Abstract:
In order to provide a high performance optical mixer having good yield, an optical mixer comprises: a first light branching means that branches a first input light into a plurality of first lights including a first output light and a second output light, and outputs the first lights; a second light branching means that branches a second input light into a plurality of second lights including a third output light and a fourth output light, and outputs the second lights; and a first light coupling and branching means and a second light coupling and branching means that couple the first and the third output lights and the second and the fourth output lights respectively, and branching the coupled lights into at least two, and outputting each of the branched lights as a coupled-and-branched light, wherein propagation paths for the third and the fourth output lights comprise widths that cause a prescribed optical path length difference to occur between the third and the fourth output lights, and propagation path lengths for the first and the second output lights are approximately equal and propagation path lengths for the third and the fourth output lights are approximately equal.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an optical mixer, an optical receiver, an optical mixing method and a production method for an optical mixer and in particular, relates to an optical mixer, an optical receiver, an optical mixing method and a production method for an optical mixer used when receiving a digital coherent signal. 
       BACKGROUND ART 
       [0002]    With a rise of transmission rate of optical communication system, investigations of the optical communication system that enables large capacity and high-speed communication more efficiently have been carried out energetically. Among them, DP-QPSK is a modulation method whose adoption is regarded as a favorite one in 100GE transmission device. DP-QPSK is an abbreviation of dual-polarization quadrature phase shift keying. Also, 100GE is an abbreviation of 100 Gigabit Ethernet (registered trademark). 
         [0003]    For demodulation of a signal light modulated by DP-QPSK, a digital coherent receiving method is used. In the digital coherent receiving method, a received signal light (received light) and a local oscillation light (local light) having optical frequency approximately the same as the received light are combined by an optical mixer called a 90-degree hybrid. Then, an output of the 90-degree hybrid is received by a light receiving element (photo diode, PD). The light receiving element outputs a beat signal of the received light and the local light to a signal processing circuit. The signal processing circuit performs calculation processing of the beat signal that PD outputted and demodulates data. 
         [0004]    In an optical receiver of the digital coherent receiving method, a light signal modulated by DP-QPSK is separated into polarized wave components crossing at right angles each other by PBS. Two received polarization-separated lights are inputted independently to the 90-degree hybrid formed out of an optical waveguide as a TE (transverse electric) signal and a TM (transverse magnetic) signal respectively. The inputted TE signal and the TM signal are mixed with the local light. 
         [0005]      FIG. 8  is a figure showing a structure of a 90-degree hybrid  10  related to the present invention. The 90-degree hybrid  10  is configured by two interferometers  11  and  12 . Both the interferometers  11  and  12  are MZI (Mach-Zehnder interferometer). 
         [0006]    In  FIG. 8 , the polarization-separated TE signal formed from the received light is inputted to an input port  31  of the 90-degree hybrid  10 . On the other hand, the polarization-separated TM signal component formed from the received light is inputted to an input port  33  of the 90-degree hybrid  10 . 
         [0007]    The local light outputted from a local oscillation light source installed outside the 90-degree hybrid is inputted to an input port  32  of the 90-degree hybrid  10 . 
         [0008]    The TE signal inputted to the input port  31  is inputted to an input light coupler  21 . The input light coupler  21  outputs the inputted TE signal to an arm  41  and an arm  42 . The TM signal inputted to the input port  33  is inputted to an input light coupler  24 . The input light coupler  24  outputs the inputted TM signal to an arm  47  and an arm  48 . 
         [0009]    The local light inputted to the input port  32  is branched into two lights and inputted to an input light coupler  22  and an input light coupler  23 . The input light coupler  22  outputs the inputted local light to an arm  43  and an arm  44 . The input light coupler  23  outputs the inputted local light to an arm  45  and an arm  46 . 
         [0010]    An output light coupler  25  couples the TE signal inputted from the arm  41  and the local light inputted from the arm  43 , and outputs the coupled signal to output ports  51  and  52 . 
         [0011]    An output light coupler  26  couples the TE signal inputted from the arm  42  and the local light inputted from the arm  44 , and outputs the coupled signal to output ports  53  and  54 . 
         [0012]    An output light coupler  27  couples the local light inputted from the arm  45  and the TM signal inputted from the arm  47 , and outputs the coupled signal to output ports  55  and  56 . 
