Abstract:
A wavelength division multiplexed optical communication system includes a plurality of optical line terminals which may be part of separate in service networks, each having a line interface and an all-optical pass-through interface including a plurality of pass-through optical ports, and each also including a plurality of local optical ports which are connectable to client equipment and an optical multiplexer/demultiplexer for multiplexing/demultiplexing optical wavelengths. The optical multiplexer/demultiplexer may include one or more stages for inputting/outputting individual wavelengths or bands of a predetermined number of wavelengths, or a combination of bands and individual wavelengths. At least one of the pass-through optical ports of an optical line terminal of one network may be connected to at least one of the pass-through optical ports of an optical line terminal of another network to form an optical path from the line interface of the optical line terminal of the one network to the line interface of the optical line terminal of the another network to form a merged network. The use of such optical line terminals allows the upgrading and merging of the separate networks while in service.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 10/737,765, filed Dec. 18, 2003, now U.S. Pat. No. 7,369,772, which is a division of application Ser. No. 09/293,775, filed Apr. 19, 1999, now U.S. Pat. No. 6,721,508, issued Apr. 13, 2004, which claims the benefit of U.S. Provisional Application No. 60/112,510, filed Dec. 14, 1998. 
    
    
     FIELD OF THE INVENTION 
     The invention is in the field of optical telecommunications, and more particularly, pertains to upgrading an in-service wavelength division multiplexed (WDM) optical communication system including a pair of optical line terminals (OLTs) that reside in the same office and are part of separate WDM networks to form an all optical pass-through from the line side of one OLT of the pair to the line side of the other OLT of the pair. 
     BACKGROUND OF THE INVENTION 
     Wavelength division multiplexing (WDM) is an approach for increasing the capacity of existing fiber optic networks. A WDM system employs plural optical signal channels, each channel being assigned a particular channel wavelength. In a WDM system optical signal channels are generated, multiplexed to form an optical signal comprised of the individual optical signal channels, transmitted over a single waveguide, and demultiplexed such that each channel wavelength is individually routed to a designated receiver. 
     SUMMARY OF THE INVENTION 
     In typical wavelength division multiplexing systems all wavelengths are constrained to pass through from a source optical node to a predetermined sink optical node. 
     In view of the above it is an aspect of the invention to selectively pass-through, add or drop individual wavelengths at selected optical nodes. 
     It is another aspect of the invention to utilize optical line terminals having all-optical pass-through interfaces that provide for continued transmission of optical signals without any intervening electro-optical conversion, and to connect two optical line terminals back-to-back at their respective pass-through interfaces to provide an optical path from the line side interface of the first optical line terminal to the line side interface of the second optical line terminal. 
     It is yet another aspect of the invention to utilize optical line terminals having a multiplexer/demultiplexer including one or more stages for inputting/outputting individual wavelengths or bands of a predetermined number of wavelengths, or a combination of bands and individual wavelengths. 
     It is a further aspect of the invention to utilize the optical line terminals to support complex mesh network structures while permitting growth of an in-service network without disrupting network service. 
     It is yet a further aspect of the invention to provide a wavelength division multiplexed optical communication system including a plurality of optical line terminals, each having a line interface and an all-optical pass-through interface including a plurality of pass-through optical ports and each also including a plurality of local optical ports and an optical multiplexer/demultiplexer for multiplexing/demultiplexing transmitted/received wavelengths. The optical multiplexer/demultiplexer may include one or more stages for inputting/outputting individual wavelengths or bands of a predetermined number of wavelengths, or a combination of bands and individual wavelengths, with at least one of the pass-through optical ports of one of the optical line terminals being connected to at least one of the pass-through optical ports of another optical line terminal to form an optical path from the line side interface of the one of the optical line terminals to the line side interface of the another optical line terminal. 
     These and other aspects and advantages of the invention will be apparent to those of skill in the art from the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an optical line terminal; 
         FIG. 2  is a flow chart of the control steps executed by the controller  10  of  FIG. 1 ; 
         FIG. 3  is a block diagram of an optical line terminal having a two-stage multiplexer/demultiplexer; 
         FIG. 4  is a schematic diagram representative of the optical line terminal of  FIG. 1  or  FIG. 3 ; 
         FIG. 5  is a schematic diagram of two optical line terminals such as in  FIG. 4  being connected back-to-back; 
         FIG. 6  is a diagram illustrating how at least two separate point-to-point WDM systems can be upgraded while in-service to form a merged point-to-point WDM system; 
         FIG. 7  is a diagram illustrating how at least two separate network WDM systems can be upgraded while in-service to form a merged network WDM system; and 
         FIG. 8  illustrates a mesh connection between a plurality of optical line terminals. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram of an optical line terminal (OLT)  2  which is the basic element of the present embodiment. The OLT  2  has an input/output line interface  4  which is connected to an external fiber facility and transmits/receives an optical signal having N optical wavelengths, for example 32 wavelengths, on a single optical fiber which is multiplexed/demultiplexed by a multiplexer/demultiplexer  6 , which outputs demultiplexed wavelengths λ 1 -λN on individual optical fibers. The respective wavelengths λ 1 -λN are sent either to a peer OLT via a pass-through port or to client equipment via a transponder and a local port. The client equipment includes SONET equipment, add/drop multiplexers, cross-connect switches, internet protocol (IP) routers, asynchronous transfer mode switches (ATM) and the like. 
