Patent Publication Number: US-2022231780-A1

Title: Identifying and monitoring connections in an optical system

Description:
RELATED APPLICATIONS 
     The present application claims priority to Chinese Patent Application No. 202110075711.5 filed on Jan. 20, 2021 which is incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     The present disclosure generally relates to optical systems, and relates more specifically to the identification and monitoring of connections between optical devices. 
     BACKGROUND ART 
     The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section. 
     Optical networks are used for many applications, such as communication, measurement, monitoring, energy delivery, and other applications. Optical networks typically offer high-speed voice, video, and data transmission between providers and homes, businesses, and other networks. In an optical network, optical links connect two or more optical devices. An optical link includes a communication medium connected to a device that enables optical communication over the communication medium, such as one or more optical fibers. 
     The optical link configurations in an optical network may become complex. For example, one optical device may be connected with one or multiple other optical devices, with one or multiple optical links between each optical device pair. Optical devices may be located in different slots of the same optical network device shelf, a different shelf of same network device rack, different locations of same site, and/or different sites. For example, some optical devices may be located remotely from a site controlled by an operator of an optical network, such as to be physically close to a user location. Optical patch panels or optical shuffle boxes may be employed in managing optical connections at a site. 
     A wavelength-division multiplexing (“WDM”) system is often employed in an optical network to handle routing. A WDM system typically multiplexes a number of optical signals with different wavelengths so that multiple distinct signals may travel over a single optical fiber. Because the fiber can simultaneously carry multiple signals, WDM can increase the complexity of optical links at a node of the network when the multiple signals are separated. 
     Optical links are often physically connected using cables on an ad hoc basis, making cable management and/or mapping difficult. Identification of a connection path, such as during device setup, configuration, and/or reconfiguration, may be a complex task. It may also be challenging to monitor the operation of optical links, such as to detect broken connections and/or degradation. 
     A typical solution may involve lasers and photodetectors with complicated algorithms in order to identify and/or monitor optical links at a remote site, which may require complex powered electrical circuits and powerful CPU to handle identification, monitoring, and/or communication with network controller. However, a remote optical device may be passive, without electrical circuitry and with no access to electrical power. For example, passive optical devices may appear at remote sites that are geographically distant from a connected site with powered optical device. 
     SUMMARY 
     The appended claims may serve as a summary. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  illustrates an optical system in an example embodiment; 
         FIG. 2  illustrates an optical system with an ID block in a source optical device and a remote ID block in a remote optical device in an example embodiment; 
         FIGS. 3A-3B  illustrate sets of WDM filters in an example embodiment; 
         FIG. 4  illustrates an optical system with a monitor block in a source optical device and a remote monitor block in a remote optical device in an example embodiment; 
         FIG. 5  illustrates an optical system with a monitor block for a source optical device and a remote monitor block for a remote optical device in an example embodiment; 
         FIG. 6  illustrates an optical system with an optical add-drop multiplexer (OADM) node in an example embodiment; 
         FIG. 7  illustrates a direction device and an add-drop group device in an OADM node in an example embodiment; 
         FIG. 8  illustrates a direction device and an add-drop group device in an OADM node that implements an ID mechanism in an example embodiment; 
         FIG. 9  illustrates a direction device and an add-drop group device in an OADM node that implements a monitor mechanism in an example embodiment; 
     
    
    
     While each of the drawing figures illustrates a particular embodiment for purposes of illustrating a clear example, other embodiments may omit, add to, reorder, or modify any of the elements shown in the drawing figures. For purposes of illustrating clear examples, one or more figures may be described with reference to one or more other figures. However, using the particular arrangement illustrated in the one or more other figures is not required in other embodiments. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. It will be apparent, however, that the present disclosure may be practiced without these specific details. The detailed description that follows describes exemplary embodiments and the features disclosed are not intended to be limited to the expressly disclosed combination(s). Therefore, unless otherwise noted, features disclosed herein may be combined to form additional combinations that were not otherwise shown for purposes of brevity. 
     It will be further understood that: the term “or” may be inclusive or exclusive unless expressly stated otherwise; the term “set” may comprise zero, one, or two or more elements; the terms “first”, “second”, “certain”, and “particular” are used as naming conventions to distinguish elements from each other, and does not imply an ordering, timing, or any other characteristic of the referenced items unless otherwise specified; the term “and/or” as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items; that the terms “comprises” and/or “comprising” specify the presence of stated features, but do not preclude the presence or addition of one or more other features. 
     This document generally describes systems, methods, devices, and other techniques for identifying and monitoring connections in an optical system. An optical system includes one or more source optical devices and one or more remote optical devices that implement an identification mechanism and/or a monitor mechanism. 
     To implement the identification mechanism, a source optical device includes an identification (ID) block comprising optical elements that perform identification of one or more connections to one or more remote optical devices. The one or more remote optical devices each include a remote ID block comprising one or more optical elements. ID signals generated at the source optical device are transmitted to the one or more remote optical devices, processed by the remote ID block, and transmitted back to the source optical device, where the ID block identifies the one or more connections based on the returned ID signals. The ID signals generated at the source optical device for identification belong to a set of ID wavelengths λ {ID} . In some embodiments, λ {ID}  does not overlap with a set of service wavelengths λ {service} , and the identification mechanism is not used during normal operation of the source optical device and the remote optical device. 
     To implement the monitor mechanism, a source optical device includes a monitor block comprising optical elements that evaluate connectivity of one or more connections between the source optical device and one or more remote optical devices. The one or more remote optical devices each include a remote monitor block comprising one or more optical elements. Monitor signals generated at the source optical device are transmitted to the one or more remote optical devices, processed by the remote monitor block, and transmitted back to the source optical device, where the monitor block evaluates the connectivity of the one or more connections based on the returned monitor signals. The monitor signals generated at the source optical device may have a reference wavelength λ r . In some embodiments, λ r  does not overlap with a set of service wavelengths λ {service} , and the monitoring mechanism is used during normal operation of the source optical device and the remote optical device. 
     In some embodiments, the remote ID block/s and/or the remote monitor block/s only include passive elements that do not require electronic elements and/or electronic power. In this manner, only purely passive optical circuits are deployed in the remote optical device/s that can be tested using the ID mechanism and/or the monitor mechanism. Additional features and advantages are apparent from the specification and the drawings. 
