JP2007025714A - Optical transmission system using raman amplification - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical transmission system reducing differences in Raman gain on an up line and a down line through easy control even in case of trouble occurrence in system constitution performing Raman amplification with exciting light supplied from an optical repeater station common to the up line and down line. <P>SOLUTION: In the optical transmission system of the present invention, the up line and down line comprise a plurality of transmission sections, and signal light is Raman-amplified and transmitted by supplying the exciting light generated by the optical repeater station 30 common to the lines to the respective transmission sections. In the optical repeater station 30, exciting light power supplied to the transmission sections of the up line, and exciting light power supplied to the transmission sections of the down line are individually settable. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical transmission system for transmitting signal light by Raman amplification, and in particular, an optical transmission system for supplying pump light generated by a common optical repeater station for uplink and downlink to perform Raman amplification of signal light. About.

  Conventionally, in a long-distance optical transmission system, an optical signal is converted into an electrical signal, and transmission is performed using an optical regenerative repeater that performs timing regeneration (retiming), waveform equalization (reshaping), and identification regeneration (regenerating). . At present, however, optical amplifiers have been put into practical use, and optical amplification repeater transmission systems using optical amplifiers as linear repeaters are being studied. By replacing the optical regenerative repeater with an optical amplifying repeater, the number of parts in the repeater can be greatly reduced, ensuring reliability and a significant cost reduction. Also, as one of the methods for realizing an increase in capacity of an optical transmission system, a wavelength division multiplexing (WDM) optical transmission system that multiplexes and transmits optical signals having two or more different wavelengths on one transmission line has attracted attention. ing.

  In the WDM optical amplifying and repeating transmission system that combines the WDM optical transmission system and the optical amplifying and repeating transmission system, it is possible to amplify optical signals having two or more different wavelengths by using an optical amplifier. With a simple configuration (economical), large capacity and long distance transmission can be realized.

FIG. 2 is a diagram illustrating a configuration example of a general WDM optical amplification repeater transmission system.
The system in FIG. 2 includes, for example, an optical transmission station 101, an optical reception station 102, an optical transmission path 103 that connects these transmission / reception stations, and a plurality of optical transmission paths 103 that are arranged at required intervals along the optical transmission path 103. And an optical repeater station 104. The optical transmission station 101 includes a plurality of optical transmitters (E / O) 101A that output a plurality of optical signals having different wavelengths, a multiplexer 101B that wavelength-multiplexes the plurality of optical signals, and a multiplexer 101B. A post-amplifier 101C that amplifies the WDM signal light to a required level and outputs it to the optical transmission line 103; The optical receiving station 102 amplifies the WDM signal light of each wavelength band transmitted via the optical transmission path 103 to a required level, and outputs light from the preamplifier 102C into a plurality of optical signals according to the wavelength. It has a branching filter 102B for dividing and a plurality of optical receivers (O / E) 102A for receiving and processing a plurality of optical signals. The optical transmission path 103 has a plurality of transmission sections that connect the optical transmitting station 101 and the optical receiving station 102 respectively. The WDM signal light transmitted from the optical transmission station 101 propagates through the optical transmission path 103, is optically amplified by the optical relay station 104 arranged for each transmission section, propagates again through the optical transmission path 103, and repeats this. To the optical receiving station 102.

  For example, an erbium-doped optical fiber amplifier (EDFA) is generally used for the optical repeater station 104 of the WDM optical amplification repeater transmission system as described above. Recently, it has been actively studied to use Raman amplification in combination with EDFA. Furthermore, a repeaterless optical transmission system that does not use an optical repeater has been proposed. In this repeaterless optical transmission system, control of distributed Raman amplification by remote-pumping has been studied.

  In Raman amplification using an optical fiber as an amplification medium, the gain is obtained in inverse proportion to the mode field diameter of the optical fiber used. Therefore, an optical fiber having a small mode field diameter is suitable for Raman amplification. For example, a negative dispersion fiber having a chromatic dispersion and a dispersion slope of opposite signs to the chromatic dispersion and dispersion slope (first derivative with respect to the wavelength of chromatic dispersion) of a 1.3 μm zero dispersion fiber has a mode field diameter of about 5 μm. Thus, a larger Raman gain can be obtained because it is smaller than the mode field diameter of a 1.3 μm zero dispersion fiber or dispersion shifted fiber (DSF, NZ-DSF) generally used as an optical transmission line.

