JPH10325913A - Dispersion compensative optical fiber and wavelength multiple light transmission line - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a dispersion compensative optical fiber connected to a single mode optical fiber having zero dispersion at a wavelength 1.31 μm and having both functions as the dispersion compensative function of the single mode optical fiber and as an optical transmission line. SOLUTION: The dispersion value of the dispersion compensative optical fiber at the wavelength 1.55 μm is made within the range of -35 ps/nm/km to -10 ps/nm/ km, and a refractive index distribution is made a W type profile. Preferably, the dispersion value at the wavelength 1.55 μm is made >=-20 ps/nm/km and <=-10 ps/nm/km, and a ratio between the dispersion value and a dispersion slope at the wavelength 1.55 μm is set so as to become opposite in a positive/negative code to and equal in an absolute value to the ratio between the dispersion value and the dispersion slope at the same wavelength 1.55 μm band. A specific refractive index difference Δ+ of a central core 1 for a clad 3 is made >=1.0% and <=1.8%, and the ratio RΔ of the specific refractive index difference Δ- of a dido core 2 for the clad 3 for the specific refractive index difference Δ+ is made <=-0.25, and the ratio Ra (Ra=a/b) of the size (a) of the central core 1 for the size (b) of a side core is made within the range of >=0.3 and <=0.4. A mode field size is made >=5.5 μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dispersion of an optical signal in a 1.55 .mu.m wavelength band which is connected to a single mode optical fiber having zero dispersion in a 1.3 .mu.m wavelength band and transmitted through the single mode optical fiber. The present invention relates to a dispersion-compensating optical fiber having both a function of compensating for the optical fiber and a function as a transmission optical fiber, and a wavelength division multiplexing optical transmission line using the optical fiber.

[0002]

2. Description of the Related Art As a transmission network for optical communication, single mode optical fibers having zero dispersion in a wavelength band of 1.3 μm are laid all over the world. Recently, with the development of the information society, the amount of communication information has tended to increase dramatically. With the increase of such information, wavelength division multiplexing transmission (WDM transmission) has been widely accepted in the communication field, and now wavelengths have been increasing. The age of multiplex transmission has been reached. Wavelength multiplex transmission is a system in which the wavelength of optical communication is not one wavelength but is divided into a plurality of wavelengths to transmit a plurality of optical signals, and is an optical transmission system suitable for large-capacity high-speed communication.

However, the existing single-mode optical fiber for transmission having zero dispersion at 1.31 μm generally has a large transmission loss. When wavelength-division multiplexed optical communication is performed using the 1.3 μm wavelength band, the conventional single-mode optical fiber has an ordinary transmission loss. The gain band of an optical amplifier 1.
Since the wavelength band of 55 μm and the wavelength band do not match, there is a problem that optical amplifiers cannot be used and hinder long-distance optical communication.Therefore, recently, zero dispersion has been applied to the existing 1.3 μm wavelength band. Wavelength-division multiplexed optical communication is performed in a 1.55 μm wavelength band using a transmission single-mode optical fiber.

However, when optical communication is performed in a wavelength band of 1.55 μm using a transmission single-mode optical fiber having a zero dispersion at 1.31 μm, the existing transmission single-mode optical fiber cannot be used in this 1.55 μm wavelength band. Since the signal has a positive dispersion and a positive dispersion slope, as the optical signal propagates through the single-mode optical fiber for transmission, the dispersion of the signal of each wavelength of wavelength division multiplexing increases, making it difficult for the receiving side to separate the signal. However, there has been a problem that the quality of the optical communication is degraded and the reliability of the optical communication is lost.

Therefore, recently, a dispersion compensating optical fiber has been developed to solve such a problem. This dispersion compensating optical fiber has a negative dispersion, and by connecting the dispersion compensating optical fiber to the receiving side of the transmission single mode optical fiber, the positive dispersion of the optical signal propagating through the transmission single mode optical fiber. Is reduced by the negative dispersion of the dispersion compensating optical fiber, and the dispersion of the optical signal on the receiving side is almost zero dispersion and received. In this way, by connecting the dispersion compensating optical fiber to the transmission single mode optical fiber, it becomes possible to separate each wavelength-division multiplexed optical signal on the receiving side, and high-quality, large-capacity, high-speed communication is expected. is there.

This kind of dispersion compensating optical fiber is modularized, and a dispersion compensating optical fiber as short as possible is generally connected to the receiving side of a single-mode optical fiber for transmission to perform dispersion compensation. .

[0007]

However, in order to modularize the dispersion compensating optical fiber and compensate for the dispersion with a short fiber length, the dispersion compensating optical fiber to be modularized has a high negative dispersion and a negative dispersion slope. It is necessary to have

However, in order to make the dispersion compensating optical fiber have a high negative dispersion and a dispersion slope, the conditions of various parameters that determine the refractive index distribution of the dispersion compensating optical fiber become very strict, and the production becomes difficult. When a refractive index structure having a high negative dispersion and a dispersion slope is used, nonlinear phenomena are inevitably inevitably generated, and the mode field diameter (MFD) of the optical fiber becomes small. When the non-linear phenomenon occurs, the signal waveform is distorted, which is a new problem in increasing the speed and increasing the capacity of the wavelength division multiplexing optical transmission.

