Abstract:
Optical-to-electrical and electrical-to-optical signal conversion is substantially eliminated for performance of monitoring and maintenance functions within a wavelength-division-multiplexed optical network having a Network Control Element by detecting optical intensities of signals conveyed over wavelength channels at the optical layer. Values associated with the detected optical intensities are generated and conveyed to the Network Control Element. The Network Control element analyzes these values for the purposes of fault detection, channel power monitoring, channel signal to noise ratio determinations, channel continuity checks, and network provisioning.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of optical fiber signal transmission, and more particularly to monitoring and maintenance functions within an optical network. 
     BACKGROUND OF THE INVENTION 
     Wavelength-division-multiplexing (WDM) optical networks are becoming increasingly complex as the number of wavelength channels transportable over a single fiber increases and as the number of channels added/dropped at various sites within the network and/or exchanged between two or more optical networks increases. Increasingly, the costs associated with such complex networks are extremely high, in large part due to the need for many necessary monitoring and maintenance functions to be performed at the electrical signal layer rather than at the optical signal layer. Performance of a particular operation or maintenance function at the electrical layer is costly because it requires conversion of the optical signal conveyed over the network to an electrical signal before performing a particular function and then reconversion back to an optical signal for further transmission over the optical network. 
     For example, prior to the development of the optical amplifier, repeater sites incorporated within optical networks were implemented utilizing regenerators. A regenerator provided optical signal amplification by first converting an optical signal to an electrical signal, amplifying the electrical signal, and then reconverting the electrical signal to an optical signal for launching back onto the optical network. Thus, adjacent repeater sites were separated by optical transmission sections, with overhead being read and recreated at each repeater site. Network monitoring was only performed at portions of the network where such optical to electrical conversions were made, and therefore management of the network and network elements was performed entirely in the electrical layer. Such network management required additional data overhead as well, in order to transmit management and monitoring data within the network. 
     Development of the optical amplifier and wavelength-division-multiplexed networks have revolutionized the management of networks: repeaters are now incorporated within the optical layer; and so are wavelength multiplexers, optical Add/Drop sites, and optical crossconnects. An increasing percentage of the optical network has become transparent, bit rate independent, and format independent. These advances have enabled increased network bandwidth and speed. Unfortunately, monitoring and management of WDM network signal processes, such as fault detection, optical signal strength detection, and optical signal to noise ratio determinations, have not been incorporated as functions within the optical layer itself. That is, such network monitoring and management requires conversion of optical signals to electrical signals prior to performing the monitoring and/or management function, and then reconversion back to an optical signal for conveyance over the optical network. 
     SUMMARY OF THE INVENTION 
     Optical-to-electrical and electrical-to-optical signal conversion is substantially eliminated, for performance of monitoring and maintenance functions within a wavelength-division-multiplexed optical network having a Network Control Element, by detecting optical intensities of signals conveyed over wavelength channels at the optical layer. Values associated with the detected optical intensities are generated and conveyed to the Network Control Element. The Network Control element analyzes these values for the purposes of fault detection, channel power monitoring, channel signal to noise ratio determinations, channel continuity checks, and network provisioning. 
