Distributed optical fiber sensing system based on AOM frequency shift and vibration positioning method

Abstract

The invention provides a distributed optical fiber sensing system based on AOM frequency shift and a vibration positioning method, and relates to the technical field of optical fiber sensing, probe light emitted by a light source is divided into two branch light beams, one branch light beam is input into a first interferometer structure after frequency shift, and the other branch light beam is input into a second interferometer structure. The two split light beams are coupled in the interferometer structure to obtain sensing light and reference light, and the sensing light is coupled with the reference light in the light demodulation element after passing through the sensing optical fiber to obtain interference light. The two paths of interference light are converted into electric signals, then the signal processing assembly is used for eliminating Rayleigh backscattering noise signals, and the vibration position in the sensing optical fiber is determined according to the phase delay inequality between the two electric signals. According to the invention, the interference of Rayleigh backscattering is avoided, the sensing distance is increased, the signal-to-noise ratio of the system is improved, an additional light source is not needed, only a single optical fiber is used for transmission, and the cost is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber sensing, and particularly relates to a distributed optical fiber sensing system based on AOM frequency shift. Background Art

[0002] Distributed optical fiber sensing systems usually utilize the scattering effect in optical fibers to detect physical quantities distributed along the optical fiber. When the optical fiber is transmitting signals, forward optical signals and backward scattering information are received at both ends simultaneously. When the optical power injected into the optical fiber is less than a certain value, the main backward scattering interference in the optical fiber is Rayleigh backward scattering. Therefore, in the case of ultra-long-distance sensing, the forward optical signal attenuates greatly in the optical fiber. In a distributed optical fiber sensing system based on forward transmission, the sensing signal is usually not received, or the sensing signal is greatly affected by Rayleigh backward scattering, resulting in system failure or low measurement accuracy.

[0003] Therefore, the prior art needs to be further improved. Summary of the Invention

[0004] In view of the above deficiencies in the prior art, the purpose of the present invention is to provide a distributed optical fiber sensing system based on AOM frequency shift and a vibration positioning method, which separates the sensing light transmitted forward from the backward scattering through AOM frequency shift, eliminates the interference of backward scattering, and improves the vibration positioning accuracy.

[0005] In a first aspect, the present application discloses a distributed optical fiber sensing system based on AOM frequency shift, which includes: a light source, a first coupler, an acousto-optic modulator, a first interferometer structure connected to a sensing optical fiber, a second interferometer structure connected to the sensing optical fiber, and a signal processing component; The light source is used to emit probe light; The first coupler, connected to the light source, is used to divide the probe light into a first sub-beam and a second sub-beam with equal amplitudes and opposite directions; The acousto-optic modulator, connected to the first coupler, is used to shift the frequency of the first sub-beam; The first interferometer structure, connected to the acousto-optic modulator, is used to receive the frequency-shifted first sub-beam and divide the first sub-beam into a first sensing light and a second reference light; the second interferometer structure, connected to the first coupler, is used to receive the second sub-beam and divide the second sub-beam into a second sensing light and a second reference light; The first interferometer structure is further used to obtain a first interference optical signal generated by the interference between the second sensing light transmitted through the sensing optical fiber and the second reference light, and output a first electrical signal corresponding to the first interference optical signal; The second interferometer structure is further configured to obtain a second interference optical signal generated by the interference between the first sensing optical signal after being transmitted through the sensing optical fiber and the first reference optical signal, and output a second electrical signal corresponding to the second interference optical signal; The information processing component is configured to receive and eliminate the Rayleigh backscattering noise signals contained in the first electrical signal and the second electrical signal, and determine the vibration positioning result.

[0006] Optionally, the first interferometer structure and the second interferometer structure are Mach-Zehnder interferometer structures, Sagnac interferometer structures or Michelson interferometer structures.

[0007] Optionally, the first interferometer structure includes: a second coupler, a first optical fiber circulator, a first optical demodulation element and a first signal detection component; the second interferometer structure includes: a third coupler, a second optical fiber circulator, a second optical demodulation element and a second signal detection component; The second coupler receives the first split optical beam, and splits the first split optical beam into a first sensing optical signal and a first reference optical signal; The first optical fiber circulator is disposed on the optical path of the first sensing optical signal, configured to receive the first sensing optical signal, transmit the first sensing optical signal to the sensing optical fiber, and input it to the second optical fiber circulator through the sensing optical fiber, and input it to the first optical demodulation element through the second optical fiber circulator, and interfere with the first reference optical signal in the first optical demodulation element, and output a first interference optical signal; The first signal detection component is configured to receive the first interference optical signal and convert the first interference optical signal into a first electrical signal; And, the third coupler receives the second split optical beam, and splits the second split optical beam into a second sensing optical signal and a second reference optical signal; The second optical fiber circulator is disposed on the optical path of the second sensing optical signal, configured to receive the second sensing optical signal, transmit the second sensing optical signal to the sensing optical fiber, and input it to the first optical fiber circulator through the sensing optical fiber, and input it to the second optical demodulation element through the first optical fiber circulator, and interfere with the second reference optical signal in the second optical demodulation element, and output a second interference optical signal; The second signal detection component is configured to receive the second interference optical signal and convert the second interference optical signal into a second electrical signal.

