Bidirectional wavelength division multiplexed self-healing ring network composed of add fiber and drop fiber

1. Field of the Invention

The present invention relates generally to a wavelength division multiplexed self-healing network, and more particularly to a bidirectional wavelength division multiplexed self-healing ring network, in which a central office and remote nodes are connected to one another using two optical fibers—add fiber and drop fiber. Optical signals are transmitted bidirectionally through the two optical fibers. And, the proposed ring network provides the self-healing function without the protection optical switch in the transmission path and doubles the transmission capacity in a normal operating state.

2. Description of the Prior Art

As the amount of the desired transmission traffic increases by the popularization of the Internet, the interest in optical networks that connect a central office to subscribers grows up.

The optical network should facilitate the provision of high-speed service to fulfill a demand for the high-speed service, and be economical to accommodate a large number of subscribers.

An optical network utilizing wavelength division multiplexing technology can transmit optical signals using various wavelengths regardless of transmission types and speeds, so communication networks can be effectively made to have a high-speed and a wide bandwidth.

In order to connect a central node to subscribers in the optical network, a remote node, which is placed near a group of subscribers, should be provided with a function of dropping and adding the signal assigned at each remote node.

Therefore, there is required a simple and economical wavelength division multiplexed optical network having add and drop functions.

The conventional wavelength division multiplexed access/Metro networks which connect a central office and remote nodes may be classified into three types: a unidirectional optical ring network using a single optical fiber, a unidirectional self-healing optical ring network using two optical fibers, and a mesh network.

In particular, an optical ring network has a relatively simple structure and can implement a self-healing function, so researches on the optical ring network have been actively carried out.

In a conventional unidirectional optical ring network, a central office and remote nodes are connected to one another by two optical fibers, one of the optical fibers transmits an optical signal in a normal operating state, and in case of a system failure, the ring network could be self-healed by detouring optical signals to the other optical fiber using an optical switch.

When optical signals are detoured through the other optical fiber in the conventional unidirectional optical ring network, the number of hops and a transmission distance of optical signals are increased in comparison with those in a normal state. Therefore, the quality of signals is deteriorated and expensive optical amplifiers may be employed to compensate for a loss due to the increased transmission distance.

Additionally, in the conventional optical ring network, signals are simultaneously added and dropped at each node, so the structure of the remote node is complicated and the requirements of the device are very strict. These features bring about increases of the system cost.

Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a bidirectional wavelength division multiplexed self-healing ring network. In the case of a system failure, the survivability of the ring network can be maintained without an additional optical switch and a protective optical fiber since wavelength division multiplexed optical signals are transmitted bidirectionally. Thus, the quality of signals can be secured and the need for an additional optical amplifier can be eliminated.

Another object of the present invention is to provide a bidirectional wavelength division multiplexed self-healing ring network, which is capable of doubling the transmission capacity by bidirectional transmission, thus facilitating system upgrade. And, one add multiplexer and one drop demultiplexer at each remote node only perform as one of the add and drop functions, respectively. And, the add multiplexer and the drop demultiplexer can be easily realized using wavelength division multiplexers and demultiplexers, or optical circulators and fiber gratings.

Still another object of the present invention is to provide a bidirectional wavelength division multiplexed self-healing ring network, which is capable of suppressing cross talk and relative intensity noise generated in the process of bidirectional transmission using inexpensive and low-grade optical components. Thus, the system can be realized cost-effectively.

In order to accomplish the above object, the present invention provides a bidirectional wavelength division multiplexed self-healing ring network, in which a central office and remote nodes are connected to one another by two optical fibers, the two optical fibers are assigned to add and drop fibers, respectively. And, optical signals can be bidirectionally transmitted between the central office and the remote nodes.

Each of the remote nodes is composed of an add multiplexer and a drop demultiplexer. The add multiplexer adds optical signals only to the add fiber, while the drop demultiplexer drops optical signals only from the drop fiber.

The add multiplexer and the drop demultiplexer can be simply constructed using wavelength division multiplexers and demultiplexers, respectively. Additionally, the add multiplexer and the drop demultiplexer can simply be constructed using optical circulators and fiber gratings.

Optical signals are bidirectionally transmitted from the central office to the remote nodes. Thus, even in the case of a system failure, the transmission path can be maintained without any increase of transmission distance and hops number.