In the following description, positive dispersion fibers such as 1.3 μm zero dispersion fiber and dispersion shifted fiber are abbreviated as + D fibers, and negative dispersion fibers as described above are −D fibers. I will abbreviate.
In the conventional WDM optical repeater transmission system, a method of managing the chromatic dispersion of the optical transmission line is used so that the deterioration of the transmission characteristics due to the nonlinear effect of the optical transmission line is reduced. For example, in Non-Patent Document 1 below, cumulative dispersion generated in a plurality of transmission sections using a mixed transmission path in which + D fibers and −D fibers are combined is dispersed in one transmission section (compensation section) using + D fibers. Compensation techniques have been proposed. Specifically, the average zero dispersion wavelength of the optical transmission line shown in this document 1 is about 1551 nm, and the signal light wavelength is 1544.5 nm to 1556.5 nm. The chromatic dispersion of each transmission section using the mixed transmission path and the compensation section using the + D fiber is about −2 ps / nm / km and about +20 ps / nm / km. According to such a configuration, the group speed of the signal light and the spontaneous emission light and the group speed of the signal lights become different, so that the interaction time of the nonlinear effect can be shortened, and four-wave mixing (Four wave) It is possible to reduce deterioration of transmission characteristics due to mixing (FWM) and cross phase modulation (XPM). In addition, since the average zero dispersion wavelength is within the signal light wavelength, it is possible to reduce deterioration of transmission characteristics due to self phase modulation (SPM) and wavelength dispersion.

  When the distributed Raman amplifier is applied to the conventional WDM optical repeater transmission system as described above, the + D fiber has a larger mode field diameter than the -D fiber, so that it is difficult to efficiently obtain the Raman gain. For this reason, in order to obtain the required Raman gain sufficient to compensate for the loss in the section using the + D fiber, a very large pump light power is required, which is disadvantageous in terms of the reliability of the pump light source. There is. In order to overcome the above-described problems, for example, a Raman amplification fiber having a smaller mode field diameter and a shorter length than that of the −D fiber is applied so that the Raman gain can be obtained more efficiently. It is conceivable to make up for losses in the fiber section.

  However, when the Raman amplification fiber having a small mode field diameter as described above is used, there arises a problem that the nonlinear effect in the signal light generated in the Raman amplification fiber is increased. In addition, since a configuration for realizing distributed Raman amplification in -D fiber and a configuration for realizing concentrated Raman amplification in Raman amplification fiber are required, the types of optical amplifiers increase. There are also drawbacks. Furthermore, there is a problem that transmission waveform distortion due to nonlinear effects in the entire optical transmission system increases.

  As another method for managing chromatic dispersion using a mixed transmission line that combines a + D fiber and a -D fiber, for example, as described in Non-Patent Document 2 below, for example, the accumulation of mixed transmission lines per transmission section A method has also been proposed in which the chromatic dispersion is made substantially zero and the accumulated dispersion generated during transmission is compensated at the terminal station.

However, when the cumulative chromatic dispersion of the mixed transmission line per transmission section is almost zero, the waveform deterioration due to SPM is reduced, but the bit arrangement between wavelengths is the same in a region that receives the same amount of nonlinear effect. Occurs in each transmission section, and waveform degradation due to XPM becomes a problem.
Therefore, the inventors of the present application, in an optical transmission system, constitutes an optical transmission line by using a mixed transmission line in which positive cumulative chromatic dispersion is generated and a mixed transmission line in which negative cumulative chromatic dispersion is generated. Has been proposed (for example, Japanese Patent Application No. 2001-077571).

