CN110864714B - Distributed sensing system based on Michelson-Sagnac fiber optic interferometer - Google Patents

Distributed sensing system based on Michelson-Sagnac fiber optic interferometer Download PDF

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CN110864714B
CN110864714B CN201911195568.2A CN201911195568A CN110864714B CN 110864714 B CN110864714 B CN 110864714B CN 201911195568 A CN201911195568 A CN 201911195568A CN 110864714 B CN110864714 B CN 110864714B
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贾波
宋秋衡
吴红艳
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Fudan University
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    • 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
    • G01D5/35306—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 an interferometer arrangement
    • G01D5/35322—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 an interferometer arrangement using interferometer with one loop with several directions of circulation of the light, e.g. Sagnac interferometer
    • 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
    • G01D5/35306—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 an interferometer arrangement
    • G01D5/35325—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 an interferometer arrangement using interferometer with two arms in reflection, e.g. Mickelson interferometer

Abstract

The invention belongs to the technical field of optical fiber sensing, and particularly relates to a distributed sensing system based on a Michelson-Sagnac optical fiber interferometer. The system takes two arms of a Michelson optical fiber interferometer as sensing arms, and the two sensing arms are distinguished through a signal analysis algorithm; the two interferometers simultaneously detect the same time-varying disturbance, the demodulated phases are added and subtracted to obtain two paths of signals with time delay, and the time delay related to the disturbance position is obtained through cross correlation, so that the disturbance position information can be calculated. The maximum detection distance of the system can reach 140km without blind areas. The system realizes distributed optical fiber sensing and positioning by using the Michelson-Sagnac hybrid optical fiber interferometer, improves the sensitivity and positioning accuracy of long-distance distributed sensing, enlarges the frequency response range of the distributed optical fiber sensing system, and solves the problem that the traditional scheme cannot position when applied to positioning of an underground optical cable.

Description

Distributed sensing system based on Michelson-Sagnac fiber optic interferometer
Technical Field
The invention belongs to the technical field of optical fiber sensing, and particularly relates to a distributed sensing system based on a Michelson-Sagnac optical fiber interferometer.
Background
Distributed optical fiber sensors are often applied to the fields of perimeter security, oil pipe leakage monitoring, power cable damage monitoring, bridge safety monitoring and the like. The optical fiber integrates sensing and transmission, can carry out continuous vibration or temperature measurement on the whole length of the optical fiber, and simultaneously obtains the specific position to be measured. The distributed optical fiber sensing system based on the Michelson optical fiber interferometer has the characteristics of two-arm sensing and high sensitivity to low-frequency signals. However, in the distributed sensing and positioning application using the structure, no matter based on time delay estimation or a notch point algorithm, the distributed sensing and positioning method is not suitable for the low-frequency vibration condition. The Sagnac-based fiber optic interferometer has a differential effect and can naturally isolate a certain degree of environmental impact. However, part of the low frequency vibration signal is also filtered out, which has certain disadvantages in the sensing and positioning application of the small disturbance signal. The invention is based on a Michelson-Sagnac fusion interferometer, and the disturbance of a sensing path is obtained and the positioning is realized by the methods of phase reduction and signal processing. The advantages of the two interferometers are fully utilized, and the function of disturbance positioning can be realized by utilizing the vibration lower than 200 Hz.
Disclosure of Invention
The invention aims to provide a distributed sensing system based on a Michelson-Sagnac optical fiber interferometer, which can improve the sensitivity and the positioning accuracy of a distributed optical fiber sensor.
The invention realizes distributed optical fiber sensing and positioning by using a Michelson-Sagnac hybrid optical fiber interferometer, and provides a sensing and positioning demodulation algorithm based on the Michelson-Sagnac optical fiber interferometer. The invention can improve the sensitivity and positioning accuracy of long-distance distributed optical fiber sensing, enlarge the frequency response range of the distributed optical fiber sensing system and solve the problem that the traditional scheme cannot be used for positioning the buried optical cable.