     Advantageously, implementation of the present invention within an optical network increases system reliability, speed, and performance, since repeated optical-to-electrical and electrical-to-optical signal conversion is not required. Moreover, capital and O&amp;M costs associated with providing and maintaining network monitoring equipment is reduced since less equipment is utilized to perform the network monitoring and maintenance functions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present invention may be obtained from consideration of the following description in conjunction with the drawings in which: 
     FIG. 1 is a block diagram of a wavelength division multiplexed (WDM) optical network incorporating a plurality of architecture topologies and utilizing a plurality of optical protocols; 
     FIG. 2 is a diagram of an exemplary embodiment of the present invention, in which a WDM optical ring network incorporates an optical monitoring unit at each wavelength Add/Drop site; and 
     FIG. 3 is a process diagram illustrating the interrelationship of the functional steps comprising an exemplary embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, there is shown an exemplary wavelength division multiplexed (WDM) optical network  100  comprising ten Add/Drop sites  110  and interconnecting optical fiber lines  120 . Each of the lines  120  connecting the sites which comprise the network may independently carry a varying number of channels and may incorporate and support a variety of data protocols and formats. For example, the illustratively thick lines interconnecting sites  1 ,  2 ,  3 , and  6  represent a dense wavelength division multiplexed (DWDM) synchronous optical network (SONET) ring connecting three SONET terminals  130 , each supporting data exchange at a channel wavelength of λ 5 . For the purpose of providing a comparative illustration, the SONET ring interconnecting sites  1 ,  2 ,  3 , and  6  may have a data capacity of 40 or more wavelength channels, whereas the remaining network optical fiber lines (between the following Add/Drop sites pairs:  2 &amp; 8 ,  8 &amp; 7 ,  7 &amp; 1 ,  1 &amp; 10 ,  6 &amp; 5 ,  5 &amp; 9 ,  5 &amp; 4 , and  4 &amp; 3 ) may be limited in capacity to 16 or fewer channels. 
     Within the optical network  100 , channels are added and dropped at various Add/Drop sites  110 . For example, SONET ring terminals  130  access optical signals operating at wavelength λ 5 , through Add/Drop sites  1 ,  2 , and  6 . Point-to-point optical connections between plesiochronous digital hierarchy (PDH) terminals  140  are also maintained. PDH terminals  140  are shown accessing the network  100  at Add/Drop sites  2 ,  4 , and  9 . A complex mesh of optical connections between ATM switches  150  is also illustrated; with ATM switches  150  connected to the network  100  at Add/Drop sites  3 ,  5 ,  8 , and  10 . Bi-directional optical fiber lines  120  between Add/Drop sites  110  are utilized to convey optical signals originating and/or terminating at the various terminals  130 ,  140  and switches  150 . 
     Although the present invention will be described within a WDM SONET optical ring network, utilization of the present invention is not to be construed as being limited to such an application. Rather, the present invention functions at the network optical layer and thus network specific protocols, data structures, and formats are transparent to its operation. Additionally, various network architectures and topologies are also compatible for utilization with the present invention. The optical network of FIG. 1 is presented here merely as an illustrative example of an optical network encompassing some of the various protocols, data structures, formats, architectures, and topologies with which the present invention is compatible, and not as an exhaustive list of its application. 
     Referring now to FIG. 2, there is illustrated a WDM SONET ring network  200 . Add/Drop sites  210  are optically coupled in a ring topology via at least one optical fiber line  220 . SONET terminals  250  are optically coupled to respective Add/Drop sites  210  via add fibers  222  and drop fibers  224 . Add/Drop sites  210  are operable to launch and retrieve a plurality of optical signal wavelengths (channels) to supply and service a corresponding plurality of SONET terminals  250 . However, for the purpose of clarity, only those SONET terminals  250  operating with a channel wavelength of 5 are shown as being optically coupled to the Add/Drop site (and therefore to the network ring) via an add fiber  222  and drop fiber  224 . Interposed along and optically coupled with the optical fiber line  220  between Add/Drop site  210 - 1  and Add/Drop site  210 - 2  are a plurality of optical amplifiers  230 . Optical amplifiers  230  are utilized to amplify the signals transmitted at the optical layer without the necessity of first converting to an electrical signal. Any number of other optical and/or electrical equipment and components including, nodes, switches, attenuators, amplifiers, regenerators and the like, may also be incorporated within an optical network  200  utilizing the present invention for optical monitoring without adversely affecting the performance of the optical monitoring system, as would be apparent to those skilled in the art. 
