Polarization state monitoring device, polarization state monitoring method, and non-transitory computer-readable medium

Abstract

A polarization state monitoring device according to the present disclosure specifies, from a target monitoring period that is a period during which the polarization state of an optical reception signal is to be monitored, a first calculation period set and a second calculation period set each being a set of a plurality of calculation periods, and calculates a polarization state vector representing the polarization state of the optical reception signal for each of one or more calculation periods included in the first calculation period set and each of one or more calculation periods included in the second calculation period set. A non-calculation period during which the polarization state vector is not calculated is present between two calculation periods adjacent to each other on a time axis. The second calculation period set is specified by shifting the first calculation period set on the time axis.

Description

Polarization state monitoring device, polarization state monitoring method, and non-temporary computer-readable medium

[0001] This disclosure relates to a polarization state monitoring device, a polarization state monitoring method, and a non-temporary computer-readable medium.

[0002] In optical communication systems, technologies have been developed to monitor changes in the polarization state of optical signals. For example, Non-Patent Document 1 discloses a technology for monitoring changes in the polarization state of an optically received signal modulated with QPSK (Quadrature Phase-Shift Keying). The system in Non-Patent Document 1 calculates multiple Jones vectors from the optically received signal and maps each calculated Jones vector to a point in Stokes space. Next, the system in Non-Patent Document 1 divides the multiple points obtained by the mapping into four groups and calculates the center point of each group. Furthermore, the system in Non-Patent Document 1 calculates a normal vector based on the four calculated center points. The system in Non-Patent Document 1 monitors the polarization state of the optically received signal based on the rotation speed of the normal vector calculated in this way.

[0003] Jingnan Li, Yangyang Fan, Zhenning Tao, Hisao Nakashima, and Takeshi Hoshida, “Polarization Change Monitor Based on Geometrical Analysis in Stokes Space”, 2021 European Conference on Optical Communication (ECOC), November 22, 2021 Bogdan Szafraniec, Todd S. Marshall, and Bernd Nebendahl, “Performance Monitoring and Measurement Techniques for Coherent Optical ”, Journal of Lightwave Technology, February 15, 2013, vol. 31, no. 4, pp. 648-663

[0004] The inventors of the present disclosure have found a new technique for monitoring the polarization state of an optical reception signal. The object of the present disclosure is to provide a new technique for monitoring the polarization state of an optical reception signal.

[0005] The polarization state monitoring device according to the present disclosure includes specifying means for specifying a first calculation period set and a second calculation period set, which are sets of a plurality of calculation periods, from a target monitoring period that is a period for monitoring the polarization state of an optical reception signal, and calculation means for calculating a polarization state vector representing the polarization state of the optical reception signal for each of one or more of the calculation periods included in the first calculation period set and each of one or more of the calculation periods included in the second calculation period set. There is a non-calculation period during which the polarization state vector is not calculated between two adjacent calculation periods on the time axis. The specifying means specifies the second calculation period set by shifting the first calculation period set on the time axis.

[0006] The polarization state monitoring method according to the present disclosure is executed by a computer. The method includes a specifying step of specifying a first calculation period set and a second calculation period set, which are sets of a plurality of calculation periods, from a target monitoring period that is a period for monitoring the polarization state of an optical reception signal, and a calculation step of calculating a polarization state vector representing the polarization state of the optical reception signal for each of one or more of the calculation periods included in the first calculation period set and each of one or more of the calculation periods included in the second calculation period set. There is a non-calculation period during which the polarization state vector is not calculated between two adjacent calculation periods on the time axis. In the specifying step, the second calculation period set is specified by shifting the first calculation period set on the time axis.

[0007] The non-temporary computer-readable medium relating to this disclosure stores a program. The program causes a computer to perform the following steps: a identification step of identifying a first set of calculation periods and a second set of calculation periods, which are sets of multiple calculation periods, from a target monitoring period, which is a period for which the polarization state of an optically received signal is to be monitored; and a calculation step of calculating a polarization state vector representing the polarization state of the optically received signal for each of the one or more calculation periods included in the first set of calculation periods and each of the one or more calculation periods included in the second set of calculation periods. Between two calculation periods adjacent to each other on the time axis, there exists a non-calculation period for which the polarization state vector is not calculated. In the identification step, the second set of calculation periods is identified by shifting the first set of calculation periods on the time axis.

[0008] This disclosure provides a new technique for monitoring the polarization state of an optically received signal.

[0009] This figure illustrates an optical transceiver system handled by the polarization state monitoring device of this disclosure. This figure illustrates an overview of the operation of the polarization state monitoring device. This figure illustrates multiple calculation period sets obtained from one target monitoring period. This is a block diagram illustrating the functional configuration of the polarization state monitoring device. This is a block diagram illustrating the hardware configuration of a computer that implements the polarization state monitoring device. This is a flowchart illustrating the processing flow performed by the polarization state monitoring device.

[0010] Embodiments of the present disclosure will be described in detail below with reference to the drawings. In each drawing, the same or corresponding elements are denoted by the same reference numerals, and redundant explanations are omitted as necessary for clarity. Unless otherwise specified, predetermined values ​​such as specified values ​​and thresholds are stored in advance in a storage device accessible from the device that uses those values. Furthermore, unless otherwise specified, the storage unit is composed of one or any number of storage devices.

[0011] [Embodiment 1] <Overview> Figure 1 is a diagram illustrating an optical transceiver system handled by the polarization state monitoring device of the present disclosure. The optical transceiver system 1 includes a transmitter 100, a receiver 200, and an optical communication channel 300. The receiver 200 receives an optical signal transmitted from the transmitter 100 via the optical communication channel 300. The optical communication channel 300 is a communication channel capable of transmitting optical signals and is configured using, for example, an optical fiber. Here, the optical signal transmitted from the transmitter 100 and the optical signal received by the receiver 200 are called the optical transmission signal 10 and the optical reception signal 20, respectively.

[0012] Communication between the transmitting device 100 and the receiving device 200 is performed, for example, as follows: The transmitting device 100 generates an optical transmission signal 10 from the data to be transmitted to the receiving device 200 (hereinafter referred to as the message). Specifically, the transmitting device 100 divides the message into multiple frames and generates a symbol sequence by encoding the data in each frame into symbols. Then, based on the symbol sequence, the transmitting device 100 generates a polarization-multiplexed optical transmission signal 10 by modulating the X polarization and Y polarization of the optical carrier wave, respectively.

[0013] The receiving device 200 reconstructs the message from the optically received signal 20. To do this, the receiving device 200 converts the optically received signal 20 into a digital signal. Furthermore, the receiving device 200 divides the digital signal into multiple frames and converts each frame into a symbol to obtain a symbol sequence. Then, the receiving device 200 decodes each symbol in the symbol sequence to obtain the message.

