Patent Publication Number: US-2022236483-A1

Title: Wavelength demultiplexer, optical transceiver front-end module, photonic circuit, and wavelength demultiplexing control method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims priority to earlier Japanese Patent Application No. 2021-010832 filed Jan. 27, 2021, which is incorporated herein by reference in its entirety. 
     FIELD 
     The present documents relate to a wavelength demultiplexer, an optical transceiver front-end module, a photonic circuit, and a wavelength demultiplexing control method. 
     BACKGROUND 
     Optical signals are suited to high-speed, high-capacity data transmission, and are widely used in the fields of data communications and data processing. In order to increase the data transmission rate, a wavelength division multiplexing (WDM) scheme for superimposing signals of different wavelengths into one optical fiber is adopted. In a WDM system, signals of multiple wavelengths are bundled or multiplexed at a transmitter side, and separated or demultiplexed at a receiver side. Various schemes of wavelength multiplexing and demultiplexing have been proposed, but the basic idea is to separate narrowly spaced wavelengths making use of interference. In general, interferometers tend to increase the device size. However, owing to the progress of silicon photonics technology, the device can be downsized. 
     Silicon photonic devices are greatly affected by manufacturing tolerances, because of their microscopic structures. In particular, it is not easy for a WDM device to achieve a desired wavelength characteristic at each of the wavelengths from the beginning in the initial state. In addition, light interference is sensitive to environmental changes including a temperature change. It is unrealistic to operate the device without any adjustment, and some tuning mechanism is required. A configuration has been proposed to monitor an optical power at a demultiplexer using an asymmetric Mach-Zehnder (AMZ) interferometer, and to separate the respective wavelengths, while compensating for manufacturing variations or characteristic fluctuation occurring due to a temperature change. See, for example, Patent Documents 1 and 2 presented below. 
     In silicon photonic devices, light behavior greatly differs between TE polarized waves and TM polarized waves, and devices are designed so as to operate for either one of the polarized waves, typically for TE polarized wave. The polarization state of light travelling through an optical fiber is unstable, and the polarized wave incident onto a silicon photonic device is not always a TE wave. In addition, fluctuation of polarization state over time occurs due to the influence of vibration of the optical fiber. 
     A technique of appropriately demultiplexing multiple wavelengths is desired, while reducing the influence of polarization. An optical device adapted to suppress the influence of the polarization state of incident light is known. See, for example, Patent Document 3. 
     Prior art document(s) described above is(are):
     Patent Document 1: JP Patent Application Laid-open Publication No. 2019-061121,   Patent Document 2: JP Patent Application Laid-open Publication No. 2019-135524, and   Patent Document 3: JP Patent Application Laid-open Publication No. 2016-18048.   

     SUMMARY 
     In an embodiment, a wavelength demultiplexer includes a photonic circuit that converts two orthogonal polarized waves contained in incident light into two same polarized waves, the photonic circuit having a first optical demultiplexing circuit and a second optical demultiplexing circuit having a same configuration and provided for the respective ones of the two same polarized waves; and a control circuit that adjusts the wavelength characteristics of the first optical demultiplexing circuit and the second optical demultiplexing circuit. The photonic circuit supplies a total output power of monitor lights extracted from the same positions in the first optical demultiplexing circuit and the second optical demultiplexing circuit to the control circuit. The control circuit controls a first wavelength characteristic of the first optical demultiplexing circuit and a second wavelength characteristic of the second optical demultiplexing circuit based on the total output power of the monitor lights. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive to the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a basic diagram of a wavelength demultiplexer having a photonic circuit to which polarization diversity is applied; 
         FIG. 2  illustrates a configuration example for combining the output signals of monitor photodetector; 
         FIG. 3A  illustrates a configuration example of a wavelength demultiplexer that separates two wavelengths, showing control on a first AMZ interferometer which is a unit circuit forming an optical demultiplexing circuit; 
         FIG. 3B  illustrates a configuration example of a wavelength demultiplexer that separates two wavelengths, showing control on a second AMZ interferometer which is also a unit circuit forming the optical demultiplexing circuit; 
         FIG. 3C  illustrates a configuration example of a wavelength demultiplexer that separates two wavelengths, showing control on a third AMZ interferometer which is also a unit circuit forming the optical demultiplexing circuit; 
         FIG. 4  illustrates an example of initial value and operating value information saved in a memory; 
         FIG. 5A  is a flowchart of determining a control direction; 
         FIG. 5B  is a flowchart of determining a control direction, performed following the operation flow of  FIG. 5A ; 
         FIG. 5C  is a flowchart of determining a control direction, performed following the operation flow of  FIG. 5B ; 
         FIG. 6  is a flowchart of setting an initial value; 
         FIG. 7  illustrates a configuration example of a wavelength demultiplexer that optically combine output powers of monitored lights; 
         FIG. 8  is a schematic diagram of a photonic circuit that separates four wavelengths; and 
         FIG. 9  is a schematic diagram of an optical transceiver front-end module using a wavelength demultiplexer. 
     
    
    
     EMBODIMENT(S) 
     Embodiments for implementing the invention will be described below with reference to the drawings. In the following description, the same elements may be denoted by the same reference numerals and redundant description may be omitted. 
       FIG. 1  illustrates a basic configuration of a wavelength demultiplexer  1  with a photonic circuit  10  to which polarization diversity is applied. The wavelength demultiplexer  1  includes a photonic circuit  10 , and a control circuit  20  that adjusts the wavelength characteristics of the photonic circuit  10 . An optical signal is input to the photonic circuit  10  through, for example, a single-mode optical fiber  31 . Light beams with a plurality of wavelengths are multiplexed in the optical signal. In the example of  FIG. 1 , the optical fiber  31  extends in a direction parallel to the surface of the photonic circuit  10 , and is coupled to a waveguide (e.g., a silicon waveguide) formed on the photonic circuit  10  using an edge coupler  32 . The edge coupler  32  may be called a spot size converter, and it can be formed on the photonic circuit  10  by a known technique. The light incident direction onto the photonic circuit  10  is not limited to the horizontal direction. The light signal may be introduced to the photonic circuit  10  from a direction perpendicular or oblique to the surface of the photonic circuit  10 . 
     The light input to the photonic circuit  10  includes both a TE polarized wave and a TM polarized wave, due to the influence of the transmission line. The TE polarized wave oscillates in a direction orthogonal to the incident axis and horizontal to the plane of the photonic circuit  10 . The TM polarized wave oscillates in a direction orthogonal to the axis of incidence and perpendicular to the plane of the photonic circuit  10 . The light incident on the photonic circuit  10  is split into the TE polarized wave and the TM polarized wave by a polarization beam splitter  33  (hereinafter abbreviated as “PBS  33 ”). The polarization plane of one of the split polarized waves, for example, the TM polarized wave is rotated by 90 degrees by a polarization rotator  34  (hereinafter, abbreviated as “PR 34 ”). As a result, two TE polarization components are generated. 
     The PBS  33  and the PR  34  may be formed by waveguides on the photonic circuit  10  by a known technique. The PBS  33  can be fabricated with, for example, a directional coupler having a tapered waveguide. The PR  34  can be fabricated using, for example, a double core structure having different refractive indexes. 
     The photonic circuit  10  has a first optical demultiplexing circuit  11  and a second optical demultiplexing circuit  12  provided for the two same polarizations. The first optical demultiplexing circuit  11  and the second optical demultiplexing circuit  12  have the same configuration. The term “same configuration” means that the basic designs are the same, and they may include a manufacturing error within an acceptable range. In  FIG. 1 , the first optical demultiplexing circuit  11  and the second optical demultiplexing circuit  12  are mirror-symmetrical, that is, symmetrically arranged with respect to the center line extending in the propagation direction of the photonic circuit  10 , but the basic design is the same. 
