Abstract:
An optical receiver including a waveguide substrate including a first waveguide that transmits a main signal beam, a second waveguide that transmits a monitoring beam that has branched from the main signal beam, and a third waveguide that transmits an amplification beam to amplify the main signal beam; a light receiving device array including, integrally formed to the same substrate, a first light receiving device that detects the main signal beam and a second light receiving device that detects the monitoring beam; and a case that houses the waveguide substrate and the light receiving device array. The first light receiving device faces toward an end of the first waveguide, and the second light receiving device faces toward an end of the second waveguide.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of International Application No. PCT/JP2012/055144, filed Feb. 29, 2012, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     FIELD 
     The embodiments discussed herein are related to an optical receiver. 
     BACKGROUND 
     There is a proposal for an optical receiver that has plural light receiving devices installed inside a sealed container (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2011-175133). 
     In optical transmission, there is recently demand, in optical receivers that split a received beam and receive the light with plural light receiving devices, for a function to detect signal interruptions, and to monitor the intensity of optical phase modulated signals input as a parameter in demodulation. There is accordingly a demand to provide inside the housing of the optical receiver both light receiving devices that measure the input main signal beam, and a light receiving device that measures a monitoring beam. It is desirable in such cases to achieve a configuration enabling good positional alignment to be attained for the light receiving device that measures the monitoring beam. 
     SUMMARY 
     According to an aspect of the embodiments, an optical receiver includes: a waveguide substrate including a first waveguide that transmits a main signal beam, a second waveguide that transmits a monitoring beam that has branched from the main signal beam, and a third waveguide that transmits an amplification beam to amplify the main signal beam; a light receiving device array including, integrally formed to the same substrate, a first light receiving device that detects the main signal beam and a second light receiving device that detects the monitoring beam; and a case that houses the waveguide substrate and the light receiving device array; wherein the first light receiving device faces toward an end of the first waveguide, and the second light receiving device faces toward an end of the second waveguide. 
     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 of the invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram to explain an example of a configuration of an optical circuit  200  of a DP-QPSK receiver section. 
         FIG. 2  is a diagram to explain structure and operation of the 90° hybrid  240  illustrated in  FIG. 1 . 
         FIG. 3  is a schematic diagram to explain an optical receiver  100  of a preferable exemplary embodiment of technology disclosed herein. 
         FIG. 4  is a diagram to explain a relationship in a DP-QPSK receptor between beam reception diameter of a light receiving device, and isolation between a monitoring beam light receiving device and a local oscillator beam. 
         FIG. 5  is a schematic perspective view to explain a light receiving device array  310 . 
         FIG. 6  is a schematic perspective view to explain an example of a coupling structure between a planar light-wave circuit  150  and light receiving device arrays  310 ,  320 . 
         FIG. 7  is a schematic perspective view to explain another example of a coupling structure between a planar light-wave circuit  150  and light receiving device arrays  310 ,  320 . 
         FIG. 8  is a schematic perspective view to explain yet another example of a coupling structure between a planar light-wave circuit  150  and light receiving device arrays  310 ,  320 . 
         FIG. 9  is a schematic perspective view to explain positional alignment of a planar light-wave circuit  150  and a light receiving device array  310 . 
         FIG. 10  is a schematic perspective view to explain positional alignment of a planar light-wave circuit  150  and a light receiving device array  310 . 
         FIG. 11  is a schematic view to explain a measurement system employed in positional alignment of a planar light-wave circuit  150  and a light receiving device array  310 . 
         FIG. 12  is a schematic perspective view to explain yet another example of a coupling structure between a planar light-wave circuit  150  and light receiving device arrays  310 ,  320 . 
         FIG. 13  is a schematic perspective view to explain yet another example of a coupling structure between a planar light-wave circuit  150  and light receiving device arrays  310 ,  320 . 
         FIG. 14  is a schematic perspective view to explain yet another example of a coupling structure between a planar light-wave circuit  150  and light receiving device arrays  310 ,  320 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Explanation next follows regarding preferable exemplary embodiments of the technology disclosed herein. 
