OPTICAL COMMUNICATION SYSTEM AND OPTICAL COMMUNICATION METHOD

Abstract

An object of the present invention is to provide an optical communication system and an optical communication method capable of achieving a long transmission distance with a passive element and obtaining redundancy of a ring topology.

The optical communication system according to the present invention is a PON system having a ring configuration, in which an unequal branch optical splitter having a left-right symmetrical configuration is disposed in a trunk fiber wired in a loop shape. An OLT and an ONU have a configuration in which two sets of Tx (transmitter) and Rx (receiver) are mounted. Two sets of Tx (transmitters) and Rx (receivers) in each of a plurality of ONUs are respectively connected to left and right symmetrical ports of one unequally branched light beam SP.

Claims

1. An optical communication system of a passive optical network (PON) in which an optical path is laid in a ring topology, the optical communication system comprising: a termination device, which is referred to as an optical line terminal (OLT), connected to the optical path so that an optical signal output is transmitted on the optical path in at least one of a clockwise direction or a counterclockwise direction; one or more unequal branch optical splitters having one primary-side main port, one secondary-side main port, a plurality of primary-side branch ports, and a plurality of secondary-side branch ports and connected to the optical path via the primary-side main port and the secondary-side main port, the one or more unequal branch optical splitters configured to branch the optical signal transmitted clockwise from the optical path, output optical signals obtained by branching from the respective secondary-side branch ports at a predetermined branching ratio, branch the optical signal transmitted counterclockwise from the optical path, and output optical signals obtained by branching from the respective primary-side branch ports at a predetermined branching ratio; and a network device, which is referred to as an optical network unit (ONU), connected to one primary-side branch port of the plurality of primary-side branch ports and a secondary-side branch port of the plurality of secondary-side branch ports corresponding to the one primary-side branch port in the unequal branch optical splitter.

2. The optical communication system according to claim 1, wherein when the ONU receives the optical signal transmitted counterclockwise and the optical signal transmitted clockwise, the ONU discards the optical signal received later.

3. The optical communication system according to claim 1, wherein the OLT and the ONU include a detection unit configured to detect disconnection of the optical path, an optical switch configured to cause the optical signal to be transmitted on the optical path either clockwise or counterclockwise, and a control unit configured to switch the optical switch when the detection unit detects the disconnection of the optical path.

4. The optical communication system according to claim 1, wherein the unequal branch optical splitter includes a cross-linking branch optical circuit that is a waveguide optical splitter having two inputs and two outputs, in which one of the two inputs of the waveguide optical splitter is connected to the primary-side main port, and one of the two outputs of the waveguide optical splitter is connected to the secondary-side main port, and an unequal branch optical circuit connected to the other of the two inputs of the waveguide optical splitter and another unequal branch optical circuit connected to the other of the two outputs of the waveguide optical splitter, and the unequal branch optical circuit connects one multiplexing port P.sub.x connected to the cross-linking branch optical circuit and m pieces of demultiplexing ports P.sub.y1 to P.sub.ym, where m is an integer of two or more, by using m-1 pieces of the waveguide optical splitters having two inputs and two outputs combined, the multiplexing port P.sub.x is connected to one of two inputs of a first stage of the waveguide optical splitter, and one of two outputs of the first stage of waveguide optical splitter, which is directly connected to the multiplexing port P.sub.x via a waveguide, is set to a first demultiplexing port P.sub.y1, one of two outputs of a (k-1)th stage of the waveguide optical splitter, which is not connected to a demultiplexing port P.sub.y(.sub.k-1), is connected to one of two inputs of a kth stage of the waveguide optical splitter, where k is an integer from two to m-2, and one of two outputs of the kth stage of the waveguide optical splitter, which is directly connected to the (k-1)th stage of the waveguide optical splitter via a waveguide, is set to a kth demultiplexing port P.sub.yk, and one of two outputs of an (m-2)th stage of the waveguide optical splitter, which is not connected to a demultiplexing port P.sub.y(.sub.m-2), is connected to one of two inputs of an (m-1)th stage of the waveguide optical circuit, one of two outputs of the (m-1)th stage of the waveguide optical splitter, which is directly connected to the (m-2)th stage of the waveguide optical splitter via a waveguide, is set to an (m-1)th demultiplexing port P.sub.y(.sub.m- .sub.1), and the other of the two outputs is set to a demultiplexing port P.sub.ym, where the (m-1)th stage of the waveguide optical splitter is provided when m is three or more, and the kth waveguide optical splitter is provided when m is four or more.

