CN105067104B - Composite optical fiber sensing system and sensing method - Google Patents
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Abstract
The invention discloses a composite optical fiber sensing system and a sensing method, which have the same positioning precision as phase-sensitive OTDR (optical time domain reflectometer), can obtain high-frequency details of vibration, further carry out accurate vibration type identification and effectively reduce the false alarm rate of the system. Since the phase-sensitive OTDR uses reverse rayleigh scattered light for detection, the intensity of reflected light is extremely low, and the power of light intensity of the interferometer is large, so that the optical signal of the phase-sensitive OTDR is easily submerged in the credit of the interferometer. The FBG is introduced, the system successfully realizes the light path combination and the optical signal separation of the interferometer and the scatterometer, so that the system has a relatively simple structure and low cost. The vibration accurate position can be obtained by carrying out an improved moving average algorithm and a wavelet information entropy algorithm on the Rayleigh scattering signal of the ultra-narrow line width laser light source. Interference signals of the narrow-linewidth laser light source contain rich frequency information, wavelet packet decomposition is carried out on the interference signals, vibration types can be distinguished through the neural network, and the false alarm rate is reduced.
Description
Technical Field
The invention relates to the field of optical fiber sensing, in particular to a composite optical fiber sensing system and a sensing method based on the principles of Bragg gratings, a Mach-Zehnder interferometer and a phase type optical time domain reflectometer.
Background
The distributed optical fiber sensing system has the characteristics of high sensitivity, no electromagnetic interference, wide detection range, low cost and the like, is widely applied to the fields of long-distance oil and gas pipeline monitoring, perimeter security, building structure health monitoring and the like, and is a research hotspot of nearly decades.
The distributed optical fiber sensing system of the Mach-Zehnder/Sagnac interferometer can well sense vibration by detecting phase difference change caused by external disturbance in two sensing light paths and positioning by a related time delay estimation method. However, the difficulty of determining the time delay by the related time delay estimation method is low, so that the method is not high in positioning precision and difficult to accurately judge the vibration point.
The coherent result of the optical pulse return light is detected by using a long coherent light source based on the coherent Rayleigh scattering phase-sensitive optical time domain reflectometry (phase-sensitive OTDR, phi-OTDR), and the interference method can effectively realize dynamic response, can simultaneously realize high positioning precision and high sensitivity detection, and especially can detect weak disturbance signals. However, since the frequency of the transmitted pulse is limited by the length of the optical fiber, the frequency response is very low, so that the vibration event cannot be effectively identified, and the false alarm rate is high.
Disclosure of Invention
The invention provides a composite optical fiber sensing system and a sensing method, which realize the positioning and identification of vibration events and effectively reduce the false alarm rate, and are described in detail in the following description:
a composite optical fiber sensing system comprising: a sensing fiber for vibration detection, further comprising: an ultra-narrow linewidth laser light source for forward injection into the sensing fiber,
continuous detection light emitted by the ultra-narrow line width laser light source is modulated into pulse light by an acousto-optic modulator, amplified by a first erbium-doped fiber amplifier and injected into the sensing fiber by a first circulator;
backward Rayleigh scattering light reversely reaches the first circulator and reaches the fiber Bragg grating from the other port through the second erbium-doped fiber amplifier and the second circulator;
the detection light emitted by the narrow linewidth laser light source is divided into 2 beams through a first 1:1 coupler, one beam is reversely injected into the sensing optical fiber through the isolator, and the other beam is reversely injected into the reference arm optical fiber through the first polarization controller;
selecting the laser frequency of the ultra-narrow linewidth laser light source and the laser frequency of the narrow linewidth laser light source, so that the laser frequency of the ultra-narrow linewidth laser light source is within the reflection band of the fiber Bragg grating, and the laser frequency of the narrow linewidth laser light source is within the pass band of the fiber Bragg grating;
the Rayleigh scattered light is reflected by the fiber Bragg grating and reenters the second circulator, and reaches the first photoelectric detector from the other outlet of the second circulator, and the optical signal is converted into an electric signal and is collected by the first collecting card;
the detection light of the narrow-linewidth laser light source and the Rayleigh scattering light with the reflection power lower than 1% pass through the fiber Bragg grating and interfere with the detection light passing through the reference arm fiber at the third 1:1 coupler;
the interference light is detected by the second photoelectric detector, converted into an optical signal, collected by the second collecting card and transmitted to the computer for processing.
