Patent Publication Number: US-2022214593-A1

Title: Pluggable optical module, optical communication system and control method of pluggable optical module

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation application of U.S. patent application Ser. No. 16/409,227 filed on May 10, 2019, which is a continuation application of U.S. patent application Ser. No. 15/559,616 filed on Sep. 19, 2017, which is issued as U.S. Pat. No. 10,331,006, which is a National Stage Entry of international application PCT/JP2016/001603 filed on Mar. 18, 2016, which claims the benefit of priority from Japanese Patent Application 2015-057345 filed on Mar. 20, 2015, the disclosures of all of which are incorporated in their entirety by reference herein. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a pluggable optical module, an optical communication system and a control method of pluggable optical module. 
     BACKGROUND ART 
     In an optical communication system, an optical module used for transmission/reception of an optical signal is embedded. In such optical module, a Mach-Zehnder type optical modulator is used (e.g. Patent Literature 1). As the Mach-Zehnder type optical modulator, one in which phase difference between two optical waveguides constituting a Mach-Zehnder interferometer changes nonlinearly with respect to an applied voltage is known. In such Mach-Zehnder type optical modulator, a bias electrode to which a bias voltage is applied and a modulation electrode to which a data signal is applied are formed on one or both of the two optical waveguides. 
     Another example where the bias voltage is applied to the optical waveguide is also known (e.g. Patent Literature 2). In this example, an input light is split to two optical waveguides and the split lights propagating through each optical waveguide are multiplexed to output a modulated light. The bias voltage supplied to at least one optical waveguide in the two optical waveguides is controlled according to a wavelength of the input light. Further, a phase of the modulated light is controlled by a phase device voltage according to the wavelength of the input light. 
     A multivalued optical signal transmitter configured by using a Mach-Zehnder modulator (Patent Literature 3) is also introduced. 
     Meanwhile, for example, in an optical communication system in conformity with standards such as SFP (Small Form-Factor Pluggable) and XFP, use of a pluggable optical module has been developed. The pluggable optical module is an optical transceiver that is insertable into and removal from a socket of an optical communication apparatus. When performing a control of the pluggable optical module, the pluggable optical module receives control information from the optical communication apparatus serving as a host side. Then, operation switching and changing of the pluggable optical module are performed according to the received control information. 
     CITATION LIST 
     Patent Literature 
     [Patent Literature 1] Japanese Unexamined Patent Application Publication No. 2014-10187 
     [Patent Literature 2] Japanese Unexamined Patent Application Publication No. 2014-160985 
     [Patent Literature 3] Japanese Unexamined Patent Application Publication No. 2005-326548 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, the inventor has found out that there is a problem in the methods described above as descried below. When the Mach-Zehnder optical module is used, a bias point of the optical modulator can be controlled by applying the bias voltage to the electrode provided on the optical waveguide of the optical modulator. However, since there are individual differences among the optical modulators embedded in the pluggable optical module, the bias voltages to be applied to shift by the same phase are different for each pluggable optical module. Therefore, the pluggable optical module that can autonomously perform a change operation of the bias point when receiving a request for changing the bias point from the optical communication apparatus is required. 
     The present invention has been made in view of the aforementioned circumstances and aims to autonomously apply an appropriate bias voltage to an optical modulator according to phase angle information provided from outside in a pluggable optical module. 
     Solution to Problem 
     An aspect of the present invention is an optical module including: a pluggable electric connector configured to be capable of communicating a communication data signal and a control signal with an optical communication apparatus, the pluggable electric connector being insertable into and removable from the optical communication apparatus; an optical signal output unit including a Mach-Zehnder type optical modulator in which a phase modulation area are provided on a waveguide and configured to output an optical signal modulated according to the communication data signal; an optical power control unit configured to be capable of controlling optical power of the optical signal; a pluggable optical receptor configured to be capable of outputting the optical signal output from the optical power control unit to an optical fiber, the optical fiber being insertable into and removable from the pluggable optical receptor; and a control unit configured to control a modulation operation of the optical signal output unit and the bias voltage applied to the phase modulation area, in which the control unit determines the bias voltage applied to the phase modulation area according to the phase angle information included in the control signal from the pluggable electric connector, and the optical signal output unit applies the bias voltage determined by the control unit to the phase modulation area. 
     An aspect of the present invention is an optical communication system including: an optical fiber configured to transmit an optical signal; a pluggable optical module configured to output the optical signal to the optical fiber, the optical fiber being insertable into and removable from the pluggable optical module; and an optical communication apparatus configured to control the pluggable optical module, the pluggable optical module being insertable into and removable from the optical communication apparatus, in which the pluggable optical module includes: a pluggable electric connector configured to be capable of communicating a communication data signal and a control signal with an optical communication apparatus, the pluggable electric connector being insertable into and removable from the optical communication apparatus; an optical signal output unit including a Mach-Zehnder type optical modulator in which a phase modulation area are provided on a waveguide and configured to output an optical signal modulated according to the communication data signal; an optical power control unit configured to be capable of controlling optical power of the optical signal; a pluggable optical receptor configured to be capable of outputting the optical signal output from the optical power control unit to the optical fiber, the optical fiber being insertable into and removable from the pluggable optical receptor; and a control unit configured to control a modulation operation of the optical signal output unit and the bias voltage applied to the phase modulation area, the control unit determines the bias voltage applied to the phase modulation area according to the phase angle information included in the control signal from the pluggable electric connector, and the optical signal output unit applies the bias voltage determined by the control unit to the phase modulation area. 