       FIG. 1  illustrates an optical system in an example embodiment. The optical system  100  includes an optical network with one or more source optical devices  102  and one or more remote optical devices  106 - 108 . As used herein, the term “optical device” refers to optical equipment with one or more optical ports to communicatively couple the optical device to another device so that optical signals can travel over a communication link between the optical devices. An optical device may be a standalone device, and/or may include two or more optical device components. 
     The source optical device  102  communicates with the one or more remote optical devices  106 - 108  via one or more optical links  1 - i . A source optical device  102  may be coupled with a particular remote optical device  106 - 108  by one or multiple optical links. An optical link may include a transmitter, receiver, and cable assembly that can transmit information between two points. An optical link may include unidirectional or bidirectional fibers. For example, optical link  1  includes two fibers used for unidirectional communication, while optical link  3  includes one fiber used for bidirectional communication. As used herein, a fiber within an optical link is referred to as an optical link component. An optical link may include one or more cables that terminate with one or more optical connectors designed to mate with an optical port of an optical device. 
     The source optical device  102  may be configured to perform one or more functions in conjunction with one or more remote optical devices  106 - 108 . The function/s are carried out by a function block  104  at the source optical device  102  and one or more function blocks  110 - 112  at the remote optical devices  106 - 108 . As used herein, an optical block, such as function blocks  104  and  110 - 112 , is a set of one or more optical elements that generate and/or process one or more optical signals related to a particular function. In some embodiments, the source optical device  102  is a direction device in an OADM node, and the remote optical devices  106 - 108  are add-drop group devices in the OADM node. 
     Function block  104  generates service signals having frequencies selected from a set of service wavelengths λ {service}  that are transmitted to one or more remote optical devices  106 - 108  over the one or more communication links  1 - i . In some embodiments, λ {service}  includes wavelengths in a particular communication band, such as the O-band, E-band, S-band, C-band, L-band, 850-nm-band, U-band, and/or another communication band. A channel refers to an optical signal transmitted at a particular wavelength. As used herein, the term “transmission path” refers to the path of service signals from a function block  104  of a source optical device  102  to a function block  112  of a remote optical device  108 . As used herein, the term “receiving path” refers to the path of service signals from a function block  112  of a remote optical device  108  to a function block  104  of a source optical device  102 . A transmission path and/or a receiving path may travel over an optical link. Transmission path i carries service signals of a particular service wavelength λ i  from W 1  to X 1  over optical link i. Receiving path i carries service signals of λ i  from Z 1  to Y 1  travel over optical link i. 
     In some embodiments, the source optical device  102  is configured to identify optical connections at one or more remote optical devices  106 - 108 . For example, the source optical device  102  may determine that signals of a particular wavelength travel over a particular optical link. In some embodiments, the source optical device  102  identifies a plurality of optical connections to a plurality of remote optical devices  106 - 108 . The source optical device  102  may include an identification (ID) block  114  that includes one or more optical elements that identify connections to one or more remote optical devices  106 - 108 . The ID block  114  transmits identification (ID) signals having frequencies selected from a set of wavelengths λ {ID}  to one or more remote optical devices  106 - 108 , such as by directing the ID signals into transmission path i at B 1 . A remote ID block  116  at a remote optical device  108  processes the ID signals from the ID block  114  and transmits the returned ID signals back to the ID block  114 . At the remote optical device  108 , ID signals from transmission path i are directed to the remote ID block  116  at G 1 , and returned ID signals from the remote ID block  116  are directed to the receiving path at H 1 . The returned ID signals are directed from the transmission path i to the ID block  114  at M 1 . 
     The term “identification mechanism” is used herein to refer to the combination of the ID block  114  at the source optical device  102  and the remote ID block  116  at one or more remote optical devices  108  optically connected to the source optical device. The identification mechanism is described in greater detail hereinafter. 
     Alternatively and/or in addition, the source optical device  102  may be configured to monitor optical connections at one or more remote optical devices  106 - 108 . For example, the source optical device  102  may include a monitor block  118  that includes one or more optical elements that monitor connections between the source optical device  102  and one or more remote optical devices  106 - 108 , such as to evaluate connectivity of optical links. The monitor block  118  transmits monitor signals having one or more reference frequencies of wavelength λ r  to one or more remote optical devices  106 - 108 , such as by directing the monitor signals into transmission path i at Pi. A remote monitor block  120  at a remote optical device  108  processes the monitor signals from the monitor block  118  and transmits the returned monitor signals back to the monitor block  118 . At the remote optical device  108 , monitor signals from transmission path i are directed to the remote monitor block  120  at E 1 , and returned monitor signals from the remote monitor block  120  are directed to the receiving path at F 1 . The returned monitor signals are directed from the transmission path i to the monitor block  118  at K 1 . 
     The source optical device  102  may include one or more microprocessors  150 . The microprocessor/s  150  may perform one or more computations required by function block  104 , ID block  114 , and/or monitor block  118 . In some embodiments, the microprocessor/s  150  execute one or more control instructions to carry out one or more control processes. The control instructions may include hard-coded instructions, firmware, and/or software. In some embodiments, the microprocessor/s  150  execute instructions for a ID control process to generate ID signals, and process measurements of returned ID signals to generate output comprising the identity one or more optical links to the remote optical devices  106 - 108 . In some embodiments, the microprocessor/s  150  execute instructions for a monitor control process to generate monitor signals, and process measurements of returned monitor signals to generate output comprising the health one or more connections to one or more remote optical devices  106 - 108 . 
     The term “monitor mechanism” is used herein to refer to the combination of the monitor block  118  at the source optical device  102  and the remote monitor block  120  at one or more remote optical devices  108  optically connected to the source optical device. The monitor mechanism is described in greater detail hereinafter. 
     In an optical system, a source optical device  102  and one or more connected remote optical devices  106 - 108  may implement both the identification mechanism and the monitor mechanism, or may independently implement either the identification mechanism or the monitor mechanism. Different source optical devices in the same optical system may implement none, one, or both of the identification mechanism and/or the monitor mechanism. In some embodiments, the source optical device  102  is a direction device in an optical add-drop multiplexer (OADM) node, and each add-drop group device in the OADM node implements the identification mechanism, the monitor mechanism, or both the identification mechanism and the monitor mechanism. 
