Abstract:
[Problem] To provide an optical communication device and the like with which the optical signals output by multiple light output units can be set with an accurate wavelength interval. 
     [Solution] An optical communication device equipped with: multiple light output units that output optical signals of mutually different wavelengths; a multiplexing unit that outputs a wavelength-multiplexed signal by multiplexing the multiple optical signals output from the multiple light output units; a reference light output unit that outputs reference light serving as a reference; a wavelength generation unit that, on the basis of the reference light, outputs multiple light beams having a wavelength interval in accordance with a prescribed frequency; and a wavelength control unit that controls the wavelength of the optical signals output by the light output units in accordance with the interference component between the wavelength-multiplexed signal and light with the wavelength of at least one of the multiple light beams.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an optical communication device, an optical communication system and an optical transmission method. 
       BACKGROUND ART 
       [0002]    There is a method of narrowing the wavelength (frequency) interval of a wavelength multiplexing optical signal to improve frequency utilization efficiency for the purpose of increasing the transmission capacity of an optical fiber transmission system. However, this method has a problem that an influence of overlapping (cross talk) of optical spectra between adjacent channels is increased since the number of wavelengths of a wavelength-multiplexed optical signal is large, thereby deteriorating the transmission quality of the wavelength multiplexing optical signal. 
         [0003]    For the above-mentioned problem, PTL 1 discloses a technique of making the wavelength of an optical signal to be output from each of a plurality of optical transmitters coincide with the wavelength of a reference wavelength optical signal, by using a transmission wavelength variable filter. The reference wavelength optical signal is one of a plurality of optical signals whose wavelength intervals are constant, which are obtained by frequency-modulating a predetermined light source with an output from a microwave oscillator. The technique described in PTL 1 makes wavelength intervals of a plurality of optical signals to be output from a plurality of optical transmitters coincide with a wavelength interval generated by the microwave oscillator. 
         [0004]    In a technique described in PTL 1, each of a plurality of optical transmitters comprises the transmission wavelength variable filter, and each of the optical transmitters controls the transmission wavelength variable filter so that the intensity of a reference wavelength optical signal becomes a maximum. The optical transmitter then controls the wavelength of an optical signal to be output so that the wavelength of the optical signal to be output becomes a maximum at the transmission wavelength variable filter. As mentioned above, the technique described in PTL 1 indirectly makes the wavelength of an optical signal to be output from each of a plurality of optical transmitters coincide with a reference wavelength optical signal by using a transmission wavelength variable filter. PTL 2 describes a frequency control method of an optical transmitter or an optical receiver used in a wavelength multiplexing optical communication network. 
       CITATION LIST 
     Patent Literature 
       [0005]    [PTL 1] Japanese Unexamined Patent Application Publication No. 2001-203643 
         [0006]    [PTL 2] Japanese Unexamined Patent Application Publication No. H08-181660 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0007]    As described above, a technique described in PTL 1 indirectly makes the wavelength contained in a reference wavelength optical signal coincide with the wavelength of an optical signal to be output from each of a plurality of optical transmitters by using a transmission wavelength variable filter. 
         [0008]    However, in the technique described in PTL 1, the wavelength of an optical signal to be adjusted depends on the accuracy of a transmission wavelength variable filter. The characteristics of a transmission wavelength variable filter change with temperature or time. This involves a shift in a wavelength of an optical signal to be adjusted and an overlap of optical spectra between adjacent channels deteriorates the transmission quality of a wavelength multiplexing optical signal, which is problematic. In addition, PTL 2 does not disclose a technique for solving such a problem. 
         [0009]    An object of the present invention is to solve the above-described problems and to provide an optical communication device or the like which can output a wavelength-multiplexed optical signal whose wavelength interval has higher accuracy. 
       Solution to Problem 
       [0010]    The optical communication device of the present invention includes: a plurality of light output unit that outputs optical signals having different wavelengths, a multiplexing unit that outputs a wavelength-multiplexed signal obtained by multiplexing a plurality of optical signals output from the plurality of light output units, a reference light output unit that outputs a reference light that is a reference, a wavelength generation unit that outputs, based on the reference light, a plurality of lights having a wavelength interval depending on a predetermined frequency, and a wavelength control unit that controls, depending on an interference component between the wavelength-multiplexed signal and a light having at least one wavelength among the plurality of lights, a wavelength of an optical signal output from the light output unit. 
         [0011]    The optical communication system of the present invention includes: an optical communication device including a plurality of light output unit that outputs optical signals having different wavelengths, a multiplexing unit that outputs a wavelength-multiplexed signal obtained by multiplexing a plurality of optical signals output from the plurality of light output unit, a reference light output unit that outputs a reference light that is a reference, a wavelength generation unit that outputs, based on the reference light, a plurality of lights having a wavelength interval depending on a predetermined frequency, and a wavelength control unit that controls, depending on an interference component between the wavelength-multiplexed signal and a light having at least one wavelength among the plurality of lights, a wavelength of an optical signal output from the light output unit; and a control device that instructs the optical communication device to control a wavelength of an optical signal output from at least one of the plurality of light output unit at a predetermined timing. 
         [0012]    The optical communication method of the present invention includes: outputting a plurality of optical signals having different wavelengths from light output unit; outputting the wavelength-multiplexed signal obtained by multiplexing a plurality of optical signals; outputting a reference light that is a reference; outputting, based on the reference light, a plurality of lights having a wavelength interval depending on a predetermined frequency; and controlling, depending on an interference component between the wavelength-multiplexed signal and a light having at least one wavelength among the plurality of lights, a wavelength of an optical signal output from the light output unit. 
         [0013]    The program recording medium for a program stores thereon a program for allowing a computer to execute: a processing of outputting optical signals having different wavelengths; a processing of outputting a wavelength-multiplexed signal obtained by multiplexing a plurality of optical signals output from the plurality of light output units; a processing of outputting a reference light that is a reference; a processing of outputting, based on the reference light, a plurality of lights having a wavelength interval depending on a predetermined frequency, and a processing of controlling, depending on an interference component between the wavelength-multiplexed signal and a light having at least one wavelength among the plurality of lights, a wavelength of an optical signal output from the light output unit. 
       Advantageous Effects of Invention 
       [0014]    The optical communication device or the like of the present invention can advantageously output a wavelength-multiplexed optical signal whose wavelength interval has a higher accuracy. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0015]      FIG. 1  is a diagram illustrating a configuration example of an optical communication device in a first exemplary embodiment of the present invention. 
           [0016]      FIG. 2  is a flow chart illustrating an operational example of an optical communication device in the first exemplary embodiment of the present invention. 
