Patent Publication Number: US-2022229238-A1

Title: Wavelength converter, optical transmission system and wavelength conversion method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2021-6318, filed on Jan. 19, 2021, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to a wavelength converter, an optical transmission system, and a wavelength conversion method. 
     BACKGROUND 
     A technique has been developed for increasing transmission capacity by increasing the number of wavelength division multiplexing (WDM) channels for continuously increasing traffic of an optical network. One such technique is wavelength conversion. In the wavelength conversion technique, an optical signal is transmitted by using a new wavelength band in a transmission line while using an existing optical transceiver used in a conventional band. In addition to a C-band, which is a conventional communication band, a communication band is extended to an L-band on a long wavelength side of the C-band and an S-band on a short wavelength side of the C-band. 
     In the wavelength conversion, light having a wavelength different from that of incident light is generated by using a non-linear optical phenomenon of a non-linear optical medium. Since the non-linear optical phenomenon has polarization dependency, a polarization diversity configuration that suppresses the polarization dependency is used. In the polarization diversity configuration, the polarization dependency is reduced by separating incident signal light into polarization components orthogonal to each other by a polarization beam splitter (PBS) and treating each polarization component independently. 
     Japanese National Publication of International Patent Application No. 2007-531900 is disclosed as related art. 
     SUMMARY 
     According to an aspect of the embodiments, a wavelength converter includes a polarization beam splitter configured to separate input light into a first polarization and a second polarization that are orthogonal to each other a non-linear optical medium configured to include a first incident end on which the first polarization separated by the polarization beam splitter is incident and a second incident end on which the second polarization separated by the polarization beam splitter is incident at a position different from a position of the first incident end, an optical multiplexer configured to multiplex the first polarization that has passed through the non-linear optical medium and the second polarization that has passed through the non-linear optical medium, and an optical element arranged between the non-linear optical medium and the optical multiplexer, and configured to correct a polarization axis of at least one of the first polarization and the second polarization incident on the optical multiplexer. 
     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 diagram illustrating temporal variation of output optical power; 
         FIG. 2  is a schematic diagram of an optical transmission system to which a wavelength converter of an embodiment is applied; 
         FIG. 3  is a diagram illustrating a basic configuration of the wavelength converter of the embodiment; 
         FIG. 4  is a schematic diagram of a wavelength converter of a first embodiment; 
         FIG. 5  is a schematic diagram of a wavelength converter of a second embodiment; 
         FIG. 6  is a schematic diagram of a wavelength converter of a third embodiment; 
         FIG. 7  is a schematic diagram of a wavelength converter of a fourth embodiment; 
         FIG. 8  is a flowchart of feedback control based on monitor light; 
         FIG. 9  is an analysis model diagram of a polarization diversity optical circuit; 
         FIG. 10  is a diagram illustrating a change in the output optical power due to polarization angle deviation in a loop; 
         FIG. 11  is a diagram illustrating a change in an interference electric field due to the polarization angle deviation in the loop when an extinction ratio of a polarization beam splitter (PBS) is 20 dB; 
         FIG. 12  is a diagram illustrating a change in the output optical power due to the polarization angle deviation in the loop when the extinction ratio of the PBS is 20 dB; 
         FIG. 13  is a diagram illustrating extinction ratio dependency of output optical power variation due to polarization angle deviation; 
         FIG. 14  is a diagram illustrating deviation angle dependency of the output optical power variation due to the extinction ratio of the PBS; 
         FIG. 15  is an analysis model diagram when a polarizer is added in the loop; 
         FIG. 16  is a diagram illustrating a change in an output electric field due to polarization angle deviation in a model of  FIG. 15 ; and 
         FIG. 17  is a diagram illustrating an effect of the addition of the polarizer. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In a case where a polarization state is not optimum in a polarization diversity optical circuit, power of output light varies with time and transmission characteristics of an optical signal deteriorate. 
     Hereinafter, embodiments of a technique capable of suppressing temporal variation of output optical power and maintaining transmission characteristics satisfactorily even when a polarization state in a polarization diversity optical circuit deviates from an optimum state will be described with reference to the drawings. 
       FIG. 1  is a diagram illustrating temporal variation of output optical power of a polarization diversity optical circuit  40 . The polarization diversity optical circuit  40  used in a wavelength converter  30  is adjusted to the optimum state at the time of shipment so as to maximize conversion efficiency. However, when a polarization in the polarization diversity optical circuit  40  deviates from the optimum state during service due to an influence of a disturbance such as vibration, temperature change, stress, or higher order polarization mode dispersion dependent which mainly induced by stress and rotate the polarization of converted light with respect to the pump and the input signal, temporal power variation occurs in output light. 
     In the polarization diversity optical circuit  40 , an input optical signal is separated into two orthogonal polarizations by a polarization beam splitter (PBS)  31 . The input optical signal has constant power averaged over time sufficiently longer than the signal modulation rate, and includes a horizontal polarization (E X1 ) and a vertical polarization (E Y1 ). The horizontal polarization transmitted through the PBS  31  is subjected to a non-linear optical effect in a first non-linear optical medium  33  (referred to as “non-linear optical medium- 1 ” in the drawing). The vertical polarization reflected by the PBS  31  is subjected to the non-linear optical effect in a second non-linear optical medium  34  (referred to as “non-linear optical medium- 2 ” in the drawing). The two polarizations, each of which is subjected to the non-linear optical effect, are multiplexed and output by a polarization beam combiner (PBC)  37 . 
     As the non-linear optical media  33  and  34 , a highly non-linear fiber (HNLF) is usually used. The HNLF includes a polarization maintaining fiber (PMF) having large difference between refractive indices with respect to a vertical polarization and a horizontal polarization in a cross section orthogonal to an optical axis, and a non-polarization maintaining fiber (NON-PMF) having substantially the same refractive indices with respect to the vertical polarization and the horizontal polarization. In the PMF type HNLF, since a polarization is maintained at input and output, a change in a polarization state due to an influence of a disturbance is very small. However, it is difficult to produce wavelength dispersion characteristics needed for wavelength conversion with good reproducibility. The NON-PMF type HNLF is commonly used for the wavelength conversion because it is possible to produce the wavelength dispersion characteristics needed for the wavelength conversion with relatively good reproducibility. However, since a polarization is not maintained in a process of propagation in a non-linear optical medium, polarization rotation occurs due to an influence of a disturbance. 
     At the time of multiplexing by the PBC  37 , when the two polarizations include only components orthogonal to each other, or when the PBC  37  is an ideal device with an infinite polarization extinction ratio, only orthogonal components are multiplexed and no optical interference occurs. In a case where a polarization of incident light on the PBC  37  is not optimum, parallel components are also multiplexed, and optical interference occurs. Even when an incident state on the PBC  37  is optimally adjusted at the time of shipment in consideration of an influence of polarization rotation by the non-linear optical media  33  and  34  in advance, an unnecessary polarization component is generated in the incident light on the PBC  37  when a rotation angle of a polarization varies due to an influence of a disturbance or the like. For example, light emitted from the first non-linear optical medium  33  includes electric field components (E X2X , E X2Y ), and light emitted from the second non-linear optical medium  34  includes electric field components (E Y2X , E Y2Y ). Among these, E X2Y  and E Y2X  cause interference. 
     In the PBC  37 , when the target component E X2X  is multiplexed with the component E Y2X  parallel to the target component E X2X , optical interference occurs. Similarly, when the target component E Y2Y  is multiplexed with the component E X2Y  parallel to the target component E Y2Y , optical interference occurs. Such optical interference is observed as temporal optical power variation when optical path lengths of the non-linear optical media  33  and  34  are long. 
     In an embodiment, even when a state of a polarization incident on the PBC  37  in the wavelength converter  30  deviates from an optimum state, unnecessary interference is avoided and temporal optical power variation is suppressed. The optimum state means a unique state (principle state of polarization) in which polarizations are orthogonal to each other. In order to put a polarization into the optimum state, optical elements for polarization correction such as a polarizer and a PBS  31  are arranged in a stage before multiplexing two polarizations subjected to a non-linear optical process, and states of the polarizations incident on the PBC  37  are optimized. In a more preferable configuration, a polarization rotator or a polarization controller is arranged between the non-linear optical media  33  and  34  and the optical element for polarization correction, and polarization axes of light emitted from the non-linear optical media  33  and  34  are aligned with a polarization axis of the optical element for polarization correction and a polarization axis of an optical multiplexer. 
     Hereinafter, a specific configuration of the embodiment will be described. In the following description, the same components may be denoted by the same reference signs and duplicate description may be omitted. Furthermore, regardless of a direction of a polarization plane, one of two polarizations separated by the PBS  31  is referred to as a first polarization, and the other is referred to as a second polarization. 
       FIG. 2  is a schematic diagram of an optical transmission system  1  in which the wavelength converter  30  of the embodiment is used. The optical transmission system  1  includes an optical communication device  10  on a transmission side, an optical communication device  20  on a reception side, and an optical transmission line  18  connecting therebetween. Both the optical communication device  10  and the optical communication device  20  have functions of both the transmission side and the reception side, and have the same configuration. However, for convenience of description, the function of the transmission side of the optical communication device  10  and the function of the reception side of the optical communication device  20  will be described as an example. 
     The optical communication device  10  includes optical transmitters  11 -L 1  to  11 -LN included in a first group, optical transmitters  11 -C 1  to  11 -CN included in a second group, and optical transmitters  11 -S 1  to  11 -SN included in a third group (hereinafter, collectively referred to as “optical transmitters  11 ” as appropriate). These optical transmitters  11  are each a photoelectric conversion front end circuit of an optical transponder, for example. The plurality of optical transmitters  11  has the same configuration, and outputs, for example, signals having a wavelength channel of a C-band (1530 to 1565 nm) (each of which is referred to as “C-band transmitter” in the drawing). 
     Output light of the optical transmitters  11 -L 1  to  11 -LN in the first group is multiplexed by a first wavelength multiplexer  12 - 1 . Output light of the optical transmitters  11 -C 1  to  11 -CN in the second group is multiplexed by a second wavelength multiplexer  12 - 2 . Output light of the optical transmitters  11 -S 1  to  11 -SN in the third group is multiplexed by a third wavelength multiplexer  12 - 3 . The first wavelength multiplexer  12 - 1 , the second wavelength multiplexer  12 - 2 , and the third wavelength multiplexer  12 - 3  have the same function and configuration, and multiplex input signals having a plurality of wavelength channels and output resultant signals. 
     Output of the first wavelength multiplexer  12 - 1  is amplified by a first optical amplifier  13 - 1 . Output of the second wavelength multiplexer  12 - 2  is amplified by a second optical amplifier  13 - 2 . Output of the third wavelength multiplexer  12 - 3  is amplified by a third optical amplifier  13 - 3 . The first optical amplifier  13 - 1 , the second optical amplifier  13 - 2 , and the third optical amplifier  13 - 3  have the same function and configuration, and amplify multiplexed optical signals of the C-band. 
     The C-band signal light amplified by the first optical amplifier  13 - 1  is subjected to wavelength conversion by a first wavelength converter  30 - 1 , and input to a wavelength multiplexer  16 . In this example, the first wavelength converter  30 - 1  collectively converts the C-band signal light to L-band signal light. The C-band signal light amplified by the third optical amplifier  13 - 3  is subjected to wavelength conversion by a second wavelength converter  30 - 2 , and input to the wavelength multiplexer  16 . In this example, the second wavelength converter  30 - 2  collectively converts the C-band signal light to S-band signal light. 
     The C-band signal light amplified by the second optical amplifier  13 - 2  is not subjected to wavelength conversion and is input to the wavelength multiplexer  16  as it is. The wavelength multiplexer  16  multiplexes the L-band signal light, the C-band signal light, and the S-band signal light, and outputs a wavelength division multiplexing (WDM) signal to the optical transmission line  18 . This WDM signal includes wavelength channels from the L-band to the S-band, and optical communication over a wide band is carried out. The WDM signal is propagated through the optical transmission line  18 , and is received by the optical communication device  20 . 
     In the optical communication device  20 , the received optical signal is demultiplexed into L-band signal light, C-band signal light, and S-band signal light by a wavelength demultiplexer  26 . The L-band signal light is converted to C-band signal light by a third wavelength converter  30 - 3 , is amplified by an optical amplifier  23 - 1 , and is demultiplexed into different wavelength channels by a first wavelength demultiplexer  22 - 1 . 
     The S-band signal light is converted to C-band signal light by a fourth wavelength converter  30 - 4 , is amplified by an optical amplifier  23 - 3 , and is demultiplexed into different wavelength channels by a third wavelength demultiplexer  22 - 3 . The C-band signal light is not subjected to wavelength conversion, is amplified by an optical amplifier  23 - 2  as it is, and is demultiplexed into different wavelength channels by a second wavelength demultiplexer  22 - 2 . 
     The optical amplifiers  23 - 1  to  23 - 3  have the same function and configuration. The wavelength demultiplexers  22 - 1  to  22 - 3  have the same function and configuration, and in this example, demultiplex C-band signal light into different wavelength channels. 
     The beams of signal light demultiplexed by the first wavelength demultiplexer  22 - 1  are supplied to optical receivers  21 -L 1  to  21 -LN in a first group. The beams of signal light demultiplexed by the second wavelength demultiplexer  22 - 2  are supplied to optical receivers  21 -C 1  to  21 -CN in a second group. The beams of signal light demultiplexed by the third wavelength demultiplexer  22 - 3  are supplied to optical receivers  21 -S 1  to  21 -SN in a third group. The optical receivers  21 -L 1  to  21 -LN, the optical receivers  21 -C 1  to  21 -CN, and the optical receivers  21 - 51  to  21 -SN are collectively referred to as “optical receivers  21 ” as appropriate. 
     The optical receivers  21  are each a photoelectric conversion front end circuit of an optical transponder, for example. The plurality of optical receivers  21  has the same configuration, and converts, for example, light having a wavelength channel of the C-band (1530 to 1565 nm) to electric signals. 
     The optical transmission system  1  does not use optical components for individual bands, but uses common optical transmitters and receivers, wavelength multiplexers and demultiplexers, optical amplifiers, and the like. By using the wavelength converters  30 - 1  to  30 - 4 , even when a polarization in the polarization diversity optical circuit deviates from the optimum state due to an influence of a disturbance such as vibration, temperature change, or generation of stress, a polarization state is controlled to the optimum polarization state, and temporal variation of output power of converted light is suppressed. With this configuration, transmission capacity may be increased while improving transmission characteristics. 
       FIG. 3  is a basic configuration diagram of the wavelength converter  30 . The wavelength converter  30  includes the PBS  31 , the PBC  37 , the non-linear optical medium  33 , an optical element  35 , the non-linear optical medium  34 , and an optical element  36 . These optical components are connected by, for example, a single mode fiber or a polarization maintaining fiber to form the polarization diversity optical circuit  40 . Instead of adjusting a polarization between the optical components with the polarization maintaining fiber, a spatial coupling system may be used to connect the optical components in a desired polarization state. It is assumed that a non-linear fiber of NON-PMF is used as the non-linear optical media  33  and  34 . 
     An optical signal having electric field components (E X1 , E Y1 ) is input to the wavelength converter  30 . This input optical signal is signal light to be subjected to wavelength conversion, and is, for example, C-band signal light. A subscript “ 1 ” of the electric field component indicates that the optical signal has not been subjected to wavelength conversion. The PBS  31  separates the input optical signal into a first polarization and a second polarization. For example, the first polarization including electric field components (E X1 , 0) transmits through the PBS  31 , propagates through a first path P 1 , and is incident on the non-linear optical medium  33 . The non-linear optical medium  33  has a first incident end  331  on which the first polarization is incident. The second polarization including electric field components (0, E Y1 ) is reflected by the PBS  31 , propagates through a second path P 2 , and is incident on the non-linear optical medium  34 . The non-linear optical medium  34  has a second incident end  341  on which the second polarization is incident. 
     As will be described later, excitation light multiplexed with the signal light is incident on the non-linear optical media  33  and  34 . The excitation light may be superposed on the signal light in a preceding stage of the PBS  31  and input to the wavelength converter  30 , or may be input to the non-linear optical medium in a succeeding stage of the PBS  31 . In a case where the excitation light is input in the preceding stage of the PBS  31 , the excitation light includes a polarization equally divided by the PBS  31 . Polarization axes of the signal light and the excitation light incident on the non-linear optical media  33  and  34  are parallel to each other in the first polarization and the second polarization, respectively. 
     In the first path P 1  from the PBS  31  to the PBC  37 , the optical element  35  for polarization correction is arranged between the non-linear optical medium  33  and the PBC  37 . In the second path P 2  from the PBS  31  to the PBC  37 , the optical element  36  for polarization correction is arranged between the non-linear optical medium  34  and the PBC  37 . 
