Fiber-optic hydrophone array based on homodyne demodulation phase response and working method

Abstract

The application provides a fiber-optic hydrophone array based on homogenous phase response of heterodyne demodulation and a working method, the array is composed of large-scale push-pull core shaft structure fiber-optic hydrophones, and the heterodyne demodulation technology is adopted; when the center optical frequency of the short-arm optical pulse of the Mach-Zehnder interferometer in the fiber-optic hydrophone array is higher than the center optical frequency of the long-arm optical pulse, the inner core shaft of the fiber-optic hydrophone is configured as the long arm, and the outer core shaft is configured as the short arm; when the center optical frequency of the short-arm optical pulse of the Mach-Zehnder interferometer is lower than the center optical frequency of the long-arm optical pulse, the inner core shaft of each fiber-optic hydrophone is defined as the short arm, and the outer core shaft is defined as the long arm. Through accurate definition of the inner and outer core shaft configurations of the fiber-optic hydrophone under different demodulation parameters, the demodulation signal of different fiber-optic hydrophones is the same as the sound pressure signal response, so that the homogeneity and accuracy of the phase response of the fiber-optic hydrophone array are ensured, and the array can accurately detect and respond to the sound pressure signal.

Description

Technical Field

[0001] The present invention mainly relates to the field of optical fiber sensing technology, in particular to an optical fiber hydrophone array based on heterodyne demodulation phase response consistency and a working method. Background Art

[0002] Fiber-optic hydrophones offer the advantages of high sensitivity, strong resistance to electromagnetic interference, and ease of long-distance transmission and large-scale array deployment. They demonstrate enormous potential for application in underwater target detection, oil and gas exploration, and seismic monitoring. Numerous studies have explored the design and application of various fiber-optic hydrophones.

[0003] Phase consistency among fiber-optic hydrophones in a large-scale array is crucial to the overall array performance. Phase mismatches can increase the sidelobe level of the beamformer, reduce the spatial signal-to-noise gain, affect detection accuracy, and potentially increase the false alarm probability of the sonar system. Therefore, ensuring phase consistency among each unit in a fiber-optic hydrophone array is key to achieving high-performance underwater acoustic wave detection.

[0004] Although the phase offset problem of fiber-optic hydrophone arrays has been studied, most of this work focuses on back-end algorithm correction, while the impact of the front-end sensor installation method on phase offset has been relatively neglected. This focus on back-end processing ignores the potential for front-end optimization, especially in large-scale arrays. The large number of hydrophones makes back-end algorithm correction extremely complex and labor-intensive. In addition, heterodyne demodulation technology, a commonly used phase signal demodulation method in fiber-optic hydrophone sensing systems, has advantages such as a large dynamic range and a simple interference signal form. During the heterodyne demodulation process, the demodulation parameters will affect the demodulation results of the fiber-optic hydrophone, resulting in inconsistent phase responses. Therefore, optimizing the hydrophone installation method and demodulation parameter matching is crucial to simplifying subsequent calibration and reducing system maintenance costs. Summary of the Invention

[0005] In view of the deficiencies of the prior art, the present invention provides a fiber optic hydrophone array and a working method based on heterodyne demodulation with consistent phase response.

[0006] Specifically, the technical solution of the present invention is:

[0007] The optical fiber hydrophone array based on heterodyne demodulation phase response consistency includes a laser generating unit, an optical fiber coupler, a first acousto-optic modulator, a second acousto-optic modulator, a total delay optical fiber, N optical fiber beam splitting couplers, N time-dividing optical fibers, and N+1 optical fiber hydrophones;

[0008] A laser generating unit, configured to output a narrow linewidth single-frequency laser signal;

[0009] A fiber coupler, a first fiber beam splitting coupler, a first acousto-optic modulator, a second acousto-optic modulator, and a total delay fiber form a Mach-Zehnder interferometer. A narrow-linewidth single-frequency laser signal output by a laser generating unit is split into a first light beam and a second light beam through the fiber coupler for output. The first light beam output by the fiber coupler is transmitted through the short arm of the Mach-Zehnder interferometer, on which the first acousto-optic modulator is disposed. The second light beam output by the fiber coupler is transmitted through the long arm of the Mach-Zehnder interferometer, on which the second acousto-optic modulator and the total delay fiber are disposed.

