Abstract:
In order to precisely know the light power of a received signal in a wide light input power range, a light reception device comprises: a reception unit that receives a coherent-modulated signal light and outputs a first electric signal to which the signal light has been converted; an amplification unit that amplifies the first electric signal and outputs the amplified electric signal as a second electric signal; and a control unit that determines the light power of the signal light on the basis of a relationship between the light power of the signal light in the reception unit and at least one of the gain of the amplification unit and the amplitude of the second electric signal.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an optical receiver, an optical receiving method and a control program of an optical receiver, and more particularly, to an optical receiver, an optical receiving method and a control program of an optical receiver that are used in a coherent optical transmission system. 
       BACKGROUND ART 
       [0002]    Practical use of a coherent optical transmission system which enables large capacity and high-speed communication is being advanced. In a coherent optical transmission system, a coherent optical receiver is used to demodulate an optical signal. In a coherent optical receiver, received signal light (a reception beam) and a LO (local oscillation) beam having an optical frequency approximately identical with that of the reception beam are combined by an optical mixer called a 90-degree hybrid. Output of a 90-degree hybrid is received by a PD (a photo diode). The PD outputs a beat signal caused between the reception beam and the LO light to a differential amplifier as photo electric current. The differential amplifier converts photocurrent outputted by the PD into a voltage signal and outputs the voltage signal to an ADC (analog-digital converter). The beat signal converted into a digital signal in the ADC is outputted to DSP (digital signal processor). The DSP performs calculation processing of the digital signal outputted from ADC to reproduce transmitted data. 
         [0003]    Related to the present invention, there is disclosed in patent literature (PTL) 1 a frequency control method having a function for holding the frequency of LO light to a numerical value just before detection of a cut off of an input signal. In PTL 2, there is disclosed a light reception circuit having a function for generating an input interruption alarm signal only at the time of an optical input interruption. 
       CITATION LIST 
     Patent Literature 
       [0004]    [PTL 1] Japanese Patent Application Laid-Open No. 1993-308325 (paragraph [0013]) 
         [0005]    [PTL 2] Japanese Patent Application Laid-Open No. 1990-105643 (page 3, lower right column to page 4, lower right column) 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0006]    In an optical receiver to receive a wavelength-multiplexed signal, only a reception beam of a selected wavelength can be received by a monitor PD by selecting by an optical filter a reception beam (reception channel) of the wavelength to be received. By such structure, input power and LOS (loss of signal) only of the reception beam in the reception channel can be detected. 
         [0007]    In contrast, in a coherent optical receiver, it is possible to receive only a reception beam of a wavelength that generates a beat signal with LO light as an electric signal. For this reason, an optical filter for selecting a reception channel is not needed for a coherent optical receiver necessarily. However, a coherent optical receiver which does not have an optical filter for selecting a reception channel cannot select a reception beam in a reception channel by an optical filter, and, therefore, its optical power cannot be measured and LOS cannot be detected. 
         [0008]    It is also possible to measure the optical power of a reception channel based on the amplitude of an electric signal converted from an optical signal in the reception channel. An automatic frequency control method disclosed in PTL 1 is equipped with a structure for suppressing a fluctuation of a frequency of LO light even if a temporary optical input interruption occurs. However, there is disclosed no structure for detecting the optical power of a reception beam in patent document 1. A light reception circuit disclosed in patent document 2 is equipped with a structure to generate an input interruption alarm based on a gain of a variable gain amplifier circuit which amplifies a received signal. However, in the structure of patent document 2, the optical power of a received signal cannot be detected correctly when an amplifier circuit is operating outside its dynamic range. 
       OBJECT OF INVENTION 
       [0009]    An object of the present invention is to provide an optical receiver, an optical receiving method and a control program of an optical receiver which can detect the optical power of a received signal correctly in a wide range of optical input power without needing an optical filter. 
