Abstract:
A planar lightwave circuit includes a substrate in which a groove being formed, the groove dividing the substrate into a first area and a second area; a first filter, a second filter, and a third filter intruded into the groove; as are formed in the first area, a first and a second waveguides formed to guide signal light and local oscillation light; a third and a fourth waveguides formed to guide signal light and local oscillation light; a first 90-degree optical hybrid; as are formed in the second area, a fifth and sixth waveguides formed to guide signal light and local oscillation light; and a second 90-degree optical hybrid.

Description:
TECHNICAL FIELD 
     The present invention relates to a planar lightwave circuit and an optical receiver which receive polarization-multiplexed signal light. 
     BACKGROUND ART 
     With the recent explosive growth in network traffic, ultra-high-speed optical transmission systems of 40 Gbit/s and more than 100 Gbit/s have been investigated. With respect to the ultra-high-speed optical transmission systems, active investigation has been performed on digital coherent communication obtained by combining a phase modulation method with coherent detection and digital signal processing technologies. The phase modulation method has better characteristics required for the long haul optical fiber transmission such as characteristics of the tolerance for signal light noise, chromatic dispersion, and polarization mode dispersion. 
     As a modulation method, Binary Phase Shift Keying (BPSK) and Quadrature Phase Shift Keying (QPSK) have attracted attention because of their excellent tolerances for the dispersion compensation. 
     Moreover, in order to expand the transmission capacity without the increase in the frequency bandwidth, the research and development have been actively performed toward the practical use on a Dual-Polarization Quadrature Phase Shift Keying (DP-QPSK) method and the like. The Dual-Polarization Quadrature Phase Shift Keying (DP-QPSK) method is a method of multiplexing QPSK signals, which are superior in the frequency utilization efficiency, by two orthogonal polarizations. 
     An optical receiver for the digital coherent communication will be described below. It will be described here using the QPSK method as an example. With reference to  FIG. 5 , a process of receiving in the digital coherent communication will be described. 
     First, an optical receiver  30000  receives signal light in which a TE wave and a TM wave are multiplexed (hereafter, referred to as “TE-wave/TM-wave multiplexed signal light”). A local oscillation light source  32000  outputs local oscillation light in which a TE wave and a TM wave are multiplexed (hereafter, referred to as “TE-wave/TM-wave multiplexed local oscillation light”). The optical reception unit  31000  receives the TE-wave/TM-wave multiplexed signal light and the TE-wave/TM-wave multiplexed local oscillation light, splits each of them depending on the polarization, and makes the separated signal light and local oscillation light interference. The optical receiver  31000  outputs four signal light components in total which are composed of the real components and the imaginary components of each of the two signal light components, each of which has the polarization state parallel to each of two orthogonal polarization axes. The four signal light components are converted into analog electrical signals by an optical detector  33000 , and then converted into digital electrical signals by an analog-to-digital converter  34000 . These digital electrical signals are transformed by a re-sampling unit (not shown in the figure) into digital electrical signals which are sampled at the symbol rate (also referred to as a baud rate) of the signal light, and then inputted into a digital signal processing unit  35000 . The digital signal processing unit  35000  has functions of compensating the chromatic dispersion, the polarization dispersion, and the phase noise and frequency deviation. For example, in compensating the optical carrier frequency deviation and optical phase deviation, the compensation is performed on a frequency deviation between the frequency of the received signal light and the frequency of the local oscillation light, and on an optical phase rotation due to an optical phase deviation, respectively. After that, each of the electrical signals is demodulated by a symbol decision unit  36000  into a bit sequence which an optical transmitter has transmitted. 
     In this way, the digital coherent detection in the ultra-high-speed optical communication system can be realized. 
     Hereinafter, the above-mentioned optical reception unit  31000  will be described in more detail. With regard to the optical reception unit, a study on the standardization has been conducted by the OIF (Optical Internetworking Forum), which is an industry organization to promote high-speed data communications, and optical reception units following the standard have been developed. There are various kinds of means for realizing the optical reception unit. 
