CN112082498A - Noise suppression sensing method based on phase measurement method OFDR strain and temperature - Google Patents
Noise suppression sensing method based on phase measurement method OFDR strain and temperature Download PDFInfo
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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
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- G—PHYSICS
- G01—MEASURING; TESTING
- 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
Abstract
The invention belongs to the technical field of optical fiber sensing, and particularly relates to a phase measurement method based on OFDR strain and temperature noise suppression sensing, which adopts a nearest neighbor analysis method to count phase measurement data during OFDR strain/temperature sensing, selects stable signals as real signals in a sliding distance window for multiple groups of signals with similar phase values, thereby suppressing random jump of phase information caused by noise influence, solving the defect of unstable signals of a phase measurement type OFDR system, and simultaneously realizing strain/temperature measurement with high stability and high spatial resolution.
Description
Technical Field
The invention belongs to the technical field of optical fiber sensing, and particularly relates to a phase measurement method-based OFDR strain and temperature noise suppression sensing method.
Background
The distributed optical fiber sensing technology attracts a great deal of attention because of the capability of simultaneously measuring the relevant physical quantities of all points along the optical fiber, and has the characteristics of electromagnetic interference resistance, intrinsic safety, high sensitivity and the like, and is widely applied to the fields of structural health monitoring, safety protection and the like. In all distributed Optical fiber sensing technologies, an Optical Frequency Domain Reflectometer (OFDR) based on the Frequency modulated continuous wave principle has the characteristic of outstanding high spatial resolution, and therefore has high application potential in specific application scenes such as Optical waveguide detection, high spatial resolution temperature, strain, vibration detection and other fields.
The method for realizing the OFDR in the distributed stress/temperature detection comprises a cross-correlation method and a phase measurement method. The basic physical principle of the cross-correlation method is that relatively stable but random Rayleigh scattering distribution in an optical fiber can be regarded as random weak optical fiber grating, and the reflection spectrum position of the weak reflection grating and the temperature and strain variation form a simple linear relation in a certain range. In actual operation, the OFDR time sequence signals before and after stress/temperature change are subjected to fast Fourier transform and windowing processing and inverse Fourier transform, so that reflection spectrum signals along the optical fiber can be obtained, cross-correlation operation is performed on the reflection spectrum signals before and after stress/temperature change, so that the spectral movement amount can be obtained, and finally, the strain/temperature change amount can be obtained by combining optical fiber parameters. In this process, if N data points are included in the window, the spatial resolution of the strain/temperature sensor is N times the spatial resolution of the system range. It can be seen that the spatial resolution of the strain/temperature sensing system cannot reach the range spatial resolution limit of the OFDR system when the cross-correlation method is used, due to the existence of the window.
Unlike cross-correlation methods, which track the phase change of scattered light caused by rayleigh scatterers along the fiber, phase measurement methods can also achieve strain/temperature measurements because strain/temperature changes cause proportional phase changes. Meanwhile, the phase measurement method can implement phase measurement along the optical fiber by taking the ranging spatial resolution of the OFDR system as a step length, so that the potential of fully utilizing the spatial resolution of the OFDR system is theoretically provided, but the phase measurement method only utilizes the information of a single scattering point, and random phase jump caused by various noises possibly causes instability of measurement, thereby causing adverse effects on the use of the method.
Disclosure of Invention
In view of the above shortcomings of the prior art, the present invention provides a noise suppression sensing method for OFDR strain and temperature based on a phase measurement method, and aims to solve the technical problem that the use of the method is adversely affected because only information of a single scattering point is utilized and random phase jump caused by various noises may cause instability of measurement in the existing OFDR strain and temperature sensing method based on a phase measurement method.
The invention provides a noise suppression sensing method of OFDR strain and temperature based on a phase measurement method, which has the following specific technical scheme:
the noise suppression sensing method based on the phase measurement method OFDR strain and temperature comprises the following steps:
s1, performing equal-period laser scanning on the optical fiber to be detected by using an optical frequency domain reflectometer, and collecting an OFDR time domain signal of the optical fiber to be detected;
s2, obtaining phase spectrum information of the OFDR time domain signal in the step S1 to obtain a phase curve phit(z), wherein t is a natural number and represents the scanned cycle number of the optical fiber to be measured, and z is the optical fiber distance;
s3, converting the phase curve phi in the step S2t(z) grouping, wherein each n groups of curves form a sequence, n is a natural number, and t in each sequence can represent a curve number less than or equal to n;
s4, obtaining a phase curve theta by taking the difference between the curve with the number t in each sequence and the curve with the same number t in the reference sequencet(z), sliding distance window is used to shift thetat(z) segmentation;
s5, calculating each theta according to formula (1) for all data in the sliding distance windowt(z) value of difference Δ Θij(z)；
ΔΘij(z)＝Θi(z)-Θj(z) (1)
Wherein i represents a natural number of 1 to n, and j represents a natural number
S6, for all delta thetaij(z) taking the variance and obtaining the phase curve sequence theta corresponding to the minimum k valuesk_1(z)、Θk_2(z)、…、Θk_2k(z), k is less than
i is a natural number;
s7, calculating the corrected phase curve in each sliding distance window according to the formula (2)
S8, curve all phases
Obtaining corrected complete phase curve end to end
The phase curve
And obtaining the stress and temperature information which is in direct proportion to different positions of the optical fiber to be measured after difference.
