CN116907627B - Optical path difference auxiliary-based large dynamic range distributed phase sensing method and device - Google Patents
Optical path difference auxiliary-based large dynamic range distributed phase sensing method and device Download PDFInfo
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Abstract
The invention discloses a large dynamic range distributed phase sensing method and device based on optical path difference assistance, which comprises an optical path difference measuring module, a phase measuring module, a multiplexing module, a scattering enhancement sensing optical fiber and a data acquisition demodulation system, wherein Optical Path Difference (OPD) demodulation with high noise and large dynamic range characteristics is combined with phase demodulation with low noise and high precision, and during data processing, phase unwrapping errors caused by pi phase principle are corrected by utilizing OPD data with large dynamic range to complete unwrapping of low noise phase data, so that phase unwrapping of large signals is realized, and the measurement dynamic range is improved while the demodulation precision of the system is maintained.
Description
Technical Field
The invention belongs to the technical field of optical fiber sensing, and particularly relates to a large dynamic range distributed phase sensing method and device based on optical path difference assistance.
Background
The distributed optical fiber acoustic wave sensing (DAS) system has the characteristics of long measurement distance, electromagnetic interference resistance, chemical corrosion resistance, good environmental suitability and the like, can realize the sensing and positioning of external acoustic waves by analyzing the back scattered light information of the detection pulse light in the optical fiber to be detected, and can continuously and non-intermittently measure the spatial temperature, strain and vibration information distributed along the optical fiber direction, thereby being widely focused and applied in the fields of structural health monitoring, perimeter security, underwater sound detection, resource exploration, seismic disaster monitoring and the like.
In various DAS systems, a phase-sensitive optical time domain reflectometry (phi-OTDR) based on coherent Rayleigh Backscattering (RBS) is a popular technology in the field of dynamic strain measurement due to high sensitivity and quick response, and the technology obtains acoustic optical fiber strain by demodulating phase information so as to realize waveform reconstruction of an acoustic event to be detected, and linear corresponding relations exist between the demodulated phase information and amplitude information of an acoustic signal, so that the change condition of the acoustic wave can be reflected well.
In the Φ -OTDR system, scattered light Phase information caused by the acoustic vibration signal to be measured is usually recovered by using an inverse trigonometric function (arctan (x)), and due to the periodicity of the inverse trigonometric function, the Phase information caused by the acoustic vibration signal is wrapped and rolled into a Phase change within [ -pi, pi ], which severely limits the measurement dynamic range of the acoustic vibration signal, and thus, a Phase Unwrapping algorithm is developed. The phase unwrapping follows the pi phase principle, i.e. requires that the absolute value of the phase difference between adjacent sample points is smaller than pi. In the algorithm, when the phase difference of adjacent points is larger than pi, the operation of adding 2 pi or subtracting 2 pi is performed according to the variation of the phase of two sampling points so as to keep the absolute value of the phase difference between the two adjacent points small pi, and further realize the expansion of the phase wrapped in [ -pi, pi ].
Although the phase Jie Juanrao algorithm realizes the improvement of the dynamic range of the system measurement, the pi phase principle also limits the further expansion of the dynamic range of the system measurement, and in practical application, the accuracy of phase unwrapping can be influenced by noise and sampling rate. When the phase change between two adjacent sampling points is larger than pi, the + -2 pi operation of the phase unwrapping algorithm can cause demodulation phase distortion, and pi phase principle, namely lifting sampling rate, is usually ensured by increasing the number of sampling points between large phase changes in practical application, which requires that the system must adopt a sampling rate much larger than the nyquist rate. However, in DAS systems, the sampling rate is an extremely precious "system resource", which cannot be lifted without limitation, and the high sampling rate aggravates the load of system data processing, which causes contradiction between the system measurement bandwidth and the dynamic range, so practical application needs to make trade-off between the measurement bandwidth and the dynamic range.
Disclosure of Invention
The invention aims to provide a large dynamic range distributed phase sensing method and device based on optical path difference assistance, which can solve the problem of limited measurement dynamic range caused by pi phase principle of phase unwrapping in the current phi-OTDR system and solve the problem of mutual restriction between measurement bandwidth and dynamic range in the system.
To achieve the above object, an embodiment provides a large dynamic range distributed phase sensing device, including:
the optical path difference measuring module is used for generating the chirp frequency pulse light and inputting the chirp frequency pulse light into the scattering enhancement sensing optical fiber, and also used for carrying out interference on first reflected light of the chirp frequency pulse light in the scattering enhancement sensing optical fiber to obtain a first interference signal for measuring the optical path difference;
the phase measurement module is used for generating another pulse light and inputting the other pulse light into the scattering enhancement sensing optical fiber, and is also used for carrying out interference on second reflected light of the other pulse light in the scattering enhancement sensing optical fiber to obtain a second interference signal for measuring phase information;
one end of the multiplexing module is connected with the optical path difference measuring module and the phase measuring module, and the other end of the multiplexing module is connected with the scattering enhancement sensing optical fiber and is used for multiplexing and de-multiplexing working channels of the optical path difference measuring module and the phase measuring module;
a scattering enhancement sensing optical fiber including scattering enhancement points having intervals for scattering enhancing the received chirped frequency pulse light and the other pulse light, respectively, and outputting first reflected light and second reflected light of the scattering enhancement points;
the data acquisition demodulation system is used for demodulating the acquired first interference signal and second interference signal respectively to obtain optical path difference data and winding phase data, and then utilizing the optical path difference data to assist the winding phase data to unwind and restore phase information.