5. An optical communication method in a passive optical network (PON) in which an optical path laid in a ring topology, the optical communication method comprising: outputting an optical signal to the optical path so that the optical signal is transmitted on the optical path in at least one of a clockwise direction or a counterclockwise direction; connecting an unequal branch optical splitter including one primary-side main port, one secondary-side main port, a plurality of primary-side branch ports, and a plurality of secondary-side branch ports, to the optical path via the primary-side main port and the secondary-side main port; by the unequal branch optical splitter, branching the optical signal transmitted counterclockwise from the optical path, outputting optical signals obtained by branching, from the respective secondary-side branch ports at a predetermined branching ratio, branching the optical signal transmitted counterclockwise from the optical path, and outputting optical signals obtained by branching, from the respective primary-side branch ports at a predetermined branching ratio; and receiving the optical signal from at least one of one primary-side branch port of the plurality of primary-side branch ports or a secondary-side branch port of the plurality of secondary-side branch ports corresponding to the one primary-side branch port in the unequal branch optical splitter.

6. The optical communication method according to claim 5, comprising: by the ONU in the PON, when the ONU in the PON receives the optical signal transmitted counterclockwise and the optical signal transmitted clockwise, discarding the optical signal received later.

7. The optical communication method according to claim 5, comprising: by the OLT and ONU in the PON, detecting disconnection of the optical path; and when the disconnection of the optical path is detected, switching an optical switch and transmitting the optical signal on the optical path either clockwise or counterclockwise.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0024] FIG. 1 is a diagram illustrating a typical double star PON system.

[0025] FIG. 2 is a diagram illustrating an issue of the present invention.

[0026] FIG. 3 is a diagram illustrating an optical communication system according to the present invention.

[0027] FIG. 4 is a diagram illustrating the optical communication system according to the present invention.

[0028] FIG. 5 is a diagram illustrating an unequal branch optical splitter included in the optical communication system according to the present invention.

[0029] FIG. 6 is a diagram illustrating an optical communication method according to the present invention.

DESCRIPTION OF EMBODIMENTS

[0030] Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention and the present invention is not limited to the embodiments described below. Note that constituent elements with the same reference signs in the specification and the drawings are assumed to be the same constituent elements.

Embodiment 1

[0031] In the present embodiment, a PON system that achieves improvement of reliability by normally transmitting light in two directions will be described.

[0032] FIG. 3 is a diagram illustrating an optical communication system 301 according to the present embodiment. The optical communication system 301 is an optical communication system of a passive optical network (PON) in which an optical path (trunk fiber) 150 has a ring topology. The optical communication system 301 includes an OLT 101, one or a plurality of unequal branch optical splitters 112, and an ONU 106. The OLT 101 is connected to the trunk fiber 150 so that an output optical signal is transmitted in at least one of a rightward direction L.sub.R and a leftward direction L.sub.L of the trunk fiber 150. The unequal branch optical splitter 112 has one primary-side main port P.sub.1, one secondary-side main port P.sub.A, a plurality of primary-side branch ports (P.sub.2 to P.sub.4), and a plurality of secondary-side branch ports (P.sub.B to P.sub.D). The unequal branch optical splitter 112 is connected to the trunk fiber 150 by the primary-side main port P.sub.1 and the secondary-side main port P.sub.A. The unequal branch optical splitter 112 branches the rightward optical signal L.sub.R from the trunk fiber 150 and outputs optical signals obtained by branching, from the respective secondary-side branch ports (P.sub.B to P.sub.D) at a predetermined branching ratio. The unequal branch optical splitter 112 branches the leftward optical signal L.sub.L from the trunk fiber 150 and outputs optical signals obtained by branching, from the respective primary-side branch ports (P.sub.2 to P.sub.4) at a predetermined branching ratio. The ONU 106 is connected to one (for example, P.sub.2) of the primary-side branch ports and the secondary-side branch port (for example, P.sub.B) corresponding to the primary-side branch port in the unequal branch optical splitter 112. Although FIG. 3 illustrates an example in which the ONU 106 is connected to the ports P.sub.2 and P.sub.B of the unequal branch optical splitter 112, the number of ONUs 106 connected to each of the unequal branch optical splitters 112 is not limited to one. The ONU may be connected to the ports P.sub.3 and P.sub.c of the unequal branch optical splitter 112, and may be connected to the ports P.sub.4 and P.sub.D. In this case, three ONUs in maximum are connected to one unequal branch optical splitter 112.