     An aspect of the present invention is a wavelength change method of a pluggable optical module including: receiving a communication data signal and a control signal from a pluggable electric connector, the pluggable electric connector being insertable into or removable from an optical communication apparatus; determining a bias voltage applied to a phase modulation area provided in a Mach-Zehnder type optical modulator based on phase angle information included in the control signal; modulating a light from a light source based on the determined bias voltage and the communication data signal to generate the optical signal; controlling optical power of the optical signal; and outputting the optical signal the optical power of which is controlled to the optical fiber via a pluggable optical receptor, the optical fiber being insertable into and removable from the pluggable optical receptor. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to autonomously apply an appropriate bias voltage to an optical modulator according to phase angle information provided from outside in a pluggable optical module. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram schematically illustrating a configuration of a pluggable optical module according to a first exemplary embodiment; 
         FIG. 2  is a block diagram illustrating a configuration example of a main part of an optical communication system in which the pluggable optical module according to the first exemplary embodiment is embedded; 
         FIG. 3  is a block diagram illustrating a configuration of a control unit according to the first exemplary embodiment; 
         FIG. 4  is a diagram illustrating contents of a table of the control unit according to the first exemplary embodiment; 
         FIG. 5  is a block diagram illustrating a configuration example of an optical signal output unit according to the first exemplary embodiment; 
         FIG. 6  is a diagram schematically illustrating a configuration of an optical modulation unit according to the first exemplary embodiment; 
         FIG. 7  is a perspective view when the pluggable optical module according to the first exemplary embodiment is observed from a side of an optical fiber; 
         FIG. 8  is a perspective view when the pluggable optical module according to the first exemplary embodiment is observed from a side of an optical communication apparatus; 
         FIG. 9  is a sequence diagram illustrating a bias point change operation of the pluggable optical module according to the first exemplary embodiment; 
         FIG. 10  is a block diagram schematically illustrating a configuration of a pluggable optical module according to a second exemplary embodiment; 
         FIG. 11  is a block diagram schematically illustrating a configuration example of an optical signal output unit according to the second exemplary embodiment; 
         FIG. 12  is a diagram schematically illustrating a configuration of an optical modulation unit according to the second exemplary embodiment; 
         FIG. 13  is a diagram illustrating a table set of a control unit according to the second exemplary embodiment; 
         FIG. 14  is a sequence diagram illustrating a bias point change operation of the pluggable optical module according to the second exemplary embodiment; 
         FIG. 15  is a block diagram schematically illustrating a configuration of a pluggable optical module according to a third 
         FIG. 16  is a block diagram illustrating a configuration example of an optical signal output unit according to the third exemplary embodiment; 
         FIG. 17  is a diagram illustrating a table set group of a control unit according to the third exemplary embodiment; and 
         FIG. 18  is a sequence diagram illustrating a wavelength change operation of the pluggable optical module according to the third exemplary embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Exemplary embodiments of the present invention will be described below with reference to the drawings. The same components are denoted by the same reference numerals throughout the drawings, and a repeated explanation is omitted as needed. 
     First Exemplary Embodiment 
     A pluggable optical module  100  according to a first exemplary embodiment will be described.  FIG. 1  is a block diagram schematically illustrating a configuration of the pluggable optical module  100  according to the first exemplary embodiment.  FIG. 2  is a block diagram illustrating a configuration example of a main part of an optical communication system  1000  in which the pluggable optical module  100  according to the first exemplary embodiment is embedded. As illustrated in  FIG. 2 , the pluggable optical module  100  is configured to cause an optical fiber with connector  91  to be insertable into and removal from the pluggable optical module  100 . For example, a LC connector and MU connector, etc. can be used as a connector of the optical fiber with connector  91 . The pluggable optical module  100  is controlled based on a control signal CON 1  input from the optical communication apparatus  92  that is a communication host. Note that the pluggable optical module  100  can also receive a modulation signal MOD that is a data signal with the control signal CON 1  from the optical communication apparatus  92 . In this case, the pluggable optical module  100  may output an optical modulation signal LS modulated based on the received modulation signal MOD. The optical communication apparatus  92  performs communication signal processing such as flaming processing of a communication data signal from the pluggable optical module  100  or a communication signal input to the pluggable optical module  100 , for example. 
     The pluggable optical module  100  includes a pluggable electric connector  11 , an optical signal output unit  13 , an optical power control unit  14 , a control unit  12 , and a pluggable optical receptor  15 . 