     In optical transmission systems, attention has recently been drawn to methods of optical phase modulation as methods to implement large capacities with low deterioration during transmission. From out of these, in particular Dual Polarization Differential Quadrature Phase Shift Keying (DP-QPSK) is progressing with standardization through the Optical Internetworking Forum (OIF), and is drawing attention as a main method for the future. 
     An example of a configuration of an optical circuit  200  of a DP-QPSK receiver section is illustrated in  FIG. 1 . In the present method, a polarization-multiplexed optical phase modulation signal beam (Sig) is polarization-beam-split by a Polarization Beam Splitter (PBS)  210 . Optical signals for each of the polarized beam split polarization beams, and a local oscillator beam (LO) that is not modulated with substantially equal light frequency, are interfered using 90° hybrids  240 ,  250  to demodulate the phase signal to an intensity signal by differential reception. In the configuration of the 90° hybrids  240 ,  250  there is a need to accurately determine beam phase relationships, and a planar light-wave circuit (PLC) is suitable from the viewpoint of manufacturing ability. The optical circuit  200  of the present exemplary embodiment is configured as a Planar Light-wave Circuit (PLC). 
     The optical circuit  200  uses a mixer circuit for coherent reception, and amplifies the reception amplitude of the optical phase modulation signal beam (Sig) using the local oscillator beam (LO). Explanation follows regarding the optical circuit  200 , with reference to  FIG. 1  and  FIG. 2 . 
     The optical circuit  200  includes two waveguides  111  and  121  on the input side. The local oscillator beam (LO) is input to the waveguide  111 , and the optical phase modulation signal beam (Sig) is input to the waveguide  121 . 
     The optical phase modulation signal beam (Sig) input to the waveguide  121  is split by the Polarization Beam Splitter (PBS)  210  into an X polarization beam and a Y polarization beam. The light of the Y polarization beam is then transmitted by a waveguide  212 , and the light of the X polarization beam is transmitted by a waveguide  214 . The light of the Y polarization beam transmitted by the waveguide  212  is input to the 90° hybrid  250 . The polarization plane of the light of the X polarization beam transmitted by the waveguide  214  is rotated by a rotator  230  to give light of a Y polarization beam, and then input to the 90° hybrid  240 . 
     The local oscillator beam (LO) input to the waveguide  111  is, in this case, a Y polarization beam. The local oscillator beam (LO) input to the waveguide  111  is split into two by a 3 dB coupler  220 , and transmitted by waveguides  222 ,  224 . The light of the Y polarization beam transmitted by the waveguide  224  is input to the 90° hybrid  240 . The light of the Y polarization beam transmitted by the waveguide  222  is input to the 90° hybrid  250 . 
     The optical phase modulation signal beam (Sig) input to the 90° hybrid  240  is amplified by the local oscillator beam (LO) input to the 90° hybrid  240 , to give beams of different phases every 90°. Beams X-Ip, X-In, X-Qp, X-Qn are then respectively output from the output side waveguides  242 ,  243 ,  244 ,  245 . 
     The optical phase modulation signal beam (Sig) input to the 90° hybrid  250  is amplified by the local oscillator beam (LO) input to the 90° hybrid  250 , to give beams of different phases every 90°. Beams Y-Ip, Y-In, Y-Qp, Y-Qn are then respectively output from the output side waveguides  252 ,  253 ,  254 ,  255 . 
     Explanation next follows regarding structure and operation of the 90° hybrid  240 . Note that the structure and operation of the 90° hybrid  250  is the same as that of the structure and operation of the 90° hybrid  240 , and so explanation thereof is omitted. 
       FIG. 2  is a diagram to explain the structure and operation of the 90° hybrid  240  illustrated in  FIG. 1 . In the coherent reception mixer circuit the reception amplitude of the optical phase modulation signal beam (Sig) is amplified by the local oscillator beam (LO). 
     The optical phase modulation signal beam (Sig) from the waveguide  214  is split by a coupler  261 , and a beam transmitted by a waveguide  271  is input to a coupler  263 , and a beam transmitted by a waveguide  272  is input to a coupler  264 . The waveguide  271  functions as a delay line. 
     The local oscillator beam (LO) from the waveguide  224  is split by a coupler  262 , and a beam transmitted by a waveguide  273  is input to a coupler  263 , and a beam transmitted by the waveguide  274  is input to a coupler  264 . 
     The beam transmitted by the waveguide  271  and the beam transmitted by the waveguide  273  are mixed together in the coupler  263 , and an Ip beam and an In beam are respectively output to the waveguide  242  and waveguide  243 . The beam transmitted by the waveguide  272  and the beam transmitted by the waveguide  274  are mixed together in the coupler  264 , and output, as a Qp beam and a Qn beam, to the waveguide  244  and the waveguide  245 , respectively. 
     The amplitude of the optical phase modulation signal beam (Sig) is denoted A, the amplitude of the local oscillator beam (LO) is denoted B, the relative phase angle between the carrier wave of the optical phase modulation signal beam (Sig) and the carrier wave of the local oscillator beam (LO) is denoted φ, the phase angle in the waveguide  271  is denoted θ, and the phase rotation angle in the couplers  263 ,  264  is denoted ξ. The complex amplitude and beam intensities of the beams Ip, In, Qp, Qn respectively output by the waveguides  242 ,  243 ,  244 ,  245  are given by the following equations. 
     Complex Amplitude
 
