JP2001298414A - Optical transmission line equalizer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide optical transmission line equalizers, by which a transmission loss in an optical communication system will not change greatly over a wide variety of wavelengths. SOLUTION: In the optical communication system, where an optical signal outputted from a transmitter 11 is transmitted through optical fibers 12a, 12b, 12c and a receiver 15 receives the signal, the optical transmission line equalizer 13a is placed between the optical fibers 12a and 12b. The equalization characteristic of the optical transmission line equalizer 13a is selected, so as to cancel the wavelength dependence of a loss caused in the optical fiber 12a, that is, so as to decrease the loss at a wavelength causing a greater loss produced in the optical fiber 12a.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an equalizer used in an optical communication system.

[0002]

2. Description of the Related Art A signal transmitted through an optical fiber is attenuated due to a loss, and the loss is dependent on the wavelength in principle. FIG. 6 is a diagram illustrating an example of the wavelength dependence of the loss of the dispersion-shifted fiber. Wavelength 15 called C band
Considering a band of 30 nm to 1560 nm and a band of a wavelength of 1560 nm to 1610 nm called an L band,
In the case of the fiber of FIG.
There is a loss difference of km. Therefore, the power of the signal in the transmission path or the signal received by the receiver differs depending on the wavelength.

On the other hand, wavelength division multiplexing (WDM) for transmitting light of a plurality of wavelengths through one optical fiber.
A method called multiplexing (transmission) has attracted attention.
In WDM transmission, the transmission capacity can be increased by increasing the number of signals having different wavelengths to be propagated in an optical fiber. For this reason, signal wavelengths that have not been used before have come to be used.

[0004]

Generally, the gain of an amplifier differs depending on the wavelength. In WDM transmission, for example, by using a gain equalizer for an amplifier, consideration has been given to maintaining flatness in which the gain does not change with wavelength.

However, the wavelength dependence of the loss has not been considered for the optical fiber as the transmission line. Therefore, even if the characteristic of the gain of the amplifier with respect to the wavelength is flattened,
The power received by the receiver differs depending on the wavelength, and there is a problem that transmission characteristics and reception characteristics are deteriorated.
Further, the difference in reception power depending on the wavelength increases as the distance over which the signal light is transmitted increases. Further, when the communication band in which signal light propagates in WDM transmission or the like is widened to cover the C band, L band, S band, and 1.3 μm band, the difference in wavelength caused by the optical fiber in the communication band increases. .

SUMMARY OF THE INVENTION The present invention provides an optical transmission line or the like that prevents the transmission loss in an optical communication system from changing significantly over a wide range of wavelengths by taking into account the wavelength dependence of the loss occurring in the optical fiber that is the transmission line. It is an object to provide a gasifier.

[0007]

According to an aspect of the present invention, there is provided an optical communication system including an optical transmission line having a plurality of sections for transmitting an optical signal. An optical transmission line equalizer having a predetermined characteristic connected to one of the sections of the optical transmission line, wherein the predetermined characteristic is such that a loss to the optical signal is a loss generated in the section of the optical transmission path. Is set so that it becomes smaller as the wavelength becomes larger.

According to the first aspect of the present invention, it is possible to reduce the wavelength dependence of the loss generated in the optical transmission line.

According to a second aspect of the present invention, there is provided a relay having an optical transmission path having a plurality of sections for transmitting an optical signal, and an amplifier connected to one of the sections of the optical transmission path and amplifying the optical signal. Used in an optical communication system including a device, an optical transmission line equalizer having predetermined characteristics included in the repeater, the predetermined characteristics, the loss for the optical signal,
It is characterized in that the characteristic is set such that the larger the loss that occurs in the section of the optical transmission path, the smaller the wavelength.

According to the second aspect of the present invention, even when a repeater having an amplifier is provided with an optical transmission line equalizer, it is possible to reduce the wavelength dependence of the loss generated in the optical transmission line.

According to a third aspect of the present invention, in the optical transmission line equalizer according to the second aspect, the repeater further includes a gain equalizer for flattening a gain characteristic with respect to a wavelength of the amplifier. Features.

According to the third aspect of the present invention, even when the optical transmission line equalizer is provided in the repeater further including the gain equalizer, the wavelength dependence of the loss generated in the optical transmission line can be reduced. .