Preferably, the optical path difference measurement module comprises a chirp frequency pulse light generation unit, a first optical fiber amplifier, a first circulator, an unbalanced optical fiber interferometer and a detection unit, wherein the chirp frequency pulse light generated by the chirp frequency pulse light generation unit is amplified by the first optical fiber amplifier and then is input into the scattering enhancement sensing optical fiber through the first circulator and the multiplexing module, first reflected light of the chirp frequency pulse light in the scattering enhancement sensing optical fiber is injected into the unbalanced optical fiber interferometer through the multiplexing module and the first circulator, and is converted into an electric signal through the detection unit after interference of the unbalanced optical fiber interferometer to obtain a first interference signal for measuring the optical path difference.
Preferably, the chirped-frequency pulse light generated by the chirped-frequency pulse light generating unit is generated by internally or externally modulating the first light source, wherein when internal modulation is adopted, the chirped-frequency pulse light generating unit comprises the first light source and a modulation driver, the modulation driver directly modulates the current or voltage of the first light source to output the chirped-frequency pulse light, when external modulation is adopted, the chirped-frequency pulse light generating unit comprises the first light source and a modulation unit, and the modulation unit modulates the intensity or phase of the output light of the first light source to obtain the chirped-frequency pulse light, and the modulation unit is composed of a single or multiple modulation devices including the first acousto-optic modulator and the electro-optic modulator.
Preferably, the combination of the unbalanced fiber interferometer and the detection unit is realized by a combination of an n×n fiber coupler and a plurality of single-ended detector structures, wherein the n×n fiber coupler is used for realizing interference, and the single-ended detector is used for converting the interfered optical signal into an electrical signal.
Preferably, the phase measurement module comprises a second light source, a first coupler, a second acoustic modulator, a second optical fiber amplifier, a second circulator and a balance detector, wherein light source light output by the second light source is divided into two paths through the first coupler, one path of light source light is modulated into another pulse light through the second acoustic modulator, the other pulse light is amplified through the second optical fiber amplifier and then is input into the scattering enhancement sensing optical fiber through the second circulator and the multiplexing module, second reflected light of the other pulse light in the scattering enhancement sensing optical fiber is injected into the second coupler through the multiplexing module and the second circulator, and in the second coupler, the second reflected light is interfered with the other path of light source light output by the first coupler and then is converted into an electric signal through the balance detector to obtain a second circulator and a second interference signal.
Preferably, the line width of the first light source in the chirped-frequency pulse light generating unitShould satisfyWherein, the method comprises the steps of, wherein,cindicating the propagation speed of light in a vacuum,nindicating the effective refractive index of the fiber core, < >>Representing the difference in physical length of the interference paths of scattered light from adjacent two scattering points.
Preferably, the chirped frequency pulse light generated in the optical path difference measurement module is equal to the pulse width of the other pulse light generated by the phase measurement module, and the repetition frequency of the chirped frequency pulse light is equal to the repetition frequency of the other pulse light.
Preferably, the pulse widthSatisfy->Wherein, the method comprises the steps of, wherein,cindicating the propagation speed of light in a vacuum,nindicating the effective refractive index of the fiber core, < >>Representing the length of the fiber at two adjacent scattering enhancement points.
Preferably, the demodulation process implemented by the data acquisition demodulation system is as follows:
step 1, performing optical path difference demodulation on a first interference signal obtained by an optical path difference measurement module to obtain OPD data, and performing noise reduction treatment on the OPD data to ensure that the noise variance is smaller than that of the OPD data；
Step 2, demodulating the second interference signal obtained by the phase measurement module to obtain winding phase dataΦ W Simultaneously selecting winding phase dataΦ W A second sampling point of (a) is used as a starting point;
step 3, for the firstiOptical path difference data of each sampling pointOPD(i) And phase dataΦ W (i) Calculate the firstiThe sampling point is the firstiOptical path difference variation between-1 pointsAnd phase change->；
Step 4, when |│<λJudging->Relative to the size of pi, if-> >+pi, then the firstiSubtracting 2 pi from each sampling point and the following sampling points; if-> <-Pi is thiAdding 2 pi to the sampling points and the following sampling points; other cases do not operate;
step 5, when |│≥λJudging at the time of/2Φ W (i) Relative to a size of 0, ifΦ W (i)>0, theniSample points and the following sample points are added with 2pi [ OPD ]i)/λ-1]The method comprises the steps of carrying out a first treatment on the surface of the If it isΦ W (i)<0, theniSample points and the following sample points are added with 2pi [ OPD ]i)/λ]Other cases do not operate;
step 6, according toiWhether or not equal to the Length of the phase dataΦ W ) Judging whether all sampling points are processed, if not, returning to the step 3; if so, then end.
