Heterodyne detection optical fiber vector hydrophone based on weak reflection grating array and demodulation system and method thereof

Abstract

The invention provides a heterodyne detection optical fiber vector hydrophone based on a weak reflection grating array and a demodulation system and method thereof, and the optical fiber vector hydrophone comprises a central mass block, and the six surfaces of the central mass block in the X-axis, Y-axis and Z-axis directions are all provided with sensing units. Each sensing unit comprises a clockwise changing cylinder structure installed on a central mass block, weak reflection optical gratings FBG are installed on the central mass block and the end portions of the six clockwise changing cylinder structures, and the weak reflection optical gratings FBG at the end portions of the six clockwise changing cylinder structures and the weak reflection optical gratings FBG located on the central mass block are arranged at equal intervals to form a weak reflection optical fiber grating array. And an optical fiber with the length of L is wound between the weak reflection grating FBG on the central mass block and the weak reflection grating FBG at the end part of the compliant cylinder. According to the optical fiber vector hydrophone, the grating is directly used as a reflection unit, the size problem caused by FRM is avoided, and meanwhile the excellent phase noise performance of the whole optical fiber vector hydrophone is guaranteed.

Description

Technical Field

[0001] The present invention belongs to the field of optoelectronic information, and particularly relates to a heterodyne detection fiber optic vector hydrophone based on a weak reflection grating array, and a demodulation system and method thereof. Background Art

[0002] By measuring vector information such as the acoustic pressure gradient of underwater sound waves or the vibration velocity, displacement, acceleration, etc. of acoustic particles, a vector hydrophone can obtain the directivity pattern of the acoustic wave signal, avoiding the drawback of a large array scale when using a scalar hydrophone array to detect the azimuth of an acoustic wave signal. Further, by combining a vector hydrophone with a scalar hydrophone, scalar and vector physical quantities such as the acoustic pressure and the vibration velocity of acoustic particles in the acoustic wave field can be obtained simultaneously and at the same point, and can be used for signal processing alone or jointly to obtain more comprehensive information about the acoustic field. Therefore, vector hydrophones have broad application prospects in the fields of ocean noise measurement, underwater and surface target detection, marine biology research, etc.

[0003] Traditional vector hydrophones mostly use piezoelectric sensitive elements as the core acoustic wave sensing unit, but this element is limited in terms of sensitivity, sensing distance, anti-electromagnetic interference, etc. Currently, fiber optic vector hydrophones based on fiber optic sensing technology are expected to break through the performance limitations of traditional piezoelectric vector hydrophones and achieve a leap in vector underwater acoustic detection technology. Currently, the main performance parameters of fiber optic vector hydrophones, such as sensitivity, working frequency band, noise floor, etc., have made great progress.

[0004] In 2012, the research group of Hu Yongming at the National University of Defense Technology proposed a vector hydrophone composed of all polarization-maintaining fibers, with an acceleration sensitivity of 32.6 dB refrad / g and a sensitivity fluctuation of less than 1.4 dB in the frequency range of 20 Hz - 2000 Hz.

[0005] In 2016, Hu Zhengliang et al. at the National University of Defense Technology developed a sonobuoy based on a fiber optic vector hydrophone. Its noise floor reached -104.3 dB refrad / sqrt(Hz) at 1 kHz, the acceleration sensitivity was 41.5 dB re rad / g, and the equivalent noise sound pressure was lower than that of the Deep Sea State Zero (DSS0). A sound source target 15 km away was detected in a lake test experiment.

[0006] In 2017, Wang Chang et al. from the Shandong Academy of Sciences realized a fiber optic vector hydrophone based on the principle of a short cavity distributed feedback (DFB) laser, with a sensitivity reaching 33 dB refpm / g, and a relatively flat frequency response range of 5 Hz - 300 Hz, suitable for detecting low-frequency acoustic wave signals.

[0007] In 2018, the research group led by Cui Hongliang from Jilin University achieved high-sensitivity (57 dBrefrad / g), low-noise (<0.2 μg), and low-frequency (0.005 Hz - 50 Hz) vector acoustic wave signal detection through push-pull structure design.

[0008] In 2021, Jiangfei Hu et al. from the 715th Institute of CSSC detailedly analyzed the sensitivity and working frequency band of fiber optic vector hydrophones based on the interferometer principle through simulation and experimental means. Its working frequency band is approximately 20 Hz - 2000 Hz, and the acceleration sensitivity is 30 dB. In the same year, Zhou Rui et al. from Northwest University etched multiple Fiber Bragg Gratings (FBGs) in multi-core optical fibers, and vector acoustic wave detection can also be achieved through signal combination, with a sensitivity of 355 pm / g and a working frequency band of 10 Hz - 220 Hz.

