Abstract:
A method and apparatus is disclosed for measurement and monitoring of in-band optical signal to noise ratio (OSNR). A two channel optical spectrum analyzer (OSA) is advantageously applied in acquiring wavelength division multiplex (WDM) signal data after it has been split according to polarization, then deriving the in-band OSNR from acquired data due to its narrow bandwidth, selective spectral shape, and capability to analyze two components of a polarized signal simultaneously. The in-band OSNR can be measured without interrupting optical transmission traffic in the network.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present invention claims priority from U.S. Provisional Patent Application No. 60/867,682 filed Nov. 29, 2006, which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure is concerned with wavelength division multiplex optical communication systems, in particular with a method for measuring in-band optical signal to noise ratio (OSNR). 
       BACKGROUND OF THE INVENTION 
       [0003]    Optical signal to noise ratio (OSNR) is a key performance parameter in wavelength division multiplex (WDM) optical networks to predict the bit error ratio (BER) of the system. It is the ratio of the useful signal to noise. Prior art OSNR measurements and calibration have made use of the interpolation method. 
         [0004]    In the interpolation method, OSNR for a given optical channel is obtained by measuring a total signal power and a noise power in a passband associated with the optical channel. Typically, the noise is dominated by amplifier spontaneous emission (ASE) noise generated within optical amplifiers in the optical network. ASE may be measured in spectral gaps either side of the optical channel, normalized to 0.1 nm bandwidth. This is called the linear interpolation method because the noise power is averaged from the values of ASE noise present to the left and to the right of the optical channel. 
         [0005]      FIG. 1  shows a representative prior art WDM spectrum comprising three 10 Gb/s optical channels. The three signal envelopes appear as optical power peaks  102   a - 102   c  on a noise background level  101 . The passband OSNR  103  is usually estimated from a ratio of an optical power reading P signal  of power peak  102   b  at the center wavelength λ i  and an average of noise power measurements P noise     —     L    104 left and P noise     —     R    104 right taken either side of the optical channel of interest  102   b.    
         [0006]    Thus the OSNR can be calculated as: 
         [0000]    
       
         
           
             OSNR 
             = 
             
               
                 P 
                 signal 
               
               
                 
                   
                     P 
                     Noise_L 
                   
                   + 
                   
                     P 
                     Noise_R 
                   
                 
                 2 
               
             
           
         
       
     
         [0007]    For WDM optical networks and all-optical networks (AON), which do not have any optical filters in their optical path, the interpolation method gives an accurate OSNR reading. 
         [0008]    However, many networks do include one or several optical filters, such as channel filters and multiplexers, with consequent distortion of the noise spectrum. In AON networks in-line optical filters built into reconfigurable optical add-drop multiplexers (ROADMs) or dispersion compensation fiber Bragg gratings (FBGs) will also suppress the noise in between optical channels. 
         [0009]    In dense WDM (DWDM) systems operating at high data rates with modulation formats like RZ, CRZ, CSRZ and similar, the modulation bandwidth can be so high that the modulation bands overlap the spectral gaps between the optical signal channels. Under such conditions the ASE noise power is not directly accessible in the inter-channel spectral gap. 
         [0010]    The measurement of the noise power in the inter-channel spectral gaps, used by the OSNR interpolation method, will not give an accurate indication of the noise present at the channel wavelength. Under such conditions the interpolation method is no longer reliable for producing accurate measurements. 
         [0011]      FIG. 2  shows a representative prior art WDM spectrum comprising two optical channels modulated at 10 Gb/s with the two signal envelopes appearing as optical power peaks  202   a  and  202   b  on a noise background level  201 . Using the interpolation method, an interpolated OSNR  204  is obtained. However, the actual noise level within the passband is given by curve  205   b  so that the actual in-band OSNR  203  is substantially smaller than the interpolated OSNR  204 . The reason is that the noise spectrum e.g. of ASE noise is no longer flat, but is distorted by cumulative filter characteristics  206   a ,  206   b  of the in-line filters mentioned above. 
         [0012]    There are a number of alternative ways for accurately determining OSNR in systems such as those mentioned above. 
         [0013]    The time resolved optical gating (TROG) method involves signal deactivation. The channel signal is switched off or blanked for a duration sufficiently short in order not to permit gain levels of optical amplifiers in the WDM optical network to be affected. During the blanking, the in-band noise level is measured. The OSNR is then derived from the in-band noise level and the power in the channel with the signal present. There are major drawbacks with this method. It cannot be performed in a live system without service interruption, which makes it unsuitable for routine monitoring. 
         [0014]    In addition, blanking the channel signal can cause instability of ASE noise, as automatic gain control in the optical amplifiers may change the ASE noise level when the signal is switched off. The noise power level reading may be rendered inaccurate in this situation, yielding an inaccurate OSNR value. 
         [0015]    Another method is based on the recognition that the noise, principally ASE noise, has a random polarization, whereas useful signals have a definite polarization. Thus, by determining the polarization of a particular signal in an optical channel, optical power measurement at the orthogonal polarization may be used to estimate the in-band noise level P Noise     —     in-band . The noise level can then be subtracted from the combined signal and noise power measured at the useful signal polarization to obtain the signal power P signal . From the two resulting values, the OSNR is calculated from the equation: 
         [0000]    
       
