Abstract:
An optical transmission apparatus includes a Mach-Zehnder modulator, transmission lines which transmit modulation data to the Mach-Zehnder modulator, a phase varying section which is connected to at least one of the transmission lines, and a phase synchronization loop which is connected to the phase varying section, and which applies a control voltage on which a dither signal is superimposed to the phase varying section. The phase varying section adjusts a skew between the transmission lines to remain constant, based on the control voltage on which the dither signal is superimposed.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to an optical transmission apparatus, and more particularly to correction of a skew between data which, in a Mach-Zehnder modulator (hereinafter, referred to as an MZ modulator), are applied in order to phase-modulate light beams that are obtained by branching in an optical path in the MZ modulator. 
       RELATED ART 
       [0002]    In the field of optical communication systems, as an optical modulation method which is suitable for a large capacity and a long transmission distance, practical application of phase modulation methods such as DPSK (Differential Phase Shift Keying) and DQPSK (Differential Quadrature Phase Shift Keying) have been studied. 
         [0003]    In such an optical communication system, as an optical modulator which phase-modulates continuous light with binarized data, an MZ modulator is used as disclosed in, for example, Patent Reference 1. An MZ modulator is an optical intensity modulator which performs ON/OFF control of light under interference conditions in the case where light that has been once split is again combined with each other, and can change the interference conditions of the combination, by applying a voltage on an electrode disposed on an optical waveguide. 
         [0004]      FIG. 6  is a block diagram showing an example of a related-art DQPSK optical transmitting apparatus using an MZ modulator. Referring to  FIG. 6 , in the MZ modulator  1 , four branched optical waveguides  1   a  are formed, and electrodes  1   b  for applying data for modulation are formed on the upper faces of the optical waveguides  1   a , respectively. 
         [0005]    Continuous light from an optical source  2  is input into one ends of the optical waveguides  1   a . While being split into four light beams, the continuous light is passed through the optical waveguides  1   a , and then again combined with each other to be output from another end as a light signal Sout. 
         [0006]    Two data Da, Db which are output from a data generator  3 , and which have the same frequency and different data arrangements are amplified to respective adequate amplitude voltages by amplifiers  4   a ,  4   b  each of which is configured so as to output signals of two positive and negative polarities, and then input to the electrodes  1   b  formed on the upper faces of the optical waveguides  1   a . The data Db are input into the amplifier  4   b  through a phase shifter  5  which is connected as phase varying section. 
         [0007]    In order to adequately set the interference conditions of the combination, a DC power source  6  applies predetermined voltages to electrodes (not shown) which are formed on the optical waveguides  1   a  of the combining portion of the MZ modulator  1 , respectively. 
         [0008]    According to the configuration, the continuous light output from the optical source  2  is passed through the MZ modulator  1  to be output as the light signal Sout which is phase-modulated with the two data Da, Db output from the data generator  3 . 
         [0009]    In the thus configured optical transmission apparatus, when a skew is generated in data transmitted between the data generator  3  and the amplifiers  4   a ,  4   b , or between the amplifiers  4   a ,  4   b  and the MZ modulator  1 , the eye pattern for demodulation is closed, and “0” or “1” of data is hardly discriminated, so that the transmission quality is lowered, thereby causing transmission degradation. In the optical transmission apparatus of  FIG. 6 , therefore, the phase shifter  5  is connected to the line of the data Db to perform the skew adjustment. 
         [0010]    Patent Reference 1 discloses the configuration of an optical device apparatus which uses an MZ modulator that can adequately control the phase shift, the DC drift, etc. 
         [0011]    [Patent Reference 1] JP-A-2007-43638 
         [0012]    Recently, a communication apparatus which is small, and in which the transmission rate is high is requested. As the transmission rate is higher, usually, electronic devices including a data generator which is a high-speed communication component consume a larger power. 
         [0013]    Usually, the data generator  3  and the amplifiers  4   a ,  4   b  are connected with each other by a coaxial cable or the like. In a DQPSK optical transmission apparatus such as shown in  FIG. 6 , in the case of a small and high-speed communication apparatus, when the optical transmission apparatus is designed to be miniaturized so that the mounting density is high, the heat distribution is largely changed depending on the arrangement of the electronic devices, and the skew between the two data Da, Db is delicately varied. 
