JPH11218632A - Dispersion shift fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a dispersion shift fiber(DSF) which is suitable for light- wavelength division multiplex transmission. SOLUTION: A refractive index distribution and a core diameter ratio are found so as to obtain an optimum mode filed diameter and an optimum dispersion gradient. In concrete, nonlinearlity and a low dispersion gradient are both obtained by setting the specific refractive index difference (Δ1 ) to the refractive index of pure silica of a center core 1 to +0.7 to +0.85%, a refractive index distribution parameter (α constant) to 1.2 to 1.8, the specific refractive index difference of a 1st side core 2 to -0.05 to +0.1%, the specific refractive index difference (Δ3 ) to +0.3 to +0.6%, the radius (a1 /2) of the 1st side core 2 to 2 to 4.5 μm, and the radius (a2 /2) of the 1st side core 2 to 7 to 12.5 μm and the radius (a3 /2) of the 2nd core 3 to 7-12.5 μm. Judging from that the nonlinearlity can be obtained, waveform distortion of signal light can be controlled for fast, large-capacity transmission and judging from the small dispersion gradient, a wide wavelength range can be used for the signal light.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dispersion-shifted fiber for transmitting an optical signal in a 1.55 .mu.m band.

[0002]

2. Description of the Related Art With the progress of high-speed and large-capacity optical transmission, wavelength division multiplexing (hereinafter referred to as WDM) transmission technology has received attention as a favorite technology. However, as a cost, non-linear phenomena in optical fibers have arisen as a new problem. For research on eliminating this nonlinear phenomenon,
Reported in OFC '97 TuN1b. The content of the technology is to reduce the amount of germanium contained in the center core and to increase the mode field diameter (MFD) of the dispersion-shifted fiber (DSF). A _eff ).

A signal distortion (hereinafter referred to as _φNL ) due to a nonlinear phenomenon of an optical fiber is generally expressed by the following equation.

## EQU1 ## Since φ _NL = (2π × n ₂ × L _eff × P) / (λ × A _eff ), the larger A _eff , the smaller φ _NL . Here, n ₂ is an index proportional to the amount of germanium contained in the center core, L _eff is the effective length of the optical fiber, and P is the input light power to the optical fiber. A _eff is

## EQU2 ## Since it can be expressed by A _eff = k × (MFD) ² , if the MFD is large, φ _NL becomes small, so that it can be understood that low nonlinearity can be achieved very efficiently (where k is a constant Is). Therefore, DSF
The expansion of the MFD in Japan is one of the most required characteristics at present.

[0004]

However, there is a problem that the expansion of the MFD is accompanied by an increase in bending loss and dispersion gradient. The increase in bending loss causes serious problems such as an increase in loss when used as an actual cable. Also,
Increasing the dispersion gradient causes an increase in dispersion difference for each wavelength when performing WDM optical transmission.

[0005]

In order to solve the above-mentioned problems, the present invention provides a refractive index distribution and a core diameter of a DSF (hereinafter, referred to as a profile together with the refractive index distribution and the core diameter). ) By optimization, the wavelength 1.55
It is an object of the present invention to provide a DSF in which a dispersion value of a DSF in a μm band is set to a small dispersion and a low nonlinearity is obtained, and further, a bending loss and a dispersion gradient do not increase.

[0006] The refractive index distribution of the DSF is defined by a relative refractive index difference (hereinafter, referred to as Δ) with respect to the refractive index of pure silica (hereinafter, referred to as silica level). The relative refractive index differences between the cores are denoted as Δ ₁ , Δ ₂ , and Δ _{3 in} the order of the center core, the first side core, and the second side core.

[0007] A first solution in the present invention is a center core, a first side core having a lower refractive index than the center core outside the center core, and a first side core having a higher refractive index than the first side core outside the first side core. In a dispersion-shifted fiber having a two-side core and a cladding having a lower refractive index than the second side core outside the second side core, a relative refractive index difference of each core with respect to pure silica is a relative refractive index difference of the center core ( Δ ₁₎ is + 0.7 + 0.
85%, the relative refractive index difference (Δ ₂ ) of the first side core is -0.0
5 to + 0.1%, relative refractive index difference of the second side core (Δ ₃ )
There is a + 0.3 + 0.6%, the refractive index distribution parameters of said center core (alpha constant) is 1.2 to 1.8, wherein the center core of the radius (a _1/2) is 2 to 4.5 [mu] m, the radius of the first side core (a _2/2) is 5 10 .mu.m, the second side core radius (a _3/2) is 7 to 12.5 is [mu] m, the dispersion value of the optical fiber, -3 to + 3 ps at 1.55μm band / nm / km (except near the dispersion value of zero), and the effective core area (A _eff )
80 μm ² or more and the absolute value of the dispersion gradient is 0.12 ps / nm ² / km
It is characterized by the following.

