JPH08136758A - Dispersion compensated optical fiber for multiwavelength transmission - Google Patents

Abstract

PURPOSE: To provide a dispersion compensated optical fiber suitable for multiwavelength optical transmission. CONSTITUTION: The distribution of refractive index of this dispersion compensated optical fiber is made to be a W-shape distribution. The fiber consists of a core 1, inner clad layer 2 set on the outside of core 1 and a pure quartz outermost clad layer is set on the outside of it. The core 1 is doped with Ge to increase the refractive index by 2.8% difference of the refractive index, while the inner clad layer is homogeneously doped with fluorine to decrease the refractive index by 0.45%. The ratio of diameters of the core 1 to the inner clad layer 2 is controlled to 1:1.5 to 1:4.0. The fiber has a high negative dispersion structure as <=-100ps/nm/km dispersion in the region where the slope of the wavelength dispersion is negative.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dispersion compensating optical fiber for wavelength division multiplex transmission, which is used for multiplex transmission of optical transmission signals of a plurality of wavelengths, particularly for wavelength division multiplex transmission at a wavelength around 1550 nm. It is about.

[0002]

2. Description of the Related Art As a fiber network for optical communication 13
00 nm zero dispersion fiber networks are known. In this fiber network, by transmitting an optical signal with a wavelength of 1300 nm, a reception wave of almost zero dispersion is obtained on the receiving side, and highly reliable optical communication is performed.

Recently, wavelength multiplexing communication for transmitting optical transmission signals (optical pulse transmission signals) of a plurality of wavelengths is being performed using the existing 1300 nm zero dispersion fiber network. 17 ps / nm / when using wavelength multiplexing near the wavelength of 1550 nm in the existing 1300 nm zero-dispersion transmission network
Dispersion of about km occurs, and long-distance transmission is impaired. Generally, there are positive dispersion and negative dispersion in chromatic dispersion, and positive dispersion has a smaller refractive index of the optical transmission line and a larger optical transmission speed as the wavelength increases. Correspondingly, it means a phenomenon in which the pulse width widens.Negative dispersion, on the contrary, as the wavelength increases, the refractive index of the optical transmission line increases and the optical transmission speed decreases. Correspondingly, it is a phenomenon in which the pulse width widens.

A conventional existing 1300 nm zero-dispersion transmission network is
In the vicinity of 1500 nm, there is a dispersion of about 17 ps / nm / km as described above, and if long distance transmission is performed with this transmission network, for example, 100 km, the dispersion of about 1700 ps / nm will occur on the receiving side. When the wavelength is subdivided near 1550 nm and high-density high-speed communication is attempted, the signal of one side wavelength and the signal of the other side are overlapped and the signal is separated due to the large dispersion as described above. Becomes difficult and the optical communication performance deteriorates.

Conventionally, in order to prevent such an increase in the dispersion amount of chromatic dispersion as much as possible, a dispersion compensating optical fiber for compensating for the dispersion amount of a specific wavelength is inserted in the optical transmission line.

This type of dispersion compensating optical fiber has a negative dispersion, and the positive dispersion of the 1300 nm zero dispersion transmission network is canceled by the negative dispersion of this dispersion compensating fiber to disperse a specific wavelength of the optical transmission. It is intended to prevent an increase in quantity.

[0007]

A dispersion compensating optical fiber has a structure as shown in JP-A-6-11620.
There are different types of refractive index structures. These five types of refractive index structures are shown in FIG. 5, and in the refractive index distributions of FIGS. 5 (a) and 5 (b), the dispersion slope (rate of change of dispersion with respect to change of wavelength) is all. It has a positive value, and when such a dispersion compensating fiber is used, dispersion compensation can be performed for a certain specific wavelength, but for other wavelengths, the dispersion amount increases, so the wavelength It is not suitable as a compensating optical fiber for performing multiplex transmission. The three kinds of fibers shown in FIGS. 5C to 5E may have a refractive index profile having a negative dispersion slope. The so-called W-type refractive index distribution structure of FIG. 5 (c) has been studied for a long time, but the conventional W-type fiber can have a negative dispersion slope but has a small negative dispersion amount, which is necessary for dispersion compensation. However, the fiber length becomes extremely large, which is not suitable for practical use. In order to increase the negative dispersion amount, it is better to reduce the core diameter, but in the W-type refractive index distribution, if the core diameter is reduced and the negative dispersion amount is increased, the dispersion from a certain point The slope is inverted from negative to positive, making it unsuitable for wavelength division multiplexing transmission.

