JPH10239550A - Dispersion shift fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a dispersion shift fiber suitable for long-distance light transmission by not making zero dispersion wavelength exist in a specific wavelength band, and setting a specific effective core sectional area. SOLUTION: The dispersion shift fiber has zero dispersion wavelength off a wavelength band of 1.53μm-1.56μm, and also has the dispersion value with the absolute value of 1.0-4.5ps/nm/dm, a dispersion slope with the absolute value of 0.13ps/nm<2> /km or less, effective core sectional area Aeff of 70μm<2> or ore, and transmission loss of 0.25B/km or less to light of a wavelength band of 1.55μ as various characteristics in 1550nm. A nonlinear optical constant (N2 /Aeff) of 5.8×10<-10> (1/W) in case of the sectional area Aeff being 55μm<2> becomes 4.6×10<-10> (1/W) if the sectional area is 70μm<2> so as to be reduced by about 20%. Wavelength multiplicity of signal light can therefore be increased in comparison to a conventional dispersion shift fiber.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a single-mode optical shift fiber (hereinafter, referred to as an S-mode optical fiber) used for optical transmission in long-distance optical communication and the like.
The present invention relates to a dispersion-shifted fiber suitable for wavelength multiplex transmission.

[0002]

2. Description of the Related Art Conventionally, in an optical communication system to which an S-mode optical fiber is applied as a transmission line, light in a 1.3 μm wavelength band or 1.55 μm wavelength band is often used as communication signal light. . However, in recent years, the use of light in a 1.55 μm wavelength band has been increasing from the viewpoint of reducing transmission loss in a transmission path. An S-mode optical fiber (hereinafter, referred to as an optical fiber) applied to such a 1.55 μm wavelength band light transmission path.
1.55 μm S-mode optical fiber).
Wavelength dispersion for light in the 55 μm wavelength band (the phenomenon that a pulse wave spreads because the light propagation speed varies depending on the wavelength)
(Zero dispersion wavelength 1.5)
5 μm dispersion-shifted fiber). As such a dispersion-shifted fiber, Japanese Patent Publication No. Hei 3-18161 discloses a dual-shape core type refractive index in which a core is composed of an inner core and an outer core having a refractive index lower than that of the inner core. Dispersion shifted fibers having a profile have been proposed. Further, JP-A-63-43107 and JP-A-2-141704 disclose, in addition to the above-mentioned double core structure, a clad having an inner clad and an outer clad having a larger refractive index than the inner clad. A dispersion-shifted fiber having a depressed clad dual shape core type refractive index profile has been proposed.

In recent years, long-distance optical transmission has become possible with the advent of wavelength division multiplexing (WDM) transmission and optical amplifiers. However, under such circumstances, the influence of the nonlinear optical effect cannot be ignored. Therefore, in order to avoid the nonlinear optical effect, there has been proposed a fiber in which the zero-dispersion wavelength is shifted to the short wavelength side or the long wavelength side of the signal wavelength band by modifying the refractive index profile (Japanese Patent Laid-Open No. 7-168).
046, U.S. Pat. No. 5,483,612). Note that the nonlinear optical effect is a phenomenon in which a signal light pulse is distorted in proportion to the density of light intensity and the like, and is a limiting factor for a transmission speed and a relay interval in a relay transmission system.

[0004]

In a conventional dispersion-shifted fiber proposed for wavelength division multiplex transmission,
By setting the zero dispersion wavelength to a value different from the wavelength value of the signal wavelength band, the expression of the nonlinear optical effect is suppressed,
The effective core area A _eff is set to about 55 μm ² .

In a conventional application example, a conventional dispersion-shifted fiber for wavelength division multiplexing transmission is sufficient. However, in view of the future progress of wavelength multiplexing with the advancement of communication, the conventional technology requires the conventional technology. It is difficult to maintain suitable transmission quality over the transmission distance.

The effective core area A _eff is _disclosed in
As shown in -248251, it is given by the following equation.

[0007]

(Equation 1)

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a dispersion-shifted fiber which can effectively suppress the occurrence of a nonlinear optical effect and is suitable for long-distance optical transmission. Aim.

[0009]

A dispersion-shifted fiber according to the present invention is an S-mode optical fiber having a core region and a cladding region and mainly composed of silica glass,
This is a dispersion-shifted fiber in which the zero-dispersion wavelength is shifted to the shorter wavelength side or the longer wavelength side of the signal light wavelength band. The transmission object of the dispersion-shifted fiber according to the present invention has a center wavelength of 1
One or more lights in the range of 500 nm to 1600 nm (signal light wavelength band). In this specification, 1.
Light in the 55 μm wavelength band is equivalent to light in this signal wavelength band. And, the dispersion-shifted fiber is 1.5
3 μm (1530 nm) to 1.56 μm (1560 n
m) having a zero-dispersion wavelength outside the wavelength band of
As various characteristics at 1550 nm, the absolute value is 1.0 to
A dispersion value of 4.5 ps / nm / km and an absolute value of 0.13
It is characterized by having a dispersion slope of ps / nm ² / km or less, an effective core area A _{eff of} 70 μm ² or more, and a transmission loss of 0.25 dB / km or less with respect to light in a 1.55 μm wavelength band. .

