CN113932726B - Low-frequency strain detection system and method based on dual-signal phase difference - Google Patents
Low-frequency strain detection system and method based on dual-signal phase difference Download PDFInfo
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- CN113932726B CN113932726B CN202111199389.3A CN202111199389A CN113932726B CN 113932726 B CN113932726 B CN 113932726B CN 202111199389 A CN202111199389 A CN 202111199389A CN 113932726 B CN113932726 B CN 113932726B
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- 239000013307 optical fiber Substances 0.000 claims description 15
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/16—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge
- G01B11/161—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge by interferometric means
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Abstract
The invention discloses a low-frequency strain detection system and a detection method based on a dual-signal phase difference, wherein the system comprises a laser, two interferometers, two phase modulators, a signal generator, two photoelectric detectors and a signal demodulation and modulation module; the two interferometers share an interference arm, which is a reference arm on which a phase modulator is arranged; the interference arm of one interferometer is used for sensing external strain and is a sensing arm; the interference arm of the other interferometer is a compensation arm, and a phase modulator is arranged on the interference arm; according to the invention, the phase compensation technology is introduced into the interference signals, so that the phase difference of two paths of interference signals is about 90 degrees, the misjudgment that the phase difference of the two signals is increased or decreased when the phase difference is 0 or 180 degrees is avoided, the dynamic range expansion of the phase difference measurement technology is realized, and the practicability of the dual-interference signal phase difference low-frequency strain detection technology is improved.
Description
Technical Field
The invention relates to the technical field of optical fiber sensing, in particular to a low-frequency strain detection system and a detection method based on a dual-signal phase difference.
Background
Optical fiber sensors are widely used in many fields with the advantages of high sensitivity, wide dynamic range, electromagnetic interference resistance and the like. The sensing field comprises temperature, refractive index, magnetic field, strain and the like; fiber optic strain sensors, particularly for the low frequency band, play a more important role in many applications, such as bridge and building health inspection, formation movement, seismic monitoring, and the like. The low-frequency strain detection technology based on the double-interference signal phase difference provides a new thought for the field of optical fiber strain measurement, the change of the double-interferometer phase difference is calculated through an ellipse fitting algorithm, and the change of the phase difference is used for representing the external strain.
The above technique avoids the limitations typically present in passive phase-shifting demodulation: first, the initial phase difference must be precisely fixed at pi/2 or 2 pi/3; secondly, a Differential Cross Multiplication (DCM) algorithm or an arctangent algorithm (ATAN) is to be performed; however, the dual-interference signal phase difference low-frequency strain detection technology has the limitation of a phase difference measurement range: when the phase difference exceeds 0 to 180 degrees, the detection system cannot distinguish the phase difference change direction of the two interference signals, namely increase or decrease, so that the practicability of the technology is limited.
Disclosure of Invention
The invention aims to provide a low-frequency strain detection system and a detection method based on a dual-signal phase difference, which solve the problem that the low-frequency strain detection technology in the prior art has the limitation of a phase difference measurement range.
The aim of the invention can be achieved by the following technical scheme:
the low-frequency strain detection system based on the double-signal phase difference comprises a laser, two interferometers, two phase modulators, a signal generator, two photoelectric detectors and a signal demodulation and modulation module;
the two interferometers share an interference arm, which is a reference arm on which a phase modulator is arranged;
the interference arm of one interferometer is used for sensing external strain and is a sensing arm;
the interference arm of the other interferometer is a compensation arm, and a phase modulator is arranged on the interference arm;
the two interferometers sense external strain by the sensing arm, and the reference arm and the compensation arm are used for isolating external strain interference;
the signal generator and the phase modulator on the reference arm introduce phase modulated signals in the two interferometers;
the photoelectric detector is connected with the two interferometers and is used for converting the two-way interference signals into two-way electric signals;
the signal demodulation and modulation module is connected with the two photoelectric detectors, performs phase difference demodulation based on an ellipse fitting algorithm on the two-way electric signal, and sends out a voltage signal to drive a phase modulator on the compensation arm according to the obtained phase difference, so that the phase difference of the two-way electric signal tends to a preset rated value;
and calculating the phase difference change of the dual interferometers, and further obtaining the length change of the optical fiber of the sensing arm.
As a further aspect of the present invention, the detection method of the detection system includes the steps of:
step one, two interferometers generate two-way interference signals;
step two, exciting a phase modulator on a reference arm by a signal generator, and introducing phase modulation signals into two interferometers;
step three, converting the double-path interference signal into a double-path electric signal through a photoelectric detector;
step four, performing phase difference demodulation on the two-way electric signal through a signal demodulation and modulation module;
step five, according to the obtained phase difference, the signal demodulation and modulation module sends out a voltage signal to drive a phase modulator on the compensation arm, so that the phase difference of the two-way electric signal tends to a preset rated value;
and step six, calculating the phase difference change of the dual interferometers, and further obtaining the length change of the optical fiber of the sensing arm.
