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

Abstract

This invention provides a distributed optical fiber sensing system and vibration localization method based on AOM frequency shifting, belonging to the field of optical fiber sensing technology. The system splits the probe light emitted from a light source into two beams. One beam is frequency-shifted and input into a first interferometer structure, while the other beam is input into a second interferometer structure. Both beams are coupled in the interferometer structures to obtain a sensing light and a reference light. The sensing light passes through the sensing fiber and is coupled with the reference light in an optical demodulation element to obtain interference light. The two interference lights are converted into electrical signals, and Rayleigh backscattering noise is eliminated using signal processing components. The vibration location in the sensing fiber is determined based on the phase delay difference between the two electrical signals. This invention avoids Rayleigh backscattering interference, increases the sensing distance, improves the system's signal-to-noise ratio, eliminates the need for an additional light source, and uses only a single optical fiber for transmission, thus reducing costs.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, and more particularly to a distributed fiber optic sensing system based on AOM frequency shifting. Background Technology

[0002] Distributed fiber optic sensing systems typically utilize the scattering effect in optical fibers to detect physical quantities distributed along the fiber. During signal transmission, both ends of the fiber simultaneously receive forward optical signals and backscattered information. When the optical power injected into the fiber is less than a certain value, the backscattering interference in the fiber is mainly Rayleigh backscattering. Therefore, in ultra-long-distance sensing, the forward optical signal attenuates significantly in the fiber. Distributed fiber optic sensing systems based on forward transmission often experience either a failure to receive the sensing signal or a significant impact from Rayleigh backscattering, leading to system failure or low measurement accuracy.

[0003] Therefore, the existing technology needs further improvement. Summary of the Invention

[0004] In view of the shortcomings of the prior art, the purpose of this invention is to provide a distributed optical fiber sensing system and vibration localization method based on AOM frequency shifting, which separates the forward-transmitted sensing light from the backscattered light by AOM frequency shifting, eliminates the interference of backscattering, and improves the accuracy of vibration localization.

[0005] In a first aspect, this application discloses a distributed optical fiber sensing system based on AOM frequency shifting, 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 a sensing optical fiber, and a signal processing component.

[0006] The light source is used to emit detection light;

[0007] The first coupler, connected to the light source, is used to split the probe light into a first beam and a second beam with equal amplitude and opposite direction;

[0008] The acousto-optic modulator is connected to the first coupler and is used to frequency shift the first split beam.

[0009] The first interferometer structure is connected to the acousto-optic modulator and is used to receive the frequency-shifted first beam and split the first beam into a first sensing beam and a second reference beam; the second interferometer structure is connected to the first coupler and is used to receive the second beam and split the second beam into a second sensing beam and a second reference beam.

[0010] The first interferometer structure is also used to obtain a first interference light signal generated by the interference between the second sensing light and the second reference light after the second sensing light is transmitted through the sensing optical fiber, and to output a first electrical signal corresponding to the first interference light signal.

[0011] The second interferometer structure is also used to obtain a second interference light signal generated by the interference between the first sensing light and the first reference light after the first sensing light is transmitted through the sensing optical fiber, and to output a second electrical signal corresponding to the second interference light signal;

[0012] The information processing component is used to receive and eliminate Rayleigh backscatter noise signals contained in the first and second electrical signals to determine the vibration positioning result.

[0013] Optionally, the first interferometer structure and the second interferometer structure are Mach-Zehnder interferometer structures, Sagna interferometer structures, or Michelson interferometer structures.

[0014] Optionally, the first interferometer structure includes: a second coupler, a first fiber optic circulator, a first optical demodulation element, and a first signal detection component; the second interferometer structure includes: a third coupler, a second fiber optic circulator, a second optical demodulation element, and a second signal detection component.

[0015] The second coupler receives the first split beam and splits it into a first sensing beam and a first reference beam;

[0016] The first fiber optic circulator is disposed in 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 fiber, and input it to the second fiber optic circulator through the sensing fiber, and input it to the first optical demodulation element through the second fiber optic circulator, where it interferes with the first reference light and outputs the first interference light signal.

[0017] 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;

[0018] Furthermore, the third coupler receives the second split beam and splits the second split beam into a second sensing beam and a second reference beam;

[0019] The second fiber optic circulator is disposed in 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 fiber, and input it to the first fiber optic circulator through the sensing fiber, and input it to the second optical demodulation element through the first fiber optic circulator. In the second optical demodulation element, it interferes with the second reference light and outputs the second interference light signal.

[0020] The second signal detection component is used to receive the second interference optical signal and convert the second interference optical signal into a second electrical signal.

[0021] Optionally, 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;

[0022] The first sensing light is input to the second fiber optic circulator via the sensing fiber, and then input to the first optical mixer via the second fiber optic circulator. In the first optical mixer, it interferes with the first reference light and outputs the first interference light signal.

