Abstract:
The present invention provides a light receiving apparatus using the DQPSK demodulation method. The light receiving apparatus comprises: one Mach-Zehnder interferometer for branching a received light signal into light signals at two arms to allow the branched two light signals to interfere with each other; one balanced photoelectric converter for converting the two interfered light signals, by using the Mach-Zehnder interferometer, into an electric signal corresponding to a difference between light intensities of the two light signals; and a phase adjuster for dynamically shifting the phase of a light signal passed through one of the two arms at the Mach-Zehnder interferometer.

Description:
[0001]    This application is based upon and claims the benefit of priority from Japanese patent application No. 2006-345649, filed on Dec. 22, 2006, the disclosure of which is incorporated herein in its entirety by reference. 
       BACKGROUND OF THE INVENTION  
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a light receiving apparatus using a DQPSK (Differential Quadrature Phase-Shift Keying) demodulation method, and a DQPSK demodulation method, and more particularly to a light receiving apparatus using a DQPSK demodulation method and a DQPSK demodulation method, which are adapted for compactly realizing, at a low cost, an apparatus or a system using a DQPSK modulation scheme. 
         [0004]    2. Description of the Related Art 
         [0005]    The DQPSK modulation/demodulation method which is one of the modulation/demodulation methods in optical communication is a method excellent in utilization efficiency of wavelength, wavelength dispersion tolerance and polarization dispersion tolerance. For this reason, as described in Non-Patent Document 1, it is expected that the DQPSK modulation/demodulation method will be applied to high speed WDM (Wavelength Division Multiplexing) optical communication systems for which these merits are required. The configurations of transmitting/receiving apparatuses of such optical communication systems are shown in  FIGS. 1 and 3  of Patent Document 1 and in  FIG. 1  of Non-Patent Document 1. 
         [0006]    [Patent Document 1] JP-A-2004-516743 ( FIG. 1  and  FIG. 3 ) 
         [0007]    [Non-Patent Document 1] R. A. Griffin, et al., Optical Differential Quadrature Phase-Shift Key (oDQPSK) for High Capacity Optical Transmission, Optical Fiber Communication Conference and Exhibit, the United States, March 2002, WX6, pp. 367-368 
         [0008]    Here, the configuration and the operation of a typical light receiving apparatus using the DQPSK demodulation method will be described with reference to  FIGS. 1 to 4 . 
         [0009]      FIG. 1  is a block diagram showing a configuration example of a typical light receiving apparatus (demodulating apparatus)  220  using a DQPSK demodulation method. 
         [0010]    Light receiving apparatus  220  shown in  FIG. 1  comprises beam splitter  228 , delay interferometers  230 ,  232 , and balanced photoelectric converters  234 ,  236 . 
         [0011]    One of two light signals branched by beam splitter  228  is inputted to delay interferometer  230 . Moreover, a light transmission path is formed within one arm  230   a  of delay interferometer  230  in order to create relative delay time τ with respect to the other arm  230   b.    
         [0012]    The other of the two light signals branched by beam splitter  228  is inputted to delay interferometer  232 . Moreover, a light transmission path is formed within one arm  232   a  of delay interferometer  232  in order to create relative delay time τ with respect to the other arm  232   b.    
         [0013]    Delay time τ is realized by allowing optical path lengths of the two arms to be different from each other, i.e., by allowing the lengths of arms  230   a ,  232   a  to be respectively physically longer than lengths of arms  230   b ,  232   b.    
         [0014]    Further, electrodes are provided at shorter arms  230   b ,  232   b  of the delay interferometers  230 ,  232 . Delay interferometers  230 ,  232  are configured so as to apply a voltage of a reasonable value to each electrode so that phase-shifts of π/4 and −π/4 are fixedly given to a light signal. 
         [0015]      FIG. 2  is an explanatory view showing an example of light signal E(t)  214  of typical DQPSK shown in  FIG. 1 .  FIG. 3  is a diagram for explaining the phase state of light signal E(t)  214  of typical DQPSK shown in  FIG. 2 . 
