CN115371716A - Distributed optical fiber sensor multi-signal detection method - Google Patents
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- G01—MEASURING; TESTING
- 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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- G—PHYSICS
- G01—MEASURING; TESTING
- 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
- G01D5/35364—Sensor working in reflection using backscattering to detect the measured quantity using inelastic backscattering to detect the measured quantity, e.g. using Brillouin or Raman backscattering
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Abstract
The invention discloses a distributed optical fiber sensor multi-signal detection method, which comprises the following specific steps: step 1): arranging a distributed optical fiber sensor, wherein laser generated by a laser generates excitation light and local oscillator light after passing through an optical coupler; modulating the excitation light according to the detection requirement to obtain sensing excitation light; step 2): the sensing excitation light is transmitted into the sensing optical fiber, and sensing detection light is obtained; step 3): the local oscillation light is subjected to frequency shift to obtain frequency shift local oscillation light, the frequency shift local oscillation light and the sensing detection light have a coherent action, and a Brillouin scattering signal and a Rayleigh scattering signal are obtained through separation of a frequency spectrum filter; step 4): and analyzing the Brillouin scattering signal and the Rayleigh scattering signal to obtain a detection result. According to the invention, the optical fiber sensor can simultaneously acquire the Brillouin scattering signal and the Rayleigh scattering signal, and respectively detect temperature, strain, vibration and the like through the signals, so that the function of simultaneously detecting various signals by one distributed optical fiber sensor is realized.
Description
Technical Field
The invention relates to the technical field of optical fiber sensor detection, in particular to a distributed optical fiber sensor multi-signal detection method.
Background
The distributed optical fiber sensor is a sensor which measures or monitors information which is distributed along the space on an optical fiber transmission path and changes along time by adopting a unique distributed optical fiber detection technology, can be used for detecting vibration, strain, temperature change and the like, and is widely used for monitoring large engineering buildings such as tunnels, bridges, dams and the like.
When the distributed optical fiber sensor is used, the distributed optical fiber sensor is usually arranged along the length direction of a measured object (such as a tunnel, a bridge, etc.), sensing excitation light is transmitted into the distributed optical fiber sensor, sensing detection light is returned to a sensing system by the distributed optical fiber sensor, and corresponding detection quantity (such as vibration, temperature, etc. at a certain position of the tunnel) is calculated by analyzing parameters such as frequency, optical power, etc. of the returned sensing detection light.
When the existing distributed optical fiber sensor is used, one distributed optical fiber sensor can only detect one signal to obtain a detection result, that is, when a plurality of physical quantities need to be detected simultaneously, for example, when the vibration and the temperature of a tunnel need to be detected simultaneously, a plurality of distributed optical fiber sensors need to be arranged, different signals are detected respectively through the plurality of distributed optical fiber sensors, so that the detection requirement of multiple signals can be met, when the mode is used for detecting multiple signals, a plurality of distributed detection sensors need to be installed, and the installation cost of the distributed optical fiber sensors is high.
Disclosure of Invention
The invention aims to solve the defects in the prior art, and provides a distributed optical fiber sensor multi-signal detection method, which can simultaneously receive, separate and detect a plurality of different signals through one distributed optical fiber sensor, thereby reducing the installation cost of the distributed optical fiber sensor.
The purpose of the invention is realized by the following technical scheme: a distributed optical fiber sensor multi-signal detection method comprises the following specific steps:
step 1): arranging a distributed optical fiber sensor, wherein laser generated by a laser generates excitation light and local oscillator light after passing through an optical coupler; modulating the excitation light according to the detection requirement to obtain sensing excitation light;
step 2): the sensing excitation light is transmitted into the sensing optical fiber, and the sensing excitation light generates Brillouin scattering and Rayleigh scattering at each position of the sensing optical fiber to form sensing detection light formed by aliasing of the Brillouin scattering light and the Rayleigh scattering light;
step 3): frequency-shifting the local oscillation light to obtain frequency-shifted local oscillation light, wherein the frequency-shifted local oscillation light has a coherent action with Brillouin scattering light and Rayleigh scattering light in the sensing detection light, so that the frequency difference between the Brillouin scattering light and the Rayleigh scattering light is improved, and a Brillouin scattering signal and a Rayleigh scattering signal are obtained through separation of a frequency spectrum filter;
and step 4): and analyzing the separated Brillouin scattering signal and Rayleigh scattering signal to obtain a detection result.