[0014] Figure 2 is a diagram illustrating an overview of the operation of the polarization state monitoring device 2000. Here, Figure 2 is a diagram intended to facilitate understanding of the overview of the polarization state monitoring device 2000, and the operation of the polarization state monitoring device 2000 is not limited to what is shown in Figure 2.

[0015] The polarization state monitoring device 2000 monitors the polarization state (SOP: State of Polarization) of the optically received signal 20. Specifically, in order to monitor changes in the polarization state of the optically received signal 20, the polarization state monitoring device 2000 identifies the polarization state of the optically received signal 20 for multiple calculation periods 80 included in the monitoring period.

[0016] The polarization state of the optical received signal 20 can be represented by a vector in Stokes space. Hereinafter, the vector in Stokes space representing the polarization state of the optical received signal 20 will be called the "polarization state vector (SOP vector)". The polarization state vector is a vector that starts at the origin of Stokes space and ends at a point in Stokes space that represents the polarization state of the optical received signal 20. Hereinafter, the point in Stokes space that represents the polarization state of the optical received signal 20 will be called the "polarization state point".

[0017] The monitoring period can be determined arbitrarily. For example, multiple monitoring periods can be obtained by dividing the period during which the polarization state monitoring device 2000 receives the optical reception signal 20 into predetermined length intervals.

[0018] Hereinafter, the monitoring period for which the polarization state monitoring device 2000 calculates the polarization state vector will be referred to as the target monitoring period. For example, suppose the polarization state monitoring device 2000 calculates the polarization state vector for the i-th monitoring period. In this case, the i-th monitoring period will be referred to as the target monitoring period. In Figure 2, the target monitoring period 70 is the first of several monitoring periods.

[0019] For example, the polarization state monitoring device 2000 treats multiple monitoring periods as target monitoring periods in chronological order. In this way, for each of the multiple monitoring periods, the polarization states of the multiple calculation periods 80 included in that monitoring period are identified sequentially.

[0020] The target monitoring period 70 includes multiple calculation periods 80. The length of the target monitoring period 70 is denoted by La. The length of each calculation period 80 is denoted by Lb. The length of the period between two adjacent calculation periods 80 (non-calculation period 85) is denoted by Lc. The number of calculation periods 80 derived from the target monitoring period 70 is represented by the integer part of La / (Lb+Lc). Using the floor function, the number of calculation periods 80 derived from the target monitoring period 70 can be expressed as floor(La / (Lb+Lc)). The period consisting of a calculation period 80 and the subsequent non-calculation period 85 is also called a subperiod.

[0021] Here, let's assume that the polarization state monitoring device 2000 calculates the polarization state vector only for the calculation period 80 shown in Figure 2. In this case, the data obtained for the optical received signal 20 during the non-calculation period 85 in Figure 2 is not used to calculate the polarization state vector. As a result, there is a possibility that the polarization state of the optical received signal 20 cannot be accurately determined.

[0022] Therefore, the polarization state monitoring device 2000 obtains multiple calculation period sets 90 from a single target monitoring period 70, as illustrated in Figure 3 below, and calculates a polarization state vector for each calculation period set 90. This reduces the amount of data that is not used for monitoring the polarization state of the optical received signal 20. As a result, the polarization state of the optical received signal 20 can be grasped more accurately.

[0023] Figure 3 illustrates multiple calculation period sets 90 obtained from a single target monitoring period 70. Figure 3 shows three calculation period sets 90 (calculation period set 90-1, calculation period set 90-2, and calculation period set 90-3) obtained from the target monitoring period 70.

[0024] Calculation period set 90-1 is a calculation period set 90 whose start time coincides with the start time of the target monitoring period 70. Calculation period set 90-2 is obtained by shifting the entire calculation period set 90-1 backward on the time axis by a predetermined length (hereinafter referred to as the step length). The step length is represented by Ld. Similarly, calculation period set 90-3 is obtained by shifting the entire calculation period set 90-1 backward on the time axis by Ld*2. In general, in the example in Figure 3, the i-th calculation period set 90 is obtained by shifting the first calculation period set 90 backward on the time axis by Ld*(i-1).

[0025] The polarization state monitoring device 2000 calculates the polarization state vector for each of at least two calculation period sets 90. To do this, for example, the polarization state monitoring device 2000 operates as follows.

[0026] First, the polarization state monitoring device 2000 identifies a first set of calculation periods 90 for the target monitoring period 70. Then, for one or more calculation periods 80 included in the first set of calculation periods 90, the polarization state monitoring device 2000 calculates a polarization state vector representing the polarization state of the optically received signal 20 during that calculation period 80.

[0027] Furthermore, the polarization state monitoring device 2000 identifies a second calculation period set 90 obtained by shifting the first calculation period set 90 on the time axis by a step length. Then, for one or more calculation periods 80 included in the second calculation period set 90, the polarization state monitoring device 2000 calculates a polarization state vector representing the polarization state of the optically received signal 20 during that calculation period 80.

[0028] <Example of Effects> According to the polarization state monitoring device 2000 of this embodiment, a plurality of calculation period sets 90 are identified from the target monitoring period 70. Calculation period sets 90 other than the first calculation period set 90 can be identified by shifting the first calculation period set 90 on the time axis. Then, for each calculation period set 90, a polarization state vector representing the polarization state of the optical received signal 20 in the calculation period 80 is calculated. In this way, the polarization state monitoring device 2000 provides a new technology for monitoring the polarization state of an optical received signal.

[0029] Here, between adjacent calculation periods 80, there exists a non-calculation period 85. However, the data obtained for the optically received signal 20 during the non-calculation period 85 is not used in the calculation of the polarization state vector. As a result, it may not be possible to accurately determine the polarization state of the optically received signal 20.

[0030] In this regard, according to the polarization state monitoring device 2000, the polarization state vector is calculated using multiple calculation period sets 90 obtained by shifting the first calculation period set 90. A non-calculation period 85 in one calculation period set 90 is highly likely to be included in the calculation period 80 in another calculation period set 90. Therefore, by monitoring the polarization state of the optical received signal 20 using multiple calculation period sets 90, more data obtained about the optical received signal 20 can be utilized compared to the case where the polarization state of the optical received signal 20 is monitored using only one calculation period set 90. Thus, the polarization state of the optical received signal 20 can be grasped more accurately.

[0031] The polarization state monitoring device 2000 of this embodiment will be described in more detail below.

[0032] <Example of Functional Configuration> Figure 4 is a block diagram illustrating the functional configuration of the polarization state monitoring device 2000. In the example in Figure 4, the polarization state monitoring device 2000 has a selection unit 2020 and a calculation unit 2040. The selection unit 2020 identifies a first calculation period set 90 from the target monitoring period 70. The selection unit 2020 also identifies a second calculation period set 90 by shifting the first calculation period set 90 by a predetermined step length.