     Output light from the non-linear optical medium  33  includes components of converted light newly generated in a non-linear optical process in addition to components of the signal light and components of the excitation light, and for the sake of simplicity of description, they are collectively treated as the first polarization. The first polarization that has exited the non-linear optical medium  33  includes electric field components (E X2X , E X2Y ) due to an influence of polarization rotation deviation or the like inside the non-linear optical medium  33 . A subscript “ 2 ” of the electric field component indicates that the optical signal has been subjected to wavelength conversion. Furthermore, X and Y after the subscript “ 2 ” indicates electric field components generated during the wavelength conversion. 
     The first polarization that has exited the non-linear optical medium  33  is incident on the optical element  35  before being multiplexed with the second polarization by the PBC  37 , and the unnecessary electric field component E X2Y  is removed by the optical element  35 . The optical element  35  is arranged so as to transmit the electric field component E X2X  and to align a polarization axis of the first polarization incident on the PBC  37  with a reflection polarization axis of the PBC  37 . For example, in a case where input and output of the optical element  35  and the PBC  37  are coupled by an optical fiber, they are connected by a polarization maintaining fiber so that a transmission polarization axis of the optical element  35  and the reflection polarization axis of the PBC  37  are aligned. Alternatively, a polarization state between the optical element  35  and the PBC  37  may be adjusted by a spatial optical system to a desired polarization state. 
     Similarly, output light from the non-linear optical medium  34  includes components of converted light newly generated in a non-linear optical process in addition to components of the signal light and components of the excitation light, and they are collectively treated as the second polarization. The second polarization that has exited the non-linear optical medium  34  includes electric field components (E Y2X , E Y2Y ) due to an influence of polarization rotation deviation or the like inside the non-linear optical medium  34 . 
     The second polarization that has exited the non-linear optical medium  34  is incident on the optical element  36  before being multiplexed with the first polarization by the PBC  37 , and the unnecessary electric field component E Y2X  is removed by the optical element  36 . The optical element  36  is arranged so as to transmit the electric field component E Y2Y  and to align a polarization axis of the second polarization incident on the PBC  37  with a transmission polarization axis of the PBC  37 . For example, in a case where input and output of the optical element  36  and the PBC  37  are coupled by an optical fiber, they are connected by a polarization maintaining fiber so that a transmission polarization axis of the optical element  36  and the transmission polarization axis of the PBC  37  are aligned. Alternatively, a polarization state between the optical element  36  and the PBC  37  may be adjusted by a spatial optical system to a desired polarization state. 
     By arranging the optical elements  35  and  36  for polarization correction in the preceding stage of the PBC  37 , even in a case where a polarization rotation angle deviates due to an influence of a disturbance or the like, the polarizations may be multiplexed in the optimum polarization state, for example, in the state in which the polarizations are orthogonal to each other. Parallel components that cause interference are removed, and temporal variation of output optical power is suppressed. 
     As will be described later, it is not always needed to use the individual non-linear optical media  33  and  34  as the non-linear optical media. A single non-linear optical medium may be used and the two polarizations separated by the PBS  31  may be incident on the non-linear optical medium from mutually different incident ends. In this case as well, two separated polarizations may be treated independently of each other. Furthermore, separating and multiplexing the polarizations may be implemented by one PBS. 
     A polarization rotator or a polarization controller may be arranged between the non-linear optical medium  33  and the optical element  35 . The polarization rotator or the polarization controller is arranged so that the axis of the first polarization emitted from the non-linear optical medium  33  is aligned with a polarization axis of the optical element  35  and a polarization axis of the PBC  37 . Similarly, a polarization rotator or a polarization controller may be arranged between the non-linear optical medium  34  and the optical element  36 . The polarization rotator or the polarization controller is arranged so that the axis of the second polarization emitted from the non-linear optical medium  34  is aligned with a polarization axis of the optical element  36  and the polarization axis of the PBC  37 . This configuration may enhance an effect of polarization axis correction by the optical elements  35  and  36 . 
     First Embodiment 
       FIG. 4  is a schematic diagram of a wavelength converter  30 A of a first embodiment. In the first embodiment, a first non-linear optical medium  33 A and a second non-linear optical medium  34 A are used as non-linear optical media. PBSs are used as optical elements  35 A and  36 A for polarization correction. Furthermore, a first polarization rotator (referred to as “PR- 1 ” in the drawing)  43  is arranged between the first non-linear optical medium  33 A and the optical element  35 A, and a second polarization rotator (referred to as “PR- 2 ” in the drawing)  44  is arranged between the second non-linear optical medium  34 A and the optical element  36 A. 
     An input optical signal having electric field components (E X1 , E Y1 ) and excitation light Lpump are superposed by a WDM coupler  42 , and incident on a PBS  31 . A laser diode (LD)  41 , which is an excitation light source, outputs a polarization equally divided by the PBS  31  as the excitation light Lpump. A polarization diversity optical circuit  40 A is formed by a first path P 1  and a second path P 2  from the PBS  31  to a PBC  37 . 
     The PBS  31  separates the input optical signal into a first polarization including the electric field component E X1  and a second polarization including the electric field component E Y1 . Furthermore, the PBS  31  divides the excitation light Lpump at a ratio of 1:1. The first polarization transmitted through the PBS  31  is multiplexed with the excitation light, propagates through the first path P 1 , and is incident on a first incident end  331  of the first non-linear optical medium  33 A. The first polarization is subjected to polarization rotation by the first polarization rotator  43  after being subjected to a non-linear optical effect in the first non-linear optical medium  33 A. By this polarization rotation, a main polarization axis of the first polarization is adjusted so as to coincide with a transmission polarization axis of the optical element  35 A and a reflection polarization axis of the PBC  37 . 
     The first polarization that has exited the first polarization rotator  43  includes electric field components (E X2X , E X2Y ) due to an influence of polarization rotation deviation or the like in a non-linear optical process. The optical element  35 A, which is the PBS, is arranged so as to transmit E X2X  of the first polarization that has been subjected to the polarization rotation and to reflect E X2Y  of the first polarization. The axis of the first polarization that exits the optical element  35 A and is incident on the PBC  37  is aligned with the reflection polarization axis of the PBC  37 . 
     The second polarization reflected by the PBS  31  is multiplexed with the excitation light, propagates through the second path P 2 , and is incident on a second incident end  341  of the second non-linear optical medium  34 A. The second polarization is subjected to polarization rotation by the second polarization rotator  44  after being subjected to a non-linear optical effect in the second non-linear optical medium  34 A. By this polarization rotation, a main polarization axis of the second polarization is adjusted so as to coincide with a transmission polarization axis of the optical element  36 A and a transmission polarization axis of the PBC  37 . 
     The second polarization that has exited the second polarization rotator  44  includes electric field components (E Y2X , E Y2Y ) due to an influence of polarization rotation deviation or the like in a non-linear optical process. The optical element  36 A, which is the PBS, is arranged so as to transmit E Y2Y  of the second polarization that has been subjected to the polarization rotation and to reflect E Y2X  of the second polarization. The axis of the second polarization that exits the optical element  36 A and is incident on the PBC  37  is aligned with the transmission polarization axis of the PBC  37 . 
     The E X2Y  component reflected by the optical element  35 A and the E Y2X  component reflected by the optical element  36 A may be monitored by a first photodetector (referred to as “PD- 1 ” in the drawing)  45  and a second photodetector (referred to as “PD- 2 ” in the drawing)  46 , respectively, and used for detecting polarization deviation. Electric output from the first photodetector  45  and the second photodetector  46  may be supplied to a monitor circuit arranged inside or outside a package of the wavelength converter  30 A, and used for monitoring polarization deviation, or the like. Alternatively, power of the light incident on each of the first photodetector  45  and the second photodetector  46  may be monitored. A configuration may be adopted in which an alarm is generated when a detected polarization deviation component exceeds a predetermined threshold. 
     In the wavelength converter  30 A, polarizations which are orthogonal to each other and each of which includes converted light, for example, the electric field components E X2X  and E X2Y  are multiplexed, and temporal variation of output optical power is suppressed. 
     Second Embodiment 
       FIG. 5  is a schematic diagram of a wavelength converter  30 B of a second embodiment. In the second embodiment, one non-linear optical medium  32  is used. An optical element  35 B for polarization correction is arranged between a PBS  31  and a first incident end  321  of the non-linear optical medium  32 . An optical element  36 B for polarization correction is arranged between the 
     PBS  31  and a second incident end  322  of the non-linear optical medium  32 . PBSs are used as the optical elements  35 B and  36 B. A polarization rotator  47  is arranged between the non-linear optical medium  32  and the optical element  35 B, or between the non-linear optical medium  32  and the optical element  36 B. A polarization diversity optical circuit  40 B is formed by a bidirectional loop starting from the PBS  31 , passing through the non-linear optical medium  32 , and returning to the PBS  31 . The PBS  31  functions as a PBS for light incident on the polarization diversity optical circuit  40 B, and functions as an optical multiplexer, for example, a PBC for a first polarization and a second polarization after circulating the loop. 
     The input optical signal in a certain wavelength band (for example, C-band) having electric field components (E X1 , E Y1 ) is superposed with excitation light Lpump by a WDM coupler  42 , is transmitted through a WDM coupler  48 , and is incident on the PBS  31 . An LD  41 , which is an excitation light source, emits a polarization transmitted through the WDM coupler  48  and equally divided by the PBS  31 , as the excitation light Lpump. 
     The PBS  31  separates the input optical signal into the first polarization including the electric field component E X1  and the second polarization including the electric field component E Y1 . Furthermore, the PBS  31  divides the excitation light Lpump at a ratio of 1:1. The first polarization transmitted through the PBS  31  propagates in the loop in a clockwise direction, is transmitted through the optical element  35 B as it is, and is incident on the first incident end  321  of the non-linear optical medium  32  to be subjected to a non-linear optical effect. The first polarization that has exited the non-linear optical medium includes electric field components (E X2X , E X2Y ) due to an influence of polarization rotation deviation or the like. The first polarization including a component of converted light is rotated by 90° by the polarization rotator  47  and is incident on the optical element  36 B. Due to this polarization rotation, an axis of the first polarization is aligned with a transmission polarization axis of the optical element  36 B, and is aligned with a reflection polarization axis of the PBS  31 . 
     The optical element  36 B, which is the PBS, is arranged so as to transmit E X2X  of the first polarization rotated by 90° and to reflect E X2Y  of the first polarization. The electric field component reflected by the optical element  36 B may be monitored by a second photodetector  46  as a polarization deviation component. 
     In the input light to the polarization diversity optical circuit  40 B, the second polarization reflected by the PBS  31  propagates in the loop in a counterclockwise direction, and is transmitted through the optical element  36 B as it is. The second polarization is rotated by 90° by the polarization rotator  47 , and is incident on the second incident end  322  of the non-linear optical medium  32 . The second polarization subjected to a non-linear optical effect includes electric field components (E Y2X , E Y2Y ) due to an influence of polarization rotation deviation or the like. 
     The optical element  35 B, which is the PBS, is arranged so as to transmit E Y2Y  of the second polarization rotated by 90° and to reflect E Y2X  of the second polarization. With this configuration, the electric field component E Y2Y  aligned with a transmission polarization axis of the PBS  31  is incident on a transmission port of the PBS  31 . The electric field component reflected by the optical element  35 B may be monitored by a first photodetector  45  as a polarization deviation component. 
     The first polarization and the second polarization after wavelength conversion are multiplexed by the PBS  31 . In the first polarization and the second polarization, which circulate in the loop of the polarization diversity optical circuit  40 B in mutually opposite directions and return to the PBS  31 , parallel components that cause interference are removed or reduced, and the orthogonal electric field components E X2X  and E Y2Y  are multiplexed. In the multiplexed light, signal light and excitation light are transmitted through the WDM coupler  48 . Converted light converted to another wavelength band (for example, L-band) is reflected by the WDM coupler  48 , and output from the wavelength converter  30 B as output light Out. 
     By the wavelength converter  30 B of the second embodiment, two polarizations orthogonal to each other are multiplexed in the optimum state, and temporal variation of output optical power is suppressed. 
     Third Embodiment 
       FIG. 6  is a schematic diagram of a wavelength converter  30 C of a third embodiment. In the third embodiment, a result of monitoring electric field components removed by optical elements  35 C and  36 C of a polarization diversity optical circuit  40 C is used for feedback control. 
     A first polarization controller (referred to as “polarization controller- 1 ” in the drawing)  53  is arranged between a first non-linear optical medium  33 C and the optical element  35 C for polarization correction. Electric output of a first photodetector  45  that detects the component removed by the optical element  35 C is fed back to a first controller (referred to as “controller- 1 ” in the drawing)  55 . The first controller  55  controls a polarization axis of the first polarization controller  53  so that polarization deviation detected by the first photodetector  45  is minimized. 
     A second polarization controller (referred to as “polarization controller- 2 ” in the drawing)  54  is arranged between a second non-linear optical medium  34 C and the optical element  36 C for polarization correction. Electric output of a second photodetector  46  that detects the component removed by the optical element  36 C is fed back to a second controller (referred to as “controller- 2 ” in the drawing)  56 . The second controller  56  controls a polarization axis of the second polarization controller  54  so that polarization deviation detected by the second photodetector  46  is minimized. 
     The first polarization controller  53  and the second polarization controller  54  may be, for example, a combination of a half-wave plate and a quarter-wave plate. The first controller and the second controller may be a microprocessor or a logic device such as a field programmable gate array (FPGA). The first controller  55  and the second controller  56  may adjust a polarization axes rotation mechanism of the half-wave plate and the quarter-wave plate on the basis of output of the first photodetector  45  and output of the second photodetector  46 . 
     By performing feedback control of the first polarization controller  53  and the second polarization controller  54 , power of E X2X  transmitted through the optical element  35 C and power of E Y2Y  transmitted through the optical element  36 C are maintained at the maximum. The feedback configuration of the third embodiment has the same effect as an active operation of the first polarization rotator  43  and the second polarization rotator  44  of the first embodiment. Other configurations are the same as those in the first embodiment, and the same components are denoted by the same reference signs and duplicate description will be omitted. 
     The wavelength converter  30 C of the third embodiment may multiplex polarizations orthogonal to each other with the maximum power even in a case where a polarization state deviates from the optimum state due to an influence of a disturbance or the like. Temporal variation of output optical power is suppressed, and output power may be maintained high. 
     Fourth Embodiment 
       FIG. 7  is a schematic diagram of a wavelength converter  30 D of a fourth embodiment. In the fourth embodiment, a result of monitoring electric field components removed by optical elements  35 D and  36 D of a polarization diversity optical circuit  40 D is used for feedback control. 
     In the polarization diversity optical circuit  40 D formed by a bidirectional loop, a polarization controller  51  is arranged between either one of the optical elements  35 D and  36 D for polarization correction and a non-linear optical medium  32 . Output of a first photodetector  45  connected to a reflection port of the optical element  35 D and output of a second photodetector  46  connected to a reflection port of the optical element  36 D are supplied to a controller  52 . 
     The controller  52  controls directions of polarization axes of optical elements constituting the polarization controller  51  so that detection results of the first photodetector  45  and the second photodetector  46  are minimized. The control may be to minimize the sum of the electric output of the first photodetector  45  and the electric output of the second photodetector  46 , or to apply a weighting coefficient to at least one of the two electric outputs. 
     By polarization control by the polarization controller  51 , an axis of a first polarization that circulates in the loop clockwise and returns to a PBS  31  is aligned with a transmission polarization axis of the optical element  36 D and is aligned with a reflection polarization axis of the PBS  31 . Furthermore, an axis of a second polarization that circulates in the loop counterclockwise and returns to the PBS  31  is aligned with a transmission polarization axis of the optical element  35 D and is aligned with a transmission polarization axis of the PBS  31 . 
     The feedback configuration of the fourth embodiment has the same effect as an active operation of the polarization rotator  47  of the second embodiment. Other configurations are the same as those in the second embodiment, and the same components are denoted by the same reference signs and duplicate description will be omitted. In the wavelength converter  30 D of the fourth embodiment, polarizations orthogonal to each other are multiplexed with high power even in a case where a polarization state deviates from the optimum state due to an influence of a disturbance or the like, and temporal variation of output optical power is suppressed.
     [Feedback Control Based on Monitor Light]   

