JPH1184159A - Dispersion flat fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber capable of suppressing the signal light distortion occurring in a wavelength dispersion characteristic and the signal light distortion occurring in a nonlinear phenomenon. SOLUTION: The refractive index distribution structure in which the relation among the refractive index n1 of a center core 3, the refractive index n2 of a side core 4 around the same, the refractive index n3 of a segment core 6 around the same and the refractive index n0 of the clad layer 5 around the same is set at n1>n3>n0>n2 is adopted. The difference (Δ1-Δ2) between the specific refractive index difference Δ1 of the center core 3 from the clad layer 5 and the specific refractive index difference Δ2 of the side core 4 from the clad layer 5 is confined to <=0.7%, Δto <=0.6% and specific refractive index difference Δ3 with the clad layer 5 of the segment core 6 is <=0.3% and the mode field diameter at a wavelength 1550 nm is specified to about >=8.6 μm, by which low linearity is attained. In addition, the absolute value of a weight average dispersion slope from a wavelength 1530 nm to 1570 nm is confined to about <=0.04 ps/nm<2> /km, by which the dispersion slope is flattened.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention is particularly applicable to a wavelength of 1.55 .mu.m.
m-band wavelength multiplexing (WDM) transmission and time multiplexing (TDM)
The present invention relates to a dispersion flat fiber suitable for high-speed large-capacity optical transmission such as transmission.

[0002]

2. Description of the Related Art With the development of the information society, the amount of communication information tends to increase dramatically, and high-speed and large-capacity optical fiber communication has become a necessary and indispensable subject. In recent years, approaches to this high-speed and large-capacity increase include, for example, an optical amplifier (EDFA) using an erbium-doped optical fiber.
Amplification of optical signals in the 1550-nm wavelength band has been used to increase the capacity of the signal light itself, and transmission methods have been studied.
Optical transmission methods such as transmission are being studied. Wavelength multiplex transmission is a method of transmitting optical signals having different wavelengths through a single optical fiber, and time multiplex transmission is a method of transmitting a plurality of digital signals by shifting the time position of each signal little by little and superimposing them. Both are optical transmission systems suitable for high-speed, large-capacity communication.

However, in the case of wavelength division multiplexing transmission, since a plurality of wavelengths are used as signal light wavelengths as described above, the wavelength dependency of the transmission optical fiber is a factor that hinders high speed optical transmission. The optical fiber is, for example, a positive dispersion slope whose dispersion value increases as the signal light wavelength goes to the longer wavelength side, and conversely, a negative dispersion slope whose dispersion value decreases as the signal light wavelength goes to the longer wavelength side The greater the absolute value of these dispersion slopes, the greater the dispersion difference between the long wavelength side and the short wavelength side of the signal light wavelength, thereby causing distortion of the signal light waveform and the wavelength. This is a factor that hinders high-speed large-capacity optical transmission by multiplex transmission. The presence of the chromatic dispersion slope of the optical fiber also causes the same signal light distortion to time multiplex transmission, which is a factor that hinders high-speed and large-capacity optical transmission by time multiplex transmission.

[0004]

Therefore, if the optical fiber whose dispersion characteristic does not depend on the wavelength, that is, the optical fiber having a small dispersion slope is applied to a transmission line such as a wavelength division multiplexing transmission or a time division multiplexing transmission, the above-mentioned wavelength can be reduced. Although it is thought that the distortion of the signal caused by dispersion can be reduced, a dispersion flat fiber having a small dispersion slope,
Conventionally, there is generally only a mode field diameter (MDF) as small as 7 μm or less. In high-speed transmission such as wavelength multiplexing transmission and time multiplexing transmission, the intensity of signal light incident on an optical fiber is strong. When an optical fiber having a small value is used for high-speed optical transmission such as wavelength-division multiplexing transmission or time-division multiplexing, nonlinear interaction increases, and a new problem arises in that the nonlinear interaction causes signal light distortion.

This distortion is called self-phase modulation (SPM) or cross-phase modulation (XPM). In general, distortion φ _nL due to a nonlinear phenomenon (nonlinear interaction) of an optical signal is expressed by the following equation (1). Is done.

Φ _nL = 2π × L _eff × P × (n ₂ / A _eff ) ÷ λ (1)

In equation (1), L _eff is the effective fiber length, P is the input light intensity, λ is the wavelength of the input light, n ₂ is the nonlinear refractive index, A _eff is the effective core area, and n ₂ /
A _eff is called a non-linear constant. The unit of this nonlinear constant is 1 / W. The effective core area A _eff can be expressed as an equation of A _eff = K × (MFD) ² where K is a coefficient (MFD: mode field diameter).

