Abstract:
A method and apparatus for determining if an optical input signals has been interrupted and responsively replacing an interrupted optical input signal with a replacement optical signal having a similar optical profile (e.g., such as a loopback between OADMs within a backbone network), thereby insuring that transient-induced high speed optical amplifier adjustments are avoided.

Description:
FIELD OF INVENTION 
     The invention relates generally to the field of communications and, more specifically, to a method and apparatus for mitigating the effect of transients in optically amplified transmission systems. 
     BACKGROUND OF INVENTION 
     Optical add-drop nodes such as optical add-drop multiplexers (OADMs) form a key functional element in dense wavelength-division multiplexed (DWDM) optical fiber networks. Optical amplifiers such as erbium doped fiber amplifiers, semi-conductor optical amplifiers, Raman amplifiers and the like are commonly deployed within such networks to overcome the attenuation of transmission fibers between nodes or the attenuation of components within network elements. The optical amplifiers typically operate in a saturated regime where, the total output power depends sub-linearly on the number of input channels or is essentially constant. If the number of input channels (i.e., wavelengths) passing through such an amplifier is suddenly reduced, the optical power of the remaining channels will be increased, potentially to the level that degrades optical quality (e.g., measured by a bit error rate) of these remaining channels. For example, in the case of an OADM node receiving a plurality of optical channels and adding a single channel, a fiber-cut upstream of the OADM node (or disconnected OADM input) will cause the sudden elimination of optical energy associated with the received ‘through’ channels, while the remaining ‘add’ channel will receive most of the total optical amplifier power that had previously been distributed among all the channels exiting the OADM. The sudden power change will have detrimental immediate effects not only on the added channel, but also on the stability of all the network elements downstream from the fiber cut. The optical power of the surviving channels can, in principle, be adjusted back to the desired value by re-adjusting the pump conditions of all optical amplifiers. However, in a large network with many WDM channels it is a very significant challenge to accomplish this in a time period sufficiently short to avoid noticeable effects on the network operation. 
     SUMMARY OF THE INVENTION 
     These and other deficiencies of the prior art are addressed by the present invention of a method and apparatus for determining if an optical input signal to an OADM has been interrupted and responsively replacing an interrupted optical input signal with a replacement optical signal having a similar optical profile, thereby suppressing transient power changes in uninterrupted ‘add’ channels without requiring any changes to the optical amplifier operating parameters. 
     A method according to one embodiment of the invention comprises monitoring at least some of a plurality of optical signals to determine if the monitored optical signals have been interrupted, where the optical signals are adapted for use by an optical amplifier and, in response to the interruption of the monitored optical signals, replacing the monitored optical signals with other optical signals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
         FIG. 1  depicts a high level block diagram of an optical add-drop multiplexer (OADM) according to an embodiment of the present invention; 
         FIG. 2  depicts a high level block diagram of a controller suitable for use in the OADM of  FIG. 1 ; 
         FIG. 3  depicts a flow diagram of a method according to an embodiment of the present invention; 
         FIGS. 4-5  depict a high level block diagram of alternate embodiments of a pair of OADMs within a bi-directional traffic environment; and 
         FIG. 6  depicts a synchronous optical network (SONET) useful in understanding an embodiment to the present invention. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The subject invention will be primarily described within the context of an optical add-drop multiplexer (OADM) which may be used in wavelength-division multiplexed (WDM) and dense WDM (DWDM) optical communications systems carrying various traffic types (e.g., SONET). However, it will be appreciated by those skilled in the art that the invention may be advantageously employed in any optical communications system in which it is desirable to avoid the need for fast optical amplifier response to transient conditions such as caused by, for example, a severed optical fiber. 
       FIG. 1  depicts a high level block diagram of an optical add-drop multiplexer (OADM) according to an embodiment of the present invention. Specifically, the OADM  100  of  FIG. 1  comprises an input selector  105 , a demultiplexer  110 , a switching fabric  120 , a multiplexer  130 , an optical amplifier  140  and a controller  150 . The OADM  100  of  FIG. 1  receives an input signal IN, illustratively a WDM or DWDM optical signal comprising N optical signals having respective wavelengths λ 1 -λ N  and transported via an optical fiber and responsively produces a corresponding output signal OUT. The OADM  100  of  FIG. 1  operates to controllably pass through, insert (add) and/or extract (drop) optical communications signals having specified wavelengths to implement thereby the known add-drop function. The OADM  100  is controlled by a network manager or element manager (not shown) via a control signal CONTROL. 
