JPH08234255A - Optical dispersion compensation circuit - Google Patents

Abstract

PURPOSE: To enable a long-distance and high-speed transmission by compensating the wavelength dispersion of optical fiber which is the cause to limit the transmission speed and transmission distance, in an optical frequency multiplex transmission system for multiplexing and transmitting light signals of plural frequencies. CONSTITUTION: The dispersion quantity for every wavelength is compensated by optical fiber in such a manner that the desired total dispersion quantity is obtd. for every wavelength by demultiplexing 8 the transmitted optical wavelength multiplex signals and the optical signals of the respective wavelengths are multiplexed. The dispersion quantity of every wavelength is otherwise adjusted by the optical fiber and the light signals thereof are respectively received.

Description

Detailed Description of the Invention

[0001]

The present invention is used in optical wavelength (frequency) multiplex communication systems. The present invention relates to an optical dispersion compensating circuit that compensates for deterioration of an optical signal due to higher-order dispersion, such as second-order dispersion, which is the slope of the group velocity dispersion value, and third-order dispersion, which is the differential value depending on its wavelength.

[0002]

2. Description of the Related Art FIG. 10 shows a configuration example of a conventional optical wavelength division multiplexing communication system. This optical WDM transmission system
In this example, an optical signal of 1.55 μm is transmitted by a 1.55 μm zero dispersion fiber.

In FIG. 10, reference numerals 1 and 2 indicate a transmitting side, reference numerals 3 and 4 indicate an optical relay section, and reference numerals 5 and 6 indicate a receiving side. That is, reference numerals 1 (1) to 1 (M) are
Optical transmitters for transmitting optical signals of different wavelengths for wavelength multiplexing, reference numeral 2 are transmitters 1 (1) to 1
It is a multiplexer for multiplexing optical signals of respective wavelengths transmitted from (M). Optical relay section 3 (1) to 3
(K) is an optical amplifier for compensating the loss of the transmission path, and 4 (1) to 4 (K) are optical fibers for transmitting an optical signal. Reference numeral 5 on the receiving side is a demultiplexer that demultiplexes the optical signals of the transmitters 1 (1) to 1 (M) that are multiplexed and transmitted, and reference numerals 6 (1) to 6 (M) are The optical receiver receives the optical signal demultiplexed by the demultiplexer 5.

FIG. 11 shows the relationship between the wavelength distribution of the conventional optical wavelength division multiplexing signal shown in FIG. 10 and the group velocity dispersion of a 1.55 μm zero-dispersion fiber. Here, the zero dispersion wavelength is 154
5 nm, the number of wavelengths and 15 from the wavelength lambda ₁ to lambda _15, lambda ₁₅ than the wavelength lambda ₁ of the optical wavelength multiplexing signal is 1 than 1531nm
It is assumed that they are arranged at 28 nm of 559 nm at 2 nm intervals.

Using a 1.55 μm zero dispersion fiber, 1.
Even when transmitting an optical signal in the 55 μm band, as shown in FIG. 11, the slope of the group velocity dispersion, that is, the second-order dispersion is 0.
Since it is about 07 ps / nm / km / nm, the group velocity dispersion value cannot be eliminated at all wavelengths of the optical wavelength division multiplexed signal.

FIG. 12 shows the total group velocity dispersion amount at each wavelength with respect to the distance in the wavelength arrangement of FIG. This FIG.
As shown in, it can be seen that the total dispersion amount, which is the integral value of the group velocity dispersion value with the distance, increases as the distance increases due to the secondary dispersion.

Therefore, in the case of transmitting an optical wavelength division multiplexed signal having a long distance and a high transmission rate, the wavelength dependency of the total dispersion amount occurs due to the influence of the secondary dispersion, and all the cases in which the optical wavelength division multiplexing communication is performed are performed. It was not possible to eliminate the signal deterioration due to group velocity dispersion at the wavelength of. The signal deterioration due to the group velocity dispersion has been a factor limiting the transmission distance. This point is related to literature: Limitation of transmission performance in optical amplification repeater Saito
Shigeru, The Institute of Electronics, Information and Communication Engineers Technical Report OCS94-26.