     The first optical demultiplexing circuit  11  is formed by a unit circuit  13 . The unit circuit  13  has an AMZ interferometer  130  and heaters  1301  and  1302  provided to the waveguides of the AMZ interferometer  130 . The second optical demultiplexing circuit  12  is formed by a unit circuit  14  having the same configuration as the unit circuit  13 , the layout being mirror-symmetric. The unit circuit  14  has an AMZ interferometer  140  having the same configuration as the AMZ interferometer  130 , and heaters  1401  and  1402  may be provided to the waveguides of the AMZ interferometer  140 . 
     The heaters  1301 ,  1302 ,  1401  and  1402  are provided to adjust the wavelength characteristics of the first optical demultiplexing circuit  11  and the second optical demultiplexing circuit  12 , and they may serve as phase shifters that change the refractive indexes of the corresponding waveguides. As long as the wavelength characteristic is adjusted, any structure may be used to adjust the refractive index of the corresponding waveguide to control the phase of light travelling through that waveguide. Accordingly, another mechanism such as a current injection structure, a voltage applying structure, a pressurizing structure, etc. may be provided, in place of the heaters. 
     In an AMZ interferometer, the wavelength shifting direction is opposite between the upper and lower waveguides (which are also called “upper and lower arms”). Accordingly, if heaters or other mechanism are provided to the both (upper and lower) waveguides for controlling the refractive index, either one of the heaters that can adjust the wavelength characteristics with less power may be selected. The two waveguides of each AMZ interferometer are split and combined by couplers (CPLs). The first optical demultiplexing circuit  11  and the second optical demultiplexing circuit  12  have wavelength separating functions, as described later. In order to simplify the illustration and facilitate understanding of the basic structure, some portions of the unit circuits  13  and  14  are omitted in  FIG. 1 . 
     In the first optical demultiplexing circuit  11  and the second optical demultiplexing circuit  12 , the optical power is monitored at the same positions. In  FIG. 1 , signal light is extracted from one of the two output ports of the AMZ interferometer  130 , and a monitor photodetector  15  is connected to the other output port, at which the optical power of the signal light is monitored. Similarly, signal light is extracted from one of the two output ports of the AMZ interferometer  140 , and a monitor photodetector  16  is connected to the other output port to monitor the optical power of the corresponding signal light. 
     If the wavelength of the incident light on the photonic circuit  10  is in the 1.31 μm band or 1.55 μm band used for optical communications, germanium photodiodes may be used as the monitor photodetectors  15  and  16 . At the germanium photodetector, the light having travelled through the silicon waveguide is absorbed, and a photocurrent corresponding to the intensity of the incident light is generated and output. 
     The outputs of the monitor photodetectors  15  and  16  are summed up and supplied to the control circuit  20 . Depending on the polarization state in the optical fiber  31 , the light quantity of polarized wave incident on one of the optical demultiplexing circuits may be insufficient. With a typical structure for acquiring the monitor results independently from the first optical demultiplexing circuit  11  and the second optical demultiplexing circuit  12 , a satisfactory monitoring result may not be obtained from one of the optical demultiplexing circuits. In such a case, the wavelength characteristic of each of the optical demultiplexing circuits may not be correctly tuned. Even if the initial state of the photonic circuit can be appropriately adjusted using an initial correction value or the like, the wavelength characteristic cannot follow the future changes likely to occur in the environment or the optical fiber. 
     With the photonic circuit  10  of the embodiment, the sum of the monitoring results of the optical powers acquired from the same positions in the first optical demultiplexing circuit  11  and the second optical demultiplexing circuit  12  having the same configuration is used, as the basis for tuning the wavelength characteristics. With this structure, the monitoring result of the optical power can be secured, regardless of the polarization state in the optical fiber  31 . When the polarization state fluctuates, the distribution ratio of light incident on the first optical demultiplexing circuit  11  and the second optical demultiplexing circuit  12  may vary, but the total monitor value does not change, and a stable monitor result can be acquired. 
     In the photonic circuit  10 , the monitoring result is shared between the first optical demultiplexing circuit  11  and the second optical demultiplexing circuit  12 . However, there may not be a guarantee that the wavelength characteristics of the AMZ interferometers having the same configuration and provided at the same positions in the first optical demultiplexing circuit  11  and the second optical demultiplexing circuit  12  are the same. This is because the wavelength characteristics fluctuate due to manufacturing tolerances, environmental change, or other factors. 
     In a configuration example, the variation in the initial state between the first optical demultiplexing circuit  11  and the second optical demultiplexing circuit  12  is measured in advance, and the measurement result is used as an initial value for correction. The initial value is used to cancel the variation or the difference in the wavelength characteristic between the two optical demultiplexing circuits. The optical power monitored in the photonic circuit is shared between the first optical demultiplexing circuit  11  and the second optical demultiplexing circuit  12 . How the initial value is set will be described later. 
     The sum of the monitoring results obtained from the photonic circuit  10  is supplied to the control circuit  20 . The control circuit  20  includes a current-voltage converter  21  (hereinafter, abbreviated as WI/V converter  21 ″), a processor  22 , and a memory  23 . The control circuit  20  adjusts the wavelength characteristics of the first optical demultiplexing circuit  11  and the second optical demultiplexing circuit  12 , based on the sum of the monitoring results. 
     As illustrated in  FIG. 2 , the I/V converter  21  can be designed as a typical negative feedback circuit. The photocurrents output from the monitor photodetectors  15  and  16  are added, and the total current is connected to one of the input terminals of the operational amplifier  211 . The other of the input terminals of the operational amplifier  211  is connected to, for example, a constant potential. The 180-degree phase inverted output of the operational amplifier  211  is connected to the input of the operational amplifier  211  via a feedback resistor  212 . The feedback resistor  212  converts the total of the output currents of the monitor photodetectors  15  and  16  into a voltage, and the voltage Vout is output from the I/V converter  21 . 
     Returning to  FIG. 1 , the output of the I/V converter  21  is connected to the input of the processor  22 . An analog-to-digital converter (ADC) may be provided before the input to the processor  22  to input a digital value of an analog voltage signal to the processor  22 , or alternatively, the processor  22  may include an ADC. 
     The processor  22  has a control direction determination unit  221  and phase control units  222  and  223  as its functions. The memory  23  has initial value information items  231  and  232  and operating value information  233 . The initial value information is used to correct the characteristic deviation or variation between interfering elements (e.g., AMZ interferometers) having the same configuration and used in the first optical demultiplexing circuit  11  and the second optical demultiplexing circuit  12 . The operating value information includes a control amount n×Δ (where n is an integer) for adjusting the wavelength characteristics of the AMZ interferometers  130  and  140  provided in the unit circuits  13  and  14 . The initial value information may be set during, for example, a pre-shipment inspection, using the operating value information. 
     The phase control units  222  and  223  control the phases of the light beams passing through the AMZ interferometers  130  and  140 , respectively, until the control direction determination unit  221  has judged that the monitored light power has increased or decreased in a desired direction. By adjusting the amount of current or the voltage level applied to the heater  1301  or  1302  of the AMZ interferometer  130 , or to the heater  1401  or  1402  of the AMZ interferometer  140 , the refractive index of the waveguide changes, and the phase of the light beam traveling through the waveguide is controlled. Based on the summed-up monitoring result of the photonic circuit  10  to which the polarization diversity is applied, the phase adjustment for the first optical demultiplexing circuit  11  and the second optical demultiplexing circuit  12  is repeated at a predetermined step size Δ of the control amount. Consequently, a plurality of wavelengths contained in the received light can be demultiplexed, while reducing the influence of polarization. 
     &lt;Separation of 2 Wavelengths&gt; 
       FIG. 3A  to  FIG. 3C  are schematic diagrams of a wavelength demultiplexer  2  that separates two wavelengths. The wavelength demultiplexer  2  includes a photonic circuit  10 A and a control circuit  20 A. The photonic circuit  10 A includes a first optical demultiplexing circuit  11 A and a second optical demultiplexing circuit  12 A having the same configuration. As in  FIG. 1 , the first optical demultiplexing circuit  11 A and the second optical demultiplexing circuit  12 A are provided mirror-symmetrically. The two wavelengths are separated in each of the first optical demultiplexing circuit  11 A and the second optical demultiplexing circuit  12 A. 