The system further comprises: a 1:99 coupler and a second polarization controller,
the ultra-narrow linewidth laser light source separates 1% laser as local oscillation light through a 1:99 coupler, and the local oscillation light reaches a second 1: and the coupler 1 interferes with Rayleigh scattered light of the ultra-narrow linewidth laser light source to form optical heterodyne detection.
A composite optical fiber sensing method, comprising the steps of:
cutting the Rayleigh scattered light into a plurality of scattering traces according to the time sequence; averaging the K scattering traces to obtain T average curves;
extracting time domain signals, and respectively calculating wavelet information entropy of each time domain signal;
taking the wavelet information entropy as comprehensive evaluation in K scattering trace time, and when the wavelet information entropy is increased, indicating that vibration occurs at a corresponding position j for determining a vibration position;
the vibration signal is classified.
The technical scheme provided by the invention has the beneficial effects that: the invention provides a distributed optical fiber vibration sensing system with high signal resolution, low noise level, high sensitivity, high positioning precision, high frequency resolution and low false alarm rate. The mixed separation problem of interferometer and scatterometer light signal in multiplexing has been solved, the signal quality of system has been improved, make the signal be convenient for survey, the stability of system has been improved, simultaneously, overcome current system can not have high positioning accuracy concurrently and the problem of high frequency difference, this system has the peculiar distributing type of distributed optical fiber supervision detecting system simultaneously, receive characteristics such as little external disturbance such as electromagnetism, and simple to operate, can be fine satisfy various vibration detection and monitoring applications, especially long distance pipeline monitoring and perimeter security protection etc..
Drawings
FIG. 1 is a schematic structural diagram of a composite optical fiber sensing system;
FIG. 2(a) is a schematic diagram of a time-domain waveform of a vibration signal;
fig. 2(b) is a schematic diagram of the wavelet packet decomposition result of the vibration signal.
In the drawings, the components represented by the respective reference numerals are listed below:
1: an ultra-narrow linewidth laser light source; 2: 1:99 coupler;
3: an acousto-optic modulator; 4: a first erbium-doped fiber amplifier;
5: a first circulator; 6: a sensing optical fiber;
7: a second erbium-doped fiber amplifier; 8: a second circulator;
9: FBG (fiber Bragg Grating); 10: a narrow linewidth laser light source;
11: a first 1:1 coupler; 12: an isolator;
13: a first polarization controller; 14: a reference arm optical fiber;
15: and thirdly, 1:1, a coupler; 16: second 1:1, a coupler;
17: a second polarization controller; 18: a first photodetector;
19: an amplitude modulation demodulation circuit; 20: a first acquisition card;
21: a second photodetector; 22: a second acquisition card;
23: and (4) a computer.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, embodiments of the present invention are described in further detail below.
The Fiber Bragg Grating (FBG) technology is characterized in that a specific position of an optical fiber is made into a grating region with a periodically distributed refractive index, light waves in a specific wavelength range are reflected, light waves with other wavelengths are transmitted, and the FBG technology has extremely high reflectivity and very narrow reflection bandwidth.
The optical fiber sensing system designed by the embodiment of the invention successfully combines the Mach-Zehnder interferometer and the phase-sensitive OTDR technology, so that the system has extremely high positioning precision and very high frequency response; the FBG is introduced to realize the optical separation of the optical signal of the interferometer and the optical signal of the OTDR, so that the quality of the two parts of optical signals is effectively improved (namely the signal to noise ratio is improved), the system can accurately position and identify the vibration event, and the false alarm rate is effectively reduced.
Example 1
A composite optical fiber sensing system based on phase-sensitive OTDR principle, Mach-Zehnder interferometer principle and FBG wavelength division multiplexing principle comprises: a sensing optical fiber 6 is laid in a range needing to detect vibration, and the sensing optical fiber 6 is used as a sensor and a signal transmission medium to realize the detection and monitoring of the vibration; an ultra narrow linewidth laser source 1 (linewidth of several hundred kHz) is used as a phase-sensitive OTDR partial light source (optical frequency v)1) Forward direction transmissionThe optical sensing fiber 6 injects probe light.
Continuous detection light emitted by the ultra-narrow linewidth laser light source 1 is modulated into pulse light through an acousto-optic modulator (AOM)3, amplified through a first erbium-doped fiber amplifier (EDFA)4 and then injected into a sensing fiber 6 through a first circulator 5. If a vibration event occurs along the sensing optical fiber 6, the optical phase of the detection optical pulse back to the rayleigh scattering light will be modulated by the vibration event, resulting in a change in the light intensity of the back rayleigh scattering light.
