Abstract:
The present invention relates to a coherent optical communication apparatus and method. According to the invention, the optical communication apparatus receives a modulated optical signal, which is generated by modulating an optical signal with a first electrical signal obtained by adding a second electrical signal carrying information to be transmitted and a reference electrical signal, and converts the modulated optical signal to a third electrical signal by coherent detection. Then the apparatus detects an amount of fluctuation of the reference electrical signal included in the third electrical signal, and compensates the second electrical signal included in the third electrical signal using the amount of fluctuation.

Description:
PRIORITY CLAIM 
     This application claims priority from Japanese patent application No. 2007-076428 filed on Mar. 23, 2007, which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a coherent optical communication apparatus and method. 
     2. Description of the Related Art 
     A coherent optical communication system is a system, in which a continuous wave signal from a laser diode is modulated with an electrical signal carrying information to be transmitted using amplitude modulation, frequency modulation and/or phase modulation at the transmitting side. The modulated optical signal from the transmitting side and a local oscillator signal are coupled for optical detection at the receiving side. In case the frequency of the local oscillator signal is the same as the optical carrier of the modulated optical signal, it is called homodyne detection, or in some cases intradyne detection. In case the frequency of the local oscillator signal is different from the one of the carrier component, it is called heterodyne detection. 
     In case of heterodyne detection, a detector outputs an electrical IF (Intermediate Frequency) signal. The IF is equal to the frequency difference between the carrier of the modulated optical signal and the local oscillator signal. In case of homodyne/intradyne detection, the detector directly outputs an electrical baseband signal. In coherent optical communication systems, it is required to synchronize the frequency of the local oscillator signal with the modulated optical signal to be received for a correct demodulation process at the receiving side. For this purpose, Christoph Wree et al, “Measured Noise Performance for Heterodyne Detection of 10-Gb/s OOK and DPSK”, IEEE PHOTONICS TECHNOLOGY LETTERS Vol. 19, No. 1, pp. 15-17, January 2007, discloses a configuration that has an AFC (Automatic Frequency Control) loop for controlling the frequency of the local oscillator signal. 
     However, it requires a high-performance AFC loop to realize the frequency control of the local oscillator signal, and therefore it makes the optical communication apparatus expensive. To solve the above mentioned problem, Satoshi Tsukamoto et al., “Coherent Demodulation of Optical Multilevel Phase-Shift-Keying Signals Using Homodyne Detection and Digital Signal Processing”, IEEE PHOTONICS TECHNOLOGY LETTERS Vol. 18, No. 10, pp. 1131-1133, May 2006, discloses a demodulation configuration without using a high-performance AFC loop or an optical PLL (Phase Lock Loop). The above document relates to a coherent optical communication system using M-ary DPSK (Differential Phase Shift Keying) modulation, where M equals to power of two. According to the above document, each symbol of an electrical signal obtained by the coherent optical detection is raised to the M-th power, the phase error is estimated using an average of several successive symbols, and then the signal is demodulated using the estimated phase error. 
     However, the average value needs to be divided by M to compensate the calculation of the M-th power. Therefore, if the phase error is not within a range of −π/M to +π/M, it is not possible to compensate it correctly. Further it can be applied only to the optical communication system, which uses M-ary DPSK modulation. 
     SUMMARY OF THE INVENTION 
     The invention has been made in view of the above-mentioned problem, and it is therefore an object of the present invention to provide a coherent optical communication apparatus and method, which require neither an AFC loop nor an optical PPL, can be used with any modulation technique, and has no phase error range limitation. 
     According to the present invention, an optical communication apparatus receives a modulated optical signal, which is generated by modulating an optical signal with a first electrical signal, where the first electrical signal is obtained by adding a second electrical signal carrying information to be transmitted and a reference electrical signal that has a predetermined frequency. The predetermine frequency includes 0 Hz. The optical communication apparatus has an optical signal generator for generating a local oscillator signal, a hybrid for coupling the modulated optical signal with the local oscillator signal, an optical electrical converter for converting an output signal from the coupler to a third electrical signal and a compensator. The compensator detects an amount of fluctuation of the reference electrical signal included in the third electrical signal, and compensates the second electrical signal included in the third electrical signal using the amount of fluctuation. 
     According to the present invention, an optical communication method includes the steps of generating a modulated optical signal by modulating an optical signal with a first electrical signal at a transmitting side, where the first electrical signal is obtained by adding an a second electrical signal carrying information to be transmitted and a reference electrical signal with predetermined frequency. At a receiving side, the method includes the steps of generating a third electrical signal by optical electrical conversion of an optical signal obtained by coupling the modulated optical signal and a local oscillator signal, detecting an amount of fluctuation of the reference electrical signal included in the third electrical signal, and compensating the second electrical signal included in the third electrical signal using the amount of fluctuation. 
     Advantageously the amount of fluctuation is based on an amplitude value and/or a phase value of the reference electrical signal. Favorably the compensation is performed by multiplying the second electrical signal by a signal, of which complex expression is based on an inverse value of the amount of fluctuation. 