FIG. 3 is a diagram illustrating a configuration example of the optical transmission system according to the above-described prior invention. In this system configuration, in each optical repeater station, the same pumping light source is used for the uplink and the downlink, and a unit system in which the upper and lower lines are set is excited by one Raman amplifier (Raman amplifier) to perform Raman amplification. Done. According to such a configuration, pump light is incident on the -D fiber in all transmission sections, so that a Raman gain can be obtained efficiently and at the same time, one type of optical amplifier can be provided.
M. Murakami et al., “Long-haul 16x10 WDM transmission experiment using higher order fiber dispersion management technique”, pp.313-314, ECOC'98, 1998. CRDavidson et al., “1800 Gb / s transmission of one hundred and eighty 10 Gb / s WDMchannels over 7, 000 km using the full EDFA C-band”, PD25, OFC2000, 2000.

  However, in the optical transmission system as shown in FIG. 3, when distributed Raman amplification of signal light is performed in each transmission section, for example, as shown in FIG. Since there are places where two common transmission sections with positive and negative average chromatic dispersion are excited using a common Raman amplifier, it is difficult to control the Raman gain in each line.

  That is, in order to adjust the chromatic dispersion, a transmission section in which the average chromatic dispersion of the section abbreviated as “+” in FIG. 4 is positive and a transmission section in which the chromatic dispersion of the section average abbreviated as “−” is negative. -D fibers are set to have different lengths. Therefore, when the Raman amplifiers for the uplink and downlink are shared by each repeater as shown in FIG. 3, the average wavelength dispersion of the section is shown as shown by the dotted line in FIG. There are places where two types of transmission sections having different positive and negative are excited using one Raman amplifier.

  FIG. 5 is an exemplary diagram showing the dotted line portion of FIG. 4 in an enlarged manner. Here, the excitation light output from the excitation light source 200 is bifurcated by the optical coupler 201, and one of the branched lights passes through the multiplexer 202A and the + D fiber 203A and the + D fiber 203A and the average wavelength dispersion becomes negative. -It is given from the -D fiber 203B side to the uplink transmission section in which the length of the D fiber 203B is adjusted. The other split light is -D with respect to the downlink transmission section in which the lengths of the + D fiber 203A and the -D fiber 203B are adjusted so that the chromatic dispersion of the section average becomes positive through the multiplexer 202B. It is given from the fiber 203B side. At this time, the Raman gain generated in each transmission section of the uplink and the downlink varies greatly depending on the length of the -D fiber 203B, and thus greatly differs between the uplink side and the downlink side. End up.

  As a specific example, in order to obtain an average chromatic dispersion of −2.7 ps / nm / km for a transmission section of 50 km, the lengths of the + D fiber 203A and the −D fiber 203B are 32.5 km and 17. It can be set to 5 km. On the other hand, in order to set the average chromatic dispersion to +2.7 ps / nm / km for the transmission section of 50 km, the lengths of the + D fiber 203A and the -D fiber 203B may be set to 36.7 km and 13.3 km, respectively. Is possible. Here, an uplink transmission section in which the section average chromatic dispersion is set to -2.7 ps / nm / km, and a downlink transmission section in which the section average chromatic dispersion is set to +2.7 ps / nm / km. And a common Raman amplifier are used, the difference between the Raman gains in the uplink and the downlink is approximately 0.5 dB when calculated using the parameters in Table 1 below.

また、上り回線および下り回線の各伝送区間を共通のラマン増幅器により励起して分布型ラマン増幅を行う光伝送システムについては、混合伝送路を適用するか否かに拘わらず、障害等の発生時におけるラマン利得の制御が難しくなってしまうという欠点がある。

For optical transmission systems that perform distributed Raman amplification by exciting each transmission section of the uplink and downlink with a common Raman amplifier, regardless of whether a mixed transmission path is applied or not There is a drawback that it becomes difficult to control the Raman gain.