[0033] The calculation unit 2040 calculates polarization state vectors for each calculation period 80 included in the first calculation period set 90. The calculation unit 2040 also calculates polarization state vectors for each calculation period 80 included in the second calculation period set 90.

[0034] <Example of Hardware Configuration> Each functional component of the polarization state monitoring device 2000 may be implemented by hardware that realizes each functional component (e.g., hardwired electronic circuits), or by a combination of hardware and software (e.g., a combination of electronic circuits and a program that controls them). The following will further explain the case in which each functional component of the polarization state monitoring device 2000 is implemented by a combination of hardware and software.

[0035] Figure 5 is a block diagram illustrating the hardware configuration of the computer 1000 that implements the polarization state monitoring device 2000. The computer 1000 is any computer. For example, the computer 1000 is a stationary computer such as a PC (Personal Computer) or a server machine. Alternatively, the computer 1000 is a portable computer such as a smartphone or a tablet terminal. Alternatively, the computer 1000 is an integrated circuit such as a SoC (System on Chip). The computer 1000 may be a dedicated computer designed to implement the polarization state monitoring device 2000, or it may be a general-purpose computer.

[0036] For example, by installing a predetermined application on computer 1000, the various functions of the polarization state monitoring device 2000 are realized on computer 1000. The above application consists of programs for realizing each functional component of the polarization state monitoring device 2000. The method of obtaining the above program is arbitrary. For example, the program can be obtained from a storage medium (such as a DVD (Digital Versatile Disk) or USB (Universal Serial Bus) memory) on which the program is stored. Alternatively, for example, the program can be obtained by downloading it from a server device that manages the storage device on which the program is stored.

[0037] Computer 1000 includes a bus 1020, a processor 1040, a memory 1060, a storage device 1080, an input / output interface 1100, and a network interface 1120. The bus 1020 is a data transmission path for the processor 1040, memory 1060, storage device 1080, input / output interface 1100, and network interface 1120 to send and receive data to and from each other. However, the method of connecting the processor 1040 and the other components is not limited to bus connection.

[0038] The processor 1040 is a variety of processors such as an MPU (Micro Processing Unit), CPU (Central Processing Unit), GPU (Graphics Processing Unit), DSP (Digital Signal Processor), or FPGA (Field-Programmable Gate Array). The memory 1060 is a main memory device implemented using RAM (Random Access Memory), etc. The storage device 1080 is an auxiliary storage device implemented using a hard disk, SSD (Solid State Drive), memory card, or ROM (Read Only Memory), etc.

[0039] The input / output interface 1100 is an interface for connecting the computer 1000 and an input / output device. For example, an input device such as a keyboard and an output device such as a display device are connected to the input / output interface 1100.

[0040] The network interface 1120 is an interface for connecting the computer 1000 to a network. This network may be a LAN (Local Area Network) or a WAN (Wide Area Network).

[0041] The storage device 1080 stores a program (a program for realizing the above-described application) for realizing each functional component of the polarization state monitoring device 2000. The processor 1040 reads out and executes this program in the memory 1060 to realize each functional component of the polarization state monitoring device 2000.

[0042] The polarization state monitoring device 2000 may be realized by one computer 1000 or by a plurality of computers 1000. In the latter case, the configurations of each computer 1000 do not need to be the same and can be different from each other.

[0043] The polarization state monitoring device 2000 may be realized integrally with the receiving device 200. In this case, each functional component of the polarization state monitoring device 2000 is mounted inside the receiving device 200. Thereby, the receiving device 200 also functions as the polarization state monitoring device 2000.

[0044] <Processing flow> FIG. 6 is a flowchart illustrating the processing flow executed by the polarization state monitoring device 2000. The processing shown in FIG. 6 is executed for each of two or more calculation period sets 90.

[0045] The specifying unit 2020 specifies the i-th calculation period set 90 from the target monitoring period 70 (S102). The calculating unit 2040 calculates a polarization state vector for each calculation period 80 included in the i-th calculation period set (S104).

[0046] <Calculation period set identification: S102> The identification unit 2020 identifies the i-th calculation period set from the target monitoring period 70 (S102). Here, it is assumed that both the length of the partial period and the step length are predetermined. A method for identifying the calculation period set 90 in the case where the length of the partial period or the step length is dynamically determined will be described later.

[0047] First, the identification unit 2020 identifies the first calculation period set 90. For example, the identification unit 2020 calculates the number of calculation periods 80 included in the first calculation period set 90, the start time of each calculation period 80, and the end time of each calculation period 80 for the first calculation period set 90, thereby identifying the first calculation period set 90. These values can be calculated, for example, using the following formula (1). M represents the number of calculation periods 80 included in the first calculation period set 90. Ts[1][j] represents the start time of the j-th calculation period 80 in the first calculation period set 90. Te[1][j] represents the end time of the j-th calculation period 80 in the first calculation period set 90. t1 represents the start time of the target monitoring period 70.

[0048] Next, the identification unit 2020 identifies the i-th calculation period set 90 based on the first calculation period set 90 (i>1). The i-th calculation period set 90 can be identified by shifting each calculation period 80 included in the first calculation period set 90 backward by Ld*(i - 1) on the time axis. That is, the i-th calculation period set 90 can be identified using the following formula (2).

[0049] The method of determining the step length Ld is arbitrary. For example, the step length Ld is determined based on the length Lb of the calculation period 80. As a specific example, it can be determined that Ld = w * Lb. w is an arbitrary real number satisfying 0 < w < 1. For example, it can be determined that w = 1 / 2. According to the method of appropriately determining the step length based on the length of the calculation period 80, it is possible to appropriately balance the accuracy of the polarization state of the optical reception signal 20 grasped by using the polarization state monitoring device 2000 and the cost (such as calculation time) of the processing by the polarization state monitoring device 2000.

[0050] <Calculation of polarization state vector: S104> The calculation unit 2040 calculates the polarization state vector for each calculation period set 90 for one or more calculation periods 80 included in the calculation period set 90 (S104). Hereinafter, several examples of the method for calculating the polarization state vector for each calculation period 80 included in one calculation period set 90 will be illustrated.

[0051] <<Example 1 of the method for calculating the polarization state vector>> For example, the calculation unit 2040 uses a polarimeter to calculate the polarization state vector for each calculation period 80. When a polarimeter is used to specify the polarization state vector, the polarimeter is pre-installed in the receiving device 200. The optical reception signal 20 is input to the polarimeter.

[0052] A polarimeter is a device that measures the polarization state of the input light. For example, when the optical reception signal 20 is input, the polarimeter outputs time-series data {S[t]} of the Stokes vector S representing the polarization state of the optical reception signal 20. Here, t represents time. The Stokes vector S is an enumeration of four types of Stokes parameters s0, s1, s2, and s3.