[0033] Differing from the unequal branch optical splitter disclosed in NPL 1, the unequal branch optical splitter 112 has a left-right symmetrical structure. In the unequal branch optical splitter 112, the trunk fiber 150 is connected to the ports P.sub.1 and P.sub.A. The port (P.sub.2, P.sub.3, P.sub.4) and the port (P.sub.B, P.sub.C, P.sub.D) are connection ports to the ONU 106. With this structure, the unequal branch optical splitter 112 outputs light to the port (P.sub.B, P.sub.C, P.sub.D) when the light is incident to the port P.sub.1. When light is incident to the port P.sub.A, the unequal branch optical splitter 112 outputs light to the port (P.sub.1, P.sub.2, P.sub.3).

[0034] In the optical communication system 301, each of the OLT 101 and the ONU 106 is equipped with two transmission and reception units TRx. The rightward optical signal L.sub.R in a downlink direction is transmitted from Tx1 of the OLT 101. A portion of the rightward optical signal is branched by the unequal branch optical splitter 112-1 and received by Rx1 of the ONU 106-1. Then, a portion of the rightward optical signal is further branched by the unequal branch optical splitter 112-2 and received by Rx1 of the ONU 106-2. The leftward optical signal L.sub.L in the downlink direction is transmitted from Tx2 of the OLT 101. A portion of the leftward optical signal is branched by the unequal branch optical splitter 112-2 and received by Rx2 of the ONU 106-2. Then, a portion of the leftward optical signal is further branched by the unequal branch optical splitter 112-1 and received by Rx2 of the ONU 106-1.

[0035] The leftward optical signal in an uplink direction is transmitted from Tx1 of the ONU 106-1 and Tx1 of the ONU 106-2, and then received by Rx1 of the OLT. The rightward optical signal in the uplink direction is transmitted from Tx2 of the ONU 106-1 and Tx2 of the ONU 106-2, and then received by Rx2 of the OLT.

[0036] As described above, the OLT 101 and the ONU 106 transmit and receive the same signal being the rightward optical signal L.sub.R and the leftward optical signal L.sub.L, which means that redundancy of the optical communication system 301 is ensured. Thus, even though a disconnection occurs at any place (for example, point X in FIG. 3) of the trunk fiber 150, the OLT 101 and the ONU 106 can receive a rightward or leftward signal. Thus, it is possible to improve the reliability of the optical communication system 301.

[0037] In addition, by adjusting a branch ratio of each waveguide optical splitter that has two inputs and two outputs and constitutes the unequal branch optical splitter 112, it is possible to adjust a branching ratio of light from the port P.sub.1 to the port (P.sub.A, P.sub.B, P.sub.C, P.sub.D) and a branching ratio of light from the port P.sub.A to the port (P.sub.1, P.sub.2, P.sub.3, P.sub.4). That is, it is possible to extend the trunk fiber 150 by reducing the branching ratio to the branch port (P.sub.B, P.sub.C, P.sub.D, P.sub.2, P.sub.3, P.sub.4) and increasing the branching ratio to the main port (P.sub.A, P.sub.1).