According to the distributed sensing system based on the Michelson-Sagnac optical fiber interferometer, two arms of the Michelson optical fiber interferometer are used as sensing arms, and the two sensing arms can be distinguished through a signal analysis algorithm; the structure of which is shown in figure 1. Wherein:
a distributed two-arm sensing system based on a Michelson interferometer comprises: the optical fiber detection device comprises a narrow-band laser, an optical isolator, a first detector, a third detector, a 3 x 3 optical fiber coupler, a first sensing optical cable, a second sensing optical cable, a first Faraday rotator mirror and a second Faraday rotator mirror; the narrow-band laser emits coherent light waves, the coherent light waves are divided into three beams of light with equal power by the 3 x 3 optical fiber coupler, and one beam of light is coupled out of the fiber core when passing through the knotted optical fiber; the other two beams of light are used as detection light waves and enter the first sensing arm 1 and the second sensing arm 2 respectively; the first detection light wave 1 and the second detection light wave 2 are respectively reflected by the first Faraday rotator mirror and the second Faraday rotator mirror, and then enter the 3 multiplied by 3 coupler again to generate interference; the effect of the external environment on the optical cable is recorded in the phase of the first detection light wave 1 and the second detection light wave 2; the phase information is presented by the voltage intensity of the first photodetector 1 and the third photodetector 3;
a distributed two-arm sensing system based on a Sagnac interferometer comprises: the optical fiber sensing device comprises a wide-spectrum laser, an optical isolator, a second optical detector, a fourth optical detector, a 3 x 3 optical fiber coupler, and an annular sensing optical cable consisting of a first sensing optical cable 1 and a second sensing optical cable 2; a polarization controller is adopted on a path; the phase information is represented by the voltage intensities of the second photodetector 2 and the fourth photodetector 4.
In the distributed sensing system based on the Michelson-Sagnac optical fiber interferometer, an optical isolator is added to prevent backward scattered light of a sensing arm and reflected signal light from entering a laser and influencing the working state of the laser; the tail ends of the sensing arms are all provided with Faraday rotation mirrors to keep the system in a polarization stable state, and a polarization controller is arranged on a path to ensure that Sagnac has a stable interference phenomenon.
In the distributed sensing system based on the Michelson-Sagnac fiber optic interferometer, both Michelson arms are used as sensing arms and spread along a path to be monitored. And demodulating a phase difference signal demodulation algorithm generated by disturbance, analyzing the positive and negative of the initial amplitude of the phase difference, distinguishing a sensing arm where the vibration is located, and calculating the disturbance position and intensity characteristics.
In the distributed sensing system based on the Michelson-Sagnac optical fiber interferometer, phase difference signals generated by disturbance are demodulated through a demodulation algorithm, the demodulated phases are added and subtracted to obtain two paths of signals with time delay, the time delay related to the disturbance position is obtained through cross correlation, and the disturbance position can be obtained.
In the distributed sensing system based on the Michelson-Sagnac fiber optic interferometer, the demodulation algorithm specifically comprises the following steps: the interference signals of SI and MI are separated by two wavelength division multiplexers, assumed at LxApplying a time-varying perturbation at the distance point of (a), then:
the two paths of interference signals of the detected MI are respectively as follows:
Figure BDA0002294578100000021
Figure BDA0002294578100000022
wherein E is01And E03Is the optical amplitude, and Δ φ (t) is the phase difference caused by the perturbation, φ01And phi01Is the initial phase difference of the 3 x 3 coupler. The two detected interference signals of the SI are respectively as follows:
Figure BDA0002294578100000023
Figure BDA0002294578100000024
wherein E is02And E04Is the optical amplitude, and Δ φ (t) is the phase difference caused by the perturbation, φ01And phi01Is the initial phase difference of the 3 x 3 coupler. Phase difference delta phi of two interferometers caused by the same disturbanceSI(t) and. DELTA.. phi. -)MI(t) are as follows:
Figure BDA0002294578100000025
Figure BDA0002294578100000026
wherein, tauxIs formed by a length LxThe time delay caused, LxIs the point in time, τ, from the point of disturbance to the TDF0Is of length L0C is the speed of light in vacuum, n is the optical fiber at wavelength λ0The refractive index of the film.
