A distributed optical fiber doppler acoustic sensing system and method

Abstract

The application relates to the technical field of optical fiber sensing, in particular to a distributed optical fiber Doppler acoustic sensing system and method. The system comprises an optical fiber Doppler laser interference device for generating a narrow-linewidth laser signal and receiving a return signal for vibration analysis; a main optical fiber connected to the interference device at one end; a plurality of branch sensing units distributed along the main optical fiber, each branch sensing unit comprising: an optical splitter with a first main connection end and a second main connection end connected to the main optical fiber respectively, the optical splitter outputting the narrow-linewidth laser signal from its splitting end and the second main connection end respectively after splitting the narrow-linewidth laser signal; and a vibrating membrane cavity sensor arranged in a measured environment and connected to the splitting end of the optical splitter through a sensing optical fiber, the vibrating membrane cavity sensor reflecting a return signal carrying a Doppler frequency shift, the return signal entering a reverse transmission path of the main optical fiber through the sensing optical fiber, the splitting end and the first main connection end, and returning to the interference device. The application realizes spatially distributed multi-point acoustic / vibration monitoring.

Description

Technical Field

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

[0002] Laser Doppler vibration measurement is a non-contact, high-precision vibration measurement method widely used in precision manufacturing, biomedical testing, and structural health monitoring. Traditional laser Doppler systems typically employ free-space optical paths, retrieving vibration information by emitting a laser and receiving the Doppler frequency shift of the scattered light from the target surface. However, such systems are usually single-point measurements, making it difficult to achieve simultaneous sensing at multiple locations, and they are sensitive to environmental disturbances, limiting their application in complex scenarios.

[0003] In recent years, with the development of fiber optic sensing technology, combining the laser Doppler principle with fiber optic systems has become a research hotspot. In existing technologies, some schemes utilize fiber optic interferometers (such as Mach-Zehnder, Michelson, or Fabry-Perot structures) for vibration detection, but these generally suffer from low signal-to-noise ratios, susceptibility to backscattering interference, and limited dynamic range. Furthermore, while traditional distributed optical fiber acoustic sensing (DAS) can continuously sense vibrations along the fiber, its sensing capability is entirely dependent on the physical orientation of the trunk fiber, preventing it from extending beyond the trunk and thus limiting its detection range. Moreover, due to its reliance on weak Rayleigh backscattered light, the signal strength is low and susceptible to inherent fiber losses and environmental noise, especially in low-frequency (<1kHz) and long-distance scenarios, where the minimum detectable sound pressure level is relatively high, making it difficult to meet the requirements for high-fidelity speech or weak sound signal detection. Summary of the Invention

[0004] This invention provides a distributed fiber optic Doppler acoustic sensing system and method for achieving highly scalable non-contact vibration and sound acquisition.

[0005] According to a first aspect of this application, a distributed fiber optic Doppler acoustic sensing system is provided, the system comprising:

[0006] A fiber optic Doppler laser interferometer is used to generate narrow linewidth laser signals and receive echo signals for vibration analysis.

[0007] The main optical fiber is connected at one end to the fiber optic Doppler laser interferometer.

[0008] Several branch sensing units are distributed along the main optical fiber, and each branch sensing unit includes:

[0009] The beam splitter has its first trunk connection end and second trunk connection end connected to the trunk optical fiber, respectively. The beam splitter splits the narrow linewidth laser signal input from the trunk optical fiber to the first trunk connection end and then outputs it from its splitting end and the second trunk connection end, respectively.

[0010] A vibrating diaphragm cavity sensor is used to be placed in the environment under test, and it is connected to the beam splitter end of the beam splitter via a sensing optical fiber.

[0011] The vibrating diaphragm cavity sensor reflects an echo signal carrying a Doppler frequency shift. The echo signal passes through the sensing fiber, the splitting end of the beam splitter, and the first trunk connection end of the beam splitter, enters the reverse transmission path of the trunk fiber, and is finally transmitted back to the fiber optic Doppler laser interferometer.

[0012] Optionally, the fiber optic Doppler laser interferometer includes:

[0013] Narrow linewidth lasers are used to generate narrow linewidth laser signals.

[0014] A first fiber coupler, the input of which is connected to the output of the narrow linewidth laser;

[0015] An optical signal enhancement module, the input of which is connected to the first output of the first optical fiber coupler;

[0016] The circulator has its first transmission end connected to the output end of the optical signal enhancement module and its second transmission end connected to the trunk optical fiber.

[0017] A dual-frequency modulation module, the input of which is connected to the second output of the first fiber optic coupler;

[0018] The second fiber optic coupler has its first input end connected to the output end of the dual-frequency modulation module and its second input end connected to the third transmission end of the circulator.

[0019] A dual-balanced photodetector, whose two input terminals are respectively connected to the two output terminals of the second fiber coupler.

[0020] The signal processing module has its input terminal connected to the output terminal of the dual-balanced photodetector.

[0021] Optionally, the dual-frequency modulation module includes:

[0022] The first acousto-optic modulator has its input end connected to the second output end of the first fiber optic coupler, and is used to perform a first-stage frequency shift on the laser signal output by the first fiber optic coupler.

