Patent Publication Number: US-8989572-B2

Title: Optical path establishing method and optical node apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2012-027808, filed on Feb. 10, 2012, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to an optical path establishing method and an optical node apparatus. 
     BACKGROUND 
       FIG. 1  illustrates a configuration of an optical network of related art. As illustrated in  FIG. 1 , control nodes (CNT)  2   a  through  2   f  are arranged on a control plane respectively for optical nodes  1   a  through  1   f , including optical cross-connect (OXC) on a data plane. 
     An optical path may be established from the optical node  1   a  to the optical node  1   f . Each control node stores detailed physical information about a link between an optical node corresponding to a source control node and an adjacent optical node. The detailed physical information may include, for each of a plurality of modulation methods, SNR (signal to noise ratio) degradations, chromatic dispersion, polarization mode dispersion (PMD), and non-linear effects such as Kerr effect of optical fiber. The control node  2   a  corresponding to the optical node  1   a  corrects the detailed physical information of links of control nodes  2   b  through  2   f  with optical nodes  1   b  through  1   f . The control node  2   a  reserves and establishes an optical path ( 1   a ,  1   b ,  1   c ,  1   e , and  1   f ) connecting the optical node  1   a  down to the optical path  1   f  denoted by a heavy solid line with respect to each selectable modulation method in accordance with parameters. The parameters include baud rate, forward error correction (FEC) overhead rate, optical signal noise ratio (OSNR) tolerance, chromatic dispersion tolerance, PMD tolerance, and non-linear effect tolerance. 
     As a related technique, a transmitter intensity-modulates (amplitude-modulates) an optical data signal with a low-speed path ID tone signal and then transmits the modulated optical data signal to a receiver. The receiver demodulates the path ID tone signal, thereby determining a modulation method and baud rate to establish a path. Related techniques are also described in U.S. Pat. No. 7,580,632. 
     As a related technique, a bias of an electroabsorption type modulator (EA modulator) at a transmitter is controlled in accordance with error rate information detected by a receiver so that chirp of an optical modulation signal is controlled to an optimum point. Transmission of a control signal from the receiver to the transmitter may be performed by digital multiplexing the control signal on an optical main signal. Related techniques are also described in Japanese Laid-open Patent Publication No. 2002-164846. 
     As a related technique, a receiver measures transmission quality with a transmission characteristic measurement unit thereof, and transmission parameters are controlled to optimum values in response to measurement results. The transmission parameters include optical transmission power, optical wavelength, dispersion compensation amount, and pre-chirp. Related techniques are also described in Japanese Laid-open Patent Publication No. 2005-223944. 
     As a related technique, a receiver measures OSNR, and bit error rate (BER) of each wavelength of a wavelength-division multiplexed (WDM) signal, and transmits measurement results in overhead information of an optical signal to a transmitter. The transmitter optimum-controls a parameter, dispersion compensation amount, and pre-emphasis of an optical transmission signal. Related techniques are also described in Japanese Laid-open Patent Publication No. 2002-57624. 
     Japanese Laid-open Patent Publication No. 8-298486 discloses a superimposed transmission technique in which an optical main signal is low-intensity-modulated with a supervisory control signal. 
     A plurality of modulation methods are characterized by a variety of parameters including baud rate, FEC overhead rate, OSNR tolerance, chromatic dispersion tolerance, PMD tolerance, and non-linear effect tolerance. Optical path establishing methods of related art perform a number of steps before establishing an optical path optimum for highly efficient data transmission. Design for establishing the optimum optical path is difficult. 
     In the related technique where the optical data signal that is optical-intensity modulated with a low-rate signal is transmitted and the receiver demodulates the low-rate signal, cross-gain modulation caused through an optical amplifier and stimulated Raman scattering causes a path ID tone signal from another channel to enter. As a result, quality degradation takes place on a main signal. 
     SUMMARY 
     According to an aspect of the embodiments, an optical node apparatus that establishes an optical path between a first optical node and a second optical node in an optical network include a frequency modulation unit that superimposes a supervisory signal on a main signal by frequency-modulating the main signal, and a frequency demodulation unit that frequency-demodulates the supervisory signal superimposed on the received main signal. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an example of an optical network of related art; 
         FIG. 2  illustrates a configuration of an optical network of an embodiment; 
         FIG. 3  illustrates multiplexing and demultiplexing of a supervisory channel in an optical node; 
         FIG. 4  is a block diagram illustrating a transmitter of an embodiment; 
         FIG. 5  is a block diagram illustrating a receiver of an embodiment; 
         FIG. 6  is a block diagram illustrating a digital frequency modulator of an embodiment; 
         FIG. 7  is a block diagram illustrating in detail the receiver of the embodiment; 
         FIG. 8  is a circuit diagram of a first frequency offset estimating unit of a first embodiment; 
         FIG. 9A  is a circuit diagram of a first frequency offset estimating unit of a second embodiment; 
         FIG. 9B  illustrates an example of a spectrum of a signal; 
         FIG. 10  is a circuit diagram of a second frequency offset estimating unit of an embodiment; 
         FIG. 11  is a flowchart illustrating an optical path establishing process of the first embodiment; 
         FIG. 12  is a flowchart illustrating an optical path establishing process of the second embodiment; and 
         FIG. 13  is a timing diagram of the optical path establishing process. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments are described below with reference to the drawings. 
