WO2001045311A1

WO2001045311A1 - Optical ring protection apparatus and methods

Info

Publication number: WO2001045311A1
Application number: PCT/US2000/033720
Authority: WO
Inventors: Ornan A. Gerstel; Rajiv R. Ramaswami
Original assignee: Tellabs Operations, Inc.
Priority date: 1999-12-14
Filing date: 2000-12-14
Publication date: 2001-06-21
Also published as: AU2092101A; CA2394163A1; CA2394163C

Abstract

A wavelength division multiplexed optical communication system protects against light path failure on a per wavelength basis. At the source optical node an output device outputs first and second multiple wavelength signals on respective first and second light paths. At the sink optical node a first demultiplexer demultiplexes the first multiple wavelength signal into separate wavelengths, and a second demultiplexer demultiplexes the second multiple wavelength signal into separate wavelengths. For each wavelength signal the sink optical node includes transponders and coupler. The coupler has inputs connected to the respective outputs of the first and second transponders, and an output for outputting an optical signal received at one of the first and second inputs. A controller determines if the first transponder is outputting the first optical signal. Inhibiting from outputting the second optical signal. If not, the second transponder is not inhibited and the coupler outputs the second optical signal.

Description

TITLE

OPTICAL RING PROTECTION APPARATUS AND METHODS

Field of the Invention

The invention is in the field of optical communications, and more particularly, pertains to a

Wavelength Division Multiplexed (WDM) optical communication system in which light path failure is protected against on a per-channel wavelength basis using passive optics such as splitters and couplers.

Background Of The Invention

In WDM links protectxon against light-path failure is typically at a facility level utilizing active optics such as optical switches. For example, as shown in Fig. 1, a double light path connection for a multiple wavelength signal, i.e. a light path connection having a working optical fiber 2 and a redundant optical fiber 4 is connected between optical nodes 6 and 8 in an optical telecommunication system.

At the node 8 of the receive side, the two optical fibers 2 and 4 are combined by an optical switch 10 via which the working optical fiber 2 is connected to the node 8 of the receive side during normal operation, when a demultiplexer 3 demultiplexes the multiple wavelength into separate individual wavelengths. If a break occurs in the working optical fiber 2, which can be identified at the switch 10 on the basis of the outage of the light transmitted over the working optical fiber 2 , the switch 10 automatically switches, so that the redundant optical fiber 4 is now connected to the node 8 of the receive side instead of the working optical fiber 2.

At the node 6 of the transmission side a multiplexer 5 multiplexes a plurality of input separate wavelengths into an optical multiple wavelength facility signal to be transmitted, which is split onto the working fiber 2 and the redundant optical fiber 4 by an optical splitter 12. Two optical switches 14 and 16, both of which are closed in normal operation, are now inserted between the optical splitter 12 and the optical fibers 2 and 4. In case of an alternate circuiting, i.e. given a switching at the receive side from the working optical fiber 2 onto the redundant fiber 4, let the node 6 of the transmission side receive a message during the course of a corresponding protocol. In response thereto that optical switch 16 of the two switches 14 and 16, which is inserted between the redundant optical fiber 4 and the optical splitter 12, continues to remain closed. In contrast, optical switch 14, which is inserted between the optical splitter 12 and the working optical fiber 2, is opened.

Thus, by way of the switching at the receive side, the interrupt time associated with the alternate circuiting continues to be kept short, on the one hand, and, on the other hand, it is assured that shortly after the interruption that the broken fiber no longer carries the optical multiple wavelength facility signal. A problem with WDM system protecting against light path failure at the facility level is that such protection protects only against breaks in the optical fiber carrying the multiple wavelength facility signal, and does not protect against failures on a per channel basis in the optical fiber, and at the respective optical nodes .

Summary of the Invention In view of the above, it is an aspect of the invention to protect against light path failures on a per-channel basis using passive optics such as splitters and couplers .