     The pluggable electric connector  11  is configured to be insertable into and removable from the optical communication apparatus  92 . The pluggable electric connector  11  receives the control signal CON 1  that is an electric signal output from the optical communication apparatus  92  and forwards the control signal CON 1  to the control unit  12 . The pluggable electric connector  11  may also receive the modulation signal MOD that is an electric signal output from the optical communication apparatus  92  and forwards it to the optical signal output unit  13 . The pluggable electric connector  11  may also forward an electric signal output from the control unit  12  to the optical communication apparatus  92 . 
     The control unit  12  controls an operation of the optical signal output unit  13  based on the control signal CON 1  input from the optical communication apparatus  92  via the pluggable electric connector  11 . The control signal CON 1  includes phase angle information representing a phase angle to be set to arms (waveguides) of a Mach-Zehnder optical modulator.  FIG. 3  is a block diagram illustrating a configuration of the control unit  12  according to the first exemplary embodiment. The control unit  12  includes a memory unit  12 A and a command unit  12 B. In the memory unit  12 A, a table TAB representing correspondence between the phase angle information and a bias voltage applied to the arms (waveguides) of the Mach-Zehnder optical modulator is stored. The command unit  12 B refers to the table TAB based on the received phase angle information and determines the corresponding bias voltage. Then, the command unit  12 B instructs a value of the determined bias voltage to the optical signal output unit  13  by the control signal CON 2 . 
       FIG. 4  is a diagram illustrating contents of the table TAB in the control unit  12  according to the first exemplary embodiment. In the table TAB, information representing correspondence between the phase angle instructed by the optical communication apparatus  92  and the bias voltage is stored. In  FIG. 4 , the phase angle information is provided by an 8-bit signal. The optical communication apparatus  92  outputs the control signal CON 1  represented by the 8-bit value to the control unit  12  according to the instructed phase angle. Here, values of the phase angle information and the control signal CON 1  may be various types such as a 16-bit signal with code, for example. 
     The control unit  12  can also control optical power of the optical modulation signal LS output from the optical power control unit  14  by the control signal CON 3  output to the optical power control unit  14 . 
     The optical signal output unit  13  includes the Mach-Zehnder type optical modulator and outputs the optical modulation signal LS modulated by a predetermined modulation method. The optical signal output unit  13  modulates the optical modulation signal LS by applying the modulation signal MOD to phase modulation areas formed on the waveguides of the Mach-Zehnder type optical modulator. The optical signal output unit  13  can also control a bias point of the Mach-Zehnder type optical modulator by applying the bias voltages to the phase modulation areas. Note that the optical signal output unit  13  can modulate the optical modulation signal LS by various modulation methods such as phase modulation, amplitude modulation and polarization modulation, or by combining the various modulation methods. Here, for example, the Mach-Zehnder type optical modulator is a semiconductor optical modulator, etc. 
     Here, the phase modulation area is an area that includes an electrode formed on the optical waveguide. An effective refractive index of the optical waveguide below the electrode is changed by applying an electric signal, e.g., a voltage signal, to the electrode. As a result, a substantial optical length of the optical waveguide in the phase modulation area can be changed. Thus, the phase modulation area can change the phase of the optical signal propagating through the optical waveguide. Then, the optical signal can be modulated by providing a phase difference between the optical signals propagating through two optical waveguides. 
     A configuration example of the optical signal output unit  13  will be described.  FIG. 5  is a block diagram illustrating the configuration example of the optical signal output unit  13  according to the first exemplary embodiment. The optical signal output unit  13  includes a light source  16  and an optical modulation unit  17 . The light source  16  is a wavelength-tunable optical module, etc. including a semiconductor laser device and a ring oscillator, for example, and outputs an output light Lorig. The output light Lorig is controlled by a control signal CON 2 . 
     The optical modulation unit  17  is a Mach-Zehnder type optical modulator, for example. Note that, although not illustrated in  FIG. 1 , the optical modulation unit  17  outputs the optical modulation signal LS generated by modulating the output light Lorig according to the modulation signal MOD corresponding to the communication data signal input from the optical communication apparatus  92  via the pluggable electric connector  11 . 
     Next, a configuration of the optical modulation unit  17  will be described.  FIG. 6  is a diagram schematically illustrating the configuration of the optical modulation unit  17  according to the first exemplary embodiment. The optical modulation unit  17  is configured as a general Mach-Zehnder type optical modulator. The optical modulation unit  17  includes an optical modulator  17 A and a driver circuit  17 B. 
     The optical modulator  17 A modulates the output light Lorig from the light source  16  to output the optical modulation signal LS. The optical modulator  17 A includes waveguides  171  to  174  and phase modulation areas PMA and PMB. The output light Lorig from the light source  16  is input to one end of the waveguide  171 . The other end of the waveguide  171  is optically connected to one end of the waveguide  172  and one end of the waveguide  173 . Thus, a light propagating through the waveguide  171  is split into the waveguide  172  and the waveguide  173 . The other end of the waveguide  172  and the other end of the waveguide  173  are connected to one end of the waveguide  174 . 