In some embodiments, in step S1, the OFDR time-domain signal is obtained by a collection device after mixing a rayleigh scattered light signal generated by the optical fiber to be measured with local oscillator light.
In some embodiments, in step S2, the phase spectrum information is sequentially subjected to frequency-distance conversion and unwrapping operation to obtain a phase curve Φt(z)。
In some embodiments, the temperature/strain experienced by the optical fiber within each sequence grouping is constant in step S3.
In some embodiments, in step S4, the reference sequence is all Φt(z) signal sequence as a reference state in the curve.
The invention has the following beneficial effects: the OFDR strain and temperature noise suppression sensing method based on the phase measurement method provided by the invention has the advantages that the same group of n phase curves phit(z) are substantially from the same groupState of the optical fiber, i.e. thetaijAnd (z) are basically consistent, and random phase jump can be picked out and removed, so that the noise influence generated by the phase random jump in the OFDR strain/temperature system based on a phase measurement method is effectively inhibited, and stable high-spatial resolution strain/temperature sensing is realized.
Drawings
FIG. 1 is a flow chart of a phase measurement based OFDR strain and temperature noise suppression sensing method provided by the present invention;
FIG. 2 is a schematic view of the structure of an optical frequency domain reflectometer in embodiment 1;
FIG. 3 is a typical power spectrum in example 1;
FIG. 4 is a phase distance curve in example 1;
FIG. 5 is a strain profile in example 1;
fig. 6 is a schematic view of the structure of the optical frequency domain reflectometer in embodiment 2.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to specific embodiments and the accompanying drawings.
Example 1
The optical frequency domain reflectometer in this embodiment comprises a noise suppression sensing system based on phase measurement OFDR strain and temperature, as shown in fig. 2, comprising a tunable laser 1, a first coupler 2, an auxiliary interferometer 3, a second coupler 4, a circulator 5, a coherent detection module 7 and a processing module 8. The tunable laser 1 outputs high-coherence continuous wave laser with scanned wavelength, the polarization state of the output light of the laser is linear polarization, and the laser can be DFB, DBR, VCSEL, ECDL and the like. The laser light is split into two beams by the first coupler 2. The weaker laser beam split by the coupler 2 is sent to the auxiliary interferometer 3 to form an external clock signal, and the auxiliary interferometer 3 can be a michelson structure interferometer or a mach-zehnder structure interferometer. The stronger part of the output of the first coupler 2 is first split into two parts again by the second coupler 4, wherein the stronger part enters from the first port of the circulator 5 and passes into the sensing fiber 6 from the second port, and the back rayleigh scattered light generated in the sensing fiber 6 enters from the second port of the circulator 5 and passes into the coherent detection module 7 from the third port together with the weaker beam of the second coupler 4. The specific configuration of the coherent detection module 7 is determined by the polarization characteristics of the system: if the system light path adopts a polarization maintaining device, that is, the rayleigh scattered light is linearly polarized light, the coherent detection module 7 can be a detection module formed by an optical coupler and a detector; if the light path of the system adopts a non-polarization-maintaining device, that is, the polarization state of the rayleigh scattered light is random, the coherent detection module 7 is a polarization-dependent detector consisting of a polarization beam splitter, a coupler and a detector. The output signal of the coherent detection module 7 and the external clock are both connected to the data acquisition and processing module 8 so as to obtain an OFDR time domain signal. The external clock signal can directly control the acquisition card to sample, and can also set the acquisition card mode as an internal clock, the nonlinear tuning quantity of the laser is corrected according to the external clock information during acquisition, and the corrected OFDR time domain signal is obtained through a secondary sampling technology.