[0053] Stokes space is a three-dimensional space defined by three axes: the s1 axis, the s2 axis, and the s3 axis. Therefore, the polarization state vector v[t] of the optical received signal 20 at time t can be expressed as v[t]=(s1[t],s2[t],s3[t]) using the three Stokes parameters s1[t], s2[t], and s3[t] output from the polarimeter into which the optical received signal 20 at time t is input.

[0054] The calculation unit 2040 then acquires time-series data of Stokes vectors output from the polarimeter provided in the receiving device 200. Furthermore, for each calculation period 80, the calculation unit 2040 calculates a polarization state vector for that calculation period 80 using the Stokes vectors at one or more time points included in that calculation period 80.

[0055] For example, the calculation unit 2040 obtains a representative Stokes vector S[rj] for the j-th calculation period 80 (hereinafter referred to as calculation period j). The time point rj is, for example, a specific time point in calculation period j (such as the start or end of calculation period j). Furthermore, the calculation unit 2040 extracts Stokes parameters s1[rj], s2[rj], and s3[rj] from the representative Stokes vector S[rj]. Then, the calculation unit 2040 calculates the vector (s1[rj], s2[rj], s3[rj]) identified by these parameters as the polarization state vector for calculation period j.

[0056] In addition, for example, the calculation unit 2040 uses the Stokes vectors of multiple time points included in the calculation period j to calculate the statistical values ​​ss1[j], ss2[j], and ss3[j] of the Stokes parameters s1, s2, and s3, respectively. The calculation unit 2040 then identifies the vectors (ss1[j], ss2[j], ss3[j]) specified by the calculated statistical values ​​as the polarization state vector for the calculation period j.

[0057] There are various ways in which the calculation unit 2040 can acquire the Stokes vector output from the polarimeter. For example, the receiving device 200 transmits the Stokes vector output from the polarimeter to the polarization state monitoring device 2000. In this case, the calculation unit 2040 acquires the Stokes vector by receiving the Stokes vector transmitted from the polarization state monitoring device 2000. Alternatively, for example, the receiving device 200 stores the Stokes vector output from the polarimeter in a storage unit accessible from the polarization state monitoring device 2000. In this case, the calculation unit 2040 acquires the Stokes vector from this storage unit.

[0058] Furthermore, if only a representative Stokes vector is used to identify the polarization state vector, the receiving device 200 may be configured to transmit only the representative Stokes vector or to store only the representative Stokes vector in the memory unit.

[0059] <<Example 2 of Method for Identifying Polarization State Vectors>> The calculation unit 2040 acquires sample data representing the polarization state of the optical received signal 20 in each frame of the optical received signal 20. For example, the sample data is a Jones vector. Furthermore, the calculation unit 2040 obtains corresponding points for each sample data by mapping the sample data of each frame to points in Stokes space.

[0060] From the sample data at time t, we can obtain the Stokes vector S[t]=(s0[t],s1[t],s2[t],s3[t]). The calculation unit 2040 then obtains the points (s1[t],s2[t],s3[t]) as corresponding points to the sample data at time t.

[0061] The Stokes vector S[t] at time t can be calculated using sample data at time t as follows: z_x[t] represents the x-polarization component of the optical received signal 20 at time t, as shown by the sample data at time t. z_y[t] represents the y-polarization component of the optical received signal 20 at time t, as shown by the sample data at time t. * represents the conjugate. j represents the imaginary unit. Here, both z_x[t] and z_y[t] are complex numbers.

[0062] Here, n is denoted as the number of frames included in the optical received signal 20 during each calculation period 80. In this case, the identification unit 2020 calculates one polarization state vector for each calculation period 80 based on the n corresponding points obtained for the n frames included in that calculation period 80. Hereafter, the set of corresponding points obtained for multiple frames included in the calculation period 80 is called the corresponding point group for that frame. The corresponding point group for each frame contains n corresponding points.

[0063] For example, the specific unit 2020 performs the following processing every 80 calculation periods. First, the specific unit 2020 calculates a plane in Stokes space that fits the corresponding point cloud obtained for the target calculation period 80 (in other words, it fits the n corresponding points included in the corresponding point cloud). The specific unit 2020 identifies a vector whose endpoint is the intersection of the normal vector of that plane and the Poincaré sphere, and whose starting point is the origin of Stokes space, as the polarization state vector.

[0064] Here, the normal vector is determined to pass through the origin of Stokes space. Furthermore, there can be two normal vectors passing through a particular point in a given plane. Therefore, a rule is predetermined for selecting which of these two normal vectors to use for calculating the polarization state point.

[0065] Here, a plane in Stokes space can be represented as follows: A, B, C, and D are real numbers.

[0066] Therefore, for example, the identification unit 2020 calculates a plane that fits the corresponding point cloud by using the corresponding point cloud to calculate A, B, C, and D that satisfy equation (4). For example, the identification unit 2020 substitutes each corresponding point included in the corresponding point cloud into equation (4) and performs singular value decomposition (SVD). As a result, A, B, C, and D in equation (4) are calculated, and a plane that fits the corresponding point cloud is calculated.

[0067] Here, when QAM (Quadrature Amplitude Modulation) modulation is used, the corresponding point cloud is located within the lens-shaped region which is a combination of the region defined by equation (5) and the region defined by equation (6). In equation (6), φ and r represent the phase angle and normalized amplitude of the optical signal, respectively. Also, φ and r satisfy 0 <= φ < 2π and 0 <= r <= 1, respectively.

[0068] The method for deriving equation (5) is disclosed in Non-Patent Document 2. In the derivation of equation (5), for the H polarization state, the point with the maximum amplitude is selected. For the V polarization state, all points within the unit circle on the imaginary plane are considered. This is represented by the following Jones vector.

[0069] Equation (5) is obtained by converting the Jones vector in equation (7) to a Stokes vector.

[0070] In contrast, in the derivation of equation (6), for the V polarization state, the point with the maximum amplitude is selected, and for the H polarization state, any point within the unit circle on the imaginary plane is considered. This is represented by the following Jones vector.

[0071] Equation (6) is obtained by converting the Jones vector in equation (8) to a Stokes vector.

[0072] From equations (5) and (6), the above lens-shaped region is a point-symmetric region centered at the origin. Therefore, the plane that fits the corresponding point cloud distributed within this lens-shaped region passes through the origin. Thus, we can assume that D=0 in equation (4).

[0073] Therefore, the specific unit 2020 may calculate a plane that fits the corresponding point cloud by calculating A, B, and C that satisfy the following conditions using the corresponding point cloud. A, B, and C are real numbers.

[0074] Furthermore, when using equation (9), just as when using equation (4), a plane that fits the corresponding point cloud can be calculated using methods such as singular value decomposition.