     The phase modulation area PMA that changes a phase of a light propagating trough the waveguide  172  is provided on the waveguide  172 . The phase modulation area PMB that changes a phase of a light propagating trough the waveguide  172  is provided on the waveguide  173 . The optical modulation signal LS is output from the other end of the waveguide  174 . 
     The driver circuit  17 B can control a bias point of the optical modulator  17 A by applying a bias voltage VBIAS to one or both of the phase modulation areas PMA and PMB according to the control signal CON 2  while controlling a modulation operation of the optical modulator  17 A. Hereinafter, a case where the driver circuit  17 B applies the bias voltage to the phase modulation area PMA will be described. Further, the driver circuit  17 B can modulate the optical modulation signal LS by applying the modulation signal MOD to one or both of the phase modulation areas PMA and PMB. In this example, the driver circuit  17 B applies a modulation signal SIG_M 1  to the phase modulation area PMA according to the modulation signal MOD. The driver circuit  17 B applies a modulation signal SIG_M 2  to the phase modulation area PMB according to the modulation signal MOD. 
     The optical power control unit  14  can control the optical power of the optical modulation signal LS by attenuating or blocking the optical modulation signal LS output from the optical signal output unit  13 . As described above, the optical power control unit  14  controls the optical power of the optical modulation signal LS according to a control signal CON 3  output from the control unit  12 . For example, an optical attenuator may be used as the optical power control unit  14 . 
     The pluggable optical receptor  15  (also referred to as a first pluggable optical receptor) is configured to cause a connector of the outside optical fiber with connector  91  (also referred to as a first optical transmission line) to be insertable into and removable from the pluggable optical receptor  15 . The pluggable optical receptor  15  transmits the optical modulation signal LS output from the optical power control unit  14  to the optical fiber  91 . 
     Appearances of the pluggable optical module  100  will be described.  FIG. 7  is a perspective view when the pluggable optical module  100  according to the first exemplary embodiment is observed from a side of the optical fiber  91 . A numerical sign  61  shown in  FIG. 7  indicates an upper surface of the pluggable optical module  100 . A numerical sign  62  shown in  FIG. 7  indicates an entry point of the pluggable optical receptor  15  into which the connector of the optical fiber  91  is inserted.  FIG. 8  is a perspective view when the pluggable optical module  100  according to the first exemplary embodiment is observed from a side of the optical communication apparatus  92 . A numerical sign  63  shown in  FIG. 8  indicates a lower surface of the pluggable optical module  100 . A numerical sign  64  shown in  FIG. 8  indicates a connection part of the pluggable electric connector  11  which is connected to the optical communication apparatus  92 . 
     Next, a bias point change operation of the pluggable optical module  100  will be described.  FIG. 9  is a sequence diagram illustrating the bias point change operation of the pluggable optical module  100  according to the first exemplary embodiment. 
     Step S 11 : Phase Angle Information Reception 
     The control unit  12  receives the control signal CON 1  including the phase angle information for the bias point change from the optical communication apparatus  92 . 
     Step S 12 : Table Reference 
     The control unit  12  refers to the table TAB based on the received phase angle information to determine the bias voltage applied to the phase modulation area PMA. 
     Step S 13 : Bias Voltage Command 
     The control unit  12  instructs the value of the determined bias voltage to the optical signal output unit  13  by the control signal CON 2 . 
     Step S 14 : Bias Voltage Application 
     The optical signal output unit  13  applies the bias voltage to the phase modulation area PMA. 
     As described above, according to the present configuration, when the bias point of the optical modulation unit  17  of the optical signal output unit  13  in the pluggable optical module  100  is changed, the external optical communication apparatus  92  may provide with the phase angle information. As the phase angle information, a general signal such as a digital signal can be used. In this exemplary embodiment, the value of the bias voltage stored in the table TAB is predetermined to control the bias point of the optical modulation unit  17  according to the instructed phase angle information. Thus, the control unit  12  refers to the table TAB to instruct the bias voltage and thereby can control the bias point of the optical modulation unit  17  according to the instructed phase angle information. 
     Therefore, according to the present configuration, the optical communication apparatus can control the bias point of the optical modulator without considering individual difference of the pluggable optical module. Further, the pluggable optical module can autonomously apply the appropriate bias voltage to the phase modulation area of the optical modulator by referring to the table according to the command from the optical communication apparatus. 
     Second Exemplary Embodiment 
     A pluggable optical module  200  according to a second exemplary embodiment will be described. The pluggable optical module  200  has a configuration enabling more dynamic phase setting while the pluggable optical module  100  according to the first exemplary embodiment corresponds to the change of the bias point.  FIG. 10  is a block diagram schematically illustrating a configuration of the pluggable optical module  200  according to the second exemplary embodiment. The pluggable optical module  200  has the configuration in which the control unit  12  and the optical signal output unit  13  of the pluggable optical module  100  according to the first exemplary embodiment are replaced with a control unit  22  and an optical signal output unit  23 , respectively. Note that the pluggable optical module  200  may also receive the modulation signal MOD corresponding to the data signal with the control signal CON 1  from the optical communication apparatus  92 . In this case, the pluggable optical module  200  may output the optical modulation signal LS modulated based on the received modulation signal MOD. As other configuration of the pluggable optical module  200  is the same as that of the pluggable optical module  100 , a description thereof will be omitted. 