     Optical Path Establishing Method 
       FIG. 2  illustrates a configuration of an optical network of an embodiment. As illustrated in  FIG. 2 , control nodes (CNT)  12   a  through  12   f  are arranged on a control plane respectively for optical nodes  11   a  through  11   f  including OXC&#39;s on a data plane. Each of the optical nodes  11   a  and  11   f  may use a plurality of modulation formats including quadrature phase shift keying (QPSK), quadrature amplitude modulation (QAM), and orthogonal frequency division multiplexing (OFDM), and thus selects as a modulation format of a main signal one of these methods. A main optical signal includes a client signal transmitted from a client. 
     An optical path may now be established from the optical node  11   a  all the way down to the optical node  11   f . Each control node collects detailed physical information about a link between an optical node corresponding to a source control node and an adjacent optical node. The detailed physical information may include, for each of a plurality of modulation formats, SNR degradations, chromatic dispersion, polarization mode dispersion (PMD), and non-linear effects. The control node  12   a  corresponding to the optical node  11   a  mutually exchanges with the control nodes  12   b  through  12   f  the detailed physical information about links thereof respectively with the optical nodes  11   b  through  11   f . The control node  12   a  reserves and establishes an optical path ( 11   a ,  11   b ,  11   c ,  11   e , and  11   f ) denoted by a heavy solid line connecting the optical node  11   a  all the way to the optical node  11   f  in accordance with parameters of unused wavelength and OSNR of a selectable modulation format. 
     The optical node  11   a  and the optical node  11   f  include supervisory controllers (SV_CNT)  13   a  and  13   f , respectively. The supervisory controllers  13   a  and  13   f  connect a supervisory channel through the optical path ( 11   a ,  11   b ,  11   c ,  11   e , and  11   f ) established between the optical nodes  11   a  and  11   f . The supervisory controllers  13   a  and  13   f  mutually exchange, through the supervisory channel, supervisory signal data such as OSNR, chromatic dispersion, PMD, and non-linear effects of the established optical path. The supervisory controllers  13   a  and  13   f  thus adjust a signal intensity, and compensation amounts for the chromatic dispersion, PMD, and non-linear effects. The supervisory controllers  13   a  and  13   f  thus optimize the established optical path. 
     Multiplexing and Demultiplexing of Supervisory Channels 
       FIG. 3  illustrates multiplexing and demultiplexing of the supervisory channel in optical nodes. As illustrated in  FIG. 3 , a transmitter (Tx)  21  in the optical node  11   a  serving as a source node of the optical path is supplied with main signal data. The transmitter  21  is also supplied with supervisory signal data generated by the supervisory controller (SV_CNT)  22  in the optical node  11   a . The supervisory controller  22  corresponds to the supervisory controller  13   a  in  FIG. 2 . 
     The transmitter  21  frequency-modulates the main signal data with the supervisory signal data, and IQ-modulates laser light with a signal resulting from superposing the supervisory signal on the main signal. An optical multiplexer  23  wavelength-multiplexes an optical signal output by the transmitter  21  and an optical signal from another transmitter, and then outputs a resulting signal as wavelength division multiplexer (WDM) signal to an optical transmission path  24  establishing the optical path. 
     The WDM signal transmitted through an optical transmission path  25  establishing the optical path is supplied to the optical node  11   f  as a terminal node of the optical path. An optical demultiplexer  26  in the optical node  11   f  demultiplexes the received WDM signal into an optical signal on a per wavelength basis. The optical signal thus demultiplexed is supplied to a receiver (Rx)  27 . The receiver  27  coherently receives the demultiplexed optical signal. The receiver  27  frequency-demodulates the coherently received signal, thereby acquiring main signal data and supervisory signal data. The main signal data are supplied to a subsequent circuit, and the supervisory signal data are supplied to a supervisory controller  28 . The supervisory controller  28  corresponds to the supervisory controller  13   f  in  FIG. 2 . 
     Similarly, a transmitter  31  in the optical node  11   f  is supplied with main signal data. Supervisory signal data generated by the supervisory controller  28  in the optical node  11   f  are supplied to the transmitter  31 . The transmitter  31  frequency-modulates the main signal data with the supervisory signal data, and IQ-modulates laser light with the frequency-modulated main signal data, and then outputs the modulated laser light. An optical multiplexer  33  multiplexes the optical signal output by the transmitter  31  and an optical signal from another transmitter, and then outputs a resulting signal as a WDM signal to an optical transmission path  34  establishing the optical path. 
     The WDM signal transmitted through an optical transmission path  35  establishing the optical path is supplied to the optical node  11   a . An optical demultiplexer  36  in the optical node  11   a  demultiplexes the received WDM signal into an optical signal of each wavelength. The optical signal thus demultiplexed is supplied to a receiver  37 . The receiver  37  coherently receives the optical signal. The receiver  37  frequency-demodulates the coherently received optical signal, thereby acquiring main signal data and supervisory signal data. The main signal data are supplied to a subsequent circuit, and the supervisory signal data are supplied to the supervisory controller  22 . 