[0008] Optionally, the first optical demodulation element is a first optical mixer, the first signal detection component includes: a first optical balanced detector and a second optical balanced detector; the second optical demodulation element is a second optical mixer, and the second signal detection component includes: a third optical balanced detector and a fourth optical balanced detector; The first sensing light is input into a second optical fiber circulator through a sensing optical fiber, and then input into a first optical mixer through the second optical fiber circulator, where it interferes with a first reference light in the first optical mixer to output a first interference light signal; The first optical balanced detector and the second optical balanced detector respectively receive the in-phase component signal and the quadrature component signal of the first interference light signal, and convert the in-phase component signal and the quadrature component signal into a first electrical signal; In addition, the second sensing light is input into a first optical fiber circulator through a sensing optical fiber, and then input into a second optical mixer through the first optical fiber circulator, where it interferes with a second reference light in the second optical mixer to output a second interference light signal; The third optical balanced detector and the fourth optical balanced detector respectively receive the in-phase component signal and the quadrature component signal of the second interference light signal, and convert the in-phase component signal and the quadrature component signal into a second electrical signal.

[0009] Optionally, the first optical demodulation element is a first 3×3 coupler, and the first signal detection component is three photodetectors; the second optical demodulation element is a second 3×3 coupler, and the second signal detection component is three photodetectors.

[0010] Optionally, the signal processing component includes: an oscilloscope, a low-pass filter, and a PC; The oscilloscope is configured to receive the first electrical signal and the second electrical signal, and transmit the first electrical signal and the second electrical signal to the low-pass filter; The low-pass filter is configured to filter out the Rayleigh backscattering noise signals contained in the first electrical signal and the second electrical signal collected by the oscilloscope; the cut-off frequency of each low-pass filter is lower than the frequency at which the acousto-optic modulator shifts the frequency of the first split light beam; The PC is configured to determine a sensing and positioning result according to the received first electrical signal and second electrical signal.

[0011] Optionally, the light source is a 1550 nm narrow linewidth laser with a linewidth less than 1 KHz; the detection light is a single-frequency continuous light; the sensing optical fiber is a single-mode optical fiber.

[0012] Optionally, a piezoelectric ceramic is connected to the sensing optical fiber; the piezoelectric ceramic is configured to output an analog vibration signal.

[0013] In a second aspect, the present application provides a vibration positioning method for a distributed optical fiber sensing system based on AOM frequency shift, which is applied to the distributed optical fiber sensing system described above; the vibration positioning method includes: The detection light emitted by the light source is input into a first coupler, and is split into a first split light beam and a second split light beam by the first coupler; The first split beam of light is input into an acousto-optic modulator, frequency shift processing is performed by the acousto-optic modulator, and the first split beam of light after frequency shift processing is input into a first interferometer structure to obtain a first interference optical signal output by the first interferometer structure; the second split beam of light is input into a second interferometer structure to obtain a second interference optical signal output by the second interferometer structure; The vibration position is located according to the first interference optical signal and the second interference optical signal.

[0014] Optionally, the step of locating the vibration position according to the first interference optical signal and the second interference optical signal includes: The first interference optical signal and the second interference optical signal are respectively converted into a first electrical signal and a second electrical signal; The first electrical signal and the second electrical signal are input into a low-pass filter, the Rayleigh backscattering noise signal is filtered by the low-pass filter, and the phase demodulation of the electrical signal after filtering the noise signal is performed to obtain phase information; The time delay difference between the first split beam of light and the second split beam of light is calculated according to the phase demodulation information, and the vibration position is located according to the time delay difference.

[0015] Advantageous effects: The present invention provides a distributed optical fiber sensing system and a vibration positioning method based on AOM frequency shift, constructs a sensing system of a forward transmission optical fiber acoustic / vibration sensor based on double-ended Mach-Zehnder interferometer demodulation and AOM frequency shift, wherein first, one optical path in the AOM double optical path is used for frequency shift, and then the Rayleigh backscattering noise is eliminated, so as to realize a long-distance sensing path without an optical amplifier. The optical fiber sensing system and the vibration positioning method provided by the present invention have a higher signal-to-noise ratio, a longer sensing distance, and the spatial resolution is not directly related to the sensing range, thereby allowing separate optimization of the sensing system and being easier to install and maintain. Description of the drawings

[0016] Figure 1 is a schematic structural diagram of the distributed optical fiber sensing system provided by the present invention; Figure 2 is a schematic structural diagram of a specific application embodiment of the distributed optical fiber sensing system provided by the present invention; Figure 3a is a schematic diagram of the transmission direction of the transmission signal in the distributed optical fiber sensing system based on AOM frequency shift of the present invention; Figure 3b is a schematic diagram of the generation of the interference optical signal generated in the first mixer in the sensing system of the present invention; Figure 3c is a schematic diagram of the generation of the interference optical signal generated in the second mixer in the sensing system of the present invention; Figure 4It is a schematic diagram of the first Mach-Zehnder structure equivalent to the system of the present invention; Figure 5 It is a schematic diagram of the second Mach-Zehnder structure equivalent to the system of the present invention; Figure 6 It is a flow chart for locating the vibration position according to the electrical signal in the embodiment of the present invention; Figure 7 It is a curve graph between the power spectral density and the frequency of the interference optical signal at the end of the sensing optical fiber in the embodiment of the present invention; Figure 8 It is a curve graph of the interference optical signal and the corresponding demodulation phase signal in the embodiment of the present invention; Figure 9 It is a step flow chart of the vibration positioning method in the embodiment of the present invention. Detailed implementation manners

[0017] To make the objectives, technical solutions and advantages of the present invention clearer and more definite, the present invention will be further described in detail below with reference to the accompanying drawings and by way of examples. It should be understood that the specific examples described herein are only used to explain the present invention and are not used to limit the present invention.