     For example, Non Patent Literature 1 describes an example of a polarization demultiplexing unit in an optical reception unit realized by using a micro-optics technology. However, if the micro-optics technology is used in that way, it is difficult to adjust positional relationships between a plurality of bulk elements. Specifically, it is necessary to align the optical axes of the plurality of bulk elements, for example. 
     It is considered, therefore, that a silica-based planar optical integrated circuit (hereafter, referred to as a “planar lightwave circuit”) has promise as a means not requiring adjustments for positional relationships. Patent Literature 1 discloses an example of an optical reception unit realized by using the planar lightwave circuit. Patent Literature 1 discloses a configuration in which a groove is formed at a part of the planar lightwave circuit, and a photonic crystal chip is inserted to intersect a waveguide in order to make the photonic crystal chip function. 
     CITATION LIST 
     Non Patent Literature 
     [NPL 1] 
     
         
         “Fully-Integrated Polarization-Diversity Coherent Receiver Module for 100G DP-QPSK,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2011), paper OML5 
       
    
     Patent Literature 
     [PTL 1] 
     
         
         Japanese Patent Application Laid-Open Publication No. 2011-76049 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     In the configuration of Patent Literature 1, however, waveguides for signal light and waveguides for local oscillation light are intersected at a plurality of points, as disclosed in FIG. 1 of Patent Literature 1. When waveguides are intersected as just described in the planar lightwave circuit, there arises a crosstalk due to leakage light or stray light from one waveguide to the other waveguide at the intersections or in their neighborhood. Because the crosstalk has a significant influence on signal quality in the coherent reception, there has been a problem that signal quality deteriorates due to a plurality of waveguides intersecting with each other at a plurality of points as mentioned above. 
     The planar lightwave circuit described in Patent Literature 1 has a problem that the polarization extinction ratio of the light inputted into a 90-degree optical hybrid is not sufficient. 
     In addition, it is necessary in the above-mentioned planar lightwave circuit to equalize the length of respective optical paths for the signal light and the local oscillation light in the path from the point where the light is split depending on the polarization to the point where the light is inputted into the 90-degree optical hybrid in order to prevent a phase difference between signal light beams and between local oscillation light beams after demultiplexing the polarization. 
     The present invention has been made in view of the above-mentioned problems, and the objective is to provide a planar lightwave circuit and an optical receiver in which, in a planar lightwave circuit including a 90-degree optical hybrid, the points at which waveguides intersect are decreased, the polarization extinction ratio of the light inputted into the 90-degree optical hybrid is improved, and it is easy to equalize the length of respective optical paths for the signal light and the local oscillation light in the path from the point where the light is split depending on the polarization to the point where the light is inputted into the 90-degree optical hybrid. 
     Solution to Problem 
     A planar lightwave circuit according to an exemplary aspect of the present invention includes a substrate in which a groove being formed, the groove dividing the substrate into a first area and a second area; a first filter intruded into the groove and performing polarization demultiplexing by transmission and reflection; a second filter included in the first area and performing polarization demultiplexing by transmission and reflection; a third filter included in the second area and performing polarization demultiplexing by transmission and reflection; as are formed in the first area, a first and a second waveguides formed to guide polarization-multiplexed signal light and polarization-multiplexed local oscillation light to the first filter, respectively, a third and a fourth waveguides formed to guide signal light and local oscillation light reflected respectively by the first filter to the second filter, and a first 90-degree optical hybrid formed to make interfere signal light and local oscillation light reflected respectively by the second filter; as are formed in the second area, a fifth and sixth waveguides formed to guide signal light and local oscillation light transmitted through the first filter to the third filter, respectively, and a second 90-degree optical hybrid formed to make interfere signal light and local oscillation light transmitted respectively through the third filter. 
