FIELD OF THE INVENTION 
   The present invention relates generally to measuring the quality of an optical signal transmitted in a photonics network, and, more particularly, to techniques for using an interferometer to separate a modulated, coherent signal from incoherent noise mixed with the signal. 
   BACKGROUND OF THE INVENTION 
   As optical transmission distances become longer and photonic networks begin to assume some of the routing functions formerly associated with electronic switching, there is a need for optical performance monitoring, and optical signal-to-noise ratio (OSNR) measurement in particular. 
   Direct spectrum measurement using tunable filters cannot distinguish between coherent signal power and incoherent noise power. It must rely on baseline measurements at signal-free wavelengths, but such baseline wavelengths may not be present in advanced networks with optical add/drop functions. 
   Polarization-based techniques offer a way to reject a signal so that the noise can be measured, but are susceptible to errors induced by partial polarization of the noise and by depolarization of the signal. 
   There remains a need for a system that is well suited to signal integrity monitoring and diagnostics, and that permits measurement of OSNR. 
   SUMMARY OF THE INVENTION 
   The present invention addresses the needs described above by providing, in one embodiment of the invention, a method for measuring the signal quality of light present in an optical communication medium, the light including at least one substantially coherent optical signal degraded by substantially incoherent optical noise. The method includes the step of passing at least a portion of the light through an interferometer capable of introducing a specified delay into at least one light beam of a plurality of beams split from the portion of the light. A light output power from the interferometer corresponding to a first specified delay setting is measured, and the coherent optical signal is distinguished from the incoherent optical noise based on the light power measurement. 
   The step of distinguishing the coherent optical signal from the incoherent optical noise may be based on the light power measured at a given optical frequency. 
   The method may further include the step of computing a numerical value representing signal quality from relative magnitudes of the coherent optical signal and the incoherent optical noise. In that case, the method may also include the step of determining a numerical value of an optical signal-to-noise ratio. 
   The optical communication medium may be an optical fiber. The step of distinguishing the coherent optical signal from the incoherent optical noise based on the light power measurement may further comprise extracting an envelope of an interferogram. The at least one coherent optical signal may be modulated with live data traffic during the measuring step. 
   The method may further comprise the steps of measuring the light output power from an interferometer at a second specified delay setting, and using power measurements corresponding to the first and second specified delay settings to determine a modulation format of the substantially coherent optical signal. Further, the first specified delay and the second specified delay may be provided by adjusting a resettable beam delayer under the control of a delay controller. 
   In another embodiment of the invention, an apparatus is provided for monitoring an optical signal quality of a light signal transmitted in an optical transmission medium. The apparatus includes a tap connected to the optical transmission medium for tapping off a portion of the light signal, and a splitter for forming two beams from the tapped portion of the light signal. The apparatus further includes a beam delayer for introducing a first specified delay into at least one of the two beams, and instrumentation for measuring a light output power from an interferogram formed from the two beams including the first specified delay. A processor is connected for receiving a measurement of the light output power from the instrumentation. A computer readable medium contains instructions that, when executed by the processor, cause the processor to extract a numerical value from the light output power measurement representing a relative magnitude of coherent and incoherent parts of the light signal. 
   The computer readable instructions may contain an algorithm for distinguishing a coherent optical signal from an incoherent optical noise based on the light power output measurement. The algorithm for distinguishing the coherent optical signal from the incoherent optical noise based on the light power may be performed on data taken at a given optical frequency. The algorithm may extract an envelope of an interferogram. 
   The computer readable instructions may further cause the processor to determine a numerical value of an optical signal-to-noise ratio. The optical communication medium may be an optical fiber. The coherent part of the light signal may be modulated with live data traffic. 
   The apparatus may further comprise a second beam delayer for introducing a second specified delay, and the instrumentation may further be for measuring a light output power from an interferogram formed from the two beams including the second specified delay. In that case, the computer readable instructions further cause the processor to use power measurements corresponding to the first and second specified delays to determine a modulation format of the coherent part of the light signal. 
   The apparatus may further include a delay controller capable of adjusting a resettable beam delayer to obtain the first specified delay and the second specified delay. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic plot of oscillations of optical power as a function of interferometer delay for a Gaussian-filtered noise source. 
       FIG. 2  is a schematic plot of oscillations and oscillation envelopes of optical power as a function of interferometer delay for a modulated signal and two broadband noise signals. 
       FIG. 3  is a plot of experimental interferogram envelopes for a modulated signal and two broadband noise signals. 
       FIG. 4  is an enlarged view of an area of the plot of  FIG. 3 . 
       FIG. 5  is a schematic diagram of an apparatus according to one embodiment of the invention. 
       FIG. 6  is a flow chart showing a method according to one embodiment of the invention. 
   