[0075] The method for calculating a single polarization state vector based on multiple corresponding points is not limited to the method of using a plane that fits the multiple corresponding points as described above. For example, the specific unit 2020 may use the method disclosed in Non-Patent Document 1 to calculate a single corresponding point based on multiple corresponding points, and then calculate a polarization state vector with the calculated corresponding point as the endpoint.

[0076] There are various ways in which the specific unit 2020 can acquire sample data. For example, the receiving device 200 is configured to generate sample data for each frame of the optical reception signal 20 and transmit the generated sample data to the polarization state monitoring device 2000. In this case, the specific unit 2020 acquires sample data by receiving the sample data transmitted from the polarization state monitoring device 2000. Alternatively, for example, the receiving device 200 is configured to store the sample data generated for each frame of the optical reception signal 20 in a storage unit accessible from the polarization state monitoring device 2000. In this case, the specific unit 2020 acquires sample data from this storage unit.

[0077] Furthermore, if the specific unit 2020 acquires sample data only for representative frames, the receiving device 200 may be configured to generate sample data only for representative frames.

[0078] <Regarding the representation of time> The length of periods such as the length La of the target monitoring period 70 and the length Lb of the calculation period 80 may be expressed using normal time units such as seconds or minutes, or they may be expressed as the number of time-series data included in the period. In the latter case, for example, the length Lb of the calculation period 80 may be expressed as the number of Stokes vectors or sample data included in that calculation period 80.

[0079] When the length of a period is expressed by the number of data points, each point in time can be represented by the index of the time-series data corresponding to that point in time. For example, suppose the Stokes vectors included in a certain calculation period 80 are S

[0010] to S

[0025] . In this case, the start and end points of this calculation period 80 are 10 and 25, respectively. The length Lb of this calculation period 80 is 16.

[0080] <How to use the polarization state vectors> The method of using the polarization state vectors calculated for each calculation period set 90 is arbitrary. The following is an example of how to use the polarization state vectors.

[0081] For example, the polarization state monitoring device 2000 calculates data representing the rate of change of the polarization state vector (hereinafter referred to as change rate data) for each calculation period set 90. The method for calculating the change rate data for one calculation period set 90 is as follows.

[0082] First, the calculation unit 2040 calculates the polarization state rotation angle for each of the multiple pairs of polarization state vectors. The polarization state rotation angle is represented by the angle between the two polarization state vectors.

[0083] A pair of polarization state vectors consists of adjacent polarization state vectors in a time series of polarization state vector data (multiple polarization state vectors arranged in a time series). For example, from the time series of polarization state vector data (v[1],v[2],v[3],...,v[m]), a set of polarization state vector pairs {(v[1],v[2]),(v[2],v[3]),...,(v[m-1],v[m])} can be obtained.

[0084] The calculation unit 2040 calculates the polarization state rotation angle for each pair included in this set. Specifically, the calculation unit 2040 calculates the polarization state rotation angle a[1] for the polarization state vector pair (v[1],v[2]), the polarization state rotation angles a[2], ... for the polarization state vector pair (v[2],v[3]), and the polarization state rotation angle a[m-1] for the polarization state rotation angle pair (v[m-1],v[m]). This gives time-series data of polarization state rotation angles (a[1],a[2],...,a[m-1]) for the set of polarization state vector pairs {(v[1],v[2]),(v[2],v[3]),...,(v[m-1],v[m])}. Here, the k-th polarization state rotation angle a[k] is calculated by determining the angle between the polarization state vectors v[k] and v[k+1].

[0085] The calculation unit 2040 calculates change rate data for each of the multiple calculation period sets 90. Then, the polarization state monitoring device 2000 uses the calculated change rate data to calculate change rate data for the target monitoring period 70. For example, the polarization state monitoring device 2000 uses the statistical value (e.g., the average value) of the change rate data calculated for each of the multiple calculation period sets 90 as the change rate data for the target monitoring period 70.

[0086] Here, there are various methods for calculating change rate data using the polarization state rotation angle. Below, we will explain, as examples, a method called the adjacent method and a method called the head-to-tail method.

[0087] <<Adjacent Method>> When using the adjacent method, the calculation unit 2040 calculates the rotation angle per unit time (i.e., rotation speed) for each of the calculated polarization state rotation angles. The calculation unit 2040 then calculates the statistical value (e.g., average value) of the calculated rotation speeds as the change rate data for the calculation period set 90.

[0088] The calculation unit 2040 calculates the rotational speed w[k] from the rotational angle a[k] of each polarization state. As a result, time-series data of rotational speed (w[1], w[2],..., w[m-1]) is obtained. The calculation unit 2040 calculates the average value wo of the rotational speeds w[1], w[2],..., w[m-1] as the change rate data for the calculation period set 90.

[0089] The rotational velocity w[k] can be calculated by dividing the polarization state rotation angle a[k] by the observation time. The observation time is expressed as La+Lb, which is the sum of the length Lb of the calculation period 80 and the length Lc of the non-calculation period 85. Therefore, w[k]=a[k] / (Lb+Lc).

[0090] <<Head-to-Tail Method>> When using the head-to-tail method, the calculation unit 2040 calculates the rate of change data for the calculation period set 90 from the polarization state vector calculated for the first calculation period 80 of the calculation period set 90 (hereinafter referred to as the first polarization state vector) and the polarization state vector calculated for the last calculation period 80 of the calculation period set 90 (hereinafter referred to as the last polarization state vector). That is, the calculation unit 2040 calculates the angle between the first polarization state vector and the last polarization state vector, and calculates the rate of change data by dividing this angle by the observation time.

[0091] Using the notation mentioned above, the leading polarization state vector and the trailing polarization state vector are represented by v[1] and v1[m], respectively. The calculation unit 2040 calculates the angle between v[1] and v[m], and calculates the rate of change data wo by dividing the calculated angle by the observation time. The observation time is represented by (Lb+Lc)*(m-1).

[0092] <Method for dynamically determining the length of a partial period> In the method for specifying the calculation period set 90 described above, the length of the partial period (i.e., the length of the calculation period 80 Lb and the length of the non-calculation period 85 Lc) is predetermined. However, the specification unit 2020 may dynamically determine the length of the partial period. Hereinafter, the length of the partial period will be expressed as Lp, where Lp = Lb + Lc.

[0093] First, the specifying unit 2020 sets a provisional value (e.g., the value of Lp used in the processing for the previous target monitoring period 70) for the length Lp of the partial period. Then, the polarization state monitoring device 2000 calculates the polarization state rotation angle for each of three cases: a first case where the length of the partial period is Lp, a second case where the length of the partial period is Lp2 which is longer than Lp, and a third case where the length of the partial period is Lp3 which is shorter than Lp. For example, the specifying unit 2020 calculates the polarization state angle in the first calculation period set 90 for each of these three cases. Then, the specifying unit 2020 determines whether the length Lp of the partial period is appropriate by comparing the polarization state rotation angles calculated for these three cases (the specific method of determination will be described later).

