BIDIRECTIONAL OPTICAL TRANSMISSION SYSTEM AND BIDIRECTIONAL OPTICAL TRANSMISSION METHOD

A bidirectional optical transmission system includes a first optical transmission line including a first repeater (30A), a second optical transmission line including a second repeater (30B), and C+L band transmitting/receiving devices (10, 20) connected to each other through these transmission lines so that they can communicate with each other. The C+L band transmitting/receiving device (10) transmits an optical signal in a C-band to the first optical transmission line and transmits an optical signal in an L-band to the second optical transmission line, and the C+L band transmitting/receiving device (20) transmits an optical signal in the C-band to the second optical transmission line and transmits an optical signal in the L-band to the first optical transmission line. The first repeater (30A) separates the optical signal in the C-band and the optical signal in the L-band bidirectionally propagating through the first optical transmission line from each other, and separately amplifies the separated optical signals, and the second repeater (30B) separates the optical signal in the C-band and the optical signal in the L-band bidirectionally propagating through the second optical transmission line from each other, and separately amplifies the separated optical signals. In this way, it is possible to expand a transmission capacity and alleviate deterioration in a transmission characteristic.

TECHNICAL FIELD

The present invention relates to a bidirectional optical transmission system and a bidirectional optical transmission method.

BACKGROUND ART

Because of the recent increase in communication traffic, it has been desired to realize a large-capacity optical transmission system. For example, in an optical submarine transmission system, large-capacity transmission is realized by using a method for increasing the number of wavelengths by reducing intervals between signal bands, or a method for increasing a transmission capacity at each wave by increasing its bit rate. However, the large-capacity transmission using these methods seems to have already been saturated. Therefore, it has been desired to further expand the transmission capacity.

At the present, optical signals are transmitted by using a C-band (a Conventional band) in an optical transmission system. However, use of an L-band (a Long wavelength band) in addition to the C-band has been studied in order to expand the transmission capacity in the future. The C-band is a band around 1550 nm and the L-band is a band around 1580 nm.

FIG. 1shows a configuration of an optical transmission system using a C+L band. This optical transmission system includes a C+L band repeater100, C+L band transmitting/receiving devices110and120, and optical fibers201to204.

The C+L band repeater100includes C-band repeaters101and102, L-band repeaters103and104, C/L band signal demultiplexers105and106, and C/L band signal multiplexers107and108. The C/L band signal demultiplexers106and105have the same structure as each other, and each of them has an input port and first and second output ports. The C/L band signal multiplexers107and108have the same structure as each other, and each of them has first and second input ports and an output port.

The input port of the C/L band signal demultiplexer105is optically coupled to the optical fiber201. The input port of the C/L band signal demultiplexer106is optically coupled to the optical fiber204. The output port of the C/L band signal multiplexer107is optically coupled to the optical fiber202. The output port of the C/L band signal multiplexer108is optically coupled to the optical fiber203. Note that “optically coupled” means that optical signals propagate through a coupling part without causing substantial losses.

The first and second output ports of the C/L band signal demultiplexer105are optically coupled to the first and second input ports, respectively, of the C/L band signal demultiplexer107through optical transmission lines. The C-band repeater101is disposed in the optical transmission line between the first output port and the first input port. The L-band repeater103is disposed in the optical transmission line between the second output port and the second input port.

The first and second output ports of the C/L band signal demultiplexer106are optically coupled to the first and second input ports, respectively, of the C/L band signal demultiplexer108through optical transmission lines. The C-band repeater102is disposed in the optical transmission line between the first output port and the first input port. The L-band repeater104is disposed in the optical transmission line between the second output port and the second input port.

The C-band optical transmitting devices1111to111noutput optical signals having mutually different wavelengths in the C-band. The L-band optical transmitting devices1121to112noutput optical signals having mutually different wavelengths in the L-band. The C/L band wavelength multiplexing unit115multiplexes the optical signals in the C-band output from the C-band optical transmitting devices1111to111nand the optical signals in the L-band output from the L-band optical transmitting devices1121to112n, and outputs a wavelength-multiplexed optical signal in a C+L band. The wavelength-multiplexed optical signal in the C+L band output from the C/L band wavelength multiplexing unit115is supplied to the C/L band signal demultiplexer105through the optical fiber201.

The C/L band signal demultiplexer105separates the wavelength-multiplexed optical signal in the C+L band into a wavelength-multiplexed optical signal in the C-band and a wavelength-multiplexed optical signal in the L-band. The wavelength-multiplexed optical signal in the C-band is amplified by the C-band repeater101and then supplied to the first input port of the C/L band signal multiplexer107. The wavelength-multiplexed optical signal in the L-band is amplified by the L-band repeater103and then supplied to the second input port of the C/L band signal multiplexer107. The C/L band signal multiplexer107multiplexes the optical signal in the C-band and the optical signal in the L-band. The C/L band signal multiplexer107outputs a wavelength-multiplexed optical signal in the C+L band. The wavelength-multiplexed optical signal in the C+L band output from the C/L band signal multiplexer107is supplied to the C/L band wavelength separating unit126through the optical fiber202.

The C/L band wavelength separating unit126separates the wavelength-multiplexed optical signal in the C+L band according to the wavelength. The C/L band wavelength separating unit126supplies the optical signals in the respective wavelengths in the C-band to the C-band optical receiving devices1231to123n. The C/L band wavelength separating unit126supplies the optical signals in the respective wavelengths in the L-band to the L-band optical receiving devices1241to124n.