     For ease of illustration, aspects described herein with respect to a particular source optical device, a particular remote optical device, and/or a particular optical link may apply to one or more other source optical devices, remote optical devices and/or optical links. For example, an optical system may include one or multiple source optical devices; a source optical device may communicate with a remote optical device over one or multiple optical links; and/or a source optical device may communicate with one or multiple remote optical devices. Furthermore, the techniques for identification and monitoring may be applied to one optical link, multiple optical links, and/or all optical links from a source optical device. While one or more specific elements may be shown in a particular embodiment, other elements and configurations may provide equivalent functionality without departing from the spirit or the scope of this disclosure. 
       FIG. 2  illustrates an optical system with an ID block in a source optical device and a remote ID block in a remote optical device in an example embodiment. The optical system  200  includes a source optical device  202  and a remote optical device  208  connected by an optical link i. A transmission path i from W 2  to X 2  carries service signals from a function block  204  of the source optical device  202  to a function block  212  of the remote optical device  208 , and a receiving path i from Z 2  to Y 2  carries service signals from the function block  212  of the remote optical device  208  to the function block  204  of the source optical device  202  using a particular service wavelength of a set of service wavelengths λ {service} . 
     The source optical device  202  includes an ID block  214  that identifies one or more connections at one or more remote optical devices. The remote optical device  208  includes a remote ID block  216 . As previously noted, a remote ID block may be present in one or multiple remote optical devices connected to the source optical device  202 . Furthermore, multiple remote ID blocks may be present in a remote optical device  208 . 
     The ID block  214  transmits ID signals over transmission path i using a set of ID wavelengths λ {ID} . At A 2 , a light source  220  generates light of the set of wavelengths λ {ID} . In some embodiments, the light source  220  includes one or more broadband light sources, one or more tunable lasers, one or more diodes such as light-emitting diodes (LEDs) and laser diodes (LDs), and/or one or more other light sources that can provide λ {ID}  light. In some embodiments, the light source  220  is a light source that exists for another purpose in the source optical device  202 , such as a light source that belongs to function block  204 . 
     The ID signals are directed into transmission path i at B 2  using one or more elements  222 - 224 . For example, element  224  may be a splitter and/or switch, a multiplexer, or another optical element. In some embodiments, the light source  220  generates λ {ID}  light that is directed into transmission paths that travel over one or more other optical links. For example, element  222  may be a switch element and/or splitter element that transmits light to one or more other transmission paths, such as but not limited to a transmission path that travels over optical link  2 , using one or more elements  242 . 
     At G 2 , the ID signals are directed into a bypass path from G 2  and H 2  by using an element  226  that can direct the ID signals into the bypass path, such as a switch, or another optical element at G 2 . In some embodiments, the bypass path is only enabled when identification of optical links is performed for the optical system  200 . In such cases, element  226  may be a switch without affecting the transmission of service signals during normal operation of function block  204  and function block  212 . 
     The ID signals transmitted over transmission path i enter the bypass path G 2 −H 2  and travel to a set of wavelength-division Multiplexing (WDM) filters  228 . Each WDM filter of the set of WDM filters  228  can either pass or block a different wavelength. The set of WDM filters  228  can be used in different combinations. When the set of WDM filters includes a maximum number of different wavelength filters l, and a maximum number of filters to “build such Optical ID Block” is k (k&lt;=l), the total number of unique identifiers (IDs) that can be created by “such Optical ID Block” will be equal to C l   k +C l   k-1 + . . . +C l   1 . For example, if the set of WDM filters has 400 GHz channel spacing in a typical C band with 4 THz total bandwidth, then the set of WDM filters can have a maximum of l=10 filters with different wavelength (4 THz/400 GHz). If only one filter is used to build the “Optical block” (k=1), then 10 optical links can be identified. If up to two filters are used to “build the Optical block (k=2), then 55 optical links can be identified (C 10   2 +C 10   1 =45+10=55). Based on the maximum connectivity of the source optical device  202 , a minimum number of filters needed can be determined to ensure every connection can be uniquely identified among all connections from the source optical device  202  to the remote optical device/s  208 . 
     In some embodiments, the set of WDM filters  228  and/or the remote ID block  216  is a pluggable component in the remote optical device  208 . When the set of WDM filters  228  and/or the remote ID block  216  is a pluggable component, the number of WDM filters can be changed, such as to accommodate a greater number of remote optical devices  208  identifiable by the source optical device  202 . 
       FIG. 3A  illustrates a configuration for a set of WDM filters (e.g. set of WDM filters  228 ) in a remote ID block (e.g. remote ID block  216 ) in an example embodiment. A set of WDM filters  328  in a bypass path (e.g. G 2 -H 2 ) includes one or multiple optical notch filters which can block signals of different wavelengths (λ i , λ j , λ k ). The filters are placed in series, and one or a series of wavelengths will be blocked if light pass through the set of WDM filters  328 . 
       FIG. 3B  illustrates a configuration for a set of WDM filters (e.g. set of WDM filters  228 ) in a remote ID block (e.g. remote ID block  216 ) in an example embodiment. A set of WDM filters  358  in a bypass path (e.g. G 2 -H 2 ) includes one or multiple optical band pass filters, which can each pass signals of different wavelengths (λ i , λ j , λ k , λ m ). The filters are cascaded together, and one or a series of wavelengths will be passed while rest will be blocked if light pass through this block. 
     Returning to  FIG. 2 , the ID signal is directed into the receiving path i at H 2 . at M 2 , one or more elements  232 - 234 , such as one or more splitters, filters, demultiplexers, and/or other optical modules, direct the returned ID signals from the receiving path i to a set of one or more elements  236 - 238  of an optical channel monitor (OCM)  240 . The OCM  240  measures properties of returned ID signals, such as the wavelength of a particular received signal. In some embodiments, the OCM  240  includes a tunable filter  236  and a photodetector  238 . The tunable filter  236  and photodetector  238  are integrated to perform optical wavelength channel monitoring. The OCM  240  allows the ID block  214  to determine which wavelength(s) of the set of ID wavelengths λ {ID}  have been blocked or passed, allowing the ID block  214  to uniquely identify optical link i. The light generated by the light source  220  passes through the path A 2 -B 2 -C 2 -D 2 -G 2 -H 2 -I 2  J 2 -M 2 -N 2 -O 2 . 