     An optical receiver according to an exemplary aspect of the present invention includes a planar lightwave circuit demultiplexing each of polarization-multiplexed signal light and polarization-multiplexed local oscillation light depending on polarization, and making the signal light and the local oscillation light interfere with respect to each polarization; a photoelectric conversion unit converting interfering light output from the planar lightwave circuit into an electrical signal; an analog-to-digital conversion unit converting the electrical signal into a digital signal; and a digital signal processing unit processing the digital signal, wherein the planar lightwave circuit includes a substrate in which a groove being formed, the groove dividing the substrate into a first area and a second area; a first filter intruded into the groove and performing polarization demultiplexing by transmission and reflection; a second filter included in the first area and performing polarization demultiplexing by transmission and reflection; a third filter included in the second area and performing polarization demultiplexing by transmission and reflection; as are formed in the first area, a first and a second waveguides formed to guide polarization-multiplexed signal light and polarization-multiplexed local oscillation light to the first filter, respectively, a third and a fourth waveguides formed to guide signal light and local oscillation light reflected respectively by the first filter to the second filter, and a first 90-degree optical hybrid formed to make interfere signal light and local oscillation light reflected respectively by the second filter; as are formed in the second area, a fifth and sixth waveguides formed to guide signal light and local oscillation light transmitted through the first filter to the third filter, respectively, and a second 90-degree optical hybrid formed to make interfere signal light and local oscillation light transmitted respectively through the third filter. 
     Advantageous Effects of Invention 
     According to the present invention, it becomes possible to provide a planar lightwave circuit and an optical receiver in which, in a planar lightwave circuit including a 90-degree optical hybrid, the points at which waveguides intersect are decreased, the polarization extinction ratio of the light inputted into the 90-degree optical hybrid is improved, and it is easy to equalize the length of respective optical paths for the signal light and the local oscillation light in the path from the point where the light is split depending on the polarization to the point where the light is inputted into the 90-degree optical hybrid. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a functional block diagram of an optical transmission system  10000  in accordance with the exemplary embodiment of the present invention. 
         FIG. 2  is a functional block diagram of an optical reception unit  31000  in accordance with the exemplary embodiment of the present invention. 
         FIG. 3  is an example of a configuration of an optical reception unit  50000  in accordance with the exemplary embodiment of the present invention. 
         FIG. 4  is an example of a transmission spectrum with a dielectric multi-layer filter used. 
         FIG. 5  is a functional block diagram of an optical receiver  30000  in accordance with the exemplary embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The exemplary embodiment of the present invention will be described with reference to drawings below. 
     First, an optical transmission system  10000  will be described using  FIG. 1 . The optical transmission system  10000  includes an optical transmitter  20000  transmitting signal light modulated by polarization-multiplexed M-level phase modulation (M representing an integer equal to or larger than two), a transmission line  40000  propagating the signal light transmitted by the optical transmitter  20000 , and an optical receiver  30000  receiving the signal light through the transmission line  40000 . 
     A single mode optical fiber can be used as the transmission line  40000 , for example. 
     The optical receiver  30000  demodulates the signal light and outputs demodulated bit sequence to the outside. An example of a configuration of the optical receiver  30000  will be described using  FIG. 5 . The demodulation using the digital signal processing will be described as an example. 
     The optical receiver  30000  includes an optical reception unit  31000 , a local oscillation light source  32000 , a photoelectric conversion unit  33000 , an analog-to-digital converter  34000 , a digital signal processing unit  35000 , and a demodulation unit  36000 . 
     The optical reception unit  31000  demultiplexes inputted polarization-multiplexed signal light and inputted polarization-multiplexed local oscillation light respectively depending on the polarization (hereafter, referred to as “polarization demultiplexing”), and makes the signal light and the local oscillation light interfere with respect to each polarization. And then the interfering light made to interfere is output to the photoelectric conversion unit  33000 . 
     The linear polarizations of TE and TM are available for the polarization to be multiplexed together. The polarization multiplexing makes it possible to double substantially a bit rate at which to be transmitted by one wavelength. 
     The local oscillation light source  32000  outputs the light with the frequency comparable to that of the signal light transmitted by the optical transmitter  20000  as the local oscillation light. If the signal light transmitted by the optical transmitter  20000  is wavelength-multiplexed, the local oscillation light source  32000  outputs the light with the frequency comparable to that of one of a plurality of wavelengths as the local oscillation light. 
     The photoelectric conversion unit  33000  converts the interfering light into electrical signals and outputs the converted electrical signals to the analog-to-digital converter  34000 . 
     The analog-to-digital converter  34000  converts the electrical signals after conversion of analog signals into digital signals. And then the digital signals are output to the digital signal processing unit  35000 . 