     As employed herein an optical signal is generally intended to encompass wavelengths in the range of approximately 300 nanometers to approximately 2000 nanometers (UV to far IR). This range of wavelengths can be accommodated by the preferred type of optical conductor (a fiber optic), which typically operates in the range of approximately 800 nanometers to approximately 1600 nanometers. 
     Consider λ 1  which is provided to a 1×2 switch  8  which is controlled by a control signal, having at least N states, from a controller  10 . The controller  10  responds to a command, from a management system (not shown), at a terminal  12  to provide the control signal at a terminal  14  and then to control terminal  16  of switch  8  to position the switch  8  in a first or second position. When in the first position, λ 1  is provided to a transponder  18  which transmits λ 1  to a client apparatus  20  via a local port  19 . When in the second position λ 1  is provided to a pass-through port  22  to a corresponding pass-through port in a peer OLT  24 . The control signal is also provided to output terminal  15 , and then to control terminal  16  of a corresponding switch  8  in peer OLT  24  to route λ 1  to the corresponding multiplexer/demultiplexer  6 . If it is desired to send λ 1  to both client apparatus  20  and peer OLT  24 , an optical splitter can be used in place of the switch  8 . 
     Switch  26  selects λ 1  coming from the opposite direction in response to a control signal at terminal  28  from controller  10  to position switch  26  in a first or second position. When in the first position, λ 1  is received from client  20  via local port  19  and transponder  18 , and when in the second position λ 1  is received from peer OLT  24  via pass-through port  22 , and then is provided to multiplexer/demultiplex  6  to be multiplexed with the other received wavelengths λ 2 -λN. 
     A wavelength can be directly passed-through to a peer OLT rather than being sent to a client apparatus. For example, λ 2  is directly sent to, and received from, peer OLT  30  via pass-through port  32 . 
     A 1×N switch can be used to send/receive a wavelength to/from one of N−1 peer OLTs or a client apparatus. For example, 1×N switch  34  under control of a control signal, having at least N states, provided to terminal  36  from controller  10  sends λ 3  to either peer OLT  38  via pass-through port  40 , or peer OLT  42  via pass-through port  44 , or peer OLT  46  via pass-through port  48  or client apparatus  50  via transponder  52  and local port  53 . Reception of λ 3  in the opposite direction is controlled by N×1 switch  54  under control of a control signal provided to terminal  56  from controller  10 , and than is provided to multiplexer/demultiplexer  6  to be multiplexed with the other received wavelengths. 
     As discussed above, a wavelength can be passed-through to a peer OLT via a pass-through port or can be optically switched to a client apparatus via a local port. λN is shown as being directly passed through to, or received from, peer OLT  60  via pass-through port  62 . 
       FIG. 2  is a flow chart of the steps performed by the controller  10  of  FIG. 1  to control the 1×2 switches  8  and  26 , and the 1×N switches  34  and  54  to route the respective wavelengths λ 1 -λN. 
     In step S 101  the controller  10  waits for a command from a management system such as a computer (not shown). At step S 102  a determination is made as to whether or not the command is a switch control signal to either pass-through the wavelength via a pass-through port to a peer OLT or drop/add the wavelength locally at/from a client apparatus via a transponder and a local port. If the answer is no, the command is handled by another interface (not shown) at step S 103 . If the answer is yes, a signal is sent to switch A (for example switch  8  or  34 ) to move switch A to transmit position X (the selected position) at step S 104 , and at S 105  a signal is sent to switch B (for example switch  26  or  54 ) to move switch B to receive position X (the selected position). At step  106  the control signal at terminal  15  of controller  10  is sent to the peer OLT to set its switches A and B in a corresponding manner. A loop-back is then made to step S 101  to wait for the next command. 
     In the multiplexer/demultiplexer  6  of  FIG. 1 , 32 wavelengths on a single optical fiber received at line interface  4  are demultiplexed into 32 individual wavelengths λ 1 -λ 32 . However, according to another aspect of the invention the 32 wavelengths can be demultiplexed into bands, for example four bands of 8 wavelengths each, by a first multiplexer, and the resultant four bands can be processed by the OLT. According to another aspect of the invention at least one of the four bands of wavelengths can be demultiplexed by a second multiplexer/demultiplexer into its individual wavelengths such that the OLT can process the individual wavelengths of the at least one band and the remaining ones of the four bands. 