[0094] Suppose it is determined that the length of the partial period is appropriate. In this case, the specifying unit 2020 does not change the length Lp of the partial period. Therefore, the specifying unit 2020 uses the current length Lp of the partial period to perform calculations such as calculating the polarization state vector for each calculation period set 90 after the second calculation period set 90.

[0095] On the other hand, when it is determined that the length of the partial period is not appropriate, the specifying unit 2020 increases or decreases the length of the partial period. The specifying unit 2020 uses the changed length of the partial period to perform calculations such as calculating the polarization state vector for each calculation period set 90.

[0096] Specifically, when it is determined that the length of the partial period is longer than the appropriate length, the specifying unit 2020 shortens the length of the partial period. For example, the specifying unit 2020 sets a value obtained by multiplying the current length Lp of the partial period by a predetermined number d1 (i.e., d1*Lp) as the new length of the partial period. d1 is a real number satisfying 0 < d1 < 1. In this case, the lengths of the new calculation period 80 and the new non-calculation period 85 are d1*Lb and d1*Lc, respectively.

[0097] If the length of the subperiod is determined to be shorter than the appropriate length, the identification unit 2020 lengthens the subperiod. For example, the identification unit 2020 sets the new subperiod length to a value obtained by multiplying the current subperiod length Lp by a predetermined number d2 (i.e., d2*Lp), where d2 is a real number greater than 1. In this case, the length of the new calculation period 80 and the length of the new non-calculation period 85 are d2*Lb and d2*Lc, respectively.

[0098] The specific unit 2020 may change the length of a partial period only once or multiple times for a single target monitoring period 70. In the latter case, for example, the specific unit 2020 repeats the above-mentioned changes to the length of the partial period until the length of the partial period becomes appropriate.

[0099] Whether the length of the partial period is appropriate can be determined, for example, as follows. First, the identification unit 2020 determines whether the length of the partial period is longer than the appropriate length. To this end, the identification unit 2020 calculates the polarization state rotation angle for each calculation period 80 in the first calculation period set 90 in the case where the length of the partial period is Lp. Furthermore, the identification unit 2020 calculates the polarization state rotation angle for each calculation period 80 in the first calculation period set 90 in the case where the length of the partial period is Lp2.

[0100] The specific unit 2020 calculates an index value I1 defined by the following formula (10). a1[k] represents the kth polarization state rotation angle calculated for the case where the length of the calculation period 80 is Lp. a2[k] represents the kth polarization state rotation angle calculated for the case where the length of the calculation period 80 is Lp2. m1 represents the total number of calculation periods 80 in the case where the length of the calculation period 80 is Lp. m2 represents the total number of calculation periods 80 in the case where the length of the calculation period 80 is Lp2.

[0101] For example, the identification unit 2020 determines whether the index value I1 is greater than or equal to a predetermined threshold α. The threshold α satisfies 0 < α < 1. If the index value I1 is greater than or equal to the threshold α, the identification unit 2020 determines that the length of the partial period is longer than an appropriate length.

[0102] Here, we assume that the value of the threshold α is predetermined. The method for determining the value of α is arbitrary. For example, the higher the accuracy of the calculation by the polarization state monitoring device 2000 (the smaller the calculation error), the closer α will be set to a value close to 0. Alternatively, α may be determined based on the length of the partial period.

[0103] If the index value I1 is less than the threshold α, the identification unit 2020 further determines whether the length of the partial period is shorter than the appropriate length. To this end, the identification unit 2020 calculates the polarization state rotation angle for each calculation period 80 in the first calculation period set 90 for the case where the length of the partial period is Lp3. Then, the identification unit 2020 calculates the index value I2 defined by the following equation (11). a3[k] represents the kth polarization state rotation angle calculated for the case where the length of the subperiod is Lp3. m3 represents the total number of calculation periods 80 in the case where the length of the calculation period 80 is Lp3.

[0104] For example, the identification unit 2020 determines whether the index value I2 is greater than or equal to a predetermined threshold β. The threshold β satisfies 0 < β < 1. The identification unit 2020 determines that the length of the partial period is appropriate if the index value I2 is greater than or equal to the threshold β. On the other hand, if the index value I2 is less than the threshold β, the identification unit 2020 determines that the length of the partial period is shorter than the appropriate length.

[0105] Assume the threshold value β is predetermined. The method for determining the value of β is arbitrary. For example, the value of β can be determined in the same way as the value of α.

[0106] When the length of a partial period is determined dynamically in this way, the specific unit 2020 may determine the step length based on the dynamically determined length of the calculation period 80. For example, as described above, the specific unit 2020 calculates the step length by multiplying the length of the calculation period 80 by a predetermined coefficient w.

[0107] <Method for dynamically determining the step length Ld> The identification unit 2020 may dynamically determine the step length Ld. In this case, for example, the identification unit 2020 calculates the step length Ld in the target monitoring period 70 using change rate data calculated for monitoring periods prior to the target monitoring period 70. Then, the identification unit 2020 uses the calculated Ld to identify each calculation period set 90.

[0108] Here, we will explain how to determine the step length Ld based on the rate of change data. For example, the specific unit 2020 determines the step length Ld by decreasing the value of the step length Ld from the value used in the previous target monitoring period 70, based on the history of the rate of change data.

[0109] For example, the specific unit 2020 determines the step length Ld by comparing a predetermined threshold with the previous rate of change data. Here, the "previous rate of change data" is the rate of change data calculated for the previous target monitoring period 70 (for example, the monitoring period that is one step prior to the monitoring period currently being treated as the target monitoring period 70).

[0110] For example, if the previous change rate data is above a threshold, the specific unit 2020 shortens the step length used in the current target monitoring period 70 compared to the step length used previously. For example, a value obtained by multiplying the previously used step length by a predetermined value greater than 0 and less than 1 is used.

[0111] On the other hand, suppose the previous change rate data is below a threshold. In this case, the specific unit 2020 uses the same step length that was used last time.

[0112] The threshold may be fixed or determined from the history of change rate data. In the latter case, for example, the threshold may be a statistical value of the change rate data calculated for each of the predetermined target monitoring periods 70 in the past.

[0113] Here, by shortening the step length when the rate of change data exceeds a threshold, the step length can be shortened when a large change occurs in the rate of change data. When the step length is set shorter, more data is used to calculate the polarization state vector, etc. Therefore, by shortening the step length when a large change occurs in the rate of change data, the polarization state vector, etc. can be calculated more accurately when a large change occurs in the polarization state. In other words, the polarization state vector, etc. can be calculated more accurately when a large change occurs in the polarization state.