   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention overcomes the above-described problems by determining the OSNR of signals propagating in photonic networks. By using an interferometer, the inventors distinguish between a coherent optical signal and an incoherent noise occupying the same optical band. 
   Interferometry of CW Light 
   When a continuous wave (CW) light with frequency ω and the power I 0  enters a Michelson interferometer, the transmitted power I T  is given by the following expression: 
   
     
       
         
           
             
               
                 
                   I 
                   T 
                 
                 = 
                 
                   
                     
                       I 
                       0 
                     
                     2 
                   
                   ⁢ 
                   
                     ( 
                     
                       1 
                       + 
                       
                         cos 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         ω 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         τ 
                       
                     
                     ) 
                   
                 
               
             
             
               
                 ( 
                 1 
                 ) 
               
             
           
         
       
     
   
   where τ is the delay between two arms of the interferometer expressed in units of time. A rigorous mathematical treatment is given in Hermann Haus, “Waves and Fields in Optoelectronics” at § 3.6, the content of which is incorporated by reference herein. Depending on the interferometer delay, the transmitted power oscillates between I 0 , corresponding to full transmission of the incident power, and zero, corresponding to total reflection of the incident light back to the input port. Those oscillations continue up to very large values of the interferometer delay, determined by the coherence time of the source (at least 100 ns for laser sources). 
   Interferometry of a Finite Spectral Width Light Source 
   The equation above can be easily generalized for a light source carrying many frequency components. Assuming that each frequency component ω k  has spectral power I k , then the transmitted power I T  is given by: 
   
     
       
         
           
             
               
                 
                   I 
                   T 
                 
                 = 
                 
                   
                     
                       1 
                       2 
                     
                     ⁢ 
                     
                       
                         ∑ 
                         k 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         I 
                         k 
                       
                     
                   
                   + 
                   
                     
                       1 
                       2 
                     
                     ⁢ 
                     
                       
                         ∑ 
                         k 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           I 
                           k 
                         
                         ⁢ 
                         cos 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           ω 
                           k 
                         
                         ⁢ 
                         τ 
                       
                     
                   
                 
               
             
             
               
                 ( 
                 2 
                 ) 
               
             
           
         
       
     
   