           [0017]      FIG. 3  is a diagram illustrating a configuration example of an optical communication device  1  in a second exemplary embodiment of the present invention. 
           [0018]      FIG. 4  is a diagram illustrating an example of a state of frequencies of optical signals output from a plurality of light output units  10  and a state of frequencies of a plurality of optical signals included in a wavelength-multiplexed signal output from the multiplexing unit  13  in the second exemplary embodiment of the present invention. 
           [0019]      FIG. 5  is a diagram illustrating a state of a frequency of a reference light output from a reference light output unit  14  and a state of frequencies of a plurality of sideband lights (optical comb) generated by an optical comb generator  11  in the second exemplary embodiment of the present invention. 
           [0020]      FIG. 6  is a diagram illustrating an example of a state of a frequency of an optical signal selected by a wavelength selection unit  18  from a plurality of optical signals generated by the optical comb generator  11  and states of a frequency of a light output unit  10 - 2  before and after adjustment of the frequency by a wavelength control unit  16 . 
           [0021]      FIG. 7  is a flow chart illustrating an operational example of the optical communication device  1  in the second exemplary embodiment of the present invention. 
           [0022]      FIG. 8  is a diagram illustrating a configuration example of the optical communication device  1  in a third exemplary embodiment of the present invention. 
           [0023]      FIG. 9  is a flow chart illustrating an operational example of the optical communication device  1  in the third exemplary embodiment of the present invention. 
           [0024]      FIG. 10  is a diagram illustrating a configuration example of a communication system in a fourth exemplary embodiment of the present invention. 
           [0025]      FIG. 11  is a diagram illustrating a configuration example of the optical communication device  1  in the fourth exemplary embodiment of the present invention. 
           [0026]      FIG. 12  is a flow chart illustrating an operational example of the optical communication device  1  in the fourth exemplary embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Exemplary Embodiment 
       [0027]    A first exemplary embodiment of the present invention will be described with reference to the Drawings. 
         [0028]      FIG. 1  is a diagram illustrating a configuration example of an optical communication device  1  in a first exemplary embodiment of the present invention. As illustrated in  FIG. 1 , the optical communication device  1  comprises a plurality of light output units  10 - 1  to  10 -N (unless there is a particular need for distinction, referred to as “light output unit  10 ”), a multiplexing unit  13 , a reference light output unit  14 , a wavelength generation unit  15 , and a wavelength control unit  16 . 
         [0029]    The plurality of light output units  10  output optical signals having different wavelengths. 
         [0030]    The multiplexing unit  13  outputs a wavelength-multiplexed signal obtained by multiplexing a plurality of optical signals output from the plurality of light output units  10  to the outside of the optical communication device  1 . The multiplexing unit  13 , for example, can branch part of a wavelength-multiplexed signal by an optical coupler or the like, and output the part of a wavelength-multiplexed signal to the wavelength control unit  16 . 
         [0031]    The reference light output unit  14  outputs a reference light which is a reference for the wavelength of an optical signal to be output from each light output unit  10 . The light output unit  10  and the reference light output unit  14  are, for example, a laser or the like which can output a coherent light. The type of the structure or the like of the laser is not restricted, and may be, for example, a variable wavelength laser or the like in which an output wavelength can be changed. 
         [0032]    The wavelength generation unit  15  outputs a plurality of lights having a wavelength interval corresponding to the frequency, for example, based on an electric signal having a frequency depending on a wavelength interval of each optical signal of a wavelength-multiplexed optical signal transmitted by a light transmission path. The wavelength generation unit  15  may output a plurality of lights, for example, by an optical comb generator which includes a combination of a phase modulator and a resonator. 
         [0033]    The wavelength control unit  16  compares a wavelength of a light from the wavelength generation unit  15  and a wavelength of a wavelength-multiplexed optical signal from the multiplexing unit  13 . The wavelength control unit  16  compares a light from the wavelength generation unit  15  and a wavelength-multiplexed optical signal from the multiplexing unit  13  by directly interfering two input lights like, for example, an optical interferometer. Here, the lights to be interfered may be, for example, part of an optical signal in a wavelength-multiplexed optical signal or part of a plurality of lights from the wavelength generation unit  15 . A light of part of wavelengths can be taken out from a light of a plurality of wavelengths by an optical filter or the like. 
         [0034]    The wavelength control unit  16  controls a wavelength of an optical signal to be output from the light output unit  10  corresponding to the compared wavelength based on the compared result. For example, the wavelength control unit  16  controls a wavelength of an optical signal output from the light output unit  10  to eliminate a wavelength shift between a light from the wavelength generation unit  15  and a wavelength-multiplexed optical signal from the multiplexing unit  13 . For example, the wavelength control unit  16  controls a wavelength of an optical signal output from the light output unit  10  to maximize the interference between two lights when a spectrointerferometer is used. The wavelength of an optical signal output from the light output unit  10  may be changed, for example, by the temperature or driving current of a laser or may be changed by controlling an oscillation wavelength in the case of a variable wavelength laser. 
         [0035]      FIG. 2  is a flow chart illustrating an operational example of an optical communication device  1  in the first exemplary embodiment of the present invention. 
         [0036]    A plurality of light output units  10  output optical signals having different wavelengths (S 101 ). 
         [0037]    The multiplexing unit  13  outputs a wavelength-multiplexed signal obtained by multiplexing a plurality of optical signals output from a plurality of light output units  10  to the outside of the optical communication device  1 , the multiplexing unit  13  branches part of the wavelength-multiplexed signal, and outputs part of the wavelength-multiplexed signal to the wavelength control unit  16  (S 102 ). 
         [0038]    The reference light output unit  14  outputs a reference light which is a reference for a wavelength of an optical signal output from each light output unit  10  (S 103 ). 
         [0039]    The wavelength generation unit  15  outputs a plurality of lights having a wavelength interval corresponding to the frequency, for example, based on an electric signal having a frequency depending on a wavelength interval of each optical signal of a wavelength-multiplexed optical signal transmitted by a light transmission path (S 104 ). 
         [0040]    The wavelength control unit  16  compares a wavelength of a light from the wavelength generation unit  15  and a wavelength of a wavelength-multiplexed optical signal from the multiplexing unit  13 , and controls a wavelength of an optical signal output from the light output unit  10  based on the compared result (S 105 ). 