Further, when the mode field diameter of the optical fiber is reduced, a problem arises that transmission loss due to bending of the optical fiber increases.

[0010] The zero-dispersion wavelength is from 1.31 µm to 1.55 µm.
There has also been proposed a method of performing optical transmission using a dispersion-shifted optical fiber shifted to a wavelength. By transmitting an optical signal at a wavelength of 1.55 μm using a dispersion-shifted optical fiber having zero dispersion at a wavelength of 1.55 μm, signal transmission without dispersion becomes possible. When wavelength multiplexed optical transmission is performed, dispersion does not occur for the wavelength of 1.55 μm, but dispersion occurs for signals of other wavelengths in the vicinity, making it suitable for large-capacity high-speed wavelength multiplexing optical communication. There is no optical transmission system. In addition, this kind of dispersion-shifted optical fiber also has a problem in that a nonlinear phenomenon is likely to occur.

In this regard, a single mode optical fiber having zero dispersion at a wavelength of 1.31 μm is very excellent in terms of low non-linearity, and therefore, has an existing wavelength of 1.11 μm as an optical transmission line.
Using a single-mode optical fiber for transmission with zero dispersion at 31 μm, and connecting a dispersion-compensating optical fiber with low nonlinearity and a large mode field diameter to this single-mode optical fiber for transmission to use a wavelength in the 1.55 μm band It would be ideal if the wavelength-division multiplexed optical transmission could be performed with almost zero dispersion.

The present inventor paid attention to this point, and changed the idea from using a dispersion compensating optical fiber simply as a modularized optical fiber dedicated to dispersion compensating, and replacing the dispersion compensating optical fiber with a single mode optical fiber for optical transmission. A dispersion-compensating optical fiber of almost the same length as the mode optical fiber is connected, and the optical signal is transmitted over a long distance while compensating for the dispersion of the optical signal propagating through the single-mode optical fiber for transmission. It is a first object of the present invention to provide a dispersion compensating optical fiber having a function of dispersion compensation and an optical transmission function, and a second object of the present invention is to provide a single mode optical fiber for optical transmission. It is an object of the present invention to provide a wavelength division multiplexing optical transmission line formed by connecting a fiber and the dispersion compensating optical fiber.

[0013]

In order to achieve the above object, the present invention takes the following measures. That is,
According to a first aspect of the present invention, there is provided a dispersion compensating optical fiber connected to a single mode optical fiber having zero dispersion in a wavelength band of 1.31 μm for transmitting an optical signal.
However, this is a means for solving the problem with a configuration in which −35 ps / nm / km ≦ σ ≦ −10 ps / nm / km.

A second invention relates to a dispersion compensating optical fiber connected to a single mode optical fiber having zero dispersion in a wavelength band of 1.31 μm for transmitting an optical signal.
The dispersion value σ in μm is -20 ps / nm / km ≦ σ ≦ −10 ps / nm
/ Km, and the ratio of the dispersion value to the dispersion slope in the 1.55 μm wavelength band is the same. The sign of the single mode optical fiber in the 1.55 μm band is opposite to the sign of the dispersion value and the absolute value of the dispersion slope. This is a means for solving the problem with a configuration in which the values are set to be substantially equal.

In a third aspect of the present invention, in the apparatus having the structure of the first or second aspect, a side core having a low refractive index is arranged around a center core having the highest refractive index, A cladding having a lower refractive index than the center core and a higher refractive index than the side cores is disposed around the center core, and the means for solving the problem is achieved by forming a refractive index distribution into a W-shaped profile.

In a fourth aspect of the present invention, the relative refractive index difference Δ + of the center core with respect to the clad is in the range of 1.0% to 1.8%, wherein the ratio of the center core to the cladding is less than 1.0%. The ratio RΔ (RΔ = Δ− / Δ +) of the relative refractive index difference Δ− to the side core cladding relative to the refractive index difference Δ + is −0.25 or less, and
Ratio Ra of center core diameter a to side core diameter b
By setting (Ra = a / b) in the range of 0.3 or more and 0.4 or less, this is a means for solving the problem.

According to a fifth aspect of the present invention, as a means for solving the problem, a mode field diameter of 5.5 μm or more is provided in the above-described first, second, third, or fourth aspect of the invention. I have.

A sixth invention is a wavelength division multiplexing optical transmission line comprising a single mode optical fiber having zero dispersion in a wavelength band of 1.31 μm and a dispersion compensating optical fiber having substantially the same length connected thereto. The optical fiber is a means for solving the problem by being constituted by the dispersion compensating optical fiber of any one of the above-mentioned inventions.