       Solution to Problem 
       [0010]    An optical receiver of the present invention includes: reception means for receiving an optical signal having undergone coherent modulation to output a first electric signal converted from the optical signal; amplifying means for amplifying the first electric signal to output the first electric signal having been amplified as a second electric signal; and control means for obtaining an optical power of the optical signal based on relation between optical power of the optical signal in the reception means and at least one of a gain of the amplifying means and amplitude of the second electric signal. 
         [0011]    An optical receiving method of the present invention includes: receiving an optical signal having undergone coherent modulation; outputting a first electric signal converted from the optical signal; amplifying the first electric signal; outputting the first electric signal having been amplified as a second electric signal; and obtaining an optical power of the optical signal based on relation between optical power when receiving the optical signal, and at least one of a gain when amplifying the first electric signal and amplitude of the second electric signal. 
         [0012]    A control program of an optical receiver of the present invention makes a computer of the optical receiver carry out: a procedure to receive an optical signal having undergone coherent modulation; a procedure to output a first electric signal converted from the optical signal; a procedure to amplify the first electric signal; a procedure to output the first electric signal having been amplified as a second electric signal; and a procedure to obtain optical power of the optical signal based on relation between optical power when receiving the optical signal, and at least one of a gain when amplifying the first electric signal and amplitude of the second electric signal. 
       Advantageous Effect of Invention 
       [0013]    An optical receiver, an optical receiving method and a control program of an optical receiver of the present invention can detect the optical power of a received signal correctly in a wide range of optical input power. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]      FIG. 1  is a block diagram showing a structure of a coherent optical receiver of a first example embodiment. 
           [0015]      FIG. 2  is a block diagram showing a structure of a differential amplifier. 
           [0016]      FIG. 3  is a diagram showing an example of relation between optical input power Pin and gain A of an AGC amplifier. 
           [0017]      FIG. 4  is a diagram showing an example of relation between optical input power Pin and amplitude V of an output signal of a differential amplifier. 
           [0018]      FIG. 5  is a flow chart showing a procedure to obtain optical input power Pin in a control circuit. 
           [0019]      FIG. 6  is a diagram showing an example of relation between optical input power Pin, amplitude V, a gain A and V/A. 
           [0020]      FIG. 7  is a block diagram showing a structure of a coherent optical receiver which is a modified example of the first example embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     The First Example Embodiment 
       [0021]    The first example embodiment of the present invention will be described with reference to a drawing.  FIG. 1  is a block diagram showing a structure of a coherent optical receiver  100  of the first example embodiment of the present invention. The coherent optical receiver  100  of the first example embodiment includes a PBS (polarization beam splitter)  1 , a BS (beam splitter)  2 , a 90-degree hybrid  3  and a LO (local oscillation) light source  4 . The coherent optical receiver  100  of the first example embodiment further includes a PD (photo diode)  5 , a differential amplifier  6 , an ADC (analog-digital converter)  7 , a DSP (digital signal processor)  8  and a control circuit  9 . Meanwhile, since the basic structure and operations of the coherent optical receiver  100  are known well, only an outline will be described about the general structure and operations below. 
         [0022]    The coherent optical receiver  100  receives an optical signal  110  for which wavelength multiplication has been performed. The received optical signal  110  (reception beam) is separated into an X-polarized wave and a Y-polarized wave crossing at right angles to each other by the PBS  1 . The separated reception beams are inputted to the different pieces of 90-degree hybrid  3 , respectively. LO light outputted from the LO light source  4  is branched by the BS  2  and inputted to the different pieces of 90-degree hybrid  3 , respectively. One piece of 90-degree hybrid  3  is provided for each of a reception beam corresponding to an X polarized wave and a reception beam corresponding to a Y polarized wave. 
         [0023]    A reception beam separated by the PBS  1  into an X-polarized wave and a Y-polarized wave is combined with LO light having an optical frequency approximately identical with that of the reception beam by the 90-degree hybrid  3 . In the 90-degree hybrid  3 , among reception beams for which wavelength multiplication have been performed, only an optical signal having a wavelength approximately identical with that of the LO light interferes with the LO light to generate a beat signal. By controlling the wavelength of the LO light, it is possible to select from reception beams an optical signal of a wavelength (reception channel) that is desired to be received to generate a beat signal. A beat signal generated by the 90-degree hybrid  3  is received by the PD  5 . 