Here, the absolute value of the dispersion value is 1.0 ps / n
If it is smaller than m / km, in long-distance optical transmission of 20 km or more, waveform distortion due to four-wave mixing or modulation instability cannot be ignored in practical use. On the other hand, if the absolute value of the dispersion value is larger than 4.5 ps / nm / km, waveform distortion due to chromatic dispersion and self-phase modulation cannot be ignored in practical use in long-distance optical transmission of 20 km or more.

In the dispersion-shifted fiber according to the present invention, the absolute value of the dispersion slope is 0.13 ps / nm ² /
km or less. Therefore, the difference in the chromatic dispersion value between each signal light is reduced, and the optical transmission in which the difference in the amount of waveform distortion due to the chromatic dispersion between each signal light is effectively reduced can be performed.

The amount of occurrence of the nonlinear optical effect is proportional to the nonlinear optical constant (N ₂ / A _eff ). Therefore, it is effective to reduce the nonlinear optical constant (N ₂ / A _eff ) in order to suppress the occurrence of the nonlinear optical effect when the conditions of the propagating light are the same. On the other hand, since the nonlinear refractive index N ₂ is almost determined by the main material of the optical fiber, for the optical fiber having the same main material, the nonlinear optical effect can be reduced by changing the nonlinear refractive index N ₂ from the conventional value. It is difficult to suppress the expression.

Therefore, the dispersion-shifted fiber according to the present invention has an effective core area A _eff of 70 μm ² from the conventional 55 μm ^2.
By increasing the size to μm ² or more, the amount of exhibiting the nonlinear optical effect is reduced by 20% or more as compared with the conventional dispersion-shifted fiber.

FIG. 1 is a graph showing the relationship between the effective core area A _eff and the nonlinear optical constant (N ₂ / A _eff ) for a dispersion-shifted fiber having a typical composition. From FIG. 1, the nonlinear optical constant (N ₂ / A _eff ), which was 5.8 × 10 ⁻¹⁰ (1 / W) when the effective core area A _eff was 55 μm ² , was changed to the effective core area A _eff 70 μm ²
Then, it becomes 4.6 × 10 ⁻¹⁰ (1 / W), which is about 20
% Reduction.

Therefore, in the dispersion-shifted fiber according to the present invention, it is possible to increase the wavelength multiplicity of the signal light as compared with the conventional dispersion-shifted fiber.

In general, the refractive index N of a medium under strong light changes depending on the light intensity. Therefore, the lowest order effect on the refractive index N is: N = N ₀ + N ₂ · E ² where N ₀ : refractive index for linear polarization N ₂ : nonlinear refractive index for tertiary nonlinear polarization E: electric field amplitude expressed. That is, under strong light, the refractive index N of the medium is given by the sum of N ₀ and an increase proportional to the square of the electric field amplitude E. In particular, the proportionality constant N ₂ (unit: m ² /
V ² ) is called the nonlinear refractive index. In addition, since the distortion of the signal light pulse is mainly affected by the nonlinear refractive index of the second term among the nonlinear refractive indexes, in this specification, the nonlinear refractive index mainly means the nonlinear refractive index.

The dispersion-shifted fiber according to the present invention can increase the incident power of signal light by about 20% (about 1 dB) as compared with the conventional dispersion-shifted fiber.
When the transmission loss is 0.2 dB / km, signal light can be transmitted over a transmission distance longer by 5 km than in the related art. As a result, for example, the distance between the conventional repeaters is 50 k
m, the number of repeaters can be reduced by about 10%.

Further, the dispersion-shifted fiber according to the present invention is characterized in that a bending loss when bent at a diameter of 32 mm (32 mmφ) is 0.5 dB / turn or less. The bending loss was measured with a mandrel (ma
ndrel) is measured with the fiber to be measured wound thereon, and the measured value is given as a value converted per turn.

In general, when the effective core area A _eff is increased, the light intensity density on the outer peripheral side is increased, and the bending loss is increased. An optical fiber having a large bending loss generates a large optical loss due to bending which is inevitably generated by a cable-forming process, cable laying, extra connection length processing, and the like. Therefore, in the dispersion-shifted fiber according to the present invention, by setting the bending loss when bent at a diameter of 32 mm to 0.5 dB / turn or less, light loss due to bending generated in a cable-forming step or the like is effectively suppressed. ing.