As a further aspect of the invention, the preset rating is 90 °.
As a further scheme of the invention, the specific steps of the fourth step and the fifth step are as follows:
the signal demodulation and modulation module is used for carrying out digital demodulation on the two-way digital signal based on an ellipse fitting algorithm to obtain the phase difference beta of the double interferometers n The amount epsilon of the phase difference deviating from 90 DEG is obtained n ＝β n -90 °, the data analog output in the signal demodulation and modulation module modulating the phase modulator on the compensation arm by Δv n To generate epsilon for the compensation arm n The phase of the two-way signal is changed to 90 deg..
As a further aspect of the present invention, in step six, the phase difference of the dual interferometers is measured to obtain β n+1 、ε n+1 The data analog output device modulates the phase modulator on the compensation arm to generate epsilon n+1 ＝β n+1 -90 DEG phase change, phase differenceThe single change cannot exceed 90 °, if it exceeds 90 °, the time interval between the two measurements is reduced; variation epsilon of phase difference according to nth and n+1th times n+1 Further calculating to obtain the change of the optical fiber length; from epsilon obtained from each test n Drawing external strain curve (alpha) n ，T n ) Wherein
The invention has the beneficial effects that:
according to the invention, the phase compensation technology is introduced into the interference signals, so that the phase difference of two paths of interference signals is about 90 degrees, the misjudgment that the phase difference of the two signals is increased or decreased when the phase difference is 0 or 180 degrees is avoided, the dynamic range expansion of the phase difference measurement technology is realized, and the practicability of the dual-interference signal phase difference low-frequency strain detection technology is improved.
Drawings
The invention is further described below with reference to the accompanying drawings.
FIG. 1 is a schematic diagram of a low frequency strain detection system based on a dual signal phase difference in an embodiment;
FIG. 2 is a phase compensation flow chart of a detection method of a low frequency strain detection system based on a dual signal phase difference in an embodiment;
FIG. 3 is a timing diagram of phase compensation of a detection method of a low frequency strain detection system based on a dual signal phase difference in an embodiment.
Detailed Description
The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
As shown in FIG. 1, the low-frequency strain detection system based on the dual-signal phase difference comprises a laser, two interferometers, two phase modulators, a signal generator, two photoelectric detectors and a signal demodulation and modulation module;
the two interferometers share an interference arm, which is a reference arm on which a phase modulator is arranged;
the interferometer is an optical fiber interferometer or a space optical interferometer, and the optical fiber interferometer comprises a Michelson interferometer, a Mach-Zehnder interferometer and a Fabry-Perot interferometer;
the interference arm of one interferometer is used for sensing external strain and is a sensing arm;
the interference arm of the other interferometer is a compensation arm, and a phase modulator is arranged on the compensation arm;
the two interferometers sense external strain by the sensing arm, and the reference arm and the compensation arm are used for isolating external strain interference;
the signal generator and the phase modulator on the reference arm introduce phase modulated signals in the two interferometers;
the photoelectric detector is connected with the two interferometers and is used for converting the two-way interference signals into two-way electric signals;
the signal demodulation and modulation module is connected with the two photoelectric detectors, performs phase difference demodulation based on an ellipse fitting algorithm on the two-way electric signal, and sends out a voltage signal to drive a phase modulator on the compensation arm according to the obtained phase difference, so that the phase difference of the two-way electric signal tends to a rated value between 0 and 180 degrees, the rated value is usually set to 90 degrees, and a sufficient adjustment space can be reserved on two sides;
and calculating the phase difference change of the dual interferometers, and further obtaining the length change of the optical fiber of the sensing arm.
The detection method of the low-frequency strain detection system based on the dual-signal phase difference comprises the following steps of:
step one, two interferometers generate two-way interference signals;
step two, exciting a phase modulator on a reference arm by a signal generator, and introducing phase modulation signals into two interferometers;
phase modulated signals having amplitude greater than pi/2 radians are preferred;
step three, converting the double-path interference signal into a double-path electric signal through a photoelectric detector;
step four, performing phase difference demodulation based on an ellipse fitting algorithm on the two-way electric signal through a signal demodulation and modulation module;
step five, according to the obtained phase difference, the signal demodulation and modulation module sends out a voltage signal to drive a phase modulator on the compensation arm, so that the phase difference of the two-way electric signal tends to a rated value, and the rated value is usually set to be 90 degrees;
and step six, calculating the phase difference change of the dual interferometers, and further obtaining the length change of the optical fiber of the sensing arm.