[0018] Optical fiber sensing technology is a detection method that studies the interaction between light and matter, and can be used to measure many physical, chemical, biological and other parameters. Distributed optical fiber vibration sensing technology is a new type of vibration sensing detection technology that integrates signal sensing and transmission. It can directly sense and detect optical wave signals to achieve continuous distributed measurement of vibration events along the optical fiber.

[0019] Distributed optical fiber sensing technology is roughly divided into two structures according to the sensing mechanism. One is a distributed optical fiber sensing system based on backscattering in the optical fiber, such as an optical time domain reflectometry system, an optical frequency domain reflectometry system, etc. The other is a distributed vibration optical fiber sensing system based on the forward transmission of light. The distributed optical fiber sensing system based on backscattering uses the scattering units in the optical fiber as sensing units, and can achieve high spatial resolution and high sensitivity sensing. However, due to the very weak backscattering in the optical fiber, the distributed optical fiber system based on backscattering has a bottleneck in sensing distance (<100Km). The distributed vibration optical fiber sensing system based on the forward transmission of light uses forward light as the detection light, and is more suitable for application in large-scale and ultra-long distance (>150Km) distributed sensing scenarios. In addition, FTDVS uses the same continuous light as the detection light source as optical fiber communication, which enables FTDVS to use existing communication optical cables as the sensing medium.

[0020] In a single-core bidirectional optical fiber transmission, forward light and backward scattering signals are received simultaneously at both ends. The backward scattering received by the photodetector generally includes: Fresnel reflection, Rayleigh backward scattering, and Brillouin scattering. When the optical power injected into the optical fiber is small, the main backward scattering interference in the optical fiber is Rayleigh backward scattering. Therefore, in the case of ultra-long-distance sensing, the forward optical signal attenuates significantly in the optical fiber. In a distributed optical fiber sensing system based on forward transmission, there is usually a phenomenon that the sensing signal cannot be received or the sensing signal is greatly affected by Rayleigh backward scattering. In the prior art, different wavelength lasers and wavelength division multiplexing methods or dual-core bidirectional transmission methods are usually used to avoid the interference of backward scattering, but these two methods increase the hardware cost and the optical fiber laying cost.

[0021] To overcome the above defects, the present invention provides a forward transmission distributed optical fiber sensing system and a vibration positioning method based on AOM frequency shift. By splitting the detection light emitted by the light source into two sub-beams, one sub-beam is frequency-shifted by an acousto-optic modulator and then used as the uplink light to input into the first interferometer structure, and the other sub-beam is used as the downlink light to input into the second interferometer structure. Sensing optical fibers are connected in both the first interferometer structure and the second interferometer structure, and the sensing light in the interferometer structure passes through the sensing optical fiber and then is coupled with the reference light in the optical demodulation element to obtain the interference light. The two interference lights output by the first interferometer and the second interferometer are converted into electrical signals, and the phase time delay difference between the two electrical signals is calculated, and then the vibration position in the sensing optical fiber is determined. The system provided by the present invention uses only a single laser as the sensing light source and uses a single optical fiber for transmission, avoiding the interference of Rayleigh backward scattering and improving the signal-to-noise ratio of the system. Moreover, the optical fiber sensing system proposed by the present invention does not require an additional light source, and at the same time uses only a single optical fiber for transmission, reducing the optical fiber laying cost.

[0022] The following further describes in more detail the forward transmission distributed optical fiber sensing system and the vibration positioning method based on AOM frequency shift provided in this embodiment with reference to the accompanying drawings.

[0023] In the first aspect, the present application also discloses a distributed optical fiber sensing system based on AOM frequency shift, as Figure 1 shown, including: a light source 10, a first coupler 20, an acousto-optic modulator 30, a first interferometer structure 40 connected with a sensing optical fiber, a second interferometer structure 50 connected with a sensing optical fiber, and an information processing component 60.

[0024] The light source 10 is used to emit detection light. In order to achieve high-power stability, long-distance, and ultra-high-resolution signal transmission, a laser light source is adopted for this light source. For example: a narrow linewidth single-frequency laser, a single-wavelength swept-frequency laser, etc. In one implementation, the light source is a 1550 nm narrow linewidth laser with a linewidth < 1 KHz; the detection light is single-frequency continuous light.

[0025] The first coupler 20 is connected to the light source and is configured to split the detection light into a first sub-beam and a second sub-beam with equal amplitudes and opposite directions. The first coupler couples the detection light emitted by the light source into the optical fiber and distributes the detection light coupled into the optical fiber in equal proportion. That is, with a splitting ratio of 1, the detection light is split into two parts with equal amplitudes and opposite propagation directions, obtaining the first sub-beam and the second sub-beam.

[0026] The acousto-optic modulator 30 is connected to the first coupler and is configured to shift the frequency of the first sub-beam. The frequency-shifted first sub-beam is input into the first interferometer structure 40 as the uplink light. The acousto-optic modulator (AOM) is based on the acousto-optic effect, that is, by the oscillating mechanical strain of the sound wave to change the refractive index of some crystal or glass materials, thereby realizing the modulation of light. Further, when the sound wave passes through the acousto-optic medium, a periodic change in the refractive index will be formed within the medium, forming a volume grating structure. When the light beam passes through this medium, the intensity, frequency, and direction of its diffracted light will change with the ultrasonic wave. In this embodiment, when the light of the first sub-beam is input into the acousto-optic modulator, the acousto-optic modulator shifts the frequency of the first sub-beam, so that its frequency increases by a certain value, thereby making the frequencies of the first sub-beam and the second sub-beam different.