     A configuration example of the optical signal output unit  23  will be described.  FIG. 11  is a block diagram schematically illustrating the configuration example of the optical signal output unit  23  according to the second exemplary embodiment. The optical signal output unit  23  has a configuration in which the optical modulation unit  17  of the optical signal output unit  13  is replaced with an optical modulation unit  27 . 
     A configuration of the optical modulation unit  27  will be described.  FIG. 12  is a diagram schematically illustrating the configuration of the optical modulation unit  27  according to the second exemplary embodiment. The optical modulation unit  27  includes an optical modulator  27 A and a driver circuit  27 B. The optical modulator  27 A has a configuration in which a plurality of general Mach-Zehnder type optical modulators are combined. In this example, the optical modulator  27 A has the configuration in which four Mach-Zehnder type optical modulators MZ 1  to MZ 4  are combined. The Mach-Zehnder type optical modulators MZ 1  to MZ 4  each have the same configuration as the optical modulation unit  17  described in the first exemplary embodiment and are arranged in parallel. 
     The output light Lorig from the light source  16  is input to an optical waveguide WG 1 . The optical waveguide WG 1  is branched into an optical waveguide WG 2  and an optical waveguide WG 3 . The optical waveguide WG 2  is branched into an optical waveguide WG 4  and an optical waveguide WGS. The optical waveguide WG 4  is connected to the input of the Mach-Zehnder type optical modulator MZ 1 . The optical waveguide WG 5  is connected to the input of the Mach-Zehnder type optical modulator MZ 2 . The optical waveguide WG 3  is branched into an optical waveguide WG 6  and an optical waveguide WG 7 . The optical waveguide WG 6  is connected to the input of the Mach-Zehnder type optical modulator MZ 3 . The optical waveguide WG 7  is connected to the input of the Mach-Zehnder type optical modulator MZ 4 . 
     The output of the Mach-Zehnder type optical modulator MZ 1  is connected to an optical waveguide WG 8 . The output of the Mach-Zehnder type optical modulator MZ 2  is connected to an optical waveguide WG 9 . The output of the Mach-Zehnder type optical modulator MZ 3  is connected to an optical waveguide WG 10 . The output of the Mach-Zehnder type optical modulator MZ 4  is connected to an optical waveguide WG 11 . The optical waveguide WG 8  and the optical waveguide WG 9  merge to be connected to an optical waveguide WG 12 . The optical waveguide WG 10  and the optical waveguide WG 11  merge to be connected to an optical waveguide WG 13 . The optical waveguide WG 12  and the optical waveguide WG 13  merge to be connected to an optical waveguide WG 14 . The optical modulation signal LS that is modulated is output from the optical waveguide WG 14 . 
     Note that, in this exemplary embodiment, the phase modulation areas PMA and PMB provided on two optical waveguides of the Mach-Zehnder type optical modulator MZ 1  are referred to as phase modulation areas PM 1  and PM 2 . The phase modulation areas PMA and PMB provided on two optical waveguides of the Mach-Zehnder type optical modulator MZ 2  are referred to as phase modulation areas PM 3  and PM 4 . The phase modulation areas PMA and PMB provided on two optical waveguides of the Mach-Zehnder type optical modulator MZ 3  are referred to as phase modulation areas PM 5  and PM 6 . The phase modulation areas PMA and PMB provided on two optical waveguides of the Mach-Zehnder type optical modulator MZ 4  are referred to as phase modulation areas PM 7  and PM 8 . Further, phase modulation areas PM 9  to PM 12  are provided on the optical waveguides WG 8  to WG 11 , respectively. 
     The driver circuit  27 B can control a modulation operation of the optical modulator  27 A and also control a bias point of the optical modulator  27 A by applying the bias voltages to the phase modulation areas PM 1  to PM 12 , respectively. The driver circuit  27 B can also modulate the optical modulation signal LS by applying the modulation signals to the phase modulation areas PM 1  to PM 12 , respectively. 
     The control unit  22  determines the bias voltages applied to the phase modulation areas PM 1  to PM 12  of the optical modulation unit  27  by referring to a table set  20 . The table set  20  includes a plurality of tables and, in the present exemplary embodiment, includes tables T 1  to T 12  corresponding to the phase modulation areas PM 1  to PM 12 , respectively.  FIG. 13  is a diagram illustrating the table set  20  of the control unit  22  according to the second exemplary embodiment. The table set  20  includes the tables T 1  to T 12  indicating values of the bias voltages to be applied to each phase area for achieving the instructed phase angle. 