     The supervisory controller  22  in the optical node  11   a  and the supervisory controller  28  in the optical node  11   f  transmit and receive the supervisory signal data. The supervisory controller  22  and the supervisory controller  28  thus exchange the supervisory signal data containing OSNR, chromatic dispersion, PMD, and non-linear effects of the established optical path. The supervisory controller  22  and the supervisory controller  28  adjust a signal intensity, and compensation amounts of chromatic dispersion, PMD, and non-linear effects. The established optical path is thus optimized. 
     Structure of Transmitter and Receiver 
       FIGS. 4 and 5  respectively illustrate a transmitter and a receiver of an embodiment. The transmitter of  FIG. 4  corresponds to the transmitters  21  and  31  of  FIG. 3 . The receiver of  FIG. 5  corresponds to the receivers  27  and  37  of  FIG. 3 . 
     As illustrated in  FIG. 4 , a transmission signal processor  41  may be at least one of digital signal processor (DSP), a field-programmable gate array (FPGA), and an application specific integrated circuit (ASIC). The transmission signal processor  41  receives main signal data. The transmission signal processor  41  also receives supervisory signal data from the supervisory controller, and control data and compensation data from an upper-level device or the supervisory controller. The transmission signal processor  41  separates, from the main signal data, two signals corresponding to polarization components X and Y orthogonal to each other. The transmission signal processor  41  constellation-maps the main signal data of two signals to electric field information in accordance with the modulation format such as QPSK, QAM, or OFDM indicated by the control data. The transmission signal processor  41  compensates for signal quality degradations of a transmission line in electric field phase indicated by the electric field information mapped to the main signal data. The transmission signal processor  41  performs fine adjustment on a carrier frequency of the optical signal by imparting a phase rotation to the electric field phase with a certain period. In this case, digital frequency modulation with the supervisory signal data is performed by imparting the phase rotation responsive to the supervisory signal data. The frequency modulation with the supervisory signal data may be performed on the electric field information of one of the polarization component X and the polarization component Y or on the electric field information of both the polarization component X and the polarization component Y. The transmission signal processor  41  compensates for signal quality degradations caused by imperfections of a transmission system including loss variation, skews, bandwidth variations, linearity between I and Q signals, in each of an in-phase (I) component and quadrature phase (Q) component of the electric field information of the polarization component X and the polarization component Y. The transmission signal processor  41  thus outputs the compensated polarization component X and polarization component Y. 
     The electric field information of the polarization component X and the electric field information of the polarization component Y output by the transmission signal processor  41  are supplied to a digital-to-analog converter (DAC)  42 . The DAC  42  digital-to-analog converts the electric field information and supplies resulting analog polarization component X and polarization component Y to a polarization multiplexing IQ modulator  43 . The polarization multiplexing IQ modulator  43  is a polarization multiplexing modulator that separately performs optical modulation on the mutually orthogonal polarization components X and Y. The polarization multiplexing IQ modulator  43  modulates output light of a laser light source (LD)  44  with the I component and the Q component of the polarization components X and Y, and the outputs a modulated signal as an optical signal through a transmission path. 
     As illustrated in  FIG. 5 , a received optical signal is supplied to a polarization diversity coherent receiver  51 . A laser light source (LD)  52  supplies the polarization diversity coherent receiver  51  with local oscillator light. The polarization diversity coherent receiver  51  separates a polarization component X and a polarization component Y from the received optical signal. An optical phase hybrid unit causes the polarization component X and the local oscillator light to interfere with each other in phase and reverse phase, resulting in a pair of output light rays. The optical phase hybrid unit causes the polarization component X and the local oscillator light to interfere with each other orthogonally (with +90 degrees) and inverse orthogonally (with −90 degrees), resulting in a pair of output light rays. A balanced photodiode differentially receives interfering light rays in phase and reverse phase, and obtains electrical signals of an in-phase interference component (I) and an orthogonal interfering component (Q) between the polarization component X and the local oscillator light. Similarly, the balanced photodiode obtains electrical signals of an in-phase interference component (I) and an orthogonal interfering component (Q) of the polarization component Y. 
     The I and Q components of the polarization components X and Y are supplied to an analog-to-digital converter (ADC)  53 . The ADC  53  analog-to-digital converts the I and Q components and supplies digital I and Q components to a received signal processor  54 . The received signal processor  54  is a DSP, for example. The received signal processor  54  frequency-demodulates a digital value of each of the I and Q components of each polarization component, thereby resulting in supervisory signal data. The received signal processor  54  also performs a chromatic dispersion compensation process on the digital values of the I and Q components of each polarization component, adaptive equalization process, frequency offset removal process, and determination process on the main signal data. The received signal processor  54  thus outputs the main signal data and the supervisory signal data. 
     Digital Frequency Modulation 
       FIG. 6  illustrates an example of a digital frequency modulator of one embodiment in the transmission signal processor  41 . The digital frequency modulator handles the polarization component X (or the polarization component Y). 
     In  FIG. 6 , a Δfa adjuster  61  generates and supplies a frequency control amount Δfa to an integrator  62 . The Ma adjuster  61  adjusts the frequency control amount Δfa to a large value during initial setting prior to synchronization establishment of a main signal. The Δfa adjuster  61  then adjusts the frequency control amount Δfa to a smaller value during an operation subsequent to the synchronization establishment of the main signal. The frequency control amount Δfa is sufficient large in comparison with a difference between a frequency of the laser light source of the transmitter and a frequency of the laser light source of the receiver. 
