Reconfigurable optical amplifier, reversible optical circulator, and optical signal transmission system

A reconfigurable optical amplifier including a first reversible optical circulator and an optical gain device is provided. The first reversible optical circulator has four I/O ports which are respectively referred to as a first terminal, a second terminal, a third terminal, and a fourth terminal. The four I/O ports sequentially transmit an optical signal in a transmission direction of a forward circulation or a backward circulation according to a control signal. The first terminal is isolated from the adjacent fourth terminal. The optical gain device is connected between the first terminal and the adjacent fourth terminal. The second terminal and the third terminal are respectively connected to a first communication node and a second communication node.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Taiwan application serial no. 97141458, filed Oct. 28, 2008. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of specification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a reconfigurable optical amplifier and applications thereof.

2. Description of Related Art

Optical communication techniques have been broadly applied to ultra-wideband networks, wherein reversible three-port or four-port optical circulators are usually used as unidirectional 1×2 and 2×2 optical switches. In addition, optical amplifiers composed of optical circulators with fixed transmission directions and optical gain devices are usually disposed on optical transmission paths for amplifying optical signals and providing network monitoring signal return paths. A conventional optical amplifier has a unidirectional design, and therefore is limited in network applications.

FIG. 1is a schematic diagram of a conventional optical communication network. Referring toFIG. 1, in the conventional optical communication network, a two-way transmission is established between two communication nodes80and90. The communication node80has a receiver Rx1and a transmitter Tx1, and the communication node90has a receiver Rx2and a transmitter Tx2respectively connected to the transmitter Tx1and the receiver Rx1. Taking that the communication node80transmits an optical signal to the communication node90as an example, the communication path is connected to the receiver Rx2of the communication node90through an optical switch100, a unidirectional optical amplifier (OA)102, an optical fiber path104, a unidirectional optical amplifier106, and an optical switch108.

However, the optical fiber path104may be broken due to some external factors. Thus, a protection path120has to be disposed, and the optical signal can be transmitted through the protection path120through the switching of the optical switches100and108. The protection path120is the same as a normal path but only served as a backup path.

Similarly, when the communication node90transmits an optical signal to the receiver Rx1of the communication node80, the communication path is connected to the receiver Rx1of the communication node80through an optical switch110, a unidirectional optical amplifier112, an optical fiber path114, a unidirectional optical amplifier116, and an optical switch118. The backup protection path122has the same function as the protection path120but a reverse transmission direction. Because optical amplifiers are usually used in unidirectional transmission only, two protection paths120and122have to be disposed. As a result, the cost of the conventional optical communication network is very high.

Even though many different designs have been provided, a more satisfactory optical communication technique is still desired.

SUMMARY OF THE INVENTION

Consistent with the invention, there is provided a reconfigurable optical amplifier including a first reversible optical circulator and an optical gain device. The first reversible optical circulator has four I/O ports which are respectively a first terminal, a second terminal, a third terminal, and a fourth terminal. The four I/O ports sequentially transmit a first optical signal in a first transmission direction of a forward circulation or a backward circulation according to a control signal. The first terminal is isolated from the adjacent fourth terminal. The optical gain device is connected between the first terminal and the adjacent fourth terminal. The second terminal and the third terminal are respectively connected to a first communication node and a second communication node.

Consistent with the invention, there is provided a reversible optical circulator including an optical circulator and a magnetic field control device. The optical circulator has four I/O ports which are respectively a first terminal, a second terminal, a third terminal, and a fourth terminal, wherein an internal magnetic unit of the optical circulator is made of a semi-hard magnetic material. The magnetic field control device generates a first direction magnetic field or a second direction magnetic field according to a control signal and supplies the first direction magnetic field or the second direction magnetic field to the optical circulator to change a magnetic polarity, so that the first terminal, the second terminal, the third terminal, and the fourth terminal sequentially transmit an optical signal in a transmission direction of a forward circulation or a backward circulation, wherein the first terminal is isolated from the fourth terminal.