Further, in the invention of claim 4, claims 1 to 3 are provided.
In the optical transmission path equalizer according to the above, the predetermined characteristic is,
The sum of the loss of the optical transmission line equalizer and the loss occurring in the section of the optical transmission line is substantially constant over a predetermined range of wavelengths of the optical signal.

According to the fourth aspect of the present invention, it is possible to substantially eliminate the wavelength dependence of the loss generated in the optical transmission line.

According to a fifth aspect of the present invention, in the optical transmission line equalizer according to the second aspect, the predetermined characteristic includes a loss of the optical transmission line equalizer and a section of the optical transmission line. The sum of the generated loss and the gain of the amplifier included in the repeater including the optical transmission line equalizer is substantially constant over a predetermined range of wavelengths of the optical signal.

According to the fifth aspect of the present invention, it is possible to substantially eliminate the wavelength dependence of the characteristic obtained by combining the loss of the optical transmission line and the gain of the amplifier.

[0017]

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

FIG. 1 is a configuration diagram of an optical communication system using an optical transmission line equalizer according to an embodiment of the present invention. The optical communication system of FIG. 1 includes a transmitter 11 and one of the optical transmission lines.
Optical fibers 12a, 12b and 12c as sections,
Optical transmission line equalizers 13a, 13b and 13c and amplifier 1
4 a and 14 b and a receiver 15. The optical communication system shown in FIG.
The data is transmitted by the optical fibers 12a, 12b, and 12c in the section, and is received by the receiver 15.

The output of the transmitter 11 is connected to one end of an optical fiber 12a, and the other end of the optical fiber 12a is connected to the input of an optical transmission line equalizer 13a. Optical transmission path equalizer 13
The output of a is connected to the input of amplifier 14a.

Similarly, the output of the amplifier 14a is connected to one end of the optical fiber 12b in the next section, and the other end of the optical fiber 12b is connected to the input of the optical transmission line equalizer 13b. The output of the optical transmission line equalizer 13b is connected to the input of the amplifier 14b, and the output of the amplifier 14b is connected to the optical fiber 12c of the next section.
Is connected to one end. The other end of the optical fiber 12c is connected to the input of the optical transmission line equalizer 13c, and the output of the optical transmission line equalizer 13c is connected to the receiver 15.

The optical fibers 12a, 12b and 12c are
For example, as shown in FIG. 6, there is a characteristic that the loss per 1 km of the length is different with respect to the wavelength. Optical fiber 12
a is 80 km, the optical fiber 12
The difference due to the wavelength of the loss occurring at a
In the range from nm to 1610 nm, it is about 0.8 dB. This difference increases as the length of the transmission path increases. The optical transmission line equalizers 13a, 13b, and 13c are optical fibers 12a, 12b
And 12c are equalized.

FIG. 2 is a diagram showing an example of characteristics of the optical fiber 12a, the optical transmission line equalizer 13a, and the optical fiber after equalization in FIG. The equalization characteristic with respect to the wavelength of the optical transmission line equalizer 13a shown in FIG.
Is set so as to cancel the wavelength dependence of the loss occurring in the optical fiber 12a, that is, to reduce the loss with respect to the wavelength where the loss occurring in the optical fiber 12a is large. Further, it is desirable that the optical fiber loss after equalization obtained as the sum of the loss of the optical fiber 12a and the loss of the optical transmission line equalizer 13a is substantially constant with respect to the wavelength. As shown in FIG. 2, the optical fiber loss after equalization by the optical transmission line equalizer 13a in FIG. 1 is substantially constant in the wavelength range of 1460 nm to 1630 nm. The connection loss due to the connection of the optical transmission line equalizer 13a is about 0.3 dB.

The optical transmission line equalizer 13 having such characteristics
a is a dielectric multilayer film, a long-period fiber grating,
By using an etalon filter or the like, it can be obtained by a known configuration.

Similarly, the optical transmission line equalizers 13b and 13c
Are designed to negate the wavelength dependence of the loss that occurs in the optical fibers 12b and 12c, respectively.

The optical transmission line equalizers 13a, 13b and 13c are connected to optical fibers 12a, 12b and 12c, respectively.
Has been described, the optical transmission line equalizers 13a, 13b, and 13c may be connected to the input sides of the optical fibers 12a, 12b, and 12c, respectively.
That is, the optical transmission line equalizer 13a is connected between the transmitter 11 and the optical fiber 12a, and the optical transmission line equalizer 13b is connected to the amplifier 1
4a and the optical fiber 12b, an optical transmission line equalizer 13
c may be provided between the amplifier 14b and the optical fiber 12c.