In the present invention, the dispersion compensating optical fiber of the present invention is connected to a single mode optical fiber having a zero dispersion in the 1.3 μm wavelength band (more specifically, having a zero dispersion in the wavelength of 1.31 μm) by almost the same length. As a result, a wavelength multiplexing optical transmission line is formed. When wavelength multiplexing optical transmission is performed using an optical signal in the 1.55 μm band using this wavelength multiplexing optical transmission line, a wavelength of 1.55 μm
Each wavelength in the band increases in positive dispersion as it travels through the single-mode fiber.

An optical signal of each wavelength of wavelength multiplexing is transmitted after being switched from a single mode optical fiber to a dispersion compensating optical fiber, and the dispersion compensating optical fiber has a dispersion value σ of −.
It has a relatively low negative dispersion value in the range of 20 ps / nm / km ≦ σ ≦ −10 ps / nm / km, and has a wavelength of 1.
1.55 wavelength with the same dispersion value and dispersion slope ratio in the 55μm band
In the μm band, the ratio between the dispersion value and the dispersion slope of the single mode optical fiber is set so that the positive and negative signs are opposite and the absolute value is almost the same, so that it increased by propagating through the single mode optical fiber. Dispersion is compensated in a direction that gradually decreases as it propagates through the dispersion compensating optical fiber.At the end of the dispersion compensating optical fiber, the dispersion of each wavelength of wavelength division multiplexing is compensated to almost zero and received. become.

As described above, the dispersion compensating optical fiber of the present invention has a relatively low negative dispersion value such as -20 ps / nm / km ≦ σ ≦ −10 ps / nm / km. The conditions for regulating the distribution are not as strict as the conventional dispersion-compensating optical fiber having a high negative dispersion slope that is modularized, and the conditions are moderate, resulting in an optical fiber structure with low nonlinearity. Thereby, it is possible to suppress the occurrence of distortion of the waveform of each wavelength of the wavelength multiplexed optical transmission, and the mode field diameter is 5.5 μm or more (more preferably, 6 μm or more).
As a result, an increase in transmission loss due to bending of the optical fiber is prevented, and high-quality, large-capacity, high-speed, wavelength-division multiplexed optical transmission with small transmission loss is made possible.

Further, the dispersion value σ at a wavelength of 1.55 μm of a dispersion compensating optical fiber connected to a single mode optical fiber having a zero dispersion in a 1.31 μm wavelength band and transmitting an optical signal is represented by −
According to the invention set in the range of 35 ps / nm / km ≦ σ ≦ −10 ps / nm / km, the dispersion compensation light having a fiber length having a dispersion of almost the same sign as the single mode optical fiber is added to the single mode optical fiber. By connecting the fiber, it becomes possible to attenuate the dispersion of the signal of a specific wavelength in the 1.55 μm wavelength band generated by propagating the single mode optical fiber to zero dispersion at the terminal side of the dispersion compensating optical fiber. Things.

Further, by forming the refractive index distribution of the dispersion compensating optical fiber into a W-shaped profile, an optical fiber having a refractive index structure satisfying the set conditions of the dispersion compensating optical fiber can be easily manufactured. And the relative refractive index difference Δ + of the center core to the cladding is 1.0% or more.
1.8% or less, and the ratio RΔ (RΔ = Δ− / Δ +) of the relative refractive index difference Δ− of the side core to the cladding relative to the relative refractive index difference Δ + of the center core to the cladding is −0.0.
By setting the ratio Ra (Ra = a / b) of the center core diameter a to the side core diameter b to be not more than 25 (Ra = a / b) in the range of 0.3 or more to 0.4 or less, the dispersion generated by propagating through the single mode optical fiber is achieved. Compensation rate close to ideal 1.
It is possible to increase the value to around 0.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows a profile of a refractive index distribution of an embodiment of the dispersion compensating optical fiber according to the present invention. As the profile of the refractive index distribution of the dispersion compensating optical fiber, a matched type profile as shown in FIG. 4A, a segment core type profile as shown in FIG. Various types of refractive index profiles such as a double core type as shown in (c) can be used, but in the present embodiment, the structure is simple.
Refractive index structure design, easy to control, low transmission loss,
In addition, a W-shaped refractive index profile as shown in FIG. 1, which is rich in the possibility of negative dispersion and negative dispersion slope, is employed.

In the refractive index structure of this W-shaped dispersion compensating optical fiber, a side core 2 having a lower refractive index than the center core 1 is arranged around the center core 1 having the highest refractive index. Surrounding the center core 1, the refractive index is higher than that of the side core 2.
The cladding 3 having a lower refractive index than that is provided, and the refractive index distribution has a W shape.

The clad 3 is formed of a layer of pure silica (SiO ₂ ), and the side core 2 is formed of pure silica (SiO ₂ ).
₂ ) is formed by doping fluorine (F) for lowering the refractive index, and the center core 1 is formed by doping pure silica with germanium (Ge) for increasing the refractive index.

In the refractive index structure shown in FIG. 1, when the refractive index of the center core 1 is n _C , the refractive index of the side core 2 is n _s , and the refractive index of the cladding 3 is n _L , the relative refraction of the center core 1 with respect to the cladding 3. The rate difference Δ + is defined by the following equation (1).