Further, in the dispersion-shifted fiber according to the present invention, the absolute value of the dispersion slope is 0.09 ps / nm ² /
km or more. That is, when the dispersion slope is small, the difference in the chromatic dispersion value between the signal lights becomes small, and the difference in the amount of waveform distortion due to the chromatic dispersion between the signal lights can be suppressed to a small value. On the other hand, the smaller the dispersion slope is, the easier it is to satisfy the phase matching condition in which four-wave mixing, which is one of the nonlinear optical phenomena, occurs. Therefore, in the dispersion-shifted fiber according to the present invention, the absolute value of the dispersion slope is 0.09 ps / nm ² / km or more and 0.1
By setting the rate to 3 ps / nm ² / km or less, not only a change in waveform distortion due to chromatic dispersion but also four-wave mixing during transmission of signal light is suppressed.

In order to realize the above characteristics, the dispersion-shifted fiber according to the present invention can be realized by a dual-shape core type or segment core type refractive index profile. Each of the refractive index profiles has a depressed cladding structure.

Here, the first embodiment of the dispersion shifted fiber according to the present invention has a depressed clad dual shape core type refractive index profile. And
The first embodiment of the dispersion-shifted fiber has an inner core having a predetermined refractive index and an outer diameter of 2a,
An outer core provided on the outer periphery of the inner core, having a lower refractive index than the inner core, and having an outer diameter of 2b;
It is characterized by comprising an inner cladding provided on the outer periphery of the outer core and having a lower refractive index than the outer core, and an outer cladding provided on the outer periphery of the inner cladding and having a higher refractive index than the inner cladding.

According to the findings obtained by the inventor from the results of the research, in a dispersion-shifted fiber having a simple dual-shaped core type refractive index profile without a depressed cladding structure, the absolute value of the dispersion value at a wavelength of 1550 nm is 1 When the effective core area A _eff is increased in the state where the power is set to 0.0 to 4.5 ps / nm / km, the cutoff wavelength becomes shorter, and the bending loss increases. Even if the cut-off wavelength is lengthened by adjusting the refractive index profile in order to reduce the bending loss, it is necessary to sufficiently improve the bending loss due to the restriction that the cut-off wavelength must be smaller than the wavelength of the signal light. Can not.

According to the knowledge of the inventor, the bending loss of the dispersion shifted fiber having the depressed clad dual shape core type refractive index profile is the same as that of the dispersion shifted fiber having the mere dual shape core type refractive index profile. Lower than bending loss. Accordingly, the optical fiber (having a depressed clad dual shape core type refractive index profile) to which the above-described structure is applied can appropriately realize the above-mentioned various characteristics and can suppress the bending loss to a predetermined value or less. .

Further, in the first embodiment of the dispersion-shifted fiber, the relative refractive index difference of the inner core to the inner cladding is Δn ₁ , the relative refractive index difference of the outer core to the inner cladding is Δn ₂ , When the relative refractive index difference is Δn ₃ , a / b ≦ 0.15 (1) 0.8% ≦ Δn ₁ ≦ 1.2% (2) 0.12% ≦ Δn ₂ ≦ 0.30% (3) Δn ₃ / Δn ₂ ≦ 0.95 (4) It is preferable to satisfy the following relationship.

According to the dispersion-shifted fiber satisfying these relations, the above-mentioned various characteristics can be suitably realized, and the absolute value of the dispersion slope can be 0.09 ps / nm ² / km or more.
In addition, it can be 0.13 ps / nm ² / km or less.

When the refractive index of the inner core is n ₁ , the refractive index of the outer core is n ₂ , the refractive index of the inner cladding is n ₃ , and the refractive index of the outer cladding is n ₄ , Relative index difference Δ of inner core to inner cladding
n ₁ , the relative refractive index difference Δ of the outer core with respect to the inner cladding
n _2, and the relative refractive index of the outer cladding difference with respect to the inner cladding [Delta] n _{3 ^{is, Δn 1 = (n 1 2}} -n 3 2) / (2n 1 2) ... (5) Δn 2 = (n 2 2 -n 3 as defined in ^{_{2) / (2n 2 2)}} ... (6) Δn 3 = (n 4 2 -n 3 2) / (2n 4 2) ... (7). Further, in this specification, each relative refractive index difference is expressed in percentage.

Further, the first embodiment of the dispersion-shifted fiber satisfies the relationship of 1.2 ≦ c / b ≦ 3.5 when the outer diameter of the inner cladding is 2c. Features.

That is, the first of the dispersion-shifted fibers
In the embodiment, if the inner cladding is too thin, the effect of reducing the bending loss, which is an effect due to the presence of the inner cladding, cannot be sufficiently exhibited. On the other hand, if the inner clad is too thick, the inner clad plays a role similar to that of a normal clad, and a depressed clad type refractive index profile that shortens the cutoff wavelength cannot be obtained. Further, when the dispersion-shifted fiber satisfies the relationship of c / b ≧ 1.2, the bending loss when bent at a diameter of 32 mm can be reduced to 0.5 dB / turn or less.

On the other hand, the first embodiment of the dispersion-shifted fiber satisfies the relationship of c / b ≦ 3.5, so that the cutoff wavelength can be suitably shortened, and the signal light capable of single mode transmission. Wavelength region can be easily secured.