The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagram showing a configuration of a bidirectional wavelength division multiplexed self-healing ring network in accordance with a first embodiment of the present invention;

FIG. 2 is a diagram showing a configuration of a bidirectional wavelength division multiplexed self-healing ring network in accordance with a second embodiment of the present invention;

FIG. 3 is a diagram showing a process of healing the self-healing ring network of FIG. 1 when the self-healing ring network of FIG. 1 fails;

FIG. 4 is a diagram showing a bidirectional wavelength division multiplexed self-healing ring network in accordance with a third embodiment of the present invention;

FIG. 5 is a diagram showing a process of healing the self-healing ring network of FIG. 4 when the self-healing ring network of FIG. 4 fails;

FIG. 6 is a diagram showing a configuration of a drop demultiplexer according to a first application of the present invention;

FIG. 7 is a diagram showing a configuration of an add multiplexer according to a first application of the present invention;

FIG. 8 is a diagram showing a configuration of a drop demultiplexer according to a second application of the present invention;

FIG. 9 is a diagram showing a configuration of an add multiplexer according to a second application of the present invention;

FIG. 10 is a diagram showing a configuration of a drop demultiplexer according to a third application of the present invention;

FIG. 11 is a diagram showing a configuration of an add multiplexer according to a third application of the present invention;

FIG. 12 is a diagram showing a configuration of a drop demultiplexer according to a fourth application of the present invention;

FIG. 13 is a diagram showing a configuration of an add multiplexer according to a fourth application of the present invention;

FIG. 14 is a diagram showing a configuration of a drop demultiplexer according to a fifth application of the present invention;

FIG. 15 is a diagram showing a configuration of an add multiplexer according to a fifth application of the present invention;

FIG. 16 is a diagram showing a configuration of a drop demultiplexer according to a sixth application of the present invention;

FIG. 17 is a diagram showing a configuration of an add multiplexer according to a sixth application of the present invention;

FIG. 18 is a diagram showing a configuration of a drop demultiplexer according to a seventh application of the present invention; and

FIG. 19 is a diagram showing a configuration of an add multiplexer according to a seventh application of the present invention.

FIG. 1 is a diagram showing a configuration of a bidirectional wavelength division multiplexed self-healing ring network in accordance with a first embodiment of the present invention. The bidirectional wavelength division multiplexed self-healing ring network includes two optical fibers that constitute an add fiber 10 and a drop fiber 12 , respectively.

A central office CO is divided into a wavelength division multiplexed optical receiving part and a wavelength division multiplexed optical transmission part. The wavelength division multiplexed optical transmission part is connected to the drop fiber 12 through its both transmission terminals, while the wavelength division multiplexed optical receiving part is connected to the add fiber 10 through its both receiving terminals.

Remote nodes RN 1 , RN 2 , RN 3 and RN 4 are each composed of an add multiplexer AM and a drop demultiplexer DD. The add fiber 10 is connected to both terminals of the add multiplexer AM, while the drop fiber 12 is connected to both terminals of the drop demultiplexer DD.

In the optical transmission part of the central office CO, an optical signal is multiplexed by a multiplexer MUX, and the weakened signals through the optical modulator and multiplexer are amplified by an erbium doped fiber amplifier EDFA. The amplified signal is divided into two signals by an optical coupler PC.

The wavelength division multiplexed optical signals at the central office CO are passed through the optical coupler PC to be divided into two signals, and the divided signals having the same wavelengths are transmitted through the drop fiber 12 in both directions.

The optical signals are transmitted to the remote nodes RN 1 , RN 2 , RN 3 and RN 4 , and only signal of a wavelength assigned to each remote node RN 1 , RN 2 , RN 3 and RN 4 are dropped to the remote node RN 1 , RN 2 , RN 3 and RN 4 , respectively.

On the contrary, each of the remote nodes RN 1 , RN 2 , RN 3 and RN 4 adds two groups of signals having the same wavelengths bidirectionally to the add fiber 10 using the add multiplexer AM connected, and transmits them to the central office CO.

An optical receiver RX of the central office CO selects and receives one of two groups of optical signals demultiplexed by two wavelength division demultiplexers DMUXs using two 1×2 optical switches SW 1 s.

Alternatively, the optical receiving part 20 of the central office CO simultaneously receives two groups of optical signals coming from both directions using two optical receivers RXs without 1×2 optical switches SW 1 s, and electrically selects one of the two groups of signals having better signal performance.

FIG. 1 shows a case where four remote nodes RN 1 , RN 2 , RN 3 and RN 4 are provided and four wavelength signals are transmitted bidirectionally from the central office CO.

