Patent Publication Number: US-9425894-B2

Title: In-band optical signal-to-noise ratio measurement

Description:
TECHNICAL FIELD 
     Various exemplary embodiments disclosed herein relate generally to optical transmission systems and performance measurement. 
     BACKGROUND 
     Optical signal-to-noise ratio (OSNR) is an important parameter of performance in optical transmission systems, as it suggests a degree of impairment when an optical signal is carried by an optical transmission system that employs optical amplifiers.  FIG. 1  illustrates the IEC 61280-2-9 standard definition of an OSNR  101 , which defines OSNR as the average between a left OSNR  102  and a right OSNR  103 , each defined as the difference in power between the peak power and the noise at half the distance between the peaks. 
       FIG. 2  illustrates an exemplary device that handles optical signals. Reconfigurable optical add/drop multiplexer (ROADM)  200  includes two arrayed wavelength gratings (AWGs)  201 - 202  to separate the multiplexed signals in the transmission fiber. In other embodiments, ROADM  200  may include a wavelength selective switch (WSS) to fulfill a similar purpose. ROADM  200  in regular operation may filter out inter-channel amplified spontaneous emission (ASE). As a result, an out-of-band measurement of the OSNR may not truly reflect the actual ASE noise power in the channel. In addition, the bandwidth of the optical signal may almost be as large as the channel filter bandwidth, which may lead to a smooth transition between the noise and the signal. Such smooth transitions may lead to out-of-band OSNR measurements being inaccurate due to the need to have clear separation between the carrier signals. 
       FIG. 3  illustrates an in-band technique for OSNR measurement, which may be commonly known as “polarization splitting.” Optical signal  300  may contain signal component  301  and a noise component  302 . Such an in-band measurement requires the transmitted optical signal  301  to be highly polarized and the ASE noise signal  302  to be randomly polarized. This in-band measurement technique also requires the optical signal to contain only one polarization and that there be a large separation between the optical signal  301  and the ASE noise signal  302  (e.g., at least 10 dB). In such a method, a polarization controller and a polarization splitter are used, as the polarization controller may be used to adjust the polarization of the signal so that all of its power will exit the polarization splitter at one port. As the ASE noise may be randomly polarized (regardless of the state of the polarization controller), approximately half of the ASE noise may exit at one port, while the remaining portion may exit at the other port. However, some devices that use dense wavelength division multiplexing (DWDM) may contain multiple signals, each of which possess a different state of polarization. As a result, this in-band measurement technique might be very time consuming. 
     SUMMARY 
     Provided are embodiments that enable accurate OSNR measurement. In particular, various embodiments enable accurate OSNR measurement for dual-polarization optical signals. 
     A brief summary of various exemplary embodiments is presented. Some simplifications and omissions may be made in the following summary, which is intended to highlight and introduce some aspects of the various exemplary embodiments, but not to limit the scope of the invention. Detailed descriptions of a preferred exemplary embodiment adequate to allow those of ordinary skill in the art to make and use the inventive concepts will follow in the later sections. 
     Various embodiments relate to a method for in-band measurement of an optical signal-to-noise ratio (OSNR) by a demodulation device. The method may comprise receiving an optical payload containing a low-frequency signal. The method may also comprise filtering a first passband of the optical payload in an optical channel. The method may also comprise converting the first passband-filtered optical signal to a first target electrical signal. The method may also comprise measuring DC and AC components of the first target electrical signal. 
     Various embodiments of the method may also comprise filtering a second passband of the optical channel within a bandwidth of the optical channel. The method may also comprise converting the second passband-filtered optical signal to a second target electrical signal. The method may also comprise measuring DC and AC components of the second target electrical signal. The method may also comprise determining the OSNR of the optical channel, wherein the OSNR of the optical channel is based on a ratio between the DC and AC components of the first and second target electrical signals. 
     Various embodiments may also relate to a demodulation device for in-band measurement of an optical signal-to-noise ratio (OSNR). The device may comprise a filter for receiving an optical payload containing a low-frequency signal. The filter may filter a first passband of the optical payload in an optical channel and filter a second passband of the optical payload in the channel. The device may also comprise a converter for converting the first passband-filtered optical signal to a first target electrical signal and a second passband-filtered signal to a second target electrical signal. The device may also comprise a measurement circuit configured to measure DC and AC components of the first and second target electrical signals and determine the OSNR of the optical channel, wherein the OSNR of the optical channel is based on a ratio between the DC and AC components of the first and second target electrical signals. 