       FIG. 3  is a block diagram illustrating a modular OLT  200  having two stages of multiplexing/demultiplexing. The operation of the OLT  200  is described with respect to the demultiplexing operation; however, it is to be understood that the multiplexing is merely the reverse operation. It is to be noted that the 1×2 switches and 1×N switches shown in  FIG. 1  are not included in  FIG. 3  in order to simplify the drawing. However, it is to be understood that in practice such switches may be utilized in the practice of the invention. The OLT terminal  200  has an input/output line interface  202  which is connected to an external fiber facility and receives on a single optical fiber N, for example 32, wavelengths which are demultiplexed by a multiplexer/demultiplexer  204 , which is situated on a first modular card  215 , into M, for example 4, bands of 8 wavelengths each. The first band  206  (λ 1 -λ 8 ) is demultiplexed into its  8  individual wavelengths by a multiplexer/demultiplexer  208 , which is situated on a second modular card  218 , with each such wavelength being provided to a pass-through port (P) or a local port (L) via transponder (T). Each of the pass-through ports (P) is situated on a different modular card (MCP), and each of the transponder (T) and its associated local port (L) are situated together on yet another modular card (MCL). Although direct connections are shown, as discussed above the respective wavelengths may be selectively switched to either of a local port (L) via transponder (T), or a pass-through port (P) as described with respect to  FIG. 1 . band  210  (λ 9 -λ 16 ) is provided directly to a pass-through port (P), and the third band  212  (λ 17 -λ 24 ) is provided directly to a pass-through port (P). 
     The fourth band  214  (λ 25 -λ 32 ) is demultiplexed into its  8  individual wavelengths by a multiplexer/demultiplexer  216 , which is situated on a modular card  217 , with each such wavelength being provided to a pass-through port (P) or a local port (L) via a transponder (T). Again, switching may be used to select a connection to either P or T. 
       FIG. 4  is a simplified schematic diagram representative of the OLT  2  shown in  FIG. 1  or the OLT  200  of  FIG. 3 . However, it is to be noted that for simplicity only 16 wavelengths are utilized. The OLT  300  interfaces and operates in a bidirectional manner as discussed in detail with respect to  FIGS. 1 and 3 . The line interface  302  is adapted for wavelength division multiplexed (WDM) optical communication signals of the highest relative order, in this example 16 wavelengths λ 1 -λ 16 , corresponding to the N optical wavelengths on a single optical fiber which are applied to input/output line interfaces  4  and  202  of OLT  2  ( FIG. 1 ) and OLT  200  ( FIG. 3 ), respectively. The pass-through interface connected to the lines WL  1 - 4 , WL  5 - 8 , WL  9 - 12  and WL  13 - 16  corresponds to the respective pass-through ports, and the local-interface connected to the lines labeled  16  local ports correspond to the local ports connected to the respective transponders, where wavelengths from or to client equipment are added or dropped. 
       FIG. 5  illustrates two OLTs  300 A and  300 B as shown in  FIG. 4  connected in a back-to-back relationship by way of their respective all-optical pass-through interfaces. Thus, it is seen that the connection results in an optical add/drop multiplexer (OADM) functionality without requiring intermediate electro-optical conversion (OEO) of the communicated optical signals. As discussed above, the add/drop feature is achieved at the 16 local ports of each OLT, where channels (wavelengths) can be added or dropped by a manual configuration, or via add/drop switching, as controlled by switches  8  and  26  of  FIG. 1 , to achieve a switchable add/drop multiplexer. 
     The pass-through may be accomplished using single conductors and/or ribbon connectors that pass multiple individual channels (wavelengths) in one cable. The pass-through connections between OLTS  300 A and  300 B is preferably made using ribbon connectors/cables. 
       FIG. 6  illustrates three separate in-service WDM point-to-point optical communication systems A, B and C which are not initially interconnected. WDM system A includes optical nodes  400  and  402  which are optically connected via their respective line interfaces, with at least optical node  402  being an OLT. WDM system B includes optical nodes  404  and  406  which are optically connected via their respective line interfaces, with at least optical node  404  being an OLT. WDM system C includes optical nodes  408  and  410  which are optically connected via their respective line interfaces, with at least optical node  408  being an OLT. 