[0008] A second solution is a center core, and
A first lower refractive index than the center core outside the center core;
A side core, and the first support outside the first side core.
A second side core having a higher refractive index than the second core;
A refractive index lower than the second side core outside the side core.
In a dispersion-shifted fiber having
The relative refractive index difference of each core with respect to pure silica is
The relative refractive index difference of the core (Δ ₁) Is +0.7 to + 0.85%, the first
The relative refractive index difference of the side core (Δ_Two) Is -0.05 ~ + 0.1%, before
The relative refractive index difference of the second side core (Δ_Three) Is +0.3 to + 0.6%
And a refractive index distribution parameter (α constant) of the center core.
Number) is 1.2 to 1.8, and the radius of the center core (a₁/
2) is 2 to 4.5 μm, the radius of the first side core (a_Two/
2) is 5 to 10 μm, the radius of the second side core (a_Three/ 2)
Is 7 to 12.5 μm, and the dispersion value of the optical fiber is
-3 to +3 ps / nm / km in the 1.55 μm band (except near the dispersion value of zero)
And the effective core area (A_eff) Is 50 μm^TwoAbove
Yes, the absolute value of the dispersion gradient is 0.065 ps / nm^Two/ km or less
It is characterized by the following.

[0009]

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing a refractive index distribution of the DSF of the present invention. The DSF includes a center core 1, a first side core 2 having a lower refractive index than the center core 1, and a first side core 2 outside the center core 1. Outside of the first
A second side core 3 having a higher refractive index than the side core 2 and a cladding 4 having a lower refractive index than the second side core 3 are provided outside the second side core 3.

In the present invention, in FIG. 1, a profile was searched by changing all parameters, and an optimum profile was obtained from a relationship between a dispersion gradient and a bending loss in the range. Specific examples are shown below.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As an embodiment, a description will first be given of searching for a range of a refractive index distribution. In the present invention, regardless of the upper limit of the MFD, the purpose is to make the refractive index distribution as large as possible, so that the refractive index distribution is clearly defined. The value of Δ ₁ is
When it is not within the range of 0.85% or less, the dispersion gradient increases when the MFD is expanded, and when it is not within the range of 0.7% or more,
This range was selected because bending loss increases and becomes weaker. Within this range, the refractive index distribution parameter (hereinafter referred to as α constant) where the dispersion gradient does not increase when the MFD is enlarged
Was found to be in the range of 1.2 to 1.8.

[0012] delta ₂ values were substantially silica level in the range of manufacturing variations, not specifically consider the adjustment of the refractive index.
If necessary, the refractive index may be adjusted to be at the silica level.

[0013] Then, as a result of examining the optimal value of Δ _3, 0.3%
In the following cases, when the variance of the DSF becomes zero dispersion, the expansion of the MFD at a small core diameter (hereinafter referred to as the small diameter domain) is insufficient, and when the dispersion becomes zero dispersion. The dispersion gradient at a large core diameter value (hereinafter referred to as a large diameter solution zone) is 0.
It does not decrease to 05ps / nm ² / km. In addition, when it is 0.6% or more, the single mode condition is not completely satisfied in the large diameter solution zone, so this range was selected.

Next, searching for the range of the core diameter ratio will be described. Center core radius, first side core radius,
Considering the relationship between the MFD and the dispersion gradient, it was found that the ratio of the second side core radius is preferably about 1: (2.1 to 2.3) :( 2.6 to 2.8), so the core diameter should be changed within that range. A search was made for a condition under which the propagation condition was good.
In other ranges, the characteristics (MFD or dispersion gradient) deteriorate, or the propagation conditions deteriorate (bending loss increases).
I understand.

In the range of the profile obtained as described above, the dispersion at 1.55 μm is in the range of −3 to 3 ps / nm / km, and in the small diameter region, the MFD is 10 μm or more and the absolute value of the dispersion gradient is A DSF with good characteristics with a value of 0.12 ps / nm ² / km or less was obtained. FIG. 2 shows an example of the relationship between the dispersion and the core diameter of the DSF created here, and FIG. 3 shows an example of the relationship between A _eff and the dispersion gradient with respect to the core diameter.

However, if the dispersion at the used wavelength is zero, there is a disadvantage that a nonlinear phenomenon called four-wave mixing is likely to occur. Therefore, it is preferable that the dispersion value has a small value of 1.55 μm. As a result of the experiment, it was found that the minute value should be 0.5 ps / nm / km or more.

Further, the value of the core diameter was further increased, and the large diameter solution zone was examined. When the core diameter is increased to have a small dispersion within the effective core area of 60 μm ² or more, the dispersion gradient decreases, and the absolute value of the dispersion gradient becomes less than 0.05 ps / nm ² / km. A solution could be found in the low dispersion gradient region.