It is another aspect of the invention to protect against light path failure from a source optical node to a sink optical node via at least one intermediate optical node on a per wavelength basis . At the source optical node an output means outputs first and second multiple wavelength signals on respective first and second light paths. The intermediate node is situated in at least one of the first and second light paths and includes an add/drop multiplexer for adding/dropping at least one wavelength to/from the first and second multiple wavelength signals. At the sink optical node a first demultiplexer demultiplexes the first multiple wavelength signal into separate wavelengths, and a second demultiplexer demultiplexes the second multiple wavelength signal into separate wavelengths. For each demultiplexed separate wavelength signal the sink optical node further includes first and second transponders and a coupler. The first transponder receives a given one of the separate wavelengths demultiplexed by the first demultiplexer, and outputs a first optical signal at an output. The second transponder receives a given one of the separate wavelengths demultiplexed by the second demultiplexer, and outputs a second optical signal at an output. The coupler has first and second inputs connected to the respective outputs of the first and second transponders, and an output for outputting an optical signal received at one of the first and second inputs thereof. A determining means determines if the first transponder is outputting the first optical signal. If so, the second transponder is inhibited from outputting the second optical signal so that the coupler outputs the first optical signal. If not, the second transponder is not inhibited and the coupler outputs the second optical signal.

Brief Description of The Drawings Fig. 1 is a schematic diagram of a prior art scheme for protecting against light path failure at a multiple wavelength facility level;

Figs. 2A and 2B when taken together as shown in Fig. 2 form a block diagram of a WDM optical communication system in which light path failure is protected against on a per-channel basis; and

Figs. 3A, 3B and 3C when taken together as shown in Fig. 3 form a flow chart of the control mechanism for protecting against light path failure on a per-channel basis .

Detailed Description Refer now to Fig. 2 which is a block diagram of a WDM optical communication system 50 for protecting against light path failure on a per-channel basis when transmitting optical signals from a source optical node 51 to a sink optical node 52 via an optical network 76. In the description that follows system operation is described for a single given wavelength in a plurality of wavelengths that are propagated from the source optical node 51 to the sink optical node 52 as a multiple wavelength facility signal, and that at least one intermediate node may be included in the light path which includes an add/drop multiplexer for adding/dropping wavelengths to/from the multiple wavelength facility signal. It is to be appreciated that the remaining ones of the plurality of wavelengths are propagated in a like manner. Likewise, it is understood that the plurality of wavelengths are propagated in the reverse direction from the sink optical node 52 to the source optical node 51 in a similar manner.

Referring to Fig. 2A, the WDM system 50 includes the source node 51 having a client equipment 53 which outputs an optical signal at a given wavelength to an optical splitter 54 which splits the input optical signal into first and second optical signals of the given wavelength which are input to Optical Line Terminals (OLT's) 56 and 58, respectively. In practice, there are other client equipments (not shown) which input other wavelengths to other optical splitters (not shown), which split the other wavelengths into respective first and second optical signals for input to OLT ' s 56 and 58, respectively. The client equipment 53 may be any one of a computer, a SONET terminal, a telephone switch, a central office switch for telephones, a digital cross-connect switch, an end device such as a terminal, or the like.

OLT 56 includes a transponder 60, a processor 62 and a multiplexer 64. It is understood that in practice, OLT 56 also includes a demultiplexer (not shown) for propagating optical signals received in the opposite direction from the sink node 52 to the source node 51. It is also understood that OLT 56 includes other transponders (not shown) for receiving other first optical signals at different wavelengths from the other optical splitters (not shown).

The processor 62 receives system protocols and

Identification Codes (ID'S) from a system manager computer (not shown) on line 66, and reports back status and the like on that line to the system manager computer. The processor 62 controls and exchanges information with transponder 60 via line 68a, and exchanges information with the other transponders (not shown), via lines 68b-68n, which provide remaining ones of the plurality of wavelengths to the multiplexer 64 on lines 70b-70n. The processor 62 communicates with a corresponding processor 80 in OLT 58 via line 72.

The first optical signal at the given wavelength is received at a Portside Input (PI) interface for transponder 60 (TI), which interface is termed PI(TI) and is output at a Lineside Output (LO) interface for transponder 60 (TI), which interface is termed LO(Tl). PI(T1) and L0(T1) serve as test points to test for the presence of the first optical signal at the input and output, respectively, of transponder 60. The first optical signal output from transponder 60 on line 70a is multiplexed with the other wavelengths on lines 70b- 70n by multiplexer 64 to form a first multiple wavelength optical facility signal which is output on optical fiber 74 to the network 76.