[0008]

As described above, conventionally, there is nothing that compensates for higher-order dispersion higher than the second-order dispersion.
When transmitting an optical wavelength division multiplexed signal over a long distance and at a high transmission speed, group velocity dispersion could not be compensated for at all wavelengths, which causes waveform deterioration due to group velocity dispersion and is a factor that limits the transmission distance. It was.

An object of the present invention is to suppress the deterioration of a signal due to higher-order dispersion when transmitting an optical wavelength division multiplexed signal at a long distance and a high transmission rate, and to transmit an optical wavelength division multiplexed signal at a long distance and a high speed. An object of the present invention is to provide an optical dispersion compensation circuit that enables the above.

Another object of the present invention is to provide an optical dispersion compensating circuit having a simple structure of the demultiplexer and the multiplexer by using one waveguide type array multiplexer / demultiplexer. It is in.

[0011]

The first aspect of the present invention is to:
In an optical dispersion compensating circuit for compensating the group velocity dispersion of an optical wavelength division multiplexed signal in which optical signals of a plurality of wavelengths are multiplexed, demultiplexing means for demultiplexing the input optical wavelength signal into respective wavelengths, and this demultiplexing means A plurality of optical fibers for inputting the optical signals of the respective wavelengths demultiplexed by the above and giving dispersions so as to obtain a desired total dispersion amount, and a multiplexing means for multiplexing the optical signals that have passed through the optical fibers. It is characterized by having.

The demultiplexing means and the demultiplexing means can be composed of one waveguide type array multiplexer / demultiplexer that performs demultiplexing.

In the waveguide type array multiplexer / demultiplexer, the optical wavelength division multiplexed signal is input to one input terminal, the multiplexed optical wavelength division multiplexed signal is output to one output terminal, and the other n wavelength division multiplexing signals are output. An output terminal (n is a natural number) and n input terminals are connected to each other by n optical fibers that provide dispersion (n
It can be a +1) × (n + 1) multiplexer / demultiplexer.

The waveguide type array multiplexer / demultiplexer is 2n
(N is a natural number) with 2 input terminals and 2 output terminals
n output terminals are connected by n optical fibers that provide dispersion, one input terminal is connected to an optical fiber that transmits dispersion-compensated optical wavelength multiplexed signals, and one input terminal is dispersion-compensated optical wavelengths. A 2n × 2n multiplexer / demultiplexer to which optical fibers for transmitting multiple signals are connected can be used.

A second aspect of the present invention is to input the demultiplexing means for demultiplexing the input optical wavelength signal into each wavelength and the optical signal of each wavelength demultiplexed by the demultiplexing means, A plurality of optical fibers for giving respective dispersions to obtain a desired total dispersion amount, and the output of each optical fiber is led to an optical receiver for receiving an optical signal of each wavelength.

The transmission line for transmitting the optical wavelength division multiplexed signal is an optical fiber having a zero dispersion wavelength of 1.55 μm, and the optical fiber for giving dispersion to an optical signal having a frequency lower than the zero dispersion wavelength is 1.3 μm. An optical fiber with zero dispersion,
The optical fiber that gives dispersion to an optical signal having a frequency higher than the zero dispersion wavelength is preferably a dispersion compensating fiber having a negative dispersion value.

[0017]

First, the operation of the invention of the first aspect will be described. The optical wavelength division multiplexed signal transmitted through the transmission line (optical fiber) is input from the input terminal of the demultiplexer, demultiplexed by the demultiplexer, and output for each wavelength. Since the dispersion of the transmission line is high-order dispersion at each wavelength, the total amount of dispersion to be compensated is different. Therefore, by connecting to the output of the demultiplexer an optical fiber having a total dispersion amount in which the positive and negative signs of the total dispersion amount of the transmission lines at each wavelength are connected, the dispersion can be compensated for in signals of all wavelengths. Then, by multiplexing the dispersion-compensated optical signals of the respective wavelengths by the multiplexer, it is possible to obtain an optical wavelength division multiplexed signal in which higher-order dispersion is compensated. By inserting this optical dispersion compensating circuit in the transmission line, it is possible to reduce signal deterioration due to high-order dispersion and enable long-distance, high-speed optical WDM transmission.