The phase difference delta phi generated by the same disturbance is processed by a simple signal processing algorithmSI(t) and. DELTA.. phi. -)MI(t) combined to produce phi with a fixed time delay1(t) and phi2(t) of (d). After the time delay is calculated by utilizing a cross-correlation algorithm, the position of the disturbance can be conveniently determined. Fig. 2 is a block diagram of the data processing algorithm. The algorithm flow is as follows:
Figure BDA0002294578100000031
Figure BDA0002294578100000032
by aligning phi1(t) and phi2(t) obtaining the time delay 2 tau by taking the cross-correlation functionxObtaining a calculation formula of the disturbance position as follows:
Lx＝cτx/2n。
in the distributed two-arm sensing system based on the Michelson-Sagnac fiber optic interferometer, the first sensing arm 1 and the second sensing arm 2 can be distinguished through an algorithm. The specific algorithm is as follows:
the expression for the Michelson interferometer phase difference is as follows:
Figure BDA0002294578100000033
wherein phi is1(t) and phi1(t) represents the phase information on the first and second sensor arms 1 and 2, respectively. Since the influence of the external temperature and the atmospheric pressure on the optical fiber is slowly changed and weak, it is assumed here that the environmental conditions of the two sensing arms are consistent, i.e. the phase difference between the two sensing arms is zero when there is no disturbance. Assuming that the signal light is disturbed at any point on the sensing arm, the phase change of the signal light caused by the photoelastic effect can be described as a superposition of cosine waves of different frequencies and amplitudes, and the expression is as follows:
Figure BDA0002294578100000034
therein, Ψi，ωiAnd phiiRepresenting the amplitude, angular frequency and phase of the signal, respectively. When external disturbance is applied to the first sensing arm 1, according to the two-arm phase:
Figure BDA0002294578100000035
the following results were obtained:
Figure BDA0002294578100000036
when external disturbance is applied to the second sensing arm 2, according to the two-arm phase:
Figure BDA0002294578100000037
the following results were obtained:
Figure BDA0002294578100000041
thus, it can be seen that when external perturbations are applied to different sensing arms, the phase difference obtained is different. According to the interference formula:
Figure BDA0002294578100000042
wherein, I0Indicates the output light intensity, phi0(t) random phase, phi, of two sensing arms brought by external environment0Representing the fixed phase introduced by the 3 x 3 coupler. After the adjusted delta phi (t), the initial direction of the phase difference amplitude is judged, and the sensing arm acted by the disturbance can be distinguished.
The maximum detection distance of the system can reach 140km without blind areas.
The system of the invention realizes distributed optical fiber sensing and positioning by using a Michelson-Sagnac hybrid optical fiber interferometer, provides a sensing and positioning demodulation algorithm based on the optical fiber interferometer, improves the sensitivity and positioning accuracy of long-distance distributed sensing, and can realize high-accuracy positioning no matter direct disturbance or indirect disturbance. Especially, compared with the traditional scheme, the distributed optical fiber sensing system has outstanding advantages in the application of underground optical cable sensing, enlarges the frequency response range of the distributed optical fiber sensing system, and solves the problem that the traditional scheme cannot be used for positioning the underground optical cable.
Drawings
FIG. 1 is a structural diagram of a distributed sensing system based on a Michelson-Sagnac fiber optic interferometer of the present invention.
FIG. 2 is a flow chart of a positioning algorithm in a distributed sensing system based on a Michelson-Sagnac fiber optic interferometer of the present invention.
FIG. 3 is a diagram showing the phase difference Δ φ demodulated by the distributed two-arm sensing system of the Michelson optical fiber interferometer for detecting the interference signals generated by MI and SISI(t) and. DELTA.. phi. -)MI(t)。
FIG. 4 is a diagram of two paths of signals phi obtained by data processing of phase differences demodulated from interference signals generated by detecting MI and SI in a distributed two-arm sensing system using a Michelson optical fiber interferometer of the present invention1(t) and phi2(t)。
FIG. 5 shows a signal phi obtained after processing when disturbance directly acts on the sensing arm 1 in the distributed sensing system based on the Michelson-Sagnac fiber optic interferometer of the present invention1(t) and phi2(t)。
FIG. 6 shows a signal phi obtained after processing when disturbance is directly applied to the sensing arm 2 in the distributed sensing system based on the Michelson-Sagnac fiber optic interferometer of the present invention1(t) and phi2(t)。
FIG. 7 shows a phase difference signal φ obtained when disturbance is respectively applied to the surface of the buried optical cable 1 based on a distributed sensing system of a Michelson-Sagnac fiber optic interferometer of the present invention1(t) and phi2(t)。
FIG. 8 shows a phase difference signal φ obtained when disturbance is respectively applied to the surface of the buried optical cable 2 in the distributed sensing system based on the Michelson-Sagnac fiber optic interferometer of the present invention1(t) and phi2(t)。
Reference numbers in the figures: 1 is a first photodetector; 2 is a second photodetector; 3 is a third photodetector; 4 is a fourth photodetector; 5 is a wide spectrum light source; 6 is a narrow spectrum light source; 7 is a first wavelength division multiplexer; 8 is a second wavelength division multiplexer; 9 is a third wavelength division multiplexer; 10 is a fourth wavelength division multiplexer; 11 is a fifth wavelength division multiplexer; 12 is a 3 × 3 fiber coupler; 13 is a polarization controller; 14 is a light delayer; 15 is a first Faraday rotator mirror; 16 is a second Faraday rotator mirror; 17 is a first sensing optical cable; 18 is a second sensing optical cable; and 19 is an optical isolator.