         [0041]    As mentioned above, the wavelength control unit  16  of the optical communication device  1  of the first exemplary embodiment of the present invention directly compares a wavelength of a light from the wavelength generation unit  15  and a wavelength of a wavelength-multiplexed optical signal from the multiplexing unit  13 , and controls a wavelength of an optical signal output from the light output unit  10  corresponding to the compared wavelength. Accordingly, the optical communication device  1  in the first exemplary embodiment of the present invention can output a wavelength-multiplexed optical signal whose wavelength interval has higher accuracy. 
       Second Exemplary Embodiment 
       [0042]    A second exemplary embodiment of the present invention will be described with reference to the Drawings. Description of a configuration in the second exemplary embodiment of the present invention similar to that of the first exemplary embodiment of the present invention will be omitted. 
         [0043]      FIG. 3  is a diagram illustrating a configuration example of the optical communication device  1  in the second exemplary embodiment of the present invention. As illustrated in  FIG. 3 , the optical communication device  1  in the second exemplary embodiment of the present invention comprises light output units  10 - 1  to  10 - 5 , a multiplexing unit  13 , a reference light output unit  14 , a wavelength generation unit  15 , a wavelength control unit  16 , a wavelength selection unit  18 , and a wavelength switching unit  19 . The number of the light output units  10  is not restricted to five, and any number of the light output units  10  may be used. The wavelength generation unit  15  includes an optical comb generator  11  and a microwave oscillator  12 . The wavelength control unit  16  comprises a comparison unit  17 . 
         [0044]    A plurality of light output units  10  output optical signals having different wavelengths. The wavelength of an optical signal output from each of the plurality of light output units  10  is set to have a predetermined interval. The wavelength of an optical signal output from each of the plurality of light output units  10 , however, may shift from a set wavelength due to device-specific optical frequency setting accuracy. 
         [0045]      FIG. 4  is a diagram illustrating an example of a state of frequencies corresponding to wavelengths of optical signals output from a plurality of light output units  10  and a state of optical spectrum of a plurality of optical signals included in a wavelength-multiplexed signal output from the multiplexing unit  13 .  FIG. 4  illustrates an example in which the frequency of an optical signal output from the light output unit  10 - 2  shifts by “+Δf”. The frequency of an optical signal output from the light output unit  10  shifts from a desired frequency by “+Δf” due to, for example, a change with time. 
         [0046]    In  FIG. 4 , a frequency f 1  of an optical signal output from the light output unit  10 - 1  is represented by “f 1 =f 0 −2×f m ”, and a frequency f 2  of an optical signal output from the light output unit  10 - 2  is represented by “f 2 =f 0 −f m +Δf”. Here, the f 0  is the frequency of a reference light output from the reference light output unit  14 . The f m  is a frequency interval of adjacent optical signals (i.e., between adjacent channels) output from the light output unit  10 . Similarly, a frequency f 3  of an optical signal output from the light output unit  10 - 3  is represented by “f 3 =f 0 ”, a frequency f 4  of an optical signal output from light output unit  10 - 4  is represented by “f 4 =f 0 +f m ”, and a frequency f 5  of an optical signal output from light output unit  10 - 5  is represented by “f 5 =f 0 +2×f m ”. 
         [0047]    As illustrated in  FIG. 4 , when the frequency of an optical signal output from the light output unit  10 - 2  shifts by “+Δf”, optical spectra overlap between adjacent channels (an optical signal output from the light output unit  10 - 2  and an optical signal output from the light output unit  10 - 3 ) in a wavelength-multiplexed signal. 
         [0048]    Accordingly, the optical communication device  1  of the second exemplary embodiment of the present invention controls the wavelength of an optical signal output from at least one of a plurality of light output units  10  to make the wavelength of an optical signal output from the plurality of light output units  10  coincide with the wavelength of at least one of lights to be generated by the optical comb generator  11 . 
         [0049]    In the second exemplary embodiment of the present invention, the plurality of light output units  10  are controlled in such a way that optical spectra are not shifted between adjacent channels to a degree to which the optical spectra overlap. The temperatures or the like of the plurality of light output units  10  are controlled. For this reason, it is sufficient to consider a shift of a wavelength (a frequency corresponding to the wavelength) by which optical spectra between the adjacent channels overlap. 
         [0050]    The multiplexing unit  13  outputs a wavelength-multiplexed signal obtained by multiplexing optical signals output from a plurality of light output units  10  to the outside of the optical communication device  1 . The multiplexing unit  13 , for example, can branch part of a wavelength-multiplexed signal by an optical coupler or the like, and output the part of a wavelength-multiplexed signal to the comparison unit  17 . 
         [0051]    The reference light output unit  14  outputs a reference light having a wavelength corresponding to a frequency f 0  which is a reference for the wavelength of an optical signal to be output from each light output unit  10 . The reference light output from the reference light output unit  14  is input to the optical comb generator  11 . 
         [0052]    A microwave oscillator  12  in the wavelength generation unit  15  outputs a clock signal (an electric signal) having a frequency F m . The frequency F m  is, for example, 50 GHz or 40 GHz. The frequency F m  of a clock signal corresponds to a wavelength interval of a plurality of optical signals output from the plurality of light output units  10 . A clock signal output from the microwave oscillator  12  is input to the optical comb generator  11 . For the microwave oscillator  12 , for example, an oscillator for modulation described in Japanese Patent No. 3444958 can be used. 
         [0053]    The optical comb generator  11  is driven by a clock signal input from the microwave oscillator  12 , and generates a plurality of sideband lights (optical comb) having wavelengths corresponding to frequencies around the frequency f 0  at a frequency interval of F m  when a reference light having a wavelength corresponding to the frequency f 0  is input. 
         [0054]    Here, for the optical comb generator  11 , an optical comb generator described in Japanese Patent No. 3444958 can be used. The optical comb generator described in Japanese Patent No. 3444958 outputs an optical comb composed of several hundreds of optical spectral lines arranged around an optical frequency of a laser beam emitted from a wavelength stabilized light source at an optical frequency interval of fm. 
         [0055]      FIG. 5  is a diagram illustrating a state of a frequency of a reference light output from the reference light output unit  14 , and a state of frequencies of an optical comb generated by the optical comb generator  11 . As illustrated in  FIG. 5 , a plurality of lights output from the optical comb generator  11  are an optical comb having a frequency interval of F m  around a frequency f 0  of a reference light. The frequency interval of the optical comb generated by the optical comb generator  11  is highly accurately controlled, and highly accurately corresponds to a frequency F m  of a clock signal input from the microwave oscillator  12 . 