     It should be apparent that, in this manner, various exemplary embodiments enable accurate in-band ONSR measurement. Particularly, by measuring the optical payload at different passbands, the demodulator device may obtain accurate measurements of the ONSR based on the relative power levels at different passbands in the optical payload. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to better understand various exemplary embodiments, reference is made to the accompanying drawings wherein: 
         FIG. 1  illustrates a prior art exemplary definition of an optical signal-to-noise ratio (OSNR); 
         FIG. 2  illustrates a prior art exemplary reconfigurable optical add/drop multiplexer (ROADM); 
         FIG. 3  illustrates a prior art in-band OSNR measurement technique for a highly-polarized optical signal with one polarization; 
         FIG. 4  illustrates an exemplary optical payload with a sub-modulation signal; 
         FIG. 5  illustrates an exemplary demodulation device for in-band OSNR measurement; 
         FIGS. 6A-6F  illustrates an exemplary in-band OSNR measurement technique for the optical payload; and 
         FIG. 7  illustrates an exemplary flowchart for measuring OSNR using an optical tunable filter. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, in which like numerals refer to like components or steps, there are disclosed broad aspects of various exemplary embodiments. 
       FIG. 4  illustrates an exemplary optical payload with a sub-modulation signal. Optical payload  400  may contain an average power signal  401  and a stable, low-frequency signal, such as a sub-modulation signal  402 . In the illustrative embodiment, for example, the optical payload  400  may contain an average power signal  401  that may be a DC power signal with a changing sub-modulation signal  402  that may include, for example, a stable AC power signal in the form of A sin wt. In some embodiments, the low-frequency signal may comprise two or more frequency tones. For example, in a network in which optical signals are modulated at high bit rates of 2.5 Gb/s, the low-frequency may comprise one or more frequency tones of approximately 10-100 kHz. The amplitude of the sub-modulation signal  402  may maintain a constant ratio with the average power signal  401 . For example, the amplitude of the sub-modulation signal  402  may maintain a 4% ratio with the average power signal  401 . 
     Optical payload  400  may be a result of an optical source, such as a laser source, being modulated with data. In some embodiments, a transmission fiber may carry a Dense Wavelength Division Multiplexing (DWDM) signal that contains multiple optical payloads  400  that are separated into a plurality of optical channels. In some instances, a demodulation device may receive the DWDM signal from the optical source without being modified by another device in the network; for example, the demodulation device may receive the DWDM signal that includes the optical payload  400  directly from the optical source through an optical transmission line. In such instances, the optical payload  400  may already contain a sub-modulation signal or a low-frequency signal that is already stable. 
     Alternatively, a modulation device may change one channel of the optical payload  400  after being created and/or modified by the optical source. In some embodiments, a system may be implemented so that multiple optical channels are modulated. This may include, for example, multiple modulation devices changing multiple channels. 
     In some embodiments, a feedback system may also be included in the transmission system to maintain a constant ratio between the sub-modulation signal and the optical mean power. For example, an exemplary feedback system for the modulation device (not shown) may tap the optical transmission line to receive an optical channel of the DWDM signal modified by the modulation device. The feedback system may then filter the optical channel and convert the filtered optical channel into an electrical signal using a P-I-N detector. The feedback system may then feed the electrical signal to a plurality of electrical filters and/or amplifiers, which may prepare one or more modified electrical signals for an analog-to-digital converter (ADC). The ADC may convert the modified electrical signals into one or more associated digital signals that are provided to a microprocessor. In some embodiments, the microprocessor may feed the digital signals to a digital-to-analog converter (DAC) and an amplifier, the output of which is provided to another input of the modulation device. In some embodiments, the digital signals produced by the ADC may be used by the microprocessor as feedback for various components of the modified electrical signal, which may then be used as feedback for the modulation device. For example, a high-pass filter (HPF) and the ADC may be used to produce a feedback signal controlling the sub-modulation signal. In such instances, the HPF and the ADC may then be used to maintain a constant ratio between the sub-modulation signal and the optical mean power. 
       FIG. 5  illustrates an exemplary demodulation device for in-band OSNR measurement. Demodulation device system  500  may be used at any point in an optical transmission network after the optical payload  400  of  FIG. 4  is created or modified to include a sub-modulation signal  402 . Demodulation device system  500  may include a demodulation device  501 , an optical tap  502 , and an optical transmission fiber  503 . The demodulation device  501  may include an optical tunable filter  510 , an optical detector  512 , an electronic amplifier and/or filter  514 , an analog-to-digital converter (ADC)  516 , and a control and process unit (CPU)  518 . 