     Interposed between each network servicing Add/Drop site  210  and its corresponding SONET terminals  250  (but illustrated in FIG. 2 only in conjunction with Add/Drop site  210 - 1  and corresponding SONET terminals  250 - 1 ) are a multiplexer  252 , demultiplexer  254 , and an optical monitoring unit  260 . SONET terminal  250 - 1  operating with a channel wavelength of λ 5  is optically coupled to provide signal data to multiplexer  252  and receive signal data from demultiplexer  254 . Similarly, SONET terminals  250 - 1  serviced by corresponding Add/Drop site  210 - 1 , but operating with channel wavelengths other than λ 5 , are also optically coupled to provide signal data to multiplexer  252  and receive signal data from demultiplexer  254 . Multiplexer  252  provides a multiplexed optical signal comprised of data from SONET terminals  250 - 1  for injection along network optical fiber lines  220  via the add fiber line  222 . Demultiplexer  254  provides demultiplexed optical data signals to a plurality of SONET terminals  250 - 1 , each demultiplexed optical signal modulated to an appropriate channel wavelength for each corresponding SONET terminal  250 - 1 . The demultiplexer receives a multiplexed signal from the network optical fiber lines  220  through Add/Drop site  210 - 1  via the drop fiber line  224 . 
     Thus, excluding an Optical Monitoring Unit (OMU)  260  (subsequently described) and the components which comprise the OMU  260  and which interconnect the OMU to the Add/Drop site  250 - 1 , FIG. 2 represents a simplified optical network model in which λ 5  is assigned to one SONET ring while sharing the fiber with other channels (wavelengths). The SONET terminals  250  at each site receive a number of dropped channels, and generate a correspondingly equal number of added channels, each SONET terminal  250  conveying signal data at a single wavelength or channel. Terminals conveying signal data at the same wavelength, for example the λ 5  SONET terminals  130  of FIG. 1, form a ring. A network may contain many such rings, each conveying a different wavelength, and each sharing the same fiber within a WDM scheme. An Optical Monitoring Unit (OMU)  260  is optically coupled to the add fiber line  222  and to the drop fiber line  224  for each Add/Drop site  210 . Each OMU  260  is comprised of an Optical Analyzer (OA)  284  which measures the optical spectrum intensity (means for measuring power across the network wavelength spectrum), an optical pilot tone generator (OPTG)  288  (means for producing and emitting optical energy having a wavelength of λ 0 , λ 0  being a wavelength within the WDM signal spectrum band), and a means for transmitting  286  the quantified data measured by the OA  284  to a Network Control Element (NCE)  240  and receiving command and control messages transmitted by the NCE  240  for OA  284  and OPTG  288  operation. The NCE  240  is a well-known apparatus providing a centralized monitor and control location for the entire network, typically operated and managed by the service provider or network manager. The NCE  240  coordinates, controls, and manages key network functions, such as network configuration, fault management, performance monitoring, accountability and security. Various means for transmitting the quantified data from the OA  284  to the NCE  240  and various means for receiving command and control messages transmitted by the NCE would be apparent to those skilled in the art. One means for transmitting/receiving data and control messages between the OA  284  to the NCE  240  is to convey the message directly over the optical layer of the network whose performance is being monitored; although if a network fault does exist, the possibility that the OA will not be able to report the fault to the NCE because of the very same detected network fault is indeed a possibility. Other means include signaling and messaging between network OA&#39;s  284  and the NCE  240  via PSTN lines, coaxial cable, wireless communication, ancillary or dedicated optical fiber, or any other well known means for the conveyance of data from one geographic location to another. 
     Several alternatives are presently available for producing a suitable OA  284 , each of which may be classified within one of a few basic categories. A first category of optical analyzer utilizes a scanning dispersive element, followed by a fixed detector, such as a scanning Fabry-Perot (FP) filter. For current applications having large numbers of closely spaced channels (50-100 GHz), optical analyzers utilizing this approach impose stringent performance specifications on the filters. Presently, commercially available bulk glass or fiber FP filters can achieve a spectral resolution of 0.07 nm (&lt;10 GHz), and a power accuracy of ±0.5 dBm. 