         [0016]    As shown in  FIGS. 2 and 3 , phase θ of light signal E(t)  214  results in one from among {π/4, 3π3/4, 5π/4, 7π/4} every symbol. An original signal of light signal E(t)  214  is encoded, and is mapped with respect to phase difference Δθ between the original signal and a signal preceding by one symbol. Δθ indicates either one of quaternary, i.e., four-value of {0, π/2, π, 3π/2}. Light signal E(t)  214  includes quaternary, i.e., 2 bits data every symbol. For this reason, in light signal E(t)  214 , in the case where the transmission capacity is B [bits/s], symbol speed becomes equal to B/2 [Symbol/s], and the symbol interval becomes equal to τ=2/B sec. 
         [0017]    The operation of light receiving apparatus  220  shown in  FIG. 1  will now be described. 
         [0018]    Received light signal E(t)  214  is branched into two light signals by beam splitter  228 . One of the branched two light signals is inputted to delay interferometer  230 , and the other light signal is inputted to delay interferometer  232 . 
         [0019]    Respective delay interferometers  230 ,  232  serve to further branch inputted light signals into two light signals. Furthermore, each of delay interferometers  230 ,  232  serves to delay the light signal of one side by τ thereafter to allow it to interfere with the light signal of the other side. Namely, each of delay interferometers  230 ,  232  allows light signals to interfere with a light signal preceding by one symbol to extract phase difference Δθ. 
         [0020]    At this time, delay interferometer  230  allow a light signal of longer arm  230   a  to interfere with a light signal obtained by shifting the phase of a light signal of shorter arm  230   b  by π/4. For this reason, light intensities I 1 , I 2  of two light output signals E 1 ( t ), E 2 ( t ) outputted from delay interferometer  230  are expressed as formula 1. 
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         [0021]    Balanced photoelectric converter  234  squares and detects two light output signals E 1 ( t ), E 2 ( t ) to convert the detected light output signals into electric signal x 1 ( t ) as shown in formula 2 in accordance with the difference between light intensities I 1 , I 2  of light output signals E 1 ( t ), E 2 ( t ). 
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         [0022]    Similarly, delay interferometer  232  allows a light signal of longer arm  232   a  to interfere with a light signal obtained by shifting the phase of a light signal of shorter arm  232   b  by −π/4. For this reason, light intensities I 3 , I 4  of light output signals E 3 ( t ), E 4 ( t ) outputted from delay interferometer  232  are expressed as formula 3. 
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         [0023]    Balanced photoelectric converter  236  squares and detects two light output signals E 3 ( t ), E 4 ( t ) to convert detected light output signals into electric signal y 2 ( t ) as shown in formula 4 in accordance with the difference between light intensities I 3 , I 4  of light output signal E 3 ( t ), E 4 ( t ). An output example of balanced photoelectric converters  234 ,  236  is shown in  FIG. 4 . 
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         [0024]    It is to be noted that, on the light transmitting apparatus side, an original electric signal is encoded so that electric signals x 1 ( t ) and y 2 ( t ) obtained above at receiving apparatus result in original electric binary signals before encoding at transmitting apparatus, and is mapped to phase difference Δθ of transmitted optical signal E(t)  214 . 
         [0025]    However, in the light receiving apparatus using the DQPSK demodulation method shown in  FIG. 1 , there are problems as described below. 
         [0026]    The first problem is that the apparatus configuration is not suitable for downsizing. The reason thereof is that two sets of optical modules (optical components) such as a delay interferometer and a balanced photoelectric converter are required for DQPSK demodulation. 
         [0027]    The second problem is that the apparatus configuration is not suitable for realization of a low-cost apparatus. The reason thereof is that since two sets of optical modules are required as stated above and that since these optical modules are constructed so that optical components are assembled, the realization of low cost based on mass-production, as in semiconductor integrated circuit production, cannot be expected in the manufacturing process for the light receiving apparatus. 
