Patent Publication Number: US-8526828-B2

Title: Optical transmitter and optical transmitter unit

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2010-269020, filed on Dec. 2, 2010, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are an optical transmitter, and an optical transmitter unit that includes a number of optical transmitters and carries out wavelength multiplexing. 
     BACKGROUND 
     In accordance with increase in data traffic, a high-capacity trunk light communication network has been demanded, and high-speed communication of 40 Gbps (Giga bit per second) or 100 Gbps are putting into practice. For high-speed communication in an optical communication network, one of the techniques recently focused is optical transmission and optical reception using digital signal processing.
     [Non-Patent Literature 1] D.-S. Ly-Gagnon, et al. “Coherent Detection of Optical Quadrature Phase-Shift Keying Signals With Carrier Phase Estimation”, IEEE JLT, vol. 24, no. 1, pp. 12-21, January 2006   [Non-Patent Literature 2] D. McGhan, “Electronic Dispersion Compensation”, OFC2006, OWK1   

     In general, a wavelength locker having, for example, an etalon filter is used to stably output light having a constant oscillation frequency (wavelength) from a transmitting light source used in a wavelength multiplexed optical transmission system. Considering aged deterioration, the accuracy of control by such a wavelength locker is several GHz. Fluctuation of this several GHz causes lowering the transmission capability in an optical transmission system and hinders high-density intervals of wavelengths to be multiplexed in a single optical fiber. In other words, frequency stability of a transmission light source is factors limiting the transmitting distance in an optical transmission system and a transmission capacity of a single optical fiber. 
     An oscillation frequency (wavelength) of a transmission light source is generally controlled by temperature, which takes time to make it difficult to flexibly change the wavelength arrangement particularly during the operation of the optical transmission system. More specifically, such control may take several minutes and the oscillation frequency to be controlled may have poor stability. In addition, it is difficult to accomplish complex and precise control while the system is working. 
     SUMMARY 
     As one aspect of the embodiments, an optical transmitter including a light source; a signal processor that carries out digital signal processing on a transmitting signal to be transmitted; an optical modulator that modulates output light from the light source in accordance with the transmitting signal subjected to the digital signal processing in the signal processor and outputs the modulated light as a light signal to a transmission path; and a carrier-wave frequency control circuit that controls a carrier-wave frequency of the light signal output from the optical modulator, the signal processor including a mapping circuit that maps the transmitting signal to electric-field information according to a modulating scheme, and a phase rotating circuit that provides a phase rotation having a constant cycle to an electric-field phase of the electric-field information to which the mapping circuit maps the transmitting signal, the carrier-wave frequency control circuit controlling the cycle of the phase rotation that the phase rotating circuit provides and thereby controlling the carrier-wave frequency of the light signal output from the optical modulator. 
     As another aspect, an optical transmitter unit including a plurality of the above optical transmitters; and an optical coupler that carries out wavelength multiplexing by combining a plurality of the light signals one output from each of the plurality of optical transmitters to the transmission path, wherein a plurality of the light sources, provided one for each of the plurality of optical transmitters, oscillate output light having a same frequency, a plurality of the carrier-wave frequency control circuits, provided one for each of the plurality of optical transmitters, fine-adjust the carrier-wave frequencies of the light signals output from the optical modulators by controlling cycles of the phase rotation that the corresponding phase rotating circuits provide. 
     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 THE DRAWINGS 
         FIG. 1  is a block diagram schematically illustrating the basic configuration of an optical transmitter; 
         FIG. 2  is a block diagram schematically illustrating the configuration of an optical transmitter according to a first embodiment; 
         FIG. 3  is a diagram illustrating a transmitting spectrum of an optical transmitter of the first embodiment; 
         FIG. 4  is a block diagram schematically illustrating the configuration of a phase rotating circuit; 
         FIG. 5  is a block diagram schematically illustrating the configuration of an IQ modulator; 
         FIG. 6  is a block diagram schematically illustrating the configuration of an optical transmitter according to a second embodiment; 
         FIG. 7  is a block diagram schematically illustrating the configuration of an optical transmitter according to a third embodiment; 
         FIG. 8  is a block diagram schematically illustrating the configuration of a polarization multiplexing IQ modulator; 
         FIG. 9  is a block diagram schematically illustrating the configuration of an optical communication system; 
         FIG. 10  is a block diagram schematically illustrating the configuration of an example of an optical receiver; 
         FIG. 11  is a block diagram schematically illustrating the configuration of another example of an optical receiver; 
         FIG. 12  is a block diagram schematically illustrating the configuration of an optical transmitter unit according to a first example; 
         FIG. 13  is a diagram illustrating a transmitting spectrum of an optical transmitter unit of  FIG. 12 ; 
         FIG. 14  is a block diagram schematically illustrating the configuration of an optical transmitter unit according to a second example; 
         FIG. 15  is a diagram illustrating a transmitting spectrum of an optical transmitter unit of  FIG. 14 ; 
         FIGS. 16A and 16B  are diagrams illustrating wavelength-arrangement change (defragmentation) of an optical transmitter unit of  FIG. 14 ; 
         FIG. 17  is a block diagram schematically illustrating the configuration of an optical transmitter unit according to a third example; 
         FIG. 18  is a diagram illustrating a transmitting spectrum of an optical transmitter unit of  FIG. 17 ; 
         FIG. 19  is a block diagram schematically illustrating the configuration of an optical transmitter unit according to a fourth example; 
         FIG. 20  is a diagram illustrating a transmitting spectrum of an optical transmitter unit of  FIG. 19 ; 
         FIG. 21  is a block diagram schematically illustrating the configuration of an optical transmitter unit according to a fifth example and an optical receiver unit; and 
         FIG. 22  is a diagram illustrating a transmitting spectrum of an optical transmitter unit of  FIG. 21 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, various embodiments will now be described with reference to the drawings. 
     (1) Basic Configuration of an Optical Transmitter: 
       FIG. 1  is a block diagram illustrating the basic configuration of an optical transmitter. The optical transmitter  1  having the basic configuration of  FIG. 1  includes a laser light source  11 , a signal processing circuit  12 , a DAC  13 , a driver  14 , an IQ modulator  15 , and a carrier-wave frequency control circuit  16 . 
     The laser light source (light source)  11  oscillates output light having a predetermined frequency f C . 
     The signal processing circuit (signal processor)  12  carries out digital signal processing on a transmitting signal which is input from an external device and which is binary data, and is exemplified by a DSP (Digital Signal Processor). The signal processing circuit  12  has functions of a modulating scheme mapping circuit  121  and a phase rotating circuit  122 . 
     The modulating scheme mapping circuit  121  constellation-maps a transmitting signal which is input from an external device and which is binary data to electric-field information in accordance with a modulating scheme such as QPSK (Quadrature Phase Shift Keying), QAM (Quadrature Amplitude Modulation), and OFDM (Orthogonal Frequency Division Multiplexing). If a modulation scheme such as RZ (Return to Zero) or NRZ (Non Return to Zero) is adopted, it is sufficient that the transmitting data in the form of binary data is simply converted into the electric-field information. 
     The phase rotating circuit  122  provides a phase rotation of a predetermined cycle to the electric-field phase of the electric-field information, to which the modulating scheme mapping circuit  121  maps the transmitting signal. Specifically, the phase rotating circuit  122  provides, upon receipt of an amount Δf a  of frequency control from the carrier-wave frequency control circuit  16 , a phase rotation θ=2πΔf a t to the electric-field phase. 
     The DAC (digital/analog converting circuit)  13  converts a digital signal from the signal processing circuit  12  into an analog signal. 
     The driver (modulator driving circuit)  14  amplifies the signal from the DAC  13 , and drives the IQ modulator  15  using the amplified signal. 
