Abstract:
Multiple signals are monitored, for example, by an instrument used for monitoring wavelength division multiplexing (WDM) transmission systems. For each monitored frequency channel, an active mask is initialized. The active mask includes an operating window defined by a minimum frequency, a maximum frequency, a minimum amplitude and a maximum amplitude. Each signal is monitored to determine when the signal strays outside the operating window. An alarm is generated when the signal strays outside the operating window.

Description:
BACKGROUND  
         [0001]    The present invention concerns signal analysis and pertains particularly to defining a channel mask for the purpose of monitoring wavelength division multiplexing (WDM) transmission systems.  
           [0002]    Wavelength division multiplexing (WDM) fiber optic transmission systems use between 4 and 500 laser signals to transmit information. Each laser signal resides within a specific frequency channel. The channel is defined by its center frequency and signal power. It is desirable to monitor the performance of each laser signal within the many channels of a WDM system.  
           [0003]    In prior art multi-wavelength meter (MWM) systems the performance of the laser signals within the channels is listed in a tabular (textual) format. The task of monitoring the channels is left to the user and his ability to analyze the data using an external computer to assess the performance of the individual laser signals relative to the channel center frequency, minimum and maximum power and frequency constraints. This method is complicated because it requires the user to develop additional software and to use additional hardware to make the measurement.  
         SUMMARY OF THE INVENTION  
         [0004]    In accordance with the preferred embodiment of the present invention, multiple signals are monitored, for example, by an instrument used for monitoring wavelength division multiplexing (WDM) transmission systems. For each monitored frequency channel, an active mask is initialized. The active mask includes an operating window defined by a minimum frequency, a maximum frequency, a minimum amplitude and a maximum amplitude. Each signal is monitored to determine when the signal strays outside the operating window. An alarm is generated when the signal strays outside the operating window or outside the active mask boundaries.  
           [0005]    In the preferred embodiment, the active mask for each monitored frequency channel additionally includes a center frequency, a minimum channel frequency limit, a maximum channel frequency limit and a power threshold limit.  
           [0006]    In the preferred embodiment, when an alarm is generated an alarm entry is made. Each alarm entry indicates channel information such as average frequency, minimum frequency, maximum frequency, minimum power, maximum power, current power, current frequency. Each alarm entry also indicates time of alarm generation, date of the alarm generation and an error code indicating a region of operation at the time of the alarm generation.  
           [0007]    A user has flexibility in selecting mask parameters. For example, the minimum frequency, the maximum frequency, the minimum amplitude and the maximum amplitude can vary between all active masks. Alternatively, active masks within specified frequency bands can have identical channel spacing with varying minimum amplitudes. Alternatively, active masks within specified frequency bands can have identical channel spacing with identical minimum/maximum amplitudes and frequencies.  
           [0008]    The present invention simplifies the process of monitoring multiple laser signals by user created masks or automatically created masks. The instrument used the mask to verify that each laser signal is within the prescribed frequency and power range. The invention also accounts for the fact that in WDM systems the noise power floor is wavelength dependent, i.e., accounts for the fact that each laser is set to a unique power level resulting in a uniform optical-signal-to-noise ratio for all laser lines/channels. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a simplified block diagram that illustrates operation of a channel analyzer.  
         [0010]    [0010]FIG. 2 shows a channel mask used to monitor wavelength division multiplexing (WDM) transmission systems in accordance with a preferred embodiment of the present invention.  
         [0011]    [0011]FIG. 3 shows active channel masks designed to account for variation in laser signal power resulting from a varying noise floor in accordance with a preferred embodiment of the present invention.  
         [0012]    [0012]FIG. 4 shows active channel masks with non-uniform channel spacing and variable amplitude threshold limit lines in accordance with a preferred embodiment of the present invention.  
         [0013]    [0013]FIG. 5 shows active channel masks where amplitude threshold and frequency limit lines are identical within a specific band of operation in accordance with a preferred embodiment of the present invention.  
         [0014]    [0014]FIG. 6 is a block diagram that shows the organization of classes that implement signal analysis and the generation of channel masks in accordance with a preferred embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]    [0015]FIG. 1 is a simplified block diagram that illustrates operation of a channel analyzer. A Michelson interferometer  11  includes a receive diode which generates analog signals forwarded to an analog-to-digital (A/D) converter  12 . A preprocessor  13  receives the analog data into a buffer. For example, the buffer is a memory that contains 256 K words of data. The data is in the time domain. Preprocessor uses a Fast Fourier Transform (FFT) to generate data in the frequency domain. For example, a memory is used to store 128 K double words of frequency domain data. Each double word contains a value in complex (r+ji) format. After performing a filter function and a magnitude calculation to generate the 128 double/real elements. The resulting data is passed to a correction block  14 .  