[0027] A second interferometer structure 50 is connected between the first coupler, and the second sub-beam output by the first coupler is directly input into the second interferometer structure 50 as the downlink light.

[0028] The first interferometer structure 40 is connected to the acousto-optic modulator 30 and is configured to receive the frequency-shifted first sub-beam and split the first sub-beam into a first sensing light and a second reference light; the second interferometer structure is connected to the first coupler and is configured to receive the second sub-beam and split the second sub-beam into a second sensing light and a second reference light. The first interferometer structure 40 is further configured to obtain a first interference optical signal generated by the interference between the second sensing light transmitted through the sensing optical fiber and the second reference light, and output a first electrical signal corresponding to the first interference optical signal.

[0029] The second interferometer structure 50 is further configured to obtain a second interference optical signal generated by the interference between the first sensing light transmitted through the sensing optical fiber and the first reference light, and output a second electrical signal corresponding to the second interference optical signal.

[0030] The information processing component 60 is configured to receive and eliminate the Rayleigh backscattering noise signals contained in the first electrical signal and the second electrical signal, and determine the vibration positioning result.

[0031] Specifically, in combination with Figure 1As shown, the first coupler 20 is connected to the light source 10, and both the acousto-optic modulator 30 and the second optical interferometer structure are connected to the first coupler 20. The first interferometer structure 40 is connected to the acousto-optic modulator 30 to receive the first split light beam with frequency shifted output by the acousto-optic modulator. The first interferometer structure 40 and the second interferometer structure 50 are respectively connected to the information processing component 60, and the information processing component 60 analyzes the electrical signals output from the first interferometer structure and the second interferometer structure to obtain the perception result of vibration positioning.

[0032] In the distributed optical fiber sensing system disclosed in this embodiment, the acousto-optic modulator is used to shift the frequency of the first split light beam, so that the interference optical signal between the backward Rayleigh scattering of the first split light beam and the reference light of the second split light beam is located in the high-frequency carrier band, thereby separating the forward-transmitted sensing light from the backward Rayleigh scattering light, and thus eliminating the interference of the backward Rayleigh scattering.

[0033] Further, the first interferometer structure includes: a second coupler, a first optical demodulation element, and a first signal detection component; the second interferometer structure includes: a third coupler, a second optical demodulation element, and a second signal detection component; the second coupler receives the first split light beam and divides the first split light beam into a first sensing light and a first reference light.

[0034] The first optical fiber circulator is arranged on the optical path of the first sensing light, and is used to receive the first sensing light, transmit the first sensing light to the sensing optical fiber, input it to the second optical fiber circulator through the sensing optical fiber, and input it to the first optical demodulation element through the second optical fiber circulator, and interfere with the first reference light in the first optical demodulation element to output a first interference optical signal.

[0035] The first signal detection component is used to receive the first interference optical signal and convert the first interference optical signal into a first electrical signal.

[0036] Moreover, the third coupler receives the downlink light and divides the downlink light into a second sensing light and a second reference light.

[0037] The second optical fiber circulator is arranged on the optical path of the second sensing light, and is used to receive the second sensing light, transmit the second sensing light to the sensing optical fiber, input it to the first optical fiber circulator through the sensing optical fiber, and input it to the second optical demodulation element through the first optical fiber circulator, and interfere with the second reference light in the second optical demodulation element to output a second interference optical signal.

[0038] The first signal detection component is used to receive the first interference optical signal and convert the first interference optical signal into a first electrical signal.

[0039] In one implementation, in combination with Figure 2As shown, the first optical demodulation element is a first optical mixer, and the first signal detection component includes: a first optical balanced detector and a second optical balanced detector; the second optical demodulation element is a second optical mixer, and the second signal detection component includes: a third optical balanced detector and a fourth optical balanced detector.

[0040] The first split light beam after frequency shifting by the acousto-optic modulator 30 is coupled by the second coupler 401 and then split into a first sensing light and a first reference light. The first sensing light is input to the second fiber optic circulator 502 through the sensing optical fiber, and then input to the first optical mixer through the second fiber optic circulator 502, where it interferes with the first reference light in the first optical mixer to output a first interference light signal.

[0041] The first optical balanced detector and the second optical balanced detector respectively receive the in-phase component signal and the quadrature component signal of the first interference light signal, and convert the in-phase component signal and the quadrature component signal into a first electrical signal.

[0042] In addition, the second split light beam output by the first coupler 20 is input to the third coupler 501, and is split into a second sensing light and a second reference light by the third coupler 501. The second sensing light is input to the first fiber optic circulator 402 through the sensing optical fiber, and then input to the second optical mixer through the first fiber optic circulator 402, where it interferes with the second reference light in the second optical mixer to output a second interference light signal.

[0043] The third optical balanced detector and the fourth optical balanced detector respectively receive the in-phase component signal and the quadrature component signal of the second interference light signal, and convert the in-phase component signal and the quadrature component signal into a second electrical signal.

[0044] Furthermore, the signal processing component includes: an oscilloscope, a low-pass filter, and a PC terminal connected in sequence.