The backscattered rayleigh scattered light then reaches the circulator 5 in a backward direction, passes through a second Erbium Doped Fiber Amplifier (EDFA)7 from the other port to a second circulator 8, and reaches a narrow band FBG of high reflectivity (e.g., 99% reflectivity, 3dB reflection wavelength 1550.0nm-1550.3nm)9 at the exit end of the second circulator 8;
on the other hand, a narrow linewidth laser light source 10 (linewidth of several MHz) is used as a light source of a Mach-Zehnder interferometer portion, the optical frequency v of which is2Let v be1With minor differences (a few tenths of a nm difference, e.g. v)1Is 1550.2nm, v21549.9nm), wherein the probe light from the ultra-narrow linewidth laser light source 1 shares the same sensing fiber 6 with the probe light from the narrow linewidth laser light source 10, while the narrow linewidth laser light source 10 uses another fiber in the same optical cable as the reference fiber. The detection light emitted by the narrow linewidth laser light source 10 is divided into 2 beams by a first 1:1 coupler 11, one beam is reversely injected into the sensing optical fiber 6 through an isolator 12, and the other beam is reversely injected into the reference arm optical fiber 14 through a first polarization controller 13. Here, the isolator 12 is used to prevent probe light from the ultra-narrow linewidth laser light source 1 from entering the narrow linewidth laser light source 10, causing device damage.
The probe light in the sensing fiber 6 is modulated by the vibration signal along the way, reaches the first circulator 5, passes through the second erbium-doped fiber amplifier 7(EDFA) and the second circulator 8, and reaches the FBG 9. At this time, the probe light of the narrow-linewidth laser light source 10 and the rayleigh scattered light of the ultra-narrow-linewidth laser light source 1 are mixed at the front end of the FBG 9.
By selecting the laser frequencies of the ultra-narrow linewidth laser light source 1 and the narrow linewidth laser light source 10, the laser frequency of the ultra-narrow linewidth laser light source 1 falls within the reflection band of the FBG9, and the laser frequency of the narrow linewidth laser light source 10 is within the pass band of the FBG 9. At this time, the rayleigh scattered light of the ultra-narrow linewidth laser light source 1 is reflected by the FBG9 and re-enters the second circulator 8, and reaches the first photodetector 18 from the other outlet of the second circulator 8, and the optical signal is converted into an electrical signal and collected by the first collection card 20.
In order to further improve the signal quality of the phase-sensitive OTDR, 1% of laser light can be separated as local oscillator light by the 1:99 coupler 2 after the ultra-narrow line-width laser light source 1, and the local oscillator light reaches the second 1: and the coupler 16 1 interferes with Rayleigh scattered light of the ultra-narrow linewidth laser light source 1 to form optical heterodyne detection. Then, the first photodetector 18 detects the beat signal generated by the interference, and the beat signal is acquired by the first acquisition card 20 after passing through the amplitude modulation demodulation circuit 19 and is sent to the computer 23 for processing.
On the other hand, the probe light of the narrow-linewidth laser light source 10 and a very small portion of the rayleigh scattered light (reflected power less than 1%) of the ultra-narrow-linewidth laser light source 1 pass through the FBG9 and interfere with the probe light passing through the reference arm optical fiber 14 at the third 1:1 coupler 15. The interference light is detected by the second photodetector 21, converted into an optical signal, collected by the second collecting card 22, and transmitted to the computer 23 for processing.
Because the detection light power of the narrow-linewidth laser light source 10 is hundreds of times of the rayleigh scattering light power of the ultra-narrow-linewidth laser light source 1, and then the rayleigh scattering light of the ultra-narrow-linewidth laser light source 1 leaked from the FBG9 is attenuated by the FBG9, the detection light intensity of the narrow-linewidth laser light source 10 will not be affected.
In summary, the embodiment of the invention designs an optical fiber sensing system organically combining a Mach-Zehnder interferometer and a phase-sensitive OTDR. The system has the same positioning precision as that of the phase-sensitive OTDR, and can obtain high-frequency details of vibration, so that the precise vibration type identification is carried out, and the false alarm rate of the system is effectively reduced. On the other hand, since the phase-sensitive OTDR uses reverse rayleigh scattered light for detection, its reflected light intensity is very low (average power is usually only several tens uW), while the interferometer light intensity power is large (usually several mW), resulting in that the optical signal of the phase-sensitive OTDR is easily buried in the credit of the interferometer. And the FBG is introduced, so that the system successfully realizes the separation of two parts of optical signals, and the signal-to-noise ratio of the system is improved. The system successfully realizes the light path combination and the optical signal separation of the interferometer and the scatterometer, so that the system has relatively simple structure and lower cost.