     In order to perform identification, light returning from the receiving path (e.g. receiving path i) of an optical link is directed through a channel monitor (e.g. OCM  240 ). The source optical device  202  may have one or multiple OCMs to test a set of optical link/s (e.g. optical link i) with remote ID block/s (e.g. remote ID block  216 ). In some embodiments, the OCM  240  is shared between two or more receiving paths such that returned ID signals returning over one or more other optical links are also directed to the OCM  240 . For example, element  244 , such as a splitter element and/or a filter element, directs light from a receiving path that travel over optical link  2  to the OCM  240 . In some embodiments, one OCM  240  is shared between all testable optical links with remote ID blocks. Alternatively and/or in addition, one or more additional OCM elements may be present in one or more connections to other remote ID blocks. The source optical device  202  may include electronic circuitry that uses the output of the OCM  240  to perform identification. In some embodiments, the ID block  214  may identify a wavelength associated with one or more optical links, one or more ports associated with a particular wavelength, or other identification information. 
     In some embodiments, each connection between the source optical device  202  and a remote optical device  208  includes a remote monitor block and a monitor block, which may include shared elements. In some embodiments, the ID block may include 214 electrical circuitry, and/or may share electrical circuitry and/or resources used by other functionality (e.g. function block  204 ) of the source optical device  202 . In some embodiments, the source optical device  202  includes one or more microprocessors (e.g. microprocessor  150 ) that executes one or more control instructions to carry out one or more identification control processes as described herein. In some embodiments, the remote ID block  216  is a passive optical block that includes only passive optical elements. 
     In some embodiments, the set of ID signal wavelengths λ {ID}  may overlap with the set of service signal wavelengths λ {service} , and the identification mechanism does not operate during normal operation of the optical system  200 . For example, the identification mechanism described herein may be used during installation, modification, testing, and/or provisioning of the source optical device  202  and the remote optical device/s  208 . In some embodiments, λ {ID}  does not overlap with λ {service} . When there is no conflict or overlap between the ID wavelengths λ {ID}  and the service wavelengths λ {service} , the identification mechanism may be used during normal operation of the source optical device  202  and the remote optical device/s  208 . 
       FIG. 4  illustrates an optical system with a monitor block in a source optical device and a remote monitor block in a remote optical device in an example embodiment. The optical system  400  includes a source optical device  402  and a remote optical device  408  connected by an optical link i. A transmission path i from W 4  to X 4  carries service signals from a function block  404  of the source optical device  402  to a function block  412  of the remote optical device  408  over optical link i using light of a particular service wavelength λ i  of a set of service wavelengths λ {service} . A receiving path i from Z 4  to Y 4  carries service signals from the function block  412  to the source optical device  402  using λ i  light. 
     The source optical device  402  includes a monitor block  418  that monitors one or more connections between the source optical device  402  and one or more remote optical devices  408 . The remote optical device  408  includes a remote monitor block  420  that is communicatively coupled with the monitor block  418 . As previously noted, a remote monitor block  420  may be present in one or multiple remote optical devices connected to the source optical device  402 . Furthermore, multiple remote monitor blocks may be present in a remote optical device  408 . 
     The monitor block  418  transmits monitor signals over transmission path i using monitor signals of a reference wavelength λ r . The monitor signals are directed into transmission path i at P 4 . For example, a light source  422  at Q 4  may generate the monitor signal. In some embodiments, the light source  422  includes one or more broadband light sources, one or more tunable lasers, one or more diodes such as light-emitting diodes (LEDs) and laser diodes (LDs), and/or one or more other light sources that can provide light of the reference wavelength λ r . In some embodiments, the light source  422  is a light source that exists for another purpose in the source optical device  402 , such as a light source that belongs to function block  404 . In some embodiments, multiple reference wavelengths and/or dynamically-selected reference wavelengths are used. 
     In some embodiments, the light source  422  generates λ r  light that is directed into transmission paths that travel over one or more other optical links. For example, element  424  may be a switch element and/or splitter element that transmits light to one or more other transmission paths, such as but not limited to a transmission path that travels over optical link  3 , using one or more elements. 
     The monitor signal is added into the transmission path i corresponding to optical link i at P 4  using one or more elements  426 . For example, element  426  may be a multiplexer (MUX) element that combines a service signal from the function block  404  with the monitor signal from the light source  422 . The monitor signal travels over a path Q 4 -P 4 -C 4 -D 4 -E 4 -F 4 -I 4 -J 4 -K 4 -L 4 . 
     At E 4 , the monitor signals are directed into a bypass path from E 4  to F 4 , such as by using element  432 . For example, the bypath path may be set up using WDM techniques, such as by using an optical demultiplexer (DEMUX) element  432  at E 4  and a MUX element  434  at F 4 . The DEMUX element  432  separates λ r  monitor signals at E 4  so that they are not received at the function block  412  of the remote optical device  408 . The MUX element  434  adds the λ r  monitor signals of wavelength λ r  to the receiving path i at F 4  so that they return to the source optical device  402  for processing. 
     At K 4 , one or more optical elements  436 - 438  direct the monitor signal from the receiving path i to a photodetector  440 . For example, a DEMUX element  436  may separate λ r  monitor signals wavelength at K 4  and direct them to the photodetector  440 . The redirected monitor signals are not received at the function block  404  of the source optical device  402 . Alternatively, other elements may be used to direct the monitor signal from the receiving path i to a photodetector  440 . The photodetector  440  evaluates returned monitor signal from the remote optical device  408 . For example, the photodetector  440  may be used to detect a power of the returned monitor signal, such as to determine an optical loss on the path C 4 -D 4 -E 4 -F 4 -I 4 -J 4 . Based on the configuration of the remote monitor block  420 , it may be assumed in one or more embodiments that the optical loss between D 4  and I 4  is negligible. The connectivity and/or health of the optical link i can be compared and continuously monitored. For example, the optical loss C 4 -D 4  and I 4 -J 4  may be compared with baseline data at factory calibration and/or provisioning. The monitoring mechanism may detect a severe fiber broken event or loss degradation issue during normal operation of the source optical device  402  and the remote optical device  408 . 
