CN114777950B - Temperature strain double-parameter sensing system and method based on dual-wavelength pulse - Google Patents
Temperature strain double-parameter sensing system and method based on dual-wavelength pulse Download PDFInfo
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- G—PHYSICS
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- G01K—MEASURING TEMPERATURE; MEASURING QUANTITY OF HEAT; THERMALLY-SENSITIVE ELEMENTS NOT OTHERWISE PROVIDED FOR
- G01K11/00—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00
- G01K11/32—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00 using changes in transmittance, scattering or luminescence in optical fibres
- G01K11/324—Measuring temperature based upon physical or chemical changes not covered by groups G01K3/00, G01K5/00, G01K7/00 or G01K9/00 using changes in transmittance, scattering or luminescence in optical fibres using Raman scattering
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- 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
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- G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
- G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
- G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
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- G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
- G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
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- G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
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- G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
- G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
- G01D5/36—Forming the light into pulses
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Abstract
The invention discloses a temperature strain double-parameter sensing system and a method based on double-wavelength pulse, and particularly relates to the technical field of optical fiber sensing, wherein the sensing system comprises a double-wavelength pulse light source module, an optical fiber coupler, a circulator, a sensing unit module, a wavelength division multiplexer, a filtering module, an adjustable optical attenuator and a signal detection and demodulation module; the dual-wavelength pulse light source module can output picosecond pulse signals with adjustable central wavelength and same intensity, and is divided into two beams through the optical fiber coupler, wherein one beam is used as a reference beam; the other beam is used as a detection beam and enters the sensing unit module after passing through the circulator, and the generated fiber bragg grating reflected light and the backward Raman anti-Stokes scattered light enter the signal detection and demodulation module after being split by the wavelength division multiplexer. The invention combines fiber grating sensing and distributed Raman temperature sensing, adopts a differential detection and single photon detection mode, can simultaneously measure temperature and strain information, and has the characteristics of high system wavelength resolution and high spatial resolution, thereby meeting various requirements in practical application.
Description
Technical Field
The invention belongs to the technical field of distributed optical fibers and quasi-distributed optical fiber grating sensing, and particularly relates to a temperature strain double-parameter sensing system and method based on dual-wavelength pulses.
Background
In recent years, as a novel sensing technology, optical fiber sensing is attracting more and more attention from researchers, wherein the optical fiber grating sensor and the distributed Raman sensor have wide application in the field of structural safety detection due to the characteristics of strong electromagnetic interference resistance, high sensitivity, corrosion resistance, good electrical insulation performance and the like. However, the fiber grating has the problem of temperature cross sensitivity during strain sensing, and the distributed raman temperature sensor is only sensitive to temperature, but the spatial resolution of the distributed raman temperature sensor is limited to the order of meters, so that the realization of high-precision and high-spatial resolution simultaneous sensing of temperature and strain is particularly important.
At present, a method for simultaneously measuring temperature and strain based on fiber grating sensing and distributed Raman sensing is realized, but the fiber grating still adopts a single-wavelength demodulation method, so that errors caused by power fluctuation of a laser cannot be eliminated, and the demodulation precision of the central wavelength of the fiber grating is reduced.
Disclosure of Invention
In order to solve the problems, the invention provides a temperature strain double-parameter sensing system and a temperature strain double-parameter sensing method based on dual-wavelength pulses, which are used for synchronously detecting reflected light of an optical fiber grating and backward Raman anti-Stokes scattered light, and improving the wavelength resolution and the spatial resolution of the system by adopting the modes of dual-wavelength pulse differential detection and single photon detection, so that the temperature strain double-parameter simultaneous sensing with high precision and high spatial resolution can be realized.
