Abstract:
A chirp switching circuit comprises a Mach-Zehnder modulator having a Y-branched part for branching an incoming optical signal into first and second optical signals and an X-branched part merging the first and second optical signals with each other, the Mach-Zehnder modulator causing phase modulation in the first and second optical signals by a modulation signal, and a directional coupling optical switch that switches first and second optical output signals output from the X-branched part of the Mach-Zehnder modulator by merging the first and second output optical signals in response to a chirp switching control signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     The present application is based on Japanese priority application No. 2005-303362 filed on Oct. 18, 2005, the entire contents of which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to chirp switching circuits and optical transmission systems and more particularly to a chirp switching circuit for switching transmission chirp and an optical transmission system that uses such a chirp switching circuit.  
         [0003]     In view of the needs of increasing transmission capacity, technical development is being undertaken with regard to optical transmission systems in the directions of achieving: higher transmission speed (increase of bit rate); wavelength multiplexing for enabling transmission of plural signals via a single optical fiber at different wavelengths; and increase of transmission distance.  
         [0004]     With increase in the transmission speed, there occurs a distortion of signal waveform in the optical signals transmitted through an optical transmission path because of spreading of the spectrum of the optical signals resulting from signal modulation and further from the dispersion pertinent to the signal transmission path. Such distortion of the signal waveform limits the distance that the signal can be transmitted. This distortion of transmitted signal waveform depends upon the mutual relationship between the chirp (time-dependent wavelength shift) that is pertinent to the transmission part and the chirp caused by the dispersion of the transmission path and non-linearity of the transmission path.  
         [0005]     In a Mach-Zehnder (MZ) modulator, it is possible to adjust the chirp (time-dependent wavelength shift) of the transmitter output by changing the ratio of phase modulation between the waveguides of two directions. The chirp caused by the transmitter induces a pulse compression or decompression in cooperation with dispersion and chirp of the transmission path, and thus, it is possible to relax the foregoing constraint imposed upon the transmission path length because of dispersion, by suitably choosing a transmission output chirp according to the transmission path condition.  
         [0006]     Meanwhile, there arises a situation in which it is desired to switch the chirp of the Mach-Zehnder modulator, as in the case of switching the signal path from the current path to a path where the transmission path conduction such as transmission distance is different.  FIGS. 1A and 1B  show such an example of switching the signal path from a current (work) path to a reserve (protection) path in an optical transmission system of ring structure.  
         [0007]     Referring to  FIGS. 1A and 1B , the optical transmission system is constructed by nodes  10   a - 10   f  connected in the form of a ring, wherein each of the nodes  10   a - 10   f  achieves optical amplification and compensation of dispersion. Thus, when an optical signal is to be transmitted from a transmission part  11  to a receiving part  12 , the node  10   b  usually chooses a current path A that reaches the node  10   b  from the node  10   a  in a clockwise direction as shown in  FIG. 1A .  
         [0008]     On the other hand, when there occurs a failure in the optical transmission path between the nodes  10   a  and  10   b , the transmission path is switched to a reserve path B that reaches from the node  10   a  to the node  10   d  in the anti-clockwise direction via the nodes  10   f ,  10   e ,  10   d  and  10   c.    
         [0009]     Here, let us assume that the path A has a negative residual dispersion and the path B has a positive residual dispersion.  
         [0010]     As can be seen from  FIG. 3  that shows the relationship between the residual dispersion and the sign of the chirp, use of positive chirp is suited in the case there is a negative residual chirp, while in the case the residual dispersion is positive, it is suitable to use a negative chirp. Thus, when there is a need to switch the transmission path from the path A to the path B, there is a need of switching the transmission chirp from positive to negative.  
         [0011]      FIG. 4  shows an example of the chirp switching circuit that uses a conventional Mach-Zehnder optical modulator.  
         [0012]     Referring to  FIG. 4 , the Mach-Zehnder optical modulator divides the light coming in from an optical source  15  into two, different optical waveguides  17  and  18  at a Y-branching part  16 , wherein the respective lights undergo phase change in the optical waveguides  17  and  18  and are merged again at a Y-branched part  19 . Thereby, On/Off control is achieved for the optical signal based on the phase difference thus induced.  
         [0013]      FIG. 5A  shows input/output characteristics of the chirp switching circuit that uses such a Mach-Zehnder optical modulator, while  FIGS. 5B-5F  show input/output signal waveforms of the chirp switching circuit.  
         [0014]     In the Mach-Zehnder modulator, the logic value of an input signal (electric signal) shown in  FIG. 5B  is inverted by a high-speed driver circuit  21  and the operational point of the Mach-Zehnder modulator is shifted by an operational point stabilization circuit  22  at the same time. With this, there is caused inversion in the transmission chirp. It should be noted that  FIG. 5B  shows the optical intensity in the optical waveguide  17  while  FIG. 5C  shows the optical intensity in the optical waveguide  18 . Further,  FIG. 5D  shows the phase of the output optical signal while  FIG. 5F  shows the chirp caused in the output optical signal.  
         [0015]     Because the optical amplifier in each of the nodes  10   a - 10   f  has a time-dependence in the response characteristics thereof, there occurs a change of output when there is caused a change in the input, wherein there are cases in which such a change is accumulated. It should be noted that such accumulated change of optical signals may cause overload or even damage in the optical components at the reception side or at the intermediate position on the signal transmission path. Thus, in the case there is provided optical amplifiers in the transmission path and in the case there is going to be made switching of chirp in correspondence to the switching of transmission path from the current transmission path to a transmission path of different path condition, it is desired that the change of optical output caused with the switching of chirp is suppressed as much as possible.  
         [0016]     Parent Reference 1 teaches the technology of obtaining a chirped optical signal of inverted polarity by changing the operational point of the Mach-Zehnder optical modulator in correspondence to the chirp polarity.  
         [0017]     PATENT REFERENCE 1 Japanese Laid-Open Patent Application 11-266200  
       SUMMARY OF THE INVENTION  
       [0018]     In order to switch the transmission chirp of a Mach-Zehnder optical modulator, there is a need of shifting the operational point of the Mach-Zehnder optical modulator simultaneously to inversion of logic value of an input signal. Because a Mach-Zehnder optical modulator has a transmission characteristic repeated periodically as shown in  FIG. 6A , there is caused a transition in the transmission chirp from a state a in which the transmission chirp is positive (α&gt;0) to a state b corresponding to the top or bottom of the transmission characteristic and further to a state c in which the transmission chirp is negative (α&lt;0). Reference should be made to  FIG. 6B .  
         [0019]     In this process, there is caused an increase (or decrease) of output from the transmission part. This means that it is inevitable that there is caused a time-dependent variation in the optical output with the switching of the transmission chirp. When this time-dependent variation of optical output is used with the optical transmission system that uses an optical amplifier in each node, there is caused an optical surge because of the variation at the input of the optical amplifier, while accumulation of such optical surge may cause error at the reception side after the reception of the optical signal.  
         [0020]     In order to avoid this problem, there is a need of suppressing the operation of the optical amplifier in each node temporarily, while such an approach increases the time needed for switching the transmission chirp.  
         [0021]     The present invention has been made in view of the foregoing problems and has its object of providing a chirp switching circuit and an optical transmission system capable of suppressing the variation of optical output associated with switching of the transmission chirp and thus capable of suppressing occurrence of error caused by optical surge.  