       FIG. 8  is a flowchart of feedback control based on monitor light. This control flow is processing in the feedback loops of  FIGS. 6 and 7 . In the first controller  55  and the second controller  56  in  FIG. 6  or the controller  52  in  FIG. 7 , an upper limit Plimit of allowable monitor power and the upper limit number of times N of a control loop are set in advance (S 11 ). 
     The upper limit of the allowable monitor power is an upper limit of power detected by the first photodetector  45  and the second photodetector  46 . When polarization deviation components detected by the first photodetector  45  and the second photodetector  46  are less than or equal to Plimit, power of converted light that is multiplexed and output by the PBC  37  or the PBS  31  may be maintained at an allowable level or above. In the following, the configuration of  FIG. 7  will be described as an example, but the same control flow applies to the feedback control of  FIG. 6 . 
     During service, power Pmon of light incident from the optical element  35 D and the optical element  36 D is measured by the first photodetector  45  and the second photodetector  46 , respectively (S 12 ), and it is determined whether or not the measured power Pmon exceeds the upper limit Plimit (S 13 ). When the power Pmon of the measurement light does not exceed the upper limit Plimit of allowable polarization deviation (No in S 13 ), it is determined that a proper operation is performed, and the processing ends until the next monitoring timing. 
     When the power Pmon of the measurement light exceeds the upper limit Plimit (Yes in S 13 ), it means that polarization deviation has increased beyond an allowable range. In this case, the controller  52  adjusts a polarization axis of the polarization controller  51  (S 14 ). After adjusting the polarization axis of the polarization controller  51 , the power Pmon of the input light to the PDs is measured again (S 15 ). Next, it is determined whether the number of control loops is less than N (S 16 ). When the number of control loops reaches N (No in S 16 ), an alert is generated because the polarization deviation is not resolved in spite of a predetermined number of times of polarization control (S 18 ). 
     When the number of control loops is less than N (Yes in S 16 ), it is determined whether or not the power Pmon of the monitor light becomes smaller than the upper limit Plimit (S 17 ), and when Pmon&lt;Plimit holds, the processing ends. In a case where the power Pmon of the monitor light does not become smaller than the upper limit Plimit (No in S 17 ), the processing returns to Step S 14 , a direction of the polarization axis of the polarization controller  51  is controlled in an opposite direction, and Steps S 15  to S 17  are repeated within a range where the number of control loops is N or less. 
     By the feedback control of  FIG. 8 , even in a case where a polarization rotation angle of the non-linear optical medium deviates due to an influence of a disturbance or the like during service, control is performed in a direction in which polarization deviation is reduced, and temporal variation of output power of converted light is suppressed.
     [Confirmation of Effects]   