[0045] The oscilloscope is used to receive the first electrical signal and the second electrical signal, and transmit the first electrical signal and the second electrical signal to the low-pass filter. Since each optical mixer has 4 outputs, namely I+, I-, Q+, and Q-. The phase difference between I+ and Q+ is 90 degrees, and the phase difference between I+ and I- is 180 degrees. Therefore, I+ and I- are connected to the same optical balanced detector, and Q+ and Q- are connected to the same optical balanced detector. Correspondingly, four optical balanced detectors are required for two optical mixers. Therefore, the electrical signals of the four optical balanced detectors received by the oscilloscope are demodulated pairwise for IQ, and the cross-correlation of the demodulated signals can obtain the time delay of the phases in two directions, and the vibration position can be calculated.

[0046] The low-pass filter is used to filter out the Rayleigh backscattering noise signals contained in the first electrical signal and the second electrical signal collected by the oscilloscope; the cut-off frequency of each low-pass filter is lower than the frequency at which the acousto-optic modulator shifts the frequency of the first split light beam.

[0047] The PC terminal is used to determine the sensing and positioning result according to the received first electrical signal and second electrical signal.

[0048] The detection light emitted by the ultra-narrow linewidth laser is split into two paths after passing through the first coupler. One path passes through the acousto-optic modulator for frequency shifting, which is defined as the first split light beam or the uplink light. The uplink light is divided into the first sensing light and the first reference light . Therefore, the reference optical field and the sensing optical field of the first split light beam have optical frequencies of . In addition, the Rayleigh backscattering optical field of the first sensing light also has an optical frequency of , where is the light source frequency, is the frequency shift occurring after AOM modulation. The other path does not pass through the acousto-optic modulator for frequency shifting and is defined as the second split light beam or the downlink light. The second split light beam is divided into the second sensing light and the second reference light . Therefore, its reference optical field and the sensing optical field as well as the Rayleigh backscattering optical field have optical frequencies of . The first sensing light and the second sensing light are transmitted to the sensing optical fiber to detect external disturbances. The first reference light and the second reference light have a transmission length much shorter than that of the first sensing light and the second sensing light, and certain heat insulation and vibration isolation are required to prevent additional interference from causing changes in their phases.

[0049] Combined with Figure 2 and Figure 3a as shown, the first sensing light is defined as the sensing light of the first interferometer structure or the uplink, is defined as the reference light of the second interferometer structure or the uplink, is defined as the corresponding Rayleigh backscattering light, with a direction opposite to and the same frequency. is defined as the sensing light of the second interferometer structure or the downlink, The reference light defined as the second interferometer structure. Defined as The corresponding Rayleigh backscattered light, with a direction opposite to and the same frequency. 、 and The optical frequencies of are 、 and The optical frequencies of . is the frequency of the light source itself, is the frequency increased relative to obtained by the acousto-optic modulator modulation.

[0050] In a specific implementation, the first circulator and the second circulator can be replaced with a coupler. The functions of the first optical mixer and the second optical mixer are to couple the sensing light and the reference light. After the sensing light and the reference light are coupled, interference will occur, and the optical mixer will output the I component (in-phase component) and the Q component (quadrature component) of the interference light. The purpose of using the optical mixer is to obtain the IQ components in order to perform IQ demodulation to obtain the phase caused by the intrusion disturbance.

[0051] Furthermore, since the sensing light and the reference light have the same frequency in the two-path interferometer structure, but the sensing light needs to pass through the sensing optical fiber and then return to the system, the sensing light will be modulated by external disturbances, causing a change in the phase of the sensing light. The transmission distance of the reference light is much shorter than that of the sensing light. At the same time, the reference light generally requires a certain degree of heat insulation and vibration isolation to make the disturbance it receives as small as possible.

[0052] Specifically, the first interferometer structure and the second interferometer structure are Mach-Zehnder interferometer structure, Sagnac interferometer structure or Michelson interferometer structure. The above interferometer structures can all achieve splitting the received split beam into two sensing lights and reference lights with the same frequency, the same vibration direction and a stable phase time delay difference, and then forming interference light from the sensing light and the reference light. The above interferometer structures can be selected accordingly according to different scenarios. For example: when used in the measurement scenarios of pressure or temperature in the atmosphere, the Mach-Zehnder interferometer structure can be used. When used in the measurement of high-sensitivity physical quantities in the fields of aviation, navigation, aerospace, etc., the Sagnac interferometer structure can be used. When used for the positioning of micro-vibrations or the detection of surface defects of optical elements, etc., the Michelson interferometer structure can be used. In this embodiment, the first interferometer structure and the second interferometer structure are equivalent to the Mach-Zehnder interferometer structure, as Figure 4 and Figure 5As shown, the distributed optical fiber sensing system provided in this embodiment is an optical fiber sensing system that combines an equivalent dual - end Mach - Zehnder interferometer structure and AOM frequency shifting, and can be applied to forward - transmission optical fiber acoustic or vibration sensors.

[0053] In one embodiment, the sensing optical fiber is arranged in a ring shape, and the first sensing light will be output to the sensing optical fiber through the first circulator, and pass through the second circulator to interfere with the first reference light in the second optical mixer. Similarly, the second sensing light will be output to the sensing optical fiber through the second circulator, and pass through the first circulator to interfere in the first optical mixer. Since there are two beams propagating in opposite directions in the sensing optical fiber, their backward scattering is in the same direction as the propagation direction of the counter - propagating beam. After passing through the circulator, they interfere with the reference light in the optical mixer, that is, the backward scattering is aliased with the forward - propagating sensing light. In the first optical mixer respectively interfere with and , the interference light intensity and the interference Rayleigh back - scattering noise are expressed by the following formulas: ; (1) ; (2) Assume that the amplitudes of the reference light and the sensing light field are equal, and are the amplitudes of the sensing light field and the Rayleigh back - scattering light field respectively. is the phase transformation generated by the external disturbance in the sensing optical fiber. In the second optical mixer respectively interfere with and , the interference light intensity and the interference Rayleigh back - scattering noise are expressed by the following formulas: ; (3) ; (4) The interference optical signals generated in the first mixer and the second mixer are as shown in Figure 3b and Figure 3c . Since the frequencies of the forward light and the reference light are the same, it can be regarded as homodyne interference. The interference optical signal is near the baseband, and the phase information carried is also near the baseband. The backward scattering has a frequency difference from the reference light in the other direction. Therefore, the backward scattering and the reference light are regarded as heterodyne interference, and the interference result has a carrier with a frequency of , and the phase information carried by the backward scattering is near the carrier.