     Optical transmission fiber  503  may carry and transmit the optical signal, such as a DWDM signal that includes the optical payload  400 . Demodulating device  501  may make an optical tap  502  of the optical transmission fiber  503  in order to receive a portion of the optical signal. The portion received by the demodulation device  501  through the optical tap  502  may be a proportional percentage of the total optical signal. For example, the demodulation device  501  may receive a sample optical payload  400  that is approximately 5% of the total optical signal. In alternative embodiments, the demodulating device  501  may receive more substantial portions of the optical signal, up to and including the entire (i.e., 100%) optical signal. 
     Optical tunable filter  510  in the demodulation device  501  may receive the sample optical signal produced by the optical tap  502  and may filter various portions of the sample optical signal. In some embodiments, the optical tunable filter  510  may include at least one filter that passes through only one target channel of a plurality of channels included in the sample optical signal. In some embodiments, the optical tunable filter  510  may also include a tunable passband filter that may pass through a segment of the target channel of the sample optical signal. In such instances, the tunable passband filter included in the optical tunable filter  510  may be controlled by the CPU  518 . Demodulation device  501  may tune the passband filter multiple times when the demodulation device  501  is making an OSNR measurement. 
     Optical detector  512  may receive the portion of the sample optical signal outputted from the optical tunable filter  510  and may convert the portion of the sample optical signal into a target electrical signal. Optical detector  512  may be, for example, a P-I-N detector that may receive the portion of the same optical signal as a series of photons and may produce an electrical signal based on the photons received. Optical detector  512  may comprise, for example, a photodiode whose detection range includes the target&#39;s optical signal. Optical detector  512  may then produce an electrical current based at least on the received target optical signal. 
     Electronic amplifier and/or filter  514  may include at least one gain amplifier or filter that receives a target electrical signal from the optical detector  512  and modifies the target electrical signal before the demodulation device  501  makes measurements to determine the OSNR of the optical channel. In some embodiments, the electronic amplifier and/or filter  514  may include a plurality of electronic filters and/or amplifiers that may modify the target electrical signal. For example, the electronic amplifier/filter stage  514  may comprise an automatic gain-controlled (AGC) amplifier in series with a high-pass filter (HPF) or low-pass filter (LPF) that may filter the target electrical signal. 
     In some embodiments, the demodulation device  501  may measure the target electrical signal between the electronic amplifier and/or filter stage  514  and the ADC stage  516 . As will be discussed below in relation to  FIGS. 6A-6F and 7 , the demodulation device may make measurements of various components of the target electrical signal in order to calculate the OSNR from those measurements. Such measurements may include, for example, the AC and DC voltages of the target electrical signal over time. Demodulation device  501  may make similar measurements at multiple times, as the optical tunable filter  510  may be tuned to different passbands. 
     Analog-to-Digital Converter (ADC)  516  may receive the modified target electrical signal from the electronic amplifier/filter stage  514  and may convert the modified target electrical signal into a digital signal. In some embodiments, the digital signal produced by the ADC  516  may be used by the CPU  518  to, for example, adjust the optical tunable filter  510  or electrical amplifier/filter stage  514 . 
     Control and process unit (CPU)  518  may receive the digital signal produced by the ADC  516  and may control the settings of the optical tunable filter  510  and/or the electronic amplifier/filter stage  514 . In some embodiments, the CPU  518  may make adjustments on the optical tunable filter  510  and the amplifier/filter stage  514  based on the received digital signal, enabling a feedback system within the demodulation device  501  based on the tapped DWDM signal. In some embodiments, the CPU  518  may adjust the passband of the optical tunable filter  510  independent of the digital signal. This may occur, for example, in order to enable the demodulation device  501  to make an ONSR calculation based on multiple measurements of the targeted optical channel. 
       FIGS. 6A-6F  illustrate an exemplary in-band OSNR measurement technique for the signal spectrum. Demodulation device  501  may employ the technique used in  FIGS. 6A-6F  when measuring and calculating the OSNR of a target optical channel. In some embodiments, the CPU  518  may calculate the OSNR based on various measurements on the modified electrical signal at the input of the ADC  516 . In some embodiments the CPU  518  may include a measurement circuit that may be configured to measure components of electrical signals in order to determine the OSNR. 
       FIG. 6A  illustrates a first measurement taken by the demodulation device  501 . CPU  518  may, for example, control the optical tunable filter  510  to a first passband  605  of the target optical channel  601 . In some embodiments, a controller may be included within the optical tunable filter  510 . In alternative embodiments, the CPU  518  may include a controller that may, for example, set the passband filter around the “center” of the target optical channel  601 . In such instances, the “center” of the target optical channel  601  may include the channel&#39;s peak power. As a result, when the demodulation device  501  subsequently measures the voltage of the resultant target electrical signal at the input of the ADC  516 , the voltage may be:
 