Next, the second embodiment of the dispersion shifted fiber according to the present invention has a depressed clad segment core type refractive index profile. The second embodiment of the dispersion-shifted fiber has a predetermined refractive index, an inner core having an outer diameter of 2a, and an outer core provided on the outer periphery of the inner core, having a lower refractive index than the inner core. An intermediate core having an outer diameter of 2b, and an outer core provided on the outer periphery of the intermediate core, having a higher refractive index than the intermediate core, and an outer core having an outer diameter of 2c, and being provided on the outer periphery of the outer core; An inner cladding having a lower refractive index than the outer core and an outer cladding provided on the outer periphery of the inner cladding and having a higher refractive index than the inner cladding are provided.

In the second embodiment of the dispersion-shifted fiber, the relative refractive index difference between the inner core and the inner core is ΔΔ.
n ₁ , the relative refractive index difference of the intermediate core with respect to the inner cladding is Δ
n ₂ , the relative refractive index difference of the outer core with respect to the inner cladding is Δ
n ₃ , relative refractive index difference of outer cladding to inner cladding
Is Δn ₄ , a / c ≦ 0.42 (9) b / c ≧ 0.60 (10) 0.5% ≦ Δn ₁ ≦ 1.1% (11) 0.2% ≦ Δn ₃ −Δn ₂ ≦ 0.7% (12) It is preferable that the following relationship is satisfied: Δn ₄ / Δn ₃ ≦ 0.95 (13)

The above equations (9) and (10) are conditions for setting the effective core area A _eff to 70 μm ² . Wavelength 1
The absolute value of the dispersion value at 550 nm is 1.0 to 4.5 p
Since it is s / nm / km, the relative refractive index difference Δn ₁ of the inner core with respect to the inner cladding is preferably 0.5% or more. Further, this relative refractive index difference Δn ₁ has a wavelength of 1550.
The absolute value of the dispersion slope in nm is 0.09 to 0.1
In order to make the range of 3 ps / nm ² / km, the content is preferably 1.1% or less. The value (Δn ₃ −Δn ₂ ) is 3
In order to suppress the bending loss when bent at 2 mm to 0.5 dB / turn or less, it is necessary to be 0.2% or more. On the other hand, to reduce the cutoff wavelength at a 2 m length to 2.2 μm or less, 0.2% or less is required. It must also be less than 7%. Note that the expression (1)
3) is to reduce the transmission loss for light in the 1.55 wavelength band to 0.2.
This is a condition for suppressing the level to 5 dB / km or less.

Further, the second embodiment of the dispersion-shifted fiber satisfies the following relationship when the outer diameter of the inner cladding is 2d: 1.2 ≦ d / c ≦ 3.5 (14) Features.

That is, when d / c ≧ 1.2, the diameter 32
The bending loss in mm is 0.5 dB / km or less, and (d
If / c) exceeds 3.5, the effect of reducing the cutoff wavelength is saturated.

[0036]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a dispersion-shifted 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.

FIG. 2 is a diagram showing a cross-sectional structure and a refractive index profile of a typical embodiment of the dispersion-shifted fiber according to the present invention. As shown in FIG. 2, the dispersion-shifted fiber 100 has a refractive index n as a maximum refractive index.
_And an inner core 1 having an outer diameter of 2a.
10 and a refractive index n ₂ provided on the outer circumference of the inner core 110.
(<N ₁ ), an outer core 120 having an outer diameter of 2b, and an inner core provided on the outer periphery of the outer core 120 and having a refractive index n ₃ (<n ₂ ) and an outer diameter of 2c. A cladding 210 and an outer cladding 2 provided on the outer periphery of the inner cladding 210 and having a refractive index n ₄ (> n ₃ ).
20. With the above configuration, the dispersion-shifted fiber 100 realizes a depressed clad dual shape core type refractive index profile 101.

Then, the dispersion-shifted fiber 100
The relative refractive index difference of the inner core 110 with respect to the inner cladding 210 is Δn ₁ , the relative refractive index difference of the outer core 120 with respect to the inner cladding 210 is Δn ₂ , and the relative refractive index difference of the outer cladding 220 with respect to the inner cladding 210 is Δn ₃ A / b ≦ 0.15 (1) 0.8% ≦ Δn ₁ ≦ 1.2% (2) 0.12% ≦ Δn ₂ ≦ 0.30% (3) Δn ₃ / Δn ₂ ≦ 0.95 (4) 1.2 ≦ c / b ≦ 3.5 (8)

FIG. 3 is a graph showing the relationship between the value (a / b) of the ratio of the outer diameter 2a of the inner core 110 to the outer diameter 2b of the outer core 120 and the effective core area _Aeff . Note that FIG.
In this example, the outer diameter 2a of the inner core 110 and the outer diameter 2b of the outer core 120 are changed so that Δn ₁ = 1.0% and Δn ₂ = 0.2%, and the zero dispersion wavelength = 1580 nm. . From FIG. 3, when (a / b) ≦ 0.15, A _eff ≧ 70
It is confirmed to be μm ² .

