A polarization dependent loss estimation method and system

Abstract

The application provides a polarization-dependent loss estimation method and system. In a transmitting end, a guard bandwidth is arranged between each subcarrier in a subcarrier multiplexing (SCM) signal; a frequency domain pilot signal (FPT) is inserted in the guard bandwidth in two polarization directions; the frequencies of the two FPTs are different and cannot be symmetrical to each other; and the PDL is estimated in a receiving end. The application has the advantages that the application can monitor the PDL of an optical fiber link, is simple and practical, and achieves very good technical effects.

Description

Technical Field

[0001] This invention relates to the field of optical communication, and in particular to a method and system for estimating polarization-dependent loss. Background Technology

[0002] With the continuous emergence of new technologies such as 5G / 6G, IoT, and cloud computing, current networks are evolving towards greater flexibility and dynamics to meet the demands of these emerging applications. New network architectures based on software-defined networking can flexibly allocate network resources according to user needs and channel parameters, achieving automated dynamic network management. In this process, to more accurately allocate system operational margins, it is necessary to accurately characterize channel performance and parameter uncertainties. Among various link parameters, polarization-dependent loss (PDL) caused by polarization-dependent devices in the link is the main impairment in polarization multiplexing (PM) systems. PDL mainly exists in components such as reconfigurable optical add-drop multiplexers (ROADMs). PDL will cause a difference in the signal-to-noise ratio (SNR) between two polarization branches, and this difference depends on the principal axis direction of the PDL. Since the polarization state of light varies randomly in the optical fiber, PDL has a random impact on channel performance. Therefore, additional operational margins need to be allocated to ensure channel availability. To reduce unnecessary system margins, the uncertainty caused by PDL can be reduced through real-time estimation of PDL.

[0003] Monitoring the polarization-dependent polarization (PDL) is an effective way to reduce the uncertainty introduced by the PDL. Amplitude-modulated pilot signals can be used to monitor the accumulated PDL from the transmitter to the receiver. However, this method monitors the orthogonal polarization power ratio introduced by the PDL, not the PDL itself. The two are only equal when the polarization direction of the signal is aligned with the principal axis of the PDL. Alternatively, the PDL can be estimated using the Stokes vector in Stokes space, but this requires additional complexity for Jones-Stokes space conversion. Furthermore, the signal-to-noise ratio (SNR) distribution caused by the PDL can be used to estimate the PDL of ROADM, but in practical links, other factors besides the PDL affect the SNR; therefore, the accuracy of this PDL estimation needs further investigation. To accurately estimate the PDL of polarization-dependent components in the link, Zhiping Jiang proposed inserting an APT detector after each PDL device and directly estimating the PDL from the APT power of the two polarizations. However, this scheme is not feasible in current networks and requires redeployment of the APT detector. A simple and practical PDL monitoring scheme is urgently needed in practical fiber optic links. Summary of the Invention

[0004] This invention provides a polarization-dependent loss estimation method. At the transmitting end, in the subcarrier multiplexed signal (SCM), a guard bandwidth is set between each subcarrier. A frequency-domain pilot signal (FPT) is inserted into the guard bandwidth in both polarization directions. The frequencies of the two FPTs are different and cannot be symmetrical. At the receiving end, the polarization-dependent loss (PDL) is estimated.

[0005] As a further improvement of the present invention, the FPT is placed within the signal spectrum band or outside the signal spectrum band.

[0006] As a further improvement to the present invention, the transmitted signal is represented as follows:

[0007]

[0008] Where E x / y (t) and S x / y (t) represents the X / Y polarized transmitted signal and the SCM signal, respectively; A, ω1 and ω2 represent the amplitude and angular frequency of the FPT, respectively; j is the imaginary unit; and t is time.

[0009] The received signal is represented as:

[0010]

[0011] Where R x / y (t), Δω, and n(t) represent the received X / Y polarization signals, the frequency offset between the transmitting laser and the local oscillator light, the carrier phase noise, and the additive white Gaussian noise, respectively; M SOP The polarization rotation and PDL matrix are represented as follows:

[0012]

[0013] Where R(θ, ε, η) is the polarization rotation matrix, θ is the polarization azimuth rotation angle, and ε and η are the phase rotation angles; D(ρ) is the PDL matrix. ρ is the splitting ratio between two consecutive polarizations, expressed in decibels (PDL) as Γ(dB) = 10log 10 (1+ρ) / (1-ρ), m 11 m 12 m 21 and m 22 It is matrix M SOP Elements; the frequency offset Δω is estimated by comparing the frequency difference between the received signal FPT and the transmitted signal FPT. After estimating the frequency offset, the frequency of FPT is shifted to zero, and a low-pass filter is used to extract FPT in the two polarization states respectively:

[0014]