         [0016]    Correction block  14  uses a correction table to correct nonlinearity in the receiving diode and to adjust diode gain as necessary to a lower gain in order to generate 128 K corrected double/real elements. From these elements are derived an input array to analyzer block  15 . From the input array, analyzer block  15  finds data for each channel, and stores the data for the channels in a channel repository. The channels are shown by a display  16 , as part of a graphical user interface (GUI). Analyzer block  14  also generates masks for each channel, as further described below.  
         [0017]    The preferred embodiment of the present invention simplifies the process of monitoring multiple laser signals by specifying a mask for each of the channels or group of channels. The channel analyzer uses the mask to verify that each laser signal is within the prescribed frequency and powers. The user can set (or the instrument can set automatically) different bandwidths and powers for different channels or groups of channels. This allows the user (or instrument) to vary the definition of the mask to take into account that in WDM systems the noise power floor is wavelength dependent. This accounts for the fact that to achieve a specific optical-signal-to-noise ratio the power of each laser is set to a unique level.  
         [0018]    For each channel, the mask defined by the user supplied frequency and amplitude values is used by the channel analyzer to verify that each laser signal is within the frequency and amplitude specified by the user. The channel analyzer monitors each laser signal and compares it to the appropriate channel masks and detects and records when the laser signal moves outside of the desired operation window.  
         [0019]    [0019]FIG. 2 illustrates a mask being used to monitor the operation of one of the many laser signals in a WDM system. The user can choose to create one mask that will be applied to all channels, or create several masks that are applied to regions of the wavelength range of interest. The mask consists of a center frequency (wavelength)  61 , a minimum operating frequency  58 , a maximum operating frequency  59 , a minimum power  55  and a maximum power  54 . A user specifies the desired operating window by specifying the above frequency and power values. The mask also includes a frequency range  43  that is the frequency range of the channel which extends between a minimum channel frequency limit  57  and a maximum channel frequency limit  60 . A power threshold limit  56  indicates a power threshold necessary to detect a signal.  
         [0020]    Within the mask, several operating regions are defined. A region  45  is the operating window of the mask. A region  47  is a region where the laser signal has an amplitude above maximum power  54 . A region  52  is a region where the laser signal has an amplitude below minimum power  55 . A region  49  is a region where the laser signal has a frequency below minimum operating frequency  58 . A region  50  is a region where the laser signal has a frequency above maximum operating frequency  59 . A region  46  is a region where the laser signal has an amplitude above maximum power  54  and has a frequency below minimum operating frequency  58 . A region  48  is a region where the laser signal has an amplitude above maximum power  54  and has a frequency above maximum operating frequency  59 . A region  51  is a region where the laser signal has an amplitude below minimum power  55  and has a frequency below minimum operating frequency  58 . A region  53  is a region where the laser signal has an amplitude below minimum power  55  and has a frequency above maximum operating frequency  59 .  
         [0021]    In FIG. 2 (as well as FIGS. 3, 4 and  5 ) a frequency axis  42  indicates the frequency, and a decibel (dBm) axis  41  indicates signal amplitude at a specified frequency.  
         [0022]    The channel analyzer monitors the laser signal relative to the mask and updates the channel information. This includes updating the following parameters: average power, average frequency, maximum frequency, minimum frequency, minimum power and maximum power. Alarms will be generated if active mask violations occur.  
         [0023]    Table 1 is an example of which information is updated by the channel analyzer for a channel.  
                                           TABLE 1                           Ave   Ave   Min   Max   Min   Max           Region   Power   Freq   Freq   Freq   Power   Power   Alarm                   45   yes   yes   yes   yes   yes   yes   no       49/50   yes   no   no   no   yes   yes   yes       47/52   no   yes   yes   yes   no   no   yes       46/48   no   no   no   no   no   no   yes       51/53   no   no   no   no   no   no   yes       other   no   no   no   no   no   no   yes                  
 
         [0024]    The ‘region’ column refers to a region within the user-defined mask shown in FIG. 2. For example “49/50” indicates the row of values applies to region  49  and region  50 . The following values are listed average power, average frequency, minimum frequency, maximum frequency, minimum power, maximum power. The ‘yes’ and ‘no’ symbols in columns headed by a value indicate which values will be updated when the laser signal is within specified regions. A ‘yes’ indicates that for a specified region the specified value will be updated. A ‘no’ indicates that for a specified region the specified value will not be updated. A ‘yes’ in the Alarm column indicates a signal in the specified region will result in an alarm being generated when violation occurs the first time. A ‘no’ in the Alarm column indicates a signal in the specified region will not result in an alarm being generated.  
         [0025]    When the laser signal violates the mask by moving outside of the desired operating window, the channel analyzer records the following channel information for future retrieval: average power, average frequency, minimum frequency, maximum frequency, minimum power, maximum power, current power, current frequency. Additionally the channel analyzer records the time/date of violation, and an error code that indicates the region of operation at the time of the alarm.  
         [0026]    In a preferred embodiment of the present invention, it is possible to account for the fact that the noise floor in a WDM system is wavelength dependent. The noise floor in a WDM system is wavelength dependent when the power of each laser signal is individually set in order to maintain a constant optical-signal-to-noise ratio (OSNR). To take into account the varying noise floor, the mask power level parameter for different masks is wavelength dependent.  
