Abstract:
A universal in-service monitoring scheme to monitor performances of optical networks, including elements that make up the optical network, is proposed. The elements may be a wavelength selective switch or an optical cross connect. In this scheme, a small tunable probe signal is injected into the optical network via an input of the network. An output signal is received at the output and processed to determine if the probe signal is contained within the output signal. The scheme is such that probe signal injection and detection devices need not be physically co-located. Thus, the scheme is useful to test even optical network that spans thousands of miles. The probe signal is non-interfering with the network traffic so the network can be providing service while its performance is monitored. As examples of the probe signal detection mechanisms include lock-in amplification and coherent detection.

Description:
PROVISIONAL APPLICATION 
   The present application claims priority under 35 U.S.C. § 120 of provisional application 60/443,908 filed on Jan. 31, 2003 and 60/499,467 filed on Sep. 3, 2003, both of which are hereby incorporated by reference in their entirety. 

   FIELD OF THE INVENTION 
   The field of the invention generally relates to optical networks. More particularly, the invention relates to monitoring performances of optical networks, including elements of the optical network such as a wavelength selective switch or an optical cross connect. 
   BACKGROUND OF THE INVENTION 
   In next generation wavelength division multiplexed (WDM) networks, an optical cross-connect (OXC) provides the capability of routing the optical path of multiple input/output fiber ports on different wavelengths or wavebands. In order for network management to control this reconfigurable function, the switching status should be set prior to each data transmission. Since the optical connections for WDM signal transmission are strongly dependent on the switching status and switching quality of OXC, switching failure or malfunction of OXC can lead the data stream to an incorrect destination, cause a collision with another signal, degrade signal performance, and cause a loss of live traffic. 
   The current state of the art on switching status monitoring includes the use of either in-band pilot tone technology (used by Nortel Networks and documented in Hamazumi, et al JLT15, p. 2197, 1997)) or local out-band ID signal generation and detecting (documented in Chang, et al., PTL6, p. 899, 1998 and Zhong et al., Digest OFC&#39;2000). 
   Wavelength selective switches (WSS)(an OXC with granularity of single wavelength), for example an N×N WSS, have been widely proposed and studied in the last few years as a cost-effective solution to provide a transparent by-pass for WDM express traffic at degree n nodes in optical networks. WSS&#39;s provide an optical cross-connect function with single channel granularity, where any WDM channel from any of the N inputs can be routed to any of the N outputs. 
   Until recently, the implementation of WSS in commercial systems was limited by the maturity of optical components and ultra-long haul optical transport technology. With these technologies now becoming available, there is a need to consider additional challenges associated with network monitoring and node management in the optical layer. 
   Current optical performance monitoring (OPM) solutions just basically monitor optical properties of existing channels along the transmission line. Monitoring of WSS, however, is more essential and demanding. Beyond the general physical layer monitoring, such as the insertion loss profile, cross-talk, etc., which affects the quality of signals passing through the WSS, there is no general capability to verify the connectivity of WSS, even before new traffic signals are provisioned in order to establish that a particular optical circuit is available and to avoid potential wavelength collisions downstream. 
   Current optical performance monitoring technology cannot satisfy these requirements, and thus an in-service, traffic signal independent monitoring schemes are desirable. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Features of the present invention will become more fully understood to those skilled in the art from the detailed description given herein below with reference to the drawings, which are given by way of illustrations only and thus are not limitative of the invention, wherein: 
       FIG. 1  illustrates a wavelength selective switch or an optical cross connect that is capable of dynamically rerouting wavelengths from different input ports to output ports according to an embodiment of the present invention; 
       FIG. 2  illustrates a wavelength selective design incorporating reconfigurable blocking filters according to an embodiment of the present invention; 
       FIG. 2A  illustrates an embodiment of the reconfigurable blocking filter according to an embodiment of the present invention; 
       FIG. 3  illustrates an apparatus to monitor performance of an optical network utilizing lock-in amplification according to an embodiment of the present invention; 
       FIG. 4  illustrates an example of a resonant photo detector according to an embodiment of the present invention; 
       FIG. 5  illustrates an apparatus to monitor performance of an optical network utilizing coherent detection according to an embodiment of the present invention; 
       FIG. 6  illustrates an apparatus to monitor performance of an optical network which includes a plurality of probe signal generators, detectors, and modulation signal generators according to an embodiment of the present invention; 
       FIG. 7  illustrates an apparatus to monitor performance of an optical network according to an embodiment of the present invention wherein the probe signal generation and detection are not co-located; 
       FIG. 8  illustrates a method to monitor the performance of an optical network according to an embodiment of the present invention; and 
       FIG. 9  illustrates steps for generating the probe signal according to an embodiment of the present invention; 
   

   DETAILED DESCRIPTION 
   For simplicity and illustrative purposes, the principles of the present invention are described by referring mainly to exemplary embodiments thereof. The same reference numbers and symbols in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. The scope of the invention is defined by the claims and equivalents thereof. 