  That is, for example, as shown in FIG. 6, assuming that a failure occurs in the optical transmission line near the optical repeater station, a required optical fiber (broken line in the figure) is inserted at the failure point for repair or the like. Sometimes. At this time, if each transmission section of the uplink and the downlink is excited by a common Raman amplifier, the Raman gain of the transmission section in which the optical fiber is inserted (uplink side in FIG. 6) and the optical fiber are inserted. The Raman gain in the transmission section (the downlink side in FIG. 6) that did not exist becomes different, and it becomes difficult to control the Raman gain in the entire optical transmission system. In addition, in order to reduce the influence on the entire system at the time of occurrence of the above-mentioned failure, for example, measures such as lowering the supply power of the Raman amplifier corresponding to the transmission section in which the optical fiber is inserted or the like are taken. If applied, there arises a problem that the optical SNR of the transmitted light deteriorates.

  The present invention has been made paying attention to the above points, and in a system configuration that performs Raman amplification with pumping light supplied from a common optical repeater station for uplink and downlink, even when a failure or the like occurs. An object of the present invention is to provide an optical transmission system capable of reducing a difference in Raman gain in each line by easy control.

  In order to achieve the above object, an optical transmission system using Raman amplification according to the present invention has an uplink and a downlink in which signal light propagates in opposite directions, and each line is composed of a plurality of transmission sections. An optical transmission system for transmitting signal light by Raman amplification by supplying pumping light generated by a common optical repeater station to each transmission line for each transmission section. The common optical repeater station can individually set the pumping light power supplied to the uplink transmission section and the pumping light power supplied to the downlink transmission section.

  In such a configuration, it becomes possible to supply pumping light having different power settings from the optical repeater station common to each line to each of the uplink transmission section and the downlink transmission section. Even when the average chromatic dispersion is different between positive and negative, or when an optical fiber is inserted when a failure occurs, the difference in Raman gain in each line can be reduced.

  As described above, according to the optical transmission system using Raman amplification of the present invention, it is possible to easily control the Raman gain even when a failure or the like occurs.

Hereinafter, the best mode for carrying out the present invention will be described with reference to the drawings.
FIG. 1 is a diagram showing a main configuration of an optical transmission system according to an embodiment of the present invention.
In FIG. 1, this optical transmission system includes an optical repeater station 30 provided with an uplink and a downlink between optical terminal stations (not shown) and having a Raman amplifier that collectively performs distributed Raman amplification of each line. A basic configuration is provided in which a plurality of light sources are arranged at regular intervals on the optical transmission line. In FIG. 1, a specific Raman amplifier configuration example for one of the plurality of optical repeaters 30 is shown in a portion surrounded by an alternate long and short dash line, and for the other optical repeaters 30, It is represented by a set of triangles drawn on the uplink and downlink.

Adjacent optical repeaters 30 are connected to each other by the optical transmission line 10 so as to correspond to the uplink and the downlink, respectively. The optical transmission line 10 used in each transmission section is preferably a mixed transmission line in which a + D fiber and a -D fiber are combined, but may be a general optical fiber transmission line.
The Raman amplifier provided in each optical repeater station 30 includes, for example, two pumping light sources 31a and 31b that generate pumping light for Raman amplification, and pumping light generated by each pumping light source 31a and 31b as an uplink and A multiplexer / demultiplexer 32 for distributing to the downlink, a multiplexer 33A for supplying one excitation light demultiplexed by the multiplexer / demultiplexer 32 to the uplink, and a multiplexer for supplying the other excitation light to the downlink. And a waver 33B.

  Each pumping light source 31a, 31b is a known light source that generates pumping light having a wavelength set according to the wavelength band of signal light transmitted through each line. The power of each pumping light generated by each pumping light source 31a, 31b can be set individually. Here, the power of the pumping light generated by the pumping light source 31a is Pa, and the pumping light source 31b The power of the generated excitation light is Pb.

The multiplexer / demultiplexer 32 multiplexes the respective excitation lights output from the respective excitation light sources 31a and 31b, branches them at a predetermined power ratio n: 1 (n ≠ 1), and branches them to the respective multiplexers 33A and 33B. Output each.
Each of the multiplexers 33A and 33B propagates the excitation light output from the multiplexer / demultiplexer 32 in the direction opposite to the propagation direction of the signal light with respect to the optical transmission line 10 connected on the upstream side. The signal light propagated through the upstream optical transmission line 10 is transmitted and transmitted to the downstream optical transmission line 10.