[0010] The first output arm of the first optical fiber beam splitting coupler is connected to the input end of the first time-delay optical fiber, the output end of the nth time-delay optical fiber is connected to the input arm of the n+1th optical fiber beam splitting coupler, and the first output arm of the n+1th optical fiber beam splitting coupler is connected to the input end of the n+1th time-delay optical fiber, where n=1, 2, ..., N-1;

[0011] The second output arm of the nth optical fiber beam splitting coupler is connected to the nth optical fiber hydrophone in the optical fiber hydrophone array; the output end of the Nth time-delay optical fiber is connected to the N+1th optical fiber hydrophone in the optical fiber hydrophone array;

[0012] The structures of N+1 fiber optic hydrophones are the same, and the fiber optic hydrophones include a fiber optic coupler, two fiber optic coils, two Faraday rotator mirrors, an inner core shaft, an outer core shaft, and two fixed ends; the inner core shaft and the outer core shaft are both made of a material that can produce elastic deformation, and the inner core shaft and the outer core shaft are both tubular, the inner core shaft is sleeved in the outer core shaft, the inner core shaft and the outer core shaft are of the same length and are coaxially arranged, and the end faces of the inner core shaft and the outer core shaft are respectively on the same plane; the ends of the inner core shaft and the outer core shaft are respectively fixedly supported by two fixed ends, and the inner core shaft and The annular gap between the outer core shafts is filled with air to form an air-back structure; two optical fiber coils are respectively wound around the outside of the inner core shaft and the outer core shaft; the two arms on the second side of the optical fiber coupler are respectively connected to one end of the two optical fiber coils; the other ends of the two optical fiber coils are respectively connected to a Faraday rotator mirror for eliminating polarization fading at the end of the interferometer; according to the different lengths of the optical fiber coils wound around the outer sides of the inner core shaft and the outer core shaft, the core shaft with the longer optical fiber coil is used as the long arm, and the other core shaft with the shorter optical fiber coil is used as the short arm.

[0013] Furthermore, the output end of the fiber coupler outputting the first light beam is connected to the input end of the first acousto-optic modulator, the output end of the first acousto-optic modulator is connected to the first input arm of the first fiber beam splitting coupler, the output end of the fiber coupler outputting the second light beam is connected to the input end of the second acousto-optic modulator, the output end of the second acousto-optic modulator is connected to the input end of the total delay fiber, and the output end of the total delay fiber is connected to the second input arm of the first fiber beam splitting coupler;

[0014] Further, the laser generating unit comprises a fiber isolator, a fiber isolator, a doped fiber amplifier, the output end of the laser is connected to the input end of the fiber isolator, the laser outputs a narrow line width single frequency laser signal into the fiber isolator, thereby avoiding the influence of backward transmission light on the light source; the output end of the fiber isolator is connected to the doped fiber amplifier, the optical signal is amplified, and the output end of the doped fiber amplifier is connected to the input end of the fiber coupler.

[0015] Further, the second output arm of the nth fiber beam splitting coupler is connected to the first arm of the first side of the nth fiber coupler of the nth fiber hydrophone, the second output arm of the Nth fiber beam splitting coupler is connected to the first arm of the first side of the Nth fiber coupler of the Nth fiber hydrophone, and the output end of the Nth delay fiber is connected to the first arm of the first side of the N+1th fiber coupler of the N+1th fiber hydrophone in the fiber hydrophone array.

[0016] Further, the application further comprises a detection unit, the detection unit comprises N fiber beam combining couplers, a photodetector, a data acquisition card and a computer, the signals detected by the N+1 fiber hydrophones are combined and output to the photodetector through the N fiber beam combining couplers, the photodetector is used for photoelectric conversion, the electrical signals output by the photodetector are collected through the data acquisition card, and the computer is used for signal processing and analysis.

[0017] The working method of the fiber hydrophone array based on the homologous phase response of the heterodyne demodulation comprises the following steps:

[0018] The center optical frequency size of the optical pulses of the long arm and the short arm of the Mach-Zehnder interferometer in the heterodyne demodulation method is acquired;

[0019] When the center optical frequency of the short arm optical pulse of the Mach-Zehnder interferometer is higher than that of the long arm optical pulse, the inner core axis of each fiber hydrophone is configured as the long arm, and the outer core axis is configured as the short arm; when the center optical frequency of the short arm optical pulse of the Mach-Zehnder interferometer is lower than that of the long arm optical pulse, the inner core axis of each fiber hydrophone is defined as the short arm, and the outer core axis is defined as the long arm;

[0020] After the inner and outer core axes of each fiber hydrophone in the fiber hydrophone array are configured, the entire fiber hydrophone array is installed and laid.

[0021] The application has the following beneficial effects:

[0022] The present invention proposes a fiber optic hydrophone array and operating method based on heterodyne demodulation phase response consistency. The fiber optic hydrophone array is composed of large-scale push-pull core-shaft fiber optic hydrophones. At the same time, heterodyne demodulation technology is used to obtain the center optical frequency of the long and short arms of the Mach-Zehnder interferometer in the heterodyne demodulation method. When the center optical frequency of the short arm optical pulse of the Mach-Zehnder interferometer is higher than the center optical frequency of the long arm optical pulse, the inner core of the fiber optic hydrophone is configured as the long arm, and the outer core is configured as the short arm. When the center optical frequency of the short arm optical pulse of the Mach-Zehnder interferometer is lower than the center optical frequency of the long arm optical pulse, the inner core of each fiber optic hydrophone is defined as the short arm, and the outer core is defined as the long arm. By precisely defining the inner and outer core configurations of the fiber optic hydrophones under different demodulation parameters, it is ensured that when each fiber optic hydrophone receives a sound pressure signal, the response relationship between its demodulated signal and the sound pressure signal remains consistent, thereby ensuring the consistency and accuracy of the phase response of the fiber optic hydrophone array, allowing the array to accurately detect and respond to the sound pressure signal.