     According to the invention, the relative fluctuation between the local oscillator signal and the modulated optical signal is detected at the receiving side using the reference electrical signal inserted at the transmitting side, and the fluctuation is compensated in electrical domain. Therefore, it is possible to demodulate the modulated optical signal correctly without controlling the local oscillator signal by the modulated optical signal. The invention has no restriction on the modulation format and compensation range as well as the electrical signal that carries information. Further it is possible to compensate the fluctuation caused by noise components included in the local oscillator signal as well. 
     Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a block diagram of a transmitting side of an optical communication apparatus according to the invention; 
         FIG. 2  is a schematic view of a signal spectrum obtained by adding a reference signal to an electrical signal; 
         FIG. 3  shows a block diagram of a receiving side of the optical communication apparatus according to the invention; 
         FIG. 4  shows a block diagram of a compensator according to the invention; 
         FIG. 5  is a schematic view of a signal spectrum obtained by adding a reference signal to an electrical signal according to another embodiment of the invention; and 
         FIGS. 6A and 6B  are explanation drawings of a method according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a block diagram of a transmitting side of an optical communication apparatus according to the invention. According to  FIG. 1 , the optical communication apparatus includes an adder  11 , a reference signal generator  12 , an optical signal generator  13 , an optical modulator  14  and an optical filter  15 . 
     The reference signal generator  12  generates a reference signal  50 , which is a sinusoidal signal, and the adder  11  adds the reference signal  50  and an electrical signal  30 , which carries information. The electrical signal  30  is, for example, a pulse signal, a sub-carriers multiplexed signal used in SCM (Sub-Carrier Multiplexing) system, a modulated signal of a single carrier converted to RF (Radio Frequency) band or an OFDM (Orthogonal Frequency Division Multiplexing) signal, and the frequency of the reference signal  50  is chosen in such a way that the reference signal  50  does not interfere with the electrical signal  30 , for example the reference signal  50  is placed outside of the electrical signal  30 . Favorably the frequency of the reference signal  50  is chosen as close as possible to the electrical signal  30  to the extent that the reference signal  50  does not interfere with the electrical signal  30 . However the DC signal can be used as the reference signal  50  as well. The reference signal  50  can be generated independently from the electrical signal  30 , i.e. the reference signal  50  is not required to synchronize with the electrical signal  30 .  FIG. 2  is a schematic view of a spectrum of the signal output by the adder  11 . 
     The optical signal generator  13  is, for example, a distributed feedback laser diode, and generates a CW (continuous wave signal)  40  with a predetermined frequency. The optical modulator  14  is, for example, a Mach-Zehnder modulator, modulates the CW  40  from the optical generator  13  with the electrical signal  35  output by the adder  11  using for example amplitude modulation technique, and outputs the modulated optical signal. 
     The optical filter  15  is provided, in case the SSB (Single Side Band) is used, and outputs a modulated optical signal, which one side band or a carrier component/one sideband is suppressed, to an optical link. Off course it is possible to generate a SSB optical signal directly by the optical modulator  14 . 
       FIG. 3  shows a block diagram of a receiving side of the optical communication apparatus according to the invention. According to  FIG. 3 , the optical communication apparatus includes an optical signal generator  21 , an optical hybrid  22 , an optical electrical converter  23 , a compensator  24  and a demodulator  25 . 
     The optical signal generator  21  is, for example, a distributed feedback laser diode, and generates a CW  41  having a predetermined frequency, which is different from the one of the CW  40 . The CW  41  is normally referred as a local oscillator signal  41 . The optical hybrid  22  couples the modulated optical signal from a remote optical communication apparatus with the local oscillator signal  41 . The optical electrical converter  23  is, for example, a photo diode, and converts an optical signal from the optical coupler  22  to a detected electrical signal  31 , which is a IF signal. That is, the optical communication apparatus according to the embodiment uses a heterodyne detection. 
     According to the invention, the CW  41  is not controlled by the modulated optical signal from the remote optical communication apparatus. That is, the optical signal generator  21  is not synchronized with the optical signal generator  13  of the remote optical communication apparatus, and the frequency difference between the CW  40  and the CW  41  vary, not constant. Further, the CWs  40  and  41  have noise components. As a result, the detected electrical signal  31  from the optical electrical converter  23  has fluctuation/variation caused by the frequency difference variation and noise components of the CWs  40  and  41 . The compensator  24  detects the fluctuation/variation, removes the fluctuation/variation, i.e. compensates the detected electrical signal  31 , and outputs the compensated electrical signal to the demodulator  25 . 
       FIG. 4  shows a block diagram of the compensator  24 . According to  FIG. 4 , the compensator  24  includes a splitter  241 , an extracting unit  242 , a fluctuation detector  243  and a fluctuation compensator  244 . 
     The splitter  241  splits the detected electrical signal  31  from the optical electrical converter  23 , and the extracting unit  242  extracts the reference signal  50  from the detected electrical signal  31 . The fluctuation detector  243  detects the fluctuation of the reference signal  50 , for example, by comparing a sinusoidal signal generated by an internal electrical oscillator in the fluctuation detector  243  with the extracted reference signal  50 , and outputs a signal indicating an amount of fluctuation. Since fluctuation of the electrical oscillator is normally too small compared to fluctuation between the optical signal generators such as laser diodes, the signal generated by the internal electrical oscillator is not required to synchronize with the receiving signal. 