         [0022]     In a first aspect, the present invention eliminates the variation of optical output at the time of switching a transmission chirp by a chirp switching circuit comprising:  
         [0023]     a Mach-Zehnder modulator having a Y-branched part for branching an incoming optical signal into first and second optical signals and an X-branched part merging said first and second optical signals with each other, said Mach-Zehnder modulator causing phase modulation in said first and second optical signals by a modulation signal; and  
         [0024]     a directional coupling optical switch that switches first and second optical output signals output from said X-branched part of said Mach-Zehnder modulator by merging said first and second output optical signals in response to a chirp switching control signal.  
         [0025]     In a preferred embodiment, said Mach-Zehnder modulator adjusts an absolute value of a chirp by carrying out said phase modulation of said first and second optical signals by changing a degree of modulation of said modulation waves in response to said chirp switching signal.  
         [0026]     In another aspect, there is provided an optical transmission system, comprising:  
         [0027]     a current path;  
         [0028]     a reserve path; and  
         [0029]     a transmission part transmitting an optical signal to said current path and said reserve path,  
         [0030]     said optical transmission system switching from said current path to said reserve path when there is a failure in said current path,  
         [0031]     said transmission part comprising a chirp switching circuit comprising:  
         [0032]     a Mach-Zehnder modulator having a Y-branched part for branching an incoming optical signal into first and second optical signals and an X-branched part merging said first and second optical signals with each other, said Mach-Zehnder modulator causing phase modulation in said first and second optical signals by a modulation signal; and  
         [0033]     a directional coupling optical switch that switches first and second optical output signals output from said X-branched part of said Mach-Zehnder modulator by merging said first and second output optical signals in response to a chirp switching control signal,  
         [0034]     said chirp switching circuit switching a transmission chirp, when it becomes necessary, concurrently to switching from said current path to said reserve path.  
         [0035]     In a preferred embodiment, a sign of said transmission chirp is predetermined for all of said current path and said reserve path.  
         [0036]     In another preferred embodiment, said Mach-Zehnder modulator and said directional coupler optical switch uses a LiNbO 3  crystal.  
         [0037]     According to the present invention, it becomes possible to suppress the variation of optical output at the time of switching the transmission chirp and it becomes possible to prevent occurrence of error caused by optical surge.  
         [0038]     Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0039]      FIGS. 1A and 1B  are diagrams explaining the switching of path from a current path to a reserve path in an optical transmission system of a conventional ring construction;  
         [0040]      FIG. 2  is a diagram showing the relationship between a transmission distance and residual dispersion;  
         [0041]      FIG. 3  is a diagram showing the relationship between a transmission distance and chirp;  
         [0042]      FIG. 4  is a diagram showing the construction of a chirp switching circuit that uses a conventional Mach-Zehnder optical modulator;  
         [0043]      FIGS. 5A-5F  are diagrams showing the input/output characteristics and input/output signal waveforms of the chirp switching circuit according to a related art that uses a Mach-Zehnder optical modulator;  
         [0044]      FIGS. 6A and 6B  are diagrams explaining the problem of output variation caused with chirp switching in the chirp switching circuit of  FIG. 4 ;  
         [0045]      FIG. 7  is a diagram explaining the occurrence of optical surge as a result of input variation in an optical amplifier;  
         [0046]      FIG. 8  is a diagram showing the construction of a chirp switching circuit according to an embodiment of the present invention;  
         [0047]      FIG. 9  is a block diagram showing the construction of a chirp switching circuit according to an embodiment of the present invention;  
         [0048]      FIG. 10  is a block diagram showing the construction of a high-speed driver circuit used with the chirp switching circuit of  FIG. 8 ;  
         [0049]      FIGS. 11A-11F  are diagrams showing the input/output characteristics and input/output signal waveforms of the chirp switching circuit according to the embodiment of  FIG. 8  that uses a Mach-Zehnder optical modulator;  
         [0050]      FIG. 12  is a diagram showing the characteristics of the directional coupler optical switch used with the chirp switching circuit of  FIG. 8 ;  
         [0051]      FIG. 13  is a diagram showing the chirp switching sequence according to the embodiment of the present invention; and  
         [0052]      FIGS. 14A and 14B  are diagrams explaining switching of the path from the current path to the reserve path in an optical transmission system of ring structure. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0053]     Hereinafter, the present invention will be explained with reference to the drawings.  
       Embodiment  
       [0054]      FIG. 8  shows the construction of a chirp switching circuit according to an embodiment of the present invention, while  FIG. 9  shows a block diagram thereof.  
         [0055]     Referring to  FIGS. 8 and 9 , a Mach-Zehnder optical modulator  30  divides an optical beam coming in from an optical source  31  at a Y-branched part  32  and directs a first optical beam to an optical waveguide  33  and a second optical beam to an optical waveguide  34 . The optical waveguides  33  and  34  include respective phase modulators  33   a  and  34   a , and the phase modulators  33   a  and  34   a  cause phase modulation respectively in the first and second optical beams traveling through the optical waveguides  33  and  34 .  
         [0056]     The first and second optical beams thus modulated are merged at a X-branched part  35 , and first and second output optical beams output from the X-branched part  35  are directed further to optical waveguides  36  and  37 . The first and second output optical beams are then injected to a directional coupler optical switch  38  from the optical waveguides  36  and  37 , wherein the optical switch  38  provides an optical output to one of output optical waveguides  39  and  40 . In the present case, the output optical beam is provided to the output optical waveguide  39 , and the output optical waveguide  40  is provided as a dummy waveguide.  
         [0057]     The phase modulators  33   a  and  34   a  of the optical waveguides  33  and  34  are provided with a modulation signal of GHz frequency from a high-speed driving circuit  42 .  
         [0058]     As shown in  FIG. 10 , the high-speed driver circuit  42  includes a logic inversion part  43  and a driver  44 , wherein the logic inversion part  43  is supplied with a modulation signal and chirp switching signal and provides the modulation signal as output without inversion when the chirp switching signal has a logic value 0, while when the chirp switching signal has a logic value 1, the logic inversion part  43  provides the modulation signal as output with logic inversion. Thereby the driver  44  amplifies the output modulation signal of the logic inversion part  43 .  
         [0059]     Further, the chirp switching signal is supplied also to a phase modulator  38   a  of the directional coupler optical switch  38 . The phase modulators  33   a ,  34   a  and  38   a  can be constructed on a crystal substrate of LiNbO 3 , GaInAsP/InP, or GaN.  
         [0060]     It should be noted that the X-branched part  35  of the Mach-Zehnder optical modulator  30  provides output optical signals of complementary phase at the output optical waveguides  36  and  37  such that when one the output optical signal in one of the optical waveguides is ON, then the output optical signal in the other of the optical waveguides is OFF. Thereby, the complementary output optical signals of the optical waveguides  36  and  37  have the same phase. Thus, mutually inverted optical outputs are obtained with the same phase with such a construction.  
         [0061]      FIG. 11A  shows the input/output characteristic of the Mach-Zehnder optical modulator  30  while  FIG. 11B  shows the waveform of the modulation signal. Further,  FIGS. 11C and 11D  show the waveforms of the output optical signals of the optical waveguides  33  and  34  respectively, while FIG.  11 E shows the phase of the output optical signals. Further,  FIG. 11F  shows the waveform of the chirp.  
         [0062]     Equation (1) represents an a parameter, which is the parameter representing the magnitude of the chirp, in terms of the optical intensity S and the optical phase φ. 
 