[0023] This front-end optimization method for matching hydrophone installation and demodulation parameters can effectively reduce the complexity of back-end algorithm calibration for large-scale arrays, lowering system maintenance costs. It also provides a more efficient and economical solution for ensuring phase alignment between hydrophones, helping to promote the development and application of fiber-optic hydrophone array technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying any creative work.

[0025] Figure 1 This is a schematic structural diagram of a fiber optic hydrophone array based on heterodyne demodulation phase response consistency provided by an embodiment;

[0026] Figure 2 This is a schematic structural diagram of a fiber optic hydrophone provided by an embodiment;

[0027] Figure 3 This is a comparison diagram of the demodulated signal and the sound pressure signal after the core shaft of the optical fiber hydrophone in one embodiment is configured using the working method of the present invention;

[0028] Figure 4 This is a comparison diagram of the demodulated signal and the sound pressure signal of the fiber optic hydrophone core shaft in one embodiment after the configuration is not performed using the working method of the present invention;

[0029] Numbers in the figure:

[0030] 1. Laser, 2. Fiber isolator, 3. Erbium-doped fiber amplifier, 4. Fiber coupler, 5-1. First acousto-optic modulator, 5-2. Second acousto-optic modulator, 6. Total delay fiber, 7-1. First fiber beam splitting coupler, 7-2. Second fiber beam splitting coupler, 7-N. Nth fiber beam splitting coupler, 8-1. First time-division delay fiber, 8-2. Second time-division delay fiber, 8-N. Nth time-division delay fiber, 9-1. First fiber hydrophone, 9-2. Second fiber hydrophone, 9-N. Nth fiber hydrophone, 9-(N+1), N+1th fiber hydrophone, 10-1. First fiber beam combining coupler, 10-2. Second fiber beam combining coupler, 10-N. Nth fiber beam combining coupler, 11. Photodetector, 12. Data acquisition card, 13. Computer;

[0031] 9-1-1, first fiber optic coupler, 9-2-1, second fiber optic coupler, 9-N-1, Nth fiber optic coupler, 9-(N+1)-1, N+1th fiber optic coupler, 9-1-2, first fiber optic coil, 9-1-3, first Faraday rotator mirror, 9-1-4, first inner core shaft, 9-1-5, first outer core shaft, 9-1-6, first fixed end. DETAILED DESCRIPTION

[0032] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0033] Reference Figure 1One embodiment of the present invention provides a fiber optic hydrophone array based on heterodyne demodulation phase response consistency, comprising a laser 1, a fiber isolator 2, an erbium-doped fiber amplifier 3, a fiber coupler 4, a first acousto-optic modulator 5-1, a second acousto-optic modulator 5-2, a total delay fiber 6, N fiber beam splitting couplers, N time-delay fibers, an N+1 fiber optic hydrophone, N fiber beam combining couplers, a photodetector 11, an acquisition card 12, and a computer 13. The N fiber beam splitting couplers are a first fiber beam splitting coupler 7-1, a second fiber beam splitting coupler 7-2, ..., and an Nth fiber beam splitting coupler 7-N. The N time-delay fibers are a first time-delay fiber 8-1, a second time-delay fiber 8-2, ..., and an Nth time-delay fiber 8-N. The N+1 fiber optic hydrophones are a first fiber optic hydrophone 9-1, a second fiber optic hydrophone 9-2, ..., an Nth fiber optic hydrophone 9-N, and an N+1th fiber optic hydrophone 9-(N+1). The N optical fiber combining couplers are respectively a first optical fiber combining coupler 10 - 1 , a second optical fiber combining coupler 10 - 2 , . . . , an Nth optical fiber combining coupler 10 -N.

[0034] The fiber coupler 4, the first fiber beam splitting coupler 7-1, the first acousto-optic modulator 5-1, the second acousto-optic modulator 5-2, and the total delay fiber 6 form a Mach-Zehnder interferometer. The narrow-linewidth single-frequency laser signal output by the laser 1 is split into a first light beam and a second light beam through the fiber coupler 4. The first light beam output by the fiber coupler 4 is transmitted through the short arm of the Mach-Zehnder interferometer, on which the first acousto-optic modulator 5-1 is provided. The second light beam output by the fiber coupler 4 is transmitted through the long arm of the Mach-Zehnder interferometer, on which the second acousto-optic modulator 5-2 and the total delay fiber 6 are provided.