     The optical modulator (optical modulating section)  15  modulates the output light from the laser light source  11  in accordance with the transmitting signal after being subjected to the digital processing by the signal processing circuit  12  and the processing by the DAC  13  and the driver  14 , and then outputs the modulated light, being regarded as a light signal, to the transmission path  3 . 
     The carrier-wave frequency control circuit  16  controls the carrier-wave frequency of the light signal output from the optical modulator  15 . Specifically, for the control of the carrier-wave frequency of the light signal output from the optical modulator  15 , the carrier-wave frequency control circuit  16  outputs an amount Δf a  of frequency control to the phase rotating circuit  122  to control the cycle of phase rotation θ=2πΔf a t that the phase rotating circuit  122  applies. The carrier-wave frequency control circuit  16  fine-adjusts the carrier-wave frequency of a light signal output from the optical modulator  15  in the electric band of the optical transmitter  1  of the first embodiment through the above frequency control. The electric band of the optical transmitter  1  depends on the band properties of the DAC  13 , the driver  14 , and the optical modulator  15 . 
     In the optical transmitter  1 , which has the above basic configuration, the carrier-wave frequency control circuit  16  outputs an amount Δf a  of frequency control to the phase rotating circuit  122 , so that the cycle of the phase rotation θ=2πΔf a t that the phase rotating circuit  122  provides is controlled. This fine-adjusts the carrier-wave frequency of the light signal output from the optical modulator  15  to a value of the sum of the frequency f C  of the output light from the laser light source  11  and an amount Δf a  of frequency control within the electric band of the optical transmitter  1 . 
     Accordingly, use of the amount Δf a  of frequency control makes the optical transmitter  1  possible to control the carrier-wave frequency of the light signal output from the optical modulator  15  more precisely and faster than the conventional method that directly controls the laser light source  11 . This ensures the stability of the oscillation frequency of the laser light source  11 , so that the transmission performance of an optical communication system using the optical transmitter  1  is improved. In addition, wavelength multiplexing intervals come to be dense to improve the rate of using the band of the transmission path (optical fiber)  3  and high-capacity transmission is realized. 
     (2) The Optical Transmitter of the First Embodiment: 
       FIG. 2  is a block diagram schematically illustrating the configuration of an optical transmitter  1 A of the first embodiment. The optical transmitter  1 A includes a laser light source  11 , a signal processing circuit  12 , DACs  13 I and  13 Q, drivers  14 I and  14 Q, an IQ optical modulator  15 A, and the carrier-wave frequency control circuit  16 . 
     The laser light source  11  oscillates output light having a predetermined frequency f C . 
     The signal processing circuit (signal processor)  12  carries out digital signal processing on a transmitting signal which is input from an external device and which is binary data. The signal processing circuit  12  has functions of a modulating scheme mapping circuit  121 , a phase rotating circuit  122 , an I-channel predistortion compensating circuit  123 I, and Q-channel predistortion compensating circuit  123 Q. 
     Likewise the optical transmitter  1  of  FIG. 1 , the modulating scheme mapping circuit  121  constellation-maps a transmitting signal which is input from an external device and which is binary data to electric-field information in accordance with a modulating scheme such as QPSK, QAM, and OFDM. The electric-field information E 1 , to which the modulating scheme mapping circuit  121  maps the transmitting signal, includes an I (In-phase) component and a Q (Quadrature-phase) component, and is expressed by Formula E 1 =I+j·Q=A(t)·exp(jθ(t)) where the term “j” represents the imaginary unit; the term “A(t)” represents an electric-field intensity (amplitude); the term “θ(t)” represents an electric-field phase; and the term “t” represents time. 
     The phase rotating circuit  122  provides a phase rotation of a predetermined cycle to the electric-field phase of the electric-field information E 1 , to which the modulating scheme mapping circuit  121  maps the transmitting signal. Specifically, the phase rotating circuit  122  applies, upon receipt of an amount Δf a  of frequency control from the carrier-wave frequency control circuit  16 , a phase rotation θ=2πΔf a t to the electric-field phase of the electric-field information E 1  similarly to the optical transmitter  1  of  FIG. 1 . 
     As illustrated in  FIG. 4 , which is a block diagram illustrating the configuration of the phase rotating circuit  122 , the phase rotating circuit  122  includes an integrator circuit  122   a  and a multiplexer  122   b . The integrator circuit  122   a  integrates, upon receipt of the amount Δf a  of frequency control from the carrier-wave frequency control circuit  16 , an amount Δω(=2πΔf a ) of phase rotation per discrete time unit T and thereby calculates a complex electric field information exp(−j(ΔωnT)) that rotates the phase by ΔωnT during a discrete time nT (here, n is an integer). The multiplexer  122   b  multiplies the complex electric field information exp(−j(ΔωnT)), which the integrator circuit  122   a  calculates, by the electric-field information E(t) (=E 1 ) from the modulating scheme mapping circuit  121 , and thereby obtains and outputs electric-field information E′(t)=E(t)*exp(jΔωnT), to which the electric-field phase of the electric-field information E(t) is rotated by ΔωnT(=2πΔf a t). 
     The I-channel predistortion compensating circuit  123 I compensates the I component of the electric-field information, to which the phase rotating circuit  122  provides the phase rotation, for prospective deterioration in signal quality of the I component of the electric-field information, which deterioration is caused by incompletion of a transmitting system corresponding to the I component. Similarly, the Q-channel predistortion compensating circuit  123 Q compensates the Q component of the electric-field information, to which the phase rotating circuit  122  provides the phase rotation, for prospective deterioration in signal quality of the Q component of the electric-field information, which deterioration is caused by incompletion of a transmitting system corresponding to the Q component. Here, an example of the transmitting system corresponding to the I component includes DAC  13 I, the driver  14 I, and a phase modulator  151 I that are to be detailed below; and an example of the transmitting system corresponding to the Q component includes the DAC  13 Q, the driver  14 Q, and a phase modulator  151 Q that are to be detailed below. The I-channel predistortion compensating circuit  123 I and the Q-channel predistortion compensating circuit  123 Q compensate for, for example, a loss variation between I and Q signals, skew, a band variation, linearity. As detailed below, the compensation is accomplished by multiplying (convolving) the electric-field information from the phase rotating circuit  122  by the reversed function of a waveform distortion of the optical transmitter  1 A (the transmitting system). 
     The DACs  13 I and  13 Q convert digital signals (I component and Q component) from the I-channel predistortion compensating circuit  123 I and the Q-channel predistortion compensating circuit  123 Q, respectively into analog signals. 
     The drivers  14 I and  14 Q amplify signals from the DACs  13 I and  13 Q, and drive the respective corresponding phase modulator  151 I and  151 Q (see  FIG. 5 ) in the IQ modulator  15 A using the amplified signals. 
     The IQ modulator (optical modulator)  15 A modulates output signal from the laser light source  11  using the transmitting signals (the I component and the Q component) processed by the DACs  13 I and  13 Q and the drivers  14 I and  14 Q after being subjected to the digital processing by the signal processing circuit  12 , and outputs the modulated light, being regarded as a light signal, to the transmission path  3 . As illustrated in  FIG. 5 , the IQ modulator  15 A includes the phase modulator  151 I for an I component, the phase modulator  151 Q for a Q component, and a phase shifter  152 .  FIG. 5  is a block diagram schematically illustrates the configuration of the IQ modulator  15 A. The phase shifter  152  provides a predetermined phase difference (π/2) between a pair of light signals propagating through the phase modulator  151 I and the phase modulator  151 Q, and is disposed on the side of the phase modulator  151 Q. The phase modulator  151 I and the phase modulator  151 Q phase-modulate the pair of light signals that have the predetermined phase difference provided on the basis of the transmitting signals (the I component and the Q component) from the drivers  14 I and  14 Q, and output the modulated signals to the transmission path  3 . 
     Likewise the optical transmitter  1  of  FIG. 1 , the carrier-wave frequency control circuit  16  controls the carrier-wave frequency of the light signal output from the IQ modulator  15 A. Specifically, for the control of the carrier-wave frequency of the light signal output from the optical modulator  15 , the carrier-wave frequency control circuit  16  outputs an amount Δf a  of frequency control to the phase rotating circuit  122  to control the cycle of phase rotation θ=2πΔf a t that the phase rotating circuit  122  applies. Through the above frequency control, the carrier-wave frequency control circuit  16  fine-adjusts the carrier-wave frequency of the light signal output from the IQ modulator  15 A within the electric band of the optical transmitter  1 A of the first embodiment as illustrated in, for example,  FIG. 3 . The electric band of the optical transmitter  1 A depends on the band properties of the DACs  13 I and  13 Q, the drivers  14 I and  14 Q, and the IQ modulator  15 A.  FIG. 3  illustrates a transmitting spectrum of the optical transmitter  1 A of the first embodiment. 