         [0013]    An output light coupler  28  couples the local light inputted from the arm  46  and the TM signal inputted from the arm  48 , and outputs the coupled signal to output ports  57  and  58 . 
         [0014]    The interferometers  11  and  12  that configure the 90-degree hybrid  10  are asymmetric MZI. That is, in the interferometer  11 , lengths of the arms  41  and  42  are the same, and length of the arm  44  is longer than that of the arm  43  by ¼ wavelength (π/2) when converted to a wavelength of the signal light that passes the interferometer  11 . And also in the interferometer  12 , length of the arms  45  and  46  are the same, and length of the arm  48  is longer than that of the arm  47  by ¼ wavelength (π/2) when converted to a wavelength of the signal light that passes interferometer  12 . 
         [0015]    Then, in the arm  44  and the arm  48 , by changing the physical lengths of waveguides from the arm  43  and the arm  47 , phase differences are caused to the propagating lights. For this reason, in the arm  44  and the arm  48 , bends  60  and  61  are installed in the arms. 
         [0016]    Patent literature (PTL) 1 related to the present invention describes phase control of an interferometer by a waveguide. The target of PTL 1 is to realize an optical filter by combining the MZI in multiple stages. Also, PTL 2 describes a 90-degree hybrid using a space optical system. PTL 2 discloses, for phase control in the space optical system, a structure for controlling a physical position or for inserting materials whose refractive index is different in an optical path. Further, PTL 3 describes a phase control method in an MZI interferometer configured by a waveguide. 
       CITATION LIST 
     Patent Literature 
       [0017]    [PTL 1] Japanese Unexamined Patent Application Publication No. 2010-134224 
         [0018]    [PTL 2] Japanese Unexamined Patent Application Publication No. 2010-237300 
         [0019]    [PTL 3] Japanese Unexamined Patent Application Publication No. 1995-281041 
       DISCLOSURE OF INVENTION 
     Technical Problem 
       [0020]    As has been explained in  FIG. 8 , in the 90-degree hybrid  10 , in order to make the physical lengths of the arm  44  and the arm  48  longer, the bends  60  and  61  are installed in the arms. And when the bends  60  and  61  are formed, a part with small radius of curvature occurs in the waveguide. 
         [0021]    However, when the bends  60  and  61  are installed in the arms  44  and  48  of the 90-degree hybrid  10  explained in  FIG. 8 , there is a case when loss of the 90-degree hybrid may increase by radiation from the part with small radius of curvature. Also, by configuring a part of the arm from the waveguide different in shape than other arms, symmetry of the structure of the optical waveguide declines, and as a result, problem occurs that there is a case that yield of the product may fall. 
         [0022]    In relation to such problems, although PTL 1 describes phase control of an interferometer by a waveguide, PTL 1 does not describe at all phase control in the 90-degree hybrid. Also, PTL 2 is one that discloses a technology that relates to a structure of the 90 degree hybrid using the space optical system, however, PTL 2 does not describe a structure that controls a phase of an optical mixer configured by a waveguide. Further, a technology described in PTL 3 does not describe at all a structure that performs phase control of the received light in the 90-degree hybrid, like PTL 1. 
         [0023]    The object of the present invention is to provide a technology for solving the problems mentioned above and for realizing an optical mixer that can be applied to the 90-degree hybrid. 
       Solution to Problem 
       [0024]    An optical mixer of the present invention includes: a first light branching means for branching a first input light into a plurality of first lights including a first output light and a second output light, and outputs the first lights; a second light branching means for branching a second input light into a plurality of second lights including a third output light and a fourth output light, and outputs the second lights; and a first light coupling and branching means and a second light coupling and branching means for coupling the first and the third output lights and the second and the fourth output lights respectively and branching the coupled lights into at least two, and outputting each of the branched lights as a coupled-and-branched light, wherein propagation paths for the third and the fourth output lights includes widths that cause a prescribed optical path length difference to occur between the third and the fourth output lights, propagation path lengths for the first and the second output lights are approximately equal and propagation path lengths for the third and the fourth output lights are approximately equal. 
         [0025]    An optical mixing method of the present invention includes: branching a first input light into a plurality of first lights including a first output light and a second output light, and outputting the first lights by a first light branching means; branching a second input light into a plurality of second lights including a third output light and a fourth output light, and outputting the second light by a second light branching means; coupling the first and the third output lights and branching the coupled lights into at least two by a first light coupling and branching means; coupling the second and the fourth output lights and branching the coupled light into at least two by a second light coupling and branching means; setting widths of propagation paths for the third and fourth output lights to cause a prescribed optical path length difference between the third and the fourth output lights; setting propagation path lengths for the first and second output lights to be approximately equal; and setting propagation path lengths for the third and the fourth output lights to be approximately equal. 
