Abstract:
A method of validating connections in an optical add/drop multiplexer (OADM) that includes a plurality of modules configured to route optical signals through the OADM, and at least one multi-fiber cable connecting modules of the OADM. A light source coupled to a first port of a first module is controlled to emit a test light. A determination is made whether or not the test light is received at a first photo-detector connected to a second port. Continuity of a connection between the first port and the second port is validated when the test light is received at the first photo-detector.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is the first application filed in respect of the present invention. 
     TECHNICAL FIELD 
     The present invention relates generally to fibre-optic communication systems, and in particular to techniques and systems for validating optical connections in a Reconfigurable Optical Add-Drop Multiplexer (ROADM) node. 
     BACKGROUND 
     Fibre optic networks often employ tunable technologies both for optical add/drop ports and for transmitters. Transmitters using tunable lasers are desirable for several reasons. First, tunable lasers reduce the number of product variants required to construct the network. For example, a Dense Wavelength Division Multiplexed (DWDM) communications band typically has 80 channels. If fixed wavelength (i.e. non-tunable) lasers are used to drive this channel band, then 80 different variants of the transmitter laser are required. This is problematic, especially for customers who must maintain an inventory of appropriate transmitter components for replacement in the case of failure (sometimes called “sparing”). The second reason is that the use of tunable lasers enables re-tuning the transmission wavelength of any given channel in the system for the purpose of reconfiguration, which in turn enables the implementation of a Reconfigurable Optical Add/Drop Multiplexer (ROADM). 
     The Add portion of a ROADM can be made tunable by including a tunable filter which is tuned in conjunction with the transmitter laser. Prior to the introduction of practical Digital Signal Processor (DSP) based coherent transmitters, it was commonplace to use Distributed Feedback (DFB) or Distributed Bragg Reflector (DBR) tunable laser designs, which have significant out of band noise in the form of side modes and spontaneous emission. This noise needed to be rejected, which drove the need for filtering the laser output light. 
     Prior to the introduction of practical DSP based coherent receivers, it was commonplace to use direct detection receivers. In a DWDM system, direct detection receivers require optical filters to separate a desired one wavelength channel from the DWDM signal, and present the separated channel light to the receiver for detection. This type of receiver can detect any wavelength which the optical filter chooses. Therefore, the drop portion of the OADM can be made tunable by including a tunable filter. 
     However, tunable filters are expensive. Reducing the number of tunable filters is advantageous. With coherent transmitters/receivers, it is possible to reduce or eliminate the filtering from the adds/drops. For example, please refer to PCT/CA2009/001455 titled COHERENT AUGMENTED OPTICAL ADD-DROP MULTIPLEXER and filed on Sep. 11, 2009 which is herein incorporated by reference in its entirety. The result is to replace the optical filters with couplers and splitters which are not wavelength selective. 
       FIG. 1  is a block diagram schematically illustrating elements of an 8-degree Colorless Directionless Contentionless (CDC) ROADM  2  known in the prior art. In the example of  FIG. 1 , the ROADM  2  generally comprises a respective Wavelength Selective Switch (WSS) module  4  for routing wavelength channels to and from each degree; MUX/DEMUX modules  6  for routing wavelength channels to and from respective transceivers  8 ; and a Fiber Interconnection Module (FIM)  10  for optically interconnecting the WSS and MCS modules. 
     Each transceiver  8  typically comprises a transmitter and a receiver (not shown in  FIG. 1 ). In some ROADM systems, the transmitter and receiver are configured to operate at the same wavelength, although this is not essential. 
     In the illustrated ROADM, each Wavelength Selective Switch (WSS) module  4  is provided as a 1×20 WSS. In the ingress direction, each WSS is capable of routing individual WDM channels received from its respective degree via its common-IN port to any of its twenty output ports. In the egress direction, the WSS can select WDM channels from twenty different input ports and couple them to its respective degree via its common-OUT port. 
     In the illustrated ROADM, each MUX/DEMUX module  6  is configured using an 8×16 Multi-Case Switch (MCS). In the ingress direction, each MCS  6  is capable of routing optical signals received through any of its eight input ports to any combination of its sixteen transceiver ports. In the egress direction, the MCS couples light received from any combination of its sixteen transceiver ports to any of its eight output ports. 