The technical scheme adopted by the invention is as follows:
the sensing system comprises a dual-wavelength pulse light source module, a first optical fiber coupler, a second optical fiber coupler, a first circulator, a sensing unit module, a wavelength division multiplexer, a filtering module, an adjustable attenuator and a signal detection and demodulation module; the dual-wavelength pulse light source module is connected with the first optical fiber coupler, the first optical fiber coupler is respectively connected with one port of the first circulator and the second optical fiber coupler, two ports and three ports of the first circulator are respectively connected with the sensing unit module and the com port of the wavelength division multiplexer, signals of 1450nm output ports of the wavelength division multiplexer enter the signal detection and demodulation module through the filtering module, and signals of 1550nm output ports of the wavelength division multiplexer enter the signal detection and demodulation module through the second optical fiber coupler and the adjustable attenuator in sequence.
The dual-wavelength pulse light source module consists of a tunable laser, an electro-optical modulator and a pulse signal generator, and can output picosecond pulse light with adjustable wavelength, and the central wavelength of the output light is positioned in the reflection spectrum of the fiber bragg grating sensor;
the first optical fiber coupler and the second optical fiber coupler are used for dividing pulse signals into reference light and detection light with the power ratio of 1:99;
the first circulator and the second circulator are used for guiding incident, reflected or scattered pulse light;
the sensing unit module is a single Gaussian fiber bragg grating or a serial or/and parallel structure formed by a plurality of Gaussian fiber bragg gratings with the same central wavelength;
the filtering module consists of a second circulator and a high-reflectivity uniform fiber bragg grating and is used for filtering noise signals of fiber bragg grating reflected light and Rayleigh scattered light which are crosstalked to an 1450nm output port of the wavelength division multiplexer;
the signal detection and demodulation unit comprises a first single photon detector, a second single photon detector, a time-to-digital converter and an upper computer. The single photon detector is composed of avalanche diode or superconductive waveguide device, the time-digital converter is composed of single chip microcomputer and/or programmable logic device, and/or digital signal processing chip, and/or embedded chip, and/or delay AND device, and the upper computer contains signal demodulation and processing program.
Further, the sensing method comprises the following steps:
s1, respectively outputting a central wavelength lambda by the dual-wavelength pulse light source module for the first time and the second time 1 、λ 2 Picosecond pulsed light, lambda 1 、λ 2 All around 1550nm with small wavelength differences, the peak power and pulse width are the same.
S2, dividing the pulse into two beams through a first optical fiber coupler, wherein one beam is used as detection light, and the detection light enters a sensing unit module through a circulator to detect the external temperature and strain information; one beam enters the second fiber coupler as reference light.
S3, enabling the reflected light of the fiber bragg grating and the reflected light of the backward Raman anti-Stokes generated by the sensing unit module to enter a wavelength division multiplexer through two ports and three ports of the circulator, dividing the reflected light of the backward Raman Stokes and the reflected light of the grating into two paths by the wavelength division multiplexer, enabling a 1450nm output end signal to enter a first single photon detector after passing through the filtering module, enabling a 1550nm output end signal to enter a second single photon detector after passing through a second fiber coupler and the adjustable optical attenuator, enabling the light reflected by the fiber bragg gratings at different positions and all positions of the sensing fiber to have different energy and propagation time, and enabling the light reflected by the fiber bragg gratings to obtain photon coincidence count values changing along with time on the time-to-digital converter after being detected by the first single photon detector and the second single photon detector;
s4, the upper computer analyzes and processes the photon to accord with the count value, so that the temperature and strain information of each point of the optical fiber can be obtained; the high-precision space positioning of the fiber bragg grating position point can be realized by analyzing the time coordinate corresponding to the photon coincidence count value and utilizing the OTDR positioning principle.
Further, the intensity of the backward raman anti-stokes scattered light generated by the pulsed light in the step S3 changes with the external temperature, and the central wavelength drift amount of the grating reflected light changes with the external temperature and the strain, so that sensing is realized;
the central wavelength of the pulse sent by the dual-wavelength pulse light source module is lambda respectively 1 And lambda (lambda) 2 When the photon coincidence count value of the reference pulse light is N 1 (λ 1 )、N 1 (λ 2 ) It can be expressed as:
wherein k is 1 Is constant and has a size ofWherein m is the power duty cycle of the reference light, P 0 For pulse peak power, η is the detection efficiency of the single photon detector, t is the measurement time, τ is the pulse width, f is the laser repetition frequency, α 0 Optical fiber attenuation constant for incident light, L 1 Is the distance of the first coupler from the single photon detector.