The C-band optical transmitting devices1211to121noutput optical signals having mutually different wavelengths in the C-band. The L-band optical transmitting devices1221to122noutput optical signals having mutually different wavelengths in the L-band. The C/L band wavelength multiplexing unit125multiplexes the wavelength-multiplexed optical signals in the C-band output from the C-band optical transmitting devices1211to121nand the wavelength-multiplexed optical signals in the L-band output from the L-band optical transmitting devices1221to122n, and outputs a wavelength-multiplexed optical signal in the C+L band. The wavelength-multiplexed optical signal in the C+L band output from the C/L band wavelength multiplexing unit125is supplied to the C/L band signal demultiplexer106through the optical fiber204.

The C/L band signal demultiplexer106separates the wavelength-multiplexed optical signal in the C+L band into a wavelength-multiplexed optical signal in the C-band and a wavelength-multiplexed optical signal in the L-band. The wavelength-multiplexed optical signal in the C-band is amplified by the C-band repeater102and then supplied to the first input port of the C/L band signal multiplexer108. The wavelength-multiplexed optical signal in the L-band is amplified by the L-band repeater104and then supplied to the second input port of the C/L band signal multiplexer108. The C/L band signal multiplexer108multiplexes the wavelength-multiplexed optical signal in the C-band and the wavelength-multiplexed optical signal in the L-band. The C/L band signal multiplexer108outputs a wavelength-multiplexed optical signal in the C+L band. The wavelength-multiplexed optical signal in the C+L band output from the C/L band signal multiplexer108is supplied to the C/L band wavelength separating unit116through the optical fiber203.

The C/L band wavelength separating unit116separates the wavelength-multiplexed optical signal in the C+L band according to the wavelength. The C/L band wavelength separating unit116supplies the optical signals in the respective wavelengths in the C-band to the C-band optical receiving devices1131to113n. The C/L band wavelength separating unit116supplies the optical signals in the respective wavelengths in the L-band to the L-band optical receiving devices1141to114n.

In the above-described optical transmission system using the C+L band, the transmission line formed by the optical fibers201and202, the C-band repeater101, the L-band repeater103, the C/L band signal demultiplexer105, and the L-band signal multiplexer107is a transmission line for upstream transmission. Meanwhile, the transmission line formed by the optical fibers203and204, the C-band repeater102, the L-band repeater104, the C/L band signal demultiplexer106, and the L-band signal multiplexer108is a transmission line for downstream transmission. The C+L band transmitting/receiving device110can transmit an optical signal in the C-band and an optical signal in the L-band by using the upstream transmission line. Further, the C+L band transmitting/receiving device120can transmit an optical signal in the C-band and an optical signal in the L-band by using the downstream transmission line. As described above, since the optical transmission system using the C+L band can transmit optical signals in the C+L band in both directions, it is possible to increase the transmission capacity in comparison to that in the optical transmission system that transmits/receives optical signals by using only the C-band.

As another optical transmission system, Patent Literature 1 discloses a bidirectional WDM (Wavelength Division Multiplexing) optical transmission system that performs bidirectional WDM optical transmission by using one optical transmission line. This bidirectional WDM optical transmission system includes a first WDM optical transmitter/receiver and a second WDM optical transmitter/receiver. The first WDM optical transmitter/receiver receives an optical signal in a C-band (a band at 1580 nm) and transmits an optical signal in an L-band (a band at 1550 nm). The second WDM optical transmitter/receiver receives an optical signal in the L-band and transmits an optical signal in the C-band.

In this bidirectional WDM optical transmission system, an optical signal in the L-band is transmitted from the first WDM optical transmitter/receiver to the second WDM optical transmitter/receiver and an optical signal in the C-band is transmitted from the second WDM optical transmitter/receiver to the first WDM optical transmitter/receiver.

As yet another optical transmission system, Patent Literature 2 discloses a single-fiber bidirectional optical wavelength multiplexing transmission system that transmits/receives an upstream wavelength-multiplexed signal and a downstream wavelength-multiplexed signal by using a single optical transmission line. The band of the upstream wavelength-multiplexed signal is different from that of the downstream wavelength-multiplexed signal.

CITATION LIST

Patent Literature

Patent Literature 1: Japanese Unexamined Patent Application Publication No. H11-284576

Patent Literature 2: Japanese Unexamined Patent Application Publication No. 2004-7146

SUMMARY OF INVENTION

Technical Problem

However, there is the following problem in the optical transmission system shown inFIG. 1.

Each of the C-band repeaters101and102and the L-band repeaters103and104cannot achieve a satisfactory amplification characteristic when its input optical signal contains a component having an out-of-band wavelength. In order to suppress the influence of this out-of-band wavelength on the amplification characteristic, each of the C/L band signal demultiplexers105and106uses a C/L demultiplexing filter having a transmission characteristic in which a guard band (an unused band) is set at the boundary between the transmission wavelength range in the C-band and that in the L-band. In this C/L demultiplexing filter, it is necessary to set a guard band having a somewhat large width in order to cut off optical signals in the L-band on a first output port (C-band transmission port) side and cut off optical signals in the C-band on a second output port (L-band transmission port) side. However, when the guard band having a large width is set, the wavelength band of the main signal is reduced. As a result, the transmission capacity is reduced.

Further, when an optical signal in the L-band remains on the C-band transmission port side of the C/L band signal demultiplexers105and106, coherent crosstalk in which the remaining optical signal in the L-band is multiplexed with other optical signals in the L-band occurs in the C/L band signal multiplexers107and108. Similarly, when an optical signal in the C-band remains on the L-band transmission port side of the C/L band signal demultiplexers105and106, coherent crosstalk in which the remaining optical signal in the C-band is multiplexed with other optical signals in the C-band occurs in the C/L band signal multiplexers107and108. Since such coherent crosstalk affects the transmission characteristic of optical signals, the quality of received optical signals may deteriorate.

