CN109459126B - Distributed optical fiber vibration sensing device and method for reducing detection dead zone probability - Google Patents
Distributed optical fiber vibration sensing device and method for reducing detection dead zone probability Download PDFInfo
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Abstract
The invention discloses a distributed optical fiber vibration sensing device for reducing the probability of detection dead zones, which comprises a pulse generator, a laser, a first coupler, a second coupler, N acousto-optic modulators (AOMs), a third coupler, an erbium-doped fiber amplifier (EDFA), an optical fiber circulator, a sensing optical fiber, a fourth coupler, a balance detector and a signal acquisition card. The invention has also disclosed the detection method of a distributed optical fiber vibration sensing device to reduce probability of detecting the dead zone, the invention has realized the multifrequency measurement through introducing the acoustooptic modulator of N different working frequencies, and its frequency interval is large enough, has guaranteed the amplitude distribution of each frequency is independent each other; and searching the signal with the best signal-to-noise ratio among the frequency signals to obtain the phase difference and backtracking the data in the previous period, so that the detection result of the phase difference is continued, the continuous measurement of the vibration signal is realized, and the probability of the system falling into the detection dead zone is effectively reduced.
Description
Technical Field
The invention relates to the technical field of optical fiber sensing, in particular to a distributed optical fiber vibration sensing device and method for reducing detection dead zone probability.
Background
In recent years, natural disasters such as earthquakes occur frequently, large-scale infrastructures in China are continuously built, early warning is carried out on the natural disasters, and large-scale infrastructures are providedHealth monitoring is becoming an active research focus. In these practical applications, long-distance and wide-range sensing under severe natural environment is often required, and phase-sensitive optical time domain reflection technology
As a distributed optical fiber sensing technology, the sensor has great advantages in the application by virtue of the advantages of vibration measurement, long measurement distance, high sensitivity, continuous distributed measurement and the like.
The phase discrimination is
The method has the advantages that the method utilizes the phase change of the backward Rayleigh scattering light to restore the vibration information, brings high sensitivity to the system, can accurately restore the waveform of external vibration, and is widely applied to natural disaster early warning, large-scale infrastructure health monitoring and the like.
In that
In the phase discrimination process, two points L on the distance axis of the back Rayleigh scattering curve need to be determined1And L2The phases of these two points are obtained by processing
And
if there is no influence of external vibration,
will remain unchanged; by monitoring under the influence of external vibrations
The vibration information can be obtained. At heterodyne coherent detection
In the method, due to the interference of the back rayleigh scattering light within one pulse width, the polarization state matching between the local oscillation light and the back rayleigh scattering light, and the like, the back rayleigh scattering curve shows random coherent growth or coherent cancellation on the distance axis. If L is measured in a certain time1And L2If any one of the positions is coherently canceled, the weak signal is submerged in the noise floor, and the measurement is made to be wrong. Therefore, the measurement accuracy of the above phase discrimination method is very dependent on L1And L2In the above process, L1And L2The power of the backward Rayleigh scattered light at any position is too low, which causes L1And L2The area in between forms a detection dead zone. The presence of detection dead zones limits phase discrimination
The accuracy of the system increases the false alarm rate of the system, and potential safety hazards may exist in practical application.
Disclosure of Invention
The technical problem to be solved by the invention is as follows:
to make use of
The invention provides a distributed optical fiber vibration sensing device and a distributed optical fiber vibration sensing method for reducing the probability of detection dead zones.