In the first remote node RN 1 , two first wavelength λ 1 signals coming from both directions are simultaneously dropped through the drop demultiplexer DD. And one of the two first wavelength λ 1 signals is selected by a 1×2 optical switch SW 1 , and received, while two first wavelength λ 1 signals are simultaneously added through the add demultiplexer AM and bidirectionally transmitted to the central office CO.

In the second remote node RN 1 , two second wavelength λ 2 signals coming from both directions are simultaneously dropped through the drop demultiplexer DD. And one of the two second wavelength λ 2 signals is selected by a 1×2 optical switch SW 1 , and received, while two second wavelength λ 2 signals are simultaneously added through the add demultiplexer AM and transmitted to the central office CO in both directions.

In the same way, the third remote node RN 3 is capable of adding and dropping third wavelength λ 3 signals, respectively, while the fourth remote node RN 4 is capable of adding and dropping fourth wavelength λ 4 signals, respectively.

FIG. 2 is a diagram showing a configuration of a bidirectional wavelength division multiplexed self-healing ring network in accordance with the second embodiment of the present invention. This drawing shows the case, where the wavelengths of signals added are different from the wavelengths of signals dropped.

The wavelengths of signals added in remote nodes RN 1 , RN 2 , RN 3 and RN 4 can be made to be different from the wavelengths of signals dropped in remote nodes RN 1 , RN 2 , RN 3 and RN 4 in such a way that wavelengths λ 1 , λ 2 , λ 3 and λ 4 of wavelength division multiplexed optical signals transmitted from the central office CO to the remote nodes RN 1 , RN 2 , RN 3 and RN 4 are made to be different from wavelengths λa, λb, λc and λd of wavelength division multiplexed optical signals transmitted from the remote nodes RN 1 , RN 2 , RN 3 and RN 4 to the central office CO.

A path of the optical signals transmitted from the remote nodes RN 1 , RN 2 , RN 3 and RN 4 to the central office CO is separated from a path of the optical signals transmitted from the central office CO to the remote nodes RN 1 , RN 2 , RN 3 and RN 4 as an add fiber 10 and a drop fiber 12 , respectively. Accordingly, the configuration of the self-healing ring network of this embodiment is not different from the one of the first embodiment in which the same wavelength signals are added and dropped.

In the first remote node RN 1 , two first wavelength signals λ 1 coming bidirectionally from the central office CO are simultaneously dropped through a drop demultiplexer DD. And one of the two first wavelength signals λ 1 is selected by a 1×2 optical switch SW 1 and received, while two first wavelength signals λa are simultaneously added through an add demultiplexer AM and bidirectionally transmitted to the central office CO.

FIG. 3 is a diagram showing the process of healing the self-healing ring network of FIG. 1 , when there exists network failure.

Since optical signals are bidirectionally transmitted, there is no trouble in communicating between the central office CO and the remote nodes RN 1 , RN 2 , RN 3 and RN 4 even in case of a system failure, for instance a sudden cutoff of the fiber 10 or 12 .

For example, if the optical fiber 10 and 12 are cut off between the first and second remote nodes RN 1 and RN 2 , optical power is not supplied to the drop channels that situated on the cutoff sides of the first, second, third and fourth remote nodes RN 1 , RN 2 , RN 3 and RN 4 , thereby monitoring the state of a system.

That is, each of the remote nodes RN 1 , RN 2 , RN 3 and RN 4 can monitor the system using optical power supplied to one of two drop demultiplexers DD near the location of the system failure.

The central office CO demultiplexes wavelength division multiplexed optical signals coming bidirectionally, and can determine whether the system fails or not by monitoring power variations at reception terminals.

If the network fails, the first remote node RN 1 drops the first wavelength λ 1 signal transmitted from the central office CO in a counterclockwise direction, and adds the first wavelength λ 1 signal to transmit it to the central office CO in a clockwise direction.

On the contrary, the second, third and fourth remote nodes RN 2 , RN 3 and RN 4 drop the second, third and fourth wavelength λ 2 , λ 3 and λ 4 signals transmitted from the central office CO in a clockwise direction, respectively, and adds the second, third and fourth wavelength λ 2 , λ 3 and λ 4 signals to transmit them to the central office CO in a counterclockwise direction, respectively.

Accordingly, even though a system failure occurs, the transmission paths of optical signals are not different from transmission paths in a normal state. And the system failure can be healed without using optical switches on optical path.

FIG. 4 is a diagram showing a bidirectional wavelength division multiplexed self-healing ring network in accordance with the third embodiment of the present invention. The wavelength division multiplexed optical signals transmitted bidirectioally have the same wavelength but are modulated with different data.

