Abstract:
An apparatus for measuring the optical-signal-to-noise ratio (OSNR) of an optical system is adapted to function in single channel or in multi-channel wavelength division multiplexed optical communication systems. An optical signal spectrum and a center frequency characterize the optical signals. A narrow-band notch filter, realized by an in-fiber Bragg grating, is utilized to remove a component of the signal so the remaining signal can be measured. When multiple channels are present, a bandpass filter is used to select the part of the multiplexed signal to be measured. Both the narrow-band notch filter and the bandpass filter can be tunable to further extend the capabilities of the system. Two detectors are utilized with the power in the channel being measured by a low-gain detector and the power in the noise being measured by a high-gain detector. A processor receives the detector outputs, calculates OSNR, and controls the tunable components.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     N/A 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     N/A 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to performance monitoring of optical networks and specifically to high resolution optical performance monitoring in wavelength division multiplexed systems. 
     Wavelength Division Multiplexing (WDM) is a technique for using an optical fiber to carry many separate and independent optical channels. Each channel is carried by a wavelength of light. The wavelengths between channels do not carry information but usually carry some noise. Dense WDM (DWDM) refers to the close spacing of the wavelengths carrying the channels. Current DWDM systems carry up to 160 channels spaced as closely as 50 Ghz apart with a channel power as low as −30 dBm before an Erbium-Doped Fiber Amplifier (EDFA). As the volume of INTERNET and other data communications traffic has increased, DWDM systems have become more in demand because of their high data carrying capacity. 
     The Bit-Error-Rate (BER) of an optical channel depends on four factors, the optical power level, non-linear optical distortion, electrical noise and distortion and the Optical-Signal-to-Noise-Ratio (OSNR). OSNR and the channel power are affected by an accumulation of factors including insertion loss, polarization dependent loss, and amplifier gain of the various in-line components in the system. OSNR is one of the most important parameters determining DWDM system performance because of its dominance in determining BER. Two DWDM channels having the same optical power but different OSNR have a significant difference in BER. Consequently, OSNR is typically monitored at each receiver site in a DWDM system and the OSNR information is used to optimize performance. 
     An additional reason to monitor OSNR in a DWDM system is the use of Optical-Add/Drop-Multiplexors (OADM). These can inject a new signal onto an unused channel of the DWDM signal or swap a new signal for an old signal in a utilized channel. When the OADM drops a signal, it drops the noise associated with that signal, reducing the noise level of the overall multiplexed signal. In addition, the signal added may have a very different power and noise level from the signal dropped. A change in the power of a channel can degrade the OSNR of other channels and the substitute wavelength may not have the needed OSNR to carry traffic if injected into routes that do not have sufficient safety margin. Each of these difficulties can be compensated for if the OSNR characteristics are measured and used to assure that the appropriate power levels are supplied. 
     One difficulty in OSNR measurement in any optical system is the narrowness of the optical channel linewidth (span of wavelengths used to carry information), requiring a very high resolution filter to distinguish the channel from the noise level. Conventional Optical Performance Monitors (OPM) have limited resolution when used in current systems, and thus can yield inaccurate OSNR measurement results and suboptimum performance of the DWDM system. In a DWDM signal, there is a noise floor above the zero power level and a set of channel powers at regular wavelength intervals. The OSNR for a signal channel is the ratio between the signal channel power and the noise power as expressed in Equation 1.                OSNR                   (   dB   )       =     10                 log                   (       P   signal       P   noise       )               Eq   .              1                                
     If the power measurement for either the channel or noise floor is incorrect, the measured OSNR will not be correct. Conventional OPMs do not measure the noise floor with sufficient resolution to provide correct readings. 
     Current detector circuits cannot measure the noise floor limit causing current OPMs to fail to measure OSNR values correctly. The information component of the signal has a relatively high power level (up to 30 dB greater than the noise) while the noise floor component has a very low power level. It is very difficult to design a circuit to measure a signal with the large dynamic range (up to 60 dB) that can occur in such a DWDM signal. 