In the bidirectional WDM optical transmission system disclosed in Patent Literature 1, the bandwidth of optical signals transmitted in the upstream direction is different from that in the downstream direction. For example, the L-band is used in the upstream direction and the C-band is used in the downstream direction. In this case, even if the transmission capacity is expanded by the method for increasing the number of wavelengths by reducing intervals between signal bands or the like, the transmission capacity in the upstream direction can be expanded only within the L-band range and the transmission capacity in the downstream direction can be expanded only within the C-band range. Therefore, in consideration of the recent increase in communication traffic, it is desired to further increase the transmission capacity.

In the single-fiber bidirectional optical wavelength multiplexing transmission system disclosed in Patent Literature 2, the bandwidth of optical signals transmitted in the upstream direction is different from that in the downstream direction. Therefore, problems similar to those that occur in the system disclosed in Patent Literature 1 occur.

An object of the present invention is to provide a bidirectional optical transmission system and a bidirectional optical transmission method capable of solving the above-described problems, and thereby expanding a transmission capacity and alleviating deterioration in a transmission characteristic.

Solution to Problem

In order to achieve the above-described object, according to an aspect of the present invention, the following bidirectional optical transmission system is provided. That is, a bidirectional optical transmission system includes:a first optical transmission line including a first repeater;a second optical transmission line including a second repeater; andfirst and second transmitting/receiving devices connected to each other through the first and second optical transmission lines so that they can communicate with each other, each of first and second transmitting/receiving devices being configured to transmit and receive an optical signal in a first band and an optical signal in a second band different from the first band; in whichthe first transmitting/receiving device transmits the optical signal in the first band to the first optical transmission line and transmits the optical signal in the second band to the second optical transmission line,the second transmitting/receiving device transmits the optical signal in the first band to the second optical transmission line and transmits the optical signal in the second band to the first optical transmission line,the first repeater separates the optical signal in the first band and the optical signal in the second band bidirectionally propagating through the first optical transmission line from each other, and separately amplifies the separated optical signals, andthe second repeater separates the optical signal in the first band and the optical signal in the second band bidirectionally propagating through the second optical transmission line from each other, and separately amplifies the separated optical signals.

According to another aspect of the invention, the following bidirectional optical transmission method is provided. That is, a bidirectional optical transmission method includes transmitting an optical signal in a first band and an optical signal in a second band different from the first band in both a first direction and a second direction opposite to the first direction by using a first optical transmission line including a first repeater and a second optical transmission line including a second repeater, the bidirectional optical transmission method further including:in the first direction, transmitting the optical signal in the first band to the first optical transmission line, transmitting the optical signal in the second band to the second optical transmission line, and in the first repeater, separating the optical signal in the first band and the optical signal in the second band bidirectionally propagating through the first optical transmission line from each other and separately amplifying the separated optical signals; andin the second direction, transmitting the optical signal in the first band to the second optical transmission line, transmitting the optical signal in the second band to the first optical transmission line, and in the second repeater, separating the optical signal in the first band and the optical signal in the second band bidirectionally propagating through the second optical transmission line from each other and separately amplifying the separated optical signals.

Advantageous Effects of Invention

According to the present invention, it is possible to expand a transmission capacity and alleviate deterioration in a transmission characteristic.

DESCRIPTION OF EMBODIMENTS

First Embodiment

FIG. 2is a block diagram showing a configuration of a bidirectional optical transmission system according to a first example embodiment of the present invention.

Referring toFIG. 2, the bidirectional optical transmission system includes C+L band transmitting/receiving devices10and20, a C+L band repeater30, and optical fibers201to204. The C+L band transmitting/receiving device10is optically coupled to the C+L band repeater30through the optical fibers201and203, and the C+L band transmitting/receiving device20is optically coupled to the C+L band repeater30through optical fibers202and204.

The optical fibers201and202form a first optical transmission line, and the optical fibers202and203form a second optical transmission line. Note that the direction from the C+L band transmitting/receiving device10to the C+L band transmitting/receiving device20is referred to as an upstream direction. The direction from the C+L band transmitting/receiving device20to the C+L band transmitting/receiving device10is referred to as a downstream direction.

The C+L band transmitting/receiving device10transmits an optical signal in a C-band to the C+L band transmitting/receiving device20through the first optical transmission line, and transmits an optical signal in an L-band to the C+L band transmitting/receiving device20through the second optical transmission line. Meanwhile, the C+L band transmitting/receiving device20transmits an optical signal in the L-band to the C+L band transmitting/receiving device10through the first optical transmission line, and transmits an optical signal in the C-band to the C+L band transmitting/receiving device10through the second optical transmission line. In the first optical transmission line, the optical signal in the C-band is transmitted in the upstream direction and the optical signal in the L-band is transmitted in the downstream direction. In the second optical transmission line, the optical signal in the L-band is transmitted in the upstream direction and the optical signal in the C-band is transmitted in the downstream direction.

Configurations of the C+L band transmitting/receiving device apparatuses10and20and the C+L band repeater30will be described hereinafter in detail.

Firstly, a configuration of the C+L band transmitting/receiving device10is described in detail.