In order to achieve the above object, an embodiment of the present invention further provides a large dynamic range distributed phase sensing method based on optical path difference assistance, where the method adopts the large dynamic range distributed phase sensing device based on optical path difference assistance, and the method includes the following steps:
detecting by using an optical path difference measuring module, a multiplexing module and a scattering enhancement sensing optical fiber to obtain a first interference signal for measuring the optical path difference;
detecting by using a phase measurement module, a multiplexing module and a scattering enhancement sensing optical fiber to obtain a first interference signal for measuring phase information;
and demodulating the first interference signal and the second interference signal by using a data acquisition demodulation system to obtain optical path difference data and winding phase data, and then using the optical path difference data to assist the winding phase data to unwind and recover the phase information.
Compared with the prior art, the invention has the beneficial effects that at least the following steps are included:
the invention combines Optical Path Difference (OPD) demodulation with high noise and large dynamic range characteristics with phase demodulation with low noise and high precision, and when data is processed, the phase unwrapping error caused by pi phase principle is corrected by utilizing the OPD data with large dynamic range to assist in completing the unwrapping of low noise phase data, thus realizing the phase unwrapping of large signals and improving the measurement dynamic range while maintaining the demodulation precision of the system;
the invention can release the contradiction between the system measurement bandwidth and the dynamic range, reduce the requirement of the system on the sampling rate and lighten the load of system data processing when realizing the demodulation of the signal with a large dynamic range.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings that are required in the embodiments or the description of the prior art will be briefly described, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
FIG. 1 is a schematic diagram of a simple structure of a large dynamic range distributed phase sensing device based on optical path difference assistance according to an embodiment;
FIG. 2 is a schematic diagram of a detailed structure of a large dynamic range distributed phase sensing device based on optical path difference assistance according to an embodiment;
FIG. 3 shows an inner modulation structure and an outer modulation structure of a chirped-frequency pulsed light generating unit according to an embodiment;
FIG. 4 is a flow chart of a phase unwrapping method based on synchronized OPD data provided by an embodiment;
FIG. 5 is large signal OPD data obtained by the optical path difference measurement module provided by the embodiment;
FIG. 6 is a wrapping phase data of a large signal obtained by the phase measurement module provided by the embodiment;
fig. 7 is a graph comparing the unwrapping effect of the phase unwrapping method based on the synchronous OPD data with that of the conventional unwrapping method on the large-signal wrapped phase data according to the embodiment.
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 detailed description is presented by way of example only and is not intended to limit the scope of the invention.
As shown in fig. 1, the large dynamic range distributed phase sensing device based on optical path difference assistance provided by the embodiment includes an optical path difference measurement module 1, a phase measurement module 2, a multiplexing module 3, a scattering enhancement sensing optical fiber 4 and a data acquisition demodulation system 5, wherein an input end of the multiplexing module 3 is respectively connected with one output end of the optical path difference measurement module 1 and one output end of the phase measurement module 2, an output end of the multiplexing module 3 is connected with the scattering enhancement sensing optical fiber 4, and the data acquisition demodulation system 5 is respectively connected with the other output ends of the optical path difference measurement module 1 and the phase measurement module 2.
The optical path difference measuring module 1 is used for generating chirped frequency pulse light and inputting the chirped frequency pulse light into the scattering enhancement sensing optical fiber 4, and is also used for carrying out interference on first reflected light of the chirped frequency pulse light in the scattering enhancement sensing optical fiber 4 to obtain a first interference signal for measuring the optical path difference; the phase measurement module 2 is used for generating another pulse light and inputting the other pulse light into the scattering enhancement sensing optical fiber 4, and is also used for carrying out interference on second reflected light of the other pulse light in the scattering enhancement sensing optical fiber 4 to obtain a second interference signal for measuring phase information; the multiplexing module 3 is used for multiplexing and de-multiplexing the working channels of the optical path difference measuring module 1 and the phase measuring module 2; the scattering enhancement sensing optical fiber 4 includes scattering enhancement points having intervals for scattering enhancing the received chirped frequency pulse light and the other pulse light, respectively, and outputting first reflected light and second reflected light of the scattering enhancement points; the data acquisition demodulation system 5 is used for demodulating the acquired first interference signal and second interference signal to obtain OPD data and winding phase data respectively, and then the OPD data is used for assisting the winding phase data to unwind, so that the phase information with high precision and large dynamic range is recovered. Each of the parts is described in detail below.
As shown in fig. 2, the optical path difference measurement module 1 includes a chirped-frequency pulsed light generation unit 11, a first optical fiber amplifier 12, a first circulator 13, an unbalanced optical fiber interferometer 14, and a detection unit 15, which are sequentially connected, and the other end of the first circulator 13 is connected to the multiplexing module 3.