The central wavelength of the pulse sent by the dual-wavelength pulse light source module is lambda respectively 1 And lambda (lambda) 2 When the photon coincidence count value of the pulse light reflected by the fiber bragg grating is N respectively 2 (λ 1 )、N 2 (λ 2 ) It can be expressed as:
wherein k is 2 Is constant and has a size of(1-m) is the spectral power ratio of the probe light, R max Is the peak reflectivity of the fiber bragg grating lambda B Is the center wavelength omega of the fiber grating B Is 3dB bandwidth of Gaussian spectrum, L 2 Is the distance from the fiber grating to the circulator, wherein the Gaussian spectrum center wavelength lambda B The amount of drift Δλ of (a) varies with the magnitude of the temperature and strain, which can be expressed as:
Δλ＝(α+β)λ B ΔT+(1-P e )λ B Δε＝Δλ T +Δλ ε (5)
wherein alpha is the thermal expansion coefficient, beta is the thermo-optic coefficient, P e For effective elastance, ΔT and ΔTEpsilon is respectively the temperature change and the axial stress change.
The central wavelength of the pulse sent by the dual-wavelength pulse light source module is lambda respectively 1 And lambda (lambda) 2 When the photon coincidence count value of backward Raman anti-Stokes scattering is N AS (λ 1 ,T)、N AS (λ 2 T), which can be expressed as:
wherein k is 2 Is constant and has a size ofWhere η is the detection efficiency of the single photon detector, Δf AS Filter bandwidth, P, for 1450nm output channel of wavelength division multiplexer 0 For peak pulse power, D is the duty cycle of the pulse, L 3 Is the distance between the position of the optical fiber point to be measured and the circulator, T is L 3 Temperature at location g R,AS Lambda is the Raman gain coefficient AS Is Raman anti-Stokes wavelength, alpha AS Fiber attenuation constant, B, for Raman anti-Stokes light AS For dark counts and background photon counts due to Rayleigh backscattering and fiber grating reflected light crosstalk, N (T) is a Raman anti-Stokes temperature modulation function with a size of +.>
Further, in the step S4, the upper computer performs differential processing on the fiber grating signal in a logarithmic manner to obtain a grating measurement signal M, which may be expressed as:
wherein S is a sensitivity coefficient, and the size isλ c The central wavelength of the pulse light with two different wavelengths is +.>The demodulated center wavelength shift Δλ can be obtained according to equation (9) and the change Δm of the grating measurement signal changes linearly, which can be expressed as:
further, in the step S4, the upper computer will have the same temperature T 0 The backward raman anti-stokes scattered photons generated at the next two wavelengths match the count N AS (λ 1 ,T 0 )、N AS (λ 2 ,T 0 ) Average value of (2)As the backward raman anti-stokes scattered photons of the current temperature conform to the count value, when the temperature changes to deltat, photons scattered according to the backward raman anti-stokes conform to the change deltan of the count value AS ＝N′ AS (T 0 +ΔT)-N′ AS (T 0 ) The temperature change delta T of the measured optical fiber can be calculated.
Further, whenIn which t is i Is L 2 The time of arrival of the reflected light of the ith grating at the single photon detector, i.e. the position L where the grating is located 2 And the position L of the optical fiber point to be measured 3 Coincidence, determining the grating wavelength drift delta lambda caused by temperature by delta T T ＝(α+β)λ B DeltaT, and according to the formula (9) and the formula (5), respectively determining the total grating wavelength drift Deltalambda and the temperature inducedGrating wavelength shift delta lambda T Calculating the wavelength shift delta lambda of the grating caused by strain ε ＝Δλ-Δλ T Thereby obtaining the strain caused by the change of the external physical information>Thereby realizing simultaneous measurement of temperature and strain information.
