# Patent

## Title
Optical line terminal arrangement, apparatus and methods

## 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.

## Background
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 λ1-λN on individual optical fibers. The respective wavelengths λ1-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 λ1 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, λ1 is provided to a transponder 18 which transmits λ1 to a client apparatus 20 via a local port 19. When in the second position λ1 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 λ1 to the corresponding multiplexer/demultiplexer 6. If it is desired to send λ1 to both client apparatus 20 and peer OLT 24, an optical splitter can be used in place of the switch 8.
Switch 26 selects λ1 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, λ1 is received from client 20 via local port 19 and transponder 18, and when in the second position λ1 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 λ2-λN.
A wavelength can be directly passed-through to a peer OLT rather than being sent to a client apparatus. For example, λ2 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 λ3 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 λ3 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 λ1-λN.
In step S101 the controller 10 waits for a command from a management system such as a computer (not shown). At step S102 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 S103. 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 S104, and at S105 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 S101 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 λ1-λ32. 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 (λ1-λ8) 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 (λ9-λ16) is provided directly to a pass-through port (P), and the third band 212 (λ17-λ24) is provided directly to a pass-through port (P).
The fourth band 214 (λ25-λ32) 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 λ1-λ16, 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 300A and 300B 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 300A and 300B 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.

## Claims

1. A wavelength division multiplexed (WDM) optical communication system comprising:
a first optical line terminal having an input line-side interface adapted to receive a WDM optical communication signal communicated to the system across an optical fiber, a first transponder, an optical splitter coupled to the first transponder, a first all-optical pass-through interface, and a second all-optical pass-through interface;
a second optical line terminal having an output line-side interface adapted to transmit a WDM optical communication signal from the system across an optical fiber, an optical switch that can be used to select a wavelength to couple to said output line-side interface, and a first all-optical pass-through interface; and
a third optical line terminal having an output line-side interface adapted to transmit a WDM optical communication signal from the system across an optical fiber, and a first all-optical pass-through interface;
the first all-optical pass-through interface of said first optical line terminal being connected to the first all-optical pass-through interface of said second optical line terminal to couple a first wavelength from the input line-side interface of said first optical line terminal through said splitter to said first transponder and, if said switch selects said wavelength, through said splitter and said switch to the output line-side interface of said second optical line terminal,
the second all-optical pass-through interface of said first optical line terminal being connected to the first all-optical pass-through interface of said third optical line terminal to couple a second wavelength from the input line-side interface of said first optical line terminal to the output line-side interface of said third optical line terminal,
said second optical line terminal further comprising a second transponder, wherein said switch couples a wavelength from said second transponder to the output line-side interface of said second optical line terminal through said switch if said switch alternatively selects said wavelength.

2. The wavelength division multiplexed optical communication system of claim 1, wherein said third optical line terminal further comprises an input line-side interface adapted to receive a WDM optical communication signal communicated to the system across an optical fiber, and a second all-optical pass-through interface connected to a second all-optical pass-through interface of said second optical line terminal to couple a wavelength from the input line-side interface of said third optical line terminal to the output line-side interface of said second optical line terminal through said optical switch if said switch alternatively selects said wavelength.

3. The wavelength division multiplexed optical communication system of claim 2, wherein said first all-optical pass-through interface of said first optical line terminal is connected to the first all-optical pass-through interface of said second optical line terminal through a first optical fiber, and said second all-optical pass-through interface of said first optical line terminal is connected to the first all-optical pass-through interface of said third optical line terminal through a second optical fiber that is not the first optical fiber, and said second all-optical pass-through interface of said third optical line terminal is connected to said second all-optical pass-through interface of said second optical line terminal through a third optical fiber that is not the first optical fiber and that is not the second optical fiber, and wherein the system is further adapted to communicate optical signals bidirectionally through each of said all-optical pass-through interfaces, and wherein an interface that is the input line-side interface of said third optical line terminal is also an output line-side interface.