     In some embodiments, the photodetector  440  is shared between two or more optical links such that monitor signals from one or more other receiving paths are also directed to the same photodetector  440 . For example, an element  438 , such as but not limited to an optical coupler or switch element, may direct light from a receiving path of optical link  3  to the photodetector  440 . In some embodiments, one photodetector  440  is shared between all monitored optical links with remote ID blocks. Alternatively and/or in addition, one or more additional photodetector elements may be present in one or more connections to other remote monitor blocks. 
     In some embodiments, each connection between the source optical device  402  and a remote optical device includes a remote monitor block and a monitor block, which may include shared elements. In some embodiments, the monitor block  418  may include electrical circuitry, and/or may share electrical circuitry and/or resources used by other functionality (e.g. function block  404 ) of the source optical device  402 . In some embodiments, the source optical device  402  includes one or more microprocessors (e.g. microprocessor  150 ) that executes one or more control instructions to carry out one or more monitor control processes as described herein. In some embodiments, the remote monitor block  420  is a passive optical block that includes only passive optical elements. 
     In some embodiments, the monitor mechanism operates during normal operation of the optical system  400 , and the reference wavelength λ r  of the monitor signals does not overlap with the wavelengths λ {service}  of the service signals. For example, λ r  may be outside of a frequency band selected for the service signals. In some embodiments, more than one reference wavelength is used. 
     In some embodiments, a source optical device is configured to independently monitor connectivity and health of a first link component  570  used by a transmission path and a second link component  572  used by a receiving path.  FIG. 5  illustrates an optical system with a monitor block for a source optical device and a remote monitor block for a remote optical device in an example embodiment. The optical system  500  includes a source optical device  502  and a remote optical device  508  connected by an optical link i. A transmission path i from W 5  to X 5  carries service signals from function block  504  of the source optical device  502  to the function block  512  of the remote optical device  508 . From C 5  to D 5 , transmission path i travels over a first link component  570  of optical link i, such as a first optical fiber. A receiving path i from Z 5  to Y 5  carries service signals from the function block  512  to function block  504 . From I 5  to J 5 , the transmission path i travels over a second link component  572  of optical link i, such as a second optical fiber. The service signals have a particular service wavelength λ i  of a set of service wavelengths λ {service} . 
     The source optical device  502  includes a monitor block  518 . One or more remote monitor blocks  520  may be present in one or multiple optical devices connected to the source optical device  502 . The monitor block  518  transmits monitor signals of a reference wavelength λ r  over one or more optical link components to be monitored. A first circuit including monitor block elements  522 - 530  and remote monitor block elements  552 - 554  is configured to monitor transmission path i, and a second circuit including monitor block elements  532 - 540  and remote monitor block elements  556 - 558  is configured to monitor receiving path i. In some embodiments, the first circuit and the second circuit operate in the same or similar fashion using elements that perform the same or similar functionality with respect to transmission path i and receiving path i. The first circuit is described in greater detail hereinafter. 
     In a first circuit associated with transmission path i, a light source  528  generates the λ r  monitor signals. The monitor signals are directed into transmission path i corresponding at P 5  using one or more elements. For example, a WDM element  524  may comprise a MUX element that adds the λ r  monitor signal to the λ i  service signal. In some embodiments, the light source  528  includes one or more broadband light sources, one or more tunable lasers, one or more diodes such as light-emitting diodes (LEDs) and laser diodes (LDs), and/or one or more other light sources that can provide λ r  light. In some embodiments, the light source  528  is a light source that exists for another purpose in the source optical device  502 , such as a light source that belongs to function block  504 . In some embodiments, the light source  528  generates monitor signals that are directed into the transmission path of one or more other optical links. For example, element  522  may be a switch element and/or splitter element that transmits light to one or more other transmission paths, such as but not limited to a transmission path that travels over optical link  3 . 
     In the remote monitor block  520  at the remote optical device  508 , the monitor signal enters a bypass path at E 5  using one or more elements. For example, a WDM element  552  may comprise a DEMUX element that separates the λ r  monitor signals at E 5  so that they are not received at function block  512 . The WDM element  552  directs the λ r  monitor signals to R 5 . At R 5 , a reflector  554  reflects the λ r  monitor signal. The λ r  monitor signal travels back to the WDM element  552 , which may comprise a MUX element that directs the reflected monitor signal back to the source optical device  502 . Although transmission path i is illustrated with arrows indicating a direction of the service signals from W 5  to X 5 , the transmission path i allows bidirectional signaling, allowing the reflected monitor signal to travel from the reflector  554  at R 5  to the WDM element  524  at P 5 . The reflected monitor signal travels to a photodetector  526  at L 5 . For example, a circulator at T 5  may direct outgoing monitor signals from the light source  528  to the WDM  524  via element  522 , and may direct incoming reflected monitor signals to the photodetector  526 . In some embodiments, a DEMUX element of the WDM element  524  at P 5  separates the returned monitor signals of wavelength λ r  so that they do not travel to function block  504 . 
     The photodetector  526  detects a power of the reflected monitor signal, such as to determine an optical loss over its path from the light source  528  to the photodetector  526 , Q 5 -T 5 -A 5 -P 5 -C 5 -D 5 -E 5 -R 5 -E 5 -D 5 -C 5 -P 5 -A 5 -T 5 -L 5 . The source optical device  502  may have one or multiple photodetectors  526  to evaluate reflected monitor signals. In some embodiments, the photodetector  526  is shared between two or more optical links such that reflected monitor signals from one or more other receiving paths are also directed to the same photodetector  526 . Alternatively and/or in addition, photodetector elements may be present in one or more other optical links. Based on the configuration of the monitor block  518  and the remote monitor block  520 , it may be assumed in one or more embodiments that the optical loss on segments outside of the first link component  570  is negligible. The connectivity and/or health of the first link component  570  can be compared and continuously monitored. For example, the optical measurements detected by the photodetector  526  may be compared with baseline data at factory calibration and/or provisioning to determine optical loss. The monitoring mechanism may detect a fiber disconnection or failure event or loss degradation issue in the first link component  570  during normal operation of the source optical device  502  and the remote optical device  508 . 