Δ + = {(n _C ² −n _L ² ) / 2n _C ² } × 100 (1)

The relative refractive index difference Δ− of the side core 2 with respect to the cladding 3 is defined by the following equation (2).

[0030] ^{_{Δ - = {(n s 2}} -n L 2) / 2n s 2} × 100 ····· (2)

In this embodiment, the concept of the dispersion compensating optical fiber is changed from a function as a conventional modularized optical fiber dedicated to dispersion compensation as in the conventional example, and is generated by propagating through a single mode optical fiber. The dispersion compensating optical fiber has a dispersion value of −20 ps / nm / km or more and −10 ps by adopting a configuration having both a function of compensating for dispersion and a function as a transmission line for propagating an optical signal. / Nm / km or less. in this way,
The dispersion value of the dispersion compensating optical fiber according to the present embodiment is reduced by the dispersion value −55 of the modularized dispersion compensating optical fiber according to the conventional example.
By setting the negative dispersion value lower than that of ps / nm / km (for example, the dispersion compensating optical fiber module disclosed in Japanese Patent Application Laid-Open No. 6-11620), the regulation of the profile design of the W-type refractive index distribution is relaxed, and the low A non-linear transmission path can be formed. A transmission single-mode optical fiber having zero dispersion at a wavelength of 1.31 μm is approximately 17 ps / nm / 1.55 μm.
km with a dispersion slope of about 0.06 ps / at the wavelength of 1.55 μm
nm with a dispersion slope of ² / km.

In this embodiment, the dispersion of the optical signal of each wavelength of wavelength multiplexing in the wavelength band of 1.55 μm propagating through the single mode optical fiber is uniformly attenuated to almost zero dispersion. The ratio of the dispersion value and the dispersion slope of the dispersion compensating optical fiber in the wavelength band of 1.55 μm is the absolute value of the sign and the sign opposite to the ratio of the dispersion value and the dispersion slope of the single mode optical fiber in the same wavelength band of 1.55 μm. Are set to be approximately equal. By setting the ratio between the dispersion value and the dispersion slope in this way, the dispersion compensating optical fiber is set to 1.
Dispersion compensation of the wavelength-division multiplexed optical signal dispersion of each wavelength in the 1.55 μm wavelength band propagating through the single-mode optical fiber by connecting the existing single-mode optical fiber with zero dispersion at 31 μm with almost the same length. At the terminal end of the optical fiber, the attenuation can be uniformly reduced to almost zero dispersion.

An important requirement for a dispersion compensating optical fiber is that when connected to a single mode optical fiber,
Low dispersion is realized in a wide range of the wavelength of 1.55 μm. Therefore, the present inventor has attempted to optimize the profile of the W-type refractive index distribution and to increase the dispersion compensation rate. The dispersion compensation rate is defined by the following equation (3).

Compensation rate = {S (DCF) / S (SMF)} / {D (DCF) / D (SMF)} (3)

In the above equation (3), S (DCF) is the average value of the dispersion slope of the dispersion compensating optical fiber in the wavelength band of 1.55 μm, and S (SMF) is the transmission single having zero dispersion at the wavelength of 1.31 μm. The average value of the dispersion slope in the 1.55 μm wavelength band of the mode optical fiber, D (DCF) is the dispersion value of the dispersion compensating optical fiber at the wavelength of 1.55 μm,
D (SMF) is a dispersion value of a transmission single mode optical fiber having a zero dispersion at a wavelength of 1.31 μm at a wavelength of 1.55 μm.

In a dispersion compensating optical fiber having a W-shaped refractive index profile, the ratio RΔ (RΔ = Δ− / R) of the relative refractive index difference Δ + of the side core 2 to the cladding 3 to the relative refractive index difference Δ + of the center core 1 to the cladding 3. It has been verified that by setting Δ +) to −0.25 or less, it is possible to obtain a dispersion slope that can compensate for low dispersion in the 1.55 μm wavelength band. The fiber has a RΔ of -0.25 or less.

Table 1 shows, as an example, R.DELTA.
Are shown in a comparison state in which the parameters of the W-shaped profile are different.

[0038]

[Table 1]

The dispersion in Table 1 is an actually measured value at a wavelength of 1.55 μm, and the dispersion slope is represented by an average value at a wavelength of 1.53 μm to 1.57 μm. The data shown in Table 1 is
This is data when the value of the ratio Ra (Ra = a / b) of the diameter a of the center core 1 to the diameter b of the side core 2 is fixed to 0.4.

As can be seen from Table 1, RΔ is -0.28
It can be seen that by setting the ratio to 5, a high dispersion compensation ratio of 92% or more and 108% or less is obtained.

Next, the present inventor determined by simulation the optimum range of Ra that can sufficiently satisfy the dispersion compensation rate under the condition that R.DELTA. Is -0.25 or less. The result of the simulation is shown in FIG. Shown in The solid line shown in FIG. 2 is a theoretical curve, and the plot points are data obtained by simulation.