[0023] The second acousto-optic modulator has its input end connected to the output end of the first acousto-optic modulator and its output end connected to the first input end of the second fiber optic coupler. It is used to perform a second-stage frequency shift on the laser signal that has undergone the first-stage frequency shift.

[0024] Optionally, the optical signal enhancement module includes:

[0025] A wavelength division multiplexer, the first input of which is connected to the first output of the first optical fiber coupler;

[0026] A pump laser, the output of which is connected to the second input of the wavelength division multiplexer;

[0027] An erbium-doped optical fiber is provided, with its input end connected to the common terminal of the wavelength division multiplexer and its output end connected to the first transmission terminal of the circulator.

[0028] Optionally, the optical signal enhancement module further includes an optical isolator and a filter, with the output end of the erbium-doped fiber connected to the input end of the optical isolator, the output end of the optical isolator connected to the input end of the filter, and the output end of the filter connected to the first transmission end of the circulator.

[0029] Optionally, the diaphragm cavity sensor is provided with a fiber optic collimator, the diaphragm cavity sensor is connected to one end of the fiber optic collimator, and the other end of the fiber optic collimator is connected to the sensing fiber.

[0030] Optionally, it also includes:

[0031] Visible lasers are used to generate visible laser light.

[0032] The third fiber coupler has its first input end connected to the second transmission end of the circulator, its second input end connected to the visible light laser, and its output end connected to the trunk fiber.

[0033] Optionally, the narrow linewidth laser is a tunable semiconductor laser.

[0034] Optionally, the diaphragm cavity sensor includes:

[0035] The cavity structure is hollow, with through holes at both ends communicating with the hollow structure. A plug-in part is provided on the side wall near one end, and the through hole at the other end is connected to the sensing optical fiber.

[0036] A metal vibrating plate with a thickness of less than 0.1 mm has a central part and a peripheral part, with a hollow part between the central part and the peripheral part. The peripheral part is installed in the plug-in part so that the hollow part and the central part are placed in the hollow structure.

[0037] According to a second aspect of this application, a distributed fiber optic Doppler acoustic sensing method is provided, applied to the distributed fiber optic Doppler acoustic sensing system described in the first aspect above, the method comprising:

[0038] Narrow linewidth laser signals are emitted from the fiber optic Doppler laser interferometer and transmitted to each of the branch sensing units via the main optical fiber.

[0039] When the narrow linewidth laser signal reaches the branch sensing unit, a portion of the optical power is coupled to the sensing fiber through the beam splitter of the branch sensing unit, and finally irradiates the vibrating diaphragm cavity sensor.

[0040] The diaphragm cavity sensor generates an echo signal carrying Doppler frequency shift information. The echo signal passes through the sensing fiber, the splitting end of the beam splitter, and the first trunk connection end of the beam splitter, and enters the reverse transmission path of the trunk fiber. Finally, it is transmitted to the fiber optic Doppler laser interferometer so that the fiber optic Doppler laser interferometer can analyze the echo signal.

[0041] Based on any of the above aspects, the distributed fiber optic Doppler acoustic sensing system and method provided in this application have the following beneficial effects:

[0042] First, by distributing multiple branch sensing units on the main optical fiber, and setting an independent diaphragm cavity sensor in each branch sensing unit, spatially distributed multi-point acoustic / vibration signal acquisition is realized. This scheme supports continuous, multi-location real-time sensing and acquisition of large-area or long-distance environments, and realizes high-sensitivity, anti-interference synchronous monitoring of acoustic and vibration signals at multiple distant distribution points, with good practicality and scalability.

[0043] Second, the diaphragm cavity sensor of this application is sensitive to external sound pressure and can efficiently convert sound waves into measurable optical phase changes, significantly improving system sensitivity.

[0044] Third, this application combines fiber optic Doppler laser vibration measurement technology, which can detect minute frequency changes (i.e., Doppler frequency shift) caused by sound waves or vibrations, thereby achieving high-precision capture of extremely weak acoustic signals. Attached Figure Description

[0045] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0046] Figure 1This is an architecture diagram of a distributed fiber optic Doppler acoustic sensing system provided in this embodiment.

[0047] Figure 2 This is a schematic diagram of the fiber optic Doppler laser interferometer device in this embodiment.

[0048] Figure 3 This embodiment presents a schematic diagram of the vibrating diaphragm cavity sensor structure.

[0049] Figure 4 This embodiment provides a schematic diagram of the structure of a metal vibrating plate. Detailed Implementation

[0050] The accompanying drawings are for illustrative purposes only and should not be construed as limiting the scope of this application. To better illustrate the following embodiments, some components in the drawings may be omitted, enlarged, or reduced, and do not represent the actual dimensions of the product; it is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0051] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort should fall within the scope of protection of the present application.

[0052] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of this application described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0053] like Figure 1 As shown, this embodiment provides a distributed fiber optic Doppler acoustic sensing system, which specifically includes:

[0054] A fiber optic Doppler laser interferometer 100 is used to generate narrow linewidth laser signals and receive echo signals for vibration analysis.