Preferably, in step 1), when the excitation light is modulated, the pulse width, the pulse and the repetition frequency of the excitation light are modulated by an acousto-optic modulator, then the optical power is amplified by an erbium-doped fiber amplifier, and then the filtering is performed by a fiber bragg grating, so as to obtain the sensing excitation light.
Preferably, the laser is a distributed feedback semiconductor laser.
Preferably, in step 3), the optical coupler distributes the shifted local oscillator light into two equal-power distributed local oscillator light, and the optical coupler distributes the sensing detection light into two equal-power distributed detection light, wherein one of the distributed local oscillator light and one of the distributed detection light are input into the first photoelectric detector and generate a coherent action, the other distributed local oscillator light and the other distributed detection light are input into the second photoelectric detector and generate a coherent action, the first photoelectric detector and the second photoelectric detector respectively generate coherent optical signals including brillouin scattering signals and rayleigh scattering signals, and the spectral filters respectively separate the coherent optical signals generated by the first photoelectric detector and the second photoelectric detector to obtain brillouin scattering signals and rayleigh scattering signals.
Preferably, the frequency change amount of the frequency-shifted local oscillator light relative to the local oscillator light is close to brillouin frequency shift.
Preferably, the brillouin scattering signal obtained after passing through the spectrum filter is multiplied and mixed with the radio frequency signal with the same frequency, and a low-frequency baseband signal is obtained through the filter; high-speed digital-analog conversion is carried out on the baseband signals, and a digital sampling accumulation averager is adopted for superposition processing to improve the signal-to-noise ratio.
Preferably, in the step 4), the measurement temperature is calculated according to the frequency shift of the brillouin signal and a temperature calculation formula; calculating according to the frequency shift of the Brillouin scattering signal and a strain calculation formula to obtain measurement strain; and obtaining the vibration of the measured object according to the Rayleigh scattering signal.
Preferably, the temperature calculation formula is:
wherein,Tin order to measure the temperature of the liquid,T r for the purpose of the reference temperature, the temperature,C T is a proportional coefficient of temperature, and is,f B (T) In order to measure the temperature of the liquid,f B (T) To measure the brillouin frequency shift at temperature,f B (T=T r ) Is the Brillouin frequency shift in a reference temperature state;
the strain calculation formula is as follows:
wherein,Ein order to measure the strain,f B (E) To measure the brillouin shift in frequency under strain,f B (E= 0) is a brillouin frequency shift when the strain is zero,C E is the strain proportionality coefficient.
The invention has the beneficial effects that: according to the invention, the Brillouin scattering signal and the Rayleigh scattering signal can be simultaneously obtained through one distributed optical fiber sensor, the separation of the signals in a frequency domain can be realized, and a plurality of physical quantities such as temperature, strain, vibration and the like are respectively detected through the signals, so that the function of simultaneously detecting a plurality of signals through one distributed optical fiber sensor is realized, and the distribution quantity and the installation cost of the distributed optical fiber sensors are reduced.
Drawings
FIG. 1 is a schematic view of the working process of the present invention.
Fig. 2 is a schematic diagram of an excitation light signal processing module.
Fig. 3 is a schematic diagram of sensing light generation.
Fig. 4 is a schematic diagram of the local oscillator optical processing module and the spectrum separation module.
Fig. 5 is a schematic diagram of optical coherent heterodyne detection to achieve spectrum separation.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived from the embodiments of the present invention by a person skilled in the art, are within the scope of the present invention.
