Optical fiber hydrophone array based on heterodyne demodulation phase response consistency and working method

Abstract

The invention provides an optical fiber hydrophone array based on heterodyne demodulation phase response consistency and a working method, the array is composed of large-scale push-pull type core shaft structure optical fiber 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 optical fiber hydrophone array is higher than the center optical frequency of the long-arm optical pulse, an inner core shaft in the optical fiber hydrophone is configured to be a long arm, and an outer core shaft is configured to be a 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 of the Mach-Zehnder interferometer, the inner core shaft in each optical fiber hydrophone is defined as a short arm, and the outer core shaft in each optical fiber hydrophone is defined as a long arm. By accurately defining the configuration of the inner core shaft and the outer core shaft of the optical fiber hydrophone under different demodulation parameters, the same response of demodulation signals and sound pressure signals of different optical fiber hydrophones is realized, so that the consistency and the accuracy of phase response of the optical fiber hydrophone array are ensured, and the array can accurately detect and respond to the sound pressure signals.

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 and a working method based on heterodyne demodulation phase response consistency. Background Art

[0002] Fiber-optic hydrophones have the advantages of high sensitivity, strong anti-electromagnetic interference ability, convenient long-distance transmission and large-scale array deployment. They have great application potential in underwater target detection, oil and gas exploration, earthquake monitoring and other fields. Many studies have explored the design and application of various fiber-optic hydrophones.

[0003] The phase consistency between fiber optic hydrophones in a large-scale array is critical to the performance of the entire array. Phase inconsistency will increase the sidelobe level of the beamformer, reduce the spatial signal-to-noise ratio gain, affect the detection accuracy, and may increase the false alarm probability of the sonar system. Therefore, ensuring the phase consistency of each unit in the fiber optic hydrophone array is the 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 the work focuses on back-end algorithm correction, while the impact of the front-end sensor installation method on phase offset has been relatively ignored. This approach that focuses on back-end processing ignores the potential for front-end optimization, especially in large-scale arrays. The large number of hydrophones makes the back-end algorithm correction extremely complex and labor-intensive. In addition, heterodyne demodulation technology, as a commonly used phase signal demodulation method in fiber optic hydrophone sensing systems, has the advantages of large dynamic range and 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 response. Therefore, optimizing the installation method of the hydrophone and the demodulation parameter matching is crucial to simplifying later corrections and reducing system maintenance costs. Summary of the invention

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

[0006] Specifically, the technical solution of the present invention is: 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; A laser generating unit, used for outputting a narrow line width single frequency laser signal; The optical fiber coupler, the first optical fiber beam splitting coupler, the first acousto-optic modulator, the second acousto-optic modulator and the total delay optical fiber form a Mach-Zehnder interferometer. The narrow linewidth single-frequency laser signal output by the laser generating unit is divided into a first light beam and a second light beam for output through the optical fiber coupler. The first light beam output by the optical fiber coupler is transmitted through the short arm of the Mach-Zehnder interferometer, and the first acousto-optic modulator is arranged on the short arm. The second light beam output by the optical fiber coupler is transmitted through the long arm of the Mach-Zehnder interferometer, and the second acousto-optic modulator and the total delay optical fiber are arranged on the long arm. 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, wherein 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 optical fiber hydrophones are the same, and the optical fiber hydrophones include an optical fiber coupler, two optical fiber coils, two Faraday rotating 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 have 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 outer sides of the inner core shaft and 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, and the other ends of the two optical fiber coils are respectively connected to a Faraday rotation 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.

[0007] Further, the output end of the optical fiber coupler outputting the first optical 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 optical fiber splitting coupler, the output end of the optical fiber coupler outputting the second optical 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 optical fiber, and the output end of the total delay optical fiber is connected to the second input arm of the first optical fiber splitting coupler; Furthermore, the laser generating unit of the present invention comprises 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 which is input into the optical fiber isolator to avoid the influence of backward transmission 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.

[0008] Furthermore, 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.

[0009] Furthermore, the present invention 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 N+1 optical fiber hydrophones are combined by the N optical fiber beam-combining couplers and output to the photoelectric detector for detection, and 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.