   The expression “optically connects” or “optically communicates” as used herein refers to any connection, coupling, link or the like by which optical signals carried by one optical element are imparted to the “connecting element.” Such “optically communicating” devices are not necessarily directly connected to one another and may be separated by intermediate optical components and/or devices. Likewise, the expressions “connection”, “operative connection”, and “optically placed” as used herein are relative terms and do not necessarily require a direct physical connection. 
   In general, an N×M WSS fabric can be expressed as an N×M wavelength transfer matrix (WTM) [W i,j (λ)] N,M  to convert the input signal spectra I i (λ) into the different output signal spectra O j (λ). In this notation, [W i,j (λ)] N,M  represents the optical properties of the WSS, which depend on the particular configuration of the switch fabric. Fundamentally, the objective of WSS monitoring can be understood as characterizing each W i,j (λ) element at any time with or without the traffic signal. 
   As noted above, capability to verify the connectivity of an optical network, such as those including wavelength selective switches (WSS) and/or optical cross connects (OXC), even before new traffic signals are provisioned is desirable. In this manner, it can be established that a particular optical circuit path is available and avoid potential wavelength collisions from occurring downstream. 
     FIG. 1  is illustrates a WSS or OXC that is capable of dynamically rerouting wavelengths from different input ports to output ports. For simplicity, an N×N WSS or OXC is assumed. As shown in  FIG. 1 , each routed output may contain cross-talk [e.g. Σε i,j I λ     i     ,j (j≠N)] from other inputs at the same wavelength (adjacent channel cross-talk is ignored and it is assumed that no multipath interferences exist). 
   In an aspect of the invention, the optical network monitoring scheme is proposed to confirm that each wavelength is correctly routed; to ensure that the optical network, including the WSS and/or the OXC, has been setup to avoid collisions from occurring while new traffic is added; and verify that the elements of the optical network (e.g. WSS, OXC) are functioning correctly and to keep the degradation in the system performance induced by each individual element at an acceptable level. 
   For example,  FIG. 2  illustrates a WSS design  200  incorporating reconfigurable blocking filters (RBF)  202 . Also,  FIG. 2A  illustrates an embodiment of the reconfigurable blocking filter. Like other blocking filters, the RBF  202  also blocks a subset of spectrum of wavelengths (channels) it receives on its input and lets other channels pass through to the output. However, as the name suggest, the channels blocked or passed through are dynamically configurable in the RBF  202  through the control signals C. In this situation, the extinction ratio of the RBFs, individually or in combination, may be monitored by the invention. 
   It should be noted that in addition to WSS or OXC, optical networks may be made of many varied elements including splitters, combiners, couplers, blocking filters, long-haul transmission fibers, etc. 
   In an embodiment of the present invention, a probe signal may be injected into the optical network from the input side and detected at the output side. The optical network may include WSS or OXC fabric. Indeed, the optical network may include multiple WSS and/or OXC fabrics combined in an arbitrary manner to meet the required services. It is desired that the probe signal be generated by a wavelength tunable source such as a tunable laser. In this manner, each working wavelength may be covered. 
   It is also desired that the probe signal power or amplitude be small to minimize interference with data-carrying traffic. For example, the probe signal may be substantially 30 dB down compared to the traffic signal, or even lower. At this level, the probe signal may be considered to be non-interfering or at least substantially non-interfering. 