From the simulation results shown in FIG. 2, it has been proved that a sufficiently satisfactory high compensation rate can be obtained by setting Ra within the range of 0.3 to 0.4.
Based on the simulation results, the value of Ra of the dispersion compensating optical fiber of this embodiment was set to a value of 0.3 or more and 0.4 or less.

Further, when the value of Ra is set to 0.3 or more and 0.4 or less and RΔ is set to −0.25 or less, according to the simulation results, the dispersion value is −10 ps / nm at −20 ps / nm / km or more. In order to realize a value of not more than / km, it has been required that the value of the relative refractive index difference Δ + of the center core 1 with respect to the cladding 3 must be at most 1.8% or less. By setting the value of Δ + to 1.8% or less,
The effect of widening the mode field diameter (MFD) of the dispersion compensating optical fiber can be obtained. In the present embodiment, the mode field diameter can be made 5.5 μm or more, more specifically, 6 μm or more. This mode field 5.5μm
The value of the above (more specifically, the diameter of 6 μm or more) is sufficiently sufficient as apparent from comparison with the value of the mode field diameter of 4.4 μm of the modularized dispersion compensating optical fiber disclosed in JP-A-6-11620. A large mode field diameter can be obtained. If the value of Δ + is too small, the layer of fluorine (F) doped into the side core 2 becomes unstable during the production of the dispersion compensating optical fiber, and the W-shaped layer shown in FIG. Since the structure of the profile cannot be achieved, it is necessary to set the value of Δ + to 1.0% or more in order to prevent this. In the present embodiment, the value of Δ + is set to 1.0% or more and 1.8% or less. doing.

That is, the dispersion compensating optical fiber of this embodiment has a W-shaped refractive index distribution profile, and the relative refractive index difference Δ + of the center core 1 with respect to the cladding 3 is 1.0%.
And not more than 1.8% and RΔ (RΔ = Δ− / Δ +) is set to −0.0.
It is set to be 25 or less, Ra is set to be 0.3 or more and 0.4 or less, the mode field diameter is 5.5 μm or more (more specifically, 6 μm or more), and the dispersion value in the wavelength band of 1.55 μm is −20 ps /. A dispersion-compensating optical fiber having a wavelength of not less than nm / km and not more than -10 ps / nm / km was obtained.

In the conventional modularized dispersion compensating optical fiber, in order to compensate the dispersion propagating through the single mode optical fiber with a short fiber length, a negative high dispersion value is used.
In pursuit of a high dispersion slope, the relative refractive index difference Δ of the center core of a dispersion compensating optical fiber having a W-shaped profile
+ Is a large value close to 2%, and the diameter of the core has to be made small (narrow). Therefore, the mode field diameter of the conventional modularized dispersion compensating optical fiber is 5μm at most
Therefore, the transmission loss due to bending becomes a large value, but the dispersion compensating optical fiber of the present embodiment has a negative low dispersion value and a low dispersion slope, so that the design of the parameters that condition the W-shaped profile is designed. , The mode field diameter can be increased and the transmission loss due to the bending of the dispersion compensating optical fiber can be suppressed.

According to the experiment of the present inventor, while maintaining the transmission loss due to bending of a diameter of 20 mm at 5 dB / m or less, the wavelength 1.
At 55 μm, it was confirmed that the condition that the mode field diameter was 5.5 μm or more could be sufficiently achieved. This value sufficiently satisfies the condition as an optical fiber for transmission that has low nonlinearity and is suitable for wavelength-division multiplexing optical transmission, which can sufficiently cope with bending as an optical transmission line.

FIG. 5 shows a wavelength-division multiplexed optical transmission line according to the present embodiment. A transmission single-mode optical fiber SMF having zero dispersion at a wavelength of 1.31 μm has a length substantially equal to that of the single-mode optical fiber SMF. The dispersion compensating optical fiber DCF of the embodiment is connected to form an optical transmission line. In FIG. 5, reference numeral 4 denotes an optical amplifier having a gain region in a wavelength band of 1.55 μm, and reference numeral 5 denotes a receiving station.

Table 2 shows the results of examining the suitability of the wavelength division multiplexing optical transmission line for various transmission lines.

[0049]

[Table 2]

The mark x in Table 2 indicates that the line is unsuitable as a wavelength division multiplexed optical transmission line, the mark Δ indicates that the line is practical but not very desirable, and the mark ○ indicates The symbol の means that the line is in a slightly better state, and the symbol ◎ means that the line is suitable for a wavelength-division multiplexed optical transmission line. This shows that the line is optimal for performing multiplex optical transmission.

The optical fiber SMF shown in Table 2 is an existing single mode optical fiber having zero dispersion at a wavelength of 1.31 μm.
DSF is a dispersion-shifted optical fiber having zero dispersion at a wavelength of 1.55 μm, and an SMF + MDCF optical fiber has a wavelength of 1.
The refractive index distribution modularized to a single mode optical fiber having zero dispersion at 31 μm means a line connecting a dispersion-compensating optical fiber of a matched type, and SMF + W
The DFCF optical fiber means an optical line formed by connecting a modularized dispersion compensating optical fiber having a W-shaped profile with a refractive index distribution to a single mode optical fiber, and the DFF optical fiber has a wavelength. This means a dispersion flat fiber having a zero dispersion at 1.55 μm and a zero dispersion slope in the wavelength band. The SMF + DFCF optical fiber is the optical transmission line in the present embodiment, and has a wavelength of 1.31 μm. Means a line in which the dispersion compensating optical fiber of the above embodiment is connected to a single mode optical fiber having zero dispersion with substantially the same length as the single mode optical fiber.