A third optical signal at the given wavelength is received at a lineside input (LI) interface for transponder 60 (TI), which interface is termed LI(T1). The third optical signal is then provided to client equipment 52 via a coupler (not shown). The third optical signal is demultiplexed from a third multiple wavelength facility signal which is provided to a demultiplexer (not shown) in OLT 56 from interface LO(Tl') of a transponder 99 in an OLT 94 at sink node 52 via the network 76. This is described in more detail with respect to Fig. 2B.

OLT 58 includes a transponder 78, a processor 80 and a multiplexer 82. It is understood that in practice, OLT 58 also includes a demultiplexer (not shown) for propagating optical signals received in the opposite direction from the sink node 53 to the source node 51. It is understood that OLT 58 includes other transponders (not shown) for receiving other second optical signals at different wavelengths from the other optical splitters (not shown).

The processor 80 receives system protocols and ID codes from the system manager (not shown) on line 82 and reports back status and the like on that line to the system manager. The processor 80 controls and exchanges information with transponder 78 via line 84a,

SUBSTITUTE SHEtr (RULE 26) and exchanges information with the other transponders (not shown), via lines 84b-84n, which provide remaining ones of the plurality of wavelengths to the multiplexer 82 on lines 86b-86n. The processor 80 communicates with processor 62 of OLT 56 via the line 72.

The second optical signal at the given wavelength is received at a portside input interface PI(T2) for transponder 78 and is output at a lineside output interface L0(T2). PI(T2) and LO(T2) serve as test points to test for the presence of the second optical signal at the input and output, respectively, of transponder 78. The second optical signal output from transponder 78 on line 86a is multiplexed with the other wavelengths on lines 86b-86n by multiplexer 82 to form a second multiple wavelength optical facility signal which is output on optical fiber 82 to the network 76.

A fourth optical signal at the given wavelength is received at a lineside input (LI) interface for transponder 78 (T2), which interface is termed LI(T2). The fourth optical signal is then provided to client equipment 53 via a coupler (not shown). The fourth optical signal is demultiplxed from a fourth multiple wavelength facility signal which is provided to a demultiplexer (not shown) in OLT 58 from interface L0(T2') of a transponder 112 in an OLT 96 at sink node 52 via the network 76. This is described in more detail with respect to Fig. 2B.

The network 76, for example, may be a point-to-point link, a point-to-multi-point link, a ring, a mesh or any other network configuration including intermediate optical nodes such as OLTs or Optical Add/Drop Multiplexers .

The network 76 then outputs the first and second multiple wavelength facility signals on optical fibers 90 and 92, respectively, to the sink node 52.

Referring to Fig. 2B, the sink node 52 includes OLT's 94 and 96 and a client equipment 98. The client equipment may be any one of a computer, a SONET terminal, a telephone switch, a central office switch for telephones, a digital cross-connect switch, an end device such as a terminal, or the like. OLT 94 includes a demultiplexer 97, a transponder 99 and a processor 100. It is understood that in practice, OLT 94 also includes a multiplexer (not shown) for propagating optical signals in the opposite direction from the sink node 52 to the source node 51.

The processor 100 receives system protocols and IDs from the system manager computer (not shown) on line 102 and reports back status and the like on that line to the system manager computer. The processor 100 controls and exchanges information with transponder 99 via line 102a, and exchanges information with other transponders (not shown), via lines 102b-102n, which receive remaining ones of the plurality of wavelengths from the demultiplexer 97 on lines 104b-104n. The processor 100 communicates with a corresponding processor in OLT 96 via line 106.

The first optical signal demultiplexed from the first facility signal by demultiplexer 97 is provided on line 104a to lineside input interface LI(Tl') of transponder 99 and is output at portside output interface PO(Tl') to an optical coupler 108. For propagation of an optical signal at the given wavelength in the opposite direction from the sink node 53 to the source node 51, the client equipment 98 provides an optical signal at the given wavelength to an optical splitter 109 which splits that signal into third and fourth optical signals at the given wavelength for provision to OLT's 94 and 96, respectively.