FIG. 4 is a graph showing the relationship between the relative refractive index difference Δn ₁ of the inner core 110 with respect to the inner cladding 210 and the absolute value | D | of the dispersion value D for light having a wavelength of 1550 nm. In FIG. 4, (a / b) = 0.13, A
The outer diameter 2a of the inner core 110, the outer diameter 2b of the outer core 120, and the inner clad 210 so as to satisfy _eff = 80 μm ^2.
Relative refractive index difference Δn ₁ of the inner core 110 to the inner cladding 210 and relative refractive index difference Δ of the outer core 120 to the inner cladding 210.
n ₂ is changed. From FIG. 4, when Δn ₁ ≧ 0.8%,
| D | ≧ 1.0 ps / nm / km, Δn ₁ ．1.
It is confirmed that | D | ≦ 4.5 ps / nm / km even at 2%.

FIG. 5 is a graph showing the relationship between the relative refractive index difference Δn ₁ of the inner core 110 with respect to the inner cladding 210 and the dispersion slope. In FIG. 5, (a / b) =
0.13, A _eff = 80 μm ² , bending loss when bent at 32 mm in diameter = 0.1 dB / turn, zero dispersion wavelength = 15
Outer diameter 2a of the inner core 110 so as to satisfy 80 nm,
The outer diameter 2b of the outer core 120, the relative refractive index difference Δn ₁ of the inner core 110 with respect to the inner cladding 210, and the relative refractive index difference Δn ₂ of the outer core 120 with respect to the inner cladding 210 are changed. From FIG. 5, it is confirmed that when Δn ₁ ≧ 1.2%, the dispersion slope is 0.13 ps / nm ² / km.

FIG. 6 is a graph showing the relationship between the relative refractive index difference Δn ₂ of the outer core 120 with respect to the inner cladding 210 and the bending loss when bent at a diameter of 32 mm. In FIG. 6, the inner core 1 with respect to the inner cladding 210 is shown.
10, a relative refractive index difference Δn ₁ = 1.0%, (a / b) = 0.
13, the outer diameter 2a of the inner core 110 and the outer core 1 so as to satisfy A _eff = 80 μm ² and zero dispersion wavelength = 1580 nm.
20 and the relative refractive index difference Δn ₂ of the outer core 120 with respect to the inner cladding 210 is changed. FIG.
This confirms that Δn ₂ ≧ 0.12% and the bending loss when bent at a diameter of 32 mm is 0.5 dB / turn or less.

FIG. 7 is a graph showing the relationship between the relative refractive index difference Δn ₂ of the outer core 120 with respect to the inner cladding 210 and the cutoff wavelength at a reference length of 2 m. In FIG. 7, the relative refractive index difference Δn ₁ of the inner core 110 with respect to the inner cladding 210 = 1.0%, (a / b) = 0.13, A _eff
= 80 μm ² , zero dispersion wavelength = 1580 nm, Δn ₃ / Δn
₂ = 0.8 so that the outer diameter of the inner core 110 is 2
a, the outer diameter 2b of the outer core 120, the relative refractive index difference Δn ₂ of the outer core 120 with respect to the inner cladding 210, and the relative refractive index difference Δn ₃ of the outer cladding 220 with respect to the inner cladding 210.

Usually, the cutoff wavelength λ of the optical fiber
_C is CCITT-G. 2m recommended by 650
It is measured by the bending method at length. On the other hand, the cutoff wavelength λ as the lower limit wavelength at which single mode transmission is possible
_C is, when the length L of the S-mode optical fiber is 2 m,
Consistent with the above measurement result, when the length L increases, it changes according to the following formula: λ _C (L) = λ _C (L ₀ = 2 m) −0.184 × log ₁₀ (L / L ₀ ) (15) Are known ("T.Kato et a
l., OECC'96 Technical Digest, July 1966, Makuhari
Messe, pp. 160-161 ”).

On the other hand, the dispersion-shifted fiber according to the present invention is assumed to transmit signal light in the 1.55 μm wavelength band (1500 nm to 1600 nm) over a long distance of 20 km or more. Therefore, it is necessary that the cutoff wavelength λ _C (L ₀ ) at a length of 2 m satisfies the following condition: λ _C (L ₀ ) ≦ 1.5 + 0.732 μm ≒ 2.2 μm (16)

FIG. 7 shows that Δn ₂ ≦ 0.30% and the cutoff wavelength λ _C (L ₀ ) ≦ 2.2 μm at a length of 2 m.

FIG. 8 is a graph showing the relationship between the value (Δn ₃ / Δn ₂ ) and the transmission loss. In FIG. 8, under the following conditions: relative refractive index difference Δn ₁ = 1.0%, relative refractive index difference Δn _{2 of} outer core 120 = 0.20% (a / b) = 0.13, inner cladding Outer cladding 22 for 210
The relative refractive index difference Δn ₃ of 0 is changed.