According to the study results in Table 2, the dispersion flat fiber DFF can also be said to be an optical transmission line suitable for wavelength division multiplexed optical transmission. However, this type of dispersion flat fiber is strictly regulated for each condition of the refractive index distribution. If the conditions are slightly deviated, the characteristics such as dispersion and dispersion slope change, so it is difficult to manufacture, and it is not necessarily a desirable line in order to make a versatile and stable wavelength multiplexing optical transmission line, It has been proved that the optical transmission line according to the present embodiment is an optimal versatile optical line, and is expected to be the most suitable next-generation wavelength multiplexing optical transmission line.

[0053]

Next, specific examples of the present invention will be described. First, as Example 1, a dispersion-compensating optical fiber having a W-type refractive index distribution shown in FIG. 1 and having Δ + of 1.44%, RΔ of −0.285, and Ra of 0.37 was prototyped. FIG. 3 shows the results of the chromatic dispersion characteristics of the dispersion compensating optical fiber of the first embodiment.

Similarly, it has a W-type refractive index distribution, and Δ +
A dispersion compensating optical fiber having 1.11%, RΔ of −0.375 and Ra of 0.33 was prototyped as Example 2. FIG. 3 similarly shows the results of the chromatic dispersion characteristics of the dispersion compensating optical fiber of the second embodiment. Note that FIG. 3 shows, as a comparative example, data obtained by multiplying the chromatic dispersion characteristic (SMF data) of a single mode optical fiber having a zero dispersion at a wavelength of 1.31 μm by −1 by −1.

As is apparent from the chromatic dispersion characteristics shown in FIG. 3, the data of the first and second embodiments both approach the data obtained by multiplying the chromatic dispersion characteristics of the single mode optical fiber by −1. By connecting the dispersion compensating optical fibers prototyped in Examples 1 and 2 to a single-mode optical fiber with almost the same length as the single-mode optical fiber, the wavelength of 1.55 μm caused by propagation through the single-mode optical fiber It can be seen that the dispersion of each wavelength can be effectively attenuated and compensated to almost zero dispersion on the terminal side of the dispersion compensating optical fiber.

Next, Table 3 shows the results of the characteristics of the dispersion compensating optical fiber according to another embodiment of the present invention in terms of the characteristics of the conventional dispersion compensating optical fiber (including those that are modularized and those that are not modularized). It is shown in a state of comparison with. In Table 3, W
Indicates that the refractive index distribution is a W-shaped profile, M indicates that the refractive index distribution is a matched type,
PMD indicates polarization dispersion. Further, the dispersion values in Table 3 are values at a wavelength of 1.55 μm, the bending loss indicates a transmission loss due to bending with a diameter of 20 mm, and the MFD indicates a mode field diameter.

[0057]

[Table 3]

The dispersion compensating optical fibers of Examples No. 2 and No. 3 in Table 3 have dispersions of -10 ps / nm / km and -15 ps / nm / k.
m and a low negative value, and the dispersion slope is -0.03 ps
/ Nm ² / km, shows -0.06ps / nm ² / km and less negative value. It can be seen that the value of the negative dispersion and the value of the negative dispersion slope are sufficiently smaller than those of the conventional dispersion compensating optical fiber modularized. Also, the bending loss has a mode field diameter of 5.5 μm or more, which is much larger than that of the conventional modularized dispersion compensating optical fiber.
It has been demonstrated that the optical line is the most suitable for high-speed and large-capacity wavelength-division multiplexing optical transmission with small transmission loss due to bending.

Further, in the case of the embodiment of No. 4 in Table 3, the dispersion value is -30 ps / nm / km, and also in the case of No. 4, the mode field diameter is as large as 5.5 μm or more, and the bending is The loss is small and good characteristics are obtained.

Table 3 shows a comparison of the characteristics of the polarization dispersion PMD. In general, when wavelength multiplexing or TDM (time division) multiplexing is performed using a wavelength multiplexing / demultiplexing device WDM or an optical circulator, a conventional method is used. When a dispersion compensating optical fiber DCF is connected to a single mode optical fiber SMF made by MCVD, the polarization dispersion PMD of the single mode optical fiber SMF
Is large, which imposes a large polarization dispersion PMD limit on the dispersion compensating optical fiber DCF. In other words, in severe cases, a specification that the polarization dispersion PMD is 0.15 or less is required.
The polarization dispersion PMD of each embodiment of the present invention shown in Table 3 is 0.
15 or less, which satisfies these specifications.

