CN111912439A - Linear frequency modulation distributed optical fiber sensing device and method - Google Patents
Linear frequency modulation distributed optical fiber sensing device and method Download PDFInfo
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Abstract
The invention belongs to the field of optical fiber sensing devices, and provides a distributed optical fiber sensing device and a distributed optical fiber sensing method for linear frequency modulation. Wherein the distributed optical fiber sensing device comprises: the laser is used for emitting linear frequency-modulated continuous light in the oblique wave signal effective interval and emitting continuous light with central frequency in the oblique wave signal ineffective interval; the conductor optical amplifier is used for converting the continuous light into pulse light under the driving of the pulse wave and outputting linear frequency modulation continuous light only when the pulse wave is at a high level; the periods and the repetition frequencies of the oblique wave and the pulse wave are consistent; the first port of the circulator receives the pulse light, the pulse light emitted from the second port enters the sensing optical fiber, and Rayleigh scattering echoes of the sensing optical fiber carrying environmental physical quantity are emitted from the second port and the third port in sequence, enter the avalanche photodiode and are converted into voltage signals; and the processor is used for obtaining the physical quantity measured value along the sensing optical fiber according to the linear relation between the deviation of the Rayleigh scattering echo waveform along the time axis and the optical fiber environment physical quantity.
Description
Technical Field
The invention belongs to the field of optical fiber sensing devices, and particularly relates to a distributed optical fiber sensing device and method for linear frequency modulation.
Background
The statements in this section merely provide background information related to the present disclosure and may not necessarily constitute prior art.
An optical fiber distributed temperature measurement system is a sensor system for measuring the spatial temperature field distribution in real time. The same optical fiber is used as a sensing and conducting medium of temperature information, the temperature field information of the optical fiber is measured by using the temperature effect of the backward Raman scattering spectrum of the optical fiber, and a measuring point is positioned by using the optical time domain reflection technology of the optical fiber. The system has the advantages of intrinsic safety, corrosion resistance, high voltage resistance, electromagnetic interference resistance, rapid multipoint measurement and positioning and the like, and has wide application field. The temperature monitoring device is used for temperature monitoring in the fields of petroleum engineering, power stations, mines, tunnels, dams and the like.
In the optical fiber distributed temperature measurement system, a laser outputs pulse light, the pulse light is injected from the initial end of an optical fiber, most energy of the pulse light is transmitted to the tail end of the optical fiber and disappears, and a small part of backscattered light waves are reflected back along the optical fiber. According to the temperature effect of Raman scattering spectra and the optical time domain reflection technology, the optical power returned to the incident end is a function of the position of the optical fiber and the ambient temperature. By utilizing the principle, the temperature of the whole optical fiber link can be measured, and meanwhile, the measuring point can be accurately positioned.
Because the raman scattering signal is very weak and is completely submerged in noise, the optical fiber distributed temperature measurement system needs to adopt a weak signal detection technology to extract a signal to be measured from the noise. Because the main component of the noise in the optical fiber distributed temperature measurement system has the statistical characteristic of zero mean value, the purpose of noise reduction can be achieved by utilizing the statistical characteristic of the noise. Therefore, in order to improve the signal-to-noise ratio, the signal processing part adopts a sampling accumulation averaging method to reduce noise, namely, N point data measured at one time is sequentially stored in a memory unit, the N point data measured at the next time is added with data of a corresponding unit of the memory, the original memory unit is put back, the cycle is repeated for M times, and then the average of each unit is calculated.