α=2 ·S (δφ/δ t )/(δ S/δt )  (1) 
 
         [0063]     Referring to Equation (1), it can be seen that, in view of the complementary relationship of the output signals in the optical waveguides  36  and  37 , the term δS/δt at the denominator indicates that the chirp in the two outputs from the X-branched part  35  are mutually inverted.  
         [0064]     It should be noted that the magnitude a of the chirp is defined as  
           E   1     ⁡     (     t   ,     z   =   0       )       =           S   o     2       ⁢     exp   ⁡     [     j   ⁡     (         ω   0     ⁢   t     +       η   a     ⁢     V   ⁡     (   t   )           )       ]             
           E   2     ⁡     (     t   ,     z   =   0       )       =           S   o     2       ⁢     exp   ⁡     [     j   ⁡     (         ω   0     ⁢   t     +       η   b     ⁢     V   ⁡     (   t   )           )       ]             
             α   =       ⁢     2   ⁢   S   ⁢         ⅆ   ϕ       ⅆ   t       /       ⅆ   S       ⅆ   t                       =       ⁢         S   o     ⁡     (     1   +     cos   ⁡     (     π   ⁢           ⁢       V   ⁡     (   t   )       /     V   π         )         )       ⁢       π   ⁢     (       η   a     -     η   b       )         2   ⁢       V   π     ⁢     (       η   a     +     η   b       )           ⁢         ⅆ     V   ⁢     (   t   )           ⅆ   t       /                       ⁢       -         S   o     ⁢   π       2   ⁢     V   π           ⁢     sin   ⁡     (       π   ⁢           ⁢     V   ⁢     (   t   )           V   π       )       ⁢       ⅆ     V   ⁡     (   t   )           ⅆ   t                     =       ⁢       -         η   a     -     η   b           η   a     +     η   b           ⁢       tan     -   1       ⁡     (       π   ⁢           ⁢     V   ⁢     (   t   )           V   π       )                   
         α   ML     =         -         η   a     -     η   b           η   a     +     η   b           ⁢           ⁢     (     0   ≤     V   ⁡     (   t   )       ≤     V   π       )     ⁢     
     ⁢     α   MZ       =       -         η   a     -     η   b           η   a     +     η   b           ⁢           ⁢     (       V   π     ≤     V   ⁡     (   t   )       ≤     2   ⁢     V   π         )             
 