The C+L band transmitting/receiving device10includes C-band optical transmitting devices111to11n, L-band optical transmitting devices121to12n, C-band optical receiving devices131to13n, L-band optical receiving devices141to14n, a C-band wavelength multiplexing unit15A, an L-band wavelength multiplexing unit15B, a C-band wavelength separating unit16A, an L-band wavelength separating unit16B, and optical circulators17and18. The number “n”, which indicates the number of devices, can be arbitrarily determined.

Each of the C-band wavelength multiplexing unit15A and the L-band wavelength separating unit16B is optically coupled to the optical fiber201through the optical circulator17. Each of the L-band wavelength multiplexing unit15B and the C-band wavelength separating unit16A is optically coupled to the optical fiber203through the optical circulator18.

Existing optical circulators such as polarization-dependent optical circulators and polarization-independent optical circulators can be used as the optical circulators17and18. The polarization-dependent optical circulator includes, for example, a Faraday rotator, two polarizers that are arranged at a relative angle of 45° so as to sandwich the Faraday rotator from both sides thereof, and a magnet covering the circumference of the Faraday rotator. The polarization-independent optical circulator includes, for example, a Faraday rotator, a ½-wavelength plate, a polarizing beam splitter, a reflection mirror, and so on. Since these optical circulators are well known, only their operations are described hereinafter while omitting descriptions of their structures.

FIG. 3Aschematically shows an image representing operations performed by the optical circulator17. As shown inFIG. 3A, the optical circulator17includes three ports P1to P3. In the optical circulator17, an optical signal input from the port P1is output from the port P2. Further, an optical signal input from the port P2is output from the port P3.

The port P1of the optical circulator17is optically coupled to the output port of the C-band wavelength multiplexing unit15A. The port P2of the optical circulator17is optically coupled to one end of the optical fiber201. The port P3of the optical circulator17is optically coupled to the input port of the L-band wavelength separating unit16B.

FIG. 3Bschematically shows an image representing operations performed by the optical circulator18. As shown inFIG. 3B, similarly to the optical circulator17, the optical circulator18also includes three ports P1to P3. In the optical circulator18, an optical signal input from the port P1is output from the port P2. Further, an optical signal input from the port P2is output from the port P3.

The port P1of the optical circulator18is optically coupled to the output port of the L-band wavelength multiplexing unit15B. The port P2of the optical circulator18is optically coupled to one end of the optical fiber203. The port P3of the optical circulator18is optically coupled to the input port of the C-band wavelength separating unit16A.

The C-band optical transmitting devices111to11noutput optical signals having mutually different wavelengths in the C-band. The C-band wavelength multiplexing unit15A multiplexes the optical signals having the respective wavelengths output from the C-band optical transmitting devices111to11n, and outputs a wavelength-multiplexed optical signal in the C-band. The wavelength-multiplexed signal in the C-band is supplied to the optical fiber201through the optical circulator17.

The L-band optical transmitting devices121to12noutput optical signals having mutually different wavelengths in the L-band. The L-band wavelength multiplexing unit15B multiplexes the optical signals having the respective wavelengths output from the L-band optical transmitting devices121to12n, and outputs a wavelength-multiplexed optical signal in the L-band. The wavelength-multiplexed signal in the L-band is supplied to the optical fiber203through the optical circulator18.

The C-band wavelength separating unit16A receives the wavelength-multiplexed optical signal in the C-band from the optical fiber203through the optical circulator18. The C-band wavelength separating unit16A separates the received wavelength-multiplexed optical signal in the C-band according to the wavelength, and supplies the separated optical signals having the respective wavelengths to the C-band optical receiving devices131to13n.

The L-band wavelength separating unit16B receives the wavelength-multiplexed optical signal in the L-band from the optical fiber201through the optical circulator17. The L-band wavelength separating unit16B separates the received wavelength-multiplexed optical signal in the L-band according to the wavelength, and supplies the separated optical signals having the respective wavelengths to the L-band optical receiving devices141to14n.

Next, a configuration of the C+L band transmitting/receiving device20is described in detail.

The C+L band transmitting/receiving device20includes C-band optical transmitting devices211to21n, L-band optical transmitting devices221to22n, C-band optical receiving devices231to23n, L-band optical receiving devices241to24n, a C-band wavelength multiplexing unit25A, an L-band wavelength multiplexing unit25B, a C-band wavelength separating unit26A, an L-band wavelength separating unit26B, and optical circulators27and28. The number “n”, which indicates the number of devices, can be arbitrarily determined. However, it is necessary to make the number of devices in the C+L band transmitting/receiving device10equal to the number of corresponding devices in the C+L band transmitting/receiving device20.

Each of the C-band wavelength multiplexing unit25A and the L-band wavelength separating unit26B is optically coupled to the optical fiber202through the optical circulator27. Each of the L-band wavelength multiplexing unit25B and the C-band wavelength separating unit26A is optically coupled to the optical fiber204through the optical circulator28.

Existing optical circulators such as polarization-dependent optical circulators and polarization-independent optical circulators can be used as the optical circulators27and28. Only operations performed by the optical circulators are described hereinafter while omitting descriptions of their structures.

FIG. 4Aschematically shows an image representing operations performed by the optical circulator27. As shown inFIG. 4A, the optical circulator27also includes three ports P1to P3. In the optical circulator27, an optical signal input from the port P1is output from the port P2. Further, an optical signal input from the port P2is output from the port P3.

The port P1of the optical circulator27is optically coupled to the output port of the L-band wavelength multiplexing unit25B. The port P2of the optical circulator27is optically coupled to the optical fiber201. The port P3of the optical circulator27is optically coupled to the input port of the C-band wavelength separating unit26A.