The chirped-frequency pulse light generating unit 11 is used for realizing pulse width ofFrequency shift is +.>The repetition frequency is->Is used for generating and outputting the chirped-frequency pulse light, and the repetition frequency of the chirped-frequency pulse light is +.>The corresponding period should be greater than the time required for the pulse to travel back and forth in the scattering enhanced sensing fiber under test. The chirped-frequency pulsed light may be generated by either internal modulation (direct modulation) or external modulation of the light source. In which, the internal modulation is to directly modulate the current or voltage of the light source by external driving to make the light source output the required chirped-frequency pulse light, as shown in fig. 3 (a), in which the chirped-frequency pulse light generating unit includes the first light source 31 and the modulation driver 32, and the modulation driver 32 directly modulates the current or voltage of the first light source 31 to output the chirped-frequency pulse light. The external modulation means that the light source output light is modulated in intensity or phase by a modulation device to generate the required chirped-frequency pulse light, and as shown in fig. 3 (b), the chirped-frequency pulse light generating unit comprises a first light source 31 and a modulation unit 33, and the modulation unit 33 performs the modulation onThe output light of the first light source 31 is intensity or phase modulated to obtain chirped frequency pulse light, and the modulation unit 33 may be preferably constituted by a single or a plurality of modulation devices in a first acousto-optic modulator, an electro-optic modulator, or the like.
The first optical fiber amplifier 12 is configured to amplify and output the chirped-frequency pulse light to the first circulator 13, and the first circulator 13 is configured to inject the chirped-frequency pulse light output from the first optical fiber amplifier 12 into the multiplexing module 3, and inject the first reflected light formed in the scattering enhanced sensing fiber output from the multiplexing module 3 into the unbalanced optical fiber interferometer 14.
The unbalanced optical fiber interferometer 14 and the detection unit 15 are configured to interfere the first reflected light and then convert the first reflected light into an electrical signal to obtain a first interference signal; the unbalanced fiber interferometer 14 is configured to receive the first reflected light of the scattering enhancement point, and utilize two-arm delay of the interferometer to make the first reflected light of two adjacent scattering enhancement points interfere. The combination of the unbalanced fiber interferometer 14 and the detection unit 15 may be implemented by a combination of structures including a 2×2 fiber coupler and a balanced detector, or may be implemented by a combination of structures including a 3×3 fiber coupler and 2 to 3 single-ended detectors, or may be extended to be implemented by a combination of structures including an n×n fiber coupler and a plurality of single-ended detectors, but the coverage area thereof is not limited to the above structures.
As shown in fig. 2, the phase measurement module 2 includes a second light source 21, a first coupler 22, a second acoustic optical modulator 23, a second optical fiber amplifier 24, a second circulator 25, a second coupler 26, and a balance detector 27, wherein the second light source 21 is connected to an input end of the first coupler 22, an output end of the first coupler 22 is sequentially connected to the second acoustic optical modulator 23, the second optical fiber amplifier 24, the second circulator 25, the second coupler 26, and the balance detector 27, and another output end of the first coupler 22 is directly connected to the second coupler 26.
After the light source light outputted from the second light source 21 is divided into two paths by the first coupler 22, one path of light source light is modulated into pulse width by the second light modulator 23Frequency shift is +.>The repetition frequency is->Is a pulse of another light of the same type. The repetition frequency of the further pulsed light +.>The corresponding period should be greater than the time required for the pulse to travel back and forth in the scattering enhanced sensing fiber under test, and the other pulse light is coupled into the second fiber amplifier 24 for amplification and output. Wherein, the repetition frequency->And (2) heavy frequency->In accordance with a pulse width of +.>Pulse width->And the optical fiber length of two adjacent scattering enhancement points +.>Satisfy the following requirements、/>，nFor the refractive index of the optical fiber,cis the vacuum light velocity.
The second circulator 25 is configured to inject another pulse light output by the second optical fiber amplifier 24 into the multiplexing module 3, and inject a second reflected light formed in the scattering enhanced sensing optical fiber output by the multiplexing module 3 into the second coupler 26, where the second reflected light interferes with the light source light output by the other path of the first coupler 22, and the balance detector 27 converts the interference result into a corresponding electrical signal to obtain a second interference signal.
The multiplexing module 3 is used for multiplexing and demultiplexing the working channels of the optical path difference measuring module 1 and the phase measuring module 2, and the multiplexing mode can be wavelength division multiplexing or other multiplexing modes such as polarization multiplexing; the wavelength division multiplexing mode, that is, the optical path difference measurement module 1 and the phase measurement module 2 have different working wavelength bands, and the optical wavelength ranges of the two output pulse lights do not overlap, and in this case, the multiplexing module 3 may be a wavelength division multiplexer whose working wavelength band covers the wavelength ranges of the two output pulse lights. The polarization multiplexing mode refers to that the pulse light as the detection light injected into the scattering enhancement sensing optical fiber 4 by the optical path difference measuring module 1 and the phase measuring module 2 respectively work in different polarization states, and in this case, the multiplexing module 3 may be a polarization multiplexer, and the scattering enhancement sensing optical fiber 4 is a polarization maintaining optical fiber.