     The digital signal processing unit  35000  extracts the information about the phase and intensity of the signal light from the digital signals. The digital signal processing unit  35000  has functions of compensating the chromatic dispersion, the polarization dispersion, and the phase noise and frequency deviation, for example. 
     The demodulation unit  36000  demodulates a bit sequence transmitted by the transmitter  20000  on the basis of the information about the phase and intensity of the signal light extracted by the digital signal processing unit  35000 . 
     Next, using  FIG. 2 , the optical reception unit  31000  will be described in more detail which is realized using a planar lightwave circuit. 
     The reception unit  31000  is composed of various components formed on a silica-based substrate  31100 . 
     In the substrate  31100 , a groove  31110  is formed which divides the surface area into an area  31120  and an area  31130 . The groove may be of any width and depth as long as a thin film filter can be intruded into and fixed in the groove. The groove  31110  described above can be easily formed by a dicing process, for example. In this case, the shape of the groove in a longitudinal direction is a linear shape. 
     There are, formed on the substrate  31100 , a signal light input unit  31131 , a local oscillation light input unit  31132 , waveguides  31133 ,  31134 ,  31135 ,  31136 ,  31121 , and  31122 , 90-degree optical hybrids  31123 ,  31137 , and output units  31138 ,  31139 ,  31124 , and  31125 . 
     The optical reception unit  31000  further includes a filter  31111  which is intruded into the groove  31110  and performs the polarization demultiplexing by transmission and reflection. The performing the polarization demultiplexing by transmission and reflection means demultiplexing the polarization by transmitting a TE wave and reflecting a TM wave, for example. The filter having such properties has already been put into practical use as a thin film filter, for example. If the thin film filter is used, it is called a TE-transmissive/TM-reflective type thin film filter. The present exemplary embodiment will be described in which the filter  31111  is such a TE-transmissive/TM-reflective type thin film filter, as an example. 
     The signal light input unit  31131  receives the polarization-multiplexed signal light transmitted from the optical transmitter  20000  through the transmission line  40000 . 
     The local oscillation light input unit  31132  receives the polarization-multiplexed local oscillation light. It is assumed that the local oscillation light source  32000  outputs local oscillation light with single polarization, for example. In this case, the local oscillation light is inputted into the local oscillation light input unit  31132  so that the polarization plane of the local oscillation light may have a predetermined angle to the polarization plane defined by the filter  31111 . This makes the polarization-multiplexed local oscillation light inputted from the local oscillation light input unit  31132  with reference to the polarization plane defined by the filter  31111 . 
     The characteristics of the filter  31111  will be described below. The filter  31111  exhibits various transmittances by polarization of the incident light. In more detail, the filter  31111  has various transmittances by polarization depending on its material or configuration, the incident angle of the incident light, or the wavelength of the incident light. The filter  31111  achieves the polarization demultiplexing function using the variation of reflection and transmission characteristics depending on the polarization. 
       FIG. 4  shows an example of a transmission spectrum of the filter  31111  described above. This example is a transmission spectrum where a dielectric multi-layer filter, which will be described below, is used as the filter  31111 . Here, the transmission spectrum is defined as a relationship between the wavelength and the transmittance of the light entering the filter  31111 . The horizontal axis in  FIG. 4  represents the wavelength of the light entering the filter  31111 , and the vertical axis represents the transmittance. One line on the graph shows the transmission spectrum of the filter  31111  for the TE wave, and the other line on the graph shows that for the TM wave. The TM wave is defined as an electromagnetic wave whose electric field component oscillates in a direction perpendicular to the surface of the substrate  31100 , and the TE wave is defined as an electromagnetic wave whose electric field component lies in the surface of the substrate  31100  and oscillates in a direction perpendicular to the travelling direction of the electromagnetic wave. For example, if the light with a wavelength in the range between the dotted lines shown in  FIG. 4  is entered into the filter  31111 , the transmitted light mainly includes the TE wave, and the reflected light mainly includes the TM wave. 
     As can be seen from the graph in  FIG. 4 , each of the transmitted light and the reflected light mainly has either polarization. That is to say, it does not necessarily follow that each of the transmitted light and the reflected light has either polarization only, but it can include both polarizations. The TE-light transmitted through the filter  31111 , therefore, also includes the TM wave in some small measure. Much the same is true on the TM-light reflected by the filter  31111 . 