In summary, by adopting the scheme, the beneficial effects of the invention are as follows:
1. the invention combines quasi-distributed fiber grating sensing and distributed Raman sensing, utilizes the characteristic that the fiber Raman temperature sensor is only sensitive to temperature, realizes temperature compensation for fiber grating stress sensing, and effectively solves the problem of temperature/strain cross sensitivity of the fiber grating sensor.
2. The invention adopts a differential detection mode to demodulate the center wavelength of the fiber bragg grating, namely, a demodulation method of dual-wavelength pulse is used for replacing the traditional single-wavelength pulse demodulation method, so that the error caused by the power fluctuation of a light source is eliminated, the wavelength resolution of the system can be effectively improved, and the measurement accuracy of the system is improved.
3. The invention introduces a structure of combining the high-reflectivity uniform fiber grating and the circulator, filters noise signals of the fiber grating reflected light and the Rayleigh scattered light of the Raman anti-Stokes channel, can inhibit crosstalk noise to the greatest extent under the condition of low cost, and improves the signal to noise ratio of a sensing system.
4. Compared with the traditional photoelectric detector, the single photon detector can realize high spatial resolution and high signal-to-noise ratio detection on single photons; high spatial resolution means that smaller spatially-spaced fiber grating sensing arrays can be detected, and high signal-to-noise ratio means that the system detection accuracy is higher.
Drawings
For a clearer description of the technical solutions of embodiments of the present invention, the drawings that are needed in the embodiments will be briefly described below, it being understood that the following drawings only illustrate some embodiments of the present invention and should not be considered limiting in scope, and other related drawings can be obtained according to these drawings without inventive effort for a person skilled in the art, wherein:
FIG. 1 is a schematic diagram of a frame of the present invention;
marked in the figure as: the device comprises a 1-dual-wavelength pulse light source module, a 2-first optical fiber coupler, a 3-second optical fiber coupler, a 4-first circulator, a 5-sensing unit module, a 6-wavelength division multiplexer, a 7-filtering module, an 8-second circulator, a 9-uniform fiber grating, a 10-adjustable attenuator, a 11-signal detection and demodulation module, a 12-first single photon detector, a 13-second single photon detector, a 14-time digital converter and a 15-upper computer.
Detailed Description
The present invention will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.
It is noted that relational terms such as "first" and "second", and the like, are used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Moreover, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one … …" does not exclude the presence of other like elements in a process, method, article, or apparatus that comprises the element.
The features and capabilities of the present invention are described in further detail below in connection with examples.
Examples
As shown in fig. 1, a temperature strain dual-parameter sensing system and method based on dual-wavelength pulse, wherein the sensing system comprises a dual-wavelength pulse light source module, a first optical fiber coupler, a second optical fiber coupler, a first circulator, a sensing unit module, a wavelength division multiplexer, a filtering module, an adjustable attenuator and a signal detection and demodulation module; the dual-wavelength pulse light source module is connected with the first optical fiber coupler, the first optical fiber coupler is respectively connected with one port of the first circulator and the second optical fiber coupler, two ports and three ports of the first circulator are respectively connected with the sensing unit module and the com port of the wavelength division multiplexer, signals of 1450nm output ports of the wavelength division multiplexer enter the signal detection and demodulation module through the filtering module, and signals of 1550nm output ports of the wavelength division multiplexer enter the signal detection and demodulation module through the second optical fiber coupler and the adjustable attenuator in sequence.