     The first optical demultiplexing circuit  11 A has a unit circuit  13 A composed of three AMZ interferometers  131 ,  132 , and  133 . The unit circuit  13 A has a tree structure in which the second AMZ interferometer  132  and the third AMZ interferometer  133  are connected to the two output ports of the first AMZ interferometer  131 . The second optical demultiplexing circuit  12 A has a unit circuit  14 A composed of three AMZ interferometers  141 ,  142 , and  143 . The unit circuit  14 A has a tree structure in which a second AMZ interferometer  142  and a third AMZ interferometer  143  are connected to the two output ports of the first AMZ interferometer  141 . The splitting and the combining regions of each of the AMZ interferometers are formed by, for example, 3 dB couplers. 
       FIG. 3A  illustrates optical power monitoring and wavelength characteristic control for the first AMZ interferometers  131  and  141  included in the unit circuits  13 A and  14 A, respectively.  FIG. 3B  illustrates optical power monitoring and wavelength characteristic control for the second AMZ interferometers  132  and  142  included in the unit circuits  13 A and  14 A, respectively.  FIG. 3C  illustrates optical power monitoring and wavelength characteristic control for the third AMZ interferometers  133  and  143  included in the unit circuits  13 A and  14 A, respectively. 
     In  FIG. 3A  to  FIG. 3C , the power monitoring and wavelength characteristic control for the three AMZ interferometers included in a unit circuit are illustrated in separate figures for the purpose of simplifying the drawings and avoiding complicated electrical wirings. In actual operations, the optical power monitoring and wavelength characteristic control for the three AMZ interferometers  131 ,  132 , and  133  included in the unit circuit  13 A and for the three AMZ interferometers  141 ,  142 , and  143  included in the unit circuit  14 A are performed simultaneously. 
     Referring to  FIG. 3A , the light incident from the optical fiber  31  onto the photonic circuit  10 A contains light components with wavelengths λ 1  and λ 2 . The incident light is subjected to polarization beam splitting at the PBS  33 . For example, the TE polarization including λ 1  and λ 2  is guided to the first optical demultiplexing circuit  11 A. The TM polarization including λ 1  and λ 2  is guided to the second light demultiplexing circuit  12 A after the polarization plane is rotated by 90 degrees at the PR  34 . 
     In the first optical demultiplexing circuit  11 A, the transmission spectrum having a peak at λ 1  and the transmission spectrum having a peak at λ 2  are separated by the difference in optical path length of the first AMZ interferometer  131 . This first AMZ interferometer  131  is referred to as “AMZ. 1   a ”. Similarly, in the second optical demultiplexing circuit  12 A, the transmission spectrum having a peak at λ 1  and the transmission spectrum having a peak at λ 2  are separated by the difference in optical path length of the first AMZ interferometer  141 . This first AMZ interferometer  141  is referred to as “AMZ. 1   b”.    
     In the first optical demultiplexing circuit  11 A, the transmission spectrum having a peak at λ 1  is incident on the second AMZ interferometer  132 , and the transmission spectrum having a peak at λ 2  is incident on the third AMZ interferometer  133 . A total of four monitor photodetectors  151 ,  152 ,  153  and  154  are provided to the output ports of the second AMZ interferometer  132  and the third AMZ interferometer  133 . The second optical demultiplexing circuit  12 A has the same configuration as the first optical demultiplexing circuit  11 A, and a total of four monitor photodetectors  161 ,  162 ,  163  and  164  are provided to the output ports of the second AMZ interferometer  142  and the third AMZ interferometer  143 . 
     In the first optical demultiplexing circuit  11 A, signal light of wavelength λ 1  is extracted from one of the two output ports of the second AMZ interferometer  132 , and the monitor photodetector  151  is connected to the other output port. The monitor photodetector  151  is used for adjusting the wavelength characteristics of the second AMZ interferometer  132 , as will be described below with reference to  FIG. 3B . 
     A portion of the signal light of wavelength λ 1  is branched and supplied to the monitor photodetector  152 . The monitor photodetector  152  is used to control the wavelength characteristics of the first AMZ interferometer  131 . This monitor photodetector  152  is referred to as “MPD. 1   a”.    
     Signal light of wavelength λ 2  is extracted from one of the two output ports of the third AMZ interferometer  133 , and a monitor photodetector  154  is connected to the other output port. The monitor photodetector  154  is used for adjusting the wavelength characteristics of the third AMZ interferometer  133 , as will be described below with reference to  FIG. 3C . A portion of the signal light of wavelength λ 2  is branched and supplied to the monitor photodetector  153 . The monitor photodetector  153  is used to control the wavelength characteristics of the first AMZ interferometer  131 . This monitor photodetector  153  is also referred to as “MPD. 1   a”.    
     The second optical demultiplexing circuit  12 A has the same configuration as the first optical demultiplexing circuit  11 A. Signal light of wavelength λ 1  is extracted from one of the two output ports of the second AMZ interferometer  142 , and a monitor photodetector  161  is connected to the other output port. The monitor photodetector  161  is used for adjusting the wavelength characteristics of the second AMZ interferometer  142 , as will be described below with reference to  FIG. 3B . 
     A portion of the signal light of wavelength λ 1  is branched and supplied to the monitor photodetector  162 . The monitor photodetector  162  is used to control the wavelength characteristics of the first AMZ interferometer  141 . This monitor photodetector  162  is referred to as “MPD. 1   b”.    
     The signal light of wavelength λ 2  is extracted from one of the two output ports of the third AMZ interferometer  143 , and the monitor photodetector  164  is connected to the other output port. The monitor photodetector  164  is used for adjusting the wavelength characteristics of the third AMZ interferometer  143 , as will be described below with reference to  FIG. 3C . A portion of the signal light of wavelength λ 2  is branched and supplied to the monitor photodetector  163 . The monitor photodetector  163  is used to control the wavelength characteristics of the first AMZ interferometer  141 . This monitor photodetector  163  is also referred to as “MPD. 1   b”.    
     The outputs of the monitor photodetectors  152  and  153  (collectively referred to as “MPD. 1   a ”) and the outputs of the monitor photodetectors  162  and  163  (collectively referred to as “MPD. 1   b ”) are summed up, and the total value is input as the first monitor value to the I/V converter  21 A of the control circuit  20 . 
     The total of the signal lights of wavelength λ 1  extracted from the second AMZ interferometer  132  of the first optical demultiplexing circuit  11 A and from the second AMZ interferometer  142  of the second optical demultiplexing circuit  12 A is detected by the λ 1  photodetector  17 . The total of the signal lights of wavelength λ 2  extracted from the third AMZ interferometer  133  of the first optical demultiplexing circuit  11 A and from the third AMZ interferometer  143  of the second optical demultiplexing circuit  12 A is detected by the λ 2  photodetector  18 . The detected signal lights of λ 1  and λ 2  are processed by a signal processor at the subsequent stage. 
     The control circuit  20 A controls the wavelength characteristics of the first AMZ interferometer  131  of the first optical demultiplexing circuit  11 A and the the first AMZ interferometer  141  of the demultiplexing circuit  12 A, by means of the control direction determination unit  221 A and the phase control units  222 A and  223 A which are implemented by the functions of the processor  22 A. Specifically, the control signal  1   a  output from the phase control unit  222 A controls a phase shifter PS provided in the AMZ interferometer  131 . The control signal  1   a  includes a compensation for the initial deviation of the characteristic of the AMZ interferometer  131 . The control signal  1   b  output from the phase control unit  223 A controls a phase shifter PS provided in the AMZ interferometer  141 . The control signal  1   b  includes a compensation for the initial deviation of the characteristic deviation of the AMZ interferometer  141 . The control circuit  20 A controls the phase of light in a direction that the first monitor value increases by the control signals  1   a  and  1   b.    