Detailed Description
The following further describes embodiments of the present invention with reference to the drawings.
The distributed sensing system based on the Michelson-Sagnac optical fiber interferometer is shown in figure 1, a narrow-band laser adopted by the distributed sensing system is a distributed feedback laser (DFB) with the center wavelength of 1550.12nm, a wide-spectrum light source adopts a super-radiation light-emitting diode (SLD), and the coupled laser is used as an injection light source after being coupled through a 200G filter with the center wavelength of 1550.12 nm. The laser light enters the 3 x 3 fiber coupler and is split into three beams of light of equal power, one of which is coupled out of the core as it passes through the knotted fiber. The optical isolator eliminates optical reflection, and four photodetectors PD1, PD2, PD3, and PD4 are used for photoelectric conversion. Between wavelength division multiplexer 2(WDM2) and wavelength division multiplexer 3(WDM3) there is a delay fiber (TDF) which is located at the centre of the fibre loop centre. This ensures that the Sagnac interferometer has a high ability to respond to common disturbances and eliminates detection dead zones. A Polarization Controller (PC) is used to ensure the interference stability of the Sagnac interferometer. The demodulation scheme using a 3 x 3 optical coupler has the advantages of passive detection and low cost because it does not require phase or frequency modulation in the reference arm or laser and therefore has no active components in the optical domain. In the fiber loop, laser light emitted from the DFB will be reflected by faraday rotating mirror 1(FRM1) and faraday rotating mirror 2(FRM2), which makes the two paths of light work as a Michelson interferometer, and interference signals are received by PD1 and PD3 and converted into electrical signals. The light emitted from the SLD is split into two paths of clockwise and counterclockwise light, and the interference signals are received and converted into electrical signals by the PD2 and the PD 4. A computer with an acquisition card with a sampling rate of 500kS/s is used for acquiring photoelectric signals and demodulating phase difference information, and the disturbance intensity and position can be judged through an algorithm in a flow chart shown in figure 2. Fig. 3 and 4 show phase difference signals of the Michelson interferometer and the Sagnac interferometer after the disturbances are demodulated. Fig. 5 and 6 show signals obtained after demodulation and processing at 40km when external disturbance directly acts on the sensing arm 1 and the sensing arm 2. Fig. 7 and 8 show signals obtained after demodulation and processing when external disturbance acts on the sensing arm 1 and the sensing arm 2 for 40km through soil layers. It can be seen that the two paths of signals have fixed time delay, and the disturbance position information can be obtained through the time delay.

Claims (4)

1. A distributed sensing system based on a Michelson-Sagnac fiber interferometer is characterized in that two arms of the Michelson fiber interferometer are used as sensing arms, and the two sensing arms are distinguished through a signal analysis algorithm; wherein:
a distributed two-arm sensing system based on a Michelson interferometer comprises: the optical fiber detection device comprises a narrow-band laser, an optical isolator, a first detector, a third detector, a 3 x 3 optical fiber coupler, a first sensing optical cable, a second sensing optical cable, a first Faraday rotator mirror and a second Faraday rotator mirror; the narrow-band laser emits coherent light waves, the coherent light waves are divided into three beams of light with equal power by the 3 x 3 optical fiber coupler, and one beam of light is coupled out of the fiber core when passing through the knotted optical fiber; the other two beams of light are used as detection light waves and enter the first sensing arm and the second sensing arm respectively; the first detection light wave and the second detection light wave are respectively reflected by the first Faraday rotator mirror and the second Faraday rotator mirror and then enter the 3 multiplied by 3 coupler again to generate interference; the effect of the external environment on the optical cable is recorded in the phases of the first detection light wave and the second detection light wave; the phase information is presented by the voltage intensity of the first photodetector and the third photodetector 3;
a distributed two-arm sensing system based on a Sagnac interferometer comprises: the optical fiber sensing device comprises a wide-spectrum laser, an optical isolator, a second optical detector, a fourth optical detector, a 3 x 3 optical fiber coupler, and an annular sensing optical cable consisting of a first sensing optical cable and a second sensing optical cable; a polarization controller is adopted on a path; the phase information is presented through the voltage intensity of the second light detector and the fourth light detector;