The dual-wavelength pulse light source module consists of a tunable laser, an electro-optical modulator and a pulse signal generator, and can output picosecond pulse light with adjustable wavelength, and the central wavelength of the output light is positioned in the reflection spectrum of the fiber bragg grating sensor;
the first optical fiber coupler and the second optical fiber coupler are used for dividing pulse signals into reference light and detection light with the power ratio of 1:99;
the first circulator and the second circulator are used for guiding incident, reflected or scattered pulse light;
the sensing unit module is a single Gaussian fiber bragg grating or a serial or/and parallel structure formed by a plurality of Gaussian fiber bragg gratings with the same central wavelength;
the filtering module consists of a second circulator and a high-reflectivity uniform fiber bragg grating and is used for filtering noise signals of fiber bragg grating reflected light and Rayleigh scattered light which are crosstalked to the 1450nm output end of the wavelength division multiplexer;
the signal detection and demodulation unit comprises a first single photon detector, a second single photon detector, a time-to-digital converter and an upper computer. The single photon detector is composed of avalanche diode or superconductive waveguide device, the time-digital converter is composed of single chip microcomputer and/or programmable logic device, and/or digital signal processing chip, and/or embedded chip, and/or delay AND device, and the upper computer contains signal demodulation and processing program.
The temperature strain double-parameter sensing system and method based on the double-wavelength pulse comprise the following specific steps:
s1, respectively outputting a central wavelength lambda by the dual-wavelength pulse light source module for the first time and the second time 1 、λ 2 Picosecond pulsed light, lambda 1 、λ 2 All around 1550nm with small wavelength differences, the peak power and pulse width are the same.
S2, dividing the pulse into two beams through a first optical fiber coupler, wherein one beam is used as detection light, and the detection light enters a sensing unit module through a circulator to detect the external temperature and strain information; one beam enters the second fiber coupler as reference light.
S3, enabling the reflected light of the fiber bragg grating and the reflected light of the backward Raman anti-Stokes generated by the sensing unit module to enter a wavelength division multiplexer through two ports and three ports of the circulator, dividing the reflected light of the backward Raman Stokes and the reflected light of the grating into two paths by the wavelength division multiplexer, enabling a 1450nm output end signal to enter a first single photon detector after passing through the filtering module, enabling a 1550nm output end signal to enter a second single photon detector after passing through a second fiber coupler and the adjustable optical attenuator, enabling the light reflected by the fiber bragg gratings at different positions and all positions of the sensing fiber to have different energy and propagation time, and enabling the light reflected by the fiber bragg gratings to obtain photon coincidence count values changing along with time on the time-to-digital converter after being detected by the first single photon detector and the second single photon detector;
s4, the upper computer analyzes and processes the photon to accord with the count value, so that the temperature and strain information of each point of the optical fiber can be obtained; the high-precision space positioning of the fiber bragg grating position point can be realized by analyzing the time coordinate corresponding to the photon coincidence count value and utilizing the OTDR positioning principle.
Further, the intensity of the backward raman anti-stokes scattered light generated by the pulsed light in the step S3 changes with the external temperature, and the central wavelength drift amount of the grating reflected light changes with the external temperature and the strain, so that sensing is realized;
the central wavelength of the pulse sent by the dual-wavelength pulse light source module is lambda respectively 1 And lambda (lambda) 2 When the photon coincidence count value of the reference pulse light is N 1 (λ 1 )、N 1 (λ 2 ) It can be expressed as:
wherein k is 1 Is constant and has a size ofWherein m is the power duty cycle of the reference light, P 0 For pulse peak power, η is the detection efficiency of the single photon detector, t is the measurement time, τ is the pulse width, f is the laser repetition frequency, α 0 Optical fiber attenuation constant for incident light, L 1 Is the distance of the first coupler from the single photon detector.
The central wavelength of the pulse sent by the dual-wavelength pulse light source module is lambda respectively 1 And lambda (lambda) 2 When the photon coincidence count value of the pulse light reflected by the fiber bragg grating is N respectively 2 (λ 1 )、N 2 (λ 2 ) It can be expressed as:
wherein k is 2 Is constant and has a size of(1-m)To detect the split power ratio of the light, R max Is the peak reflectivity of the fiber bragg grating lambda B Is the center wavelength omega of the fiber grating B Is 3dB bandwidth of Gaussian spectrum, L 2 Is the distance from the fiber grating to the circulator, wherein the Gaussian spectrum center wavelength lambda B The amount of drift Δλ of (a) varies with the magnitude of the temperature and strain, which can be expressed as:
Δλ＝(α+β)λ B ΔT+(1-P e )λ B Δε＝Δλ T +Δλ ε (5)
wherein alpha is the thermal expansion coefficient, beta is the thermo-optic coefficient, P e For effective elastance, ΔT and Δε are the magnitudes of temperature change and axial stress change, respectively.