     The Fiber Interconnection Module (FIM)  10  is typically provided as a patch panel providing all of the necessary interconnections between the WSSs  4  and MCSs  6 . By means of optical connections within the FIM  10 , every WSS  4  is connected to every other WSS  4  and to every MCS  6 . For example, the FIM  10  provides connections for coupling each of the eight output ports of an MCS  6  to an input port of each of the eight WSSs  4 , so that optical signals from all eight degrees can be coupled to any transceiver  8  subtending any one of the twelve MCSs  6 . In addition, the FIM  10  provides connections for coupling each of the eight WSS&#39;s to all of the other WSS&#39;s, so that an optical signal received from one degree can be optically routed to another degree. For the sake of clarity of illustration  FIG. 1  only shows optical connections through the FIM  10  between the WSS module  4  on Degree #1 and each of the twelve MCS modules  6  and the respective WSS modules on each of the other degrees. It will be appreciated that each of the optical connections shown in  FIG. 1  would normally be implemented as a pair of optical paths to enable bi-directional optical signal flow, and that a corresponding set of connections would be provided for connecting to WSS modules  4  on all of the other degrees both to each other and to each of the MCS modules  6 . 
     Because of the large number of fiber interconnections between the FIM  10  and each of the WSSs  4  and MCSs  6 , it is advantageous to make use of multi-fiber patch-cord such as Multiple-Fiber Push-On/Pull-off (MPO) connectors or Multiple-Fiber Push-On (MTP) cables in order to simplify the cabling process. In the example of  FIG. 1 , each 1×20 WSS  4  includes a conventional duplex Local Connector (LC)  12  to connect its common-IN and common-OUT ports to the fiber pair corresponding to a particular degree, as well as 4 MPO connectors  14 , each with 12 optical fibers. Each WSS  4  uses 2×20=40 of the 48 available fibers, leaving 8 unused fibers across the 4 MPO connectors  14 . Similarly, each MCS module  6  has 2 MPO terminations  16  (for a total of 24 fibers) carrying 2×8=16 active fibers and 8 unused fibers. Each MCS  6  also has 16 transceiver-facing duplex LCs  18  to connect to up to sixteen individual transceivers  8 . 
     One challenge of using multi-fiber connectors like MPO&#39;s is that the user must be able to confirm during the initial installation that all fibers within a particular MPO have continuity and acceptable losses, including fibers that may not be in use initially but that could become active after a node upgrade. The reason for this requirement is that the use of an MPO cable makes it impossible to service individual fibers, i.e. if a problem is found on a previously unused fiber at the time of the node upgrade, all 12 fibers must be disconnected at once to service the cable, which could be interrupting traffic in a section of the ROADM that was already up and running. 
     It is, therefore, desirable to provide a connection validation technique capable of validating all fibers of a multi-fiber cable, including fibers that are not currently in use. 
     SUMMARY 
     Aspects of the present invention provide methods and systems to validate physical connections within an optical add/drop multiplexer (OADM). 
     According to one aspect, the present invention provides a method of validating connections in an optical add/drop multiplexer (OADM) that includes a plurality of modules configured to route optical signals through the OADM, and at least one multi-fiber cable connecting modules of the OADM. A light source coupled to a first port of a first module is controlled to emit a test light. A determination is made whether or not the test light is received at a first photo-detector connected to a second port. Continuity of a connection between the first port and the second port is validated when the test light is received at the first photo-detector. 
     Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Representative embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which: 
         FIG. 1  is a block diagram schematically illustrating elements of a conventional 8-degree Colorless Directionless Contentionless (CDC) Reconfigurable Optical Add-Drop Multiplexer (ROADM) known in the art; 
         FIGS. 2A-2E  are block diagrams schematically illustrating respective different scenarios for verifying connections in the ROADM of  FIG. 1 , in accordance with a representative embodiment of the present invention; 
         FIG. 3  is a block diagram schematically illustrating the ROADM of  FIG. 1  with loop-back connectors on un-used terminals of the Fiber Interconnection Module (FIM), in accordance with a representative embodiment of the present invention; 
         FIGS. 4A-4B  are block diagrams schematically illustrating the loop-back connectors of  FIG. 3  in greater detail; 
         FIGS. 5A-5E  illustrate respective different scenarios for verifying connections in the ROADM of  FIG. 3 , in accordance with a representative embodiment of the present invention; and 
         FIG. 6  is a block diagram illustrating use of an MCS module of  FIG. 3  in another scenario for verifying connections in the ROADM of  FIG. 3 , in accordance with a representative embodiment of the present invention. 