The OFDR strain and temperature noise suppression sensing method based on the phase measurement method provided by the embodiment has the following specific technical scheme:
s1, performing equal-period laser scanning on the optical fiber to be detected by using an optical frequency domain reflectometer, and collecting an OFDR time domain signal of the optical fiber to be detected;
s2, the OFDR time domain signal in the step S1 is taken as phase spectrum information, and the phase spectrum information is subjected to frequency-distance conversion and unwrapping operation in sequence to obtain a phase curve phit(z), wherein t is a natural number and represents the scanned cycle number of the optical fiber to be measured, and z is the optical fiber distance;
s3, converting the phase curve phi in the step S2t(z) grouping, wherein the temperature/strain of the optical fiber in each sequence group is basically constant, each n groups of curves form a sequence, n is a natural number, and t in each sequence can represent a curve number less than or equal to n;
s4, obtaining a phase curve theta by taking the difference between the curve with the number t in each sequence and the curve with the same number t in the reference sequencet(z), sliding distance window is used to shift thetat(z) segmentation in which the reference sequence is all Φt(z) a signal sequence in the curve as a reference state;
s5, calculating each theta according to formula (1) for all data in the sliding distance windowt(z) value of difference Δ Θij(z)；
ΔΘij(z)＝Θi(z)-Θj(z) (1)
Wherein i represents a natural number of 1 to n, and j represents a natural number
S6, for all delta thetaij(z) taking the variance and obtaining the phase curve sequence theta corresponding to the minimum k valuesk_1(z)、Θk_2(z)、…、Θk_2k(z), k is less than
i is a natural number;
s7, calculating the corrected phase curve in each sliding distance window according to the formula (2)
S8, curve all phases
Obtaining corrected complete phase curve end to end
The phase curve
And obtaining the stress and temperature information which is in direct proportion to different positions of the optical fiber to be measured after difference.
As an example of high spatial resolution strain and temperature sensing, under the condition that the optical fiber to be measured, namely the sensing optical fiber 6 is 1m, the distance resolution is about 0.67mm, the total OFDR point number of the optical fiber is 1500, and two strong reflection points are arranged at the positions of 0.4 meter and 0.9 meter as marks, the wide position at the position of 0.68m is used as a mark in simulationA strain of 100u was applied to a 0.67mm fiber span and a typical power spectrum of the resulting signal is shown in fig. 3. The optical frequency domain reflectometer provided by this embodiment is used to perform the laser scanning for 20 times, and perform phase analysis on the time domain signal after the external clock is corrected to obtain a phase curve Φ1(z)、Φ2(z)、Φ3(z)、…、Φ20(z), the fiber strain is loaded between 10 th and 11 laser scans.
Will phase curve phi1(z)、Φ2(z)、Φ3(z)、…、Φ20(z) are divided into 2 groups, the first 10 groups are reference sequences, and the last 10 groups are measurement sequences. The measured sequence was compared with the reference sequence to obtain a phase curve Θ1(z)、Θ2(z)、Θ3(z)、…、Θ10(z). The raw measurement results are the thinner lines in fig. 4, from which it can be seen that Θ is due to the influence of various types of noiset(z) a random phase jump occurs, the jump value of which is about 2 pi, thereby affecting the measurement stability of the system. Using a sliding distance window of 50 points in width will Θ1(z)、Θ2(z)、Θ3(z)、…、Θ10(z) dividing, and calculating each theta according to a formula (1) for data in all distance windowst(z) value of difference Δ Θij(z), for all Δ Θij(z) take the variance and find the phase curve sequence Θ corresponding to the smallest k (in the example k takes 2) valuesk_1(z)、Θk_2(z)、…、Θk_2k(z) taking the value of thetak_1(z)、Θk_2(z)、…、Θk_2k(z) the average value is the corrected phase curve in the distance window
Namely to obtain
Will be provided with
The complete phase curve can be obtained by connecting the head and the tail
The thicker line in fig. 4 shows that the position of each phase jump has been correctly identified and corrected. The strain position in fig. 4 is enlarged and shown in a small graph, and the corrected point-shaped curve is observed to be in a predicted step shape, and the rising edge is only 1-2 points, which proves that the method obtains the strain measurement capability with the spatial resolution being the OFDR ranging resolution.
Finally, for the phase curve in FIG. 4
The difference is processed and converted to strain coordinates and the result is shown in fig. 5, where the strain data is also magnified and shown in a small graph. It can be seen that the main strain 60u occurs at only one spatial location, with a small amount of strain being shared by other peripheral points, a phenomenon caused by the randomness of the rayleigh scattering points in the fiber.
Example 2
The optical frequency domain reflectometer in this embodiment comprises a noise suppression sensing system based on phase measurement OFDR strain and temperature, as shown in fig. 6, comprising a laser 11, an adjustable frequency shifter 9, a signal generator 10, a second coupler 4, a circulator 5, a coherent detection module 7 and a processing module 8. The laser 11 outputs high-coherence continuous wave laser, the polarization state of the output light of the laser is linear polarization, and the laser can be DFB, DBR, VCSEL, ECDL and the like. The amount of frequency shift by the adjustable frequency shifter 9 is determined by the signal provided by the signal generator 3. The laser passes through the adjustable frequency shifter 9 and then is divided into two parts by the second coupler 4, wherein the stronger part enters from the first port of the circulator 5 and is transmitted into the sensing fiber 6 from the second port, and the backward rayleigh scattered light generated in the sensing fiber 6 enters from the second port of the circulator 5 and is transmitted into the coherent detection module 7 from the third port together with the weaker light beam of the second coupler 4. The output signals of the coherent detection modules 7 are all connected to the data acquisition and processing module 8 so as to obtain the OFDR time domain signal.