The central wavelength of the pulse sent by the dual-wavelength pulse light source module is lambda respectively 1 And lambda (lambda) 2 When the photon coincidence count value of backward Raman anti-Stokes scattering is N AS (λ 1 ,T)、N AS (λ 2 T), which can be expressed as:
wherein k is 2 Is constant and has a size ofWhere η is the detection efficiency of the single photon detector, Δf AS Filter bandwidth, P, for 1450nm output channel of wavelength division multiplexer 0 For peak pulse power, D is the duty cycle of the pulse, L 3 Is the distance between the position of the optical fiber point to be measured and the circulator, T is L 3 Temperature at location g R,AS For the raman gain coefficient, alpha AS Optical fiber attenuation constant, lambda, for raman anti-stokes light AS For the Raman anti-Stokes wavelength, B AS For dark counting and Rayleigh back dispersionThe number of background photons caused by the crosstalk of reflected light of the optical fiber grating, N (T) is a Raman anti-Stokes temperature modulation function, and the size of the background photons is +.>
Further, in the step S4, the upper computer performs differential processing on the fiber grating signal in a logarithmic manner to obtain a grating measurement signal M, which may be expressed as:
wherein S is a sensitivity coefficient, and the size isλ c The central wavelength of the pulse light with two different wavelengths is +.>The demodulated center wavelength shift Δλ can be obtained according to equation (9) and the change Δm of the grating measurement signal changes linearly, which can be expressed as:
further, in the step S4, the upper computer will have the same temperature T 0 The backward raman anti-stokes scattered photons generated at the next two wavelengths match the count N AS (λ 1 ,T 0 )、N AS (λ 2 ,T 0 ) Average value of (2)As the backward raman anti-stokes scattered photons of the current temperature conform to the count value, when the temperature changes to deltat, photons scattered according to the backward raman anti-stokes conform to the change deltan of the count value AS ＝N′ AS (T 0 +ΔT)-N AS (T 0 ) The temperature change delta T of the measured optical fiber can be calculated.
Further, whenIn which t is i Is L 2 The time of arrival of the reflected light of the ith grating at the single photon detector, i.e. the position L where the grating is located 2 And the position L of the optical fiber point to be measured 3 Coincidence, determining the grating wavelength drift delta lambda caused by temperature by delta T T ＝(α+β)λ B DeltaT, and according to formula (9) and formula (5), total grating wavelength drift Deltalambda and temperature induced grating wavelength drift Deltalambda, respectively T Calculating the wavelength shift delta lambda of the grating caused by strain ε ＝Δλ-Δλ T Thereby obtaining the strain caused by the change of the external physical information> Thereby realizing simultaneous measurement of temperature and strain information.
The above description is not intended to limit the scope of the invention, but is intended to cover any modifications, equivalents, and improvements within the spirit and scope of the present invention.