Consistent with the invention, there is provided an optical signal transmission system including a first communication terminal, a second communication terminal, an optical first path, an optical second path, and a protection path. The first communication terminal has a first optical switch unit connected to a first receiver and a first transmitter. The second communication terminal has a second optical switch unit connected to a second receiver and a second transmitter. The optical first path is connected between the first receiver and the second transmitter through the first optical switch unit and the second optical switch unit. The optical second path is connected between the first transmitter and the second receiver and/or between the second receiver and the first transmitter through the first optical switch unit and the second optical switch unit. The protection path is connected between the first receiver and the second transmitter through the first optical switch unit and the second optical switch unit, and the protection path includes at least one reconfigurable optical amplifier. The reconfigurable optical amplifier includes a first reversible optical circulator and an optical gain device. The first reversible optical circulator has four I/O ports which are respectively a first terminal, a second terminal, a third terminal, and a fourth terminal. The four I/O ports sequentially transmit a first optical signal in a first transmission direction of a forward circulation or a backward circulation according to a control signal. The first terminal is isolated from the adjacent fourth terminal. The optical gain device is connected between the first terminal and the adjacent fourth terminal. The second terminal and the third terminal are respectively connected to a first node and a second node, wherein the first node and the second node are respectively connected to the first communication terminal and the second communication terminal either directly or through a second reversible optical circulator.

DESCRIPTION OF THE EMBODIMENTS

In exemplary embodiments consistent with the present invention, there is provided a reconfigurable optical amplifier, wherein a reversible optical circulator is adopted to allow an optical transmission path to be reversible, and an optical gain device is also adopted. As a result, the number of protection paths in an optical communication network is reduced and accordingly the design of the optical communication network is simplified.

FIG. 2AandFIG. 2Bare state diagrams of a reversible optical circulator according to an embodiment of the present invention. All the optical circulators adopted in the present invention are the reversible optical circulator150, but with a specific isolated position. Taking the reversible optical circulator150having four I/O ports152(port1-port4) as an example, the reversible optical circulator150may be in a clockwise forward circulation as denoted by the solid arrow inFIG. 2Aor an anti-clockwise backward circulation as denoted by the hollow arrow inFIG. 2B. However, regardless of forward circulation or backward circulation, the port1and the adjacent port4are isolated from each other, as denoted by the gap on the arrow. It should be noted that the sequence of the four I/O ports152is not limited herein, and it is within the scope of the present invention as long as the same section is isolated in both the forward circulation and the backward circulation.

Regarding the sequence illustrated inFIG. 2A, a signal input from the port1is sequentially output from the next I/O port, namely, the port2. A signal input from the port2is sequentially output from the next I/O port, namely, the port3. A signal input from the port3is sequentially output from the next I/O port, namely, the port4. However, a signal input from the port4is not output or is output through an external path. The output direction inFIG. 2Bis reverse to that inFIG. 2A; however, the same mechanism is adopted. The design of the reversible optical circulator150will be described below with reference toFIG. 4.

FIG. 3is a schematic diagram of a reconfigurable optical amplifier according to an embodiment of the present invention. Referring toFIG. 3, the reconfigurable optical amplifier200has a reversible optical circulator150as illustrated inFIG. 2AandFIG. 2B. Generally speaking, the reconfigurable optical amplifier200includes the reversible optical circulator150and an optical gain device202. The reversible optical circulator150has four I/O ports which are respectively referred to as a first terminal (1), a second terminal (2), a third terminal (3), and a fourth terminal (4). The first terminal (1), the second terminal (2), the third terminal (3), and the fourth terminal (4) of the reversible optical circulator150sequentially transmit an optical signal in a transmission direction of a forward circulation (as denoted by the solid arrow) or a backward circulation (as denoted by the hollow arrow), namely, in two switchable directions, according to a control signal supplied to the magnetic field control device154. In the present embodiment, the first terminal (1) and the adjacent fourth terminal (4) are isolated from each other; however, the present invention is not limited thereto. The optical gain device202is connected between the first terminal (1) and the adjacent fourth terminal (4). The second terminal (2) and the third terminal (3) are respectively connected to two communication nodes80and90, wherein the communication nodes80and90are respectively denoted as terminal-1and terminal-2.

