CN110793558A - Coherent detection type phi-OTDR system and self-checking method - Google Patents
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- G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
- G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
- G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
- G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
- G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
- G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
- G01D5/35338—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using other arrangements than interferometer arrangements
- G01D5/35354—Sensor working in reflection
- G01D5/35358—Sensor working in reflection using backscattering to detect the measured quantity
- G01D5/35361—Sensor working in reflection using backscattering to detect the measured quantity using elastic backscattering to detect the measured quantity, e.g. using Rayleigh backscattering
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- G01D18/00—Testing or calibrating apparatus or arrangements provided for in groups G01D1/00 - G01D15/00
Abstract
The invention discloses a self-checking method of a coherent detection type phi-OTDR system, and a system device comprises a laser, a first coupler, an adjustable optical attenuator (OVA), an acousto-optic modulator (AOM), an erbium-doped fiber amplifier (EDFA), a fiber circulator, a second coupler, a balanced detector, an adjustable gain Low Noise Amplifier (LNA), a signal acquisition card, a sensing fiber device box and a pulse generator. Operating the sensing system to obtain Rayleigh backscattering signals of the optical fibers in the optical fiber box; analyzing the intensity and the frequency domain of the original signal, wherein if the results tend to be stable normal values, the system works normally, otherwise, the system is in a fault state; under the normal working condition, the EDFA is adjusted to appropriate parameters to improve the signal intensity of the system; and adjusting the sizes of the OVA and the LNA to improve the signal-to-noise ratio of the system to obtain the optimal performance of the system. The system self-checking method disclosed by the invention can quickly detect whether the system normally operates or not, optimizes the system performance and has practical application value.
Description
Technical Field
The invention belongs to the technical field of optical fiber sensing, and particularly relates to a coherent detection type phi-OTDR system and a self-checking method.
Background
The research on effective monitoring systems for the structural health of long-distance, large-scale infrastructures has become a hot spot of research. The distributed optical fiber sensing system has the advantages of high measurement accuracy, electromagnetic interference resistance, corrosion resistance, long-distance distributed sensing realization and the like, and is applied to the field of structural health detection. In 1993, taylor henry F in the united states first proposed the use of coherent fading effect of rayleigh scattered light in optical fiber for sensing, which was the earliest Φ -OTDR. The phi-OTDR can detect and position disturbance events applied to the optical fiber to be detected, and is widely applied to health monitoring of large-scale structures.
The Φ -OTDR detection technique can be classified into a direct detection mode and a coherent detection mode. The structure of the self-heterodyne coherent detection phi-OTDR system is shown in figure 1, a part of continuous light emitted by a laser is divided as local reference light, the detection light and the local reference light are subjected to beat frequency through a coupler and then input into a photoelectric detector for detection, so that the evolution of the product of the intermediate frequency alternating current component detected by the detector and the local oscillator optical power and the signal optical power is in direct proportion, rather than only in direct proportion to the signal optical power, noise in a circuit can be well suppressed, and extremely high detection sensitivity and signal-to-noise ratio are obtained.
The performance of the phi-OTDR system will change due to different parameter settings of system devices and the influence of environmental factors. When the machine is debugged manually, the judgment of the working state of the machine is influenced by subjective factors, and the debugging of the machine by experience is not always close to the optimal setting method. In order to quickly judge whether a machine is in a normal working state and effectively debug a device to obtain a better parameter combination so as to improve the detection capability of a system, a system self-checking method is provided, an optical fiber sensing device box is designed, the performance of the system can be quickly judged, the better parameter combination can be obtained, and the application value is realized. By using the self-checking method, the sensing system of the type can be subjected to power-on self-checking, and the working condition of a machine can be rapidly and accurately mastered.
Disclosure of Invention
The purpose of the invention is as follows: aiming at the technical problem to be solved by the invention, the invention provides a coherent detection phi-OTDR system and a self-checking method, which can quickly judge the performance of the system and obtain better parameter combination.