     In some embodiments, the first circuit associated with transmission path i has additional components to improve health and connectivity monitoring. For example, a photodetector  530  may be used to monitor the health of the light source  528 . Light travels from the light source  528  to the photodetector  530  without traveling over any optical links. For example, light may travel from the light source  528  to the photodetector  530  via an element  529 , such as but not limited to an optical splitter or switch element that directs light away from the path Q 5 -T 5  to the photodetector  530 . The photodetector  530  may determine a current output of the light source  528  and compare the current output of the light source to the baseline data at factory calibration to determine a health of the light source  528 . In some embodiments, the optical measurements detected by the photodetector  526  are compared to the current output of the light source as detected by the photodetector  530  to determine optical loss over transmission path i. 
     In some embodiments, each connection between the source optical device  502  and a remote optical device includes a remote monitor block and a monitor block, which may include shared elements. 
     In some embodiments, the monitor mechanism operates during normal operation of the optical system  500 , and the reference wavelength λ r  does not overlap with the wavelengths λ {service}  of the service signals. For example, λ r  may be outside of a frequency band selected for the service signals. In some embodiments, more than one reference wavelength is used. 
     In some embodiments, the monitor mechanism operates during normal operation of the optical system  500 , and the wavelength λ r  of the monitor signals does not overlap with the wavelengths λ {service}  of the service signals. For example, the reference wavelength λ r  may be outside of a frequency band selected for the service signals. In some embodiments, more than one reference wavelength is used. 
     In some embodiments, the monitor block  518  may include electrical circuitry, and/or may share electrical circuitry and/or resources used by other functionality (e.g. function block  504 ) of the source optical device  502 . In some embodiments, the source optical device  502  includes one or more microprocessors (e.g. microprocessor  150 ) that executes one or more control instructions to carry out one or more monitor control processes as described herein. In some embodiments, the remote monitor block  520  is a passive optical block that includes only passive optical elements. 
     An optical add-drop multiplexer (OADM) is an optical device used in wavelength-division multiplexing (WDM) systems for multiplexing and routing different wavelengths of light into or out of a single fiber. This allows multiple communication channels with different wavelengths to travel over a fiber. An OADM device generally includes an optical demultiplexer (DEMUX), an optical multiplexer (MUX), a method of reconfiguring the paths between the optical demultiplexer and the optical multiplexer, as well as a set of ports for adding and dropping signals. OADMs are often used in telecommunications networks. An OADM may refer to both a fixed optical add-drop multiplexer (FOADM) and/or a reconfigurable optical add-drop multiplexer (ROADM). 
       FIG. 6  illustrates an optical system with an OADM node in an example embodiment. The optical system  600  includes a plurality of OADM nodes including OADM node  608 . The OADM node  608  is coupled to a plurality of other nodes in the optical system  600  by at least one inter-node optical link  620 - 626 . An inter-node optical link  620 - 626  includes at least one optical fiber for transmission of multiple wavelength signals in a unidirectional and/or bidirectional manner to and from the OADM node  608 . Typically, one or more OADM nodes  608  are arranged in a bus, ring, star, mesh, or hybrid topology arrangement. An OADM node  608  may be a terminal node in the optical system  600 , such as when the OADM node  608  is connected to only one inter-node optical link  620 - 626 . 
     The OADM node  608  includes at least one direction device  610 - 616 . A direction device  610 - 616  routes signals received over a corresponding inter-node optical link  620 - 626  to other components within the OADM node  608 , such as but not limited to one or more add-drop group devices  602 - 606  and/or one or more other direction devices  610 - 616 . For example, the OADM node  608  may include one or more express communication links that directly transmit and receive service signals between direction devices  610 - 616  without adding or dropping any channels. 
     A direction device  610  may be coupled to one or more add-drop group devices  602 - 606  with one or more optical links. An add-drop group device  602 - 606  may perform add-drop functionality for signals a different set of wavelengths. For example, a particular direction device  610  may communicate signals with a first set of wavelengths with a first add-drop group device  602 , signals with a second set of wavelengths with a second add-drop group device  604 , and signals with a third set of wavelengths with a third add-drop group device  606 . In some embodiments, the signals assigned to a particular add-drop group device  60  are a sub-band of band of frequencies used by the optical system  600 . In some embodiments, an OADM node  608  only has one add-drop group device  602 , and a direction device  610 - 616  transmits the entire band of service signals to the single add-drop group device  602 . 
     An add-drop group device  602 - 606  separates and combines individual channels of particular wavelengths in the received service signals. For example, an add-drop group device  602  may drop or separate signals of wavelength λ x , transmit the λ x  signals to a device  628  over an optical link  630  coupling the device  628  and the add-drop group device  602 - 606 , receive λ x  signals from the device  628  over the optical link  630 , and add the received λ x  signals to a combined outgoing signal comprising outgoing signals of multiple wavelengths from one or more other devices. The device  628  may be an optical device, electrical device, and/or electro-optical device. One or more transponders, receivers, transceivers, and/or other optical-electrical and electrical-optical devices may be employed to communicate with the device  628 . 
     The add-drop group device  602  may drop and add signals of a plurality of wavelengths (such as but not limited to λ x ) and may communicate individual wavelength signals with a plurality of devices (such as but not limited to device  628 ). The add-drop group device  602  transmits the combined signal comprising multiple channels assigned to the add-drop group device  602  to one or more direction devices  610 - 616 . 
     Although the OADM node  608  is illustrated as a logical device, the components of the OADM node  608  may be deployed separately. Add-drop group devices  602 - 606  are often physically deployed separately and independently from direction devices  610 - 616 . For example, one or more add-drop group devices  602 - 606  may be located in different slots of the same optical network device shelf as one or more direction devices  610 - 616 , one or more different shelves of same network device rack, one or more different locations at the same site, and/or remotely from a site comprising one or more direction devices  610 - 616 . In some embodiments, one or more add-drop group devices  602 - 606  are located close to a location of one or more end-users. In some embodiments, one or more optical links between an add-drop group device  602 - 606  and a direction device  610 - 616  go through one or more optical cabling systems, such as but not limited to one or more optical patch panels and/or optical shuffle boxes. 