     The integrator  62  integrates a phase rotation amount Δω (=2πΔfa) every discrete time unit T, resulting in a phase rotation amount ΔωnT throughout discrete time nT. The integrator  62  then outputs the phase rotation amount ΔωnT to a sign addition unit  63 . 
     The sign addition unit  63  attaches to the phase rotation amount ΔωnT output by the integrator  62  a sign responsive to the supervisory signal data. If the supervisory signal data are a value 1, the sign addition unit  63  outputs the phase rotation amount +ΔωnT. If the supervisory signal data are a value 0, the sign addition unit  63  outputs the phase rotation amount −ΔωnT. An output of the sign addition unit  63  is supplied to a sine unit  65  and a cosine unit  66 , forming a phase rotation unit  64 . 
     The sine unit  65  in the phase rotation unit  64  calculates and outputs a sine function of the supplied phase rotation amount, and the cosine unit  66  in the phase rotation unit  64  calculates and output a cosine function of the supplied phase rotation amount. A multiplier  67  multiplies an output of the sine unit  65  by the Q component of the main signal, and a multiplier  68  multiplies the output of the sine unit  65  by the I component of the main signal. The multiplier  70  multiplies an output of the cosine unit  66  by the I component of the main signal, and the multiplier  71  multiplies the output of the cosine unit  66  by the Q component of the main signal. A subtracter  72  subtracts an output of the multiplier  67  from an output of the multiplier  70  and outputs a resulting difference. An adder  73  adds an output of the multiplier  68  to an output of the multiplier  71 , and then outputs a resulting sum. Let α represent the phase rotation amount, and an output of the subtracter  72  is I cos α−Q sin α. An output of the adder  73  is I sin α+Q cos α. The I component and the Q component of the main signal rotate by a phase rotation amount α (=+ΔωnT or −ΔωnT) during the discrete time nT. More specifically, an optical signal output by the polarization multiplexing IQ modulator  43  is a frequency-modulated signal represented by f T ±Δfa where f T  represents a center frequency, and nT represent unit time. 
     Structure of Receiver 
       FIG. 7  illustrates the polarization diversity coherent receiver  51  of  FIG. 5  in detail as one embodiment.  FIG. 7  illustrates only the polarization diversity coherent receiver  51  and subsequent elements thereof that processes the polarization component X (or the polarization component Y), and elements identical to those illustrated in  FIG. 5  are designated with the same reference numerals. 
     As illustrated in  FIG. 7 , a received optical signal is supplied to the polarization diversity coherent receiver  51 . The polarization diversity coherent receiver  51  is supplied with local oscillator light from a laser light source  52 . The polarization diversity coherent receiver  51  separates a polarization component X and a polarization component Y from the received optical signal. An optical phase hybrid unit causes the polarization component X and the local oscillator light to interfere with each other in phase and reverse phase, resulting in a pair of output light rays. The optical phase hybrid unit causes the polarization component X and the local oscillator light to interfere with each other orthogonally (with +90 degrees) and inverse orthogonally (with −90 degrees), resulting in a pair of output light rays. A balanced photodiode differentially receives interfering light rays in phase and reverse phase, and obtains electrical signals of an in-phase interference component (I) and an orthogonal interfering component (Q) between the polarization component X and the local oscillator light. 
     The I and Q components of the polarization component X output by the polarization diversity coherent receiver  51  are respectively analog-to-digital converted through AD converters (ADC)  53   a  and  53   b  to digital I and Q components. The digital I and Q components are then supplied to a received signal processor  54   a , and to a first frequency offset estimating unit  81 . The first frequency offset estimating unit  81  estimates a frequency offset amount caused by a frequency difference between the laser light sources of the transmitter and the receiver in an initial setting before the main signals are synchronized. In accordance with the estimated offset amount, the first frequency offset estimating unit  81  demodulates the supervisory signal data superimposed in digital frequency modulation by the transmitter, and then supplies demodulated supervisory signal data to a supervisory controller (SV_CNT)  90 . 
     A chromatic dispersion compensator  82  in the received signal processor  54   a  compensates for chromatic dispersion in the supplied I and Q components. An adaptive equalizer  83  performs an equalization process on I and Q components output by the chromatic dispersion compensator  82 , i.e., compensates for a mixture of X polarization and Y polarization. Equalized I and Q components are then supplied to a second frequency offset estimating unit  84  and a frequency compensator  85 . 
     The second frequency offset estimating unit  84  estimates a frequency offset amount caused by a frequency difference between the laser light sources of the transmitter and the receiver during an operation after the main signals are synchronized. In accordance with the estimated offset amount, the second frequency offset estimating unit  84  demodulates the supervisory signal data superimposed in digital frequency modulation by the transmitter, and then supplies demodulated supervisory signal data to the supervisory controller  90 . 
     The frequency compensator  85  compensates for the frequency offset amount estimated by the second frequency offset estimating unit  84  in accordance with the I and Q components output by the adaptive equalizer  83 . A phase-locked loop unit  86  synchronizes I and Q components output by the frequency compensator  85  with a phase of the light output by the laser light source in the transmitter. A determining unit  87  determines a value of each of I and Q components output by the phase-locked loop unit  86  in accordance with each of the modulation formats, demodulates the main signal data and outputs demodulated main signal data. 