     Three devices have traditionally been used to perform optical power measurements: the optical spectrum analyzer (OSA), an optical grating plus a detector array analyzer and the filter analyzer. The optical spectrum analyzer is a piece of laboratory equipment, large, bulky and expensive. It accomplishes bandpass filtering or splitting of the signals using a detraction grating to separate wavelengths, and a detector which measures the power in the wavelength that the signal has been broken into. The OSA can be highly accurate if enough time is allowed for enough energy to impinge on the detector. Because of the size, cost and time needed, it is not practical to utilize OSAs in a DWDM system. 
     The detector array analyzer uses a bulk grating and a detector array. This device satisfies the size and cost requirements for multiple deployments in a DWDM system, but has limitations as to resolution. The filter analyzer is based on a Fabry-Perot filter to determine the wavelength to be measured by the detector. If the spacing of the detector array is narrow enough, the difference between the noise and the channel can be measured. However, because the filter is designed to span multiple channels, the optical resolution is limited. Both the bulk grating and the Fabry-Perot filter can be made small and inexpensive enough to be used in multiple locations in a DWDM system, but they can only measure OSNR to 20 to 25 dB when the DWDM channel spacing is 50 Ghz or less. This limitation results in the measurement error described above and the attendant system inefficiency. 
     As the channel spacing decreases with increasing system capacity, it becomes more necessary to use the OSNR measurement. The best system performance can be realized by equalizing OSNR rather than power. With a built-in optical channel monitor, OSNR can be measured in realtime in the system. For long-haul systems, the OPM facilitates balancing of the optical power to minimize the effects of fiber amplifier gain non-uniformity. In addition, as an increasing number of vendors and service providers come into the DWDM market, it is desirable to use equipment (such as transmitters, optical amplifiers, and receivers) from multiple vendors in the same DWDM system. A small an economical OPM provides a useful tool for system turn-up, operation and troubleshooting in such a mixed vendor environment. Consequently, there is a need for a small, economical high resolution optical monitor that can be utilized and mounted with circuit boards implementing a DWDM system. 
     BRIEF SUMMARY OF THE INVENTION 
     A high resolution optical performance monitor measures the Optical-Signal-to-Noise-Ratio (OSNR) of an optical signal, having channel and noise components. The high resolution optical performance monitor uses a notch filter to separate signal from noise, and two detectors, one for signal and one for noise measurement. The high resolution optical performance monitor is compact and economical to produce, allowing it to be used at each receiver in an optical system. When an in-fiber Bragg grating (FBG) implements the notch filter, the filter response can be made narrow-band and shaped like the optical signal thereby increasing the accuracy of separation of signal and noise. The resolution of each of the detectors can be tailored to the expected power of the channel components, thereby increasing the resolution of the optical performance monitor. The high resolution optical performance monitor provides the resolution needed for DWDM systems with channel spacing down to 50 Ghz and smaller. 
     A high resolution optical performance monitor tailored for single channel operation requires relatively inexpensive components. One high resolution optical performance monitor able to handle the multiplexed signals of a DWDM system utilizes a tunable notch filter in conjunction with a bandpass filter. Alternative implementations incorporate cascaded notch filters and tunable bandpass filters. The narrow-band FBG notch filter is well matched to the tolerances needed to separate signal from noise in a DWDM system. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The invention will be understood from the following detailed description in conjunction with the drawings, of which: 
     FIG. 1 is an illustration of a hypothetical DWDM signal waveform, the associated true OSNR values, and the OSNR values as measured by prior art optical power monitors; 
     FIG. 2 is a diagram of a DWDM signal; 
     FIG. 3 is a block diagram of a DWDM transmission system; 
     FIG. 4 is an illustration of the waveform existing at an optical add/drop channel multiplexor as shown in FIG. 3; 
     FIG. 5 is block diagram of the components used in prior art bandpass filter spectrum analysis; 
     FIGS. 6 a,    6   b  and  6   c  are illustrations of the waveform existing at an optical add/drop channel multiplexor as shown in FIG. 3; 
     FIG. 7 is a block diagram of the basic configuration of one tunable high resolution optical power monitor in accordance with the present invention; 