[0054] After the interference optical signal is converted into an electrical signal by an optical balance detector and received by an oscilloscope, the electrical signal received by the oscilloscope is filtered through a low-pass filter, and then the Rayleigh backscattering contained in the electrical signal can be filtered out. Figure 7 It is the power spectral density PSD of the interference optical signal at the end of the 20Km sensing optical fiber. It can be seen that there are two peaks near the baseband and 200MHz, and the difference between the two is 22.62dB, indicating that in the distributed vibration optical fiber sensing system with a ring structure, one of the paths can successfully separate the forward-transmitted sensing light from the backscattering after frequency shifting by the AOM, thus eliminating the interference of the backscattering.

[0055] In another implementation, the first optical demodulation element is a first 3×3 coupler, and the first signal detection component is three photodetectors; the second optical demodulation element is a second 3×3 coupler, and the second signal detection component is three photodetectors. The received signal is demodulated by using a differential cross-multiplication algorithm or an arctangent algorithm.

[0056] Specifically, the sensing optical fiber is a single-mode optical fiber. In this embodiment, since the used sensing optical fiber is a single-mode optical fiber and the connecting lines between the components are also single-mode optical fibers, while avoiding the interference of Rayleigh backscattering, no additional light source is added, and the laying cost of the system is reduced.

[0057] Further, a piezoelectric ceramic is connected to the sensing optical fiber. The piezoelectric ceramic is used to output an analog vibration signal. The piezoelectric ceramic can generate corresponding expansion according to the input driving voltage signal, and the optical fiber can be wound around the piezoelectric ceramic to obtain an analog vibration.

[0058] In a second aspect, the present application provides a vibration positioning method for a distributed optical fiber sensing system based on AOM frequency shifting, which is applied to the distributed optical fiber sensing system as Figure 9 shown, and the vibration positioning method includes: Step S1: The detection light emitted by the light source is input to the first coupler, and is split into a first split light beam and a second split light beam by the first coupler.

[0059] Step S2: The first split light beam is input to the acousto-optic modulator, and is frequency-shifted by the acousto-optic modulator, and the frequency-shifted first split light beam is input to the first interferometer structure to obtain a first interference optical signal output by the first interferometer structure; the second split light beam is input to the second interferometer structure to obtain a second interference optical signal output by the second interferometer structure.

[0060] Step S3: The vibration position is located according to the first interference optical signal and the second interference optical signal.

[0061] Specifically, the step of locating the vibration position according to the first interference optical signal and the second interference optical signal includes: Converting the first interference optical signal and the second interference optical signal into a first electrical signal and a second electrical signal respectively.

[0062] Inputting the first electrical signal and the second electrical signal into a low-pass filter, filtering out the Rayleigh backscattering noise signal through the low-pass filter, and performing phase demodulation on the electrical signal after filtering out the noise signal to obtain phase information.

[0063] Calculating the time delay difference between the first sub-beam and the second sub-beam according to the phase demodulation information, and locating the vibration position according to the time delay difference.

[0064] When vibration occurs on a single-mode optical fiber, the refractive index, length, and diameter in the optical fiber will all change, which results in a change in the signal phase. This means that the forward light in the optical fiber will be modulated by the strain effect, elasto-optic effect, and Poisson effect. The modulated phase Can be expressed as: ;(5) Where Is the length of the single-mode optical fiber, n is the refractive index of the optical fiber core, Is the propagation coefficient of the single-mode optical fiber. In formula (5) , And Respectively represent the phase change values caused by the strain effect, elasto-optic effect, and Poisson effect. They can be respectively expressed by the following formulas: ;(6) ;(7) ;(8) Where, Is the strain tensor along the longitudinal direction of the optical fiber, Is the Poisson coefficient in the optical fiber, And Respectively represent the elasto-optic coefficients, Is the wave number, Is the axial pressure, Represents Young's modulus of elasticity. In the phase change generated by vibration, Only accounts for 3%, so the phase change is approximately equal to: ;(9) It can be seen from the above formula that the phase change is proportional to the longitudinal pressure . Based on this criterion, vibration sensing can be achieved by demodulating the phase change of the interference light.