     
    
    
     It will be noted that throughout the appended drawings, like features are identified by like reference numerals. 
     DETAILED DESCRIPTION 
     Generally, the present invention provides a method and system for connection testing and/or validation of a multi-fiber patch-cord or cable in a Reconfigurable Optical Add-Drop Multiplexer (ROADM). 
     In general terms, the present technique a connection validation (CV) procedure that is capable of confirming continuity and losses on every individual fiber connection within the ROADM node. This method relies on the ability to generate and detect light on every possible connection, including future connections in a node that is not yet fully populated. The present technique is both efficient and cost effective, largely reusing components already found in the ROADM elements, with the addition of a few items. 
     In some embodiments, the present technique may be implemented by means of software instructions stored on a suitable memory, for execution by either a management processor of a node or a computer connected to the node. This enables an automated process for validating connections in the node. 
     In embodiments of the present technique, a connection may be validated by inserting test light into the connection and detecting that light at a downstream location. If the light is detected, then optical continuity it between the source and the detector is confirmed. For example, light may be inserted into a connection within an MCS module. Detection of the inserted light by a photodetector in a WSS module confirms optical continuity of the connections between the MCS and the WSS modules via the FIM. In embodiments of the present technique, the following hardware elements may be used, either alone or in any suitable combination: 
     1/ One or more Light sources which can be coupled to each output port of a WSS module  4  or an MCS module  6  connected (via an MPO connector) to the Fiber Interconnection Module (FIM)  10 ; 
     2/ Photodetectors which can detect the presence (and optionally power levels) of optical signals at each input port of a WSS module  4  or an MCS module  6  connected (via an MPO connector) to the FIM  10 ; and 
     3/ Loopback connectors at an unused port of the FIM  10 . 
     There are a number of options for implementing the light sources and detectors within the WSS and MCS modules  4  and  6 , some of which are shown in  FIGS. 2A to 2E . In the example of  FIG. 2A , a dedicated light source  20 ,  22  is multiplexed into each WSS and MCS output port through a WDM filter  24 ,  26 . In some embodiments a 1310 nm source may be used but other wavelengths are possible. Preferably, the wavelength of the light source  20 ,  22  is selected to avoid overlap with the signal band. Each WSS and MCS input port has a tap and a photodetector (PD)  28 ,  30  capable of detecting light from the dedicated light source  20 ,  22 . It would also be possible to use a WDM filter instead of a tap. When multi-fiber cables are installed between the WSS  4  and MCS  6  modules and the FIM  10 , the light sources  20 ,  22  may be activated and the photodetector signals analysed to verify continuity through the FIM  10 . In embodiments in which the output power of each light source  20 ,  22  is known, the photodetector signals may also be analysed to determine insertion loss of the path through the FIM  10 . In some embodiments, calibration of a light source may be achieved by providing a tap photodetector (not shown) at the output of each light source  20 ,  22  to measure the launch power. There is also an option to implement a signaling protocol, for example by modulating a light source  20 ,  22  with data to provide a unique identifier corresponding to a particular port on a particular card, for example. This information can then be detected in the photodetector signals to allow confirmation of the point of origin of any given signal, which can allow the ROADM to “auto-discover” its topology. 
     One disadvantage of the embodiment of  FIG. 2A  is that it requires a large number of additional optical components. For example, twenty light sources  20  and WDM filters  24  are required in each WSS module  4  and eight light sources  22  and WDM filters  26  are needed in each MCS module  6 . These additional light sources and filters increase cost and complexity of the WSS and MCS modules. 
       FIG. 2B  shows an alternative arrangement with functionality similar to that of  FIG. 2A , but using with shared light sources. In this example, a single wavelength DFB laser  32  is coupled through a WDM filter  34  to the common input port of the WSS. The wavelength of this source needs to be within the transmission band of the WSS, but is preferably outside the signal band. One implementation example would be to have the DFB laser  32  at 1528.0 nm in a system with a signal band ranging from 1528.8 nm to 1566.7 nm, with a WSS capable of routing wavelengths from 1528.0 to 1566.7 nm. In this case, the WSS can switch the DFB signal to any of its 20 output ports, which allows the system software to perform CV on all the corresponding connections by cycling through them one by one. Because the CV wavelength does not overlap with WDM traffic, this function can be done in service. A similar approach can be used in the MCS module  6 , however in this case an additional 1×8 switch  36  must be provided to couple the DFB source  38  to any one of the 8 possible outputs going to the FIM  10 . 