     FIG. 8 a  is a signal spectrum into a first detector in the high resolution optical power monitor of FIG. 7; 
     FIG. 8 b  is the signal spectrum into a second detector in the high resolution optical power monitor of FIG. 7; 
     FIG. 9 is a schematic of an alternative tunable high resolution optical power monitor for multiple channels in accordance with the present invention; 
     FIG. 10 a  is a signal spectrum into a first detector in the high resolution optical power monitor of FIG. 9; 
     FIG. 10 b  is a signal spectrum as measured by a second detector in the high resolution optical power monitor of FIG. 9; 
     FIG. 11 is a block diagram of an alternative high resolution optical power monitor in accordance with the present invention; 
     FIG. 12 a  is a signal containing information and noise as provided to a first detector in the high resolution optical power monitor of FIG. 11; 
     FIG. 12 b  is a signal spectrum of noise only as provided to a second detector in the high resolution optical power monitor of FIG. 11; and 
     FIG. 13 is a block diagram of a wide range high resolution optical performance monitor. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An idealized multiplexed optical signal  100  is shown in FIG.  1 . The peaks of the waveforms  101  represent the information carrying wavelengths, while the flats  105  between the peaks represents the noise floor of the signal. The true OSNR is the ratio of the channel power to the noise power as represented by arrow  104 . An OPM with insufficient resolution does not detect the true signal  100 , but rather a lower resolution signal represented by the dashed line  102 . While the low resolution signal  102  represents the channel power  101  correctly, its noise floor  103 , is not represented accurately. Consequently, the OPM measures a false OSNR  106 . 
     A Dense Wavelength Division Multiplexing system places optical information channels at separate wavelengths in an optical signal where the wavelengths used are closely packed (separations as close as 50 GHz are currently used). FIG. 2 shows a power spectrum over a frequency range for a DWDM signal. Peaks  10 ,  12 ,  14  and  16  represent the center wavelengths used to carry four separate information channels. The portion of the waveform  18  elevated from a baseline  20  represents the noise floor of the DWDM system due to the use of optical amplifiers. 
     In order to reliably carry communications the channel OSNR must be maintained at a desirably high level. When the OSNR is accurately measured, an optical DWDM system can be balanced to maximize the number of channels carried in the system and the distance between re-amplifications. If the OSNR is inaccurately measured, the power levels of some information channels may be set unnecessarily high, wasting some of the power spectrum that would be otherwise available for use by other channels in the system. Alternately, the power of some channel may be set too low so the OSNR is insufficient to provide error-free link performance. The inaccurate OSNR measurement can also result in an un-optimized system turn-up that may not be able to meet the design target. Such an un-optimized system may give false alarms to the network management system during operation. 
     FIG. 3 illustrates the typical elements of an optical network in a DWDM system. The transmitter signals  22 ,  24 , and  26  are combined by a multiplexor  28  onto one fiber that carries all the channels. Optical amplifiers  30 ,  42 ,  46  and  50  assure that a signal of adequate power is transmitted over the span and adequate power is delivered to demultiplexor  52 . Channels are dropped and added by an add/drop multiplexor  44 . The use of amplifiers  42  and  46  before and after the OADM compensate for the OADM loss. Once the signal is received by the demultiplexor  52 , it is broken into its component channels, which are then delivered to the respective receivers,  54 ,  56  and  58 . 
     The amplifiers  30 ,  42 ,  46  and  50 , used in the DWDM system are typically erbium doped fiber amplifiers. These amplifiers amplify the multiplexed optical signals, but also inject noise into the signal across the wavelength spectrum. Therefore, at least four places  60 ,  62 ,  64  and  66  in the DWDM system are necessarily monitored using OPMs coupled to the optical fiber as is known in the art. The output of the multiplexor  28  is monitored to determine whether each channel signal suffers unexpected loss between transmitter and amplifier. An input to the multiplexor can be adjusted to compensate for problems, such as signal strength, at monitoring point  60 . Measuring the signal going into the add/drop multiplexor  44  allows the system to be tuned to compensate for a channel being added or dropped. For instance, if the channel carrying signal  45 , which is dropped at the OADM  44 , suffers from low OSNR or power, the signal  45  can be adjusted at the transmitter and/or amplifier site to produce a higher OSNR or power. 