         
           
             OSNR 
             = 
             
               
                 P 
                 signal 
               
               
                 P 
                 
                   Noise_in 
                    
                   
                     - 
                   
                    
                   band 
                 
               
             
           
         
       
     
         [0016]    In effect, the optical signal is suppressed to permit the noise power at the signal-wavelength to be measured. This in-band OSNR testing principle is sometimes referred to as polarization controlling or nulling. 
         [0017]    Prior art OSNR monitoring apparatus making use of this principle has been disclosed by Chung (US Patent Application 20040114923), as shown in  FIG. 3 . 
         [0018]    An optical WDM signal is introduced into a polarization controller  22 , from where it is passed through a tunable optical bandpass filter  24  and split into two components by a polarization separator  42  along paths Path 3  and Path 4 . Each component is converted to an electric signal by photodetectors  30   a  and  30   b  whose output is converted into digital form by analog-digital converters  32   a  and  32   b  respectively, which feed into a power calculator  34  followed by an OSNR calculator  36 . 
         [0019]    For each wavelength setting of the tunable optical bandpass filter  24  a corresponding polarization state of a signal can be determined by varying the state of the polarization controller  22 , thereby permitting appropriate polarization splitting to be accomplished. 
         [0020]    The apparatus of Chung presents certain disadvantages. Since it is difficult to make a tunable filter with high dynamic range that has a single mode fiber (SMF) output and narrow bandwidth, in practice SMF pigtailed elements such as polarization controller  22 , polarization separator  42  or photodiodes  30   a ,  30   b  cannot be readily coupled to the output of tunable optical bandpass filter  24  without incurring considerable insertion loss. 
       SUMMARY OF THE INVENTION 
       [0021]    An object of the present invention is to provide an optical signal to noise ratio (OSNR) monitoring method and apparatus for a wavelength division multiplex (WDM) optical transmission system, based on a dual-channel tunable filter using a polarization splitting principle for scanning a range of optical wavelengths. 
         [0022]    The dual-channel tunable filter is disposed after a polarization splitter within an optical train of the OSNR monitoring apparatus, thereby permitting free-space coupling of the dual-channel tunable filter to two photodiodes at the end of the optical train. Because the tunable filter has to pass light of only one given polarization orientation, the effect of already small polarization dependent losses of the tunable filter are further reduced. 
         [0023]    A further object of the present invention is to provide an OSNR monitoring apparatus and method capable of determining the OSNR with improved accuracy using optical power measurements at different polarizations and wavelengths by a suitable choice of wavelength offsets from channel center wavelengths. 
         [0024]    In accordance with one aspect of the present invention, a polarization controller at an input of the OSNR monitoring apparatus enables a rotation of the polarization of the input signal over all polarization orientations. The polarization orientation angle is varied by a central processing unit (CPU) for each wavelength scan of the dual-channel tunable filter. 
         [0025]    Another aspect of the present invention provides for a polarization splitter to separate a WDM signal and noise into two lightpaths, P and S, which are detected simultaneously after passing through the dual-channel tunable filter. By processing signals from the P and S lightpaths for several wavelength scans of the dual-channel tunable filter, a value for in-band OSNR in each signal channel of the WDM signal can be determined. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein: 
           [0027]      FIG. 1  is a prior art optical WDM spectrum illustrating the linear interpolation method; 
           [0028]      FIG. 2  is a prior art optical WDM spectrum showing the effect of filtered noise, which results in a different noise power within the signal bandwidth than in the gaps between optical channels; 
           [0029]      FIG. 3  is a prior art schematic diagram of an OSNR monitoring apparatus; 
           [0030]      FIG. 4  is a schematic diagram of an OSNR monitoring apparatus according to the present invention; 
           [0031]      FIG. 5  is a more detailed schematic diagram of an OSNR monitoring apparatus according to the present invention; 
           [0032]      FIGS. 6   a - 6   c  illustrate the choice of wavelength offset for reducing OSNR measurement error due to polarization cross-talk; 
           [0033]      FIG. 7  is a block diagram an OSNR test setup; 
           [0034]      FIG. 8  shows OSNR measurement results performed at each access point of the test setup obtained by three different methods; and 
           [0035]      FIG. 9  shows measurement error at each access point of the test setup for three different OSNR measurement methods. 
       