     The control illustrated in  FIG. 3A  is a control for increasing the peak powers at the wavelengths λ 1  and λ 2 . The center wavelength of the peak of the transmission spectrum whose peak is located near λ 1  is brought closer to λ 1 , and the center wavelength of the peak of the transmission spectrum whose peak is located near λ 2  is brought closer to λ 2 , thereby increasing the peak intensities of the both wavelengths. 
     Because the sum of the optical powers monitored at the same positions in the first optical demultiplexing circuit  11 A and the second optical demultiplexing circuit  12 A which have the same configuration is used, stable monitoring results can be acquired, regardless of the polarization state in the optical fiber  31 . This configuration can improve the accuracy of control on the wavelength characteristics of the AMZ interferometers  131  and  141 . 
       FIG. 3B  illustrates control on the wavelength characteristics of the second AMZ interferometer  132  of the first optical demultiplexing circuit  11 A and the second AMZ interferometer  142  of the second optical demultiplexing circuit  12 A. The second AMZ interferometer  132  of the first optical demultiplexing circuit  11 A is referred to as “AMZ. 2   a ”, and the second AMZ interferometer  142  of the second optical demultiplexing circuit  12 A is referred to as “AMZ. 2   b”.    
     In the second AMZ interferometer  132 , the monitor photodetector  151  is connected to an output port, which is provided on the other side of a signal light output port of wavelength λ 1 . Ideally, only the signal light of wavelength λ 1  is extracted from the second AMZ interferometer  132 . However, due to the influence of changes in the refractive index caused by manufacturing errors, environmental changes, etc., other wavelength components may be contained in the transmission spectrum having a peak at λ 1 . The monitor photodetector  151  is used to detect other wavelength components included in the transmission spectrum incident on the second AMZ interferometer  132 . This monitor photodetector  151  is referred to as “MPD. 2   a”.    
     Also in the second optical demultiplexing circuit  12 A, the ideal is that only the signal light of wavelength λ 1  is extracted from the second AMZ interferometer  142 . However, the transmission spectrum with a peak at λ 1  may contain other wavelength components due to the influence of changes in the refractive index caused by manufacturing errors, environmental changes, etc. The monitor photodetector  161  is used to detect other wavelength components included in the transmission spectrum incident on the second AMZ interferometer  142 . This monitor photodetector  161  is referred to as “MPD. 2   b”.    
     The output of the monitor photodetector  151  (i.e., “MPD. 2   a ”) and the output of the monitor photodetector  161  (i.e., “MPD. 2   b ”) are summed up, and the total is input as the second monitor value to the I/V converter  21 A of the control circuit  20 A. 
     The control circuit  20 A controls the wavelength characteristics of the AMZ interferometer  132  and the AMZ interferometer  142 , by means of the control direction determination unit  221 A and the phase control units  222 A and  223 A which are implemented as the functions of the processor  22 A. Specifically, the control signal  2   a  output from the phase control unit  222 A controls a phase shifter PS provided in the AMZ interferometer  132 . The control signal  2   a  includes a compensation for the initial deviation of the characteristic of the AMZ interferometer  132 . The control signal  2   b  output from the phase control unit  223 A controls a phase shifter PS provided in the AMZ interferometer  142 . The control signal  2   b  includes a compensation for the initial deviation of the characteristic of the AMZ interferometer  142 . The control circuit  20 A controls the phase of light in a direction that the second monitor value decreases by the control signals  2   a  and  2   b.    
     The control illustrated in  FIG. 3B  is a control for reducing the side lobe of the transmission spectrum having a peak at λ 1 . Because the sum of the optical powers monitored at the same positions in the first optical demultiplexing circuit  11 A and the second optical demultiplexing circuit  12 A having the same configuration is used, stable monitoring results can be acquired, regardless of the polarization state in the optical fiber  31 , thereby improving the control accuracy on the wavelength characteristics of the AMZ interferometers  132  and  142 . 
       FIG. 3C  illustrates control on the wavelength characteristics of the third AMZ interferometer  133  of the first optical demultiplexing circuit  11 A and the third AMZ interferometer  143  of the second optical demultiplexing circuit  12 A. The third AMZ interferometer  133  of the first optical demultiplexing circuit  11 A is referred to as “AMZ. 3   a ”, and the third AMZ interferometer  143  of the second optical demultiplexing circuit  12 A is referred to as “AMZ. 3   b”.    
     In the third AMZ interferometer  133 , the monitor photodetector  154  is connected to an output port provided on the other side of the signal light output port of wavelength λ 2 . Ideally, only the signal light of wavelength λ 2  is extracted from the third AMZ interferometer  133 . However, due to the influence of changes in the refractive index caused by manufacturing errors, environmental changes, etc., other wavelength components may be contained in the transmission spectrum having a peak at λ 2 . The monitor photodetector  154  is used to detect other wavelength components included in the transmission spectrum incident on the AMZ interferometer  133 . This monitor photodetector  153  is referred to as “MPD. 3   a”.    
     Also in the second optical demultiplexing circuit  12 A, the ideal is that only the signal light having a wavelength of λ 2  is extracted from the third AMZ interferometer  143 . The transmission spectrum with a peak at λ 2  may contain other wavelength components due to the influence of changes in the refractive index caused by manufacturing errors, environmental changes, etc. The monitor photodetector  164  is used to detect other wavelength components included in the transmission spectrum incident on the AMZ interferometer  143 . This monitor photodetector  164  is referred to as “MPD. 3   b”.    
     The output of the monitor photodetector  154  (i.e., “MPD. 3   a ”) and the output of the monitor photodetector  164  (i.e., “MPD. 3   b ”) are summed up, and the total is input as the third monitor value to the I/V converter  21 A of the control circuit  20 A. 
     The control circuit  20 A controls the wavelength characteristics of the AMZ interferometers  133  and  143 , by means of the control direction determination unit  221 A and the phase control units  222 A and  223 A which are implemented by the functions of the processor  22 A. Specifically, the control signal  3   a  output from the phase control unit  222 A controls a phase shifter PS provided in the AMZ interferometer  133 . The control signal  3   a  includes a compensation for the initial deviation of the characteristic of the AMZ interferometer  133 . The control signal  3   b  output from the phase control unit  223 A controls a phase shifter PS provided in the AMZ interferometer  143 . The control signal  3   b  includes a compensation for the initial deviation of the characteristic of the AMZ interferometer  143 . The control circuit  20 A controls the phase of light in a direction that the third monitor value decreases by the control signals  3   a  and  3   b.    
     The control illustrated in  FIG. 3C  is a control for reducing the side lobe of the transmission spectrum having a peak at λ 2 . Because the sum of the optical powers monitored at the same positions in the first optical demultiplexing circuit  11 A and the second optical demultiplexing circuit  12 A having the same configuration is used, stable monitoring results can be acquired, regardless of the polarization state in the optical fiber  31 . This configuration can improve the control accuracy on the wavelength characteristics of the AMZ interferometers  133  and  143 . 
     When the controls illustrated in  FIG. 3A  to  FIG. 3C  are performed at the same time, three I/V converters  21 A are provided corresponding to the first monitor value, the second monitor value, and the third monitor value. Three logical blocks inside the processor  22 A may individually determine the control direction based on the first monitor value, the second monitor value, and the third monitor value. 
       FIG. 4  illustrates examples of initial value information  231 A and  232 A and an example of operation value information  233 A, saved in the memory  23 A. Initial value information items [ 1   a ], [ 1   b ], [ 2   a ] [ 2   b ], [ 3   a ], and [ 3   b ] used for initial correction are recorded in the initial value memory area of the memory  23 A. The initial values [ 1   a ], [ 2   a ], and [ 3   a ] are for correcting the initial characteristics of the three AMZ interferometers  131 ,  132 , and  133  (AMZ. 1   a , AMZ. 2   a , and AMZ. 3   a ) of the first optical demultiplexing circuit  11 A, and are saved as initial value information  231 A. The initial values [ 1   b ], [ 2   b ] and [ 3   b ] are for correcting the initial characteristics of the first to third AMZ interferometers  141  to  143  (AMZ. 1   b  to AMZ. 3   b ) of the second optical demultiplexing circuit  12 A, and are saved as the initial value information  232 A. 