 Ip:Ae   (i(φ+θ))   +Be   (i(ξ))   Equation (1-1)
 
 In:Ae   (i(φ+θ+ξ))   +Be   (i(0))   Equation (1-2)
 
 Qp:Ae   (i(φ))   +Be   (i(ξ)   Equation (1-3)
 
 Qn:Ae   (i(φ+ξ))   +Be   (i(0))   Equation (1-4)
 
     Light Intensity
 
 Ip:A   2   +B   2 +2 AB  cos(φ+θ−ξ)  Equation (2-1)
 
 In:A   2   +B   2 +2 AB  cos(φ+θ+ξ)  Equation (2-2)
 
 Qp:A   2   +B   2 +2 AB  cos(φ−ξ)  Equation (2-3)
 
 Qn:A   2   +B   2 +2 AB  cos(φ+ξ)  Equation (2-4)
 
     Taking the differential reception between the Ip and the In, and between the Qp and the Qn, gives I output and Q output according to the following equations.
 
 I=Ip−In= 4 AB  sin(φ+θ)sin(ξ)  Equation (3-1)
 
 Q=Qp−Qn= 4 AB  sin( 100 )sin(ξ)  Equation (3-2)
 
     In the Equations (3-1) and (3-2), due to setting ξ=90° and θ=90° in a 90° hybrid, the intensities of the I output and the Q output are given by the following equations.
 
 I=Ip−In= 4 AB  cos(φ)  Equation (4-1)
 
 Q=Qp−Qn= 4 AB  sin(φ)  Equation (4-2)
 