FIG. 4Bschematically shows an image representing operations performed by the optical circulator28. As shown inFIG. 4B, the optical circulator28also includes three ports P1to P3. In the optical circulator28, an optical signal input from the port P1is output from the port P2. Further, an optical signal input from the port P2is output from the port P3.

The port P1of the optical circulator18is optically coupled to the output port of the C-band wavelength multiplexing unit25A. The port P2of the optical circulator28is optically coupled to the optical fiber204. The port P3of the optical circulator28is optically coupled to the input port of the L-band wavelength separating unit26B.

The C-band optical transmitting devices211to21noutput optical signals having mutually different wavelengths in the C-band. The C-band wavelength multiplexing unit25A multiplexes the optical signals having the respective wavelengths output from the C-band optical transmitting devices211to21n, and outputs a wavelength-multiplexed optical signal in the C-band. The wavelength-multiplexed signal in the C-band is supplied to the optical fiber204through the optical circulator28.

The L-band optical transmitting devices221to22noutput optical signals having mutually different wavelengths in the L-band. The L-band wavelength multiplexing unit25B multiplexes the optical signals having the respective wavelengths output from the L-band optical transmitting devices221to22n, and outputs a wavelength-multiplexed optical signal in the L-band. The wavelength-multiplexed signal in the L-band is supplied to the optical fiber202through the optical circulator27.

The C-band wavelength separating unit26A receives the wavelength-multiplexed optical signal in the C-band from the optical fiber202through the optical circulator27. The C-band wavelength separating unit26A separates the received wavelength-multiplexed optical signal in the C-band according to the wavelength, and supplies the separated optical signals having the respective wavelengths to the C-band optical receiving devices231to23n.

The L-band wavelength separating unit26B receives the wavelength-multiplexed optical signal in the L-band from the optical fiber204through the optical circulator28. The L-band wavelength separating unit26B separates the received wavelength-multiplexed optical signal in the L-band according to the wavelength, and supplies the separated optical signals having the respective wavelengths to the L-band optical receiving devices241to24n.

The C+L band repeater30includes a first repeater unit30A disposed between the optical fibers201and202, and a second repeater unit30B disposed between the optical fibers203and204.

Next, a configuration of the C+L band repeater30is described in detail.

The first repeater unit30A includes a C-band repeater31, an L-band repeater32, and optical circulators35and36.

The C-band repeater31is an optical amplifier that amplifies an optical signal in the C-band. An input port of the C-band repeater31is optically coupled to the optical fiber201through the optical circulator35. An output port of the C-band repeater31is optically coupled to the optical fiber202through the optical circulator36. For example, an optical amplifier such as an EDFA (Erbium Doped Fiber Amplifier) may be used as the C-band repeater31.

The L-band repeater32is an optical amplifier that amplifies an optical signal in the L-band. An input port of the L-band repeater32is optically coupled to the optical fiber202through the optical circulator36. An output port of the L-band repeater32is optically coupled to the optical fiber201through the optical circulator35. For example, an optical amplifier such as an EDFA may be used as the L-band repeater32.

Existing optical circulators such as polarization-dependent optical circulators and polarization-independent optical circulators can be used as the optical circulators35and36. Only operations performed by the optical circulators are described hereinafter while omitting descriptions of their structures.

FIG. 5Aschematically shows an image representing operations performed by the optical circulator35. As shown inFIG. 5A, the optical circulator35includes three ports P1to P3. In the optical circulator35, an optical signal input from the port P1is output from the port P2. Further, an optical signal input from the port P2is output from the port P3.

The port P1of the optical circulator35is optically coupled to the output port of the L-band repeater32. The port P2of the optical circulator35is optically coupled to the optical fiber201. The port P3of optical circulator35is optically coupled to the input port of the C-band repeater31.

FIG. 5Bschematically shows an image representing operations performed by the optical circulator36. As shown inFIG. 5B, the optical circulator36also includes three ports P1to P3. In the optical circulator36, an optical signal input from the port P1is output from the port P2. Further, an optical signal input from the port P2is output from the port P3.

The port P1of the optical circulator36is optically coupled to the output port of the C-band repeater31. The port P2of the optical circulator36is optically coupled to the optical fiber202. The port P3of the optical circulator36is optically coupled to the input port of L-band repeater32.

In the above-described first repeater unit30A, a wavelength-multiplexed optical signal in the C-band output from the C-band wavelength-multiplexing unit15A is supplied to the port P2of the optical circulator35. In the optical circulator35, the wavelength-multiplexed optical signal in the C-band input from the port P2is output from the port P3. The wavelength-multiplexed optical signal in the C-band output from the port P3of the optical circulator35is amplified by the C-band repeater31and then supplied to the port P1of the optical circulator36. In the optical circulator36, the wavelength-multiplexed optical signal in the C-band input from the port P1is output from the port P2.

Meanwhile, the wavelength-multiplexed optical signal in the L-band output from the L-band wavelength-multiplexing unit25B is supplied to the port P2of the optical circulator36. In the optical circulator36, the wavelength-multiplexed optical signal in the L-band input from the port P2is output from the port P3. The wavelength-multiplexed optical signal in the L-band output from the port P3of the optical circulator36is amplified by the L-band repeater32and then supplied to the port P1of the optical circulator35. In the optical circulator35, the wavelength-multiplexed optical signal in the L-band input from the port P1is output from the port P2.

The second repeater unit30B includes a C-band repeater33, an L-band repeater34, and optical circulators37and38.