[0055] The main optical fiber 200 is connected at one end to the fiber optic Doppler laser interferometer 100;

[0056] Several branch sensing units 300 are distributed along the trunk optical fiber 200, and each branch sensing unit 300 includes:

[0057] The beam splitter 301 has its first trunk connection end and second trunk connection end connected to the trunk fiber 200 respectively. The beam splitter 301 splits the narrow linewidth laser signal input from the trunk fiber 200 to the first trunk connection end and then outputs it from its splitting end and second trunk connection end respectively.

[0058] A diaphragm cavity sensor 302 is used to be placed in the environment under test, and it is connected to the beam splitter end of the beam splitter 301 via a sensing optical fiber 400.

[0059] The diaphragm cavity sensor 302 reflects an echo signal carrying a Doppler frequency shift. The echo signal passes through the sensing fiber 400, the splitting end of the beam splitter 301, and the first trunk connection end of the beam splitter 301, enters the reverse transmission path of the trunk fiber 200, and is finally transmitted back to the fiber Doppler laser interferometer 100.

[0060] In this embodiment, the fiber optic Doppler laser interferometer 100 provides a stable narrow-linewidth laser emission source, outputting a narrow-linewidth laser signal to the trunk fiber 200 to achieve Doppler frequency shift detection and demodulation. Specifically, the narrow-linewidth laser signal is emitted from the fiber optic Doppler laser interferometer 100 and transmitted through the trunk fiber 200 to each of the branch sensing units 300. When the narrow-linewidth laser signal reaches a branch sensing unit 300, a portion of the optical power is coupled to the sensing fiber 400 by the beam splitter 301 of the branch sensing unit 300, ultimately illuminating the diaphragm cavity sensor 302. When external sound waves or vibrations act on the diaphragm cavity sensor 302, its diaphragm will undergo a slight displacement, causing a change in the phase of the reflected light, thereby generating a Doppler frequency shift. The echo signal carrying Doppler frequency shift information passes through the sensing fiber 400, the splitting end of the beam splitter 301, and the first trunk connection end of the beam splitter 301, and enters the reverse transmission path of the trunk fiber 200, and is finally transmitted to the fiber optic Doppler laser interferometer 100. Since the diaphragm cavity sensors 302 of each branch sensing unit 300 can be distributed in different locations in the environment, they can all transmit the corresponding echo signals. This allows the fiber optic Doppler laser interferometer 100 to receive multiple echo signals, realizing the acquisition of spatially distributed multi-point acoustic / vibration signals.

[0061] Furthermore, after the fiber optic Doppler laser interferometer 100 acquires the echo signal, it interferes the echo signal with the reference light separated from the narrow linewidth laser signal, demodulates the interfered signal, and finally performs vibration analysis.

[0062] In practice, the echo signals from each of the diaphragm cavity sensors 302 are ultimately transmitted back to the fiber optic Doppler laser interferometer 100 via the main optical fiber 200. The echo signals generated by each diaphragm cavity sensor 302 return to the fiber optic Doppler laser interferometer 100 at different times depending on their distance, allowing the fiber optic Doppler laser interferometer 100 to acquire the echo signals from several diaphragm cavity sensors 302 in parallel, and then analyze them independently, achieving "one machine, multiple points" acquisition and monitoring.

[0063] In the specific implementation process, such as Figure 2 As shown, the fiber optic Doppler laser interferometer 100 may include:

[0064] Narrow linewidth laser 110 is used to generate narrow linewidth laser signals;

[0065] The first fiber coupler 120 has its input end connected to the output end of the narrow linewidth laser 110;

[0066] The optical signal enhancement module 130 has its input end connected to the first output end of the first optical fiber coupler 120;

[0067] The circulator 140 has its first transmission end connected to the output end of the optical signal enhancement module 130, and its second transmission end connected to the trunk optical fiber 200.

[0068] The dual-frequency modulation module 150 has its input end connected to the second output end of the first fiber optic coupler 120;

[0069] The second fiber optic coupler 160 has its first input end connected to the output end of the dual-frequency modulation module 150, and its second input end connected to the third transmission end of the circulator 140.

[0070] The dual-balanced photodetector 170 has its two input terminals connected to the two output terminals of the second fiber coupler 160, respectively.

[0071] The signal processing module 180 has its input terminal connected to the output terminal of the dual-balanced photodetector 170.

[0072] In one embodiment, the narrow linewidth laser 110 can be implemented using a 1550nm narrow linewidth laser.

