CN113701793B - Method for realizing phase locking without hardware in phase-sensitive optical time domain reflection system - Google Patents
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Abstract
The invention discloses a method for realizing non-hardware phase locking in a phase-sensitive optical time domain reflection system, which is characterized in that the acquired intermediate frequency signals are subjected to adjacent point sliding filtering to obtain data with high similarity, the signal-to-noise ratio of the intermediate frequency signals obtained by filtering is obviously improved, meanwhile, the interference frequency caused by hardware phase locking is avoided or moved away from a region to be observed, the interference of the signals to be observed is avoided, and finally, the obtained M groups of phase-locked phase-demodulation data can also be directly subjected to power spectral density superposition, so that higher sensitivity is obtained. The invention can realize the initial phase locking of the collected intermediate frequency signal by performing data extraction on the intermediate frequency signal at a certain sampling interval on the premise of not changing a hardware structure for a phi-OTDR system in which three signal clocks of a carrier signal in a driving source of an acousto-optic modulator, a pulse modulation signal of the driving source of the acousto-optic modulator and a trigger signal of a data acquisition card are not locked.
Description
Technical Field
The invention belongs to the technical field of optical fiber sensing, and particularly relates to a method for realizing non-hardware phase locking in a phase-sensitive optical time domain reflection system.
Background
The Phase-Sensitive Optical Time Domain Reflectometry (phi-OTDR) technology can implement long-distance, distributed, real-Time and quantitative monitoring of strain caused by dynamic disturbance along an Optical fiber, and is applied to various fields such as structural health monitoring of large facilities, perimeter security, seismic wave detection, power transmission line monitoring, underwater cable monitoring and the like. In order to obtain dynamic disturbance information along an optical fiber, a phi-OTDR system is realized by utilizing interference between a narrow-linewidth pulse laser and reference Light and a backward Rayleigh scattering (RBS) generated inside the optical fiber, but the strength and the phase of the RBS have certain randomness, so that coherent fading noise is inevitably generated when the signal to noise ratio is improved by directly superposing the RBS, the signal to noise ratio of the system is rapidly deteriorated under the influence of the coherent fading noise, and the external disturbance signal obtained by reconstruction is severely distorted.
However, because three signal clock sources, namely a carrier signal in a driving source of an acoustic-optical modulator (AOM), a pulse modulation signal of the driving source of the acoustic-optical modulator (AOM), and a trigger signal of a data acquisition card, are independent from each other, the initial phase of an original intermediate frequency signal acquired by the data acquisition card continuously changes, so that the correlation between adjacent curves of the intermediate frequency signal which is continuous along with time is gradually reduced along with time, when coherent fading noise is suppressed in a sliding average manner, false medium-high frequency signal interference generated by phase demodulation is caused, and periodic inherent frequency which does not exist in the actual environment can be observed on a spectrogram of an RBS signal after IQ demodulation is performed on the RBS signal.
In order to suppress the influence of coherent fading noise by means of moving average, hardware modification and upgrade can be generally performed only on the structure of the phi-OTDR system, so that the clocks of the three signals come from the same clock source, however, the existing method needs to generate a synchronous clock signal by means of a function generator outside the system. Because the existing phi-OTDR system is not provided with a hardware phase locking structure generally, and has the advantages of complex installation process, high cost and certain occupied space volume, the system can achieve the effect similar to hardware phase locking by extracting intermediate frequency signal data.
Disclosure of Invention
The purpose of the invention is as follows: aiming at the problems, the invention provides a method for realizing phase locking by non-hardware in a phase-sensitive optical time domain reflection system, which can realize the effect of approximate hardware phase locking by a data extraction mode, improve the signal-to-noise ratio of an intermediate frequency signal in a phi-OTDR system and obtain higher sensitivity.