It will be understood by those skilled in the art that in the present disclosure, the terms "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," and the like are used in an orientation or positional relationship indicated in the drawings for ease of description and simplicity of description, and do not indicate or imply that the referenced devices or components must be constructed and operated in a particular orientation and thus are not to be considered limiting.
It is understood that the terms "a" and "an" should be interpreted as meaning that a number of one element or element is one in one embodiment, while a number of other elements is one in another embodiment, and the terms "a" and "an" should not be interpreted as limiting the number.
As shown in fig. 1 to fig. 5, a distributed optical fiber sensor multi-signal detection method is characterized by comprising the following specific steps:
step 1): arranging a distributed optical fiber sensor, wherein laser generated by a laser generates excitation light and local oscillator light after passing through an optical coupler; and modulating the excitation light according to the detection requirement to obtain the sensing excitation light.
In this step, a distributed feedback semiconductor laser (DFB-LD) with a linewidth of less than 1MHz is used as a light source, and the laser frequency generated by the laserf P 193THz, laser light passes through the optical couplerThe frequency of the local oscillator light and the frequency of the excitation light are distributedf P 。
As shown in fig. 1, the excitation light enters an excitation light signal processing module and modulates the excitation light, the excitation light signal processing module includes an acousto-optic modulator, an erbium-doped fiber amplifier, a fiber circulator a, and a fiber bragg grating, an excitation light pulse with a pulse width of 50ns and a repetition frequency of 2kHz is formed by the acousto-optic modulator (AOM), and the pulse width is greater than 2 times of the transmission time of the light in the fiber, so that only the excitation light pulse signal with a single frequency is transmitted in the sensing fiber.
And an acousto-optic modulator (AOM) simultaneously performs pulse modulation on the excitation light pulses, and each light pulse signal corresponds to each electric pulse signal in the Brillouin sensing detection signal processing module and the phase sensitive sensing detection signal processing module one by one, so that light pulse frequency sweeping is realized.
The modulated excitation light pulse is transmitted into an erbium-doped fiber amplifier (EDFA) to amplify the light power and then filtered by a Fiber Bragg Grating (FBG); the fiber bragg grating can reflect optical signals in a specific wavelength range and transmit signals of other wavelengths, so that the function of an optical filter is realized.
In the excitation light signal processing module, a port 1 on an optical fiber circulator A is connected with an erbium-doped optical fiber amplifier in series, a port 3 on the optical fiber circulator A is connected with an optical Fiber Bragg Grating (FBG) in series, and a port 2 on the optical fiber circulator A outputs processed sensing excitation light.
The excitation light amplified by the erbium-doped fiber amplifier is input from a port 1 of the circulator and reaches a Fiber Bragg Grating (FBG) on a port 3; the Fiber Bragg Grating (FBG) reflects the optical signal with the frequency near fP and transmits the light with the rest frequency along the other direction, thereby realizing the filtration of the spontaneous radiation noise of the amplifier generated by the erbium-doped fiber amplifier; the reflected light of the Fiber Bragg Grating (FBG) reaches the port 2 from the port 3 of the circulator and outputs the modulated sensing excitation light outwards.
Step 2): the sensing excitation light is transmitted into the sensing optical fiber, and the sensing excitation light generates brillouin scattering and rayleigh scattering at each position of the sensing optical fiber to form sensing detection light formed by aliasing of the brillouin scattering light and the rayleigh scattering light.
As shown in fig. 2, in this step, the sensing excitation light is transmitted to the sensing optical fiber on the distributed optical fiber sensor through the optical splitting module. The optical branching module is an optical fiber circulator B, sensing excitation light obtained after modulation processing in step 1) is input from a port 1 on the optical fiber circulator B, a port 2 on the optical fiber circulator B is connected with a sensing optical fiber in series, and a port 3 on the optical fiber circulator B is connected with the spectrum separation module.