     The direction devices  610 - 616  may be well equipped with powered electrical elements, such as light sources (such as photodiodes, laser diodes, and/or other light sources) and/or optical channel monitors (OCMs). Furthermore, the direction devices  610 - 616  may be closely linked a to powered optical network device and/or network controllers, making their optical connectivity simpler to identify and/or monitor during provisioning and/or operation. Alternatively, one or more add-drop group devices  602 - 606  may have complex connection paths to the direction devices  610 - 616  and/or other devices in the OADM node  608 . Furthermore, one or more add-drop group devices  602 - 606  may be passive, having no electrical circuitry and no powered optical element. 
       FIG. 7  illustrates a direction device and an add-drop group device in an OADM node in an example embodiment. An OADM node  700  includes one or more direction devices  760  and one or more add-drop group devices  762 . The direction device may transmit and receive signals over one or more optical links  718 - 720  with one or more other direction devices (e.g. direction devices  610 - 616 ). For clarity in explanation, one direction device  760  and one add-drop group device  762  are described in greater detail hereinafter; one or more described features may apply to one or more other direction devices and/or add-drop group devices within the OADM node  700 . In some embodiments, one or more direction devices  760  are source optical devices (e.g. source optical device  102 ,  202 ,  402 ,  502 ) that include one or more identification blocks and/or one or more monitor blocks. In some embodiments, one or more add-drop group devices  762  are remote optical devices (e.g. remote optical device  108 ,  208 ,  408 ,  508 ) that include one or more remote identification blocks and/or one or more remote monitor blocks. Specific examples are described in  FIGS. 8-9  without limiting the disclosure to the example embodiments. 
     A direction device  760  may include a DEMUX element  704  to separate signals so that a particular sub-band assigned to a particular add-drop group device  762  can be directed to the particular add-drop group device  762 . The DEMUX element  704  separates service signals of a set of wavelengths λ {service}  received over communication link  720  into one or more signal subsets and transmits the separated signals to one or more corresponding add-drop group devices  762  over one or more optical links  722 - 726 . For example, signals of wavelengths λ {i}  are directed from DEMUX element  704  to add-drop group device  762  over communication link  722 ; signals of wavelengths λ {j}  are directed from DEMUX element  704  to another add-drop group device over communication link  724 ; and signals of wavelengths λ {k}  are directed from DEMUX element  704  to another add-drop group device over communication link  726 . 
     The direction device  760  may include a MUX element  702  to combine signals from one or more add-drop group devices (e.g. add-drop group devices  602 - 606 ) so that the combined signals can be transmitted to one or more direction devices (e.g. direction devices  610 - 616 ) over one or more communication links  718 - 720 . For example, the MUX element  702  may combine returned λ {i}  signals from add-drop group device  762  over communication link  722 ; returned λ {j}  signals from another add-drop group device over communication link  724 ; and returned λ {k}  signals from another add-drop group device over communication link  726 . 
     A direction device  760  may include one or more powered electrical and/or optical elements, such as a pre-amplifier  708 , an optical channel monitor  710 , a booster amplifier  706 , a photodiode  712 , an optical supervisory channel  714 , a variable optical attenuator, a light source, a power source, electronic circuitry, a processor, and/or other elements, including powered elements, that can be used by an ID block (e.g. ID block  114 ,  214 ) and/or a monitor block (e.g. monitor block  118 ,  418 ,  518 ) in one or more embodiments. 
     In the add-drop group device  762 , the DEMUX element  736  separates signals based on wavelength and directs the separated signals to a plurality of single-wavelength optical links  728 - 732 . Each single-wavelength optical link  728 - 732  may carry signals of a particular wavelength (e.g. λ a , λ b , λ c ) between the add-drop group device  762  and a device (e.g. device  628 ). The MUX element  734  combines returned signals received over the single-wavelength optical links  728 - 732  so that the combined returned signals can be transmitted to the direction device  760  over communication link  722 . 
     An add-drop group device  762  may be connected to one or multiple direction devices  760 . For example, add-drop group device  762  may be connected to one or more other direction devices (e.g. direction devices  610 - 616 ) by one or more optical links  752 - 754 . For example, add-drop group device  762  may also receive λ {i}  signals from other direction devices over optical links  752 - 754 . In some embodiments, signals from two or more direction devices may be directed to the MUX element  734  and the DEMUX element  736  in an add-drop group device  762 . Alternatively and/or in addition, signals from a direction device may have its own MUX element  734  and DEMUX element  736 . For example, the combined signals may also be transmitted from MUX element  734  to one or more other direction devices over communication links  752 - 754 . 
       FIG. 8  illustrates a direction device and an add-drop group device in an OADM node that implements an ID mechanism in an example embodiment. An OADM node  800  includes one or more direction devices  860  and one or more add-drop group devices  862 . A direction device  860  may transmit and receive signals over one or more optical links  818 - 820  to/from one or more other direction devices (e.g. direction devices  610 - 616 ). For clarity in explanation, one direction device  860  and one add-drop group device  862  are described in greater detail hereinafter; one or more described features may apply to one or more direction devices and/or add-drop group devices within the OADM node  800 . In some embodiments, the OADM node  800 , one or more direction devices  860  and/or one or more add-drop group devices  862  include one or more elements described with respect to one or more other embodiments described herein. 
     The direction device  860  includes one or more ID block components, such as a light source  850  upstream of DEMUX element  804  and/or a light source  856  downstream of DEMUX element  804 . The DEMUX element  804  separates service signals of a set of wavelengths λ {service}  received over communication link  820  into one or more signal subsets and transmits the separated signals to one or more corresponding add-drop group devices  862  over one or more optical links  822 - 826 . 