The technical scheme is as follows: the invention relates to a method for realizing non-hardware phase locking in a phase sensitive optical time domain reflection system, which specifically comprises the following steps:
(1) rayleigh back scattering light generated in a sensing optical fiber by a detection optical pulse obtained by a phi-OTDR system based on coherent detection and a beat frequency signal of reference light;
(2) the beat frequency signal is subjected to intermediate frequency filtering through a band-pass filter, broadband noise is suppressed, and an intermediate frequency signal is obtained and is acquired by a data acquisition card and input into a computer;
(3) selecting continuous intermediate frequency signal data in a quiet state as a sample, taking a time domain oscillogram at any sampling moment as a template, continuously drawing the time domain oscillogram of the sample to be compared with the template, observing the phase drift of the time domain oscillogram, and counting a period M when the phase of the time domain oscillogram and the template are approximately coincident each timetMultiple observations and statistics of MtA value;
(4) finding MtTaking the k value obtained by rounding the positive integral multiple of the mean value as a sampling interval, and simultaneously extracting M groups of intermediate frequency signal data with consistent sampling intervals in the sampling interval;
(5) IQ demodulation is respectively carried out on each group of extracted intermediate frequency signal data: dividing the intermediate frequency signal into two paths to carry out frequency mixing with sine and cosine digital signals, and then removing a high-frequency part through a low-pass filter to obtain I \ Q two paths of outputs so as to obtain phase information of the intermediate frequency signal;
(6) and performing phase unwrapping processing on the obtained phase data to obtain M groups of phase-locked phase discrimination data containing the signals to be detected.
Further, the intermediate frequency signal in step (2) is:
wherein E isR(n) amplitude intensity of scattered light, EL(n) is the amplitude intensity of the reference light, fmThe quantitative optical frequency shift quantity introduced for the acousto-optic modulator,
to be the initial phase of the scattered light,
referring to the initial phase of the reference light, N is any sampling point, and N is the total number of samples.
Further, said step (3) said observing and counting MtThe number of values is at least 100.
Further, the step (4) is realized as follows:
to avoid the occurrence of frequency difference f between the quantitative optical shift frequency and the sampling rate of the data acquisition cardnIf the phase drift of the intermediate frequency signal is to be avoided and the hardware phase is not locked, the mean value M is usedtWhen the rounded k value of the positive integer multiple is used as the sampling interval, k is:
k＝N1Mt (14)
wherein N is1In practice, it should be preferred that when sampling with k, the phase result is:
due to the mean value MtFor observed values, there is an error Δ M from the true valuestAnd the positive integer multiple of the integer can not be taken as a positive integer k but a decimal k1Rounding off Δ k1K is then obtained, which is actually:
k＝N1(Mt+△Mt)+△k1 (16)
when sampling is carried out by taking the k value as a sampling interval, an error exists, and the approximate error is as follows:
wherein N is1As an optimal value, k should be made to meet the requirement on the premise of ensuring that the sampling rate meets the requirement1As close to k as possible.
Further, the M groups of phase-locked phase detection data in step (6) can improve the signal-to-noise ratio by the superposition of power spectral density, and obtain higher sensitivity.
Furthermore, the sampling rate of the data acquisition card is higher than the frequency of the signal to be detected by more than k times.
Has the advantages that: compared with the prior art, the invention has the beneficial effects that: 1. under the condition of not locking a hardware phase and on the premise of not changing the structure of a traditional phi-OTDR system, the initial phase of the collected backward Rayleigh scattering light is locked by extracting data of the intermediate frequency signal at a certain sampling interval; 2. the acquired intermediate frequency signals are subjected to inter-near point sliding filtering, so that the obtained data have high similarity, the signal-to-noise ratio of the intermediate frequency signals obtained by filtering is obviously improved, the interference frequency caused by hardware phase locking can be avoided or moved away from the area to be observed, and the interference of the signals to be observed is avoided; 3. the obtained M groups of phase-locked phase detection data can be directly superposed with power spectral density, so that higher sensitivity is obtained.
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FIG. 1 is a flow chart of the present invention;
FIG. 2 is a superimposed graph of 100 continuous time domain waveforms at a quiet time when the conventional φ -OTDR system does not perform hardware structure phase locking;
FIG. 3 is a diagram showing an overlay of 100 continuous time domain waveforms after intermediate frequency signal data extraction is performed at a quiet time when a conventional phi-OTDR system is not performing hardware structure phase locking;
FIG. 4 is a graph of the spectrum of an intermediate frequency signal detected by a balanced detector when a 720Hz interference signal is externally applied when a conventional phi-OTDR system is not phase locked by a hardware structure;
FIG. 5 is a frequency spectrum diagram of an intermediate frequency signal detected by a balanced detector after performing intermediate frequency signal data extraction when a 720Hz interference signal is externally applied when a conventional phi-OTDR system does not perform hardware structure phase locking.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings.