     In accordance with an error signal from an optical receiver  2 A or  2 B (see  FIGS. 9-11 ) in communication with the optical transmitter  1 A, the carrier-wave frequency control circuit  16  fine-adjusts the carrier-wave frequency within the electric band of the optical transmitter  1 A by adjusting the rotating frequency that the phase rotating circuit  122  applies so that the error at the receiver end is resolved. Here, an error signal is related to the result of quality of a received signal received by the optical receiver  2 A or  2 B, and represents occurrence of deterioration in quality of received signal in the receiver end, as to be detailed below with reference to  FIGS. 10 and 11 . Such an error signal may be transmitted from the optical receiver  2 A or  2 B to the optical transmitter  1 A through a reverse-direction channel; through a control channel of the optical communication system; or through frequency modulating (to be detailed below), that is a novel function of the optical transmitter  1 A. Alternatively, the carrier-wave frequency may be fine-adjusted on the basis of a light spectrum or a light frequency measured by means of a transmitter output, a relay, or a receiver. 
     Further alternatively, the carrier-wave frequency control circuit  16  may superimpose a pilot signal onto a frequency adjusting value Δf a . Specifically, the carrier-wave frequency control circuit  16  may have a function of superimposing a pilot signal onto a carrier-wave frequency of a light signal that the IQ modulator  15 A outputs through frequency modulation by controlling the cycle of the phase rotation θ=2πΔf a t that the phase rotating circuit  122  applies to thereby control the carrier-wave frequency of the light signal output from the IQ modulator  15 A, and transmits the superimposed signal to the receiver end through the transmission path  3 . Besides, the carrier-wave frequency control circuit  16  may superimpose a result of detecting a quality of a received signal of the reverse-direction channel, specifically being the above error signal and serving as a pilot signal, onto the carrier-wave frequency through frequency modulation. If such frequency modulation is carried out, the light receiver  2 B should have a function of demodulating a pilot signal subjected to the frequency modulation from the carrier wave as to be detailed below with reference to  FIG. 11 . 
     Furthermore, the carrier-wave frequency control circuit  16  may superimpose dither for detecting quality of a received signal onto the carrier-wave frequency of a light signal that the IQ modulator  15 A outputs by controlling the cycle of the phase rotation θ=2πΔf a t that the phase rotating circuit  122  applies and transmit the superimposed dither to the receiver end (the optical receiver  2 A or  2 B) through the transmission path  3 . 
     When the carrier-wave frequency is to be adjusted beyond the electric band of the transmitter, the carrier-wave frequency control circuit  16  may carry out control using the phase rotating circuit  122  and control of the oscillation frequency of the laser light source  11  in combination with each other. Specifically, the carrier-wave frequency control circuit  16  may use both the fine adjustment that controls the carrier-wave frequency of a light signal output from the IQ modulator  15 A through controlling the cycle of the phase rotation that the phase rotating circuit  122  applies and rough adjustment that directly controls the frequency of the output light from the laser light source  11 . The oscillation frequency of the laser light source  11  generally used in a wavelength multiplexed optical system is controllable by means of oscillation frequency adjustment based on changing an oscillation frequency grid and controlling the temperature. The carrier-wave frequency control circuit  16  accomplishes the above rough adjustment using oscillation frequency adjustment based on changing an oscillation frequency grid and controlling the temperature. 
     Likewise the optical transmitter  1  of  FIG. 1 , in the above optical transmitter  1 A of the first embodiment, the carrier-wave frequency control circuit  16  outputs an amount Δf a  of frequency control to the phase rotating circuit  122 , so that the cycle of the phase rotation θ=2πΔf a t that the phase rotating circuit  122  provides is controlled. This fine-adjusts the carrier-wave frequency of the light signal output from the IQ modulator  15 A to a value of the sum of the frequency f C  of the output light from the laser light source  11  and an amount Δf a  of frequency control within the electric band of the optical transmitter  1 A. 
     Accordingly, use of the amount Δf a  of frequency control makes the optical transmitter  1 A possible to control the carrier-wave frequency of the light signal output from the IQ modulator  15 A more precisely and faster than the conventional method which directly controls the laser light source  11 . This ensures the stability of the oscillation frequency of the laser light source  11 , so that the transmission performance of an optical communication system using the optical transmitter  1 A is improved. In addition, wavelength multiplexing intervals come to be dense to improve the rate of using the band of the transmission path (optical fiber)  3  and high-capacity transmission is realized. 
     In addition, differently from a conventional method that directly controls the laser light source  11 , the optical transmitter  1 A can carry out complex frequency control such as superimposing a pilot signal onto a carrier-wave frequency by means of frequency control, and superimposing dither for detecting quality of a received signal onto the carrier-wave frequency. This makes it possible to transmit a pilot signal (e.g., an error signal) without using a control channel, and to ensure various advantages, such as improvement in sensitivity of error detection at a receiver end with the aid of dither as detailed below with reference to  FIGS. 10 and 11 . 
     In addition, since the optical transmitter  1 A of the first embodiment has functions of the I-channel predistortion compensating circuit  123 I and the Q-channel predistortion compensating circuit  123 Q, deterioration of signal quality caused by incompletion of the DACs  13 I and  13 Q, the drivers  14 I and  14 Q, and the phase modulators  151 I and  151 Q is compensated beforehand. This enables the optical transmitter  1 A to accomplish high-quality light transmission. 
     Furthermore, since the carrier-wave frequency control circuit  16  concurrently uses the fine adjustment that controls the carrier-wave frequency by the phase rotating circuit  122  and rough adjustment to directly control the frequency of the output light from the laser light source  11 , the optical transmitter  1 A can precisely adjusts the carrier-wave frequency even beyond the electric band of the transmitter. 
     (3) Optical Transmitter of the Second Embodiment: 
       FIG. 6  is a block diagram schematically illustrating an optical transmitter  1 B according to a second embodiment. The optical transmitter  1 B of  FIG. 6  is similar in configuration with the optical transmitter  1 A except for the signal processing circuit  12  having a function of a transmission-path predistortion compensating circuit  124 .  FIG. 6  omits the laser light source  11 , the drivers  14 I and  14 Q, and the IQ modulator  15 A. In  FIG. 6 , parts and elements having the same reference numbers as the foregoing description represent the same or similar parts and elements, so repetitious description is omitted here. 
     The transmission-path predistortion compensating circuit  124  is arranged between the  121  and the phase rotating circuit  122 , and compensates the electric-field information E 1 , to which the modulating scheme mapping circuit  121  maps the transmission signal, for prospective deterioration in signal quality due to transmission through the transmission path (optical fiber)  3 . As detailed below, the compensation is accomplished by multiplying (convolving) the electric-field information E 1  from the modulating scheme mapping circuit  121  by the reversed function of a waveform distortion due to the optical transmission through the transmission path  3 . 
     Accordingly, the optical transmitter  1 B of the second embodiment ensures the same effects as those of the optical transmitter  1 A of the first embodiment. In addition, since the signal processing circuit  12  has a function of the transmission-path predistortion compensating circuit  124 , the waveform distortion caused by optical transmission through the transmission path  3  can be compensated beforehand, so that the optical transmitter  1 B of the second embodiment can attain higher-quality optical transmission. 
     Hereinafter, description will now be made in relation to the principle of the signal processing in the optical transmitter  1 B of  FIG. 6 , that is, controlling the carrier-wave frequency by the phase rotating circuit  122  with reference to following Formulae (1) through (5). 
     As described above, the electric-field information E 1  after the mapping of the transmitting signal by the modulating scheme mapping circuit  121  is represented by Formula (1).
 