     As indicated by Equations (4-1) and (4-2), it is clear that the signal output can be increased by increasing the amplitude B of the local oscillator beam, enabling an improvement in the signal noise ratio to be achieved. 
     Explanation next follows regarding an optical receiver  100  of a preferable exemplary embodiment of the technology disclosed herein, with reference to  FIG. 3 . 
     The optical receiver  100  of the present exemplary embodiment includes a planar light-wave circuit (PLC)  150 , a light receiving device array  310 , and a light receiving device array  320 . The optical receiver  100  includes plural waveguides on the input side. In the present exemplary embodiment explanation is given of a case in which there are two waveguides  101 ,  102  present on the input side. The local oscillator beam (LO) is input to the waveguide  101 , and the optical phase modulation signal beam (Sig) is input to the waveguide  102 . 
     The local oscillator beam (LO) input to the waveguide  101  is split into two by a coupler  110 . The beam propagating through the waveguide  111  is input to the above optical circuit  200 , and the beam propagating through a waveguide  112  is employed as a monitoring beam. The optical phase modulation signal beam (Sig) input to the waveguide  102  is split into two by a coupler  120 . The beam propagating through the waveguide  121  is input to the optical circuit  200 , and the beam propagating through the waveguide  122  is employed as monitoring beam. 
     The beams X-Ip, X-In, X-Qp, X-Qn are respectively output from the waveguides  242 ,  243 ,  244 ,  245  on the output side of the optical circuit  200 . The beams Y-Ip, Y-In, Y-Qp, Y-Qn are respectively output from the output side waveguides  252 ,  253 ,  254 ,  255 . The monitoring beam of the local oscillator beam (LO) is output from the  112 , and the monitoring beam of the optical phase modulation signal beam (Sig) is output from the waveguide  122 . 
     The light receiving device array  310  includes light receiving devices  311  to  315 . The light receiving device  311  is the light receiving device employed for the monitoring beam of the local oscillator beam (LO). The light receiving devices  312  to  315  are light receiving devices for receiving the signal beams respectively emitted from the waveguides  242 ,  243 ,  244 ,  245 . The light receiving device  311  is provided facing toward the waveguide  112  that is exposed at an end face  151  of the planar light-wave circuit (PLC)  150 . The light receiving devices  312  to  315  are respectively provided facing toward the waveguides  242 ,  243 ,  244 ,  245  that are exposed at the end face  151  of the planar light-wave circuit (PLC)  150 . The light receiving devices  311  to  315  are integrally formed to the same substrate. The light receiving devices  311  to  315  are photodiodes. 
     The light receiving device array  320  includes light receiving devices  321  to  325 . The light receiving device  321  is the light receiving device employed for the monitoring beam of the optical phase modulation signal beam (Sig). The light receiving devices  322  to  325  are light receiving devices for receiving the signal beams respectively emitted from the waveguides  252 ,  253 ,  254 ,  255 . The light receiving device  321  is provided facing toward the waveguide  122  that is exposed at the end face  151  of the planar light-wave circuit (PLC)  150 . The light receiving devices  322  to  325  are respectively provided facing toward the waveguides  252 ,  253 ,  254 ,  255  that are exposed at the end face  151  of the planar light-wave circuit (PLC)  150 . The light receiving devices  321  to  325  are integrally formed to the same substrate. The light receiving devices  321  to  325  are photodiodes. 
     In the thus configured optical receiver  100 , as control of the reception system, there is demand for a function to monitor the intensity of an input optical phase modulation signal, to detect signal interruption and as a parameter in demodulation. In order to miniaturize the device, to improve fiber yield, and to improve packaging properties, preferably such a monitoring function is built into the optical receiver  100 . As stated above, monitoring of the optical input signal is performed by using the couplers  110 ,  120  and the like in the input waveguides  101 ,  102  to extract (branch) the beam. In such cases, the planar light-wave circuit (PLC)  150 , the light receiving devices  312  to  315 ,  322  to  325  employed for signal reception, and the light receiving devices  311 ,  321  employed for monitoring beam detection, are preferably housed in the same case. 
     From the perspective of high speed signal reception, the beam reception diameter of the light receiving devices  312  to  315 ,  322  to  325  employed for signal reception is extremely small. For example, a beam reception diameter of about 15 μm to 25 μm is required to receive 100 Gigabit DP-QPSK transmissions (25 to 30 Gigabits baud rate), requiring high precision adjustment for beam coupling with output of the planar light-wave circuit (PLC)  150 . 
     In the present exemplary embodiment, as indicated by Equations (4-1), (4-2), the signal output is increased by increasing the amplitude B of the local oscillator beam, reducing the relative thermal noise, enabling an improvement in signal noise ratio, and enabling an improvement in reception sensitivity. Thus a specification of the OIF anticipates cases in which the intensity of the local oscillator beam is a maximum of 34 dB greater than the intensity of the signal beam. However, making the intensity of the local oscillator beam as large as this leads to an unwanted increase in noise, due to such factors as stray light caused by the local oscillator beam. 