         [0024]    Four pieces of PD  5  are provided for the output of one piece of 90-degree hybrid  3 . Two among the four pieces of PD  5  output a beat signal having a phase of I (inphase) component as a differential signal (photo electric current). The other two pieces of PD  5  output a beat signal having a phase of Q (quadrature) component as a differential signal. 
         [0025]    The differential signal outputted from the PD  5  is inputted to the differential amplifier  6 . One piece of differential amplifier  6  is provided for each of signals of an X-polarized wave I component (XI), an X-polarized wave Q component (XQ), a Y-polarized wave I component (YI) and a Y-polarized wave Q component (YQ). 
         [0026]      FIG. 2  is a block diagram showing a structure of the differential amplifier  6 . 
         [0000]    The differential amplifier  6  includes a TIA (trans-impedance amplifier)  61 , an AGC (automatic gain control) amplifier  62 , a buffer  63 , an offset detector  64  and a peak detector  65 . Photocurrent outputted from the PD  5  is converted into a voltage signal by the TIA  61 , and is inputted to the AGC amplifier  62 . The peak detector  65  detects a peak value of amplitude of a signal outputted from the buffer  63 , and a gain of the AGC amplifier  62  is controlled in such a way as to make the amplitude of a detected signal be within a fixed range (AGC control). The amplitude of a signal outputted from the buffer  63  (output amplitude) and a gain of the AGC amplifier  62  are inputted to the control circuit  9 . In this example embodiment, the gain of the buffer  63  is made to be 1. That is, the output amplitude of the AGC amplifier  62  and the output amplitude of the buffer  63  are equal. The maximum value of a gain of the AGC amplifier  62  is A 0 . Even when a gain beyond A 0  is needed by AGC control, a gain of the AGC amplifier  62  is set to A 0 . In the coherent optical receiver  100 , the optical power of an optical signal in a reception channel of the coherent optical receiver  100  is estimated based on a gain of the AGC amplifier  62  used for feedback control and the amplitude of an output signal of the buffer  63 . 
         [0027]    The differential amplifier  6  converts photocurrent outputted by the PD  5  into a voltage signal and outputs it to the ADC  7 . The reception signal converted into a digital signal by the ADC  7  is outputted to the DSP  8 . The DSP  8  performs calculation processing of the digital signal outputted from the ADC  7  and reproduces transmitted data. 
         [0028]    After having been attenuated by optical loss in the PBS  1 , the 90-degree hybrid  3  and on the path connecting those, the optical signal in a reception channel is mixed with the LO light and converted into a photo electric current by the PD  5 . 
         [0029]    Here, the amplitude of photo electric current outputted from the PD  5  is determined by the optical power of an optical signal in the reception channel inputted to the PD  5 , the optical power of the LO light, and the quantum efficiency of the PD  5  (a conversion factor from a signal light to an electric signal). The attenuation of the PBS  1 , the BS  2  and the 90-degree hybrid  3  and the quantum efficiency of the PD  5  are of a fixed nature, and thus these can be measured in advance. In addition, the optical power of LO light outputted from the LO light source  4  is easy to be measured in advance, or to be controlled in a desired numerical value also. The current-voltage transfer characteristics in the TIA  61  can be deemed to be fixed also. 
         [0030]    On the other hand, the amplitude of a signal inputted to the AGC amplifier  62  can be known from output amplitude V of the differential amplifier  6  and gain A of the AGC amplifier  62 . Then, the amplitude of the photo electric current outputted from the PD  5  can be obtained based on the amplitude of a signal inputted to the AGC amplifier  62  and the current-voltage transfer characteristics of the TIA  61 . That is, relation between gain A of the differential amplifier  6  and output amplitude V of the AGC amplifier  62 , and the optical power of an optical signal in a reception channel inputted to the PD  5  can be obtained. Then, finally, the optical power of an optical signal in a reception channel at the time when it has been inputted to the coherent optical receiver  100  can be obtained from output amplitude V of the differential amplifier  6  and gain A of the AGC amplifier  62  using: the above-mentioned relation; the optical power of the LO light outputted from the LO light source  4 ; and the respective attenuation of the PBS  1 , the BS  2  and the 90-degree hybrid  3 . 