     A second category of optical analyzer for use in conjunction with the present invention utilizes a fixed dispersive element, for example a dispersion grating or a waveguide router, followed by an array of detectors. An example of this type of optical analyzer is described by J. L. Wagener et al. in  Proceedings of the ECOC  &#39;97, Vol. 5, pg. 65, in which a monitor composed of a blazed and chirped fiber Bragg grating is utilized. A 256 element detector array is coupled to the fiber Bragg grating. The device delivers performance equivalent to a single pass optical spectrum analyzer over the entire spectral range of the device, with 0.1 nm (12.5 GHz) resolution and a power detection accuracy of ±0.5 dBm. Other examples of a second category optical analyzer include: (i) a device comprised of a bulk diffraction grating followed by a 256 p-i-n photodiode array, utilized to control the power of channels entering an Add/Drop node, as described by K. Otsuka et al. in  Proceedings of ECOC  &#39;97, Vol. 2, pg. 147; (ii) a waveguide router with 18 arms followed by a 36 pixel detector array, used to control the wavelength of eight lasers acting as regenerators in an optical crossconnect with wavelength conversion, as described by M. Teshima et al., in  Proceedings of ECOC  &#39;97, Vol. 3, pg. 59; and (iii) an optical analyzer device used to perform network fault detection, such as reporting loss of wavelength, loss of signal, and optical signal degradation in an optical crossconnect, as described by H. Takeshita et al., in  Proceedings of ECOC  &#39;97, Vol. 3, pg. 335. 
     A third category of optical analyzer for use in conjunction with the present invention is based on mapping of wavelength into time delay groups by using group-velocity dispersion in fiber, as described in “Optical Monitoring using Data Correlation for WDM Systems,” L. E. Nelson, S. T. Cundiff, and C. R. Giles,  IEEE Photonics Technology Letters , Vol. 10, No. 7, July 1998, pp. 1030-32. 
     Utilizing this method, a multi-wavelength input is passed through a series of fiber gratings, which temporally separate the various wavelengths. Different wavelength groups are identified and quantified by the time delay associated with each particular wavelength group in arriving at a fixed detector. Identification of individual channels may be performed either by asynchronously modulating each at a low duty cycle, or by utilizing data correlation to determine the time shift of the channels. 
     Yet other devices for performing multi-channel, wide-spectrum optical analysis may also be utilized in conjunction with the present invention, as would be readily apparent to those skilled in the art. 
     At each Add/Drop site  210 , the Optical Analyzer (OA)  284  is optically coupled to the add fiber line  222  and to the drop fiber line  224  to selectively provide the OA with light from each. It should be noted that although the description included herein refers solely to optically coupling an OA (and therefore the corresponding OMU) at various network Add/Drop sites, the present invention is equally applicable and useful when optically coupled or tapped at any location within the optical network; since the present invention detects signals at the optical layer without first requiring conversion to an electrical signal. In the instant embodiment of the present invention, light from the drop fiber line  224  is conveyed to the OA  284  over a drop monitoring line  262  which is optically tapped to the drop fiber line  224  at a first end and optically coupled to a first input of a two input optical switch  280  at its second end. The output of switch  280  is then optically coupled as an input to the OA  284 . Light from the add fiber line  222  is conveyed to the OA  284  over an add test line  264  which is optically tapped to the add fiber line  222  at a first end and optically coupled to the distributing input of a 1×2 optical splitter  266  at its second end. A first distributed output of the 1×2 optical splitter  266  is then optically coupled via an add monitoring line  270  to a second input of the two input optical switch  280 . Switch  280  is utilized to select whether the desired location which the OA is to monitor is the add fiber line  222  or the drop fiber line  224 . 
     A second distributed output of the 1×2 optical splitter  266  is optically coupled to the optical pilot tone generator  288 . The optical pilot tone generator  288  is a light emitting device, such as a laser, operable to provide a test signal having a wavelength λ 0  as an output. Light emitting devices other than lasers may also be utilized as an optical pilot tone generator, as would be apparent to those skilled in the art. The optical pilot tone generator  288  is coupled to provide an optical test signal through splitter  266 , over add test line  264 , onto add fiber line  222  and onto the optical fiber line  220  which comprises the SONET ring via Add/Drop site  210 - 1 . 