From FIG. 8, it is confirmed that when (Δn ₃ / Δn ₂ )> 0.95, the transmission loss exceeds 0.25 dB / km and rapidly increases.

FIG. 9 is a graph showing the relationship between the value (c / b) and the bending loss when bending at a diameter of 32 mm.
In FIG. 9, the following conditions: relative refractive index difference Δn ₁ = 1.0% relative refractive index difference Δn ₂ = 0.20% relative refractive index difference Δn ₃ = 0.12% radius a = 2.1 μm radius b The outer diameter 2c of the inner cladding 210 is changed under = 16.0 μm.

FIG. 9 shows that (c / b) <1.2 and the diameter 3
It is confirmed that the bending loss when bent at 2 mm exceeds 0.5 dB / turn and sharply increases.

FIG. 10 is a graph showing the relationship between the value (c / b) and the cutoff wavelength at a length of 2 m. Note that FIG.
Then, the following conditions: relative refractive index difference Δn ₁ = 1.0% relative refractive index difference Δn ₂ = 0.20% relative refractive index difference Δn ₃ = 0.12% radius a = 2.1 μm radius b = 16.0 μm Below, the outer diameter 2c of the inner cladding 210 is changed.

From FIG. 10, it is confirmed that the effect of reducing the cutoff wavelength is saturated when (c / b)> 3.5.

That is, when the dispersion-shifted fiber according to the present invention has a depressed clad / dual shape core type refractive index profile, a / b ≦ 0.15 (1) 0.8% ≦ Δn ₁ ≦ 1 .2% (2) 0.12% ≦ Δn ₂ ≦ 0.30% (3) Δn ₃ / Δn ₂ ≦ 0.95 (4) 1.2 ≦ c / b ≦ 3.5 (8) Satisfies the following relationship:
The absolute value of the dispersion value is 1.0 to 4.5 ps / nm / km, the absolute value of the dispersion slope is 0.13 ps / nm ² / km or less, the effective core area is 70 μm ² or more, and 1.55
the transmission loss of light in the μm wavelength band is 0.25 dB / km or less,
0.5dB bending loss when bent at 32mm diameter
/ Turns or less is preferably satisfied.

Further, it is possible to effectively suppress the occurrence of the nonlinear optical effect, and to realize a dispersion-shifted fiber suitable for long-distance optical transmission.

On the other hand, the present invention is not limited to a dispersion shifted fiber having a depressed clad dual shape core type refractive index profile as described above, and various modifications are possible. For example, the dispersion shifted fiber according to the present invention may be a dispersion shifted fiber having a depressed clad segment core type refractive index profile.

A dispersion-shifted fiber having a depressed clad segment core type refractive index profile is as follows:
An inner core having a predetermined refractive index and having an outer diameter of 2a; an intermediate core provided on the outer periphery of the inner core and having a lower refractive index than the inner core and having an outer diameter of 2b; An outer core provided on the outer periphery of the core and having a higher refractive index than the intermediate core and having an outer diameter of 2c, and an outer core provided on the outer periphery of the outer core and having a lower refractive index than the outer core and It has an inner cladding having a diameter of 2d and an outer cladding provided on the outer periphery of the inner cladding and having a higher refractive index than the inner cladding.

Accordingly, in the dispersion-shifted fiber having the above-mentioned various characteristics at a wavelength of 1550 nm, the relative refractive index difference of the inner core with respect to the inner cladding is Δn ₁ , the relative refractive index difference of the intermediate core with respect to the inner cladding is Δn ₂ , and the relative refractive index difference with respect to the inner cladding. When the relative refractive index difference of the outer core is Δn ₃ and the relative refractive index difference of the outer cladding relative to the inner cladding is Δn ₄ , a / c ≦ 0.42 (9) b / c ≧ 0.60 (10) 0.5% ≦ Δn ₁ ≦ 1.1% (11) 0.2% ≦ Δn ₃ −Δn ₂ ≦ 0.7% (12) Δn ₄ / Δn ₃ ≦ 0.95 (13) 2 ≦ d / c ≦ 3.5 (14)

In the case of a depressed clad / segment core type refractive index profile, the relative refractive index difference of each glass region is defined as follows.

[0059] ^{_{Δn x = (n x 2 -n}} cld 2) / (2n x 2) x: 1,2,3,4 where, n ₁ is the refractive index of the inner core, n ₂ is the refractive index of the intermediate core , N ₃ represents the refractive index of the outer core, n ₄ represents the refractive index of the outer cladding, and n _cld represents the refractive index of the inner cladding.