         [0056]    The wavelength selection unit  18  inputs, among a plurality of lights included in an optical comb generated by the optical comb generator  11 , a light whose wavelength has been specified by the wavelength switching unit  19  to the comparison unit  17 . For the wavelength selection unit  18 , for example, a wavelength selection switch described in Japanese Patent No. 4748514 can be used. The wavelength selection switch can output a light having a desired wavelength among a plurality of input lights. The wavelength selection switch comprises an optical waveguide circuit formed on a substrate and control means for adding thermal variations to the optical waveguide circuit and performing switching control of an optical signal path, and can precisely control the wavelength of a light to be output. 
         [0057]    The comparison unit  17  compares a wavelength of a light from the wavelength generation unit  15  and a wavelength of a wavelength-multiplexed optical signal from the multiplexing unit  13 . Specifically, the comparison unit  17  compares a frequency “f 2 =f 0 −f m +Δf” corresponding to a wavelength of an optical signal output from the light output unit  10 - 2  included in the wavelength-multiplexed signal and a frequency “f 0 −2×F m ” corresponding to a wavelength of a light input from the wavelength selection unit  18 . For the comparison unit  17 , an optical signal detection device described in Japanese Patent No. 2758556 can be used. The optical signal detection device described in Japanese Patent No. 2758556 measures an interference intensity (interference component) which is a difference between input two optical signals. As a result of comparison, the comparison unit  17  detects a difference of wavelengths corresponding to a difference (“Δf”) between both of the optical signals. 
         [0058]    The wavelength control unit  16  controls the wavelength of an optical signal output from the light output unit  10  in such a way that the difference (shift) detected by the comparison unit  17  is compensated. Here, for the light output unit  10 , for example, a wavelength variable laser device described in Japanese Patent No. 3197869 can be used. In the wavelength variable laser described in Japanese Patent No. 3197869, a diffraction grating is used for a wavelength selecting element, and a wavelength to be output (to be selected) can be changed by adjusting the angle of the diffraction grating. 
         [0059]    The wavelength switching unit  19  specifies a wavelength to be selected by the wavelength selection unit  18 , and requests the wavelength selection unit  18  to output a light having the specified wavelength. The wavelength switching unit  19  specifies a light output unit  10  which outputs an optical signal having the specified wavelength, and notifies the specified light output unit  10  (for example, a light output unit  10 - 2 ) to the wavelength control unit  16 . 
         [0060]      FIG. 6  is a diagram illustrating a state of a frequency corresponding to the wavelength of a light selected by the wavelength selection unit  18  (specified by the wavelength switching unit  19 ) among a plurality of lights generated by the optical comb generator  11 .  FIG. 6  is a diagram illustrating states of a frequency corresponding to a wavelength of an optical signal output from the light output unit  10 - 2  before and after controlling a shift of a wavelength by the wavelength control unit  16 . 
         [0061]      FIG. 6  is an example in which an optical signal output from the light output unit  10 - 2  shifts by a wavelength corresponding to a frequency Δf, and the wavelength control unit  16  compensates a wavelength component corresponding to the frequency Δf. 
         [0062]    As illustrated in  FIG. 6 , the wavelength control unit  16  compensates a shift “Δf” (a shift of wavelength correspond thereto) of a frequency of an optical signal output from the light output unit  10 - 2 . The wavelength control unit  16  compensates a shift between both of the wavelengths by controlling to make a frequency of an optical signal output from the light output unit  10 - 2  coincide with a frequency “f 0 −F m ” of a light selected by the wavelength selection unit  18 . 
         [0063]    The wavelength switching unit  19  may control a wavelength of an optical signal to be output for all the light output units  10  while switching light output units  10 - 1  to  10 - 5  to be controlled and lights to be selected by the wavelength selection unit  18  in turn. By this, the optical communication device  1  can make a wavelength interval of an optical signal output from each of the light output units  10 - 1  to  10 - 5  correspond to a frequency interval “F m ” of a plurality of sideband lights (optical comb) generated by the optical comb generator  11 . 
         [0064]      FIG. 7  is a flow chart illustrating an operational example of the optical communication device  1  in the second exemplary embodiment of the present invention.  FIG. 7  is an example in which a wavelength of an optical signal output from the light output unit  10 - 2  shifts by a wavelength corresponding to a frequency “+Δf”, and the wavelength of an optical signal output from the light output unit  10 - 2  is compensated. 
         [0065]    A plurality of light output units  10  output optical signals having different wavelengths (S 201 ). 
         [0066]    A wavelength-multiplexed signal obtained by multiplexing a plurality of optical signals output from a plurality of light output units  10  is output to the outside of the optical communication device  1 , part of the wavelength-multiplexed signal is branched, and the part of the wavelength-multiplexed signal is output to the comparison unit  17  (S 202 ). 
         [0067]    The reference light output unit  14  outputs a reference light having a frequency f 0  which is a reference of a wavelength of an optical signal output from each light output unit  10  (S 203 ). 
         [0068]    The wavelength generation unit  15  generates, based on a reference light having a wavelength corresponding to a frequency f 0  output from the reference light output unit  14 , a plurality of lights having wavelengths around the frequency f 0  at a frequency interval of F m  (S 204 ). 
         [0069]    The wavelength selection unit  18  inputs, among the plurality of generated optical signals, a light whose wavelength has been specified by the wavelength switching unit  19  into the comparison unit  17  in the wavelength control unit  16  (S 205 ). 
         [0070]    The comparison unit  17  compares a wavelength of a light from the wavelength generation unit  15  and a wavelength of a wavelength-multiplexed optical signal from the multiplexing unit  13 , and as a result of the comparison, detects a difference between wavelengths corresponding to a difference (“Δf”) of both of the frequencies (S 206 ). 
         [0071]    The wavelength control unit  16  controls a wavelength of an optical signal output from the light output unit  10 - 2  in such a way that a difference (shift) between wavelengths detected by the comparison unit  17  is compensated (S 207 ). 
         [0072]    As mentioned above, the wavelength control unit  16  of the optical communication device  1  of the second exemplary embodiment of the present invention directly compares a wavelength of a light from the wavelength generation unit  15  and a wavelength of a wavelength-multiplexed optical signal from the multiplexing unit  13 , and controls a wavelength of an optical signal output from the light output unit  10  corresponding to the compared wavelength. Accordingly, the optical communication device  1  in the first exemplary embodiment of the present invention can output a wavelength-multiplexed optical signal whose wavelength interval has higher accuracy. 
       Third Exemplary Embodiment 
       [0073]    A third exemplary embodiment of the present invention will be described with reference to the Drawings. 