[0023] The first optical balance detector and the second optical balance 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;

[0024] Furthermore, the second sensing light is input to the first fiber optic circulator via the sensing fiber, and then input to the second optical mixer via the first fiber optic circulator. In the second optical mixer, it interferes with the second reference light and outputs the second interference light signal.

[0025] The third and fourth optical balance detectors respectively receive the in-phase and quadrature component signals of the second interference light signal and convert the in-phase and quadrature component signals into a second electrical signal.

[0026] 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.

[0027] Optionally, the signal processing components include: an oscilloscope, a low-pass filter, and a PC.

[0028] The oscilloscope is used to receive the first electrical signal and the second electrical signal, and to transmit the first electrical signal and the second electrical signal to a low-pass filter.

[0029] The low-pass filter is used to filter out Rayleigh backscatter noise signals contained in the first and second electrical signals acquired by the oscilloscope; the cutoff frequency of each of the low-pass filters is lower than the frequency at which the acousto-optic modulator shifts the first beam splitter.

[0030] The PC terminal is used to determine the sensing and positioning results based on the received first and second electrical signals.

[0031] Optionally, the light source is a 1550nm narrow linewidth laser with a linewidth of less than 1kHz; the probe light is a single-frequency continuous light; and the sensing fiber is a single-mode fiber.

[0032] Optionally, a piezoelectric ceramic is connected to the sensing optical fiber; the piezoelectric ceramic is used to output a simulated vibration signal.

[0033] Secondly, this application provides a vibration localization method for a distributed optical fiber sensing system based on AOM frequency shifting, wherein the method is applied to the distributed optical fiber sensing system; the vibration localization method includes:

[0034] The probe light emitted by the light source is input to the first coupler, and is split into a first beam and a second beam by the first coupler;

[0035] The first beam splitter is input to the acousto-optic modulator, where it undergoes frequency shifting. The frequency-shifted first beam splitter is then input to the first interferometer structure to obtain the first interference light signal output by the first interferometer structure. The second beam splitter is input to the second interferometer structure to obtain the second interference light signal output by the second interferometer structure.

[0036] The vibration position is determined by locating the first and second interference light signals.

[0037] Optionally, the step of locating the vibration position based on the first interference light signal and the second interference light signal includes:

[0038] The first interference optical signal and the second interference optical signal are converted into a first electrical signal and a second electrical signal, respectively.

[0039] The first and second electrical signals are input to a low-pass filter, which filters out Rayleigh backscatter noise signals. The electrical signals after the noise signals are filtered out are then demodulated to obtain phase information.

[0040] The time delay difference between the first and second beams is calculated based on the phase demodulation information, and the vibration location is determined based on the time delay difference.

[0041] Beneficial effects:

[0042] This invention provides a distributed fiber optic sensing system and vibration localization method based on AOM frequency shifting. It constructs a sensing system based on a forward-transmission fiber optic acoustic / vibration sensor using a dual-ended Mach-Zehnder interferometer for demodulation and AOM frequency shifting. First, one optical path in the AOM dual-path is used for frequency shifting, and then Rayleigh backscattering noise is eliminated, thus achieving a long-distance sensing path without the need for an optical amplifier. The fiber optic sensing system and vibration localization method provided by this invention have a higher signal-to-noise ratio, a longer sensing distance, and spatial resolution is not directly related to the sensing range, allowing for individual optimization of the sensing system and making it easier to install and maintain. Attached Figure Description

[0043] Figure 1 This is a schematic diagram of the structure of the distributed optical fiber sensing system provided by the present invention;

[0044] Figure 2 This is a schematic diagram of a specific application embodiment of the distributed optical fiber sensing system provided by the present invention;

[0045] Figure 3a This is a schematic diagram of the signal transmission direction in the distributed optical fiber sensing system based on AOM frequency shifting according to the present invention.

[0046] Figure 3b This is a schematic diagram illustrating the generation of the interference light signal in the first mixer of the sensing system of the present invention;

[0047] Figure 3c This is a schematic diagram illustrating the generation of the interference light signal in the second mixer of the sensing system of the present invention;

[0048] Figure 4 This is a schematic diagram of the first Mach-Zehnder structure equivalent to the system of the present invention;

[0049] Figure 5 This is a schematic diagram of the equivalent second Mach-Zehnder structure of the system of the present invention;

[0050] Figure 6 This is a flowchart illustrating how the vibration location is determined based on an electrical signal in an embodiment of the present invention.

[0051] Figure 7 This is a graph showing the relationship between the power spectral density and frequency of the interference optical signal at the end of the sensing fiber in this embodiment of the invention.