The C-band repeater33is similar to the C-band repeater31. An input port of the C-band repeater33is optically coupled to the optical fiber203through the optical circulator37. An output port of the C-band repeater33is optically coupled to the optical fiber204through the optical circulator38.

The L-band repeater34is similar to the L-band repeater32. An input port of the L-band repeater34is optically coupled to the optical fiber204through the optical circulator38. An output port of the L-band repeater34is optically coupled to the optical fiber203through the optical circulator37.

Existing optical circulators such as polarization-dependent optical circulators and polarization-independent optical circulators can be used as the optical circulators37and38. Only operations performed by the optical circulators are described hereinafter while omitting descriptions of their structures.

FIG. 6Aschematically shows an image representing operations performed by the optical circulator37. As shown inFIG. 6A, the optical circulator37includes three ports P1to P3. In the optical circulator37, an optical signal input from the port P1is output from the port P2. Further, an optical signal input from the port P2is output from the port P3.

The port P1of the optical circulator37is optically coupled to the output port of the C-band repeater33. The port P2of the optical circulator37is optically coupled to the optical fiber203. The port P3of the optical circulator37is optically coupled to the input port of the L-band repeater34.

FIG. 6Bschematically shows an image representing operations performed by the optical circulator38. As shown inFIG. 6B, the optical circulator38also includes three ports P1to P3. In the optical circulator38, an optical signal input from the port P1is output from the port P2. Further, an optical signal input from the port P2is output from the port P3.

The port P1of the optical circulator38is optically coupled to the output port of the L-band repeater34. The port P2of the optical circulator38is optically coupled to the optical fiber202. The port P3of the optical circulator38is optically coupled to the input port of the C-band repeater33.

In the above-described second repeater unit30B, a wavelength-multiplexed optical signal in the C-band output from the C-band wavelength-multiplexing unit25A is supplied to the port P2of the optical circulator38. In the optical circulator38, the wavelength-multiplexed optical signal in the C-band input from the port P2is output from the port P3. The wavelength-multiplexed optical signal in the C-band output from the port P3of the optical circulator38is amplified by the C-band repeater33and then supplied to the port P1of the optical circulator37. In the optical circulator37, the wavelength-multiplexed optical signal in the C-band input from the port P1is output from the port P2.

Meanwhile, the wavelength-multiplexed optical signal in the L-band output from the L-band wavelength-multiplexing unit15B is supplied to the port P1of the optical circulator37. In the optical circulator37, the wavelength-multiplexed optical signal in the L-band input from the port P1is output from the port P2. The wavelength-multiplexed optical signal in the L-band output from the port P2of the optical circulator37is amplified by the L-band repeater34and then supplied to the port P1of the optical circulator38. In the optical circulator38, the wavelength-multiplexed optical signal in the L-band input from the port P1is output from the port P2.

Next, operations performed by the bidirectional optical transmission system according to this example embodiment will be described in a specific manner.

Firstly, an operation for transmitting an optical signal in the upstream direction (the UP stream) is described.

In the C+L band transmitting/receiving device10, the C-band wavelength multiplexing unit15A outputs a wavelength-multiplexed optical signal in the C-band and the L-band wavelength multiplexing unit15B outputs a wavelength-multiplexed optical signal in the L-band.

The wavelength-multiplexed optical signal in the C-band output from the C-band wavelength-multiplexing unit15A is supplied to the first repeater30A through the optical circulator17and the optical fiber201. In the first repeater30A, the wavelength-multiplexed optical signal in the C-band is supplied to the C-band repeater31through the optical circulator35. The wavelength-multiplexed optical signal in the C-band is amplified by the C-band repeater31and then supplied from the optical circulator36to the optical fiber202.

The wavelength-multiplexed optical signal in the C-band is supplied to the C+L band transmitting/receiving device20through the optical fiber202. In the C+L band transmitting/receiving device20, the wavelength-multiplexed optical signal in the C-band is supplied to the C-band wavelength separating unit26A through the optical circulator27.

Meanwhile, the wavelength-multiplexed optical signal in the L-band output from the L-band wavelength-multiplexing unit15B is supplied to the second repeater30B through the optical circulator18and the optical fiber203. In the second repeater30B, the wavelength-multiplexed optical signal in the L-band is supplied to the L-band repeater34through the optical circulator37. The wavelength-multiplexed optical signal in the L-band is amplified by the L-band repeater34and then supplied from the optical circulator38to the optical fiber204.

The wavelength-multiplexed optical signal in the L-band is supplied to the C+L band transmitting/receiving device20through the optical fiber204. In the C+L band transmitting/receiving device20, the wavelength-multiplexed optical signal in the L-band is supplied to the L-band wavelength separating unit26B through the optical circulator28.

Next, an operation for transmitting an optical signal in the downstream direction (the Down stream) is described.

In the C+L band transmitting/receiving device20, the C-band wavelength multiplexing unit25A outputs a wavelength-multiplexed optical signal in the C-band and the L-band wavelength multiplexing unit25B outputs a wavelength-multiplexed optical signal in the L-band.

The wavelength-multiplexed optical signal in the C-band output from the C-band wavelength-multiplexing unit25A is supplied to the second repeater30B through the optical circulator28and the optical fiber204. In the second repeater30B, the wavelength-multiplexed optical signal in the C-band is supplied to the C-band repeater33through the optical circulator38. The wavelength-multiplexed optical signal in the C-band is amplified by the C-band repeater33and then supplied from the optical circulator37to the optical fiber203.

