CN108489640B - Distributed dynamic stress frequency measurement method based on white light interference - Google Patents

Abstract

The invention relates to a distributed dynamic stress frequency measurement method based on white light interference, which is realized by a distributed polarization coupling test system based on white light interference, and includes the following steps: applying dynamic stress to polarization-maintaining fibers by using piezoelectric ceramics; The polarization coupling system measures the white light interferogram under dynamic stress to obtain the interference pattern; intercepts the data of the interference fringe part of the dynamic coupling point in the interferogram received by the photodetector; performs Hilbert transform on the dynamic coupling point data to obtain the coupling point envelope curve ; Perform wavelet time-frequency analysis on the envelope curve of the dynamic coupling point to obtain a time-frequency distribution diagram, which contains the frequency information of the dynamic stress.

Description

A distributed dynamic stress frequency measurement method based on white light interference

technical field

The invention relates to the field of optical fiber sensing, in particular to a frequency measurement method of dynamic stress.

Background technique

In recent years, the research of distributed optical fiber vibration sensing technology has received extensive attention. Due to its advantages of high sensitivity, large dynamic range, anti-electromagnetic interference, small size and easy networking, it is widely used in residential areas, schools, airports, etc. Regional intrusion monitoring; health monitoring of infrastructure such as large buildings and underground transmission systems. Has broad application prospects.

At present, the distributed optical fiber vibration sensing technology can be divided into distributed optical fiber vibration sensing based on the interference principle and distributed optical fiber vibration sensing based on the backscattering principle according to the principle. Interferometric fiber optic sensors include Sagnac interferometers, Michelson interferometers, Mach-Zehnder interferometers, and composite interference structures based on the above three structures. For example, in the invention patent "An Optical Fiber Vibration Sensing System for Pipeline Leak Detection" with the application publication number of CN102313141A, a 3×3 coupler is used to introduce a fixed phase offset in the sensing system based on the Sagnac interference principle, and 2 ×2 couplers to enhance the optical power returned to the detector for long-distance vibration sensing of the pipeline. Distributed optical fiber vibration sensing technologies based on the principle of backscattering include: phase-sensitive optical time domain reflectometry (j-OTDR), polarization-sensitive optical time domain reflectometry (P-OTDR), Brillouin optical time domain analysis Technology (B-OTDA), etc. For example, the invention patent "a vibration frequency measurement method" with the application publication number of CN104596634A converts the backscattered Rayleigh optical power signal and the Fresnel reflected optical power signal returned by the link into random signals through the time-domain reflection optical fiber link of polarized light. The time-varying light intensity signal is obtained, and the Fourier transform is performed on the light intensity signal to obtain the frequency domain spectrum of each reflection point, and the vibration frequency is obtained. Application publication number CN106768277A The invention patent "A Distributed Optical Fiber Vibration Sensing Device Based on Coherent Phase Detection" utilizes the light source module to generate two continuous narrow linewidth lasers, one is input to the coherent light receiving module, and the other is modulated by the light. After being modulated into a short pulse sequence, it enters the sensing fiber. After the photon sequence is reflected by the weakly reflective Bragg grating array, it interferes with the first light in the coherent light receiving module, and the vibration can be demodulated from the multiple beat frequency subsequences of the interference signal. The location and waveform of the signal. However, distributed optical fiber vibration sensors based on the principle of backscattering usually have very weak backscattered light power and long average detection time, which greatly limits the detection sensitivity and frequency response range of the system.

The distributed polarization coupling system based on white light interference has many advantages such as simple structure, low cost, and insensitivity to electromagnetic interference. It can measure a series of physical parameters such as stress, dispersion, displacement, temperature, fiber beat length, and birefringence. For example, in the invention patent with the application publication number CN104006948A "Method for Demodulating the Position of the Polarization Coupling Point of a Polarization-Maintaining Fiber Based on Multi-peak Splitting Period", the multi-peak splitting phenomenon is used to realize the demodulation of the position of the coupling point in the fiber, but it is only suitable for static stress situation.