Two groups of optical signals transmitted bidirectionally from the central office CO are allowed to have the same wavelengths, and modulated with different data by two kinds of optical modulators (not shown). A pair of differently modulated signals having the same wavelength are applied to two terminals of a 2×2 optical switch SW 2 , and two other terminals of the 2×2 optical switch SW 2 are connected to the assigned input terminals of two multiplexers MUXs, respectively. The 2×2 optical switch SW 2 establishes a path of optical signals so that wavelength signals modulated with the higher priority data may be transmitted in a counterclockwise direction in a normal state.

Accordingly, two groups of optical signals transmitted bidirectionally from the central office CO have the same wavelength but are modulated with different data. Of these optical signals, optical signals modulated into the higher priority data are transmitted through the drop fiber 12 in a counterclockwise direction, while the other signals are transmitted in a clockwise direction.

Since two groups of optical signals entering each of the remote nodes RN 1 , RN 2 , RN 3 and RN 4 have the same wavelengths but are modulated with different data, the two groups of optical signals having entered each of the remote nodes RN 1 , RN 2 , RN 3 and RN 4 are passed through the 2×2 optical switch SW 2 , and received by two different optical receivers RXs. Since the optical signals transmitted to each of the remote nodes RN 1 , RN 2 , RN 3 and RN 4 in a counterclockwise direction are modulated with higher priority data, the optical switches SW 2 s establish a path of the optical signals so that the optical signals modulated with higher priority data can be received by higher priority optical receivers RXs.

Two groups of optical signals bidirectionally added at the remote nodes RN 1 , RN 2 , RN 3 and RN 4 have the same wavelengths but are modulated with different data. The two differently modulated signals having the same wavelengths are bidirectionally transmitted through the add fiber. A path of the differently modulated signals having the same wavelengths is established by optical switches SW 2 s so that the higher priority signals may be transmitted in a clockwise direction.

In the optical receiving part of the central office CO, two groups of optical signals are demultiplexed by two demultiplexers DMUXs, and are received by the optical receivers RXs, respectively, because of the differently modulated two groups of optical signals. In this case, a path of the optical signals modulated with higher priority data is established by the optical switches SW 2 s so that these signals may be received by the higher priority optical receivers RXs.

In a normal state, the central office CO and each of the remote nodes RN 1 , RN 2 , RN 3 and RN 4 transmit different optical signals bidirectionally, so the bidirectional ring network can double the transmission capacity compared to the unidirectional ring network.

Alternatively, as shown in FIG. 2 , the wavelengths of signals added are made to be different from those of signals dropped. Thus the wavelengths of signals passed through the add fiber are made to be different from those of signals passed through the drop fiber.

FIG. 5 is a diagram showing the process of healing the self-healing ring network of FIG. 4 when there exists network failure.

If the bidirectional ring network fails, one of two groups of optical signals are lost. Accordingly, the central node CO and each of the remote nodes RN 1 , RN 2 , RN 3 and RN 4 can monitor the failure and the position of optical fibers through the power of optical signals entering its receiving terminal. Additionally, the central node CO and each of the remote nodes RN 1 , RN 2 , RN 3 and RN 4 change the path of optical signals using 2×2 switches so as to survive higher priority optical signals, which are the more important than any one of the two groups of optical signals.

FIG. 5 shows the process of reestablishing paths of optical signals using 2×2 optical switches SW 2 s, when the optical fibers 10 and 12 are cut off between the first and second remote nodes RN 1 and RN 2 . Here, in a normal state, the optical signals which the central office CO transmits in a counterclockwise direction and the optical signals which each of the remote nodes RN 1 , RN 2 , RN 3 and RN 4 transmits in a clockwise direction have had the higher priority.

The first remote node RN 1 cannot receive optical signals in a clockwise direction, thus it receives higher priority first wavelength λ 1 signals in a counterclockwise direction. On the other hand, the first remote node RN 1 transmits first wavelength λ 1 signals modulated with higher priority data to the central office CO in a clockwise direction. In the first remote node, the establishment of the path of the optical switch SW 2 s is not changed.

However, since the second, third and fourth nodes RN 2 , RN 3 and RN 4 cannot receive higher priority optical signals, the central office CO switches the path of optical signals using the optical switches SW 2 s of its receiving part so that the higher priority optical signals to be transmitted in a counterclockwise direction in a normal state may be transmitted in a clockwise direction. Optical switches SW 2 s connected to drop demultiplexers DD of the second, third and fourth nodes RN 2 , RN 3 and RN 4 change the path of the higher priority optical signals so that the higher priority optical signals transmitted from the central office CO in a clockwise direction can enter higher priority optical receivers RXs of the second, third and fourth nodes RN 2 , RN 3 and RN 4 .