     The fluctuation compensator  244  compensates frequency fluctuation of the detected electrical signal  31  based on the signal indicating the amount of fluctuation output by the fluctuation detector  243 . As a result, a signal output by the fluctuation compensator  244  has less fluctuation. That is, the signal output by the fluctuation compensator  244  equivalents to a signal obtained by using an AFC or an optical PPL as the prior art does, and it is possible to use conventional demodulators for the demodulator  25 . In another embodiment, the fluctuation compensator  244  compensates the electrical signal  30  included in the detected electrical signal  31 , which is obtained by filtering out the reference signal  50  from the detected electrical signal  31 , using the amount of the fluctuation. 
     In the embodiment, the fluctuation compensation is preformed in IF band, however, the invention is not limited to the embodiment, and it is possible to perform the fluctuation compensation against a baseband signal after frequency conversion of the detected electrical signal  31  in IF band. Further, the invention is not limited to the heterodyne detection. As the person in the art can easily understand, the invention can be applied to homodyne/intradyne detection as well. In this case, the fluctuation compensation is also preformed against a baseband signal. 
     Preferably, the detected electrical signal  31  is converted to a digital signal by an analog digital converter, and the compensator  24  and the demodulator  25  are realized in digital domain, for example, by use of a DSP (Digital Signal Processor). 
       FIGS. 6A and 6B  are explanation drawings of a method according to the invention.  FIG. 6A  shows plots on the complex plane of the reference signal  50  observed at the receiving side using the internal electrical oscillator of the receiving side as a reference.  FIG. 6B  is an enlarged view of one portion of the  FIG. 6A . If the CWs  40  and  41  are synchronized, and have no phase noise, all plots are located on the same one point, for example a point  60  in  FIG. 6A . However, the CW  41  is not synchronized with the CW  40  according to the invention. As a result, phase change occurs, and plots are distributed around a circle. Further, due to frequency difference fluctuation between the CWs  40  and  41  as well as the phase noise of the CWs  40  and  41 , the phase and amplitude of the reference signal  50  observed in the receiving side is irregularly changing. 
     For example, the fluctuation detector  243  periodically performs discrete Fourier transform of the reference signal  50 , calculates an amplitude and a phase of the reference signal  50  in each period, and outputs the amplitude α and the phase θ as the amount of fluctuation to the fluctuation compensator  244 . More specifically, if the amplitude of the in-phase component of the reference signal  50  is A, and the amplitude of the quadrature component of the reference signal  50  is B in a period, then the amount of fluctuation of the period is A+jB, and A+jB is notified to the fluctuation compensator  244 . Here, α 2 =A 2 +B 2 , and tan θ=B/A. Also it is possible to use only the phase θ for the amount of fluctuation. 
     The fluctuation compensator  244  compensates the detected electrical signal  31  in a period using a value, which is an inverse of the amount of fluctuation in the same period informed by the fluctuation detector  243 . More precisely, if A+jB is informed from the fluctuation detector  243  in a period, the fluctuation compensator  244  multiply a signal, which complex expression is A/(A 2 +B 2 )−jB/(A 2 +B 2 ), i.e. inverse of A+jB, by the detected electrical signal  31  in the same period. It is also possible to modify the inverse value, for example based on the frequency difference between the reference signal  50  and the electrical signal  30 . That is, a value based on the inverse of the amount of fluctuation can be used for compensation. 
     By the processes described above, it is possible to remove fluctuation of the detected electrical signal  31  caused by the local oscillator signal  41 , which have phase noise and is generated asynchronously from the modulated optical signal. A sampling clock signal used to digitize the detected electrical signal  31  is also generated independently from the modulated optical signal, because fluctuation of the electrical oscillator, which generates the sampling clock, is normally too small compared to the one of the optical signal generators. 
     In case the electrical signal  30  is an OFDM signal, one sub-carrier in the OFDM signal can be used as the reference signal  50 , which is used to detect relative fluctuation between the CW  40  and the CW  41 .  FIG. 5  is a schematic view of a signal spectrum output by the adder  11 , in case the electrical signal  30  is an OFDM signal of RF band. Here, the center sub-carrier of the OFDM signal is used as the reference signal  50 . In other word, a DC position of the baseband OFDM signal is used as the reference signal  50 . In this case, the reference signal  50  can be added to the OFDM signal  30  by applying a DC offset to the baseband OFDM signal  30 . It is also possible to add the reference signal  50  to the OFDM signal in inverse discrete Fourier transform operation. 
     Further, if the electrical signal  30  includes a plurality of sub-carriers, such as SCM or OFDM, the fluctuation compensator  244  preferably modifies the inverse value of the amount of fluctuation for each sub-carrier based on the frequency position of each sub-carrier. Further, if the electrical signal  30  is a SCM signal, it is preferable to add the reference signal  50  for each sub-carrier. 
     Many modifications and variations will be apparent those of ordinary skilled in the art. The embodiments was chosen and described in order to best explain the principles of the invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.