         [0014]    In the case of a super high-speed signal of 100 Gbps, for example, it is assumed that the half period of one bit is 5 ps, devices are connected to each other by a cable having a length of 50 mm, the core wires of the coaxial cables are made of pure copper, and the temperature difference between the coaxial cables for the data Da, Db is 5° C. In this case, the skew between the data Da, Db is indicated by the following expression, the coaxial length difference is about 4 μm, and a skew of about 0.02 ps is generated, 
         [0000]    
       
      
       L 
       skew 
       =α·L 
       0 
       ·Δt  
      
       
         
           
             α: coefficient of linear expansion (in the case of copper: α=16.5 e-6) 
             L 0 : coaxial length 
             Δt: temperature difference 
           
         
       
     
         [0018]    Furthermore, sometimes, the skew between the data Da, Db is varied by degradation with time of devices. 
         [0019]    When focusing attention on the lengths of the cables which are used for transmitting the data Da, Db, production dispersion of about 1 mm is produced. In a super high-speed transmission system of, for example, 100 Gbps, the production dispersion exerts a large influence in view of influences such as transmission degradation. 
         [0020]    When a skew due to such a temperature change, degradation with time of devices, or the like is changed, transmission quality degradation is caused as described above. Therefore, it is preferable that the state of a skew is always monitored by using any means, and, when a skew is generated, the skew is immediately adjusted. 
         [0021]    A skew due to production dispersion of the cables must be eliminated in a stage of assembling and adjusting the optical transmission apparatus. 
         [0022]    In the related-art configuration shown in  FIG. 6 , in the adjustment of the skews, for example, it is necessary that an instrument for observing an eye pattern is connected to the optical transmission apparatus, and the phase shifter  5  is adjusted on the basis of the demodulated eye pattern, or that the BERT (Bit Error Rate Test) is performed on the demodulated signal, and the phase shifter  5  is adjusted while checking the transmission quality. Therefore, there is a problem in that many work hours are required. 
         [0023]    In an optical transmission apparatus which is being practically used, even when a skew due to a temperature change, degradation with time of devices, or the like is generated, it is difficult to immediately adjust the skew. 
       SUMMARY 
       [0024]    Exemplary embodiments of the present invention provide an optical transmission apparatus in which a skew is always monitored, and, when a skew is generated, the skew can be immediately eliminated. 
         [0025]    An optical transmission apparatus according to an exemplary embodiment of the invention, comprises: 
         [0026]    a Mach-Zehnder modulator; 
         [0027]    transmission lines which transmit modulation data to the Mach-Zehnder modulator; 
         [0028]    a phase varying section which is connected to at least one of the transmission lines; and 
         [0029]    a phase synchronization loop which is connected to the phase varying section, and which applies a control voltage on which a dither signal is superimposed to the phase varying section, 
         [0030]    wherein the phase varying section adjusts a skew between the transmission lines to remain constant, based on the control voltage on which the dither signal is superimposed. 
         [0031]    The phase synchronization loop may have 
         [0032]    a dither generating source which generates a dither signal, 
         [0033]    a mixer which multiples the dither signal output from the dither generating source with a signal that is phase-adjusted and photoelectrically converted, 
         [0034]    a frequency detector which detects an arbitrary frequency band signal from an output signal from the mixer, 
         [0035]    a controller which receives a synchronous detection output from the frequency detector, and which changes a control signal on the basis of a value which is synchronous-detected; and 
         [0036]    an adder which adds the control voltage output from the controller, to the dither signal. 
         [0037]    Phase varying sections may be connected respectively to the transmission lines. 
         [0038]    A part of the phase synchronization loop may be formed by an FPGA or an ASIC. 
         [0039]    The phase varying section may be controlled so that a synchronous-detected value is “0”. 
         [0040]    According to the configuration, it is possible to always monitor a skew, and, when a skew is generated between transmission lines, the skew can be immediately eliminated. 
         [0041]    Other features and advantages may be apparent from the following detailed description, the accompanying drawings and the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0042]      FIG. 1  is a block diagram showing an embodiment of the invention. 