     Subsequently, a bias point change operation of the pluggable optical module  200  will be described.  FIG. 14  is a sequence diagram illustrating the bias point change operation of the pluggable optical module  200  according to the second exemplary embodiment. 
     Step S 21 : Phase Angle Information Reception 
     The control unit  22  receives the control signal CON 1  including phase angle information corresponding to the phase modulation areas PM 1  to PM 12  for changing the bias point from the optical communication apparatus  92 . 
     Step S 22 : Bias Voltage Determination 
     The control unit  22  refers to the tables T 1  to T 12  based on the phase angle information corresponding to the phase modulation areas PM 1  to PM 12  to determine the bias voltages applied to the phase modulation areas PM 1  to PM 12 . 
     Step S 23 : Bias Voltage Command 
     The control unit  22  instructs values of the determined bias voltages applied to the phase modulation areas PM 1  to PM 12  to the optical signal output unit  13  by the control signal CON 2 . 
     Step S 24 : Bias Voltage Application 
     The optical signal output unit  23  applies the determined bias voltages to the phase modulation areas PM 1  to PM 12 , respectively. 
     As described above, according to the present configuration, in the pluggable optical module  200  including the optical modulator in which a plurality of the Mach-Zehnder optical modulators are combined, the bias voltages suitable for achieving the instructed phase angle can be autonomously applied to each phase modulation area of each Mach-Zehnder optical modulator. Additionally, each phase modulation area can be controlled so that flexible phase setting can be dynamically achieved. 
     Third Exemplary Embodiment 
     A pluggable optical module  300  according to a third exemplary embodiment will be described. The pluggable optical module  300  is a modified example of the pluggable optical module  200  according to the second exemplary embodiment, and is configured to change the wavelength of the light output from the light source. There may be a case where the optical communication apparatus requests the pluggable optical module to change the wavelength. When complying with the request for the wavelength change, the pluggable optical module is required to autonomously perform and complete the change operation. 
       FIG. 15  is a block diagram schematically illustrating a configuration of the pluggable optical module  300  according to the third exemplary embodiment. The pluggable optical module  300  has the configuration in which the control unit  12  and the optical signal output unit  13  of the pluggable optical module  200  according to the second exemplary embodiment are replaced with a control unit  32  and an optical signal output unit  33 , respectively. 
     The optical signal output unit  33  is configured to be capable of changing the wavelength of the optical modulation signal LS. Thus, the optical signal output unit  33  outputs the optical modulation signal LS of a single wavelength within a tunable wavelength range according to the control signal CON 2  output from the control unit  32 . The optical signal output unit  33  may modulates the optical modulation signal LS by various types of modulation methods such as phase modulation, amplitude modulation and polarization modulation or by combining the various types of modulation methods as in the case of the optical signal output unit  23 . 
     A configuration example of the optical signal output unit  33  will be described.  FIG. 16  is a block diagram illustrating the configuration example of the optical signal output unit  33  according to the third exemplary embodiment. The optical signal output unit  13  includes a wavelength-tunable light source  36  and the optical modulation unit  27 . As the optical modulation unit  27  is the same as that in the second exemplary embodiment, a description thereof will be omitted. 
     The wavelength-tunable light source  36  includes a semiconductor laser and wavelength tuning means such as a ring oscillator, for example, and outputs the output light Lorig. The wavelength of the output light Lorig is controlled by the control signal CON 2  from the control unit  32 . 
     Similarly to the control unit  22  according to the second exemplary embodiment, the control unit  32  controls the modulation operation and the operating point of the optical modulation unit  27  of the optical signal output unit  33 . Further, the control unit  32  can control the wavelength of the output light Lorig of the wavelength-tunable light source  36  by the control signal CON 2 . 
     As described above, the pluggable optical module  300  is configured to be capable of changing the wavelength of the optical modulation signal LS. However, when the wavelength of the optical signal is different, the bias voltage applied to the phase modulation area for achieving the instructed phase angle also varies. Thus, for each changeable wavelength, a table set storing information of the bias voltage applied to each phase modulation area is required. 
       FIG. 17  is a diagram illustrating a table set group  30  of the control unit  32  according to the third exemplary embodiment. It is a block diagram illustrating a configuration of the control unit  32  according to the third exemplary embodiment. The control unit  32  refers to any of table sets  30 A to  30 C provided for each changeable wavelength included in the table set group  30  to determine the bias voltages applied to the phase modulation areas PM 1  to PM 12  of the optical modulation unit  27 . The table sets  30 A to  30 C include a plurality of tables similarly to the table set  20  according to the second exemplary embodiment. In this embodiment, the table sets  30 A to  30 C include tables T 1  to T 12  corresponding to the phase modulation areas PM 1  to PM 12 , respectively. 
     Subsequently, a wavelength change operation of the pluggable optical module  300  will be described.  FIG. 18  is a sequence diagram illustrating the wavelength change operation of the pluggable optical module  300  according to the third exemplary embodiment. 
