CN111103122A - Method for extracting weak coupling point for polarization maintaining fiber distributed polarization coupling detection - Google Patents
Method for extracting weak coupling point for polarization maintaining fiber distributed polarization coupling detection Download PDFInfo
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Abstract
The invention discloses a weak coupling point extraction method for polarization maintaining optical fiber distributed polarization coupling detection, which comprises the following steps of 1: fourier transform is carried out on the polarization maintaining optical fiber distributed polarization coupling detection signal as an input signal; step 2: carrying out variation modal decomposition on the distributed polarization coupling detection signal as an input signal, and respectively outputting K eigenmodes as u1、u2、…、ui、…uKRespectively carrying out Fourier transform on the output eigenmodes and observing a frequency spectrum; and step 3: eliminating noise signals from decomposed eigenmodes, and reconstructing input signals; and 4, step 4: computing a reconstructed signal
The coupling strength of (c); and 5: and 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 canThe signal-to-noise ratio of the original signal of the polarization coupling detection can be improved, the purpose of noise reduction is achieved, and the measurement precision is greatly improved.
Description
Technical Field
The invention relates to the field of optical fiber sensing and the technical field of optical interference signal data processing, in particular to a method for extracting a weak coupling point for polarization maintaining optical fiber distributed polarization coupling detection based on a white light interference method.
Background
The polarization maintaining fiber is a special single mode fiber, can keep the polarization state of linearly polarized light transmitted along a certain main shaft of the polarization maintaining fiber unchanged, and is widely applied to the fields of temperature measurement, stress measurement, fiber optic gyroscopes and the like.
Polarization coupling phenomena occur due to imperfections in the structure of the polarization maintaining fiber and external perturbations. That is, at the point of perturbation, the coupling of optical energy to another principal axis orthogonal to its principal axis of propagation can degrade the polarization-maintaining ability of the polarization-maintaining fiber, thereby affecting the performance of the measurement system.
The white light has wide and continuous spectrum range and short coherence length, and only when the optical path difference is small, the interference can occur. When the optical path difference is zero, the two beams of each spectral line in the white light spectrum are completely overlapped, the lights with various wavelengths are overlapped to form a central zero-order fringe with the maximum contrast, namely the 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. The measurement of polarization coupling quantity of the polarization maintaining fiber is realized by scanning and compensating optical path difference through a scanning arm of the Michelson interferometer. The scanning arm introduces vibration disturbance in the scanning process, and the vibration disturbance and other external disturbances increase the background noise of the system. Some weak coupling points are submerged in the noise floor of the system, so that the detail 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 characteristic parameters of the polarization maintaining optical fiber.
Currently, several methods have been proposed to achieve denoising of optical signals, so as to improve the measurement accuracy of measured parameters. Such as: chinese patent of invention publication No. CN102095538A, "data demodulation method for polarization maintaining fiber stress sensing", indicates that collected photovoltage data is preprocessed by an averaging algorithm, and then photovoltage data signals are decomposed into a plurality of eigenmode functions and a margin by an empirical mode; finding out a base component to realize the identification of the small coupling point; for example, the invention patent of china with application number 201611096167.8, a perimeter warning method for collecting and denoising optical fiber vibration signals, denoises optical fiber vibration signals by a wavelet threshold denoising method, and eliminates redundant noise information carried by the optical fiber vibration signals, thereby realizing feature extraction and classification and identification of the signals.
Disclosure of Invention
The invention aims to provide a weak coupling point extraction method for polarization maintaining fiber distributed polarization coupling detection, which decomposes an original signal into a plurality of eigenmodes through variational mode decomposition, proposes a noise mode, selects a useful mode to reconstruct the signal, then calculates the coupling strength, achieves the aim of reducing noise and realizes the extraction of the weak coupling point submerged in background noise.