     Advantageously, the present invention enables performance of in-service and out-of-service network management and maintenance functions at the network optical layer, and more specifically enables features not currently available under the SONET standard. For example, in-service maintenance capabilities provided by the present invention include fault isolation and optical performance monitoring. Out-of-service testing capabilities provided by the present invention include channel continuity checks, channel power delivery performance checks, and Optical Signal to Noise Ratio (OSNR) checks. The above recited list of maintenance and management functions and features are not intended as an exhaustive list, but merely as illustrative. Still other tests, checks, measurements, and functions are enabled for performance with the present invention, as would be known to those skilled in the art. 
     Referring now to FIG. 3, there is illustrated a functional block diagram  300  for performance of the aforementioned in-service (IS) and out-of-service (OOS) network management and maintenance functions. Commencing at step  305 , the Network Control Element (NCE)  240  for the network illustrated in FIG. 2 determines whether OOS or IS testing and/or monitoring is to be performed. As previously described, each of the Add/Drop sites ( 210 - 1  through  210 - 4 ) of FIG. 2 is equipped with and coupled to an Optical Monitoring Unit (OMU)  260 . The NCE  240  coordinates and controls each OMU  260  within the network, and the associated components within each OMU, via data messaging signals. Return messages from each OMU  260  to the NCE  240  provide a mechanism for reporting Add/Drop site status and OA  284  measured performance and test data to the NCE. 
     If OOS tests are to be performed, the NCE  240  signals the optical pilot tone generator (OPTG)  288  associated with the appropriate Add/Drop site  210  to inject an optical signal at wavelength λ 0  over the optical fiber line  220  of the network, in accordance with step  310 . For purposes of clarity of description, it is assumed that the NCE signals the OPTG  288  associated with Add/Drop site  210 - 1 . However, since the OMU  260  associated with every other Add/Drop site  210  is also equipped with an OPTG  288 , the present description of system operation applies equally well when the NCE signals OPTG&#39;s associated with other network Add/Drop sites ( 210 - 2  through  210 - 4 ) to inject an optical signal. 
     The OPTG  288 , when triggered, launches an optical test signal at wavelength λ 0  through the 1×2 optical splitter  266 , over add test line  264 , over add fiber line  222 , through Add/Drop site  210 - 1  and onto the optical fiber line  220  of the network. In accordance with step  315 , the Optical Monitoring Unit (OMU)  260  for each Add/Drop site  210  within the network measures the power received for a plurality of wavelength channels across the network&#39;s WDM spectrum with its respective Optical Analyzer  284 . Each OA within the network verifies that an optical pilot tone (λ 0 ) is received, in accordance with step  320 . If one or more OA&#39;s within the network have not received an optical pilot tone, then a continuity failure has occurred within the network, in accordance with step  325 , and the respective OA(s) not receiving an optical pilot tone transmit a message to the NCE  240  that there is a network continuity failure. For example, if OA corresponding to Add/Drop site  210 - 3  does not receive a transmitted optical pilot tone, but the OA corresponding to Add/Drop site  210 - 2  does receive a transmitted optical pilot tone, the NCE determines that there is a network continuity failure and that the failure has occurred along the optical fiber line  220  between Add/Drop site  210 - 2  and Add/Drop site  210 - 3 . 
     Referring to step  330 , the peak power spectrum profile determined by the OA  284  corresponding to each Add/Drop site  210  within the network is reported via message to the NCE  240 . The NCE compares the reported power spectrum profile for each reporting OA with an expected power spectrum profile for each reporting OA (based upon known losses along the various paths from OPTG  288  to each reporting OA), in accordance with step  335 . If the magnitude of the difference between a reported profile and a corresponding expected profile (PWR 13  CHK_SUM) is greater than a predetermined threshold value (PWR_CHK_THRESH), then in accordance with step  340 , a network segment fault is indicated. The NCE  240  isolates the segment fault to a segment of optical fiber line  220  coupling two Add/Drop sites  210 , one Add/Drop site  210  having an OA  284  reporting a PWR_CHK_SUM magnitude less than the PWR_CHK_THRESH value and the second Add/Drop site  210  having an OA  284  reporting a PWR_CHK_SUM magnitude greater than the PWR_CHK_THRESH value. Utilizing a laser (OPTG  288 ) having a λ 0  at the center of the WDM signal band ensures that reported information is representative of any channel that may be transmitted over the network. 