[0010] The above-mentioned working method of the optical fiber hydrophone array based on the consistent heterodyne demodulation phase response comprises the following steps: Obtain the central optical frequency of the optical pulses of the long arm and short arm of the Mach-Zehnder interferometer in the heterodyne demodulation method; When the central optical frequency of the short-arm optical pulse of the Mach-Zehnder interferometer is higher than the central optical frequency of the long-arm optical pulse, the inner core axis in each optical fiber hydrophone is configured as a long arm, and the outer core axis is configured as a short arm; when the central optical frequency of the short-arm optical pulse of the Mach-Zehnder interferometer is lower than the central optical frequency of the long-arm optical pulse, the inner core axis in each optical fiber hydrophone is defined as a short arm, and the outer core axis is defined as a long arm; After configuring the inner and outer core shafts of each optical fiber hydrophone in the optical fiber hydrophone array, the entire optical fiber hydrophone array is installed and deployed.

[0011] The beneficial effects of the present invention are: The present invention proposes a fiber optic hydrophone array and a working method based on consistent heterodyne demodulation phase response. The fiber optic hydrophone array is composed of large-scale push-pull core shaft structure fiber optic hydrophones, and heterodyne demodulation technology is used to obtain the center optical frequency of the optical pulses of the long arm and short arm 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 shaft in the fiber optic hydrophone is configured as a long arm, and the outer core shaft is configured as a 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 in each fiber optic hydrophone is defined as a short arm, and the outer core shaft is defined as a long arm. By accurately defining the inner and outer core shaft configurations of the fiber optic hydrophone under different demodulation parameters, it is ensured that when each fiber optic hydrophone receives a sound pressure signal, the response relationship between its demodulation signal and the sound pressure signal remains consistent, thereby ensuring the consistency and accuracy of the phase response of the fiber optic hydrophone array, so that the array can accurately detect and respond to the sound pressure signal.

[0012] The front-end optimization method for matching hydrophone installation and demodulation parameters provided by the present invention can effectively reduce the complexity of back-end algorithm correction in large-scale arrays and reduce the maintenance cost of the system. It provides a more efficient and economical solution to ensure phase consistency between hydrophones, which is helpful to promote the development and application of fiber-optic hydrophone array technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. 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 creative work.

[0014] Figure 1 It is a structural schematic diagram of a fiber optic hydrophone array based on heterodyne demodulation phase response consistency provided by an embodiment; Figure 2 is a schematic diagram of the structure of a fiber optic hydrophone provided by an embodiment; Figure 3 It 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; Figure 4 It is a comparison diagram of the demodulated signal and the sound pressure signal of the core shaft of the optical fiber hydrophone in one embodiment after the configuration is not performed using the working method of the present invention; Numbers in the figure: 1. Laser, 2. Optical fiber isolator, 3. Erbium-doped fiber amplifier, 4. Optical fiber coupler, 5-1. First acousto-optic modulator, 5-2. Second acousto-optic modulator, 6. Total delay optical fiber, 7-1. First optical fiber beam splitting coupler, 7-2. Second optical fiber beam splitting coupler, 7-N. Nth optical fiber beam splitting coupler, 8-1. First time-division optical fiber, 8-2. Second time-division optical fiber, 8-N. Nth time-division optical fiber, 9-1. First optical fiber hydrophone, 9-2. Second optical fiber hydrophone, 9-N. Nth optical fiber hydrophone, 9-(N+1), N+1th optical fiber hydrophone, 10-1. First optical fiber beam combining coupler, 10-2. Second optical fiber beam combining coupler, 10-N. Nth optical fiber beam combining coupler, 11. Photoelectric detector, 12. Data acquisition card, 13. Computer; 9-1-1, the first optical fiber coupler, 9-2-1, the second optical fiber coupler, 9-N-1, the Nth optical fiber coupler, 9-(N+1)-1, the N+1th optical fiber coupler, 9-1-2, the first optical fiber coil, 9-1-3, the first Faraday rotation mirror, 9-1-4, the first inner core shaft, 9-1-5, the first outer core shaft, 9-1-6, the first fixed end. DETAILED DESCRIPTION

[0015] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0016] Reference Figure 1An embodiment of the present invention provides a fiber optic hydrophone array based on heterodyne demodulation phase response consistency, including 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-dividing fibers, N+1 fiber optic hydrophones, N fiber beam combining couplers, a photodetector 11, an acquisition card 12, and a computer 13. The N fiber beam splitting couplers are respectively 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-dividing fibers are respectively a first time-dividing fiber 8-1, a second time-dividing fiber 8-2, ..., and an Nth time-dividing fiber 8-N. The N+1 fiber optic hydrophones are respectively 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.