     Operating value information is recorded in the operating value memory area of the memory  23 A. The operating value information includes an operating value [V 1 ] for controlling the phase shifters PSs of the first AMZ interferometers  131  and  141 , an operating value [V 2 ] for controlling the phase shifters PSs of the second AMZ interferometers  132  and  142 , and an operating value [V 3 ] for controlling the phase shifters PSs of the third AMZ interferometers  133  and  143 . 
       FIG. 5A  to  FIG. 5C  are flowcharts of wavelength characteristic control using initial value information and operating value information. These control flows are executed by the processor  22 A of the control circuit  20 A. In the flowcharts of  FIG. 5A  to  FIG. 5C , tuning of the three AMZ interferometers constituting one unit circuit  13 A or  14 A is sequentially illustrated for convenience of illustration and explanation. However, in actual operations, tuning of the three AMZ interferometers composing one unit circuit  13 A or  14 A may be simultaneously performed in parallel. 
       FIG. 5A  illustrates a process of increasing the peak levels of the target wavelengths λ 1  and λ 2 . The processor  22 A acquires a first monitor value X 1  (S 11 ). The first monitor value X 1  represents the total of the monitoring result from the monitor photodetectors  152  and  153  (i.e., “MPD. 1   a ”) of the first optical demultiplexing circuit  11 A and the monitoring result from the monitor photodetectors  162  and  163  (i.e., “MPD. 1   b ”) of the second optical demultiplexing circuit  12 A. Then, the operating value [V 1 ] for the first AMZ interferometer  131  and  141  is increased by a predetermined step size Δ (S 12 ). 
     Using the incremented operating value [V 1 ]′ to which Δ has been added, and the initial value [ 1   a ], the phase shifter PS of the first AMZ interferometer  131  (“AMZ. 1   a ”) of the first optical demultiplexing circuit  11 A is controlled (S 13 ). The initial value [ 1   a ] is a value set in advance in the AMZ interferometer  131  in order to compensate for or cancel the characteristic deviation or variation among the AMZ interferometers due to manufacturing errors or the like. 
     Similarly, using the incremented operating value [V 1 ]′ to which Δ has been added, and the initial value [ 1   b ], the phase shifter PS of the first AMZ interferometer  141  (“AMZ. 1   b ”) of the second optical demultiplexing circuit  12 A is controlled (S 14 ). The initial value [ 1   b ] is a value set in advance in the AMZ interferometer  141  in order to compensate for or cancel the characteristic deviation or variation among the AMZ interferometers due to a manufacturing error or the like. Steps S 13  and S 14  may be performed simultaneously, or in any order. 
     Then, a first monitor value Y 1  is acquired after the phase adjustment (S 15 ). This first monitor value Y 1  represents the sum of the monitoring result from the monitor photodetectors  152  and  153  (“MPD. 1   a ”) of the first optical demultiplexing circuit  11 A, and the monitoring result from the monitor photodetectors  162  and  163  (“MPD. 1   b ”) of the second optical demultiplexing circuit  12 A, acquired after the phase adjustment. 
     It is determined whether the first monitor value Y 1  after the phase adjustment is greater than the previous first monitor value X 1  (S 16 ). If the currently acquired first monitor value Y 1  is greater than the previously acquired first monitor value X 1  (Yes in S 16 ), then the process proceeds to control on the second AMZ interferometer illustrated in  FIG. 5B . If the first monitor value Y 1  is not greater than the previous first monitor value X 1  (No in S 16 ), the control direction may not be correct. In this case, the operating value [V 1 ] for the first AMZ interferometers  131  and  141  is decreased by 2Δ (S 17 ). 
     Using the updated operating value [V 1 ]″ reduced by 2Δ and the initial value [ 1   a ], the phase of the first AMZ interferometer  131  (“AMZ. 1   a ”) of the first optical demultiplexing circuit  11 A is controlled (S 18 ). Similarly, using the updated operating value [V 1 ]″ reduced by 2Δ and the initial value [ 1   b ], the phase of the first AMZ interferometer  141  (“AMZ. 1   b ”) of the second optical demultiplexing circuit  12 A is controlled (S 19 ). Subsequently, the process proceeds to control on the second AMZ interferometer illustrated in  FIG. 5B . 
       FIG. 5B  illustrates a process continued from the node A of  FIG. 5A . The process in  FIG. 5B  is a control for reducing the side lobe of the target wavelength λ 1 . The processor  22 A acquires the second monitor value X 2  (S 21 ). The second monitor value X 2  represents the sum of the monitoring result of the monitor photodetector  151  (“MPD. 2   a ”) of the first optical demultiplexing circuit  11 A and the monitoring result of the monitor photodetector  161  (“MPD. 2   b ”) of the second optical demultiplexing circuit  12 A. Then, the operating value [V 2 ] for the second AMZ interferometers  132  and  142  is increased by a predetermined step size Δ (S 22 ). The step size Δ for changing (increasing or decreasing) the operating value [V 2 ] may be the same as or different from the step size Δ for increasing or decreasing the operating value [V 1 ]. 
     Using the updated operating value [V 2 ]′ to which Δ has been added, and the initial value [ 2   a ], the phase shifter PS of the second AMZ interferometer  132  (“AMZ. 2   a ”) of the first optical demultiplexing circuit  11 A is controlled (S 23 ). The initial value [ 2   a ] is a value set in advance in the AMZ interferometer  132  in order to compensate for or cancel the characteristic deviation or variation among the AMZ interferometers due to a manufacturing error or the like. 
     Similarly, the phase shifter PS of the second AMZ interferometer  142  (“AMZ. 2   b ”) of the second optical demultiplexing circuit  12 A is controlled using the initial value [ 2   b ] and the updated operating value [V 2 ]′ to which Δ has been added (S 24 ). The initial value [ 2   b ] is a value set in advance in the AMZ interferometer  142  in order to compensate for or cancel the characteristic deviation or variation among the AMZ interferometers due to a manufacturing error or the like. Steps S 23  and S 24  may be performed simultaneously, or in any order. 
     Then, a second monitor value Y 2  is acquired after the phase adjustment (S 25 ). This monitor value Y 2  represents the sum of the monitoring result of the monitor photodetector  151  (“MPD. 2   a ”) of the first optical demultiplexing circuit  11 A and the monitoring result of the monitor photodetector  161  (“MPD. 2   b ”) of the second optical demultiplexing circuit  12 A, acquired after the phase adjustment. 
     It is determined whether the second monitor value Y 2  after the phase adjustment is smaller than the previous second monitor value X 2  (S 26 ). If the current second monitor value Y 2  is smaller than the previous second monitor value X 2  (Yes in S 26 ), the process proceeds to the control on the third AMZ interferometer illustrated in  FIG. 5C . If the second monitor value Y 2  is not smaller than the previous second monitor value X 2  (No in S 26 ), the control direction may not be correct. In this case, the operating value [V 2 ]′ for the second AMZ interferometers  132  and  142  is decreased by 2Δ (S 27 ). 
     Using the updated operating value [V 2 ]″ reduced by 2Δ and the initial value [ 2   a ], the phase shifter PS of the second AMZ interferometer  132  (“AMZ. 2   a ”) of the first optical demultiplexing circuit  11 A is controlled (S 28 ). Similarly, using the operating value [V 2 ]″ reduced by 2Δ and the initial value [ 2   b ], the phase shifter PS of the second AMZ interferometer  142  (“AMZ. 2   b ”) of the second optical demultiplexing circuit  12 A is controlled (S 29 ). Subsequently, the process proceeds to control on the third AMZ interferometer illustrated in  FIG. 5C . 
       FIG. 5C  illustrates a process continued from the node B of  FIG. 5B .  FIG. 5C  is a control for reducing the side lobe of the target wavelength λ 2 . The processor  22 A acquires the third monitor value X 3  (S 31 ). The third monitor value X 3  represents a sum of the monitoring result of the monitor photodetector  154  (“MPD. 3   a ”) of the first optical demultiplexing circuit  11 A and the monitoring result of the monitor photodetector  164  (“MPD. 3   b ”) of the second optical demultiplexing circuit  12 A. 