The third optical signal at the given wavelength is received at a portside input port interface PI(TI') of transponder 99 and is output at a lineside output port interface LO(Tl') thereof for provision via the network 76 to a multiplexer (not shown) in OLT 94 which generates a third multiple wavelength facility signal for provision to interface LI(T1) of OLT 56 of source node 51 via the network 76.

Operability of transponder 99 is determined by testing for the presence of the first optical signal at interface LI(Tl') and PO(Tl'), and by testing for the presence of the third optical signal at interface PI(Tl') and LO(Tl'). This is explained in more detail with respect to Fig. 3. OLT 96 includes a demultiplexer 110, a transponder 112 and a processor 114. It is understood that in practice, OLT 96 also includes a multiplexer (not shown) for propagating optical signals in the opposite direction from the sink node 52 to the source node 51.

The processor 114 receives system protocols and ID'S from the system manager computer (not shown) on line 116 and reports back status and the like on that line to the system manager computer. The processor 114 controls and exchanges information with transponder 112 via line 118a, and exchanges information with other transponders (not shown), via lines 118b-118n, which receive remaining ones of the plurality of wavelengths from the demultiplexer 110 on lines 120b-120n. The processor 114 communicates with processor 100 in OLT 94 via the line 106.

The second optical signal demultiplexed from the second facility signal by demultiplexer 110 is provided on line 120a to lineside input interface LI(T2') of transponder 112 and is output at portside output interface PO(T2') to the output coupler 108. The fourth optical signal at the given wavelength is received from splitter 108 at a portside input interface PI(T2') of transponder 112 and is output at a lineside output port interface L0(T2*) thereof for provision via network 76 to a multiplexer (not shown) in OLT 96 which generates a fourth multiple wavelength facility signal for provision to interface LI(T2) of OLT 58 of source node 51 via the network 76.

As discussed above, the coupler 108 is connected to interface PO(Tl') of transponder 99 of OLT 94 and interface PO(T2') of transponder 112 of OLT 96 for receiving either the first optical signal or the second optical signal as controlled by processor 100 of OLT 94 according to the control flow chart of Fig. 3 for outputting the received optical signal to client equipment 98. If it is determined that transponder 99 is transmitting the first and third optical signals, transponder 112 of OLT 96 is inhibited from outputting the second optical signal and coupler 108 only receives the first optical signal from transponder 99 of OLT 94, which in turn is provided to client equipment 98. On the other hand, if it is determined that transponder 99 is not transmitting either one of the first and third optical signals, transponder 99 in OLT 94 is inhibited from outputting the first optical signal and the coupler 108 only receives the second optical signal from transponder 112 OF OLT 96, which in turn is provided to client equipment 98. This is explained in more detail below with respect to Fig. 3.

In practice, there are other couplers (not shown), each receiving other first and second optical signals from the other transponders (not shown) for outputting one of the other first and optical signals to other client equipment (not shown). Likewise, couplers (not shown) are used to source node 51 for coupling respective wavelengths received from sink node 52 via the network 76 to other client equipment.

Also in practice, there are other splitters (now shown) for coupling other individual wavelengths from other client equipment (not shown) to the other transponders (not shown) in OLT's 94 and 96.

Figure 3 is a block diagram of the control protocol for protecting against light path failures on a per-channel basis. For purposes of explanation, the control protocol is described as being run on processor 100 of OLT 94. However, it is to be appreciated that a like protocol is run on processor 114 of OLT 96, and is also run on processor 62 of OLT 56 and processor 80 of OLT 58.

Referring to Fig. 3A, at step SI processor 100 of OLT 94 gets an ID of 100 from the system manager computer (not shown) via the system management interface, such as CMIPT, SNMP, or TL1, and likewise the processor 114 of OLT 96 gets an ID of 20. Likewise, the system management interface provides IDs for processors 62 and 82 of OLTs 56 and 58, respectively. These ID'S are unique, and for purposes of explanation the ID of OLT 94 (OLT 1') is 100 and the ID of OLT 96 (OLT 2') is 20. Therefore, the ID of OLT 1' > ID of OLT 2'. The ID of OLT 56 (OLT 1) is 10 and the ID of OLT 58 (OLT 2) is 2. Therefore, the ID of OLT 1 > ID of OLT 2.