[0052] Figure 8 This is a graph showing the interference optical signal and the corresponding demodulated phase signal in an embodiment of the present invention;

[0053] Figure 9 This is a flowchart of the vibration positioning method in an embodiment of the present invention. Detailed Implementation

[0054] To make the objectives, technical solutions, and advantages of this invention clearer and more explicit, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0055] Fiber optic sensing technology is a detection method for studying the interaction between light and matter, and can be used to measure many parameters in physics, chemistry, biology, and other fields. Distributed fiber optic vibration sensing technology is a novel vibration sensing and detection technology that integrates signal sensing and transmission. It can achieve continuous distributed measurement of vibration events along the fiber optic cable by directly sensing and detecting optical wave signals.

[0056] Distributed fiber optic sensing technology can be broadly categorized into two structures based on sensing mechanisms: one is a backscattering-based distributed fiber optic sensing system, such as optical time-domain reflectometry (OTDR) systems and optical frequency-domain reflectometry (FDR) systems; the other is a forward-propagation-based distributed vibration fiber optic sensing system. Backscattering-based systems utilize scattering units within the fiber as sensing units, achieving high spatial resolution and high sensitivity. However, due to the extremely weak backscattering in fiber optics, these systems face a limitation in sensing distance (<100 km). Forward-propagation-based distributed vibration fiber optic sensing systems use forward light as the probe light, making them more suitable for large-scale, ultra-long-distance (>150 km) distributed sensing applications. Furthermore, FTDVS uses the same continuous light source as fiber optic communication as its probe light source, allowing it to utilize existing communication optical cables as the sensing medium.

[0057] In single-core bidirectional fiber optic transmission, both ends simultaneously receive forward and backscattered signals. The backscattering received by the photodetector is generally Fresnel reflection, Rayleigh backscattering, and Brillouin scattering. When the optical power injected into the fiber is low, the backscattering interference in the fiber is mainly Rayleigh backscattering. Therefore, in ultra-long-distance sensing, the forward light signal attenuates significantly in the fiber. Distributed fiber optic sensing systems based on forward transmission often experience the phenomenon of not receiving sensing signals or the sensing signals being significantly affected by Rayleigh backscattering. Existing technologies typically use lasers of different wavelengths and wavelength division multiplexing (WDM) or dual-core bidirectional transmission to avoid backscattering interference, but these two methods increase hardware and fiber optic laying costs.

[0058] To overcome the aforementioned shortcomings, this invention provides a forward transmission distributed fiber optic sensing system and vibration localization method based on AOM frequency shifting. The system splits the probe light emitted from the light source into two beams. One beam is frequency-shifted by an acousto-optic modulator and used as the uplink light input to a first interferometer structure. The other beam is used as the downlink light input to a second interferometer structure. Both the first and second interferometer structures are connected to sensing fibers. The sensing light in the interferometer structure is coupled to a reference light at a demodulation element after passing through the sensing fiber, thus obtaining interference light. The two interference beams output from the first and second interferometers are converted into electrical signals. The phase delay difference between the two electrical signals is calculated to determine the vibration location in the sensing fiber. The system provided by this invention uses only a single laser as the sensing light source and a single fiber for transmission, avoiding Rayleigh backscattering interference and improving the system's signal-to-noise ratio. Furthermore, the fiber optic sensing system proposed in this invention does not require an additional light source and uses only a single fiber for transmission, reducing fiber optic laying costs.

[0059] The following description, in conjunction with the accompanying drawings, provides a more detailed account of the forward transmission distributed optical fiber sensing system and vibration localization method based on AOM frequency shifting provided in this embodiment.

[0060] Firstly, this application also discloses a distributed optical fiber sensing system based on AOM frequency shifting, such as... Figure 1 It includes: a light source 10, a first coupler 20, an acousto-optic modulator 30, a first interferometer structure 40 connected to a sensing optical fiber, a second interferometer structure 50 connected to a sensing optical fiber, and an information processing component 60.

[0061] The light source 10 is used to emit probe light. To achieve high power stability, long distance, and ultra-high resolution signal transmission, the light source is a laser source. For example, a narrow-linewidth single-frequency laser, a single-wavelength swept-frequency laser, etc. In one implementation, the light source is a 1550nm narrow-linewidth laser with a linewidth <1kHz; the probe light is a single-frequency continuous-wave light.

[0062] The first coupler 20, connected to the light source, is used to split the probe light into a first beam and a second beam with equal amplitude and opposite direction. The first coupler couples the probe light emitted from the light source into the optical fiber and distributes the probe light coupled into the optical fiber proportionally. That is, with a splitting ratio of 1, the probe light is split into two parts with equal amplitude and opposite propagation direction to obtain the first beam and the second beam.

[0063] The acousto-optic modulator 30, connected to the first coupler, is used to frequency-shift the first split beam. The frequency-shifted first split beam is then input as the uplink light to the first interferometer structure 40. The acousto-optic modulator is based on the acousto-optic effect, that is, it modulates light by changing the refractive index of certain crystal or glass materials through the mechanical strain of sound wave oscillations. Furthermore, when sound waves pass through an acousto-optic medium, a periodic change in refractive index is formed within the medium, creating a volume grating structure. When the beam passes through this medium, the intensity, frequency, and direction of its diffracted light will change with the ultrasound. In this embodiment, when the light from the first split beam is input to the acousto-optic modulator, the modulator frequency-shifts the first split beam, increasing its frequency by a certain value, thus making the frequencies of the first and second split beams different.