The wavelength-multiplexed optical signal in the C-band is supplied to the C+L band transmitting/receiving device10through the optical fiber203. In the C+L band transmitting/receiving device10, the wavelength-multiplexed optical signal in the C-band is supplied to the C-band wavelength separating unit16A through the optical circulator18.

Meanwhile, the wavelength-multiplexed optical signal in the L-band output from the L-band wavelength-multiplexing unit25B is supplied to the first repeater30A through the optical circulator27and the optical fiber202. In the first repeater30A, the wavelength-multiplexed optical signal in the L-band is supplied to the L-band repeater32through the optical circulator36. The wavelength-multiplexed optical signal in the L-band is amplified by the L-band repeater32and then supplied from the optical circulator35to the optical fiber201.

The wavelength-multiplexed optical signal in the L-band is supplied to the C+L band transmitting/receiving device10through the optical fiber201. In the C+L band transmitting/receiving device10, the wavelength-multiplexed optical signal in the L-band is supplied to the L-band wavelength separating unit16B through the optical circulator17.

The above-described bidirectional optical transmission system according to this example embodiment provides the following functions and effects. In the following descriptions, a transmission line formed by the optical fibers201and202and the first repeater30A is referred to as a first optical transmission line. Further, a transmission line formed by the optical fibers203and204and the second repeater30B is referred to as a second optical transmission line.

The C+L band transmitting/receiving device10transmits an optical signal in the C-band to the C+L band transmitting/receiving device20through the first optical transmission line and transmits an optical signal in the L-band to the C+L band transmitting/receiving device20through the second optical transmission line. Meanwhile, the C+L band transmitting/receiving device20transmits an optical signal in the L-band to the C+L band transmitting/receiving device10through the first optical transmission line and transmits an optical signal in the C-band to the C+L band transmitting/receiving device10through the second optical transmission line.

In the first optical transmission line, the optical signal in the C-band is transmitted in the upstream direction and the optical signal in the L-band is transmitted in the downstream direction. According to the configuration in which an optical signal in the C-band and an optical signal in the L-band are transmitted in opposite directions as described above, the optical signal in the C-band and the optical signal in the L-band can be separated from each other by using the optical circulators35and36without using the C/L demultiplexing filter. Further, since only optical signals in the C-band are supplied to the C-band repeater31, there is no need to cut off optical signals in the L-band in the upstream transmission line on the C-band repeater31side. Further, since only optical signals in the L-band are supplied to the L-band repeater32, there is no need to cut off optical signals in the C-band in the downstream transmission line on the L-band repeater32side. Therefore, since there is no need to secure a guard band, the problem that the wavelength band of the main signal is reduced due to the guard band does not occur. Therefore, according to the bidirectional optical transmission system in accordance with this example embodiment, the transmission capacity can be expanded as compared to that in the bidirectional optical transmission system shown inFIG. 1.

In addition, an optical signal in the C-band amplified by the C-band repeater31and an optical signal in the L-band amplified by the L-band repeater32are transmitted in directions opposite to each other, and these optical signals are not multiplexed. According to the above-described configuration, the coherent crosstalk, which is one of the problems in the bidirectional optical transmission system shown inFIG. 1, does not occur. Therefore, according to the bidirectional optical transmission system in accordance with this example embodiment, the deterioration in the transmission characteristic can be alleviated as compared to the bidirectional optical transmission system shown inFIG. 1.

In the bidirectional optical transmission system according to this example embodiment, each of the optical circulators17,18,27,28and35-38can be referred to as an optical coupler. The C+L band transmitting/receiving devices10and20can be referred to as first and second transmitting/receiving devices, respectively. The transmission line formed by the optical fibers201and202and the first repeater30A may be referred to as a first optical transmission line. Further, the transmission line formed by the optical fibers203and204and the second repeater30B may be referred to as a second optical transmission line.

In the first optical transmission line, the C-band repeater31may be referred to as a first optical amplifier and the transmission line including this C-band repeater31may be referred to as a first branch transmission line. Further, the L-band repeater32may be referred to as a second optical amplifier and the transmission line including this L-band repeater32may be referred to as a second branch transmission line.

In the second optical transmission line, the C-band repeater33may be referred to as a third optical amplifier the transmission line including this C-band repeater31may be referred to as a third branch transmission line. Further, the L-band repeater34may be referred to as a fourth optical amplifier and the transmission line including this L-band repeater32may be referred to as a fourth branch transmission line.

Second Embodiment

FIG. 7is a block diagram showing a configuration of a bidirectional optical transmission system according to a second example embodiment of the present invention.

The bidirectional optical transmission system shown inFIG. 7has a configuration similar to that of the bidirectional optical transmission system according to the first example embodiment, except that it includes optical couplers45to48in place of the optical circulators35to38. Note that only parts of the configuration different from those of the first example embodiment are described while omitting descriptions of the same parts thereof.

Each of the optical couplers45to48includes three ports P1to P3. In the optical coupler45, the port P1is optically coupled to the output port of the L-band repeater32and the port P2is optically coupled to the optical fiber201. Further, the port P3is optically coupled to the input port of the C-band repeater31. The optical coupler45supplies a wavelength-multiplexed signal in the C-band output from the C+L band transmitting/receiving device10to the C-band repeater31and supplies a wavelength-multiplexed signal in the L-band output from the L-band repeater32to the C+L band transmitting/receiving device10.

In the optical coupler46, the port P1is optically coupled to the output port of the C-band repeater31and the port P2is optically coupled to the optical fiber202. Further, the port P3is optically coupled to the input port of the L-band repeater32. The optical coupler46supplies a wavelength-multiplexed signal in the L-band output from the C+L band transmitting/receiving device20to the L-band repeater32and supplies a wavelength-multiplexed signal in the C-band output from the C-band repeater31to the C+L band transmitting/receiving device10.

