CN111103122B - Polarization maintaining fiber distributed polarization coupling detection weak coupling point extraction method - Google Patents
Polarization maintaining fiber distributed polarization coupling detection weak coupling point extraction method Download PDFInfo
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Abstract
The invention discloses a method for extracting weak coupling points for polarization-maintaining fiber distributed polarization coupling detection, which comprises the following steps of: performing Fourier transform on the polarization-maintaining fiber distributed polarization coupling detection signal serving as an input signal; step 2: performing variation modal decomposition on a distributed polarization coupling detection signal serving as an input signal, respectively outputting K eigenmodes u 1、u2、…、ui、…uK, respectively performing Fourier transformation on the output eigenmodes, and observing a frequency spectrum; step 3: removing noise signals from the decomposed eigenmodes, and reconstructing an input signal; step 4: calculating a reconstructed signalIs a coupling strength of (a); step 5: judging whether weak coupling points appear or not through the coupling strength graph until the weak coupling points appear in the coupling strength calculation graph. The invention can improve the signal-to-noise ratio of the original signal detected by polarization coupling, achieves the purpose of noise reduction, and is greatly beneficial to improving the measurement precision.
Description
Technical Field
The invention relates to the technical fields of optical fiber sensing and optical interference signal data processing, in particular to an extraction method of a polarization maintaining optical fiber distributed polarization coupling detection weak coupling point based on a white light interferometry.
Background
The polarization maintaining fiber is a special single-mode fiber, can keep the polarization state of linearly polarized light propagating along a certain main axis unchanged, and is widely applied to the fields of temperature measurement, stress measurement, fiber optic gyroscopes and the like.
Polarization coupling phenomenon occurs due to imperfections in the structure of the polarization maintaining fiber and external disturbances. I.e. at the point of disturbance, the coupling of light energy to another principal axis orthogonal to its propagation principal axis reduces the polarization maintaining capacity of the polarization maintaining fiber, thereby affecting the performance of the measurement system.
The spectrum of white light is wide, continuous, and the coherence length is short, and only when the optical path difference is small, the interference occurs. When the optical path difference is zero, the double light beams of each spectral line in the white light spectrum are completely overlapped, and the light with various wavelengths are overlapped to form a central zero-order stripe with the maximum contrast, namely an optimal interference position, and the measured parameters are measured through the interference phenomenon.
Common white light interferometers include spatial light Michelson interferometers and fiber-optic Michelson interferometers. And the optical path difference is compensated by scanning through a scanning arm of the Michelson interferometer, so that the polarization coupling amount of the polarization maintaining optical fiber is measured. The scanning arm introduces vibration disturbance in the scanning process, and increases the noise floor of the system together with other disturbance outside. Some weak coupling points are submerged in the noise floor of the system, so that the detailed information of polarization coupling of the polarization maintaining fiber cannot be acquired. Therefore, the weak coupling point is extracted from the background noise, and the method has very important significance for high-precision measurement of the polarization maintaining fiber characteristic parameters.
Currently, several methods have been proposed to implement denoising of an optical signal to improve the measurement accuracy of a measured parameter. Such as: the Chinese patent with publication number CN102095538A discloses a data demodulation method of polarization maintaining fiber stress sensing, which is to preprocess the collected photovoltage data through an average algorithm, and then decompose the photovoltage data signal into a plurality of eigen-mode functions and a margin through an empirical mode; finding out a base component, and realizing the identification of small coupling points; in another example, the Chinese patent application number 201611096167.8 is a method for collecting and denoising perimeter early-warning optical fiber vibration signals, which is used for denoising the optical fiber vibration signals by a wavelet threshold denoising method, so as to eliminate redundant noise information carried by the optical fiber vibration signals, thereby realizing characteristic extraction and classification identification of the signals.
Disclosure of Invention
The invention aims to provide a method for extracting weak coupling points for polarization-maintaining fiber distributed polarization coupling detection, which decomposes an original signal into a plurality of eigenmodes through variation mode decomposition, provides a noise mode, selects a useful mode for signal reconstruction, calculates coupling strength, achieves the purpose of noise reduction, and realizes extraction of the weak coupling points submerged in noise floor.
The invention discloses a method for extracting weak coupling points for polarization-maintaining fiber distributed polarization coupling detection, which comprises the following steps:
Step 1: performing Fourier transform on a polarization-maintaining fiber distributed polarization coupling detection signal serving as an input signal, estimating the number K of modes of the polarization-maintaining fiber distributed polarization coupling detection signal based on the frequency spectrum of the input signal, and setting a fidelity coefficient alpha to 2000;
Step 2: performing variation modal decomposition on a distributed polarization coupling detection signal serving as an input signal, wherein the output K eigenmodes are u 1、u2、…、ui、…uK respectively, i represents the serial number of the eigenmodes, and the initial value of the serial number is set to be 2; fourier transforming the output eigenmodes respectively, and observing the frequency spectrum;
step 3: noise signals are removed from the decomposed eigenmodes, an input signal is reconstructed, and the reconstruction process is as follows:
the formula for reconstructing the signal is:
Wherein u i represents the i-th eigenmode;
Step 4: calculating a reconstructed signal Is a coupling strength of (a);
step 5: judging whether weak coupling points appear or not through the coupling intensity diagram, and stopping operation if so; if not, making i=i+1, and returning to the step 3 until weak coupling points appear; when i=k, if no weak coupling point is present, modifying the values of K and α, and returning to step 2; until weak coupling points appear in the coupling strength calculation map, and then the operation is stopped.