[0015]

[0016] Where H{·} represents the low-pass filter operation, yielding fiber polarization rotation and an estimate of the PDL matrix:

[0017]

[0018] The constant term A in the formula is eliminated by power normalization. It can be seen from equation (6) that, in the presence of PDL, the combined matrix of polarization rotation and PDL can still be accurately estimated using FPT. After obtaining the estimated matrix W of polarization rotation and PDL, the eigenvalue of the product of matrix W and its transpose conjugate matrix is ​​calculated. The PDL can be estimated from this eigenvalue. Let T = W·W H ,in{·} H For the transpose conjugate operation, let λ1 and λ2 be the eigenvalues ​​of matrix T, then the estimated PDL is expressed as: PDL es =|10lg(λ1 / λ2)|.

[0019] As a further improvement of the present invention, the low-pass filter operation is implemented using a sliding window averaging method to reduce computational complexity.

[0020] This invention also provides a polarization-dependent loss estimation system, including a transmitter DSP, a root-raised cosine filter, a digital-to-analog converter (DAC), a dual-polarization IQ modulator, a dual-polarization coherent optical receiver, an analog-to-digital converter (ADC), and a digital signal processing module. The transmitter DSP generates independent random bit sequences loaded onto each subcarrier, which are mapped to signals of the desired modulation format. The DAC then shapes the signals. After shaping, the signals undergo row subcarrier multiplexing (SCM), with a reserved guard bandwidth between adjacent subcarriers to prevent overlap. One free-pole transducer (FPT) is inserted into each of the two polarization guard bandwidths; the FPTs of the two polarizations have different and asymmetrical frequencies. The DAC converts the signals into analog signals and drives the dual-polarization IQ modulator. The modulated signal is loaded onto a standard single-mode optical fiber and transmitted to the receiver. The signal light and local oscillator light are received by the dual-polarization coherent optical receiver. The received signals are converted into digital signals by the DAC, and the resulting electrical signals are processed by the digital signal processing module to estimate the polarization-dependent loss (PDL) of the optical fiber link.

[0021] As a further improvement of the present invention, the modulation format includes, but is not limited to, QASK, 16QAM, and 64QAM.

[0022] As a further improvement of the present invention, the digital signal processing module first compensates for the IQ delay and imbalance of the receiving device. Then, it estimates the frequency offset by calculating the deviation between the frequencies of the two FPTs transmitted at their respective frequencies. Subsequently, it extracts all FPTs on the two polarizations to obtain the combined matrix W of the estimated polarization rotation and PDL. A new matrix T is obtained by multiplying W and its transpose and conjugate. The eigenvalues ​​of the new matrix T are calculated, and the PDL of the optical fiber link is directly estimated from the eigenvalues ​​of the new matrix T.

[0023] As a further improvement of the present invention, all FPTs on the two polarizations are extracted by a low-pass filter.

[0024] As a further improvement of the present invention, the extracted FPT is normalized to obtain the estimated signal polarization rotation and PDL combination matrix W.

[0025] The beneficial effects of this invention are: this invention can monitor the PDL of optical fiber links, is simple and practical, and has achieved very good technical results. Attached Figure Description

[0026] Figure 1(a) and Figure 1(b) are the spectrum diagrams of the transmitted signal, where Figure 1(a) is the spectrum diagram of the X-polarized signal; and Figure 1(b) is the spectrum diagram of the Y-polarized signal.

[0027] Figure 2 This is a schematic diagram of a coherent optical communication model;

[0028] Figure 3 This is a flowchart of PDL estimation. Detailed Implementation

[0029] This invention discloses a polarization-dependent loss estimation method and system, which is a PDL estimation scheme applicable to optical fiber links in coherent optical communication systems.

[0030] Figures 1(a) and (b) show the transmitted signal spectrum of one embodiment of the present invention. In the subcarrier multiplexing (SCM) signal, a guard bandwidth is provided between each subcarrier. A frequency-domain pilot signal (FPT) is inserted into the guard bandwidth in both polarization directions. To avoid crosstalk of the FPT caused by transmitter-end impairment, the frequencies of the two FPTs are different and cannot be symmetrical. This scheme is also applicable to single-carrier systems, in which case the FPT is placed outside the signal spectrum band. The transmitted signal can be represented as:

[0031]

[0032] Where E x / y (t), S x / y(t) represents the X / Y polarized transmitted signal and the SCM signal, respectively; A, ω1, and ω2 represent the amplitude and angular frequency of the FPT, respectively; j is the imaginary unit, and t is time. The added power of the FPT depends on the pilot signal power ratio (PSR).