The invention adopts the following technical scheme for solving the technical problems:
the invention provides a distributed optical fiber vibration sensing device for reducing the probability of detection dead zones, which comprises a pulse generator, a laser, a first coupler, a second coupler, N acousto-optic modulators, a third coupler, a erbium-doped optical fiber amplifier, an optical fiber circulator, a sensing optical fiber, a fourth coupler, a balance detector and a signal acquisition card, wherein the pulse generator is connected with the first coupler; the laser generates continuous mode narrow linewidth laser, and the narrow linewidth laser is divided into two paths through the first coupler; one path of the light is used as initial detection light and input to the second coupler, and the other path of the light is used as local oscillation light and input to the fourth coupler;
the pulse generator respectively outputs N paths of same modulation pulses to N acousto-optic modulators and outputs synchronous pulses synchronous with the modulation pulses to the signal acquisition card;
dividing the initial detection light into N paths by a second coupler, converting the continuous mode narrow linewidth laser into N paths of synchronous pulse light by N acousto-optic modulators respectively, and enabling the N paths of pulse light to generate different frequency shifts;
the third coupler combines N paths of pulse light into one path and outputs the path of pulse light to the erbium-doped fiber amplifier, the pulse light is amplified and then output to the first port of the fiber circulator and is injected into the sensing fiber from the second port of the fiber circulator, the sensing fiber generates backward Rayleigh scattering light, the backward Rayleigh scattering light is input to the second port of the fiber circulator and is output to the fourth coupler from the third port of the fiber circulator;
the fourth coupler mixes the backward Rayleigh scattering light with the local oscillator light to form coherent light, the coherent light is output to two input ends of a balance detector by a first output end and a second output end of the fourth coupler to be detected, the balance detector obtains an intermediate-frequency electric signal, and the intermediate-frequency electric signal is output to a signal acquisition card by an output end of the balance detector.
The distributed optical fiber vibration sensing device for reducing the detection dead zone probability further comprises that the N acousto-optic modulators have different frequency shift amounts for the detection optical pulse, and the frequency difference between the frequency shift amounts is greater than the reciprocal of the detection optical pulse width.
The invention also provides a method of the distributed optical fiber vibration sensing device based on the detection dead zone probability reduction, which comprises the following steps:
step one, enabling a laser to generate continuous mode narrow linewidth laser; setting the pulse width and the pulse period of a pulse generator; n acousto-optic modulators convert N paths of continuous probe light into pulse light and generate corresponding frequency shift quantity delta fNThe frequency shift quantity generated by each acousto-optic modulator is different;
step two, designing N band-pass filtersThe center frequency of the N band-pass filters and the frequency shift amount Deltaf of the N acousto-optic modulatorsNRespectively corresponding; filtering the digital signals acquired by the signal acquisition card by the N band-pass filters to obtain time domain signal curves corresponding to the N frequencies respectively;
selecting m reference points at equal intervals along the horizontal axis of the time domain signal curve;
step four, selecting the frequency with the best signal-to-noise ratio at the two points from the N frequencies for the two adjacent reference points, and obtaining the intermediate frequency signal of the frequency at t through phase demodulationnPhase of time at the two points
And
and difference thereof
Step five, the intermediate frequency signal of the frequency in the step four is traced back forward for a period to obtain the frequency at tn-1Time of phase difference of the frequency intermediate frequency signal at the two points
And step six, subtracting the phase difference obtained by tracing back a cycle forwards in the step four from the phase difference obtained by tracing back a cycle forwards in the step five to obtain the variation of the phase difference
Step seven, assigning n +1 to n, and returning to the step four; when n reaches the signal acquisition number set in advance, entering step eight;
step eight, pair
Performing phase unwrapping to obtain continuous
And (4) a curve, so that continuous measurement of the vibration signal is realized.
The method for reducing the probability of detecting the dead zone based on the distributed optical fiber vibration sensing device is characterized in that the strategy for selecting the frequency signal with the best signal-to-noise ratio of the two determined points is as follows: at tnThe intensity of each frequency at the two points is multiplied, and the frequency with the maximum intensity product of the two points is regarded as the frequency with the best signal-to-noise ratio at the moment.
Compared with the prior art, the invention adopting the technical scheme has the following technical effects:
1. the apparatus and method are now known
On the basis of a vibration sensing system, N acousto-optic modulators with different frequency shift amounts are introduced, N-section frequency shift of final detection light is achieved, backward Rayleigh scattering signals with different frequency shift amounts are filtered and selected in the phase discrimination process, and the probability of weak signals at the selected position is greatly reduced. If the probability of the detection dead zone in the traditional phase discrimination method is P, the probability of the detection dead zone in the device and the method is PN. Therefore, the device and the method can effectively inhibit the detection dead zone, enhance the reliability of the system and reduce the false alarm rate of the system;
2. the method introduces the concept of backtracking the previous period and obtaining the phase difference variable quantity, and effectively solves the problem that the phase difference cannot be directly continued due to the switching of different frequencies.