     The input selector  105 , in response to a control signal SW produced by the controller  150 , responsively couples one of the input signal IN and a replacement signal REP to an input of the demultiplexer  110 . The replacement signal REP comprises a signal having optical characteristics similar to those of the input signal IN. Specifically, in the embodiment of  FIG. 1  the replacement signal REP comprises a DWDM signal comprising the same number of multiplexed channels (i.e., λ 1 -λ N ) where each of the multiplexed channels has optical power characteristics similar to those of the channels they are replacing. In this manner, channel equalization and amplification characteristics used by the optical amplifier  140  are acceptable for processing optical signals derived from either the input signal N or replacement signal REP. In this manner, transitioning from the input signal N to replacement signals REP will not induce a large transient adjustment problem for the optical amplifier  140 , and in particular will not cause a significant power excursion of any channels added via the switch fabric  120 . 
     The demultiplexer  110  demultiplexes the selected DWDM input signal IN (or REP) to extract therefrom a plurality of optical signals having respective wavelengths denoted as λ 1 , λ 2 , and so on up to λ N , which optical signals (channels) are coupled to respective inputs of the switch fabric  120 . The switching fabric  120  also receives up to N “add” wavelengths or channels, denoted as ADD at respective inputs. The switch fabric  120 , illustratively an M×M switch fabric, provides at its output ports up to N respective signals or channels selected from the selected input channels IN/REP (pass through mode) and/or additional channels ADD. Additionally, the switch fabric  120  couples to a second group of output ports up to N channels to be dropped. 
     The optical signals selected for propagation as part of the multiplexed output signal OUT are coupled from the switch fabric  120  to the multiplexer  130 , where they are multiplexed to form an optical signal corresponding in form to the input signal IN (i.e., a DWDM optical signal comprising up to N wavelengths). The multiplexed signal produced by the multiplexer  130  is then amplified by optical amplifier  140  to produce the output signal OUT. 
     The optical amplifier may comprise an erbium doped fiber amplifier, semi-conductor optical amplifier, Raman amplifier and the like. Such optical amplifiers typically include various modes of operation adapted to insure appropriate amplification for each of the wavelengths within a DWDM optical signal. The optical amplifier  140  of  FIG. 1  may perform various equalization calculations, spectral analysis calculations, spectral analysis calculations, individual and aggregate power calculations and the like among the various wavelengths (channels) within the multiplexed DWDM optical signal. It is noted that such operations are typically not performed rapidly enough to avoid transient errors caused by, for example, an upstream cut fiber condition. However, the controller  150  operates to cause the input selector  105  to provide the replacement signal REP in place of the input signal IN should such a cut fiber condition (or OADM input open condition) be detected. Thus, the relatively slow adaptations made by the optical amplifier  140  to insure appropriate amplification are sufficient to keep the output signal OUT valid for at least those wavelengths including valid data. 
     The controller  150  controls various operations within the OADM  100  of  FIG. 1 . For example, in one embodiment the switch fabric  120  includes an input monitor  125  that monitors various parameters associated with the input signals (λ 1 -λ N ) to the switch fabric  120 . The input monitor  125  is capable of determining that the power level and/or data within the received wavelengths indicates that an upstream cut fiber condition or OADM open input condition exists. In response to the determination of such condition, the controller  150  causes the input selector switch  105  to route the replacement optical signal to the demultiplexer  110  instead of the input signal IN. Contemporaneously, the controller  150  causes the optical amplifier  140  to begin the equalization and power distribution calibration processes used to equalize optical energy between channels within the DWDM signal it is processing. An embodiment of the controller  150  will be discussed in more detail below with respect to  FIG. 2 . 
     In one embodiment of the invention, the input monitor  125  monitors a plurality of the individual wavelengths λ 1 -λ N  and determines thereby whether a condition associated with one of a fiber cut or disconnected input is present. In one embodiment of the invention, in response to this condition the controller  150  causes all of the wavelengths within the input signal IN to be replaced by selecting, using the input selector  105 , a corresponding DWDM signal including replacement wavelength channels. In an alternate embodiment of the invention, the replacement signals REP are inserted via the add input signals. In this embodiment of the invention, individual wavelengths λ 1 -λ N  within the input signal IN may be selectively replaced by adding corresponding replacement signals and dropping those signals determined by the input monitor  125  to be defective or otherwise inappropriate. 
     In one embodiment of the invention, a portion (e.g., 1% power) of the input signal IN is sampled by a power detector (PD)  102  comprising an optical splitter and a photo detector to produce thereby a power detection signal PD. The power detection signal PD is coupled to the controller  150 , which responsively determines if the power detection signal is above a threshold level indicative of a non-severed upstream fiber condition (or a non-open OADM input condition). If the power level is below the threshold level, then an error condition is assumed and the switch  105  adapted accordingly. It is noted that the power detector  102  may be used in any of the other embodiments of the invention discussed herein with respect to  FIGS. 1-6 . 