Therefore, the larger the nonlinear refractive index, the longer the effective fiber length, the stronger the input light intensity, and the smaller the effective core area, the greater the distortion due to the nonlinear phenomenon of the signal light. That is, the smaller the mode field diameter, the greater the distortion due to the nonlinear phenomenon.

Therefore, a dispersion-shifted optical fiber formed by enlarging the mode field diameter in order to reduce the distortion due to the non-linear phenomenon has been proposed. Since the dispersion slope becomes large, signal distortion due to dispersion occurs as described above. From the above, conventionally, an optical fiber whose chromatic dispersion characteristics do not depend on wavelength and whose mode field diameter is large has not been developed. Therefore, signal light distortion due to nonlinear phenomena and signal light distortion due to chromatic dispersion characteristics have been developed. An optical fiber in which both were small could not be realized.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to reduce both signal light distortion caused by nonlinear phenomena occurring in an optical fiber and signal light distortion caused by dispersion characteristics. It is an object of the present invention to provide a dispersion flat fiber suitable for high-speed and large-capacity optical transmission such as transmission and time multiplex transmission.

[0011]

In order to achieve the above-mentioned object, the present invention has the following structure to solve the problem. That is, the first invention has a wavelength of 15
This is a means for solving the problem with a configuration in which the mode field diameter at 50 nm is about 8.6 μm or more, and the absolute value of the average dispersion slope from wavelength 1530 nm to 1570 nm is about 0.04 ps / nm ² / km or less.

In the second invention, the nonlinear constant at a wavelength of 1550 nm is set to about 4.5 × 10 ⁻¹⁰ or less, and
Absolute value of average dispersion slope up to 1570nm is about 0.04ps / nm
It is a means to solve the problem with a configuration of ² / km or less.

Further, according to a third aspect of the present invention, in addition to the configuration of the first or second aspect, the center core is covered by a side core concentrically with the center core, and the periphery of the side core is concentric with the side core. And a cladding layer around the segment core, and a refractive index n1 of the center core and a refractive index n2 of the side core.
And a configuration in which the relationship between the refractive index n3 of the segment core and the refractive index n0 of the cladding layer is set as n1>n3>n0> n2 as means for solving the problem.

According to a fourth aspect of the invention, in addition to the configuration of the first aspect, the second aspect, or the third aspect, the center core is covered with a side core concentrically with the center core.
The periphery of the side core is covered with a segment core concentrically with the side core, the periphery of the segment core is covered with a cladding layer, and the relative refractive index between the side core and the cladding layer is determined from the relative refractive index difference between the center core and the cladding layer. The object is solved by a configuration in which the value obtained by subtracting the difference is set to 0.7% or less, the relative refractive index of the center core to the cladding layer is set to 0.6% or less, and the relative refractive index difference of the segment core to the cladding layer is set to 0.3% or less. Means.

As described above, the distortion due to the non-linear phenomenon of the optical signal is expressed by the above equation (1). The smaller the non-linear constant of the optical fiber is, the smaller the mode field diameter is. The dispersion flat fiber of the present invention having the above configuration has a mode field diameter at a wavelength of 1550 nm as large as about 8.6 μm or more,
Since the nonlinear constant at the same wavelength is a small value of about 4.5 × 10 ⁻¹⁰ or less, it is possible to suppress the distortion due to the nonlinear phenomenon of the optical signal, and the low nonlinearity of the optical transmission is ensured.

The distortion due to the dispersion characteristics of the optical signal decreases as the dispersion slope of the optical fiber decreases. Conventionally used wavelength 1.30
An optical fiber called a μm-band zero-dispersion single-mode optical fiber, a dispersion-shifted optical fiber, or a non-zero dispersion-shifted optical fiber whose zero-dispersion wavelength is out of the usable wavelength band has a dispersion slope of at least 0.05 ps / nm ² / km In contrast, the dispersion flat fiber of the present invention has a dispersion slope of 0.04 ps / nm ² / km or less, which is a small value of about half or less of a general dispersion shift optical fiber. It is possible to suppress distortion due to the wavelength dispersion characteristics of the fiber.

As described above, in the present invention, 1550 nm
By increasing the mode field diameter or reducing the nonlinear constant in the above, signal light distortion due to nonlinear phenomena is suppressed, and signal light distortion due to chromatic dispersion characteristics is suppressed by reducing the dispersion slope. Therefore, it is possible to provide an optical fiber suitable for high-speed and large-capacity optical transmission such as wavelength multiplex transmission or time multiplex transmission, and the above-mentioned problem is solved.