  In each optical repeater station 30 having the above-described configuration, the excitation light with the power Pa output from the excitation light source 31a and the excitation light with the power Pb output from the excitation light source 31b are combined by the multiplexer / demultiplexer 32. After being waved, it is branched to n: 1 and sent to the multiplexers 33A and 33B. At this time, the power of the pumping light sent from the multiplexer / demultiplexer 32 to the upstream side multiplexer 33A is Pa × n + Pb, and the pumping light sent from the multiplexer / demultiplexer 32 to the downstream side multiplexer 33B is transmitted. The power is Pa + Pb × n. Therefore, by appropriately adjusting the power Pa and Pb of each pump light output from each pump light source 31a and 31b, the power of the pump light supplied to the optical transmission line 10 on the uplink side and the uplink side It is possible to set the power of the pumping light supplied to the optical transmission line 10 independently.

  As a result, for example, a mixed transmission line combining + D fiber and −D fiber is used as the optical transmission line 10 for the uplink and the downlink, and the transmission average in which the average wavelength dispersion is positive and the average average wavelength dispersion are negative. For the system configuration in which the transmission sections to be appropriately arranged are the codes that supply the pumping light to two types of transmission sections in which the average chromatic dispersion is different between positive and negative in the uplink and the downlink In the optical repeater station 30 of the difference, the pump light having different power settings is given to the mixed transmission line of each line by the Raman amplifier common to each line. For this reason, even if the length of the -D fiber is different between the positive mixed transmission line and the negative mixed transmission line as in the prior art, the excitation light supplied to each line according to the difference in the lengths. By adjusting the respective powers, the Raman gains generated in the respective mixed transmission lines can be made substantially equal.

  Further, not only when a mixed transmission line is used as the optical transmission line 10, but also when a general optical fiber transmission line is used, a repair light can be used when a failure or the like as shown in FIG. Depending on whether or not the fiber is inserted into the corresponding transmission section, the power of the pumping light supplied to the transmission section can be adjusted for each line, so that the difference in Raman gain between the uplink and the downlink can be reduced. Become.

  In the above embodiment, the power Pa and Pb of the pumping light generated by the pumping light sources 31a and 31b is individually set, so that the power of each pumping light supplied to the uplink and the downlink is independent. However, the present invention is not limited to this. For example, the excitation light generated by the excitation light source using a known device capable of adjusting the power branching ratio as the multiplexer / demultiplexer 32 is supplied to each line. The pump light power may be controlled independently. In this case, it is also possible to distribute the pumping light generated by one pumping light source to each line by a duplexer with a variable power branching ratio.

It is a figure which shows the principal part structure of the optical transmission system by one Embodiment of this invention. It is a figure which shows the structural example of a general WDM optical amplification repeater transmission system. It is a figure which shows the structural example of the optical transmission system concerning prior invention. It is a figure for demonstrating the fault regarding the optical transmission system of FIG. It is the illustration which expanded and showed the dotted-line part in FIG. It is a figure for demonstrating the problem at the time of the occurrence of a failure etc. in the conventional system configuration.

Explanation of symbols

10, 10P, 10N Optical transmission line 30 Optical repeater station 31a, 31b Excitation light source 32 Multiplexer / demultiplexer 33A, 33B Multiplexer

Claims (2)

The signal light has an uplink and a downlink that propagate in opposite directions, each of which is composed of a plurality of transmission sections, and each transmission section is generated by an optical repeater common to each line. An optical transmission system for transmitting signal light by Raman amplification by supplying pumping light,
The optical amplification station common to each line can individually set the pumping light power supplied to the uplink transmission section and the pumping light power supplied to the downlink transmission section. Optical transmission system using An optical transmission system using Raman amplification according to claim 1,
The optical repeater station common to each line has a plurality of pumping light sources capable of individually setting the power of the pumping light to be generated and 2 at a predetermined power ratio after combining the pumping lights generated by the pumping light sources. An optical transmission system using Raman amplification, characterized by having a multiplexer / demultiplexer that branches and distributes to an uplink and a downlink.

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