     The output of the add/drop multiplexor  64  is monitored to measure the channel power levels. The OADM  44  can inject added signals at different power levels than the express channels (those channels passing through the OADM  44  without modification) in the multiplexed signal. This measurement is very difficult to make with a high degree of accuracy because of the low noise levels that may be present at the output of the OADM. A detector that can measure the signal power may have too gross a resolution to distinguish the low noise levels. FIG. 4 illustrates an exemplary OADM output. In FIG. 4, a channel has been dropped and a new one added at the third peak  274 . The noise level associated with the old channel has been dropped, moving the noise level near the channel  274  from the level at  284  to the level at  272 . The power of the new channel  274  is added to this new noise level  272 . Therefore, the new channel  274  does not exhibit the same power level as other channels in the multiplexed signal. Because prior art OPMs cannot measure the lower noise level at  272 , they report the OSNR represented by arrow  282  rather than the actual OSNR represented by arrow  276 . Because the disclosed high resolution optical power monitor can measure noise floor  272  correctly, it can report the correct OSNR. 
     Just before the combined signals are demultiplexed by demultiplexor  52 , the signal is also monitored to find weak signals that need compensation. Compensation can be applied to signals with weak OSNR or power by increasing the power in their transmitter. After the adjustment, all signal channels will have similar OSNR and every channel will be able to meet its expected BER target. 
     A prior art circuit for measuring optical power is illustrated in FIG.  5 . An isolator  80  prevents any reflections created by tunable bandpass filter  82  from disturbing the actual signal being monitored. The optical output of the bandpass filter  82  is fed to a detector  84 . An electrical signal representing the power in the optical signal is fed to a processor  86 . The processor  86  generates a control signal control a tuner  88  for the tunable bandpass filter  86 . The bandpass filter  82  allows only the wavelengths within the bandpass region to pass through it. When the bandpass filter  82  is centered on a channel, then the detector  84  measures the power of the channel. When the bandpass filter  82  is centered away from any channels, then the detector  84  measures the power of the noise floor. The optical detector  84  converts the optical power into an electrical signal which can be digitized and manipulated by the processor. The detector  84  has only one output electrical range, which is used for both the channel power and the noise power measurements. Once measurement at a particular bandpass center frequency has been taken by the processor  86 , the processor  86  retunes the center frequency of the bandpass filter  82  for the next measurement. 
     The traditional steps for measuring OSNR using the circuit of FIG. 5 are illustrated in FIG.  6 . FIG. 6 a  shows a signal  91  to be measured. The signal  91  has a base power level  92 , a noise floor  94  and a number of channels  96 . In order to measure the OSNR across the frequency spectrum, the bandpass filter  82  is tuned to sequentially select portions of the frequency range  90 ,  90 ′,  90 ″ etc. The signal after passing through the bandpass filter  82  in region  90  is illustrated in FIG. 6 b,  and in range  90 ′ is illustrated in FIG. 6 c.    
     FIG. 6 c  illustrates the results of applying the bandpass filter  82  to the noise floor  94 . The bandpass filter  82  when set to range  90 ′, does not completely isolate the noise floor  94  from the channel signal  96 . Power from tails  106  of the channels  96  is also passed when the noise floor  94  is selected in operating range  90 ′. The resulting a waveform  104 , fed to the detector  84 , has too much power. This extra power distorts the accuracy of the noise floor readings. 
     Selecting a width for bandpass filter  82  is difficult. One requirement is that the filter should be wide enough to cover the whole modulated signal linewidth for an accurate signal power measurement. The competing requirement is that the filter should be narrow enough to reject the signal power when measuring the noise. A conventional OPM is unable to meet both needs sufficiently to obtain accurate OSNR measurements in DMDW systems. 
     FIG. 7 shows one implementation of an improved OPM utilizing a narrow-band notch filter that is implemented as an in-fiber-Bragg grating (FBG). The OPM is first isolated from the main transmission path by an isolator  120 . The optical signal passes through a narrow-band notch filter  122  and a tunable bandpass filter  124 . Depending on whether the power in the channel or the noise is to be measured, a switch  126  directs the optical signal to either a first detector  128  or a second detector  130 . The electrical outputs of the detectors are received by controller/processor  132  which cycles the tuning of the FGB filter  122 , the tuning of the bandpass filter  124  and the setting of the switch  126  for further measurements across a frequency band of interest. 