         [0074]    In a third exemplary embodiment of the present invention, the optical communication device  1  comprises a coherent detection unit. By using interference characteristics between a wavelength-multiplexed signal and a reference light, a frequency of an optical signal output from an LD  101  in the light output unit  10  is controlled. 
         [0075]      FIG. 8  is a diagram illustrating a configuration example of the optical communication device  1  in the third exemplary embodiment of the present invention. As illustrated in  FIG. 8 , the optical communication device  1  in the third exemplary embodiment of the present invention comprises a coherent detection unit  20  and a wavelength shift detection unit  21  in place of the comparison unit  17 . The optical communication device  1  comprises a branching device  22 . The optical communication device  1  comprises LDs (Laser Diodes)  101 - 1  to  101 - 5  (unless there is a particular need for distinction, referred to as “LD  101 ”) and optical modulators  102 - 1  to  102 - 5  (unless there is a particular need for distinction, referred to as “optical modulator  102 ”). 
         [0076]    The LD  101  outputs an optical signal having a predetermined frequency. A wavelength of an optical signal output from each of the plurality of LDs  101  is set to have a predetermined interval. Here, the wavelength of an optical signal output from the LD  101  may have a predetermined shift with respect to the set wavelength due to device-specific optical frequency setting accuracy of the LD  101 . 
         [0077]    The optical modulator  102  performs data modulation on an optical signal output from the LD  101 , and outputs the signal to the multiplexing unit  13 . 
         [0078]    The multiplexing unit  13  outputs a wavelength-multiplexed signal obtained by multiplexing a plurality of optical signals output from a plurality of light output units  10  to the branch unit  22 . 
         [0079]    The branching device  22  branches a wavelength-multiplexed signal output from the multiplexing unit  13 , and outputs one branched signal to a transmission path, and the other to the coherent detection unit  20 . 
         [0080]    The wavelength selection unit  18  inputs, among a plurality of lights (sideband light) included in an optical comb generated by the optical comb generator  11 , a light whose wavelength has been specified by the wavelength switching unit  19  into the coherent detection unit  20 . 
         [0081]    The coherent detection unit  20  coherently detects a wavelength-multiplexed signal input from the branching device  22  and a light having a predetermined wavelength input from the wavelength selection unit  18 . The coherent detection unit  20  outputs an interference signal between a wavelength-multiplexed signal input from the branching device  22  and the light input from the wavelength selection unit  18  to the wavelength shift detection unit  21 . 
         [0082]    The coherent detection unit  20  comprises a 90-degree hybrid mixer (not illustrated) which is called a coherent mixer in which a wavelength-multiplexed signal input from the branching device  22  interferes with a light input from the wavelength selection unit  18 . The coherent mixer outputs a signal (an interference signal) obtained when a wavelength-multiplexed signal input from the branching device  22  interferes with a light input from the wavelength selection unit  18 . 
         [0083]    The wavelength shift detection unit  21  detects, based on an input interference signal, a difference (shift) of wavelengths between at least one optical signal included in a wavelength-multiplexed signal input from the wavelength selection unit  18  among wavelength-multiplexed signals and a light input from the wavelength selection unit  18 . 
         [0084]    Here, for the wavelength shift detection unit  21 , a frequency offset estimator in a frequency offset compensation device described in Japanese Patent No. 5146285 can be used. A frequency offset is a difference of frequencies between an input light and a local light. A baseband electric signal from a front-end circuit is input to the frequency offset estimator, then the frequency offset estimator estimates a frequency offset included in the baseband electric signal, and outputs the estimated frequency offset to a device in a latter step as an electric signal. In the third exemplary embodiment of the present invention, the wavelength shift detection unit  21  outputs a difference (shift) of wavelengths corresponding to the estimated frequency offset as an electric signal. 
         [0085]    The wavelength control unit  16  adjusts a wavelength of an optical signal output from the light output unit  10  specified by the wavelength switching unit  19  among a plurality of light output units  10  in such a way that a difference (shift) of wavelengths detected by the wavelength shift detection unit  21  is compensated. Similarly to the second exemplary embodiment, for the plurality of light output units  10 , a wavelength variable laser device described in Japanese Patent No. 3197869 can be used. 
         [0086]      FIG. 9  is flow chart illustrating an operational example of the optical communication device  1  in the third exemplary embodiment of the present invention.  FIG. 9  is an example in which a wavelength of an optical signal output from the light output unit  10 - 2  shifts by a wavelength corresponding to a frequency “+Δf”, and the wavelength of an optical signal output from the light output unit  10 - 2  is compensated. 
         [0087]    The LDs  101  in the plurality of light output units  10  output optical signals having different wavelengths (S 301 ). 
         [0088]    Each of the optical modulator  102  performs data modulation on an optical signal output from the LD  101 , and outputs the signal to the multiplexing unit  13  (S 302 ). 
         [0089]    The multiplexing unit  13  multiplexes optical signals output from the plurality of light output units  10 , and outputs a wavelength-multiplexed signal (S 303 ). 
         [0090]    The branching device  22  branches a wavelength-multiplexed signal output from the multiplexing unit  13 , and outputs one branched signal to a transmission path, and the other to the coherent detection unit  20  (S 304 ). 
         [0091]    The reference light output unit  14  outputs a reference light having a frequency f 0  which is a reference for a wavelength of an optical signal output from each light output unit  10  (S 305 ). 
         [0092]    The wavelength generation unit  15  generates, based on a reference light having a wavelength corresponding to a frequency f 0  output from the reference light output unit  14 , a plurality of lights having wavelengths around the frequency f 0  at a frequency interval of F m  (S 306 ). 
         [0093]    The wavelength selection unit  18  inputs, among the plurality of generated optical signals, a light whose wavelength has been specified by the wavelength switching unit  19  into the coherent detection unit  20  (S 307 ). 
         [0094]    The coherent detection unit  20  outputs a signal (an interference signal) obtained when a wavelength of a light from the wavelength selection unit  18  interferes with a wavelength of a wavelength-multiplexed optical signal from the multiplexing unit  13  to the wavelength shift detection unit  21  (S 308 ). 
         [0095]    The wavelength shift detection unit  21  detects, based on the interference signal, a difference (shift) of wavelengths between at least one optical signal included in a wavelength-multiplexed signal and a light input from the wavelength selection unit  18  (S 309 ). 
         [0096]    The wavelength control unit  16  controls a wavelength of an optical signal output from an LD  101 - 2  in the light output unit  10 - 2  in such a way that the difference (shift) of wavelengths detected by the wavelength shift detection unit  21  is compensated (S 310 ). 