FIG. 4 shows an example of the evaluation results of the wavelength characteristics of the transmission loss and the chromatic dispersion of a DSF designed so that the core diameter has a small dispersion in the 1.55 μm band. The value of △ ₁ at this time is
The value of 0.78%, α was 1.5, the value of △ ₂ was 0% (silica level), the value of △ ₃ was 0.41%, and the core diameter ratio was 1: 2.2: 2.8.

Further, more trial productions were conducted within the above-mentioned range of the refractive index distribution, and it was confirmed that the values of the bending loss and the dispersion gradient did not become too large even in the range where the effective core area was 90 μm ² or more. . Table 1 shows an example of the characteristics of the prototype DSF. In Table 1, seg is a segment (profile form). Specifically, in the large-diameter solution zone, an MFD of about 9.0 μm (effective core area of 60 μm ² or more), a low bending loss, and an ultra-low dispersion gradient of 0.04 ps / nm ² / km or less were obtained. .

[0020]

[Table 1]

[0021]

As described above, according to the present invention, the dispersion in the 1.55 μm wavelength band of the DSF is set to a minute dispersion, and the profile is set to the optimum range, so that the low nonlinearity and the low dispersion gradient can be simultaneously achieved. DSF can be obtained. Obtaining low non-linearity means that waveform distortion of signal light can be controlled even in high-speed and large-capacity transmission, and small dispersion gradient means that a wide range of wavelength ranges can be used as signal light. Means

That is, by designing with a profile as in the present invention, a low-nonlinear DS suitable for wavelength division multiplexing transmission is provided.
F can be made.

[Detailed description of drawings]

FIG. 1 is a diagram showing an example of a refractive index distribution of a dispersion-shifted fiber used in the present invention.

FIG. 2 is a diagram illustrating an example of a relationship between a dispersion value and an outer diameter value of an outermost core of a dispersion-shifted fiber used in the present invention.

FIG. 3 is a diagram showing an example of the relationship between A _eff and the dispersion gradient with respect to the outer diameter value of the outermost core of the dispersion-shifted fiber used in the present invention.

FIG. 4 is a diagram showing an example of transmission loss and wavelength characteristics of chromatic dispersion of the dispersion-shifted fiber used in the present invention.

[Explanation of symbols]

 Reference Signs List 1 center core 2 first side core 3 second side core 4 clad

Claims (2)

[Claims] 1. A center core, a first side core having a lower refractive index than the center core outside the center core, a second side core having a higher refractive index than the first side core outside the first side core, and the second side core. In the dispersion-shifted fiber having a cladding having a lower refractive index than the second side core, a relative refractive index difference of each core with respect to pure silica is such that a relative refractive index difference (Δ ₁ ) of the center core is +0.7 to + 0.85%, the relative refractive index difference (Δ ₂ ) of the first side core is −0.05 to + 0.1%,
The relative refractive index difference (Δ ₃ ) of the second side core is +0.3 to +0.6
%, And the refractive index distribution parameter (α constant) of the center core is 1.
2 is 1.8, the center core of the radius (a _1/2) is 2 to 4.5 [mu] m, the first side core radius (a _2/2) is 5 10 .mu.m, the second side core radius (a _3/2 ) Is 7 to 12.5 μm, and the dispersion value of the optical fiber is −3 to +3 in the 1.55 μm band.
ps / nm / km (except near zero dispersion value), effective core area (A _eff ) is 80 μm ² or more, and absolute value of dispersion gradient is 0.12 ps / nm ² / km or less And dispersion shifted fiber. 2. A center core, a first side core having a lower refractive index than the center core outside the center core, a second side core having a higher refractive index than the first side core outside the first side core, and the second side core. In the dispersion-shifted fiber having a cladding having a lower refractive index than the second side core, a relative refractive index difference of each core with respect to pure silica is such that a relative refractive index difference (Δ ₁ ) of the center core is +0.7 to + 0.85%, the relative refractive index difference (Δ ₂ ) of the first side core is −0.05 to + 0.1%,
The relative refractive index difference (Δ ₃ ) of the second side core is +0.3 to +0.6
%, And the refractive index distribution parameter (α constant) of the center core is 1.
2 is 1.8, the center core of the radius (a _1/2) is 2 to 4.5 [mu] m, the first side core radius (a _2/2) is 5 10 .mu.m, the second side core radius (a _3/2 ) Is 7 to 12.5 μm, and the dispersion value of the optical fiber is −3 to +3 in the 1.55 μm band.
ps / nm / km (except near zero dispersion value), effective core area (A _eff ) is 50 μm ² or more, and absolute value of dispersion gradient is 0.065 ps / nm ² / km or less And dispersion shifted fiber.