         [0031]    Or, relation between output amplitude V of the differential amplifier  6  and gain A of the AGC amplifier  62 , and the optical power of an optical signal in a reception channel may be measured when the coherent optical receiver  100  is produced to store the measured data in the control circuit  9 . Such measurement may be performed under different operating conditions taking the optical characteristics and the electrical characteristics of constituent elements of the coherent optical receiver  100 , such as loss of optical parts and the power of LO light, as parameters. Then, the optical power of an optical signal in a reception channel can be obtained also by referring to measured data at the time of production by a numerical value of output amplitude V of the differential amplifier  6  and gain A of the AGC amplifier  62 . 
         [0032]    That is, the coherent optical receiver  100  can come to know the optical power of an optical signal in a reception channel based on an operation state of the differential amplifier  6  without selecting a received wavelength by an optical filter. 
         [0033]      FIG. 3  is a diagram showing an example of relation between optical input power Pin and gain A of the AGC amplifier  62  in this example embodiment. Optical input power Pin is optical power of an optical signal in a reception channel at the time of being inputted to the coherent optical receiver  100 . As mentioned above, the relation of  FIG. 3  may be calculated based on the electrical characteristics or optical characteristics of the elements constituting the coherent optical receiver  100 , or it may be measured when the coherent optical receiver  100  is produced. 
         [0034]    P 0  is the smallest optical input power required in order to obtain an output signal of a fixed amplitude (that is, a set value of an output amplitude under AGC control) V 0  by AGC control of the AGC amplifier  62 . In the area of Pin&lt;P 0 , since the amplitude of a signal inputted to the AGC amplifier  62  is small, the amplitude of an output signal of the AGC amplifier  62  does not reach V 0  although the AGC amplifier  62  operates at the maximum gain A 0 . In the region of Pin≧P 0 , the AGC amplifier  62  operates within its dynamic range. 
         [0035]      FIG. 4  is a diagram showing an example of relation between optical input power Pin and amplitude V of an output signal of the differential amplifier  6  in this example embodiment. Optical input power Pin is the optical power of a reception channel received by the coherent optical receiver  100 . Like  FIG. 3 , relation of  FIG. 4  may be calculated based on numerical values of the characteristics of elements constituting the coherent optical receiver  100 , or may be measured when the coherent optical receiver  100  is produced. 
         [0036]    In  FIG. 4 , since gain A of the AGC amplifier  62  will be the maximum value A 0  when optical input power Pin is less than P 0 , output amplitude V declines along with a fall of optical input power Pin. On the other hand, when optical input power Pin is no smaller than P 0 , the amplitude of an output signal of the AGC amplifier  62  will be a constant value V 0  due to AGC control. That is, when Pin≧P 0 , optical input power Pin cannot be obtained using  FIG. 4 . However, in the case when Pin≧P 0 , optical input power Pin can be acquired from the relation between gain A and optical input power Pin shown in  FIG. 3 . 
         [0037]    In addition, when optical input power Pin falls to less than P 0 , gain A of the AGC amplifier  62  will be a constant value of the maximum value A 0  as shown in  FIG. 3 . That is, when it is Pin&lt;P 0 , optical input power P cannot be obtained using  FIG. 3 . However, in the case when Pin&lt;P 0 , optical input power Pin can be obtained from the relation between output amplitude V and optical input power Pin shown in  FIG. 4 . 
         [0038]    Accordingly, by using both of  FIG. 3  and  FIG. 4 , optical input power Pin can be obtained from numerical values of output amplitude V and gain 
         [0039]    A of the AGC amplifier  62  during operation. For example, when optical input power Pin is less than P 0 , optical input power Pin can be obtained from output amplitude V and the relation of  FIG. 4 . Alternatively, optical input power Pin can be obtained by calculation using output amplitude V, gain A 0  and numerical values of the electrical characteristics or optical characteristics of elements constituting the coherent optical receiver  100 . 