[0035] The output of laser 1 is connected to the input of fiber isolator 2. Laser 1 outputs a narrow-linewidth, single-frequency laser signal that is input into fiber isolator 2 to prevent the influence of backward-propagating light on the light source. The output of fiber isolator 2 is connected to an erbium-doped fiber amplifier 3 to amplify the optical signal. The output of erbium-doped fiber amplifier 3 is connected to the input of fiber coupler 4. The amplified light beam from erbium-doped fiber amplifier 3 is split into a first light beam and a second light beam for output via fiber coupler 4. The output of the first light beam from fiber coupler 4 is connected to the input of a first acousto-optic modulator 5-1. The output of the first acousto-optic modulator 5-1 is connected to the first input arm of a first fiber beam splitter coupler 7-1. The output of the second light beam from fiber coupler 4 is connected to the input of a second acousto-optic modulator 5-2. The output of the second acousto-optic modulator 5-2 is connected to the input of a total delay fiber 6. The output of the total delay fiber 6 is connected to the second input arm of the first fiber beam splitter coupler 7-1. The first output arm of the first optical fiber splitting coupler 7-1 is connected to the input end of the first time-delay optical fiber 8-1, the output end of the nth time-delay optical fiber 7-n is connected to the input arm of the n+1th optical fiber splitting coupler 7-(n+1), and the first output arm of the n+1th optical fiber splitting coupler 7-(n+1) is connected to the input end of the n+1th delay optical fiber 8-(n+1), where n=1,2,......N-1.

[0036] The second output arm of the nth optical fiber beam splitting coupler 7-n is connected to the nth optical fiber hydrophone 9-n in the optical fiber hydrophone array, where n = 1, 2, ..., N-1. The output end of the Nth time-delay optical fiber 8-N is connected to the N+1th optical fiber hydrophone 9-(N+1) in the optical fiber hydrophone array.

[0037] The signals detected by the N+1 fiber optic hydrophones are combined by N fiber optic combining couplers and output to the photoelectric detector 11 for detection. The photoelectric conversion is performed by the photoelectric detector 11. The electrical signal output by the photoelectric detector 11 is collected by the data acquisition card 12 and processed and analyzed by the computer 13.

[0038] The narrow-linewidth, single-frequency laser signal output by laser 1 is input into fiber isolator 2. The configuration of fiber isolator 2 effectively prevents the influence of backward-transmitted light on laser 1, thereby preventing damage to laser 1. The output of fiber isolator 2 is connected to an erbium-doped fiber amplifier 3, which amplifies the optical signal. The output of erbium-doped fiber amplifier 3 is connected to the input of fiber coupler 4. The amplified light beam by erbium-doped fiber amplifier 3 is then split into two output beams by fiber coupler 4. The two beams pass through a first acousto-optic modulator 5-1 and a second acousto-optic modulator 5-2, respectively. A total delay fiber 6 is provided after the second acousto-optic modulator to separate the two heterodyne optical pulses in the time domain. Through N fiber beam splitting couplers, the optical pulse pair is input into N+1 fiber hydrophones in the fiber hydrophone array via N time-delay fibers. The N+1 fiber hydrophones detect the underwater acoustic signal. The optical pulse pairs carrying the sound pressure information are then fed into the corresponding fiber optic beam-combining couplers. The signals detected by the N+1 fiber optic hydrophones are combined and output by the N fiber optic beam-combining couplers. The signals are detected by the photodetector 11, which converts the detected optical signals into electrical signals for output. The electrical signals output by the photodetector 11 are collected by the data acquisition card 12 and processed and analyzed by the computer 13.

[0039] Reference Figure 2 In one embodiment, a fiber optic hydrophone is provided, which includes a fiber optic coupler, a fiber optic coil, a Faraday rotator mirror, an inner core shaft, an outer core shaft, and a fixed end. Figure 1 In the optical fiber hydrophone array based on the consistent heterodyne demodulation phase response provided by the embodiment shown, each optical fiber hydrophone has the same structure. The structure of each optical fiber hydrophone can be Figure 2 The fiber optic hydrophone structure provided in the illustrated embodiment. Figure 2The fiber optic hydrophone with a core structure shown in the figure features a simple design, is easy to array, and exhibits excellent pressure resistance. Specifically, it consists of two cores, an inner and outer core, with an air cavity formed between them. In this push-pull core structure, each arm of the fiber optic hydrophone functions as an independent sensing arm for detecting underwater acoustic signals. When fiber optic hydrophones are arrayed and beamforming technology is employed, they can accurately acquire the position of underwater targets. In theory, increasing the size of the array can improve target directionality. However, differences in the positions of the two sensing arms around the core can cause phase shifts in the fiber optic hydrophones, leading to phase mismatches between the fiber optic hydrophones. Phase consistency between the fiber optic hydrophones in a large array is crucial to the performance of the entire array. Phase mismatches can increase the sidelobe level of the beamformer, reduce the spatial signal-to-noise ratio gain, affect detection accuracy, and potentially increase the false alarm probability of the sonar system. Therefore, ensuring phase consistency among the elements in the fiber optic hydrophone array is key to achieving high-performance underwater acoustic wave detection. In the present invention, the N+1 optical fiber hydrophones have the same structure, and the structure of each optical fiber hydrophone is optimized as follows:

[0040] The structure of each fiber optic hydrophone is described using the first fiber optic hydrophone 9-1 as an example. The first fiber optic hydrophone 9-1 includes a first fiber optic coupler 9-1-1, a first fiber optic coil 9-1-2, a first Faraday rotator mirror 9-1-3, a first inner core shaft 9-1-4, a first outer core shaft 9-1-5, and a first fixed end 9-1-6. The first inner core shaft 9-1-4 and the first outer core shaft 9-1-5 are both made of a material capable of elastic deformation, such as elastic metal, plastic, or rubber. Both the first inner core shaft 9-1-4 and the first outer core shaft 9-1-5 are tubular and sleeved within the first outer core shaft 9-1-5. The first inner core shaft 9-1-4 and the first outer core shaft 9-1-5 are of the same length and are coaxially arranged. The end surfaces of the first inner core shaft 9-1-4 and the first outer core shaft 9-1-5 are respectively located on the same plane. The ends of the first inner core shaft 9-1-4 and the first outer core shaft 9-1-5 are fixedly supported by two first fixed ends 9-1-6, respectively. The annular space between the first inner core shaft 9-1-4 and the first outer core shaft 9-1-5 is filled with air, forming an air-back structure. Two first optical fiber coils 9-1-2 are wound around the outside of the first inner core shaft 9-1-4 and the outside of the first outer core shaft 9-1-5, respectively. The two arms on the second side of the first optical fiber coupler 9-1-1 in the first optical fiber hydrophone 9-1 are connected to one end of each of the two first optical fiber coils 9-1-2. The other ends of each of the two first optical fiber coils 9-1-2 are connected to a first Faraday rotator mirror 9-1-3, which is used to eliminate polarization fading at the end of the interferometer. Based on the different lengths of the optical fiber coils wound around the first inner core shaft 9-1-4 and the first outer core shaft 9-1-5, the longer optical fiber coil is designated as the long arm, while the shorter optical fiber coil is designated as the short arm.

[0041] Figure 1 In particular, the second output arm of the nth fiber splitter coupler 7-n is connected to the first arm of the first side of the nth fiber hydrophone 9-n in the fiber hydrophone array through the nth fiber coupler 9-n-1, where n = 1, 2, …, N-1. The second output arm of the Nth fiber splitter coupler 7-N is connected to the first arm of the first side of the Nth fiber hydrophone 9-N in the fiber hydrophone array through the Nth fiber coupler 9-N-1. The output end of the Nth delay fiber 8-N is connected to the first arm of the first side of the N+1th fiber hydrophone 9-(N+1) in the fiber hydrophone array through the N+1th fiber coupler 9-(N+1)-1.

[0042] Figure 1 In particular, the second arm of the first side of the N+1th fiber coupler 9-(N+1)-1 is connected to the two input arms of the input side of the Nth fiber combiner coupler 10-N through the second arm of the first side of the Nth fiber coupler 9-N-1, and the combiner arm of the output side of the Nth fiber combiner coupler 10-N is connected to the two input arms of the input side of the N-1th fiber combiner coupler 10-(N-1) through the second arm of the first side of the N-1th fiber coupler 9-(N-1)-1, and so on, until the combiner arm of the output side of the second fiber combiner coupler 10-2 is connected to the two input arms of the input side of the first fiber combiner coupler 10-1 through the second arm of the first side of the first fiber coupler 9-1-1, and the combiner arm of the output side of the first fiber combiner coupler 10-1 is combined and output to the photodetector 11 for detection.

[0043] The working principle of each fiber hydrophone is illustrated by taking the first fiber hydrophone 9-1 as an example: the optical pulse is input through the first arm of the first side of the first fiber coupler 9-1-1 of the first fiber hydrophone 9-1, and is divided into two beams by the first fiber coupler 9-1-1 and is output from the two arms of the second side of the first fiber coupler 9-1-1, one of which is emitted to the long arm and the other to the short arm. Under the action of the underwater acoustic pressure wave, the first inner core shaft 9-1-4 expands radially outward under tensile strain, while the first outer core shaft 9-1-5 contracts radially inward under compressive strain. This push-pull structure causes the light phase of the fiber coil wound on the first inner core shaft 9-1-4 and the first outer core shaft 9-1-5 to change out of phase. The light beams transmitted by the two fiber coils are reflected by the corresponding first Faraday mirror 9-1-3, and then return to the first fiber coupler 9-1-1 to form interference light again, which is output through the second arm of the first side of the first fiber coupler 9-1-1. This light can be monitored by a photodetector. The information of the underwater acoustic pressure signal is contained in the light phase change, which can be demodulated by heterodyne detection technology.