     In one embodiment of the invention, the optical amplifier A is not used to amplify the DWDM signal provided by the multiplexer  130 . Rather, each of the individual wavelengths λ 1 -λ N  provided by the switch fabric  120  are individually amplified by respective amplifiers (not shown). In this embodiment, the controller  150  communicates with each of the individual wavelength amplifiers to effect thereby an initiation of a recalibration process. It is also noted that where individual switch fabric outputs are amplified, the controller  150  may selectively cause only those optical amplifiers operative to process replacement signals to enter a calibration process. 
     In one embodiment of the invention, the power detector  102 , input monitor  125  and related logic within the controller  150  may be implemented as a stand-alone detector/control function in which detection of a power level below an appropriate threshold level causes a change in state of a control signal (e.g., SW), thereby cause a replacement of an input WDM or output WDM signal with a replacement signal. In any event, the signal provided at the output will comprise a replacement signal in the case of an inappropriate input signal power level. In embodiments discussed below (e.g.,  FIGS. 2-6 ) the replacement signal may comprise a “looped back” signal provided by a physically proximate OADM servicing a second traffic path (e.g., in and East/West traffic path topology). 
       FIG. 2  depicts a high level block diagram of a controller suitable for use in the OADM of  FIG. 1 . Specifically, the controller  200  of  FIG. 2  is suitable for implementing the controller  150  within the OADM  100  of  FIG. 1 . 
     The controller  200  of  FIG. 2  comprises a processor  230  as well as memory  240  for storing various control programs and other programs  242 . The processor  230  cooperates with conventional support circuitry  220  such as power supplies, clock circuits, cache memory and the like as well as circuits that assist in executing the software routines stored in the memory  340 . As such, it is contemplated that some of the steps discussed herein as software processes may be implemented within hardware, for example as circuitry that cooperates with the processor  230  to perform various steps. The controller  200  also contains input/output (I/O) circuitry  210  that forms an interface between the various functional elements communicating with the controller  200 . 
     Although the controller  200  of  FIG. 2  is depicted as a general purpose computer that is programmed to perform various control functions in accordance with the present invention, the invention can be implemented in hardware as, for example, an application specific integrated circuit (ASIC) or a field programmable gate array (FPGA). As such, the process steps described herein are intended to be broadly interpreted as being equivalently performed by hardware, software, or a combination thereof. 
     The memory  240  is used to store various software instructions including those useful in implementing different embodiments of the present invention, such as the steps discussed below with respect to  FIG. 3 . For example, in one embodiment of the invention the switch fabric  120  includes an input monitor  125  that monitors some or all of the signals (λ 1  through λ N ) received by the switch fabric  120 . In response to a loss of signal indicative of, for example, a severed upstream fiber optic cable or a disconnected OADM input, the input monitor  125  provides indicia of the options of one or more input signals λ 1  through λ N  to the controller  150 . The controller  150  responsively causes one or more replacement signals REPLACE to be inserted in lieu of the missing input signals such that the optical amplifiers  130  will continue to correctly process the output signal OUT. 
       FIG. 3  depicts a flow diagram of a method according to an embodiment of the present invention. Specifically,  FIG. 3  depicts a flow diagram of a method  300  suitable for use in the OADM  100  of  FIG. 1  for identifying a loss of input signal condition and responsively providing a replacement signal such that optical amplification of one or more added signals is performed in an appropriate manner. 
     The method  300  of  FIG. 3  is entered at step  310 , where the input signals (i.e., λ 1  through λ N ) are monitored. Referring to box  315 , the group power of the input signals (i.e., the aggregate powers of signals λ 1  through λ N ) may be monitored, individual input signals may be monitored or other monitoring techniques may be employed. At step  320 , a query is made as to whether the monitored input signals are in a “good” status (i.e., no apparent fiber cut or OADM input disconnect condition). If the monitored input signals appear satisfactory, the method continues to monitor and query at steps  310  and  320 . In the event of a non-satisfactory condition, the method  300  proceeds to step  330 . 
     At step  330 , a determination is made as to whether all or some of the input signals are unsatisfactory. Referring to box  335 , this determination may be made with respect to the entirety of the input signals, individual groups of input signals, or each input signal individually, as well as other techniques. 