[0009] In the above research on fiber optic vector hydrophones, the acoustic sensing unit mostly adopts a fiber optic Michelson interferometer with a Faraday Rotation Mirror (FRM), or an active grating with a gain medium. In the acoustic sensing unit based on a fiber optic interferometer, the external acoustic pressure acts on the sensing arm of the fiber optic interferometer, causing a change in the phase of the light wave transmission in the sensing arm relative to the reference arm, and causing a change in the received light intensity through the optical interference effect. At the receiving end, the change in light intensity can be used, combined with various demodulation algorithms, to recover the phase change suffered by the sensing arm of the fiber optic Michelson interferometer, and further the detected acoustic wave signal can be obtained. In this scheme, the FRM plays the role of reflecting light waves and can also be used to eliminate the polarization fading effect. However, the introduction of the FRM will inevitably increase the sensing unit of the fiber optic vector hydrophone. For the latter technical scheme, it mainly relies on the change in the wavelength of the light wave radiated by the gain medium for acoustic wave detection. Its working principle is roughly as follows: The external acoustic wave will cause changes in the length, refractive index, etc. between the two gratings, resulting in a change in the wavelength of the light wave radiated by the gain medium between the two gratings. This change in wavelength can be demodulated and tracked by the interferometer at the back end, thereby restoring the information of the external acoustic wave. Although this scheme avoids the use of the FRM, due to relying on the laser radiation of the active medium, it is easily interfered by external non-vibration factors such as temperature, and at the same time, the phase noise of the radiated light source is often large, restricting the noise performance of acoustic wave detection.

[0010] On the other hand, how to demodulate the optical signals returned by the fiber optic vector hydrophone to obtain the acoustic wave information contained in the light wave is also a key content in the research of fiber optic vector hydrophones. Currently, most of the signal demodulation methods for fiber optic vector hydrophones are the Phase-generated Carrier (PGC) scheme. This scheme can introduce a carrier into the signal returned by the fiber optic vector hydrophone by directly sinusoidally modulating the driving current of the laser. After receiving the interference signal, by mixing it with the first harmonic carrier and the second harmonic carrier respectively and then performing low-pass filtering, the in-phase component and the quadrature component containing the acoustic wave signal can be obtained. Then, by using differential cross multiplication or arctangent operation, the phase signal linearly related to the acoustic wave information can be recovered. This method has advantages such as simple structure and low demodulation noise floor, and has become the mainstream means for signal demodulation of phase modulation type fiber optic sensors, and has been widely used in various fiber optic sensing systems. At present, many research groups are still conducting in-depth research on its demodulation performance limit, etc. Further, by measures such as modulating continuous light into pulsed light and introducing multi-band laser light sources, the PGC scheme can also be combined with Time-domain Division Multiplexing (TDM) technology, Wavelength-domain Division Multiplexing (WDM) technology, etc., to become an efficient demodulation method for large-scale fiber optic sensor arrays or networks.

[0011] In the current sensing unit schemes of fiber optic vector hydrophones, the mainstream Fiber Michelson interferometer scheme based on FRM will cause an increase in the volume of the acoustic sensing unit, which is not conducive to the miniaturization design and fabrication of fiber optic vector hydrophones. And the fiber grating scheme based on active media will introduce relatively serious light source phase noise during use.

[0012] In the signal demodulation method of fiber optic vector hydrophones, although the current mainstream PGC method has advantages such as simple system structure and low demodulation noise floor, there are still deficiencies in indicators such as signal dynamic range and harmonics that are more concerned in underwater acoustic detection

[20] . Moreover, there are also problems with the adjustment of additional parameters such as carrier depth and carrier delay in the PGC scheme, which makes parameter calibration and calibration necessary during use, and there are many inconveniences in use. Summary of the Invention

[0013] The object of the present invention is to provide a heterodyne detection fiber optic vector hydrophone based on a weak reflection grating array, its demodulation system and method. By directly using a grating as a reflection unit, the volume problem caused by the FRM is avoided. At the same time, the grating used here is a passive device, which has a limited impact on the phase noise of the system, and can ensure relatively excellent phase noise performance of the entire fiber optic vector hydrophone.