   It is further desired that the frequency of the probe-signal be set to a frequency different than those from the standard International Telecommunications Union (ITU) grid. In this manner, coherent cross-talk may be mitigated. For example, the probe signal may be set at a frequency that is 12.5 GHz away from the ITU grid. 
   On the output side, output signals of the optical network are analyzed to detect the presence of the probe signal. For example, coherent detection or a lock-in amplification mechanism maybe utilized for detection. In this manner, state of the optical network may be accurately characterized including determining whether or not WDM channels are present or not. 
   It should be noted that the probe signal injection and the probe signal detection need not be geographically co-located, particularly while lock-in amplification mechanism is used. For example, the optical network to be tested may include a long-haul transmission line such that the inputs and outputs are thousands of miles apart. 
   An example of an apparatus  300  to monitor performance of an optical network  302  is shown in  FIG. 3 . In this instance, the optical network  302  is shown as a wavelength selective switch with N inputs and N outputs (an N×N WSS). However, it should be noted that the number of inputs and outputs need not be the same, i.e. the optical network  302  may be an N×M WSS. It should also be noted that the optical network  302  may be an N×M optical cross connect. In  FIG. 3 , a path from input  1  of the WSS  302  to the output  2  is tested and monitored. However, it should be noted that any path may be tested and monitored. 
   Each input to the WSS  302  may be optically connected to an input optical coupler  304 . Each input optical coupler  304  may include a plurality of inputs and at least one output. Each input of the input optical coupler  304  is capable of receiving optical signals from an optical signal source. In this manner, each input coupler  304  may receive optical signals from multiple sources and output the optical signals to the corresponding input of the WSS  302 . 
   In a mirror fashion, each output from the WSS  302  may be connected to an output optical coupler  306 . Each output optical coupler  306  may include at least one input and a plurality of outputs. The input of the output optical coupler may receive output signals from the corresponding output of the WSS  302  and output them to its plurality of outputs. 
   The use of the input and output optical couplers  304 ,  306  enables the performance monitoring to take place without interfering with normal data traffic. For example, optical taps which only tap a small percentage of the input signal helps to minimize or prevent interference. The input and output couplers  304 ,  306  may be part of the optical network  302  or part of the monitoring apparatus  300 . 
   The performance monitoring apparatus  300  may include at least one probe signal generator  310  configured to generate a probe signal. In an embodiment, the probe signal generator  310  is a tunable laser. As noted above, by using a tunable laser, the frequency of the probe signal as well as the strength of the probe signal may be adjusted as desired. It should be noted that other devices, such as a broadband white light source plus a tunable filter, may be used to generate the probe signal. 
   The apparatus  300  may also include an ingress switch  312 . The ingress switch  312  receives the probe signal from the probe signal generator  310  and selectively directs the probe signal to one or more inputs of the WSS  302 . For example, as shown in  FIG. 3 , the outputs of the ingress switch  302  are optically connected to the input optical couplers  304  and the ingress switch  302  selectively directs the probe signal to any of the desired input optical coupler  304 . 
   In this instance, the ingress switch  312  is depicted as a 1×N switch (one input, N outputs). While not shown, it should be noted that the ingress switch  312  may include multiple inputs, i.e. a K×N switch. The K×N switch may be capable of directing probe signal received on each of its K inputs to any of its N outputs independently of and simultaneously with directing a probe signal received on any of its other inputs. As will be discussed below, this is useful where multiple probe signals are utilized. 
   Conversely, the apparatus  300  may include an egress switch  314 . Each input of the egress switch  314  receives an output signal from the corresponding output of the WSS  302  and selectively directs the one or more output signals from the WSS  302  to its input. For example, as shown in  FIG. 3 , the inputs of the egress switch  314  are optically connected to the output optical couplers  306  and the egress switch  314  selectively directs the optical signals from the desired output coupler(s)  306  to its input. 
   In an embodiment, the egress switch  314  is a N×1 switch (N inputs, one output). While not shown, it should be noted that the egress switch  314  may include multiple outputs, i.e. a N×L switch. The N×L egress switch  314  may be capable of directing the output signal received on each of its input to any of its L outputs independently of and simultaneously with directing any output signal received on any of its other inputs. Again, this is useful in situations where multiple probe signals are being detected simultaneously. 