[0064] The second interferometer structure 50 is connected to the first coupler, and the second beam output from the first coupler is directly input to the second interferometer structure 50 as the downlink light.

[0065] The first interferometer structure 40 is connected to the acousto-optic modulator 30 and is used to receive the frequency-shifted first beam and split the first beam into a first sensing beam and a second reference beam; the second interferometer structure is connected to the first coupler and is used to receive the second beam and split the second beam into a second sensing beam and a second reference beam.

[0066] The first interferometer structure 40 is also used to obtain a first interference light signal generated by the interference between the second sensing light and the second reference light after the second sensing light is transmitted through the sensing optical fiber, and to output a first electrical signal corresponding to the first interference light signal.

[0067] The second interferometer structure 50 is also used to obtain a second interference light signal generated by the interference between the first sensing light and the first reference light after the first sensing light is transmitted through the sensing optical fiber, and to output a second electrical signal corresponding to the second interference light signal.

[0068] The information processing component 60 is used to receive and eliminate Rayleigh backscatter noise signals contained in the first and second electrical signals to determine the vibration positioning result.

[0069] Specifically, in combination 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 and receives the first split beam output by the acousto-optic modulator after frequency shifting. The first interferometer structure 40 and the second interferometer structure 50 are respectively connected to the information processing component 60. The information processing component 60 analyzes the electrical signals output from the first and second interferometer structures to obtain the vibration localization sensing results.

[0070] In the distributed optical fiber sensing system disclosed in this embodiment, the first split beam is frequency-shifted by an acousto-optic modulator so that the interference light signal of the backscattered Rayleigh light of the first split beam and the reference light of the second split beam is located in the high-frequency carrier band, thereby separating the forward-transmitted sensing light from the backscattered Rayleigh light and eliminating the interference of backscattered Rayleigh light.

[0071] Furthermore, 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 beam and splits the first split beam into a first sensing beam and a first reference beam.

[0072] The first fiber optic circulator is disposed in 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 fiber, and input it to the second fiber optic circulator through the sensing fiber, and input it to the first optical demodulation element through the second fiber optic circulator, where it interferes with the first reference light and outputs the first interference light signal.

[0073] 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.

[0074] Furthermore, the third coupler receives the downlink light and splits the downlink light into a second sensing light and a second reference light.

[0075] The second fiber optic circulator is disposed in 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 fiber, and input it to the first fiber optic circulator through the sensing fiber, and input it to the second optical demodulation element through the first fiber optic circulator. In the second optical demodulation element, it interferes with the second reference light and outputs the second interference light signal.

[0076] 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.

[0077] In one implementation, combined with Figure 2As shown, 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.

[0078] After being frequency-shifted by the acousto-optic modulator 30, the first beam is coupled by the second coupler 401 and split into a first sensing beam and a first reference beam. The first sensing beam is input to the second fiber optic circulator 502 through the sensing fiber and then input to the first optical mixer through the second fiber optic circulator 502. In the first optical mixer, it interferes with the first reference beam and outputs a first interference light signal.

[0079] The first optical balance detector and the second optical balance 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 the first electrical signal.

[0080] Furthermore, the second split beam output from the first coupler 20 is input to the third coupler 501, and is split into a second sensing beam and a second reference beam by the third coupler 501. The second sensing beam is input to the first fiber optic circulator 402 through the sensing fiber, and is input to the second optical mixer through the first fiber optic circulator 402. In the second optical mixer, it interferes with the second reference beam and outputs a second interference light signal.

[0081] The third and fourth optical balance detectors respectively receive the in-phase and quadrature component signals of the second interference light signal and convert the in-phase and quadrature component signals into a second electrical signal.

[0082] Furthermore, the signal processing components include: a connected oscilloscope, a low-pass filter, and a PC.

[0083] The oscilloscope receives the first and second electrical signals and transmits them to a low-pass filter. Each optical mixer has four outputs: I+, I-, Q+, and Q-. I+ and Q+ are 90 degrees out of phase, and I+ and I- are 180 degrees out of phase. 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, two optical mixers require four optical balanced detectors. Therefore, the electrical signals received by the oscilloscope from the four optical balanced detectors are demodulated pairwise using IQ modulation. The cross-correlation of the demodulated signals yields the time delay of the phase in both directions, allowing the calculation of the vibration position.

[0084] The low-pass filter is used to filter out Rayleigh backscatter noise signals contained in the first and second electrical signals acquired by the oscilloscope; the cutoff frequency of each of the low-pass filters is lower than the frequency at which the acousto-optic modulator shifts the first beam.