           [0043]      FIG. 2  is a graph showing correlation between a skew between data and a synchronous detection output in the embodiment of the invention. 
           [0044]      FIG. 3  is a block diagram showing another embodiment of the invention. 
           [0045]      FIG. 4  is a block diagram showing a further embodiment of the invention. 
           [0046]      FIG. 5  is a block diagram showing a still further embodiment of the invention. 
           [0047]      FIG. 6  is a block diagram showing an example of a related-art configuration. 
       
    
    
     DETAILED DESCRIPTION 
       [0048]    Hereinafter, the optical transmission apparatus of the invention will be described with reference to the drawings.  FIG. 1  is a block diagram showing an embodiment of the invention. In the figure, components common with  FIG. 6  are denoted by the same reference numerals. The configuration of  FIG. 1  is different from that of  FIG. 6  in that a phase synchronization loop configured by an optical distributor  7 , a photodetector  8 , and a synchronous detection circuit  9  is connected to the phase shifter  5 . 
         [0049]    Light from the optical source  2  is incident on the optical waveguides  1   a  of the MZ modulator  1 . The one data Da output from the data generator  3  is input into the data amplifier  4   a , and the other data Db is input into the data amplifier  4   b  via the phase shifter  5  in which the phase of the data is adjusted. The data are amplified to an adequate amplitude voltage, and then applied to the electrodes  1   b  which are disposed on the upper faces of the optical waveguides  1   a  of the MZ modulator  1 . 
         [0050]    Predetermined voltages from the DC power source  6  are applied to the electrodes (not shown) which are formed on the upper face of the combining portion of the MZ modulator  1 , respectively, thereby modulating the intensity of the light. A light signal in which the light intensity is modulated by the MZ modulator  1  is input into the optical distributor  7  to be split into the signal Sout to be output to the outside, and a signal to be input into the photodetector  8 . 
         [0051]    In  FIG. 1 , for example, a configuration where the electrical length is mechanically changed by a motor or the like to control the phase, or where a voltage is applied to a device that is configured by a varactor diode or the like, and that varies a delay time, to generate a delay time, and a delay of the electrical phase is used is employed as the phase shifter  5 . In the case where an emitter follower circuit is used in the output stage of the data generator, the power source voltage of the emitter follower circuit may be changed to vary the delay time. When an adequate control voltage from the synchronous detection circuit  9  constituting the phase synchronization loop is applied to the phase shifter  5  by such means as described later, a skew between transmission lines can be eliminated, and the transmission quality can be maintained. 
         [0052]    Namely, a low-frequency dither from a dither generating source  91  is superimposed on the electric signal output from the data generator, by the phase shifter  5 , and the signal is then applied to the MZ modulator. The continuous light from the optical source  2  is subjected to lightwave modulation by the MZ modulator. A weak dither is superimposed on the lightwave modulation signal by the phase shifter  5 . The dither is split by the optical distributor  7 , photoelectrically converted by the photodetector  8 , and then input into the synchronous detection circuit  9 . When a weak dither is applied to the phase shifter  5 , an envelope curve appears in the light modulated output. The envelope curve is converted from light to an electric signal by the photodetector  8 . 
         [0053]    The synchronous detection circuit  9  is configured by the low-frequency dither generating source  91 , a mixer  92 , a low-pass filter (LPF)  93 , a controller  94 , and an adder  95 . 
         [0054]    In the synchronous detection circuit  9 , the signal which is photoelectrically converted by the photodetector  8  is multiplied with the low-frequency dither by the mixer  92 , and the output signal of the mixer  92  is input into the controller  94  as a synchronous detection output, via the LPF  93 . Based on the obtained synchronous detection output, the controller  94  applies an optimum control voltage to the phase shifter  5  via the adder  95 . Also the low-frequency dither is input from the dither generating source  91  into the adder  95 . The adequate phase voltage means a voltage at a point where, in a correlation chart of the synchronous detection output and a skew between data shown in  FIG. 2 , the synchronous detection output is “0”. 