the two Michelson arms are used as sensing arms and spread along a path to be monitored; demodulating a phase difference signal generated by disturbance through a demodulation algorithm, analyzing the positive and negative of the initial amplitude of the phase difference, distinguishing a sensing arm where the vibration is located, and calculating the disturbance position and intensity characteristics;
demodulating the phase difference signal generated by the disturbance through a demodulation algorithm, which specifically comprises the following steps: the interference signals of SI and MI are separated by two wavelength division multiplexers, assumed at LxApplying a time-varying perturbation at the distance point of (a), then:
the two paths of interference signals of the detected MI are respectively as follows:
Figure FDA0003099886570000011
Figure FDA0003099886570000012
wherein E is01And E03Is the amplitude of the light, and,
Figure FDA0003099886570000013
is the phase difference caused by the disturbance,
Figure FDA0003099886570000014
and
Figure FDA0003099886570000015
is the initial phase difference of the 3 x 3 coupler; the two detected interference signals of the SI are respectively as follows:
Figure FDA0003099886570000016
Figure FDA0003099886570000017
wherein E is02And E04Is the amplitude of the light, and,
Figure FDA0003099886570000018
is the phase difference caused by the disturbance,
Figure FDA0003099886570000019
and
Figure FDA00030998865700000110
is the initial phase difference of the 3 x 3 coupler; phase difference of two interferometers caused by the same disturbance
Figure FDA00030998865700000111
And
Figure FDA00030998865700000112
respectively as follows:
Figure FDA00030998865700000113
Figure FDA0003099886570000021
wherein, tauxIs formed by a length LxThe time delay caused, LxIs the point in time, τ, from the point of disturbance to the TDF0Is of length L0C is the speed of light in vacuum, n is the optical fiber at wavelength λ0Refractive index of time;
using signal processing algorithm to correct the phase difference generated by the same disturbance
Figure FDA0003099886570000022
And
Figure FDA0003099886570000023
combined to produce phi with a fixed time delay1(t) and phi2(t); using cross-correlationThe time delay is calculated, so that the position of the disturbance can be determined.
2. The distributed sensing system according to claim 1, wherein the optical isolator is configured to prevent backscattered light from the sensing arm and reflected signal light from entering the laser and affecting its operating state; the two Faraday rotation mirrors are positioned at the tail ends of the two sensing arms to keep the system in a polarization stable state, and a polarization controller is adopted on a path to ensure that Sagnac has a stable interference phenomenon.
3. The distributed sensing system based on the Michelson-Sagnac fiber optic interferometer of claim 1, wherein the algorithm for determining the location of the disturbance proceeds as follows:
Figure FDA0003099886570000024
Figure FDA0003099886570000025
by aligning phi1(t) and phi2(t) obtaining the time delay 2 tau by taking the cross-correlation functionxObtaining a calculation formula of the disturbance position as follows:
Lx＝cτx/2n。
4. the distributed sensing system based on the Michelson-Sagnac fiber optic interferometer of one of claims 1-3, wherein the first sensing arm and the second sensing arm are distinguished by an algorithm, the specific algorithm being as follows:
the expression for the Michelson interferometer phase difference is as follows:
Figure FDA0003099886570000026
wherein the content of the first and second substances,
Figure FDA0003099886570000027
and
Figure FDA0003099886570000028
respectively representing phase information on the first sensing arm and the second sensing arm, and assuming that environmental conditions of the two sensing arms are consistent, namely phase difference of the two sensing arms is zero when no disturbance exists; when the phase of the signal light is disturbed at any point on the sensing arm, the phase change of the signal light caused by the photoelastic effect can be described as the superposition of cosine waves with different frequencies and amplitudes, and the expression is as follows:
Figure FDA0003099886570000029
therein, Ψi，ωiAnd phiiRespectively representing the amplitude, angular frequency and phase of the signal; when external disturbance is applied to the first sensing arm, according to the two-arm phase:
Figure FDA00030998865700000210
obtaining:
Figure FDA0003099886570000031
when external disturbance is applied to the second sensing arm, according to the two-arm phase:
Figure FDA0003099886570000032
obtaining:
Figure FDA0003099886570000033
therefore, when external disturbance action is applied to different sensing arms, the obtained phase difference is different; according to the interference formula:
Figure FDA0003099886570000034
wherein, I0Which is indicative of the intensity of the output light,
Figure FDA0003099886570000035
representing the random phase, phi, of the two sensing arms brought by the external environment0Represents the fixed phase introduced by the 3 x 3 coupler; called out
Figure FDA0003099886570000036
Then, the initial direction of the phase difference amplitude is judged, and the sensing arm acted by the disturbance can be distinguished.
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