Also, the case where three optical transmission line equalizers are connected to the optical fiber has been described. In an optical communication system having optical fibers in more sections, the characteristics of the optical fibers in each section are described. An optical transmission path equalizer for equalization may be provided.

If the difference in signal magnitude depending on the wavelength is small,
There are the following advantages. 1. In an optical communication system that performs WDM transmission, the size of a signal of each wavelength received by a receiver is substantially equal. For this reason, the difference in reception characteristics between signals having different wavelengths is reduced, and it is not necessary to adjust the sensitivity or the like of the light receiving element for each wavelength after demultiplexing. Therefore, the receiving unit for each wavelength can be shared, and the configuration of the receiver can be simplified. 2. The effect of the non-linear effect generated in the transmission path can be relatively reduced. Considering cross-phase modulation as an example of the non-linear effect, when there are two lights having different amplitudes propagating in an optical fiber, the effects of cross-phase modulation are equal when both amplitudes are equal, but the amplitudes of both are equal. This is because, when they are not equal, light having a small amplitude is subjected to larger cross-phase modulation than light having a large amplitude. 3. A fiber amplifier such as an erbium-doped fiber amplifier (EDFA) utilizing the stimulated emission phenomenon can be used with desired characteristics. Since the stimulated emission increases as the signal input increases, it is possible to prevent the expected performance of the fiber amplifier from being fully exerted due to the difference in signal magnitude between signals of different wavelengths.

In an amplifier utilizing a stimulated emission phenomenon, such as an EDFA, if the signal magnitude differs depending on the wavelength, when the signal is amplified, the difference in the signal magnitude due to the wavelength is emphasized, and the amplification is increased. If repeated, the difference will gradually increase. As explained above,
Such a phenomenon can be avoided by using an optical transmission line equalizer that reduces the difference in signal magnitude depending on the wavelength during signal transmission.

(First Modification) FIG. 3 is a configuration diagram of an optical communication system using an optical transmission line equalizer according to a first modification of the present embodiment. The optical communication system shown in FIG.
1 and an optical fiber 1 as one section of the optical transmission line, respectively.
2a, 12b and 12c, and repeaters 20a and 20b
, An optical transmission path equalizer 13c, and a receiver 15. In the optical communication system of FIG. 3, in the optical communication system of FIG. 1, the optical transmission line equalizer 13a and the amplifier 14a are replaced by a repeater 20a, and the optical transmission line equalizer 13b and the amplifier 14b are replaced by a repeater 20b. Things.

The repeater 20a has an amplifier 21a and an optical transmission line equalizer 13a, and the repeater 20b has an amplifier 21b and an optical transmission line equalizer 13b. In the repeater 20a, the optical fiber 12a is connected to the input of the amplifier 21a. The output of the amplifier 21a is the optical transmission line equalizer 13a
The output of the optical transmission line equalizer 13a is connected to the optical fiber 12b. The same applies to the repeater 20b.

Optical transmission line equalizers 13a, 13b and 13c
Are the same as those shown in FIG. 1, and these equalization characteristics are set so as to cancel the wavelength dependence of the loss occurring in the optical fibers 12a, 12b and 12c, respectively.

As described above, the optical transmission line equalizers 13a and 13a
3b, the optical fibers 12a and 12b may be incorporated in the repeaters 20a and 20b, respectively, as in FIG.
Can cancel the wavelength dependence of the loss caused by the above.

In this modification, in the repeater 20a,
Although the case where the optical fiber 12a is connected to the input of the amplifier 21a and the output of the amplifier 21a is input to the optical transmission line equalizer 13a has been described, the amplifier 21a and the optical transmission line equalizer 13a are exchanged. You may.

When the optical fiber 12a is connected to the input of the amplifier 21a, the noise characteristic of the repeater 20a can be made relatively good. Further, the amplifier 21a
Is reduced by the optical transmission line equalizer 13a.
Non-linear optical effects depending on the magnitude of the optical signal are less likely to occur.