SUMMARY OF THE INVENTION

The present invention proposes a method capable of realizing dynamic stress frequency measurement, and the technical scheme is as follows:

1. a distributed dynamic stress frequency measurement method based on white light interference, utilizes the distributed polarization coupling test system based on white light interference to realize, comprising the steps:

1) Using piezoelectric ceramics to apply dynamic stress to the polarization-maintaining fiber;

2) Using the distributed polarization coupling system to measure the white light interferogram under dynamic stress to obtain the interference pattern;

3) Intercept the data of the interference fringe part of the dynamic coupling point in the interferogram received by the photodetector;

4) Hilbert transform is performed on the dynamic coupling point data to obtain the coupling point envelope curve;

5) Perform wavelet time-frequency analysis on the envelope curve of the dynamic coupling point to obtain a time-frequency distribution diagram, which contains the frequency information of the dynamic stress.

2. The method according to claim 1, wherein in step 1), the Gaussian light emitted by the light source becomes the slow axis of the linearly polarized light coupled to the polarization-maintaining fiber through the polarizer, and the Gaussian light generated by the piezoelectric ceramic is passed through. During dynamic stress, part of the light is coupled to the fast axis of the fiber, and at the output end of the fiber, the light becomes spatially parallel light after passing through the beam expander; Adjusted to 45°, the analyzer projects the light of the fast and slow axes to the same direction; then the light enters the Michelson interferometer and moves through the arm of the Michelson interferometer to scan the optical path difference generated in the polarization maintaining fiber; At the output end of the Michelson interferometer, the photodetector receives the scanned interferogram, and the received signal is transmitted to the computer through the data acquisition card.

The invention uses a white light interference polarization coupling test system to measure the interferogram of the polarization-maintaining fiber under dynamic stress, intercepts the interference fringes of the dynamic coupling point from the measured interferogram, performs Hilbert transformation on it to obtain the envelope of the coupling point, and calculates the coupling point envelope. The frequency of the dynamic stress is demodulated by time-frequency analysis of the envelope of the coupling point. The frequency range of the dynamic stress that this measurement method can measure is from a few hertz to several hundred hertz.

Description of drawings

Fig. 1 is the flow chart of measuring dynamic stress frequency by white light interferometry;

2 is a schematic diagram of a measurement system device;

Figure 3 shows the measurement results of a single dynamic stress;

Figure 4 is the extraction result of a single dynamic coupling point envelope;

Fig. 5 is the wavelet time-frequency distribution diagram of the envelope of a single dynamic coupling point;

Figure 6 is a schematic diagram of the relationship between the envelope modulation frequency and the dynamic stress frequency;

Figure 7 shows the results of two dynamic stress measurements;

Figure 8 shows the extraction results of two dynamic coupling point envelopes;

Figure 9 is a wavelet time-frequency distribution diagram of the envelopes of two dynamic coupling points.

Detailed ways

Specific embodiments of the present invention are described in detail below in conjunction with the accompanying drawings:

Figure 2 is a schematic diagram of a polarization-maintaining fiber dynamic stress measurement system based on white light interference. The superluminescent diode (SLD) emits a central wavelength of 1310 nm, a half-height width of 18.7 nm, and a coherence length of 42 μm. The polarizer becomes linearly polarized light coupling to the slow axis of the PM fiber. At a distance of 47.3cm from the fiber exit end, a piezoelectric ceramic with a free stroke of 8 mm, an external dimension of 3.4x 4.8x 9.0mm, and a driving voltage range of 0-75V exerts dynamic stress on the polarization-maintaining fiber. The magnitude of the dynamic stress is It is determined by the output voltage of the signal generator that drives the piezoelectric ceramics. At the point of force, a part of the light is coupled to the fast axis of the fiber. Due to the birefringence between the fast and slow axes of the fiber, there is a certain optical path difference between the two polarization modes at the output end of the fiber. Then, the light is adjusted to space light by the beam expander, and the maximum visibility is obtained by the rotatable half-wave plate, and the analyzer adjusts the light of the two polarization states to the same direction. In MI, the moving mirror scans the optical path difference of the two polarization states, and the interference light intensity is received by the photodetector after passing through the converging lens, and transmitted to the computer through the data acquisition card.