    
    
     DETAILED DESCRIPTION 
       [0036]    To achieve the objects of the present invention, an apparatus and method is disclosed based on a principle of polarization splitting or nulling with the advantage that it is suitable for live signal in-band OSNR monitoring without the need for service interruption. 
         [0037]    A first embodiment is shown in a simplified block diagram in  FIG. 4 . The in-band OSNR monitor  400  comprises a polarization controller  402  for receiving a WDM signal via input port  401 . The polarization controller  402 , controlled by a central processing unit (CPU)  420  is capable of varying the polarization orientation of the WDM signal over all polarization orientations. 
         [0038]    From the polarization controller  402  the WDM signal is passed to a polarization filter or splitter  403 , where it is divided into two orthogonally polarized WDM signals  431   p  and  431   s . Each of the orthogonally polarized WDM signals  431   p  and  431   s  is input into an optical spectrum analyzer (OSA)  404  where they are filtered by a narrow passband filter with variable center wavelength and converted simultaneously to electrical signals  411   p  and  411   s  by means of suitable photodetectors. The two electrical signals  411   p  and  411   s  correspond to an optical power in the respective orthogonally polarized WDM signals  431   p  and  431   s , respectively. The CPU  420  can also be used to control scanning of the center wavelength of the narrow passband filter in the OSA  404 , which can be a standard OSA capable of measuring the optical power of two input signals simultaneously rather than sequentially. For example, a suitable dual-port OSA has been disclosed by Benzel et al. (U.S. Pat. No. 6,690,468), which is included here by way of reference. The bandwidth of the narrow passband filter in the OSA  404  is preferably narrower than the signal channel bandwidth to enable more than one measurement to be taken for each signal channel. 
         [0039]    A signal processing unit  405  receives the electrical signals  411   p  and  411   s . Typically the center wavelength of the narrow passband filter in the OSA  404  is repeatedly scanned over a wavelength range of interest while stepping through many different polarization rotation settings of the polarization controller  402 . Alternatively, the polarization rotation settings of the polarization controller  402  may be repeatedly scanned over a Poincare polarization space while stepping through different values of center wavelength of the narrow passband filter in the OSA  404  over a wavelength range of interest. In both cases, values of the electrical signals  411   p  and  411   s  are compiled in a suitable form, such as a table in digital memory, for each scanned center wavelength and polarization rotation setting of the polarization controller  402 . 
         [0040]    The signal processing unit  405  applies mathematical algorithms to the compiled values to extract required features of the WDM signals, such as signal summation, ratio, running minima and/or maxima, for calculating a signal power  441  and noise power  442 , from which the OSNR of signal channels within the scanned wavelength range may be obtained. 
         [0041]    The advantage of locating the OSA  404  after the polarization splitter  403  lies in the freedom to couple the two orthogonally polarized WDM signals  431   p  and  431   s  after being filtered by the narrow passband filter directly or via large diameter fiber to the last elements in the optical train, the photodetectors which output electrical signals  411   p  and  411   s . Thus while the polarization controller  402  and polarization splitter  403  can be standard single mode fiber (SMF) coupled elements, the output from the dual-port OSA  404  does not need to go back to a single mode fiber. It is difficult to make a tunable filter with high dynamic range that has a SMF output and narrow bandwidth. So it is easier to pass signals through the simpler elements without internal mechanisms first before passing them through the more complicated element, namely the OSA  404  (particularly if it is a dual-port OSA), to the photodetector. While it may not be obvious that employing a dual channel filter carries no cost penalty, the present invention capitalizes on proprietary technology which provides a dual channel filter at the same cost as a single channel filter. Using two separate filters would, of course, create much more costs with little or no advantage. 
         [0042]    The measurement principle is based on the fact that optical transmission signals are polarized in an arbitrary orientation, whereas noise such as amplifier spontaneous emission (ASE) noise is randomly polarized. 