On the other hand, when the optical fiber 12a is connected to the input of the optical transmission line equalizer 13a, the output of the amplifier 21a is not attenuated by the optical transmission line equalizer 13a. The output of the repeater 20a can be increased. Further, the magnitudes of the optical signals for each wavelength input to the amplifier 21a become uniform, and the performance of the fiber amplifier such as the EDFA can be sufficiently exhibited.

(Second Modification) FIG. 4 is a configuration diagram of an optical communication system using an optical transmission line equalizer according to a second modification of the present embodiment. The optical communication system shown in FIG.
1 and an optical fiber 1 as one section of the optical transmission line, respectively.
2a, 12b and 12c, and repeaters 30a and 30b
, An optical transmission path equalizer 13c, and a receiver 15. The optical communication system of FIG. 4 is different from the optical communication system of FIG.
a and 30b.

The repeater 30a includes an amplifier 31a, a gain equalizer (GEQ) 32a, and an optical transmission line equalizer 13a. The repeater 30b includes an amplifier 31b, a gain equalizer 3a.
2b and an optical transmission path equalizer 13b. Repeater 3
At 0a, optical fiber 12a is connected to the input of amplifier 31a. The output of the amplifier 31a is input to the gain equalizer 32a, and the output of the gain equalizer 32a is input to the optical transmission line equalizer 13a. The output of the optical transmission line equalizer 13a is connected to the optical fiber 12b. The same applies to the repeater 30a.

As the amplifiers 31a and 31b, for example, an EDFA is used. The gain equalizers 32a and 32b equalize the gain characteristics of the amplifiers 31a and 31b, that is, make the gain characteristics with respect to wavelength substantially constant in a certain range of wavelengths.

FIG. 5 is a diagram showing an example of the gain characteristic of the amplifier (EDFA) 31a, the transmission characteristic of the gain equalizer 32a, and the amplifier gain characteristic after equalization. In FIG.
The gain characteristic of the amplifier 31a is indicated by ■. The gain equalizer 32a has three long-period fiber gratings FG1 to FG3 connected in series. The dotted line indicates the respective characteristics of the long-period fiber gratings FG1 to FG3, and the solid line indicates the transmission characteristics of the gain equalizer 32a obtained by adding these characteristics.

The gain characteristic of the amplifier 31a after equalization, that is, the output characteristic of the gain equalizer 32a
The sum of the gain characteristic of the gain equalizer 1a and the transmission characteristic of the gain equalizer 32a is indicated by squares. As described above, in the case of FIG. 5, the characteristics of the amplifier 31a after the equalization are almost constant in the wavelength range of about 1535 nm to about 1565 nm, and have flattened gain characteristics.

Optical transmission line equalizers 13a, 13b and 13c
Are the same as those shown in FIG. 1, and these equalization characteristics are set so as to cancel the wavelength dependence of the loss occurring in the optical fibers 12a, 12b and 12c, respectively.

As described above, the optical transmission line equalizers 13a and 13a
Even if 3b is incorporated in repeaters 30a and 30b provided with gain equalizers 32a and 32b, respectively, it is possible to cancel the wavelength dependence of the loss occurring in the optical fibers 12a and 12b as in the case of FIG.

The amplifiers 31a and 31b are EDF
A was explained using A, but PDFA (praseodymium-doped
fiber amplifier), TDFA (thulium-doped fiber)
amplifier). Further, the gain equalizers 32a and 32b can be configured by an etalon filter or the like.

In place of the gain equalizer 32a and the optical transmission line equalizer 13a, these may be integrated into an optical transmission line equalizer 33a. That is, the wavelength dependence of the loss of the optical fiber 12a shown in FIG. 2 and the wavelength dependence of the gain of the amplifier 31a shown in FIG. 5 may be collectively equalized by the optical transmission line equalizer 33a. Good.

In this case, the characteristics of the optical transmission line equalizer 33a are determined by the loss of the optical transmission line equalizer 33a and the optical fiber 1
2a plus the gain of amplifier 31a, ie,
The combined loss and gain characteristics are set to be substantially constant with changes in wavelength. For example, the characteristics of the optical transmission line equalizer 33a are the same as those of the equalizer of FIG. 2 and the GEQ of FIG.
It is assumed that the characteristic and the characteristic are added.

Similarly, instead of the gain equalizer 32b and the optical transmission line equalizer 13b, these may be integrated into an optical transmission line equalizer 33b.