 E   1   =A ( t )·exp( j θ( t ))  (1)
 
     where, the term “j” represents the imaginary unit; the term “A(t)” represents an electric-field intensity (amplitude); the term “θ(t)” represents an electric-field phase; and the term “t” represents time. 
     The electric-field information E 2  after the transmission-path predistortion compensating circuit  124  compensates for distortion of the electric-field information E 1  is expressed by following Formula (2).
 
 E   2   =h   1 ( t )* E   1   (2)
 
     where, the symbol “*” represents calculation of convolving; the term “h 1 (t)” represents the reversed function of waveform distortion of the transmission path  3 . 
     Electric-field information E 3  obtained through the phase rotation based on the amount Δf a  of frequency control on the electric-field information E 2  by the phase rotating circuit  122  is expressed following Formula (3)
 
 E   3 =exp( j 2 πΔf   a   t )· E   2   (3)
 
     Electric-field information E 4  obtained through compensation on the electric-field information E 3  by the I-channel predistortion compensating circuit  123 I and the Q-channel predistortion compensating circuit  123 Q is expressed following Formula (4).
 
 E   4   =h   2 ( t )* E   3   (4)
 
     where, the term “h 2 (t)” represents a reversed function of a waveform distortion of the optical transmitter  1 B (transmitting system) of the second embodiment. 
     The intensity P sig  of a signal obtained by processing on the electric-field information E 4  by the DACs  13 I and  13 Q, the drivers  14 I and  14 Q, and the phase modulators  151 I and  151 Q is expressed by following Formula (5) 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     where, the term “f c ” represents the frequency of output light that the laser light source  11  oscillates; the term “P” represents a photoelectric-field intensity; and the term “h 2 (t)′” represents a function of the waveform distortion of the optical transmitter  1 B (transmitting system) and establishes the relationship h 2 (t)′*h 2 (t)=1. 
     (4) Optical Transmitter According to a Third Embodiment 
       FIG. 7  is a block diagram schematically illustrating the configuration of the optical transmitter  1 C according to the third embodiment. The optical transmitter  1 C adopts a polarization multiplexing scheme, and includes the laser light source  11 , the signal processing circuit  12 , the DACs  13 XI,  13 XQ,  13 YI, and  13 YQ, the drivers  14 XI,  14 XQ,  14 YI, and  14 YQ, a polarization multiplexed IQ modulator  15 C, and the carrier-wave frequency control circuit  16 . 
     The laser light source  11  oscillates output light having a predetermined frequency f C . 
     The signal processing circuit  12  carries out digital signal processing on a transmitting signal which is input from an external device and which is binary data, and has functions of a 1:2 DMUX  120 , modulating scheme mapping circuits  121 X and  121 Y, the phase rotating circuits  122 X and  122 Y, the I-channel predistortion compensating circuits  123 XI and  123 YI, and the Q-channel predistortion compensating circuits  123 XQ and  123 YQ. 
     The 1:2 DMUX  120  serves as a divider that separates the transmitting signal in the form of binary data input from an external device into two branch signals corresponding one to each of polarization components X and Y orthogonal to each other. 
     The modulating scheme mapping circuits  121 X and  121 Y correspond to the polarization components X and Y separated by the 1:2 DMUX  120 , respectively. 
     The modulating scheme mapping circuit  121 X constellation-maps a signal of the polarization component X to electric-field information according to a modulation scheme such as QPSK, QAM, and OFDM. The electric-field information E x1 , to which the modulating scheme mapping circuit  121 X maps the transmitting signal, includes an I component and a Q component, and is expressed by Ex 1 =xI+j·xQ. Similarly, the modulating scheme mapping circuit  121 Y constellation-maps a signal of the polarization component Y to electric-field information according to a modulation scheme such as QPSK, QAM, and OFDM. The electric-field information Ey 1 , to which the modulating scheme mapping circuit  121 Y maps the transmitting signal, includes an I component and a Q component, and is expressed by Ey 1 =yI+j·yQ. 
     The phase rotating circuits  122 X and  122 Y are arranged downstream modulating scheme mapping circuits  121 X and  121 Y, respectively, and correspond to the polarization components X and Y, respectively. 
     The phase rotating circuit  122 X applies phase rotation having a predetermined cycle to an electric-field phase of the electric-field information Ex 1 , to which the modulating scheme mapping circuit  121 X maps the transmitting signal. Specifically, likewise the optical transmitters  1  and  1 A of  FIGS. 1 and 2 , the phase rotating circuit  122 X applies, upon receipt of an amount Δf a  of frequency control from the carrier-wave frequency control circuit  16 , phase rotation θ=2πΔf a t to the electric-field phase of the electric-field information Ex 1 . Similarly, the phase rotating circuit  122 Y applies phase rotation having a predetermined cycle to an electric-field phase of the electric-field information Ey 1 , to which the modulating scheme mapping circuit  121 Y maps the transmitting signal. Specifically, the phase rotating circuit  122 Y applies, upon receipt of an amount Δf a  of frequency control from the carrier-wave frequency control circuit  16 , phase rotation θ=2πΔf a t to the electric-field phase of the electric-field information Ey 1 . The phase rotating circuits  122 X and  122 Y each have the same configuration as the phase rotating circuit  122  of  FIG. 4 . 
     The I-channel predistortion compensating circuit  123 XI and the Q-channel predistortion compensating circuit  123 XQ are arranged the downstream of the phase rotating circuit  122 X and correspond to the polarization component X. 
     The I-channel predistortion compensating circuit  123 XI compensates the I component of the electric-field information, to which the phase rotating circuit  122 X provides the phase rotation, for prospective deterioration in signal quality of the I component of the polarization component X, which deterioration is caused by incompletion of a transmitting system corresponding to the I component of the polarization component X. Similarly, the Q-channel predistortion compensating circuit  123 XQ compensates the Q component of the electric-field information, to which the phase rotating circuit  122 X provides the phase rotation, for prospective deterioration in signal quality of the Q component of the polarization component X, which deterioration is caused by incompletion of a transmitting system corresponding to the Q component of the polarization component X. Here, an example of the transmitting system corresponding to the I component of the polarization component X includes the DAC  13 XI, the driver  14 XI, and a phase modulator  151 XI that are to be detailed below; and an example of the transmitting system corresponding to the Q component of the polarization component X includes the DAC  13 XQ, the driver  14 XQ, and a phase modulator  151 XQ that are to be detailed below. As detailed below, the compensation is accomplished by multiplying (convolving) the electric-field information from the phase rotating circuit  122 X by the reversed function of a waveform distortion of the optical transmitter  1 C (the transmitting system). 
     The I-channel predistortion compensating circuit  123 YI and the Q-channel predistortion compensating circuit  123 YQ are arranged the downstream of the phase rotating circuit  122 Y and correspond to the polarization component Y. 
     The I-channel predistortion compensating circuit  123 YI compensates the I component of the electric-field information, to which the phase rotating circuit  122 Y provides the phase rotation, for prospective deterioration in signal quality of the I component the polarization component Y, which deterioration is caused by incompletion of a transmitting system corresponding to the I component of the polarization component Y. Similarly, the Q-channel predistortion compensating circuit  123 YQ compensates the Q component of the electric-field information, to which the phase rotating circuit  122 Y provides the phase rotation, for prospective deterioration in signal quality of the Q component of the polarization component Y, which deterioration is caused by incompletion of a transmitting system corresponding to the Q component of the polarization component Y. Here, an example of the transmitting system corresponding to the I component of the polarization component Y includes the DAC  13 YI, the driver  14 YI, and a phase modulator  151 YI that are to be detailed below; and an example of the transmitting system corresponding to the Q component of the polarization component Y includes the DAC  13 YQ, the driver  14 YQ, and a phase modulator  151 YQ that are to be detailed below. As detailed below, the compensation is accomplished by multiplying (convolving) the electric-field information from the phase rotating circuit  122 Y by the reversed function of a waveform distortion of the optical transmitter  1 C (the transmitting system). 
     The DACs  13 XI and  13 XQ correspond to the polarization component X, and convert digital signals (I component and Q component) from the I-channel predistortion compensating circuit  123 XI and the Q-channel predistortion compensating circuit  123 XQ, respectively, into analog signals. Similarly, the DACs  13 YI and  13 YQ correspond to the polarization component Y, and convert digital signals (I component and Q component) from the I-channel predistortion compensating circuit  123 YI and the Q-channel predistortion compensating circuit  123 YQ, respectively, into analog signals. 
     The divers  14 XI and  14 XQ correspond to the polarization component X, and amplify signals from the DACs  13 XI and  13 XQ and drive the respective corresponding phase modulator  151 XI and  151 XQ (see  FIG. 8 ) in the polarization multiplexed IQ modulator  15 C using the respective amplified signals. Similarly, the divers  14 YI and  14 YQ correspond to the polarization component Y, and amplify signals from the DACs  13 YI and  13 YQ and drive the respective corresponding phase modulator  151 YI and  151 YQ (see  FIG. 8 ) in the polarization multiplexed IQ modulator  15 C using the respective amplified signals. 
     The polarization multiplexed IQ modulator (optical modulator)  15 C is a polarization multiplexed modulator that modulates the polarization component X and Y, which are orthogonal each other, independently of each other. The polarization multiplexed IQ modulator  15 C modulates the output light from the laser light source  11  in accordance with the I component and Q component of two branch signals (polarization components X and Y) subjected to digital processing in the signal processing circuit  12 , and outputs the modulated light, regarded as a light signal, to the transmission path  3 . 