       SUMMARY OF THE INVENTION  
       [0028]    In view of the above, an object of the present invention is to provide a light receiving apparatus using a DQPSK demodulation method and a DQPSK demodulation method, which are adapted for realization of downsizing and low cost of the apparatus. 
         [0029]    A first light receiving apparatus of the present invention using a DQPSK demodulation method, comprises: 
         [0030]    one Mach-Zehnder interferometer for branching a received light signal into light signals at two arms to allow the branched two light signals to interfere with each other; 
         [0031]    one balanced photoelectric converter for converting the two interfered light signals, by using the Mach-Zehnder interferometer, into an electric signal corresponding to the difference between light intensities of the two light signals; and 
         [0032]    a phase adjuster for dynamically shifting the phase of the light signal passed through one of the two arms at the Mach-Zehnder interferometer. 
         [0033]    A second light receiving apparatus using a DQPSK demodulation method, comprises: 
         [0034]    one Mach-Zehnder interferometer for branching a received light signal into light signals at two arms to allow the branched two light signals to interfere with each other; 
         [0035]    one balanced photoelectric converter for converting the two interfered light signals, by using the Mach-Zehnder interferometer, into an electric signal corresponding to a difference between light intensities of the two light signals; 
         [0036]    a clock extractor for extracting a clock signal of a symbol period from the electric signal outputted from the balanced photoelectric converter; and 
         [0037]    a wiring for delivering, to the Mach-Zehnder interferometer, an electric signal based on the clock signal which has been extracted by the clock extractor in order to shift the phase of a light signal passed through one of the two arms at the Mach-Zehnder interferometer. 
         [0038]    A first DQPSK demodulation method of the present invention for demodulating a DQPSK-modulated light signal, comprises: 
         [0039]    branching a received light signal into two light signals; 
         [0040]    shifting the phase of one of the branched two light signals thereafter to allow the two light signals to interfere with each other; and 
         [0041]    converting the two interfered light signals into an electric signal corresponding to a difference between light intensities of the two light signals. 
         [0042]    A second DQPSK demodulation method of the present invention for demodulating a DQPSK-modulated light signal, comprises: 
         [0043]    branching a received light signal into two light signals by a delay interferometer; 
         [0044]    allowing the branched two light signals to interfere with each other; 
         [0045]    converting the two interfered light signals into an electric signal corresponding to a difference between light intensities of the two light signals; 
         [0046]    extracting a clock signal of a symbol period of a light signal from the converted electric signal; 
         [0047]    delivering the extracted clock signal to the delay interferometer; and 
         [0048]    allowing phase-shifts of π/4 and −π/4 to periodically take place at one of the two light signals by the delay interferometer in accordance with the clock signal. 
         [0049]    The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0050]      FIG. 1  is a block diagram showing a configuration example of a typical light receiving apparatus using a DQPSK demodulation method; 
           [0051]      FIG. 2  is a diagram for explaining an example of a light signal of a typical DQPSK shown in  FIG. 1 ; 
           [0052]      FIG. 3  is a diagram for explaining a phase state of a light signal of a typical DQPSK shown in  FIG. 2 ; 
           [0053]      FIG. 4  is a diagram for explaining an output example of the balanced photoelectric converter shown in  FIG. 1 ; 
           [0054]      FIG. 5  is a block diagram showing the configuration of a light receiving apparatus using the DQPSK demodulation method of an exemplary embodiment of the present invention; 
           [0055]      FIG. 6  is a diagram for explaining the operation of the light receiving apparatus shown in  FIG. 5 ; and 
           [0056]      FIG. 7  is a block diagram showing the configuration of a practical example of the light receiving apparatus shown in  FIG. 5 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0057]    An Exemplary embodiment of the present invention will now be described in detail with reference to the attached drawings. 
         [0058]      FIG. 5  is a block diagram showing the configuration of a light receiving apparatus using a DQPSK demodulation method (hereinafter referred to as DQPSK receiving apparatus) of the exemplary embodiment of the present invention. 
         [0059]    The DQPSK receiving apparatus shown in  FIG. 5  comprises one delay interferometer  130 , and one balanced photoelectric converter  134 . 
