm or more, this dispersion flat optical fiber has excellent bending characteristics. Therefore, this dispersion flat optical fiber is suitable for time division multiplex transmission and wavelength multiplex soliton transmission using an optical amplifier.

When the effective area is 70 μm ² or more, the power density of the signal light in the dispersion-flattened optical fiber can be further suppressed, and the occurrence of the nonlinear optical phenomenon can be suppressed more effectively. The dispersion flat optical fiber is more suitable for time division multiplex transmission and wavelength multiplex soliton transmission using an optical amplifier.

[Brief description of the drawings]

FIG. 1A is a view showing a cross-sectional structure of a dispersion flat optical fiber having a ring core structure according to the present invention;
(B) is a refractive index profile shown along a line L1 in FIG.

FIG. 2 is a graph showing dispersion characteristics of the dispersion flat optical fiber according to the first embodiment (ring core structure) having the refractive index profile shown in FIG. 1 (b).

FIG. 3 is a refractive index profile of a dispersion flat optical fiber according to a second embodiment of the present invention (ring core structure + depressed clad structure).

FIG. 4 is a graph showing dispersion characteristics of the dispersion flat optical fiber of the second embodiment having the refractive index profile shown in FIG.

FIG. 5A is a diagram showing a cross-sectional structure of a dispersion flat optical fiber having a three-layer core structure according to the present invention, and FIG.
Is a refractive index profile shown along a line L2 in FIG.

FIG. 6 is a graph showing dispersion characteristics of the dispersion flat optical fiber according to the third embodiment (three-layer core structure + depressed clad structure) having the refractive index profile shown in FIG. 5B.

FIG. 7 is a refractive index profile of a dispersion flat optical fiber according to a fourth embodiment (three-layer core structure) of the present invention.

FIG. 8 is a graph showing dispersion characteristics of the dispersion flat optical fiber of the fourth embodiment having the refractive index profile shown in FIG.

FIG. 9 is a table summarizing various characteristics of each of the dispersion flat optical fibers according to the first to fourth embodiments.

FIG. 10 shows the ratio (b2 / c2) of the outer diameter of the second core to the outer diameter of the third core of samples 1 to 5 having the refractive index profile of FIG. 5B and the second clad (outer clad).
13 is a table summarizing the measurement results of the relative refractive index difference (Δn3) of the third core with respect to FIG.

FIG. 11 shows a dispersion slope (ps / nm2 / km) with respect to the outer diameter of the core region (outer diameter c2 of the third core) for Samples 1 to 5 having the refractive index profile of FIG.
6 is a graph showing a change in the graph.

12 is a graph showing a change in cutoff wavelength (μm) with respect to the outer diameter of the core region (outer diameter c2 of the third core) for Samples 1 to 5 having the refractive index profile of FIG. 5B.

FIG. 13 is a modification (refractive index profile) of the refractive index profile of FIG. 5B.

FIG. 14 is a modification (refractive index profile) of the refractive index profile of FIG. 5B.

FIG. 15 is a table summarizing the results of measuring various characteristics of each sample 6 to 8 having the refractive index profiles shown in FIGS. 13 (a), 13 (b) and 14;

FIG. 16 is a graph showing changes in various characteristics with respect to changes in the outer diameter of the core region (outer diameter c2 of the third core) for Sample 7 having the refractive index profile shown in FIG. 13B. In particular, (a) shows the relationship between the outer diameter (μm) of the core region and the effective sectional area (μm ² ), and (b) shows the bending loss (dB) when bent at the outer diameter (μm) of the core region and a diameter of 20 mm. / M), (c) is the outer diameter of the core region (μm)
Relationship of the relationship between the dispersion slope ^{(ps / nm 2 / km)} , (d) is dispersed to the outer diameter of the core region (μm) (ps / n
m / km).

FIG. 17 is a graph showing the relationship between the variation in the outer diameter of the core region and the variation in various characteristics for Sample 8. In particular,
(A) shows the outer diameter of the core region and the dispersion (ps / nm / k).
m) and the dispersion slope (ps / nm ² / km),
(B) shows the relationship between the variation amount (ps / nm ² / km /%) of the dispersion slope and the variation rate (%) of the outer diameter of the core region.

FIG. 18A shows a bending diameter (mm) and a bending loss (dB).
/ M), and (b) is a graph showing the relationship between the fiber length (km) and the cutoff wavelength (μm).

FIG. 19A is a graph showing the relationship between the profile volume (% · μm 2) of the third core and the dispersion slope (ps / nm ² / km) for Samples 1 to 5, and FIG. 9 is a graph showing the relationship between the profile volume (% · μm ² ) of the third core and the cutoff wavelength (μm) for Samples 1 to 5.