[0018]

Embodiments of the present invention will be described below with reference to the drawings. The dispersion flat fiber of this embodiment has a mode field diameter of about 8.6 μm or more at a wavelength of 1550 nm (1.55 μm), a non-linear constant of about 4.5 × 10 ⁻¹⁰ or less at the same wavelength, and a wavelength of 1530 nm.
The absolute value of the average dispersion slope from nm to 1570 nm (wavelength 1550 nm band) is about 0.04 ps / nm ² / km, and has the refractive index distribution and the cross-sectional structure shown in FIG.

In FIG. 1, the periphery of the center core 3 is covered by a side core 4 concentrically with the center core 3, and the periphery of the side core 4 is concentric with the side core 4 by a segment core 6.
, And a dispersion flat fiber is formed by covering the periphery of the segment core 6 with the cladding layer 5, and the refractive index n 1 of the center core 3, the refractive index n 2 of the side core 4, the refractive index n 3 of the segment core 6, and the refractive index of the cladding layer 5. The relationship between the refractive indices n0 is such that n1>n3>n0> n2.

As shown in FIG. 1A, a duplicated layer formed by a side core 4 is provided around a center core 3 and a segment core 6 is provided around a duprested layer. The refractive index distribution structure as shown in FIG. As described above, in the present embodiment, the dispersion characteristic of the optical fiber is changed with respect to the wavelength by forming the refractive index distribution structure of the dispersion flat fiber into the segment structure with the duprested and forming the duprested layer around the center core 3. To reduce the chromatic dispersion slope, thereby making the chromatic dispersion characteristics independent of wavelength, and to reduce the bending loss caused by the reduction of the chromatic dispersion slope, the area around the dumpled layer is reduced. The segment core 6 is provided to suppress an increase in bending loss.

In addition, in order to study conditions for obtaining a structure capable of increasing the mode field diameter in such an optical fiber having a segmented structure with a dupressed structure, the applicant of the present application has examined the ratio of the center core 3 to the cladding layer 5. The relationship between the difference (Δ1−Δ2) between the refractive index difference Δ1 and the relative refractive index difference Δ2 between the cladding layer 5 of the side core 4 and the mode field diameter of the dispersion flat fiber was examined. The result of the study is shown in FIG.

In this specification, the relative refractive index difference between the center core 3 and the cladding layer 5 is defined as Δ1, and is defined by the following equation (2).

Δ1 = {(n1 ² −n0 ² ) / 2n0 ² } × 100 (2)

The relative refractive index difference between the side core 4 and the cladding layer 5 is defined as Δ2, and Δ2 is defined by the following equation (3).

Δ2 = {(n2 ² −n0 ² ) / 2n0 ² } × 100 (3)

Further, the cladding layer 5 of the segment core 6
Is defined as Δ3, and Δ3 is defined by the following equation (4).

Δ 3 = {(n 3 ² −n 0 ² ) / 2n 0 ² } × 100 (4)

As apparent from FIG. 2, the difference (Δ1-Δ2) between the relative refractive index difference Δ1 between the center core 3 and the cladding layer 5 and the relative refractive index difference Δ2 between the side core 4 and the cladding layer 5 is shown in FIG. In the range shown (about 0.5 to about 0.11%), (Δ1
The mode field diameter tends to increase as the value of -Δ2) decreases.
If it is less than 7%, the mode field diameter is about 8.6 μm
It turns out that it becomes above.

However, when the absolute value of the relative refractive index difference Δ2 between the side core 4 and the cladding layer 5 is not more than 0.1, that is, the relative refractive index difference Δ1 between the center core 3 and the cladding layer 5 is smaller than 0.1.
If it is not less than 0.6%, it is confirmed that the effect of flattening the dispersion slope by the dupressed layer of the side core 3 becomes small, and the dispersion slope of the optical fiber exceeds 0.04 ps / nm ² / km.

Further, the present applicant has examined the relationship between the relative refractive index difference Δ3 between the segment core 6 and the cladding layer 5 and the cutoff wavelength of the optical fiber, and the results shown in FIG. 3 are obtained. If it exceeds 0.3%, the cutoff wavelength becomes 1550
It turned out to exceed nm. That is, Δ3 is 0.
Exceeding 3% indicates that single mode transmission cannot be maintained at a wavelength of 1550 nm, and therefore,
In order to perform high-speed and large-capacity optical transmission such as wavelength multiplex transmission or time multiplex transmission at a wavelength of 1550 nm, the relative refractive index difference Δ3 between the segment core 6 and the cladding layer 5 needs to be 0.3% or less.