     As a material of the filter  31111  described above, there is a dielectric multi-layer filter, for example. The dielectric multi-layer filter can be produced by laminating a plurality of films having various transmittance values and thicknesses. 
     Next, the shape of the filter will be described. The filter  31111  has a finite thickness in the light incident direction. It is only necessary for the light incidence plane of the filter  31111  to have an area almost equal to or larger than the area of light irradiation. 
     The after-mentioned filters  31141  and  31151  also have characteristics, materials, and shapes similar to those of the filter  31111 . 
     The waveguide  31133  is formed to guide the signal light, which is polarization-multiplexed and inputted from the signal light input unit  31131 , to the filter  31111 . The waveguide  31133  is formed so that the optical axis may be angled at a non-right angle to the reflection surface of the filter  31111 . This can prevent the reflected light from moving backward toward the waveguide  31133 . The reflection surface is a surface facing a side surface of the groove  31110  in the filter  31111  region. 
     Next, a structure of the waveguide  31133  and a method for forming the waveguide  31133  on the substrate  31100  will be described. As to the structure of the waveguide  31133 , the refractive index of the core layer is set to be about 1.5% larger than that of the cladding layer surrounding the core layer. By the difference in refractive index, the waveguide  31133  confines the light in the planar direction of the substrate  31100 . The waveguide  31133  is formed on the substrate  31100  made of silicon (Si) by CVD (Chemical Vapor Deposition) or the like. The waveguide structure and the method for forming the waveguide on the substrate described above are similar to other waveguides described below. 
     The waveguide  31134  is formed to guide the local oscillation light, which is inputted from the local oscillation light input unit  31132  and polarization-multiplexed, to the filter  31111 . The waveguide  31134  is also formed so that the optical axis may be angled at a non-right angle to the reflection surface of the filter  31111 . The waveguide  31121  is formed to guide the transmitted light, which is output from the filter  31111  and included in the polarization-multiplexed signal light, to the 90-degree optical hybrid  31123 . The waveguide  31135  is formed to guide the reflected light, which is reflected by the filter  31111  and included in the polarization-multiplexed signal light, to the 90-degree optical hybrid  31137 . The waveguide  31122  is formed to guide the transmitted light, which is output from the filter  31111  and included in the polarization-multiplexed local oscillation light, to the 90-degree optical hybrid  31123 . The waveguide  31136  is formed to guide the reflected light, which is reflected by the filter  31111  and included in the polarization-multiplexed local oscillation light, to the 90-degree optical hybrid  31137 . 
     As shown in  FIG. 2 , the waveguides  31133 ,  31134 ,  31135 , and  31136  are formed in the area  31130  on the substrate  31100 . The waveguides  31121  and  31122  are formed in the area  31120  on the substrate  31100 . 
     The 90-degree optical hybrid  31123  is formed in the area  31120  to make the signal light and the local oscillation light interfere which are transmitted through the filter  31111 . The 90-degree optical hybrid  31137  is formed in the area  31120  to make the signal light and the local oscillation light interfere which are reflected by the filter  31111 . 
     The 90-degree optical hybrid  31123  extracts real components and imaginary components of the guided signal light by making the signal light and the local oscillation light interfere which are guided by the waveguide  31121  and the waveguide  31122 . The 90-degree optical hybrid  31137  extracts real components and imaginary components of the guided signal light by making the signal light and the local oscillation light interfere which are guided by the waveguide  31135  and the waveguide  31136 . 
     The output unit  31124  outputs to the outside the real components of the signal light which have been extracted by the 90-degree optical hybrid  31123 . The output unit  31125  outputs to the outside the imaginary components of the signal light which have been extracted by the 90-degree optical hybrid  31123 . The output unit  31138  outputs to the outside the real components of the signal light which have been extracted by the 90-degree optical hybrid  31137 . The output unit  31139  outputs to the outside the imaginary components of the signal light which have been extracted by the 90-degree optical hybrid  31137 . 
     An example of the configuration of the planar lightwave circuit  31000  has been described above. 