In the above device, the optical path difference measurement module 1 is used for detecting OPD data, in the optical path difference measurement module 1, the chirped frequency pulse light generated by the chirped frequency pulse light generating unit 11 is injected into the scattering enhancement sensing optical fiber 4, when the chirped frequency pulse light is transmitted in the scattering enhancement sensing optical fiber 4, the generated backward rayleigh scattered light is received as first reflected light by the unbalanced optical fiber interferometer 14, by controlling the arm length difference of the unbalanced optical fiber interferometer 14, the backward rayleigh scattered light of two adjacent scattering enhancement points in the scattering enhancement sensing optical fiber 4 realizes an interference spectrum with smaller optical path difference in the unbalanced optical fiber interferometer 14, and finally, the detection unit 15 converts the interference light signals under different wavelengths into electric signals as first interference signals, and processes the first interference signals to realize demodulation of the optical path difference.
In the scattering enhancement sensing optical fiber 4, the optical fiber length of two adjacent scattering enhancement points isThe arm length difference of the unbalanced fiber interferometer is +.>. The interference output by unbalanced fiber interferometer 14Light sourceICan be expressed as:
（1）
wherein A represents the DC component of the interference spectrum, B represents the contrast of the interference spectrum,λindicating the wavelength of chirped frequency pulses input to the scatter enhanced sensing fiber as the probe light,nindicating the effective refractive index of the fiber core,representing the difference in physical length of the interference paths of scattered light of two adjacent scattering points, < >>。
When (when)λWhen the interference light signal intensity changes, the interference light signal intensity also changes. For wavelengths of light that vary within a sufficient range, the optical path difference OPD between two adjacent scattering enhancement points can be expressed as:
（2）
wherein the method comprises the steps ofλ m The peak wavelength of the mth-order interference fringe.
Therefore, by scanning the light wavelength within a certain range, a signal of interference light intensity changing along with the cosine of the light wavelength is obtained, and the peak wavelength of interference fringes within the wavelength scanning range is searched by utilizing data processingλ m And the light intensity change along with the wavelength can be reversely deduced to obtain the optical path difference of the interference spectrum. The optical path difference is used as the demodulation quantity of the distributed acoustic wave sensing system, the one-to-one correspondence relation between the optical path difference and the acoustic wave amplitude is established, the OPD data is directly obtained by searching the number of interference fringes, and the winding condition does not exist in the demodulation process, so that the direct demodulation of the signal with a large dynamic range can be realized. However, the OPD parametric demodulation based on the interference fringe search has disadvantages in demodulation accuracy and noise with respect to the phase demodulation capable of subdividing the interference fringe, and cannot achieve high accuracy and low noiseAnd demodulating the acoustic signal.
In the above device, the phase measurement module 2 is configured to detect phase data, in the phase measurement module 2, a continuous optical signal output by the second light source is split into two paths through the first coupler 22, one path is used as detection light, the other path is used as local oscillation light, wherein the detection light is modulated into another pulse light used as detection through the second acoustic optical modulator 23 and then is injected into the scattering enhancement sensing optical fiber 4, when the other pulse light is transmitted through the scattering enhancement sensing optical fiber 4, the backward rayleigh scattered light generated by the other pulse light is returned as second reflected light and then interferes with the other path of local oscillation light, and finally, the balanced photoelectric detector 27 converts the interference optical signal into an electrical signal as the second interference signal, and processes the second interference signal to implement phase demodulation with high precision and low noise.
In distributed sensing systems based on phase demodulation, the phase information is typically demodulated using an arctan (x) function, since the arctan function is [ -pi, pi]Periodic function in, when continuous phase informationΦ U (i) Exceeding [ -pi, pi]When the phase obtained by the arctangent function is wound, i.e. jump of + -2 pi occurs, so that the demodulated phase information is always wrapped in [ -pi, pi]And (3) inner part. To take up phase informationΦ W (i) Restoring continuous phase informationΦ U (i) Form, phase information of the winding must be takenΦ W (i) The 2 pi jump that occurs in (a) is removed, a process called phase unwrapping. At present, the traditional phase unwrapping method follows pi phase principle, namely, the absolute value of the phase difference between adjacent sampling points is required to be smaller than pi, and specifically comprises the following steps: calculating the difference between the current sampling point and the immediately adjacent previous sampling point, and subtracting 2 pi from the sampling point and the sampling point after the current sampling point when the difference is greater than +pi; when the difference between the two is smaller than-pi, the sampling point and the following sampling points are added with 2 pi, and the specific process can be expressed as a formula (3).
（3）
Wherein,irepresenting the index of the sample point,the representation is from the firstiData from sample point to end.