Distributed optical fiber sensing technology based on Raman scattering and Brillouin scattering generally needs to utilize multiple accumulation average suppression noise and frequency scanning (Brillouin scattering) to obtain frequency domain information, has long measuring time and is only suitable for measuring slowly-varying physical quantities such as temperature. Compared with Raman scattering and Brillouin scattering, the distributed optical fiber sensing technology based on Rayleigh scattering requires less accumulated average times and is more suitable for measuring dynamic physical quantities such as stress, vibration and the like. In particular phase sensitive optical time domain reflectometry
The technical Rayleigh scattering distributed optical fiber sensor can realize high-precision measurement of physical quantities such as temperature, stress, vibration and the like along the sensing optical fiber with the resolution up to several kilometers. However, there is no linear relationship between the rayleigh scattering intensity and the physical quantities such as the ambient temperature and the stress, and theoretically, the measured values of the physical quantities such as the temperature, the stress and the vibration cannot be obtained by detecting the rayleigh scattering intensity of the sensing fiber. The inventors have found, albeit in
In the technology, the measured value of the physical quantity can be obtained by detecting the Rayleigh scattering phase distribution of the sensing optical fiber, extra steps of digital orthogonal demodulation, phase unwrapping calculation and the like are needed, the complexity of the system is increased, correlation calculation of a plurality of pulse periods is needed, the measuring time is prolonged, and the measurable frequency range of the vibration signal is limited.
Disclosure of Invention
In order to solve the above problems, a first aspect of the present invention provides a chirped distributed optical fiber sensing apparatus, which uses chirped pulsed light based on a rayleigh scattering phase-sensitive optical time domain reflection method to obtain a physical quantity measurement value along a sensing optical fiber in a single acquisition, and can greatly shorten a measurement time.
In order to achieve the purpose, the invention adopts the following technical scheme:
a chirped distributed optical fiber sensing apparatus comprising:
the laser is used for emitting linear frequency-modulated continuous light in a ramp signal effective interval and emitting continuous light with a central frequency in a ramp signal ineffective interval;
the semiconductor optical amplifier is used for converting continuous light into pulse light under the driving of pulse waves, outputting linear frequency modulation continuous light only when the pulse waves are at a high level, and not outputting light waves when the pulse waves are at a low level; wherein, the periods and the repetition frequencies of the oblique wave and the pulse wave are consistent;
the first port of the circulator receives the pulse light, the pulse light emitted from the second port enters the sensing optical fiber, and the Rayleigh scattering echo of the sensing optical fiber carrying the environmental physical quantity sequentially exits from the second port and the third port and enters the avalanche photodiode to be converted into a voltage signal;
and the processor is used for obtaining the physical quantity measured value along the sensing optical fiber according to the linear relation between the deviation of the Rayleigh scattering echo waveform along the time axis and the optical fiber environment physical quantity.
In order to solve the above problems, a second aspect of the present invention provides a working method of a distributed fiber sensing apparatus with chirp, which is based on a rayleigh scattering phase-sensitive optical time domain reflection mode, and uses chirp pulsed light to obtain a physical quantity measurement value along a sensing fiber in a single acquisition, so as to greatly shorten the measurement time.
In order to achieve the purpose, the invention adopts the following technical scheme:
a working method of a distributed optical fiber sensing device with linear frequency modulation comprises the following steps:
the laser and the semiconductor optical amplifier respectively correspond to a ramp wave signal and a pulse wave signal with consistent receiving period and repetition frequency;
the laser emits linear frequency-modulated continuous light in the oblique wave signal effective interval, and emits central frequency continuous light in the oblique wave signal ineffective interval;
under the drive of pulse waves, the semiconductor optical amplifier converts continuous light output by the laser into pulse light, outputs linear frequency modulation continuous light only at the high level of the pulse waves, and does not output light waves at the low level of the pulse waves;
the first port of the circulator receives the pulse light, the pulse light emitted from the second port enters the sensing optical fiber, and Rayleigh scattering echoes of the sensing optical fiber carrying environmental physical quantity are sequentially emitted from the second port and the third port to enter the avalanche photodiode and are converted into voltage signals;
and the processor obtains the physical quantity measured value along the sensing optical fiber according to the linear relation between the deviation of the Rayleigh scattering echo waveform along the time axis and the optical fiber environment physical quantity.
