CN108489640B - Distributed dynamic stress frequency measurement method based on white light interference - Google Patents
Distributed dynamic stress frequency measurement method based on white light interference Download PDFInfo
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- CN108489640B CN108489640B CN201810043277.0A CN201810043277A CN108489640B CN 108489640 B CN108489640 B CN 108489640B CN 201810043277 A CN201810043277 A CN 201810043277A CN 108489640 B CN108489640 B CN 108489640B
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
- G01L1/00—Measuring force or stress, in general
- G01L1/10—Measuring force or stress, in general by measuring variations of frequency of stressed vibrating elements, e.g. of stressed strings
- G01L1/103—Measuring force or stress, in general by measuring variations of frequency of stressed vibrating elements, e.g. of stressed strings optical excitation or measuring of vibrations
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
- G01L5/00—Apparatus for, or methods of, measuring force, work, mechanical power, or torque, specially adapted for specific purposes
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Abstract
The invention relates to a distributed dynamic stress frequency measurement method based on white light interference, which is realized by using a distributed polarization coupling test system based on white light interference and comprises the following steps: applying dynamic stress to the polarization maintaining fiber by utilizing piezoelectric ceramics; measuring a white light interference pattern under dynamic stress by using a distributed polarization coupling system to obtain an interference pattern; intercepting data of a dynamic coupling point interference fringe part in an interference pattern received by a photoelectric detector; performing Hilbert transformation on the dynamic coupling point data to obtain a coupling point envelope curve; and performing wavelet time-frequency analysis on the dynamic coupling point envelope curve to obtain a time-frequency distribution graph, wherein the time-frequency distribution graph contains frequency information of dynamic stress.
Description
Technical Field
The invention relates to the field of optical fiber sensing, in particular to a frequency measurement method of dynamic stress.
Background
In recent years, research on distributed optical fiber vibration sensing technology is widely concerned, and intrusion monitoring in important areas such as residential districts, schools, airports and the like is realized due to the advantages of high sensitivity, large dynamic range, electromagnetic interference resistance, small size, easiness in networking and the like; the health monitoring of infrastructure such as large-scale building, underground transmission system, etc. Has wide application prospect.
The existing 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. The interference type optical fiber sensor comprises a Sagnac interferometer, a Michelson interferometer, a Mach-Zehnder interferometer and a composite type interference structure based on the three structures. In the invention of CN102313141A patent application publication No. 3 × 3, a 3 × 3 coupler is used to introduce a fixed phase offset in a fiber vibration sensing system for detecting leakage in a pipeline, and a 2 × 2 coupler is used to enhance the optical power returned to a detector, so as to perform long-distance vibration sensing on the pipeline. The distributed optical fiber vibration sensing technology based on the backscattering principle comprises the following steps: a phase-sensitive optical time domain reflectometer (j-OTDR), a polarization-sensitive optical time domain reflectometer (P-OTDR), a Brillouin optical time domain analysis technique (B-OTDA), and the like. For example, in patent application publication No. CN104596634A, "a vibration frequency measurement method" of the invention, a polarized light time domain reflection optical fiber link is used to convert a backscattered rayleigh optical power signal and a fresnel reflected optical power signal returned from the link into a light intensity signal varying with time, and the light intensity signal is subjected to fourier transform to obtain a frequency domain spectrum of each reflection point, so as to obtain a vibration frequency. The invention patent with application publication number CN106768277A, a distributed optical fiber vibration sensing device based on coherent phase detection, utilizes a light source module to generate two continuous narrow linewidth lasers, one is input to a coherent light receiving module, the other is modulated into a short pulse sequence by a light modulation module and then enters a sensing optical fiber, after the photon sequence is reflected by a weak reflection bragg grating array, the coherent light receiving module interferes with the first path of light, and the position and waveform of a vibration signal can be demodulated from a plurality of beat frequency subsequences of the interference signal. However, distributed fiber-optic vibration sensors based on the backscatter principle generally have 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 the advantages of simple structure, low cost, insensitivity to electromagnetic interference and the like, and can realize measurement of a series of physical parameters such as stress, dispersion, displacement, temperature, optical fiber beat length, birefringence and the like. In the invention patent "method for demodulating polarization-maintaining fiber polarization coupling point position based on multimodal splitting period" with application publication No. CN104006948A, the demodulation of the coupling point position in the fiber is realized by using multimodal splitting phenomenon, but is only suitable for static stress condition.
