Distributed fiber optic acoustic sensor

Abstract

The application provides a kind of distributed optical fiber acoustic sensor, comprising: narrow line width laser, pulse light modulator, erbium-doped fiber amplifier, circulator, filter, photodetector, analog-to-digital converter, demodulation system and fluorescent optical fiber;Fluorescent optical fiber is doped with fluorescent substance in the core;Narrow line width laser is modulated into pulse light after emitting continuous laser, and the pulse light enters the circulator through the first port after passing through erbium-doped fiber amplifier and is injected into fluorescent optical fiber through the second port;Pulse light excites fluorescent substance to produce fluorescence, and the fluorescence returned along the fluorescent optical fiber forms fluorescent back light;The pulse light injected into the fluorescent optical fiber produces back Rayleigh scattering light;Fluorescent back light and back Rayleigh scattering light beat in the fluorescent optical fiber, forming beat signal.The distributed optical fiber acoustic sensor provided by the application is based on the similar structure as the direct detection method, realizes coherent detection, does not need frequency shift device and coupler, simplifies the device structure, and improves the sensor quality.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, and more specifically to a distributed fiber optic acoustic sensor. Background Technology

[0002] Distributed fiber optic acoustic sensors (DAS), in addition to possessing the advantages of ordinary fiber optic sensors such as resistance to electromagnetic interference, good concealment, corrosion resistance, and insulation, also offer numerous outstanding advantages, including distributed and quantitative detection of dynamic strain, simple structure, long detection distance, high adaptability to complex terrain, and no need for external power supply. They are widely used in many defense and industrial fields such as perimeter security, resource exploration, oil and gas pipeline inspection, and communication line inspection. Distributed fiber optic acoustic sensing systems based on phase-sensitive optical time-domain reflectometry (Φ-OTDR) utilize the phase change of backscattered Rayleigh light during fiber transmission to achieve distributed measurement and reconstruction of acoustic signals. Generally, distributed fiber optic vibration sensors (DVS) can only detect the location of vibration events along the entire fiber optic link, but cannot quantitatively measure the amplitude and phase of the vibration. DAS, in addition to possessing the advantages of DVS, can achieve real-time, multi-point, and quantitative detection of the amplitude and phase of acoustic signals, and then demodulate the magnitude and frequency of the acoustic information, thus having a broader application prospect.

[0003] Please see Figure 1 Early DAS employed a direct detection method, where continuous light emitted from a narrow-linewidth laser (NLL) was modulated into pulsed light by an acousto-optic modulator (AOM). This pulsed light was amplified by an erbium-doped fiber amplifier (EDFA) and then entered the sensing fiber (SF) via a circulator (C). The pulsed light generated backscattered Rayleigh light in the SF. Acoustic signals from the sound source were coupled into this backscattered light, forming a modulated signal. This modulated signal was transmitted through the circulator (C) to a photodetector (PD), where it was converted into an electrical signal. This signal was then processed by an analog-to-digital converter (ADC) to obtain a digital signal. Finally, a demodulation system (DS) demodulated this digital signal to obtain the position, amplitude, frequency, and phase information of the acoustic wave. The modulation principle of the acoustic signal on the backscattered Rayleigh light is as follows: when an acoustic signal acts on the sensing fiber (SF), according to Hooke's law and the photoelastic effect of the fiber itself, the fiber length and refractive index at that location will change, leading to a change in the optical path length of the transmitted light wave at that location. Ultimately, due to interference effects, the intensity of the backscattered Rayleigh light changes. During demodulation, information about the sound wave can be obtained by analyzing the changes in the intensity of the backscattered Rayleigh light.