The following steps determine the operability of transponder (Tl') 99 of OLT 94 based on the failure to detect light at the respective interfaces of transponder 99. At step S2, if transponder (Tl') 99 is OFF, it is turned ON. At step S3 a determination is made as to whether or not there has been a failure to detect light at interface PI(TI'). If light is detected at step S3, at step S4 a determination is made as to whether or not interface PI(Tl') doesn't receive a SONET frame. If PI(Tl') does receive a SONET frame, at step S5 a determination is made as to whether or not transponder (Tl') 99 has failed. If transponder 99 hasn't failed, control proceeds to step S7 (Fig. 3B) .

If the answer is Yes at any one of steps S3, S4 or S5, at step S6 a SONET Alarm Indication Signal (AIS) is transmitted from interface LO(Tl') of transponder 99 of OLT 94 at sink node 52 to transponder 60 of OLT 56 of source node 51 via the network 76, and control proceeds to step S7 (Fig. 3B).

Referring to Fig. 3B, at step S7 a determination is made as to whether or not there has been a failure to detect light at interface LI(Tl'). If light is detected, a determination is made at step S8 as to whether or not interface LI(Tl') doesn't receive a SONET frame. If LI(Tl') does receive a SONET frame, at step S9 a determination is made as to whether a SONET AIS frame is received at interface LI(Tl') from

SUBSTITUTE SHEET (RULE 25) source node 51, which is indication of a failure at the source node 51. If not, control proceeds to step Sll (Fig. 3C).

If the answer is YES at any one steps S7, S8 or S9, at step S10 the transponder (Tl') 99 of OLT 94 is turned OFF and a return is made to step S2 of Fig. 3A via Al . Since only the transponder (T2') 112 of OLT 96 is ON, coupler 108 provides the second optical signal to client equipment 98.

Referring to Fig. 3C, at step Sll a determination is made as to whether or not transponder 99 (Tl') of OLT 94 has failed. If it has failed, a return is made to step S9 of Fig. 3B via A , and transponder 99 (Tl') of OLT 94 is turned OFF. If transponder 99 (Tl") hasn't failed, at step S12 processor 100 of OLT 94 sends a message via line 106 to processor 114 of OLT 96 to turn OFF transponder 112 (T2'), so that coupler 108 only receives the first optical signal which is then received by client equipment 98. At step S13 a determination is made as to whether or not processor 100 in OLT 94 (OLT 1' ) is receiving a message from processor 114 in OLT 96 OLT2 ' ) and if the ID of the received message from processor 114 of OLT 96 is greater than the ID of processor 100 of OLT 94. That is, is the ID (OLT 2') > ID (OLT 1'). If the answer is YES a return is made to step S10 of Fig. 3B via A4 and transponder 99 (Tl') of OLT 94 is turned OFF. If the answer is NO a return is made to S2 via A2 and the procedure is repeated. In this instance, since the answer is No, the return is made to S2 via A2.

Thus, according to the control protocol, protection is provided against lightpath failure, and coupler 108 only receives either the first or second optical signal at any given time for provision to client equipment 98.

In summary, in the apparatus of the present invention in a WDM optical communication system, light path failure is protected against on a per-channel wavelength basis using passive optics such as splitters and couplers which are less susceptible to failure than active devices such as switches.

Although certain embodiments of the invention have been described and illustrated herein, it will be readily apparent to those of ordinary skill in the art that a number of modifications and substitutions can be made to the preferred example methods and apparatus disclosed and described herein without departing from the true spirit and scope of the invention.