Additionally, optical switches SW 2 s connected to add multiplexers AM of the second, third and fourth nodes RN 2 , RN 3 and RN 4 change the path of the higher priority optical signals so that the higher priority optical signals can be transmitted to the central office CO in a counterclockwise direction.

The receiving part of the central office CO demultiplexes optical signals transmitted to its both terminals, and allows the demultiplexed higher priority optical signals to be received by higher priority optical receivers RXs using optical switches SW 2 s.

As a result, if the bidirectional ring network fails, the capacity of transmission is reduced to a half. However, the higher priority optical signals of the ring network can be easily survived.

FIGS. 6 and 7 are diagrams showing the configuration of a drop demultiplexer and the configuration of an add multiplexer in accordance with the first application of the present invention, respectively. The drop demultiplexer and the add multiplexer are implemented in the case, where the wavelengths of signals transmitted from each remote node are the same as the wavelengths of signals received by the remote node as shown in FIG. 1 .

FIG. 6 shows the drop demultiplexer DD that is constructed by disposing two four-port optical circulators 14 a and 14 b on both sides and connecting two same fiber gratings 16 a and 16 b parallel between the optical circulators 14 a and 14 b.

Tunable fiber gratings can be employed as the fiber gratings 16 a and 16 b to tune the wavelengths of signals dropped from both sides.

When two same groups of wavelength division multiplexed optical signals enter the optical circulators 14 a and 14 b connected to a drop fiber 12 , two optical signals assigned to each remote node are reflected by the upper and lower same fiber gratings 16 a and 16 b and come out through drop terminals connected to the optical circulators 14 a and 14 b.

In such a case, optical signals except for the optical signals dropped are bidirectionally passed through the optical circulators 14 a and 14 b.

In FIG. 6 , an dropped signal of optical signals entering from left to right is selected by the lower fiber grating 16 a , while the other dropped signal from optical signals entering from right to left is selected by the upper fiber grating 16 b.

The dropped optical signals are passed through a 1×2 optical switch SW 1 and received by an optical receiver RX. The optical receiver RX selects one having a higher sensitivity of the two optical signals.

FIG. 7 shows the add multiplexer AM that is constructed by disposing two four-terminal optical circulators 14 a and 14 b on both sides and connecting two same fiber gratings 16 a and 16 b parallel between the optical circulators 14 a and 14 b.

Tunable fiber gratings can be employed as the fiber gratings 16 a and 16 b to tune the wavelengths of signals added to both sides.

In the add multiplexer AM, an optical signal is divided into two by an optical coupler PC, and the two optical signals having the same wavelength are sent to the optical circulators 14 a and 14 b through add terminals.

An optical signal entering through the left add terminal is varied in its path by the optical circulator 14 a , reflected by the lower fiber grating 16 b , and transmitted to the left through the optical circulator 14 a.

In the same way, an optical signal entering through the right add terminal is varied in its path by the optical circulator 14 b , reflected by the upper fiber grating 16 a , and sent to the right through the optical circulator 14 b.

FIGS. 8 and 9 are diagrams showing the configuration of a drop demultiplexer and an add multiplexer in accordance with the second application of the present invention, respectively. The drop demultiplexer and the add multiplexer are implemented in the case where the wavelengths of signals transmitted from each remote node are the same as the wavelengths of signals received by the remote node as shown in FIG. 1 .

FIG. 8 shows the drop demultiplexer DD that is constructed using two same wavelength division demultiplexers WDD 1 and WDD 2 .

Tunable wavelength division demultiplexers can be employed as these wavelength division demultiplexers WDD 1 and WDD 2 .

These two same three-port wavelength division multiplexers WDD 1 and WDD 2 are disposed on both sides with their pass ports connected to each other and their common ports connected to a drop fiber 12 .

When two same groups of wavelength division multiplexed optical signals enter through the drop fiber 12 , two optical signals assigned to each remote node are selected by these wavelength division multiplexers WDD 1 and WDD 2 , and come out through reflection ports.

In such a case, optical signals except for the dropped optical signals are passed through the pass ports of these wavelength division multiplexers WDD 1 and WDD 2 .

The dropped optical signals are passed through a 1×2 optical switch SW 1 and received by an optical receiver RX. The optical receiver RX selects one having a higher sensitivity of the two optical signals.