[Explanation of symbols]

100, 500: dispersion flat optical fiber, 200, 6
00: core region, 300, 700: cladding region, 20
1, 601: first core, 202, 602: second core, 6
03: third core, 301, 701: first clad, 30
2, 702... Second cladding.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Eiji Sasaoka, Inventor 1 Tayacho, Sakae-ku, Yokohama-shi, Kanagawa Prefecture Sumitomo Electric Industries, Ltd. Yokohama Works (72) Inventor Masayuki Nishimura 1-Tagamachi, Sakae-ku, Yokohama, Kanagawa Prefecture Sumitomo Electric Ki Kogyo Co., Ltd.

Claims (8)

[Claims] 1. A dispersion-flattened optical fiber comprising a core region extending along a predetermined axis and a cladding region provided on an outer periphery of the core region, wherein the core region has a predetermined refractive index. A first core, a second core provided on the outer periphery of the first core and having a lower refractive index than the first core, and a refraction provided on the outer periphery of the second core and higher than the second core A third core having a ratio of 5 ps / nm / km at a wavelength of 1550 nm.
Dispersion at 45 nm, an effective area of 45 μm ² or more at a wavelength of 1550 nm, and 0.03 ps / nm ² / km at a wavelength of 1550 nm.
A dispersion flat optical fiber having the following dispersion slope, a transmission loss of 0.5 dB / turn or less when bent to a diameter of 32 mm at a wavelength of 1550 nm, and a cutoff wavelength of 1.0 μm or more at a length of 2 m. 2. A dispersion-flattened optical fiber having a core region extending along a predetermined axis and a cladding region provided on an outer periphery of the core region, wherein the core region has a predetermined refractive index. A first core, a second core provided on the outer periphery of the first core and having a lower refractive index than the first core, and a refraction provided on the outer periphery of the second core and higher than the second core A third core having a ratio of 5 ps / nm / km at a wavelength of 1550 nm.
Dispersion at 45 nm, an effective area of 45 μm ² or more at a wavelength of 1550 nm, and 0.03 ps / nm ² / km at a wavelength of 1550 nm.
The following dispersion slope, a transmission loss of 0.5 dB / turn or less when bent to a diameter of 32 mm at a wavelength of 1550 nm, and a cutoff wavelength of 1.0 μm or more at a length of 2 m. When the outer diameter is b, the outer diameter of the third core is c, and the relative refractive index difference of the third core with respect to the reference region of the cladding region is Δn ₃ , Δn ₃ ≧ 0.25% 0.40 ≦ b /C≦0.75 A dispersion flat optical fiber satisfying the following relationship: 3. A dispersion-flattened optical fiber comprising a core region extending along a predetermined axis and a cladding region provided on an outer periphery of the core region, wherein the core region has a predetermined refractive index. A first core, a second core provided on an outer periphery of the first core and having a lower refractive index than the first core, and a refraction provided on an outer periphery of the second core and higher than the second core A third core having a ratio of 5 ps / nm / km at a wavelength of 1550 nm.
Following dispersion and is, the effective area 45 [mu] m ² or more at the wavelength of 1550nm, 0.03ps / nm ² / km at a wavelength of 1550nm
The following dispersion slope, a transmission loss of 0.5 dB / turn or less when bent to a diameter of 32 mm at a wavelength of 1550 nm, and a cutoff wavelength of 1.0 μm or more at a length of 2 m, When a relative distance between the cladding region and the reference region at a portion at a distance r from the center to the reference region at a portion at a distance r from the center is Δn (r), A dispersion flat optical fiber that satisfies the following relationship. As characteristics at 4. Wavelength 1550 nm, the effective cross-sectional area, 45 [mu] m ² or more and 58 .mu.m ² dispersion-flattened optical fiber according to any one of claims 1 to 3, wherein the less. 5. As various characteristics at a wavelength of 1550 nm, the effective area is 45 μm ² or more and 53.9 μm.
dispersion-flattened optical fiber according to claim 4, characterized in that m ² or less. 6. The apparatus according to claim 1, wherein the cutoff wavelength at a length of 2 m is at least 1.4 μm.
4. The dispersion flat optical fiber according to claim 3. 7. As various characteristics at a wavelength of 1550 nm, the effective sectional area is 45 μm ² or more, and the dispersion slope is 0.02 ps / nm ² / km or less. 4. The dispersion flat optical fiber according to claim 3. 8. The cladding region is provided on the outer periphery of the third core and has a first cladding having a lower refractive index than the third core, and a region corresponding to the reference region. Provided on the outer periphery of the clad and the first
A second cladding having a higher refractive index than the cladding, wherein the outer diameter of the first core is a, the outer diameter of the second core is b, the outer diameter of the third core is c, First
When the relative refractive index difference of the core is Δn ₁ and the relative refractive index difference of the first cladding with respect to the second cladding is Δn ₄ , 0.40% ≦ Δn ₁ ≦ 0.90% Δn ₄ ≦ −0.02 %. 0.20 ≦ a / c ≦ 0.35 20 μm ≦ c ≦ 30 μm The dispersion flat optical fiber according to claim 1, wherein the following relationship is satisfied.