The sensing excitation light is input from a port 1 on the optical fiber circulator B, then reaches a port 2 and further enters the sensing optical fiber, the sensing excitation light generates Brillouin scattering and Rayleigh scattering at each position in the sensing optical fiber to form sensing detection light formed by aliasing of the Brillouin scattering light and the Rayleigh scattering light, and the sensing detection light returns to the port 2 and then reaches a spectrum separation module on a port 3.
The brillouin scattered light includes temperature/strain information, and the rayleigh scattered light includes vibration information. The brillouin scattering process in the optical fiber can be described as the nonlinear interaction of the excitation light wave and the scattering light wave through the sound wave. The frequency shift of the Brillouin scattering light is related to a scattering angle, the scattering angle is determined by temperature and strain, and only two transmission directions, namely a front transmission direction and a rear transmission direction, exist in a single-mode optical fiber; normally, the brillouin scattering is only back scattering, with its brillouin frequency shiftf B Comprises the following steps:
in the formula,v A is the speed of sound of the acoustic wave in the sensing fiber,nin order to sense the refractive index of the optical fiber,λis the wavelength of the incident light; if getv A =5.96km/s, for silica fibern=1.45, when wavelength of excitation lightλBrillouin frequency shift in the neighborhood of =1.55 μmf B The value is approximately equal to 11GHz; since the frequency of the exciting light isf P Brillouin frequency shift off B Self-emitting fiber in optical fiberThe Brillouin backscattering light generated by the Brillouin scattering has an optical frequency off P －f B 。
Rayleigh scattering is elastic scattering generated by medium refractive index fluctuation caused by medium density fluctuation in the optical fiber; since it is elastic scattering, the frequency of Rayleigh scattering light is the same as that of the excitation light and bothf P 。
Rayleigh scattering occurs at each position of the sensing fiber, and when a certain point position of the sensing fiber vibrates, the rayleigh scattering light is subjected to phase modulation, so that the amplitude of the rayleigh scattering signal light changes significantly, and therefore the rayleigh backscattered light includes vibration information of each position of the sensing fiber.
The method adopts a single light source and a single optical fiber to simultaneously detect two kinds of scattered light at each position of the photosensitive fiber, and the frequency of the Rayleigh scattered light isf P The frequency of the Brillouin scattered light isf P －f B Therefore, the difference between the two scattering frequencies is Brillouin frequency shiftf B About 11Ghz, and both scattered lights are high frequency lights of several hundred terahertz, so the frequency difference between the two scattered lights is much smaller than the self frequency of the two scattered lights.
Step 3): the method comprises the steps of obtaining frequency-shifted local oscillator light after frequency shifting of the local oscillator light, enabling the frequency-shifted local oscillator light to be in a coherent action with Brillouin scattering light and Rayleigh scattering light in sensing detection light, increasing frequency difference of the Brillouin scattering light and the Rayleigh scattering light, and separating through a spectrum filter to obtain Brillouin scattering signals and Rayleigh scattering signals.
The step is realized by a frequency spectrum separation module and a local oscillator optical signal processing module. The local oscillator optical signal processing module is a microwave electro-optical modulator (EOM), the frequency spectrum separation module comprises an optical coupler, a balance detector and a frequency spectrum separator, the balance detector comprises a first photoelectric detector (PD 1), a second photoelectric detector (PD 1) and a capacitor, and the capacitor is a blocking capacitor.
