Abstract:
An optical communication system comprising plural optical communication paths, a passive optical coupler, and at least one optical node. The passive optical coupler has a plurality of inputs coupled through the passive optical coupler to an output of the passive optical coupler, and each input is coupled in a corresponding one of the optical communication paths. Each optical node comprises plural optical line terminals, each of which is interposed in a corresponding at least one of the optical communication paths. Each optical line terminal is operable to either (a) substantially prevent an optical signal propagating in that corresponding at least one optical communication path from reaching a corresponding input of the passive optical coupler through the at least one optical communication path, or (b) permit the optical signal to propagate in that corresponding at least one optical communication path towards the corresponding input of the passive optical coupler.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/112,412, filed Dec. 14, 1998. 
    
    
     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 protection 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 ,  3 B and  3 C 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&#39;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&#39;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&#39;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  68   a , and exchanges information with the other transponders (not shown), via lines  68   b - 68   n , which provide remaining ones of the plurality of wavelengths to the multiplexer  64  on lines  70   b - 70   n . 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  (T 1 ), which interface is termed PI(TI) and is output at a Lineside Output (LO) interface for transponder  60  (T 1 ), which interface is termed LO(T 1 ). PI(T 1 ) and LO(T 1 ) 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  70   a  is multiplexed with the other wavelengths on lines  70   b - 70   n  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  (T 1 ), which interface is termed LI(T 1 ). 
     The third optical signal is then provided to client equipment  53  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(T 1 ′) 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.  2 B. 
     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  52  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  84   a , and exchanges information with the other transponders (not shown), via lines  84   b - 84   n , which provide remaining ones of the plurality of wavelengths to the multiplexer  82  on lines  86   b - 86   n . 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(T 2 ) for transponder  78  and is output at a lineside output interface LO(T 2 ). PI(T 2 ) and LO(T 2 ) 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  86   a  is multiplexed with the other wavelengths on lines  86   b - 86   n  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  (T 2 ), which interface is termed LI(T 2 ). 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 LO(T 2 ′) 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.  2 B. 
     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&#39;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  102   a , and exchanges information with other transponders (not shown), via lines  102   b - 102   n , which receive remaining ones of the plurality of wavelengths from the demultiplexer  97  on lines  104   b - 104   n . 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  104   a  to lineside input interface LI(T 1 ′) of transponder  99  and is output at portside output interface PO(T 1 ′) 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&#39;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(T 1 ′) 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(T 1 ) 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(T 1 ′) and PO(T 1 ′), and by testing for the presence of the third optical signal at interface PI(T 1 ′) and LO(T 1 ′). 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&#39;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  118   a , and exchanges information with other transponders (not shown), via lines  118   b - 118   n , which receive remaining ones of the plurality of wavelengths from the demultiplexer  110  on lines  120   b - 120   n . 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  120   a  to lineside input interface LI(T 2 ′) of transponder  112  and is output at portside output interface PO(T 2 ′) to the output coupler  108 . 
     The fourth optical signal at the given wavelength is received from splitter  108  at a portside input interface PI(T 2 ′) of transponder  112  and is output at a lineside output port interface LO(T 2 ′) 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(T 2 ) of OLT  58  of source node  51  via the network  76 . 
     As discussed above, the coupler  108  is connected to interface PO(T 1 ′) of transponder  99  of OLT  94  and interface PO(T 2 ′) 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  941  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 (not shown) for coupling other individual wavelengths from other client equipment (not shown) to the other transponders (not shown) in OLT&#39;s  94  and  96 . 
       FIG. 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 S 1  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 TL 1 , 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&#39;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 ′&gt;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 &gt;ID of OLT  2 . 
     The following steps determine the operability of transponder (T 1 ′)  99  of OLT  94  based on the failure to detect light at the respective interfaces of transponder  99 . At step S 2 , if transponder (T 1 ′)  99  is OFF, it is turned ON. At step S 3  a determination is made as to whether or not there has been a failure to detect light at interface PI(T 1 ′). If light is detected at step S 3 , at step S 4  a determination is made as to whether or not interface PI(T 1 ′) doesn&#39;t receive a SONET frame. If PI(T 1 ′) does receive a SONET frame, at step S 5  a determination is made as to whether or not transponder (T 1 ′)  99  has failed. If transponder  99  hasn&#39;t failed, control proceeds to step S 7  (FIG.  3 B). 
     If the answer is Yes at any one of steps S 3 , S 4  or S 5 , at step S 6  a SONET Alarm Indication Signal (AIS) is transmitted from interface LO(T 1 ′) 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 S 7  (FIG.  3 B). 
     Referring to  FIG. 3B , at step S 7  a determination is made as to whether or not there has been a failure to detect light at interface LI(T 1 ′). If light is detected, a determination is made at step S 8  as to whether or not interface LI(T 1 ′) doesn&#39;t receive a SONET frame. If LI(T 1 ′) does receive a SONET frame, at step S 9  a determination is made as to whether a SONET AIS frame is received at interface LI(T 1 ′) from source node  51 , which is indication of a failure at the source node  51 . If not, control proceeds to step S 11  (FIG.  3 C). 
     If the answer is YES at any one steps S 7 , S 8  or S 9 , at step S 10  the transponder (T 1 ′)  99  of OLT  94  is turned OFF and a return is made to step S 2  of  FIG. 3A  via A 1 . Since only the transponder (T 2 ′)  112  of OLT  96  is ON, coupler  108  provides the second optical signal to client equipment  98 . 
     Referring to  FIG. 3C , at step S 11  a determination is made as to whether or not transponder  99  (T 1 ′) of OLT  94  has failed. If it has failed, a return is made to step S 9  of  FIG. 3B  via A 3 , and transponder  99  (T 1 ′) of OLT  94  is turned OFF. If transponder  99  (T 1 ′) hasn&#39;t failed, at step S 12  processor  100  of OLT  94  sends a message via line  106  to processor  114  of OLT  96  to turn OFF transponder  112  (T 2 ′), so that coupler  108  only receives the first optical signal which is then received by client equipment  98 . At step S 13  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  (OLT 2 ′) 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 ′)&gt;ID (OLT  1 ′). If the answer is YES a return is made to step S 10  of  FIG. 3B  via A 4  and transponder  99  (T 1 ′) of OLT  94  is turned OFF. If the answer is NO a return is made to S 2  via A 2  and the procedure is repeated. In this instance, since the answer is No, the return is made to S 2  via A 2 . 
     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.