[0114] Here, it is conceivable that repeated changes to the step length could result in the step length deviating significantly from its initial value. Therefore, for example, the specific unit 2020 may reset the step length to its initial value if the situation where "the previous change rate data is below the threshold" continues for a certain period of time. For example, if the specific unit 2020 determines that "the previous change rate data is below the threshold" for a predetermined number of consecutive times, it sets the initial value of the step length to be used for the current target monitoring period 70. By doing this, the step length can be reset to its normal value when it is predicted that the polarization state has returned to its normal state.

[0115] Here, the longer the step length, the less data is used to calculate the polarization state vector and other parameters. And when the amount of data used to calculate the polarization state vector and other parameters decreases, the computing resources used to calculate them also decrease. For this reason, when the polarization state is in its normal state, the amount of computing resources used can be reduced by resetting the step length to its initial value.

[0116] When the step length is determined dynamically in this way, the specific unit 2020 may determine the length of the partial period based on the dynamically determined step length. For example, as mentioned above, the step length Ld can be defined as Ld = w * Lb. Therefore, the specific unit 2020 calculates the length of the calculation period 80 as Lb = Ld / w.

[0117] Furthermore, the specific unit 2020 determines the length of the non-calculation period 85 based on the length of the calculation period 80. For example, suppose the ratio between the length of the calculation period 80 and the length of the non-calculation period 85 is predetermined. In this case, the specific unit 2020 calculates the length of the non-calculation period 85 based on this ratio and the length of the calculation period 80 calculated based on the step length.

[0118] The specific unit 2020 may independently and dynamically determine the length of the partial period and the step length. In this case, the specific unit 2020 determines the length of the partial period using the aforementioned index values ​​I1 and I2. The specific unit 2020 also determines the step length using the history of the rate of change data.

[0119] Although the present disclosure has been described above with reference to embodiments, the present disclosure is not limited to the embodiments described above. Various modifications to the structure and details of the present disclosure can be made as can be understood by those skilled in the art within the scope of the present disclosure. Furthermore, each embodiment can be combined with other embodiments as appropriate.

[0120] Each drawing is merely illustrative to illustrate one or more embodiments. Each drawing may be associated with one or more other embodiments, rather than being associated with only one specific embodiment. As those skilled in the art will understand, various features or steps described with reference to any one drawing can be combined with features or steps shown in one or more other drawings, for example, to create embodiments not explicitly shown or described. Not all features or steps shown in any one drawing to illustrate an exemplary embodiment are necessarily required, and some features or steps may be omitted. The order of steps described in any of the drawings may be changed as appropriate.

[0121] In the above examples, the program includes a set of instructions (or software code) that, when loaded into a computer, cause the computer to perform one or more of the functions described in the embodiments. The program may be stored on a non-temporary computer-readable medium or a physical storage medium. Examples, but not limited to, include random-access memory (RAM), read-only memory (ROM), flash memory, solid-state drive (SSD) or other memory technologies, CD-ROM, digital versatile disc (DVD), Blu-ray® disc or other optical disc storage, magnetic cassette, magnetic tape, magnetic disk storage or other magnetic storage devices. The program may be transmitted over a temporary computer-readable medium or a communication medium. Examples, but not limited to, include temporary computer-readable medium or a communication medium that includes electrical, optical, acoustic or other forms of propagating signals.

[0122] Some or all of the above embodiments may also be described as follows, but are not limited to the following: (Note 1) A polarization state monitoring device comprising: a identification means for identifying a first set of calculation periods and a second set of calculation periods, which are sets of a plurality of calculation periods, from a target monitoring period, which is a period for which the polarization state of an optically received signal is to be monitored; and a calculation means for calculating a polarization state vector representing the polarization state of the optically received signal for each of the one or more calculation periods included in the first set of calculation periods and each of the one or more calculation periods included in the second set of calculation periods, wherein there exists a non-calculation period for which the polarization state vector is not calculated between two calculation periods adjacent to each other on the time axis, and the identification means identifies the second set of calculation periods by shifting the first set of calculation periods on the time axis. (Note 2) The polarization state monitoring device according to Note 1, wherein the identification means identifies the first set of calculation periods based on the start time of the first set of calculation periods, a predetermined length of the calculation period, and a predetermined length of the non-calculation period. (Note 3) The polarization state monitoring device according to Note 2, wherein the identifying means identifies the second calculation period set by shifting the first calculation period set by a predetermined step length. (Note 4) The polarization state monitoring device according to Note 1, wherein the identifying means calculates a first statistical value of the rotation angle of the polarization state vector for the calculation period set in which the sum of the length of the calculation period and the length of the non-calculation period is a first predetermined value; calculates a second statistical value of the rotation angle of the polarization state vector for the calculation period set in which the sum of the length of the calculation period and the length of the non-calculation period is greater than the first predetermined value; and determines the length of the calculation period and the length of the non-calculation period by determining whether or not to increase the length of the calculation period and the length of the non-calculation period based on the first statistical value and the second statistical value.(Note 5) The polarization state monitoring device according to Note 1, wherein the identifying means calculates a first statistical value of the rotation angle of the polarization state vector for a set of calculation periods where the sum of the length of the calculation period and the length of the non-calculation period is a first predetermined value; calculates a second statistical value of the rotation angle of the polarization state vector for a set of calculation periods where the sum of the length of the calculation period and the length of the non-calculation period is a second predetermined value smaller than the first predetermined value; and determines the length of the calculation period and the length of the non-calculation period by determining whether or not to reduce the length of the calculation period and the length of the non-calculation period based on the first statistical value and the second statistical value. (Note 6) The polarization state monitoring device according to Note 1, wherein the calculation means calculates first change rate data representing the rate of change of the polarization state of the optically received signal in the first calculation period set using a plurality of polarization state vectors calculated for the first calculation period set, calculates second change rate data representing the rate of change of the polarization state of the optically received signal in the second calculation period set using a plurality of polarization state vectors calculated for the second calculation period set, and calculates third change rate data representing the rate of change of the polarization state of the optically received signal in the target monitoring period using the first change rate data and the second change rate data. (Note 7) The polarization state monitoring device according to Note 6, wherein the identification means calculates the third change rate data for each of the plurality of target monitoring periods, and reduces the amount by which the first calculation period set is shifted in order to identify the second calculation period set when the third change rate data calculated for the monitoring target period prior to the current target monitoring period is greater than or equal to a threshold. (Note 8) The polarization state monitoring device according to Note 7, wherein the identifying means sets the statistical value of the third change rate data calculated for each of the multiple monitoring target periods prior to the current target monitoring period as the threshold.(Note 9) A polarization state monitoring method performed by a computer, comprising: a identification step of identifying a first set of calculation periods and a second set of calculation periods, which are sets of a plurality of calculation periods, from a target monitoring period, which is a period for which the polarization state of an optically received signal is to be monitored; and a calculation step of calculating a polarization state vector representing the polarization state of the optically received signal for each of the one or more calculation periods included in the first set of calculation periods and each of the one or more calculation periods included in the second set of calculation periods, wherein there is a non-calculation period for which the polarization state vector is not calculated between two calculation periods that are adjacent to each other on the time axis, and the second set of calculation periods is identified by shifting the first set of calculation periods on the time axis in the identification step. (Note 10) A non-temporary computer-readable medium that stores a program that causes a computer to perform the following steps: a identification step of identifying a first set of calculation periods and a second set of calculation periods, which are sets of multiple calculation periods, from a target monitoring period, which is a period for which the polarization state of an optically received signal is to be monitored; and a calculation step of calculating a polarization state vector representing the polarization state of the optically received signal for each of the one or more calculation periods included in the first set of calculation periods and each of the one or more calculation periods included in the second set of calculation periods, wherein there is a non-calculation period between two calculation periods adjacent to each other on the time axis for which the polarization state vector is not calculated, and the second set of calculation periods is identified by shifting the first set of calculation periods on the time axis in the identification step.