The invention has the beneficial effects that:
the invention uses the linear frequency modulation pulse light to enter the sensing optical fiber, and converts the physical quantity changes of temperature, stress, vibration and the like along the optical fiber into the deviation of the waveform of the Rayleigh scattering echo signal along the time axis. And then, performing cross-correlation calculation by using Rayleigh scattering echoes of two adjacent pulse periods, and performing reverse estimation to obtain the measured values of physical quantities such as temperature, stress, vibration and the like along the sensing optical fiber according to the deviation of the echo waveform along a time axis.
Compared with the conventional
Compared with the sensor, the complexity and the cost of the device of the invention are not obviously increased, and the invention not only reserves
The inherent advantages of the technology: can obtain the physical quantity measured values of temperature, stress, vibration and the like along a dozen kilometers long sensing optical fiber, the spatial resolution is as high as several meters,the sensitivity is superior to that of the common sensing technology; and each pulse period can obtain the physical quantity measured value along the sensing optical fiber, thereby greatly shortening the measuring time.
Drawings
The accompanying drawings, which are incorporated in and constitute a part of this specification, are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the invention and together with the description serve to explain the invention and not to limit the invention.
Fig. 1 is a schematic structural diagram of a distributed fiber optic sensing device with linear frequency modulation according to an embodiment of the present invention;
FIG. 2(a) is a diagram of ramp pulse signal propagation according to an embodiment of the present invention;
FIG. 2(b) is a diagram of a square wave pulse signal propagation according to an embodiment of the present invention;
FIG. 2(c) is a diagram of the electric field waveform of the continuous light wave emitted by the laser according to the embodiment of the present invention;
FIG. 2(d) is a diagram of the electric field waveform of the pulsed light wave output by the semiconductor optical amplifier according to the embodiment of the present invention;
FIG. 3 is a schematic diagram of a pulsed light propagating along a sensing fiber according to an embodiment of the present invention;
fig. 4 is a schematic view of another pulsed light propagating along a sensing fiber according to an embodiment of the present invention.
Detailed Description
The invention is further described with reference to the following figures and examples.
It is to be understood that the following detailed description is exemplary and is intended to provide further explanation of the invention as claimed. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of exemplary embodiments according to the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof, unless the context clearly indicates otherwise.
In the present invention, terms such as "upper", "lower", "left", "right", "front", "rear", "vertical", "horizontal", "side", "bottom", and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings, and are only terms of relationships determined for convenience of describing structural relationships of the parts or elements of the present invention, and are not intended to refer to any parts or elements of the present invention, and are not to be construed as limiting the present invention.
In the present invention, terms such as "fixedly connected", "connected", and the like are to be understood in a broad sense, and mean either a fixed connection or an integrally connected or detachable connection; may be directly connected or indirectly connected through an intermediate. The specific meanings of the above terms in the present invention can be determined according to specific situations by persons skilled in the relevant scientific or technical field, and are not to be construed as limiting the present invention.
The present embodiment is directed to distributed fiber optic sensing technology, particularly based on Rayleigh scattering
The technology uses linear frequency modulation pulse light to obtain the distributed measurement results of physical quantities such as temperature, stress, vibration and the like of the sensing optical fiber in a single acquisition. Compared with the conventional
Compared with the sensor, the complexity and the cost of the device of the invention are not obviously increased, and the invention not only reserves
The inherent advantages of the technology: physical quantity measurement values of temperature, stress, vibration and the like along a dozen kilometers long sensing optical fiber can be obtained, the spatial resolution is as high as several meters, and the sensitivity is superior to that of the common sensing technology; and each pulse period can obtain the physical quantity measured value along the sensing optical fiber, thereby greatly shortening the measuring time.
Referring to fig. 1, the structure of the distributed fiber sensing device with chirp in this embodiment includes a laser, an optical isolator, a signal generator, a semiconductor optical amplifier, an erbium-doped fiber amplifier, an optical circulator, a sensing fiber, an avalanche photodiode, a data acquisition card, and a processor.