FIG. 9 shows the add multiplexer AM that is constructed using two same wavelength division multiplexers WDD 1 and WDD 2 .

Tunable wavelength division demultiplexers can be employed as these wavelength division demultiplexers WDD 1 and WDD 2 .

The two same three-port wavelength division multiplexers WDD 1 and WDD 2 are disposed on both sides with their pass ports connected to each other and their common ports connected to an add fiber 10 .

In the add multiplexer AM, an optical signal is divided into two by an optical coupler PC, and the two optical signals having the same wavelength are added through these wavelength division multiplexers WDD 1 and WDD 2 and bidirectionally transmitted through the add fiber 10 connected to the common ports of these wavelength division multiplexers WDD 1 and WDD 2 .

FIGS. 10 and 11 are diagrams showing the configuration of a drop demultiplexer and an add multiplexer in accordance with the third application of the present invention, respectively. The drop demultiplexer and the add multiplexer are implemented using optical circulators and fiber gratings in the case where the wavelengths of signals transmitted from each remote node are allowed to be the same as the wavelengths of signals received by the remote node, but the former and the latter are modulated with different data as shown in FIG. 4 .

FIG. 10 shows the drop demultiplexer DD that is constructed by disposing two four-port optical circulators 14 a and 14 b on both sides, and connecting two same fiber gratings 16 a and 16 b parallel between the optical circulators 14 a and 14 b.

Tunable fiber gratings can be employed as the fiber gratings 16 a and 16 b to tune the wavelengths of signals dropped from both sides.

Optical signals are dropped as shown in FIG. 6 , but received by two different optical receivers RXs because the dropped optical signals are modulated with different data.

A 2×2 optical switch SW 2 disposed between the optical circulators 14 a and 14 b and the optical receivers RXs switches the paths of optical signals to receive higher priority optical signals first, in the case of a system failure.

FIG. 11 shows the add multiplexer AM that is constructed by disposing two four-port optical circulators 14 a and 14 b on both sides, and connecting two same fiber gratings 16 a and 16 b parallel between the optical circulators 14 a and 14 b.

Tunable fiber gratings can be employed as the fiber gratings 16 a and 16 b to tune the wavelengths of signals added to both sides.

In the add multiplexer AM, two same wavelength optical signals modulated with different data are added through two add ports in the same way shown in FIG. 7 , and transmitted in both directions.

A 2×2 switch SW 2 disposed between optical sources and the optical circulators 14 a and 14 b switches the paths of the optical signals to transmit higher priority optical signals first in the case of a system failure.

FIGS. 12 and 13 are diagrams showing the configuration of a drop demultiplexer and an add multiplexer in accordance with the fourth application of the present invention, respectively. The drop demultiplexer and the add multiplexer are implemented in the case where the wavelengths of signals transmitted from each remote node are the same as the wavelengths of signals received by the remote node, but the former and the latter are modulated with different data as shown in FIG. 4 .

FIG. 12 shows the drop demultiplexer DD that is constructed using two same wavelength division demultiplexers WDD 1 and WDD 2 .

Tunable wavelength division demultiplexers can be employed as the wavelength division demultiplexers WDD 1 and WDD 2 .

The configuration of the drop demultiplexer DD is the same as that shown in FIG. 8 , but the dropped optical signals are received by two optical receivers RXs because the dropped optical signals are modulated with different data.

A 2×2 switch SW 2 disposed between the wavelength division demultiplexers WDD 1 and WDD 2 and the optical receivers RXs switches the paths of the optical signals to transmit higher priority optical signals first in the case of a system failure.

FIG. 13 shows the add multiplexer AM that is constructed using two same wavelength division multiplexers WDD 1 and WDD 2 .

Tunable wavelength division demultiplexers can be employed as these wavelength division demultiplexers WDD 1 and WDD 2 .

The configuration of the add multiplexer AM is similar to that shown in FIG. 9 , but optical signals to be added to the add ports of the wavelength division multiplexers WDD 1 and WDD 2 are modulated with different data.

A 2×2 switch SW 2 disposed between optical sources and the wavelength division demultiplexers WDD 1 and WDD 2 switches the paths of the optical signals to transmit higher priority optical signals first in the case of a system failure.

FIGS. 14 and 15 are diagrams showing the configuration a drop demultiplexer and an add multiplexer in accordance with the fifth application of the present invention, respectively. The drop demultiplexer and the add multiplexer are implemented using optical circulators and fiber gratings in the case where a plurality of wavelengths are bidirectionally transmitted and received at each of remote nodes in the bidirectional self-healing ring network.