Examples:
as shown in fig. 1, the low-frequency strain detection system based on the dual-signal phase difference comprises a laser, an optical beam splitter, a signal demodulation and modulation module, two phase modulators, two photodetectors, three optical couplers and a signal generator;
the data processing module comprises a data acquisition card, a data simulation output device and computer software calculation;
the following describes in detail a detection method of the low-frequency strain detection system based on dual-signal phase difference according to the present invention with reference to fig. 1 to 3, the detection method comprising the steps of:
the first step, the laser is divided into three paths of light beam signals after passing through an optical beam splitter, and the first light beam signal reaches a photoelectric detector 1 through an optical coupler 1; the second light beam signal is respectively injected into the optical couplers 1 and 3 after passing through the phase modulator 1 and the optical coupler 2; the third light beam signal reaches the photoelectric detector 2 through the phase modulator 2 and the optical coupler 3; the first beam signal and the second beam signal interfere at the optical coupler 1, and the third beam signal and the second beam signal interfere at the optical coupler 3 to form two interference signals;
the optical fiber and the optical device used in the whole optical path are all polarization-maintaining.
Secondly, exciting the phase modulator 1 by a signal generator, and introducing a phase modulation signal into the dual interferometer;
step three, converting the two-way interference signal into a two-way electric signal through a two-way photoelectric detector;
wherein the initial strength of the two electrical signals can be expressed as:
wherein a is 1 And a 2 Is the DC value of the initial signal, b 1 And b 2 For fringe visibility, n is the effective index of the fiber (n=1.46 for SM fiber), λ is the wavelength of the laser, L 1 And L 2 The length of the sensing arm, phi, of interferometer 1 and interferometer 2, respectively 1 And phi 2 Initial phase differences of interferometer 1 and interferometer 2, respectively, are set to phi for ease of analysis 1 Equal to phi 2 Beta is phi 1 And phi 2 The difference, also referred to as the phase difference of the dual interferometers, is caused by the difference in fiber lengths of the sensor arms 1 and 2, θ s Fiber length variation S for reference arm t The resulting phase change has an amplitude greater than pi/2;
step four, converting the two-way electric signal into a two-way digital signal through a data acquisition card in the signal demodulation and modulation module;
step five, the computer in the signal demodulation and modulation module is used for carrying out digital demodulation on the two-way digital signal based on an ellipse fitting algorithm to obtain a phase difference beta of the dual interferometers n The amount epsilon of the phase difference deviating from 90 DEG is obtained n ＝β n The data analog output in the-90 ° signal demodulation and modulation module modulates the phase modulator on the compensation arm by Δv n To generate epsilon for the compensation arm n The phase of the two-way signal is changed to 90 deg..
Step six, continuously measuring the phase difference of the double interferometers to obtain beta n+1 、ε n+1 Data analog output device pair compensation armThe phase modulator modulates to make the compensating arm generate epsilon n+1 ＝β n+1 -a phase change of 90 °, further a single change of phase difference cannot exceed 90 °, if 90 ° is exceeded, the time interval between the two measurements is reduced;
step seven, changing epsilon according to the phase difference between the nth and n+1th times n+1 Further calculating to obtain the change of the optical fiber length;
step eight, according to epsilon obtained by each test n Drawing external strain curve (alpha) n ，T n ) Wherein
Specifically, by introducing an ellipse fitting algorithm into the phase demodulation technique, the phase difference β of the dual interferometers can be calculated in real time using the ellipse fitting algorithm, and in the general form of the ellipse equation, equation 1 can be expressed as:
set two constants V 1 And V 2 Calculating the coefficient A, B, C, D, E of the elliptic function by using a least square fitting method, and then calculating an elliptic correction parameter beta;
from the resulting change in phase difference, the fiber length difference Δl of the sensor arm can be directly calculated.
ΔL may be expressed as
ΔL＝(Δβ×λ)/(180×4×n)，
Where Δβ is the variation value of the phase difference β.
In the description of the present invention, it should be understood that the terms "upper," "lower," "left," "right," and the like indicate an orientation or a positional relationship based on that shown in the drawings, and are merely for convenience of description and for simplifying the description, and do not indicate or imply that the apparatus or element in question must have a specific orientation, as well as a specific orientation configuration and operation, and thus should not be construed as limiting the present invention. Furthermore, the terms "first," "second," and the like, are used for descriptive purposes only and are not to be construed as indicating or implying a relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defining "a first" or "a second" may explicitly or implicitly include one or more such feature. In the description of the present invention, unless otherwise indicated, the meaning of "a plurality" is two or more.