[0038] Thus, the optical communication system 301achieves a long transmission distance between passive elements and obtain the redundancy of the ring topology.

[0039] In the optical communication system 301, when the fiber disconnection does not occur, the same optical signal is input to the single ONU 106 from two directions of the rightward direction and the leftward direction. Thus, the ONU 106 uses the optical signal (packet) that has arrived first for communication and discards the optical signal (packet) that has arrived second.

[0040] In the optical communication system 301, the two unequal branch optical splitters are disposed in the trunk fiber 150, but to obtain the above effect, the number of unequal branch optical splitters is not limited to two and may be equal to or more than one.

Embodiment 2

[0041] In the present embodiment, a PON system that achieves improvement of reliability by detecting fiber disconnection and performing switching between a normal system and an abnormal system will be described.

[0042] FIG. 4 is a diagram illustrating an optical communication system 302 according to the present embodiment. The optical communication system 302 is different from the optical communication system 301 described in Embodiment 1 in the following points. Each of an OLT 101 and an ONU 106 in the optical communication system 302 include a detection unit (11, 21) that detects a disconnection of a trunk fiber 150, an optical switch (13, 23) that causes an optical signal to be transmitted in any of a rightward direction and a leftward direction of the trunk fiber 150, and a control unit (12, 22) that switches the optical switch (13, 23) when the detection unit (11, 21) detects the disconnection of the trunk fiber 150.

[0043] The structure of an unequal branch optical splitter 112 is similar to that described in Embodiment 1.

[0044] In the optical communication system 302, the trunk fiber 150 of the ring topology is divided into a normal system and an abnormal system, and the OLT 101 and the ONU 106 perform switching between the normal system and the abnormal system by the optical switches 13 and 23. That is, differing from the optical communication system 301 in Embodiment 1, the optical communication system 302 transmits an optical signal in any one system.

[0045] The OLT 101 and the ONU 106 include detection units mPD (monitor photodiodes) 11 and 21 that detect occurrence of a disconnection in the trunk fiber 150, and control units 12 and 22 that receive a notification that the disconnection has been detected, from a MAC unit when the detection units 11 and 21 detect the disconnection, and output a switching command to the optical switches 13 and 23. With this configuration, when the disconnection occurs in the trunk fiber 150, the OLT 101 and the ONU 106 perform switching between a normal system and an abnormal system by the optical switches 13 and 23.

[0046] As described above, even though the disconnection occurs at any place (for example, point X in FIG. 4) of the trunk fiber 150, the OLT 101 and the ONU 106 can receive either the rightward signal or a leftward signal by switching the system using the optical switch (13, 23). Thus, it is possible to improve the reliability of the optical communication system 302.

[0047] In addition, it is possible to extent the trunk fiber 150 by adjusting the branching ratio of the unequal branch optical splitter 112 as described above.

[0048] Thus, the optical communication system 302 achieves a long transmission distance between passive elements and obtain the redundancy of the ring topology.

Third Embodiment

[0049] In Embodiments 1 and 2, the unequal branch optical splitter 112 has four ports (P.sub.1, P.sub.2, P.sub.3, P.sub.4) on the primary side and four ports (P.sub.A, P.sub.B, P.sub.C, P.sub.D) on the secondary side, but the unequal branch optical splitter in the present invention is not limited thereto.

[0050] In the present embodiment, a configuration in which the number of ports of an unequal branch optical splitter 112 is n (n is an integer of 2 or more) on both the primary side and the secondary side will be described.

[0051] FIG. 5 is a diagram illustrating a configuration of the unequal branch optical splitter 112. The unequal branch optical splitter 112 includes a cross-linking branch optical circuit 31 and two unequal branch optical circuits (32a and 32b). The cross-linking branch optical circuit 31 is a waveguide optical splitter having two inputs and two outputs. One of the two inputs of the waveguide optical splitter is connected to the primary-side main port P.sub.1, and one of the two outputs of the waveguide optical splitter is connected to the secondary-side main port P.sub.A. The two unequal branch optical circuits (32a and 32b) are respectively connected to the other of the two inputs and the other of the two outputs of the waveguide optical splitter.