[0014] The object of the present invention is achieved by the following technical solutions:

[0015] A heterodyne detection fiber optic vector hydrophone based on a weak reflection grating array, comprising: a central mass block, six surfaces in the X-axis, Y-axis, and Z-axis directions of the central mass block are each mounted with a sensing unit, the sensing unit includes a compliant cylinder structure mounted on the central mass block, weak reflection gratings FBG are mounted on the central mass block and at the ends of the 6 compliant cylinder structures, the weak reflection gratings FBG at the ends of the 6 compliant cylinders and the weak reflection grating FBG located on the central mass block are arranged at equal intervals to form a weak reflection fiber grating array, and a fiber with a length of L is wound between the weak reflection grating FBG located on the central mass block and the weak reflection grating FBG at the end of the compliant cylinder.

[0016] The present invention further includes:

[0017] A demodulation system for a heterodyne detection fiber optic vector hydrophone based on a weak reflection grating array, comprising: a coupler 1, an optical pulse modulator, the fiber optic vector hydrophone, the coupler 1 is respectively connected to the optical pulse modulator and an optical frequency shifter, the optical pulse modulator is connected to a driver 1, the driver 1 is connected to a pulse generator, the optical frequency shifter is connected to a driver 2, the optical pulse modulator is connected to a circulator, the circulator is respectively connected to the fiber optic vector hydrophone and an erbium-doped fiber amplifier, the erbium-doped fiber amplifier is connected to an optical bandpass filter, the optical frequency shifter and the optical bandpass filter are respectively connected to a coupler 2, the coupler 2 is connected to a data acquisition system, and the data acquisition system is connected to a computer.

[0018] Further, the trigger pulse and the working clock of the pulse generator are synchronized with the driver 2 and the data acquisition system.

[0019] The present invention may further include:

[0020] A demodulation method for a vector hydrophone demodulation system, which uses the above signal demodulation system, and specifically includes the following steps:

[0021] The narrow linewidth laser provides a high-quality continuous optical wave, which is split into two beams by a coupler. Among them, the upper half of the continuous optical wave is modulated into pulsed light by the optical pulse modulator. The optical frequency of this pulsed light is denoted as f1, the time width of the pulsed light is denoted as w, and the pulse repetition frequency is denoted as T rep , and the complex amplitude expression of the electric field of the pulsed light is written as:

[0022]

[0023] In the formula: E0 is the electric field strength of the output optical pulse, R w represents a rectangular window function with a width of w, k represents the serial number of the optical pulse, and t is the time variable;

[0024] The amplified optical pulse sequence is output from port 1 of the circulator to port 2 and enters the weak reflection fiber grating array of a fiber optic vector hydrophone. The optical pulse sequence reflected from the weak reflection grating array arrives at port 3 of the circulator from port 2 of the circulator, and then is amplified by an erbium-doped fiber amplifier to compensate for the optical power loss. The amplified optical signal also needs to pass through an optical bandpass filter to filter out the spontaneous emission noise to obtain an optical pulse sequence. If the noise introduced by EDFA and the like is not considered, the optical pulse sequence at this time is written as

[0025]

[0026] Among them, η m is the optical field amplitude scaling factor considering factors such as grating reflectivity, optical link loss, and EDFA gain, τ m is the delay introduced by the mth FBG, φ FBGm is the phase information carried in the optical pulse returned by the mth FBG. This phase information contains the phase change information generated by the action of the external sound field signal on the sensing fiber with a length of L.

[0027] The other part of the light split from the coupler 1, that is, the lower half of the continuous optical wave, after passing through the optical frequency shifter, shifts its optical frequency domain to f2. Here, the continuous light with an optical frequency of f2 is the local oscillator light, and its complex amplitude of the electric field is written as:

[0028] E LO (t, k) = E′0exp[j2πf2(t + kT rep )] (3)

[0029] The pulse sequence represented by formula (2) and the local oscillator light represented by formula (3) are mixed in coupler 2, and the formed interference signal is then received by a balanced photodetector. The balanced detection method is used to effectively suppress the direct current component in the interference signal, and the obtained photocurrent signal is written as:

[0030]

[0031] Among them, Δf = f1 - f2 is the heterodyne frequency, which is the difference in the optical frequency shift of the light beams on the two arms after the first coupler, and φ 0m is the DC component in the phase of the optical pulse reflected back by each grating;

[0032] The photocurrent signal I(t,k) is converted into a voltage signal through the transimpedance amplifier circuit inside the balanced photodetector, and further can be collected by the data acquisition system and converted into a digital signal, and finally sent to the computer for corresponding data processing;

[0033] In the computer, through the IQ demodulation algorithm and the time division process, the phase information φ FBGm (k) in the cosine expression of each interference signal in each formula (4) can be restored. The influence of the independent variable t has been eliminated in the process of the time division algorithm, and the phase information sampling rate corresponding to each grating return pulse is restored, that is, the corresponding pulse repetition frequency f rep f rep = 1 / T rep .