Claims

What Is Claims Is:

1. In a wavelength division multiplexed communication system, apparatus for protection against a lightpath failure from a source optical node to a sink optical node, comprising: said source optical node having first and second outputs for respectively outputting single wavelength first and second optical signals; a first lightpath, with said first lightpath being coupled to the first output of said source optical node for receiving the first optical signal; a second light path, with said second lightpath being coupled to the second output of said source optical node for receiving the second optical signal; said sink optical node having first and second inputs and first and second outputs, with the first input being coupled to said first lightpath for receiving the first optical signal and for outputting the first optical signal at the first output, with the second input being coupled to said second lightpath for receiving the second optical signal and for outputting the second optical signal at the second output; coupling means having first and second inputs and an output, with the first and second inputs being coupled to the first and second outputs of said sink optical node, respectively, and for outputting at the output an optical signal received at one of the first and second inputs; and determining means for determining if said sink optical node is outputting the first optical signal, and if so, inhibiting said sink optical node from outputting the second optical signal so that said coupling means outputs the first optical signal, and if not, not inhibiting said sink optical node from outputting the second optical signal so that said coupling means outputs the second optical signal.

2. The wavelength division multiplexed communication system according to Claim 1, wherein the first and second optical signals have a same wavelength.

3. The wavelengtn division multiplexed communication system according to Claim 1, wherein at least one of said first and second lightpaths includes at least one intermediate optical node.

SUBSTITUTE SHEET (RULE 25)

4. The wavelength division multiplexed communication system according to Claim 3, wherein said at least one intermediate optical node comprises one of an optical line terminal and an add/drop multiplexer.

5. The wavelengtn division multiplexed communication system according to Claim 1, wherein said first and second light paths are part of an optical network.

6. The wavelength division multiplexed communication system according to Claim 1, wherein said coupling means is a passive coupler.

7. The wavelength division multiplexed communication system according to Claim 1, further comprising: an input means for inputting a third optical signal to a third input of said sink optical node, for outputting the third optical signal at a third output of said sink optical node for transmission to said source optical node, with said determining means further determining if said sin]; optical node is outputting the third optical signal in addition to the first optical signal, and if so, inhibiting said sink optical node from outputting the second optical signal so that said coupling means outputs the first optical signal, and if not, not inhibiting said sink optical node from outputting the second optical signal so that said coupling means outputs the second optical signal.

8. In a wavelength division multiplexed communication system, a method of protecting against a lightpath failure from a source optical node to a sink optical node, comprising: an outputting step at said source optical node of outputting onto a first lightpath a single wavelength first optical signal, and of outputting onto a second lightpath a single wavelength second optical signal; a receiving step at said sink optical node of receiving from the first light path at a first input the first optical signal, and of receiving from the second light path at a second input the second optical signal; an outputting step at said sink optical node of outputting at a first output the first optical

SUBSTITUTE SHtEr (HULE 26) signal, and of outputting at a second output the second optical signal; a coupling step at said sink, node of coupling the first and second outputs of said sink node to first and second inputs of a coupling means, with the coupling means having an output for outputting an optical signal coupled to one of the first and second inputs thereof; and a determining step of determining if said sink optical node is outputting the first optical signal, and if so, inhibiting said sink optical node from outputting the second optical signal so that said coupling means outputs the first optical signal, and if not, not inhibiting saic sink optical node from outputting the second optical signal so that said coupling means outputs the second signal.

9. The wavelength division multiplexed communication system method according to Claim 8, wherein the first and second optical signals have a same wavelength.

10. The wavelength division multiplexed communication system method according to Claim 8, wherein at least one of said first and second lightpaths includes at least one intermediate optical node .

11. The wavelength division multiplexed communication system method according to Claim 10, wherein said at least one intermediate optical node comprises one of an optical line terminal and an add/drop multiplexer.

12. The wavelength division multiplexed communication system metnod according to Claim 8, wherein said first and second light paths are part of an optical network.

13. The wavelength division multiplexed communication system metnod according to Claim 8, wherein said coupling me .ns is a passive coupler.

14. The wavelength division multiplexed communication system method according to Claim 8, further comprising: an input step of inputting a third optical signal to a third input of said sink optical node, for outputting the third optical signal at a third output of said sink optical node for transmission to said source optical node, with said determining step further determining if said sink optical node is outputting the third optical signal in addition to the first optical signal, and if so, inhibiting said sink optical node from outputting the second optical signal so that said coupling means outputs the first optical signal, and if not, not inhibiting said sink optical node from outputting the second optical signal so that said coupling means outputs tr.e second optical signal.