         [0040]    On the other hand, when optical input power Pin is no smaller than P 0 , optical input power Pin can be obtained from gain A acquired from the peak detector  65  and the relation of  FIG. 3 . Otherwise, optical input power Pin can be obtained by calculation using gain A, a setting value of amplitude V 0 , and numerical values of the electrical characteristics or optical characteristics of elements constituting the coherent optical receiver  100 . 
         [0041]    Whether optical input power Pin is larger than P 0  or not can be judged by comparing gain A of the AGC amplifier  62  with the maximum value A 0  of a gain. That is, when A&lt;A 0 , it is judged that an optical input power P exceeds P 0  because the output of the AGC amplifier  62  does not reach the maximum value. On the other hand, when A=A 0 , optical input power Pin is judged as being no more than P 0 . 
         [0042]    Or, whether optical input power Pin is larger than P 0  or not can be judged also by comparing output V of the AGC amplifier  62  and V 0  which is the default of the output amplitude under AGC control. That is, when V=V 0 , optical input power Pin is judged as being no smaller than P 0 . On the other hand, when V&lt;V 0 , optical input power Pin is judged as being less than P 0 . 
         [0043]    Meanwhile, the scale of each axis of  FIG. 3  and  FIG. 4  is arbitrary, and the oblique line portions of the graphs are not ones indicating linear relation between the variables necessarily. 
         [0044]    The procedure to obtain optical input power Pin described in  FIG. 3  and  FIG. 4  is carried out in the control circuit  9 . Gain A of the AGC amplifier  62  and output amplitude V of the differential amplifier  6  are inputted to the control circuit  9 . The control circuit  9  stores the maximum gain A 0  of the AGC amplifier  62  and a setting value V 0  of an output amplitude. The control circuit  9  obtains optical input power Pin by the above-mentioned procedure based on relation between gain A or output amplitude V and optical input power Pin. 
         [0045]      FIG. 5  is a flow chart showing an example of a procedure for the control circuit  9  to obtain optical input power Pin using the relation of FIG. 
         [0046]      3  and  FIG. 4 . The control circuit  9  acquires a numerical value of gain A (Step S 1  of  FIG. 5 ), and compares the inputted gain A with the stored maximum gain A 0  (S 2 ). Then, when A&lt;A 0  (in S 2 : A&lt;A 0 ), the control circuit  9  obtains optical input power Pin from gain A using the relation of  FIG. 3  because optical input power Pin exceeds P 0  (S 3 ). When A=A 0  (in S 2 : A=A 0 ), the control circuit  9  acquires output amplitude V and obtains optical input power Pin from the output amplitude V using the relation of  FIG. 4  because optical input power Pin is no more than P 0  (S 4 ). 
         [0047]    If whether optical input power Pin is larger than P 0  or not is judged by comparison of output amplitude V and a setting value V 0 , Step S 1  of  FIG. 5  is changed to a procedure to acquire output amplitude V, and the procedure of Step S 2  is changed to a procedure to compare V and V 0 . Then, when V=V 0  in the changed Step S 2 , the control circuit  9  acquires gain A and advances to Step S 3 , and when V&lt;V 0 , advances to Step S 4 . 
         [0048]    The control circuit  9  may include a CPU (central processing unit)  91  and a memory  92 . The memory  92  is a non-volatile storage medium to store a program fixedly, and is a non-volatile semiconductor memory, for example, but not limited to this. The CPU  91  may perform the function of the coherent optical receiver  100  mentioned above by executing a program stored in the memory  92 . The memory  92  may memorize measured data or calculation result of relation between output amplitude V of the differential amplifier  6  and gain A of the AGC amplifier  62 , and optical power Pin of an optical signal in a reception channel. In addition, the memory  92  may memorize the setting value V 0  of the output amplitude of the differential amplifier  6  on the occasion of AGC control and the maximum gain A 0  of the AGC amplifier  62 . 