         [0060]    At shorter arm  130   b  in delay interferometer  130 , phase modulation of light signal E(t)  214  is performed on the basis of inputted electric signal Vc(t). Description will be made on the assumption that phase adjuster  131  is provided at shorter arm  130   b.    
         [0061]    Moreover, in delay interferometer  130 , a light transmission path is formed within one arm  130   a  in order to create delay time τ with respect to the other arm  130   b . Delay time τ is equal to a symbol interval of light signal E(t)  214 . 
         [0062]    The operation of the DQPSK receiving apparatus shown in  FIG. 5  will now be described with reference to  FIG. 6 . 
         [0063]    Received light signal E(t)  214  is inputted to delay interferometer  130 . 
         [0064]    Delay interferometer  130  serves to branch inputted light signal E(t)  214  into two light signals at arms  130   a ,  130   b . The light signal of longer arm  130   a  is delayed by τ, and then interferes with the light signal of shorter arm  130   b.    
         [0065]    Moreover, suitable voltage Vc(t) (a voltage with an amplitude to permit the shift of phase of light signal E(t)  214 ) is applied to an electrode provided at phase adjuster  131  of shorter arm  130   b . Voltage Vc(t) is applied to the electrode in synchronism at the time when the symbol at light signal E(t)  214  is switched. Here, Vc(t) is assumed to be V 1  or V 2 . As stated above, at phase adjuster  131 , since voltage Vc(t) of V 1  or V 2  is periodically applied to the electrode, the phase of light signal E(t)  214  is shifted by −π/4 or π/4. 
         [0066]    As shown in  FIG. 6 , voltage Vc(t) is switched from V 1  to V 2 , or from V 2  to V 1  at period τ/2 which is one half of symbol interval τ of light signal E(t)  214 . As a result of the fact that voltage Vc(t) is switched, shift quantity φ(t) of the phase at phase adjuster  131  is also alternately switched to values of π/4 and −π/4 at a period of τ/2. 
         [0067]    In the state where shift quantity φ(t) of the phase at phase adjuster  131  is π/4, signal components of light output signals E 1 ( t ), E 2 ( t ) of delay interferometer  230  shown in  FIG. 1  are obtained as interferometer output signals (light output signals) E 5 ( t ), E 6 ( t ). On the other hand, in the state where shift quantity φ(t) of the phase at phase adjuster  131  is −π/4, signal components of light output signals E 3 ( t ), E 4 ( t ) of delay interferometer  232  shown in  FIG. 1  are obtained as interferometer output signals (light output signals) E 5 ( t ), E 6 ( t ). 
         [0068]    Accordingly, as shown in  FIG. 6 , electric signal x 1 ( t ) and electric signal y 2 ( t ) which are shown in  FIG. 4  are alternately obtained as electric signal z(t) which is an output of balanced photoelectric converter  134 . 
         [0069]    In the present embodiment, in DQPSK demodulation, only one delay interferometer  130  and only one balanced photoelectric converter  134  are used. Accordingly, the number of optical components can be reduced to one-half as compared to the apparatus shown in  FIG. 1 . Thus, the apparatus can be downsized as compared to the apparatus shown in  FIG. 1 . In addition, since the number of optical components, whose costs are difficult to reduce, are decreased, the cost can be suppressed as compared to the apparatus shown in  FIG. 1 . 
       PRACTICAL EXAMPLE S 
       [0070]    A practical example of the present invention will now be described with reference to the attached drawing. 
         [0071]      FIG. 7  is a block diagram showing the configuration of a practical example of the DQPSK receiving apparatus of the present invention shown in  FIG. 5 .  FIG. 7  shows, in a practical manner, the configuration of the DQPSK receiving apparatus shown in  FIG. 5  in connection with the case where the transmission capacity of light signal E(t) is expressed as B=40 G bps. 