         [0097]    As mentioned above, the optical communication device  1  of the third exemplary embodiment of the present invention comprises the coherent detection unit  20 , and controls the LD  102  in the light output unit  10  using interference characteristics by making a wavelength-multiplexed signal directly interfere with a reference light. Accordingly, the optical communication device  1  in the third exemplary embodiment of the present invention can output a wavelength-multiplexed optical signal whose wavelength interval has higher accuracy. 
       Fourth Exemplary Embodiment 
       [0098]    A fourth exemplary embodiment of the present invention will be described with reference to the Drawings. 
         [0099]    A fourth exemplary embodiment of the present invention comprises a control device which instructs timing for controlling wavelength to the optical communication device  1 . 
         [0100]      FIG. 10  is a diagram illustrating a configuration example of a communication system in a fourth exemplary embodiment of the present invention. As illustrated in  FIG. 10 , the communication system in the fourth exemplary embodiment of the present invention comprises optical communication devices  1 - 1  to  1 - 4  (unless there is a particular need for distinction, referred to as “optical communication device  1 ”), a transmission path  2 , and a control device  3 . In an example of  FIG. 10 , there are four optical communication devices  1 , the number of the optical communication devices  1  is not limited to four, and any number of the optical communication devices  1  may be used. 
         [0101]    The transmission path  2  is constituted of optical fibers, and may be configured, for example, by bundling a plurality of optical fibers. 
         [0102]    The control device  3  instructs the optical communication device  1  to control a wavelength of an optical signal output from at least one of a plurality of light output units  10  at a predetermined timing. The timing for controlling wavelength is, for example, a cycle which is determined in advance based on a change of the LD  101  with time. 
         [0103]    The control device  3  may specify the light output unit  10  which controls a wavelength. In this case, the control device  3  specifies the light output unit  10  which controls a wavelength to the wavelength switching unit  19 . The optical communication device  1  then controls a wavelength of an optical signal output from the light output unit  10  specified by the control device  3 . 
         [0104]    A wavelength may be controlled when assignment of a wavelength to the light output unit  10  is changed (assignment of a channel is changed) in the optical communication device  1 . Different wavelengths are assigned to a plurality of light output units  10 , and data modulation is performed on an optical signal having the assigned wavelength, and an optical signal having the wavelength is output. For example, the optical communication device  1  changes assignment of a wavelength to the light output unit  10  when a channel interval is changed (for example, when a channel interval is changed from 50 GHz to 40 GHz). When assignment of a wavelength to the light output unit  10  is changed, a wavelength of an optical signal generated by the LD  101  (for example, a wavelength variable laser) in the light output unit  10  is changed, and therefore, the changed wavelength of an optical signal output from the LD  101  may shift from a desired wavelength. Accordingly, the control device  3  allows the optical communication device  1  to control a wavelength to cancel the wavelength shift. 
         [0105]    The timing for controlling wavelength is not limited to the above-described example. For example, a wavelength may be controlled when a new light output unit  10  is added to the optical communication device  1 . 
         [0106]      FIG. 11  is a diagram illustrating a configuration example of the optical communication device  1  in the fourth exemplary embodiment of the present invention.  FIG. 11  is an example in which the light output unit  10  controls a wavelength to be output when a channel interval is changed (for example, changed from 50 GHz to 40 GHz). 
         [0107]    As illustrated in  FIG. 11 , when a channel interval is changed, the light output units  10  of the optical communication device  1  are notified of a newly assigned wavelength from the control device  3 . Each of the light output units  10  changes a wavelength of an optical signal to be output based on the notification from the control device  3 . 
         [0108]    The microwave oscillator  12  of the optical communication device  1  is requested by the control device  3  to change a frequency F m  of a clock signal to be output. The microwave oscillator  12  changes the frequency F m  of a clock signal (for example, from 50 GHz to 40 GHz) when requested by the control device  3 . Since the optical comb generator  11  is driven by a clock signal input from the microwave oscillator  12 , a frequency interval F m  of a frequency of an optical comb output from the optical comb generator  11  changes (for example, a frequency interval changes from 50 GHz to 40 GHz). 
         [0109]    The wavelength switching unit  19  of the optical communication device  1  is notified by the control device  3  of a wavelength (the wavelength of an optical signal output from each of the plurality of light output units  10 ) which is set to each of the plurality of light output units  10 . The wavelength switching unit  19  specifies a wavelength of a light selected by the wavelength selection unit  18  based on a wavelength notified by the control device  3 . The wavelength selection unit  18  selects a light having the wavelength specified by the wavelength switching unit  19  among a plurality of lights generated by the wavelength generation unit  15 , and inputs the light into the coherent detection unit  20 . The wavelength switching unit  19  specifies a light output unit  10  which outputs an optical signal corresponding to the specified wavelength based on the notification from the control device  3 , and notifies the specified light output unit  10  to the wavelength control unit  16 . The wavelength control unit  16  controls the specified wavelength of an optical signal output from the light output unit  10  based on the notification from the wavelength switching unit  19 . 
         [0110]      FIG. 12  is a flow chart illustrating an operational example of the optical communication device  1  in the fourth exemplary embodiment of the present invention.  FIG. 12  is an example in which the light output unit  10  controls a wavelength output from the light output unit  10  when a channel interval is changed (for example, from 50 GHz to 40 GHz). 
         [0111]    Each LD  101  in the plurality of light output units  10  changes a wavelength of an optical signal to be output based on the change of assignment of a wavelength notified by the control device  3  (S 401 ). 
         [0112]    Each of the optical modulator  102  performs data modulation on an optical signal output from the LD  101 , and outputs the signal to the multiplexing unit  13  (S 402 ). 
         [0113]    The multiplexing unit  13  multiplexes optical signals output from the plurality of light output units  10 , and outputs a wavelength-multiplexed signal (S 403 ). 
         [0114]    The branching device  22  branches a wavelength-multiplexed signal output from the multiplexing unit  13 , and outputs one branched signal to a transmission path, and the other to the coherent detection unit  20  (S 404 ). 
         [0115]    The reference light output unit  14  outputs a reference light having a frequency f 0  which is a reference for a wavelength of an optical signal output from each light output unit  10  (S 405 ). 
         [0116]    The microwave oscillator  12  changes a frequency F m  of a clock signal to be output to the optical comb generator  11  based on a request from the control device  3  (S 406 ). 
         [0117]    The optical comb generator  11  generates, based on a reference light having a wavelength corresponding to a frequency f 0  output from the reference light output unit  14 , a plurality of lights having wavelengths around the frequency f 0  at a frequency interval of F m  (S 407 ). 