         [0055]    Namely, the control signal of the controller  94  controls the phase shifter  5  so that the synchronous-detected value output from the low-pass filter  93  is “0”. 
         [0056]    The adequate control voltage from the controller  94  in the synchronous detection circuit  9  is applied to the phase shifter  5 , and therefore a skew between the transmission lines can be eliminated, and the transmission quality of the optical transmission apparatus can be maintained. 
         [0057]    Although not shown in  FIG. 1 , in the synchronous detection circuit  9 , a phase shifter is connected between the low-frequency dither generating source  91  and the mixer  92 , and adjusted so that the mixer output is obtained at the maximum value. Furthermore, for example, amplifying means such as a trans-impedance amplifier is connected between the photodetector  8  and the mixer  92  to amplify the input to the mixer  92  to an adequate signal level. 
         [0058]    The light signal which is split by the optical distributor  7  to be input into the photodetector  8  is photoelectrically converted. The electric signal which is photoelectrically converted is input into the synchronous detection circuit  9 , and the skew between the two data Da, Db output from the data generator  3  is made constant. 
         [0059]    In the low-frequency dither generating source  91 , usually, a sinusoidal wave is used, or alternatively a rectangular wave may be used. The optical distributor  7  and the photodetector  8  may be mounted in the MZ modulator  1 . 
         [0060]    As described above, the phase synchronization loop that applies the control voltage onto which the dither signal is superimposed, to the phase shifter  5 , and that performs the synchronous detection is disposed, whereby it is possible to immediately cope with the situation that a skew is generated between the modulation data to be applied to the MZ modulator  1 . Therefore, the skew adjusting time can be remarkably shortened as compared with the related art, and the transmission quality can be always maintained constant. 
         [0061]    When a part of the phase synchronization loop is formed by, for example, an FPGA (Field Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit), the optical transmission apparatus can be miniaturized, and the power consumption can be reduced. 
         [0062]      FIG. 3  is a block diagram showing another embodiment of the invention. In the figure, components common with  FIG. 1  are denoted by the same reference numerals. The configuration of  FIG. 3  is different from that of  FIG. 1  in that the data Da, Db output from the data generator  3  are transmitted to the data amplifiers  4   a ,  4   b  via phase shifters  5   a ,  5   b , respectively, and that the control voltage of the controller  94  on which the low-frequency dither is superimposed is input into the one phase shifter  5   b  via the adder  95 , and the control voltage of the controller  94  in the synchronous detection circuit  9  is input into the other phase shifter  5   a  directly or without superimposing the low-frequency dither on the control voltage. 
         [0063]    In the synchronous detection circuit  9 , the signal which is photoelectrically converted is multiplied with the signal output from the low-frequency dither generating source  91  by the mixer  92 , and the output signal of the mixer  92  is input into the controller  94  as a synchronous detection output, via the low-pass filter  93  in order to prevent a transient response from occurring. Based on the obtained synchronous detection output from the low-pass filter  93 , the optimum control voltage output from the low-pass filter  93 , and the low-frequency dither signal output from the low-frequency dither generating source  91  are added to each other by the adder  95 , so that the optimum control voltage on which the low-frequency dither signal is superimposed is applied to the phase shifter  5   b . The controller  94  directly applies the optimum control voltage to the other phase shifter  5   a.    
         [0064]    In the configuration, an optimum control voltage from the controller  94  in the synchronous detection circuit  9  is applied to the phase shifters  5   a ,  5   b , and hence the transmission quality can be maintained. 
         [0065]    The phase shifters  5   a ,  5   b  are instructed to operate in the following various methods, by the controller  94 : 
         [0066]    a) a method in which both the phase shifters  5   a ,  5   b  advance the respective phases; 
         [0067]    b) a method in which the data phase of one of the data Da, Db output from the data generator  3  is advanced; 
         [0068]    c) a method in which the phases of both the data Da, Db output from the data generator  3  are retarded; 
         [0069]    d) a method in which the phases of both the data Da, Db output from the data generator  3  are retarded, and the phase of one of them is advanced; 
         [0070]    e) a method in which the phases of the data Da, Db output from the data generator  3  are advanced in respective opposite directions; and 
         [0071]    f) a method in which the phases of the data Da, Db output from the data generator  3  are retarded in respective opposite directions, and the phase of one of them is advanced. 