     The supervisory controller  90  corresponds to each of the supervisory controllers  22  and  28  in  FIG. 3 . The supervisory controller  90  retrieves from the first frequency offset estimating unit  81  the supervisory signal data indicating the modulation format of the main signal, for example, and then notifies the determining unit  87  in the received signal processor  54   a  of the supervisory signal data. The supervisory controller  90  also retrieves, from the second frequency offset estimating unit  84 , the supervisory signal data that compensate for OSNR, chromatic dispersion, PMD, non-linear effects, and the like. The supervisory controller  90  then controls the chromatic dispersion compensator  82 , the adaptive equalizer  83 , the frequency compensator  85 , the phase-locked loop unit  86 , and the determining unit  87  in the received signal processor  54   a.    
       FIG. 8  is a circuit diagram of the first frequency offset estimating unit  81  of a first embodiment. As illustrated in  FIG. 8 , a digital value of the I component output by the ADC  53   a  is supplied to a multiplier  112 . Also, the digital value of the I component output by the ADC  53   a  is delayed by unit time t by a delay element  115  before being supplied to a multiplier  113 . A digital value of the Q component output by the ADC  53   b  is supplied to the multiplier  113 . The digital value of the Q component output by the ADC  53   b  is delayed by unit time τ by a delay element  114  before being supplied to the multiplier  112 . The multiplier  112  multiplies the I component by the delayed Q component, and the multiplier  113  multiplies the Q component by the delayed I component. A subtracter  116  subtracts the output of the multiplier  112  from the output of the multiplier  113 , thereby determining a difference therebetween. An average operation unit  117  calculates an average value of differences of n times, for example, output by the subtracter  116 , and then outputs the resulting average value as a frequency offset. For example, n may be 100 or so. 
     The received optical signal supplied to the polarization diversity coherent receiver  51  may be represented by expression s(t)exp(jωt). Let s(t) represent a modulation baseband data signal, and ω represent a frequency of a carrier wave. The local oscillator light from the laser light source  52  is represented by expression exp(jωLt). In this case, the I and Q components are represented by I+jQ=s(t)exp(jδωt). Here, δω=ω−ωL holds, and symbols “&lt; &gt;” represent an operation of averaging, and symbol “*” represents complex conjugate. The first frequency offset estimating unit  81  is an autocorrelation calculation unit, and a signal output by the average operation unit  117  is represented by expression (1). 
     
       
         
           
             
               
                 
                   
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     Here, term &lt;s(t)s*(t−τ)&gt; represents an autocorrelation coefficient, and is a specific positive value. In expression (1), δωτ is in the vicinity of 0 (−π/2&lt;δωτ&lt;π/2). If δωτ is positive, sin(εωτ)&lt;s(t)s*(t−τ)&gt; is positive. If δωτ is negative, sin (δωτ)&lt;s(t)s*(t−τ)&gt; is negative. 
     Whether the received optical signal is offset from the center frequency f T  into a positive side or a negative side is determined from the output of the first frequency offset estimating unit  81  represented by expression (1). The supervisory signal data are thus frequency demodulated from the received optical signal. 
       FIG. 9A  is a circuit diagram of the first frequency offset estimating unit  81  as a second embodiment. As illustrated in  FIG. 9A , a fast Fourier transform (FFT) unit  121  receives a complex signal having as a real part thereof the I component output by the ADC  53   a  and as an imaginary part thereof the Q component output by the ADC  53   b . The FFT unit  121  performs fast Fourier transform on the supplied signal, and then supplies the transformed signal to a center of spectrum calculation unit  122 . 
     The center of spectrum calculation unit  122  calculates a spectrum of the spectrum from the FFT unit  121 . The center of spectrum is a center frequency of a frequency range of the spectrum where signals are distributed. The center of spectrum calculation unit  122  calculates the center of spectrum in accordance with expression (2) 
     
       
         
           
             
               
                 
                   
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     Here, i represents an index indicating a frequency in the spectrum, P(i) represents power of the spectrum on the frequency indicated by the index, and N represents an FFT size of Fourier transform of the FFT unit  121 . Furthermore, Δ is fs/(N−1), fs is a sampling frequency of the ADCs  53   a  and  53   b , and fb is a baud rate of the signal light received by the polarization diversity coherent receiver  51 . 
       FIG. 9B  illustrates an example of the signal spectrum. In  FIG. 9B , the abscissa represents frequency and index i responsive to the frequency. More specifically, −N/2+1 through N/2 of index i correspond to frequency −fs/2+Δ through fs/2. The ordinate represents power P at a frequency component. The spectrum denoted by a solid line  400  is the spectrum supplied by the FFT unit  121 . 
     The center of spectrum denoted by a broken line  401  is calculated by the center of spectrum calculation unit  122 . The center of spectrum  401  is shifted from the center frequency at 0 in response to a magnitude of the frequency offset. 
     Whether the received optical signal is offset from the center frequency f T  into a positive side or a negative side is determined by estimating the frequency offset based on spectrum II. The supervisory signal data are thus frequency demodulated from the received optical signal. 