[0073] In another embodiment, the narrow-linewidth laser 110 can be a tunable semiconductor laser. Using a narrow-linewidth tunable semiconductor laser, the system can control its output wavelength to switch or scan multiple discrete wavelengths as needed, with each wavelength corresponding to one of the branch sensing units 300. When a light signal of a specific wavelength is transmitted to a branch sensing unit 300, it is coupled into the sensing fiber 400 via the corresponding beam splitter 301, and finally illuminates the diaphragm cavity sensor 302 whose wavelength is matched. At this time, several diaphragm cavity sensors 302 can be configured to match different wavelengths. The diaphragm cavity sensors 302 undergo micro-vibration under the action of the acoustic field, and the echo signal carries Doppler frequency shift information and returns along the original path, coupling back to the trunk fiber 200 via the sensing fiber 400 and the beam splitter 301, ultimately reaching the second input end of the second fiber coupler 160 via the circulator 140. Meanwhile, the second output of the first fiber coupler 120 outputs a local reference light. The dual-frequency modulation module 150 performs a two-stage offset of the local reference light at a fixed frequency to ensure that the interference signal is located in the high signal-to-noise ratio intermediate frequency region. The echo signals from different branches are naturally isolated due to their different carrier wavelengths, achieved through wavelength division multiplexing—that is, each wavelength channel is orthogonal in the optical domain, allowing for parallel processing without time gating. Furthermore, all echo signals interfere with the corresponding wavelength frequency-shifted reference light in the second fiber coupler 160, generating a heterodyne signal, which is then converted into an electrical signal by the dual-balanced photodetector 170. At this point, the signal processing module 180 can demodulate based on frequency-modulated continuous wave (FMCW). Specifically, it can use Hilbert transform to calculate the phase difference of each channel, effectively suppressing the inherent phase noise of the narrow-linewidth laser. Finally, it synchronously extracts the vibration and acoustic information of multiple branch sensing units 300, realizing high-density, low-crosstalk, parallel fiber Doppler acoustic sensing based on wavelength division multiplexing.

[0074] In practical implementation, the first fiber coupler 120 splits the narrow-linewidth laser signal output by the narrow-linewidth laser 110 into two paths—signal light and local reference light—thereby constructing the optical path foundation for a heterodyne interferometry system. Specifically, the first fiber coupler 120 can be implemented using a fiber coupler with a 90:10 splitting ratio.

[0075] Specifically, the optical signal enhancement module 130 performs online optical amplification of the signal light to compensate for transmission losses caused by the structure of the trunk fiber 200 and the plurality of branch sensing units 300, ensuring that each branch sensing unit 300 receives sufficient excitation optical power and improving the echo signal strength and system signal-to-noise ratio. In one specific embodiment, the optical signal enhancement module 130 includes:

[0076] The wavelength division multiplexer 131 has its first input terminal connected to the first output terminal of the first fiber optic coupler 120.

[0077] Pump laser 132, the output of which is connected to the second input of wavelength division multiplexer 131;

[0078] Erbium-doped fiber 133 has its input end connected to the common terminal of wavelength division multiplexer 131 and its output end connected to the first transmission terminal of circulator 140.

[0079] In the specific implementation process, the wavelength division multiplexer 131 performs low-loss, unidirectional beam combining of the signal light from the narrow linewidth laser 110 and the pump light from the pump laser 132 on the main optical path, thereby realizing online optical amplification of the excitation signal light and improving the system's detection sensitivity and operating distance.

[0080] Specifically, the pump laser 132 uses a laser with a different wavelength than the narrow linewidth laser 110, such as a 980nm pump laser. Its function is to provide excitation energy for the optical signal enhancement module 130, amplify the signal light through stimulated emission, and improve the system's detection sensitivity, dynamic range, and maximum operating distance. In this embodiment, the backbone fiber 200, the beam splitter 301 in the branch sensing unit 300, etc., all introduce optical losses. Especially when there are many branch sensing units 300 or they are far apart, the optical power at the end may not be sufficient to excite the vibrating diaphragm cavity sensor 302 to generate an effective echo. The pump laser 132 enhances the signal light power in advance by supporting the operation of the optical signal enhancement module 130, ensuring that the far-end sensor still has sufficient excitation light.

[0081] In specific operation, under the excitation of the pump laser 132, the erbium ions in the erbium-doped fiber 133 form population inversion. When the signal light passes through the erbium-doped fiber 133, stimulated emission is triggered, generating photons with the same frequency, phase, and polarization as the original signal light, thereby significantly amplifying the signal light.

[0082] More preferably, the optical signal enhancement module 130 further includes an optical isolator 134 and a filter 135. The output end of the erbium-doped fiber 133 is connected to the input end of the optical isolator 134, the output end of the optical isolator 134 is connected to the input end of the filter 135, and the output end of the filter 135 is connected to the first transmission end of the circulator 140. The optical isolator 134 prevents reverse light reflection from entering the erbium-doped fiber 133. While amplifying the signal light, the erbium-doped fiber 133 spontaneously generates broadband optical noise, known as amplified spontaneous emission. The filter 135 can filter out this amplified spontaneous emission noise.

[0083] In the specific implementation process, the circulator 140 injects the amplified signal light unidirectionally into the trunk optical fiber 200 and guides the echo signals from each of the vibrating diaphragm cavity sensors 302 to the dual-frequency modulation module 150, while isolating the interference of the reverse light on the optical signal enhancement module 130.

[0084] In the specific implementation process, the input end of the dual-frequency modulation module 150 is connected to the second output end of the first fiber coupler 120, and a fixed and stable frequency offset is applied to the local reference light output from the first fiber coupler 120 so that a controllable beat frequency signal is generated between it and the echo signal light, thereby realizing heterodyne interference detection, avoiding low-frequency noise interference, and improving the system sensitivity and dynamic range.