     As illustrated in  FIG. 8  that illustrates the configuration of the polarization multiplexed IQ modulator  15 C, the polarization multiplexed IQ modulator  15 C includes the phase modulator  151 XI for the I component, the phase modulator  151 XQ for the Q component, and a phase shifter  152 X that deal with the polarization component X; and the phase modulator  151 YI for the I component, the phase modulator  151 YQ for the Q component, and a phase shifter  152 Y that deal with the polarization component Y; and a polarization coupler  153 . It is to be noted that  FIG. 8  is a block diagram schematically illustrating the configuration of the polarization multiplexed IQ modulator  15 C. 
     The phase shifter  152 X provides a predetermined phase difference (e.g., π/2) between a pair of light signals propagating through the phase modulator  151 XI and the phase modulator  151 XQ, and is disposed on the side of the phase modulator  151 XQ. The phase modulator  151 XI and the phase modulator  151 XQ perform phase modulation based on the transmitting signals (I component and Q component) from the drivers  14 XI and  14 XQ on the pair of light signals to which the predetermined phase difference is provided. 
     The phase shifter  152 Y provides a predetermined phase difference (e.g., π/2) between a pair of light signals propagating through the phase modulator  151 YI and the phase modulator  151 YQ, and is disposed on the side of the phase modulator  151 YQ. The phase modulator  151 YI and the phase modulator  151 YQ perform phase modulation based on the transmitting signals (I component and Q component) from the drivers  14 YI and  14 YQ on the pair of light signals to which the predetermined phase difference is provided. 
     The Polarization Beam Combiner (PCB)  153  combines the modulated signal of the polarization component X from the phase modulators  151 XI and  151 XQ and the modulated signal of the polarization component Y from the phase modulators  151 YI and  151 YQ, and outputs the combined signal to the transmission path  3 . 
     Similarly to the optical transmitters  1 , 1 A, and  1 B, the carrier-wave frequency control circuit  16  controls the carrier-wave frequency of the light signal (X polarization and Y polarization) output from the polarization multiplexed IQ modulator  15 C. The carrier-wave frequency control circuit  16  outputs an mount Δf a  of frequency control to the phase rotating circuits  122 X and  122 Y to control the cycles of the phase rotation θ=2πΔf a t that the phase rotating circuits  122 X and  122 Y apply, so that the carrier-wave frequency of the light signal (X polarization and Y polarization) output from the polarization multiplexed IQ modulator  15 C can be controlled. As a result of the above frequency control, the carrier-wave frequency control circuit  16  fine-adjusts the carrier-wave frequency of the light signal (X polarization and Y polarization) output from the polarization multiplexed IQ modulator  15 C within the electric band of the optical transmitter  1 C. The electric band of the optical transmitter  1 C depends on the band properties of the DACs  13 XI,  13 XQ,  13 YI, and  13 YQ, the drivers  14 XI,  14 XQ,  14 YI, and  14 YQ, and the polarization multiplexed IQ modulator  15 C. 
     Similarly to the first embodiment, the carrier-wave frequency control circuit  16  fine-adjusts the frequency of the carrier wave within the electric band of the optical transmitter  1 C by adjusting the rotating frequencies that the phase rotating circuits  122 X and  122 Y apply in accordance with an error signal from an optical receiver  2 A or  2 B (see  FIGS. 9-11 ) in communication with the optical transmitter  1 C, so that the error at the receiver end is resolved. 
     Alternatively, the carrier-wave frequency control circuit  16  may superimpose a result (i.e., the above error signal) of detecting a quality of a pilot signal exemplified by a received signal onto the carrier-wave frequency of the light signal (X polarization and Y polarization) output from the polarization multiplexed IQ modulator  15 C through frequency modulation similarly to the first embodiment. In the optical transmitter  1 C of  FIG. 7 , the carrier-wave frequency control circuit  16  outputs the same amount Δf a  and Δf b  of frequency control to the phase rotating circuits  122 X and  122 Y. Alternatively, different amounts Δf a  of frequency control may be provided to the phase rotating circuits  122 X and  122 Y. 
     Similarly to the first embodiment, the carrier-wave frequency control circuit  16  may superimpose dither for detecting quality of a received signal onto the carrier-wave frequency of a light signal (X polarization and Y polarization) that the polarization multiplexed IQ modulator  15 C outputs by controlling the cycles of the phase rotations θ=2πΔf a t that the phase rotating circuits  122 X and  122 Y apply to control the carrier-wave frequency of a light signal (X polarization and Y polarization) that the polarization multiplexed IQ modulator  15 C outputs, and transmit the superimposed dither to the receiver end (the optical receiver  2 A or  2 B) through the transmission path  3 . 
     Similarly to the first embodiment, when the carrier-wave frequency is to be adjusted beyond the electric band of the transmitter, the carrier-wave frequency control circuit  16  may carry out control using the phase rotating circuits  122 X and  122 Y and control of the oscillation frequency of the laser light source  11  in combination with each other. Specifically, the carrier-wave frequency control circuit  16  uses both the fine adjustment that controls the carrier-wave frequency of a light signal (X polarization and Y polarization) output from the polarization multiplexed IQ modulator  15 C through controlling the cycles of the phase rotations that the phase rotating circuits  122 X and  122 Y apply, and rough adjustment that directly controls the frequency of the output light from the laser light source  11 , so that the carrier-wave frequency of a light signal output from the polarization multiplexed IQ modulator  15 C can be controlled. 
     Likewise the optical transmitter  1  of  FIG. 1 , in the above optical transmitter  1 C of the third embodiment, the carrier-wave frequency control circuit  16  outputs an amount Δf a  of frequency control to the phase rotating circuits  122 X and  122 Y, so that the cycle of the phase rotation θ=2πΔf a t that the phase rotating circuits  122 X and  122 Y apply is controlled. This fine-adjusts the carrier-wave frequency of the light signal (X polarization and Y polarization) output from the polarization multiplexed IQ modulator  15 C to a value of the sum of the frequency f C  of the output light from the laser light source  11  and an amount Δf a  of frequency control in the electric band of the optical transmitter  1 C. 
     Accordingly, use of the amount Δf a  of frequency control makes the optical transmitter  1 C possible to control the carrier-wave frequency of the light signal output from the polarization multiplexed IQ modulator  15 C more precisely and faster than the conventional method which directly controls the laser light source  11 , similarly to the optical transmitter  1 A of the first embodiment. This ensures the stability of the oscillation frequency of the laser light source  11 , so that the transmission performance of an optical communication system using the optical transmitter  1 C is improved. In addition, wavelength multiplexing intervals come to be dense to improve the rate of using the band of the transmission path (optical fiber)  3 , and high-capacity transmission is realized. In particular, the optical transmitter  1 C, which adopts the polarization multiplexing scheme, can double the bit-rate per baud rate in the optical communication system. 
     In addition, differently from a conventional method that directly controls the laser light source  11 , also the optical transmitter  1 C can carry out complex frequency control such as superimposing a pilot signal onto a carrier-wave frequency by means of frequency control, superimposing dither for detecting quality of a received signal onto the carrier-wave frequency. This makes it possible to transmit a pilot signal (e.g., an error signal) without using a control channel, and to ensure various advantages, such as improvement in sensitivity of error detection at the receiver end with the aid of dither as detailed below with reference to  FIGS. 10 and 11 . 
     Besides, the optical transmitter  1 C of the third embodiment, the carrier-wave frequency control circuit  16  may output different amounts Δf a  and Δf b  of frequency control to the phase rotating circuits  122 X and  122 Y, as described above. Thereby, different pilot signals can be superimposed onto the polarization components X and Y through frequency modulation, so that an amount information to be transmitted by the pilot signals can be doubled. 
     Also in the optical transmitter  1 C, since the signal processing circuit  12  has functions of the I-channel predistortion compensating circuits  123 XI and  123 YI, and the Q-channel predistortion compensating circuit  123 XQ and  123 YQ, it is possible to compensate for distortion of signal quality due to incompletion of the DACs  13 XI,  13 XQ,  13 YI, and  13 YQ; the drivers  14 XI,  14 XQ,  14 YI, and  14 YQ, and the phase modulators  151 XI,  151 XQ,  151 YI, and  151 YQ beforehand. This enables the optical transmitter  1 C to accomplish high-quality light transmission. 
     Furthermore, since the carrier-wave frequency control circuit  16  uses both the fine adjustment that controls the carrier-wave frequency by the phase rotating circuits  122 X and  122 Y and rough adjustment that directly controls the oscillation frequency of the output light from the laser light source  11 , the optical transmitter  1 C can precisely adjusts the carrier-wave frequency even beyond the electric band of the transmitter. 
     Also in the optical transmitter  1 C, transmission-path predistortion compensating circuits being the same as the transmission-path predistortion compensating circuit  124  of the optical transmitter  1 B illustrated in  FIG. 6  may be disposed between the modulating scheme mapping circuit  121 X and the phase rotating circuit  122 X and between the modulating scheme mapping circuit  121 Y and the phase rotating circuit  122 Y. The transmission-path predistortion compensating circuits compensate the electric-field information, to which the modulating scheme mapping circuit  121 X or  121 Y maps the transmitting signal, for deterioration in signal quality, which deterioration is caused when transmission through the transmission path  3 . This configuration compensates for distortion in waveform caused by optical transmission through the transmission path  3  beforehand, and higher-quality light transmission can be realized. 
     The signal processing in the optical transmitter  1 C, which adopts a polarization multiplexing scheme, is carried out along Formulae (1) through (5) described above for the signal processing in the second embodiment. It should be noted that, when a polarization multiplexing scheme is adopted, the electric-field information E in Formula (1) through (5) uses E x  and E y  serving as information of X polarization and Y polarization as taught in following Formula (6), and the function and the reversed function h(=h 1 (t), h 2 (t) or h 2 (t)′) related to distortion in waveform is replaced with the following function considering polarization as Formula (7) below. 
     