     Step S 31 : Control Information Reception 
     In a state where the pluggable optical module  300  outputs the optical modulation signal LS of the wavelength λ1 to the optical fiber  91 , the control unit  32  receives the control signal CON 1  including control information from the optical communication apparatus  92 . This control information includes a wavelength change command of the optical signal and phase angle information corresponding to the phase modulation areas PM 1  to PM 12  for the bias point change from the optical communication apparatus  92 . 
     Step S 32 : Optical Signal Block Operation 
     The control unit  32  performs a block operation of the optical signal based on the wavelength change command. Specifically, the control unit  32  instructs the optical power control unit  14  to block the optical modulation signal LS using the control signal CON 3 . The optical power control unit  14  blocks the optical modulation signal LS according to the control signal CON 3 . The control unit  32  may also perform the block operation of the optical signal LS by instructing the optical signal output unit  33  to stop outputting the optical modulation signal LS using the control signal CON 2 . Note that the control unit  32  may also instruct the optical power control unit  14  to block the optical modulation signal LS and instruct the optical signal output unit  33  to stop outputting the optical modulation signal LS in parallel when performing the block operation of the optical signal. 
     Step S 33 : Wavelength Change Command 
     The control unit  32  instructs the optical signal output unit  33  to change the wavelength of the optical modulation signal LS from λ 1  to λ 2  (λ 1 ≠λ 2 ) based on the wavelength change command. Thus, the optical signal output unit  33  changes the wavelength of the optical modulation signal LS from λ 1  to λ 2 . In this case, the optical signal output unit  33  may perform the operation for changing the wavelength from λ 1  to μ 2  after stopping the output of the optical modulation signal LS. 
     Step S 34 : Table Set Reference 
     The control unit  32  refers to the table set  30 B corresponding to the phase modulation areas PM 1  to PM 12  at the wavelength λ 2  after the wavelength change based on the phase angle information to determine the bias voltages applied to the phase modulation areas PM 1  to PM 12 . 
     Step S 35 : Bias Voltage Command 
     The control unit  32  instructs the values of the determined bias voltages applied to the phase modulation areas PM 1  to PM 12  to the optical signal output unit  33  by the control signal CON 2 . 
     Step S 36 : Bias Voltage Application 
     The optical signal output unit  33  applies the determined bias voltage to the phase modulation areas PM 1  to PM 12 , respectively. 
     Step S 37 : Output Restart Command 
     After the wavelength change is finished, the control unit  32  performs an operation for restarting the output of the optical modulation signal LS. Specifically, the control unit  32  controls the optical power control unit  14  to control the optical modulation signal LS to predetermined optical power. Thus, the optical modulation signal LS of the wavelength λ 2  is output to the optical fiber  91 . 
     The control unit  32  may control the optical signal output unit  13  to output the optical modulation signal LS of the wavelength λ 2  after the wavelength change before the control of the optical power control unit  14  in the Step S 14  when the output of the optical signal of the optical signal output unit  33  has been stopped in the Step S 32  or S 33 . 
     As described above, the present configuration robustly blocks the output of the optical signal when the pluggable optical module changes the wavelength of the optical signal according to the command of the optical communication apparatus  92 . Therefore, transmission of an instable optical signal during the wavelength change can be prevented. Then, the optical signal is transmitted after the wavelength change so that the optical signal having the desired wavelength and stability can be transmitted from the pluggable optical module. 
     Thus, the appropriate bias voltage corresponding to the wavelength after the change can be applied. In other words, in each of the changeable plurality, the appropriate bias voltages for achieving the instructed phase angle can be applied to the phase modulation areas provided in the modulation unit. 
     Other Exemplary Embodiments 
     The present invention is not limited to the above-described exemplary embodiments, and can be modified as appropriate without departing from the scope of the invention. For example, in the exemplary embodiments described above, the optical communication apparatus  92  also may perform a status request on the pluggable optical module. In this case, the control unit of the pluggable optical module receives the status request from the optical communication apparatus  92  via the pluggable electric connector  11 . The control unit of the pluggable optical module notifies the optical communication apparatus  92  via the pluggable electric connector  11  of the operation state of the pluggable optical module when receiving the status request. Specifically, the control unit of the pluggable optical module notifies the optical communication apparatus  92  whether or not the bias point operation or the wavelength change operation is running. The control unit of the pluggable optical module may also notify the optical communication apparatus  92  which processing stage of each step illustrated in  FIG. 10 ,  FIG. 14  and  FIG. 18  is running during the wavelength change operation. Further, it is possible to notify the optical communication apparatus  92  of operation stability information of the wavelength-tunable light source and the optical modulation unit included in the optical signal output unit  13 . 