         [0026]    A production method of an optical mixer of the present invention includes: a step for forming a first clad layer on a substrate; a step for laminating a core layer on the first clad layer; a step for patterning the core layer and forming a core; and a step for covering the core by a second clad layer having a same refractive index as the first clad; wherein the patterning of the core layer uses a mask pattern forming a waveguide whose structure includes: a first light branching means for branching a first input light into a plurality of first lights including a first output light and a second output light and outputs the first lights; a second light branching means for branching a second input light into a plurality of second lights including a third output light and a fourth output light, and outputs the second lights; and a first light coupling and branching means and a second light coupling and branching means for coupling the first and the third output lights and the second and the fourth output lights respectively and branching the coupled lights into at least two, and outputting each of the branched lights as a coupled-and-branched light; and wherein propagation paths for the third and the fourth output lights include widths that cause a prescribed optical path length difference to occur between the third and fourth output lights and propagation path lengths for the first and the second output lights are approximately equal and propagation path lengths for the third and the fourth output lights are approximately equal. 
       Advantageous Effects of Invention 
       [0027]    The present invention has an effect that a high-performance optical mixer whose production is easy can be realized. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0028]      FIG. 1  A figure showing a structure of an optical mixer of the first exemplary embodiment 
           [0029]      FIG. 2  A figure showing calculation results of respective amount of change of equivalent refractive index difference and phase difference in case width of a waveguide is changed 
           [0030]      FIG. 3  A figure showing a structure of an optical mixer of the second exemplary embodiment 
           [0031]      FIG. 4  A figure showing a structure of an arm of an optical mixer of a modified example of the second exemplary embodiment 
           [0032]      FIG. 5  A figure showing a structure of an optical mixer of the third exemplary embodiment 
           [0033]      FIG. 6  A figure showing a structure of an optical mixer of the fourth exemplary embodiment 
           [0034]      FIG. 7  A figure showing a structure of an optical mixer of the fifth exemplary embodiment 
           [0035]      FIG. 8  A figure showing a structure of a 90-degree hybrid related to the present invention 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0036]    Generally, phase of a light after passing an optical waveguide changes depending on a wavelength of the light that passes the optical waveguide, an equivalent refractive index of the optical waveguide or an optical path length of the optical waveguide. Also, the equivalent refractive index of the optical waveguide changes depending on a width of the waveguide. In each exemplary embodiment explained below, an optical mixer will be explained that changes an optical path length of an optical waveguide utilizing a change in an equivalent refractive index caused by changing a width of an arm and, as a result, enables control of phase of the light that passes the optical waveguide. 
       The First Exemplary Embodiment 
       [0037]      FIG. 1  is a figure showing a structure of the first exemplary embodiment of an optical mixer of the present invention. An optical mixer  1  includes a same structure as the optical mixer  11  except for including an arm  49  in place of the arm  44  in the optical mixer  11  explained in  FIG. 8 . Further, in  FIG. 1 , elements including the same function and structure as  FIG. 8  are assigned the identical reference signs. 
         [0038]    In the optical mixer  1 , a first input light is inputted to the input port  31 , and a second input light is inputted to an input port  34 . The first input light is branched in the input light coupler  21  and propagates in the arms  41  and  42 , and inputted to the output light coupler  25  and the optical coupler  26  respectively. The second input light is branched in the input light coupler  22  and propagates in the arm  43  and the arm  49 , and inputted to the output light couplers  25  and  26  respectively. 
         [0039]    The output light coupler  25  combines the first input light that propagated in the arm  41  and the second input light that propagated in the arm  43 , and outputs first and second output lights to the output ports  51  and  52 . 
         [0040]    The output light coupler  26  combines the first input light that propagated in the arm  42  and the second input light that propagated in the arm  49 , and outputs third and fourth output lights to the output ports  53  and  54 . 
         [0041]    In the optical mixer  1  shown in  FIG. 1 , lengths of the arm  41  and the arm  42  are equal, and lengths of the arm  43  and the arm  49  are equal. And the optical mixer  1 , by making a width of the arm  49  different from a width of the arm  43 , causes a phase difference to occur between the arm  43  and the arm  49  for the second input light. 