Example 2
At the computer 23, a higher sampling frequency (e.g., 100MHz) is selected. Respectively obtaining Rayleigh scattering signals S of the ultra-narrow linewidth laser light source 1 from the first acquisition card 20 and the second acquisition card 222And interference detection signal S of narrow linewidth laser light source 101The Rayleigh scattering signal S is converted into a Rayleigh scattering signal S according to the repetition frequency of the modulation signal of the acousto-optic modulator 32Time-sequentially truncating into multiple scatter traces r ═ r1,r2,r3,…,ri,…,rk}。
In order to realize the positioning of the vibration event, an improved moving average algorithm is used for reducing signal noise, a wavelet information entropy method is used for eliminating the influence of attenuation along the sensing optical fiber 6 and quickly and accurately positioning the vibration occurrence position. The method specifically comprises the following steps:
101: averaging the K scattering traces to obtain T average curves;
k scattering traces r ═ r are selected1,r2,r3,…,ri,…,rkSelecting an average time M (recommended M)>50) Interval parameter n (recommended n is 5), for K scattering traces: the 1 st to M th averaging, and the nth to M-n +1 th averaging … … result in T ═ int ((K-M)/n) +1 average curves, i.e.:
wherein R isiFor the ith moving average resulting curve, int () is a rounding-down operation.
102: extracting the time-domain signal tj＝{xijRespectively calculating the wavelet information entropy of each time domain signal as an evaluation index of a detection point corresponding to the jth signal point;
wherein x isijIs RiThe jth value of (a). Here j represents the spatial domain, corresponding to the position information.
The wavelet information entropy provides detail information of the measured signal under different frequency scales and time scales for wavelet analysis, and the Shannon information entropy calculation rule is used for estimating the uncertainty of the measured signal. Each detection point is taken as an independent system, the vibration event increases the chaos degree of the corresponding detection point, and the distribution of signal energy is changed, so that the wavelet information entropy value of the detection point is increased. When the value increases, it is determined that a vibration event has occurred for the detected position. The specific calculation process is as follows:
firstly, to the time domain signal tjPerforming wavelet decomposition on N layers to obtain wavelet components of each layer,
here, DkFor high-frequency wavelet components obtained by wavelet decomposition of the k-th layer, ANAnd the low-frequency wavelet components obtained by the wavelet decomposition of the Nth layer. For the sake of simplicity and clarity, the above formula is uniformly expressed as,
wherein, CkRespectively are k +1 wavelet components obtained after wavelet decomposition of N layers.
Then, the energy E at each scale is calculatedk，
Ek＝∑|Ck|2
Then, the entropy of the wavelet information at each detection position is calculated:
wherein,
wherein, SwtjThe entropy of the wavelet information at the j point; p is a radical ofkIs the proportion of the kth wavelet component to the total energy; etotalIs the total energy of the signal.
103: entropy Swt of wavelet informationjAs a comprehensive evaluation of the time of K scattering traces on the sensing fiber 6, when the value increases, it indicates that vibration occurs at the corresponding position j, and the formula a is equal to (n)ccj)/(2fs) the vibration position can be determined;
where c is the speed of light, ncFs is the sampling rate of the first acquisition card 20, which is the refractive index of the optical fiber.
104: the vibration signal is classified.
In order to reduce the false alarm rate of the system, after the vibration is positioned, the vibration details are analyzed and identified, and the vibration type is judged. When wavelet information entropy SwtjWhen vibration is generated, an interference detection signal S of the narrow linewidth laser light source 10 in the time corresponding to the K scattering traces is extracted1. Detecting signal S due to interference1In order to perform a continuous light detection,the frequency resolution is determined by the sampling rate of the second acquisition card 22. If the sampling rate of the second acquisition card 22 is 100MHz, the frequency resolution is 50 MHz. After obtaining the vibration detail information, there are many methods to classify the signals, and the method is only exemplified by the wavelet packet energy method. The method specifically comprises the following steps:
on an oil pipeline, the frequency of a vibration event is usually below 20KHz, and an interference detection signal S is firstly detected1Down-sampling is performed to reduce the sampling rate to 40 KHz. And then 4 layers of wavelet packet decomposition are carried out to obtain 16 wavelet packet components, and the first 8 components are selected as the input feature vector of the BP neural network for event identification as shown in fig. 2 (b).