But when external disturbance causes the change delta of the optical fiber length between two adjacent sampling pointslAt-λ/4n，+λ/4n) Otherwise, the phase difference between two adjacent sampling points may reach ±pi or more, and the phase unwrapping algorithm erroneously determines the sampling point as phase wrapping, and the errors are continuously accumulated, which affects all the sampling points after the sampling point, in which case the phase unwrapping algorithm cannot correctly restore the phase information. Thus, for a system with a sampling rate of Fs, it is determined by the phase unwrapping algorithm that the frequency isf 0 When the sinusoidal signal of (1) is unwound, its theoretical maximum undistorted signal amplitude A max The method comprises the following steps:
（4）
as can be seen from equation (4), the system measurement frequency and the dynamic range are in a mutually restricted relationship, and the dynamic range can be increased by increasing the system sampling rate, but increasing the sampling rate increases the system hardware requirement and aggravates the system data processing load.
In the embodiment of the invention, the optical path difference measurement module 1 and the phase measurement module 2 can realize synchronous demodulation of OPD and phase change of two adjacent scattering enhancement points in the scattering enhancement sensing optical fiber. In theory, the conversion can be performed by the formula (5), but the direct conversion of the data of the two cannot be performed due to the difference in demodulation accuracy between the two and the presence of the unwinding problem. But the linear correspondence between the two makes it possible to assist in achieving high-precision, large dynamic range phase unwrapping based on OPD data.
（5）
The basic principle of the phase unwrapping method based on the OPD data is to judge the reason of the phase difference of two adjacent points in the wrapping phase through the OPD data and restore the wrapping phase by taking the OPD data as a reference. In the wrapping phase data, the phase variation of two adjacent sampling points can be divided into the following three cases, namely, the normal continuous phase variation without wrapping, the phase wrapping with a single 2 pi period, and the phase wrapping with a plurality of 2 pi periods. In the first two cases, the corresponding sampling point OPD data changes less thanλ2, in this case, the normal unwinding flow can be adopted for phase recovery; for the third case, the corresponding sampling point OPD data changes more thanλIn this case,/2, the number of 2 pi cycles of the phase wrap is determined based on the OPD data, and the wrap phase is recovered.
Therefore, the phase unwrapping method based on OPD data is: when the OPD variation between two adjacent sampling points is smaller thanλAnd (2) the phase unwrapping mode is consistent with the traditional phase unwrapping method; when the OPD variation between two adjacent sampling points exceedsλAnd/2, rounding the phase of the winding at the point according to the 2 pi phase delay number generated by the OPD at the point. The specific process can be expressed as formula (6).
（6）
Symbol in the above []From the above, it can be seen that the OPD data has a main role in the process of assisting in determining, and that the recovery of the signal is achieved based on the phase data, so that the demodulation accuracy of the system is also mainly dependent on the phase demodulation accuracy. However, when the signal is recovered, the selection of the phase unwrapping method and the number of phase unwrapping cycles are determined using OPD data, and an excessive noise in the OPD data affects the calculation result of the number of phase unwrapping cycles and the determination of the phase unwrapping method, resulting in an error in the phase unwrapping method based on the OPD data. This puts demands on the noise level of the OPD data, which should ideally be less than. Therefore, noise reduction processing needs to be performed on the OPD data in advance in the actual processing process to ensure that the noise level thereof can satisfy the calculation condition.
In summary, in the data acquisition and demodulation system, the unwrapping work of the wrapped phase data is mainly completed with the assistance of the OPD data, and the complete phase information is recovered, as shown in fig. 4, and the phase unwrapping method based on the synchronized OPD data specifically includes the following steps:
step 1, performing optical path difference demodulation on a first interference signal obtained by an optical path difference measurement module to obtain OPD data, and performing noise reduction treatment on the OPD data to ensure that the noise variance is smaller than that of the OPD data；
Step 2, demodulating the second interference signal obtained by the phase measurement module to obtain winding phase dataΦ W Simultaneously selecting winding phase dataΦ W A second sampling point of (a) is used as a starting point;
step 3, for the firstiOptical path difference data of each sampling pointOPD(i) And phase dataΦ W (i) Calculate the firstiThe sampling point is the firstiOptical path difference variation between-1 pointsAnd phase change->；
Step 4, when |│<λJudging->Relative to the size of pi, if-> >+pi, then the firstiSubtracting 2 pi from each sampling point and the following sampling points; if-> <-Pi is thiAdding 2 pi to the sampling points and the following sampling points; other cases do not operate;
step 5, when |│≥λJudging at the time of/2Φ W (i) Relative to a size of 0, ifΦ W (i)>0, theniSample points and the following sample points are added with 2pi [ OPD ]i)/λ-1]The method comprises the steps of carrying out a first treatment on the surface of the If it isΦ W (i)<0, theniSample points and the following sample points are added with 2pi [ OPD ]i)/λ]Other cases do not operate;
step 6, according toiWhether or not equal to the Length of the phase dataΦ W ) Judging whether all sampling points are processed, if not, returning to the step 3; if so, then end.