         [0049]      FIG. 6  is a diagram showing an example of relation between optical input power Pin, and amplitude V, gain A and a numerical value (V/A), which is obtained by dividing amplitude by a gain, in the first example embodiment. The thick dashed lines of  FIG. 6  indicate amplitude V and gain A, and the solid line indicates V/A.  FIG. 6  indicates diagrams shown in  FIG. 3  and  FIG. 4  in one diagram, and also describes numerical values (V/A) obtained by dividing amplitude V by gain A. The scale of each axis of  FIG. 6  is arbitrary, and oblique line portions of the graph are not necessarily ones indicating linear relation between the variables. 
         [0050]    By storing the data of the solid line (output amplitude/gain) of  FIG. 6  in the control circuit  9 , and, obtaining V/A from amplitude V and gain A obtained at the time of use and referring to the data of the solid line of  FIG. 6 , optical power Pin of an optical signal in a reception channel can be acquired. The data of the solid line of  FIG. 6  may be calculated based on the optical characteristics and electrical characteristics of elements constituting the coherent optical receiver  100  like the relation of  FIG. 3  and  FIG. 4 , or may be obtained from the relation between amplitude V, gain A and optical input power Pin measured when the coherent optical receiver  100  is produced. 
         [0051]    By such procedure, the coherent optical receiver  100  of the first example embodiment can measure optical input power Pin of an optical signal in a reception channel even when the AGC amplifier circuit  62  is operating outside its dynamic range, that is, even when optical input power Pin is in a low level that is an out-of-bounds of AGC control. The reason of this is that, when optical input power Pin is in a low level that is an out-of-bounds of AGC control, the coherent optical receiver  100  obtains optical input power Pin using output amplitude V of the AGC amplifier  62 . 
         [0052]    Accordingly, the coherent optical receiver  100  of the first example embodiment exerts an effect that the optical power of a received signal can be detected correctly in a wide range of optical input power. The coherent optical receiver  100  of the first example embodiment can measure the optical power of a received signal without selecting a reception channel using an optical filter. 
         [0053]    As a warning of an optical receiver, LOS (loss of signal) is used. When the optical power of an optical signal is less than a predetermined optical power, LOS is sent. Since the coherent optical receiver  100  of the first example embodiment can measure the input level of an optical signal over a wide range, it also has an effect that a detection range of LOS can be expanded. 
       (Modification of the First Example Embodiment) 
       [0054]      FIG. 7  is a block diagram showing a structure of a coherent optical receiver  101  which is a modification of the first example embodiment. In the coherent optical receiver  100  of the first example embodiment, output amplitude V of the differential amplifier  6  is used. However, the DSP  8  may output the data of output amplitude used inside the DSP  8  to the control circuit  9 . The same effect as that of the first example embodiment is obtained by the control circuit  9  using data inputted from the DSP  8  by converting it into output amplitude V. 
       The Second Example Embodiment 
       [0055]    An optical receiver of the second example embodiment includes a reception unit, an amplifying unit and a control unit. An optical receiver of the second example embodiment includes a part of the structure of the coherent optical receiver  100  of the first example embodiment shown in  FIG. 1 . 
         [0056]    The reception unit receives an optical signal to which coherent modulation has been performed, and outputs a first electric signal converted from the optical signal. For example, the function of the reception unit is performed by the part including the PBS  1 , the BS  2 , the 90-degree hybrid  3 , the LO light source  4  and the PD  5  of  FIG. 1 . The reception unit receives an optical signal to which coherent modulation has been performed, and makes the optical signal interfere with LO light to output a beat signal generated by the interference to the amplifying unit as a first electric signal. 
         [0057]    The first electric signal is amplified by the amplifying unit. The amplifying unit amplifies the first electric signal and outputs the first electric signal that has been amplified as a second electric signal. 