4. An optical wavelength division multiplexed (WDM) add/drop multiplexing system, comprising:
a first input line-side interface and an output line-side interface;
a first transponder to which a first optical path of the system extends from the first input line-side interface, wherein the first transponder is an electro-optical conversion (OEO) device configured to receive a network-side optical signal and in response output a client-side optical signal, and wherein the system is operable to communicate a wavelength along the first optical path from the first input line-side interface to the first transponder to drop the wavelength, and wherein along the first optical path, between the first input line-side interface and the first transponder, the first optical path is established by at least an optical splitter;
a second transponder, wherein the second transponder is an electro-optical conversion (OEO) device configured to receive a client-side optical signal and in response output a network-side optical signal;
a second optical path, wherein in a first state of the system the second optical path extends from the first input line-side interface to the output line-side interface and, along the second optical path between the first input line-side interface and the output line-side interface, is established by at least the optical splitter and an optical switch, and wherein in the first state of the system the system is operable to communicate a wavelength along the second optical path from the first input line-side interface to the output line-side interface without any intervening electro-optical conversion (OEO) to pass the wavelength;
and wherein in a second state of the system the second optical path alternatively extends from the second transponder to the output line-side interface and, along the second optical path between the second transponder and the output line-side interface, is established by at least the optical switch, and wherein in the second state of the system the system is operable to communicate a wavelength along the second optical path from the second transponder to the output line-side interface to add the wavelength;
and wherein the optical splitter is configured to receive an optical signal comprising at least one wavelength, and to output at least two optical signals each comprising the at least one wavelength, wherein one of the at least two optical signals is output on the first optical path, and another of the at least two optical signals is output on an optical path that is the second optical path in at least the first state of the system and is not the first optical path.

5. The optical add/drop multiplexing system of claim 4, further comprising a second input line-side interface that is not the first input line-side interface, and wherein in a third state of the system the second optical path alternatively extends from the second input line-side interface to the output line-side interface and, along the second optical path between the second input line-side interface and the output line-side interface, is established by at least the optical switch, and wherein in the third state of the system the system is operable to communicate a wavelength along the second optical path from the second input line-side interface to the output line-side interface without any intervening electro-optical conversion (OEO) to pass the wavelength.

6. An optical wavelength division multiplexed (WDM) add/drop multiplexing system, comprising:
a first input line-side interface;
an output line-side interface;
an optical demultiplexer configured to receive a plurality of wavelengths multiplexed together as an optical signal, and to output each of the plurality of wavelengths as an individual wavelength;
an optical multiplexer configured to receive a plurality of individual wavelengths, and to output the plurality of wavelengths multiplexed together as an optical signal;
a first transponder to which a first optical path of the system extends from the first input line-side interface, wherein the first transponder is an electro-optical conversion (OEO) device configured to receive a network-side optical signal and in response output a client-side optical signal, and wherein the system is operable to communicate a wavelength along the first optical path from the first input line-side interface to the first transponder to drop the wavelength, and wherein along the first optical path, between the first input line-side interface and the first transponder, the first optical path is established by at least an optical splitter;
a second transponder, wherein the second transponder is an electro-optical conversion (OEO) device configured to receive a client-side optical signal and in response output a network-side optical signal;
an optical switch;
a second optical path, wherein in a first state of the system:
the second optical path extends from the first input line-side interface to the output line-side interface and is established by at least the optical splitter, the optical demultiplexer, the optical switch and the optical multiplexer;
along the second optical path, between the first input line-side interface and the optical switch, the second optical path is established by at least the optical splitter;
along the second optical path, between the optical splitter and the optical multiplexer and also between the optical demultiplexer and the optical multiplexer, the second optical path is established by at least the optical switch;
along the second optical path, between the optical switch and the output line-side interface, the second optical path is established by at least the optical multiplexer;
along the second optical path, between the first input line-side interface and the optical switch, the second optical path is established by at least the optical demultiplexer; and
the system is operable to communicate a wavelength along the second optical path from the first input line-side interface to the output line-side interface without any intervening electro-optical conversion (OEO) to pass the wavelength;
and wherein in a second state of the system:
the second optical path alternatively extends from the second transponder to the output line-side interface and is established by at least the optical switch and the optical multiplexer;
along the second optical path, between the second transponder and the optical multiplexer, the second optical path is established by at least the optical switch;
along the second optical path, between the optical switch and the output line-side interface, the second optical path is established by at least the optical multiplexer; and
the system is operable to communicate a wavelength along the second optical path from the second transponder to the output line-side interface to add the wavelength;
and wherein the optical splitter is not the optical demultiplexer, and wherein the optical splitter is configured to receive an optical signal comprising at least one wavelength, and to output at least two optical signals each comprising the at least one wavelength, wherein one of the at least two optical signals is output on the first optical path, and another of the at least two optical signals is output on an optical path that is the second optical path in at least the first state of the system and is not the first optical path.

7. The optical add/drop multiplexing system of claim 6, further comprising a second input line-side interface that is not the first input line-side interface, and wherein in a third state of the system:
the second optical path alternatively extends from the second input line-side interface to the output line-side interface and is established by at least the optical switch and the optical multiplexer;
along the second optical path, between the second input line-side interface and the optical multiplexer, the second optical path is established by at least the optical switch;
along the second optical path, between the optical switch and the output line-side interface, the second optical path is established by at least the optical multiplexer; and
the system is operable to communicate a wavelength along the second optical path from the second input line-side interface to the output line-side interface without any intervening electro-optical conversion (OEO) to pass the wavelength.