The present embodiment has a refractive index distribution structure of a segment structure with a duprested structure shown in FIG. 1 based on the above-described examination results, and a relative refractive index difference Δ1 of the center core 3 with respect to the cladding layer 5 and the side core 4 The difference (Δ1−Δ2) between the relative refractive index difference Δ2 and the cladding layer 5 is set to 0.7% or less, the relative refractive index difference Δ1 of the center core 3 to the cladding layer 5 is set to 0.6% or less. By giving the optimum profile condition of the refractive index distribution with the relative refractive index difference Δ3 from the cladding layer 5 being 0.3% or less, the mode field diameter at a wavelength of 1550 nm is about 8.6 μm or more, and the nonlinear constant at the wavelength is about 4.5 ×. The value was 10 ⁻¹⁰ or less, and the absolute value of the average dispersion slope from a wavelength of 1530 nm to 1570 nm was able to be about 0.04 ps / nm ² / km or less.

According to this embodiment, the optimum profile condition of the refractive index distribution is given as described above, and the wavelength of 1550 nm
The mode field diameter at is set to a large value of about 8.6 μm or more, and the nonlinear constant at the same wavelength is set to about 4.5 × 10 ^-10
By setting the following small values, it is possible to suppress distortion caused by the nonlinear phenomenon of the optical fiber, and it is possible to ensure low nonlinearity of optical transmission. Also, the wavelength
Approximate the absolute value of the average dispersion slope from 1530 nm to 1570 nm.
By setting the value to a small value of 0.04 ps / nm ² / km or less, it is possible to suppress the distortion caused by the chromatic dispersion characteristics of the optical fiber. An excellent dispersion flat fiber that can suppress both distortions can be obtained.

Further, according to the present embodiment, as described above, the relative refractive index difference Δ
By setting 3 to 0.3% or less, the cutoff wavelength can be reduced to 15%.
Since it can be 50 nm or less, single mode transmission at a wavelength of 1550 nm can be reliably maintained.

Therefore, by applying the dispersion flat fiber of this embodiment to high-speed and large-capacity optical transmission such as wavelength division multiplexing transmission and time division multiplexing transmission using a wavelength band of 1550 nm, signal distortion can be suppressed. High-quality, high-speed, large-capacity optical transmission can be performed.

(Embodiment) Next, a specific embodiment of the dispersion-shifted optical fiber of the present invention will be described. The present inventor has a refractive index distribution of the segment structure with depressed based on the above-mentioned examination result, and Δ1−Δ2 ≦ 0.7%, and sets the value of Δ1 to 0.45% and the value of Δ2 to −0.15%. Dispersion flat fibers # 1 and # 2 were experimentally manufactured. Note that these # 1 and # 2
The prototype optical fiber is a clad layer 5 of a segment core 6.
Is less than 0.3%, that is, # 1
0.13% of the optical fiber of # 2, Δ3 of the optical fiber of # 2
Is set to 0.25%.

Table 1 shows the conditions of the refractive index distribution structure and the fiber characteristics of the prototype dispersion-shifted optical fibers # 1 and # 2, and a comparison between a normal dispersion-shifted optical fiber (normal DSF) and a comparative example 2. This is shown in comparison with an optical fiber (dispersion flat fiber) having a W-type refractive index distribution structure that does not have the segment core 6.

[0037]

[Table 1]

In Table 1, the dispersion value, the dispersion slope, the mode field diameter (MFD), and the transmission loss are as follows.
It is a measurement result when an optical signal of 55 μm is propagated.

As is apparent from Table 1, the dispersion flat fiber of this embodiment is a conventional dispersion-shifted optical fiber or a dispersion-type fiber having a W-type refractive index distribution structure without the segment core 6. Compared to a flat fiber, the mode field diameter can be greatly increased, the nonlinear constant can be significantly reduced, and the absolute value of the dispersion slope can be extremely small, 0.005 or 0.025 ps / nm ² / km. did it. Further, the transmission loss of the light could be made as small as the dispersion-shifted optical fiber and the dispersion flat fiber of the comparative example.

Further, although not shown in Table 1, as shown in Comparative Example 2, in a dispersion flat fiber having a W-type refractive index distribution structure having no segment core 6, for example, the mode field diameter is enlarged. In this case, bending loss will be caused accordingly, but in the dispersion flat fiber of the present embodiment, by providing the segment core 6 on the outer peripheral side of the side core 4, the mode field diameter can be increased as described above. Nevertheless, the bending loss can be made comparable to, for example, a conventional dispersion-shifted optical fiber.