It should be noted that, using one waveguide type array multiplexer / demultiplexer as a demultiplexer and a multiplexer, an optical wavelength division multiplexed signal input from one input terminal is demultiplexed to a plurality of output terminals, and By connecting an optical fiber having a total dispersion amount in which the positive and negative values of the total dispersion amount of each wavelength are opposite to each other, and inputting again to the waveguide array multiplexer / demultiplexer and multiplexing to one terminal, It is possible to obtain an optical wavelength division multiplexed signal in which the following dispersion is compensated. As a result, since the single demultiplexer and the multiplexer can be used, the setting of the demultiplexing mechanism and the multiplexing mechanism can be simplified, and the configuration of the optical dispersion compensation circuit can be simplified.

The invention of the second aspect is to remove the multiplexer,
An optical signal in which higher-order dispersion is compensated by an optical fiber for each wavelength is directly guided to an optical receiver for each wavelength. That is, the optical wavelength division multiplexed signal transmitted through the transmission line is input from the input terminal of the demultiplexer, demultiplexed by the demultiplexer, and output for each wavelength. Since the dispersion of the transmission line is high-order dispersion at each wavelength, the total amount of dispersion to be compensated is different. Therefore, by connecting to the output of the demultiplexer an optical fiber having a dispersion amount of positive and negative, which compensates the dispersion amount of the transmission line at each wavelength, it is possible to compensate the dispersion of signals of all wavelengths. The signals of the respective wavelengths that have compensated for this dispersion are guided to the optical receiver and received.

As an optical fiber having a dispersion amount of positive and negative, which compensates for the dispersion amount of the transmission line at each wavelength,
When using a 55 μm zero-dispersion fiber as a transmission line,
For optical signals with a frequency lower than the zero-dispersion wavelength, 1.3
For the optical signal with a frequency higher than the zero-dispersion wavelength, use a dispersion-compensating fiber having a negative dispersion value of 1.55 μm and pass a fiber having a length that gives a desired dispersion amount. By compensating the dispersion generated in the transmission line, the total dispersion amount of each wavelength can be easily suppressed.

[0021]

Embodiments of the present invention will be described below with reference to the drawings.

(First Embodiment) FIG. 1 is a block diagram showing the arrangement of an optical dispersion compensating circuit according to the first embodiment of the present invention. The optical dispersion compensating circuit according to the first embodiment of the present invention uses an optical dispersion compensating circuit for compensating for group velocity dispersion of an optical wavelength multiplexed signal in which optical signals of a plurality of n wavelengths are multiplexed, A demultiplexer 8 as demultiplexing means for demultiplexing into each wavelength and a plurality of optical signals of respective wavelengths demultiplexed by the demultiplexing means are inputted and a plurality of dispersions are given so as to obtain desired total dispersion amounts. Optical fibers 9 (1) to 9 for dispersion compensation as optical fibers
(N) and the optical fibers 9 (1) to 9 for dispersion compensation
And a multiplexer 10 as a multiplexer for multiplexing the optical signals that have passed through (n). Reference numerals 7 and 11 are optical fibers for transmitting the optical wavelength division multiplexed signal.

The dispersion compensating operation in this optical dispersion compensating circuit will be described. It is assumed that n-wave multiplexed signals of wavelengths λ ₁ to λ _n are transmitted by the optical fiber 7 that transmits the optical wavelength multiplexed signal. This n-wave multiplexed signal is input to the input terminal of the demultiplexer 8, and the demultiplexer 8 demultiplexes the signals of each wavelength from λ ₁ to λ _n to output terminals 8 (1) to 8 (n). Is output. Dispersion compensating optical fibers 9 (1) to 9 (n) connected to the output terminals for each wavelength.
Thus, the total dispersion amount is compensated for each wavelength. The dispersion compensating optical fibers 9 (1) to 9 (n) are connected to a multiplexer 10, and optical signals of λ ₁ to λ _n are multiplexed by the multiplexer to form an optical fiber 11 forming a transmission line. Is transmitted as an optical wavelength division multiplexed signal.

The demultiplexer 8 and the demultiplexer 10 are specifically composed of multi-stage Mach-Zehnder type demultiplexers, diffraction gratings, waveguide type array demultiplexers, and the like.