         [0043]    The polarization controller  402  and polarization splitter  403  can separate the polarized signal from the randomly polarized ASE noise. Depending on the setting of the polarization controller  402 , the polarization splitter  403  will divide the optical channel power according to polarization state and transmit each part to its appropriate output. On the other hand the polarization splitter  403  will always pass half of the randomly polarized ASE noise to each of its outputs. 
         [0044]    A special processing unit, not shown, evaluates the ASE noise power and signal power. The measurement of the total channel power and the calculation of the ASE noise power within the optical system filter bandwidth gives the ‘in-band’ OSNR. The method, called optical polarization splitting (OPS-OSA) method, assumes that ASE noise induced in the system by optical amplifiers has random polarization. 
         [0045]    A second embodiment of the present invention is shown in  FIG. 5 . The in-band OSNR monitor  500  comprises a polarization controller  502  for receiving a WDM signal  506  (only 3 representative channels shown) via an input port  501 . The polarization controller  502 , controlled by a central processing unit (CPU), not shown, is capable of rotating the polarization orientation of the WDM signal  506  over all polarization orientations. 
         [0046]    From the polarization controller  502  the WDM signal  506  is passed to a polarization splitter  503 , where it is divided into two orthogonally polarized WDM signals p and s. Each of the orthogonally polarized WDM signals p and s is input into a dual-port OSA  504 , comprising a dual-channel tunable filter  506  whose output is fed into two photodetectors  507   p  and  507   s , respectively. The photodetectors  507   p  and  507   s  produce corresponding electrical signals at photodetector outputs  517   p  and  517   s , respectively. The bandwidth of the dual-channel tunable filter  506  is narrower than the signal channel bandwidth to enable more than one measurement to be taken for each signal channel. 
         [0047]    In operation, the dual-port OSA  504  is made to scan over a predetermined range of wavelengths by scan controller  520 , while polarization orientation is rotated by means of the polarization controller  502  for each wavelength scan of the dual-port OSA  504 . Preferably for each wavelength scan of the OSA  504 , only one polarization setting of the polarization controller  502  is used. In general, to cover all the required polarization orientations for a complete OSNR measurement of the WDM signal  506 , a sufficient number of scans is required, in practice exceeding about 100 scans. It is important to measure both signals at the outputs  517   p  and  517   s  of the two photodetectors  507   p  and  507   s  simultaneously while scanning over the wavelength range, as the power in the orthogonally polarized WDM signals p and s can change with time. Sequential measurement could therefore introduce errors. 
         [0048]    When the polarization controller  502  is adjusted so that only the ASE noise  517   p  appears at an output of one of the photodetectors, for instance photodetector  507   p , at the output of the other photodetector  507   s  a channel signal  517   s  will become available. 
         [0049]    A summation unit  508  performing an addition of the electrical signals P e  and S e  at the outputs  517   p  and  517   s  of photodetectors  507   p  and  507   s  in the dual-port OSA  504  is used to display a trace of the total signal on a display unit  512 , which is also controlled by the scan controller  520  (similar to a conventional OSA): 
         [0000]      Signal=( P   e   +S   e ) 
         [0050]    A ‘minimum-hold’ function  509  in the dual-port OSA  504  will internally detect and store the minimum values P min  and S min  for P e  and for S e , respectively, to finally display the minimum values for all the polarization states as adjusted by the polarization controller  502 . The minimum of both P min  and S min  will display a trace with maximally suppressed optical signal channels on the display unit  512 . 
         [0051]    A subtraction unit  510  is used to calculate a ratio by performing division in the logarithmic domain. 
         [0052]    The standard method to measure OSNR can now be applied by measuring the noise power at a wavelength offset left and right of the channel center wavelength 
         [0000]      Noise=min( P   min   ,S   min ) 
         [0053]    Finally, in-band OSNR is calculated using the approximate equation: 
         [0000]    
       