       FIG. 10  is a circuit diagram of the second frequency offset estimating unit  84  as an embodiment. As illustrated in  FIG. 10 , the second frequency offset estimating unit  84  includes argument calculation unit  131 , subtracter  132 , tentative determining unit  133 , subtracter  134 , delay element  135 , subtracter  136 , loop filter  137 , adder  138 , delay element  139 , and determining unit  140 . 
     The second frequency offset estimating unit  84  recursively estimates the frequency offset of a signal input thereto. More specifically, the second frequency offset estimating unit  84  estimates the frequency offset by feeding back an output of the loop filter  137  as a calculation result of frequency offset. 
     A complex signal having as a real part the I component output by the adaptive equalizer  83  and as an imaginary part the Q component of the adaptive equalizer  83  is supplied to the argument calculation unit  131 . The argument calculation unit  131  calculates a phase angle indicated by the complex signal input thereto. The argument calculation unit  131  supplies a signal indicating the calculated phase angle to the subtracter  132  and the subtracter  134 . The subtracter  132  subtracts an output signal of the delay element  139  from the output signal of the argument calculation unit  131 . The subtracter  132  supplies a subtraction result to the tentative determining unit  133 . 
     The tentative determining unit  133  tentatively determines (identifies) the subtraction result from the subtracter  132 . The tentative determining unit  133  supplies a signal indicating a tentative determination result to the subtracter  134 . The subtracter  134  subtracts the output signal of the tentative determining unit  133  from the output signal of the argument calculation unit  131 . The subtracter  134  supplies a subtraction result to the delay element  135 , the subtracter  136 , and the adder  138 . 
     The delay element  135  delays the signal output by the subtracter  134  by unit time τ, i.e., one symbol, before supplying the signal output by the subtracter  134  to the subtracter  136 . The subtracter  136  subtracts the output signal of the delay element  135  from the output signal of the subtracter  134 . The subtracter  136  supplies a subtraction result to a determining unit  140 . If an absolute value of a phase angle change during one symbol is equal to or lower than π, the determining unit  140  determines that the subtraction result is normal, and then supplies the subtraction result to the loop filter  137 . 
     The loop filter  137  averages an output signal of the determining unit  140 . The loop filter  137  is an infinite impulse response (IIR) filter having infinite response. The loop filter  137  outputs an averaged signal as the frequency offset value. The loop filter  137  also supplies the averaged signal to the adder  138 . 
     The adder  138  sums the output signal of the subtracter  134  and the output signal of the loop filter  137 . The adder  138  supplies a resulting sum to the delay element  139 . The delay element  139  delays the output signal of the adder  138  by unit time τ. The delay element  139  supplies the delayed output signal to the subtracter  132 . 
     A signal output by the argument calculation unit  131  at time t+1 is a phase angle θ(t+1). Phase angle θ(t+1) is represented by θd(t+1)+θfo(t+1)+θPN(t+1). Let θd(t+1) represent a modulation component, θfo(t+1) represent a phase rotation amount caused by the frequency offset, and θPN(t+1) represent phase noise. 
     The signal output by the tentative determining unit  133  indicates a tentative determination result of a modulation component, and is thus approximately equal to θd(t+1). The signal output by the subtracter  134  is represented by θfo(t+1)+θPN(t+1). The signal output by the delay element  135  is represented by θfo(t)+θPN(t). 
     The signal output by the subtracter  136  is represented by θfo(t+1)−θfo(t)+θPN(t+1)−θPN(t). If θPN(t+1)=θPN(t), the signal output by the subtracter  136  is represented by Δfo=θfo(t+1)−θfo(t). Δfo is an amount of change in the phase rotation amount between symbols, and thus indicates a frequency offset. The loop filter  137  acquires the frequency offset value by averaging the frequency offset Δfo. The loop filter  137  averages the frequency offset Δfo by as many as 10 times. 
     The signal output by the adder  138  is θfo(t+1)+θPN(t+1)+Δfo. The signal output by the delay element  139  is θfo(t)+θPN(t)+Δfo. Here, θfo(t)=θfo(t+1)−Δfo. Also, expression θPN(t)=θPN(t+1) may hold. The signal output by the delay element  139  may be θfo(t+1)+θPN(t+1), and the signal output by the subtracter  132  is a modulation component θd(t+1). 
     Whether the received optical signal is offset from the center frequency f T  into a positive side or a negative side is determined from the frequency offset value output by the loop filter  137 . The supervisory signal data are thus frequency demodulated from the received optical signal. 
     Supervisory Channel 
     A supervisory signal is superimposed on a main signal by frequency-modulating main signal data with supervisory signal data in a supervisory channel. In the supervisory channel, a maximum frequency shift and communication speed during initial setting prior to establishment of synchronization of the main signals are different from those during operation subsequent to the establishment of the synchronization of the main signals. During the initial setting, the maximum frequency shift is set to be as large as the center frequency f T  (several THz, for example) of the optical signal ± several 100 MHz. In other words, a frequency bandwidth is set to be wider. The communication speed is set to be low so that OSNR exceeds a specific value. With the maximum frequency shift set to be large and the communication speed set to be low, reliable demodulation may be performed even before the establishment of the synchronization of the main shifts. A maximum frequency shift affects the main signal, and is not used after the establishment of synchronization. Since a low communication speed reduces an amount of transmission, only limited information, such as the modulation format, is transmitted. 