The invention provides a method for realizing non-hardware phase locking in a phase sensitive optical time domain reflection system, which is used for realizing initial phase locking of an acquired intermediate frequency signal by performing data extraction on the intermediate frequency signal at certain sampling intervals on the premise of not changing a hardware structure for a phi-OTDR system in which three signal clocks of a carrier signal in an Acoustic Optical Modulator (AOM) driving source, a pulse modulation signal in the Acoustic Optical Modulator (AOM) driving source and a trigger signal of a data acquisition card are not locked; the acquired intermediate frequency signals are subjected to inter-near point sliding filtering, the obtained data have high similarity, the signal-to-noise ratio of the intermediate frequency signals obtained by filtering is obviously improved, and meanwhile, the interference frequency caused by hardware phase locking is avoided or moved away from the area to be observed, so that the interference of the signals to be observed is avoided; the finally obtained M groups of phase-locked phase discrimination data can also be directly superposed with power spectral density, thereby obtaining higher sensitivity; as shown in fig. 1, the method specifically comprises the following steps:
step 1: rayleigh backscattered light (RBS) generated in the sensing fiber by the probe light pulses obtained by a coherent detection based phi-OTDR system and a beat signal of reference light.
Step 2: and (3) performing intermediate frequency filtering on the beat frequency signal through a band-pass filter, inhibiting broadband noise to obtain an intermediate frequency signal, and acquiring and inputting the intermediate frequency signal into a computer by a data acquisition card (DAQ).
Suppose that the quantitative optical frequency shift quantity introduced by an acousto-optic modulator (AOM) of the traditional coherent detection type phi-OTDR system is fmAmplitude intensity of scattered light is ER(n) amplitude intensity of the reference light is EL(n) initial phase of scattered light is
Reference lightHas an initial phase of
And any sampling point is set as N, and the total number of samples is N. The intermediate frequency signals collected by the data acquisition card can be expressed as:
for the traditional coherent detection type phi-OTDR system, IQ demodulation is generally directly carried out after an intermediate frequency signal is obtained because phase locking on hardware is not carried out, and the amplitude intensity of sine and cosine signals in the IQ demodulation is assumed to be represented as ATFrequency of fTInitial phase of
The functional expression can be expressed as:
intermediate frequency signal I0(n) are respectively related to sine and cosine signals T1、T2Mixing:
wherein: a (n) ═ ER(n)EL(n) (6)
In ideal condition fm＝fTAnd removing the high-frequency part of the two paths of signals through a low-pass filter to obtain I \ Q two paths of outputs:
however, the carrier signal in the drive source of the acousto-optic modulator (AOM), the pulse modulation signal of the drive source of the acousto-optic modulator (AOM) and the trigger signal of the data acquisition card come from different clock sources, are independent of each other, the phase relation is not locked, and the influence of external temperature change and environment causes fmAnd fTFrequency difference f between themnAnd corresponding phase error
So that there is a phase shift in the intermediate frequency signal, the I, Q signal at this time can be actually expressed as:
wherein f issThe sampling rate of the data acquisition card is higher than the frequency of the signal to be detected by more than k times. This will cause an error in the phase demodulation result finally obtained by the system, and the phase demodulation result with a certain error can be expressed as:
wherein m is a positive integer, since fnDue to the existence of the phase, the finally obtained phase result cannot correctly represent the actual condition of RBS signal phase change caused by external intrusion disturbance, so that the detection result is distorted, and the signal-to-noise ratio is reduced.
And step 3: selecting 10s continuous intermediate frequency signal data in a quiet state as a sample, taking a time domain oscillogram at any sampling moment as a template, continuously drawing the time domain oscillogram of the sample to be compared with the template, observing the phase drift of the time domain oscillogram, and counting a period M when the phase of the time domain oscillogram and the template are approximately coincident each timetMultiple observations and statistics of MtThe value is obtained. Observe and count MtThe number of values is at least 100.
And 4, step 4: finding MtAnd taking the k value obtained by rounding the positive integral multiple of the mean value as a sampling interval, and simultaneously extracting M groups of intermediate frequency signal data with consistent sampling intervals in the sampling interval.