The invention discloses a weak coupling point extraction method for polarization maintaining optical fiber distributed polarization coupling detection, which comprises the following steps:
step 2: carrying out variation modal decomposition on the distributed polarization coupling detection signal as an input signal, and respectively outputting K eigenmodes as u1、u2、…、ui、…uKI represents the sequence number of the eigenmode, and the initial value thereof is set to 2; fourier transform is respectively carried out on the output eigenmodes, and frequency spectrum is observed;
and step 3: and removing noise signals from the decomposed eigenmodes, and reconstructing an input signal, wherein the reconstruction process comprises the following steps:
the formula for the reconstructed signal is:
wherein u isiRepresents the ith bookCarrying out mold characterization;
and 4, step 4: computing a reconstructed signal
The coupling strength of (c);
and 5, judging whether weak coupling points appear or not through the coupling strength graph, if so, stopping the operation, if not, enabling i to be i +1, returning to the step 3 until the weak coupling points appear, if not, modifying the values of K and α, returning to the step 2, if not, then, stopping the operation until the weak coupling points appear in the coupling strength computation graph.
Compared with the prior art, the method has the advantages that the signal-to-noise ratio of the original signal of the polarization coupling detection can be improved, the purpose of noise reduction is achieved, and the method is greatly beneficial to improving the measurement precision.
Drawings
FIG. 1 is a flowchart of the whole method for extracting weak coupling points in polarization maintaining fiber distributed polarization coupling detection according to the present invention;
FIG. 2 is a diagram of a polarization maintaining fiber distributed polarization coupling detection experiment system;
FIG. 3 is a simulated diagram of an original interference pattern for distributed polarization coupled detection;
FIG. 4 is a graph illustrating raw coupling strength curves for 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 spectrogram of an interference pattern after noise addition;
FIG. 8 is a graph of the eigenmodes after metamorphic mode decomposition;
FIG. 9 is a graph illustrating the coupling strength curves of the reconstructed signals;
reference numerals:
1. the device comprises a light source, 2, a polarizer, 3, a polarization maintaining optical fiber, 4, a beam expanding collimating lens, 5, an analyzer, 6, a static arm, 7, a scanning arm, 8, a beam splitting prism, 9, a motor, 10 and a Michelson interferometer
Detailed Description
The technical solution of the present invention is described in detail below with reference to the accompanying drawings and examples.
Fig. 2 shows a system diagram of a polarization maintaining fiber distributed polarization coupling detection experiment based on white light interferometry. The polarization maintaining optical 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 optical fiber and generating coupling points in interference patterns.
The light emitted by the SLD white light source 1 is changed into linearly polarized light through the polarizer 2, and the linearly polarized light is aligned with the fast axis of the polarization maintaining optical fiber 3 and is incident into the polarization maintaining optical fiber 3; when an external force acts on a point P on the polarization maintaining optical fiber 3, the linear polarization light is transmitted to the external force, a polarization coupling phenomenon occurs, and partial energy of an excitation mode transmitted along a fast axis is interfered to a slow axis to form a coupling mode; birefringence Δ n due to fiber modebThe excitation mode and the coupling mode generate a certain optical path difference delta Z-delta n at the emergent end of the optical fiberbl, l is the distance between the coupling point and the exit end of the optical fiber; the beam is expanded by a beam expanding collimating lens 4, and then the beam is expanded by an analyzer 5, and then the excitation mode and the coupling mode are projected onto a transmission axis of the analyzer 5 and then are incident to a spatial light Michelson interferometer 10. The beam splitter prism 8 splits the light into two beams, one beam being directed towards the stationary arm 6 and the other beam being directed towards the scanning arm 7. The stepping motor 9 drives the scanning arm 7 of the michelson interferometer 10 to scan, so that the optical path difference is compensated, and an interference pattern is generated. The scanning arm 7 introduces vibration noise in the scanning process, and the vibration noise and other external disturbances increase the background noise of the system. The photoelectric detector 11 converts the optical signal into an electrical signal, and then the data acquisition circuit 12 performs signal acquisition and inputs the signal into a computer for signal processing. Some of the weakly coupled points are buried in the noise floor of the system and cannot be detected.