8. A method for an optical add/drop multiplexing system in an optical wavelength division multiplexed (WDM) network environment, comprising the steps of:
establishing a first optical path extending from a first input line-side interface of the system to a first transponder, wherein the first transponder is an electro-optical conversion (OEO) device configured to receive a network-side optical signal and in response output a client-side optical signal, and wherein along the first optical path, between the first input line-side interface and the first transponder, the first optical path is established by at least an optical splitter;
establishing a second optical path, wherein in a first state of the system:
the second optical path extends from the first input line-side interface to an output line-side interface and is established by at least the optical splitter, a first optical port, a second optical port that is not the first optical port, an optical demultiplexer, an optical switch and an optical multiplexer, and wherein the optical demultiplexer is configured to receive a plurality of wavelengths multiplexed together as an optical signal and to output each of the plurality of wavelengths as an individual wavelength, and wherein the optical multiplexer is configured to receive a plurality of individual wavelengths and to output the plurality of wavelengths multiplexed together as an optical signal;
along the second optical path, between the first input line-side interface and the first optical port, the second optical path is established by at least the optical splitter;
along the second optical path, between the optical splitter and the second optical port, the second optical path is established by at least the first optical port;
along the second optical path, between the first optical port and the optical switch, the second optical path is established by at least the second optical port;
along the second optical path, between the second optical port and the optical multiplexer and also between the optical demultiplexer and the optical multiplexer, the second optical path is established by at least the optical switch;
along the second optical path, between the optical switch and the output line-side interface, the second optical path is established by at least the optical multiplexer;
along the second optical path, between the first input line-side interface and the optical switch, the second optical path is established by at least the optical demultiplexer;
the second optical path extends from the first input line-side interface to the optical splitter in common with the first optical path; and
the second optical path extends from the optical splitter to the output line-side interface not in common with the first optical path;
and wherein in a second state of the system:
the second optical path alternatively extends from a second transponder to the output line-side interface and is established by at least the optical switch and the optical multiplexer, wherein the second transponder is an electro-optical conversion (OEO) device configured to receive a client-side optical signal and in response output a network-side optical signal;
along the second optical path, between the second transponder and the optical multiplexer, the second optical path is established by at least the optical switch; and
along the second optical path, between the optical switch and the output line-side interface, the second optical path is established by at least the optical multiplexer;
and wherein in a third state of the system:
the second optical path alternatively extends from a second input line-side interface to the output line-side interface and is established by at least the optical switch and the optical multiplexer, wherein the second input line-side interface is not the first input line-side interface;
along the second optical path, between the second input line-side interface and the optical multiplexer, the second optical path is established by at least the optical switch; and
along the second optical path, between the optical switch and the output line-side interface, the second optical path is established by at least the optical multiplexer;
communicating along the first optical path, from the first input line-side interface to the first transponder, a wavelength of a WDM optical signal received by the system at the first input line-side interface, to drop the wavelength;
in the first state of the system, communicating without any intervening electro-optical conversion (OEO) along the second optical path a wavelength of the WDM optical signal received by the system at the first input line-side interface, to pass the wavelength to a WDM optical signal transmitted by the system from the output line-side interface;
in the second state of the system, alternatively communicating along the second optical path a wavelength to add the wavelength to a WDM optical signal transmitted by the system from the output line-side interface;
in the third state of the system, alternatively communicating along the second optical path a wavelength of a WDM optical signal received by the system at the second input line-side interface, to pass the wavelength to a WDM optical signal transmitted by the system from the output line-side interface; and
wherein the optical splitter is not the optical demultiplexer, and wherein the optical splitter is configured to receive an optical signal comprising at least one wavelength, and to output at least two optical signals each comprising the at least one wavelength, wherein one of the at least two optical signals is output on the first optical path, and another of the at least two optical signals is output on another optical path that is the second optical path in at least the first state of the system.

9. The method of claim 8, wherein at least a portion of the first optical path extends from one modular card to another modular card, and wherein the system is further adapted to communicate optical signals bidirectionally through at least the first and second optical ports.

10. The method of claim 8, wherein at least a portion of the second optical path extends from one modular card to another modular card in at least the first state of the system.

11. The method of claim 10, wherein at least a portion of the second optical path extends from one modular card to another modular card in each of the second state of the system and the third state of the system.