[0004] To improve the signal-to-noise ratio, the coherent detection method, which was subsequently developed, introduced a local reference light based on the direct detection method. The principle is that the backscattered Rayleigh light beats with the local reference light, which has a fixed frequency difference, forming a beat frequency signal. The demodulation system then extracts the amplitude, phase, and frequency information of the acoustic signal from this beat frequency signal. (See also...) Figure 2 The principle of the distributed fiber optic acoustic wave sensor based on heterodyne modulation is as follows: Continuous light emitted from a narrow-linewidth laser (NLL) is split into two paths by the first coupler (OC1). One path serves as the local reference light (LO) and enters the second coupler (OC2), while the other path serves as the probe light. The probe light passes through an acousto-optic modulator (AOM) and an erbium-doped fiber amplifier (EDFA) before entering the sensing fiber (SF), generating backscattered Rayleigh light coupled with the acoustic wave signal. The probe light is typically frequency-shifted by a waveform generator (WG) to ensure a fixed frequency difference between the probe light and the local reference light. The local reference light (LO) and the backscattered Rayleigh light beat in the second coupler (OC2), forming a beat signal. The beat signal is converted into an electrical signal by a photodetector (PD) and processed by an analog-to-digital converter (ADC) to obtain a digital signal. Finally, the demodulation system (DS) demodulates this digital signal to obtain the position, amplitude, frequency, and phase information of the acoustic wave.

[0005] Compared to direct detection, coherent detection demodulates the beat frequency signal, thus maintaining constant phase sensitivity, strong resistance to external interference, and a high signal-to-noise ratio, making it more widely applicable. However, coherent detection requires frequency-shifting devices such as waveform generators and several couplers, increasing structural complexity and sensor cost. Furthermore, background noise generated by the waveform generators and other frequency-shifting devices, as well as poor coupling between the couplers and the sensing fiber, can degrade sensor performance. Summary of the Invention

[0006] The present invention aims to solve the problems of complex structure and low performance of existing distributed fiber optic acoustic sensors.

[0007] To address the aforementioned issues, this invention provides a distributed fiber optic acoustic wave sensor that enables heterodyne modulation while avoiding the problems of complex structure, frequency shifting devices, and couplers affecting sensor performance in existing heterodyne-modulated distributed fiber optic acoustic wave sensors.

[0008] The distributed fiber optic acoustic sensor provided by this invention includes:

[0009] Narrow linewidth lasers, pulsed light modulators, erbium-doped fiber amplifiers, circulators, filters, photodetectors, analog-to-digital converters, demodulation systems, and fluorescent fibers;

[0010] Fluorescent optical fibers contain fluorescent substances in their cores;

[0011] The circulator has a first port, a second port, and a third port;

[0012] A narrow-linewidth laser emits continuous laser light, which is modulated into pulsed light by a pulsed light modulator. The pulsed light passes through an erbium-doped fiber amplifier, enters a circulator through the first port, and is injected into a fluorescent fiber through the second port.

[0013] Pulsed light injected into a fluorescent fiber excites a fluorescent material to produce fluorescence, and the fluorescence returning along the fluorescent fiber forms a fluorescent backlight.

[0014] The pulsed light injected into the fluorescent fiber generates backscattered Rayleigh light;

[0015] Fluorescent backlight and back Rayleigh scattering light beat in the fluorescent fiber to form a beat frequency signal, and external acoustic wave signals are coupled into the beat frequency signal to form a modulated beat frequency signal.

[0016] The modulated beat frequency signal enters the filter through the third port to remove noise, and is then converted into an electrical signal by the photodetector. The electrical signal is converted into a digital signal by the analog-to-digital converter, and the demodulation system demodulates the digital signal to obtain the external sound wave signal.

[0017] Preferably, the fluorescent material includes neodymium ions, europium ions, and samarium ions.

[0018] Preferably, the fluorescent material includes a fluorescent dye, and the fluorescent dye includes a rhodamine series dye.

[0019] Preferably, the fluorescent material is uniformly distributed in the fiber core.

[0020] Preferably, the core material comprises a polymer material.

[0021] Preferably, the photodetector includes a balanced photodetector.

[0022] Preferably, the narrow linewidth laser is used to continuously output single-frequency laser, and the linewidth of the single-frequency laser does not exceed 1MHz.

[0023] Preferably, the pulsed light modulator includes an electro-optic modulator, an acousto-optic modulator, or a semiconductor optical amplifier.

[0024] Preferably, the fluorescence and the pulsed light form zero-difference interference.