[0085] Specifically, the dual-frequency modulation module 150 is used to perform two frequency shifts on the local reference light, thereby achieving a net frequency shift with high stability and low phase noise, such as... Figure 2 As shown, the dual-frequency modulation module 150 includes a first acousto-optic modulator 151 and a second acousto-optic modulator 152. The input end of the first acousto-optic modulator 151 is connected to the second output end of the first fiber optic coupler 120, and is used to perform a first-stage frequency shift on the local reference light output by the first fiber optic coupler 120. The input end of the second acousto-optic modulator 152 is connected to the output end of the first acousto-optic modulator 151, and its output end is connected to the first input end of the second fiber optic coupler 160, and is used to perform a second-stage frequency shift on the local reference light that has undergone the first-stage frequency shift. Preferably, the first acousto-optic modulator 151 can be an 80MHz acousto-optic modulator, and the second acousto-optic modulator 152 can be a 40MHz acousto-optic modulator. The local reference light first passes through the first acousto-optic modulator 151, introducing a fixed frequency offset (e.g., +80 MHz), and then passes through the second acousto-optic modulator 152 again, introducing a second frequency offset (e.g., +40 MHz). This ensures that the interference signal is located in the mid-frequency region with a high signal-to-noise ratio, so that the local reference light and the echo signal have the same frequency scanning characteristics, forming a differential heterodyne structure for the signal processing module 180 to receive and process. Preferably, after receiving the signal, the signal processing module 180 can calculate the phase difference using Hilbert transform, effectively suppressing the phase noise of the light source.

[0086] In specific implementation, the second fiber coupler 160 combines the echo signal light with the dual-frequency modulated local reference light and generates interference, producing a heterodyne interference signal. This signal is then converted into an electrical signal by the dual-balanced photodetector 170, realizing the conversion from optical domain phase change to electrical domain signal. Specifically, the second fiber coupler 160 can be implemented using a 50:50 splitting ratio fiber coupler.

[0087] In the specific implementation process, the dual-balanced photodetector 170 converts the interference signal output by the second fiber coupler 160 into an electrical signal, and suppresses common-mode intensity noise through a differential balance structure while retaining the interference term, thereby achieving high-sensitivity and low-noise photoelectric conversion.

[0088] In specific implementation, the signal processing module 180 receives the electrical signal output by the dual-balanced photodetector 170. After receiving the electrical signal, it can perform vibration analysis, which may include phase demodulation of the heterodyne interference signal, extraction of phase change information of the echo signal, and analysis of the phase information, such as differential operations and time-frequency analysis, to calculate the displacement, velocity, and acceleration information of the vibration diaphragm cavity sensor 302, thereby achieving high-precision reconstruction of the sensed vibration signal. In specific implementation, the signal processing module 180 can be implemented using an FPGA chip circuit, or other implementation methods, which are not limited here.

[0089] In specific implementation, the diaphragm cavity sensor 302 is correspondingly provided with an optical fiber collimator 303. One end of the diaphragm cavity sensor 302 is connected to the optical fiber collimator 303, and the other end of the optical fiber collimator 303 is connected to the sensing optical fiber 400. The function of the optical fiber collimator 303 is to collimate the optical signal in the sensing optical fiber 400 into parallel light, so that it can be stably transmitted within the diaphragm cavity sensor 302 and efficiently reflected back into the optical fiber, thereby improving the signal-to-noise ratio, stability, and sensor sensitivity of the FP interference signal.

[0090] More preferably, this embodiment also includes:

[0091] Visible laser 500, used to generate visible laser light;

[0092] The third fiber coupler 600 has its first input end connected to the second transmission end of the circulator 140, its second input end connected to the visible light laser 500, and its output end connected to the trunk fiber 200. The visible light laser 500 and the third fiber coupler 600 form a visible light injection and monitoring channel, which can be used to confirm the optical path connection during on-site installation. Simultaneously, the visible laser generated by the visible light laser 500 and the narrow-linewidth laser signal are combined through the third fiber coupler 600 and finally enter the fiber collimator 303. Thus, visible light can be used as guiding light to achieve visual alignment of the signal light in the fiber collimator 303, ensuring that the signal light is accurately focused on each of the vibrating diaphragm cavity sensors 302. Preferably, the visible light laser 500 can be a visible-red laser, such as a 650nm wavelength visible-red laser.

[0093] Based on the scheme of this embodiment, the narrow-linewidth laser 110 of this application generates a narrow-linewidth laser signal. The narrow-linewidth laser signal enters the first fiber coupler 120 and is split into two paths: one path serves as the signal light (approximately 50%), which continues to propagate forward and is ultimately injected into the trunk fiber 200; the other path serves as the reference light (approximately 50%), which is guided to the dual-frequency modulation module 150 and used as a local reference light for subsequent interference. The signal light enters the wavelength division multiplexer 131, where it undergoes wavelength multiplexing with the pump light output from the pump laser 132. After beam combining, they are jointly injected into the erbium-doped fiber 133. Under the excitation of the pump light, stimulated emission amplification occurs in the signal light, resulting in a significant increase in optical power and compensating for subsequent long-distance transmission and splitting losses. The amplified signal light first passes through the optical isolator 134 and filter 135 and then enters the circulator 140. It is output from the second transmission end of the circulator 140 and is ready to be injected into the trunk fiber 200. When the echo signal returns, it enters from the splitter end of the beam splitter 301, is output from its first trunk connection end and enters the trunk fiber 200, thereby guiding it to the second fiber coupler 160.