     Then, the operating value [V 3 ] of the third AMZ interferometer  133  and  143  is increased by a predetermined step size Δ (S 32 ). The step size Δ for increasing or decreasing the operating value [V 3 ] may be the same as or different from the step size Δ for increasing or decreasing the operating value [V 2 ], and it may be the same as or different from the step size Δ for increasing or decreasing the operating value [V 1 ]. 
     Using the updated operating value [V 3 ]′ to which Δ has been added and the initial value [ 3   a ], the phase shifter PS of the third AMZ interferometer  133  (“AMZ. 3   a ”) of the first optical demultiplexing circuit  11 A is controlled (S 33 ). The initial value [ 3   a ] is a value set in advance in the AMZ interferometer  133  in order to compensate for or cancel the characteristic deviation or variation among the AMZ interferometers due to a manufacturing error or the like. 
     Similarly, using the updated operating value [V 3 ]′ to which Δ has been added and the initial value [ 3   b ], the phase shifter PS of the third AMZ interferometer  143  (“AMZ. 3   b ”) of the second optical demultiplexing circuit  12 A is controlled (S 34 ). The initial value [ 3   b ] is a value set in advance in the AMZ interferometer  143  in order to compensate or cancel the characteristic deviation or variation among the AMZ interferometers due to a manufacturing error or the like. Steps S 33  and S 34  may be performed simultaneously, or in any order. 
     Then a third monitor value Y 3 , which is a total of the monitoring results of “MPD. 3   a ” and “MPD. 3   b ” after the phase adjustment, is acquired (S 35 ), and it is determined whether Y 3  is smaller than the previously acquired third monitor value X 3  (S 36 ). If the third monitor value Y 3  is smaller than the previous third monitor value X 3  (Yes in S 36 ), the process returns to  FIG. 5A , and the processes of  FIG. 5A  to  FIG. 5C  are repeated. If the third monitor value Y 3  is not smaller than the previous third monitor value X 3  (No in S 36 ), the control direction may not be correct. In this case, the operating value [V 3 ]′ of the third AMZ interferometers  133  and  143  is reduced by 2Δ (S 37 ). 
     Using the updated operating value [V 3 ]″ reduced by 2Δ and the initial value [ 3   a ], the phase of the third AMZ interferometer  133  (“AMZ. 3   a ”) of the first optical demultiplexing circuit  11 A is controlled (S 38 ). Similarly, using the operating value [V 3 ]″ reduced by 2Δ and the initial value [ 3   b ], the phase of the third AMZ interferometer  143  (“AMZ. 3   b ”) of the second optical demultiplexing circuit  12 A is controlled (S 39 ). Subsequently, the process returns to  FIG. 5A , and the processes of  FIG. 5A  to  FIG. 5C  are repeated. 
     Even if the environment changes during the service, the wavelength characteristics of the first optical demultiplexing circuit  11 A and the second optical demultiplexing circuit  12 A of the photonic circuit  10 A can be controlled so as to follow the environmental change by repeating the control operations of  FIG. 5A  to  FIG. 5C . 
       FIG. 6  is a flowchart of setting the initial value. The control process illustrated in  FIG. 5A to 5C  is a control performed during the actual demultiplexing in service, in which the wavelength characteristics of the first AMZ interferometers  131  and  132 , the second AMZ interferometers  132  and  142 , and the third AMZ interferometers  133  and  143  are simultaneously or sequentially controlled using the preset initial values, while compensating for the characteristic deviation or variation among the AMZ interferometers. When the initial value is set in  FIG. 6 , a light beam containing approximately the same quantities of two orthogonal polarization components, such as random polarized waves or circularly polarized waves, is input as a test light. Phase adjustment is performed individually for the first optical demultiplexing circuit  11 A and the second optical demultiplexing circuit  12 A. Light of λ 1  and light of λ 2  are successively incident on each of the first optical demultiplexing circuit  11 A and the second optical demultiplexing circuit  12 A for the phase adjustment. Subsequently, the lights of λ 1  and λ 2  may be introduced at the same time to determine an ultimate initial value. 
     First, the initial value information and the operating value information are initialized (S 51 ). At this point of time, the phase control operation is suspended (S 52 ). Then, one of the optical demultiplexing circuits which deals with either one of the polarizations is adjusted (S 53 ). The polarized wave “a” is, for example, TE polarization contained in the incident light on the photonic circuit  10 A. The first optical demultiplexing circuit  11 A is to be adjusted first. During the adjustment of the first optical demultiplexing circuit  11 A, phase adjustment of the second optical demultiplexing circuit  12 A is not performed. 
     Only the light of λ 1  is introduced into the photonic circuit  10 A, and incidence of λ 2  light is stopped (S 54 ). The phase shifters PSs of all the AMZ interferometers located between the first input port for inputting the λ 1  light and the last output port for outputting the λ 1  light in the unit circuit  13 A are controlled, until the output light of λ 1  becomes the steady state (the loop of “No” in S 55 ). This example focuses on the TE polarization, namely, the first optical demultiplexing circuit  11 A. Accordingly, the first input port of the unit circuit  13 A to which λ 1  is input is the input side coupler of the first AMZ interferometer  131 , and the last output port from which λ 1  is output is a signal light output port of the second AMZ interferometer  132 . The phase shifters PSs of the first AMZ interferometer  131  and the second AMZ interferometer  132  are controlled. The stability of the output light of λ 1  can be determined by monitoring the output of the monitor photodetector  152  or the output of the λ 1  photodetector  17 . 
     When the output of λ 1  becomes a steady state (Yes in S 55 ), the incidence of the λ 1  light is stopped and the light of λ 2  is introduced (S 56 ). The phase shifters PSs of all the AMZ interferometers located between the first input port for inputting the λ 2  light and the last output port for outputting the λ 2  light are controlled, until the output light of λ 2  becomes a steady state (the loop of “No” in S 57 ). The first input port of the unit circuit  13 A to which λ 2  is input is the input side coupler of the first AMZ interferometer  131 . The last output port from which λ 2  is output is the signal light output port of the third AMZ interferometer  133 . Therefore, the phase shifters of the first AMZ interferometer  131  and the third AMZ interferometer  133  are controlled. The stability of the λ 2  light is determined by monitoring the output of the monitor photodetector  153  or the output of the λ 2  photodetector  18 . 
     When the output of λ 2  becomes a steady state (Yes in S 57 ), the light of λ 1  and the light of λ 2  are simultaneously input (S 58 ), and the phase shifters PSs of all the AMZ interferometers through which the λ 1  light and the λ 2  light travel are controlled, until the lights become in the steady states (the loop of “No” in S 59 ). When the output levels of the λ 1  light and the λ 2  light become steady states (Yes in S 59 ), the operating values [V 1 ], [V 2 ], and [V 3 ] at that time are written as the initial values [ 1   a ], [ 2   a ], and [ 3   a ] of the AMZ interferometers  131 ,  132 , and  133  (S 60 ). Then, the initial phase control on the polarization “a”, namely, the first optical demultiplexing circuit  11 A is terminated (S 61 ), and initial phase control on the polarization “b”, namely, the second demultiplexing circuit  12 A which deals with the TE polarization obtained by rotating the polarization plane of the TM polarization is started (S 62 ). 
     First, only the light of λ 1  is introduced into the photonic circuit  10 A, and incidence of light of λ 2  is suspended (S 63 ). The phase shifters PSs of all the AMZ interferometers located between the first input port where λ 1  is input and the last output port where λ 1  is output in the unit circuit  14 A are controlled, until the output light of λ 1  becomes a steady state (the loop of “No” in S 64 ). In this example, the phase shifters of the first AMZ interferometer  141  and the second AMZ interferometer  142  are controlled. 