[0065] In the present invention, since an acousto-optic modulator (AOM) is used to perform frequency shift processing on the up-link optical signal, Rayleigh backscattering noise can be eliminated by passing the optical intensity signals received by four optical balanced detectors through a low-pass filter. The cut-off frequency of the low-pass filter should be lower than the frequency shift generated by the AOM. 。

[0066] As can be seen from Equations (1) and (2), the interference optical signal can be expressed as , ignoring the DC component can be expressed as 。The interference optical intensity can obtain in-phase components with equal amplitudes and a 90° phase difference and quadrature components through a 90° optical mixer. The phase change of the external disturbance can be obtained using the I / Q demodulation algorithm: ; (10) A piezoelectric ceramic (PZT, Piezoelectric Transducer) is connected to the sensing optical fiber, and a sinusoidal signal with a frequency of 500 Hz and an amplitude of 4.5 V is input to it to simulate an external single-frequency vibration. The interference optical signal received by the BPD optical balanced detector and the corresponding demodulated signal Figure 8 are shown as

[0067] The FTDVS proposed in the present invention consists of a dual Mach-Zehnder interferometer (DMZI) structure. The key step in locating vibrations in the DMZI structure is to calculate the time delay between two interference signals modulated by the disturbance, because the location algorithm highly depends on the time delay. Since there is a non-linear relationship between the optical intensity signal at the receiving end of the Mach-Zehnder interferometer (MZI) and the external vibration signal, and there is a linear relationship between the external vibration signal and the phase change in the optical field caused by the external vibration, the location algorithm based on the DMZI structure usually needs to demodulate the optical intensity signal to obtain , and then use the cross-correlation algorithm to calculate the phase time delay difference between the two receiving ends of the MZI to obtain the location where the vibration occurs. The specific process is as follows: Assume that the vibration occurs at position Z in the sensing optical fiber, the length of the sensing optical fiber is L, the refractive index of the optical fiber is n, and the speed of light in vacuum is c. Since the sensing optical fields and propagate in opposite directions in the sensing optical fiber, the relative time for the phase change caused by the vibration intrusion to reach the two optical mixers is different. Therefore, Equations (1) and (2) can be rewritten as the following equations: ; (11) where , Let be the phase time delay difference of the optical intensity signals at the two MZI receivers, then can be expressed as: ; (12) Thus, the vibration position Z can be obtained from the following formula: ; (13) The phase and are obtained by the above I / Q demodulation algorithm. By correlation analysis, the time-domain similarity degree of the two signals and is compared, so as to calculate . The cross-correlation function can be expressed as: ; (14) The cross-correlation function has an obvious maximum value at . Therefore, from the peak position of the relationship curve, the time delay value can be determined, that is, the maximum value is determined by the peak-seeking algorithm, and the index at which this value is located is the time delay between the two phases. The actual operation process is as Figure 6 shown. After the interference optical signals received by the 4 photoelectric balanced detectors are transmitted into the computer and pass through the low-pass filter, IQ demodulation is performed pairwise, and the time delay of the phases in two directions can be obtained by performing cross-correlation on the demodulated signals. The vibration position Z can be calculated through formula (13).

[0068] The present invention provides a distributed optical fiber sensing system and a vibration positioning method based on AOM frequency shift, constructs a sensing system of a forward transmission optical fiber acoustic / vibration sensor based on the demodulation of a dual-ended Mach-Zehnder interferometer and AOM frequency shift. Among them, first, one optical path in the AOM dual optical path is used for frequency shift, and then the Rayleigh backscattering noise is eliminated, so as to realize a long-distance sensing path without an optical amplifier. The optical fiber sensing system and the vibration positioning method provided by the present invention have a higher signal-to-noise ratio, a longer sensing distance, and the spatial resolution is not directly related to the sensing range, thus allowing separate optimization of the sensing system and being easier to install and maintain.

[0069] After considering the specification and practicing the invention disclosed herein, those skilled in the art will readily conceive of other embodiments of the present invention. The present invention is intended to cover any variations, uses, or adaptations of the present invention, which follow the general principles of the present invention and include the common general knowledge or conventional technical means in the technical field not disclosed in this application. The specification and examples are only regarded as exemplary, and the true scope and spirit of the present invention are pointed out by the following claims.

[0070] In the description of this specification, the description with reference to terms such as "one embodiment", "some embodiments", "example", "specific example", or "some examples", etc. means that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of this application. In this specification, the schematic expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in any one or N embodiments or examples in a suitable manner. In addition, without contradiction, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples.

[0071] It can be understood that the above embodiments are exemplary and should not be construed as limiting this application. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of this application.

Claims

1. A distributed optical fiber sensing system based on AOM frequency shift, characterized in that: include: A light source, a first coupler, an acousto-optic modulator, a first interferometer structure connected to a sensing optical fiber, a second interferometer structure connected to a sensing optical fiber, and an information processing component; The light source is used to emit detection light; The first coupler is connected to the light source and is used to split the detection light into a first sub-beam and a second sub-beam of equal amplitude and opposite direction; The acousto-optic modulator is connected to the first coupler and is used to shift the frequency of the first split light beam; The first interferometer structure is connected to the acousto-optic modulator and is used to receive the first frequency-shifted sub-beam and split the first sub-beam into a first sensing light and a second reference light; the second interferometer structure is connected to the first coupler and is used to receive the second sub-beam and split the second sub-beam into a second sensing light and a second reference light; The first interferometer structure is further used to obtain a first interference light signal generated by the interference between the second sensing light and the second reference light after being transmitted through the sensing optical fiber, and output a first electrical signal corresponding to the first interference light signal; The second interferometer structure is further used to obtain a second interference light signal generated by the first sensing light interfering with the first reference light after being transmitted through the sensing optical fiber, and output a second electrical signal corresponding to the second interference light signal; The information processing component is used to receive and eliminate the Rayleigh backscatter noise signal contained in the first electrical signal and the second electrical signal to determine the vibration positioning result.

2. The distributed optical fiber sensing system based on AOM frequency shift according to claim 1, characterized in that: The first interferometer structure and the second interferometer structure are Mach-Zehnder interferometer structures, Sagner interferometer structures or Michelson interferometer structures.