       FIG. 9  is an analysis model diagram for confirming effects of the configurations of the embodiments. The analysis model diagram of  FIG. 9  simulates a polarization diversity optical circuit without an optical element for polarization correction. Input light is demultiplexed into two polarizations by a PBS- 1 . A first polarization is subjected to polarization rotation by a PR- 1  in a path A, and a second polarization is subjected to polarization rotation by a PR- 2  in a path B. An electric field component E A  (E A =E AX +E AY ) of the light incident in the path A and an electric field component E B  (E B =E BX +E BY ) of the light incident in the path B are multiplexed and output by a PBS- 2 . 
     The paths A and B of the analysis model of  FIG. 9  are represented by Jones matrices of  FIG. 9 . In the paths A and B, matrices on the right side represent the PBS- 1 , matrices on the left side represent the PBS- 2 , and matrices in the middle represent the polarization rotators PR. In the matrices, “ER” indicates a polarization extinction ratio of the PBS. Subscripts “R” and “T” represent reflection and transmission of the PBS. 
     In the path A, the polarization reflected by the PBS- 1  is rotated by 90° in a negative direction by the PR- 1  and incident on a transmission port of the PBS- 2 . In the path B, the polarization transmitted through the PBS- 1  is rotated by 90° in a positive direction by the PR- 2  and incident on a reflection port of the PBS- 2 . 
     The light incident on the PBS- 2  in the path A may also include a component reflected by a fourth port P 4  of the PBS- 2 . The light incident on the PBS- 2  in the path B may also include a component transmitted through the fourth port P 4  of the PBS- 2 . The light output from the fourth port P 4  of the PBS- 2  is an interference light component unnecessary for wavelength conversion using the polarization diversity optical circuit. 
       FIG. 10  illustrates a change in output optical power due to polarization angle deviation in the analysis model of  FIG. 9 . A solid line is power of output light in which E A  and E B  are multiplexed by the PBS- 2 . In  FIG. 10 , the power is referred to as “PBS reflection output”, focusing on the path B. A broken line is output power of the fourth port P 4  of the PBS- 2 . The PBS reflection output is high when polarization angle deviation in a loop is small. However, as the polarization angle deviation increases, power of the target light decreases, and when the polarization angle deviation is π/2 radians, the PBS reflection output becomes zero. 
     On the other hand, the unnecessary component output from the fourth port P 4  increases as the polarization angle deviation in the loop increases, and becomes maximum at the polarization angle deviation of π/2 radians. An intersection of the solid line and the broken line, for example, the polarization angle deviation when the target output light and the unnecessary component observed at the fourth port P 4  are at the same level is π/4 radians. 
     According to the verification so far, even when a polarization state of a polarization diversity optical circuit is optimally adjusted at the time of shipment, it is estimated that polarization angle deviation of about 0.2 to 0.5 radians occurs due to an influence of a disturbance or the like during use. When a polarization maintaining (PM) non-linear fiber is used, the polarization angle deviation does not become so large. However, since a non-polarization maintaining (NON-PM) non-linear fiber that may produce required wavelength dispersion characteristics with good reproducibility is used, the polarization angle deviation becomes large. 
     In the embodiments, an optical element for polarization correction is arranged in front of the PBS- 2  to reduce the polarization angle deviation in the loop. To maximize the power of the target output light, it is desirable that the polarization angle deviation be made smaller than 0.2 radians (for example, power deviation be made less than 0.2 dB). 
     Focusing on the curve of the solid line in  FIG. 10 , it is difficult to monitor an output port and control the polarization angle deviation to less than 0.2 radians. Since a change in the unnecessary component of the broken line is larger, it is possible to control the polarization angle deviation more accurately by monitoring the output light from the fourth port P 4 . On the basis of this analysis result, in the configurations of  FIGS. 4 to 7 , the PBSs are used as the optical elements, and the photodetectors (PDs) are arranged at the reflection ports of the PBSs to detect the polarization angle deviation. 
       FIG. 11  illustrates a change in an interference electric field due to the polarization angle deviation in the loop when an extinction ratio of the PBS is 20 dB.  FIG. 12  illustrates a change in output optical power due to the polarization angle deviation in the loop when the extinction ratio of the PBS is 20 dB. In  FIG. 11 , a horizontal axis is an angle (radian) and a vertical axis is an output electric field normalized to “1”. An alternate long and short dash line is electric field components E AX  and E BY  of a target polarization extracted from the PBS- 2 . A solid line is unnecessary electric field components E AY  and E BX  output from the fourth port of the PBS- 2 . Subscripts A and B indicate the paths A and B of the analysis model of  FIG. 9 , and X and Y indicate polarization components. 
     In the PBS with a finite polarization extinction ratio, E AX  and E BX  interfere with each other, and E AY  and E BY  interfere with each other. When a polarization rotation angle deviates, a degree of interference, for example, a difference between the alternate long and short dash line and the solid line itself varies. The change in the degree of interference causes temporal variation of output optical power of the wavelength converter. 
     The electric field component E A  transmitted through the path A is more accurately represented by the Jones matrices in  FIG. 9  as follows. 
     