[0085] The PC terminal is used to determine the sensing and positioning results based on the received first and second electrical signals.

[0086] The probe beam emitted by the ultra-narrow linewidth laser is split into two paths after passing through the first coupler. One path is frequency-shifted by an acousto-optic modulator and is defined as the first split beam or uplink beam. The uplink beam is split into the first sensing beam by the second coupler. and the first reference light Therefore, the reference light field of the first beam and sensing light field optical frequency is In addition, the first light sensor Rayleigh backscattered light field The optical frequency is also for ,in For the frequency of the light source, This refers to the frequency shift that occurs after AOM modulation. The other path, which does not pass through the acousto-optic modulator, undergoes frequency shifting and is defined as the second split beam or downlink beam. The second split beam is then split into the second sensing beam via a third coupler. Second reference light Therefore, its reference light field and sensing light field as well as Rayleigh backscattered light field The optical frequencies are all First light sensor Second photosensitive External disturbances are detected by transmitting the signal to the sensing fiber optic cable. First reference light. Second reference light The transmission length is much shorter than that of the first and second sensor light, and certain heat and vibration insulation is required to prevent additional interference from causing changes in its phase.

[0087] Combination Figure 2 and Figure 3a As shown, the first light sensor Defined as the sensing light of the first interferometer structure or the uplink. Defined as the reference light for the second interferometer structure or the uplink. Defined as The corresponding Rayleigh backscattered light, the direction of which is the same as Conversely, they have the same frequency. Defined as the sensing light of the second interferometer structure or downlink. The reference light is defined as the second interferometer structure. Defined as The corresponding Rayleigh backscattered light, the direction of which is the same as Conversely, they have the same frequency. , and optical frequency is ; , and optical frequency is . The frequency of the light source itself, The result obtained by modulation by an acousto-optic modulator relative to Increased frequency.

[0088] In practical implementation, the first and second circulators can be replaced with couplers. The function of the first and second optical mixers is to couple the sensing light and the reference light. After coupling, interference will occur between the sensing light and the reference light. 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 an optical mixer is to obtain the IQ components in order to perform IQ demodulation to obtain the phase caused by the intrusion disturbance.

[0089] Furthermore, since the sensing light and reference light have the same frequency in the two-way interferometer structure, but the sensing light needs to return to the system after passing through the sensing fiber, 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, and the reference light generally requires certain heat and vibration isolation to minimize the disturbances it is subjected to.

[0090] Specifically, the first and second interferometer structures are Mach-Zehnder interferometers, Sagna interferometers, or Michelson interferometers. All of these interferometer structures can split the received beam into two paths—a sensing beam and a reference beam—with the same frequency, the same vibration direction, and a stable phase delay difference, thereby forming interference light. The interferometer structure can be selected according to different scenarios. For example, a Mach-Zehnder interferometer structure can be used for measuring atmospheric pressure or temperature. A Sagna interferometer structure can be used for measuring highly sensitive physical quantities in fields such as aviation, navigation, and aerospace. A Michelson interferometer structure can be used for locating minute vibrations or detecting surface defects in optical components. In this embodiment, the first and second interferometer structures are equivalent to a Mach-Zehnder interferometer structure, such as... Figure 4 and Figure 5As shown, the distributed fiber optic sensing system provided in this embodiment is a fiber optic sensing system that combines an equivalent double-ended Mach-Zehnder interferometer structure with an AOM frequency shifter, and can be applied to forward transmission fiber optic acoustic or vibration sensors.

[0091] In one embodiment, the sensing fiber is arranged in a loop, and the first sensing fiber... The light will be output through the first circulator to the sensing fiber, and then through the second circulator to the first reference light. Interference occurs in the second optical mixer. Similarly, the second sensing light... The output will be sent to the sensing fiber via the second circulator, and then through the first circulator and... Interference occurs in the first optical mixer. Since two beams propagating in opposite directions exist simultaneously in the sensing fiber, their backscattering propagates in the same direction as the opposing beam. After passing through the circulator, they interfere with the reference light in the optical mixer; that is, the backscattered light and the forward-propagating sensing light aliased. In the first optical mixer... respectively with and The interference that occurs, the intensity of the interference light and interferometric Rayleigh backscattering noise It can be expressed by the following formula:

[0092] (1)

[0093] (2)

[0094] Assuming the amplitudes of the reference light and the sensing light field are equal, and These are the amplitudes of the sensing light field and the Rayleigh backscattered light field, respectively. This refers to the phase transformation caused by external disturbances in the sensing fiber. In the second optical mixer... respectively with and The interference that occurs, the intensity of the interference light and interferometric Rayleigh backscattering noise It can be expressed by the following formula:

[0095] (3)

[0096] (4)

[0097] The interference light signals generated in the first and second mixers are as follows: Figure 3b and Figure 3c As shown, since the forward beam and the reference beam have the same frequency, they can be considered as zero-difference interference. The interference signal is located near the baseband, and the phase information it carries is also located near the baseband. The backscattered beam, however, interacts with the reference beam in the other direction. The frequency difference means that the backscattered light and the reference light are considered heterodyne interference, and the interference result has a frequency of . The phase information carried by the backscattering of the carrier is located near the carrier.