     In the add-drop group device  862 , the DEMUX element  836  separates signals based on wavelength and directs the separated signals to a plurality of single-wavelength optical links  828 - 832 , which may couple the add-drop group device  862  to one or more devices. The MUX element  834  combines returned signals received over the single-wavelength optical links  828 - 832  so that the combined returned signals can be transmitted to the direction device  860  over optical link  822 . 
     The ID signals are added to a transmission path of service signals transmitted from the direction device  860  to the add-drop group device  862 . The add-drop group device  862  includes one or more remote ID block components, such as elements  866 - 870 . For example, element  866  may direct the ID signals into a bypass path that includes a set of WDM filters  868 , and element  870  may direct the ID signals into a receiving path for returned service signals from the add-drop group device  862  to the direction device  860 . 
     In the direction device  860 , the returned ID signals are directed to an optical channel monitor (OCM)  810 . The OCM  810  measures properties of returned ID signals, such as the wavelength of a particular returned ID signal. The OCM  810  allows the direction group  860  to determine which wavelength(s) of the set of ID wavelengths λ {ID}  have been blocked or passed, allowing identification of a corresponding optical link  822 . In some embodiments, the OCM  810  receives the returned ID signals after a MUX element  802  combines received signals from one or more add-drop group devices  862  over one or more optical links  822 - 826 . 
     An add-drop group device  862  may be connected to one or multiple direction devices  860 . For example, add-drop group device  862  may be connected to one or more other direction devices (e.g. direction devices  610 - 616 ) by one or more optical links  852 - 854 . The add-drop group device  862  may also receive ID signal and/or service signals from other direction devices over optical links  852 - 854 . 
     In the add-drop group device  862 , the bypass path with the set of WDM filters may be present in each connection path between each direction device and each add-drop group device. For example, ID signals from optical links  852 - 854  may pass through elements  866 - 870 , or may pass through one or more similar set of elements. Furthermore, the bypass path/s for each direction device of the OADM node  800  may be present in one or more other add-drop group devices of the OADM node  800 . 
     In some embodiments, the direction device  860  includes one or more microprocessors (e.g. microprocessor  150 ) that executes one or more control instructions to carry out one or more identification control processes as described herein. In some embodiments, the add-drop group device  862  is a passive optical device that includes only passive optical elements. 
       FIG. 9  illustrates a direction device and an add-drop group device in an OADM node that implements a monitor mechanism in an example embodiment. An OADM node  900  includes one or more direction devices  960  and one or more add-drop group devices  962 . A direction device  960  may transmit and receive signals over one or more optical links  918 - 920  to/from one or more other direction devices (e.g. direction devices  610 - 616 ). For clarity in explanation, one direction device  960  and one add-drop group device  962  are described in greater detail hereinafter; one or more described features may apply to one or more other direction devices and/or add-drop group devices within the OADM node  900 . In some embodiments, the OADM node  900 , one or more direction devices  960 , and/or one or more add-drop group devices  962  include one or more elements described with respect to one or more other embodiments described herein. 
     The direction device  960  includes one or more monitor block components, such as a light source  940  for generating monitor signals of a reference wavelength λ r . A MUX element  942  may add the λ r  monitor signals to one or more service signals, such as a λ {i}  service signals having wavelengths in a set of wavelengths assigned to a particular add-drop group device  962 . The monitor signals added are transmitted from the direction device  960  to the add-drop group device  962  over optical link  922 . A same or similar mechanism may add reference signals to services signals transmitted to one or more other add-drop group devices over one or more other optical links  924 - 926 . 
     The add-drop group device  962  includes one or more remote monitor block components, such as elements  944 - 946 . For example, element  944  may direct the monitor signals into a bypass path, such as by using a DEMUX element  944  to drop the λ r  monitor signals from a transmission path from the direction device  960  and a MUX element  946  to add the λ r  monitor signals to a receiving path to the direction device  960 . The remaining service signal is processed by the add-drop group device  962 , such as by DEMUX element  936  and MUX element  934  to separate signals transmitted to optical links  928 - 932  and combine signals received from optical links  928 - 932 . 
     At the direction device  960 , the returned λ r  monitor signals are evaluated. For example, a DEMUX element  948  may separate λ r  monitor signals from the receiving path and direct them to a photodetector  950 . The photodetector  950  evaluates returned monitor signal from add-drop group device  962 . For example, the photodetector  950  may be used to detect a power of the returned monitor signal, such as to determine an optical loss over optical link  922 . 
     An add-drop group device  962  may be connected to one or multiple direction devices  960 . For example, add-drop group device  962  may be connected to one or more other direction devices (e.g. direction devices  610 - 616 ) by one or more optical links  952 - 954 . The add-drop group device  962  may also receive monitor signal and/or service signals from other direction devices over optical links  952 - 954 . 
     In the add-drop group device  962 , the bypass path may be present in each connection path between each direction device and each add-drop group device. For example, monitor signals from optical links  952 - 954  may pass through elements  944 - 946 , or may pass through one or more similar set of elements. Furthermore, the bypass path/s for each direction device of the OADM node  900  may be present in one or more other add-drop group devices of the OADM node  900 . 
     In some embodiments, the direction device  960  includes one or more microprocessors (e.g. microprocessor  150 ) that executes one or more control instructions to carry out one or more monitor control processes as described herein. In some embodiments, the add-drop group device  962  is a passive optical device that includes only passive optical elements. 
     The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. The sole and exclusive indicator of the scope of the disclosure, and what is intended by the applicants to be the scope of the disclosure, is the literal and equivalent scope of the set of claims that issue from this application, in the specific form in which such claims issue, including any subsequent correction. 
     In the foregoing specification, embodiments are described with reference to specific details that may vary from implementation to implementation. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. The examples set forth above are provided to those of ordinary skill in the art as a complete disclosure and description of how to make and use the embodiments, and are not intended to limit the scope of what the inventor/inventors regard as their disclosure. Modifications of the above-described modes for carrying out the methods and systems herein disclosed that are obvious to persons of skill in the art are intended to be within the scope of the present disclosure and the following claims. The sole and exclusive indicator of the scope of the disclosure, and what is intended by the applicants to be the scope of the disclosure, is the literal and equivalent scope of the set of claims that issue from this application, in the specific form in which such claims issue, including any subsequent correction.