       
         
           
             
               
                 
                   E 
                   = 
                   
                     [ 
                     
                       
                         
                           
                             E 
                             x 
                           
                         
                       
                       
                         
                           
                             E 
                             y 
                           
                         
                       
                     
                     ] 
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
             
               
                 
                   h 
                   = 
                   
                     [ 
                     
                       
                         
                           
                             h 
                             xx 
                           
                         
                         
                           
                             h 
                             yx 
                           
                         
                       
                       
                         
                           
                             h 
                             xy 
                           
                         
                         
                           
                             h 
                             yy 
                           
                         
                       
                     
                     ] 
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     (5) Optical Communication System and Optical Receiver: 
     Next, referring to  FIG. 9 , description will now be made in relation to an optical communication system adopting the above optical transmitters  1 , and  1 A through  1 C, and referring to  FIGS. 10 and 11 , description will now be made in relation to the configuration of the optical receivers  2 A and  2 B that the optical communication system adopts.  FIG. 9  is a block diagram schematically illustrating the configuration of the optical communication system of this embodiment;  FIG. 10  is a block diagram schematically illustrating the configuration of the optical receiver  2 A; and  FIG. 11  is a block diagram schematically illustrating the configuration of the optical receiver  2 B. 
     As illustrated in  FIG. 9 , the optical communication system adopting the optical transmitters  1 , and  1 A through  1 C includes an optical transmitter Tx, an optical receiver Rx, the transmission path  3 , and a number (four in  FIG. 9 ) of repeaters  4 . Here, the optical transmitter Tx is one of the above optical transmitters  1 , and  1 A through  1 C, and the optical receiver Rx is the optical receiver  2 A or  2 B that are to be detailed below with reference to  FIGS. 10 and 11 . The transmission path  3  (optical fiber) connects the optical transmitter Tx and the optical receiver Rx, which are interposed by two or more repeaters  4 . 
     As illustrated in  FIG. 10 , the optical receiver (an example of the optical receiver)  2 A includes an OE  21  and a decision/error-correcting circuit  22 . 
     The OE (optical/electric converting circuit)  21  receives a light signal that the optical transmitter Tx outputs through the transmission path  3 , and converts the light signal into an electric signal. 
     The decision/error-correcting circuit  22  recognizes the electric signal from the OE  21 , and monitors the number of error corrections of the electric signal. 
     The number of error corrections monitored by the decision/error-correcting circuit  22  is regarded as an error signal described above, which is transmitted from the optical receiver Rx ( 2 A) to the optical transmitter through a reverse-direction channel, through the control channel of the optical communication system, regarding the error signal as a pilot signal, or through the use of the above frequency modulation. In the optical transmitter Tx, which receives the number of error corrections from the optical receiver Rx ( 2 A) in the above manner, the carrier-wave frequency control circuit  16  adjusts the rotation frequencies of the phase rotating circuits  122 ,  122 X, and  122 Y such that the number of error corrections is minimized for fine-adjustment of the carrier-wave frequency within the electric band of the optical receiver Tx. 
     As illustrated in  FIG. 11 , the optical receiver (another example of the optical receiver, a digital coherent receiver)  2 B includes a local oscillator light source  23 , a 90-degree hybrid circuit  24 , an OE  25 , an ADC  26 , and a signal processing circuit  27 . 
     The local oscillator light source  23  oscillates local oscillation light, and outputs the oscillated light. 
     The 90-degree hybrid circuit  24  combines the local oscillated light from the local oscillator light source  23  and the light signal that the optical transmitter Tx transmit through the transmission path  3 , and outputs two pairs of light signals having light phases having a phase shift of 90 degrees. 
     The OE (optical/electric converting circuit)  25  converts the two pairs of light signals from the 90-degree hybrid circuit  24  into electric signals. 
     The ADC (analog/digital converting circuit)  26  converts the electric signals from the OE  25  into digital signals. 
     The signal processing circuit  27  carries out the digital signal processing on the two pairs of digital signals from the ADC  26 , and has functions of a waveform distortion compensating circuit  271 , a carrier-wave phase synchronizing circuit  272 , a decision circuit  273 , and the signal quality monitor  274 . 
     The waveform distortion compensating circuit  271  compensates for distortion in waveform of the digital signal from the ADC  26 . 
     The carrier-wave phase synchronizing circuit  272  has a function of compensating for a frequency difference (offset) and phase difference between the received optical signal and the local oscillator light-source (see, J. C. Rasmussen et al., “Digital Coherent Receive Technology for 100-Gps Optical Transport System”, FUJITSU, vol. 60, no. 5, p. 476-483, September, 2009); and demodulating a frequency. 
     The function of compensating for a frequency difference (offset) and phase difference between the received optical signal and the local oscillator light-source carries out compensation for the frequency difference (offset) and phase difference on the digital signal whose distortion in waveform is compensated by the waveform distortion compensating circuit  271 . There is a possibility of occurring a frequency offset in the range of a wavelength accuracy between the laser light source  11  in the optical transmitter Tx and the local oscillator light source  23  of the light receiver  2 B. An ordinary light source for wavelength multiplexing may have a frequency offset as large as several GHz at maximum. A large frequency offset makes it difficult to demodulate a received signal. Therefore, the function of compensating for frequency offset obtains an estimated value of a frequency offset, feeds back the estimated value based on which the oscillation frequency of the local oscillator light source  23  is fine-adjusted or compensated for through digital processing, so that the frequency offset is compensated for. 
     A phase difference between the laser light source  11  of the optical transmitter Tx and the local oscillator light source  23  of the optical receiver  2 B is left after the compensation for the frequency offset between the laser light source  11  and the local oscillator light source  23 . The function of estimating the phase of a carrier estimates the phase difference of the carrier wave and compensates for the phase difference in order to prepare for data decision. 
     The function of demodulating a frequency demodulates, when the pilot signal is superimposed through the above frequency modulation in the optical transmitter Tx, the pilot signal from the carrier wave. Consequently, the carrier-wave phase synchronizing circuit  272  can obtain an estimated value of the frequency offset through the function of compensating for a frequency offset, so that the offset value of the light-source frequency can demodulate a pilot signal superimposed through the frequency modulation by the optical transmitter Tx. 
     The decision circuit  273  carries out data decision on a received signal on the basis of the signal obtained by the carrier-wave phase synchronizing circuit  272 , and outputs the result of the decision as a data signal. 
     The signal quality monitor  274  monitors the quality of a signal obtained by the carrier-wave phase synchronizing circuit  272 . 
     The signal quality monitored by the signal quality monitor  274  is regarded as the above error signal, which is transmitted from the optical transmitter Rx ( 2 B) to the optical transmitter Tx through a reverse-direction channel, through the control channel of the optical communication system, regarding the error signal as a pilot signal, or through the use of the frequency modulation. In the optical transmitter Tx, which receives the signal quality from the optical receiver Rx ( 2 B) adjusts the rotation frequency of the phase rotating circuits  122 ,  122 X, and  122 Y such that the signal quality comes to the best and fine-adjusts the carrier-wave frequency within the electric band of the optical transmitter Tx. 
     In order to easily find the optimum carrier-wave frequency, it is effective to transmit a signal obtained by superimposing a dither onto a carrier-wave frequency by the carrier-wave frequency control circuit  16 . In this case, the dither superimposed to the carrier-wave frequency is monitored on the basis of the number of error corrections obtained by the decision/error-correcting circuit  22  or of the quality information obtained by the signal quality monitor  274 . Then, the carrier-wave frequency control circuit  16  adjusts the rotation frequency of the phase rotating circuits  122 ,  122 X, and  122 Y such that the amplitude of the dither monitored comes to be the minimum, and fine-adjusts the carrier-wave frequency within the electric band of the optical transmitter Tx. Concurrently, the amplitude of the monitored dither is regarded as the error signal, which is transmitted from the optical receiver Rx to the optical transmitter Tx. 