Drawings
FIG. 1 is a system block diagram of the present invention.
FIG. 2 is a system configuration diagram of the experiment of the present invention.
FIG. 3 is a schematic diagram of the pulses output by the pulse generator in the apparatus of the present invention.
FIG. 4 is a three-way pulse optical spectrum after modulation by an acousto-optic modulator.
FIG. 5 is a spectrum of coherent light with a beat frequency.
FIG. 6 shows the time domain signal of 40MHz measured by the device and method of the present invention.
FIG. 7 shows the 80MHz time domain signal measured by the apparatus and method of the present invention.
FIG. 8 shows the 150MHz time domain signal measured by the apparatus and method of the present invention.
Detailed Description
The technical scheme of the invention is further explained in detail by combining the attached drawings:
it will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the prior art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
Fig. 1 is a system structure diagram of the present invention, which includes a pulse generator, a laser, a first coupler, a second coupler, N acousto-optic modulators (N ═ 1, 2, 3 … …), a third coupler, an erbium-doped fiber amplifier, a fiber circulator, a sensing fiber, a fourth coupler, a balanced detector and a signal acquisition card; wherein the content of the first and second substances,
a pulse generator for generating modulation pulses, synchronization pulses: the modulation pulse is output to N acousto-optic modulators, and the synchronous pulse is output to a signal acquisition card;
a laser for outputting continuous narrow linewidth laser light to the first coupler;
the first coupler is used for dividing the narrow linewidth laser into two paths of 90% and 10%: outputting 90% of the first path to a second coupler by using the initial detection light, and outputting 10% of the first path to a fourth coupler by using the local oscillation light;
the second coupler is used for dividing the initial detection light into N paths and inputting the N paths of the initial detection light into the N acousto-optic modulators respectively;
the acousto-optic modulator is used for converting the continuous detection light into pulse light according to the received modulation pulse and outputting the pulse light to the third coupler;
the third coupler is used for combining the N paths of pulse light into one path of detection pulse light;
the erbium-doped optical fiber amplifier is used for amplifying the detection pulse light and outputting the amplified detection pulse light to the first port of the circulator;
the optical fiber circulator is used for inputting the final detection light from a first port of the optical fiber circulator and injecting the final detection light into the sensing optical fiber from a second port of the optical fiber circulator;
the sensing optical fiber is used for generating back scattered light when receiving the final detection light, inputting the back scattered light to the second port of the optical fiber circulator and outputting the back scattered light to the fourth coupler from the third port of the optical fiber circulator;
the fourth coupler is used for mixing the backward Rayleigh scattering light output by the third port of the optical fiber circulator with the local oscillator light and outputting coherent light to the balance detector;
the balance detector is used for converting coherent light into an electric signal and outputting the electric signal to the signal acquisition card;
and the signal acquisition card is used for converting the electric signal output by the balance detector into a digital signal for subsequent processing according to the synchronous pulse.
The frequency shift amount of the N acousto-optic modulators to the detection light is different, and the frequency difference of each acousto-optic modulator is larger than the reciprocal of the pulse width of the detection light.
And selecting the frequency signal with the best signal-to-noise ratio to obtain the phase difference and backtrack the data of the previous period, and carrying out differential operation on the phase difference between the current moment and the previous moment again to obtain the variation of the phase difference and carrying out unwrapping on the variation.
The strategy for selecting the frequency signal with the best signal-to-noise ratio of the two determined points is as follows: at tnThe intensity of each frequency at the two points is multiplied, and the frequency with the maximum intensity product of the two points can be regarded as the frequency with the best signal-to-noise ratio at the moment.
In the experiment, N is 3. The system structure diagram of the specific experiment is shown in FIG. 2.
Using device performance: the laser is an RIO laser, the wavelength of the laser is 1550nm, the line width is 3kHz, and the output optical power is 11 dBm; the model of the first acousto-optic modulator is Gooch & Housego, and the frequency of 40MHz can be shifted up; the model of the second sound optical modulator is Gooch & Housego, and the frequency of 80MHz can be shifted up; the model of the third acousto-optic modulator is Gooch & Housego, and the frequency of 150MHz can be shifted up; the EDFA adopts an Amonics amplifier, the center frequency is 1550nm, and the constant power gain can reach 23 dBm; the balanced probe is model PDB430C from ThorLab corporation, 350MHz bandwidth, 40dB amplification.