The technical scheme is as follows: in order to realize the purpose of the invention, the technical scheme adopted by the invention is as follows: a coherent detection type phi-OTDR system comprises a laser, a first coupler, an adjustable optical attenuator, an acousto-optic modulator, an erbium-doped fiber amplifier, a fiber circulator, a second coupler, a balance detector, an adjustable gain low-noise amplifier, a signal acquisition card, a sensing fiber device box and a pulse generator; wherein:
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: the two paths of light are respectively input to an acousto-optic modulator and a variable optical attenuator;
the variable optical attenuator is used for properly attenuating 10% of continuous light output by the first coupler and then inputting the continuous light to the second coupler;
the acousto-optic modulator is used for modulating the continuous light output by the first coupler to generate pulse light and inputting the pulse light to the erbium-doped fiber amplifier;
the erbium-doped optical fiber amplifier is used for amplifying the input pulse light and outputting the amplified pulse light to the optical fiber circulator;
the fiber circulator is used for inputting amplified pulse light from a 1 st port of the fiber circulator and injecting the amplified pulse light into an accessed sensing fiber device box from a 2 nd port of the fiber circulator, and backward Rayleigh scattering light generated by the fiber is output to the second fiber coupler from a 3 rd port of the fiber circulator;
the second optical fiber coupler is used for mixing the Rayleigh back scattering light input from the 3 rd port of the optical fiber circulator with the local oscillator light input by the first coupler and outputting coherent light to the balance detector;
the balance detector is used for converting the frequency mixing optical signal into an electric signal and outputting the electric signal to the adjustable gain low noise amplifier;
the adjustable gain low-noise amplifier is used for amplifying and inputting the electric signal to the signal acquisition card;
the signal acquisition card is used for converting the electric signal into a digital signal for subsequent processing according to the trigger pulse;
and the synchronous pulse signal generator is used for outputting a synchronous pulse signal to the acousto-optic modulator and the data acquisition card, the acousto-optic modulator modulates continuous light emitted by the laser into pulsed light under the action of a pulse electrical signal, and the data acquisition card triggers an acquisition signal according to the synchronous pulse signal.
The invention also provides a self-checking method of the coherent detection type phi-OTDR system based on the device, which comprises the following steps:
acquiring original signal intensity of Rayleigh back scattering optical signals of optical fibers in a sensing optical fiber device box for a period of time by a coherent detection type phi-OTDR system and a data acquisition card;
analyzing and processing the acquired original signals, and making variance of the accumulated value of the intensity of the original signals per minute to obtain the variation condition of the variance V value of the original data along with the system operation time t;
thirdly, phase demodulation is carried out on the original signal, and then Fourier transform is carried out to obtain frequency domain information so as to obtain a spatial frequency distribution map;
step four, if the variance value V and the signal frequency domain value f of the intensity gradually tend to stable values and are smaller than a threshold value, the phi-OTDR system device can be considered to be in a normal working state, otherwise, the system is judged to be in a fault state;
analyzing whether more data acquired by the data acquisition card exceeds the range of the acquisition card or not under the condition that the system works normally; when a considerable proportion (95%) of the collected data does not exceed the range, the original signal intensity is considered to be appropriate;
regulating the pumping current of the erbium-doped fiber amplifier to enhance the intensity of the original Rayleigh scattered light signal and improve the dynamic range of the system;
and step seven, simultaneously adjusting the sizes of the variable optical attenuator and the variable gain low noise amplifier, calculating the signal-to-noise ratio of the signals obtained under various parameter combinations, and comparing to obtain the parameter combination with the highest signal-to-noise ratio so as to optimize the performance of the sensing system.
As a further preferable scheme of the self-checking method of the coherent detection type Φ -OTDR system according to the present invention, in the fourth step, a specific process of determining the working state of the sensing system is as follows:
step 1: after the system starts to work, data are continuously collected for a period of time. The variance V of the signal intensity in each minute is obtained by calculation, if the system works normally and the optical fiber box is not interfered by the outside, the variance of the signal is gradually stable and is less than the empirical low value VnEmpirically, t is usually runnAfter time, the signal variance is less than Vn. If the variance value of the system signal exceeds the expected time and cannot be stably lower than VnThen the system is abnormal, wherein tnAnd VnThe value of (a) is derived from empirical values measured a number of times during normal operation of the system.