The signal generator is designed and realized by a double-channel direct digital frequency synthesizer (DDS), outputs two paths of analog signals of ramp waves and pulse waves, and the two paths of signals share a high-stability clock source. The oblique wave drives the laser and the pulse wave drives the semiconductor optical amplifier. The repetition frequencies of the two paths of signals are consistent, and the ramp wave has the lowest value v at the rising edge of the pulse wavemin-5V, having a maximum value V at the falling edge of the pulse wavemaxAt the time of the center of the high level of the pulse wave and at the time of the low level of the pulse wave, having an intermediate value of 0V, ramp wave and rectangular wave waveforms are shown in fig. 2(a) and 2(b), respectively.
Specifically, the laser adopts an external cavity technology narrow linewidth laser of RIO company in America, emits linear frequency modulation narrow linewidth continuous light under the excitation of an oblique wave signal, and the central frequency of a light wave is v0193.5THz, the frequency range is from
Become to
Where the frequency bandwidth v is 400 MHz. And the amplitude of the ramp wave is VminWhen corresponding to a light wave frequency of
Amplitude of the ramp wave is VmaxWhen corresponding to a light wave frequency of
When the amplitude of the oblique wave is 0V, the corresponding frequency of the light wave is V0. That is, in the effective interval of the ramp signal, the laser emits continuous light with chirp; in the inactive interval of the ramp signal, the laser emits continuous light at the center frequency. LaserThe waveform of the electric field of the light emitted by the optical device is shown in FIG. 2 (c).
The laser outputs linear frequency modulation continuous light under the drive of oblique waves, and the electric field of the light waves is as follows:
wherein E is0Represents the amplitude, v0Which represents the center frequency of the signal and,vrepresenting the frequency variation range, i.e. bandwidth, of the signalpRepresents the effective time width of the ramp, i.e. the time width of the chirp, T represents the time, T represents the ramp period, and k represents a non-negative integer including zero. Thus, ElaserOnly at τ of each cyclepOutputting a chirp signal over a length of time, the signal having a frequency of
Become to
The other time outputs a continuous signal with a constant signal frequency.
The linear frequency-modulated continuous light emitted by the laser enters the optical isolator. The optical isolator plays a role in isolating the front-stage optical device from the rear-stage optical device, and reflected light of the rear-stage optical device is prevented from damaging the laser.
The continuous light of linear frequency modulation of laser instrument output, through optical isolator, semiconductor optical amplifier, converts linear frequency modulation's pulse light into, and the electric field of pulse light wave is:
wherein pi () represents a rectangular function, when kT ≦ t ≦ kT + τpWhen k is equal to 1, k is equal to T + TpWhen T is less than or equal to (k +1) T, the value is zero.
The linear frequency-modulated continuous light output by the optical isolator enters the semiconductor optical amplifier. The semiconductor optical amplifier converts continuous light into pulsed light by driving with a pulse wave, outputs chirped continuous light only at a high level of the pulse wave, does not output light at a low level of the pulse wave, and outputs a pulse light wave having an electric field waveform as shown in fig. 2 (d).
Pulsed light output by the semiconductor optical amplifier is subjected to power amplification through the erbium-doped optical fiber amplifier, then enters a port 1 of the circulator and is emitted through a port 2 of the circulator to enter the sensing optical fiber, vibration measurement information along the optical fiber is obtained, backward Rayleigh scattered light which carries environmental physical quantity information and is generated in the sensing optical fiber passes through the port 2 of the circulator again and is emitted from a port 3 of the circulator to enter the avalanche photodiode.
The linear frequency modulation pulse light output by the semiconductor optical amplifier enters the long-distance sensing optical fiber through the optical circulator. Regardless of the intensity attenuation, the electric field of the chirped light propagating along the sensing fiber can be expressed as:
wherein z represents the distance along the sensing fiber, the starting end connected with the optical circulator is marked as 0 meter, n (z) represents the refractive index of the fiber at the distance z of the sensing fiber, and c represents the speed of light.
When the pulse light propagates along the sensing optical fiber, an infinitesimal part of the pulse light generates Rayleigh scattering light waves opposite to the propagation direction at each moment. At a certain time t, the optical circulator receives echo optical signals generated by superposition of different parts of rayleigh scattering echoes corresponding to different moments. The electric field of the rayleigh scattered echo optical signal can be expressed as:
wherein r (z) represents the rayleigh scattering profile function of the sensing fiber along the line, and n represents the sensing fiber bulk average refractive index.