Disclosure of Invention
The invention provides a method capable of realizing dynamic stress frequency measurement, which adopts the following technical scheme:
1. a distributed dynamic stress frequency measurement method based on white light interference is realized by using a distributed polarization coupling test system based on white light interference, and comprises the following steps:
1) applying dynamic stress to the polarization maintaining fiber by utilizing piezoelectric ceramics;
2) measuring a white light interference pattern under dynamic stress by using a distributed polarization coupling system to obtain an interference pattern;
3) intercepting data of a dynamic coupling point interference fringe part in an interference pattern received by a photoelectric detector;
4) performing Hilbert transformation on the dynamic coupling point data to obtain a coupling point envelope curve;
5) and performing wavelet time-frequency analysis on the dynamic coupling point envelope curve to obtain a time-frequency distribution graph, wherein the time-frequency distribution graph contains frequency information of dynamic stress.
2. The method according to claim 1, wherein in step 1), the gaussian light emitted by the light source is linearly polarized by the polarizer and coupled to the slow axis of the polarization maintaining fiber, when passing through the dynamic stress generated by the piezoelectric ceramic, a part of the light is coupled to the fast axis of the fiber, and at the exit end of the fiber, the light becomes spatially parallel light after passing through the beam expander; the rotatable half-wave plate adjusts the included angle between the light of the fast and slow axes and the transmission axis of the analyzer to be 45 degrees, and the analyzer projects the light of the fast and slow axes to the same direction; then the light enters a Michelson interferometer, and the optical path difference generated in the polarization maintaining fiber can be scanned through the movement of one arm of the Michelson interferometer; at the emergent end of the Michelson interferometer, an interference pattern obtained by scanning is received by the photoelectric detector, and a received signal is transmitted to a computer through a data acquisition card.
The invention uses a white light interference polarization coupling test system to measure an interference pattern when a polarization maintaining optical fiber is subjected to dynamic stress, intercepts dynamic coupling point interference fringes from the measured interference pattern, performs Hilbert transformation on the interference fringes to obtain a coupling point envelope, and performs time-frequency analysis on the coupling point envelope to demodulate the frequency of the dynamic stress. The frequency range of dynamic stress that this measurement method can measure is several hertz to several hundred hertz.
Drawings
FIG. 1 is a flow chart of measuring dynamic stress frequency by white light interferometry;
FIG. 2 is a schematic view of a measurement system apparatus;
FIG. 3 is a single dynamic stress measurement;
FIG. 4 shows the envelope extraction result of a single dynamic coupling point;
FIG. 5 is a wavelet time-frequency distribution diagram of a single dynamic coupling point envelope;
FIG. 6 is a diagram illustrating the relationship between envelope modulation frequency and dynamic stress frequency;
FIG. 7 is two dynamic stress measurements;
FIG. 8 shows the envelope extraction results of two dynamic coupling points;
fig. 9 is a wavelet time-frequency distribution diagram of envelopes of two dynamic coupling points.
Detailed Description
Specific embodiments of the present invention are described in detail below with reference to the accompanying drawings:
FIG. 2 is a schematic diagram of a system for measuring dynamic stress of a polarization maintaining optical fiber based on white light interference, wherein a central wavelength emitted by a super-luminescent diode (SLD) is 1310nm, a full width at half maximum is 18.7nm, a coherence length is 42 μm, and the light is converted into linearly polarized light through a polarizer and is coupled to a slow axis of the polarization maintaining optical fiber. And at a position 47.3cm away from the exit end of the optical fiber, applying dynamic stress to the polarization maintaining optical fiber by piezoelectric ceramics with a free stroke of 8 mm, an overall dimension of 3.4x 4.8x 9.0mm and a driving voltage range of 0-75V, wherein the magnitude of the dynamic stress is determined by the magnitude of the output voltage of a signal generator for driving the piezoelectric ceramics. At the stress point, a part of light is coupled to the fast axis of the optical fiber, and due to the birefringence between the fast axis and the slow axis of the optical fiber, a certain optical path difference is generated between two polarization modes at the emergent end of the optical fiber. Then, the light is adjusted into space light through the beam expander, the maximum visibility is obtained through the rotatable half-wave plate, and the light in two polarization states is adjusted to the same direction through the analyzer. In MI, the moving reflector scans the optical path difference between two polarization states, and the interference light intensity is received by the photoelectric detector after passing through the converging lens and transmitted to the computer through the data acquisition card.