For example, an optical signal204is input to the third terminal (3) of the reversible optical circulator150from the communication node90(the terminal-2). The reversible optical circulator150is controlled to circulate clockwise. Since the first terminal (1) is isolated from the fourth terminal (4), the optical signal204enters the optical gain device202and is amplified into a signal206, namely, the optical gain device202produces an optical signal gain. The signal206then enters the first terminal (1) of the reversible optical circulator150and output as a signal208from the second terminal (2) to the communication node80. In addition, even though there may be a signal210reflected by the communication node80back to the second terminal (2), the reflected signal210will not enter the optical gain device202again to produce any noise.

FIG. 4is a schematic diagram of a reversible optical circulator according to an embodiment of the present invention. Referring toFIG. 4, the reversible optical circulator150includes an optical circulator and a magnetic field control device154controlled by a control signal. As described above, the optical circulator has four I/O ports which are respectively referred to as a first terminal (port1), a second terminal (port2), a third terminal (port3), and a fourth terminal (port4), wherein an internal magnetic unit of the optical circulator is made of a semi-hard magnetic material. The magnetic field control device154(for example, a coil) generates a first direction magnetic field or a second direction magnetic field according to a control signal and supplies the first direction magnetic field or the second direction magnetic field to the optical circulator to change a magnetic polarity, so that the first terminal, the second terminal, the third terminal, and the fourth terminal sequentially transmit an optical signal in a forward direction or a backward direction. Besides, the first terminal is isolated from the fourth terminal; however, the present invention is not limited thereto.

The major optical components in the optical circulator include a polarization beam splitter (PBS), a Faraday rotator, and a polarization beam combiner (PBC), and the operations of these optical components will be described below. The Faraday rotator operates according to the Faraday Magneto-optical Effect. The polarized rotation angle of the Faraday rotator changes along with the intensity of an external magnetic field, and the polarized rotation angle of an input light wave is changed by the Faraday rotator. After that, the light wave passes through the PBC and then is output to an optical fiber path. Since the input light wave is split into two light beams of TE and TM by the PBS and the polarization state of each light beam is changed by the Faraday rotator, a linear polarization filter effect is produced on the PBC and accordingly the intensity of the light wave is changed. When the polarized rotation angle of the Faraday rotator is large enough to reverse the optical transmission path in the optical circulator, the input light wave in the original direction is almost completely blocked. In other words, the optical transmission direction is reversed.

Taking a coil design as an example, the optical circulator is wrapped with a coil of an appropriate number of turns, and the coil is covered with a magneto-conductive material in order to reduce the requirement to the driving current. If a forward current pulse passes through the coil, the semi-hard magnetic material in the optical circulator induces the corresponding magnetic field and retains its magnetic force. As a result, the rotation direction of the Faraday rotator is affect so that the optical transmission direction is reversed. If a backward current pulse passes through the coil, the semi-hard magnetic material in the optical circulator induces the corresponding magnetic field and retains its magnetic force. As a result, the rotation direction of the Faraday rotator is affected so that the optical transmission direction is restored. As described above, the optical transmission direction is changed by supplying a current of different directions.

In the present embodiment, regardless of whether the optical circulator is in the forward circulation or backward circulation, the same section of the optical circulator is isolated so that a reconfigurable optical amplifier can be accomplished. Since the optical circulator has to be switched between the forward circulation and the backward circulation during the actual operation, the trigger time has to be short enough to transmit data quickly.FIG. 5is a diagram illustrating the inspection result of the trigger time according to an embodiment of the present invention. Referring toFIG. 5, the forward circulation is inspected as a signal is transmitted from the second terminal (port2) to the third terminal (port3), wherein the signal280is a trigger signal. After the negative edge triggering the forward circulation (port2to port3) is reversed at about 0.8 ms. In addition, the backward circulation is inspected as a signal is transmitted from the second terminal (port2) to the first terminal (port1). The backward circulation (port2to port1) is started about 1.5 ms after the negative edge trigger is applied. The trigger time is also applicable.