         [0072]    In the method in which the data Da, Db output from the data generator  3  are advanced in respective opposite directions, for example, the operation speed can be improved, and the phase control region can be widened. 
         [0073]    In the case other than the method in which the data Da, Db output from the data generator  3  are advanced in respective opposite directions, the phase control region can be widened. 
         [0074]      FIG. 4  is a block diagram showing a further embodiment of the invention. In the figure, components common with  FIG. 3  are denoted by the same reference numerals. The configuration of  FIG. 4  is different from that of  FIG. 3  in that two synchronization detection circuits  9   a ,  9   b  are included in the synchronization detection circuit  9 , that a dither is superimposed on both the phase shifters  5   a ,  5   b , and that band-pass filters (BPFs)  10   a ,  10   b  the number of which is equal to that of the synchronization detection circuits  9   a ,  9   b  are disposed between the photodetector  8  and the synchronization detection circuit  9 . The basic operation is identical with that of  FIG. 3 . 
         [0075]    The signal which is photoelectrically converted by the photodetector  8  is input into the band-pass filters  10   a ,  10   b , so that only frequencies coincident with frequency ranges which are set in the respective closed loops are passed through the filters, and then input into mixers  92   a ,  92   b  in the synchronization detection circuit  9  corresponding to the respective band-pass filters  10   a ,  10   b.    
         [0076]    The two synchronization detection circuits  9   a ,  9   b  make the skew of the data Da, Db output from the data generator  3  constant, apply respective optimum control voltages to the phase shifters  5   a ,  5   b , and superimpose respective low-frequency dither signals on the phase shifters  5   a ,  5   b  via adders  95   a ,  95   b.    
         [0077]    Namely, the signals which are photoelectrically converted by the photodetector  8 , and the signals output from low-frequency dither generating sources  91   a ,  91   b  are multiplied with each other in the respective mixers  92   a ,  92   b . Then, the resulting signals are input into the controller  94  as synchronous detection outputs, via low-pass filters  93   a ,  93   b  in order to prevent a transient response from occurring. 
         [0078]    Based on the respective obtained synchronous detection outputs from the low-pass filters  93   a ,  93   b , the controller  94  supplies the optimum control voltages output from the low-pass filters  93   a ,  93   b , to one input terminals of the adders  95   a ,  95   b . The low-frequency dither signals output from the low-frequency dither generating sources  91   a ,  91   b  are supplied to the other input terminals of the adders  95   a ,  95   b . The optimum control voltages output from the low-pass filters  93   a ,  93   b , and the low-frequency dither signals are added to each other by the adders  95   a ,  95   b , respectively, and the optimum control voltages on which the low-frequency dither signals are respectively superimposed are applied to the phase shifters  5   a ,  5   b , respectively. 
         [0079]    Since the adequate control voltages from the controller  94  in the synchronization detection circuit  9  are applied to the phase shifters  5   a ,  5   b , the transmission quality can be maintained. 
         [0080]    Dithers are superimposed on both the phase shifters  5   a ,  5   b . The dither frequencies are set to values which are not integer multiples of the respective counter frequencies. When the dither signals are to be applied, the application may be controlled in a time-division manner. 
         [0081]    The band-pass filters  10   a ,  10   b  allow only frequencies coincident with frequency ranges which are set in the respective closed loops, to pass through the filters, under the conditions that they are not affected by the respective other dither frequencies. 
         [0082]    In the case where the same dither frequency is input into the phase shifters  5   a ,  5   b , the application may be performed in a time-division manner, and only one of the synchronization detection circuits  9   a ,  9   b  may be used. 
         [0083]    The embodiment can achieve the same effects as the embodiment of  FIG. 3 . Namely, when the data Da, Db output from the data generator  3  are advanced in respective opposite directions, the operation speed can be improved, and the phase control region can be widened. 
         [0084]    In the case other than the method in which the data Da, Db output from the data generator  3  are advanced in respective opposite directions, the phase control region can be widened. 