[0052] Each of the unequal branch optical circuits (32a and 32b) connects one multiplexing port P.sub.x connected to the cross-linking branch optical circuit 31 to m pieces of demultiplexing ports (P.sub.y1 to P.sub.ym), where m is an integer of two or more, by using m-1 pieces of waveguide optical splitters having two inputs and two outputs combined. The multiplexing port P.sub.x is connected to one of two inputs of a first stage waveguide optical splitter 33-1, and one of two outputs of the first stage waveguide optical splitter 33-1, which is directly connected to the multiplexing port P.sub.x via a waveguide, is set to a first demultiplexing port P.sub.y1. One of two outputs of a (k-1)th stage waveguide optical splitter 33-(k-1), which is not connected to the demultiplexing port P.sub.y(.sub.k-1), is connected to one of two inputs of a kth stage waveguide optical splitter 33-k, where k is an integer of two to m-2, and one of two outputs of the kth stage waveguide optical splitter 33-k, which is directly connected to the (k-1)th stage waveguide optical splitter 33-(k-1) via a waveguide, is set to the kth demultiplexing port P.sub.yk. One of two outputs of an (m-2)th stage waveguide optical splitter 33-(m-2), which is not connected to a demultiplexing port P.sub.y(.sub.m-2), is connected to one of two inputs of an (m-1)th stage waveguide optical splitter 33-(m-1), which is directly connected to an (m-2)th stage waveguide optical splitter 33-(m-2) via a waveguide, is set to an (m-1)th demultiplexing port P.sub.y(.sub.m-1), and the other of the two outputs is set to a demultiplexing port P.sub.ym.

[0053] When m is three or more, the (m-1)th waveguide optical splitter 33-(m-1) is provided. When m is four or more, the kth waveguide optical splitter 33-k is provided. In addition, n is m + 1.

[0054] The cross-linking branch optical circuit 31 is a waveguide optical splitter having two inputs and two outputs, in which the branch ratio at a wavelength λ1 is xo:100-x.sub.0. In the cross-linking branch optical circuit 31, when the input is set to the primary side, one of the two inputs is connected to the main port P.sub.1, and the other is connected to the multiplexing-side port P.sub.x of the unequal branch optical circuit 32a. When the output of the cross-linking branch optical circuit 31 is set to the secondary side, one of the two outputs is connected to the main port P.sub.A, and the other is connected to the multiplexing-side port P.sub.x of the unequal branch optical circuit 32b.

[0055] The unequal branch optical circuit 32a and the unequal branch optical circuit 32b have the same structure. The circuit configuration will be described by using the unequal branch optical circuit 32b.

[0056] In the unequal branch optical circuit 32b, combinations of (m-1) pieces of waveguide optical splitters having two inputs and two outputs are connected between one multiplexing port P.sub.x connected to the cross-linking branch optical circuit 31 and m pieces of demultiplexing ports (P.sub.y1 to P.sub.ym), where m is an integer of 2 to (n-1). The demultiplexing ports (P.sub.y1 to P.sub.ym) are connected to the branch ports (P.sub.B to P.sub.N), respectively. N is an integer, similar to n.

[0057] In the first waveguide optical splitter 33-1, the multiplexing-side port P.sub.x is connected to the one of the two inputs. In the waveguide optical splitter 33-1, one of the two outputs, which is directly connected to the multiplexing port P.sub.X by a waveguide is set to the first demultiplexing port P.sub.y1, and the other is connected to one of the two inputs of the second waveguide optical splitter 33-2.

[0058] In the kth waveguide optical splitter 33-k, one of the two inputs is connected to one of the two outputs of the waveguide optical splitter 33-(k-1) at the previous stage, which is not connected to the demultiplexing port P.sub.y(.sub.k-1). In the waveguide optical splitter 33-k, one of the two outputs, which is directly connected to the waveguide optical splitter 33-(k-1) at the previous stage by a waveguide is set to the kth demultiplexing port P.sub.yk, and the other is connected to one of the two inputs of the waveguide optical splitter 33-(k+1) at the next stage.