         [0118]    The wavelength selection unit  18  inputs, among the plurality of optical signals generated by the optical comb generator  11 , a light whose wavelength has been specified by the wavelength switching unit  19  into the coherent detection unit  20  (S 408 ). 
         [0119]    The coherent detection unit  20  outputs a signal (an interference signal) obtained when a wavelength of a light from the wavelength selection unit  18  interferes with a wavelength of a wavelength-multiplexed optical signal from the multiplexing unit  13  to the wavelength shift detection unit  21  (S 409 ). 
         [0120]    The wavelength shift detection unit  21  detects, based on the interference signal, a difference (shift) of wavelengths between at least one optical signal included in a wavelength-multiplexed signal and a light input from the wavelength selection unit  18  (S 410 ). 
         [0121]    The wavelength control unit  16  controls a wavelength of an optical signal output from an LD  101 - 2  in the light output unit  10 - 2  in such a way that the difference (shift) of wavelengths detected by the wavelength shift detection unit  21  (S 411 ). 
         [0122]    As mentioned above, the fourth exemplary embodiment of the present invention comprises the control device  3  which instructs timing for controlling wavelength to the optical communication device  1 , and the optical communication device  1  controls a wavelength of an optical signal output from the light output unit  10  based on the instruction from the control device  3 . Accordingly, the optical communication device  1  can control a wavelength by a timing instructed by the control device  3 , which makes it possible to flexibly control timing for controlling wavelength. 
       Fifth Exemplary Embodiment 
       [0123]    A fifth exemplary embodiment of the present invention will be described. In the fifth exemplary embodiment, a computer, a CPU (Central Processing Unit), an MPU (Micro-Processing Unit) and the like of the optical communication device  1  executes a software (a program) which realizes a function described in each of the above-described exemplary embodiments. 
         [0124]    In the fifth exemplary embodiment of the present invention, the optical communication device  1  acquires a software (a program) which realizes a function of the above-described exemplary embodiments via a variety of storage media or a network, which is non-transitory one, such as a CD-R (Compact Disc Recordable). A program which the optical communication device  1  acquires or a storage medium on which the program is stored constitutes the present invention. The software (program) may be stored in a predetermined storage unit, which is non-transitory one, included in the optical communication device  1 . 
         [0125]    A computer, a CPU, an MPU, or the like of the optical communication device  1  reads a program code from the acquired software (program) and execute the program code. In other words, the optical communication device  1  executes a processing identical to the processing of the optical communication device  1  in each of the above-described exemplary embodiments. 
         [0126]    The fifth exemplary embodiment of the present invention can be applied to a use such as a program realized by a computer, a CPU, or an MPU of the optical communication device  1 . 
         [0127]    A part or the whole of the above-described exemplary embodiment can be described as in the following Supplementary notes, but is not limited thereto. 
         [0128]    [Supplementary Note 1] 
         [0129]    An optical communication device comprising: 
         [0130]    a plurality of light output units for outputting optical signals having different wavelengths, 
         [0131]    a multiplexing unit for outputting a wavelength-multiplexed signal obtained by multiplexing a plurality of optical signals output from the plurality of light output units, 
         [0132]    a reference light output unit for outputting a reference light which is a reference, 
         [0133]    a wavelength generation unit for outputting, based on the reference light, a plurality of lights having a wavelength interval depending on a predetermined frequency, and 
         [0134]    a wavelength control unit for controlling, depending on an interference component between the wavelength-multiplexed signal and a light having at least one wavelength among the plurality of lights, a wavelength of an optical signal output from the light output unit. 
         [0135]    [Supplementary Note 2] 
         [0136]    The optical communication device according to Supplementary note 1, wherein the wavelength control unit comprises a comparison unit for comparing the wavelength-multiplexed signal and a light having at least one wavelength among the plurality of lights. 
         [0137]    [Supplementary Note 3] 
         [0138]    The optical communication device according to Supplementary note 1 or 2, wherein 
         [0139]    the wavelength control unit comprises:
       a coherent detection unit for coherently detecting the wavelength-multiplexed signal and a light having at least one wavelength among the plurality of lights; and   a wavelength shift detection unit for detecting, based on an interference signal input from a coherent detection unit, a difference between a wavelength of at least one optical signal among the optical signals included in the wavelength-multiplexed signal and a wavelength of at least one light among the plurality of lights.       
 
         [0142]    [Supplementary Note 4] 
         [0143]    The optical communication device according to any one of Supplementary notes 1 to 3, comprising a wavelength selection unit for selecting a light having at least one wavelength among a plurality of lights generated by the wavelength generate unit. 
         [0144]    [Supplementary Note 5] 
         [0145]    The optical communication device according to Supplementary note 4, comprising a wavelength switching unit for instructing the at least one wavelength to be selected to the wavelength selection unit. 
         [0146]    [Supplementary Note 6] 
         [0147]    The optical communication device according to any one of Supplementary notes 1 to 5, wherein 
         [0148]    the wavelength generate unit comprises:
       a microwave oscillator which outputs a clock signal having a predetermined frequency; and   an optical comb generator which outputs, based on the reference light, a plurality of lights having a wavelength interval depending on the predetermined frequency.       
 
         [0151]    [Supplementary Note 7] 
         [0152]    The optical communication device according to Supplementary note 6, wherein 
         [0153]    the wavelength switching unit notifies, based on at least one wavelength which has been instructed to the wavelength selection unit, a light output unit for controlling a wavelength of an optical signal to be output among the plurality of light output units to the wavelength control unit, and 
         [0154]    the wavelength control unit controls a wavelength of an optical signal output from the light output unit notified by the wavelength switching unit. 
         [0155]    [Supplementary Note 8] 
         [0156]    The optical communication device according to Supplementary note 6 or 7, wherein 
         [0157]    the wavelength switching unit changes at least one wavelength which is instructed to the wavelength selection unit and a light output unit which is notified to the wavelength control unit in turn, and 
         [0158]    the wavelength control unit controls a wavelength of an optical signal output from the light output unit notified by the wavelength switching unit in turn. 
         [0159]    [Supplementary Note 9] 
         [0160]    An optical communication system comprising: 
         [0161]    an optical communication device comprising:
       a plurality of light output units for outputting optical signals having different wavelengths;   a multiplexing unit for outputting a wavelength-multiplexed signal obtained by multiplexing a plurality of optical signals output from the plurality of light output units;   a reference light output unit for outputting a reference light which is a reference;   a wavelength generation unit for outputting, based on the reference light, a plurality of lights having a wavelength interval depending on a predetermined frequency; and   a wavelength control unit for controlling, depending on an interference component between the wavelength-multiplexed signal and a light having at least one wavelength among the plurality of lights, a wavelength of an optical signal output from the light output unit; and       
 
         [0167]    a control device which instructs the optical communication device to control a wavelength of an optical signal output from at least one of the plurality of light output units at a predetermined timing. 