     Using of the optical transmitters  1 , 1 A through  1 C in combination with the optical receivers  2 A and  2 B, the carrier wave frequency can be precisely controlled at a high-speed such that the number of error corrections of a received signal is minimized, that the quality of a received signal is best, or that the dither amplitude is minimized. 
     (6) First Example of an Optical Transmitter Unit: 
     Hereinafter, a description will now be made in relation to examples (first through fifth example) of an optical communication unit that accomplishes high-density wavelength multiplexing using the optical transmitters  1 ,  1 A through  1 C having the above configuration. 
     To begin with, an optical transmitter unit according to the first example will now be described with reference to  FIGS. 12 and 13 .  FIG. 12  is a block diagram illustrating the configuration of an optical transmitter unit of the first example; and  FIG. 13  is a diagram illustrating a transmission spectrum of the optical transmitter unit of  FIG. 12 . 
     As illustrated in  FIG. 12 , the optical transmitter unit  100 A of the first example includes a number (n in  FIG. 12 , where n is a natural number) of optical transmitters Tx 1  through Tx n  and an optical coupler  101 . The optical transmitters Tx 1  through Tx n  are the same one of the optical transmitters  1 ,  1 A through  1 C illustrated in  FIGS. 1-8 . The optical transmitters Tx 1  through Tx n  in  FIG. 12  is the optical transmitter  1  illustrated in  FIG. 1 , and  FIG. 12  omits the DAC  13  and the carrier-wave frequency control circuit  16 . 
     The optical coupler  101  combines light signals from optical transmitters Tx 1  through Tx n  for wavelength multiplexing and outputs the wavelength multiplexed signal to the transmission path  3 . 
     The respective laser light sources  11  of the optical transmitters Tx 1  through Tx n  oscillate output light having the same frequency f c . 
     The respective carrier-wave frequency control circuits  16  of the optical transmitters Tx 1  through Tx n  fine-adjust the carrier-wave frequency of light signals output from the optical modulators  15 ,  15 A, or  15 C by controlling the cycles of the phase rotations that the phase rotating circuits  122 ,  122 X, or  122 Y apply. 
     The respective carrier-wave frequency control circuits  16  of the optical transmitters Tx 1  through Tx n  output different amounts Δf a1  through Δf an  of frequency control to the phase rotating circuits  122 ,  122 X, or  122 Y. As illustrated in  FIG. 13 , the center frequencies of the carrier waves output from the optical modulators  15 ,  15 A, or  15 C of the optical transmitters Tx 1  through Tx n  are adjusted to f C +Δf a1 , f C +Δf a2 , . . . , f C +Δf an  in the electric bandwidth. 
     According to the optical transmitter unit  100 A of the first example, use of the functions of the respective carrier-wave frequency control circuits  16  of the optical transmitters Tx 1  through Tx n  can precisely adjust the carrier-wave frequencies of the light signals output from the Tx 1  through Tx n  within the electric bandwidth at a high speed. Thereby, the intervals of wavelength multiplexing come to be highly dense to improve the using rate of the band of the transmission path (optical fiber)  3 , so that high-capacity transmission can be realized. 
     A single laser light source  11  may be shared by the optical transmitters Tx 1  through Tx n , which makes it possible to further simplify the configuration of the optical transmitter unit  100 A. 
     (7) Second Example of the Optical Transmitter Unit: 
     The optical transmitter unit according to a second example will now be described with reference to  FIGS. 14 through 16B .  FIG. 14  is a block diagram illustrating the configuration of an optical transmitter unit of the second example;  FIG. 15  is a diagram illustrating a transmission spectrum of the optical transmitter unit of  FIG. 14 ; and  FIGS. 16A and 16B  are diagrams explaining wavelength arrangement change (defragmentation) of the optical transmitter unit of  FIG. 14 . 
     As illustrated in  FIG. 14 , the optical transmitter unit  100 B of the second example includes a number (n in  FIG. 14 ) of optical transmitters Tx 1  through Tx n , and a variable-band optical coupler  102 . 
     The optical transmitters Tx 1  through Tx n  are the same one of the optical transmitters  1 ,  1 A through  1 C illustrated in  FIGS. 1-8 . 
     The variable-band optical coupler  102  combines light signals from the optical transmitters Tx 1  through Tx n  for wavelength multiplexing and outputs the wavelength multiplexed signal to the transmission path  3 , and can vary the band. 
     In the optical transmitter unit  100 B of the second example, the respective carrier-wave frequency control circuits  16  of optical transmitters Tx 1  through Tx n  control the cycles of the phase rotations that phase rotating circuit  122  applies, so that wavelength arrangement can be adjusted in accordance with the bit rate of the transmitting signal (binary data). 
     Accordingly, when signals having different bit rates are to be transmitted through a single optical fiber, the functions of the carrier-wave frequency control circuits  16  of the optical transmitters Tx 1  through Tx n  precisely fine-adjust the arrangement of wavelengths corresponding to the bit rates of optical path P 1  through P 6  (i.e., the transmitting signal from the optical transmitters Tx 1  through Tx n ) at high speed as illustrated in  FIG. 15 . Thereby, the intervals of wavelength multiplexing come to be highly dense to improve the using rate of the band of the transmission path (optical fiber)  3 , so that high-capacity transmission can be realized. 
     At that time, the carrier-wave frequency control circuits  16  of the optical transmitters Tx 1  through Tx n  may adjust the wavelength arrangement by using both the above fine adjustment function and a rough adjustment function that directly controls the frequency of the output light from the laser light source  11 . As detailed above, the rough adjustment is accomplished by adjusting the oscillation frequency by means of changing the oscillation frequency grid or controlling the temperature. Such a combination of the fine adjustment and rough adjustment can precisely adjust the carrier-wave frequency within a large width exceeding the electric band of the optical transmitters. 
     Here, after an optical network having high-dense wavelength arrangement as illustrated in  FIGS. 15 and 16A  are practically used for years, demands and requests of the respective optical paths may be changed. In this case, correction and change of the wavelength arrangement of the optical network can improve the using efficiency of wavelength resources. 
     In the optical transmitter unit  100 B of the second example can correct and change the wavelength arrangement of the optical network using the above fine and rough adjustment functions. If an optical path is moved and changed within at least the electric band, the use of the fine adjustment of the carrier-wave frequency control circuits  16  can precisely change the wavelength arrangement hitlessly (that is, without disconnection of the optical path) at a high speed. 
     For example, in an optical network being in a state of wavelength arrangement of optical paths P 1  through P 6  as illustrated in  FIG. 16A , the optical paths P 2  and P 3  come to be not required any longer while the optical path P 4  comes to require a broader band. In this case, as illustrated in  FIG. 16B , for example, after the bit rate of the optical path P 4  is changed, the optical paths P 4  through P 6  are moved by the above fine and rough adjustment functions, so that the optical paths P 1 , P 4  through P 6  are high-densely arranged. In addition, in the example of  FIG. 16B , new optical paths P 7  and P 8  are added in a frequency band subsequent to the optical path P 6  and high-densely arranged by the above fine and rough adjustment functions. 
     According to the optical transmitter unit  100 B of the second example, the fine and rough adjustment functions of the respective carrier-wave frequency control circuits  16  of the optical transmitters Tx 1  through Tx n  precisely change the wavelength arrangement at a high speed. Even if the demand and the requirement of each optical path changes after a long-term operation of the optical network, the wavelength arrangement of the optical network is corrected and changed, so that the using rate of wavelength resource can be improved. 
     (8) Third Example of an Optical Transmitter Unit: 
     A description will now be made in relation to an optical transmitter unit according to a third example with reference to  FIGS. 17 and 18 .  FIG. 17  is a block diagram schematically illustrating an optical transmitter unit of the third example; and  FIG. 18  is a diagram illustrating a transmitting spectrum of the optical transmitter unit of  FIG. 17 . 
     As illustrated in  FIG. 17 , an optical transmitter unit  100 C of the third example includes a number (n in  FIG. 17 ) of optical transmitters Tx 1  through Tx n  and an optical coupler  101 , similarly to the optical transmitter unit  100 A of the first example illustrated in  FIG. 12 . 
     The optical transmitters Tx 1  through Tx n  are the same one of the optical transmitters  1 ,  1 A through  1 C illustrated in  FIGS. 1-8 . The optical transmitters Tx 1  through Tx n  in  FIG. 17  is the optical transmitter  1  illustrated in  FIG. 1  likewise  FIG. 12 , and  FIG. 17  omits the DAC  13  and the carrier-wave frequency control circuit  16 . 
     The optical coupler  101  combines light signals from optical transmitters Tx 1  through Tx n  for wavelength multiplexing and outputs the wavelength multiplexed signal to the transmission path  3 . 
     The respective laser light sources  11  of the optical transmitters Tx 1  through Tx n  are different light source out of frequency synchronization with one another for each sub-channel or each sub-channel group. 