The specific procedure for binding the experimental parameters was as follows:
step one, a laser generates continuous mode narrow linewidth laser with initial frequency f0The narrow linewidth laser is divided into two paths by a 90/10 coupler: inputting 90% of one path as initial detection light to a second coupler (1 × 3 coupler), and inputting 10% of one path as local oscillation light to a fourth coupler (2 × 2 coupler);
setting the pulse width and the pulse period of a pulse generator, wherein the pulse width of a modulation pulse is 100ns, the period is 1ms, outputting three paths of same modulation pulses to a first acousto-optic modulator, a second acousto-optic modulator and a third acousto-optic modulator, and outputting synchronous pulses synchronous with the modulation pulses to a signal acquisition card, wherein the synchronous relation of the pulses is shown in figure 3;
step three, the initial detection light is divided into three paths by the second coupler, the continuous mode narrow linewidth laser is converted into three paths of synchronous pulse light by the first acousto-optic modulator, the second acousto-optic modulator and the third acousto-optic modulator respectively, and the three paths of pulse light generate different frequencies and move upwards by delta f1＝40MHz、Δf2＝80MHz、Δf3The frequency diagram of three-way pulse light is shown in fig. 4 at 150 MHz;
combining three paths of pulse light into one path through a third coupler (1 x 3 coupler), outputting the path to an erbium-doped fiber amplifier, outputting the pulse light amplified until the peak value is 21dBm to a first port of an optical fiber circulator, injecting the pulse light into a sensing optical fiber from a second port of the optical fiber circulator, receiving the final pulse light by the sensing optical fiber, generating back scattering light, inputting the back scattering light to a second port of the optical fiber circulator, and outputting the back scattering light to a fourth coupler from a third port of the optical fiber circulator;
fifthly, after mixing the backward Rayleigh scattered light and the local oscillation light, the fourth coupler outputs coherent light to a balance detector for detection to obtain intermediate frequency electric signals containing frequency components of 40MHz, 80MHz and 150MHz, wherein the frequency spectrum schematic diagram is shown in FIG. 5, and the signals are output to a signal acquisition card for acquisition;
step six, designing band-pass filters with center frequencies of 40MHz, 80MHz and 150MHz to respectively filter the acquired digital signals to respectively obtain time domain signals corresponding to 40MHz, 80MHz and 150MHz, wherein each frequency corresponds to an independent band
Fig. 6, 7 and 8 are respectively time domain signals of 40MHz, 80MHz and 150MHz measured by the device and method of the present invention, and the amplitude value of the time domain signal processed by the method of taking the best signal-to-noise ratio at any reference point is not less than the original time domain signals of 40MHz, 80MHz and 150 MHz;
seventhly, selecting m reference points at equal intervals along the distance axis, wherein the distance between every two adjacent reference points is the system spatial resolution, and for any 2 adjacent reference points L1And L2At L1And L2At 40MHz, the intensity of the curve is P11And P1280MHz corresponds to a curve intensity of P21And P22150MHz corresponds to a curve intensity of P31And P32At tnTime of day calculation P1＝P11*P12,P2＝P21*P22,P3＝P31*P32Selecting P1、P2、P3Obtaining a corresponding intermediate frequency signal t with the maximum median value by phase demodulationnWhen is at L1And L2Phase of
And
and difference thereof
Then, the intermediate frequency signal corresponding to the frequency is traced back forward for one period to obtain the signal at tn-1Phase difference of time
Finally obtaining the variation of the phase difference
This is cycled through each pulse period. In this embodiment, the number of signal acquisitions set in advance is 1000, that is, when n is 1000, the loop is ended. And to
By phase unwrapping, continuous
And (4) a curve, so that continuous measurement of the vibration signal is realized.
The foregoing is only a partial embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.