[0034] Further, the modulation signal of the optical pulse modulator comes from the pulse generator and also needs to be adjusted by the first driver for power amplification, impedance matching, etc. to correctly drive the optical pulse modulator.

[0035] Further, the amplified optical pulse sequence is output from port 1 of the circulator to port 2 and enters a weak reflection fiber grating array. The weak reflection fiber grating array is composed of 7 equally spaced weak reflection gratings FGB. The sensing fiber with a length of L is between two adjacent weak reflection gratings FGB. The following relationship should be satisfied between L and the width w of the optical pulse:

[0036]

[0037] Among them, c is the propagation speed of light in vacuum, and n is the equivalent refractive index of the sensing fiber.

[0038] Further, the optical frequency shifter adjusts the heterodyne frequency so that there are at least 3 - 5 heterodyne signal waveforms in each pulse of the obtained interference photocurrent signal I, so that the subsequent algorithm can demodulate the phase information carried in each pulse. The formula is expressed as:

[0039]

[0040] Further, if only the optical pulse modulator exists, formula (6) can be satisfied, that is, f2 = 0, then the optical frequency shifter in the signal demodulation mechanism can be omitted.

[0041] Further, when the power of the returned pulse sequence is sufficient, the erbium-doped fiber amplifier and the optical bandpass filter are omitted.

[0042] The beneficial effects of the present invention are as follows:

[0043] The present invention uses an optical grating array to replace the FRM structure in the traditional fiber Michelson interferometer, which can significantly reduce the structural complexity and volume of the fiber optic vector hydrophone. Generally, the size of the FRM is at least above 2.5 mm. At the same time, the fiber Michelson interferometer composed of it also includes a coupler, and generally its length will be above 20 mm. The diameter of the fiber grating can be kept the same as that of the fiber, generally 250 μm, and the grating area length can be controlled within 10 mm, effectively reducing the volume and complexity of the sensing unit.

[0044] The present invention uses a weakly reflective fiber grating array and combines time-division multiplexing technology to form each sensing unit of the vector hydrophone. By directly using the grating as the reflection unit, the volume problem brought by the FRM is avoided; at the same time, the grating used here is a passive device, and the influence on the phase noise of the system is limited, which can ensure relatively excellent phase noise performance of the entire fiber optic vector hydrophone. In addition, this scheme uses an improved heterodyne detection scheme as the optical signal modulation and demodulation scheme, which will make the overall scheme have the advantages of large dynamic range and flexible use, and there is no need to estimate and calibrate parameters such as the carrier modulation depth and carrier phase delay.

[0045] The demodulation method proposed by the present invention does not involve the adjustment of delay parameters such as carrier depth and carrier phase, which is beneficial to the stable demodulation of the fiber optic vector hydrophone signal. Description of the Drawings

[0046] Att Figure 1 is a schematic structural diagram of the demodulation system of the fiber optic vector hydrophone of the present invention.

[0047] Att Figure 2 is a schematic structural diagram of the fiber optic vector hydrophone of the present invention.

[0048] Att Figure 3 is a position diagram of the weakly reflective grating FBG in the fiber optic vector hydrophone of the present invention.

[0049] In the drawings: 1, central mass block; 2, compliant cylinder structure; 3, weakly reflective grating FBG; 4, optical fiber. Detailed Embodiments

[0050] The present invention will be further described below with reference to the drawings.

[0051] The present invention provides a heterodyne detection fiber optic vector hydrophone based on a weakly reflective grating array, as shown in Att Figure 1-2As shown in the figure, it includes: a central mass block 1, and six faces of the central mass block 1 in the X-axis, Y-axis, and Z-axis directions are all equipped with sensing units. The sensing unit includes a compliant cylinder structure 2 installed on the central mass block. Weak reflection gratings FBG are installed on the central mass block 1 and at the ends of the 6 compliant cylinder structures 2. The weak reflection gratings FBG at the ends of the 6 compliant cylinders 2 and the weak reflection gratings FBG located on the central mass block 1 are arranged at equal intervals to form a weak reflection fiber grating array. An optical fiber 4 with a length of L is wound between the weak reflection gratings FBG located on the central mass block 1 and the weak reflection gratings FBG at the ends of the compliant cylinders.