     The respective carrier-wave frequency control circuits  16  of the optical transmitters Tx 1  through Tx n  fine-adjust the carrier-wave frequency of light signals output from the optical modulators  15 ,  15 A, or  15 C by controlling the cycles of the phase rotations that the phase rotating circuits  122 ,  122 X, or  122 Y apply. 
     The respective carrier-wave frequency control circuits  16  of the optical transmitters Tx 1  through Tx n  output different amounts Δf a1  through Δf an  of frequency control to the phase rotating circuits  122 ,  122 X, or  122 Y. As illustrated in  FIG. 18 , the intervals of the center frequencies of the light signals output from the optical modulators  15 ,  15 A, or  15 C are adjusted to a value of constant times the symbol rate (N×symbol rate, where N is a natural number). 
     Accordingly, even if the oscillation frequency of the laser light sources  11  are out of synchronization in the optical transmitter unit  100 C, the use of the functions of the respective carrier-wave frequency control circuits  16  can adjust the intervals of the center frequencies of the light signals output from the optical transmitters Tx 1  through Tx n  to a value of constant times the symbol rate. Thereby, the frequencies of light signals output from the optical transmitters Tx 1  through Tx n  are synchronized, so that OFDM (Orthogonal Frequency Division Multiplexing) can be realized. 
     (9) Fourth Example of an Optical Transmitter Unit: 
     A description will now be made in relation to the optical transmitter unit according to a fourth example with reference to  FIGS. 19 and 20 .  FIG. 19  is a block diagram schematically illustrating the configuration of the optical transmitter unit of the fourth example; and  FIG. 20  is a diagram illustrating a transmitting spectrum of the optical transmitter unit of  FIG. 20 . 
     As illustrated in  FIG. 19 , the optical transmitter unit  100 D of the fourth example includes a number (n+1 in  FIG. 19 ) of Tx 1 , Tx 2 , . . . , Tx m , Tx m ′, . . . , and Tx n , an AWG  103 , and the coupler  104 . Here, the number “m” represents a natural number of n or less. 
     The optical transmitters Tx 2 , Tx 2 , . . . , Tx m , Tx m ′, . . . , Tx n  is the same of the optical transmitters  1 ,  1 A through  1 C illustrated in  FIGS. 1 through 8 . 
     The AWG (Arrayed Waveguide Gratings)  103  is an optical coupler having fixed filtering bands that includes n ports  1  through n and that combines light signals input through the ports  1  through n from the optical transmitters Tx 2  through Tx n  for wavelength multiplexing and outputs the wavelength multiplexed signal to the transmission path  3 . As illustrated in  FIG. 20 , filtering bands are fixedly allocated one to each of the ports  1  through n. 
     Here, the optical transmitters Tx m  and Tx m ′ transmit signals having bit rates lower than those of the remaining optical transmitters, and are connected to a port m of the AWG  103  via a coupler  104  that combines signals from the optical transmitters Tx m  and Tx m ′. 
     The optical transmitters Tx 1  through Tx m−1 , and Tx m+1  through Tx n  are connected to ports  1  through m−1, abd m+1 through n of the AWG  103 , respectively. 
     The carrier-wave frequency control circuits  16  of the optical transmitters Tx 1  through Tx m−1 , and Tx m+1  through Tx n  controls the cycles of the phase rotations that the phase rotating circuits  122 ,  122 X, or  122 Y apply, so that one of the filtering bands of the AWG  103  is shared by two or more of the optical transmitters. Specifically, in the optical transmitter unit  100 D of  FIG. 19 , the functions of the carrier-wave frequency control circuits  16  precisely adjust the carrier-wave frequencies of two optical transmitters Tx m  and Tx m ′ on a particular path input into the port m at a high speed. Thereby, the filtering band corresponding to the port m of the AWG  103  is shared by two optical transmitters Tx m  and Tx m ′, as illustrated in  FIG. 20 . 
     Accordingly, in the optical transmitter unit  100 D of the fourth example, the use of the AWG  103  makes it possible to combine two or more optical transmitters Tx m  and Tx m ′ having low bit rates by the coupler  104 , and the functions of the respective carrier-wave frequency control circuits  16  control the carrier-wave frequencies of the optical transmitters Tx 1  through Tx n . Thereby, one of filtering bands of the AWG  103  is shared by such two or more optical transmitters Tx m  and Tx m ′ having low bit rates, so that the respective filtering band of the AWG  103  can be efficiently used. 
     The example of  FIGS. 19 and 20  assume that one of the filtering bands of the AWG  103  is shared by two optical transmitters Tx m  and Tx m ′. However, the number of optical transmitters should by no means be limited to two. Alternatively, the carrier-wave frequencies of the optical transmitters Tx 1  through Tx n  may be controlled by the functions of the carrier-wave frequency control circuits  16  such that one of the filtering bands is shared by three or more optical transmitters. 
     (10) Fifth Example of an Optical Transmitter Unit: 
     Description will now be made in relation to an optical transmitter unit according to a fifth example, and an optical receiver unit corresponding to the optical transmitter unit with reference to  FIGS. 21 and 22 .  FIG. 21  is a block diagram illustrating the configurations of the optical transmitter unit of the fifth example and the corresponding optical receiver unit, and  FIG. 22  is a diagram illustrating the transmission spectrum of the optical transmitter unit of  FIG. 21 . 
     As illustrated in  FIG. 21 , an optical transmitter unit  100 E of the fifth example includes a number (n in  FIG. 21 ) of optical transmitters Tx 1  through Tx n , an AWG  103 , and couplers  105  and  106 . 
     The optical receiver unit  200  corresponding to the optical transmitter unit  100 E is connected to the optical transmitter unit  100 E via the transmission path (optical fiber)  3 , and includes a number (n in  FIG. 21 ) optical receivers Rx 1  through R n , an optical branching filter  201 , and couplers  202  and  203 . 
     The optical transmitters Tx 1  through Tx n  are the same one of the optical transmitters  1 ,  1 A through  1 C of  FIGS. 1 through 8 . 
     The AWG  103  is an optical coupler having fixed filtering bands that includes n ports  1  through n and that combines light signals input via the couplers  105  and  106  from the optical transmitters Tx 1  through Tx n  for wavelength multiplexing and outputs the wavelength multiplexed signal to the transmission path  3 . As illustrated in  FIG. 22 , filtering bands are fixedly allocated one to each of the ports  1  through n. 
     The modulation scheme of the modulating scheme mapping circuits  121  of the optical transmitters Tx 1  through Tx n  in the optical transmitter unit  100 E adopt an Orthogonal Frequency Division Multiplexing (OFDM) scheme (see  FIG. 22 ). 
     If a light signal from one of the optical transmitters Tx 1  through Tx n  (in the example of  FIG. 22 , the optical transmitters Tx 2  and Tx 5 ) extends two filtering band of the AWG  103 , the carrier-wave frequency control circuits  16  of the optical transmitters Tx 2  and Tx 5  conduct the following control. 
     Specifically, the carrier-wave frequency control circuits  16  control the cycles of the phase rotations that the phase rotating circuits  122 ,  122 X, or  122 Y provide, and thereby control the carrier-wave frequencies of the light signals output from the optical modulators  15 ,  15 A or  15 C of the optical transmitters Tx 2  and Tx 5  such that the particular sub-carrier of the light signals from the optical transmitters Tx 2  and Tx 5  position at the guard band between the two filtering bands as illustrated in  FIG. 22 . At that time, the optical transmitters Tx 2  and Tx 5  prohibit use of the sub-carrier disposed at the guard bands. 
     The optical receivers Rx 1  through Rx n  in the optical receiver unit  200  is, for example, the same one of the optical receivers  2 A and  2 B of  FIGS. 10 and 11 . 
     The optical branching filter  201  receives a wavelength multiplexed light signal from the optical transmitter unit  100 E through the transmission path  3 , separates the received wavelength multiplexed light signal into n signals, and outputs the n signals one from each of the ports  1  through n to the optical receivers Rx 1  through Rx n  via the couplers  202  and  203 . 
     The coupler  202  corresponds to the coupler  106  of the transmitter end, and divides a signal from the port  1  of the optical branching filter  201  into two signals, one of which is output to the optical receiver Rx 1 . The coupler  203  corresponds to the coupler  105  of the transmitter end, combines the other signal from the coupler  202 , the signal from the port  2  of the branching filter  201 , and outputs the combined signal to the optical receiver Rx 2 . 
     According to the optical transmitter unit  100 E of the fifth example, the optical transmitters Tx 1  through Tx n  each carry out high-dense multiplexing through an OFDM scheme regardless of the filtering bandwidth of the AWG  103 , and precisely control the carrier-wave frequencies of the respective channels (optical transmitters Tx 1  through Tx n ) at a high speed such that only particular sub-carrier of each particular channel is disposed at the position of the guard band. Then, use of the particular sub-band positioning at the guard band is prohibited. Thereby, it is possible to surely avoid vain transmission processing through the sub-carrier of the guard band, so that transmission efficiency by OFDM in the optical network can be further enhanced. 
     (11) Others; 
     The preferable embodiments of the present invention are described as above. The present invention should by no means be limited to the foregoing embodiments, and various changes and modifications can be suggested without departing from the gist of the present invention. 
     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 embodiment has 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.