Claims (3)
1. A distributed optical fiber vibration sensing method for reducing detection dead zone probability is characterized in that: the distributed optical fiber vibration sensing device for reducing the probability of the detection dead zone comprises a pulse generator, a laser, a first coupler, a second coupler, N acousto-optic modulators, a third coupler, a erbium-doped optical fiber amplifier, an optical fiber circulator, a sensing optical fiber, a fourth coupler, a balance detector and a signal acquisition card;
the laser generates continuous mode narrow linewidth laser, and the narrow linewidth laser is divided into two paths through the first coupler; one path of the light is used as initial detection light and input to the second coupler, and the other path of the light is used as local oscillation light and input to the fourth coupler;
the pulse generator respectively outputs N paths of same modulation pulses to N acousto-optic modulators and outputs synchronous pulses synchronous with the modulation pulses to the signal acquisition card;
dividing the initial detection light into N paths by a second coupler, converting the continuous mode narrow linewidth laser into N paths of synchronous pulse light by N acousto-optic modulators respectively, and enabling the N paths of pulse light to generate different frequency shifts;
the third coupler combines N paths of pulse light into one path and outputs the path of pulse light to the erbium-doped fiber amplifier, the pulse light is amplified and then output to the first port of the fiber circulator and is injected into the sensing fiber from the second port of the fiber circulator, the sensing fiber generates backward Rayleigh scattering light, the backward Rayleigh scattering light is input to the second port of the fiber circulator and is output to the fourth coupler from the third port of the fiber circulator;
the fourth coupler mixes the back Rayleigh scattering light with the local oscillator light to form coherent light, the coherent light is output to two input ends of a balance detector from a first output end and a second output end of the fourth coupler to be detected, the balance detector obtains an intermediate-frequency electric signal, and the intermediate-frequency electric signal is output to a signal acquisition card from the output end of the balance detector;
the method comprises the following steps:
step one, enabling a laser to generate continuous mode narrow linewidth laser; setting the pulse width and the pulse period of a pulse generator; n acousto-optic modulators convert N paths of continuous probe light into pulse light and generate corresponding frequency shift quantity delta, wherein the frequency shift quantities generated by the acousto-optic modulators are different;
designing N band-pass filters, wherein the center frequencies of the N band-pass filters correspond to the frequency shift quantity delta of the N acousto-optic modulators respectively; the digital signals acquired by the signal acquisition card are filtered by the N band-pass filters to obtain time domain signal curves corresponding to N frequencies respectively;
selecting m reference points at equal intervals along the horizontal axis of the time domain signal curve;
step four, selecting the frequency with the best signal-to-noise ratio at two points from the N frequencies for two adjacent reference points, and obtaining the frequency signal in the frequency through phase demodulationPhase of the signal at two points at time tn
And
and difference thereof
Step five, the intermediate frequency signal of the frequency in the step four is traced back forward for a period to obtain the frequency at tn-1Time of phase difference of the frequency intermediate frequency signal at two points
Sixthly, the phase difference of the tn time in the step four and t obtained by backtracking the time in the step five forwards for one periodn-1The phase differences of the moments in time are subtracted,
obtaining the variation of the phase difference
Step seven, assigning n +1 to n, and returning to the step four; when n reaches the signal acquisition number set in advance, entering step eight;
step eight, pair
Performing phase unwrapping to obtain continuous
And (4) a curve, so that continuous measurement of the vibration signal is realized.
2. The distributed optical fiber vibration sensing method for reducing the probability of detecting the dead zone according to claim 1, wherein the strategy for selecting the frequency signal with the best signal-to-noise ratio of the two determined points is as follows: at tnThe intensity of each frequency at the two points is multiplied, and the frequency with the maximum intensity product of the two points is regarded as the frequency with the best signal-to-noise ratio at the moment.
3. The distributed optical fiber vibration sensing method for reducing the probability of detecting the dead zone according to claim 1, wherein the N acousto-optic modulators have different frequency shift amounts for the detection light, and the frequency difference between the frequency shift amounts is larger than the reciprocal of the detection light pulse width.
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| CN114111860B (en) * | 2021-12-03 | 2022-08-30 | 北京科技大学 | Distributed phi-OTDR sensing method and system based on multi-frequency pulse coding |
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