FIG. 14 shows the drop demultiplexer DD, which is constructed by disposing two four-port optical circulators 14 a and 14 b on both sides, and connecting two groups of fiber gratings 16 a , 16 b , 16 c and 16 d parallel between the optical circulators 14 a and 14 b . The fiber gratings whose number is equal to the number of wavelengths to be dropped are connected in series.

The fiber gratings 16 a and 16 c connected in series on a lower side reflect only signals of wavelengths λi and λj desired to be dropped, respectively. The number of the fiber gratings 16 a and 16 c is equal to the number of the wavelengths λi and λj desired to be dropped.

In the same way, the fiber gratings 16 b and 16 d connected in series on a upper side reflect only signals of wavelengths λi and λj desired to be dropped, respectively, and the number of the fiber gratings 16 b and 16 d is equal to the number of the wavelengths λi and λj desired to be dropped.

In the case where two groups of optical signals bidirectionally transmitted are the same, the optical signals are received by the optical receiver RX through the 1×2 optical switch SW 1 as shown in FIG. 6 ; while in the case where two groups of optical signals bidirectionally transmitted have the same wavelengths but are modulated with different data, the 2×2 optical switch SW 2 which is connected between the optical circulators 14 a and 14 b and the optical receivers RXs switches the path of optical signals in the case of a system failure, so as to transmit higher priority optical signals first.

Referring to FIG. 15 , two four-port optical circulators 14 a and 14 b are disposed on both sides, respectively, and two groups of fiber gratings 16 a , 16 b , 16 c and 16 d are connected in parallel between the optical circulators 14 a and 14 b . The fiber gratings whose number is equal to the number of wavelengths to be added are connected in series.

Fiber gratings 16 a and 16 c , and 16 b and 16 d of each group are connected in series to each other, and the two groups of fiber gratings 16 a and 16 c , and 16 b and 16 d are connected in parallel between the optical circulators 14 a and 14 b on upper and lower sides. Accordingly, the signals of wavelengths λi and λj desired to be added to the left are reflected by the lower fiber gratings 16 b and 16 d , while signals of wavelengths λi and λj desired to be added to the right are reflected by the upper fiber gratings 16 a and 16 c , thus being added bidirectionally.

In the case where two groups of optical signals to be bidirectionally added are the same, the two groups of optical signals are added using the optical coupler PC as shown in FIG. 7 ; while in the case where two groups of optical signals bidirectionally transmitted have the same wavelengths but are modulated with different data, the 2×2 optical switch SW 2 which is connected between the optical sources and the optical circulators 14 a and 14 b switches the path of optical signals in the case of a system failure, so as to transmit higher priority optical signals first.

FIGS. 16 and 17 are diagrams showing the configuration a drop demultiplexer and an add multiplexer in accordance with the sixth application of the present invention, respectively. The drop demultiplexer and the add multiplexer are implemented as wavelength division demultiplexers/multiplexers in the case where a plurality of wavelengths are bidirectionally transmitted and received at each of remote nodes in the bidirectional self-healing ring network of the present invention.

FIG. 16 shows the drop demultiplexer DD that is constructed by connecting a plurality of wavelength division demultiplexers WDD 1 , WDD 2 , WDD 3 and WDD 4 in series.

Wavelength division multiplexers WDD 1 and WDD 3 , or WDD 2 and WDD 4 whose number is equal to the number of wavelengths to be dropped are connected in series, so as to simultaneously drop a plurality of wavelengths.

In the case where two groups of optical signals bidirectionally transmitted are the same, the optical signals are received by the optical receiver RX through the 1×2 optical switch SW 1 as shown in FIG. 8 ; while in the case where two groups of optical signals bidirectionally transmitted have the same wavelengths but are modulated with different data, the 2×2 optical switch SW 2 which is connected between the wavelength division demultiplexers WDD 1 , WDD 2 , WDD 3 and WDD 4 and the optical receivers RXs switches the path of optical signals in the case of a system failure, so as to transmit higher priority optical signals first.

FIG. 17 shows the add multiplexer AM that is constructed by connecting a plurality of wavelength division multiplexers WDM 1 , WDM 2 , WDM 3 and WDM 4 in series.

Wavelength division multiplexers WDM 1 and WDM 3 , or WDM 2 and WDM 4 whose number is equal to the number of wavelengths to be dropped are connected in series to form a group of wavelength division multiplexers, so as to simultaneously add a plurality of wavelengths.