[0098] After the interference light signal is converted into an electrical signal by the optical balance detector, it is received by the oscilloscope. The electrical signal received by the oscilloscope is filtered by a low-pass filter, which can remove the Rayleigh backscattering contained in the electrical signal. Figure 7 The power spectral density (PSD) of the interference light signal at the end of the 20km sensing fiber is shown. It can be seen that there are two peaks near the baseband and 200MHz, with a difference of 22.62dB. This indicates that in the distributed vibration fiber sensing system with a ring structure, one of the paths can be successfully separated from the backscattered light after frequency shifting by AOM, thereby eliminating the interference of backscattering.

[0099] In another implementation, the first optical demodulation element is a first 3×3 coupler, and the first signal detection component consists of three photodetectors; the second optical demodulation element is a second 3×3 coupler, and the second signal detection component consists of three photodetectors. The received signal is demodulated using a differential cross-multiplication algorithm or an arctangent algorithm.

[0100] Specifically, the sensing fiber is a single-mode fiber. In this embodiment, because the sensing fiber used is a single-mode fiber, and the connecting lines between the various components are also single-mode fibers, Rayleigh backscattering interference is avoided, and no additional light source is required, thus reducing the system's deployment cost.

[0101] Furthermore, a piezoelectric ceramic is connected to the sensing optical fiber. The piezoelectric ceramic is used to output simulated vibration signals. The piezoelectric ceramic expands accordingly based on the input driving voltage signal; simulating vibration can be obtained by winding the optical fiber around the piezoelectric ceramic.

[0102] Secondly, this application provides a vibration localization method for a distributed optical fiber sensing system based on AOM frequency shifting, wherein the method is applied to the distributed optical fiber sensing system, such as... Figure 9 As shown, the vibration positioning method includes:

[0103] Step S1: The probe light emitted by the light source is input to the first coupler and split into a first beam and a second beam by the first coupler.

[0104] Step S2: The first beam splitter is input to the acousto-optic modulator, undergoes frequency shifting processing, and the frequency-shifted first beam splitter is input to the first interferometer structure to obtain the first interference light signal output by the first interferometer structure; the second beam splitter is input to the second interferometer structure to obtain the second interference light signal output by the second interferometer structure.

[0105] Step S3: Locate the vibration position based on the first interference light signal and the second interference light signal.

[0106] Specifically, the step of locating the vibration position based on the first interference light signal and the second interference light signal includes:

[0107] The first interference optical signal and the second interference optical signal are converted into a first electrical signal and a second electrical signal, respectively.

[0108] The first and second electrical signals are input to a low-pass filter. The Rayleigh backscatter noise signal is filtered out by the low-pass filter, and the phase demodulation is performed on the electrical signal after the noise signal is filtered out to obtain the phase information.

[0109] The time delay difference between the first and second beams is calculated based on the phase demodulation information, and the vibration location is determined based on the time delay difference.

[0110] When vibration occurs in a single-mode fiber, the refractive index, length, and diameter of the fiber all change, leading to a change in the signal phase. This means that the forward light in the fiber is modulated by strain, elasto-optical effects, and the Poisson effect. Its modulated phase... It can be represented as:

[0111] (5)

[0112] in Where n is the length of the single-mode fiber, and n is the refractive index of the fiber core. is the propagation coefficient of a single-mode fiber. In formula (5) , and Let represent the phase changes caused by strain, elasto-optical effect, and Poisson effect, respectively. They can be expressed by the following formulas:

[0113] (6)

[0114] (7)

[0115] (8)

[0116] in, It is the strain tensor along the longitudinal direction of the optical fiber. It is the Poisson coefficient in optical fiber. and They represent the elastic coefficients, For wave number, For axial pressure, This represents Young's elastic modulus. In the phase change produced by vibration, It accounts for only 3%, therefore the phase change is approximately equal to:

[0117] (9)

[0118] According to the above formula, the phase change is related to the longitudinal pressure. It is directly proportional. Based on this principle, vibration sensing can be achieved by demodulating the phase change of the interference light.

[0119] In this invention, because AOM (Optical Optical Array) is used to perform frequency shifting on the uplink light, Rayleigh backscattering noise can be eliminated by passing the light intensity signals received by the four optical balance detectors through a low-pass filter. The cutoff frequency of the low-pass filter should be lower than the frequency shifting frequency generated by the AOM. .