[0033] Considering random polarization rotation and polarization correlation loss, the received signal can be expressed as:

[0034]

[0035] Where R x / y (t), Δω, and n(t) represent the received X / Y polarization signals, the frequency offset between the transmitting laser and the local oscillator light, the carrier phase noise, and the additive white Gaussian noise, respectively; M SOP The polarization rotation and PDL matrix can be represented as:

[0036]

[0037] Where R(θ, ε, η) is the polarization rotation matrix, θ is the polarization azimuth rotation angle, and ε and η are the phase rotation angles; D(ρ) is the PDL matrix. ρ is the splitting ratio between two consecutive polarizations. The PDL, expressed in decibels (dB), can be represented as Γ(dB) = 10log 10 (1+ρ) / (1-ρ), m 11 m 12 m 21 and m 22 It is matrix M SOP Elements. The frequency offset Δω is estimated by comparing the frequency difference between the received signal FPT and the transmitted signal FPT. After estimating the frequency offset, the frequency of FPT is shifted to zero, and a low-pass filter is used to extract FPT in both polarization states:

[0038]

[0039]

[0040] Where H{·} denotes the low-pass filter operation. Correspondingly, the fiber polarization rotation and the estimation of the PDL matrix can be obtained:

[0041]

[0042] The constant term A in the formula is eliminated by power normalization. It can be seen from equation (6) that, even with the presence of PDL, the combined matrix of polarization rotation and PDL can still be accurately estimated using FPT. After obtaining the estimated matrix W of polarization rotation and PDL, the eigenvalues ​​of the product of matrix W and its transpose conjugate matrix are calculated. These eigenvalues ​​can then be used to estimate PDL. Let T = W·WH ,in{·} H For the transpose conjugate operation, let λ1 and λ2 be the eigenvalues ​​of matrix T, then the estimated PDL can be expressed as: PDL es =|10lg(λ1 / λ2)|. Through the above process, without the need for complex adaptive equilibration, PDL can be directly estimated from FPT, thus realizing simple and practical PDL monitoring.

[0043] The low-pass filter operation is implemented using a sliding window averaging method to reduce computational complexity.

[0044] Figure 2 This embodiment of the invention provides a model for coherent optical communication, applicable to digital subcarrier multiplexing scenarios. Details are as follows:

[0045] The transmitter's DSP generates independent random bit sequences for each subcarrier, which are mapped to the desired modulation format signal, including but not limited to QASK, 16QAM, and 64QAM. Root-raised cosine filters are used to shape the signals. The shaped signals are then subjected to row subcarrier multiplexing (SCM), with the number of subcarriers for each polarization designed according to requirements. A protection bandwidth is reserved between adjacent subcarriers in the SCM signal. This protection bandwidth is set based on the subcarrier baud rate and the shaping filter, ensuring that adjacent subcarriers do not overlap. One free-pole transducer (FPT) is inserted into each of the two polarization protection bandwidths. The FPTs for the two polarizations have different and asymmetrical frequencies. The spectrum of the generated signal is shown in Figure 1. The signal is then converted to an analog signal via digital-to-analog conversion and used to drive a dual-polarization optical IQ modulator. The modulated signal is loaded onto a standard single-mode fiber and transmitted to the receiver. The signal light and local oscillator light are received by a dual-polarization coherent optical receiver. The received signal is converted to a digital signal by an analog-to-digital converter, and the resulting electrical signal is processed by a subsequent offline digital signal processing module. The digital signal processing module flow is as follows: Figure 3 As shown, the digital signal processing module first compensates for the IQ delay and imbalance of the receiving device. Then, it estimates the frequency offset by calculating the deviation between the frequencies of the two transmitted FPTs. Subsequently, a low-pass filter extracts all FPTs on both polarizations, and the extracted FPTs are normalized to obtain the combined matrix W of the estimated polarization rotation and PDL. A new matrix T is obtained by multiplying W and its transpose and conjugate. The eigenvalues ​​of matrix T are calculated, and the PDL of the fiber optic link is directly estimated from the eigenvalues ​​of matrix T. The flowchart of this process is shown below. Figure 3 As shown.

[0046] This invention also applies to single-carrier systems. For single-carrier systems, the process is the same as for subcarrier systems, except that there is no carrier multiplexing and demultiplexing process. The FPT can be placed inside or outside the signal spectrum. When the FPT is placed outside the signal spectrum, signal interference can be avoided and estimation accuracy can be improved, but some spectrum resources will be wasted.

[0047] This invention can monitor the PDL of fiber optic links, is simple and practical, and has achieved very good technical results.

[0048] The above description, in conjunction with specific preferred embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such modifications and substitutions should be considered within the scope of protection of the present invention.