[0094] When a visible light laser 500 (e.g., 650 nm red light) is provided, the second transmission end of the circulator 140 is connected to the first input end of the third fiber coupler 600, and the visible light laser 500 is connected to the second input end of the third fiber coupler 600. The third fiber coupler 600 combines the signal light and the visible light, enabling co-fiber transmission. The visible light does not participate in sensing and is only used for on-site installation, debugging, and fault location. The combined optical signal (including the main signal light and the visible light) enters the trunk fiber 200 and is transmitted along the trunk fiber 200 to the remote branch sensing unit 300. The trunk fiber 200 connects to the beam splitters 301 of each branch sensing unit 300, dividing the optical signal transmitted on the trunk fiber 200 equally or proportionally to multiple sensing fibers 400, achieving distributed or multi-point parallel sensing. The beam splitter 301 of the branch sensing unit 300 guides the light through the sensing optical fiber 400, and finally reaches the vibrating diaphragm cavity sensor 302 at the end.

[0095] The diaphragm cavity sensor 302 is a reflective Fabry-Perot structure. When an external sound field acts on the diaphragm cavity sensor 302, the diaphragm surface inside undergoes a slight displacement, causing a change in the cavity length, thereby altering the phase of the reflected light and forming an echo signal carrying acoustic information, which returns along the original optical path. After being reflected from the diaphragm cavity sensor 302, the echo signal returns to the beam splitter 301 via the sensing fiber 400, is output from the first trunk connection end of the beam splitter 301, enters the trunk fiber 200, and is transmitted back to the fiber optic Doppler laser interferometer 100. The echo signal enters the receiving optical path via the circulator 140 and reaches the second fiber coupler 160. Simultaneously, the local reference light split from the first fiber coupler 120 passes successively through the first acousto-optic modulator 151 and the second acousto-optic modulator 152 of the dual-frequency modulation module 150, generating a fixed frequency shift each time (e.g., +80 MHz and +40 MHz, for a total frequency shift of +120 MHz). The echo signal (unshifted) and the dual-frequency modulated local reference light (e.g., +120 MHz) are simultaneously injected into the second fiber coupler 160. Since they satisfy the coherence condition, they interfere within the second fiber coupler 160, generating a heterodyne beat frequency signal. The light intensity changes periodically with the phase difference. The two output ports of the second fiber coupler 160 are respectively connected to the two input ports of the dual-balanced photodetector 170. Through differential amplification, common-mode noise is suppressed, and the output is an electrical signal containing only the interference term, superimposed with the phase modulation caused by acoustic vibration. The electrical signal is received by the signal processing module 180, obtaining the electrical signals corresponding to multiple branch sensing units 300, realizing multi-point acoustic / vibration acquisition. Furthermore, the signal processing module 180 can perform vibration analysis on the received electrical signals, ultimately outputting distributed acoustic / vibration sensing results.

[0096] The entire system of this application realizes a complete closed loop from narrow linewidth laser output, sequentially passing through optical path beam splitting and amplification, signal injection, acoustic sensing, echo reception, heterodyne interference, and photoelectric conversion. It uses the aforementioned diaphragm cavity sensor 302 to sense acoustic vibrations in the environment, forming a Doppler laser microphone architecture based on heterodyne interference. A fixed intermediate frequency is introduced through the dual-frequency modulation module 150, shifting the interference signal to a high-frequency region far from 1 / f noise. Combined with the dual-balanced photodetector 170, intensity noise and environmental interference are effectively suppressed. Compared with traditional beat-frequency interferometry schemes, the dynamic range of this application's system is improved by approximately 20 dB, and the minimum detectable sound pressure level is significantly reduced. It achieves high-fidelity, long-distance, all-weather sensing of weak acoustic signals and is suitable for long-distance, multi-point vibration and acoustic signal detection.

[0097] In the specific implementation process, the vibrating diaphragm cavity sensor 302 uses sound waves to cause the vibrating diaphragm inside to vibrate, causing the diaphragm surface to shift and changing the length of the Fabry-Perot cavity, resulting in the modulation of the phase or interference state of the reflected light. The original sound pressure signal can be restored by demodulating the optical signal change through the signal processing module 180.

[0098] Specifically, such as Figure 3 and 4 As shown, the specific structure of the vibrating diaphragm cavity sensor 302 can be as follows:

[0099] The cavity structure 3021 is a hollow structure 30211, with through holes 30212 at both ends communicating with the hollow structure 30211. A plug-in part is provided on the side wall near one end, and the through hole 30212 at the other end is connected to the sensing optical fiber 400.