Claims (3)
1. The temperature strain double-parameter sensing system based on the dual-wavelength pulse is characterized by comprising a dual-wavelength pulse light source module (1), a first optical fiber coupler (2), a second optical fiber coupler (3), a first circulator (4), a sensing unit module (5), a wavelength division multiplexer (6), a filtering module (7), a second circulator (8), a high-reflectivity uniform fiber grating (9) adjustable attenuator (10) and a signal detection and demodulation unit (11); the dual-wavelength pulse light source module is connected with the first optical fiber coupler (2), the first optical fiber coupler (2) is respectively connected with one port of the first circulator (4) and the second optical fiber coupler (3), two ports and three ports of the first circulator (4) are respectively connected with the sensing unit module (5) and the com port of the wavelength division multiplexer (6), signals of 1450nm output ports of the wavelength division multiplexer enter the signal detection and demodulation unit (11) through the filtering module (7), and signals of 1550nm output ports of the wavelength division multiplexer enter the signal detection and demodulation unit (11) through the second optical fiber coupler (3) and the adjustable attenuator (10) in sequence;
the dual-wavelength pulse light source module (1) consists of a tunable laser, an electro-optical modulator and a pulse signal generator, and can output picosecond pulse light with adjustable wavelength, and the central wavelength of the output light is positioned in the reflection spectrum of the fiber bragg grating sensor;
the first optical fiber coupler (2) and the second optical fiber coupler (3) are used for dividing pulse signals into reference light and detection light with the power ratio of 1:99;
the first circulator (4) and the second circulator (8) are used for guiding incident, reflected or scattered pulse light;
the sensing unit module is a single Gaussian fiber bragg grating or a serial or/and parallel structure formed by a plurality of Gaussian fiber bragg gratings with the same central wavelength;
the filtering module (7) consists of a second circulator (8) and a high-reflectivity uniform fiber bragg grating (9) and is used for filtering noise signals of fiber bragg grating reflected light and Rayleigh scattered light which are crosstalked to the 1450nm output end of the wavelength division multiplexer;
the signal detection and demodulation unit (11) comprises a first single photon detector (12), a second single photon detector (13), a time-digital converter (14) and an upper computer (15), wherein the first single photon detector (12) and the second single photon detector (13) are composed of avalanche diodes or superconducting waveguide devices, the time-digital converter (14) is composed of a digital signal processing chip, and the upper computer (15) contains a signal demodulation and processing program.
2. A sensing method of a dual wavelength pulse based temperature strain dual parametric sensing system according to claim 1, wherein the sensing method comprises the steps of:
s1, respectively outputting a central wavelength lambda by the dual-wavelength pulse light source module for the first time and the second time 1 、λ 2 Picosecond pulsed light, lambda 1 、λ 2 The peak power and the pulse width are the same, and the peak power and the pulse width are all around 1550nm and have small wavelength difference;
s2, dividing the pulse into two beams through a first optical fiber coupler, wherein one beam is used as detection light, and the detection light enters a sensing unit module through a circulator to detect the external temperature and strain information; one beam enters the second optical fiber coupler as reference light;
s3, enabling the reflected light of the fiber bragg grating and the reflected light of the backward Raman anti-Stokes generated by the sensing unit module to enter a wavelength division multiplexer through two ports and three ports of the circulator, dividing the reflected light of the backward Raman Stokes and the reflected light of the grating into two paths by the wavelength division multiplexer, enabling a 1450nm output end signal to enter a first single photon detector after passing through the filtering module, enabling a 1550nm output end signal to enter a second single photon detector after passing through a second fiber coupler and the adjustable optical attenuator, enabling the light reflected by the fiber bragg gratings at different positions and all positions of the sensing fiber to have different energy and propagation time, and enabling the light reflected by the fiber bragg gratings to obtain photon coincidence count values changing along with time on the time-to-digital converter after being detected by the first single photon detector and the second single photon detector;
s4, the upper computer analyzes and processes the photon to accord with the count value, so that the temperature and strain information of each point of the optical fiber can be obtained; the high-precision space positioning of the fiber bragg grating position point can be realized by analyzing the time coordinate corresponding to the photon coincidence count value and utilizing the OTDR positioning principle.