15. In a wavelength division multiplexed communication system, apparatus for protecting against a failure in a light patn from a source optical node to a sink optical node on a per-channel basis, the combination comprising: input means for inputting a single wavelength optical signal; an optical splatter for splitting the input single wavelength optical signal into first and second optical signals; said source optical node comprising: multiplexing means for separately multiplexing the first and second optical signals with other optical signals of different wavelengths for respectively outputting on first and second light paths first and second multiple wavelength signals; said sink optical node comprising: demultiplexing means for separately demultiplexing the first and second multiple wavelength signals into separate wavelengths including the first and second optical signals; first and second transponders for receiving the first and second optical signals at respective inputs thereof, with said first and second transponders each having an output at which the first and second optical signals are provided; a coupler having first and second inputs connected to the outputs of said first and second transponders, respectively, and an output for outputting an optical signal received at one of the first and second inputs thereof; and determining means for determining if said first transponder is outputting the first optical signal, and if so, inhibiting said second transponder from outputting the second optical signal so that said

SUBSTITUTE SHEET (RULE 25) coupler outputs the first optical signal, and if not inhibiting said second transponder so that said coupler outputs the second optical signal.

16. The wavelength division multiplexed communication system according to Claim 15, wherein the first and second outicai signals have a same wavelength.

17. The wavelength division multiplexed communication system according to Claim 15, wherein at least one of said first and second lightpaths includes at least one intermediate optical node.

18. The wave division multiplexed communication system according to Claim 17, wherein said at least one intermediate optical noci comprises one of an optical line terminal and an add/drop multiplexer.

19. The wavelength division multiplexed communication system according to Claim 15, wherein said first and second l_rht paths are part of an optical network.

20. The wavelength division multiplexed communication system according to Claim 15, wherein said coupling means is a passive coupler.

21. The combination claimed in Claim 15, wherein said second optical node further comprises: an input means for inputting a third optical signal at the given wavelength to another input of said first transponder :or outputting at another output of said first transponder for transmission to said first optical node, with saiα determining means further determining if said first transponder is outputting the third optical signal in addition to the first optical signal, and if so, inhibiting said second transponder from outputting the second optical signal so that said coupler outputs the first optical signal, and if not, not inhibiting said second transponder so that said coupler outputs the second optical signal.

22. In a wavelength division multiplexed communication system, apparatus for protecting against a failure in a light path from a source optical node to a sink optical node, including at least one intermediate optical node, comprising: output means at said source optical node for outputting first and second multiple wavelength signals on respective first and second light paths; said at least one intermediate node including an add/drop multiplexer for adding/dropping at least one wavelength to/rrom the first and second multiple wavelength signals; said sink optical node comprising: a first demultiplexer for demultiplexing the first multiple wavelength signal into separate wavelengths ; a second demultiplexer for demultiplexing the second multiple wavelength signal into separate wavelengths ; said sin - optical node further comprising, for each separate wavelength: a first transponder for receiving at an input a given one of the separate wavelengths demultiplexed by said first demultiplexer, and for outputting a first optical signal at ^~ τι output; a second transponder for receiving at an input a given one of the separate wavelengths demultilexed by said second demultiplexer, and for outputting a seconα optical signal at an output; a coupler having first and second inputs connected to the outputs of said first and second transponders, respectively, and an output for outputting an optical signal received at one of the first and second inputs tnereof; and determining means for determining if said first transponder is outputting the first optical signal, and if so, inhibiting said second transponder from outputting the second optical signal so that said coupler outputs the first optical signal, and if not, not inhibiting said second transponder so that said coupler outputs the second optical signal.

23. The wavelength division multiplexed communication system according to Claim 22, wherein the first and second optical signals have a same wavelength.

24. The wavelength division multiplexed communication system according to Claim 22, wherein said coupler is a passive coupler.

25. The combination claimed in Claim 22, wherein said sink optical node further comprises: an input means for inputting a third optical signal at the given wavelength to another input of said first transponder tor outputting at another output of said first transponder for transmission to said source optical node, with said determining means further determining if said first transponder is outputting the third optical signal in addition to the first optical signal, and if so, inhibiting said second transponder from outputting the second optical signal so that said coupler outputs the first optical signal, and if not, not inhibiting said second transponder so that said coupler outputs the second optical signal.