In the description of the present invention, it should be noted that, unless explicitly specified and limited otherwise, the terms "mounted," "connected," and the like are to be construed broadly and may be, for example, fixedly connected, detachably connected, or integrally connected; can be mechanically or electrically connected; can be directly connected or indirectly connected through an intermediate medium, and can be communication between two elements. The specific meaning of the above terms in the present invention will be understood in specific cases by those of ordinary skill in the art.
The foregoing describes one embodiment of the present invention in detail, but the description is only a preferred embodiment of the present invention and should not be construed as limiting the scope of the invention. All equivalent changes and modifications within the scope of the present invention are intended to be covered by the present invention.
Claims (4)
1. The low-frequency strain detection system based on the double-signal phase difference comprises a laser, two interferometers, two phase modulators, a signal generator, two photoelectric detectors and a signal demodulation and modulation module, and is characterized in that:
the two interferometers share an interference arm, which is a reference arm on which a phase modulator is arranged;
the interference arm of one interferometer is used for sensing external strain and is a sensing arm;
the interference arm of the other interferometer is a compensation arm, and a phase modulator is arranged on the interference arm;
the two interferometers sense external strain by the sensing arm, and the reference arm and the compensation arm are used for isolating external strain interference;
the signal generator and the phase modulator on the reference arm introduce phase modulated signals in the two interferometers;
the photoelectric detector is connected with the two interferometers and is used for converting the two-way interference signals into two-way electric signals;
the signal demodulation and modulation module is connected with the two photoelectric detectors, performs phase difference demodulation based on an ellipse fitting algorithm on the two-way electric signal, and sends out a voltage signal to drive a phase modulator on the compensation arm according to the obtained phase difference, so that the phase difference of the two-way electric signal tends to a preset rated value;
calculating to obtain the phase difference change of the double interferometers, and further obtaining the length change of the sensing arm optical fiber;
the detection method of the detection system comprises the following steps:
step one, two interferometers generate two-way interference signals;
step two, exciting a phase modulator on a reference arm by a signal generator, and introducing phase modulation signals into two interferometers;
step three, converting the double-path interference signal into a double-path electric signal through a photoelectric detector;
step four, performing phase difference demodulation on the two-way electric signal through a signal demodulation and modulation module;
step five, according to the obtained phase difference, the signal demodulation and modulation module sends out a voltage signal to drive a phase modulator on the compensation arm, so that the phase difference of the two-way electric signal tends to a preset rated value;
and step six, calculating the phase difference change of the dual interferometers, and further obtaining the length change of the optical fiber of the sensing arm.
2. The dual signal phase difference based low frequency strain detection system of claim 1 wherein the predetermined nominal value is 90 °.
3. The dual signal phase difference based low frequency strain detection system of claim 1, wherein the specific steps of step four and step five are:
the signal demodulation and modulation module is used for carrying out digital demodulation on the two-way digital signal based on an ellipse fitting algorithm to obtain the phase difference beta of the double interferometers n The amount epsilon of the phase difference deviating from 90 DEG is obtained n ＝β n -90 °, the data analog output in the signal demodulation and modulation module modulating the phase modulator on the compensation arm by Δv n To generate epsilon for the compensation arm n The phase of the two-way signal is changed to 90 deg..
4. The dual signal phase difference based low frequency strain sensing system of claim 1 wherein the phase difference of the dual interferometers is measured in step six to obtain β n+1 、ε n+1 The data analog output device modulates the phase modulator on the compensation arm to generate epsilon n+1 ＝β n+1 -a phase change of 90 °, a single change of phase difference not exceeding 90 °, if exceeding 90 °, a time interval between two measurements is reduced; variation epsilon of phase difference according to nth and n+1th times n+1 Further calculating to obtain the change of the optical fiber length; from epsilon obtained from each test n Drawing external strain curve (alpha) n ，T n ) Wherein
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| CN113108710A (en) * | 2021-04-14 | 2021-07-13 | 安徽大学 | Optical low-frequency strain detection system and detection method based on ellipse fitting |
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| CN101485122A (en) * | 2006-04-27 | 2009-07-15 | 克雷斯特姆公司 | Method and apparatus for adaptively controlling signals |
| EP1985967A1 (en) * | 2007-04-26 | 2008-10-29 | Nxtar Technologies, Inc. | Fiber interferometric sensor and phase compensation method of PGC demodulator |
| CN108092932A (en) * | 2017-12-25 | 2018-05-29 | 中国科学技术大学 | It is a kind of that microwave source is exported based on double sideband modulation device and the frequency-adjustable multi-frequency of frequency multiplier nonlinear effect |
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