FIG. 3 is a simulated diagram of a typical distributed polarization coupling detection interference pattern, the coupling strength of which is shown in the graph of the original coupling strength curve 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 the inaccurate alignment, and the middle position of the optical fiber to be measuredThe method comprises the steps of firstly carrying out Fourier transformation on the interference pattern after noise addition shown in figure 5, and carrying out frequency spectrum graph of the interference pattern after noise addition shown in figure 7. the graph 7 shows that two frequency components with relatively larger amplitudes appear, so that the number K value can be set to be 2, the fidelity coefficient α value can be set to be 2000, then carrying out variational decomposition on an input signal, and outputting a variational mode graph after the variational mode decomposition shown in figure 8, and two eigenmodes u values can be set to be 20001And u2Occurrence of u1Has a center frequency of 100Hz, is a low-frequency vibration disturbance, u2Has a central frequency of 384Hz, is an interference signal, and the visible interference signal and the noise signal are separated in space without modal aliasing. Then u is put1As noise rejection, u2As reconstructed input signal, based on u2The coupling strength is calculated, and fig. 9 is a graph showing the coupling strength curve of the reconstructed signal. The noise floor is-98 dB, and the noise floor is reduced by 19dB compared with that of the figure 6. The weak coupling point appears, the coupling strength is-82.8 dB, the coupling strength is reduced by 0.2dB compared with the true value of-82.6 dB, 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 signal is decomposed into 2 modes, but in other occasions, the signal may be decomposed into a plurality of eigenmodes, and the noise signal needs to be removed from the plurality of eigenmodes according to prior knowledge, so as to reconstruct the input signal according to the overall flow chart of the method for extracting the weak coupling point for polarization maintaining fiber distributed polarization coupling detection of the invention as shown in fig. 1.
As shown in fig. 1, an overall flowchart of a method for extracting weak coupling points in polarization maintaining fiber distributed polarization coupling detection specifically includes the following steps:
step 2: carrying out variation mode decomposition on the distributed polarization coupling detection signal as an input signal, and outputting K eigenmodes, wherein K is the total number of decomposed eigenmodes and is u1、u2、…、ui、…uKRespectively carrying out Fourier transform on the output eigenmodes and observing a frequency spectrum;
and step 3: and removing noise signals from the decomposed eigenmodes, and reconstructing an input signal, wherein the reconstruction process comprises the following steps:
the formula for the reconstructed signal is:
wherein u isiRepresents the ith eigenmode; setting the initial value of i to 2;
and 4, step 4: computing a reconstructed signal
The coupling strength of (c);
and 5, judging whether weak coupling points appear or not through the coupling strength graph, if so, stopping operation, if not, enabling i to be i +1, wherein i represents the serial number of the eigenmode, the initial value of the serial number is 2, then returning to the step 3 until the weak coupling points appear, when i is K, K is the total number of the decomposed eigenmodes, and when no weak coupling point appears, modifying the values of K and α, returning to the step 2, until the weak coupling points appear in the coupling strength computation graph, and then stopping operation.
After the original experimental data are processed by the 5 steps, the signal-to-noise ratio is greatly improved, the effect is obvious, the extraction of the weak coupling point of the polarization maintaining optical fiber distributed polarization coupling detection is realized, and the measurement precision is greatly improved.
Claims (1)
1. A weak coupling point extraction method for polarization maintaining fiber distributed polarization coupling detection is characterized by comprising the following steps:
step 1, carrying out Fourier transform on a polarization maintaining fiber distributed polarization coupling detection signal serving as an input signal, estimating the modal number K of the input signal based on the frequency spectrum of the input signal, and setting a fidelity coefficient α to 2000;
step 2: carrying out variation modal decomposition on the distributed polarization coupling detection signal as an input signal, and respectively outputting K eigenmodes as u1、u2、…、ui、…uKI represents the sequence number of the eigenmode, and the initial value thereof is set to 2; fourier transform is respectively carried out on the output eigenmodes, and frequency spectrum is observed;
and step 3: and removing noise signals from the decomposed eigenmodes, and reconstructing an input signal, wherein the reconstruction process comprises the following steps: the formula for the reconstructed signal is:
wherein u isiRepresents the ith eigenmode;
and 4, step 4: computing a reconstructed signal
The coupling strength of (c);
and 5, judging whether weak coupling points appear or not through the coupling strength graph, if so, stopping the operation, if not, enabling i to be i +1, returning to the step 3 until the weak coupling points appear, if not, modifying the values of K and α, returning to the step 2, if not, then, stopping the operation until the weak coupling points appear in the coupling strength computation graph.
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