The comparative examples of No. 1, No. 5, and No. 6 in Table 3 show line-type (non-module type) dispersion compensating optical fibers. And the dispersion slope values are both too small, and the function of dispersion compensation is insufficient. Also,
Nos. 5 and 6 do not satisfy both the bending loss condition and the mode field condition, have large polarization dispersion, and have not obtained the characteristics of each embodiment of the present invention. Since the dispersion compensating optical fiber of the embodiment and each example according to the present invention is a low-loss transmission line, it is suitable for use as a laid cable.

FIG. 6 shows a wavelength division multiplexing optical transmission line in which the above-mentioned dispersion compensating optical fiber DCF according to the present invention and single mode optical fibers SMF1 and SMF2 having zero dispersion in the 1.31 μm wavelength band are connected in series. In this embodiment, a single-mode optical fiber SMF1, a dispersion-compensating optical fiber DCF, and a single-mode optical fiber SMF2 are arranged in this order between a wavelength multiplexer / demultiplexer WDM to which a transmitter and a receiver are connected. Connected, and the overall dispersion (total dispersion) of these lines is adjusted to zero (including almost zero), or the dispersion slope of these lines as a whole is adjusted to zero (including almost zero). It is.

In the dispersion-compensating optical fiber DCF as the cable for laying according to the present invention, the cable having a small bending loss can lengthen the cable laid, and the cable having a large mode field diameter has a non-linear effect (non-linear effect). ) Is desirable because it is small. Generally, a single mode optical fiber SM
The mode field diameters of F1 and SMF2 are larger than the mode field diameter of dispersion-compensating optical fiber DCF.

Therefore, by connecting in the order shown in the embodiment shown in FIG. 6, when an input signal is input from SMF1 or SMF2 of a single mode optical fiber,
In these single mode optical fibers, the input signal level is attenuated to some extent and then input to the dispersion compensating optical fiber DCF, and since the mode field diameter of the dispersion compensating optical fiber DCF of the present invention is large, the dispersion compensating optical fiber DCF
Non-linear effects generated in F can be reduced. That is,
The wavelength division multiplexing optical transmission line of this embodiment is suitable for two-way communication.

When a dispersion-compensating optical fiber DCF input signal is input from the beginning without passing through the single-mode optical fiber SMF, a high-power optical signal is input to the dispersion-compensating optical fiber DCF. A nonlinear effect occurs in the fiber DCF, and the input power cannot be increased. In this regard, in the case of the present embodiment, the input signal is first transmitted to the single mode optical fiber SM even in the bidirectional case.
After passing through F, where the signal power attenuates to some extent,
Since the light reaches the dispersion compensating optical fiber DCF having the same level of attenuation as that of a normal transmission optical fiber, the input power can be increased and the communication distance can be increased.

[0066]

According to the present invention, the dispersion value at a wavelength of 1.55 μm is-
The dispersion value and the dispersion of a single mode optical fiber in the 1.55 μm wavelength band are in the range of greater than 20 ps / nm / km and less than −10 ps / nm / km, and the ratio of the dispersion value and the dispersion slope in the 1.55 μm wavelength band is the same. Since the sign is opposite to the slope ratio and the absolute value is set to be almost equal,
By connecting the dispersion compensating optical fiber of the present invention having substantially the same length as the single mode optical fiber to the single mode optical fiber, the wavelength generated by propagating through the single mode optical fiber at the end side of the dispersion compensating optical fiber. The dispersion of the signal of each wavelength of the wavelength multiplexed optical transmission in the 1.55 μm band can be almost uniformly reduced to zero dispersion.

The dispersion value σ at a wavelength of 1.55 μm of a dispersion compensating optical fiber connected to a single mode optical fiber having zero dispersion in a wavelength band of 1.31 μm and transmitting an optical signal is represented by −
In the invention set in the range of 35 ps / nm / km ≦ σ ≦ −10 ps / nm / km, the wavelength generated by propagating through the single mode optical fiber at the terminal side of the dispersion compensating optical fiber.
The dispersion of a signal of a specific wavelength in the 1.55 μm band can be attenuated and compensated to zero (including almost zero) dispersion.

Further, the dispersion compensating optical fiber of the present invention is not a conventional modularized short optical fiber, but functions as an optical cable for optical transmission as well as compensating for the dispersion of a single mode optical fiber. ,
As described above, the dispersion value at a wavelength of 1.55 μm has a relatively small negative value such as −35 ps / nm / km or greater than −20 ps / nm / km and less than −10 ps / nm / km. Can be relaxed, and the mode field diameter can be increased accordingly, so that it has low nonlinearity. Therefore, distortion of the wavelength multiplexed light transmission waveform can be suppressed. As described above, since the dispersion compensating optical fiber of the present invention is a low-loss transmission line, it is suitable for use in a laid cable.

In particular, in the configuration in which the refractive index distribution is a W-shaped profile, the profile shape is simple, so that the design is easy, the transmission loss is small, and the negative dispersion and the negative dispersion slope can be easily reduced. The dispersion compensating optical fiber having the excellent properties of the present invention and the wavelength multiplexed light using the dispersion compensating optical fiber of the present invention can be manufactured more easily. It is possible to provide a transmission path at low cost.