Inputting the local oscillation light into a microwave electro-optical modulator (EOM), and performing frequency shift processing on the local oscillation light by the microwave electro-optical modulator (EOM)So that the frequency of the local oscillator light is controllably shifted to obtain frequency-shifted local oscillator light, wherein the frequency-shifted local oscillator light has a frequency change amount relative to the original local oscillator lightf m， So that the frequency of the frequency-shifted local oscillator light isf P －f m (ii) a Wherein, the frequency shift local oscillator light changes relative to the frequency of the original local oscillator lightf m Close to brillouin frequency shiftf B (ii) a In the present invention,f m is set to be different from 11GHz by tens MHz to hundreds MHz, so that the frequency of the coherent Brillouin scattering signal isf m －f B A lower frequency value; then the frequency shift local oscillator light is transmitted into an optical coupler, the optical coupler distributes the frequency shift local oscillator light into two parts of distributed local oscillator light with equal power, and the optical power of the distributed local oscillator light is 50 percent of the optical power of the original frequency shift local oscillator light; meanwhile, the sensing detection light is also transmitted into the optical coupler, and the sensing detection light is distributed into two equal-power distribution detection lights through the optical coupler, wherein the optical power of the distribution detection light is 50 percent of the optical power of the original sensing detection light. One part of the frequency shift local oscillator light and one part of the sensing detection light are input into a first photoelectric detector and have a coherent action, the other part of the distribution local oscillator light and the other part of the distribution detection light are input into a second photoelectric detector and have a coherent action, and the first photoelectric detector and the second photoelectric detector respectively generate coherent light signals containing Brillouin scattering signals and Rayleigh scattering signals.
The Brillouin scattering light in the distributed detection light and the distributed local oscillator light generate a coherent action to obtain a coherent Brillouin scattering signal, the Rayleigh scattering light in the distributed detection light and the distributed local oscillator light generate a coherent action to obtain a coherent Rayleigh scattering signal, and the frequency of the coherent Brillouin scattering signal isf m －f B The frequency of the coherent Rayleigh scattering signal isf m . Wherein, the frequency change amount of the frequency shift local oscillator light relative to the local oscillator light is close to Brillouin frequency shiftf B . Using this embodiment as an example, brillouin frequency shiftf B About 11GHz and a laser generating frequency off P About 193THz, the frequency of Rayleigh scattered light before coherence occurs isf P The frequency of the Brillouin scattering light isf P －f B Since the frequencies of the rayleigh scattered light and the brillouin scattered light are both close to 193THz, the difference in frequency between the two is largef B The rayleigh scattering signal and the brillouin scattering signal are both high-frequency signals, which are much lower than their own frequencies, and thus are difficult to separate in the frequency domain.
After the coherent processing, the frequency of the Brillouin scattering signal isf m －f B The frequency of the Rayleigh scattering signal isf m Although the frequency difference between the Brillouin scattering signal and the Rayleigh scattering signal is unchanged after the coherent processing, the Brillouin scattering signal still has Brillouin frequency shiftf B However, the frequencies of the brillouin scattering signal and the rayleigh scattering signal become respectivelyf m －f B Andf m the Rayleigh scattering signal and the Brillouin scattering signal can be separated in the frequency domain by using the band-pass filter.
Coherent light signals respectively generated by the first photoelectric detector and the second photoelectric detector reach the spectrum filter after passing through the blocking capacitors, and brillouin scattering signals (BOTDR signals) and rayleigh scattering signals (phi-OTDR signals) are respectively obtained by separating the coherent light signals generated by the first photoelectric detector and the second photoelectric detector through the spectrum filter.
Step 4): and analyzing the separated Brillouin scattering signal and Rayleigh scattering signal to obtain a detection result.
In this step, after the brillouin scattering signal and the rayleigh scattering signal obtained in step 3) are separated, the brillouin scattering signal is introduced into a brillouin sensing detection signal processing module for detection, and the rayleigh scattering signal is introduced into a phase sensitive sensing detection signal processing module for detection.
The Brillouin scattering signal obtained through the frequency spectrum separation in the step 3) is a band-pass amplitude modulation signal and contains abundant noise, and before the Brillouin scattering signal enters the Brillouin sensing detection signal processing module for detection, the Brillouin scattering signal obtained after passing through the frequency spectrum filter and the radio frequency signal with the same frequency are subjected to multiplication mixing and then are filtered to obtain a low-frequency baseband signal; high-speed digital-analog conversion is carried out on the baseband signals, and a digital sampling accumulation averager is adopted for superposition processing to improve the signal-to-noise ratio.