     For example, in the third exemplary embodiment, it can be assumed that the optical communication apparatus  92  instructs the pluggable optical module to stop the output of the optical signal while the pluggable optical module is performing the wavelength change operation. In this case, the control unit of the pluggable optical module receives the command to stop the output of the optical signal from the optical communication apparatus  92  via the pluggable electric connector  11 . However, since the pluggable optical module is under the wavelength change operation, the optical module may reject the command to stop the output of the optical signal. Therefore, occurrence of malfunction due to the overlapped operation requests can be prevented. Further, when receiving the command to stop the optical signal output from the optical communication apparatus  92  via the pluggable electric connector  11 , the optical signal output may be stopped after the wavelength change is completed. Therefore, the overlapped operation requests can be processed in order and the desired operation required by the optical communication apparatus  92  can be robustly performed. Note that it goes without saying that the optical communication apparatus  92  can appropriately instruct the pluggable optical module to start the output of the optical signal and to stop the output of the optical signal. 
     In the exemplary embodiments described above, it is described that the control unit of the pluggable optical module controls the wavelength-tunable light source, the optical modulation unit and the optical power control unit according to the control signal CON 1  from the optical communication apparatus  92 . However, it is merely an example. The control unit of the pluggable optical module can autonomously control the wavelength-tunable light source, the optical modulation unit and the optical power control unit regardless of the control signal from outside. 
     In the exemplary embodiments described above, the communication of the control signal via the pluggable electric connector  11  can be achieved applying the technologies such as a MDIO (Management Data Input/Output) or an I2C (Inter-Integrated Circuit). 
     In the exemplary embodiments described above, the power of the optical signal output from the optical signal output unit may be monitored and, for example, the optical output power of the wavelength-tunable light source provided in the optical signal output unit may be feedback-controlled. In this case, a part of the optical signal output from the optical signal output unit is branched by such as an optical demultiplexer and the branched optical signal is monitored by a light receiving device such as a photodiode. Then, the control unit can feedback-control the power of the optical signal output from optical signal output unit by notifying the control unit of the monitoring result. 
     In the exemplary embodiments described above, the power of the optical signal output from the optical power control unit is monitored and, for example, the optical power of the optical power control unit and the optical output power of the wavelength-tunable light source provided in the optical signal output unit may be feedback-controlled. In this case, a part of the optical signal output from the optical power control unit is branched by such as the optical demultiplexer and the branched optical signal is monitored by the light receiving device such as the photodiode. Then, the control unit can feedback-control one or both of the power of the optical signal output from optical signal output unit and the optical power controlled by the optical power control unit by notifying the control unit of the monitoring result. 
     In the exemplary embodiments described above, although it is described that the configuration of the transmission side of the pluggable optical module, it goes without saying that the pluggable optical module may include a reception unit receiving the optical signal from the outside and demodulating the received optical signal. 
     The pluggable optical module described above may set a phase difference at the output of the optical modulator at the time of no modulation to zero (phase difference minimum) or 180 degrees (phase difference maximum) by controlling the bias point. Further, the pluggable optical module may set the phase difference at the output of the optical modulator at the time of no modulation to any phase difference such as 30 degrees, 45 degrees and 60 degrees by controlling the bias point. 
     The present invention has been described above with reference to the exemplary embodiments, but the present invention is not limited to the above exemplary embodiments. The configuration and details of the present invention can be modified in various ways which can be understood by those skilled in the art within the scope of the invention. 
     This application is based upon and claims the benefit of priority from Japanese patent application No. 2015-57345, filed on Mar. 20, 2015, the disclosure of which is incorporated herein in its entirety by reference. 
     REFERENCE SIGNS LIST 
     
         
           11  PLUGGABLE ELECTRIC CONNECTOR 
           12 ,  22 ,  32  CONTROL UNITS 
           12 A MEMORY UNIT 
           12 B COMMAND UNIT 
           13 ,  23 ,  33  OPTICAL SIGNAL OUTPUT UNITS 
           14  OPTICAL POWER CONTROL UNIT 
           15  PLUGGABLE OPTICAL RECEPTOR 
           16  LIGHT SOURCE 
           17 ,  27  OPTICAL MODULATION UNITS 
           17 A,  27 A OPTICAL MODULATOR 
           17 B,  27 B DRIVER CIRCUITS 
           20 ,  30 A TO  30 C TABLE SETS 
           30  TABLE SET GROUP 
           36  WAVELENGTH-TUNABLE LIGHT SOURCE 
           91  OPTICAL FIBER 
           92  OPTICAL COMMUNICATION APPARATUS 
           100 ,  200 ,  300  PLUGGABLE OPTICAL MODULES 
           171  TO  174  OPTICAL WAVEGUIDES 
           1000  OPTICAL COMMUNICATION SYSTEM 
         CON 1  TO CON 3  CONTROL SIGNALS 
         Lorig OUTPUT LIGHT 
         LS OPTICAL MODULATION SIGNAL 
         MOD MODULATION SIGNAL 
         MZ 1  TO MZ 4  MACH-ZEHNDER TYPE OPTICAL MODULATORS 
         PM 1  OT PM 12 , PMA, PMB PHASE MODULATION AREAS 
         T 1  TO T 12  TABLES 
         TAB TABLE 
         WG 1  TO WG 14  OPTICAL WAVEGUIDES