Further, it has a W-type refractive index profile, and the relative refractive index difference Δ + of the center core to the clad is 1.
By setting it to 8% or less, the wavelength of the dispersion compensating optical fiber becomes 1.55
-35ps / nm / km or -20ps dispersion value in μm
/ Nm / km or more and -10 ps / nm / km or less, and by setting the relative refractive index difference Δ + to 1.0% or more, stabilization of the fluorine (F) layer doped into the side core can be achieved. Can be achieved. Also, if the value of Δ +
By suppressing it to 1.8% or less, an effect of expanding the mode field diameter of the dispersion compensating optical fiber can be obtained. According to the dispersion compensating optical fiber of the present invention, the mode field diameter can be reduced.
It can be 5.5 μm or more (particularly preferably 6 μm or more), and this value is sufficiently large as compared with the mode field diameter of the conventional modularized dispersion compensating optical fiber. An increase in transmission loss can be effectively prevented.

Furthermore, the ratio R.DELTA. Of the relative refractive index difference .DELTA.- of the side core to the cladding of the center core relative to the cladding R.sub.- is -0.25 or less, and the ratio Ra of the diameter a of the center core to the diameter b of the side core is Ra. (Ra
= A / b) in the range of 0.3 or more to 0.4 or less, thereby sufficiently increasing the dispersion compensation rate for signals of each wavelength in wavelength-multiplexed optical transmission in the 1.55 μm band generated by propagating through a single-mode optical fiber. Therefore, by using the wavelength-division multiplexed optical transmission line of the present invention, transmission loss due to bending is reduced, and high-speed, high-capacity, high-quality, high-capacity signal distortion free from signal distortion due to low nonlinearity in the wavelength band of 1.55 μm. It enables wavelength-division multiplexed optical communication and can be sufficiently used as a next-generation wavelength-division multiplexed optical line.

[Brief description of the drawings]

FIG. 1 is a diagram showing a profile of a refractive index distribution of an embodiment of a dispersion compensating optical fiber according to the present invention.

FIG. 2 illustrates the R of the dispersion compensating optical fiber according to the embodiment.
It is a figure of the simulation which shows the relationship between the value of a and a compensation rate.

FIG. 3 is a graph showing the chromatic dispersion characteristics of Examples 1 and 2 of the present invention in comparison with data obtained by multiplying the chromatic dispersion characteristic of a single mode optical fiber by −1.

FIG. 4 is an explanatory view showing another example of the profile of the refractive index distribution of the dispersion compensating optical fiber.

FIG. 5 is an explanatory diagram of a wavelength division multiplexed optical transmission line according to the embodiment.

FIG. 6 is an explanatory diagram of one embodiment of a wavelength division multiplexing optical transmission line.

[Explanation of symbols]

 1 center core 2 side core 3 clad

Claims (6)

[Claims] 1. A dispersion compensating optical fiber connected to a single mode optical fiber having a zero dispersion in a wavelength band of 1.31 μm and transmitting an optical signal, the dispersion value σ at a wavelength of 1.55 μm.
Is set in the range of −35 ps / nm / km ≦ σ ≦ −10 ps / nm / km. 2. In a dispersion compensating optical fiber connected to a single mode optical fiber having zero dispersion in a wavelength band of 1.31 μm and transmitting an optical signal, a dispersion value σ at a wavelength of 1.55 μm is obtained.
Is in the range of −20 ps / nm / km ≦ σ ≦ −10 ps / nm / km, and the dispersion value of the single mode optical fiber in the 1.55 μm wavelength band has the same ratio of the dispersion value and the dispersion slope in the 1.55 μm wavelength band. A dispersion compensating optical fiber characterized in that the sign of the sign is opposite to that of the ratio of the dispersion slope to the dispersion slope, and the absolute values are substantially equal. 3. A side core having a lower refractive index is disposed around a center core having the highest refractive index, and a cladding having a lower refractive index than the center core and having a higher refractive index than the side core is disposed around the side core. 3. The dispersion-compensating optical fiber according to claim 1, wherein the refractive index distribution has a W-shaped profile. 4. The relative refractive index difference Δ + of the center core to the cladding is in the range of 1.0% to 1.8%, and the ratio RΔ of the relative refractive index difference Δ− of the side core to the cladding relative to the relative refractive index difference Δ + of the center core to the cladding. (RΔ =
Δ− / Δ +) is -0.25 or less, and the ratio Ra of the center core diameter a to the side core diameter b (Ra = a /
4. The dispersion compensating optical fiber according to claim 3, wherein b) is in the range of 0.3 to 0.4. 5. The dispersion compensating optical fiber according to claim 1, wherein the mode field diameter is 5.5 μm or more. 6. A wavelength division multiplexing optical transmission line comprising a single mode optical fiber having zero dispersion in a wavelength band of 1.31 μm and a dispersion compensating optical fiber having substantially the same length connected thereto. A wavelength division multiplexing optical transmission line comprising the dispersion compensating optical fiber according to any one of claims 1 to 5.