Compared with the prior art, the method has the advantages that the signal to noise ratio of the original signal detected by polarization coupling can be improved, the purpose of noise reduction is achieved, and the measurement accuracy is greatly improved.
Drawings
FIG. 1 is a flowchart of an overall method for extracting weak coupling points of polarization-maintaining fiber distributed polarization coupling detection according to the present invention;
FIG. 2 is a diagram of a distributed polarization coupling detection experiment system of a polarization maintaining fiber;
FIG. 3 is a simulation of the original interference pattern for distributed polarization coupling detection;
FIG. 4 is a graph showing the original coupling strength curve of distributed polarization coupling detection;
FIG. 5 is a schematic diagram of an interference pattern after noise addition;
FIG. 6 is a graph showing the coupling strength after noise addition;
FIG. 7 is a spectral diagram of a noisy interference pattern;
FIG. 8 is a graph of eigenmodes after decomposition of the variation mode;
FIG. 9 is a schematic diagram of a coupling strength curve of a reconstructed signal;
reference numerals:
1. the light source, 2, polarizer, 3, polarization-preserving fiber, 4, beam-expanding collimating lens, 5, analyzer, 6, stationary arm, 7, scanning arm, 8, beam-splitting prism, 9, motor, 10, michelson interferometer
Detailed Description
The technical scheme of the invention is described in detail below with reference to the accompanying drawings and examples.
FIG. 2 is a diagram of a system for testing distributed polarization coupling of polarization maintaining fiber based on white light interferometry. The polarization-maintaining fiber distributed polarization coupling measurement system based on the white light interferometry can be used for detecting the polarization coupling phenomenon of the polarization-maintaining fiber and generating coupling points in the interference pattern.
The light emitted by the SLD white light source 1 is changed into linear polarized light after passing through the polarizer 2, and the linear polarized light is aligned with the fast axis of the polarization maintaining fiber 3 and is incident into the polarization maintaining fiber 3; when external force acts on the P point on the polarization maintaining optical fiber 3, the linear polarized light is transmitted to the external force, polarization coupling phenomenon occurs, and part of energy of an excitation mode propagating along a fast axis is crossly connected to a slow axis to form a coupling mode; because of the existence of the optical fiber mode double refractive index delta n b, a certain optical path difference delta Z=delta n b l is generated at the optical fiber emergent end by the excitation mode and the coupling mode, and l is the distance between the coupling point and the optical fiber emergent end; the beam is firstly expanded through the beam expansion collimating lens 4, then is projected onto the transmission axis of the analyzer 5 through the excitation mode and the coupling mode after passing through the analyzer 5, and then is incident on the spatial light Michelson interferometer 10. The beam splitting prism 8 splits the light into two beams, one beam directed to the stationary arm 6 and the other beam directed to the scanning arm 7. The stepping motor 9 drives the scanning arm 7 of the Michelson interferometer 10 to scan, so that the compensation of the optical path difference is realized, and an interference pattern is generated. The scanning arm 7 introduces vibration noise during scanning, and together with other external disturbances, increases the noise floor of the system. The photodetector 11 converts the optical signal into an electrical signal, and the data acquisition circuit 12 acquires the signal and inputs the signal to a computer for signal processing. Some weak coupling points are submerged in the noise floor of the system and cannot be detected.