     In some embodiments, each output port of an MCS module  6  includes a respective Erbium Doped Fiber Amplifier (EDFA)  40 . In such cases, the dedicated light source of  FIGS. 2A and 2B  may be eliminated and the EDFA used as the light source instead. This arrangement is shown in  FIG. 2C . In the absence of an input optical signal, EDFA&#39;s can easily be turned into ASE (amplified spontaneous emission) sources by running the pump lasers of the EDFA under constant current. Many conventional MCS module designs require amplification and in the example shown in  FIG. 2C , each output port of the MCS  6  has an EDFA  40  which can be used as a controllable ASE source. Note that an EDFA is a broadband source that overlaps with the signal band, so this option can only be enabled with MCS ports that are out of service, which is still very useful for connection validation.  FIG. 2C  also shows a similar ASE source  42  in the WSS module  4 , but in this case it is coupled to the signal path via a narrow-band WDM filter  44 , with the filter wavelength selected to be outside the signal band, e.g. at 1528.0 nm. This allows a single shared ASE source to be switched to the different WSS output ports without affecting the WDM traffic that may already be going through the node. Although EDFA ASE sources cannot be modulated at high speed, low-speed signaling is still possible by turning the EDFA&#39;s on and off to create data patterns. 
       FIG. 2D  shows an example in which the EDFA used as an ASE source within the WSS can also provide an amplified loopback path. In this case, the common output port of the WSS  4  is coupled to the input of the EDFA via a WDM filter  46  at the dedicated CV wavelength (e.g. 1528.0 nm). As shown in  FIG. 2D , this makes it possible to configure the ROADM node in a loopback mode where a local transceiver  8  can tune to the CV wavelength and then be routed through the WSS  4  and back onto itself. This allows the system software to verify that a) the local transceiver  8  is operational and b) that it is properly connected to the ROADM. This type of feature is particularly important in colorless ROADMs to avoid wavelength collisions between mis-provisioned or mis-connected channels. 
       FIG. 2E  illustrates an example in which each WSS module  4  is implemented as a 2×20 WSS, which operates to couple wavelength channels from any of its 20 input ports to either one of its 2 common-OUT ports, and in the ingress direction it can couple wavelength channels from any of its 2 common-IN ports to any of its 20 output ports. In the example of  FIG. 2E , one each of the common-IN and common-OUT ports are used as loopback ports. This arrangement removes the need to use a WDM filter to couple Connection Validation (CV) signalling through the loopback path, which has the advantage of reducing the insertion loss for the WDM traffic going through the node. Another advantage is both the signal path and the loopback path are now more broadband, which provides more flexibility, for example allowing multiple XCVR&#39;s to be looped back simultaneously through the same WSS using different wavelengths. 
     In accordance with the present technique, out-of service (un-used) connections can be validated by means of loopback connectors at each un-used port of the FIM. As mentioned above, the FIM is provisioned with a pair of optical paths connecting each degree port to all of the other degree ports and all of the MCS ports.  FIG. 3  illustrates the connections between three degree ports and three MCS ports of the FIM  10 . The other WSS and MSC ports (and their associated connections) are not illustrated for the sake of clarity of the drawing, but can be readily constructed from the information presented in  FIG. 3 . In the illustration of  FIG. 3 , one of the degree ports and one of the MCS ports are unused, and so are coupled to respective loop-back connectors  46 . It is contemplated that all unused WSS and MSC ports of the FIM  10  would be coupled to suitable loop-back connectors. As may be seen in  FIGS. 4A and 4B , each loop-back connector  46  is configured to optically connect corresponding pairs of fibers, so that an optical signal will be routed back to the particular module from which that signal was sent. 
       FIG. 5A  illustrates the signal paths through the FIM between the WSS module on degree 1 and the WSS module on degree 8. This enables the connections between these two modules to be validated using light sources and detectors within each WSS module as described above with reference to  FIGS. 2A-E   
       FIG. 5B  shows a similar example where the connections between the WSS module on degree 1 and a loop-back connector on degree port  4  are validated. In this case, the light sources and detectors in the WSS module are used to validate the connections in both directions (to and from the loop-back connector). In so doing, the unused fibers in the MPO connectors between the WSS module and the FIM can be validated, even though they are currently unused. 