[0052] The 7 weak reflection gratings FBG are FBG1, FBG2, FBG3, FBG4, FBG5, FBG6, and FBG7.

[0053] Furthermore, there are 6 sensing units in 3 directions in this structure, that is, 6 fiber optic accelerometers. There are two sensing units in a push-pull mode in each direction, namely X1 and X2, Y1 and Y2, Z1 and Z2. These 6 sensing units all adopt a compliant cylinder structure and share a common mass block in the middle. The middle 6 sections of sensing optical fibers among the gratings arranged at equal intervals are respectively wound around the compliant cylinders of these 6 leaflet units in sequence. Taking the X direction as an example, the optical fiber with a length of L between FBG1 and FBG2 is wound around a compliant cylinder at one end in the X direction to form a fiber optic accelerometer, that is, the sensing unit X1. And another section of optical fiber with a length of L between FBG2 and FBG3 is wound around the compliant cylinder at the other end in the X direction to form the sensing unit X2. When detecting the acoustic wave field, when there is an external acoustic wave signal acting on the central mass block, if it has a squeezing effect on X1, then it has a stretching effect on X2 (vice versa). Therefore, the phase change magnitudes felt by X1 and X2 are equal and opposite in direction. The total phase change in the X direction is:

[0054]

[0055] And φ X1 (k), that is, the phase change felt by the sensing optical fiber between FBG1 and FBG2, is the difference in the phase information carried in the optical pulse returns of FBG1 and FBG2, that is

[0056]

[0057] According to formulas (7) and (8), it can be seen that as long as the phase information carried in the return pulse of each grating in the grating array can be obtained, the vibration information of this fiber optic vector hydrophone in the X direction can be obtained. By analogy, the vibration information of this fiber optic vector hydrophone in the Y and Z directions can be continuously obtained, so as to obtain the vector vibration information of the sound field.

[0058] In this embodiment, the optical frequency shifter is optional. Its purpose is to adjust the heterodyne frequency so that there are at least 3 to 5 heterodyne signal waveforms within each pulse of the obtained interference photocurrent signal I, so that the subsequent algorithm can demodulate the phase information carried in each pulse. This empirical relationship is expressed by the formula:

[0059]

[0060] If only the optical pulse modulator formula (6) can be satisfied (i.e., f2 = 0), the optical frequency shifter in the system structure can be omitted. At the same time, the EDFA and the optical bandpass filter are also optional. When the power of the returned pulse sequence is sufficient, these two devices can be omitted.

[0061] The present invention also proposes a signal demodulation mechanism for a heterodyne detection fiber optic vector hydrophone based on a weak reflection grating array, as shown in the appendix Figure 1 shown, including: a first coupler, an optical pulse modulator, and the above-mentioned fiber optic vector hydrophone. The first coupler is respectively connected to the optical pulse modulator and the optical frequency shifter. The optical pulse modulator is connected to a first driver. The first driver is connected to a pulse generator. The optical frequency shifter is connected to a second driver. The optical pulse modulator is connected to a circulator. The circulator is respectively connected to the fiber optic vector hydrophone and an erbium-doped fiber amplifier. The erbium-doped fiber amplifier is connected to an optical bandpass filter. The optical frequency shifter and the optical bandpass filter are respectively connected to a second coupler. The second coupler is connected to a data acquisition system. The data acquisition system is connected to a computer.

[0062] The trigger pulse and the working clock of the pulse generator are synchronized with the second driver and the data acquisition system.

[0063] Preferably, the light source uses an L-band laser with an output power of 10 mW and an output wavelength of 1550.12 nm (corresponding to f0 = 193.5 THz). The first coupler uses a splitting ratio of 90:10, and 10% of the continuous light enters the optical pulse modulator. In this embodiment, an acousto-optic modulator (AOM) with a center frequency of 200 MHz is used as the optical pulse modulator, so the output optical frequency f1 = 193.5 THz + 200 MHz. At the same time, under the control of the pulse generator and the first driver, the output optical pulse width is set to w = 100 ns, and the pulse period T rep = 5 μs.