         [0065]     wherein E 1 (t, z=0) represents the electric field strength of the optical waveguide  33 , E 2 (t, z=0) represents the electric field strength of the optical waveguide  34 , S o  represents the incident optical intensity, η a  represents the phase modulation efficiency of the optical waveguide  33  and η b  represents the phase modulation efficiency of the optical waveguide  34 . Further, it should be noted that ω o  is the angular frequency of the light, V(t) is the voltage, V n  is the V pi voltage of the Mach-Zehnder modulator, while α MZ  is the α parameter of the Mach-Zehnder modulator.  
         [0066]     Next, the characteristics of the directional coupler optical switch  38  will be examined.  
         [0067]     The directional coupler optical switch  38  is a device that causes a phase difference between the optical signals in the two optical waveguides  36  and  37  and selects one of the optical output waveguides  39  and  40  to which the optical output is to be provided. Thereby, the total optical intensity of the two output optical waveguides  39  and  40  is held equal to the total optical intensity of the optical waveguides  36  and  37 .  
         [0068]     Because the output optical waveguides  36  and  37 , extending out from the X-branch  35  of the Mach-Zehnder optical modulator  30 , serve for the input optical waveguides of the optical directional coupler  38 , a constant total optical intensity equal to the total optical intensity of the optical waveguides  36  and  37  is maintained for the optical output signals in the optical waveguides  39  and  40 . Only the ratio of the optical intensity is changed between the optical waveguides  39  and  40 .  
         [0069]     Here, it should be noted that, while the chirp output from the X-branched part  35  are inverted with each other between the output waveguides  36  and  37 , average of the optical output of the two optical waveguides  36  and  37  becomes constant and unchanged. Thus, the optical output in the optical waveguide  39  is held constant during the switching operation of the directional coupler optical switch  38 . In other words, there is no fluctuation of optical output during the switching operation as shown in  FIG. 12 .  
         [0070]      FIG. 12  represents the optical intensity component of the optical waveguide  36  in the output optical waveguide  40  during the switching operation by a broken line and the optical intensity component of the optical waveguide  37  in the output optical waveguide  40  during the switching operation by a continuous line.  
         [0071]     Designating the optical intensities in the optical waveguides  36  and  37  as Iin 1  and Iin 2  and the optical intensities in the optical waveguides  40  and  39  as Iout 1  and Iout 2  respectively, there hold the equations 
 