     Next, the operation of the planar lightwave circuit  31000  shown in  FIG. 2  will be described. 
     First, the polarization-multiplexed signal light transmitted through the transmission line  40000  is inputted into the signal light input unit  31131 . On the other hand, the polarization-multiplexed local oscillation light from the local oscillation light source  32000  is inputted into the local oscillation light input unit  31132 . 
     The waveguide  31133  guides the polarization-multiplexed signal light to the filter  31111 . On the other hand, the waveguide  31134  guides the polarization-multiplexed local oscillation light to the filter  31111 . 
     The waveguide  31121  guides the transmitted light of the signal light through the filter  31111  to the 90-degree optical hybrid  31123 . On the other hand, the waveguide  31122  guides the transmitted light of the local oscillation light through the filter  31111  to the 90-degree optical hybrid  31123 . 
     The waveguide  31135  guides the reflected light of the signal light from the filter  31111  to the 90-degree optical hybrid  31137 . On the other hand, the waveguide  31136  guides the reflected light of the local oscillation light from the filter  31111  to the 90-degree optical hybrid  31137 . 
     The 90-degree optical hybrid  31123  makes the light interfere which is composed of the signal light guided by the waveguide  31121  and the local oscillation light guided by the waveguide  31122 . On the other hand, the 90-degree optical hybrid  31137  makes the light interfere which is composed of the signal light guided by the waveguide  31135  and the local oscillation light guided by the waveguide  31136 . 
     And then, the output unit  31124  and the output unit  31125  output the interfering light having interfered in the 90-degree optical hybrid  31123  to the photoelectric conversion unit  33000 . The output unit  31138  and the output unit  31139  output the interfering light having interfered in the 90-degree optical hybrid  31137  to the photoelectric conversion unit  33000 . 
     The operation of the planar lightwave circuit  31000  shown in  FIG. 2  has been described above. 
     As described above, in the present exemplary embodiment, the 90-degree optical hybrid  31123  is formed in the area  31120  on the substrate  31100 , and the 90-degree optical hybrid  31137  is formed in the area  31130  on the substrate  31100 . In addition, by intruding into the groove  31110  the filter  31111  which splits the polarization-multiplexed signal light depending on the polarization, the waveguides connected to the 90-degree optical hybrid  31123  are separated from the waveguides connected to the 90-degree optical hybrid  31137  by the groove  31110 . This makes it possible to decrease the points at which waveguides  31121 ,  31135 ,  31122 , and  31136  intersect and guide the polarization-demultiplexed signal light and local oscillation light to the 90-degree optical hybrids  31123  and  31137 . 
     It becomes possible to form the waveguides  31121  and  31135 , the waveguides  31122  and  31136 , and the 90-degree optical hybrids  31123  and  31137  symmetrically with respect to the groove  31110 . This makes it possible to easily equalize the length of the waveguides  31121  and  31135 , and the length of the waveguides  31122  and  31136 . As a result, it becomes possible to easily equalize the length of respective optical paths for the signal light and the local oscillation light in the path from the point where the light is split depending on the polarization to the point where the light is inputted into the 90-degree optical hybrid. 
     Preferably, the waveguides  31121  and  31135  are formed with their lengths equal to each other. This equalizes the optical path length through which the signal light interfering with the local oscillation light in the 90-degree optical hybrid  31123  is transmitted, to the optical path length through which the signal light interfering with the local oscillation light in the 90-degree optical hybrid  31137  is transmitted. As a result, it is possible to reduce a skew occurring between the two signal light beams and reduce the deterioration of the signal quality. 
     More preferably, the waveguides  31122  and  31135  are formed with their lengths equal to each other. 
     Next, a modified example will be described using  FIG. 3  in which an optical reception unit  50000  is realized by a planar lightwave circuit. The identical reference signs are attached to the same configurations as those of the planar lightwave circuit  31000  shown in  FIG. 2 , and their descriptions will be omitted for simplification. 
     In the planar lightwave circuit  50000 , a groove  31140  is additionally formed in the area  31120 , and a groove  31150  is additionally formed in the area  31130 . The planar lightwave circuit  50000  includes a filter  31141  and a filter  31151 . 