     The isolator  120 , as previously described, prevents reflections from the monitor back to the multiplexed channel. The FBG narrow-band notch filter  122  permits all wavelengths to pass through it undistorted except for a narrow-band around the wavelength where it is centered. It is much easier to build a notch filter with a very narrow pass band than it is to build a very narrow passband bandpass filter. A FBG filter can be fixed or be tunable within a range of wavelengths. In applications in DWDM systems, the ideal shape of the optical spectrum of the notch filter cancels the optical spectrum of a signal. 
     This is illustrated in FIGS. 8 a  and  8   b,  in which the solid line indicates a waveform after passing through the narrow-band notch filter  122  and the dotted line indicates a frequency range placement of the bandpass filter  124 . When the notch filter  122  is centered in the noise region, FIG. 8 a,  it filters out some noise from the noise floor  140 , as shown at  142 . However, the information channel  144 , passes through unattenuated by the notch filter  122 . When the notch filter  122  is centered on a channel, FIG. 8 b,  it filters out both the channel and any associated noise as shown at  154 . 
     The bandpass filter typically has a response as illustrated by the dotted line  146  in FIG. 8 a.  Note that when the bandpass filter  124  is centered on the information channel  144 , the information channel  144  and some portion of the noise floor  140  passes through. The bandpass filter  124  selects the frequency range to be fed to the detectors  128  and  130 . When the channel  144  is to be measured FIG. 8 a,  the tunable bandpass filter  124  is set to encompass the channel  144  and the notch filter is set away from the channel  144 . When the noise  140  is to be measured FIG. 8 b,  the tunable bandpass filter  124  is set to either of the positions  150  or  152  and the notch filter  122  is set the center of the signal spectrum to cancel the channel power  144  as shown at  154 . While the noise power could be measured at either of positions  150  or  152  alone, measurement accuracy is improved by measuring at both positions and averaging. The unflattened EDFA noise floor  140  may cause the noise power to the left of the channel  144  to be different from the noise power to the right of the channel  144 . In this case, Equation 2 is used by the processor  132  to calculate OSNR.                OSNR                   (   dB   )       =     10                 log                         (       P   sig         1   2          (       P       noise   –        left       +     P       noise   –        right         )         )                     Eq   .              2                                
     If more rapid measurement is desired, the improved accuracy of measuring on both sides of the channel can be sacrificed by measuring on only one side. The OSNR measurement for each of the channel carriers is carried out sequentially. The power for each channel is measured and the noise around the channel is measured. 
     After the signal has passed through the notch filter  122  and the bandpass filter  124 , the power in the signal is measured by a photodetector. Since the channel power can be 30 dB greater than the noise power, two detectors  128  and  130  having different resolutions are used. Signal detector  128  is a low gain detector used to measure channel power. Its full resolution is used to measure the higher power information component of the signal, so one step in its range may represent 0.1 watts of power for example. Noise detector  130 , is a high gain detector used to measure noise power. Its full resolution is used to measure the low power noise component of the signal, so one step in its range may represent 0.001 watts of power for example. The processor  132  normalizes the output of detector  128  the higher resolution of the noise measurement, so the OSNR reading can be expressed at the higher resolution. 
     Controller/processor  132  coordinates the components in making the measurements. Therefore, when measuring channel power, the controller  132  tunes the notch filter  122  to allow the channel to pass, tunes the bandpass filter  124  to select the channel, sets the switch  126  to feed the resultant optical signal to the low-gain detector  128  and normalizes the output of the low-gain detector  128 . When the noise measurement is in progress, the controller  132  centers the notch filter  122  on the signal, tunes the bandpass filter  124  to either side of the channel, and sets the switch  126  to feed the resultant optical signal to the high-gain detector  130 . 
     The circuit of FIG. 7 can be used at monitor points  60 ,  62 ,  64  and  66  of FIG.  3 . Additionally, in a completely monitored system, this circuit can be used to measure OSNR for each channel at the receivers  54 ,  56  and  58 . 