[0064] The following will describe the demodulation system of the heterodyne detection fiber optic vector hydrophone based on the weak reflection grating array with reference to the appendix Figure 1 as follows:

[0065] The light source is a narrow linewidth laser, which provides high-quality continuous light waves for the overall fiber optic vector hydrophone system, and the output light frequency is denoted as f0. Subsequently, the continuous light wave is split into two beams by a coupler, and the splitting ratio of the coupler can be adjusted according to the actual situation.

[0066] The upper part of the continuous light wave split into two beams by the coupler is modulated into a pulsed light by an optical pulse modulator. The optical frequency of the pulsed light is denoted as f1, and the time width of the pulsed light is denoted as w, and the pulse repetition frequency is denoted as T. rep . The complex amplitude expression of the electric field of the pulsed light can be written as:

[0067]

[0068] where E0 is the electric field strength of the output optical pulse, R w represents a rectangular window function with a width of w, k represents the serial number of the optical pulse, which can range from 1 to infinity. t is the time variable. Generally, we only consider the situation of the time variable t within one pulse period, that is, 0 ≤ t ≤ T. rep . The optical pulse modulator can be implemented by various different types of devices, including acousto-optic modulators (AOM), electro-optic modulators (EOM), and semiconductor optical amplifiers (SOA). The appropriate optical pulse modulator can be selected according to the optical power, pulse rise / fall time, etc. Note that the modulation signal of the optical pulse modulator comes from a pulse generator and also needs to be adjusted by a driver one for power amplification, impedance matching, etc. to correctly drive the optical pulse modulator. The trigger pulse and working clock of the pulse generator need to be synchronized with the subsequent driver two and data acquisition system.

[0069] In this embodiment, the amplified optical pulse sequence is output from port 1 of the circulator to port 2 and enters a weak reflection fiber grating array. The weak reflection fiber grating array is composed of 7 weak reflection gratings (FGB1~FBG7) at equal intervals, and the sensing fiber with a length of L is between adjacent two gratings. The relationship between L and the width w of the optical pulse should satisfy the following:

[0070]

[0071] where c is the propagation speed of light in vacuum, and n is the equivalent refractive index of the sensing fiber. Formula (5) can ensure that the optical pulse sequences returned from the grating array do not overlap and interfere with each other. On the other hand, in order to ensure that the optical powers returned from multiple gratings are as consistent as possible, and at the same time to avoid crosstalk caused by multiple reflections, the reflectivity of the grating should be as low as possible, generally designed to be less than 5%.

[0072] In this embodiment, the optical pulse sequence reflected from the weak reflection grating array reaches the port 3 of the circulator from the port 2 of the circulator, and then is amplified by an Erbium-doped Fiber Amplifier (EDFA) to compensate for the optical power loss. The amplified optical signal also needs to pass through an optical bandpass filter to filter out the spontaneous emission noise. If the noise introduced by the EDFA and the like is not considered, the optical pulse sequence at this time can be written as

[0073]

[0074] where η m is the optical field amplitude scaling factor after comprehensively considering factors such as grating reflectivity, optical link loss, and EDFA gain. τ m is the delay introduced by the m-th FBG. φ FBGm is the phase information carried in the optical pulse returned by the m-th FBG, and this phase information contains the phase change information generated by the action of the external sound field signal on the sensing optical fiber with a length of L.

[0075] Another part of the light split from the coupler 1, that is, the optical signal in the lower half, after passing through an optical frequency shifter, shifts its optical frequency domain to f2. Note that common devices used as optical frequency shifters include AOM, EOM, etc. The optical frequency shifter needs to be driven by driver 2, and this drive also needs to be synchronized with driver 1 and the data acquisition system in terms of clock. The continuous light with an optical frequency of f2 here is also called the Local Oscillator (LO), and its complex electric field amplitude can be written as:

[0076] E LO (t, k) = E′0exp[j2πf2(t + kT rep )] (3)

[0077] The pulse sequence represented by formula (2) and the local light represented by formula (3) are mixed in coupler 2, and the formed interference signal is then received by a Balanced Photo Detector (BPD). The use of the balanced detection method can effectively suppress the direct current component in the interference signal, and the obtained photocurrent signal can be written as:

[0078]

[0079] where Δf = f1 - f2 is the heterodyne frequency, which is the difference in the optical frequency shift amounts of the light beams on the two arms after coupler 1. φ 0mIt is the DC component in the phase of the light pulse reflected back by each grating. The photocurrent signal I(t,k) is converted into a voltage signal by the transimpedance amplifier circuit inside the BPD, and further can be collected by the data acquisition system and converted into a digital signal, and finally sent to the computer for corresponding data processing.