     In addition to OOS testing for network continuity (per step  320 ) and network segment faults (per step  340 ), the present invention is also operable to perform system OOS optical signal to noise ratio (OSNR) checks. Since the OPTG  288  injects an optical pilot tone signal at wavelength λ 0  over the network optical fiber line  220  in accordance with step  310 , and since the peak power spectrum profile determined by the OA  284  corresponding to each Add/Drop site  210  within the network is reported to the NCE in accordance with step  330 , an OSNR is readily determined by the NCE for each network Add/Drop site. In accordance with step  350 , the NCE  240  accesses the data received from each OA  284  and isolates and quantifies the spectral power reported at wavelength λ 0  and sums the total out-of-band (OOB) spectral power (at wavelengths other than λ 0 ), in accordance with step  350 . The NCE  240  then calculates the OSNR for the network signal at each Add/Drop site  210  by comparing the calculated OSNR to a threshold value, in accordance with step  360 . Referring to step  370 , an OSNR below the threshold value indicates a satisfactory network physical layer. However, in accordance with step  365 , a degradation of the OSNR beyond the threshold value is indicative of a network fault or an unacceptable network performance degradation. 
     Referring now to step  372 , if in service (IS) monitoring or tests are to be performed, then injection of an optical pilot tone signal at each OMU ( 260 ) is not required. Rather, IS tests utilize the OA  284  incorporated within each OMU  260  to conduct passive checks and monitoring of system optical data signals to determine the network health. In step  375 , the OA&#39;s  284  at each Add/Drop site  210  report to the NCE the wavelength, peak power and OSNR for each channel. Performance is monitored at the NCE  240  in accordance with step  380 . As previously described in conjunction with OOS testing, any degradation of the OSNR is reported to the NCE and can be isolated to the appropriate faulty optical fiber segment or component within the network. 
     Furthermore, since the OA  284  monitors the spectrum of dropped and added channels at each Add/Drop site  210  within the network, in accordance with step  384 , the present invention also provides aid in further isolating a network fault to an add fiber segment or drop fiber segment. In step  386 , an OA  284  monitors for a loss of a detected in service (IS) optical data signal at the add segment of a particular Add/Drop site  210 . If the optical data signal is lost along an add segment, then a segment failure is indicated between the associated terminal  250  and its respective Add/Drop site  210 , in accordance with step  388 . In step  390 , the optical data signal at the drop segment for a particular Add/Drop site  210  is monitored. If no loss of an optical data signal is detected by the OA  284  along the drop segment, then no fault is indicated at that Add/Drop site  210 , in accordance with step  394 . If however, a loss of an optical data signal is detected by the OA  284  along the drop segment of a corresponding Add/Drop site  210 , then a prior segment fault is indicated, in accordance with step  392 . For example, assume that an optical data signal is not detected (while IS monitoring is activated) at the drop fiber line  224  corresponding to Add/Drop site  210 - 1 . If it is also assumed that an optical data signal is detected at the drop fiber line  224  corresponding to Add/Drop site  210 - 4 , then the fault is isolated to a intermediate location between Add/Drop site  210 - 1  and Add/Drop site  210 - 4 . Fault determination and isolation occurs at the NCE  240  since monitoring data from each of the OA&#39;s  284  is conveyed to the NCE via messaging signals. 
     Numerous modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of the foregoing description. For example, several currently available alternatives are described within this specification for producing an Optical Analyzer suitable for performing its intended function within the Optical Monitoring Unit. As advances in optical networking technologies and materials continue, it is anticipated that other means and methods for producing viable Optical Analyzers will emerge. Their incorporation within the present invention as a means for optical spectrum analysis would therefore be apparent to those skilled in the art as functional equivalents. 
     Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention and is not intended to illustrate all possible forms thereof. It is also understood that the words used are words of description, rather than limitation, and that details of the structure may be varied substantially without departing from the spirit of the invention and the exclusive use of all modifications which come within the scope of the appended claims are reserved.