[0017] The optical fiber coupler 4, the first optical fiber beam splitting coupler 7-1, the first acousto-optic modulator 5-1, the second acousto-optic modulator 5-2 and the total delay optical fiber 6 form a Mach-Zehnder interferometer. The narrow linewidth single-frequency laser signal output by the laser 1 is divided into a first light beam and a second light beam for output through the optical fiber coupler 4; the first light beam output by the optical fiber coupler 4 is transmitted through the short arm of the Mach-Zehnder interferometer, and the first acousto-optic modulator 5-1 is arranged on the short arm; the second light beam output by the optical fiber coupler 4 is transmitted through the long arm of the Mach-Zehnder interferometer, and the second acousto-optic modulator 5-2 and the total delay optical fiber 6 are arranged on the long arm.

[0018] The output end of the laser 1 is connected to the input end of the optical fiber isolator 2. The laser 1 outputs a narrow linewidth single-frequency laser signal and inputs it into the optical fiber isolator 2 to avoid the influence of the backward transmission light on the light source. The output end of the optical fiber isolator 2 is connected to the erbium-doped fiber amplifier 3 to amplify the optical signal. The output end of the erbium-doped fiber amplifier 3 is connected to the input end of the optical fiber coupler 4. The light beam amplified by the erbium-doped fiber amplifier 3 is divided into a first light beam and a second light beam through the optical fiber coupler 4 for output. The output end of the first light beam of the optical fiber coupler 4 is connected to the input end of the first acousto-optic modulator 5-1, and the output end of the first acousto-optic modulator 5-1 is connected to the first input arm of the first optical fiber splitting coupler 7-1. The output end of the second light beam of the optical fiber coupler 4 is connected to the input end of the second acousto-optic modulator 5-2, and the output end of the second acousto-optic modulator 5-2 is connected to the input end of the total delay optical fiber 6. The output end of the total delay optical fiber 6 is connected to the second input arm of the first optical fiber splitting coupler 7-1. The first output arm of the first optical fiber beam splitter 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 beam splitter coupler 7-(n+1), and the first output arm of the n+1th optical fiber beam splitter 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.

[0019] 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.

[0020] The signals detected by N+1 optical fiber hydrophones are combined by N optical fiber combining couplers and output to the photoelectric detector 11 for detection, and 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 the signal is processed and analyzed by the computer 13.

[0021] The narrow linewidth single-frequency laser signal output by the laser 1 is input to the optical fiber isolator 2. The setting of the optical fiber isolator 2 can effectively avoid the influence of the backward transmission light on the laser 1 and avoid damage to the laser 1. The output end of the optical fiber isolator 2 is connected to the erbium-doped fiber amplifier 3 to amplify the optical signal. The output end of the erbium-doped fiber amplifier 3 is connected to the input end of the optical fiber coupler 4. The light beam amplified by the erbium-doped fiber amplifier 3 is divided into two light beams for output through the optical fiber coupler 4. The two light beams pass through the first acousto-optic modulator 5-1 and the second acousto-optic modulator 5-2 respectively, wherein a total delay optical fiber 6 is set after the second acousto-optic modulator to separate the two heterodyne optical pulses in the time domain. Through N optical fiber beam splitting couplers, the optical pulse pair is input into the N+1 optical fiber hydrophones in the optical fiber hydrophone array through the N time-delay optical fibers, and the N+1 optical fiber hydrophones detect the hydroacoustic signal. The optical pulse pair carrying the sound pressure information is then sent to the corresponding optical fiber beam-combining coupler. The signals detected by N+1 optical fiber hydrophones are combined and output by N optical fiber beam-combining couplers and detected by the photoelectric detector 11. The photoelectric detector 11 converts the detected optical signal into an electrical signal for output. 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.