[0100] A metal vibrating plate 3022 with a thickness of less than 0.1 mm has a central portion 30221 and a peripheral portion 30222. A hollow portion 30223 is provided between the central portion 30221 and the peripheral portion 30222. The peripheral portion 30222 is installed in the insertion portion so that the hollow portion 30223 and the central portion 30221 are placed in the hollow structure 30211.

[0101] Specifically, the cavity structure 3021 can be configured as a hollow columnar or tubular structure, with through holes 30212 at both ends to allow light to pass through.

[0102] The insertion part is at a certain distance from the end face of one end of the cavity structure 3021, so that after the metal vibrating plate 3022 is installed in the insertion part, there is a certain distance between it and the end face of one end of the cavity structure 3021. This gap is set to form a controllable and closed back cavity between the metal vibrating plate 3022 and the end face of the cavity structure 3021, which serves as an acoustic impedance matching cavity to adjust the frequency response characteristics of the diaphragm cavity sensor 302 and to provide buffer space during large amplitude vibrations, preventing the metal vibrating plate 3022 from contacting the cavity. At the same time, this gap ensures that the length of the Fabry-Perot optical resonator is controllable, avoiding optical short circuits and improving the sensor's sensitivity, linearity, and environmental adaptability.

[0103] In one embodiment, the plug-in portion can be configured as an annular groove, and the metal vibrating plate 3022 can be fixed by adhesive bonding when it is installed into the plug-in portion, so that the metal vibrating plate 3022 is sealed and fixed on the plug-in portion.

[0104] Specifically, the metal vibrating plate 3022 has a thickness of less than 0.1 mm, preferably 0.05 mm, making it an ultra-thin flexible diaphragm that improves the response sensitivity to weak sound pressure. Its outer periphery 30222 can be configured as an annular region for mechanical fixation, insertion, and installation in the insertion portion of the cavity structure 3021. The hollow portion 30223 is located between the outer periphery 30222 and the central portion 30221, and can be configured as several arc-shaped regions; the central portion 30221 is located at the very center of the metal vibrating plate 3022, serving as an optical reflecting surface and directly participating in Fabry-Perot cavity (FP) interference.

[0105] In the diaphragm cavity sensor 302, the optical path process is as follows: the sensing fiber 400 guides the laser into the cavity structure 3021 of the diaphragm cavity sensor 302, which then illuminates the center part 30221 of the metal vibrating plate 3022 and is reflected to form an echo signal. The echo signal returns through the through hole 30212. The acoustic path process is as follows: external sound waves pass through the through hole 30212 and act on the outer surface of the center part 30221, causing the center part 30221 to shift, resulting in a change in the FP cavity length, thereby modulating the phase of the reflected light and realizing high-sensitivity acoustic sensing.

[0106] The diaphragm cavity sensor 302 described in this embodiment improves high-frequency response speed and dynamic characteristics through its miniaturized design. It utilizes the acoustic cavity resonance mechanism to achieve adaptive enhancement of acoustic signals in the low-to-mid frequency range, enabling it to not only achieve wide frequency coverage in the entire audible frequency range of 20 Hz – 20 kHz, but also possess excellent response characteristics of high sensitivity, low distortion, and high dynamics.

[0107] This application embodiment also provides a distributed fiber optic Doppler acoustic sensing method, applied to the distributed fiber optic Doppler acoustic sensing system described in this embodiment, the method comprising:

[0108] Narrow linewidth laser signals are emitted from the fiber optic Doppler laser interferometer and transmitted to each of the branch sensing units via the main optical fiber.

[0109] When the narrow linewidth laser signal reaches the branch sensing unit, a portion of the optical power is coupled to the sensing fiber through the beam splitter of the branch sensing unit, and finally irradiates the vibrating diaphragm cavity sensor.

[0110] The diaphragm cavity sensor generates an echo signal carrying Doppler frequency shift information. The echo signal passes through the sensing fiber, the splitting end of the beam splitter, and the first trunk connection end of the beam splitter, and enters the reverse transmission path of the trunk fiber. Finally, it is transmitted to the fiber optic Doppler laser interferometer so that the fiber optic Doppler laser interferometer can analyze the echo signal.

[0111] The method in this embodiment supports continuous, multi-location real-time sensing of large-area or long-distance environments, achieving highly sensitive and interference-resistant synchronous acquisition of acoustic and vibration signals from multiple distant distribution points, demonstrating good practicality and scalability. Furthermore, this application, combined with fiber optic Doppler laser vibrometer technology, can detect minute frequency changes (i.e., Doppler shifts) caused by sound waves or vibrations, thereby achieving high-precision capture of extremely weak acoustic signals. Moreover, the diaphragm cavity sensor in this embodiment is sensitive to external sound pressure, efficiently converting sound waves into measurable optical phase changes, significantly improving system sensitivity.

[0112] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the technical solution of the present invention, and are not intended to limit the specific implementation of the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the claims of the present invention should be included within the protection scope of the claims of the present invention.