     When the output of λ 1  becomes a steady state (Yes in S 64 ), the incidence of the λ 1  light is suspended, and λ 2  light is introduced (S 65 ). The phase shifters PSs of all the AMZ interferometers located between the first input port where λ 2  is input and the last output port where λ 2  is output in the unit circuit  14 A are controlled, until the output of the λ 2  light becomes a steady state (the loop of “No” in S 66 ). In this example, the phase shifters of the first AMZ interferometer  141  and the third AMZ interferometer  143  are controlled. 
     When the output of λ 2  becomes a steady state (Yes in S 66 ), the light of λ 1  and the light of λ 2  are simultaneously introduced (S 67 ), and the phase shifters of all the AMZ interferometers through which the λ 1  light and the λ 2  light travel in the unit circuit  14 A are controlled, until the output lights become in the steady states (the loop of “No” in S 68 ). When the outputs of the λ 1  light and the λ 2  light become steady states (S 68 ), the operating values [V 1 ], [V 2 ], and [V 3 ] at that time are written as the initial values [ 1   b ], [ 2   b ], and [ 3   b ] of the AMZ interferometers  141 ,  142 , and  143  (S 69 ). Then, the setting of the initial values is terminated. 
     By setting the initial values as described above, variation in the initial characteristics of the AMZ interferometers, which are fabricated as minute silicon photonics devices, can be compensated for or cancelled. 
     &lt;Modified Example of Totalizing Monitoring Results&gt; 
       FIG. 7  illustrate a configuration example of a wavelength demultiplexer  3  which optically sums up the powers of the monitored lights. The wavelength demultiplexer  3  has a photonic circuit  10 B which includes a first optical demultiplexing circuit  11 B, a second optical demultiplexing circuit  12 B, and a control circuit  20 B that controls the demultiplexing operations of the photonic circuit  10 B. 
     In the wavelength demultiplexer  2  illustrated in  FIG. 3A  to  FIG. 3C , the total of eight monitor photodetectors are used, four on the output side of the unit circuit  13 A, and four on the output side of the unit circuit  14 A. The total photocurrent is supplied to the control circuit  20 A. In  FIG. 7 , the light beams output from the same positions of the first optical demultiplexing circuit  11 B and the second optical demultiplexing circuit  12 B are guided to the same monitor photodetector, at which the output light beams are optically added and detected. 
     Separation of two wavelengths λ 1  and λ 2  is described as an example. In the photonic circuit  10 B, a two-dimensional grating coupler  35  is used as an interface with the optical fiber, in place of the edge coupler  32 . The input light is incident onto the circuit at an angle perpendicular or oblique to the surface of the circuit. The two-dimensional grating coupler  35  has a plurality of light scatterers arranged in, for example, a matrix at the intersection of the tapered waveguides  351  and  352  extending in the directions of the first light demultiplexing circuit  11 B and the second light demultiplexing circuit  12 B, respectively. The two-dimensional grating coupler  35  can extract two orthogonal polarizations as two TE polarizations, and accordingly, a polarization beam splitter or a polarization rotator is unnecessary. If edge connection is desirable depending on the configuration, the edge coupler  32  may be used, as illustrated in  FIG. 3A  to  FIG. 3C . 
     In the unit circuit  13 B, the waveguide WG 1   a , which includes a waveguide branched from the signal light output port of λ 1  of the second AMZ interferometer  132  and a waveguide branched from the signal light output port of λ 2  of the third AMZ interferometer  133 , is connected to a monitor photodetector MPD. 1 . In the unit circuit  14 B, the waveguide WG 1   b , which includes a waveguide branched from the signal light of λ 1  of the second AMZ interferometer  142  and a waveguide branched from the output signal light of λ 2  of the third AMZ interferometer  143 , is also connected to the monitor photodetector MPD. 1 . The optical power obtained from the monitor photodetector MPD. 1  is the total monitor power used for phase adjustment of the first AMZ interferometer  131  of the unit circuit  13 B and the first AMZ interferometer  141  of the unit circuit  14 B. 
     At the second AMZ interferometer  132  of the unit circuit  13 B, the other output port provided on the other side of the signal light output port of λ 1  leads to a waveguide WG 2   a , which is connected to the monitor photodetector receiver MPD. 2 . At the second AMZ interferometer  142  of the unit circuit  14 B, the other output port provided on the other side of the signal light output port of λ 1  leads to a waveguide WG 2   b , which is also connected to the monitor photodetector MPD. 2 . The optical power obtained from the MPD. 2  is the total optical power used for the phase adjustment of the second AMZ interferometers  132  and  142 . 
     At the third AMZ interferometer  133  of the unit circuit  13 B, the other output port provided on the other side of the signal light output port of λ 2  leads to a waveguide WG 3   a , which is connected to the monitor photodetector MPD. 3 . At the third AMZ interferometer  143  of the unit circuit  14 B, the other output port provided on the other side of the signal light output port of λ 2  leads to a waveguide WG 3   b , which is also connected to the monitor photodetector MPD. 3 . The optical power obtained from the monitor photodetector MPD. 3  is the total monitor power used for the phase adjustment of the third AMZ interferometers  133  and  143 . 
     The phase adjustment using the outputs from the monitor photodetectors MPD. 1 , MPD. 2 , and MPD. 3  is the same as that described with reference to  FIG. 5A to 5C . The signal light of λ 1  output from the AMZ interferometer  132  of the unit circuit  13 B and the signal light of λ 1  output from the AMZ interferometer  142  of the unit circuit  14 B are detected by the photodetector  17  for λ 1 , and the detected signal is processed in the subsequent stage. The signal light of λ 2  output from the AMZ interferometer  133  of the unit circuit  13 B and the signal light of λ 2  output from the AMZ interferometer  143  of the unit circuit  14 B are detected by the photodetector  18  for λ 2 , and the detected signal is processed in the subsequent stage. 
     The configuration of  FIG. 7  can achieve phase adjustment using only three monitor photodetectors in the photonic circuit  10 B used for separation of two wavelengths. Compared with the configurations of  FIG. 3A  to  FIG. 3C , the number of monitor photodetectors can be reduced to less than half. By configuring the first optical demultiplexing circuit  11 B and the second optical demultiplexing circuit  12 B in a mirror symmetry, the waveguides WG 1   a , WG 1   b , WG 2   a , WG 2   b , WG 3   a , and WG 3   b  can be connected to the associated monitor photodetectors without crossing one another. 
     &lt;Separation of 4 Wavelengths&gt; 
       FIG. 8  is a schematic diagram of a photonic circuit  10 C that separates four wavelengths. The photonic circuit  10 C includes a first optical demultiplexing circuit  11 C and a second optical demultiplexing circuit  12 C having the same configuration, but arranged mirror-symmetrically. 
     The first optical demultiplexing circuit  11 C has three unit circuits  13 C- 1 ,  13 C- 2 , and  13 C- 3  connected in a tree structure. The second optical demultiplexing circuit  12 C has three unit circuits  14 C- 1 ,  14 C- 2 , and  14 C- 3  connected in a tree structure. 
     Each of the unit circuits  13 C- 1 ,  13 C- 2 , and  13 C- 3  has the same structure as the unit circuits  13 A and  13 B, in which three AMZ interferometers are connected in a tree. Each of the unit circuits  14 C- 1 ,  14 C- 2 , and  14 C- 3  has the same structure as the unit circuits  14 A and  14 B, in which three AMZ interferometers are connected in a tree. 
     From the two-dimensional grating coupler  36  provided on the surface of the photonic circuit  10 C, a WDM signal which contains multiplexed signal lights of λ 1 , λ 2 , λ 3 , and λ 4  is input to the photonic circuit  10 C. By means of the two-dimensional grating coupler  36 , the TE-polarized wave and the TM-polarized wave orthogonal to each other are guided as two TE polarized waves to the first optical demultiplexing circuit  11 C and the second optical demultiplexing circuit  12 C. 