Claims

1. A method for estimating polarization-dependent loss, characterized in that: At the transmitting end, in the subcarrier multiplexed SCM signal, a guard bandwidth is set between each subcarrier; in both polarization directions, a frequency domain pilot signal FPT is inserted into the guard bandwidth; the frequencies of the two FPTs are different and cannot be symmetrical to each other; At the receiving end, the PDL is estimated; First, the IQ delay and imbalance of the receiving device are compensated. Then, the frequency offset is estimated by calculating the deviation between the frequencies of the transmitted FPTs of the two FPTs. Subsequently, all FPTs on the two polarizations are extracted to obtain the estimated polarization rotation and PDL combination matrix. W ; Depend on W The product of its transpose and conjugate yields a new matrix. T Calculate the new matrix T The eigenvalues ​​are derived from the new matrix. T The eigenvalues ​​can be used to directly estimate the PDL of the fiber optic link.

2. The polarization-dependent loss estimation method according to claim 1, characterized in that: The FPT can be placed within or outside the signal spectrum band.

3. The polarization-dependent loss estimation method according to claim 1 or 2, characterized in that: The transmitted signal is represented as: ， in and These represent the X / Y polarized transmitted signal and the SCM signal, respectively. , and Let j represent the amplitude and angular frequency of FPT, respectively, where j is the imaginary unit and t is time. The received signal is represented as: ， in , , n(t) represents the received X / Y polarization signal, the frequency offset between the transmitting laser and the local oscillator light, the carrier phase noise, and the additive white Gaussian noise, respectively. The polarization rotation and PDL matrix are represented as follows: ， in, For polarization rotation matrix, The polarization azimuth rotation angle. The phase rotation angle; For PDL matrix, , The splitting ratio between two polarizations, expressed in decibels (PDL). m 11 m 12 m 21 and m 22 It is a matrix M SOP element; Frequency offset is estimated by comparing the frequency difference between the received signal FPT and the transmitted signal FPT. After estimating the frequency offset, the frequency of the FPT is shifted to zero, and a low-pass filter is used to extract the FPT in the two polarization states respectively: ， ， in, This represents the operation of a low-pass filter, yielding fiber polarization rotation and an estimate of the PDL matrix: ， constant term in the formula A By eliminating the power normalization, it can be seen from equation (6) that, in the presence of PDL, the combined matrix of polarization rotation and PDL can still be accurately estimated using FPT; then the estimated matrix of polarization rotation and PDL is obtained. Then, calculate the matrix. The eigenvalues ​​of the product ... ,in For the transpose conjugate operation, let and For matrix If the eigenvalues ​​are such that the estimated PDL is expressed as: .

4. The polarization-dependent loss estimation method according to claim 3, characterized in that: The low-pass filter operation is implemented using a sliding window averaging method to reduce computational complexity.

5. A polarization-dependent loss estimation system, characterized in that: The system includes a transmitter DSP, a root-raised cosine filter, a digital-to-analog converter (DAC) module, a dual-polarization IQ modulator, a dual-polarization coherent optical receiver, an analog-to-digital converter (ADC), and a digital signal processing module. The transmitter DSP generates independent random bit sequences loaded onto each subcarrier, which are mapped to the desired modulation format signal. The DAC then shapes the signals. After shaping, the signals undergo row subcarrier multiplexing (SCM), with a reserved guard bandwidth between adjacent subcarriers to prevent overlap. One free-pole transducer (FPT) is inserted into each of the two polarization guard bandwidths; the FPTs of the two polarizations have different and asymmetrical frequencies. The DAC module converts the signal into an analog signal, which drives the dual-polarization IQ modulator. The modulated signal is then loaded onto a standard single-mode fiber and transmitted to the receiver. The signal light and local oscillator light are received by the dual-polarization coherent optical receiver. The received signal is converted into a digital signal by the DAC, and the resulting electrical signal is processed by the digital signal processing module to estimate the power ductility limit (PDL) of the fiber optic link. The digital signal processing module first compensates for the IQ delay and imbalance of the receiving device. Then, it estimates the frequency offset by calculating the deviation between the frequencies of the two transmitted FPTs. Subsequently, it extracts all FPTs on both polarizations to obtain the estimated polarization rotation and PDL combination matrix. W ; Depend on W The product of its transpose and conjugate yields a new matrix. T Calculate the new matrix T The eigenvalues ​​are derived from the new matrix. T The eigenvalues ​​can be used to directly estimate the PDL of the fiber optic link.

6. The polarization-dependent loss estimation system according to claim 5, characterized in that: Modulation formats include, but are not limited to, QASK, 16QAM, and 64QAM.

7. The polarization-dependent loss estimation system according to claim 5, characterized in that: All FPTs on both polarizations are extracted using a low-pass filter.

8. The polarization-dependent loss estimation system according to claim 7, characterized in that: The extracted FPT, after normalization, yields the combined matrix of the estimated signal polarization rotation and PDL. W .

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