Step 2: and carrying out phase demodulation on the original data, and carrying out Fourier transform on a phase demodulation result to obtain frequency domain information. Also, the low frequency part changes drastically when the sensing system is just turned on. After a period of time, the obtained frequency domain information tends to be stable and is lower than the empirical value fnAt this time, the sensing system is considered to be in a normal operating state.
And step 3: the Rayleigh scattering signal obtained by using the sensing system is gradually stabilized and smaller than an empirical value V if the signal variance value and the frequency domain information are simultaneously satisfied within a preset timenAnd fnIf not, judging that the coherent detection type phi-OTDR system works normally, otherwise, judging that the system is abnormal.
As a further preferable scheme of the self-checking method of the coherent detection type Φ -OTDR system according to the present invention, in the seventh step, the method for improving the signal-to-noise ratio of the system specifically includes:
step 1: respectively adjusting the sizes of an Optical Variable Attenuator (OVA) and a gain-adjustable Low Noise Amplifier (LNA) to obtain a groupSignal to noise ratio results of the system signal. The calculation method for defining the signal-to-noise ratio comprises the following steps:
PSis the average power, P, of the Rayleigh scattered signal of the systemNIs the average power of the background noise.
Step 2: and setting a plurality of groups of parameter combinations of the variable optical attenuator and the variable gain low noise amplifier to obtain the parameter combination when the signal-to-noise ratio of the signal is highest, and optimizing the system performance at the moment.
Has the advantages that: compared with the prior art, the technical scheme of the invention has the following beneficial technical effects:
(1) the working state of the optical fiber sensing system can be judged quickly and effectively;
(2) the parameters of each device can be reasonably adjusted under the condition that the system normally operates, the overall performance of the system is improved, and a higher signal-to-noise ratio is provided;
(3) the invention is intelligent and convenient, can be used for automatically calculating and judging the performance of the optical fiber sensing system and optimizing parameters, is more convenient and effective than the conventional method of debugging the system parameters by experience, and has practical value.
Drawings
FIG. 1 is a system block diagram of the present invention;
FIG. 2 is a flow chart of a method of the present invention;
FIG. 3 is a block diagram of a sensing fiber optic device cartridge designed according to the present invention;
FIG. 4 is a graph of the trend of the variance of the original signal over time;
FIG. 5 is a plot of the power of frequency components of a signal at a time;
fig. 6 is a raw signal acquired.
Detailed Description
The technical scheme of the invention is further explained in detail by combining the attached drawings:
FIG. 1 shows a system structure diagram of the present invention, which includes a laser, a first coupler, an adjustable optical attenuator, an acousto-optic modulator, an erbium-doped fiber amplifier, a fiber circulator, a second coupler, a balanced detector, an adjustable gain low noise amplifier, a signal acquisition card, a sensing fiber device box, and a pulse generator; wherein:
using device performance: the laser is a narrow linewidth laser, and the linewidth and the central wavelength of the laser are respectively 3.7kHz and 1550.12 nm; the model of the acousto-optic modulator is Gooch & Housego, and the frequency shift of the AOM is 200 MHz; the EDFA adopts an Amonics amplifier, the center frequency is 1550nm, and the constant power gain can reach 23 dBm.