FIG. 3 is an interference diagram of a single sinusoidal dynamic stress of a polarization maintaining fiber, and a sinusoidal signal with a peak-to-peak voltage of 1.5V and a peak-to-peak voltage of 50Hz is output by a signal generator. Fig. 4 is a captured interference fringe of a dynamic coupling point in an interferogram, and Hilbert transform is performed on data to obtain an envelope of the coupling point. FIG. 5 is a time-frequency distribution diagram obtained by performing wavelet transform on the coupled point envelope curve, the adopted wavelet basis function is cmor3-3 wavelet, the length of the scale sequence is 4096, and there is a frequency component of 49.77Hz in the time-frequency distribution diagram. Fig. 6 is a simulation and experiment result of the relationship between the envelope modulation frequency and the dynamic stress frequency under different driving voltages, where the envelope modulation frequency and the dynamic stress frequency are in a linear relationship, and the envelope modulation frequency is independent of the amplitude of the driving voltage of the piezoelectric ceramic.
FIG. 7 is an interference diagram of the polarization maintaining fiber under two dynamic stresses, the frequency of the two dynamic stresses (A, B) is 30Hz, the amplitude of the output voltage of the signal generator is 2V and 500mV, and two dynamic coupling points exist in the interference diagram. Fig. 8 is an envelope curve extracted by interference fringes of two intercepted dynamic coupling points and a Hilbert transform of the interference fringes. FIG. 9 is a time-frequency distribution diagram of wavelet transform of an envelope curve, in which two frequency components of 30.05Hz and 29.11Hz are respectively corresponding to dynamic stresses A and B.
Claims (1)
1. A distributed dynamic stress frequency measurement method based on white light interference is realized by using a distributed polarization coupling test system based on white light interference, and comprises the following steps:
1) the method for applying dynamic stress to the polarization maintaining fiber by utilizing the piezoelectric ceramics comprises the following steps: gaussian light emitted by a light source becomes linearly polarized light through a polarizer and is coupled to a slow axis of a polarization maintaining optical fiber, when the dynamic stress is generated by piezoelectric ceramics, a part of light is coupled to a fast axis of the optical fiber, and at an emergent end of the optical fiber, the light becomes space parallel light after passing through a beam expander; the rotatable half-wave plate adjusts the included angle between the light of the fast and slow axes and the transmission axis of the analyzer to be 45 degrees, and the analyzer projects the light of the fast and slow axes to the same direction; then, the light enters a Michelson interferometer, and the optical path difference generated in the polarization maintaining fiber is scanned through the movement of one arm of the Michelson interferometer; at the exit end of the Michelson interferometer, an interferogram obtained by receiving and scanning by a photoelectric detector is transmitted to a computer through a data acquisition card, and 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 a polarization maintaining optical fiber at a position 47.3cm away from the exit end of the optical fiber, wherein the size of the dynamic stress is determined by the size of the output voltage of a signal generator for driving the piezoelectric ceramic;
2) measuring a white light interference pattern under dynamic stress by using a distributed polarization coupling system to obtain an interference pattern;
3) intercepting the dynamic coupling points in the interference graph, carrying out Hilbert transformation on data to obtain envelopes of the coupling points, and then carrying out wavelet transformation on envelope curves of the coupling points to obtain a time-frequency distribution graph, wherein the adopted wavelet basis function is cmor3-3 wavelet, the length of a scale sequence is 4096, and the time-frequency distribution graph contains frequency information of dynamic stress.
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| JP2559248B2 (en) * | 1987-03-13 | 1996-12-04 | トムソン−セエスエフ | Detector for localized polarization coupling in birefringent optics |
| CN102095538A (en) * | 2011-02-25 | 2011-06-15 | 天津大学 | Data demodulation method for polarization maintaining fiber stress sensing |
| CN103353389A (en) * | 2013-06-21 | 2013-10-16 | 天津大学 | Frequency-domain interference spectrum demodulation method |
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| US8085407B2 (en) * | 2009-08-12 | 2011-12-27 | Honeywell International Inc. | Resonator optical gyroscope having input beam modulation optimized for high sensitivity and low bias |
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|---|---|---|---|---|
| JP2559248B2 (en) * | 1987-03-13 | 1996-12-04 | トムソン−セエスエフ | Detector for localized polarization coupling in birefringent optics |
| CN102095538A (en) * | 2011-02-25 | 2011-06-15 | 天津大学 | Data demodulation method for polarization maintaining fiber stress sensing |
| CN103353389A (en) * | 2013-06-21 | 2013-10-16 | 天津大学 | Frequency-domain interference spectrum demodulation method |
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| 分布式保偏光纤偏振耦合应力传感系统的实现;陈信伟;《中国激光》;20100630;第37卷(第6期);第1468页第3-5段，图1 * |
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