By the steps 1-6, the wound phase information can be obtainedΦ W (i) Restoring continuous phase informationΦ U (i) And the demodulation dynamic range is not limited by pi phase principle, and the complete demodulation and recovery of large signal phase with any amplitude can be realized.
In the embodiment of the present invention, in order to ensure that the above process can be implemented, specific parameters of specific components need to be limited, including:
in order to ensure that the backscattering light of two adjacent scattering enhancement points can interfere in the unbalanced optical fiber interferometer in the optical path difference measurement module 1, the line width of the first light sourceThe following should be satisfied:
（7）
wherein,cindicating the propagation speed of light in vacuum.
To ensure that the spatial resolution of the distributed sensing system is less than the length of the optical fiber at two adjacent scattering enhancement points, the pulse width of the pulsed lightThe following should be satisfied:
（8）
meanwhile, in order to ensure that the optical path difference measurement module and the phase measurement module can realize synchronous demodulation of OPD and phase change of two adjacent scattering enhancement points in the scattering enhancement sensing optical fiber, the width and the pulse repetition frequency of modulated pulse light of the light source output light of the two light sources are consistent.
In the embodiment of the invention, the method (OPD-Unwrapping) and the traditional unwinding method (Unwrapping) are used for unwinding the large-signal winding phase data, the unwinding effect is as shown in fig. 7, the analysis of fig. 7 can be obtained, the traditional unwinding method is used for unwinding the large-signal winding phase data, the obvious distortion condition of the phase number after the unwinding occurs, and the real phase data is well recovered by adopting the method of the invention due to the pi phase principle limitation in the traditional unwinding method.
The embodiment of the invention provides a solution to the problem that the demodulation dynamic range in the current DAS system is limited by phase unwrapping, combines the distributed sensing technology based on OPD demodulation and the distributed sensing technology based on phase demodulation, realizes simultaneous demodulation of two parameters of OPD and phase in the same set of system, and realizes demodulation of signals with high precision and large dynamic range by using OPD data to assist in completing phase data unwrapping. The OPD demodulation technology has the characteristics of high noise and large dynamic range, and the phase demodulation technology has the characteristics of low noise and high precision. The low-noise phase data wrapped in [ -pi, pi ] is unwound with the assistance of the OPD data with a large dynamic range, so that the phase recovery of a large signal can be realized while the demodulation precision and the measurement noise of the system are maintained, and the measurement dynamic range of the system is improved.
The foregoing detailed description of the preferred embodiments and advantages of the invention will be appreciated that the foregoing description is merely illustrative of the presently preferred embodiments of the invention, and that no changes, additions, substitutions and equivalents of those embodiments are intended to be included within the scope of the invention.
Claims (8)
1. A large dynamic range distributed phase sensing device based on optical path difference assistance, comprising:
the optical path difference measuring module is used for generating the chirp frequency pulse light and inputting the chirp frequency pulse light into the scattering enhancement sensing optical fiber, and also used for carrying out interference on first reflected light of the chirp frequency pulse light in the scattering enhancement sensing optical fiber to obtain a first interference signal for measuring the optical path difference;
the phase measurement module is used for generating another pulse light and inputting the other pulse light into the scattering enhancement sensing optical fiber, and is also used for carrying out interference on second reflected light of the other pulse light in the scattering enhancement sensing optical fiber to obtain a second interference signal for measuring phase information;
one end of the multiplexing module is connected with the optical path difference measuring module and the phase measuring module, and the other end of the multiplexing module is connected with the scattering enhancement sensing optical fiber and is used for multiplexing and de-multiplexing working channels of the optical path difference measuring module and the phase measuring module;
a scattering enhancement sensing optical fiber including scattering enhancement points having intervals for scattering enhancing the received chirped frequency pulse light and the other pulse light, respectively, and outputting first reflected light and second reflected light of the scattering enhancement points;
the data acquisition demodulation system is used for demodulating the acquired first interference signal and second interference signal respectively to obtain optical path difference data and winding phase data, then utilizing the optical path difference data to assist the winding phase data to unwind and restore phase information, and specifically comprises the following steps:
step 1, carrying out optical path difference demodulation on a first interference signal obtained by an optical path difference measurement module to obtain an OPD numberAccording to the above, noise reduction processing is carried out on the OPD data so that the noise variance is smaller than，λThe chirp frequency pulse wavelength of the input scattering enhancement sensing optical fiber as the detection light is represented;
step 2, demodulating the second interference signal obtained by the phase measurement module to obtain winding phase dataΦ W Simultaneously selecting winding phase dataΦ W A second sampling point of (a) is used as a starting point;
step 3, for the firstiOptical path difference data of each sampling pointOPD(i) And phase dataΦ W (i) Calculate the firstiThe sampling point is the firstiOptical path difference variation between-1 pointsAnd phase change->；
Step 4, when |│<λJudging->Relative to the size of pi, if->>+pi, then the firstiSubtracting 2 pi from each sampling point and the following sampling points; if-> <Pi, theniAdding 2 pi to the sampling points and the following sampling points; other cases do not operate;
step 5, when |│≥λJudging at the time of/2Φ W (i) Relative to a size of 0, ifΦ W (i)>0, theniSample points and the following sample points are added with 2pi [ OPD ]i)/λ-1]The method comprises the steps of carrying out a first treatment on the surface of the If it isΦ W (i)<0, theniSample points and the following sample points are added with 2pi [ OPD ]i)/λ]Other cases do not operate;
step 6, according toiWhether or not equal to the Length of the phase dataΦ W ) Judging whether all sampling points are processed, if not, returning to the step 3; if yes, ending;
the optical path difference measuring module comprises a chirp frequency pulse light generating unit, a first optical fiber amplifier, a first circulator, an unbalanced optical fiber interferometer and a detecting unit, wherein the chirp frequency pulse light generated by the chirp frequency pulse light generating unit is amplified by the first optical fiber amplifier and then is input into the scattering enhancement sensing optical fiber through the first circulator and the multiplexing module, first reflected light of the chirp frequency pulse light in the scattering enhancement sensing optical fiber is injected into the unbalanced optical fiber interferometer through the multiplexing module and the first circulator, and is converted into an electric signal through the detecting unit after interference of the unbalanced optical fiber interferometer, so that a first interference signal for measuring the optical path difference is obtained.