Figure 3 is the interference diagram of the polarization-maintaining fiber under a single sinusoidal dynamic stress. The signal generator outputs a sinusoidal signal with a peak-to-peak voltage of 1.5V and 50Hz. Fig. 4 shows the interference fringes of the dynamic coupling point in the intercepted interferogram, and the Hilbert transform is performed on the data to obtain the envelope of the coupling point. Figure 5 is a time-frequency distribution diagram obtained by wavelet transformation of the envelope curve of the coupling point. The wavelet base function used is cmor3-3 wavelet, the scale sequence length is 4096, and there is a frequency component of 49.77Hz in the time-frequency distribution diagram. Figure 6 shows the simulation and experimental results of the relationship between the envelope modulation frequency and the dynamic stress frequency under different driving voltages. The envelope modulation frequency has a linear relationship with the dynamic stress frequency, and the envelope modulation frequency is related to the piezoelectric ceramic driving voltage. The magnitude is irrelevant.

Figure 7 is the interference diagram of the polarization-maintaining fiber under two dynamic stresses at the same time. The frequencies of the two dynamic stresses (A and B) are 30Hz, respectively, and the amplitudes of the output voltages of the signal generator are 2V and 500mV, respectively. Two dynamic coupling points. Figure 8 shows the intercepted interference fringes of two dynamic coupling points and their envelope curves extracted by Hilbert transform. Fig. 9 is a time-frequency distribution diagram of wavelet transform of the envelope curve. There are two frequency components, 30.05 Hz and 29.11 Hz, corresponding to the dynamic stress A and B respectively.

Claims (1)

1. A distributed dynamic stress frequency measurement method based on white light interference is realized by a distributed polarization coupling test system based on white light interference, comprising the following steps: 1) Using piezoelectric ceramics to apply dynamic stress to the polarization-maintaining fiber, the method is as follows: Gaussian light emitted by the light source becomes linearly polarized light through the polarizer and is coupled to the slow axis of the polarization-maintaining fiber. , a part of the light is coupled to the fast axis of the fiber, and at the output end of the fiber, the light becomes spatially parallel light after passing through the beam expander; the rotatable half-wave plate adjusts the angle between the light of the fast and slow axis and the transmission axis of the analyzer to 45 °, the analyzer projects the light of the fast and slow axes to the same direction; then the light enters the Michelson interferometer and moves through the arm of the Michelson interferometer to scan the optical path difference generated in the polarization maintaining fiber; in the Michelson interferometer At the output end of the instrument, the photoelectric detector receives the interferogram obtained by scanning, and the received signal is transmitted to the computer through the data acquisition card. At a position of 47.3 cm from the fiber output end, there is a free stroke of 8 mm and an external dimension of 3.4 x 4.8 x 9.0 mm, the piezoelectric ceramic with a driving voltage range of 0-75V applies dynamic stress to the polarization-maintaining fiber, and the dynamic stress is determined by the output voltage of the signal generator driving the piezoelectric ceramic; 2) Using the distributed polarization coupling system to measure the white light interferogram under dynamic stress to obtain the interference pattern; 3) Intercept the dynamic coupling points in the interferogram, and perform Hilbert transform on the data to obtain the envelope of the coupling point, and then perform the wavelet transform on the envelope curve of the coupling point to obtain the time-frequency distribution diagram. The wavelet basis function used is cmor3 -3 wavelet, the length of the scale sequence is 4096, and the frequency information of dynamic stress is included in the time-frequency distribution map.

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