     During the operation, the maximum frequency shift is set to be as small as the center frequency f T  (several THz, for example) of the optical signal ± several 10 MHz. In other words, a frequency bandwidth is set to be narrower. The communication speed is set to be higher. If the modulation format of the main signal is known, demodulation may be performed even if the maximum frequency shift is small. With the maximum frequency shift set to be small, effects on the main signal are sufficiently small. With the communication speed set to be high, a high-speed communication is enabled. A large quantity of information including the supervisory signal data to compensate for OSNR, chromatic dispersion, PMD, and non-linear effects is transmitted. 
       FIG. 11  is a flowchart illustrating an optical path establishing process. Each of a control node that is requested to establish an optical path and an optical node corresponding to the control node includes at least one processor and a memory corresponding to the processor. The processor performs the optical path establishing process. 
     In step S 1  in  FIG. 11 , each source node in an optical network collects detailed physical information about at least one link connecting an optical node corresponding to the source node and an adjacent optical node. The detailed physical information may include unused wavelength, and OSNR, chromatic diversion, PMD, and non-linear effects such as Kerr effect of optical fiber for each of a plurality of modulation formats including QPSK, QAM, and OFDM. In step S 2 , each control node advertises the collected detail physical information of each link to the other control nodes in the optical network. Steps S 1  and S 2  are performed beforehand as a pre-process. 
     The optical path establishing process starts in response to an optical path setting request. When a control node receives the optical path setting request, the control node sets a route of the optical path from a source node to a terminal node of the optical path setting request, and wavelength to be used, a modulation format, baud rate, and FEC overhead rate in step S 3 . The control node that receives the optical path setting request is typically but not necessarily the source node. In step S 3 , a small quantity of information, such as the unused wavelength and OSNR, out of the detailed physical information of the link is used to set the route of the optical path, and wavelength to be used, a modulation format, baud rate, and FEC overhead rate. 
     In step S 4 , the control node of the source node having received the optical path setting request establishes an optical path on the control node of the terminal node using the route of the optical path, wavelength to be used, a modulation format, baud rate, and FEC overhead rate of the optical path. The control node as the source node sets a supervisory channel during the initial setting on the established optical path. In this state, a carrier wave of the main signal on which the supervisory signal is superimposed is present, but the main signal has no meaning. 
     In step S 5 , the set modulation format, baud rate, and FEC overhead rate are transmitted and received between the source node and the terminal node as optical nodes via the set initial setting supervisory channel. 
     In step S 6 , the supervisory channel is switched from an initial setting supervisory channel used during the initial setting to an operational supervisory channel used during operation. In step S 7 , OSNR, chromatic dispersion, PMD, and non-linear effect are measured by the source node and the terminal node as the optical nodes. Measurement results are mutually exchanged using the operational supervisory channel. The optical path is optimized by changing chromatic dispersion compensation amount, PMD compensation amount, and non-linear effect compensation amount in the optical nodes as the source node and the terminal node. Meaningful main signal may be transmitted or received via the optical path established in steps S 6  and S 7 . 
     It is determined in step S 8  whether OSNR, chromatic dispersion, PMD, and non-linear effect of the optimized optical path satisfy desired performance. The desired performance is that OSNR is equal to or above a preset threshold value, that residual chromatic dispersion is equal to or below a preset threshold value, that PMD is equal to or below a preset threshold value, and that non-linear distortion is equal to or below a preset threshold value. 
     If the desired performance is not satisfied, the modulation format, wavelength to be used, baud rate, and FEC overhead rate are modified in step S 9 . Processing then returns to step S 4  to repeat steps S 4  through S 8 . If the desired performance is satisfied, processing proceeds from steps S 8  to S 10 . 
     In step S 10 , clock recovery, phase locked loop, and dispersion compensation are performed on the main signal to establish the main signal. The main signal is transmitted and received. OSNR, chromatic dispersion, PMD, and non-linear effect are measured periodically by the optical nodes as the source node and the terminal node. Measurement results are mutually exchanged over the operational supervisory channel. The optical path is optimized by changing the chromatic dispersion compensation amount, PMD compensation amount, and non-linear effect compensation amount in the optical nodes as the source node and the terminal node. The optical path is optimized in response to a dynamic change in OSNR, chromatic dispersion, PMD, and non-linear effect. 
       FIG. 12  is a flowchart illustrating an optical path establishing process of a second embodiment. Each of a control node that is requested to establish an optical path and an optical node corresponding to the control node includes at least one processor and a memory corresponding to the processor. The processor performs the optical path establishing process. 
     In step S 21  in  FIG. 12 , each source node in an optical network collects detailed physical information about at least one link connecting an optical node corresponding to the source node and an adjacent optical node. The detailed physical information may include unused wavelength, and OSNR, chromatic diversion, PMD, and non-linear effects such as Kerr effect of optical fiber for each of a plurality of modulation formats including QPSK, QAM, and OFDM. In step S 22 , each control node advertises the collected detail physical information of each link to the other control nodes in the optical network. Steps S 21  and S 22  are performed beforehand as a pre-process. 