[0044] In particular, the phase expression of the first pulse light signal returned from the long arm end is:

[0045] (1)

[0046] The phase expression of the second pulsed light signal returned from the short arm end is:

[0047] (2)

[0048] wherein, and are the amplitudes of the two pulsed light signals, and are the frequencies of the two pulsed light signals, and are the initial phases of the two light fields and the phase drifts caused by environmental disturbances, and are the phase changes of the two light fields caused by the external signal to be measured, the expression of the interference light field is:

[0049] (3)

[0050] wherein, represents the complex conjugate of , , represent the complex conjugates of , , is the carrier frequency of the heterodyne interference signal, referred to as the heterodyne frequency. The initial phase of the heterodyne interference signal and the phase drift introduced by environmental disturbances are included. The final purpose of heterodyne detection is to obtain the sensing phase information . Usually, the reference signal and its quadrature signal with the same frequency as the heterodyne frequency are used for quadrature demodulation calculation to extract . Obviously, when > 0, is equal to the optical phase change caused by the underwater pressure wave in the long arm minus the optical phase change in the short arm:

[0051] (4)

[0052] However, when the heterodyne frequency < 0, the opposite will be obtained. Wherein the radial stress and the hoop stress of the first inner mandrel 9-1-4 and the first outer mandrel 9-1-5 can be represented as:

[0053] (5)

[0054] wherein, is the radial distance of the mandrel, and is a constant determined by the boundary conditions of the mandrel, = 1 and 2 correspond to the first inner core shaft 9-1-4 and the first outer core shaft 9-1-5, respectively. The inner diameter of the first inner core shaft 9-1-4 and the outer diameter of the first outer core shaft 9-1-5 are respectively loaded with the acoustic pressure signal P. Therefore, the boundary conditions for the radial pressure of the first inner core shaft 9-1-4 and the first outer core shaft 9-1-5 are:

[0055] (6)

[0056] (7)

[0057] in, is the inner radius of the mandrel, is the outer radius of the core shaft. Substituting equation (5) into equations (6) and (7), we get the solution of the equation, which is , , , Four constants:

[0058] (8)

[0059] (9)

[0060] Substituting it back into equation (1), we can get the radial and hoop stress expressions of the core shaft. According to Hooke's law, the hoop strain of the core shaft at the outer radius can be obtained ( )for:

[0061] (10)

[0062] in, and are Young's modulus and Poisson's ratio of the core axis respectively. The phase of light propagating through the optical fiber is defined as:

[0063] (11)

[0064] in, is the propagation constant of light, is the refractive index of the fiber core, λ is the wavelength of light in free space, is the length of the fiber around the core. Since β is the refractive index of the fiber core and the fiber diameter D, so the change in the optical phase in the fiber can be written as:

[0065] (12)

[0066] in, is the change in fiber length around the core, is the change in the propagation constant of light, The last term represents the waveguide mode dispersion effect caused by the change in fiber diameter, which can be ignored compared with the first two terms. Changes can be written as:

[0067] (13)

[0068] in, and are the radial strain and longitudinal strain of the optical fiber, respectively. and is the strain optical coefficient of the optical fiber. Therefore, by using equation (13), equation (12) can be written as:

[0069] (14)

[0070] in is the Poisson's ratio of the optical fiber. Assuming that the optical fiber is perfectly bonded to the outer surface of the core shaft, the longitudinal strain of the optical fiber is equal to the hoop strain of the pipe. Therefore, the phase changes of the optical fibers wrapped around the first inner core shaft 9-1-4 and the first outer core shaft 9-1-5 are:

[0071] (15)

[0072] (16)

[0073] When the first inner core shaft 9-1-4 is a long arm, substituting equations (15) and (16) into equation (4), the sensor output after heterodyne demodulation can be written as:

[0074] (17)

[0075] Consider another case, when the first outer core shaft 9-1-5 is a long arm, the demodulation result is:

[0076] (18)

[0077] According to the working principle of the above-mentioned fiber optic hydrophone, it is known that matching the configuration of the inner and outer core shafts with the demodulation parameters is crucial to ensure the accuracy of the demodulated signal. As a preferred embodiment, in one embodiment, based on Figure 1 The embodiment shown provides a fiber optic hydrophone array structure based on a consistent heterodyne demodulation phase response, wherein each fiber optic hydrophone has the same structure, and the structure of each fiber optic hydrophone adopts Figure 2The fiber optic hydrophone structure provided in the illustrated embodiment further proposes a method for configuring each core axis in each fiber optic hydrophone as a long arm and a short arm to obtain the center optical frequency of the optical pulses of the long and short arms of the Mach-Zehnder interferometer in a heterodyne demodulation method. When the center optical frequency of the optical pulse of the short arm of the Mach-Zehnder interferometer is higher than the center optical frequency of the optical pulse of the long arm, the inner core axis of the fiber optic hydrophone is configured as a long arm, and the outer core axis is configured as a short arm. When the center optical frequency of the optical pulse of the short arm of the Mach-Zehnder interferometer is lower than the center optical frequency of the optical pulse of the long arm, the inner core axis of each fiber optic hydrophone is defined as a short arm, and the outer core axis is defined as a long arm.