[0022] Reference Figure 2 In one embodiment, a fiber optic hydrophone is provided, which includes a fiber optic coupler, a fiber optic coil, a Faraday rotation mirror, an inner core shaft, an outer core shaft, and a fixed end. Figure 1 In the optical fiber hydrophone array based on heterodyne demodulation phase response consistency provided by the illustrated embodiment, each optical fiber hydrophone has the same structure, and the structure of each optical fiber hydrophone can be adopted. Figure 2 The fiber optic hydrophone structure provided in the illustrated embodiment. Figure 2The fiber optic hydrophone with a core shaft structure shown in the figure has a simple structural design, is easy to array, and has good pressure resistance. Specifically, it is composed of two core shafts, an inner layer and an outer layer, and an air cavity is formed between the two. For this push-pull core shaft structure fiber optic hydrophone, each arm is used as an independent sensing arm to detect underwater acoustic signals. When the fiber optic hydrophones are arrayed and beamforming technology is used, the azimuth information of underwater targets can be accurately obtained. In theory, the expansion of the array scale can improve the directional accuracy of the target. However, the difference in the position of the two sensing arms wrapped around the core shaft will cause the phase offset of the fiber optic hydrophone, resulting in phase inconsistency between the fiber optic hydrophones. The phase consistency between the fiber optic hydrophones in a large-scale array is crucial to the performance of the entire array. Phase inconsistency will lead to an increase in the sidelobe level of the beamformer, reduce the spatial signal-to-noise ratio gain, affect the detection accuracy, and may increase the false alarm probability of the sonar system. Therefore, ensuring the phase consistency of each unit in the fiber optic hydrophone array is the key to achieving high-performance underwater acoustic wave detection. In the present invention, the structures of N+1 optical fiber hydrophones are completely the same, and the structure of each optical fiber hydrophone is optimized and designed, as follows: Taking the first fiber optic hydrophone 9-1 as an example, the structure of each fiber optic hydrophone is described. The first fiber optic hydrophone 9-1 includes a first fiber coupler 9-1-1, a first fiber 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 producing elastic deformation, such as elastic metal, plastic, or rubber material, and the first inner core shaft 9-1-4 and the first outer core shaft 9-1-5 are both tubular, the first inner core shaft 9-1-4 is sleeved in 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 have the same length and are coaxially arranged, and the end faces of both ends of the first inner core shaft 9-1-4 and the first outer core shaft 9-1-5 are respectively on the same plane. The two 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 to form an air back structure. The two first optical fiber coils 9-1-2 are respectively 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. 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 respectively connected to one end of the two first optical fiber coils 9-1-2, and the other ends of the two first optical fiber coils 9-1-2 are respectively connected to a first Faraday rotation mirror 9-1-3 for eliminating polarization fading at the end of the interferometer. According to the different lengths of the optical fiber coils wound around the outside of the first inner core shaft 9-1-4 and the first outer core shaft 9-1-5, the longer optical fiber coil is used as the long arm, and the shorter optical fiber coil is used as the short arm.

[0023] Figure 1 In the embodiment, the second output arm of the nth optical fiber beam splitting coupler 7-n is connected to the first arm of the first side of the nth optical fiber coupler 9-n-1 of the nth optical fiber hydrophone 9-n in the optical fiber hydrophone array, wherein n=1,2,...N-1. The second output arm of the Nth optical fiber beam splitting coupler 7-N is connected to the first arm of the first side of the Nth optical fiber coupler 9-N-1 of the Nth optical fiber hydrophone 9-N in the optical fiber hydrophone array. The output end of the Nth time-delay optical fiber 8-N is connected to the first arm of the first side of the N+1th optical fiber hydrophone 9-(N+1)-1 of the N+1th optical fiber coupler 9-(N+1)-1 in the optical fiber hydrophone array.

[0024] Figure 1 In the figure, the second arm on the first side of the N+1th fiber coupler 9-(N+1)-1 and the second arm on the first side of the Nth fiber coupler 9-N-1 are connected to the two input arms on the input side of the Nth fiber beam combining coupler 10-N, the beam combining arm on the output side of the Nth fiber beam combining coupler 10-N and the second arm on the first side of the N-1th fiber coupler 9-(N-1)-1 are connected to the two input arms on the input side of the N-1th fiber beam combining coupler 10-(N-1)... and so on, until the beam combining arm on the output side of the second fiber beam combining coupler 10-2 and the second arm on the first side of the first fiber coupler 9-1-1 are connected to the two input arms on the input side of the first fiber beam combining coupler 10-1, and the beam combining arm on the output side of the first fiber beam combining coupler 10-1 is combined and output to the photodetector 11 for detection.

[0025] Taking the first fiber optic hydrophone 9-1 as an example, the working principle of each fiber optic hydrophone is explained: the optical pulse is input through the first arm of the first side of the first fiber optic coupler 9-1-1 of the first fiber optic hydrophone 9-11, and is divided into two beams through the first fiber optic coupler 9-1-1 and output by the two arms of the second side of the first fiber optic coupler 9-1-1, wherein one optical pulse is emitted to the long arm and the other optical pulse is emitted to the short arm. Under the action of the hydroacoustic pressure wave, the first inner core shaft 9-1-4 expands radially outward under the action of tensile strain, while the first outer core shaft 9-1-5 contracts radially inward under the action of compressive strain. This push-pull structure causes the optical phase of the fiber optic coils 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 optic coils are reflected by the corresponding first Faraday reflector 9-1-3, and then return to the first fiber optic coupler 9-1-1 to form interference light again, which is output through the second arm of the first side of the first fiber optic coupler 9-1-1. This light can be monitored by a photoelectric detector. The information of the hydroacoustic pressure signal is contained in the optical phase change and can be demodulated by heterodyne detection technology.