In the optical coupler47, the port P1is optically coupled to the input port of the C-band repeater33and the port P2is optically coupled to the optical fiber203. Further, the port P3is optically coupled to the input port of the L-band repeater34. The optical coupler47supplies a wavelength-multiplexed signal in the L-band output from the C+L band transmitting/receiving device10to the L-band repeater34and supplies a wavelength-multiplexed signal in the C-band output from the C-band repeater33to the C+L band transmitting/receiving device10.

In the optical coupler48, the port P1is optically coupled to the input port of the C-band repeater33and the port P2is optically coupled to the optical fiber204. Further, the port P3is optically coupled to the output port of the L-band repeater34. The optical coupler48supplies a wavelength-multiplexed signal in the C-band output from the C+L band transmitting/receiving device20to the C-band repeater33and supplies a wavelength-multiplexed signal in the L-band output from the L-band repeater34to the C+L band transmitting/receiving device20.

Couplers capable of multiplexing or demultiplexing an optical signal in the C-band and an optical signal in the L-band, for example, directional couplers such as 3 dB couplers, can be used as the optical couplers45to48. In this case, the optical couplers45to48may be formed by C/L demultiplexing filters, but there is no need to secure a guard band. As for the C/L demultiplexing filter, for example, those using an AWG (Arrayed Waveguide Grating) element or those using a multi-layer interference film made of a semiconductor material having a periodic structure in which a refractive index changes in a continuous and periodic manner may be used.

The bidirectional optical transmission system according to this example embodiment also provides functions and effects similar to those in the first example embodiment.

In the bidirectional optical transmission system according to this example embodiment, the C+L band transmitting/receiving devices10and20may also be referred to as first and second transmitting/receiving devices, respectively. The transmission line formed by the optical fibers201and202and the first repeater30A may be referred to as a first optical transmission line. Further, the transmission line formed by the optical fibers203and204and the second repeater30B may be referred to as a second optical transmission line.

In the first optical transmission line, the C-band repeater31may be referred to as a first optical amplifier and the transmission line including this C-band repeater31may be referred to as a first branch transmission line. Further, the L-band repeater32may be referred to as a second optical amplifier and the transmission line including this L-band repeater32may be referred to as a second branch transmission line.

In the second optical transmission line, the C-band repeater33may be referred to as a third optical amplifier the transmission line including this C-band repeater31may be referred to as a third branch transmission line. Further, the L-band repeater34may be referred to as a fourth optical amplifier and the transmission line including this L-band repeater32may be referred to as a fourth branch transmission line.

The above-described bidirectional optical transmission systems according to the respective example embodiments are merely examples of the present disclosure. Further, modifications and improvements that can be understood by those skilled in the art can be made to their configuration and operations without departing from the scope and spirit of the invention.

For example, an optical submarine transmission system is required to be used for a long period of time, i.e., 25 years or longer. However, losses of optical signals increase due to cable repairs and aging deterioration. As a result, spectrum deviations occur in some cases. For example, a spectrum fluctuation in which a gain on the short wave side of the wavelength region of the main signal increases due to increased losses. As a result, the flat gain characteristic may deteriorate.

In the bidirectional optical transmission system according to the first or second example embodiment, in order to reduce the aforementioned spectrum fluctuation, dummy light may be inserted on the short wave side of the C-band optical signal or the L-band optical signal, or each of the C-band optical signal and the L-band optical signal in the C+L band transmitting/receiving devices10and20.

FIG. 8shows an example of an L-band optical signal in which dummy light is inserted. This example corresponds to the bidirectional optical transmission system according to the first example embodiment. In the C+L band transmitting/receiving device10, the L-band wavelength multiplexing unit15B outputs an L-band optical signal in which dummy light is inserted on the short wave side thereof. InFIG. 8, an L-band optical signal with no dummy light inserted therein is shown in an upper part and an L-band optical signal with dummy light inserted therein is shown in a lower part.

A gain on the short wave side of the L-band optical signal (with no dummy light) output from the L-band repeater34increases and hence the flat gain characteristic deteriorates. By inserting dummy light on the short wave side outside the L-band, fluctuations on the short wave side thereof can be suppressed.

A C-band optical signal propagating in the upstream direction and an L-band optical signal propagating in the downstream direction are separately supplied to the C-band repeater31and the L-band repeater32, respectively. A C-band optical signal propagating in the downstream direction and an L-band optical signal propagating in the upstream direction are separately supplied to the C-band repeater33and the L-band repeater34, respectively. Since optical signals supplied to respective repeaters are separated from each other as described above, it is possible to easily insert dummy light for reducing a spectrum deviation into each of the C-band wavelength multiplexing unit15A, the L-band wavelength multiplexing unit15B, the C-band wavelength multiplexing unit25A, and the L-band wavelength multiplexing unit25B.

The above-described insertion of dummy light can be applied to the second example embodiment in a similar manner.

Further, in the bidirectional optical transmission systems according to the respective example embodiments, the number of C+L band repeaters30is not limited to one. A plurality of C+L band repeaters30may be disposed between the C+L band transmitting/receiving devices10and20through optical fibers.

This application is based upon and claims the benefit of priority from Japanese patent application No. 2017-082077, filed on Apr. 18, 2017, the disclosure of which is incorporated herein in its entirety by reference.

REFERENCE SIGNS LIST