2. The optical path difference auxiliary-based large dynamic range distributed phase sensing apparatus according to claim 1, wherein the chirped frequency pulse light generated by the chirped frequency pulse light generating unit is generated by internally or externally modulating the first light source, wherein when internal modulation is adopted, the chirped frequency pulse light generating unit comprises the first light source and a modulation driver, the modulation driver directly modulates the current or voltage of the first light source to output the chirped frequency pulse light, when external modulation is adopted, the chirped frequency pulse light generating unit comprises the first light source and a modulation unit, the modulation unit modulates the intensity or phase of the output light of the first light source to obtain the chirped frequency pulse light, and the modulation unit is composed of a single or multiple modulation devices in the first acousto-optic modulator and the electro-optic modulator.
3. The optical path difference-assisted high dynamic range distributed phase sensing apparatus of claim 1, wherein the combination of unbalanced fiber interferometers and detection units is implemented by a combination of an nxn fiber coupler for effecting interference and a number of single-ended detector structures for converting the interfered optical signals into electrical signals.
4. The optical path difference auxiliary-based large dynamic range distributed phase sensing device according to claim 1, wherein the phase measuring module comprises a second light source, a first coupler, a second acoustic optical modulator, a second optical fiber amplifier, a second circulator and a balance detector, wherein light source light output by the second light source is divided into two paths through the first coupler, one path of light source light is modulated into another pulse light through the second acoustic optical modulator, the other pulse light is amplified through the second optical fiber amplifier and then is input into the scattering enhancement sensing optical fiber through the second circulator and the multiplexing module, second reflected light of the other pulse light in the scattering enhancement sensing optical fiber is injected into the second coupler through the multiplexing module and the second circulator, and the second reflected light is interfered with the other path of light source light output by the first coupler and then is converted into an electric signal through the balance detector to obtain a second interference signal for measuring phase information.
5. The optical path difference assist-based high dynamic range distributed phase sensing apparatus according to claim 1, wherein the line width of the first light source in the chirped-frequency pulsed light generation unitShould satisfy->Wherein, the method comprises the steps of, wherein,cindicating the propagation speed of light in a vacuum,nindicating the effective refractive index of the fiber core, < >>Representing the difference in physical length of the interference paths of scattered light from adjacent two scattering points.
6. The optical path difference-assisted high dynamic range distributed phase sensing apparatus according to claim 1, wherein the chirped frequency pulse light generated in the optical path difference measurement module is equal to the pulse width of the other pulse light generated in the phase measurement module, and the repetition frequency of the chirped frequency pulse light is equal to the repetition frequency of the other pulse light.
7. The optical path difference assist based high dynamic range distributed phase sensing apparatus of claim 6, wherein pulse widthSatisfy->Wherein, the method comprises the steps of, wherein,cindicating the propagation speed of light in a vacuum,nindicating the effective refractive index of the fiber core, < >>Representing the length of the fiber at two adjacent scattering enhancement points.
8. A large dynamic range distributed phase sensing method based on optical path difference assistance, characterized in that the method adopts the large dynamic range distributed phase sensing device based on optical path difference assistance as claimed in any one of claims 1 to 7, and comprises the following steps:
detecting by using an optical path difference measuring module, a multiplexing module and a scattering enhancement sensing optical fiber to obtain a first interference signal for measuring the optical path difference;
detecting by using a phase measurement module, a multiplexing module and a scattering enhancement sensing optical fiber to obtain a first interference signal for measuring phase information;
and demodulating the first interference signal and the second interference signal by using a data acquisition demodulation system to obtain optical path difference data and winding phase data, and then using the optical path difference data to assist the winding phase data to unwind and recover the phase information.
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