         [0042]    Setting procedure of phase difference in the arm  49  of the optical mixer will be explained below. Generally, if a wavelength of light that propagates in an arm is λ, then phase difference Δφ between an arm of MZI of length L 1  and an equivalent refractive index n 1  and an arm of MZI of length L 2  and an equivalent refractive index n 2  can be obtained by the following formula. 
         [0000]      Δφ=2π( n 1× L 1− n 2× L 2)/λ  (1)
 
         [0043]    In formula (1), if a difference between the equivalent refractive indices n 1 −n 2  is made Δn, and the arm lengths L 1  and L 2  that configure an interferometer are made equal, that is, L 1 =L 2 =L, then the following formula (2) is obtained. 
         [0000]      Δφ=2π(Δ n×L )/λ  (2)
 
         [0044]    In this case, the difference Δn between the equivalent refractive indices of waveguides necessary to cause a phase change of π/2 can be obtained from the following formula derived from formula (2). 
         [0000]      Δ n=λ/ 4 L   (3)
 
         [0045]    For example, if L=2 mm, Δn is obtained from formula (2) as 1.94×10 −4  for a wavelength of 1.55 μm. 
         [0046]    Accordingly, in order to cause the phase difference of, for example, π/2 between the arm  43  and the arm  49  of an optical interferometer  1 , it can be understood that waveguides should be made so that the difference between the equivalent refractive indices of the arm  43  and the arm  49  will be about 1.94×10 −4 . 
         [0047]    Relation between a width of a waveguide and an equivalent refractive index can be obtained by numerical calculation.  FIG. 2  is a graph of relation between the width of a waveguide and, changes of the equivalent refractive index difference and the phase difference that occurs in the waveguide for a case of the wavelength of 1.55 μm, obtained by numerical calculation. In  FIG. 2 , a horizontal axis is the width (μm) of a waveguide, and a vertical axis is an amount of change of the equivalent refractive index difference and an amount of change of the phase difference (deg.).  FIG. 2  shows, by making a waveguide with an width of 4 μm as a standard (amount of changes of equivalent refractive index difference and phase difference=0), calculation result of respective amount of changes of the equivalent refractive index difference and the phase difference in case the width of the waveguide is changed between 3.9 μm and 4.1 μm. A dotted line of  FIG. 2  shows the amount of change of the equivalent refractive index difference. Also, four solid lines a to d of  FIG. 2  show calculation results of the phase difference in case the lengths of the waveguide are 1800 μm (a), 2000 μm (b), 2200 μm (c) and 2400 μm (d) respectively. 
         [0048]    From  FIG. 2 , it can be found that, for example, when the solid line b is focused, for a waveguide of the length of 2 mm (2000 μm) and the width of 4 μm, the width of a waveguide of the same length and that causes a phase difference of 90 degrees (π/2) is about 4.04 μm or about 3.96 μm. That is, for example, in the optical mixer  1  shown in  FIG. 1 , the lengths of the arms  41  to  43  and  49  are all set to 2 mm, the width of the arms  41  to  43  are set to 4 μm, and the width of the arm  49  is set to 4.04 μm. By configuring as above, the difference between the phase at the output light coupler  26  of the light that propagates in the arm  49  and the phase at the output light coupler  25  of the light that propagates in the arm  43  can be made π/2. 
         [0049]    Here, when the width of the arm  49  is set to 4 μm and the width of the arm  43  is set to 3.96 μm, the phase difference of π/2 can be caused between the light that propagates in the arm  49  and the light that propagates in the arm  43 . 
         [0050]    Also, in order to cause a prescribed phase difference, waveguides may be formed so that the width of the arm  49  will be narrower than the width of the arm  43 . That is, even when the width of the arm  43  is set to 4 μm and the width of the arm  49  is set to 3.96 μm, the phase difference of π/2 can be caused between the light that propagates in the arm  49  and the light that propagates in the arm  43 . 
         [0051]    In the optical mixer  1 , as shown in the example of computation mentioned above, the lengths of the arms  41  to  43  and the arm  49  may all be made equal. And by forming waveguides so that the width will be made different from the width of the arm  43  only for the arm  49  to which the phase difference is to be given to the light that passes, it becomes possible to configure an asymmetric MZI that includes the same function as the optical mixer  11  explained in  FIG. 8 . 