     The unit circuit  13 C- 1  transmits a transmission spectrum having peaks at λ 1  and λ 3  through to the unit circuit  13 C- 2 , and transmits a transmission spectrum having peaks at λ 2  and λ 4  through to the unit circuit  13 C- 3 . The signal light of λ 1  and the signal light of λ 3  are output from the two AMZ interferometers of the second-stage unit circuit  13 C- 2 , respectively. The signal light of λ 2  and the signal light of λ 4  are output from the two AMZ interferometers of the second-stage unit circuit  13 C- 3 , respectively. 
     The unit circuit  14 C- 1  transmits a transmission spectrum having peaks at λ 1  and λ 3  through to the unit circuit  14 C- 2 , and transmits a transmission spectrum having peaks at λ 2  and λ 4  through to the unit circuit  14 C- 3 . The signal light of λ 1  and the signal light of λ 3  are output from the two AMZ interferometers, respectively, provided in the second-stage unit circuit  14 C- 2 . The signal light of λ 2  and the signal light of λ 4  are output from the two AMZ interferometers, respectively, provided in the second-stage unit circuit  14 C- 3 . 
     The light of λ 1  separated by the unit circuit  13 C- 2  and the light of λ 1  separated by the unit circuit  14 C- 2  are detected by the photodetector  17  for λ 1 . The detected λ 1  signal undergoes signal processing in the subsequent stage. The light of λ 3  separated by the unit circuit  13 C- 2  and the light of λ 3  separated by the unit circuit  14 C- 2  are detected by the photodetector  27  for λ 3 . The detected λ 3  signal undergoes signal processing in the subsequent stage. 
     The light of λ 2  separated by the unit circuit  13 C- 3  and the light of λ 2  separated by the unit circuit  14 C- 3  are detected by the photodetector  18  for λ 2 , and the detected λ 2  signal undergoes signal processing in the subsequent stage. The light of λ 4  separated by the unit circuit  13 C- 3  and the light of λ 4  separated by the unit circuit  14 C- 3  are detected by the photodetector  28  for λ 4 , and the detected λ 4  signal undergoes signal processing in the subsequent stage. 
     The photonic circuit  10 C has nine monitor photodetectors MPDs, each of which is connected commonly to waveguides extending from the same positions of the first optical demultiplexing circuit  11 C and the second optical demultiplexing circuit  12 C. Monitor photodetector MPD.C 1  monitors a portion of the transmission spectrum having peaks of all the wavelengths and extracted from the unit circuit  13 C- 1 , and a portion of the transmission spectrum having peaks of all the wavelengths and extracted from the unit circuit  14 C- 1 . The monitoring result of the monitor photodetector MPD.C 1  is used to control the wavelength characteristic of the first AMZ interferometer of the unit circuit  13 C- 1 , and the wavelength characteristic of the first AMZ interferometer of the unit circuit  14 C- 1 . 
     Monitor photodetector MPD.C 2  monitors a portion of the transmission spectrum having peaks at λ 1  and λ 3  and extracted from the unit circuit  13 C- 2 , and a portion of the transmission spectrum having peaks at λ 1  and λ 3  and extracted from the unit circuit  14 C- 2 . The monitoring result of the monitor photodetector MPD.C 2  is used to control the wavelength characteristic of the first AMZ interferometer of the unit circuit  13 C- 2 , and the wavelength characteristic of the first AMZ interferometer of the unit circuit  14 C- 2 . 
     Monitor photodetector MPD.C 3  monitors a portion of the transmission spectrum having peaks at λ 2  and λ 4  and extracted from the unit circuit  13 C- 3 , and a portion of the transmission spectrum having peaks at λ 2  and λ 4  and extracted from the unit circuit  14 C- 3 . The monitoring result of the monitor photodetector MPD.C 3  is used to control the wavelength characteristic of the first AMZ interferometer of the unit circuit  13 C- 3 , and the wavelength characteristic of the first AMZ interferometer of the unit circuit  14 C- 3 . 
     In addition to the monitor photodetectors MPD.C 1 , MPD.C 2  and MPD.C 3 , monitor photodetectors MPD. 4  to MPD. 9  are provided. Each of the MPD. 4  to MPD. 9  is connected to two waveguides extending from the same positions of the first optical demultiplexing circuit  11 C and the second optical demultiplexing circuit  12 C, so as to monitor the total optical power. The way of performing the phase control on each of the AMZ interferometers using the total monitor power detected by the associated monitor photodetector is the same as that described with reference to  FIG. 5A  to  FIG. 5C . 
     If the configuration of  FIG. 3A  to  FIG. 3C  is employed to merge photocurrents for separation of 4 wavelengths, a total of 24 monitor photodetectors are required. By employing the configuration of  FIG. 8  in which the monitored light components are optically added up, the number of monitor photodetectors can be reduced to about one third (⅓). The configuration of  FIG. 8  can achieve separation of four wavelengths, while reducing the influence of polarization. 
     &lt;Application to Optical Transceiver Frontend Module&gt; 
       FIG. 9  is a schematic diagram of an optical transceiver front-end module  100  to which the wavelength demultiplexer of an embodiment is applied. The optical transceiver front-end module  100  is, for example, a 4-channel WDM transceiver front-end module. The optical transceiver front-end module  100  includes an optical transmitter front-end circuit TX and an optical receiver front-end circuit RX. The optical transmitter front-end circuit TX includes lasers  2011 ,  2012 ,  2013  and  2014 , modulators  2031 ,  2032 ,  2033  and  2034 , and driver circuits  2021 ,  2022 ,  2023  and  2024 . The lasers  2011  to  2014  output light beams of the respective wavelengths. The driver circuits  2021  to  2024  drive the associated modulators  2031  to  2034 . The optical transmitter front-end circuit TX also has a photonic circuit  250  for multiplexing light signals of multiple wavelengths. The four optical signals having different wavelengths and modulated by the modulators  2031 ,  2032 ,  2033 , and  2034  are multiplexed by the photonic circuit  250  and output to the optical transmission line  102  through a connector  104 . The optical transmission line  102  is, for example, a single mode fiber optic cable. 
     The optical receiver front-end circuit RX includes a wavelength demultiplexer  5  and an optical-to-electric converter (denoted as “O/E” in the  FIG. 150 . The wavelength demultiplexer  5  includes a photonic circuit  10 C for wavelength demultiplexing, and a control circuit  20 C for controlling the demultiplexing operation of the photonic circuit  10 C. The photonic circuit  10 C may be one illustrated in  FIG. 8  adapted to separation of four wavelengths. The control circuit  20 C controls the wavelength characteristics of the photonic circuit  10 C, based on the total monitor power of the lights extracted from the same positions in the first optical demultiplexing circuit  11 C and the second optical demultiplexing circuit  12 C (see  FIG. 8 ) of the photonic circuit  10 C. 
     The photo receivers  111 ,  112 ,  113 , and  114  used in the O/E converter  150  are, for example, photodiodes (PDs), which correspond to the photodetector  17  for λ 1 , the photodetector  18  for λ 2 , the photodetector  27  for λ 3 , and the photodetector  28  for λ 4  illustrated in  FIG. 8 . The photocurrents output from the respective photo receivers are converted into voltage signals by the corresponding transimpedance amplifiers (TIAs)  121 ,  122 ,  123 , and  124 , and output to a signal processor of the subsequent stage. 
     By using the wavelength demultiplexer  5  in the optical transceiver front-end module  100 , the influence of polarization is suppressed, and wavelength characteristic deviation due to manufacturing variation and environmental change is corrected. The optical signals of the respective wavelengths can be appropriately separated. 
     Although the specific configuration examples have been described above, the invention is not limited to the above-described examples. In all the configuration examples, a WDM signal may be introduced to the photonic circuit  10  (or  10 A to  10 C) using either the edge coupler  32  of a horizontal connection type or the two-dimensional grating coupler  35  of a vertical connection type. The phase shifter PS used in each AMZ interferometer may be provided to only one of the two waveguide arms of the AMZ interferometer. The control circuit  20  may be composed of a field programmable gate array or another logic device having a built-in memory. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of superiority or inferiority of the invention. Although the embodiments of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the scope of the invention.