In the first and second modifications, the optical transmission line equalizers 13a and 13b shown in FIGS.
The optical fibers 12a and 12a on the transmitter side as viewed from each
Although the case where the characteristics of b are equalized has been described,
The optical transmission line equalizers 13a and 13b may equalize the characteristics of the optical fibers 12b and 12c on the receiver side as viewed from each. In this case, the optical transmission line equalizer 13c becomes unnecessary. Instead, an optical transmission line equalizer for equalizing the characteristics of the optical fiber 12a is inserted between the transmitter 11 and the optical fiber 12a, Transmission path equalizer 13
It is desirable that a also equalizes the characteristics of the optical fiber 12a.

In the above embodiment, the loss characteristic of the optical fiber is equalized by using the optical transmission line equalizer, but the wavelength dependence of the optical fiber itself may be flattened without using this. This can be realized by using an absorber or a scatterer for light having a relatively small attenuation in a predetermined range of wavelengths. In addition, a grating may be applied to a part of an optical fiber as an optical transmission line to have a function as an equalizer.

The optical communication system is not limited to one that performs WDM transmission, but may be any one that transmits a wideband optical signal.

[0050]

As described above, according to the optical transmission line equalizer of the present invention, it is possible to prevent the transmission loss in the optical communication system from largely changing with respect to the wavelength. Therefore, the configuration of the receiver can be simplified, and the performance of the fiber amplifier can be sufficiently exhibited.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an optical communication system using an optical transmission line equalizer according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating an example of characteristics of the fiber, the optical transmission line equalizer, and the fiber after equalization in FIG. 1;

FIG. 3 is a configuration diagram of an optical communication system using an optical transmission line equalizer according to a first modification.

FIG. 4 is a configuration diagram of an optical communication system using an optical transmission line equalizer according to a second modification.

FIG. 5 is a diagram illustrating an example of a gain characteristic of an amplifier (EDFA), a transmission characteristic of a gain equalizer, and an amplifier gain characteristic after equalization.

FIG. 6 is a diagram illustrating an example of the wavelength dependence of the loss of a dispersion-shifted fiber.

[Explanation of symbols]

11 Transmitter 12a, 12b, 12c Optical fiber (optical transmission line) 13a, 13b, 13c, 33a, 33b Optical transmission line equalizer 14a, 14b, 21a, 21b, 31a, 31b Amplifier 15 Receiver 20a, 20b, 30a , 30b Repeaters 32a, 32b Gain equalizer

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04B 10/17 10/16 (72) Inventor Kazuo Imamura 4-3 Ikejiri, Itami-shi, Hyogo Mitsubishi Electric Cable Industrial Stock F term in the Itami Works (reference) 5K002 CA01 CA10 DA02 FA01 5K046 AA07 EE12

Claims (5)

[Claims] 1. An optical communication system having an optical transmission line having a plurality of sections for transmitting an optical signal and having a predetermined characteristic connected to one of the sections of the optical transmission path. An optical transmission line, wherein the predetermined characteristic is set such that a loss with respect to the optical signal becomes smaller as a wavelength at which a loss occurring in a section of the optical transmission line becomes larger. Equalizer. 2. An optical communication system comprising: an optical transmission path having a plurality of sections for transmitting an optical signal; and a repeater connected to one of the sections of the optical transmission path and having an amplifier for amplifying the optical signal. An optical transmission line equalizer used in a system and having a predetermined characteristic included in the repeater, wherein the predetermined characteristic is such that a loss to the optical signal is large in a loss occurring in a section of the optical transmission line. An optical transmission line equalizer characterized in that characteristics are set such that the characteristics become smaller as the wavelength becomes longer. 3. The optical transmission line equalizer according to claim 2, wherein the repeater further includes a gain equalizer that flattens a gain characteristic of the amplifier with respect to a wavelength. Chemist. 4. The optical transmission line equalizer according to claim 1, wherein the predetermined characteristic is a sum of a loss of the optical transmission line equalizer and a loss occurring in a section of the optical transmission line. Is substantially constant over a predetermined range of wavelengths of the optical signal. 5. The optical transmission line equalizer according to claim 2, wherein the predetermined characteristics are a loss of the optical transmission line equalizer, a loss occurring in a section of the optical transmission line, and the optical transmission line. An optical transmission line equalizer, wherein the sum of the gain of an amplifier included in the repeater including the line equalizer is substantially constant over a predetermined range of wavelengths of the optical signal.