       
         
           
             
               
                 
                   
                     
                       E 
                       A 
                     
                     = 
                     
                       
                         [ 
                         
                           
                             
                               
                                 
                                   1 
                                   - 
                                   
                                     ER 
                                     
                                       T 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       2 
                                     
                                   
                                 
                               
                             
                             
                               0 
                             
                           
                           
                             
                               0 
                             
                             
                               
                                 
                                   ER 
                                   
                                     T 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     2 
                                   
                                 
                               
                             
                           
                         
                         ] 
                       
                       ⁢ 
                       
                         
                             
                             
                         
                         ⁡ 
                         
                           [ 
                           
                             
                               
                                 
                                   cos 
                                   ⁡ 
                                   
                                     ( 
                                     
                                       
                                         - 
                                         θ 
                                       
                                       - 
                                       
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                                         2 
                                       
                                     
                                     ) 
                                   
                                 
                               
                               
                                 
                                   - 
                                   
                                     sin 
                                     ⁡ 
                                     
                                       ( 
                                       
                                         
                                           - 
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                                         - 
                                         
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                                           2 
                                         
                                       
                                       ) 
                                     
                                   
                                 
                               
                             
                             
                               
                                 
                                   sin 
                                   ⁡ 
                                   
                                     ( 
                                     
                                       
                                         - 
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                                     ) 
                                   
                                 
                               
                               
                                 
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                                   ⁡ 
                                   
                                     ( 
                                     
                                       
                                         - 
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                           ] 
                         
                       
                       ⁢ 
                       
                         
                             
                             
                         
                         ⁡ 
                         
                           [ 
                           
                             
                               
                                 
                                   
                                     ER 
                                     
                                       R 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       1 
                                     
                                   
                                 
                               
                               
                                 0 
                               
                             
                             
                               
                                 0 
                               
                               
                                 
                                   
                                     1 
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                                       ER 
                                       
                                         R 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         1 
                                       
                                     
                                   
                                 
                               
                             
                           
                           ] 
                         
                       
                       ⁢ 
                       
                         E 
                         I 
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       E 
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                     = 
                     
                         
                       
                         
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                                     - 
                                     
                                       
                                         1 
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                                           ER 
                                           
                                             T 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             2 
                                           
                                         
                                       
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       ER 
                                       
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                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
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                                   ⁢ 
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                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
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                                       1 
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                                           ⁢ 
                                           
                                               
                                           
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                                         ⁢ 
                                         
                                             
                                         
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                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
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                                         ER 
                                         
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                                           ⁢ 
                                           
                                               
                                           
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                                   ⁢ 
                                   
                                     
                                       1 
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                                           R 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
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                                   ⁢ 
                                   sin 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
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                         ⁢ 
                         
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                           I 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     Here, E 1  is an input electric field to the polarization diversity optical circuit of the analysis model, ER R1  is a polarization extinction ratio of a reflection port of the PBS- 1 , θ is a polarization rotation amount of the first polarization rotator, and ER T2  is a polarization extinction ratio of the transmission port of the PBS- 2 . Similarly, the electric field component E B  transmitted through the path B is more accurately represented as follows. 
     