[0026] Among them, the phase expression of the first pulse light signal returned from the long arm end is: (1) The phase expression of the second pulse optical signal returned from the short arm end is: (2) in, and are the amplitudes of the two pulse light signals, and are the frequencies of the two pulse light signals, and are the initial phases of the two light fields and the phase drift caused by environmental disturbances, and are the phase changes of the two light fields caused by the external signal to be measured, then the expression of the interference light field is: (3) in, express The complex conjugate of , Respectively , The complex conjugate of The carrier frequency of the heterodyne interference signal is called the heterodyne frequency. Including the initial phase of the heterodyne interference signal and the phase drift introduced by environmental disturbance. The ultimate goal of heterodyne detection is to obtain sensor phase information Usually, a reference signal with the same frequency as the heterodyne frequency and its orthogonal signal are used for orthogonal demodulation calculation. Obviously, when >0 o'clock, It is equal to the phase change of light in the long arm caused by the hydroacoustic pressure wave minus the phase change of light in the short arm: (4) However, when the heterodyne frequency <0, the opposite Among them, the radial stress and the hoop stress of the first inner core shaft 9-1-4 and the first outer core shaft 9-1-5 can be expressed as: (5) in, 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 sound pressure signal P. Therefore, the boundary conditions of the radial pressure of the first inner core shaft 9-1-4 and the first outer core shaft 9-1-5 are: (6) (7) in, is the inner radius of the mandrel, is the outer radius of the mandrel. Substituting equation (5) into equations (6) and (7), we get the solution of the equation, namely , , , Four constants: (8) (9) Substituting it back into equation (1), we can get the radial and hoop stress expressions of the mandrel. According to Hooke's law, the hoop strain of the mandrel at the outer radius can be obtained ( )for: (10) 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: (11) 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: (12) in, is the change in fiber length around the mandrel, is the change in the propagation constant of light, is the change in fiber diameter. 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: (13) 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: (14) in is the Poisson's ratio of the optical fiber. Assuming that the optical fiber is perfectly combined with the outer surface of the core shaft, the longitudinal strain of the optical fiber is equal to the circumferential strain of the pipe. Therefore, the phase changes of the optical fiber wrapped around the first inner core shaft 9-1-4 and the first outer core shaft 9-1-5 are: (15) (16) 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: (17) Consider another case, when the first outer core shaft 9-1-5 is a long arm, the demodulation result is: (18) According to the working principle of the above-mentioned optical fiber 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 demodulation signal. As a preferred implementation, 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 2 The fiber optic hydrophone structure provided in the illustrated embodiment. A method for configuring each core axis in each fiber optic hydrophone as a long and short arm is further proposed to obtain the central optical frequency of the optical pulses of the long arm and the short arm of the Mach-Zehnder interferometer in the heterodyne demodulation method; when the central optical frequency of the short arm optical pulse of the Mach-Zehnder interferometer is higher than the central optical frequency of the long arm optical pulse, the inner core axis in the fiber optic hydrophone is configured as a long arm and the outer core axis is configured as a short arm; when the central optical frequency of the short arm optical pulse of the Mach-Zehnder interferometer is lower than the central optical frequency of the long arm optical pulse, the inner core axis in each fiber optic hydrophone is defined as a short arm and the outer core axis is defined as a long arm.

[0027] During the installation of the entire array, the long and short arms of the core shaft of each fiber optic hydrophone are precisely configured to ensure that the demodulated signals of different fiber optic hydrophones are consistent with the responses of the actual sound pressure signals.

[0028] Figure 3 The present invention is a comparison diagram of a demodulated signal and a sound pressure signal after the core shaft of an optical fiber hydrophone is configured using the working method of the present invention. When the central optical frequency of the short-arm optical pulse of the Mach-Zehnder interferometer is higher than the central optical frequency of the long-arm optical pulse, the inner core shaft of the optical fiber hydrophone is configured as a long arm and the outer core shaft is configured as a short arm, and the demodulated signal and the sound pressure signal remain in phase.