         [0052]    Thus, the optical mixer of the first exemplary embodiment controls, by changing the width of a waveguide of an arm and controlling the equivalent refractive index, the phase of the light that passes the arm concerned. For this reason, compared with a structure that causes a phase difference by including a bend in an arm, it has an effect that, without increasing optical loss by a steep curve of a waveguide, a high-performance optical mixer can be realized. Also, the optical mixer of the first exemplary embodiment can make the lengths of the arms all equal. Accordingly, since symmetry of the construction of the optical mixer increases compared with the structure including the bend in the arm, the optical mixer of the first exemplary embodiment also has an effect that the production yield improves. 
         [0053]    Further, the optical mixer explained in the first exemplary embodiment can operate as a 90-degree hybrid of a digital coherent receiver by inputting a QPSK-modulated light signal as the first input light, and inputting a local oscillation light as the second input light. 
         [0054]    Also, an optical receiver may be configured by adding PD, ADC (analog to digital converter) and a signal processing circuit to the optical mixer  1 . The PD receives each of the output lights outputted by the optical mixer  1  to the output ports  51  to  54  and outputs the received signals as electric signals. ADC applies analog-to-digital conversion to the electric signals outputted by the PDs. The signal processing circuit performs calculation processing to an output of ADC and demodulates data included in the electric signal. 
         [0055]    Further, the optical mixer explained in the first exemplary embodiment can be produced by the following procedure. That is, a first clad layer is formed on a substrate, and a core layer is laminated on the first clad layer. And by a mask pattern of the structure explained in  FIG. 1 , the core layer is patterned and a core is formed. Further, the core is covered with a second clad layer having the same refractive index as the first clad. 
       The Second Exemplary Embodiment 
       [0056]    In the optical interferometer of the first exemplary embodiment, in case a waveguide width of an arm is increased or decreased compared with widths of other arms, the waveguide width may not be changed over a full length of the arm. In case a prescribed phase difference is obtained, the waveguide width may be changed only for a part of the arm in longitudinal direction. 
         [0057]      FIG. 3  is a figure showing a structure of an optical mixer  2  of the second exemplary embodiment of the present invention. The optical mixer  2  includes an arm  80  in place of the arm  50  compared with the optical mixer  1  explained in the first exemplary embodiment. In the optical mixer  2  shown in  FIG. 3 , the identical reference signs are assigned to the elements including the same function and structure as the optical mixer  1  shown in  FIG. 1 . 
         [0058]    As for the arm  80  included in the optical mixer  2  shown in  FIG. 3 , only an arm central part  81  has a width different from the arm  43  and end parts of the arm  80  have widths identical with the arm  43 . 
         [0059]      FIG. 4  is a figure showing a structure of the arm  80  of a modified example of the optical mixer of the second exemplary embodiment. By making the difference between the widths of the waveguides larger, it is possible to obtain the change of the identical phase difference by making the length of the arm central part  81  shorter. 
         [0060]    The optical mixers of the second exemplary embodiment and of the modified example thus configured, like the optical mixer of the first exemplary embodiment, by changing the width of the waveguide of the arm and controlling the equivalent refractive index, control the phase of the light that passes the arm concerned. For this reason, compared with a structure that causes a phase difference by including a bend in an arm, the optical loss does not increase by a steep curve of a waveguide. And the optical mixers of the second exemplary embodiment and of the modified example have an effect that, compared with the structure including the bend in the arm, the symmetry of the construction of the optical mixer increases and the production yield of the optical mixer improves. 
       The Third Exemplary Embodiment 
       [0061]      FIG. 5  is a figure showing a structure of an optical mixer  3  of the third exemplary embodiment of the present invention. In the optical mixer  3  shown in  FIG. 5 , the identical reference signs are assigned to the elements including the same function and structure as the optical mixers  1  and  2  shown in  FIG. 1  and  FIG. 3 . 
         [0062]    The optical mixer  3  includes an arm  82  in place of the arm  80  compared with the optical mixer  2  explained in the second exemplary embodiment. In the arm  82 , a central part  83  of the arm  82  and end parts  85  of the arm  82  are connected using a tapered waveguide  84 . Accordingly, the optical mixer of the third exemplary embodiment has, in addition to the effects explained in the first and the second exemplary embodiments, an further effect that the optical mixer can reduce optical loss accompanied by a steep change of a waveguide width. 