[0025] Preferably, the fluorescence and the pulsed light form heterodyne interference.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] The distributed fiber optic acoustic wave sensor provided by this invention generates fluorescence backlight in the core of a fluorescent fiber by exciting it with pulsed light. The fluorescence backlight and backscattered Rayleigh light beat in the fluorescent fiber to form a beat frequency signal, which is then modulated by an external acoustic wave signal to obtain a modulated beat frequency signal. This distributed fiber optic acoustic wave sensor, based on a structure similar to direct detection, achieves coherent detection; it eliminates the need for frequency shifting devices and couplers, simplifying the structure of existing coherent detection-based distributed fiber optic acoustic wave sensors, improving sensor quality, and reducing sensor cost. Attached Figure Description

[0028] Figure 1 The diagram shows a block diagram of an existing distributed fiber optic acoustic sensor based on zero-difference modulation.

[0029] Figure 2 The diagram shows a structural block diagram of an existing distributed fiber optic acoustic sensor based on heterodyne modulation.

[0030] Figure 3 The diagram shown is a structural block diagram of a distributed fiber optic acoustic sensor in one embodiment.

[0031] Figure 4 The diagram shown is a cross-sectional view of the fluorescent optical fiber in one embodiment.

[0032] The reference numerals in the attached figures are explained as follows:

[0033] 1-Narrow linewidth laser; 2-Pulse light modulator; 3-Erbium-doped fiber amplifier; 4-Circulator; 5-Filter; 6-Photodetector; 7-Analog-to-digital converter; 8-Demodulation system; 9-Fluorescent fiber; 901-Fiber core; 902-Cladding; 903-Fluorescent material; 10-Fluorescent backscattering; 11-Backscattered Rayleigh light; 12-Sound source. Detailed Implementation

[0034] To make the objectives, advantages, and features of the present invention clearer, the distributed fiber optic acoustic sensor provided by the present invention will be further described in detail below with reference to the accompanying drawings. It should be noted that the drawings are all in a very simplified form and use non-precise proportions, and are only used to facilitate and clarify the explanation of the embodiments of the present invention.

[0035] Please see Figure 3 This embodiment provides a distributed fiber optic acoustic sensor, comprising:

[0036] 1. Narrow linewidth laser; 2. Pulsed light modulator; 3. Erbium-doped fiber amplifier; 4. Circulator; 5. Filter; 6. Photodetector; 7. Analog-to-digital converter; 8. Demodulation system; and 9. Fluorescent fiber.

[0037] The fluorescent optical fiber 9 has a fluorescent substance 903 doped into its core 901;

[0038] The circulator 4 has a first port, a second port, and a third port;

[0039] The narrow linewidth laser 1 emits continuous laser light, which is modulated into pulse light by the pulse light modulator 2. The pulse light passes through the erbium-doped fiber amplifier 3, enters the circulator 4 through the first port, and is injected into the fluorescent fiber 9 through the second port.

[0040] The pulsed light injected into the fluorescent fiber 9 excites the fluorescent material 903 to produce fluorescence, and the fluorescence returning along the fluorescent fiber 9 forms the fluorescence backlight 10.

[0041] The pulsed light injected into the fluorescent fiber 9 generates backscattered Rayleigh light 11;

[0042] The fluorescent backlight 10 and the backward Rayleigh scattering light 11 beat in the fluorescent optical fiber 9 to form a beat frequency signal, and an external acoustic wave signal is coupled into the beat frequency signal to form a modulated beat frequency signal.

[0043] The modulated beat frequency signal enters the filter 5 through the third port to filter out noise, and is then converted into an electrical signal by the photodetector 6. The electrical signal is converted into a digital signal by the analog-to-digital converter 7, and the demodulation system 8 demodulates the digital signal to obtain an external sound wave signal.

[0044] The fluorescent material 903 includes rare earth ions, such as neodymium ions (Nd). 3+ Europium ions (Eu) 3- and samarium ions Sm 3+ The fluorescent material 903 can also be a fluorescent dye, such as a rhodamine series dye. Preferably, the fluorescent material 903 is uniformly distributed in the fiber core 901. Preferably, the material of the fiber core 901 includes a polymer material to facilitate the uniform incorporation of different fluorescent materials. The polymer material is, for example, polymethyl methacrylate (PMMA).