The OFDR strain and temperature noise suppression sensing method based on the phase measurement method provided by the embodiment has the following specific technical scheme:
s1, performing equal-period laser scanning on the optical fiber to be detected by using an optical frequency domain reflectometer, and collecting an OFDR time domain signal of the optical fiber to be detected;
s2, the OFDR time domain signal in the step S1 is taken as phase spectrum information, and the phase spectrum information is subjected to frequency-distance conversion and unwrapping operation in sequence to obtain a phase curve phit(z), wherein t is a natural number and represents the scanned cycle number of the optical fiber to be measured, and z is the optical fiber distance;
s3, converting the phase curve phi in the step S2t(z) grouping, wherein the temperature/strain of the optical fiber in each sequence group is basically constant, each n groups of curves form a sequence, n is a natural number, and t in each sequence can represent a curve number less than or equal to n;
s4, obtaining a phase curve theta by taking the difference between the curve with the number t in each sequence and the curve with the same number t in the reference sequencet(z), sliding distance window is used to shift thetat(z) segmentation;
s5, calculating each theta according to formula (1) for all data in the sliding distance windowt(z) value of difference Δ Θij(z)；
ΔΘij(z)＝Θi(z)-Θj(z) (1)
Wherein i represents a natural number of 1 to n, and j represents a natural number
S6, for all delta thetaij(z) taking the variance and obtaining the phase curve sequence theta corresponding to the minimum k valuesk_1(z)、Θk_2(z)、…、Θk_2k(z), k is less than
i is a natural number;
s7, calculating the corrected phase curve in each sliding distance window according to the formula (2)
S8，Curve all phases
Obtaining corrected complete phase curve end to end
The phase curve
And obtaining the stress and temperature information which is in direct proportion to different positions of the optical fiber to be measured after difference.
The above description is only for the purpose of illustrating preferred embodiments of the present invention and is not to be construed as limiting the invention, and the present invention is not limited to the above examples, and those skilled in the art should also be able to make various changes, modifications, additions or substitutions within the spirit and scope of the present invention.
Claims (5)
1. The noise suppression sensing method based on the OFDR strain and temperature of the phase measurement method is characterized by comprising the following steps of:
s1, performing equal-period laser scanning on the optical fiber to be detected by using an optical frequency domain reflectometer, and collecting an OFDR time domain signal of the optical fiber to be detected;
s2, obtaining phase spectrum information of the OFDR time domain signal in the step S1 to obtain a phase curve phit(z), wherein t is a natural number and represents the scanned cycle number of the optical fiber to be measured, and z is the optical fiber distance;
s3, converting the phase curve phi in the step S2t(z) grouping, wherein each n groups of curves form a sequence, n is a natural number, and t in each sequence can represent a curve number less than or equal to n;
s4, obtaining a phase curve theta by taking the difference between the curve with the number t in each sequence and the curve with the same number t in the reference sequencet(z), sliding distance window is used to shift thetat(z) segmentation;
s5, calculating each theta according to formula (1) for all data in the sliding distance windowt(z) value of difference Δ Θij(z)；
ΔΘij(z)＝Θi(z)-Θj(z) (1)
Wherein i represents a natural number of 1 to n, and j represents a natural number
S6, for all delta thetaij(z) taking the variance and obtaining the phase curve sequence theta corresponding to the minimum k valuesk_1(z)、Θk_2(z)、…、Θk_2k(z), k is less than
A natural number of (2);
s7, calculating the corrected phase curve in each sliding distance window according to the formula (2)
S8, curve all phases
Obtaining corrected complete phase curve end to end
The phase curve
And obtaining the stress and temperature information which is in direct proportion to different positions of the optical fiber to be measured after difference.
2. The noise suppression sensing method based on the strain and temperature of the OFDR of the phase measurement method of claim 1, wherein in step S1, the OFDR time domain signal is obtained by a collection device after mixing a rayleigh scattered light signal generated by an optical fiber to be measured and local oscillator light.
3. The noise suppression sensing method based on the strain and temperature of the OFDR of claim 1, wherein in step S2, the phase spectrum information is sequentially processed by frequency-distance conversion and unwrapping operation to obtain a phase curve Φt(z)。
4. The phase measurement OFDR strain and temperature based noise suppression sensing method of claim 1, wherein in step S3, the temperature/strain experienced by the optical fiber within each sequence group is substantially constant.
5. The phase measurement method OFDR strain and temperature based noise suppression sensing method of claim 1, wherein in step S4, the reference sequence is all Φt(z) signal sequence as a reference state in the curve.
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