Next, FIG. 2 shows the wavelength arrangement of the optical wavelength division multiplexed signal and the total group velocity dispersion amount of each wavelength when a 1.55 μm zero-dispersion fiber is transmitted for 500 km as the optical fiber 7 of the transmission line of FIG. Here, the zero-dispersion wavelength is 1545 nm, the number of wavelengths is 15, and the wavelength division multiplexed optical signal is 2 nm between 28 nm of 1531 nm to 1559 nm from λ ₁ to λ _15.
Spaced, higher order dispersion is 0.07 ps / nm / k
The secondary dispersion of m / nm is taken into consideration.

Here, as shown in FIG.
-490 ps / nm in nm, 490 in wavelength 1559 nm
There is a total group velocity dispersion of ps / nm. In order to compensate for this, as compensation optical fibers 9 (1) to 9 (15), a 1.3 μm zero-dispersion fiber having a length shown in FIG.
By connecting a dispersion compensating fiber having a dispersion value of an optical fiber of 55 μm and then multiplexing by a multiplexer 10, an optical wavelength division multiplexed signal in which higher-order dispersion is compensated at each wavelength as shown in FIG. 4 is obtained. It is possible to obtain. As shown in FIG. 4, the total dispersion amount within the 2 nm band of the optical demultiplexer 8 is ± 35 ps.
/ Nm can be suppressed, and the signal transmission band is ± 10 GHz
The total dispersion amount can be suppressed to ± 2.8 ps / nm. In this way, the dispersion compensating optical fibers 9 (1) to 9 (1
The length of 5) can be linearly determined by the wavelength.

Here, 1. of the 1.3 μm zero dispersion fiber is used.
The dispersion value at 55 nm is 17 ps / nm / km, and the dispersion compensating fiber is described in Reference: Improving the figure of merit of dispersion compensating fiber Masashi Onishi et al., 1994 Spring Meeting of the Institute of Electronics, Information and Communication Engineers C-366. The dispersion value was -82 ps / nm / km.

This optical dispersion compensating circuit is arranged in the optical repeater section of the optical wavelength division multiplexing communication system shown in FIG. Also,
It is arranged at the end of the transmission line to equalize the dispersion generated in the transmission line.

(Second Embodiment) Next, a second embodiment will be described with reference to FIG. In the second embodiment, as shown in FIG. 5, one (n + 1) × the demultiplexer 8 and the multiplexer 10 of the first embodiment are used.
It is replaced with the (n + 1) waveguide type array multiplexer / demultiplexer 13. That is, the optical fiber 7 for transmitting an optical wavelength division multiplexed signal in which n wavelengths are multiplexed is guided to one input terminal 13 (1) i of the waveguide type array multiplexer / demultiplexer 13 and one output terminal 13 (1) The optical wavelength division multiplexed signal multiplexed from o is transmitted through the dispersion compensating optical fiber 11. Λ ₁ is output to the output terminal 13 (2) o and is connected to the input terminal 13 (n + 1) i by the optical fiber 9 (1) for dispersion compensation. The output terminal and the input terminal are sequentially connected, and λ _n
Up to the input terminal 13 with an optical fiber 9 (n) for dispersion compensation
(N) is connected to i.

FIG. 6 shows this waveguide type array multiplexer / demultiplexer 13
2 shows the multiplexing / demultiplexing characteristic of As shown in FIG. 6, the optical wavelength multiplexed signals λ ₁ to λ _n input from the input terminal 13 (n + 1) o are output terminals 13 (2) o to 13 (n +), respectively.
1) Output from o. Output terminal 13 (2) o to 13
Optical fibers 9 (1) to 9 (n) for dispersion compensation for compensating for higher-order dispersion are connected to (n + 1) o, and connected to input terminals 13 (n + 1) i to 13 (2) i, respectively. The signals λ ₁ to λ _n are recombined by
It is output from the output terminal 13 (1) o. Through such a process, it is possible to obtain an optical wavelength division multiplexed signal in which higher-order dispersion is compensated for at each wavelength.

(Third Embodiment) Next, a third embodiment of the present invention will be described with reference to FIG. In the third embodiment, similar to the second embodiment, the demultiplexer 8 and the multiplexer 10 of the first embodiment are 2n ×.
It is replaced with a 2n waveguide type array multiplexer / demultiplexer 14.