     The filter  31141  is included in the area  31120  and performs the polarization demultiplexing by transmission and reflection. On the other hand, the filter  31151  is included in the area  31130  and performs the polarization demultiplexing by transmission and reflection. Here, the filter  31141  is included in the area  31120  by being intruded into the groove  31140 , and the filter  31151  is included in the area  31130  by being intruded into the groove  31150 . The present exemplary embodiment will be described using TE-transmissive/TM-reflective type thin film filters as the filters  31141  and  31151 . 
     In the present exemplary embodiment, the waveguide  31121  is formed to guide to the filter  31141  the transmitted light which is output from the filter  31111  and included in the polarization-multiplexed signal light, and guide the transmitted light output from the filter  31141  to the 90-degree optical hybrid  31123 . The waveguide  31122  is formed to guide to the filter  31141  the transmitted light which is output from the filter  31111  and included in the polarization-multiplexed local oscillation light, and guide the transmitted light output from the filter  31141  to the 90-degree optical hybrid  31123 . Here, the waveguides  31121  and  31122  are formed so that the guided light may be entered with its optical axis angled at a non-right angle to the reflection surface of the filter  31141 . 
     The waveguide  31135  is formed to guide the reflected light, which is reflected by the filter  31111  and included in the polarization-multiplexed signal light, to the filter  31151 , and guide the reflected light reflected by the filter  31151  to the 90-degree optical hybrid  31137 . The waveguide  31136  is formed to guide the reflected light, which is reflected by the filter  31111  and included in the polarization-multiplexed signal light, to the filter  31151 , and guide the reflected light reflected by the filter  31151  to the 90-degree optical hybrid  31137 . Here, the waveguides  31135  and  31136  are also formed so that the guided light may be entered with its optical axis angled at a non-right angle to the reflection surface of the filter  31151 . 
     An example of the configuration of the planar lightwave circuit  50000  has been described above. 
     Next, the operation of the planar lightwave circuit  50000  shown in  FIG. 3  will be described. 
     First, the signal light transmitted through the transmission line  40000  is inputted into the signal light input unit  31131 . The polarization-multiplexed local oscillation light from the local oscillation light source  32000  is inputted into the local oscillation light input unit  31132 . 
     The waveguide  31133  guides the polarization-multiplexed signal light to the filter  31111 . On the other hand, the waveguide  31134  guides the polarization-multiplexed local oscillation light to the filter  31111 . 
     Next, the waveguide  31121  guides the transmitted light, which is output from the filter  31111  and included in the polarization-multiplexed signal light, to the filter  31141 . And it guides the transmitted light output from the filter  31141  to the 90-degree optical hybrid  31123 . On the other hand, the waveguide  31122  guides the transmitted light, which is output from the filter  31111  and included in the TE-wave/TM-wave multiplexed local oscillation light, to the filter  31141 . And it guides the transmitted light output from the filter  31141  to the 90-degree optical hybrid  31123 . 
     The waveguide  31135  guides the reflected light, which is reflected by the filter  31111  and included in the polarization-multiplexed signal light, to the filter  31151 . And it guides the reflected light reflected by the filter  31151  to the 90-degree optical hybrid  31137 . On the other hand, the waveguide  31136  guides the reflected light, which is reflected by the filter  31111  and included in the polarization-multiplexed local oscillation light, to the filter  31151 . And it guides the reflected light reflected by the filter  31151  to the 90-degree optical hybrid  31137 . 
     The subsequent operations are the same as those of the planar lightwave circuit  31000 , and accordingly their descriptions are omitted. 
     As mentioned above, the modified example, compared with the planar lightwave circuit  31000 , further includes the filter  31141  which is included in the area  31120  and performs the polarization demultiplexing by transmission and reflection, and the filter  31151  which is included in the area  31130  and performs the polarization demultiplexing by transmission and reflection. 
     It is possible to further transmit, through the filter  31141 , the signal light and local oscillation light transmitted through the filter  31111  by including multiple-stage filters in the path through which the signal light and the local oscillation light are guided as mentioned above. This makes it possible to further improve the polarization extinction ratio of the light inputted into the 90-degree optical hybrid  31123 . It is also possible to make the filter  31151  further reflect the signal light and the local oscillation light having been reflected by the filter  31111 . This makes it possible to further improve the polarization extinction ratio of the light inputted into the 90-degree optical hybrid  31137 . 