At time t, the Rayleigh scattering echo optical signal Escatter(t) may be pulsed light of a chirp typeThe convolution of the backward rayleigh scattered light produced by the length of sensing fiber. Assuming that the length of fiber is z1To z3The frequency of the pulsed light corresponding to the position is v4To v1And is and
get near z1At a certain position of the optical fiber, denoted as z2And z is2At z1To z3And z is2The frequency of the pulsed light at the corresponding position is denoted as v3Fig. 3 is a schematic diagram illustrating the currently pulsed light propagating along the sensing fiber.
At time t + Δ t, the Rayleigh scattering echo optical signal Escatter(t + Δ t) may be obtained by convolution of the backscattered rayleigh light generated by the chirped light on a section of sensing fiber. Assuming that the length of fiber is z2To z4The frequency of the pulsed light corresponding to the position is v4To v1And is and
get near z4At a certain position of the optical fiber, denoted as z3And z is3At z2To z3And z is3The frequency of the pulsed light at the corresponding position is denoted as v2Fig. 4 is a schematic diagram illustrating the currently pulsed light propagating along the sensing fiber.
If Δ t is taken small enough, z1To z2And z, and3to z4Is much smaller than z1To z4The distance of (c). Thus, z1To z2Rayleigh scattered light pair E within rangescatter(t) contribution, and z3To z4Rayleigh scattered light pair E within rangescatterThe contribution of (t + Δ t) is negligible. Further, Escatter(t) and Escatter(t + Δ t) may be represented by z2To z3The rayleigh scattering light within the distance. However, for Escatter(t) and Escatter(t + Δ t), the frequency components of the pulsed light propagating through the sensing fibers within the same distance are different, and are each v1To v3And v and2to v4There is a frequency difference Δ v.
If z is1To z2The refractive index of the sensing fiber is kept constant, then Escatter(t) and Escatter(t + Δ t) is not the same, but if the refractive index of the sensing fiber from z1 to z2 is changed by a change in physical quantity such as temperature, stress, vibration, etc., and the change in refractive index due to the change in physical quantity exactly compensates for the frequency difference Δ v, Escatter(t) and Escatter(t + Δ t) are the same. Therefore, by observing the waveform of the rayleigh scattering echo signal of different pulse periods, if a certain portion of the waveform is found to be shifted along the time axis, it can be concluded that there is a change in physical quantity such as temperature, stress, vibration, etc. at the sensing fiber corresponding to the portion. And because the waveform of the Rayleigh scattering echo signal deviates along the time axis and has a linear relation with the change of physical quantities such as temperature, stress, vibration and the like, the measured value of the physical quantity can be calculated according to the time deviation.
The avalanche photodiode converts weak Rayleigh scattering echoes of the sensing optical fiber into voltage signals, and the voltage signals are sent to the data acquisition card. The sampling rate of the data acquisition card is larger than 2 times of the linear frequency modulation bandwidth v, and the voltage signal is converted into a digital signal and sent to the processor. The processor performs cross-correlation calculation by using the Rayleigh scattering echo data of two adjacent periods, and obtains the measured values of physical quantities such as temperature, stress, vibration and the like along the sensing optical fiber according to the deviation of the echo waveform along the time axis.
For physical quantities such as temperature, stress, non-high-frequency vibration and the like along the sensing optical fiber, similarity necessarily exists between the Rayleigh scattering echo data of two adjacent periods. If the similarity is lower than a preset similarity degree (such as a correlation coefficient threshold value), the instantaneous sudden change of the physical quantity along the sensing optical fiber is indicated, and the situation is beyond the measuring range of the device. The purpose of the cross-correlation calculation is to obtain "the shift of the echo waveform along the time axis", and then further obtain "the measured values of physical quantities such as temperature, stress, vibration, etc. along the sensing fiber".