This 2n × 2n waveguide array multiplexer / demultiplexer 1
FIG. 8 shows the multiplexing / demultiplexing characteristic of No. 4. As shown in this FIG.
The wavelength division multiplexed signals λ ₁ to λ _n input from the input terminal 14 (1) i are output terminal 14 (1) o to output terminal 1 respectively.
4 (n) o. Optical fibers 9 (1) to 9 (n) for dispersion compensation for compensating for higher-order dispersion are connected from the output terminal 14 (1) o to the output terminal (n) o, and the output terminals 14 (n + 1) are respectively connected. output terminal 14 (2
n) from signal λ ₁ to signal λ by connecting to o
_n is multiplexed again, and is output to the optical fiber 11 from the output terminal 13 (n + 1) i as an optical wavelength division multiplexed signal. Through such a process, it is possible to obtain an optical wavelength division multiplexed signal in which higher-order dispersion is compensated for at each wavelength.

(Fourth Embodiment) Next, a fourth embodiment of the book is shown in FIG.
Will be explained. This fourth embodiment is basically shown in FIG.
The same dispersion compensation as in the first embodiment shown in
Instead of multiplexing the demultiplexed optical wavelength division multiplexed signals, the total amount of dispersion is compensated by the dispersion compensating optical fibers 9 (1) to 9 (n) for each wavelength, and then the optical receiver 12 (1) is used as it is.
This is a configuration leading to ~ 12 (n). That is, the signals λ _{1 to} λ _n of the respective wavelengths demultiplexed by the demultiplexer 8 are guided to the dispersion compensating fiber and the 1.3 μm zero-dispersion fiber having different lengths, and the respective signals are optically multiplexed without being multiplexed. Receiver 1
It is received at 2 (1) to 12 (n). Also in this case, by adjusting the lengths of the dispersion compensating fiber and the 1.3 μ-zero dispersion fiber respectively connected to the demultiplexer 8, it is possible to receive the optical signal in which higher-order dispersion is compensated. .

The optical dispersion compensation circuit of the fourth embodiment is shown in FIG.
It is inserted in place of the demultiplexer 5 of the optical wavelength division multiplexing communication system shown in 0 to compensate for the dispersion generated in the transmission line.

In the above embodiments, the case where the zero-dispersion wavelength is set in the center of the optical wavelength division multiplexing signal wavelength band has been described. However, when the zero-dispersion wavelength is not in the signal wavelength band, 1.3 μm is set.
By adjusting the length of the zero-dispersion fiber or the dispersion-compensating fiber, it is possible to compensate higher-order dispersion.

[0036]

As described above, according to the present invention, the total amount of dispersion due to the difference in the group velocities of the optical fibers can be compensated, so that the deterioration of the signal due to the higher order dispersion can be suppressed.
Therefore, it is possible to perform optical wavelength division multiplexing transmission capable of transmitting an optical wavelength division multiplexing signal over a long distance at a high transmission rate.

Further, by using one waveguide type array multiplexer / demultiplexer, the configurations of the demultiplexer and the multiplexer can be simplified, and an optical dispersion compensation circuit having an all-optical configuration can be provided. .

Further, since the total dispersion amount can be compensated for each wavelength of the input signal of the optical receiver, it is possible to receive the signal with good reception quality.

Furthermore, since the dispersion amount provided by the dispersion compensating fiber can be adjusted for each wavelength only by its length, the total dispersion amount of all wavelengths transmitted through the transmission path can be suppressed to a small value.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an optical dispersion compensation circuit according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating a relationship between a total dispersion amount of a transmission line and a signal arrangement according to the first embodiment.

FIG. 3 is a diagram illustrating the length of a dispersion compensating optical fiber according to the first embodiment.

FIG. 4 is a diagram showing group velocity dispersion characteristics obtained by the dispersion compensation circuit of the first embodiment.

FIG. 5 is a block diagram showing a configuration of an optical dispersion compensation circuit according to a second embodiment of the present invention.

FIG. 6 is a diagram showing a multiplexing / demultiplexing characteristic of the waveguide array multiplexer / demultiplexer according to the second embodiment of the present invention.

FIG. 7 is a block diagram showing a configuration of an optical dispersion compensation circuit according to a third embodiment of the present invention.

FIG. 8 is a diagram showing a multiplexing / demultiplexing characteristic of the waveguide array multiplexer / demultiplexer according to the third embodiment of the present invention.