[0060]

FIG. 11 is a diagram showing a cross-sectional structure and a refractive index profile of a first embodiment of the dispersion-shifted fiber according to the present invention. As shown in FIG. 11, the dispersion-shifted fiber has a depressed clad / dual-shaped core type refractive index profile 201, an inner core 111 having an outer diameter 2a of 4.2 μm, and an inner core 111 having an outer diameter 2a of 4.2 μm. An outer core 121 provided on the outer circumference and having an outer diameter 2b of 32 μm; an inner clad 211 provided on the outer circumference of the outer core 121 and having an outer diameter 2c of 63 μm;
Outer cladding 2 provided on the outer periphery of inner cladding 211
21.

The dispersion-shifted fiber 200 according to the first embodiment satisfies the above relational expressions (1) to (4) and (8) as follows.

A / b = 0.13 <0.15 0.8% <Δn ₁ = 0.98% <1.2% 0.12% <Δn ₂ = 0.20% <0.30% Δn ₃ = 0.12% Δn ₃ / Δn ₂ = 0.6 <0.95 1.2 <c / b = 1.97 <3.5 Further, at a wavelength of 1550 nm, the characteristics of the dispersion-shifted fiber of the first embodiment. As a result of the measurement, zero dispersion wavelength = 1585 nm, dispersion value at 1550 nm = −3.8 ps / nm / km, dispersion slope = 0.111 ps / nm ² / km, effective core area A _eff = 78.2 μm ² , The bending loss was 0.1 dB / turn and the transmission loss was 0.21 dB / km when the filter was bent at a cutoff wavelength of 1.59 μm and a diameter of 32 mm.

Next, FIG. 12 is a view showing a sectional structure and a refractive index profile of a second embodiment of the dispersion shifted fiber according to the present invention. As shown in FIG. 12, the second embodiment of the dispersion-shifted fiber has a depressed clad segment core type refractive index profile 310.
And an inner core 301 having an outer diameter 2a of 7.0 μm and an outer core 2
b is 13.4 μm, and an intermediate core 302
And an outer core 303 having an outer diameter 2c of 19.2 μm, an inner clad 304 having an outer diameter 2d of 38.4 μm, and an outer core 303 having an outer diameter 2c of 38.4 μm. Outer cladding 305
And

The dispersion-shifted fiber 300 of the second embodiment satisfies the above relational expressions (9) to (14) as follows.

A / c = 0.36 <0.42 b / c = 0.7> 0.60 0.5% <Δn ₁ = 0.72% <1.1% 0.2% <(Δn ₃ −Δn ₂ ) = 0.4% <0.7% Δn ₄ / Δn ₃ = 0.15 <0.95 1.2 <d / c = 2 <3.5 Furthermore, at a wavelength of 1550 nm, As a result of measuring the characteristics of the dispersion-shifted fiber of the example, zero dispersion wavelength = 1567 nm, dispersion value at 1550 nm = −1.8 ps / nm / km, dispersion slope = 0.110 ps / nm ² / km, effective core area A _eff = 81.8 μm ² , cut-off wavelength = 1.74 μm, bending loss when bent at a diameter of 32 mm = 0.1 dB / turn, and transmission loss = 0.21 dB / km.

The refractive index profile of the depressed clad / segment core type can be variously modified as shown in FIG. For example, FIG.
FIG. 10 shows a first application example of the depressed clad / segment core type refractive index profile when the refractive index of the intermediate core 302 is higher than the refractive index of the inner cladding 304 (Δn ₂ > 0). 13 (b) shows the intermediate core 3
13 shows a second application example of the depressed cladding / segment core type refractive index profile when the refractive index of No. 02 is lower than the refractive index of the inner cladding 304 (Δn ₂ <0).

[0067]

As described above in detail, according to the dispersion-shifted fiber according to the present invention, the zero-dispersion wavelength does not exist at least in the wavelength range of 1.53 to 1.56 μm, and the effective core cutoff at the wavelength of 1550 nm. Area 7
Since the thickness is set to 0 μm ² or more, the development of the nonlinear optical effect is effectively suppressed. Therefore, the development of the nonlinear optical effect can be effectively suppressed, and a dispersion-shifted fiber suitable for long-distance optical transmission can be suitably realized.

Further, according to the dispersion-shifted fiber of the present invention, since it has a depressed clad / dual-shape core type refractive index profile, the bending loss can be reduced and the above-mentioned 1550 n
Various characteristics at m can be suitably realized.

[Brief description of the drawings]

FIG. 1 shows the effective core area A _eff and the nonlinear optical constant (N ₂ /
6 is a graph showing an example of the relationship with A _eff ).

FIG. 2 is a diagram showing a cross-sectional structure and a refractive index profile (depressed clad dual shape core type) of a representative embodiment of the dispersion-shifted fiber according to the present invention.

FIG. 3 is a graph showing a relationship between a value (a / b) and an effective core area A _eff .

FIG. 4 is a graph showing a relationship between a relative refractive index difference Δn ₁ and an absolute value | D | of a dispersion value D at a wavelength of 1550 nm.