To avoid generating fnIt is necessary to avoid the phase drift of the intermediate frequency signal as much as possible, and to use the mean value M without performing hardware phase lockingtWhen the rounded k value of positive integer multiple is taken as the sampling interval, k can be expressed as:
k＝N1Mt (14)
it should be noted that N is used herein1Actually, it should be the preferred value, and when sampling with k value, f can be effectively avoided to some extentnThe phase value demodulated at this time can obtain the actual correct phase result after unwrapping:
but in general, due to the mean value MtFor observed values, there is an error Δ M from the true valuestAnd its positive integer multiple passOften, an integer k cannot be obtained, but a decimal k1Rounding off Δ k1K is then obtained, which is actually:
k＝N1(Mt+△Mt)+△k1 (16)
therefore, there is an error in sampling at sampling intervals of k value, and the approximation error is:
δk＝(k-k1)/k1×100％ (17)
simplifying to obtain:
δk＝△k1/N1(Mt+△Mt)×100％ (18)
due to the approximation error, the interference of the periodic natural frequency cannot be completely eliminated, and when the intermediate frequency signal is sampled by taking the k value as the sampling interval, a frequency f is remained in the frequency of the periodic natural interference signallWhen k is1As close to k as possible, flWill be moved away from the original frequency point, thereby avoiding the interference of the signal to be observed. At the same time, since the extraction of the intermediate frequency signal data is actually a down-sampling process, N1The value should not be too large, otherwise the sampling rate of the data acquisition card (DAQ) would be required to be too high, so N1As an optimal value, k must be set to satisfy the sampling rate1As close to k as possible.
And 5: IQ demodulation is respectively carried out on each group of extracted intermediate frequency signal data: the intermediate frequency signal is divided into two paths to be mixed with sine and cosine digital signals, and then the high frequency part is removed through a low-pass filter to obtain I \ Q two paths of outputs, so that the phase information of the intermediate frequency signal is obtained.
Step 6: and performing phase unwrapping processing on the obtained phase data to obtain M groups of phase-locked phase discrimination data. The phase discrimination data of M groups of phase locking can be directly superposed with power spectral density, thereby improving the signal to noise ratio and obtaining higher sensitivity.
In the present embodiment, the center frequency of the band-pass filter is specified to be 200MHz and the bandwidth to be 20MHz, according to the frequency of the beat signal of 200MHz and the pulse width of the probe optical pulse of 100 ns. Will beat the frequency signalAnd performing intermediate frequency filtering through a band-pass filter, inhibiting broadband noise to obtain an intermediate frequency signal, and acquiring and inputting the intermediate frequency signal into a computer by a data acquisition card (DAQ), wherein the sampling rate of the data acquisition card is set to be 20 kHz. Selecting intermediate frequency signal data in a 10s quiet state as a sample, taking a time domain oscillogram at a first sampling moment as a template, continuously drawing the time domain oscillogram of the sample to be compared with the template, observing the phase drift of the time domain oscillogram, and counting a period M when the phase of the time domain oscillogram and the template are approximately coincident each timetObserve and count MtValue 300 times, most of MtThe value is 6, and a few are 5. Finding MtHas an average value of 5.929, with a preferred value of N1If 1 is selected, k is 6 as the sampling interval, 6 groups of intermediate frequency signal data with the same sampling interval are simultaneously extracted in the sampling interval, and the sampling rate is reduced to 3373Hz through data extraction. IQ demodulation is respectively carried out on each group of extracted intermediate frequency signal data: the intermediate frequency signal is divided into two paths to be mixed with sine and cosine digital signals, and then the high frequency part is removed through a low-pass filter to obtain I \ Q two paths of outputs, so that the phase information of the intermediate frequency signal is obtained. And performing phase unwrapping processing on the obtained phase data to obtain 6 groups of phase-locked phase discrimination data.
The experiment shows that hardware phase locking is carried out on a clock source of a carrier signal which is not in a driving source of the acousto-optic modulator, a clock source of a pulse modulation signal which is used for being input to the AOM driving source and a clock source of a trigger signal which is used for a data acquisition card, when an observed signal is a 720Hz vibration signal, the method is shown in the step 2 that 100 continuous time domain oscillograms of intermediate frequency signal samples detected by a balance detector in a certain quiet state are superposed, and the observation that phase information among the intermediate frequency signals at different moments has no correlation due to phase drift can be observed; fig. 3 is a graph showing that after 100 continuous time domain waveform diagrams obtained by data extraction of intermediate frequency signal samples detected by a balanced detector in a quiet state are superimposed, it can be observed that phase drift is avoided by the data extraction mode, and phase information among intermediate frequency signals at different times has strong correlation. By comparing fig. 2 and fig. 3, it can be illustrated that the method of the present invention can lock the initial phase of the collected backward rayleigh scattering light when the conventional coherent detection type phi-OTDR is not hardware phase-locked.