       
         
           
             
               
                 
                   
                     
                       E 
                       B 
                     
                     = 
                     
                       
                         [ 
                         
                           
                             
                               
                                 
                                   ER 
                                   
                                     R 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     2 
                                   
                                 
                               
                             
                             
                               0 
                             
                           
                           
                             
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                                       ⁢ 
                                       
                                           
                                       
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                       ⁢ 
                       
                         
                             
                             
                         
                         ⁡ 
                         
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                                   ⁡ 
                                   
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                                     ⁡ 
                                     
                                       ( 
                                       
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                       ⁢ 
                       
                         
                             
                             
                         
                         ⁡ 
                         
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                                         ⁢ 
                                         
                                             
                                         
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                                       T 
                                       ⁢ 
                                       
                                           
                                       
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                           ] 
                         
                       
                       ⁢ 
                       
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                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       E 
                       B 
                     
                     = 
                     
                         
                       
                         
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                                           ⁢ 
                                           
                                               
                                           
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                                             ⁢ 
                                             
                                                 
                                             
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                                       - 
                                       
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                                   ⁢ 
                                   
                                       
                                   
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                                       ER 
                                       
                                         R 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
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                                   ⁢ 
                                   
                                     
                                       ER 
                                       
                                         T 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         1 
                                       
                                     
                                   
                                   ⁢ 
                                   cos 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
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                                     - 
                                     
                                       
                                         1 
                                         - 
                                         
                                           ER 
                                           
                                             R 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             2 
                                           
                                         
                                       
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       ER 
                                       
                                         T 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         1 
                                       
                                     
                                   
                                   ⁢ 
                                   sin 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   θ 
                                 
                               
                             
                           
                           ] 
                         
                         ⁢ 
                         
                           E 
                           I 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   ] 
                 
               
             
           
         
       
     
     ER T1  is a polarization extinction ratio of a transmission port of the PBS- 1 , θ is a polarization rotation amount of the second polarization rotator, and RR R2  is a polarization extinction ratio of the reflection port of the PBS- 2 . Assuming that the input electric field E 1  is a linear polarization tilted by 45 degrees with respect to a polarization axis of the PBS- 1 , E A  and E B  are represented as follows. 
     
       
         
           
             
               
                 
                   
                     
                       
                         E 
                         A 
                       
                       = 
                     
                       
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       
                         1 
                         
                           2 
                         
                       
                       [ 
                       
                           
                       
                       ⁢ 
                       
                         
                           
                             
                               
                                 
                                   - 
                                   
                                     
                                       1 
                                       - 
                                       
                                         ER 
                                         
                                           T 
                                           ⁢ 
                                           
                                               
                                           
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                                           2 
                                         
                                       
                                     
                                   
                                 
                                 ⁢ 
                                 
                                   
                                     ER 
                                     
                                       R 
                                       ⁢ 
                                       
                                           
                                       
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                                 ⁢ 
                                 
                                     
                                 
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                               + 
                               
                                 
                                   
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                                         T 
                                         ⁢ 
                                         
                                             
                                         
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                                 ⁢ 
                                 
                                   
                                     1 
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                                 ⁢ 
                                 
                                     
                                 
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                                       ER 
                                       
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                                         ⁢ 
                                         
                                             
                                         
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                                 ⁢ 
                                 
                                   
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                                       ⁢ 
                                       
                                           
                                       
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                                 ⁢ 
                                 
                                     
                                 
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                       ⁢ 
                       
                           
                       
                       ] 
                     
                     ⁢ 
                     
                       E 
                       I 
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       E 
                       B 
                     
                     = 
                     
                         
                     
                     ⁢ 
                     
                       
 
                     
                     ⁢ 
                     
                       
                         
                           1 
                           
                             2 
                           
                         
                         ⁡ 
                         
                           [ 
                           
                             
                               
                                 
                                   
                                     
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                                             R 
                                             ⁢ 
                                             
                                                 
                                             
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                                     ⁢ 
                                     
                                       
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                                     ⁢ 
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                                   - 
                                   
                                     
                                       
                                         1 
                                         - 
                                         
                                           ER 
                                           
                                             R 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             2 
                                           
                                         
                                       
                                     
                                     ⁢ 
                                     
                                       
                                         1 
                                         - 
                                         
                                           ER 
                                           
                                             T 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             1 
                                           
                                         
                                       
                                     
                                     ⁢ 
                                     cos 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     θ 
                                   
                                 
                               
                             
                             
                               
                                 
                                   
                                     
                                       
                                         ER 
                                         
                                           R 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           2 
                                         
                                       
                                     
                                     ⁢ 
                                     
                                       
                                         ER 
                                         
                                           T 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           1 
                                         
                                       
                                     
                                     ⁢ 
                                     cos 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     θ 
                                   
                                   - 
                                   
                                     
                                       
                                         1 
                                         - 
                                         
                                           ER 
                                           
                                             R 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             2 
                                           
                                         
                                       
                                     
                                     ⁢ 
                                     
                                       
                                         ER 
                                         
                                           T 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           1 
                                         
                                       
                                     
                                     ⁢ 
                                     sin 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     θ 
                                   
                                 
                               
                             
                           
                           ] 
                         
                       
                       ⁢ 
                       
                         E 
                         I 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ] 
                 
               
             
           
         
       
     
       FIG. 11  is a plot of electric field intensity of each component with respect to angular deviation of polarization rotation (referred to as “polarization deviation angle”), assuming that ER R1 =ER T2 =ER T1 =ER R2 =0.01. 
     When the polarization deviation angle is θ=0, which is the optimum state, the unnecessary components E AY  and E BX  are not generated, but as θ increases, the unnecessary components E AY  and E BX  increase. 
     In  FIG. 12 , a solid line indicates strengthening interference |E A −E B |, and a broken line indicates weakening interference |E A +E B |. As in  FIG. 11 , the optimum state is obtained when θ=0, where there is no difference in optical power due to the interference state, but as θ increases, a difference in optical power due to the interference state increases. Therefore, an amplitude of optical power variation generated by the interference becomes large. 
       FIG. 13  illustrates extinction ratio dependency of output optical power variation due to the polarization angle deviation. A vertical axis indicates the output optical power variation (dB). The output optical power variation is represented by 10 log(|(E A −E B )/(E A +E B )|). A range of angular deviation of polarization rotation is set to 0 to π/4 radians. 
     The lower an extinction ratio, the greater the output optical power variation. An effective extinction ratio of the PBS, which is said to have a relatively high polarization extinction ratio, is 29 to 34 dB for transmission and 24 to 30 dB for reflection. Focusing on a broken line of 30 dB, the output optical power variation becomes remarkable when the angular deviation of the polarization rotation exceeds 0.5 radians. 
       FIG. 14  illustrates polarization deviation angle dependency of the output optical power variation due to the extinction ratio of the PBS. A horizontal axis is the polarization extinction ratio of the PBS. In a case where the polarization extinction ratio is 20 dB and the deviation angle of the polarization rotation is 0.5 radians, the output optical power variation exceeds 1 dB. Even in the same PBS, the output optical power variation may be reduced to less than half by setting the deviation angle of the polarization rotation to 0.2 radians. This tendency is true even in a case where the polarization extinction ratio of the PBS is high. Thus, in the embodiments, regardless of the types of the PBSs used in the polarization diversity optical circuit, the optical elements for polarization correction are used to reduce the angular deviation of the polarization rotation. 
       FIG. 15  is an analysis model diagram of an embodiment in which polarizers are inserted in front of an optical multiplexer (PBC). Input light is demultiplexed into two polarizations by a PBS- 1 . A first polarization is subjected to polarization rotation by a PR- 1  in a path A, a polarization axis of the first polarization is corrected by a first polarizer, and the first polarization is incident on a PBS- 2 . By the first polarizer, a main polarization axis of the first polarization incident on the PBS- 2  is aligned with a transmission polarization axis of the PBS- 2 . 
     A second polarization is subjected to polarization rotation by a PR- 2  in a path B, a polarization axis of the second polarization is corrected by a second polarizer, and the second polarization is incident on the PBS- 2 . By the second polarizer, a main polarization axis of the second polarization is aligned with a reflection polarization axis of the PBS- 2 . The first polarizer and the second polarizer may be PBSs. In this configuration, unnecessary E AY  is removed in the path A and unnecessary E BX  is removed in the path B. An electric field component E A  of the first polarization included in output light multiplexed by the PBS- 2  becomes a target electric field component E AX , and an electric field component E B  of the second polarization included in the output light becomes a target electric field component E BY . 
     In the analysis model of  FIG. 15 , the path A is represented by the Jones matrices as follows. 
     