     At step  340 , at least the unsatisfactory input signal or signals are replaced by alternative or replacement optical signals having at last a similar optical power and spectral characteristics. In the embodiment  100  of  FIG. 1 , the input monitor  125  and controller  150  operate to replace all of the wavelengths within the input signal IN with corresponding wavelengths within a DWDM replacement signal REP using the input selector switch  105 . Referring to box  345 , the replacement signals may comprise one or more dummy signals, loop back signals from another OADM (or portions of the OADM  100  of  FIG. 1  that are not shown) or other signal sources. In any case, the replacement optical signals comprise optical signals having characteristics similar to the missing optical signals such that the optical amplifiers used within the OADM are not subjected to a fast transient change condition. In this manner, those optical signals added by the OADM  100  of  FIG. 1  are appropriately amplified and tend to avoid any increase in bit error rate (BER) or other quality of service (QoS) degradation due to the sudden loss of received input signal IN. 
     At step  350 , software adjustment procedures for the optical amplification stage are initiated. That is, in one embodiment the controller  128  of the switch fabric  120  indicates to the optical amplifier stage that certain input signals are not present and that the optical amplifier should perform various readjustment procedures such as channel equalization and the like as performed within standard optical amplifier adjustment processes. 
     The method  300  of  FIG. 3  is primarily described within the context of a particular OADM topology (i.e., that described above with respect to  FIG. 1 ). However, modifications to the method  300  may be made by those skilled in the art and informed by the teachings of the present invention to enable similar methodology within the context of the OADMs of  FIGS. 4-6 . 
     The present invention is especially well suited to optical backbone networks (as opposed to access networks), which are typically symmetrical and bi-directional networks providing an “East” connection to complement a “West” connection having a substantially equal bandwidth. Backbone networks are typically implemented as two fiber links between nodes using one fiber-optic cable (i.e., a cable including at least two fibers). The architecture and configuration of an OADM node in this environment may be symmetric or asymmetric with respect to add-channels, drop-channels and through-channels, though a symmetric configuration is more common. Therefore, through-traffic in the “west” direction will have the same channel count and spectral distribution as through-traffic in the “east” direction. The inventors have determined that the through channels in either direction tend to be equalized to a desired power level at the OADM, and that such redirected or replacement channels provide an excellent substitute (with respect to optical amplifier performance considerations) for lost channels, such that there is no immediate need to reconfigure any optical amplifiers within the network. 
       FIG. 4  depicts a high level block diagram of a pair of OADMs within a bi-directional traffic environment. In the embodiment of  FIG. 4 , to suppress optical amplifier transients and stabilize the optical layer downstream of a cable cut, the upstream traffic is looped back and inserted into the downstream direction via a fast optical selector switch. That is, the replacement channels REP discussed above with respect to  FIG. 1  are derived from the opposite direction traffic channels. 
     Specifically, referring to  FIG. 4 , a pair of OADMs  100   W  and  100   E  are shown. Each of the East  100   E  and West  100   W  OADMs includes a respective input switch  105 , a respective add-drop multiplexer core ( 110 - 150 ) and a respective splitter  107 . Each of the OADMs receives a respective input DWDM signal IN at a first input of its respective selector switch  105 , and a split portion of the opposed OADM output at a second input of its respective selector switch  105 . The output of each selector switch is coupled to the input of its respective OADM core. 
     Each ADM core comprises the functionality of the demultiplexer  110 , switch fabric  120 , multiplexer  130 , optical amplifier  140  and controller  150  described above with respect to  FIG. 1 . The output signal produced by each ADM core is coupled to its respective splitter  107 , which splits the output signal into two reduced power (e.g., −3 dB) signals. The first split signal is provided as a respective output signal OUT for the next node in the network, while the second split signal REP is provided as the second input signal (i.e., the replacement signal) to the switch  105  of the opposite direction OADM. 
     In operation, referring to the East signal path, an input signal IN E  comprises a DWDM signal including up to N individual wavelengths. One of the East input signal IN E  and a portion of the West output signal OUT W  is selectively coupled to the East ADM core via the East OADM input switch  105   E  in response to a switch control signal SW E  provided by the controller  150  within the East ADM core. The East ADM core responsively adds up to N signals ADD E  and drops up to N signals DROP E  as previously discussed and produces a DWDM output signal which is coupled to the East splitter  107   E . 
     Similarly, referring to the West signal path, an input signal IN W  comprises a DWDM signal including up to N individual wavelengths. One of the West input signal IN W  and the portion of the East output signal OUT E  is selectively coupled to the West ADM core via the West OADM input switch  105   W  in response to a switch control signal SW W  provided by the controller  150  within the West ADM core. The West ADM core responsively adds up to N signals ADD W  and drops up to N signals DROP W  as previously discussed and produces a DWDM output signal which is coupled to the West splitter  107   W . 