FIGS. 6A-6Bare diagrams illustrating examples of different operations according to an embodiment of the present invention. Referring toFIG. 6A, the reconfigurable optical amplifier may allow an optical signal to be transmitted bi-directionally at the same time; however, the optical transmission pattern of the backward circulation can be determined according to the actual requirement. When the reversible optical circulator150is set to forward circulation, the optical signal is transmitted from the communication node80to the communication node90via a path300, but the optical signal is not amplified. If the optical signal is transmitted from the communication node90to the communication node80via a path302, the optical signal is amplified by the optical gain device202. The optical gain device202is represented by OA. Referring toFIG. 6B, when the reversible optical circulator150is set to backward circulation, the optical signal is transmitted from the communication node90to the communication node80via a path304, but the optical signal is not amplified. If the optical signal is transmitted from the communication node80to the communication node90via a path306, the optical signal is amplified by the optical gain device202.

FIGS. 7A-7Dare diagrams illustrating examples of different operations according to another embodiment of the present invention. Referring toFIG. 7A, the reconfigurable optical amplifier may be further connected to at least another reversible optical circulator160. The reversible optical circulator160has the same structure as the reversible optical circulator150. In the present embodiment, both the two reversible optical circulators150and160are set to forward circulation. The second terminal (2) and the third terminal (3) of the reversible optical circulator150are respectively connected to the first terminal (1) and the fourth terminal (4) of the reversible optical circulator160. In other words, the reversible optical circulator150is connected between two isolated terminals of the reversible optical circulator160. Thus, an optical signal transmitted from the communication node80to the communication node90is transmitted via the path300, namely, via the same path. However, the transmission path302of the optical signal from the communication node90to the communication node80passes through the reconfigurable optical amplifier illustrated inFIG. 6A. As can be understood, the connection between the terminals can have various manners. Here in general, a communication node153is indicated between the first terminal (1) of the reversible optical circulator160and the second terminal (2) of the reversible optical circulator150. Likewise, the communication node155is also indicated between the fourth terminal (4) and the third terminal (3) of two reversible optical circulators150,160. The communication nodes153,155are used for indicating a general connection to any other subsequently connected device without a specific manner.

In addition, referring toFIG. 7B, the reversible optical circulator160is set to backward circulation. Thus, the optical signal is transmitted from the communication node80to the communication node90via the path306, and the optical signal passes through the reversible optical circulator150but not the optical gain device202. Meanwhile, the optical signal is transmitted from the communication node90to the communication node80via the path304, and the optical signal enters the reversible optical circulator160through the third terminal (3) and is output to the communication node80from the second terminal (2).

Referring toFIG. 7C, another variation is based on the settings illustrated inFIG. 6B. Accordingly, the reversible optical circulator150is set to backward circulation, but the reversible optical circulator160is set to forward circulation. Thus, the optical signal is transmitted from the communication node80to the communication node90via the path300without being passed through the reversible optical circulator150. The optical signal is transmitted from the communication node90to the communication node80via the path302, and the optical signal passes through the reversible optical circulator150but is not amplified by the optical gain device202.

Referring toFIG. 7D, both the reversible optical circulator150and the reversible optical circulator160are set to backward circulation. Thus, the optical signal is transmitted from the communication node90to the communication node80via the path304without being passed through the reversible optical circulator150. The optical signal is transmitted from the communication node80to the communication node90via the path306, and the optical signal passes through the reversible optical circulator150and is amplified by the optical gain device202.

In the embodiment illustrated inFIGS. 7A-7D, the optical signal can be transmitted bi-directionally at the same time. However, the connections of the reversible optical circulator150and the reversible optical circulator160can be simply changed if the optical signal is to be transmitted unidirectionally according to the actual requirement.

FIGS. 8A-8Dare diagrams illustrating examples of different operations according to yet another embodiment of the present invention. Referring toFIG. 8A, the second terminal (2) and the third terminal (3) of the reversible optical circulator150are respectively connected to the second terminal (2) and the third terminal (3) of the reversible optical circulator160. The first terminal (1) and the fourth terminal (4) of the reversible optical circulator160are respectively connected to the communication node80and the communication node90. The reversible optical circulator150is set to the forward circulation, and the reversible optical circulator160is set to the forward circulation. Thus, the optical signal is transmitted from the communication node80to the communication node90via a path400, and the optical signal passes through the reversible optical circulator150but is not amplified by the optical gain device202. Besides, the optical signal transmitted from the communication node90to the communication node80enters the fourth terminal (4) of the reversible optical circulator160; however, since the fourth terminal (4) is isolated from the first terminal (1), the optical signal is only transmitted unidirectionally.