       
         
           
             
               
                 
                   
                     
                       E 
                       A 
                     
                     = 
                     
                       
                         
                           [ 
                           
                             
                               
                                 
                                   
                                     1 
                                     - 
                                     
                                       ER 
                                       
                                         T 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         2 
                                       
                                     
                                   
                                 
                               
                               
                                 0 
                               
                             
                             
                               
                                 0 
                               
                               
                                 
                                   
                                     ER 
                                     
                                       T 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       2 
                                     
                                   
                                 
                               
                             
                           
                           ] 
                         
                         ⁡ 
                         
                           [ 
                           
                             
                               
                                 
                                   
                                     1 
                                     - 
                                     
                                       ER 
                                       Ta 
                                     
                                   
                                 
                               
                               
                                 0 
                               
                             
                             
                               
                                 0 
                               
                               
                                 
                                   
                                     ER 
                                     Ta 
                                   
                                 
                               
                             
                           
                           ] 
                         
                       
                       ⁢ 
                       
                         
                             
                             
                         
                         ⁡ 
                         
                           [ 
                           
                             
                               
                                 
                                   cos 
                                   ⁡ 
                                   
                                     ( 
                                     
                                       
                                         - 
                                         θ 
                                       
                                       - 
                                       
                                         π 
                                         2 
                                       
                                     
                                     ) 
                                   
                                 
                               
                               
                                 
                                   - 
                                   
                                     sin 
                                     ⁡ 
                                     
                                       ( 
                                       
                                         
                                           - 
                                           θ 
                                         
                                         - 
                                         
                                           π 
                                           2 
                                         
                                       
                                       ) 
                                     
                                   
                                 
                               
                             
                             
                               
                                 
                                   sin 
                                   ⁡ 
                                   
                                     ( 
                                     
                                       
                                         - 
                                         θ 
                                       
                                       - 
                                       
                                         π 
                                         2 
                                       
                                     
                                     ) 
                                   
                                 
                               
                               
                                 
                                   cos 
                                   ⁡ 
                                   
                                     ( 
                                     
                                       
                                         - 
                                         θ 
                                       
                                       - 
                                       
                                         π 
                                         2 
                                       
                                     
                                     ) 
                                   
                                 
                               
                             
                           
                           ] 
                         
                       
                       ⁢ 
                       
                         
                             
                             
                         
                         ⁡ 
                         
                           [ 
                           
                             
                               
                                 
                                   
                                     ER 
                                     
                                       R 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       1 
                                     
                                   
                                 
                               
                               
                                 0 
                               
                             
                             
                               
                                 0 
                               
                               
                                 
                                   
                                     1 
                                     - 
                                     
                                       ER 
                                       
                                         R 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         1 
                                       
                                     
                                   
                                 
                               
                             
                           
                           ] 
                         
                       
                       ⁢ 
                       
                         E 
                         I 
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       E 
                       A 
                     
                     = 
                     
                         
                       
                         
                           [ 
                           
                               
                           
                           ⁢ 
                           
                             
                               
                                 
                                   
                                     - 
                                     
                                       
                                         1 
                                         - 
                                         
                                           ER 
                                           
                                             T 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             2 
                                           
                                         
                                       
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       1 
                                       - 
                                       
                                         ER 
                                         Ta 
                                       
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       ER 
                                       
                                         R 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         1 
                                       
                                     
                                   
                                   ⁢ 
                                   sin 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   θ 
                                 
                               
                               
                                 
                                   
                                     
                                       1 
                                       - 
                                       
                                         ER 
                                         
                                           T 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           2 
                                         
                                       
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       1 
                                       - 
                                       
                                         ER 
                                         Ta 
                                       
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       1 
                                       - 
                                       
                                         ER 
                                         
                                           R 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           1 
                                         
                                       
                                     
                                   
                                   ⁢ 
                                   cos 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   θ 
                                 
                               
                             
                             
                               
                                 
                                   
                                     - 
                                     
                                       
                                         ER 
                                         
                                           T 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           2 
                                         
                                       
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       ER 
                                       Ta 
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       ER 
                                       
                                         R 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         1 
                                       
                                     
                                   
                                   ⁢ 
                                   cos 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   θ 
                                 
                               
                               
                                 
                                   
                                     - 
                                     
                                       
                                         ER 
                                         
                                           T 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           2 
                                         
                                       
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       ER 
                                       Ta 
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       1 
                                       - 
                                       
                                         ER 
                                         
                                           R 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           1 
                                         
                                       
                                     
                                   
                                   ⁢ 
                                   sin 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   θ 
                                 
                               
                             
                           
                           ⁢ 
                           
                               
                           
                           ] 
                         
                         ⁢ 
                         
                           E 
                           I 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     4 
                   
                   ] 
                 
               
             
           
         
       
     
     E 1  is an input electric field to the polarization diversity optical circuit of the analysis model, ER R1  is a polarization extinction ratio of a reflection port of the PBS- 1 , θ is a polarization rotation amount of the first polarization rotator, ER Ta  is a polarization extinction ratio of the added first polarizer, and ER T2  is a polarization extinction ratio of a transmission port of the PBS- 2 . Similarly, the path B is represented by the Jones matrices as follows. 
     
       
         
           
             
               
                 
                   
                     
                       E 
                       B 
                     
                     = 
                     
                       
                         [ 
                         
                           
                             
                               
                                 
                                   ER 
                                   
                                     R 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     2 
                                   
                                 
                               
                             
                             
                               0 
                             
                           
                           
                             
                               0 
                             
                             
                               
                                 
                                   1 
                                   - 
                                   
                                     ER 
                                     
                                       R 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       2 
                                     
                                   
                                 
                               
                             
                           
                         
                         ] 
                       
                       ⁢ 
                       
                         
                             
                             
                         
                         ⁡ 
                         
                           [ 
                           
                             
                               
                                 
                                   cos 
                                   ⁡ 
                                   
                                     ( 
                                     
                                       θ 
                                       + 
                                       
                                         π 
                                         2 
                                       
                                     
                                     ) 
                                   
                                 
                               
                               
                                 
                                   - 
                                   
                                     sin 
                                     ⁡ 
                                     
                                       ( 
                                       
                                         θ 
                                         + 
                                         
                                           π 
                                           2 
                                         
                                       
                                       ) 
                                     
                                   
                                 
                               
                             
                             
                               
                                 
                                   sin 
                                   ⁡ 
                                   
                                     ( 
                                     
                                       θ 
                                       + 
                                       
                                         π 
                                         2 
                                       
                                     
                                     ) 
                                   
                                 
                               
                               
                                 
                                   cos 
                                   ⁡ 
                                   
                                     ( 
                                     
                                       θ 
                                       + 
                                       
                                         π 
                                         2 
                                       
                                     
                                     ) 
                                   
                                 
                               
                             
                           
                           ] 
                         
                       
                       ⁢ 
                       
                         
                             
                             
                         
                         ⁡ 
                         
                           [ 
                           
                             
                               
                                 
                                   
                                     1 
                                     - 
                                     
                                       ER 
                                       
                                         T 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         1 
                                       
                                     
                                   
                                 
                               
                               
                                 0 
                               
                             
                             
                               
                                 0 
                               
                               
                                 
                                   
                                     ER 
                                     
                                       T 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       1 
                                     
                                   
                                 
                               
                             
                           
                           ] 
                         
                       
                       ⁢ 
                       
                         E 
                         I 
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       E 
                       B 
                     
                     = 
                     
                         
                       
                         
                           [ 
                           
                               
                           
                           ⁢ 
                           
                             
                               
                                 
                                   
                                     - 
                                     
                                       
                                         ER 
                                         
                                           R 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           2 
                                         
                                       
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       1 
                                       - 
                                       
                                         ER 
                                         
                                           T 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           1 
                                         
                                       
                                     
                                   
                                   ⁢ 
                                   sin 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   θ 
                                 
                               
                               
                                 
                                   
                                     - 
                                     
                                       
                                         1 
                                         - 
                                         
                                           ER 
                                           
                                             R 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             2 
                                           
                                         
                                       
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       1 
                                       - 
                                       
                                         ER 
                                         
                                           T 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           1 
                                         
                                       
                                     
                                   
                                   ⁢ 
                                   cos 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   θ 
                                 
                               
                             
                             
                               
                                 
                                   
                                     
                                       ER 
                                       
                                         R 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         2 
                                       
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       ER 
                                       
                                         T 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         1 
                                       
                                     
                                   
                                   ⁢ 
                                   cos 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   θ 
                                 
                               
                               
                                 
                                   
                                     - 
                                     
                                       
                                         1 
                                         - 
                                         
                                           ER 
                                           
                                             R 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             2 
                                           
                                         
                                       
                                     
                                   
                                   ⁢ 
                                   
                                     
                                       ER 
                                       
                                         T 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         1 
                                       
                                     
                                   
                                   ⁢ 
                                   sin 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   θ 
                                 
                               
                             
                           
                           ⁢ 
                           
                               
                           
                           ] 
                         
                         ⁢ 
                         
                           E 
                           I 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     5 
                   
                   ] 
                 
               
             
           
         
       
     
     E 1  is an input electric field to the polarization diversity optical circuit of the analysis model, ER T1  is a polarization extinction ratio of a transmission port of the PBS- 1 , θ is a polarization rotation amount of the second polarization rotator, ER Ra  is a polarization extinction ratio of the added second polarizer, and ER R2  is a polarization extinction ratio of a reflection port of the PBS- 2 . 
     Assuming that the input electric field E 1  is a linear polarization tilted by 45 degrees with respect to a polarization axis of the PBS- 1 , E A  and E B  are represented as follows. 
     