As shown in FIG. 3, an interference pattern simulation diagram of a typical distributed polarization coupling detection is shown, with the coupling strength shown in the original coupling strength curve diagram of the distributed polarization coupling detection of FIG. 4. The corresponding noise floor is-103 dB. The polarizer 1 is connected with the incident end of the optical fiber to be measured through a flange, coupling points are generated due to misalignment of the axis, and a weak coupling point of-82.6 dB exists in the middle position of the optical fiber to be measured. However, the scanning arm 7 introduces vibrational disturbances during scanning, which, together with other disturbances from the outside, increases the noise floor of the system. Through simulation, gaussian white noise and low-frequency vibration disturbance are added in the interference pattern. As shown in fig. 5, a schematic diagram of the interference pattern after noise addition is shown. As shown in fig. 6, a schematic diagram of the coupling strength curve after noise addition is shown. The corresponding noise floor is about-79 dB. The weak coupling point in fig. 6 is submerged in the noise floor of the system and cannot be observed. Weak coupling points are extracted by the method of the invention. First, fourier transform is performed on the interference pattern after noise addition shown in fig. 5, and a spectrum diagram of the interference pattern after noise addition is shown in fig. 7. Fig. 7 shows that two frequency components with relatively large amplitudes occur, so the number K of modes can be set to 2 and the fidelity coefficient α to 2000. And then carrying out variation modal decomposition on the input signal, outputting an eigenvector graph after the variation modal decomposition shown in fig. 8, wherein two eigenvectors u 1 and u 2 appear, the center frequency of u 1 is 100Hz, the center frequency of u 2 is 384Hz, the interference signal is the interference signal, the visible interference signal and the noise signal are separated in space, and modal aliasing does not occur. Then u 1 is taken as noise elimination, u 2 is taken as a reconstructed input signal, and the coupling strength is calculated based on u 2, and the coupling strength curve diagram of the reconstructed signal is shown in fig. 9. The noise floor was-98 dB, which was reduced by 19dB compared to fig. 6. The weak coupling point appears, the coupling strength is-82.8 dB, compared with the true value-82.6 dB, the coupling strength is reduced by 0.2dB, the error is less than 0.5%, the high-precision extraction of the weak coupling point is realized, and the effect is obvious. In this example, the noise-added signal is decomposed into 2 modes, but in other cases, the signal may be decomposed into a plurality of eigenmodes, the noise signal needs to be removed from the plurality of eigenmodes according to priori knowledge, and the input signal is reconstructed according to the overall flowchart of the polarization-maintaining fiber distributed polarization coupling detection weak coupling point extraction method shown in fig. 1.
As shown in fig. 1, an overall flowchart of a method for extracting weak coupling points of polarization-maintaining fiber distributed polarization coupling detection is provided, which specifically includes the following steps:
Step 1: performing Fourier transform on a polarization-maintaining fiber distributed polarization coupling detection signal serving as an input signal, estimating the mode number K (K=1, 2 … n, wherein n is a positive integer) of the polarization-maintaining fiber distributed polarization coupling detection signal based on the frequency spectrum of the input signal, and setting the fidelity coefficient alpha to 2000;
step 2: performing variation modal decomposition on a distributed polarization coupling detection signal serving as an input signal, outputting K eigenmodes, wherein K is the total number of the decomposed eigenmodes and u 1、u2、…、ui、…uK, performing Fourier transformation on the output eigenmodes respectively, and observing a frequency spectrum;
step 3: noise signals are removed from the decomposed eigenmodes, an input signal is reconstructed, and the reconstruction process is as follows:
the formula for reconstructing the signal is: Wherein u i represents the i-th eigenmode; setting the initial value of i to 2;
Step 4: calculating a reconstructed signal Is a coupling strength of (a);
Step 5: judging whether weak coupling points appear or not through the coupling intensity diagram, and stopping operation if the weak coupling points appear; if not, let i=i+1, i denote the serial number of the eigenmode, whose initial value is 2; then returning to the step 3 until weak coupling points appear; when i=k, K is the total number of decomposed eigenmodes, and when no weak coupling point exists, modifying the values of K and α, and returning to step 2; until weak coupling points appear in the coupling strength calculation map, and then the operation is stopped.
After the original experimental data are processed by the steps 5, the signal to noise ratio is greatly improved, the effect is remarkable, the extraction of weak coupling points of polarization-maintaining fiber distributed polarization coupling detection is realized, and the measurement accuracy is greatly improved.
Claims (1)
1. The method for extracting the weak coupling point of the polarization-maintaining optical fiber distributed polarization coupling detection is characterized by comprising the following steps of:
Step 1: performing Fourier transform on a polarization-maintaining fiber distributed polarization coupling detection signal serving as an input signal, estimating the number K of modes of the polarization-maintaining fiber distributed polarization coupling detection signal based on the frequency spectrum of the input signal, and setting a fidelity coefficient alpha to 2000;
Step 2: performing variation modal decomposition on a distributed polarization coupling detection signal serving as an input signal, wherein the output K eigenmodes are u 1、u2、…、ui、…uK respectively, i represents the serial number of the eigenmodes, and the initial value of the serial number is set to be 2; fourier transforming the output eigenmodes respectively, and observing the frequency spectrum;
step 3: noise signals are removed from the decomposed eigenmodes, an input signal is reconstructed, and the reconstruction process is as follows: the formula for reconstructing the signal is:
Wherein u i represents the i-th eigenmode;
Step 4: calculating a reconstructed signal Is a coupling strength of (a);
step 5: judging whether weak coupling points appear or not through the coupling intensity diagram, and stopping operation if so; if not, making i=i+1, and returning to the step 3 until weak coupling points appear; when i=k, if no weak coupling point is present, modifying the values of K and α, and returning to step 2; until weak coupling points appear in the coupling strength calculation map, and then the operation is stopped.
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