     In order to detect beam signal interruption, monitoring needs to detect beam intensity at levels further extracted from a smaller beam input than the minimum input intensity of the signal beam. If this setting value is denoted a dB, then when a monitoring light receiving device is housed in the same case, since stray light and the like caused by the local oscillator beam becomes a noise component of the light receiving device, isolation of 34 dB+α dB or greater needs to be secured between the local oscillator beam and the monitoring light receiving device. 
     One conceivable method to secure such a level of isolation is to make the monitoring beam reception diameter small. In such cases, although the required beam reception diameter depends on packaged state and the required setting value α, the setting value α is, for example, about 10 dB, and in consideration of inter-device variation, it is desirable to secure isolation of about 50 dB. 
       FIG. 4  is a graph illustrating a relationship in a DP-QPSK receiver between the beam reception diameter of the light receiving devices, and the isolation between the monitoring light receiving device and the local oscillator beam. For example, a beam reception diameter of 20 μm or less is required in order to secure isolation of 50 dB or greater, this being a size that is substantially the same as the beam reception diameter of the light receiving device employed for signal beam reception. 
     Returning to  FIG. 3 , in the present exemplary embodiment, the light receiving device  311  that is a monitoring light receiving device, and the light receiving devices  312  to  315  that are signal beam-receiving light receiving devices, are integrally formed on the same substrate. The light receiving devices  311  to  315  are provided so as to respectively face toward the waveguides  112 ,  242  to  245  exposed at the end face  151  of the planar light-wave circuit  150 . The relative positions of the waveguides  112 ,  242  to  245  are determined by the precision of process masks during manufacturing the waveguides  112 ,  242  to  245 . The relative positions of the light receiving devices  312  to  315  are also determined by the precision of process masks during manufacturing the light receiving devices  312  to  315 . Consequently, by determining the relative positions (beam coupling conditions) between the signal beam waveguides  242  to  245 , and the signal beam light receiving devices  312  to  315 , the positional relationship between the monitoring waveguide  112  and the monitoring light receiving device  311  is also adjusted at the same time, in an automatic determination. As a result, the need to adjust the monitoring waveguide  112  and the monitoring light receiving device  311  is eliminated, and coupling can be achieved with good precision between the monitoring waveguide  112  and the monitoring light receiving device  311 . Consequently, the present exemplary embodiment enables positional alignment between the monitoring waveguide  112  and the monitoring light receiving device  311  to be performed simply and with good precision. 
     Moreover, the light receiving device  321  that is a light receiving device employed for a monitoring beam and the light receiving devices  322  to  325  that are light receiving devices employed for receiving signal beams are integrally formed on the same substrate. The light receiving devices  321  to  325  are provided so as to respectively face toward the waveguides  122 ,  252  to  255  exposed at the end face  151  of the planar light-wave circuit  150 . The relative positions of the waveguides  122 ,  252  to  255  are determined by the precision of process masks during manufacturing the waveguides  122 ,  252  to  255 . The relative positions of the light receiving devices  321  to  325  are also determined by the precision of process masks during manufacturing the light receiving devices  321  to  325 . Consequently, by determining the relative positions (beam coupling conditions) between the signal beam waveguides  252  to  255 , and the signal beam light receiving devices  322  to  325 , the positional relationship between the monitoring waveguide  122  and the monitoring light receiving device  321  is also adjusted at the same time, in an automatic determination. As a result, the need to adjust the monitoring waveguide  122  and the monitoring light receiving device  321  is eliminated, and coupling can be achieved with good precision between the monitoring waveguide  122  and the monitoring light receiving device  321 . Consequently, the present exemplary embodiment enables positional alignment between the monitoring waveguide  122  and the monitoring light receiving device  321  to be performed simply and with good precision. 
     Note that if the monitoring light receiving device  311  and the signal beam reception light receiving devices  312  to  315  are not integrally formed on the same substrate, then a need arises to separately determine the positional alignment between the monitoring waveguide  112  and the monitoring light receiving device  311 . In such cases, a large packaging tolerance is permitted as long as the beam reception diameter of the monitoring light receiving device  311  is, for example, 300 μm or larger. This results in the ability to perform positional alignment between the monitoring waveguide  112  and the monitoring light receiving device  311  easily and at low cost. However, in the present exemplary embodiment, the beam reception diameter of the monitoring light receiving device  311  needs to be, for example, 30 μm or smaller, and preferably 20 μm or smaller. Such cases result in the need to adjust the positional alignment between the monitoring waveguide  112  and the monitoring light receiving device  311  with high precision, with an accompanying rise in the cost of the optical receiver  100 . 