   The first term of that equation is half of the total spectral power, and the second term, viewed as a function of the interferometer delay τ, is simply half of the real Fourier transform of the incident spectrum. Thus the shape of the interferogram can be used to determine the complete optical power spectrum. 
   The result of equation (2) can be easily illustrated for a special case of a source whose width δω is much smaller than its center frequency ω 0 .  FIG. 1  is a theoretical plot  100  showing normalized power  110  as a function of interferometer delay  120 . The transmitted power I T  still has rapid oscillations with the interferometer delay τ at the center frequency ω 0 , but this oscillatory pattern is confined within a bellshape-like envelope  130  of the width about 1/δω as shown schematically in the plot of  FIG. 1 . While the exact shape of the envelope depends on the shape of the incident spectrum, the transmitted power I T , in contrast to the monochromatic case, asymptotically approaches a constant level of I 0 /2 (shown at  140 ) for sufficiently large values of the interferometer delay τ (greater then the inverse width of the incident spectrum 1/δω). 
   Note that the “broadband” terminology used herein may be confusing. Optical noise sources that are conventionally called “broadband” in the telecommunication community have a width of about 10 nm (1.25 THz), and are centered around 1550 nm (193.5 THz). So all these sources have, in fact, a rather narrow width when it is compared with their center frequency. Thus, the special case considered above fully describes situations that are of interest herein. 
   Note that numerical calculations of the Fourier transform of a narrow power spectrum centered on high frequency are cumbersome. However, it follows from the convolution theorem that the envelope alone can be obtained by the Fourier transformation of the power spectrum shifted to zero frequency. 
   Interferometry of a Modulated Signal 
   In telecom systems, digital information is encoded in the optical signal by modulating a coherent CW laser light with a sequence of 0&#39;s and 1&#39;s. That modulation causes spectral broadening of the initial laser line. Although a modulated carrier may produce a power spectrum that is quite similar to that of a filtered noise source, its interferogram will, in general, be different. The discussion below considers the most widely-deployed modulation format: simple NRZ OOK modulation. The analysis can be easily extended to other modulation formats. 
   Coherent and incoherent signals display very different interferogram envelopes. An envelope  210  for a 10 Gb/s signal with pseudorandom OOK modulation is presented in the plot  200  of  FIG. 2 . For comparison, there is also plotted the interferograms from a 20 GHz wide square spectrum of incoherent light (line  220 ) and from an incoherent Gaussian spectrum (line  230 ) with a full width of 14 GHz. Comparison of the two broadband noise interferograms reveals that the sharp edges of the square spectrum give rise to ripples in the interferogram. The key difference between the modulated signal interferogram and both broadband noise interferograms is that the latter two have finite width, while the former continues practically indefinitely, similar to the purely monochromatic case of equation (1) above, albeit at a lower level (eventually it will be limited by the coherence time of the laser). The explanation for this is rather simple: various 1&#39;s, even when separated by more than a bit period in time, are coherent because of the underlying carrier frequency. Thus, even if the spectra of a modulated signal and incoherent noise may be of a similar width and appear almost identical when observed with a limited spectral resolution, the interferometer can distinguish between the two because of the signal&#39;s fundamental coherence. 
   Another instructive way to determine the envelope of a modulated signal is to repeat the arguments that led to equation (1), but assume that electric fields are modulated by a slow function of time s(t). This produces the following expression for the transmitted power: 
   
     
       
         
           
             
               
                 
                   I 
                   T 
                 
                 = 
                 
                   
                     
                       I 
                       0 
                     
                     2 
                   
                   ⁢ 
                   
                     ( 
                     
                       1 
                       + 
                       
                         cos 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         ω 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         τ 
                       
                     
                     ) 
                   
                   ⁢ 
                   
                     
                       〈 
                       
                         
                           
                             s 
                             * 
                           
                           ⁡ 
                           
                             ( 
                             t 
                             ) 
                           
                         
                         ⁢ 
                         
                           s 
                           ⁡ 
                           
                             ( 
                             
                               t 
                               + 
                               τ 
                             
                             ) 
                           
                         
                       
                       〉 
                     
                     t 
                   
                 
               
             
             
               
                 ( 
                 3 
                 ) 
               
             
           
         
       
     
   