3. The sensing method according to claim 2, wherein the intensity of the backward raman anti-stokes scattered light generated by the pulsed light in the step S3 varies with the external temperature, and the center wavelength shift of the reflected light of the grating varies with the external temperature and the strain, so as to implement sensing;
the central wavelength of the pulse sent by the dual-wavelength pulse light source module is lambda respectively 1 And lambda (lambda) 2 When the photon coincidence count value of the reference pulse light is N 1 (λ 1 )、N 1 (λ 2 ) It can be expressed as:
wherein k is 1 Is constant and has a size ofWherein m is the power duty cycle of the reference light, P 0 For pulse peak power, η is the detection efficiency of the single photon detector, t is the measurement time, τ is the pulse width, f is the laser repetition frequency, α 0 Optical fiber attenuation constant for incident light, L 1 Distance from the first coupler to the single photon detector;
the central wavelength of the pulse sent by the dual-wavelength pulse light source module is lambda respectively 1 And lambda (lambda) 2 When the photon coincidence count value of the pulse light reflected by the fiber bragg grating is N respectively 2 (λ 1 )、N 2 (λ 2 ) It can be expressed as:
wherein k is 2 Is constant and has a size of(1-m) is the spectral power ratio of the probe light, R max Is the peak reflectivity of the fiber bragg grating lambda B Is the center wavelength omega of the fiber grating B Is 3dB bandwidth of Gaussian spectrum, L 2 For fibre-optic gratings to circulatorsDistance, where Gaussian spectrum center wavelength lambda B The amount of drift Δλ of (a) varies with the magnitude of the temperature and strain, which can be expressed as:
Δλ＝(α+β)λ B ΔT+(1-P e )λ B Δε＝Δλ T +Δλ ε (5)
wherein alpha is the thermal expansion coefficient, beta is the thermo-optic coefficient, P e For effective elastance, delta T and delta epsilon are respectively the magnitudes of temperature change and axial stress change, delta lambda T As the temperature-induced grating wavelength drift, Δλ ε The grating wavelength drift amount caused by strain;
the central wavelength of the pulse sent by the dual-wavelength pulse light source module is lambda respectively 1 And lambda (lambda) 2 When the photon coincidence count value of backward Raman anti-Stokes scattering is N AS (λ 1 ,T)、N AS (λ 2 T), which can be expressed as:
wherein k is 3 Is constant and has a size ofWhere η is the detection efficiency of the single photon detector, Δf AS Filter bandwidth, P, for 1450nm output channel of wavelength division multiplexer 0 For peak pulse power, D is the duty cycle of the pulse, L 3 Is the distance between the position of the optical fiber point to be measured and the circulator, T is L 3 Temperature at location g R,AS Lambda is the Raman gain coefficient AS Is Raman anti-Stokes wavelength, alpha AS Fiber attenuation constant, B, for Raman anti-Stokes light AS For dark counts, the number of background photons due to rayleigh backscattering and fiber grating reflected light crosstalk, N (T) is the raman anti-stokes temperature modulation function,the size is as follows
In the step S4, the upper computer performs differential processing on the fiber grating signal in a logarithmic manner to obtain a grating measurement signal M, which may be expressed as:
wherein S is a sensitivity coefficient, and the size isλ c The central wavelength of the pulse light with two different wavelengths is +.>The demodulated center wavelength shift Δλ can be obtained according to equation (9) and the change Δm of the grating measurement signal changes linearly, which can be expressed as:
the same temperature T is set in the step S4 0 The backward raman anti-stokes scattered photons generated at the next two wavelengths match the count N AS (λ 1 ,T 0 )、N AS (λ 2 ,T 0 ) Average value of (2)As the backward raman anti-stokes scattered photons of the current temperature conform to the count value, when the temperature changes to deltat, photons scattered according to the backward raman anti-stokes conform to the change deltan of the count value AS ＝N' AS (T 0 +ΔT)-N' AS (T 0 ) The temperature change delta T of the measured optical fiber can be calculated; when->In which t is i Is L 2 The time of arrival of the reflected light of the ith grating at the single photon detector, i.e. the position L where the grating is located 2 And the position L of the optical fiber point to be measured 3 Coincidence, the delta T determines the grating wavelength drift delta lambda caused by temperature T ＝(α+β)λ B DeltaT, the formula (9) and the formula (5) respectively determine the total grating wavelength drift Deltalambda and the temperature-induced grating wavelength drift Deltalambda T Calculating the wavelength shift delta lambda of the grating caused by strain ε ＝Δλ-Δλ T Thereby obtaining the strain caused by the change of the external physical information>Thereby realizing simultaneous measurement of temperature and strain information.
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