The temperature was measured as follows: and calculating to obtain the measured temperature according to the frequency shift of the Brillouin scattering signal and a temperature calculation formula, wherein the temperature calculation formula is as follows:
wherein,Tin order to measure the temperature of the liquid,T r for the purpose of the reference temperature, the temperature,C T is a temperature proportionality coefficient, is a constant,C T =9.6×10 -5 /K，f B (T) In order to measure the temperature of the liquid,f B (T) To measure the brillouin frequency shift at temperature,f B (T=T r ) Is the brillouin shift at the reference temperature. In the actual detection, a reference temperature can be set in advanceT r， And measuring the Brillouin frequency shift at the reference temperaturef B (T=T r ) And will obtainT r Andf B (T=T r ) Substituting the constant value into the above formula, only Brillouin frequency shift is measured when measuring temperaturef B (T) So that the measured temperature can be calculated
。
The strain measurement method is as follows: and calculating to obtain the measured strain according to the frequency shift of the Brillouin scattering signal and a strain calculation formula, wherein the strain calculation formula is as follows:
wherein,Ein order to measure the strain,f B (E) To measure the brillouin shift in frequency under strain,f B (E= 0) brillouin shift when strain is zero,C E is a strain proportionality coefficient, is a constant,C E =4.61/LE. In actual detection, the Brillouin frequency shift when the strain is zero can be measured in advancef B (E= 0), and measuredf B (E= 0) as a constant, so that only the brillouin frequency shift needs to be measured when measuring strainf B (E) So that the measurement strain can be calculatedE。
The vibration borne by the measured object is obtained through Rayleigh scattering signals, and the principle is as follows:
because the sensing excitation light is pulsed light, when the position Z vibrates to cause the amplitude of Rayleigh backward scattering light signals to change remarkably, the time interval t from the emitting of the sensing excitation light to the receiving of the Rayleigh scattering light signals generated at the position Z is detected, and the propagation speed of the light in the optical fiber is combinedcThe distance traveled by the light transmission process can be obtainedL z And then determining the position Z; the formula is as follows:
L z =ct；
due to the loss in the sensing optical fiber, the power of the optical wave is attenuated continuously when the optical wave is transmitted in the optical fiber, and the optical power of the received Rayleigh scattered light signalP R Comprises the following steps:
in the formula:P 0 in order to sense the peak power of the excitation light,eis a natural constant and is a natural constant,awhich is the attenuation coefficient of the optical fiber,
for the back-scattered light power capture factor,wis a gaussian-shaped effective field mode area,α s is a coefficient of the rayleigh scattering,Wthe line width of the sensing excitation light is.
According to the formula, the optical power curve of the Rayleigh backscattering signals along the optical fiber is an exponentially decaying Lorentz-shaped curve, and the curve shows the loss condition of the optical fiber along the line; the position and power information of the vibration of the sensing optical fiber can be obtained by analyzing the curve through a computer.
According to the invention, the Brillouin scattering signal and the Rayleigh scattering signal can be simultaneously obtained by one distributed optical fiber sensor, the separation of the signals in a frequency domain can be realized, and a plurality of physical quantities such as temperature, strain, vibration and the like can be respectively detected by the signals, so that the function of simultaneously detecting a plurality of signals by one distributed optical fiber sensor is realized, and the arrangement number and the installation cost of the distributed optical fiber sensors are reduced.
The present invention is not limited to the above-mentioned preferred embodiments, and any other products in various forms can be obtained by anyone in the light of the present invention, but any changes in the shape or structure thereof, which have the same or similar technical solutions as those of the present application, fall within the protection scope of the present invention.