       The Fourth Exemplary Embodiment 
       [0063]      FIG. 6  is a figure showing a structure of an optical mixer  4  of the fourth exemplary embodiment of the present invention. The optical mixer  4  differs, compared with the optical mixer  1  explained in the first exemplary embodiment, in a point that the optical mixer  4  includes a multimode interference element  62  as the input light coupler  22 . 
         [0064]    In  FIG. 6 , the multimode interference element  62  transmits the second input light inputted from the input port  34  to the arms  43  and  49 . By an action of the multimode interference element  62 , the lights outputted to the arm  43  and the arm  49  have a prescribed phase difference. Accordingly, in case the multimode interference element  62  outputs the lights to the arms  43  and  49  with exactly the phase difference of π/2, there is no need to add the phase difference by the arm  49  in order to cause the phase difference of π/2 to occur at the output light couplers  25  and  26  for the light inputted from the input port  34 . 
         [0065]    However, by variation of characteristics of the multimode interference element  62 , there is a case when the phase difference between the lights outputted from the multimode interference element  62  to the arm  43  and the arm  49  may not be exactly π/2. Such variation of characteristics of the multimode interference element  62  is occurred, for example, by an error in the production. 
         [0066]    For this reason, in the fourth exemplary embodiment, the optical mixer  4  adjusts the phase of the light that passes the arm  49  so that the phase difference of the light inputted from the input port  34  will be a prescribed value at the output light coupler  25  and the output light coupler  26 . 
         [0067]    For example, at an output of the multimode interference element  62 , suppose that the phase of the light outputted to the arm  49  advances by (π/2)+Δθ (Δθ&gt;0) compared with the phase of the light outputted to the arm  43  due to the variation of characteristics of the multimode interference element  62 . Δθ is a phase error of the multimode interference element  62 . In this case, by delaying the phase of the light at the arm  49  by only Δθ, the phase difference between the lights at the output light couplers  25  and  26  can be made π/2. 
         [0068]    Thus, by further adjusting the phase of the light outputted from the multimode interference element by the arm, the optical mixer of the fourth exemplary embodiment can match the phase difference between the lights inputted to the output light couplers with a prescribed value exactly. Accordingly, the optical mixer of the fourth exemplary embodiment has, in addition to the effect of the optical mixer of the first exemplary embodiment, an effect that the optical mixer can reduce influence of the phase error caused by the variation in the production of the multimode interference element. 
         [0069]    In the fourth exemplary embodiment, a case when the multimode interference element is employed as the input light coupler in the optical mixer of the first exemplary embodiment has been explained. And also in the optical mixers explained in the second and the third exemplary embodiments, the multimode interference element can be employed as the input light coupler. And in case a multimode interference element is employed as the input light coupler in the second or the third exemplary embodiment, in addition to the effect of each of the exemplary embodiments, the same effect as the fourth exemplary embodiment that the influence of the phase error of the multimode interference element can be reduced, is obtained. 
       The Fifth Exemplary Embodiment 
       [0070]      FIG. 7  is a figure showing a structure of an optical mixer of the fifth exemplary embodiment of the present invention. The optical mixer  5  shown in  FIG. 7  is one that arranges two optical mixers  1  explained in the first exemplary embodiment in parallel as optical mixers  6  and  7  and configured them as a 90-degrees hybrid used for demodulation of DP-QPSK signal. 
         [0071]    In the optical mixer  5 , a polarization-separated TE signal formed from a received light is inputted to the input port  31 , and a local light is inputted to the input port  32 . Also, a polarization-separated TM signal formed from the received light is inputted to the input port  33 . 
         [0072]    The TE signal is branched in the input light coupler  21  and each of the branched signals propagates in the arm  41  or the arm  42 , and is inputted to the output light coupler  25  or the output light coupler  26  respectively. The TM signal is branched in an input light coupler  122  and each of the branched signals propagates in an arm  143  or an arm  149 , and is inputted to an output light coupler  125  or an output light coupler  126  respectively. 
         [0073]    The local light is branched in the input light coupler  22  and an input light coupler  121 . The local lights branched in the input light coupler  22  propagate in the arm  43  and the arm  49 , and are inputted to the output light coupler  25  and the output light coupler  26  respectively. The local lights branched in the input light coupler  121  propagate in an arm  141  and an arm  142 , and are inputted to the output light coupler  125  and the output light coupler  126  respectively. 