       FIG. 5C  illustrates the signal paths through the FIM between the WSS module on degree 1 and the MCS module #1. This enables the connections between these two modules to be validated using light sources and detectors within each module as described above with reference to  FIGS. 2A-E   
       FIG. 5D  shows how a connection from a WSS module to a missing MCS module can be validated using the loopback connector provisioned at the MCS port. In this example, the WSS module on degree 1 is sending a signal from Demux port  5  and detecting that same signal at Mux port  5 , after it loops back through the FIM. This ensures that the dark fibers that will be used at a later date to provision a new MCS have continuity and sufficiently low losses.  FIG. 5D  shows a similar example where a connection between MCS module #1 and a missing WSS module (in this case on degree 4) can be validated by means of light sources and detectors in the MCS module and a loop-back connector on the degree port of the FIM. 
       FIG. 6  illustrates an alternative arrangement in which a DFB laser  50  and a photodetector  52  are connected to a respective port of an MCS module  6 . In some embodiments, the DFB laser and photodetector may be external to the MCS module, as shown in  FIG. 6 . In other embodiments, the DFB laser and photodetector may be internal to the MCS module  6 . In both of these embodiments, the number of transceiver ports of the MCS module  6  that are available for connection to transceivers is reduced (by one). If desired, the number of transceiver-facing ports of the MCS module  6  may be increased to compensate for this reduction. For example, in the embodiment of  FIGS. 2, 3 and 5 , each MCS  6  is configured as an 8×16 MUX/DEMUX, having a total of 16 transceiver ports, and therefore capable of supporting up to 16 transceivers. If desired, the MCS  6  may be reconfigured as an 8×17 (or, more generally, an N×M+1) MUX/DEMUX, capable of supporting a total of 17 transceiver ports. With this arrangement, one of the possible transceiver ports is dedicated (either internally or externally of the MCS module) to the connection validation DFB laser and photodetector, while the remaining 16 transceiver ports remain available for connection to transceivers. This arrangement enables the validation of connections through the MCS module  6 , and between the MCS module  6  and the FIM  10  and WSS modules  4 , without requiring a transceiver  8  to generate and detect the test signal. 
     In the preceding description, embodiments are described in the context of an example embodiment ROADM made up of WSS modules  4 , a FIM  10  and MCS modules  8 . It will be appreciated that ROADMs may be configured using different architectures, and different combinations of modules, and that the present techniques may be equally applied in such alternative architectures. For example, in some cases, a plurality of EDFAs may be provided in one or more modules coupled between e.g. a FIM and an MCS module. In such cases, the present techniques may be used to control the EDFAs connected in un-used optical paths to provide test signals for validating connectivity of those paths. In embodiments in which an EDFA module also contains photodetectors, these may be used in the present technique, as described above with reference to  FIGS. 2A-2E . 
     In the preceding description, for purposes of explanation, numerous details are set forth in order to provide a thorough understanding of the embodiments of the invention. However, it will be apparent to one skilled in the art that these specific details are not required in order to practice the invention. In other instances, well-known electrical structures and circuits are shown in block diagram form in order not to obscure the invention. For example, specific details are not provided as to whether the embodiments of the invention described herein are implemented as a software routine, hardware circuit, firmware, or a combination thereof. 
     Embodiments of the invention can be represented as a software product stored in a machine-readable medium (also referred to as a computer-readable medium, a processor-readable medium, or a computer usable medium having a computer-readable program code embodied therein). The machine-readable medium can be any suitable tangible medium, including magnetic, optical, or electrical storage medium including a diskette, compact disk read only memory (CD-ROM), memory device (volatile or non-volatile), or similar storage mechanism. The machine-readable medium can contain various sets of instructions, code sequences, configuration information, or other data, which, when executed, cause a processor to perform steps in a method according to an embodiment of the invention. Those of ordinary skill in the art will appreciate that other instructions and operations necessary to implement the described invention can also be stored on the machine-readable medium. Software running from the machine-readable medium can interface with circuitry to perform the described tasks. 
     The above-described embodiments of the invention are intended to be examples only. Alterations, modifications and variations can be effected to the particular embodiments by those of skill in the art without departing from the scope of the invention, which is defined solely by the claims appended hereto.