   The apparatus  300  may further include at least one probe signal detector  316 . The probe signal detector  316  receives the output signal directed by the egress switch  314  and detects the presence of the probe signal in the optical signal. 
   In the particular embodiment shown in  FIG. 3 , a lock-in amplification mechanism is used to detect the presence of the probe signal. Lock-in amplification is particularly useful in detecting a weak signal (probe signal) mixed in with a strong signal (data traffic) in the output signal. In this instance, the strength of the probe signal may be as much as 30 dB below the strength of the data traffic of a single WDM channel (or even lower). It should be noted that when the strength of the probe signal is so low as compared to the data traffic, the interference is so minimal such that the probe signal may be considered to be non-interfering. 
   The apparatus  300  may include a modulation signal generator  318 , which generates a modulation signal of a predetermined modulation frequency, for example at 1 kHz. The generator  318  may be connected to the probe signal generator  310 , electrically or optically, such that the probe signal is modulated based on the modulation signal. It should be noted that the modulation signal, including the modulation frequency, may be adjusted as desired. 
   The probe signal detector  316  may include a resonant photo detector  320  and a lock-in amplifier  322 . The resonant photo detector  320  detects the presence of the probe signal within the output signal and the lock-in amplifier  322  amplifies the probe signal detected by the photo detector  320 . 
   As shown in  FIG. 4 , the resonant photo detector  320  may be a combination of a photo detector  402  configured to receive the optical signal and a resonant amplification circuit  404  configured to filter out data signal and noise outside of a resonant band from the output signal. The probe signal detection is enhanced when the resonant amplification circuit  404  resonates at a frequency that is substantially equal to the frequency of the modulation signal from the modulation signal generator  318 . 
   Referring back to  FIG. 3 , the lock-in amplifier  322  amplifies the probe signal based on the modulation signal from the signal generator  318 . The lock-in amplifier  322  may be an analog amplifier or a digital signal processing (DSP) amplifier. One advantage of a DSP amplifier over an analog amplifier is that the DSP amplifier provides a greater dynamic range. Experiments conducted by the inventors indicate that a DSP based lock-in amplification probe signal detector is able to detect modulated probe signal down to −85 dBm. If the probe signal is at −30 dBm, then the provided dynamic range is substantially 55 dB. 
   The probe signal detector  316  may also include a tunable filter  324  that receives the output signal from the egress switch  314 . The tunable filter  324  is designed to filter out noise contributions from other WDM channels to the output signal. The filtered output is then sent to the resonant photo detector  316 . This enables easier detection of the presence of the probe signal in the output signal. 
   In addition to the lock-in amplification, coherent detection may also be used to detect the presence of the probe signal within the output signal.  FIG. 5  illustrates an embodiment of a performance monitoring apparatus  500  using this scheme. The apparatus  500  includes devices that are similar to those included in apparatus  300  and these will not be discussed. 
   The apparatus  500  does differ from the apparatus  300  in that there is no modulation signal generator  318 . Instead, the output of the probe signal generator  310  is received by a phase modulator  510  (through a coupler  520  for example). The phase modulator  510  generates a substantially constant modulated signal based on the probe signal. The output of the phase modulator  510  is connected to a polarization scrambler  512 , which reduces the fluctuations caused in the output of the phase modulator. 
   In this manner, the polarization of the output of the polarization scrambler  512  is fairly uniformly distributed. This is useful to enhance the possibility of detection of the probe signal, since the probe signal sent through the WSS  302  and received at one of the outputs undergoes a random polarization. 
   The probe signal detector  512  includes a tunable filter  516  working in concert with a photo detector  518  to detect the presence of the probe signal, while beating with the output of the polarization scrambler  512 . 
   It has been indicated above that the invention is not limited to a single probe signal generator nor is it limited to a single probe signal detector.  FIG. 6  shows a performance monitoring apparatus  600  that illustrates this point. The apparatus  600  is much like the apparatus  300  of  FIG. 3 , but includes a plurality of probe signal generators  310 , a plurality of modulation signal generators  318 , and a plurality of probe signal detectors  316 . The plurality of probe signals may be generated and sent substantially simultaneously by the probe signal generators  310 . Also each of the plurality of probe signal detectors  316  may detect the presence of its respective probe signal substantially simultaneously with other probe signal detectors  316 . 