   In this expression, the oscillatory transmitted power from equation (1) is modified by the autocorrelation function of the modulation signal s(t). Naturally,            s(t)s * (t+τ)          gives the envelope  210  of the modulated signal shown in  FIG. 2 .
   Measured Interferograms 
   The inventors have measured three interferogram envelopes with various sources, using an Exfo model IQ230 interferometer. For broadband noise sources the inventors used ASE from an SOA, which was filtered using commercially available filters from JDS Uniphase: type TB9 diffraction grating filter with bandwidth 75 GHz and type TB4500 angle-tuned thin-film filter with bandwidth 160 GHz. The third source was an unfiltered laser signal with 10 Gb/s OOK modulation, using a pseudorandom data pattern of length 2 23 −1. The results are shown in the plot  300  of  FIG. 3  and are in excellent agreement with the simulations discussed above. All curves were normalized, so that the peak power is equal to 1 on each one of them. In particular, the modulated signal  310  is easily distinguished from the shaped noise  320 ,  330  at larger interferometer delays. 
   The plot  400  of  FIG. 4  is an enlarged view of an area of the plot of  FIG. 3 , showing the wings of the envelopes to show the magnitude of the ripples  410  observed for those two real filters. 
   DESCRIPTION OF EMBODIMENTS 
   A schematic diagram of an apparatus  500  according to one embodiment of the invention is shown in  FIG. 5 . The apparatus  500  for monitoring an optical signal quality of a light signal transmitted in an optical transmission medium  505  such as an optical fiber, includes a tap  506  connected to the optical transmission medium  505  for tapping off a portion of the light signal. The tap  506  may, for example, be a partially silvered mirror or other similar device for re-routing a small percentage of the light signal in the medium  505  to form a tapped beam  535 . 
   The tapped beam  535  enters an interferometer  510  that may include a splitter  515 , a delay means  520  and a detector  525 . The splitter  515  may be another partially silvered mirror for forming two beams from the tapped beam  535 . 
   A first portion  516  of the tapped beam travels directly to a detector  525 . A second portion  517  of the beam travels first through a beam delayer  520  for introducing a delay into the beam  517 . In its simplest form, the beam delayer may be a length of optical fiber or planar wave guide providing a fixed delay. Other means known in the art, such as thermo-optic techniques or a free space moving mirror, may be used to introduce an adjustable delay into the beam  517 . 
   Output power from the recombined beams  516 ,  517  is measured using instrumentation  530  that includes a detector  525 . The beams form an interferogram (not shown) that is a function of the delay, as well as a function of the characteristics of the tapped beam  535 , as discussed above. If the tapped beam  535  is a combination of a coherent modulated signal and incoherent noise, then power output at larger interferometer delays will be close to that of the coherent modulated signal alone. Additional power output measured at zero delay may thereby be attributed to incoherent noise. 
   A processor  540  receives and processes measurement data of the light output power from the instrumentation  530 . In one embodiment, the processor is a component of a computer (not shown). In addition to the processor  540 , the computer may include memory, a reader for reading computer executable instructions on computer readable media, a common communication bus, a communication suite with external ports, a network protocol suite with external ports and a graphical user interface, as is well known in the art. 
   The processor includes or is connected to one or more computer readable media  550 , such as a hard or floppy disk in a disk drive, a magnetic tape in a tape drive, a non-volatile programmable ROM chip such as an EPROM, or volatile computer memory. 
   The computer readable medium  550  contains instructions that, when executed by the processor, cause the processor to compute a numerical value using the light output power measurement. That numerical value represents a relative magnitude of coherent and incoherent parts of the tapped light signal  535 . 
   A method  600  according to one embodiment of the invention is shown in  FIG. 6 . The method measures a signal quality of light present in an optical communication medium. The light includes at least one substantially coherent optical signal degraded by substantially incoherent optical noise. 
   At least a portion of the light is passed (step  610 ) through an interferometer. The interferometer is capable of introducing a specified delay into at least one light beam of a plurality of beams split from the portion of the light. A light output power from the interferometer, corresponding to a first specified delay setting, is measured (step  620 ). 
   The coherent optical signal is then distinguished (step  630 ) from the incoherent optical noise based on the light power measurement. The signal may be distinguished by computing (step  640 ) a numerical value representing signal quality from relative magnitudes of the coherent optical signal and the incoherent optical noise. 
   CONCLUSION 
   In sum, the inventors have shown that an interferogram envelope from a modulated signal extends far beyond that of shaped broadband noise with similar bandwidth. Thus, by measuring that envelope for the interferometer delay values greater than a bit period of the modulation, it is possible to extract the signal power alone even when the signal is corrupted with optical noise. That capability makes possible new OSNR measurement techniques that are well-suited to signal integrity monitoring and diagnostics in emerging photonic networks. 
   The foregoing Detailed Description is to be understood as being in every respect illustrative and exemplary, but not restrictive, and the scope of the invention disclosed herein is not to be determined from the Detailed Description, but rather from the claims as interpreted according to the full breadth permitted by the patent laws. For example, while the method of the invention is described herein with respect to the measurement of signal-to-noise ratio in an optical telecommunications network, the method and apparatus of the invention may be used in any situation where a modulated wave signal must be separated from an unmodulated wave signal. It is to be understood that the embodiments shown and described herein are only illustrative of the principles of the present invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention.
Differences in the interferometric patterns of modulated telecommunication signals and broadband optical noise sources are identified and are exploited in measuring the optical signal-to-noise measurements in reconfigurable photonic networks. A light output power from said interferometer corresponding to a specified delay setting in the interferometer is measured, and a coherent optical signal is distinguished from the incoherent optical noise based on the light output power measurement.