     The planar lightwave circuit  50000  in accordance with the modified example has the characteristics of the planar lightwave circuit  31000  in accordance with the above-mentioned exemplary embodiment as well. It is possible, therefore, in the planar lightwave circuit including the 90-degree optical hybrid, to decrease the points at which waveguides intersect and to improve the polarization extinction ratio of the light inputted into the 90-degree optical hybrid. In addition, it becomes possible to easily equalize the length of respective optical paths for the signal light and the local oscillation light in the path from the point where the light is split depending on the polarization to the point where the light is inputted into the 90-degree optical hybrid. 
     Although the present invention has been described above using the exemplary embodiments as an example, the present invention is not limited to the above-described exemplary embodiments and can be variously modified and implemented within the technological scope of the present invention. 
     For example, although the above-described exemplary embodiment has been described using M-level phase shift keying, it is also acceptable to use Amplitude Phase Shift Keying (APSK) and M-level Quadrature Amplitude Modulation (QAM) which is one of APSK methods. It is also acceptable to employ Orthogonal Frequency Division Multiplexing (OFDM) as transmission methods, and to employ the polarization-multiplexed M-level phase shift keying or the like for at least one of the subcarriers. 
     With regard to the configuration of the transmission line  40000 , it is also acceptable to use a multimode optical fiber in place of a single mode optical fiber. 
     Although the example has been shown in the above-described exemplary embodiment in which the local oscillation light source  32000  is included in the optical receiver  30000 , it is also acceptable to have the local oscillation light source  32000  outside the optical receiver  30000 . In that case, the optical receiver  30000  further includes an input unit into which the local oscillation light from the local oscillation light source  32000  is inputted. 
     The shape of the grooves  31110 ,  31140  and  31150  is not necessarily a linear shape in the longitudinal direction of the substrate  31100 , but it is also acceptable to be a curved shape in the longitudinal direction. These grooves can be formed traversing the substrate completely from end to end. It is not excluded, however, to form the grooves stopped in the middle of the substrate. If the grooves are formed stopped in the middle of the substrate, it can be assumed to separate the area  31120  and the area  31130  by a hypothetical extended line. The grooves can be formed by methods other than a dicing process. 
     Although the example has been described in which the dielectric multi-layer filter is used as the material for the filters  31111 ,  31141 , and  31151 , a photonic crystal polarizer can be also used. 
     The above-described exemplary embodiment has been described using the filters having the properties of transmitting a TE wave and reflecting a TM wave as the filters  31131 ,  31141 , and  31151 . It is also acceptable, however, to use a filter having the properties of transmitting a TM wave and reflecting a TE wave as the filters  31131 ,  31141 , and  31151 . Such a filter can be called a TE-reflective/TM-transmissive type thin film filter, and is realized by changing the material and configuration depending on the wavelength and the incident angle of the incident light. 
     Although the above-described exemplary embodiment has been described in which the filter  33151  is intruded into the groove  31150 , the filter  33151  can be attached on the side of the substrate  31100  in the area  31130 . In that case, there is no need to form the groove  31150  in the area  31130 . 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2012-048681, filed on Mar. 6, 2012, the disclosure of which is incorporated herein in its entirety by reference. 
     [Reference Signs List] 
     
         
           10000  optical transmission system 
           20000  optical transmitter 
           40000  transmission line 
           30000  optical receiver 
           31000 ,  50000  optical reception unit 
           32000  local oscillation light source 
           33000  photoelectric conversion unit 
           34000  analog-to-digital converter 
           35000  digital signal processing unit 
           36000  demodulation unit 
           31100  substrate 
           31110 ,  31140 ,  31150  groove 
           31120 ,  31130  area 
           31131  signal light input unit 
           31132  local oscillation light input unit 
           31133 ,  31134 ,  31135 ,  31136 ,  31121 ,  31122  waveguide 
           31111 ,  31141 ,  31151  filter 
           31137 ,  31123  90-degree optical hybrid 
           31124 ,  31125 ,  31138 ,  31139  output unit