       FIG. 5  is schematic perspective diagram to explain the light receiving device array  310 . The light receiving device array  320  is configured the same as the light receiving device array  310  and so explanation thereof is omitted. A first main face  331  of a semiconductor substrate  330 , such as InP, is selectively implanted with impurities to form the light receiving devices  311  to  315 , such as photodiodes. A light-blocking mask  340  is provided on the other main face  332  of the semiconductor substrate  330 , on the opposite side to that of the first main face  331 . The beams from the waveguides  112 ,  242  to  245  are incident from the light-blocking mask  340  side. The light-blocking mask  340  is, for example, formed by providing transparent windows  341  to  345  in a gold film. The transparent windows  341  to  345  are provided so as to respectively face toward the light receiving devices  311  to  315 . The size of each of the transparent windows  341  to  345  is about the same as the beam diameter that passes through to the light receiving devices  311  to  315  side, for example 30 μm or less. The characteristics of the light receiving devices  311  to  315  are advantageous in cases in which there is a side peak in peripheral sensitivity. 
     Note that a light receiving device array  310  that does not employ the light-blocking mask  340  may also be employed. In such cases, the beam reception diameter of the light receiving devices  311  to  315  is, for example, 30 μm or smaller. In such cases, the beams from the waveguides  112 ,  242  to  245  are made incident from the side of the light receiving devices  311  to  315 . 
       FIG. 6  is a schematic perspective view to explain an example of a coupling structure between the planar light-wave circuit (PLC)  150  and the light receiving device arrays  310 ,  320 . The planar light-wave circuit (PLC)  150  includes a silicon substrate  160 , and a SiO 2  layer  162  formed on the silicon substrate  160 . Portions with a higher refractive index than SiO 2  is provided within the SiO 2  layer  162  to configure waveguides. The light receiving device arrays  310 ,  320  are fixed to the end face  151  of the planar light-wave circuit  150 . A glass structural member  401  is employed on the planar light-wave circuit  150  during fixing. An end face  403  of the structural member  401  is in the same plane as the end face  151  of the planar light-wave circuit  150 , and the light receiving device arrays  310 ,  320  are attached and fixed to the end face  151  of the planar light-wave circuit  150 , and to the end face  403  of the structural member  401 . This structure has few members employed to couple together the planar light-wave circuit (PLC)  150  and the light receiving device arrays  310 ,  320 , and is advantageous cost-wise. 
     The light receiving device arrays  310 ,  320  employ photodiodes as light receiving devices. The light receiving device arrays  310 ,  320  accordingly have transimpedance amplifiers (TIA)  351 ,  352  respectively disposed immediately after the light receiving devices to convert current signals generated in the light receiving devices into voltage signals. The light receiving device arrays  310 ,  320  and the transimpedance amplifiers (TIA)  351 ,  352  are connected together, such as by gold wire, and it is accordingly important in high speed signal reception to suppress the parasitic capacity and inductance between the light receiving devices and the TIA by using such a placement. 
     In a DP-QPSK, due to treating four outputs as a single group for each polarization beam (see  FIG. 1  to  FIG. 3 ), it is effective to integrate together the respective four devices in order to suppress the variation in device characteristics between the light receiving devices and the transimpedance amplifiers corresponding to each of the outputs. Thus in the present exemplary embodiment, five individual light receiving devices are integrated together, the four individual light receiving devices corresponding to the four outputs for each polarization beam and the monitoring light receiving device, to make up the smallest unit. 
     Note that structural members  412 ,  422  are fixed to the bottom face on the end face  151  side of the planar light-wave circuit  150 . The transimpedance amplifiers  351 ,  352  are respectively fixed to the structural members  412 ,  422 . The structural members  412 ,  422  are employed as height adjustment members during fixing of the transimpedance amplifiers  351 ,  352 . 
     A structural member  420  is fixed to the bottom face of the planar light-wave circuit  150 . The structural member  420  is employed as a height adjusting member during fixing of the planar light-wave circuit  150 . Optical fibers  411 ,  412  are fixed to an end face  152 , on the opposite side of the end face  151  side of the planar light-wave circuit  150 , with fiber fixing members  413 ,  414  interposed therebetween. A glass structural member  402  is employed on the planar light-wave circuit  150 . An end face  404  of the structural member  402  is set in the same plane as the end face  152  of the planar light-wave circuit  150 , and is employed as a support member when attaching the fiber fixing members  413 ,  414 . 
       FIG. 7  is a schematic perspective view to explain another example of a coupling structure between the planar light-wave circuit (PLC)  150  and the light receiving device arrays  310 ,  320 . In the structure of  FIG. 6 , the light receiving device arrays  310 ,  320  are fixed to the end face  151  of the planar light-wave circuit  150 . In contrast thereto, in the structure of  FIG. 7 , the light receiving device arrays  310 ,  320  are attached to the top face of the planar light-wave circuit  150 , with the top faces of the light receiving device arrays  310 ,  320  and the top faces of the transimpedance amplifiers  351 ,  352  set in the same plane as each other. Other points regarding the structure of  FIG. 7  are the same as those of the structure of  FIG. 6 . Setting the top faces of the light receiving device arrays  310 ,  320  and the top faces of the transimpedance amplifiers  351 ,  352  in the same plane as each other facilitates connection between the light receiving device arrays  310 ,  320  and the transimpedance amplifiers (TIA)  351 ,  352 , such as by gold wire. Note that the output beam of the planar light-wave circuit  150  needs to face the top face of the planar light-wave circuit  150  in the vicinity of the end face  151  in order to attach the light receiving device arrays  310 ,  320  to the top face of the planar light-wave circuit  150 . Thus, for example, fabrication, such as providing an up-throwing mirror, is performed in the vicinity of the end face  151  of the planar light-wave circuit  150 . Structural members  423 ,  424  are fixed to the end face  151  of the planar light-wave circuit  150 , the transimpedance amplifiers  351 ,  352  are respectively attached on the structural members  423 ,  424 , and height adjustment is performed during fixing of the transimpedance amplifiers  351 ,  352 . 