         [0058]    The control unit obtains the optical power of the optical signal based on the relation between optical power of an optical signal in the reception unit, and a gain of the amplifying unit and amplitude of the second electric signal. Relation between the optical power of an optical signal in the reception unit, and a gain of the amplifying unit and amplitude of the second electric signal is measured when the optical receiver is produced, and the measurement result has been stored in the control unit. Alternately, relation between the optical power of an optical signal in a reception unit, and a gain of an amplifying unit and amplitude of the second electric signal may be obtained by calculation using optical characteristics and the electrical characteristics (such as loss, output light power of LO light source, conversion efficiency of a light receiving element and amplifying characteristics of an amplifier) of elements of which an optical receiver is composed. Or, the control unit may acquire a gain of the amplifying unit and amplitude of the second electric signal from the amplifying unit, and divide the amplitude of the second electric signal by the gain of the amplifying unit to obtain the amplitude of the first electric signal. Then, the control unit may obtain the optical power of an optical signal from the relation between the optical power of an optical signal and amplitude of the first electric signal measured in advance. 
         [0059]    An optical receiver of the second example embodiment having such structure can come to know the optical power of a received signal correctly within a wide range of optical input power. The reason of this is that the control unit obtains the optical power of an optical signal based on at least one of the amplitude of the second electric signal outputted from an amplifying unit and a gain of the amplifying unit regardless of the amplitude of the first electric signal being within or an out-of-bounds of the range of the dynamic range of the amplifying unit. 
       The Third Example Embodiment 
       [0060]    An optical receiver of the third example embodiment includes a reception unit, an amplifying unit and a control unit. In an optical receiver of the third example embodiment, an optical signal received by the reception unit is not limited to an optical signal for which coherent modulation has been performed. 
         [0061]    That is, a reception unit of the optical receiver of the third example embodiment receives an optical signal and outputs a first electric signal converted from the optical signal. For example, the function of the reception unit is performed by PD. The structure of an optical receiver  3  of the third example embodiment besides above is similar to an optical receiver of the second example embodiment. 
         [0062]    An optical receiver of the third example embodiment having such structure can also come to know the optical power of a received signal correctly within a wide range of optical input power. The reason of this is that the control unit obtains the optical power of an optical signal based on at least one of amplitude of the second electric signal outputted from the amplifying unit and a gain of the amplifying unit regardless of the amplitude of the first electric signal being within or an out-of-bounds of the range of the dynamic range of the amplifying unit. 
         [0063]    Although the present invention has been described with reference to the example embodiments above, the present invention is not limited to the above-mentioned example embodiments. Various changes which a person skilled in the art can understand can be made in the composition and details of the present invention within the scope of the present invention. 
         [0064]    For example, in the first and second example embodiments, there have been described example embodiments in which the present invention is applied to a coherent optical receiver. However, as is the third example embodiment, the present invention is also applied to an optical receiver besides a coherent optical receiver. As a result, the present invention also exerts an effect that even a general optical receiver can come to know the optical power of a received signal correctly in a wide range of optical input power. 
         [0065]    This application claims priority based on Japanese application Japanese Patent Application No. 2014-121490, filed on Jun. 12, 2014, the disclosure of which is incorporated herein in its entirety by reference. 
       REFERENCE SIGNS LIST 
       [0066]      100 ,  101  Coherent optical receiver 
         [0067]      110  Optical signal 
         [0068]      1  PBS (polarization beam splitter) 
         [0069]      2  BS (beam splitter) 
         [0070]      3  90-degree hybrid 
         [0071]      4  LO (local oscillation) light source 
         [0072]      5  PD (photo diode) 
         [0073]      6  Differential amplifier 
         [0074]      61  TIA (trans-impedance amplifier) 
         [0075]      62  AGC (automatic gain control) amplifier 
         [0076]      63  Buffer 
         [0077]      64  Offset detector 
         [0078]      65  Peak detector 
         [0079]      7  ADC (analog-digital converter) 
         [0080]      8  DSP (digital signal processor) 
         [0081]      9  Control circuit 
         [0082]      91  CPU (central processing unit) 
         [0083]      92  Memory