         
           
             OSNR 
             = 
             
               
                 Signal 
                 Noise 
               
               = 
               
                 
                   P 
                   + 
                   S 
                 
                 
                   min 
                    
                   
                     ( 
                     
                       
                         P 
                         min 
                       
                       , 
                       
                         S 
                         min 
                       
                     
                     ) 
                   
                 
               
             
           
         
       
     
         [0054]    In a third embodiment, the dual-port OSA  504  is replaced by two single-port optical spectrum analyzers, whose wavelength scanning is arranged to permit simultaneous measurement of the two orthogonally polarized WDM signals p and s. Simultaneous measurements have the advantage of reducing errors in the OSNR measurement arising from the effects of polarization mode dispersion. 
         [0055]    Alternatively, in a fourth embodiment, the dual-port OSA  504  can be replaced by a separate dual-channel tunable filter  506  whose output is fed into two photodetectors  507   p  and  507   s.    
         [0056]    The sum of the power represented at the outputs  517   p  and  517   s  of photodetectors  507   p  and  507   s  is at every moment equal to the total power (Signal+ASE) at any instantaneous wavelength and independent of the SOP setting of the polarization controller  502 , whereby the spectral information is immediately available with the first scan, as with a conventional optical spectral analyzer. 
         [0057]    Measurement errors due to polarization cross-talk require special attention. By avoiding measurements at a peak power of channel signals this problem can be largely mitigated. A method to achieve this is illustrated in the spectrum shown in  FIG. 6   a.    
         [0058]    A spectral envelope of optical signal  630 , centered on a center wavelength λ c , is enclosed by a channel filter spectral envelope  620  resulting from the various filters within the optical transmission system as mentioned before. A filter characteristic  611  of the dual-channel tunable filter  506  from  FIG. 5  is shown positioned at the center wavelength λ c  at a measurement point  601 . Since the optical signal  630  is at a peak value at this point, it may produce a noticeable distortion of the ASE background (for instance  205   b  in  FIG. 2 ) through polarization cross-talk. By offsetting the filter characteristic  612  of the dual-channel tunable filter  506  to a measurement point  602 , the effects of polarization cross-talk can be substantially reduced. 
         [0059]    A further illustration of this technique appears in  FIG. 6   b . A spectrum of a first polarization peak  631   p  of the optical signal  630  measured at the center wavelength λ c  measurement point  601  and at a maximum setting is represented superimposed on a first ASE power spectrum  640   p  in a plane perpendicular to direction  600 . Ideally a corresponding orthogonally polarized spectrum parallel to direction  600  would show only the ASE power spectrum  640   s . However, due to polarization cross-talk, the first polarization peak  631   p  produces an orthogonally polarized peak  631   s  on top of the ASE power spectrum  640   s.    
         [0060]    When the filter characteristic  612  of the dual-channel tunable filter  506  is offset to a measurement point  602 , a spectrum of the first polarization peak  632   p  of the optical signal  630  at a maximum setting again appears superimposed on the first ASE power spectrum  640   p . However, in this case the corresponding orthogonally polarized spectrum shows only the ASE spectrum  640   s , since the polarization cross-talk peak  632   s  is below the ASE power spectrum  640   s , as illustrated in  FIG. 6   c.    
         [0061]      FIG. 7  shows a simplified block diagram of test access points A-G after each amplifier section. Reference OSNR values were measured with the JDSU Inc. TROG method with high speed optical gating. An acousto-optic-modulator (AOM) chopper modulated at 1 MHz at the transmitter (Tx) site was used to switch a 10 Gb/s signal on and off. A second AOM in front of a standard OSA was synchronously triggered. Synchronizing the second AOM to the ON-state made the standard OSA indicate the signal power (P signal ), whereas synchronizing it to the OFF-state produced an indication of the noise power, which is equal to the in-band noise power (P Noise     —   in-band). 
         [0062]    The high chopper frequency of 1 MHz prevented the optical amplifiers from exhibiting any automatic gain control and amplifier relaxation effects. With this method accurate OSNR measurements could be achieved according to the following formula: 
         [0000]    
       
         
           
             OSNR 
             = 
             
               
                 P 
                 signal 
               
               
                 P 
                 
                   Noise_in 
                    
                   
                     - 
                   
                    
                   band 
                 
               
             
           
         
       
     
         [0063]      FIG. 8  is a graph of measured OSNR in dB versus test access point for three different measurement methods: standard OSA (interpolation method), TROG method with a standard OSA and OPS-method using a JDSU Inc. OPS prototype. The time resolved optical gating method (TROG) was taken as the reference for the ‘true’ OSNR value. Note—this method can only be applied in systems out of service. 
         [0064]      FIG. 9  is a graph of OSNR measurement accuracy in dB versus test access point, referred to the TROG method. The test results show that the standard OSA will always show OSNR values which are too high. This method is based on the noise power in the gaps between the channels which is suppressed by in-line optical filtering. The error can be as high as 9 dB to 10 dB, depending on the system configuration 
         [0065]    The OPS-OSA method shows very accurate conformance to the TROG reference method. The error was typically in the range &lt;±0.5 dB. 
         [0066]    In summary, the measurements of OSNR with the interpolation method used by standard OSAs does not provide accurate measurement results in an AON with in-line optical filters (ROADMs, optical cross-connects (OXC), etc.). The error can be as high as 10 dB. 
         [0067]    The new JDSU Inc. OPS-OSA apparatus and method based on the principle of optical polarization splitting for signal elimination has proven that OSNR measurements with a high accuracy of &lt;±0.5 dB can be attained. A major benefit from this method is that it can be used in monitoring live optical systems without the need of service interruption.