The circulator emits Rayleigh scattering echo optical signals, the Rayleigh scattering echo optical signals enter the avalanche photodiode to be converted into electric signals, then the electric signals enter the data acquisition card to be converted into digital signals, and then the digital signals are sent into the processor. The processor performs cross-correlation calculation by using the Rayleigh scattering echo data of two adjacent periods, and obtains the measured values of physical quantities such as temperature, stress, vibration and the like along the sensing optical fiber according to the deviation of the echo waveform along the time axis. And, in order to guarantee the precondition that the delta t is small enough, the sampling rate of the data acquisition card is at least 2 times larger than the linear frequency modulation bandwidth v.
The distributed optical fiber sensing device of the embodiment comprises a laser, an optical isolator, a signal generator, a semiconductor optical amplifier, an erbium-doped optical fiber amplifier, an optical circulator, a sensing optical fiber, an avalanche photodiode, a data acquisition card and a processor. The signal generator outputs two paths of analog signals: ramp waves and pulse waves. The oblique wave drives the laser to generate linear frequency modulation continuous light; the pulse wave drives the semiconductor optical amplifier to convert the continuous light into pulse light. The linear frequency modulation pulse light is transmitted along the sensing optical fiber, the physical quantity changes of temperature, stress, vibration and the like along the optical fiber are converted into the deviation of the radiated Rayleigh scattering echo signal along the time axis, and the deviation of the radiated Rayleigh scattering echo signal along the time axis is reserved
The inherent advantages of the technology: physical quantity measurement values of temperature, stress, vibration and the like along a dozen kilometers long sensing optical fiber can be obtained, the spatial resolution is as high as several meters, and the sensitivity is superior to that of the common sensing technology; and each pulse period can obtain the physical quantity measured value along the sensing optical fiber, thereby greatly shortening the measuring time.
At time t, the Rayleigh scattered echo signal Escatter(t) and EscatterThe (t + Δ t) can be obtained by convolution of backward Rayleigh scattering light generated by the linear frequency modulation pulse light in a certain section of sensing optical fiber. If Δ t is taken small enough, Escatter(t) and EscatterBoth (t + Δ t) are considered to be determined by rayleigh scattered light within the same distance. However, for Escatter(t) and Escatter(t + Δ t), the frequency components of the pulsed light propagating through the sensing fibers within the same distance are different, and a frequency difference Δ v exists. And due to Rayleigh scattering of the echo signal wavesThe deviation of the waveform along the time axis has a linear relation with the change of the physical quantity such as temperature, stress, vibration and the like, and the measured value of the physical quantity such as temperature, stress, vibration and the like can be calculated according to the time deviation amount of the waveform of the Rayleigh scattering echo signal between different pulse periods.
The working principle of the distributed optical fiber sensing device with linear frequency modulation of the embodiment is as follows:
the laser and the semiconductor optical amplifier respectively correspond to a ramp wave signal and a pulse wave signal with consistent receiving period and repetition frequency;
the laser emits linear frequency-modulated continuous light in the oblique wave signal effective interval, and emits central frequency continuous light in the oblique wave signal ineffective interval;
under the drive of pulse waves, the semiconductor optical amplifier converts continuous light output by the laser into pulse light, outputs linear frequency modulation continuous light only at the high level of the pulse waves, and does not output light waves at the low level of the pulse waves;
the first port of the circulator receives the pulse light, the pulse light emitted from the second port enters the sensing optical fiber, and Rayleigh scattering echoes of the sensing optical fiber carrying environmental physical quantity are sequentially emitted from the second port and the third port to enter the avalanche photodiode and are converted into voltage signals;
and the processor obtains the physical quantity measured value along the sensing optical fiber according to the linear relation between the deviation of the Rayleigh scattering echo waveform along the time axis and the optical fiber environment physical quantity.
And performing cross-correlation calculation by using Rayleigh scattering echoes of two adjacent pulse periods, and performing reverse estimation to obtain the measured values of physical quantities such as temperature, stress, vibration and the like along the sensing optical fiber according to the deviation of the echo waveform along a time axis. The physical quantity measured value along the sensing optical fiber can be obtained in each pulse period, and the measuring time is greatly shortened.