FIG. 4 is a graph of the spectrum of a 250s IF signal detected by a balanced detector when a 720Hz interference signal is externally applied without hardware phase lock for a conventional φ -OTDR system. In a spectrogram, the influence of natural frequencies such as 622Hz, 780.7Hz, 880Hz and the like exists, the signal-to-noise ratio is about 2.5dB, and the noise spectrum is uneven; fig. 5 is a frequency spectrum diagram of 250s intermediate frequency signals detected by a balanced detector when a 720Hz interference signal is externally applied after data extraction and phase locking is realized on software, the power spectrum density of the diagram is already superposed on 6 groups of sampled data, and it can be seen that the natural interference frequency around 720Hz is shifted away from the frequency band, the signal-to-noise ratio is 12.3842dB, the improvement is greatly achieved, and the noise spectrum is flattened. By comparing fig. 4 and fig. 5, it can be seen that when the conventional coherent detection type phi-OTDR does not perform hardware phase locking, the invention can avoid or move away the interference frequency caused by the hardware phase locking from the region to be observed, so as to avoid the interference of the signal to be observed; meanwhile, the collected intermediate frequency signals are subjected to near point sliding filtering, the obtained data have high similarity, and the signal-to-noise ratio of the intermediate frequency signals obtained through filtering is obviously improved, so that high sensitivity is obtained.
The foregoing is a more detailed description of the invention in connection with specific preferred embodiments and it is not intended that the invention be limited to these specific details. For those skilled in the art to which the invention pertains, several simple deductions or substitutions can be made without departing from the spirit of the invention, and all should be considered as belonging to the protection scope of the invention.
Claims (4)
1. A method for realizing phase locking in a non-hardware way in a phase sensitive optical time domain reflection system is characterized by comprising the following steps:
(1) rayleigh back scattering light generated in a sensing optical fiber by a detection optical pulse obtained by a phi-OTDR system based on coherent detection and a beat frequency signal of reference light;
(2) the beat frequency signal is subjected to intermediate frequency filtering through a band-pass filter, broadband noise is suppressed, and an intermediate frequency signal is obtained and is acquired by a data acquisition card and input into a computer;
(3) selecting continuous intermediate frequency signal data in a quiet state as a sample, taking a time domain oscillogram at any sampling moment as a template, continuously drawing the time domain oscillogram of the sample, comparing the time domain oscillogram with the template, observing the phase drift of the time domain oscillogram, and counting a period M when the phase of the time domain oscillogram and the template are approximately coincident each timetMultiple observations and statistics of MtA value;
(4) finding MtTaking the k value obtained by rounding the positive integral multiple of the mean value as a sampling interval, and simultaneously extracting M groups of intermediate frequency signal data with consistent sampling intervals in the sampling interval;
(5) IQ demodulation is respectively carried out on each group of extracted intermediate frequency signal data: dividing the intermediate frequency signal into two paths to be mixed with sine and cosine digital signals, and removing a high-frequency part through a low-pass filter to obtain two paths of I and Q outputs so as to obtain phase information of the intermediate frequency signal;
(6) and performing phase unwrapping processing on the obtained phase data to obtain M groups of phase-locked phase discrimination data containing the signals to be detected.
2. The method according to claim 1, wherein the intermediate frequency signal in step (2) is:
wherein E isR(n) amplitude intensity of scattered light, EL(n) is the amplitude intensity of the reference light, fmThe amount of optical frequency shift introduced for the acousto-optic modulator,
in order to be able to initiate the phase of the scattered light,
n is any sampling point, and N is the total number of samples.
3. The method of claim 1, wherein the observing and counting M in step (3) is performed according to the non-hardware phase locking method in the phase-sensitive optical time domain reflectometry systemtThe number of values is at least 100.
4. The method according to claim 1, wherein the phase detection data of the M groups of phase locking in step (6) are obtained by superimposing power spectral densities to improve a signal-to-noise ratio and obtain a high sensitivity.
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