Claims

1. A distributed fiber optic Doppler acoustic sensing system, characterized in that, The system includes: A fiber optic Doppler laser interferometer is used to generate narrow linewidth laser signals and receive echo signals for vibration analysis. The main optical fiber is connected at one end to the fiber optic Doppler laser interferometer. Several branch sensing units are distributed along the main optical fiber, and each branch sensing unit includes: The beam splitter has its first trunk connection end and second trunk connection end connected to the trunk optical fiber, respectively. The beam splitter splits the narrow linewidth laser signal input from the trunk optical fiber to the first trunk connection end and then outputs it from its splitting end and the second trunk connection end, respectively. A vibrating diaphragm cavity sensor is used to be placed in the measured environment, and it is connected to the beam splitter end of the beam splitter through a sensing optical fiber; the vibrating diaphragm cavity sensor is a reflective Fabry-Perot structure; The vibrating diaphragm cavity sensor reflects an echo signal carrying a Doppler frequency shift. The echo signal passes through the sensing fiber, the splitting end of the beam splitter, and the first trunk connection end of the beam splitter, enters the reverse transmission path of the trunk fiber, and is finally transmitted back to the fiber optic Doppler laser interferometer. The fiber optic Doppler laser interferometer includes: Narrow linewidth lasers are used to generate narrow linewidth laser signals. A first fiber coupler, the input of which is connected to the output of the narrow linewidth laser; An optical signal enhancement module, the input of which is connected to the first output of the first optical fiber coupler; The circulator has its first transmission end connected to the output end of the optical signal enhancement module and its second transmission end connected to the trunk optical fiber. A dual-frequency modulation module, the input of which is connected to the second output of the first fiber optic coupler; The second fiber optic coupler has its first input end connected to the output end of the dual-frequency modulation module and its second input end connected to the third transmission end of the circulator. A dual-balanced photodetector, whose two input terminals are respectively connected to the two output terminals of the second fiber coupler. The signal processing module has its input terminal connected to the output terminal of the dual-balanced photodetector; The dual-frequency modulation module includes: The first acousto-optic modulator has its input end connected to the second output end of the first fiber optic coupler, and is used to perform a first-stage frequency shift on the laser signal output by the first fiber optic coupler. The second acousto-optic modulator has its input end connected to the output end of the first acousto-optic modulator and its output end connected to the first input end of the second fiber optic coupler. It is used to perform a second-stage frequency shift on the laser signal that has undergone the first-stage frequency shift.

2. The system according to claim 1, characterized in that, The optical signal enhancement module includes: A wavelength division multiplexer, the first input of which is connected to the first output of the first optical fiber coupler; A pump laser, the output of which is connected to the second input of the wavelength division multiplexer; The erbium-doped fiber has its input end connected to the common terminal of the wavelength division multiplexer and its output end connected to the first transmission terminal of the circulator.

3. The system according to claim 2, characterized in that, The optical signal enhancement module also includes an optical isolator and a filter. The output end of the erbium-doped fiber is connected to the input end of the optical isolator, the output end of the optical isolator is connected to the input end of the filter, and the output end of the filter is connected to the first transmission end of the circulator.

4. The system according to any one of claims 1 to 3, characterized in that, The diaphragm cavity sensor is equipped with a fiber optic collimator. The diaphragm cavity sensor is connected to one end of the fiber optic collimator, and the other end of the fiber optic collimator is connected to the sensing fiber.

5. The system according to claim 4, characterized in that, Also includes: Visible lasers are used to generate visible laser light. The third fiber coupler has its first input end connected to the second transmission end of the circulator, its second input end connected to the visible light laser, and its output end connected to the trunk fiber.

6. The system according to any one of claims 1 to 3, characterized in that, The narrow linewidth laser is a tunable semiconductor laser.

7. The system according to any one of claims 1 to 3, characterized in that, The vibrating diaphragm cavity sensor includes: The cavity structure is hollow, with through holes at both ends communicating with the hollow structure. A plug-in part is provided on the side wall near one end, and the through hole at the other end is connected to the sensing optical fiber. A metal vibrating plate with a thickness of less than 0.1 mm has a central part and a peripheral part, with a hollow part between the central part and the peripheral part. The peripheral part is installed in the plug-in part so that the hollow part and the central part are placed in the hollow structure.

8. The system according to claim 7, characterized in that, The cavity structure is configured as a hollow tubular structure with through holes at both ends.

9. The system according to claim 7, characterized in that, The insertion part is configured as an annular slot.

10. A distributed fiber optic Doppler acoustic sensing method, characterized in that, The method, applied to the distributed fiber optic Doppler acoustic sensing system according to any one of claims 1-9, comprises: Narrow linewidth laser signals are emitted from the fiber optic Doppler laser interferometer and transmitted to each of the branch sensing units via the main optical fiber. When the narrow linewidth laser signal reaches the branch sensing unit, a portion of the optical power is coupled to the sensing fiber through the beam splitter of the branch sensing unit, and finally irradiates the vibrating diaphragm cavity sensor. The diaphragm cavity sensor generates an echo signal carrying Doppler frequency shift information. The echo signal passes through the sensing fiber, the splitting end of the beam splitter, and the first trunk connection end of the beam splitter, and enters the reverse transmission path of the trunk fiber. Finally, it is transmitted to the fiber optic Doppler laser interferometer so that the fiber optic Doppler laser interferometer can analyze the echo signal.

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