   It has also been indicated above that performance monitoring apparatus may be such that all parts need not be geographically co-located. An example of this is shown in  FIG. 7 . As shown, the optical network performance monitoring apparatus  700  monitors the performance of the optical network  702 . For example, the optical network  702  may include ultra long haul optical transmission systems such that the input and the output of the network  702  are thousands of miles apart. In this situation, the probe signal generator  710  and the probe signal detector  712  are not co-located. 
   While only a single instances of the probe signal generator  710  and the probe signal detector  712  are shown, it should be noted that multiple generators  710  and detectors  712  can be included. 
   The probe signal generator  710  may be a tunable laser wherein the probe signal is modulated based on an input modulation signal generated by the input modulation signal generator  714  that generates the input modulation signal at a predetermined input modulation frequency. The connection between the probe signal generator  710  and the input modulation signal generator  714  may be electrical or optical. 
   The apparatus  700  may also include an output modulation signal generator  716  optically or electrically connected to the probe signal detector  712  and may generate a reference output modulation signal at a reference output frequency. The output signal is modulated within the probe signal detector  712  base on the output modulation signal. 
   The probe signal detector  712  may modulate the output signal as follows. Based on the reference output frequency, an upper limit modulation frequency and a lower limit modulation frequency is determined. For example, the lower and upper limits may be 10% below and above the reference frequency. Taking this example further, if the reference output frequency is 1 kHz, then the lower limit may be 900 Hz and the upper limit may be 1.1 kHz. As the example indicates, the reference output frequency is included in the range of modulation frequencies. 
   The output signal is modulated with frequencies within the range specified by the upper and lower modulation frequency limits and the results are processed to detect the probe signal. For example, the modulation results may be added and if a strength of a signal at a particular frequency is above a certain threshold, it may be determined that the probe signal of the particular frequency has been detected. Typically, the input modulation frequency and the output modulation frequency is the same or substantially the same. 
   The probe signal detector  712  may be a lock-in amplifier based detector such as the detector  316  shown in  FIG. 3 . It is very likely that the long haul optical network performance monitoring apparatus  700  will be less sensitive than the apparatus  300  shown in  FIG. 3 . However, dynamic range of the apparatus  700  is sufficient, i.e. more than 30 dB, such that detection is possible, especially when DSP lock-in amplifiers are utilized. 
     FIG. 8  illustrates a method  800  to monitor the performance of an optical network, or optical elements such as a WSS or OXC. As shown, a probe signal is generated (step  802 ). The probe signal is directed to a particular input of the optical network (step  804 ). Then an output signal is received from the network (step  806 ) and the output signal is processed to detect the presence of the probe signal within the output signal (step  808 ). 
   Again, as noted above, it is desired that the probe signal be non-interfering with the traffic on the optical network. The probe signal may be tuned to a desired frequency and modulated at the predetermined modulation frequency. If multiple probe signals are generated at step  802 , then each probe signal may be individually modulated as well. In this manner, multiple paths of the optical network may be tested simultaneously. 
   In step  808 , either lock-in amplification or the coherent detection scheme may be used to detect the presence of the probe signal. The output signal may be modulated at the predetermined modulation frequency. If multiple probe signals are to be detected, then each output signal may be modulated individually related to the modulation of the correspondingly generated probe signal. In this manner, multiple probe signals may detected simultaneously. 
   As shown in  FIG. 9 , the probe signal generation step  802  may include tuning the frequency of the probe signal (step  902 ), generating a modulation signal at a predetermined modulation frequency (step  904 ), and modulating the probe signal based on the modulation signal (step  906 ). It should be noted that steps  902  and  904  may be performed in an order other than shown in  FIG. 9 . 
   With the various embodiments of the present invention, the performance of an optical network, or any generic configuration, may be monitored effectively, without being limited geographically, and at low cost. 
   While the invention has been described with reference to the exemplary embodiments thereof, it is to be understood that various modifications may be made to the described embodiments without departing from the spirit and scope of the invention thereof. The terms as descriptions used herein are set forth by way of illustration only and are not intended as limitations.