       
         
           
             
               
                 
                   
                     E 
                     A 
                   
                   = 
                   
                     
                       1 
                       
                         2 
                       
                     
                     ⁢ 
                     
                         
                       
                         
                           
                             [ 
                             
                                 
                             
                             ⁢ 
                             
                               
                                 
                                   
                                     
                                       
                                         - 
                                         
                                           
                                             1 
                                             - 
                                             
                                               ER 
                                               
                                                 T 
                                                 ⁢ 
                                                 
                                                     
                                                 
                                                 ⁢ 
                                                 2 
                                               
                                             
                                           
                                         
                                       
                                       ⁢ 
                                       
                                         
                                           1 
                                           - 
                                           
                                             ER 
                                             Ta 
                                           
                                         
                                       
                                       ⁢ 
                                       
                                         
                                           ER 
                                           
                                             R 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             1 
                                           
                                         
                                       
                                       ⁢ 
                                       sin 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       θ 
                                     
                                     + 
                                     
                                       
                                         
                                           1 
                                           - 
                                           
                                             ER 
                                             
                                               T 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               2 
                                             
                                           
                                         
                                       
                                       ⁢ 
                                       
                                         
                                           1 
                                           - 
                                           
                                             ER 
                                             Ta 
                                           
                                         
                                       
                                       ⁢ 
                                       
                                         
                                           1 
                                           - 
                                           
                                             ER 
                                             
                                               R 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               1 
                                             
                                           
                                         
                                       
                                       ⁢ 
                                       cos 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       θ 
                                     
                                   
                                 
                               
                               
                                 
                                   
                                     
                                       
                                         - 
                                         
                                           
                                             ER 
                                             
                                               T 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               2 
                                             
                                           
                                         
                                       
                                       ⁢ 
                                       
                                         
                                           ER 
                                           Ta 
                                         
                                       
                                       ⁢ 
                                       
                                         
                                           ER 
                                           
                                             R 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             1 
                                           
                                         
                                       
                                       ⁢ 
                                       cos 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       θ 
                                     
                                     - 
                                     
                                       
                                         
                                           ER 
                                           
                                             T 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             2 
                                           
                                         
                                       
                                       ⁢ 
                                       
                                         
                                           ER 
                                           Ta 
                                         
                                       
                                       ⁢ 
                                       
                                         
                                           1 
                                           - 
                                           
                                             ER 
                                             
                                               R 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               1 
                                             
                                           
                                         
                                       
                                       ⁢ 
                                       sin 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       θ 
                                     
                                   
                                 
                               
                             
                             ⁢ 
                             
                                 
                             
                             ] 
                           
                           ⁢ 
                           
                             E 
                             I 
                           
                           ⁢ 
                           
                             
 
                           
                           ⁢ 
                           
                             E 
                             B 
                           
                         
                         = 
                         
                             
                         
                         ⁢ 
                         
                           
 
                         
                         ⁢ 
                         
                           
                             
                               1 
                               
                                 2 
                               
                             
                             ⁡ 
                             
                               [ 
                               
                                 
                                   
                                     
                                       
                                         
                                           - 
                                           
                                             
                                               ER 
                                               
                                                 R 
                                                 ⁢ 
                                                 
                                                     
                                                 
                                                 ⁢ 
                                                 2 
                                               
                                             
                                           
                                         
                                         ⁢ 
                                         
                                           
                                             ER 
                                             Ra 
                                           
                                         
                                         ⁢ 
                                         
                                           
                                             1 
                                             - 
                                             
                                               ER 
                                               
                                                 T 
                                                 ⁢ 
                                                 
                                                     
                                                 
                                                 ⁢ 
                                                 1 
                                               
                                             
                                           
                                         
                                         ⁢ 
                                         sin 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         θ 
                                       
                                       - 
                                       
                                         
                                           
                                             1 
                                             - 
                                             
                                               ER 
                                               
                                                 R 
                                                 ⁢ 
                                                 
                                                     
                                                 
                                                 ⁢ 
                                                 2 
                                               
                                             
                                           
                                         
                                         ⁢ 
                                         
                                           
                                             1 
                                             - 
                                             
                                               ER 
                                               Ra 
                                             
                                           
                                         
                                         ⁢ 
                                         
                                           
                                             1 
                                             - 
                                             
                                               ER 
                                               
                                                 T 
                                                 ⁢ 
                                                 
                                                     
                                                 
                                                 ⁢ 
                                                 1 
                                               
                                             
                                           
                                         
                                         ⁢ 
                                         cos 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         θ 
                                       
                                     
                                   
                                 
                                 
                                   
                                     
                                       
                                         
                                           
                                             ER 
                                             
                                               R 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               2 
                                             
                                           
                                         
                                         ⁢ 
                                         
                                           
                                             ER 
                                             Ra 
                                           
                                         
                                         ⁢ 
                                         
                                           
                                             ER 
                                             
                                               T 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               1 
                                             
                                           
                                         
                                         ⁢ 
                                         cos 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         θ 
                                       
                                       - 
                                       
                                         
                                           
                                             1 
                                             - 
                                             
                                               ER 
                                               
                                                 R 
                                                 ⁢ 
                                                 
                                                     
                                                 
                                                 ⁢ 
                                                 2 
                                               
                                             
                                           
                                         
                                         ⁢ 
                                         
                                           
                                             1 
                                             - 
                                             
                                               ER 
                                               Ra 
                                             
                                           
                                         
                                         ⁢ 
                                         
                                           
                                             ER 
                                             
                                               T 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               1 
                                             
                                           
                                         
                                         ⁢ 
                                         sin 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         θ 
                                       
                                     
                                   
                                 
                               
                               ] 
                             
                           
                           ⁢ 
                           
                             E 
                             I 
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ] 
                 
               
             
           
         
       
     
       FIG. 16  is a plot of electric field intensity of each component with respect to angular deviation of polarization rotation (referred to as “polarization deviation angle”), assuming that ER R1 =ER T2 =ER T1 =ER R2 =ER Ta =ER Ra =0.01. 
     In  FIG. 16 , θ=0 is the optimum state, an unnecessary component E BX  is hardly generated, and a target electric field component E AX  is the maximum. As θ increases, the target electric field component E AX  decreases, but the unnecessary component E BX  is suppressed small. Focusing on the path B, an effect similar to that in  FIG. 16  may be obtained. 
       FIG. 17  is a plot of a ratio of optical power between a strengthening interference state |E A −E B | and a weakening interference state |E A +E B | with respect to an angular deviation amount. Output optical power variation (dB) on a vertical axis is represented by 10 log(|(E A −E B )/(E A +E B )|). A broken line is a characteristic of a conventional configuration in which no polarizer is added, and a solid line is a characteristic of the configuration of the embodiment in which the polarizers are arranged in front of the optical multiplexer (or PBC). 
     By adding the polarizers in a preceding stage of the PBS- 2  for multiplexing light, even in a case where angular deviation of polarization rotation increases, unnecessary interference components are removed and temporal power variation of output light may be suppressed. As in the configuration examples of  FIGS. 4 to 7 , in a case where a PBS is used as an optical element for polarization correction, a polarization deviation component may be monitored by using a relatively inexpensively available 3-port PBS, and a polarization axis of each polarization may be corrected. 
     The configurations of the embodiments and the analysis model of  FIG. 15  have meanings different from simply adding a filter to a wavelength converter. When the number of filters is simply increased, there is no dependency on the order or arrangement positions of filters, which are passive elements, so an optical element for polarization correction (polarizer or PBS) may be arranged before or after the PBS- 2 . However, even when the polarizer or PBS is arranged in a succeeding stage of the PBS- 2 , it is not possible to remove interference components (E AY  in E A  and E BX  in E B ) because interference has already occurred. An absolute value of the interference component may be small, but a ratio to target light does not change. 
     In the analysis model, calculation is performed by setting a non-diagonal component of the Jones matrix of the polarizer to zero. However, in reality, slight polarization rotation and depolarization occur, so even when polarizers with an extinction ratio of 20 dB are layered in two stages, it is not possible to obtain an extinction ratio of 40 dB. In contrast, in the embodiments, the polarizer (optical element for polarization correction) is arranged between the output end of the non-linear optical medium and the PBS or PBC for multiplexing light. By the polarizer, an interference component generated by deviation of a polarization rotation amount is removed and an axis of a polarization incident on the optical multiplexer is aligned with a polarization axis of the optical multiplexer, so that an effect equivalent to improving a polarization extinction ratio may be obtained. 
     Non-linear optical phenomena in the wavelength converters of the embodiments include common optical parametric amplification, including four-wave mixing, sum or difference frequency generation, harmonic generation, and optical parametric oscillation, and the number of excitation light beams and a relative relationship of polarization states of the excitation light and the signal light is optional. Regardless of which phenomenon is used, by arranging the optical element for polarization correction between the emission end of the non-linear optical medium and the optical multiplexer, even when a polarization rotation amount varies due to an influence of a disturbance or the like, temporal power variation of output light may be suppressed. 
     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.