FIG. 5 is a graph showing a relationship between a relative refractive index difference Δn ₁ and a dispersion slope.

FIG. 6 is a graph showing a relationship between a relative refractive index difference Δn ₂ and a bending loss when bent at a diameter of 32 mm.

FIG. 7 is a graph showing a relationship between a relative refractive index difference Δn ₂ and a cutoff wavelength at a reference length of 2 m.

FIG. 8 is a graph showing a relationship between a value (Δn ₃ / Δn ₂ ) and transmission loss.

FIG. 9 is a graph showing a relationship between a value (c / b) and a bending loss in the case of bending at a diameter of 32 mm.

FIG. 10 is a graph showing a relationship between a value (c / b) and a cutoff wavelength at a reference length of 2 m.

FIG. 11 is a diagram showing a cross-sectional structure and a refractive index profile (depressed clad dual shape core type) of the first embodiment of the dispersion shifted fiber according to the present invention.

FIG. 12 is a diagram showing a cross-sectional structure and a refractive index profile (depressed clad segment core type) of a second embodiment of the dispersion-shifted fiber according to the present invention.

FIG. 13 is a diagram showing a modification of the refractive index profile in the dispersion-shifted fiber of the second embodiment shown in FIG.

[Explanation of symbols]

110, 111, 301 ... inner core, 302 ... intermediate core, 120, 121, 303 ... outer core, 210, 21
1, 304 ... inner cladding, 220, 221, 305 ...
Outer cladding.

Claims (9)

[Claims] 1. It has a zero-dispersion wavelength out of a wavelength band of 1.53 to 1.56 μm and a wavelength of 1550 nm.
As the various characteristics in the above, a dispersion having an absolute value of 1.0 to 4.5 ps / nm / km, a dispersion slope having an absolute value of 0.13 ps / nm ² / km or less, an effective core area of 70 μm ² or more, A dispersion-shifted fiber having a transmission loss of 0.25 dB / km or less with respect to light in a 1.55 μm wavelength band. 2. The dispersion-shifted fiber according to claim 1, wherein a bending loss when bent at a diameter of 32 mm is 0.5 dB / turn or less. 3. An absolute value of the dispersion slope is 0.09.
^2. The dispersion-shifted fiber according to claim 1, wherein the dispersion-shifted fiber is at least ps / nm ² / km. 4. An inner core having a predetermined refractive index and an outer diameter of 2a, provided on an outer periphery of the inner core, having a lower refractive index than the inner core and having an outer diameter of 2b. An outer clad provided on the outer periphery of the outer core and having a lower refractive index than the outer core; and an outer clad provided on the outer periphery of the inner clad and having a higher refractive index than the inner clad. The dispersion-shifted fiber according to claim 1, further comprising: 5. A relative refractive index difference of the inner core with respect to the inner cladding is Δn ₁ , a relative refractive index difference of the outer core with respect to the inner cladding is Δn ₂ , and a relative refractive index difference of the outer cladding with respect to the inner cladding is Δn ₁ . When Δn ₃ , the following relationship is satisfied: a / b ≦ 0.15 0.8% ≦ Δn ₁ ≦ 1.2% 0.12% ≦ Δn ₂ ≦ 0.30% Δn ₃ / Δn ₂ ≦ 0.95 5. The dispersion-shifted fiber according to claim 4, wherein: 6. The dispersion-shifted fiber according to claim 5, wherein when the outer diameter of the inner cladding is 2c, a relationship of 1.2 ≦ c / b ≦ 3.5 is satisfied. 7. An inner core having a predetermined refractive index and an outer diameter of 2a, provided on the outer periphery of the inner core, having a lower refractive index than the inner core and having an outer diameter of 2b. An outer core provided on the outer periphery of the intermediate core, having a higher refractive index than the intermediate core, and having an outer diameter of 2b; and an outer core provided on the outer periphery of the outer core, 2. The dispersion-shifted fiber according to claim 1, comprising: an inner cladding having a lower refractive index than the outer cladding; and an outer cladding provided around the inner cladding and having a higher refractive index than the inner cladding. 8. A relative refractive index difference of the inner core with respect to the inner cladding is Δn ₁ , a relative refractive index difference of the intermediate core with respect to the inner cladding is Δn ₂ , and a relative refractive index difference of the outer core with respect to the inner cladding is Δn ₁ . Δn ₃ , when the relative refractive index difference between the outer cladding and the inner cladding is Δn ₄ , a / c ≦ 0.42 b / c ≧ 0.60 0.5% ≦ Δn ₁ ≦ 1.1% 8. The dispersion-shifted fiber according to claim 7, wherein a relationship of 2% ≦ Δn ₃ −Δn ₂ ≦ 0.7% Δn ₄ / Δn ₃ ≦ 0.95 is satisfied. 9. The dispersion-shifted fiber according to claim 8, wherein when the outer diameter of the inner cladding is 2d, a relationship of 1.2 ≦ d / c ≦ 3.5 is satisfied.