 I in1 +I in2 =I out1 +I out2 
 
 I out1 =a·I in1 +( 1 −a )· I in2 
 
 I out2=(1 −a )· I in1 +a·I in2, 
 
 wherein a represents the mixing ratio of the optical directional coupler  38 . 
 
         [0072]     Thus, in the case of Iin 2 =Iin 1 , there holds the relationship 
 
Iout2=Iin1 
 
 for arbitrary value of the mixing ratio a. 
 
         [0073]     Strictly speaking, Iin 1  and Iin 2  changes with time because of the modulation. However, the speed of modulation is sufficiently high, and it is possible to consider the distribution of the optical power in terms of averaged optical power.  
         [0074]      FIG. 13  shows a chirp switching sequence.  
         [0075]     Referring to  FIG. 13 , the logic value of the chirp switching signal causes transition from 0 to 1 (or from 1 to 0) at a time t 1 .  
         [0076]     Thereby, the optical directional coupler switch  38  starts the switching operation at a time t 2  and the logic inversion part  43  starts logic inversion at a time t 3 .  
         [0077]     Thereafter, the logic inversion part  43  completes the logic inversion at a time t 5 , and the directional optical coupler switch  38  completes the switching operation at a time t 6 . The duration from t 2  to t 6  is in the order of ten milliseconds to several ten milliseconds and falls in the time interval allowed in an optical transmission system for path switching.  
         [0078]     Thus, with he present embodiment, it becomes possible to eliminate the time-dependent variation of optical signal strength at the time of inverting the transmission chirp of the optical signal output to the output optical waveguide  39 .  
         [0079]     Next, switching from the current path to the reserve path will be explained for the case of the optical transmission system of the ring structure with reference to  FIGS. 14A and 14B .  
         [0080]     Referring to  FIGS. 14A and 14B , there is formed an optical transmission system by nodes  50   a - 50   f  that are connected to form a ring structure, wherein it should be noted that each of the nodes  50   a - 50   f  performs optical amplification and dispersion compensation.  
         [0081]     In the case of transmitting an optical signal from a transmission part  51  to a reception part  52 , it is usual that the node  50   b  selects a current path A that reaches the node  50   b  from the node  50   a  in a clockwise direction as shown in  FIG. 14A . In this normal situation, the transmission part provides a negative transmission chirp α.  
         [0082]     Now, when there is caused a failure in the optical transmission path between the node  50   a  and the node  50   b , the node  50   b  detects the loss of signal and notifies this to the node  50   a  and the transmission part  51  via an optical supervisory channel (OSC)  53 .  
         [0083]     In response thereto, the transmission part  51  switches the transmission chirp α to positive, and the node  50   a  switches the path to a reserve path B that reaches the node  50   b  via the nodes  50   f ,  50   e ,  50   d  and  50   c  in the anticlockwise direction as shown in  FIG. 14B . Further, the node  50   b  also selects the reserve path B that reaches the node  50   b  from the node  50   a  in the clockwise direction.  
         [0084]     Thereby, it should be noted that the optical supervisory channel  53  holds the positive and negative sings of the transmission chirp determined optimally for all of the transmission paths.