         [0085]      FIG. 5  is a block diagram showing a configuration example of a 16QAM (Quadrature Amplitude Modulation) which is a still further embodiment of the invention. In the figure, components common with  FIG. 3  are denoted by the same reference numerals. The configuration of  FIG. 5  is different from that of  FIG. 3  in that four synchronization detection circuits  9   a  to  9   d  are included in the synchronization detection circuit  9 , that the data generator  3  outputs data Da, Db, Dc, Dd, that low-frequency dither signals are superimposed on phase shifters  5   a  to  5   d , respectively, and that band-pass filters (BPFs)  10   a  to  10   d  the number of which is equal to that of the synchronization detection circuits  9   a  to  9   d  are disposed between the photodetector  8  and the synchronization detection circuit  9 . 
         [0086]    The signal which is photoelectrically converted by the photodetector  8  is input into the band-pass filters  10   a  to  10   d , so that only frequencies coincident with frequency ranges which are set in the respective closed loops are passed through the filters, and then input into mixers  92   a  to  92   d  in the synchronization detection circuit  9  corresponding to the respective band-pass filters  10   a  to  10   d.    
         [0087]    The four synchronization detection circuits  9   a  to  9   d  make the skew of the data Da to Dd output from the data generator  3  constant, apply respective optimum control voltages to the phase shifters  5   a  to  5   d , and superimpose respective low-frequency dither signals on the phase shifters  5   a  to  5   d.    
         [0088]    Namely, the signals which are photoelectrically converted by the photodetector  8 , and the signals output from low-frequency dither generating sources  91   a  to  91   d  are multiplied with each other in the mixers  92   a  to  92   d , respectively. Then, the resulting signals are input into the controller  94  as synchronous detection outputs, via low-pass filters  93   a  to  93   d  in order to prevent a transient response from occurring. 
         [0089]    Based on the obtained synchronous detection outputs from the low-pass filters  93   a  to  93   d , the controller  94  supplies the optimum control voltages output from the low-pass filters  93   a  to  93   d , to one input terminals of the adders  95   a  to  95   d . The low-frequency dither signals output from the low-frequency dither generating sources  91   a  to  91   d  are supplied to the other input terminals of the adders  95   a  to  95   d . The optimum control voltages output from the low-pass filters  93   a  to  93   d , and the low-frequency dither signals are added to each other by the adders  95   a  to  95   d , respectively, and the optimum control voltages on which the low-frequency dither signals are respectively superimposed are applied to the phase shifters  5   a  to  5   d , respectively. 
         [0090]    Dithers are superimposed on the phase shifters  5   a  to  5   d . The dither frequencies are set to values which are not integer multiples of the respective counter frequencies, with respect to relationships between the data Da and Db, and between the data Dc and Dd. When the dither signals are to be applied, the application may be controlled in a time-division manner. 
         [0091]    The band-pass filters  10   a  to  10   d  allow only frequencies coincident with frequency ranges which are set in the respective closed loops, to pass through the filters, under the conditions that the band-pass filters  10   a  to  10   d  are not affected by the other dither frequencies. 
         [0092]    In the case where the same dither frequency is input into the phase shifters  5   a  to  5   d , the application is performed in a time-division manner, and only three of the synchronization detection circuits  9   a  to  9   d  are used. 
         [0093]    In  FIG. 5 , in the case where one of the phase shifters  5   a  to  5   d  is omitted, the data of the omitted one of the phase shifters  5   a  to  5   d  may be set as phase reference data. 
         [0094]    The embodiment can achieve the same effects as the embodiments of  FIGS. 3 and 4 . Namely, when the two data Da, Db, and two data Dc, Dd output from the data generator  3  are advanced in respective opposite directions, the operation speed can be improved, and the phase control region can be widened. 
         [0095]    In the case other than the method in which two signals output from the data generator  3  are advanced in respective opposite directions, the phase control region can be widened. 
         [0096]    As described above, according to the invention, it is possible to realize an optical transmission apparatus in which a skew is always monitored, and, when a skew is generated, the skew can be immediately eliminated. Therefore, the skew adjusting time can be remarkably shortened as compared with the related art, and the transmission quality can be always maintained constant. 
         [0097]    While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.