[0080] In the computer, through the IQ demodulation algorithm and the time-division process, the phase information φ in the cosine expression of each interference signal in each formula (4) can be restored. FBGm (k), where the influence of the independent variable t has been eliminated in the process of the time-division algorithm, and the phase information sampling rate corresponding to each grating return pulse is restored, that is, the corresponding pulse repetition frequency f. rep (f rep = 1 / T rep ).

[0081] In this embodiment, the pulse sequence is incident on the weak reflection fiber grating array through the circulator. The array contains 7 weak reflection fiber gratings. The reflectivity of each grating is set to 1‰, the 3dB bandwidth is 0.1nm, and the reflection center wavelength is also set to 1550.12nm. The spacing between two gratings is set to 20m, which satisfies the constraint conditions of formula (5). The peak power of the pulse reflected back from the grating is 1μW. If the voltage amplification factor of the subsequent balanced photodetector is considered to be 20kΩ, the voltage amplitude is 20mW, which is slightly insufficient. An EDFA can be considered to provide a gain of 10 - 100 times. According to different EDFA models, its gain current can be set to 10 - 100mA. The bandwidth of the optical bandpass filter after the EDFA can be selected as 0.8nm or 0.4nm to filter out the amplifier spontaneous-emission noise in the EDFA.

[0082] In this embodiment, the frequency shift frequency generated by the optical pulse frequency shifter is relatively high, which puts forward higher requirements for the bandwidth of the balanced photodetector. In order to reduce the demand for the bandwidth of the photodetector, an optical frequency shifter can be inserted in one path with a 90% splitting ratio of the coupler to reduce the frequency of the optical signal after beating. An AOM with a center frequency of 150MHz can be selected as the optical frequency shifter, that is, f2 = 193.5THz + 150MHz, then its heterodyne frequency is Δf = f1 - f2 = 50MHz, and formula (6) can also be well satisfied. After the above design, the heterodyne frequency is reduced to 50MHz. In each optical pulse, there are 5 carrier oscillation periods, and the phase demodulation task can be well completed. Through the above reasonable design, the design requirements of the fiber optic vector hydrophone based on heterodyne detection can be achieved.

[0083] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. An optical fiber vector hydrophone for heterodyne detection based on a weak reflection grating array, characterized in that Comprising: A central mass block (1), on six faces of the central mass block (1) in the X-axis, Y-axis, and Z-axis directions, sensing units are installed. The sensing unit includes a compliant cylinder structure (2) installed on the central mass block. Weak reflection gratings FBG (3) are installed on the central mass block (1) and at the ends of the 6 compliant cylinder structures (2). The weak reflection gratings FBG at the ends of the 6 compliant cylinders (2) and the weak reflection grating FBG (3) located on the central mass block (1) are arranged at equal intervals to form a weak reflection fiber grating array. A fiber (4) with a length of L is wound between the weak reflection grating FBG (3) located on the central mass block (1) and the weak reflection grating FBG at the end of the compliant cylinder.

2. A demodulation system for a heterodyne detection fiber optic vector hydrophone based on a weak reflection grating array, characterized in that, Comprising: A coupler one, an optical pulse modulator, and the fiber optic vector hydrophone as described in claim 1. The coupler one is respectively connected to the optical pulse modulator and the optical frequency shifter. The optical pulse modulator is connected to a driver one. The driver one is connected to a pulse generator. The optical frequency shifter is connected to a driver two. The optical pulse modulator is connected to a circulator. The circulator is respectively connected to the fiber optic vector hydrophone and an erbium-doped fiber amplifier. The erbium-doped fiber amplifier is connected to an optical bandpass filter. The optical frequency shifter and the optical bandpass filter are respectively connected to a coupler two. The coupler two is connected to a data acquisition system. The data acquisition system is connected to a computer.

3. The demodulation system of the heterodyne detection fiber optic vector hydrophone based on a weak reflection grating array according to claim 2, characterized in that, The trigger pulse and the working clock of the pulse generator are synchronized with the driver two and the data acquisition system.