The present invention has been made in order to solve the above-mentioned conventional problems, and an object thereof is to utilize an existing 1300 nm zero-dispersion transmission network to compensate for dispersion in the vicinity of 1550 nm in a wide wavelength range, In order to provide a dispersion compensating fiber for wavelength division multiplexing that enables multiplex transmission, it has both a negative dispersion slope having an effective magnitude and a negative dispersion value having an effective magnitude in the W-type refractive index distribution. It is to provide a reliable structure.

[0009]

The present invention takes the following means in order to achieve the above object. That is,
In the first invention, when the core radius of the optical fiber is a, the chromatic dispersion is σ, and the wavelength of the optical transmission signal is λ, the core radius at which the chromatic dispersion slope (dσ / dλ) is zero is a _0, and dσ is
/ Dλ = -0.28ps / nm ² / km, where a ₁ is the core radius, the chromatic dispersion σ is σ ≤-within the range of a ₀ ≤ a ≤ a _1.
The configuration with a range of 100 ps / nm / km is used as a means to solve the problem.

The second invention is such that, when the core radius of the optical fiber is a, the chromatic dispersion is σ, and the wavelength of the optical transmission signal is λ, the core radius at which the chromatic dispersion slope (dσ / dλ) becomes zero is and _{a 0, dσ / dλ = -0.28ps} / nm 2 / when km to become the core radius was defined as _{a 1, a 0 ≦ a ≦} a ≦ wavelength dispersion sigma sigma within _a range -100 ps / nm / In the range of km, the refractive index structure of the optical fiber has a W-shaped refractive index distribution, the inner clad layer is formed outside the core, and the outer clad layer is the outermost clad layer. The dopant to be lowered is doped to have a relative refractive index difference of −0.45%,
The outermost clad layer is made of pure quartz, and the core is doped with a dopant for increasing the refractive index so that the relative refractive index difference is + 2.8%.

A third aspect of the invention is based on the structure of the first aspect, wherein the refractive index structure of the optical fiber has a W-shaped refractive index distribution, and an inner cladding layer is formed outside the core. , The outer cladding layer is the outermost cladding layer, the inner cladding layer is doped with a dopant that lowers the refractive index so that the relative refractive index difference is -0.45%, the outermost cladding layer is pure quartz, and the core is As a means to solve the problem, a dopant that raises the refractive index is doped so that the relative refractive index difference becomes + 2.8%, and the diameter ratio between the core and the inner cladding layer is set in the range of 1: 1.5 to 1: 4.0. There is.

Furthermore, a fourth invention is based on the configuration of the third invention, further, in which the wavelength is 1550 nm, the dispersion wavelength slope is in the negative region, and the core diameter is larger than 2.1 μm.
In the range smaller than 3 μm, the wavelength dispersion value at the light wavelength of 1550 nm is smaller than −100 ps / nm / km and −170 ps / nm
It is a means to solve the problem with a configuration larger than / km.

In the present invention having the above configuration, for example, when the dispersion compensating optical fiber for wavelength division multiplexing transmission of the present invention is inserted into an existing 1300 nm zero dispersion transmission network and wavelength division multiplexing optical communication is performed using wavelengths around 1550 nm. , The optical signal of each wavelength reaching the terminal end through the 1300 nm zero-dispersion transmission network has a large amount of chromatic dispersion, but the dispersion-compensating optical fiber of the present invention has negative high dispersion and negative dispersion slope at the same time. The large positive dispersion amount generated by passing through the 1300 nm zero dispersion transmission network is effectively compensated by the large negative dispersion of the dispersion compensating optical fiber and the dispersion amount due to the negative dispersion slope. The dispersion amount of the optical signal of each wavelength that has passed through the optical fiber has a value close to zero. As a result, signal separation of each wavelength is clearly performed on the receiving side, and reliable, high-density, high-speed, wavelength-multiplexed optical transmission communication becomes possible.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows the configuration of an embodiment of a dispersion compensating optical fiber for wavelength division multiplexing transmission according to the present invention.

In the dispersion compensating optical fiber of this embodiment, the refractive index structure has a W-shaped structure, and the core 1 has a refractive index difference Δ of Δ = 2.8%. Is doped with germanium Ge which enhances the conductivity. The outer side of the core 1 is an inner clad layer 2. The inner clad layer 2 is uniformly doped with fluorine F which lowers the refractive index so that the relative refractive index difference Δ is −0.45%. There is. The outer side of the inner clad layer 2 is an outermost clad layer (not shown) of pure quartz. The diameter ratio a / D between the core and the inner cladding layer is set in the range of 1: 1.5 to 1: 4.0.