         [0168]    [Supplementary Note 10] 
         [0169]    The optical communication system according to Supplementary note 9, wherein the control device specifies a light output unit for controlling a wavelength of the optical signal, and notifies the specified light output unit to the optical communication device. 
         [0170]    [Supplementary Note 11] 
         [0171]    The optical communication system according to Supplementary note 9 or 10, wherein the predetermined timing is when the control device changes assignment of a wavelength of an optical signal output from the plurality of light output units. 
         [0172]    [Supplementary Note 12] 
         [0173]    The optical communication system according to Supplementary notes 9 to 11, wherein the predetermined timing is when the control device changes a wavelength interval of an optical signal output from the plurality of light output units. 
         [0174]    [Supplementary Note 13] 
         [0175]    The optical communication system according to Supplementary note 12, wherein the control device instructs the optical communication device to change a wavelength interval of a plurality of lights generated by the wavelength generation unit when the wavelength interval is changed. 
         [0176]    [Supplementary Note 14] 
         [0177]    An optical communication method comprising: 
         [0178]    outputting an optical signal having a different wavelength; 
         [0179]    outputting the wavelength-multiplexed signal obtained by multiplexing a plurality of optical signals; 
         [0180]    outputting a reference light which is a reference; 
         [0181]    outputting, based on the reference light, a plurality of lights having a wavelength interval depending on a predetermined frequency; and 
         [0182]    controlling, depending on an interference component between the wavelength-multiplexed signal and a light having at least one wavelength among the plurality of lights, a wavelength of the output optical signal. 
         [0183]    [Supplementary Note 15] 
         [0184]    The optical communication method according to Supplementary note 14, wherein the wavelength-multiplexed signal and a light having at least one wavelength among the plurality of lights are compared. 
         [0185]    [Supplementary Note 16] 
         [0186]    The optical communication method according to Supplementary note 14 or 15, wherein 
         [0187]    the wavelength-multiplexed signal and a light having at least one wavelength among the plurality of lights are coherently detected, and 
         [0188]    a difference between a wavelength of at least one optical signal among the optical signals included in the wavelength-multiplexed signal and a wavelength of at least one light among the plurality of lights is detected based on the coherently detected interference signal. 
         [0189]    [Supplementary Note 17] 
         [0190]    The optical communication method according to any one of Supplementary notes 14 to 16, wherein at least one of light having a wavelength among the generated plurality of lights is selected. 
         [0191]    [Supplementary Note 18] 
         [0192]    The optical communication method according to Supplementary notes 17, wherein the at least one wavelength to be the selected is instructed. 
         [0193]    [Supplementary Note 19] 
         [0194]    The optical communication method according to any one of Supplementary notes 14 to 18, wherein 
         [0195]    a clock signal having a predetermined frequency is output, and 
         [0196]    a plurality of lights having a wavelength interval depending on the predetermined frequency is output based on the reference light. 
         [0197]    [Supplementary Note 20] 
         [0198]    A program for allowing a computer to execute: 
         [0199]    a processing of outputting optical signals having different wavelengths; 
         [0200]    a processing of outputting a wavelength-multiplexed signal obtained by multiplexing a plurality of optical signals output from the plurality of light output units; 
         [0201]    a processing of outputting a reference light which is a reference; 
         [0202]    a processing of outputting, based on the reference light, a plurality of lights having a wavelength interval depending on a predetermined frequency, and 
         [0203]    a processing of controlling, depending on an interference component between the wavelength-multiplexed signal and a light having at least one wavelength among the plurality of lights, a wavelength of the output optical signal. 
         [0204]    [Supplementary Note 21] 
         [0205]    The program according to Supplementary note 20, comprising a processing of comparing the wavelength-multiplexed signal and a light having at least one wavelength among the plurality of lights. 
         [0206]    [Supplementary Note 22] 
         [0207]    The program according to Supplementary note 20 or 21, comprising: 
         [0208]    a processing of coherently detecting the wavelength-multiplexed signal and a light having at least one wavelength among the plurality of lights; and 
         [0209]    a processing of detecting a difference between a wavelength of at least one optical signal among the optical signals included in the wavelength-multiplexed signal and a wavelength of at least one light among the plurality of lights, based on the coherently detected interference signal. 
         [0210]    [Supplementary Note 23] 
         [0211]    The program according to any one of Supplementary notes 20 to 22, comprising a processing of selecting a light having at least one wavelength among the generated plurality of lights. 
         [0212]    [Supplementary Note 24] 
         [0213]    The program according to Supplementary note 23, comprising a processing of instructing the at least one wavelength to be selected. 
         [0214]    [Supplementary Note 25] 
         [0215]    The program according to any one of Supplementary notes 20 to 24, comprising: 
         [0216]    a processing of outputting a clock signal having a predetermined frequency; and 
         [0217]    a processing of outputting, based on the reference light, a plurality of lights having a wavelength interval depending on the predetermined frequency. 
         [0218]    The present invention has been described with reference to the exemplary embodiments. However, applicable aspects of the present invention are not limited to the above-described exemplary embodiments. The configuration or a detailed description of the present invention may be modified in various ways which can be understood by those skilled in the art within a scope of the present invention. 
         [0219]    This application claims the priority based on Japanese Patent Application No. 2014-065011 filed on Mar. 27, 2014, the entire disclosure of which is incorporated herein by reference. 
       REFERENCE SIGNS LIST 
       [0000]    
       
         
           
               1 ,  1 - 1 ,  1 - 2 ,  1 - 3 ,  1 - 4  Optical communication device 
               2  Transmission path 
               3  Control device 
               10 ,  10 - 1 ,  10 - 2 ,  10 - 3 ,  10 - 4 ,  10 - 5  Light output unit 
               11  Optical comb generator 
               12  Microwave oscillator 
               13  Multiplexing unit 
               14  Reference light output unit 
               15  Wavelength generation unit 
               16  Wavelength control unit 
               17  Comparison unit 
               18  Wavelength selection unit 
               19  Wavelength switching unit 
               20  Coherent detection unit 
               21  Wavelength shift detection unit 
               22  Branching device 
               101  LD 
               102  Optical modulator