[0029] Figure 4The figure is a comparison diagram of a demodulated signal and a sound pressure signal after the core shaft of an optical fiber hydrophone is not configured using the working method of the present invention. When the central optical frequency of the short-arm optical pulse of the Mach-Zehnder interferometer is higher than the central 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 are in opposite phase.

[0030] 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 comprises the following steps: Obtain the central optical frequency of the optical pulses of the long arm and short arm of the Mach-Zehnder interferometer in the heterodyne demodulation method; When the central optical frequency of the short-arm optical pulse of the Mach-Zehnder interferometer is higher than the central optical frequency of the long-arm optical pulse, the inner core axis in each optical fiber hydrophone is configured as a long arm, and the outer core axis is configured as a short arm; when the central optical frequency of the short-arm optical pulse of the Mach-Zehnder interferometer is lower than the central optical frequency of the long-arm optical pulse, the inner core axis in each optical fiber hydrophone is defined as a short arm, and the outer core axis is defined as a long arm; After configuring the inner and outer core shafts of each optical fiber hydrophone in the optical fiber hydrophone array, the entire optical fiber hydrophone array is installed and deployed.

[0031] The present invention achieves the same response of demodulation signals and sound pressure signals of different optical fiber hydrophones by accurately defining the inner and outer core shaft configurations of the optical fiber hydrophones under different heterodyne demodulation parameters, thereby ensuring the consistency and accuracy of the phase response of the optical fiber hydrophone array, so that the array can accurately detect and respond to the sound pressure signal.

[0032] Matters not covered by the present invention are known technologies.

[0033] The technical features of the above embodiments may 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.

[0034] The above-described embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention. It should be pointed out that, for a person of ordinary skill in the art, several modifications and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the attached claims.

[0035] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A fiber optic hydrophone array based on heterodyne demodulation with consistent phase response, characterized in that: It includes a laser generating unit, a fiber coupler, a first acousto-optic modulator, a second acousto-optic modulator, a total delay fiber, N fiber beam splitting couplers, N time-delay fibers, and N+1 fiber hydrophones; A laser generating unit, used for outputting a narrow line width single frequency laser signal; The optical fiber coupler, the first optical fiber beam splitting coupler, the first acousto-optic modulator, the second acousto-optic modulator and the total delay optical fiber form a Mach-Zehnder interferometer. The narrow linewidth single-frequency laser signal output by the laser generating unit is divided into a first light beam and a second light beam for output through the optical fiber coupler. The first light beam output by the optical fiber coupler is transmitted through the short arm of the Mach-Zehnder interferometer, and the first acousto-optic modulator is arranged on the short arm. The second light beam output by the optical fiber coupler is transmitted through the long arm of the Mach-Zehnder interferometer, and the second acousto-optic modulator and the total delay optical fiber are arranged on the long arm. 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, wherein 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 optical fiber hydrophones are the same, and the optical fiber hydrophones include an optical fiber coupler, two optical fiber coils, two Faraday rotating 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 have 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 outer sides of the inner core shaft and 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, and the other ends of the two optical fiber coils are respectively connected to a Faraday rotation 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.

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 optical 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 optical fiber splitting coupler, the output end of the optical 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 optical fiber, and the output end of the total delay optical fiber is connected to the second input arm of the first optical fiber 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 optical 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 the backward transmitted light on the light source. The output end of the optical fiber isolator is connected to the erbium-doped optical fiber amplifier to amplify the optical signal, and the output end of the erbium-doped optical 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 detection unit also 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.

6. The working method of the optical fiber hydrophone array based on heterodyne demodulation phase response consistency according to claim 1, characterized in that: The steps include: 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 of the long arm and short arm of the Mach-Zehnder interferometer in the heterodyne demodulation method; When the central optical frequency of the short-arm optical pulse of the Mach-Zehnder interferometer is higher than the central optical frequency of the long-arm optical pulse, the inner core axis in each optical fiber hydrophone is configured as a long arm, and the outer core axis is configured as a short arm; when the central optical frequency of the short-arm optical pulse of the Mach-Zehnder interferometer is lower than the central optical frequency of the long-arm optical pulse, the inner core axis in each optical fiber hydrophone is defined as a short arm, and the outer core axis is defined as a long arm; After the inner and outer mandrels of each fiber optic hydrophone are precisely configured, the entire array is installed and deployed.

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