Before application, interference detection signals of various events collected in advance are used as a training and verification set, and referring to fig. 2(a), the interference detection signals are used as input quantities of a BP neural network to train and verify the BP neural network.
The trained neural network is used to realize the discrimination of common events, such as manual excavation and mechanical excavation aiming at pipeline application.
In summary, the embodiment of the present invention can obtain the vibration accurate position by performing an improved moving average algorithm and a wavelet information entropy algorithm on the rayleigh scattering signal of the ultra-narrow line width laser light source 1. The interference signal of the narrow linewidth laser light source 10 contains rich frequency information, wavelet packet decomposition is carried out on the interference signal, the vibration type can be distinguished through the neural network, and the false alarm rate is reduced.
Those skilled in the art will appreciate that the drawings are only schematic illustrations of preferred embodiments, and the above-described embodiments of the present invention are merely provided for description and do not represent the merits of the embodiments.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.
Claims (3)
1. A composite optical fiber sensing system comprising: a sensing fiber for vibration detection, further comprising: the ultra-narrow linewidth laser light source of the forward injection sensing optical fiber is characterized in that,
continuous detection light emitted by the ultra-narrow line width laser light source is modulated into pulse light by an acousto-optic modulator, amplified by a first erbium-doped fiber amplifier and injected into the sensing fiber by a first circulator;
backward Rayleigh scattering light reversely reaches the first circulator and reaches the fiber Bragg grating from the other port through the second erbium-doped fiber amplifier and the second circulator;
the detection light emitted by the narrow linewidth laser light source is divided into 2 beams through a first 1:1 coupler, one beam is reversely injected into the sensing optical fiber through the isolator, and the other beam is reversely injected into the reference arm optical fiber through the first polarization controller;
selecting the laser frequency of the ultra-narrow linewidth laser light source and the laser frequency of the narrow linewidth laser light source, so that the laser frequency of the ultra-narrow linewidth laser light source is within the reflection band of the fiber Bragg grating, and the laser frequency of the narrow linewidth laser light source is within the pass band of the fiber Bragg grating;
the Rayleigh scattered light is reflected by the fiber Bragg grating and reenters the second circulator, and reaches the first photoelectric detector from the other outlet of the second circulator, and the optical signal is converted into an electric signal and is collected by the first collecting card;
the detection light of the narrow-linewidth laser light source and the Rayleigh scattering light with the reflection power lower than 1% pass through the fiber Bragg grating and interfere with the detection light passing through the reference arm fiber at the third 1:1 coupler;
the interference light is detected by the second photoelectric detector, converted into an optical signal, collected by the second collecting card and transmitted to the computer for processing.
2. A composite optical fiber sensing system according to claim 1, further comprising: a 1:99 coupler and a second polarization controller,
the ultra-narrow linewidth laser light source separates 1% laser as local oscillation light through a 1:99 coupler, and the local oscillation light reaches a second 1: and the coupler 1 interferes with Rayleigh scattered light of the ultra-narrow linewidth laser light source to form optical heterodyne detection.
3. A sensing method for a hybrid fiber optic sensing system according to any of claims 1-2, comprising the steps of:
cutting the Rayleigh scattered light into a plurality of scattering traces according to the time sequence; averaging the K scattering traces to obtain T average curves;
extracting time domain signals, and respectively calculating wavelet information entropy of each time domain signal;
taking the wavelet information entropy as comprehensive evaluation in K scattering trace time, and when the wavelet information entropy is increased, indicating that vibration occurs at a corresponding position j for determining a vibration position;
the vibration signal is classified.
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| JP2006208080A (en) * | 2005-01-26 | 2006-08-10 | Hitachi Cable Ltd | Optical fiber vibration sensor |
| CN102506912A (en) * | 2011-09-29 | 2012-06-20 | 北京航空航天大学 | Optical fiber distributed disturbance sensor |
| CN103033842A (en) * | 2012-12-18 | 2013-04-10 | 电子科技大学 | Time division multiplexing array type fiber bragg grating seismic wave real-time monitoring system |
| CN104568120A (en) * | 2015-01-14 | 2015-04-29 | 天津大学 | Composite principle optical fiber sensing system and sensing method |
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