FIG. 4 shows a conventional general simple step type fiber structure as a comparative example. In this comparative example, the core 1 has a relative refractive index difference of 2.8%. It is doped with germanium to increase the refractive index,
The clad outside the core 1 is uniformly doped with fluorine F which lowers the refractive index so that the relative refractive index difference becomes −0.45%.

Under the fiber structure shown in FIG. 1, the dispersion value σ and the dispersion slope δ at each core diameter are calculated by using the diameter ratio between the core and the inner cladding layer as parameters, and as shown in Tables 1 to 3. become.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

[Table 3]

The following equation was used as the arithmetic expression of the dispersion value σ and the dispersion slope δ.

Σ = (K / C) dM2 / dK + (K / C) (d (M1−M2) / dK) d (V · b) / dV + {(M1−M2) / C} Vd ² (V · b) / dV ² (1)

Δ = dσ / dλ (2)

The symbols used in the calculation represent the following. C: speed of light, K: wave number of medium space, n1: refractive index of core, n2: refractive index of outer cladding, Δ: relative refractive index difference between core and outer cladding, V: (K · n1 · a (2
Δ)) ^1/2 , a: core diameter, M1: d (K · n1) / d
K, M2: d (K · n2) / dK, b: normalized variable.

Table 1 shows the calculation results when the diameter ratio between the core diameter and the inner cladding is 1: 2.5, and Table 2 shows the calculation results when the diameter ratio between the core and the inner cladding is 1: 1.5. Table 3 shows that the diameter ratio of the core to the inner cladding is 1: 4.0.
The calculation result in the case of is shown.

As is clear from these calculation results, the dispersion slope is inverted from positive to negative at a certain core diameter. In the data of Table 1, the dispersion slope is reversed at the core diameter of 2.1 μm, and in the data of Table 2, the core diameter is 2.00.
The dispersion slope is inverted at a certain core diameter between ˜2.17, and in the data in Table 3, the dispersion slope is inverted at a certain core diameter between 2.1 μm and 2.2 μm. Therefore, the structure of the dispersion compensating optical fiber having a high negative dispersion can be specified in the negative range of the dispersion slope based on these calculation data.

Focusing on this point, the present embodiment provides a dispersion compensating optical fiber having a negative dispersion slope and a high negative dispersion. In the present embodiment, the core of the optical fiber is provided. When the radius is a, the wavelength dispersion is σ, and the wavelength is λ, the dispersion slope (dσ / dλ) is -0.28ps / nm ²
When the core radius that becomes / km is a ₁ , a ₀ ≤ a ≤ a
_A dispersion compensating optical fiber having a chromatic dispersion σ of σ ≦ −100 ps / nm / km within the range of ₁ is obtained.
The condition is satisfied in the core diameter range of 2.1 to 2.3,
In the data in Table 2, the core diameter is 2.17 μm, and in the data in Table 3, the conditions are satisfied in the core diameter range of 2.2 to 2.4 nm.

There is a slight difference between the calculation results shown in the above tables and the actually measured values of the dispersion-compensating optical fiber actually manufactured. For example, the diameter ratio of the core diameter to the inner cladding was 1: 2.5. The measured value of the dispersion compensating optical fiber is
2.124 μm, dispersion value is -163.3 ps / nm / km, dispersion slope is −0.129 ps / nm ² / km, and core diameter is 2.184.
In μm, the dispersion value is −152 ps / nm / km, the dispersion slope is −0.249 ps / nm ² / km, and the negative slope is −100 ps / nm / km or less within the range of the negative dispersion slope. A fiber having a high dispersion can be obtained by referring to the value obtained by calculation (aiming at the calculated value and correcting if necessary).

FIG. 2 shows the measurement results of the dispersion slope and the dispersion value for each core diameter of the dispersion compensating optical fiber in this embodiment, using the diameter ratio of the core and the inner cladding as a parameter. Based on the data of this actual measurement result, in the range where the dispersion slope is 0 or less, the chromatic dispersion has a negative high dispersion of -100 ps / nm / km or less. The diameter of the inner cladding layer is determined, and a dispersion compensating optical fiber having the fiber structure shown in FIG. 1 and excellent chromatic dispersion compensating performance can be obtained. In addition, in FIG. 2, the actual measurement data of the conventional comparative example fiber of FIG. 4 is shown for reference.