     In one embodiment of the invention, the portion of the output signals diverted to the input of the opposing ADM cores is scrambled by either an East  410   E  or West  410   W  scrambler. In this manner, the physical layer error of a fiber cut or open input is correctly interpreted as an error by a higher logical layer of, for example, a SONET system. That is, the rerouted data will likely not cause a loss of signal (LOS) error within the system, even thought the data is invalid. To induce an error (since the rerouted or replacement data is not valid to the link(s) into which it is routed), the scrambling of such data will likely cause at least loss of frame (LOF) error. 
       FIG. 5  depicts a high-level block diagram of a pair of OADMs within a bi-directional traffic environment. Specifically, the OADM pair  500  of  FIG. 5  comprises a modified version of the OADM pair  400  of  FIG. 4 . That is, each of an East  100   E  and West  100   W  OADMs of  FIG. 5  includes a respective output switch  105  (rather than an input switch), a respective add-drop multiplexer core ( 110 - 150 ), a respective input splitter  107  (rather than an output splitter) and an optional scrambler  510 . In  FIG. 5 , the position of the switches  105  and splitters  107  is reversed from that of  FIG. 4  such that the looped back replacement signal(s) comprises and input signal IN rather than an output signal OUT. 
     In operation, referring to the East signal path, an input signal IN E  comprises a DWDM signal including up to N individual wavelengths. The East splitter  107   E  provides one portion (e.g., half the power) of the input signal IN E  to the East ADM core and the other portion to an input of the West OADM output switch  105   W . The East ADM core responsively adds up to N signals ADD E  and drops up to N signals DROP E  as previously discussed and produces a DWDM output signal which is coupled to the East output switch  105   E . In response to a switch control signal SW E  provided by the controller  150  within the East ADM core, the East output switch  105   E  couples one or the output of the East ADM core and a portion of the West input signal IN W  to its output as the east output signal OUT E . The West signal path works in a similar manner. 
     In an alternate embodiment of the OADM pairs of  FIGS. 4 and 5 , the 1×2 switches and splitter used for each of the East and West paths may be replaced with a 2×2 switch. Specifically, rather than splitting the input (or output) optical signals, a 2×2 switch is used to nominally route each respective input signal IN through its respective ADM core and to its respective output signal path out. In case of a fiber cut condition, the 2×2 switch operates to route active data signals as output signals OUT. 
     Within the context of a synchronous optical network (SONET) system, such as a ring network, a loss of signal (LOS) on the line side of an OADM does not automatically result in a LOS for SONET client equipment (i.e., the client side laser and modulator are not switched off immediately). Where an automatic protection switch (APS) is used, it is especially important to give error signaling to the network manager and such that the network traffic may be routed via an alternate path. 
       FIG. 6  depicts a synchronous optical network (SONET) useful in understanding an embodiment to the present invention. Specifically,  FIG. 6A  depicts a SONET at work comprising a first SONET network element  610  communicating with a second SONET network element  670  via East and West communications paths including, in the order named, a first optical translator (OT) that creates a suitable DWDM wavelenth signal, a plurality of optical ADMs  630 - 650  and a second OT.  FIG. 6B  depicts a fiber cut occurring between OADMs  640  and  650 . In this instance, it is noted that the East bound signal provided by SONET element  610  is guided back towards SONET element  610  via a loop back element  645  within OADM  640 . Similarly, a West bound signal provided by SONET element  670  is routed back to SONET element  670  via a loop back element  655  within OADM  650 . 
     The system  600  of  FIG. 6  uses a two step process to handle the fiber cut. As a first step, the optical amplifier transient avoidance scheme discussed above with respect to  FIGS. 1-5  is employed, thereby ensuring appropriate optical amplification within the various OADMs and OTs as the various optical amplifiers avoid transient-induced error and refine their operation under software control. In this manner, signals added and dropped between OADMs having paths that avoid the fiber cut are still valid and usable. Optionally, by scrambling loop back channels, loss of frame (LOF) errors may be generated to inform the SONET management system that a major signal degradation has occurred, and thereby to induce automatic protection switching at the SONET layer. 
     The scramblers discussed above with respect to the various figures may be implemented by using a Lyot depolarizer which comprises, in part, a high second order PMD element. This may be implemented using approximately 80 meters of PM fiber in two sections spliced under 45 degree angles. An alternate embodiment comprises an all pass filter loop, in which a 3DBM coupler has one output spliced back into its inputs. 
     Although various embodiments that incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.