[0045] Please see Figure 4The fluorescent optical fiber 9 may include a core 901 and a cladding 902. The cladding 902 may have a multi-layer structure, and a sleeve may also be provided outside the cladding 902. In the core 901, the fluorescent material 903 can absorb the pulsed light, thereby exciting itself and emitting fluorescence in various directions. The fluorescence whose radiation direction satisfies the total internal reflection condition at the interface between the core 901 and the cladding 902 will be transmitted along the optical fiber axis and eventually received by the third port. This part of the fluorescence is the fluorescence return light 10, and the fluorescence return light 10 and the Rayleigh scattered light from the same excitation position arrive at the third port connected to the circulator 4 simultaneously. The fluorescence in other radiation directions will be absorbed by the cladding 902 or rapidly lost through the cladding 902. Therefore, the fluorescence in other radiation directions will not reach the third port and be received by the photoelectric sensor. In the core 901, the pulsed light can also be scattered by the core 901 to form Rayleigh scattered light, which is transmitted backward along the optical fiber axis to form the backward Rayleigh scattered light 11. The fluorescent backlight 10 and the backscattered Rayleigh light 11 can beat in the fluorescent optical fiber 9 to form a beat frequency signal. Specifically, in the fluorescent optical fiber 9, the fluorescent backlight 10 and the backscattered Rayleigh light 11, generated at the same excitation position, have a fixed frequency ratio and amplitude ratio and the same propagation direction, so they can beat and generate a distinct beat frequency signal. The acoustic wave signal generated by the external sound source 12 can be coupled to the beat frequency signal to form a modulated beat frequency signal. The distributed fiber optic acoustic wave sensor achieves coherent detection without the need for frequency shifting devices and couplers, avoiding the performance degradation problem caused by the inclusion of frequency shifting devices and couplers in existing distributed fiber optic acoustic wave sensors.

[0046] By selecting a narrow-linewidth laser 1 with a specific frequency and a fluorescent fiber 9 with specific parameters (including the type and concentration of fluorescent substance 903), the ratio of the center frequency of the fluorescence to that of the pulsed light can be controlled. When the center frequencies of the fluorescence and the pulsed light are the same, the fluorescence and the pulsed light form zero-difference interference (or zero-difference modulation); when the center frequencies of the fluorescence and the pulsed light are different and have a fixed frequency difference, the fluorescence and the pulsed light form heterodyne interference (or heterodyne modulation). Therefore, the distributed fiber acoustic sensor provided in this embodiment can perform modulation and demodulation based on both zero-difference interference and heterodyne interference, improving its applicability and practicality. Furthermore, the concentration of fluorescent substance 903 is proportional to the fiber loss generated by the fluorescent substance; by adjusting the concentration of fluorescent substance 903, the fiber loss of the fluorescent fiber 9 can be adjusted. In this embodiment, preferably, the fiber loss generated by the fluorescent material does not exceed 10 times the fiber loss generated by Rayleigh scattering, so as to ensure that the fluorescent backlight 10 and the backscattered Rayleigh light 11 have a comparable intensity ratio.

[0047] In this embodiment, the narrow linewidth laser 1 is used to continuously output single-frequency laser light, and the linewidth of the single-frequency laser light does not exceed 1 MHz. The pulsed light modulator 2 includes an acousto-optic modulator (AOM), an electro-optic modulator (EOM), or a semiconductor optical amplifier (SOA).

[0048] The filter 5 is used to filter out noise and retain the optical signal of the center wavelength; the optical signal is converted into an electrical signal by the photodetector 6, which can be a balanced photodetector, and the electrical signal can also be amplified by a signal amplifier; the electrical signal is converted into a digital signal by the analog-to-digital converter 7.

[0049] The demodulation system 8 is used to demodulate the digital signal. The demodulation process of the digital signal may include: the demodulation system 8 preprocesses the digital signal (e.g., digital filtering, cumulative averaging) and then displays it to obtain the distribution curve of the intensity of the modulated beat frequency signal on the optical fiber link along the time axis. Then, by replacing the time scale on the time axis with the corresponding optical fiber position, a test curve is obtained. Finally, corresponding time domain analysis, frequency domain analysis, etc. are performed to obtain the position, frequency, phase and amplitude information of the sound source.

[0050] In summary, the distributed fiber optic acoustic wave sensor provided in this embodiment generates fluorescence backlight in the core of a fluorescent fiber by exciting it with pulsed light. The fluorescence backlight and backscattered Rayleigh light beat in the fluorescent fiber to form a beat frequency signal, which is then modulated by an external acoustic wave signal to obtain a modulated beat frequency signal. This distributed fiber optic acoustic wave sensor, based on a structure similar to direct detection, achieves coherent detection; it eliminates the need for frequency shifting devices and couplers, simplifying the structure of existing coherent detection-based distributed fiber optic acoustic wave sensors, improving sensor quality, and reducing sensor cost.