[0078] During the installation of the entire array, by precisely configuring the length and short arms of the core shaft of each fiber optic hydrophone, it can be ensured that the demodulated signals of different fiber optic hydrophones remain consistent with the responses of the actual sound pressure signals.

[0079] Figure 3 This is a comparison diagram of the demodulated signal and the sound pressure signal after the core shafts of an optical fiber hydrophone are configured using the working method of the present invention. When the center optical frequency of the short-arm optical pulse of the Mach-Zehnder interferometer is higher than the center optical frequency of the long-arm optical pulse, the inner core shaft of the optical fiber hydrophone is configured as the long arm and the outer core shaft is configured as the short arm, and the demodulated signal and the sound pressure signal remain in phase.

[0080] Figure 4 This is a comparison diagram of the demodulated signal and the sound pressure signal of an optical fiber hydrophone in an embodiment, in which the core shafts are not configured using the working method of the present invention. When the center optical frequency of the short-arm optical pulse of the Mach-Zehnder interferometer is higher than the center optical frequency of the long-arm optical pulse, if the inner core shaft of the optical fiber hydrophone is configured as a short arm and the outer core shaft is configured as a long arm, the demodulated signal and the sound pressure signal have opposite phases.

[0081] Furthermore, the working method of the optical fiber hydrophone array based on the consistent heterodyne demodulation phase response proposed in any of the above embodiments includes the following steps:

[0082] Obtain the central optical frequency of the optical pulses in the long and short arms of the Mach-Zehnder interferometer in the heterodyne demodulation method;

[0083] When the center optical frequency of the short-arm optical pulse of the Mach-Zehnder interferometer is higher than the center optical frequency of the long-arm optical pulse, the inner core axis of each optical fiber hydrophone is configured as the long arm and the outer core axis is configured as the short arm; when the center optical frequency of the short-arm optical pulse of the Mach-Zehnder interferometer is lower than the center optical frequency of the long-arm optical pulse, the inner core axis of each optical fiber hydrophone is defined as the short arm and the outer core axis is defined as the long arm;

[0084] After configuring the inner and outer core shafts of each fiber optic hydrophone in the fiber optic hydrophone array, the entire fiber optic hydrophone array is installed and deployed.

[0085] The present invention achieves the same response between the demodulation signals and the sound pressure signals of different fiber optic hydrophones by precisely defining the configuration of the inner and outer core axes of the fiber optic hydrophones under different heterodyne demodulation parameters, thereby ensuring the consistency and accuracy of the phase response of the fiber optic hydrophone array, enabling the array to accurately detect and respond to the sound pressure signal.

[0086] Matters not covered by the present invention are known technologies.

[0087] The technical features of the above embodiments can be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0088] The above-described embodiments merely represent several implementation methods of the present application. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that a person of ordinary skill in the art may make various modifications and improvements without departing from the spirit of the present application, and such modifications and improvements are intended to fall within the scope of protection of the present application. Therefore, the scope of protection of the present application shall be determined by the appended claims.