         [0074]    The output light coupler  25  combines the TE signal that propagated in the arm  41  and the local light that propagated in the arm  43 , and outputs an output light to the output ports  51  and  52 . 
         [0075]    The output light coupler  26  combines the TE signal that propagated in the arm  42  and the local light that propagated in the arm  49 , and outputs an output light to the output ports  53  and  54 . 
         [0076]    The output light coupler  125  combines the TE signal that propagated in the arm  141  and the local light that propagated in the arm  143 , and outputs an output light to the output ports  151  and  152 . 
         [0077]    The output light coupler  126  combines the TE signal that propagated in the arm  142  and the local light that propagated in the arm  149 , and outputs an output light to the output ports  153  and  154 . 
         [0078]    In the optical mixer  5  shown in  FIG. 7 , lengths of the arm  41  and the arm  42  are equal, and the lengths of the arm  43  and the arm  49  are equal. Further, lengths of the arm  141  and the arm  142  are equal, and lengths of the arm  143  and the arm  149  are equal. Additionally, lengths of all the arms may be made equal. 
         [0079]    And widths of the arms  49  and  149  are defined so that at the output light coupler  26  and the output light coupler  126 , a phase difference between the TE signal or the TM signal and the local light will be π/2 respectively. The widths of the arm  49  and the arm  149  are determined by the procedure explained in the first exemplary embodiment. 
         [0080]    By including such a structure, the optical mixer  5  generates mixed signals of the local light, and the polarization-separated TM signal or the polarization-separated TE signal formed from the DP-QPSK modulated received light at the output light couplers. 
         [0081]    That is, the output light couplers  25  and  26  mix the TE signal and the local light. And phases of the local light against the TE signal are different by π/2 between the output light coupler  25  and the output light coupler  26 . Similarly, the output light couplers  125  and  126  mix the TM signal and the local light. And phases of the local light against the TM signal are different for π/2 at the output light coupler  125  and the output light coupler  126 . 
         [0082]    The optical mixer of the fifth exemplary embodiment explained above controls, like the optical mixer of the first exemplary embodiment, by changing the width of the waveguide of the arm and controlling the equivalent refractive index, the phase of the light that passes the arm concerned. For this reason, compared with a structure that causes a phase difference by including a bend in an arm, the optical loss does not increase by a steep curve of a waveguide. 
         [0083]    As a result, the optical mixer of the fifth exemplary embodiment can realize a high performance optical mixer having good yield for making the signal for which DP-QPSK modulation is performed and the local light interfere. 
         [0084]    Incidentally, an optical receiver may be configured by adding PD, ADC and a signal processing circuit to the optical mixer  5 . The PD receives each of the output lights outputted to the output ports  51 - 54  and  151 - 154  by the optical mixer  5  and outputs the received signals as electric signals. ADC applies analog-to-digital conversion to the electric signals outputted by the PD. The signal processing circuit performs calculation processing to an output of ADC and demodulates data included in the electric signal. 
         [0085]    Incidentally, in the fifth exemplary embodiment, the optical mixers  6  and  7  may be replaced by any one of the optical mixers  2  to  4  explained in the second to the fourth exemplary embodiments. In this case, it is clear that any of the effect that has been explained in the second to the fourth exemplary embodiments corresponding to the replaced optical mixer is obtained together. 
         [0086]    As above, although exemplary embodiments of the present invention have been explained with reference to the first to the fifth exemplary embodiments, embodiments to which the present invention is applicable are not limited to the exemplary embodiments mentioned above. Various changes that a person skilled in the art can understand within the scope of the present invention can be performed in the structure and detail explanation of the present invention. 
         [0087]    This application claims priority based on Japanese Patent Application No. 2011-106390 filed on May 11, 2011 and the disclosure thereof is incorporated herein in its entirety. 
       REFERENCE SIGNS LIST 
       [0000]    
       
         
           
               1 - 7  Optical mixer 
               21 - 24 ,  121 ,  122  Input light coupler 
               25 - 28 ,  125 ,  126  Output light coupler 
               31 - 34  Input port 
               41 - 49 ,  80 ,  82 ,  141 - 143 ,  149  Arm 
               51 - 58 ,  151 - 154  Output port 
               62  Multimode interference element 
               81 ,  83  Arm central part 
               84  Tapered waveguide 
               85  Arm end part