In the case where two groups of optical signals bidirectionally added are the same, two groups of optical signals having the same wavelengths are added using the optical coupler PC as shown in FIG. 9 ; while in the case where two groups of optical signals bidirectionally added have the same wavelengths but are modulated with different data, the 2×2 optical switch SW 2 which is connected between the optical sources and the wavelength division multiplexers WDM 1 , WDM 2 , WDM 3 , WDM 3 and WDM 4 switches the path of optical signals in the case of a system failure, so as to transmit higher priority optical signals first.

FIGS. 18 and 19 are diagrams showing the configuration a drop demultiplexer and an add multiplexer in accordance with the seventh application of the present invention, respectively. The drop demultiplexer and the add multiplexer are implemented as multi-channel wavelength division demultiplexers/multiplexers, in the case where a plurality of wavelengths are bidirectionally transmitted and received at each remote nodes in the bidirectional self-healing ring network of the present invention.

FIG. 18 shows the drop demultiplexer DD that is constructed using two multi-channel wavelength division demultiplexers WDD 5 and WDD 6 each of which can simultaneously drop signals having plural wavelengths λi and λj.

Of wavelength division multiplexed optical signals bidirectionally transmitted, only signals having plural wavelengths λi and λj assigned to each remote node can be simultaneously dropped using the multi-channel wavelength division demultiplexers WDD 5 and WDD 6 .

In the case where two groups of optical signals bidirectionally transmitted are the same, the optical signals are received by the optical receiver RX through the 1×2 optical switch SW 1 as shown in FIG. 8 ; while in the case where two groups of optical signals bidirectionally transmitted have the same wavelengths but are modulated with different data, the 2×2 optical switch SW 2 which is connected between the multi-channel wavelength division demultiplexers WDD 5 and WDD 6 and the optical receivers RXs switches the path of optical signals in the case of a system failure, so as to transmit higher priority optical signals first.

FIG. 19 shows the add multiplexer AM that is constructed using two multi-channel wavelength division multiplexers WDM 5 and WDM 6 each of which can simultaneously add signals having plural wavelengths λi and λj.

The signals having plural wavelengths λi and λj desired to be added can be simultaneously added by allowing the signals to enter the plural add ports of each of the multi-channel wavelength division multiplexers WDM 5 and WDM 6 .

In the case where two groups of optical signals bidirectionally added are the same, two groups of optical signals having the same wavelengths are added using the optical coupler PC as shown in FIG. 9 ; while in the case where two groups of optical signals bidirectionally added have the same wavelengths but are modulated with different data, the 2×2 optical switch SW 2 which is connected between the optical sources and the multi-channel wavelength division multiplexers WDM 5 and WDM 6 switches the path of optical signals in the case of a system failure, so as to transmit higher priority optical signals first.

Optical components such as a grating structure or multi-layer thin film elements can be used instead of the fiber gratings 16 a , 16 b , 16 c and 16 d . Additionally, arrayed-waveguide grating multiplexers can be used instead of the multi-channel wavelength division multiplexers/demultiplexers WDM 5 , WDM 6 , WDD 5 and WDD 6 . This use of a grating structure, the multi-layer thin film elements and the arrayed-waveguide grating multiplexers falls under the scope of the present invention.

As described above, the present invention provides a bidirectional wavelength division multiplexed self-healing ring network in which optical fibers connecting a central office and remote nodes are assigned to an add fiber and a drop fiber. The optical signals are added to the add fiber and dropped from the drop fiber at the remote nodes. Accordingly, the bidirectional wavelength division multiplexed self-healing ring network transmits wavelength division multiplexed optical signals bidirectionally, so the survivability of the ring network can be maintained without an additional optical switch and a protective optical fiber in the case of a system failure. Thus, the quality of signals can be secured and the need for an additional optical amplifier can be eliminated.

In addition, in accordance with the bidirectional wavelength division multiplexed ring network of the present invention, the transmission capacity can be doubled by bidirectional transmission, thus facilitating system upgrade. And, one add multiplexer and one drop demultiplexer in each remote node only perform as one of the add and drop functions, respectively. And, the add multiplexer and the drop demultiplexer can be easily realized using wavelength division multiplexers and demultiplexers, or optical circulators and fiber gratings.

In addition, in accordance with the bidirectional wavelength division multiplexed ring network of the present invention, cross talk and relative intensity noise generated in the process of bidirectional transmission can be suppressed using inexpensive and low-grade optical components. Thus, the system can be realized cost-effectively.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.