 V   ADC-DC1   =V   Sig-DC   +V   ASE  
 
     Where V ADC-DC1  is the measured DC voltage of the target electrical signal when the filter is in its first passband  605 . In some embodiments, the measured DC voltage may be equal to the average voltage of the target electrical signal. The measured DC voltage V ADC-DC1  may be the sum of the DC voltage of the signal, V Sig-DC , and the voltage of the ASE noise, V ASE . 
       FIG. 6B  illustrates the components of the target optical signal selected by the optical tunable filter  510  for the first passband  605 . The components may comprise a signal portion  602  and an Amplified Spontaneous Emission (ASE) noise portion  603 . Demodulation device  501  may attempt to measure the ratio between the signal and the noise by measuring various component values for the target optical signal  601 .  FIG. 6C  illustrates the target optical signal  601  as captured from the first passband  605 . Average power signal  604  may be the average power of the selected optical signal as passed through the first passband  605 . Its value equals:
 
 V   ADC-DC1   =V   Sig-DC   +V   ASE  
 
     This value may be equal to the sum of the signal mean and the ASE noise mean without any sub-modulation signal  607 . Accordingly, the value of the line  603  remains as the voltage of the noise, V ASE , while the sub-modulation signal  607  has a voltage equal to the AC component, V Sig-AC . 
       FIGS. 6D-6F  illustrate another portion of the target optical channel when measured at a second passband  610 . CPU  518  may reposition the optical tunable filter  510  to a second passband  610  after the demodulation device  501  made measurements at the first passband  605 . In some embodiments, the CPU  518  may control the optical tunable filter  510  to a second passband  610 , where the CPU  518  chooses the second passband  610  based on the relative power of the optical channel. For example, in the illustrative embodiment, the second passband  610  may be positioned where the power of the target optical channel is at least 10% below peak power. 
     In other embodiments, the CPU  518  may choose the second passband  610  based on the location of the first passband  605 ; for example, the CPU  518  may move the optical tunable filter  510  to center around a second passband  610 , where the second passband  610  is at a defined distance away from the first passband  605 . For example, in a 50 GHz Grid system, the tuning may have a maximum distance of 25 GHz. In such instances, the CPU  518  may use a defined distance of 15 GHz when tuning to subsequent passbands. While  FIGS. 6A-6F  illustrate the demodulation device  501  making a measurement where the first passband  605  is centered around the peak power of the optical signal, alternative embodiments may have the demodulation device  501  position the passband filter of the optical tunable filter  510  to passbands that are not at peak power. Similarly, in some alternative embodiments, the second passband  610  may have a higher power than the first passband  605 . 
       FIG. 6E  illustrates the target optical channel at the second passband  610 . In the illustrative embodiment, the ASE noise signal  613  may maintain a similar value to the noise measurement at the first passband  605 , where V ASE1 ≈V ASE2 . However, the total optical signal  612  may be measurably less than the total optical signal  602  of the target optical signal at the first passband  605 , where the voltage measured at the input of the ADC  516  equals:
 