[0059] In the final waveguide optical splitter 33-(m-1), one of the two inputs is connected to one of the two outputs of the waveguide optical splitter 33-(m-2) at the previous stage, which is not connected to the demultiplexing port P.sub.y(.sub.m-2). In the waveguide optical splitter 33-(m-1), one of the two outputs, which is directly connected to the waveguide optical splitter 33-(m-2) at the previous stage by a waveguide is set to the (m-1)th demultiplexing port P.sub.y(.sub.m-1), and the other is set to the demultiplexing port P.sub.ym.

[0060] The branching ratios at the wavelength λ1 in the waveguide optical splitters 33-1 to 33-(m-1) in the unequal branch optical circuit 32a are (33-1) x.sub.1:100-x.sub.1, (33-k) x.sub.k:100-x.sub.k, (33-(m-1)) X.sub.m-1: 100- X.sub.m-1, and X.sub.1 to X.sub.m-1 > 0.

[0061] In addition, the branching ratios at the wavelength λ1 in the waveguide optical splitters 33-1 to 33-(m-1) in the unequal branch optical circuit 32b are (33-1)x.sub.1':100-x.sub.1', (33-k) x.sub.k':100-x.sub.k', (33-(m-1)) x.sub.m-1': 100- x.sub.m-1', and X.sub.1' to x.sub.m-1' > 0.

[0062] The branch ratios (for example, X.sub.1 and X.sub.1', x.sub.k and .sub.Xk', x.sub.m-1 and x.sub.m-1') of the waveguide optical splitters corresponding to both the unequal branch optical circuits may be the same or different depending on the configuration of the optical communication system.

Fourth Embodiment

[0063] FIG. 6 is a flowchart illustrating an optical communication method in the optical communication system described in Embodiments 1 and 2. The present optical communication method includes outputting an optical signal to an optical path (trunk fiber) 150 so that the optical signal is transmitted in at least one of a rightward direction and a leftward direction of the optical path (Step S01), connecting an unequal branch optical splitter 112 having one primary-side main port P.sub.1, one secondary-side main port P.sub.A, a plurality of primary-side branch ports (P.sub.2 to P.sub.n), and a plurality of secondary-side branch ports (P.sub.B to P.sub.N) to the optical path by the main port P.sub.1 and the main port P.sub.A (Step S02), by the unequal branch optical splitter 112, branching the rightward optical signal from the optical path and outputting the signal obtained by branching, from each of the branch ports (P.sub.B to P.sub.N) at a predetermined branching ratio, and branching the leftward optical signal from the optical path and outputting the signal obtained by branching, from each of the branch ports (P.sub.2 to P.sub.n) at a predetermined branching ratio (Step S03), and receiving, by the same ONU 106, the optical signal output from at least one (both in the case of the optical communication system 301, and either in the case of the optical communication system 302) of one (for example, P.sub.k) of branch ports of the unequal branch optical splitter 112 and a branch port (for example, P.sub.K) corresponding to the one branch port (Step S04).

[0064] Performing such an optical communication method achieves a long transmission distance between the passive element and to obtain redundancy of the ring topology.

REFERENCE SIGNS LIST

[0065] 11, 21: Detection unit

[0066] 12, 22: Control unit

[0067] 13, 23: Optical switch unit

[0068] 31: Cross-linking branch optical circuit

[0069] 32a, 32b: Unequal branch optical circuit

[0070] 33-1, 33-2, ..., 33-k, ..., 33-(m-1): waveguide optical splitter

[0071] 100: Telecommunications carrier station

[0072] 101: Optical line termination (OLT)

[0073] 102: m:1 optical splitter

[0074] 103: Primary subscriber optical fiber line

[0075] 104: n:1 optical splitter

[0076] 105: Branched subscriber optical fiber line

[0077] 106-1 to 106-n: Optical network unit (ONU)

[0078] 112: Unequal branch optical splitter

[0079] 150: Trunk fiber