Referring toFIG. 8B, different fromFIG. 8A, the reversible optical circulator150is in forward circulation and the reversible optical circulator160is in backward circulation. Thus, the optical signal cannot be transmitted from the communication node80to the communication node90so that only a unidirectional transmission is accomplished. Besides, the optical signal transmitted from the communication node90to the communication node80via a path402passes through the reversible optical circulator150and amplified by the optical gain device202.

Referring toFIG. 8C, different fromFIG. 8A, the reversible optical circulator150is in backward circulation and the reversible optical circulator160is in forward circulation. Thus, the optical signal is transmitted from the communication node80to the communication node90via the path400, and the optical signal passes through the reversible optical circulator150and amplified by the optical gain device202. Besides, the optical signal cannot be transmitted from the communication node90to the communication node80.

Referring toFIG. 8D, different fromFIG. 8A, both the reversible optical circulator150and the reversible optical circulator160are in backward circulation. Thus, the optical signal cannot be transmitted from the communication node80to the communication node90so that a unidirectional transmission is accomplished. The optical signal is transmitted from the communication node90to the communication node80via the path402, and the optical signal passes through the reversible optical circulator150but not amplified by the optical gain device202.

Similarly, more reversible optical circulators can be adopted according to the actual requirement.

The reconfigurable optical amplifier described above can be applied in an actual optical signal transmission system. Besides being directly disposed between two communication nodes, the reconfigurable optical amplifier in the present invention may also be used for replacing a conventional protection path.FIG. 9is a diagram of an optical signal transmission system according to an embodiment of the present invention. Referring toFIG. 9, the optical signal transmission system is for transmitting an optical signal between two communication nodes80and90. The communication node80has optical switches100and118connected to a transmitter Tx1and a receiver Rx1. The optical switches100and118form an optical switch unit which is used for switching an optical path to a protection path508. The communication node90has optical switches108and110connected to a receiver Rx2and a transmitter Tx2. The optical switches108and110form an optical switch unit which is used for switching an optical path to the protection path508. An optical path104is connected between the transmitter Tx1and the receiver Rx2through the optical switches100and108. An optical path114is connected between the receiver Rx1and the transmitter Tx2through the optical switches118and110. The protection path508is connected to the paths502and500via the optical couplers504and512through the switching of the optical switches100,108,110, and118. The protection path508includes at least one reconfigurable optical amplifier. For example, the protection path508includes two reconfigurable optical amplifiers506and510. The reconfigurable optical amplifiers506and510have the structure and mechanism as described above. Thereby, the protection path508is simplified into an optical amplifying path with switchable transmission directions. As a result, the cost of the protection path508is reduced.

A protected system structure is to preserve a protection path in the entire system. If an error occurs in the system, the system can automatically determines the error and switches the protection path to an uplink path or a downlink path in order to ensure the stability of the entire system. The important components of the protection path include optical switches and reconfigurable optical amplifiers. When a central terminal transmits a data to a user terminal through an optical fiber downlink path and an error occurs on the downlink path, a monitoring device detects the error and changes the amplification direction of the optical switches and the reconfigurable optical amplifiers to turn the protection path into a downlink path. When a user terminal transmits a data to a central terminal through an optical fiber uplink path and an error occurs on the uplink path, the monitoring device detects the error and changes the amplification direction of the optical switches and the reconfigurable optical amplifiers to turn the protection path into an uplink path. If an error occurs on an optical fiber network, the network connection can be instantly switched to the protection path so that the network connection can be retained anytime and the cost of the network can be greatly reduced.

The reversible optical circulator provided by the present invention can be applied to a device with reversible optical transmission direction, wherein the optical transmission direction can be switched in the device, and the device can be applied to an optical fiber network system as an important device for protecting the network system.