 V   ADC-DC2   =V   Sig-DC   *R+V   ASE  
 
     Where V ADC-DC2  is the measured DC voltage of the electrical signal at the second passband  610 . In this equation, R is equal to a reduction value. In the illustrative embodiment, R may be a value between 0 and 1, due to the rolloff characteristics of the payload spectrum as shown in  FIG. 6A . In alternative embodiments, R may be greater than 1. This may occur, for example, when the measured power in the second passband  610  is higher than the measured power in the first passband  605 . 
       FIG. 6F  illustrates the target optical signal as captured by the passband filter. In the illustrative embodiment, the target optical signal  601  may include the portion of the tapped optical channel that passed through the second passband  610 . In such instances, the sub-modulation signal  617  may also be attenuated by the same reduction value R, while the ASE noise signal  613  may remain constant. 
     CPU  518  may use the measured values for the target electrical signal at both the first passband  605  and the second passband  610  to calculate the OSNR of the optical signal. As:
 
 V   ADC-DC1   =V   Sig-DC   +V   ASE  
 
 V   ADC-DC2   =V   Sig-DC   *R+V   ASE  
 
     Subtracting these equations from each other may produce:
 
 V   ADC-DC1   −V   ADC-DC2 =(1− R ) V   Sig-DC  
 
     Similarly, the voltage of the sub-modulation signals  607 ,  617  may be expressed as:
 
 V   ADC-AC1   =V   Sig-AC  and
 
 V   ADC-AC2   =V   Sig-AC   *R  
 
     Where V ADC-AC1  and V ADC-AC2  may be measured as AC voltages at the first passband  605  and the second passband  610 , respectively. Dividing the two previous equations may then produce: 
     
       
         
           
             
               
                 V 
                 
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                     AC 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                 
               
               / 
               
                 V 
                 
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                     ⁢ 
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                       AC 
                     
                   
                   * 
                   R 
                 
                 
                   V 
                   
                     Sig 
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                     AC 
                   
                 
               
               = 
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     Using these equations, the ratio of signal power to in-band ASE noise power, P sig /P ASE , may be expressed as the ratio of voltages V sig-DC /V ASE , where: 
     
       
         
           
             
               
                 V 
                 
                   Sig 
                   - 
                   DC 
                 
               
               
                 V 
                 ASE 
               
             
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                           ⁢ 
                           
                               
                           
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                           ⁢ 
                           
                               
                           
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                 ⁢ 
                 
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                     ADC 
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                       1 
                     
                   
                 
               
               
                 
                   
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                         DC 
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     The power ratio may be simplified to: 
     
       
         
           
             
               
                 P 
                 Sig 
               
               / 
               
                 P 
                 ASE 
               
             
             = 
             
               
                 1 
                 - 
                 
                   R 
                   DC 
                 
               
               
                 
                   R 
                   DC 
                 
                 - 
                 
                   R 
                   AC 
                 
               
             
           
         
       
     