     The optical path establishing process starts in response to an optical path setting request. When a control node receives the optical path setting request, the control node sets a route of the optical path from a source node to a terminal node of the optical path setting request, and wavelength to be used, a modulation format, baud rate, and FEC overhead rate in step S 23 . The control node that receives the optical path setting request is typically but not necessarily the source node. In step S 23 , a small quantity of information, such as the unused wavelength and OSNR, out of the detailed physical information of the link is used to set the route of the optical path, and wavelength to be used, a modulation format, baud rate, and FEC overhead rate. 
     In step S 24 , the control node of the source node having received the optical path setting request establishes an optical path on the control node of the terminal node using the route of the optical path, wavelength to be used, a modulation format, baud rate, and FEC overhead rate of the optical path. The control node as the source node sets a supervisory channel during the initial setting on the established optical path. In this state, a carrier wave of the main signal on which the supervisory signal is superimposed is present, but the main signal has no meaning. 
     In step S 25 , the set modulation format, baud rate, and FEC overhead rate are transmitted and received between the source node and the terminal node as optical nodes via the set initial setting supervisory channel. 
     In step S 26 , the supervisory channel is switched from an initial setting supervisory channel used during the initial setting to an operational supervisory channel used during operation. In step S 27 , OSNR, chromatic dispersion, PMD, and non-linear effect are measured by the source node and the terminal node as the optical nodes. Measurement results are mutually exchanged using the operational supervisory channel. The optical path is optimized by changing the chromatic dispersion compensation amount, PMD compensation amount, and non-linear effect compensation amount in the optical nodes as the source node and the terminal node. Meaningful main signal may be transmitted or received via the optical path established in steps S 26  and S 27 . 
     It is determined in step S 28  whether OSNR, chromatic dispersion, PMD, and non-linear effect of the optimized optical path satisfy desired performance. The desired performance is that OSNR is equal to or above a preset threshold value, that residual chromatic dispersion is equal to or below a preset threshold value, that PMD is equal to or below a preset threshold value, and that non-linear distortion is equal to or below a preset threshold value. 
     If the desired performance is not satisfied, the modulation format, wavelength to be used, baud rate, and FEC overhead rate are modified in step S 29 . In step S 30 , the set modulation format, baud rate, and FEC overhead rate are transmitted and received between the optical nodes as the source node and the terminal node using the operational supervising channel. Processing proceeds to step S 27 . Steps in S 27  and S 28  are repeated. If the desired performance is satisfied, processing proceeds from steps S 28  to S 31 . 
     In step S 31 , clock recovery, phase locked loop, and dispersion compensation are performed on the main signal to establish the main signal. The main signal is transmitted and received. OSNR, chromatic dispersion, PMD, and non-linear effect are measured periodically by the optical nodes as the source node and the terminal node. Measurement results are mutually exchanged over the operational supervisory channel. The optical path is optimized by changing the chromatic dispersion compensation amount, PMD compensation amount, and non-linear effect compensation amount in the optical nodes as the source node and the terminal node. The optical path is optimized in response to a dynamic change in OSNR, chromatic dispersion, PMD, and non-linear effect. 
     Timing Diagram 
       FIG. 13  is a timing diagram of the optical path establishing process. During period T 1 , the transmitter, i.e., the control node serving as a source node sets a modulation format. The optical node of the source node establishes a supervisory channel (SV) in the initial setting during period T 2 , and starts transmitting an optical signal during period T 3 . 
     The receiver, i.e., the control node serving as a terminal node starts receiving the optical signal during period T 4 , and demodulates the supervisory channel (SV) in the initial setting during period T 5 . The optical node serving as the terminal node recognizes the modulation format of the main signal during period T 6 . 
     The optical node of the source node receives from the optical node of the terminal node a response that the modulation format of the main signal has been received, and establishes an operational supervisory channel (SV) during period T 7  subsequent to period T 6 . The optical node of the source node transmits the main signal and the operational supervisory channel (SV) during period T 8 . The optical node of the terminal node receives the main signal during period T 8  while also demodulating the operational supervisory channel (SV). 
     The main signal is frequency-modulated with the supervisory signal in accordance with the embodiments, different from related art where the main signal is intensity-modulated with the supervisory signal. The embodiments control cross-gain modulation caused through an optical amplifier and stimulated Raman scattering. With quality degradation reduced on the main signal, the supervisory data are exchanged between optical nodes. Without preknowledge of the detailed physical information of each link including OSNR, chromatic dispersion, polarization mode dispersion (PMD), and non-linear effects, the optical path is established and optimized by exchanging the supervisory signal superimposed on the main signal between the optical nodes. An optimum light waveform matching the optical path is set. Time to establish the optical path is shortened. 
     The channels of two types, the initial setting supervisory channel and the operational supervisory channel, are switchably used. A continuous operation is maintained during transition from supervisory signal exchange prior to the establishment of the main signals to supervisory signal exchange that does not affect the main signal in operation. 
     Since superposition of the supervisory signal to the main signal (frequency modulation) and extraction of the supervisory signal (frequency demodulation) are performed through digital signal processing, an addition of a new optical component becomes unnecessary. Large-size design of an optical node apparatus is thus controlled. 
     As described above, the supervisory signal is superimposed on the main signal through frequency modulation. If a modulation format is different from a modulation format of the main signal, the supervisory signal may be superimposed on the main signal through phase modulation. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.