3. The distributed optical fiber sensing system based on AOM frequency shift according to claim 1, characterized in that: The first interferometer structure includes: a second coupler, a first optical fiber circulator, a first optical demodulation element and a first signal detection component; the second interferometer structure includes: a third coupler, a second optical fiber circulator, a second optical demodulation element and a second signal detection component; The second coupler receives the first sub-beam and splits the first sub-beam into a first sensing light and a first reference light; The first optical fiber circulator is arranged on the optical path of the first sensing light, and is used to receive the first sensing light, transmit the first sensing light to the sensing optical fiber, and input the first sensing light to the second optical fiber circulator through the sensing optical fiber, and input the first sensing light to the first optical demodulation element through the second optical fiber circulator, interfere with the first reference light in the first optical demodulation element, and output a first interference light signal; The first signal detection component is used to receive a first interference light signal and convert the first interference light signal into a first electrical signal; and, the third coupler receives the second sub-beam and splits the second sub-beam into a second sensing light and a second reference light; The second optical fiber circulator is arranged on the optical path of the second sensing light, and is used to receive the second sensing light, transmit the second sensing light to the sensing optical fiber, and input the second sensing light to the first optical fiber circulator through the sensing optical fiber, and input the second sensing light to the second optical demodulation element through the first optical fiber circulator, interfere with the second reference light in the second optical demodulation element, and output a second interference light signal; The second signal detection component is used to receive a second interference light signal and convert the second interference light signal into a second electrical signal.

4. The distributed optical fiber sensing system based on AOM frequency shift according to claim 3 is characterized in that: The first optical demodulation element is a first optical mixer, and the first signal detection component includes: a first optical balance detector and a second optical balance detector; the second optical demodulation element is a second optical mixer, and the second signal detection component includes: a third optical balance detector and a fourth optical balance detector; The first sensing light is input to the second optical fiber circulator through the sensing optical fiber, and is input to the first optical mixer through the second optical fiber circulator, where it interferes with the first reference light and outputs a first interference light signal; The first optical balance detector and the second optical balance detector respectively receive the in-phase component signal and the orthogonal component signal of the first interference optical signal, and convert the in-phase component signal and the orthogonal component signal into a first electrical signal; And, the second sensing light is input to the first optical fiber circulator through the sensing optical fiber, and is input to the second optical mixer through the first optical fiber circulator, and interferes with the second reference light in the second optical mixer to output a second interference light signal; The third optical balanced detector and the fourth optical balanced detector respectively receive the in-phase component signal and the orthogonal component signal of the second interference optical signal, and convert the in-phase component signal and the orthogonal component signal into a second electrical signal.

5. The distributed optical fiber sensing system based on AOM frequency shift according to claim 3, characterized in that: The first optical demodulation element is a first 3×3 coupler, and the first signal detection component is three photodetectors; the second optical demodulation element is a second 3×3 coupler, and the second signal detection component is three photodetectors.

6. The distributed optical fiber sensing system based on AOM frequency shift according to claim 4 or 5, characterized in that: The signal processing component includes: an oscilloscope, a low-pass filter and a PC terminal; The oscilloscope is used to receive the first electrical signal and the second electrical signal, and transmit the first electrical signal and the second electrical signal to a low-pass filter; The low-pass filter is used to filter out the Rayleigh backscattering noise signal contained in the first electrical signal and the second electrical signal collected by the oscilloscope; the cutoff frequency of each low-pass filter is lower than the frequency at which the acousto-optic modulator shifts the frequency of the first sub-beam; The PC end is used to determine the sensing positioning result according to the received first electrical signal and the second electrical signal.

7. The distributed optical fiber sensing system based on AOM frequency shift according to claim 1, characterized in that: The light source is a 1550nm narrow linewidth laser with a linewidth less than 1KHz; the detection light is single-frequency continuous light; and the sensing optical fiber is a single-mode optical fiber.

8. The distributed optical fiber sensing system based on AOM frequency shift according to claim 1, characterized in that: The sensing optical fiber is connected with a piezoelectric ceramic; The piezoelectric ceramic is used to output analog vibration signals.

9. A vibration positioning method of a distributed optical fiber sensing system based on AOM frequency shift, characterized in that: Applied to the distributed optical fiber sensing system according to any one of claims 1 to 8; the vibration positioning method comprises: The detection light emitted by the light source is input to the first coupler and is divided into a first split light beam and a second split light beam by the first coupler; The first split light beam is input to an acousto-optic modulator, subjected to frequency shift processing by the acousto-optic modulator, and the first split light beam after frequency shift processing is input to a first interferometer structure to obtain a first interference light signal output by the first interferometer structure; the second split light beam is input to a second interferometer structure to obtain a second interference light signal output by the second interferometer structure; The vibration position is located according to the first interference light signal and the second interference light signal.

10. The vibration positioning method according to claim 9, characterized in that: The step of locating the vibration position according to the first interference light signal and the second interference light signal comprises: Converting the first interference light signal and the second interference light signal into a first electrical signal and a second electrical signal respectively; Inputting the first electrical signal and the second electrical signal into a low-pass filter, filtering out the Rayleigh backscattering noise signal through the low-pass filter, and performing phase demodulation on the electrical signal after the noise signal is filtered out to obtain phase information; The time delay difference between the first sub-beam and the second sub-beam is calculated according to the phase demodulation information, and the vibration position is located according to the time delay difference.

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