         [0027]    For example, FIG. 3 illustrates that the amplitude limit can be individually adjusted for each mask. A wave form  90  represents a noise floor in a WDM system. An active mask  91  is shown within a channel  71 . An active mask  92  is shown within a channel  72 . An active mask  93  is shown within a channel  73 . An active mask  94  is shown within a channel  74 . An active mask  95  is shown within a channel  75 . An active mask  96  is shown within a channel  76 . An active mask  97  is shown within a channel  77 . An active mask  98  is shown within a channel  78 . An active mask  99  is shown within a channel  79 . An active mask  100  is shown within a channel  80 . An active mask  101  is shown within a channel  81 . Active masks  91  through  101  are designed to account for the variation in laser signal power which is driven by the varying noise floor and the need to maintain a constant OSNR. FIG. 3 also illustrates that the frequency limits of masks can be varied to take into account varying signal bandwidths.  
         [0028]    It is common that the channel bandwidth and channel center frequency spacing is uniform over the operating wavelength range of a WDM system. However, some WDM system manufactures use non-uniform channel spacing. And some systems have separate bands where the spacing is dependent on the band of operation. FIG. 4 illustrates how the definition of the mask can be varied to take this into account.  
         [0029]    In FIG. 4, a wave form  110  represents a noise floor in a WDM system. In FIG. 4, an active mask  111 , an active mask  112 , an active mask  113 , an active mask  114  all have the same channel spacing but a different power threshold line. Likewise, an active mask  115 , an active mask  116 , an active mask  117 , an active mask  118 , active mask  119 , an active mask  120  and an active mask  121  also all have the same channel spacing but a different power threshold line. Thus, the user may specify non-uniform channel spacing and variable amplitude threshold limit lines.  
         [0030]    A typical use is where the amplitude and frequency limit line settings are identical within a specific band of operation. For example, FIG. 5 illustrates a dual band system with different data rates and signal strengths.  
         [0031]    Particularly, in FIG. 5, a waveform  130  represents a noise floor in a WDM system. An active mask  131 , an active mask  132  and an active mask  133  all have the same channel spacing and the same power threshold line. Likewise, an active mask  134 , an active mask  135 , an active mask  136 , an active mask  137  and an active mask  138  all have the same channel spacing and the same power threshold line. Thus FIG. 5 illustrates the most common use of the mask where, within one or more bands, all of the amplitude threshold levels are set to the same value. In FIG. 5 there are two WDM bands each with unique data rates and hence unique channel spacing and bandwidth.  
         [0032]    [0032]FIG. 6 is a block diagram that illustrates functionality within analyzer  15 . Each block in FIG. 6 represents a class used to implement a particular feature within analyzer  15 .  
         [0033]    User preferences are communicated to analyzer  15  by way of a preference interface block, an alarm interface block, a mask interface block and a channel interface block. Preference interface block  21  includes methods that provide all get and set functionality for the analyzer. Alarm interface block  22  includes methods available for the alarm container. Mask interface block  23  includes methods available for the mask container. Channel interface  24  includes methods available for the channel container. The outside calling software can get access to the analyzer data with the help of standard template library (STL) iterators supported within most C++compilers.  
         [0034]    Analyzer block  20  includes analyze methods that take the input array as specified by methods within preferences interface block  21 . Methods within the analyzer block  20  maintain or generate all channels from the input array.  
         [0035]    Sweep data block  30  handles all data that used to update a channel. The data is mainly extracted from the input array. Sweep data repository  25  provides a container for all sweep data.  
         [0036]    Channel block  31  is the class for channels. A channel is a signal in the input spectrum which is defined mainly by frequency and power. Each Channel has an active mask. This mask (called active mask) defines the frequency range within which the channel will be recognized. The active mask is assigned to the channel in the first sweep or in each sweep. The channel is updated using methods within channel block  31 . Channel repository  26  provides a container for all channels.  
         [0037]    Mask Repository  27  contains masks for the whole frequency range. A new mask can be added by a user (or by another software module) using mask interface block  23 . After a channel has been found the active mask information is updated with the mask for the current frequency of the channel. This update can happen each sweep or only the first sweep (that leads to an active mask that moves within the channel). After the assignment of the mask to the channel, the channel is identified by the higher and lower active mask frequency boundaries.  
         [0038]    Mask block  32  is the class for masks. Active mask block  34  is the assigned frequency range for the channel. This information will be generated from mask repository  27  because mask repository  27  holds user-defined masks for the whole frequency range.  
         [0039]    Alarm block  33  is the class for alarms. Alarm repository  27  provides all violation information collected in time.  
         [0040]    Container template  29  is the base class for each of the repositories. The base class for the repositories is based on STL container types. Suitable containers are list or vector. The base class contains basic methods for each repository.  
         [0041]    The foregoing discussion discloses and describes merely exemplary methods and embodiments of the present invention. As will be understood by those familiar with the art, the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.