       FIG. 8  is a schematic perspective view to explain yet another example of a coupling structure between the planar light-wave circuit  150  and the light receiving device array  310  ( 320 ). The positional alignment between the light receiving devices and the planar waveguides needs to be at a precision of several microns or higher. In order to achieve positional alignment precision of this order, in the coupling structure in  FIG. 6  and  FIG. 7 , normally electrical connection is made to the light receiving devices, such as by probe, the photoelectric current monitored, and the light receiving device arrays  310 ,  320  fixed where the photoelectric current is greatest, however the procedures and equipment involved are complicated. In the coupling structure of the present example, it is possible to implement positional alignment between the light receiving devices and the planar light-wave circuit without electrical connection to the light receiving devices. 
     A planar light-wave circuit  150  includes a silicon substrate  160 , and a SiO 2  layer  162  serving as a cladding layer formed on the silicon substrate  160 . Waveguide cores  163  are provided in the SiO 2  layer  162  with a refractive index higher than that of SiO 2 . The light receiving device array  310  is fixed to an end face  151  of the planar light-wave circuit  150 . A glass structural member  405  is employed on the planar light-wave circuit  150 . An end face  406  of the structural member  405  is set in the same plane as the end face  151  of the planar light-wave circuit  150 , and the light receiving device array  310  is attached and fixed to the end face  151  of the planar light-wave circuit  150  and the end face  406  of the structural member  405 . 
     In the light receiving device array  310 , light receiving devices  311  to  315 , such as photodiodes, are formed by selectively implanting impurities into a first main face  331  of a semiconductor substrate  330 , such as InP. A reflective light blocking mask  350  is then provided to the other main face  332  of the semiconductor substrate  330 , on the opposite side to the first main face  331 . Apertures  351  to  355 , serving as transparent windows, are provided in the reflective light blocking mask  350  so as to align with the waveguide cores  163 . The size of the apertures  351  to  355  is substantially the same as the cross-sectional area of the waveguide core  163 . The apertures  351  to  355  are respectively provided so as to face the light receiving devices  311  to  315 . 
     The positional alignment precision between the light receiving devices  311  to  315  side and the reflective light blocking mask  350  side is of the order of a few microns, according to the precision of the processing device employed to produce the light receiving devices  311  to  315 . In such a case, the intensity of the reflected beam is low when the positions of the apertures  351  to  355  are aligned with respect to the waveguide core  163 , as illustrated in  FIG. 9 , and the intensity of the reflected beam is high when the positions of the apertures  351  to  355  are not aligned with respect to the waveguide core  163 , as illustrated in  FIG. 10 . Thus the relationship between the intensity of the reflected beam and the coupling state enables positional alignment to be implemented between the light receiving devices  311  to  315  and the waveguide cores  163  without electrical connection to the light receiving devices  311  to  315 . Note that the positional alignment is performed in the X axis, the Y axis and by rotation (θ). 
       FIG. 11  is a schematic diagram illustrating a measurement system employed in such positional alignment. A beam from a light source  510  is made incident to the planar light-wave circuit  150  through an optical fiber  516 , an optical circulator  514 , and an optical fiber  518 . The beam is then reflected by the reflective light blocking mask  350  provided to the light receiving device array  310 , and made incident to an optical power meter  512  through the optical circulator  514  and an optical fiber  520 . The intensity of the reflected light is measured by the optical power meter  512 . 
       FIG. 12  is a schematic perspective view to explain yet another coupling structure between a planar light-wave circuit  150  and light receiving device arrays  310 ,  320 . In the structure of  FIG. 6 , the light receiving device arrays  310 ,  320  are attached to the end face  151  of the planar light-wave circuit  150 , and in the structure of  FIG. 7  the light receiving device arrays  310 ,  320  are attached to the top face of the planar light-wave circuit  150 . In contrast thereto, in the structure of  FIG. 12 , the planar light-wave circuit  150  and the light receiving device arrays  310 ,  320  are coupled together by lenses  431  to  434 . 
     The light emitted from the end face  151  of the planar light-wave circuit  150  is made incident to a bending optical system  441  through the lenses  431  to  433 , the beams are bent toward the top face by the bending optical system  441 , and made incident to the back face side of the light receiving device array  310 . The light emitted from the end face  151  of the planar light-wave circuit  150  is made incident to a bending optical system  442  through the lenses  432 ,  434 , the beams are bent toward the top face by the bending optical system  442 , and made incident to the back face side of the light receiving device array  320 . 