[0123] Some or all of the elements (e.g., configuration and function) described in Appendices 2 to 8 that are dependent on Appendice 1 {e.g., apparatus} may also be dependent on Appendice 9 {e.g., method} and Appendice 10 {e.g., non-temporary computer-readable medium} in the same way as in Appendices 2 to 8. Some or all of the elements described in any appendice may be applied to various hardware, software, recording means, systems, and methods for recording software.

[0124] 1 Optical Transmitting and Receiving System 10 Optical Transmitting Signal 20 Optical Receiving Signal 70 Target Monitoring Period 80 Calculation Period 85 Non-Calculation Period 90 Calculation Period Set 100 Transmitter 200 Receiving Device 300 Optical Communication Channel 1000 Computer 1020 Bus 1040 Processor 1060 Memory 1080 Storage Device 1100 Input / Output Interface 1120 Network Interface 2000 Polarization Status Monitoring Device 2020 Specific Unit 2040 Calculation Unit

Claims

1. A polarization state monitoring device comprising: a identification means for identifying a first set of calculation periods and a second set of calculation periods, which are sets of multiple calculation periods, from a target monitoring period, which is a period for which the polarization state of an optically received signal is to be monitored; and a calculation means for calculating a polarization state vector representing the polarization state of the optically received signal for each of the one or more calculation periods included in the first set of calculation periods and each of the one or more calculation periods included in the second set of calculation periods, wherein there exists a non-calculation period between two calculation periods adjacent to each other on the time axis for which the polarization state vector is not calculated, and the identification means identifies the second set of calculation periods by shifting the first set of calculation periods on the time axis.

2. The polarization state monitoring device according to claim 1, wherein the identifying means identifies the first calculation period set based on the start time of the first calculation period set, a predetermined length of the calculation period, and a predetermined length of the non-calculation period.

3. The polarization state monitoring device according to claim 2, wherein the identifying means identifies the second calculation period set by shifting the first calculation period set by a predetermined step length.

4. The polarization state monitoring device according to claim 1, wherein the identifying means calculates a first statistical value of the rotation angle of the polarization state vector for a set of calculation periods where the sum of the length of the calculation period and the length of the non-calculation period is a first predetermined value; calculates a second statistical value of the rotation angle of the polarization state vector for a set of calculation periods where the sum of the length of the calculation period and the length of the non-calculation period is greater than the first predetermined value; and determines the length of the calculation period and the length of the non-calculation period by determining whether or not to increase the length of the calculation period and the length of the non-calculation period based on the first statistical value and the second statistical value.

5. The polarization state monitoring device according to claim 1, wherein the identifying means calculates a first statistical value of the rotation angle of the polarization state vector for a set of calculation periods where the sum of the length of the calculation period and the length of the non-calculation period is a first predetermined value; calculates a second statistical value of the rotation angle of the polarization state vector for a set of calculation periods where the sum of the length of the calculation period and the length of the non-calculation period is a second predetermined value smaller than the first predetermined value; and determines the length of the calculation period and the length of the non-calculation period by determining whether or not to reduce the length of the calculation period and the length of the non-calculation period based on the first statistical value and the second statistical value.

6. The polarization state monitoring device according to claim 1, wherein the calculation means calculates first change rate data representing the rate of change of the polarization state of the optically received signal in the first calculation period set using a plurality of polarization state vectors calculated for the first calculation period set; calculates second change rate data representing the rate of change of the polarization state of the optically received signal in the second calculation period set using a plurality of polarization state vectors calculated for the second calculation period set; and calculates third change rate data representing the rate of change of the polarization state of the optically received signal in the target monitoring period using the first change rate data and the second change rate data.

7. The polarization state monitoring device according to claim 6, wherein the identifying means calculates a third change rate data for each of the plurality of target monitoring periods, and when the third change rate data calculated for a monitoring target period prior to the current target monitoring period is greater than or equal to a threshold, the magnitude by which the first calculation period set is shifted in order to identify the second calculation period set is reduced.

8. The polarization state monitoring device according to claim 7, wherein the identifying means sets the statistical value of the third change rate data calculated for each of the plurality of monitoring target periods prior to the current target monitoring period as the threshold.

9. A polarization state monitoring method performed by a computer, comprising: a identification step of identifying a first set of calculation periods and a second set of calculation periods, which are sets of multiple calculation periods, from a target monitoring period, which is a period for which the polarization state of an optically received signal is to be monitored; and a calculation step of calculating a polarization state vector representing the polarization state of the optically received signal for each of the one or more calculation periods included in the first set of calculation periods and each of the one or more calculation periods included in the second set of calculation periods, wherein there is a non-calculation period for which the polarization state vector is not calculated between two calculation periods that are adjacent to each other on the time axis, and the second set of calculation periods is identified by shifting the first set of calculation periods on the time axis in the identification step.

10. A non-temporary computer-readable medium that stores a program causing a computer to perform the following steps: a identification step of identifying a first set of calculation periods and a second set of calculation periods, which are sets of multiple calculation periods, from a target monitoring period, which is a period for which the polarization state of an optically received signal is to be monitored; and a calculation step of calculating a polarization state vector representing the polarization state of the optically received signal for each of the one or more calculation periods included in the first set of calculation periods and each of the one or more calculation periods included in the second set of calculation periods, wherein there is a non-calculation period between two calculation periods adjacent to each other on the time axis for which the polarization state vector is not calculated, and the second set of calculation periods is identified by shifting the first set of calculation periods on the time axis in the identification step.

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