[0120] From equations (1) and (2), it can be seen that the interference light signal can be expressed as: Ignore DC component It can be represented as Interference light intensity can be converted into in-phase components with equal amplitude and 90° phase difference by using a 90° optical mixer. and orthogonal components Phase changes of external disturbances This can be obtained using an I / Q demodulation algorithm:

[0121] (10)

[0122] A piezoelectric transducer (PZT) is inserted into 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 external single-frequency vibration. The interference light signal received by the BPD photoelectric balance detector... and the corresponding demodulated signal like Figure 8 As shown.

[0123] The FTDVS proposed in this invention consists of a DMZI structure. A key step in locating vibrations within the DMZI structure is calculating the time delay between two interference signals modulated by the interference, as the localization algorithm is highly dependent on this time delay. This is because the optical intensity signal at the MZI (Mach-Zehnder interferometer) receiver... There is a nonlinear relationship between the external vibration signal and the phase change in the optical field caused by the external vibration. The relationship between them is linear; therefore, positioning algorithms based on DMZI structures typically require demodulation of the light intensity signal to obtain the signal. Then, the phase delay difference between the two MZI receivers is calculated using a cross-correlation algorithm to determine the location of the vibration. The specific process is as follows:

[0124] Let the vibration occur at position Z in the sensing fiber, the length of the sensing fiber be L, the refractive index of the fiber be n, and the speed of light in vacuum be c. Due to the sensing optical field... and In the sensing fiber, transmission is reciprocal. The phase change caused by vibration intrusion arrives at the two optical mixers at different relative times. Therefore, equations (1) and (2) can be rewritten as follows:

[0125] (11)

[0126] in , .set up Let be the phase delay difference between the optical intensity signals at the two MZI receivers, then It can be represented as:

[0127] (12)

[0128] Therefore, the vibration position Z can be obtained from the following formula:

[0129] (13)

[0130] phase and The result is obtained using the I / Q demodulation algorithm described above. Correlation analysis is then used to compare the two signals. and The degree of temporal similarity, thus calculating Cross-correlation function It can be represented as:

[0131] (14)

[0132] Cross-correlation function exist There is a clear maximum value at that point, therefore, by The peak position of the relationship curve determines the time delay value; that is, the maximum value is determined through a peak-finding algorithm. The index of this value represents the time delay between the two phases. The actual operation process is as follows: Figure 6 As shown. The interference light signals received by the four photoelectric balance detectors are transmitted to the computer and then demodulated in pairs through a low-pass filter. The time delay of the phase in the two directions can be obtained by cross-correlation of the demodulated signals. The vibration position Z can be calculated by formula (13).

[0133] This invention provides a distributed fiber optic sensing system and vibration localization method based on AOM frequency shifting. It constructs a sensing system based on a forward-transmission fiber optic acoustic / vibration sensor using a dual-ended Mach-Zehnder interferometer for demodulation and AOM frequency shifting. First, one optical path in the AOM dual-path is used for frequency shifting, and then Rayleigh backscattering noise is eliminated, thereby achieving a long-distance sensing path without the need for an optical amplifier. The fiber optic sensing system and vibration localization method provided by this invention have a higher signal-to-noise ratio, a longer sensing distance, and spatial resolution is not directly related to the sensing range, thus allowing for individual optimization of the sensing system and making it easier to install and maintain.

[0134] Other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein. The specification and examples are to be considered exemplary only, and the true scope and spirit of the invention are indicated by the following claims.

[0135] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0136] It is understood that the above embodiments are exemplary and should not be construed as limiting the present application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present application.

Claims

1. An AOM frequency-shift-based distributed optical fiber sensing system, characterized in that, The application relates to a vibration positioning system, which comprises a light source, a first coupler, an acousto-optic modulator, a first interferometer structure connected with a sensing optical fiber, a second interferometer structure connected with the sensing optical fiber and an information processing assembly. The sensing optical fiber is a single-core optical fiber. The light source is used for emitting probe light. The first coupler is connected with the light source and is used for dividing the probe light into first and second light beams with equal amplitude and opposite directions. The acousto-optic modulator is connected with the first coupler and is used for frequency-shifting the first light beam. The first interferometer structure is connected with the acousto-optic modulator and is used for receiving the frequency-shifted first light beam and dividing the first light beam into first sensing light and second reference light. The second interferometer structure is connected with the first coupler and is used for receiving the second light beam and dividing the second light beam into second sensing light and second reference light. The first interferometer structure is further used for obtaining first interference light signals generated by interference between the second sensing light transmitted through the sensing optical fiber and the second reference light, and outputting first electric signals corresponding to the first interference light signals. The second interferometer structure is further used for obtaining second interference light signals generated by interference between the first sensing light transmitted through the sensing optical fiber and the first reference light, and outputting second electric signals corresponding to the second interference light signals. The information processing assembly is used for receiving and eliminating Rayleigh backscattering noise signals contained in the first and second electric signals, and determining a vibration positioning result. The first sensing light, the first reference light, and the light frequency of the Rayleigh backscatter light field corresponding to the first sensing light are The second sensing light, the second reference light, and the light frequency of the Rayleigh backscatter light field corresponding to the second sensing light are The first sensing light, the first reference light, and the light frequency of the Rayleigh backscatter light field corresponding to the first sensing light are The frequency of the light source itself is The frequency shift occurring after AOM modulation is The interference light signal of the back Rayleigh scattering of the first split light beam and the second split light beam reference light is located in the high-frequency carrier band, thereby separating the forward transmission sensing light and the back Rayleigh scattering light.