The specific steps for combining device parameters are as follows:
step one, opening the coherent detection type phi-OTDR system, connecting a sensing optical fiber device box, and obtaining original data of Rayleigh back scattering optical signals of optical fibers for a period of time. FIG. 3 is a structural diagram of a sensing optical fiber device box, in which a single-mode optical fiber with a length of 200 meters is packaged, and which can be used for shock insulation and heat insulation;
step two, analyzing and processing the acquired original data, and making the accumulated value of the original signal intensity per minute as variance to obtain the change condition of the system signal variance V value along with the running time t of the sensing system, wherein the variance value is stable and reduced to V within 5 minutes during normal worknAs shown in fig. 4;
and step three, carrying out phase demodulation on the data, and then carrying out Fourier transform to obtain frequency domain information so as to obtain a frequency component distribution diagram. As shown in FIG. 5, the power of the frequency component below 2Hz gradually decreases after a period of time, and the power of each frequency component is stable and less than an empirical value fn。
Step four, if the variance value V and the signal frequency f of the intensity gradually tend to stable values and are smaller than a threshold value, the sensing system can be considered to be in a normal working state, otherwise, the system is judged to be in a fault state;
and step five, analyzing whether more data acquired by the data acquisition card exceeds the range of the acquisition card or not under the condition that the sensing system works normally. When a significant proportion (95%) of the collected data does not exceed the range, the parameter settings are considered appropriate. FIG. 6 shows that there is a strong local signal that exceeds the acquisition range of the data acquisition card (shown in the rectangular bar). At this time, the ratio of the excess data is counted, and if more than 5% of the data exceeds the measurement range, it is considered to be inappropriate.
And step six, improving the pumping current of the EDFA, improving the original signal intensity and improving the dynamic range of the sensing system. The pumping current is adjusted to a suitable parameter.
And step seven, simultaneously adjusting the sizes of the OVA and the LNA, calculating the signal-to-noise ratio of the system signals under various parameter combinations, and comparing to obtain the parameter combination with the highest signal-to-noise ratio of the system, so that the system performance is optimized.
The signal-to-noise ratio calculation method comprises the following steps:
PSis the average power, P, of the Rayleigh scattered signal of the systemNIs the average power of the background noise and fig. 6 designates the signal portion and the noise portion.
Table 1 is a table of the designed OVA and LNA parameter combinations. Two groups of values are selected in a proper parameter range respectively, and the signal-to-noise ratio of the system under the combination is calculated. And obtaining the parameter combination when the signal-to-noise ratio is maximum so as to optimize the system performance. Table 1 is a table of designed LNA and OVA parameter set combinations from which the parameter settings are modified. The SNR is the signal-to-noise ratio of the sensing system under the parameter combination.
TABLE 1
| LNA1 | LNA2 | LNA3 | LNA4 | |
| OVA1 | SNR11 | SNR12 | SNR13 | SNR14 |
| OVA2 | SNR21 | SNR11 | SNR23 | SNR24 |
| OVA3 | SNR31 | SNR32 | SNR33 | SNR34 |
| OVA4 | SNR41 | SNR42 | SNR43 | SNR44 |
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. Several simple deductions or substitutions should be considered as belonging to the protection scope of the present invention.
Claims (4)
1. A coherent detection type phi-OTDR system is characterized by comprising a laser, a first coupler, an adjustable optical attenuator, an acousto-optic modulator, an erbium-doped optical fiber amplifier, an optical fiber circulator, a second coupler, a balance detector, an adjustable gain low-noise amplifier, a signal acquisition card, a sensing optical fiber device box and a pulse generator, wherein:
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: the two paths of light are respectively input to an acousto-optic modulator and a variable optical attenuator;
the variable optical attenuator is used for properly attenuating 10% of continuous light output by the first coupler and then inputting the continuous light to the second coupler;
the acousto-optic modulator is used for modulating the continuous light output by the first coupler to generate pulse light and inputting the pulse light to the erbium-doped fiber amplifier;
the erbium-doped optical fiber amplifier is used for amplifying the input pulse light and outputting the amplified pulse light to the optical fiber circulator;
the fiber circulator is used for inputting amplified pulse light from a 1 st port of the fiber circulator and injecting the amplified pulse light into an accessed sensing fiber device box from a 2 nd port of the fiber circulator, and backward Rayleigh scattering light generated by the fiber is output to the second fiber coupler from a 3 rd port of the fiber circulator;
the second optical fiber coupler is used for mixing the Rayleigh back scattering light input from the 3 rd port of the optical fiber circulator with the local oscillator light input by the first coupler and outputting coherent light to the balance detector;
the balance detector is used for converting the frequency mixing optical signal into an electric signal and outputting the electric signal to the adjustable gain low noise amplifier;
the adjustable gain low-noise amplifier is used for amplifying and inputting the electric signal to the signal acquisition card;
the signal acquisition card is used for converting the electric signal into a digital signal for subsequent processing according to the trigger pulse;
and the synchronous pulse signal generator is used for outputting a synchronous pulse signal to the acousto-optic modulator and the data acquisition card, the acousto-optic modulator modulates continuous light emitted by the laser into pulsed light under the action of a pulse electrical signal, and the data acquisition card triggers an acquisition signal according to the synchronous pulse signal.