     Where R DC  and R AC  are the ratios of the DC voltages and the AC voltages at the second and first passbands, respectively. This ratio of P sig /P ASE  may be a quantity that compares the signal power to the in-band ASE noise power, where P sig  is the signal power within the passband of the optical filter  510  and P ASE  is the ASE noise power within the passband of the optical filter  510 , respectively. Knowledge of the passband and the format of the optical payload  400  may then be used to determine the optical power with the optical payload spectrum  601 . Similarly, knowledge of the bandwidth of the passband may also be used to determine the ASE noise power within a 0.1 nm bandwidth. As the target electrical signal is related to the optical signal, the OSNR may be determined based on the power ratio. For example, knowledge of the bandwidth of the passband and the format of the optical payload  400  may be used to determine the OSNR of the optical payload  400  from the measurements of the associated electrical signal. 
       FIG. 7  illustrates an exemplary flowchart for measuring OSNR using the optical tunable filter  510 . Demodulation device  501  may employ method  700  at any point upon tapping the optical transmission fiber  503  to measure the OSNR of an optical channel. Method  700  may be similar to steps illustrated in  FIGS. 6A-6F , where the demodulation device may use the CPU  518  to tune a passband filter in the optical tunable filter  510  to different portions of the target optical channel  601  in order to make measurements to calculate the ONSR. 
     Method  700  begins at step  701  and proceeds to step  703 , where the demodulation device  501  receives the optical signal. In some embodiments, the optical tunable filter  510  of the demodulation device  501  may receive the optical signal. In some embodiments, the optical tunable filter  510  may receive a tapped optical signal from the optical tap  502  located on the optical transmission fiber  503 . 
     Demodulation device  501  may then proceed to step  705 , where the passband filter within the optical tunable filter  510  is set to a first passband  605 . In some embodiments, the CPU  518  may control the tuning of the passband filter of the optical tunable filter  510 . In some embodiments, the passband filter may be tuned to a first passband  605  in which the passband filter is centered on the tapped optical payload at peak power. For example, the CPU  518  may tune the passband filter of the optical tunable filter  510  around the center of the tapped optical payload spectrum  601 . 
     In step  707 , the demodulation device may convert the received optical payload into an electrical signal. In some embodiments, the demodulation device  501  may use an optical detector  512 , such as a P-I-N detector to detect optical levels of the optical signal transmitted from the optical tunable filter  510  and convert these optical levels into a related electrical signal. In some embodiments, the optical signal transmitted from the optical tunable filter  510  may only include portions that passed through the passband filter. 
     In step  709 , the demodulation device  501  may measure the electrical signal. In some embodiments, the demodulation device  501  may use a measurement circuit included in the CPU  518  to measure the electrical signal after traversing through both the optical detector  512  and an amplifier/filter stage  514 . In some embodiments, the demodulation device  501  may measure the electrical signal immediately after being produced by the optical detector  512 . Demodulation device may measure multiple components of the electrical signal over time, such as the voltage of the electrical signal over a defined period. From this voltage measurement, the demodulation device may separate the voltage into separate DC and AC voltages, which may represent the average power signal and sub-modulation signal, respectively. In some embodiments, the measured DC voltage may include both the voltage of the signal and the voltage of the noise in the tapped optical signal. In some embodiments, the CPU  518  may make the measurements of the components in the electrical signal. In some embodiments, the CPU  518  may store the measured values and related calculated values in a system memory. 
     Demodulation device  501  may then proceed to step  711 , where the passband filter may be detuned to a second passband  610 . In some embodiments, the passband filter may maintain the same characteristics as for the first passband  605 , such as, for example, maintaining the same bandwidth. In some embodiments, the CPU  518  may control the tuning of the passband filter of the optical tunable filter  510  so that it is centered on a different location after measuring an electrical signal associated with the first passband  605 . In some embodiments, the CPU  518  may detune the passband filter of the optical tunable filter  510  so that it is centered on a location that has a power that has a defined difference from the peak power. For example, the CPU  518  may detune the passband filter so that it is centered on a second passband  610  that is 10% below the peak power of the first passband  605 . In alternative embodiments, the CPU  518  may detune the passband filter to a defined distance away from the first passband  605 . In step  712 , the demodulation device may convert the second filtered optical signal to a second electrical signal, in a similar manner to that of step  707 . 