[0089] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that the present invention is susceptible to various modifications and variations. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A fiber optic hydrophone array with consistent phase response based on heterodyne demodulation, characterized in that: It includes a laser generating unit, an optical fiber coupler, a first acousto-optic modulator, a second acousto-optic modulator, a total delay optical fiber, N optical fiber beam splitting couplers, N time-delay optical fibers, and N+1 optical fiber hydrophones; A laser generating unit, configured to output a narrow linewidth single-frequency laser signal; A fiber coupler, a first fiber beam splitting coupler, a first acousto-optic modulator, a second acousto-optic modulator, and a total delay fiber form a Mach-Zehnder interferometer. A narrow-linewidth single-frequency laser signal output by a laser generating unit is split into a first light beam and a second light beam through the fiber coupler for output. The first light beam output by the fiber coupler is transmitted through the short arm of the Mach-Zehnder interferometer, on which the first acousto-optic modulator is disposed. The second light beam output by the fiber coupler is transmitted through the long arm of the Mach-Zehnder interferometer, on which the second acousto-optic modulator and the total delay fiber are disposed. The first output arm of the first optical fiber beam splitting coupler is connected to the input end of the first time-delay optical fiber, the output end of the nth time-delay optical fiber is connected to the input arm of the n+1th optical fiber beam splitting coupler, and the first output arm of the n+1th optical fiber beam splitting coupler is connected to the input end of the n+1th time-delay optical fiber, where n=1, 2, ..., N-1; The second output arm of the nth optical fiber beam splitting coupler is connected to the nth optical fiber hydrophone in the optical fiber hydrophone array; the output end of the Nth time-delay optical fiber is connected to the N+1th optical fiber hydrophone in the optical fiber hydrophone array; The structures of N+1 fiber optic hydrophones are the same, and the fiber optic hydrophones include a fiber optic coupler, two fiber optic coils, two Faraday rotator mirrors, an inner core shaft, an outer core shaft, and two fixed ends. The inner core shaft and the outer core shaft are both made of a material that can produce elastic deformation, and the inner core shaft and the outer core shaft are both tubular. The inner core shaft is sleeved inside the outer core shaft. The inner core shaft and the outer core shaft have the same length and are coaxially arranged. The end faces of the inner core shaft and the outer core shaft are respectively on the same plane. The ends of the inner core shaft and the outer core shaft are respectively fixedly supported by two fixed ends, and the annular gap between the inner core shaft and the outer core shaft is filled with air. An air-back structure is formed; two optical fiber coils are respectively wound around the outside of the inner core shaft and the outside of the outer core shaft; two arms on the second side of the optical fiber coupler are respectively connected to one end of the two optical fiber coils; the other ends of the two optical fiber coils are respectively connected to a Faraday rotator mirror for eliminating polarization fading at the end of the interferometer; according to the different lengths of the optical fiber coils wound around the outer sides of the inner core shaft and the outer core shaft, the core shaft with the longer optical fiber coil is used as the long arm, and the core shaft with the shorter optical fiber coil is used as the short arm. The inner core shaft and the outer core shaft of each optical fiber hydrophone in the optical fiber hydrophone array are configured as follows: Obtain the central optical frequency of the optical pulses in the long and short arms of the Mach-Zehnder interferometer in the heterodyne demodulation method; When the center optical frequency of the short-arm optical pulse of the Mach-Zehnder interferometer is higher than the center optical frequency of the long-arm optical pulse, the inner core axis of each optical fiber hydrophone is configured as the long arm and the outer core axis is configured as the short arm; when the center optical frequency of the short-arm optical pulse of the Mach-Zehnder interferometer is lower than the center optical frequency of the long-arm optical pulse, the inner core axis of each optical fiber hydrophone is defined as the short arm and the outer core axis is defined as the long arm; After precisely configuring the inner and outer cores of each fiber optic hydrophone, the entire array is installed and deployed.

2. The optical fiber hydrophone array based on heterodyne demodulation phase response consistency according to claim 1, characterized in that: The output end of the fiber coupler outputting the first light beam is connected to the input end of the first acousto-optic modulator, the output end of the first acousto-optic modulator is connected to the first input arm of the first fiber beam splitting coupler, the output end of the fiber coupler outputting the second light beam is connected to the input end of the second acousto-optic modulator, the output end of the second acousto-optic modulator is connected to the input end of the total delay fiber, and the output end of the total delay fiber is connected to the second input arm of the first fiber beam splitting coupler.

3. The optical fiber hydrophone array based on heterodyne demodulation phase response consistency according to claim 1, characterized in that: It also includes an optical fiber isolator, an optical fiber isolator, and an erbium-doped fiber amplifier. The output end of the laser is connected to the input end of the optical fiber isolator. The laser outputs a narrow linewidth single-frequency laser signal and inputs it into the optical fiber isolator to avoid the influence of backward transmitted light on the light source; the output end of the optical fiber isolator is connected to the erbium-doped fiber amplifier to amplify the optical signal, and the output end of the erbium-doped fiber amplifier is connected to the input end of the optical fiber coupler.

4. The optical fiber hydrophone array based on heterodyne demodulation phase response consistency according to claim 1, 2 or 3, characterized in that: The second output arm of the nth optical fiber splitting coupler is connected to the first arm of the first side of the nth optical fiber coupler of the nth optical fiber hydrophone, and the second output arm of the Nth optical fiber splitting coupler is connected to the first arm of the first side of the Nth optical fiber coupler of the Nth optical fiber hydrophone; the output end of the Nth time-delay optical fiber is connected to the first arm of the first side of the N+1th optical fiber coupler of the N+1th optical fiber hydrophone in the optical fiber hydrophone array.

5. The optical fiber hydrophone array based on heterodyne demodulation phase response consistency according to claim 4, characterized in that: The system also includes a detection unit, which includes N optical fiber beam-combining couplers, a photoelectric detector, a data acquisition card and a computer. The signals detected by the N+1 optical fiber hydrophones are beam-combined by the N optical fiber beam-combining couplers and output to the photoelectric detector for detection. The photoelectric detector is used for photoelectric conversion. The electrical signal output by the photoelectric detector is collected by the data acquisition card and the computer performs signal processing and analysis.

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