2. The AOM frequency-shift based distributed optical fiber sensing system of claim 1, wherein, The first and second interferometer structures are Mach-Zehnder interferometer structures, Sagnac interferometer structures or Michelson interferometer structures.

3. The AOM frequency-shift based distributed optical fiber sensing system of claim 1, wherein, The first interferometer structure comprises a second coupler, a first optical fiber circulator, a first optical demodulation element and a first signal detection assembly, and the second interferometer structure comprises a third coupler, a second optical fiber circulator, a second optical demodulation element and a second signal detection assembly. The second coupler receives the first light beam and divides the first light beam into the first sensing light and the first reference light. The first optical fiber circulator is arranged in the light path of the first sensing light, receives the first sensing light, transmits the first sensing light to the sensing optical fiber, inputs the first sensing light to the second optical fiber circulator through the sensing optical fiber, inputs the first sensing light to the first optical demodulation element through the second optical fiber circulator, and outputs first interference light signals generated by interference between the first sensing light and the first reference light in the first optical demodulation element. The first signal detection assembly receives the first interference light signals and converts the first interference light signals into first electric signals. The third coupler receives the second light beam and divides the second light beam into the second sensing light and the second reference light. The second optical fiber circulator is arranged in the light path of the second sensing light, receives the second sensing light, transmits the second sensing light to the sensing optical fiber, inputs the second sensing light to the first optical fiber circulator through the sensing optical fiber, inputs the second sensing light to the second optical demodulation element through the first optical fiber circulator, and outputs second interference light signals generated by interference between the second sensing light and the second reference light in the second optical demodulation element. 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.

4. The AOM frequency-shift based distributed optical fiber sensing system of claim 3, wherein, The first optical demodulation element is a first optical frequency 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 frequency 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 to the second optical fiber circulator through the sensing optical fiber, is input to the first optical frequency mixer through the second optical fiber circulator, and is interfered with the first reference light in the first optical frequency mixer to output a first interference optical signal; The first optical balanced detector and the second optical balanced detector respectively receive in-phase component signals and quadrature component signals of the first interference optical signal, and convert the in-phase component signals and the quadrature component signals into a first electrical signal; The second sensing light is input to the first optical fiber circulator through the sensing optical fiber, is input to the second optical frequency mixer through the first optical fiber circulator, and is interfered with the second reference light in the second optical frequency mixer to output a second interference optical signal; The third optical balanced detector and the fourth optical balanced detector respectively receive in-phase component signals and quadrature component signals of the second interference optical signal, and convert the in-phase component signals and the quadrature component signals into a second electrical signal.

5. The AOM frequency-shift based distributed optical fiber sensing system of claim 3, wherein, 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 AOM frequency-shift based distributed optical fiber sensing system of claim 4 or 5, wherein, The information processing component includes an oscilloscope, a low-pass filter, and a PC terminal. 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 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 beam; The PC terminal is configured to determine a sensing positioning result according to the received first electrical signal and second electrical signal.

7. The AOM frequency-shift based distributed optical fiber sensing system of claim 1, wherein, The light source is a 1550nm narrow linewidth laser with a linewidth less than 1 KHz; the probe light is single-frequency continuous light; and the sensing optical fiber is a single-mode optical fiber.

8. The AOM frequency-shift based distributed optical fiber sensing system of claim 1, wherein, The sensing optical fiber is connected to a piezoelectric ceramic; The piezoelectric ceramic is configured to output an analog vibration signal. 9.A vibration positioning method of a distributed optical fiber sensing system based on AOM frequency shift, characterized in that, The vibration positioning method is applied to the distributed optical fiber sensing system of any one of claims 1-8. The probe light emitted by the light source is input to the first coupler, and is split into a first split beam and a second split beam by the first coupler; The first split beam is input to the acousto-optic modulator, is subjected to frequency shifting processing by the acousto-optic modulator, and is input to the first interferometer structure after the frequency shifting processing, to obtain a first interference optical signal output by the first interferometer structure; the second split beam is input to the 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.

10. The vibrational positioning method of claim 9, wherein, 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 electric signal and a second electric signal, respectively; inputting the first electric signal and the second electric signal into a low-pass filter, filtering out Rayleigh backscattering noise signals through the low-pass filter, and performing phase demodulation on the electric signals after filtering out the noise signals to obtain phase information; calculating a time delay difference between the first split light beam and the second split light beam according to the phase demodulation information, and locating the vibration position according to the time delay difference.

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