     In step  713 , the demodulation device  501  may measure a second electrical signal. The second measured electrical signal may be produced from the tapped optical payload transmitted from the optical tunable filter  510  at the second passband  610 . The measured components of the second electrical signal may also be separated into DC and AC components, where the differing values from the DC and AC components in the first measurement may be a function of reduction values R DC  and R AC , respectively. In some embodiments, the demodulation device  501  may tune the passband filter to other passbands and may make subsequent measurements at each new passband. 
     In step  714 , the bandwidth of the passband filter may be determined. In some embodiments, the CPU  518  may save the value of the bandwidth of the passband filter when setting the passband filter in the first passband  605  and the second passband  610 . In alternative embodiments, the CPU  518  may determine the bandwidth after making the measurements at the first passband  605  and the second passband  610 . In some embodiments, the format of the optical payload spectrum  601  may be based, for example, on known information in the optical transmission system. In some embodiments, the passband bandwidth and the optical payload spectrum format may be used by the demodulation device  501  to determine the total OSNR from the measured electrical signals in the first passband  605  and the second passband  610 . 
     Demodulation device  501  may then proceed to step  715 , where it determines the OSNR of the tapped optical channel. In some embodiments, the CPU  518  of the demodulation device  501  may determine the OSNR of the tapped optical channel based on the measurements of the first and the second electrical signals in steps  709  and  713 . From these measurements, the CPU  518  may, for example, determine the OSNR of the tapped optical channel as being equivalent to the voltage ratio of the signal and the ASE noise. In such instances, the CPU  518  may determine the signal voltage  602 ,  612  as a function of the DC and the AC voltages of the first and the second electrical signals. In some embodiments, the CPU  518  may determine the DC and the AC voltages based on subsequent measurements made when the passband filter is tuned to additional passbands. CPU  518  may similarly determine the voltage of the ASE noise  603 ,  613  as a function of the DC and the AC voltages of the first and the second electrical signals. 
     In some embodiments, the CPU  518  may determine the ONSR by dividing the calculated signal voltage  602 ,  612  by the calculated ASE noise voltage  603 ,  613 . In such instances, the ONSR may be a function of the DC and the AC voltages of the first and the second electrical signals. CPU  518  may also determine the ONSR as a function of the reduction values of the DC and the AC voltages between the first and the second electrical signals. In some embodiments, the CPU  518  may use the passband bandwidth and the optical payload spectrum format determined in step  714  to determine the OSNR from the first and the second electrical signals measured in the first passband  605  and the second passband  610 . After calculating the ONSR from the measured values of the first and the second electrical signals, method  700  may end at step  717 . 
     It should be apparent from the foregoing description that various exemplary embodiments of the invention may be implemented in hardware and/or firmware. Furthermore, various exemplary embodiments may be implemented as instructions stored on a tangible machine-readable storage medium, which may be read and executed by at least one processor to perform the operations described in detail herein. A tangible machine-readable storage medium may include any mechanism for storing information in a form readable by a tangible machine, such as a personal or laptop computer, a server, or other computing device. Thus, a tangible machine-readable storage medium may include a read-only memory (ROM), a random-access memory (RAM), magnetic disk storage media, optical storage media, flash-memory devices, and similar storage media. 
     It should be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative circuitry embodying the principals of the invention. Similarly, it will be appreciated that any flow charts, flow diagrams, state transition diagrams, pseudo code, and the like represent various processes which may be substantially represented in machine readable media and so executed by a computer or processor, whether or not such computer or processor is explicitly shown. 
     Although the various exemplary embodiments have been described in detail with particular reference to certain exemplary aspects thereof, it should be understood that the invention is capable of other embodiments and its details are capable of modifications in various obvious respects. As is readily apparent to those skilled in the art, variations and modifications can be affected while remaining within the spirit and scope of the invention. Accordingly, the foregoing disclosure, description, and figures are for illustrative purposes only and do not in any way limit the invention, which is defined only by the claims.