FIG. 9 is a block diagram showing a configuration of an optical dispersion compensation circuit according to a fourth embodiment of the present invention.

FIG. 10 is a diagram showing a configuration of a conventional optical wavelength multiplexing communication system.

FIG. 11: 1.55μ with a zero dispersion wavelength of 1545 nm
The figure explaining the relationship between the group velocity dispersion characteristic of m zero dispersion fiber, and arrangement | positioning of an optical wavelength signal.

FIG. 12 is a diagram showing a total dispersion amount with respect to a distance when a conventional optical fiber is used.

[Explanation of symbols]

1 (1) to 1 (M) Optical transmitter 2, 5, 10 Multiplexer 3 (1) to 3N (n), 31 (1) to 3N (n) Optical amplifier 4 (1) to 4 (K) Optical fiber 6 (1) to 6 (M), 12 (1) to 12 (n) Optical receiver 7, 11 Optical fiber for transmission 8 Demultiplexer 8 (1) to 8 (n) Output of multiplexer Terminals 9 (1) to 9 (n) Dispersion compensation optical fibers 13 and 14 Waveguide type array multiplexer / demultiplexer, 13 (1) i to 13 (n + 1) i, 14i to 14 (n +)
1) i input terminal 13 (1) o to 13 (n + 1) o, 14o to 14 (n +
1) o Output terminal

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroshi Toba 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (6)

[Claims] 1. An optical dispersion compensating circuit for compensating for group velocity dispersion of an optical wavelength division multiplexed signal in which optical signals of a plurality of wavelengths are multiplexed, and demultiplexing means for demultiplexing the input optical wavelength signal into respective wavelengths. , The optical signals of the respective wavelengths demultiplexed by the demultiplexing means are input, and a plurality of optical fibers that give respective dispersions to obtain a desired total dispersion amount and the optical signals that have passed through the optical fibers are multiplexed. An optical dispersion compensating circuit comprising: a multiplexing unit. 2. The optical dispersion compensating circuit according to claim 1, wherein the demultiplexing means and the demultiplexing means are composed of one waveguide type array multiplexer / demultiplexer that performs demultiplexing. 3. A waveguide type array multiplexer / demultiplexer receives an optical wavelength division multiplexed signal at one input terminal, outputs the optical wavelength division multiplexed signal at one output terminal, and outputs n other optical wavelength division multiplexed signals. An output terminal (n is a natural number) and n input terminals give variance n
Connected by optical fiber giving book dispersion (n +
The optical dispersion compensating circuit according to claim 2, wherein the optical dispersion compensating circuit is 1) × (n + 1) multiplexer / demultiplexer. 4. A waveguide type array multiplexer / demultiplexer has 2n (n is a natural number) input terminals and output terminals, and 2n output terminals are connected by an optical fiber giving n dispersions. A 2n × 2n multiplexer / demultiplexer in which one input terminal is connected to an optical fiber that transmits a dispersion-compensated optical wavelength division multiplexed signal, and one input terminal is connected to an optical fiber that transmits a dispersion-compensated optical wavelength division multiplexed signal The optical dispersion compensation circuit according to claim 2. 5. An optical dispersion compensating circuit for compensating for group velocity dispersion of an optical wavelength division multiplexed signal in which optical signals of a plurality of wavelengths are multiplexed, and demultiplexing means for demultiplexing the input optical wavelength signal into respective wavelengths. The optical signal of each wavelength demultiplexed by the demultiplexing means is input, and a plurality of optical fibers are provided which respectively provide dispersion so as to obtain a desired total dispersion amount, and the output of each optical fiber is An optical dispersion compensating circuit which is guided to an optical receiver for receiving an optical signal. 6. A transmission line for transmitting an optical wavelength division multiplexed signal is 1.
An optical fiber having a zero-dispersion wavelength of 55 μm, and an optical fiber giving dispersion to an optical signal having a frequency lower than the zero-dispersion wavelength is an optical fiber having a zero-dispersion wavelength of 1.3 μm, and an optical fiber having a frequency higher than the zero-dispersion wavelength. 6. The optical dispersion compensating circuit according to claim 1, wherein the optical fiber that gives dispersion to the signal is a dispersion compensating fiber having a negative dispersion value.