     An alternative OSNR measuring system is illustrated in FIG.  9 . Here, the single tunable FBG notch filter  122  of FIG. 7 is replaced by a set of tunable cascaded FBG notch filters  170 . The center frequencies of the cascade are spaced such that the notch flters  170  can filter out a number of sequential channels in the system. The remaining components are as in FIG. 7, except that controller/processor  132  is programmed differently. The cascaded FGB notch filters  170  are centered to allow a set of channel signals to pass through. The bandpass filter  124  is centered on the channel whose power is to be measured, and the switch  126  is set to direct the filtered signal to the low gain detector  128 . The signal power for that one channel is measured and registered in the processor  132 . The bandpass filter  124  is then tuned to the next channel for the next power measurement. Channel power is measured for a sequence of channels up to the number of notches in the cascaded notch filters  170 . Each one of the cascaded notch filters can be tuned independently. Therefore, the cascaded notch filters have a much wider tuning range than the single filter. 
     After the set of channel powers has been measured, the noise powers are measured. The cascaded notch filter  170  is tuned to cancel out several signal channels, and noise measurements are taken on each side of the channels by successively tuning the bandpass filter  124 . In particular, during the measurement of channel power shown in FIG. 10 a,  the notches are placed at locations  190  in the noise region to allow the information channels  188  to pass. The bandpass filter  124  is set sequentially at locations  180 ,  182 ,  184 , etc. The switch  126  directs the output of the bandpass filter  124  to the first detector  128  to measure the power in the respective channel. 
     After the signal powers have been measured as illustrated by FIG. 10 a,  the cascaded notch filter  170  is tuned to cancel out several of the signals as shown in FIG. 10 b.  The switch  126  is set to direct the signals to the second detector  130 . The tunable bandpass filter  124  is centered, for instance, to the left of the first channel as shown in FIG. 10 b  position  202  and the noise at this position is measured. The bandpass filter is then moved sequentially to positions  204 ,  206 , etc. and the noise power for the respective channels are measured. Eq. 1 above is used by the processor/controller  132  to calculate the OSNRs. If it is desired to average the noise on either side of the channels, then the tunable bandpass filter  124  is centered the same distance to the right and left of the channel and the measurements are made. Equation 2 is then used to calculate the OSNR of each of the channels. FIG. 11 illustrates a high resolution OSNR monitor that can be used on a single optical channel, as illustrated at  45 ,  54 ,  56  or  58  in FIG.  3 . As shown, a splitter  210  splits the signal. The power splitting ratio can be 50/50 or other value depending on the detector sensitivities for the signal and noise measurement. Part of the signal is sent to a notch filter  214  and part directly to a first detector  218 . A second detector  216  measures the optical output of the notch filter  214 , and a processor  220  collects the readings from both detectors. An isolator  212  prevents reflections from the notch filter  214  from feeding back to the main path. The notch filter  214  is centered on the signal channel being measured, so the signal out of the notch filter  214  carries only the noise component of the signal as illustrated in FIG. 12 b.  Second detector  216  is a high gain detector used to measure the noise power. The low gain detector  218  is used to measure the power of both the channel and the noise. Equation 3 is used by the processor  220  to calculate OSNR, where P X  represents the power measured by detector x.                OSNR                   (   dB   )       =     10                 log                   (         P   1     -     P   2         P   2       )               Eq   .              3                                
     The circuit of FIG. 11 can be less expensive than the circuits of FIGS. 7 and 9, because the notch filter  214  does not need to be tunable and there is no bandpass filter or switch. 
     An extension of the circuit of FIG. 11 is illustrated in FIG.  13 . Here, the signal passes through a tunable bandpass filter  250  before being split between a cascaded notch filter  256  and a detector  262 . The circuit of FIG. 13 can be used on a multiplexed signal such as a DWDM signal. The tunable bandpass filter  250  selects a region surrounding the region encompassing one channel, blocking all other channels. The cascaded notch filter  256  has a notch for each information channel in the multiplexed signal. For each measurement, the tunable bandpass filter  250  is set to the respective channel. The low gain detector  262  measures channel and noise powers, while the high gain detector  258  measures only the noise for the selected channel. Equation 3 is used to determine the OSNR for each channel. 
     It will be apparent to those of ordinary skill in the art that other embodiments incorporating the disclosed concepts may be used. Accordingly, it is submitted that the invention should not be limited by the described embodiments but rather should encompass the spirit and full scope of the appended claims.