     The transimpedance amplifiers  351 ,  352  are respectively attached on structural members  425 ,  426 , and height adjustment is performed during fixing of the transimpedance amplifiers  351 ,  352 . Connection is thereby facilitated between the light receiving device arrays  310 ,  320  and the transimpedance amplifiers  351 ,  352 , such as by gold wire. 
     In lens coupling, in contrast to the configurations of  FIG. 6  and  FIG. 7 , it is possible to adjust coupling while monitoring the light reception intensity, and lens coupling is applicable in cases that require even smaller beam reception diameters. In this example, the bending optical systems  441 ,  442  are employed in front of the light receiving device arrays  310 ,  320  to apply lens coupling, however the bending optical systems  441 ,  442  are redundant in cases in which beams are received by the light receiving device arrays  310 ,  320  from an end face (chip side face). 
       FIG. 13  is a schematic perspective diagram to explain yet another example of a coupling structure between the planar light-wave circuit  150  and the light receiving device arrays  310 ,  320 . In the structure of  FIG. 12 , the planar light-wave circuit  150  and the light receiving device arrays  310 ,  320  are coupled together by the lenses  431  to  434 . In contrast thereto, in the structure of  FIG. 13 , the planar light-wave circuit  150  and the light receiving device arrays  310 ,  320  are coupled together by microlens arrays  451  to  454 . 
     The microlens arrays  451 ,  452  are fixed to the end face  151  of the planar light-wave circuit  150 . A glass structural member  401  is employed on the planar light-wave circuit  150 . An end face  403  of the structural member  401  is set in the same plane as the end face  151  of the planar light-wave circuit  150 , and the microlens arrays  451 ,  452  are attached and fixed to the end face  151  of the planar light-wave circuit  150  and the end face  403  of the structural member  401 . 
     The microlens array  453  is attached to the side face of the bending optical system  441 , and the light receiving device array  310  is attached to the top face of the bending optical system  441 . The microlens array  454  is attached to the side face of the bending optical system  442 , and the light receiving device array  320  is attached to the top face of the bending optical system  442 . The light emitted from the end face  151  of the planar light-wave circuit  150  is made incident to the bending optical system  441  through the microlens arrays  451 ,  453 , the beams are bent toward the top face by the bending optical system  441 , and made incident to the light receiving device array  310  from the back face side. The light emitted from the end face  151  of the planar light-wave circuit  150  is made incident to the bending optical system  442  through the microlens arrays  452 ,  454 , the beams are bent toward the top face by the bending optical system  442 , and made incident to the light receiving device array  320  from the back face side. 
     The transimpedance amplifiers  351 ,  352  are respectively attached to structural members  425 ,  426 , and height adjustment is performed during fixing of the transimpedance amplifiers  351 ,  352 . Connection is thereby facilitated between the light receiving device arrays  310 ,  320  and the transimpedance amplifiers  351 ,  352 , such as by gold wire. 
     In the present example, explanation has been given of a two lens optical system with high flexibility for adjustment, however implementation may be using a single lens optical system. 
       FIG. 14  is a schematic perspective view to yet explain another example of a coupling structure between the planar light-wave circuit  150  and the light receiving device arrays  310 ,  320 . In the sealed structure of  FIG. 14 , the planar light-wave circuit  150 , the light receiving device arrays  310 ,  320 , and the transimpedance amplifiers  351 ,  352  are all housed in the same case  600 . A lid (top)  610  is attached to the case  600  to give a sealed structure. The present structure enables a more compact device to be achieved overall, without having separate individual cases for each of the elements. Note that the structure of  FIG. 14  is the structure of  FIG. 6 , housed in the case  600 , however any of the structures from  FIG. 7  to  FIG. 13  may also be employed as the structure housed in the case  600 . 
     The above optical waveguides may be configured by any of a quartz-based, silicon-based, InP-based, LiNO 3 -based, resin-based optical waveguide, or the like, and is not limited by waveguide configuration material. Although a single core fiber connection input is employed for the optical phase modulation signal beam (Sig) and the local oscillator beam (LO), micro-optics using a multi-core fiber array, lenses etc. may be employed, and the coupling method is not limited. 
     The above optical receiver  100  is preferably employed in an optical transmission device provided with the optical receiver  100 . 
     As explained above, the technology disclosed herein enables necessary monitoring operations to be implemented without increasing the effort to package the monitoring light receiving devices. 
     All publication, patent applications and technical standards mentioned in the present specification are incorporated by reference in the present specification to the same extent as if the individual publication, patent application, or technical standard was specifically and individually indicated to be incorporated by reference. 
     Various typical exemplary embodiments have been illustrated and explained above, however the present invention is not limited by these exemplary embodiments. The scope of the present invention is only limited by the scope of the following claims. 
     All examples and conditional language provided herein are intended for the pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.