As mentioned above, when the dispersion slope is in the negative range,
By inserting a dispersion-compensating optical fiber with a high negative dispersion, which has a chromatic dispersion of -100 ps / nm / km or less, into the optical transmission line with positive dispersion, the large positive dispersion produced in the optical transmission line is effective. It is possible to compensate the dispersion of each wavelength to a small value (desirably, a dispersion of substantially zero) on the receiving side. Since an ordinary optical transmission line has a positive dispersion slope, by using the dispersion compensating optical fiber having the negative dispersion slope of the present embodiment example, variations in chromatic dispersion on the receiving side of each wavelength can be prevented. There is also an effect that the wavelength dispersion can be suppressed and the wavelength dispersion can be made uniform within a range of small variation.

When wavelength multiplexing transmission around 1550 nm is performed using the existing 1300 nm zero dispersion fiber network, the dispersion slope has a negative region near the wavelength 1550 nm and the core diameter is 2.1 μm or less. By setting the selection condition of the dispersion compensating optical fiber in the range of less than 2.3 μm, the dispersion value at the wavelength of 1550 nm is −100 ps / nm / km.
A smaller negative high dispersion fiber is chosen. This optical fiber is inserted into an existing 1300 nm zero-dispersion fiber network as a dispersion compensating optical fiber, and wavelength-multiplexed optical transmission is performed using wavelengths near 1550 nm, thereby effectively compensating for the dispersion of the optical transmission line. It is possible to receive an optical signal having a small dispersion.

FIG. 3 shows measured data of stimulated Brillouin scattering of the dispersion compensating optical fiber manufactured in this embodiment. This stimulated Brillouin scattering measurement device is shown inside the graph for reference. In this figure, the horizontal axis represents the input power level of light and the vertical axis represents the power level of backscattered light. As is clear from this experimental data, when the input power level is 8 dBm, stimulated Brillouin scattering occurs, and the threshold value of 8 dBm at which this stimulated Brillouin scattering occurs is the conventional dispersion-compensated light with a positive dispersion slope. The same as the fiber, the dispersion-compensating optical fiber of the present embodiment example, the stimulated Brillouin scattering is not worse than the conventional one, it is possible to sufficiently increase the input power in the range that does not generate backscattered light, It can be seen that the practicality is fully satisfied.

The present invention is not limited to the above embodiment, but various embodiments can be adopted.

[0034]

According to the present invention, it becomes possible to provide an unprecedented dispersion compensating optical fiber having a negative dispersion slope and a high negative dispersion, and having the negative dispersion slope, Moreover, by using the optical fiber of the present invention having a high negative dispersion as a dispersion compensating optical fiber for wavelength division multiplexing optical communication in an optical transmission line having a positive dispersion, It is possible to effectively reduce the amount of dispersion and form a reception wave with small chromatic dispersion on the receiving side, and thus it is possible to significantly improve the reliability of high-density and high-speed wavelength-multiplexed optical communication.

Further, since the dispersion compensating optical fiber of the present invention has a high negative dispersion, even if a large positive dispersion occurs in the optical transmission signal passing through the optical transmission line, the positive dispersion can be compensated with a short fiber length. become. Therefore, even when the dispersion compensating optical fiber is housed in a package, the package does not need to be small in size and is not bulky, which makes the package very practical.

Furthermore, in the existing 1300 nm zero dispersion transmission network, 15
When performing wavelength division multiplexing optical communication using wavelengths around 50 nm, by inserting and using the dispersion compensating optical fiber of the present invention in the optical transmission line, similarly, at the receiving side of the transmission optical signal of each wavelength. Since the chromatic dispersion can be effectively canceled and compensated, highly reliable high-density and high-speed WDM communication in the vicinity of 1550 nm becomes possible by using the existing transmission network.

By the way, the optical fiber has a condition for effectively propagating light. This light propagation condition depends on the propagation refractive index (β / K) of the optical fiber. Here, β is a propagation coefficient, a value obtained by dividing each frequency ω of the wave by the phase velocity of the wave, and K is the wave number of the medium space.