[0051] Furthermore, it is understood that although the present invention has been disclosed above with reference to preferred embodiments, these embodiments are not intended to limit the present invention. For any person skilled in the art, many possible variations and modifications can be made to the technical solutions of the present invention based on the disclosed technical content, or equivalent embodiments with equivalent changes, without departing from the scope of the present invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the present invention are still within the scope of protection of the present invention. It should also be understood that the present invention is not limited to the specific methods, compounds, materials, manufacturing techniques, uses, and applications described herein; these can vary. It should also be understood that the terminology described herein is used only to describe specific embodiments and is not intended to limit the scope of the invention. It must be noted that the singular forms “a,” “an,” and “the” used herein and in the appended claims include plural bases unless the context clearly indicates otherwise. Thus, for example, a reference to “a step” means a reference to one or more steps, and may include secondary steps. All conjunctions used should be understood in the broadest sense. Therefore, the word "or" should be understood as having a logical definition of "or," not a logical definition of "exclusive or," unless the context explicitly indicates the opposite. The structure described here will be understood as also referencing its functional equivalent. Language that can be interpreted as approximate should be understood in that way, unless the context explicitly indicates the opposite.

Claims

1. A distributed fiber optic acoustic sensor, characterized in that, include: Narrow linewidth lasers, pulsed light modulators, erbium-doped fiber amplifiers, circulators, filters, photodetectors, analog-to-digital converters, demodulation systems, and fluorescent fibers; The fluorescent optical fiber has a fluorescent substance incorporated into its core. The circulator has a first port, a second port, and a third port; The narrow linewidth laser emits continuous laser light, which is modulated into pulsed light by the pulsed light modulator. The pulsed light passes through the erbium-doped fiber amplifier, enters the circulator through the first port, and is injected into the fluorescent fiber through the second port. The pulsed light injected into the fluorescent optical fiber excites the fluorescent material to produce fluorescence, and the fluorescence returning along the fluorescent optical fiber forms a fluorescent backlight; The pulsed light injected into the fluorescent fiber generates backscattered Rayleigh light; The fluorescent backlight and the back Rayleigh scattered light beat in the fluorescent optical fiber to form a beat frequency signal, and an external acoustic wave signal is coupled into the beat frequency signal to form a modulated beat frequency signal. The modulated beat frequency signal enters the filter through the third port to remove noise, and is then converted into an electrical signal by the photodetector. The electrical signal is converted into a digital signal by the analog-to-digital converter, and the demodulation system demodulates the digital signal to obtain the external sound wave signal.

2. The distributed fiber optic acoustic sensor as described in claim 1, characterized in that, The fluorescent substances include neodymium ions, europium ions, and samarium ions.

3. The distributed fiber optic acoustic sensor as described in claim 1, characterized in that, The fluorescent material includes fluorescent dyes, and the fluorescent dyes include rhodamine series dyes.

4. The distributed fiber optic acoustic sensor as described in claim 1, characterized in that, The fluorescent material is uniformly distributed in the fiber core.

5. The distributed fiber optic acoustic sensor as described in claim 1, characterized in that, The core material includes polymer materials.

6. The distributed fiber optic acoustic sensor as described in claim 1, characterized in that, The photodetector includes a balanced photodetector.

7. The distributed fiber optic acoustic sensor as described in claim 1, characterized in that, The narrow linewidth laser is used to continuously output single-frequency laser light, and the linewidth of the single-frequency laser light does not exceed 1 MHz.

8. The distributed fiber optic acoustic sensor as described in claim 1, characterized in that, The pulsed light modulator includes an electro-optic modulator, an acousto-optic modulator, or a semiconductor optical amplifier.

9. The distributed fiber optic acoustic sensor as described in claim 1, characterized in that, The fluorescence and the pulsed light form zero-difference interference.

10. The distributed fiber optic acoustic sensor as described in claim 1, characterized in that, The fluorescence and the pulsed light form heterodyne interference.

Images