     As discussed above, the three separate WDM systems are not initially interconnected. However, any two of the three WDM systems, or all three of the WDM systems, may be interconnected by connecting respective OLTs of the separate WDM system back-to-back at respective pass-through ports as shown in  FIG. 5 , without disrupting service. For example, WDM system A may be connected to WDM system B by directly optically connecting pass-through ports of the OLT of node  402  to pass-through ports of the OLT of node  404  via optical fibers  416  and  418 . WDM system A may also be connected to WDM system C by directly optically connecting pass-through optical ports of the OLT of node  402  to pass-through ports of the OLT of node  408  via optical fibers  420  and  422 . Thus, an all optical path is provided from optical node  400  of WDM system A to optical node  406  of WDM system B, and likewise an all optical path is provided from optical node  400  of WDM system A to optical node  410  of WDM system C, resulting in a merger of WDM systems A, B and C without disrupting service. At the back-side of the respective optical nodes, lines with a box are indicative of local ports (L) to which client equipment is normally connected. 
       FIG. 7  illustrates three separate in-service WDM network optical communication systems D, E and F which are not initially interconnected. WDM system D includes optical nodes  500  and  502  which are optically connected via their respective line interfaces through an optical network  503 , with at least optical node  502  being an OLT. WDM system E includes optical nodes  504  and  506  which are optically connected via their respective line interfaces through an optical network  507 , with at least optical node  504  being an OLT. WDM system F includes optical nodes  508  and  510  which are optically connected via their respective line interfaces through an optical network  511 , with at least optical node  508  being an OLT. 
     As discussed above, the three separate WDM optical networks are not initially interconnected. However, any two of the three WDM optical networks, or all three of the WDM optical networks may be interconnected by connecting respective OLTs of the separate WDM optical networks back-to-back at respective pass-through ports as shown in  FIG. 5 , without disrupting service. For example, WDM optical network D may be connected to WDM optical network E by directly optically connecting pass-through ports of the OLT of node  502  to pass-through ports of the OLT of node  504  via optical fibers  516  and  518 . WDM system D may also be connected to WDM optical network F by directly optically connecting pass-through optical ports of the OLT of node  502  to pass-through ports of the OLT of node  508  via optical fibers  520  and  522 . Thus, an all optical path is provided from optical node  500  of WDM optical network D to optical node  506  of WDM optical network E, and likewise an all optical path is provided from optical node  500  of WDM optical network D to optical node  510  of WDM optical network F, resulting in a merger of WDM network optical communication systems D, E and F without disrupting service. At the back-side of the respective optical nodes, lines with a box are indicative of local ports (L) to which client equipment is normally connected. 
       FIG. 8  illustrates how OLTs can be connected in more complex ways to achieve greater functionality, such as, for example, limited cross-connection capabilities. Specifically, OLT  600  and OLT  602  are connected back-to-back to form a first OADM, OLT  604  and OLT  606  are connected back-to-back to form a second OADM, OLT  600  and OLT  606  are connected back-to-back to form a third OADM and OLT  602  and OLT  604  are connected back-to-back to form a fourth OADM. OLT  600 , OLT  602  and OLT  604  each have add/drop switching capability, whereas OLT  606  has no switching capability. 
     The arrangement shown in  FIG. 8  illustrates how a group of OLTs in an office, which may be part of separate WDM networks, can be coupled to form different OADMs on an individual channel or per band basis. Wavelengths  1 ,  2 ,  3  and  4  (channels  1 ,  2 ,  3  and  4 ) are connected between pass-through optical ports of OLT  600  and OLT  602  via optical fiber  603  and are also connected between pass-through optical ports of OLT  604  and OLT  606  via optical fiber  607 . Wavelengths  5 ,  6 ,  7  and  8  (channels  5 ,  6 ,  7  and  8 ) are connected between pass-through optical ports of OLT  600  and OLT  606  via optical fiber  608  and are also connected between pass-through optical ports of OLT  602  and OLT  604  via optical fiber  609 . Wavelengths  9 ,  10 ,  11  and  12  (channels  9 ,  10 ,  11  and  12 ) can be separated into individual channels that are connected between local ports of the respective OLTs. For example, channel  9  is directly connected between a local port of OLT  600  and a local port of OLT  602  via optical fiber  610 , and channel  10  is directly connected between a local port of OLT  600  and a local port of OLT  606  via optical fiber  612 . To simplify the drawing, no connections are shown for wavelengths  11  and  12 ; however, they may be connected in a like manner. The local ports may also be connected to client equipment as discussed above. It is to be noted that the connection configuration of  FIG. 8  does not constitute a plain patch-panel form of connectivity, insofar as it allows for switching of channels without manual reconfigurations. 
     In summary, the methods and apparatus of the present invention allow upgrading of a wavelength division multiplexed optical communication system including a pair of OLTs that reside in the same office or facility and are part of separate WDM networks (whether point-to-point links or more advanced networks) to form an OADM. Such upgrade is accomplished without service disruption to the network by appropriate connection of the OLTs through the pass-through interfaces. 
     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.