         [0072]    The DQPSK receiving apparatus shown in  FIG. 7  comprises delay interferometer  330  supplied with received light signal E(t)  314  and functions as a demodulator, a balanced PD (Photo Detector)  334 , TIA (Transimpedance Amplifier)  336 , and 1:16 DMUX (Demultiplexer) with CDR (Clock and Data Recovery)  338  function to divide an electric signal of 40 G bps that TIA  336  outputs into 16 signals. 
         [0073]    Delay interferometer  330  is a Mach-Zehnder interferometer (MZI). The MZI is ordinarily formed of gallium arsenic or lithium niobate. 
         [0074]    In delay interferometer  330 , the length of arm  330   a  is longer than that of arm  330   b . Thus, a light signal which has been passed through arm  330   a  is delayed by 50 ps as compared to a light signal which has been passed through arm  330   b . This is because in the case where the transmission capacity of light signal E(t)  314  is 40 G bps, the symbol speed is 20 G Symbol/s and the symbol interval is 50 ps as shown in  FIG. 6 . 
         [0075]    Phase adjuster  331  at delay interferometer  330  is provided at one arm  330   b  of delay interferometer  330 . Moreover, phase adjuster  331  is of the structure in which an electrode is provided. When voltage Vc(t) is applied to this electrode, the refractive index of the light transmission path is changed. As a result, the phase of light signal E(t)  314  is changed. The amplitude value of voltage Vc(t) is a value to change the refractive index of the light transmission path so that the shift quantity of the phase of light signal E(t)  314  becomes equal to π/4 or −π/4. 
         [0076]    Balanced PD  334  converts two light output signals E 5 ( t ), E 6 ( t ) outputted from delay interferometer  330  into a current signal with a level proportional to a difference between the light intensities of the two light output signals E 5 ( t ), E 6 ( t ). 
         [0077]    TIA  336  converts a current signal converted at balanced PD  334  into a voltage signal to amplify the voltage signal. The signal thus amplified is inputted to 1:16 DMUX with CDR  338  function. 
         [0078]    1:16 DMUX with CDR  338  function is a clock extractor for extracting a clock signal with 20 GHz of symbol rate from the signal amplified at TIA  336 . 
         [0079]    A clock signal with 20 GHz extracted by 1:16 DMUX with CDR  338  function is applied, as voltage Vc(t), to phase adjuster  331  at delay interferometer  330  through wiring  339 . At this time, the timing at which the voltage value of a clock signal is switched from the minimum value to the maximum value and the timing at which the voltage value of the clock signal is switched from the maximum value to the minimum value, are caused to be in correspondence with the timing at which a symbol of light signal E(t)  314  inputted to delay interferometer  330  is switched. 
         [0080]    Accordingly, a binary signal appears every  25  ps at light output signal E 5 ( t ) and light output signal E 6 ( t ). Further, electric signal z(t) proportional to a difference between the light intensities of light output signals E 5 ( t ) and E 6 ( t ) is obtained as an output of balanced PD  334 . Electric signal z(t) is a binary signal every 25 ps, i.e., one 40 Gbps binary signal. 
         [0081]    In this case, voltage Vc(t) of a clock signal delivered to phase adjuster  331  is adjusted in advance within 1:16 DMUX with CDR  338  function, or by external voltage regulating means (not shown). 
         [0082]    As described above, in accordance with the exemplary embodiment, advantages as described below can be obtained. 
         [0083]    The first advantage is to have the ability to downsize the light receiving apparatus using the DQPSK modulation method. The reason thereof is that two delay interferometers for DQPSK demodulation and two balanced photoelectric converters are required in the apparatus shown in  FIG. 1 , whereas a DQPSK-modulated light signal can be demodulated by one delay interferometer and one balanced photoelectric converter. 
         [0084]    The second advantage is to have the ability to reduce the cost of the light receiving apparatus using a DQPSK demodulation method. The reason thereof is that because two sets of optical components are required in the apparatus shown in  FIG. 1 , whereas since it is sufficient to only use one set of optical components, the number of optical components, whose costs are difficult to decrease, can be reduced to one-half. 
         [0085]    While the invention has been particularly shown and described with reference to exemplary embodiments thereof, the invention is not limited to these embodiments. It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the claims.