In the optical fiber having a [0038] W-type refractive index distribution of the propagation refractive index of the light of the relative refractive index difference delta relative refractive index difference delta ⁺ and inner cladding of the core ^- is dependent on the values of these ratios It is necessary to find the optimum combination of refractive index differences Δ ⁺ and Δ ⁻ .

According to a study made by the present inventor, the larger the relative refractive index difference Δ ^{+ of the} core and the smaller the relative refractive index difference Δ ^{− of} the inner cladding, the more easily the propagation conditions tend to be satisfied. The combination with the index difference Δ ⁺ of ⁺ 2.8% and the relative refractive index difference Δ ⁻ of the inner cladding of −0.45% gives the optimum light propagation conditions. As the optimum propagation conditions are deviated, the light propagation performance deteriorates. For ^{example, Δ + = + 2.8%,} Δ - = -0.7
%, The optical fiber has too large Δ ⁻ and the light propagation performance is deteriorated, and Δ ⁺ = + 2.1% and Δ ⁻ = −0.35% are too small Δ ^{+, the same} is true. The light propagation performance deteriorates.

As in the present invention, Δ ⁺ = + 2.8%, Δ ^- =-
By giving a value of 0.45%, the optimum propagation refractive index for light propagation can be obtained.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a fiber configuration showing an embodiment of a dispersion compensating optical fiber for wavelength division multiplexing transmission according to the present invention.

FIG. 2 is a graph of various measured data of dispersion slope and dispersion of the dispersion compensating optical fiber of the present embodiment example.

FIG. 3 is a graph of measured data of stimulated Brillouin scattering of the dispersion compensating optical fiber of the present embodiment example.

FIG. 4 is an explanatory diagram of a conventional general simple step type optical fiber structure.

FIG. 5: 5 shown in JP-A-6-11620
It is explanatory drawing of a kind of refractive index structure.

[Explanation of symbols]

 1 core 2 inner clad layer

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical indication location H04B 10/18 H04J 14/00 14/02 (72) Inventor Kunio Ogura 2 Marunouchi, Chiyoda-ku, Tokyo No. 6-1 Furukawa Electric Co., Ltd.

Claims (4)

[Claims] 1. When the core radius of the optical fiber is a, the chromatic dispersion is σ, and the wavelength of the optical transmission signal is λ, the core radius at which the chromatic dispersion slope (dσ / dλ) becomes zero is a _0, and d σ is
/Dλ=−0.28ps/nm ² / km, where a ₁ is the core radius, chromatic dispersion σ is σ ≦ −1 within the range of a ₀ ≦ a ≦ a _1.
A dispersion compensating optical fiber for wavelength division multiplexing transmission in the range of 00 ps / nm / km. 2. When the core radius of the optical fiber is a, the chromatic dispersion is σ, and the wavelength of the optical transmission signal is λ, the core radius at which the chromatic dispersion slope (dσ / dλ) is zero is a _0, and dσ is
/Dλ=−0.28ps/nm ² / km, where a ₁ is the core radius, chromatic dispersion σ is σ ≦ −1 within the range of a ₀ ≦ a ≦ a _1.
In the range of 00 ps / nm / km, the refractive index structure of the optical fiber has a W-shaped refractive index distribution, the inner clad layer is formed outside the core, and the outer clad layer is the outermost clad layer. The layer is doped with a dopant that lowers the refractive index so that the relative refractive index difference is −0.45%, the outermost cladding layer is pure quartz, and the core has a refractive index increasing dopant of +2 due to the relative refractive index difference. Dispersion compensating optical fiber for WDM transmission doped to 8%. 3. The refractive index structure of an optical fiber has a W-shaped refractive index distribution, an inner clad layer is formed on the outside of the core, and the outer side is the outermost clad layer, and the inner clad layer has a refractive index. The dopant to be lowered is doped to have a relative refractive index difference of −0.45%, the outermost cladding layer is made of pure quartz, and the core has a refractive index difference of + 2.8% to increase the refractive index difference. 2. The diameter ratio of the core to the inner cladding layer doped in the range of 1: 1.5 to 1: 4.0.
A dispersion compensating optical fiber for wavelength-division multiplexing transmission. 4. The wavelength dispersion value at a light wavelength of 1550 nm is −100 ps / nm / in the range where the wavelength is 1550 nm and the dispersion wavelength slope is in the negative region and the core diameter is larger than 2.1 μm and smaller than 2.3 μm. The dispersion compensating optical fiber for wavelength division multiplex transmission according to claim 3, which is smaller than km and larger than -170 ps / nm / km.