4. A demodulation method for a demodulation system of a vector hydrophone, characterized in that This method uses the demodulation system as described in any one of claims 2-3, specifically including the following steps: The narrow linewidth laser provides a high-quality continuous light wave, which is split into two beams by a coupler. Among them, the upper half of the continuous light wave is modulated into a pulsed light by the optical pulse modulator. The optical frequency of the pulsed light is denoted as f1, the time width of the pulsed light is denoted as w, and the pulse repetition frequency is denoted as T rep , and the expression of the complex amplitude of the electric field of the pulsed light is written as: where: E0 is the electric field strength of the output optical pulse, R w represents a rectangular window function with width w, k represents the serial number of the optical pulse, and t is the time variable; The amplified optical pulse sequence is output from port 1 of the circulator to port 2 and enters the weak reflection fiber grating array of one of the fiber optic vector hydrophones. The optical pulse sequence reflected from the weak reflection grating array arrives at port 3 of the circulator from port 2 of the circulator, and then is amplified by the erbium-doped fiber amplifier to compensate for the optical power loss. The amplified optical signal also needs to pass through the optical bandpass filter to filter out the spontaneous emission noise to obtain an optical pulse sequence. If the noise introduced by the EDFA and the like is not considered, the optical pulse sequence at this time is written as Among them, η m is the optical field amplitude scaling factor after comprehensively considering factors such as grating reflectivity, optical link loss, and EDFA gain. τ m is the delay introduced by the m-th FBG, and φ FBGm is the phase information carried in the optical pulse returned by the m-th FBG. This phase information contains the phase change information generated by the action of the external sound field signal on the sensing optical fiber with a length of L. Another part of the light split from the coupler one, that is, the continuous light wave in the lower half, after passing through the optical frequency shifter, shifts its optical frequency domain to f2. The continuous light with an optical frequency of f2 here is the local oscillator light, and its complex amplitude of the electric field is written as: E LO (t, k) = E′0exp[j2πf2(t + kT rep )] (3) The pulse sequence represented by formula (2) and the eigen light represented by formula (3) are mixed in the coupler two, and the formed interference signal is then received by a balanced photodetector. The direct current component in the interference signal is effectively suppressed by adopting the balanced detection method, and the obtained photocurrent signal is written as: where Δf = f1 - f2 is the heterodyne frequency, which is the difference in the optical frequency shifts of the light beams on the two arms after coupler 1, and φ 0m is the DC component in the phase of the optical pulse reflected by each grating; The photocurrent signal I(t,k) is converted into a voltage signal through the transimpedance amplifier circuit inside the balanced photodetector of the detector, and further can be collected by the data acquisition system and converted into a digital signal, and finally sent to the computer for corresponding data processing; In a computer, through the IQ demodulation algorithm and the time-division process, the phase information φ in each cosine expression of the interference signal in each formula (4) can be restored. FBGm (k), where the influence of the independent variable t has been eliminated in the process of the time-division algorithm, and the phase information sampling rate corresponding to each grating return pulse is restored, that is, the corresponding pulse repetition frequency f. rep f rep = 1 / T rep .

5. The demodulation method of the vector hydrophone demodulation system according to claim 4, characterized in that, The modulation signal of the optical pulse modulator comes from a pulse generator and needs to be adjusted by Driver 1 for power amplification, impedance matching, etc. before it can correctly drive the optical pulse modulator.

6. The demodulation method of the vector hydrophone demodulation system according to claim 5, characterized in that The amplified optical pulse sequence is output from Port 1 of the circulator to Port 2 and enters a weak reflection fiber grating array. The weak reflection fiber grating array is composed of 7 equally spaced weak reflection gratings FGB. The sensing fiber with a length of L is between two adjacent weak reflection gratings FGB. The following relationship should be satisfied between L and the width w of the optical pulse: where c is the propagation speed of light in a vacuum and n is the equivalent refractive index of the sensing fiber.

7. The demodulation method of the vector hydrophone demodulation system according to claim 6, characterized in that The optical frequency shifter adjusts the heterodyne frequency so that there are at least 3 - 5 heterodyne signal waveforms in each pulse of the obtained interference photocurrent signal I, so that the subsequent algorithm can demodulate the phase information carried in each pulse. It is expressed by the formula:

8. The demodulation method of the vector hydrophone demodulation system according to claim 7, characterized in that If only the optical pulse modulator exists, formula (6) can be satisfied, that is, f2 = 0, then the optical frequency shifter in the signal demodulation mechanism can be omitted.

9. The demodulation method of the vector hydrophone demodulation system according to claim 7, characterized in that When the power of the returned pulse sequence is sufficient, the erbium-doped fiber amplifier and the optical bandpass filter can be omitted.

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