[0041]

According to the present invention, the absolute value of the average dispersion slope in the wavelength band of 1550 nm (from 1530 nm to 1570 nm), which is the gain band of an optical amplifier (EDFA) using an erbium-doped optical fiber, is about 0.04 ps /. It is a small value of nm ² / km or less, and the mode field diameter at a wavelength of 1550 nm is a large value of about 8.6 μm or more, and the nonlinear constant at a wavelength of 1550 nm is a small value of about 4.5 × 10 ^-10 or less. Therefore, it is possible to obtain an excellent dispersion flat fiber that can suppress both the signal light distortion due to the wavelength dispersion characteristics of the optical fiber and the signal light distortion due to the non-linear phenomenon. Therefore, by applying the dispersion flat fiber of the present invention as a transmission path for high-speed and large-capacity optical transmission of wavelength multiplexing transmission or time-division multiplexing transmission provided with, for example, an EDFA, high-quality high-speed and large-capacity optical transmission can be performed. An optical transmission system can be constructed.

In particular, the periphery of the center core is covered by a side core concentrically with the center core, the periphery of the side core is covered by a segment core concentrically with the side core, the periphery of the segment core is covered by a cladding layer, and the center core is bent. The refractive index n1, the refractive index n2 of the side core, the refractive index n3 of the segment core, and the refractive index n of the cladding layer
By setting the relationship of 0 to n1>n3>n0> n2, it is possible to easily configure a dispersion flat fiber that can suppress signal light distortion caused by the nonlinear phenomenon and chromatic dispersion characteristics.

The value obtained by subtracting the relative refractive index difference between the side core and the clad layer from the relative refractive index difference between the center core and the clad layer is set to 0.7% or less, thereby obtaining a wavelength of 15% or less.
The mode field diameter at 50 nm can be set to a large value of about 8.6 μm or more. In addition, by setting the relative refractive index difference between the center core and the cladding layer to 0.6% or less, the average dispersion slope from the wavelength of 1530 nm to 1570 nm can be flattened as described above, and the difference between the center core and the cladding layer of the segment core can be reduced. By setting the relative refractive index difference to 0.3% or less, the cutoff wavelength of the optical fiber can be 1550 nm or less, and single mode transmission in the 1550 nm band can be reliably maintained.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a refractive index distribution structure (a) and a cross-sectional structure (b) of a dispersion flat fiber according to the present invention.

FIG. 2 shows a relationship between a value (Δ1−Δ2) obtained by subtracting a relative refractive index difference between a side core and a cladding layer from a relative refractive index difference between the center core and a cladding layer in the embodiment, and a mode field diameter of an optical fiber. FIG.

FIG. 3 is a graph showing a relationship between a relative refractive index difference between a segment core and a cladding layer and a cutoff wavelength of an optical fiber in the embodiment.

FIG. 4 is an explanatory view showing a step-type refractive index distribution structure.

FIG. 5 is an explanatory diagram showing a W-type refractive index distribution structure.

[Explanation of symbols]

 3 Center core 4 Side core 5 Clad layer 6 Segment core

Claims (4)

[Claims] 1. A dispersion flat fiber, wherein a mode field diameter at a wavelength of 1550 nm is about 8.6 μm or more, and an absolute value of an average dispersion slope from a wavelength of 1530 nm to 1570 nm is about 0.04 ps / nm ² / km or less. . 2. The non-linear constant at a wavelength of 1550 nm is about 4.5.
Dispersion flat fiber, characterized in that it is less than × 10 ^-10 and the absolute value of the average dispersion slope from wavelength 1530 nm to 1570 nm is less than about 0.04 ps / nm ² / km. 3. The center core is covered by a side core concentrically with the center core, the periphery of the side core is covered by a segment core concentrically with the side core, the periphery of the segment core is covered by a cladding layer, and the center core is bent. 3. The relationship among the refractive index n1, the refractive index n2 of the side core, the refractive index n3 of the segment core, and the refractive index n0 of the cladding layer is n1>n3>n0> n2. The dispersion flat fiber as described. 4. The center core is covered with a side core concentrically with the center core, the periphery of the side core is covered with a segment core concentrically with the side core, and the periphery of the segment core is covered with a cladding layer. The value obtained by subtracting the relative refractive index difference between the side core and the cladding layer from the relative refractive index difference from the cladding layer is 0.7% or less. The relative refractive index between the center core and the cladding layer is 0.6% or less. 4. The dispersion flat fiber according to claim 1, wherein the relative refractive index difference from the layer is 0.3% or less.