2. A self-test method implemented by a coherent probing-type Φ -OTDR system according to claim 1, characterized in that it comprises the following steps:
acquiring original signal intensity of Rayleigh back scattering optical signals of optical fibers in a sensing optical fiber device box for a period of time by a coherent detection type phi-OTDR system and a data acquisition card;
analyzing and processing the acquired original signals, and making variance of the accumulated value of the intensity of the original signals per minute to obtain the variation condition of the variance V value of the original data along with the system operation time t;
thirdly, phase demodulation is carried out on the original signal, and then Fourier transform is carried out to obtain frequency domain information so as to obtain a spatial frequency distribution map;
step four, if the variance value V of the intensity and the signal frequency domain value f are gradually stable in the preset time and are smaller than a preset threshold value, the phi-OTDR system is considered to be in a normal working state, otherwise, the system is judged to be in a fault state;
under the condition that the system works normally, analyzing whether the data acquired by the data acquisition card exceeds the range of the acquisition card in a certain proportion, judging whether the original strength is proper or not according to the analysis, and if so, performing the step six;
regulating the pumping current of the erbium-doped fiber amplifier to enhance the intensity of the original Rayleigh scattered light signal and improve the dynamic range of the system;
and step seven, adjusting the sizes of the variable optical attenuator and the variable gain low noise amplifier simultaneously, setting a plurality of groups of parameters, calculating the signal-to-noise ratio of the signals obtained under various parameter combinations, and comparing to obtain the parameter combination with the highest signal-to-noise ratio so as to optimize the performance of the sensing system.
3. A self-test method of a coherent detection type Φ -OTDR system according to claim 2, characterized in that: in the fourth step, the concrete steps for judging whether the system works normally are as follows
Step 1: continuously collecting data for a period of time by the system, calculating to obtain the variance V of the signal intensity in each minute, and if the system works normally, the variance of the signal is at the preset tnGradually stabilizes and is less than a threshold value V in timen；
Step 2: carrying out phase demodulation on the original data, carrying out Fourier transform on the phase demodulation result to obtain frequency domain information, and if the frequency domain information is at preset tmTends to stabilize in time and falls below a threshold fn；
And step 3: if the Rayleigh scattering signal obtained by using the sensing system meets the condition that both the signal variance value and the frequency domain result are gradually stable within the preset time and are smaller than the threshold value VnAnd fnIf not, judging that the coherent detection type phi-OTDR system works normally, otherwise, judging that the system is abnormal.
4. A self-test method of a coherent detection-type Φ -OTDR system according to claim 2 or 3, characterized in that: the method for calculating the signal-to-noise ratio in the seventh step comprises the following steps:
PSis the average power, P, of the Rayleigh scattered signal of the systemNIs the average power of the background noise.
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| CN112051031A (en) * | 2020-08-25 | 2020-12-08 | 中电科仪器仪表有限公司 | Testing device and testing method for loss distribution of optical fiber with ultra-large dynamic range |
| CN113406656A (en) * | 2021-06-18 | 2021-09-17 | 中国航空工业集团公司西安飞行自动控制研究所 | Device and method for improving signal-to-noise ratio of laser Doppler coherent velocity measurement system |
| WO2023019720A1 (en) * | 2021-08-19 | 2023-02-23 | 深圳供电局有限公司 | Distributed detection method and detection system for partial discharge of power cable |
| CN116015462A (en) * | 2023-02-27 | 2023-04-25 | 中国科学院国家授时中心 | EDFA optical gain setting method applied to optical fiber time transmission |
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