The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (10)
1. A chirped distributed optical fiber sensing apparatus, comprising:
the laser is used for emitting linear frequency-modulated continuous light in a ramp signal effective interval and emitting continuous light with a central frequency in a ramp signal ineffective interval;
the semiconductor optical amplifier is used for converting continuous light into pulse light under the driving of pulse waves, outputting linear frequency modulation continuous light only when the pulse waves are at a high level, and not outputting light waves when the pulse waves are at a low level; wherein, the periods and the repetition frequencies of the oblique wave and the pulse wave are consistent;
the first port of the circulator receives the pulse light, the pulse light emitted from the second port enters the sensing optical fiber, and the Rayleigh scattering echo of the sensing optical fiber carrying the environmental physical quantity sequentially exits from the second port and the third port and enters the avalanche photodiode to be converted into a voltage signal;
and the processor is used for obtaining the physical quantity measured value along the sensing optical fiber according to the linear relation between the deviation of the Rayleigh scattering echo waveform along the time axis and the optical fiber environment physical quantity.
2. A chirped distributed fibre optic sensing apparatus according to claim 1, wherein an optical isolator is further connected in series between the laser and the semiconductor optical amplifier.
3. A chirped distributed fibre optic sensing apparatus according to claim 1, wherein an erbium doped fibre optic amplifier is further connected in series between the semiconductor optical amplifier and the first port of the circulator, the erbium doped fibre optic amplifier is used for power amplification of the pulsed light output from the semiconductor optical amplifier.
4. A chirped distributed fibre optic sensing apparatus according to claim 1, wherein said avalanche photodiode is further connected to a data acquisition card for converting the voltage signal to a digital signal for feeding to the processor.
5. A chirped distributed fibre optic sensing device according to claim 4, wherein the sampling rate of said data acquisition card is at least twice the bandwidth of the chirp.
6. A chirped distributed fibre optic sensing apparatus according to claim 1, wherein the ramp wave has a lowest value at a rising edge of the pulsed wave, a highest value at a falling edge of the pulsed wave, and an intermediate value of 0V at the centre of the high level of the pulsed wave and at the low level of the pulsed wave.
7. A chirped distributed fibre optic sensing apparatus according to claim 1, wherein the ramp wave and the pulsed wave are both generated by a signal generator, and the signal generator is connected to the processor.
8. A chirped distributed optical fiber sensing apparatus according to claim 1, wherein in said processor the offset of the rayleigh scattered echo waveform along the time axis is obtained from the cross-correlated rayleigh scattered echo voltage signals of two adjacent cycles.
9. A chirped distributed fibre optic sensing apparatus according to claim 1, wherein the sensing fibres carrying the environmental physical quantities rayleigh scatter back is the convolution of the back rayleigh scattered light produced by the chirped light at the respective sensing fibres.
10. A method of operating a chirped distributed fibre optic sensing device according to any one of claims 1 to 9, comprising:
the laser and the semiconductor optical amplifier respectively correspond to a ramp wave signal and a pulse wave signal with consistent receiving period and repetition frequency;
the laser emits linear frequency-modulated continuous light in the oblique wave signal effective interval, and emits central frequency continuous light in the oblique wave signal ineffective interval;
under the drive of pulse waves, the semiconductor optical amplifier converts continuous light output by the laser into pulse light, outputs linear frequency modulation continuous light only at the high level of the pulse waves, and does not output light waves at the low level of the pulse waves;
the first port of the circulator receives the pulse light, the pulse light emitted from the second port enters the sensing optical fiber, and Rayleigh scattering echoes of the sensing optical fiber carrying environmental physical quantity are sequentially emitted from the second port and the third port to enter the avalanche photodiode and are converted into voltage signals;
and the processor obtains the physical quantity measured value along the sensing optical fiber according to the linear relation between the deviation of the Rayleigh scattering echo waveform along the time axis and the optical fiber environment physical quantity.
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