Claims (8)
1. A distributed optical fiber sensor multi-signal detection method is characterized by comprising the following specific steps:
step 1): arranging a distributed optical fiber sensor, wherein laser generated by a laser generates excitation light and local oscillator light after passing through an optical coupler; modulating the excitation light according to the detection requirement to obtain sensing excitation light;
step 2): the sensing excitation light is transmitted into the sensing optical fiber, and the sensing excitation light generates Brillouin scattering and Rayleigh scattering at each position of the sensing optical fiber to form sensing detection light formed by aliasing of the Brillouin scattering light and the Rayleigh scattering light;
step 3): frequency shifting is carried out on the local oscillation light to obtain frequency-shifted local oscillation light, the frequency-shifted local oscillation light and Brillouin scattering light and Rayleigh scattering light in the sensing detection light are subjected to coherent action, the frequency difference between the Brillouin scattering light and the Rayleigh scattering light is improved, and a Brillouin scattering signal and a Rayleigh scattering signal are obtained through frequency spectrum filter separation;
and step 4): and analyzing the separated Brillouin scattering signal and Rayleigh scattering signal to obtain a detection result.
2. The method for detecting multiple signals of a distributed optical fiber sensor according to claim 1, wherein in step 1), when the excitation light is modulated, the pulse width, the pulse and the repetition frequency of the excitation light are modulated by an acousto-optic modulator, then the optical power is amplified by an erbium-doped fiber amplifier, and then the sensing excitation light is obtained by filtering through a fiber bragg grating.
3. The distributed fiber optic sensor multi-signal detection method according to claim 1, wherein the laser is a distributed feedback semiconductor laser.
4. The method according to claim 1, wherein in step 3), the optical coupler distributes the frequency-shifted local oscillator light into two equal-power distributed local oscillator lights, the optical coupler distributes the sensing detection light into two equal-power distributed detection lights, one of the distributed local oscillator lights and one of the distributed detection lights are input into the first photodetector and generate a coherent action, the other of the distributed local oscillator lights and the other of the distributed detection lights are input into the second photodetector and generate a coherent action, the first photodetector and the second photodetector respectively generate coherent optical signals including brillouin scattering signals and rayleigh scattering signals, and the spectral filters respectively separate the coherent optical signals generated by the first photodetector and the second photodetector to obtain brillouin scattering signals and rayleigh scattering signals.
5. The method for detecting the multiple signals of the distributed optical fiber sensor according to claim 4, wherein the frequency change amount of the frequency-shifted local oscillator light relative to the local oscillator light is close to Brillouin frequency shift.
6. The method for detecting multiple signals of a distributed optical fiber sensor according to claim 1, wherein the brillouin scattering signal obtained after passing through the spectrum filter is multiplied and mixed with the radio frequency signal with the same frequency, and a low-frequency baseband signal is obtained through the filter; high-speed digital-analog conversion is carried out on the baseband signals, and a digital sampling accumulation averager is adopted for superposition processing to improve the signal-to-noise ratio.
7. The multi-signal detection method of the distributed optical fiber sensor according to claim 1, wherein in step 4), the measured temperature is calculated according to the frequency shift of the brillouin scattering signal and a temperature calculation formula; calculating according to the frequency shift of the Brillouin scattering signal and a strain calculation formula to obtain measurement strain; and obtaining the vibration of the measured object according to the Rayleigh scattering signal.
8. The distributed optical fiber sensor multi-signal detection method according to claim 7, wherein the temperature calculation formula is:
wherein,Tin order to measure the temperature of the liquid,T r for the purpose of the reference temperature, the temperature,C T in order to be the temperature proportionality coefficient,f B (T) In order to measure the temperature of the liquid,f B (T) To measure the brillouin frequency shift at temperature,f B (T=T r ) Is the Brillouin frequency shift in a reference temperature state;
the strain calculation formula is as follows:
wherein,Ein order to measure the strain,f B (E) To measure the brillouin shift in frequency under strain,f B (E= 0) brillouin shift when strain is zero,C E is the strain proportionality coefficient.
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