A high-sensitivity and low-noise hydrophone system structure based on four-core optical fiber

Abstract

The present invention discloses a high-sensitivity, low-noise hydrophone system structure based on four-core optical fiber, including a laser, an optical modulation and beam splitting module, a four-core optical fiber hydrophone and a phase demodulation and noise suppression processing module. The four-core optical fiber hydrophone includes a first reference optical fiber unit, a first cascade sensitivity enhancement channel, a second reference optical fiber unit, and a second cascade sensitivity enhancement channel. The first reference unit and the first cascade sensitivity enhancement channel respectively output interference optical signals. I 1 , I 2 The second reference unit and the second cascade sensitivity enhancement channel output interference light signals respectively I 3 , I 4 The present invention is applied to the field of hydrophones, using four-core optical fibers to construct an ultra-long cyclotron sensitivity enhancement optical path, and then using the noise correlation between channels to construct a multi-channel interference complementary optical path. Through joint processing, sensitivity improvement and self-suppression of common-mode noise are achieved simultaneously, without the need to introduce additional fiber interferometers, and the noise is independent of the transmission optical path, reducing the complexity of the system and the background noise.

Description

Technical Field

[0001] The invention relates to the technical field of hydrophones, in particular to a high-sensitivity and low-noise hydrophone system structure based on four-core optical fibers. Background Art

[0002] Fiber optic hydrophones are based on the principle of optical coherence detection. They convert weak sound field information in seawater into hydrophone interference phase information, and then obtain useful information of underwater sound field after phase demodulation and signal processing. They can be applied to underwater target detection, submarine oil exploration, and marine sound field environment monitoring. Compared with traditional piezoelectric detector systems, fiber optic hydrophones have the advantages of small size, light weight, and good adaptability. They can easily form various underwater fiber optic sensor networks, providing an effective technical approach to solving problems such as long-distance target detection and high-resolution marine energy exploration.

[0003] As energy exploration and other application needs continue to expand to the deep sea, higher requirements are placed on the overall performance of the fiber optic hydrophone system. First, the deep sea environment is quieter than the shallow sea, and the background noise in the marine environment is low. Therefore, it is necessary to further reduce the equivalent noise pressure of the hydrophone system to fully utilize the advantages of the deep sea to obtain weaker sound field information and better detection performance; secondly, as the working depth of the hydrophone element increases, its pressure resistance performance also needs to be improved simultaneously to adapt to the long-term working requirements in the deep sea high hydrostatic pressure and high corrosion environment. In view of the above requirements, in theory, the reduction of equivalent noise pressure can be achieved by increasing the element sensitivity and reducing the system self-noise, while the improvement of the hydrophone pressure resistance performance can be achieved by strengthening the hardness and thickness of the element structure. However, the above scheme has the following problems in specific implementation: (1) The acoustic sensitivity of the hydrophone element is inversely proportional to the structural hardness. The improvement of pressure resistance performance is often at the expense of sensitivity. Therefore, high pressure resistance and high sensitivity are a pair of contradictory indicators, which are usually difficult to achieve at the same time; (2) In addition to the sensing element, the fiber optic hydrophone system also contains a relatively complex optical transmission and modulation and demodulation unit. When used in deep sea, the system background noise is above a certain threshold due to the limitations of optical path and device noise such as the disturbance noise of the long link transmission optical cable, the frequency and phase noise of the laser, the spontaneous emission noise of the optical amplifier, and the shot noise of the detection. And with the increase of the array multiplexing scale and the transmission distance, the system noise continues to increase, and it is increasingly difficult to suppress the self-noise only through device selection, vibration reduction and noise reduction. In summary, when the fiber optic hydrophone is used in the deep sea, it is affected by comprehensive factors such as working depth, sensitivity and optical noise. The insufficient equivalent noise sound pressure of the system is one of the important factors restricting its high-performance application.

[0004] At present, there are two main solutions to reduce the equivalent noise pressure of deep-sea fiber-optic hydrophones: (1) Ultra-long thin straight fiber optic sensitivity enhancement solution. Under the limitation of deep-sea materials and structural dimensions, a single-core thin-diameter ultra-long optical fiber is wound on an elastic sensitivity enhancement cylinder in order to simultaneously improve the sensitivity and pressure resistance of the fiber-optic hydrophone, that is, to minimize the outer diameter of the wound optical fiber and increase the length of the wound optical fiber to achieve sensitivity enhancement. At present, the outer diameter of the hydrophone wound optical fiber has been reduced from the ordinary 250μm to 80μm, and the length has been increased from 100m to 400m or even higher. It is difficult to further improve the sensitivity. In addition, as the outer diameter of the optical fiber is reduced and the length is increased, many problems such as increased difficulty in winding process, increased element loss, decreased reliability and consistency are brought about, and it is often difficult to achieve the expected effect. (2) Reference interferometer noise cancellation solution. This solution adds a non-acoustically sensitive reference interferometer to the hydrophone system to subtract the phase noise of the conventional hydrophone and the reference interferometer, in order to suppress the system's self-noise. The disadvantage of this scheme is that it needs to introduce a reference interferometer that is not related to sensing, which increases the complexity of the system. In addition, the phase noise of the reference interferometer and the conventional hydrophone needs to be highly correlated to achieve a good suppression effect. However, in actual systems, due to the influence of various differences such as polarization, delay, and loss of different optical paths, the two are often partially correlated, so the reference interferometer noise cancellation scheme also has great limitations. Summary of the invention

[0005] In view of the deficiencies in the above-mentioned prior art, the present invention provides a high-sensitivity, low-noise hydrophone system structure based on four-core optical fiber. The structure utilizes the multi-channel cascade sensing effect of a single four-core optical fiber to improve the sensitivity of the elastomer. Compared with the conventional single-core optical fiber winding scheme, under the same hydrophone pressure-resistant structure size and sensitivity requirements, the optical fiber length can be greatly shortened, and the difficulty of hydrophone winding can be reduced; on this basis, the noise correlation between the four-core optical fiber channels is utilized to construct a multi-channel interference complementary optical path, and the common mode noise is self-suppressed through joint processing. Compared with the conventional noise elimination scheme, there is no need to introduce additional optical fiber interferometers, and the noise is independent of the transmission optical path, thereby reducing the complexity of the system and the background noise. That is, the present invention uses a single optical fiber to construct a special optical fiber hydrophone system structure to achieve efficient acoustic sensitivity enhancement and noise suppression at the same time. The dual approach has great advantages in equivalent noise pressure suppression compared to the original scheme.

[0006] To achieve the above object, the present invention provides a high-sensitivity, low-noise hydrophone system structure based on four-core optical fiber, comprising:

[0007] A laser, used for outputting laser light;

[0008] An optical modulation and beam splitting module, used for receiving the laser output by the laser device and outputting two modulated equal-amplitude optical signals;

[0009] The four-core optical fiber hydrophone is used to receive the two optical signals output by the optical modulation and beam splitting module and output four interference optical signals. I 1. I 2. I 3. I 4, and I 1 and I 2 constitutes a set of complementary interference light units, I 3 and I 4 constitutes another set of complementary interference light units;

[0010] Four photoelectric detectors, used to receive the four interference light signals respectively and output four analog electrical signals;

[0011] Four analog-to-digital converters, used to receive the four analog electrical signals respectively and output four digital signals;

[0012] A phase demodulation and noise suppression processing module, used for receiving the four-channel digital signals and performing phase demodulation and noise suppression processing;

[0013] The four-core optical fiber hydrophone comprises a first reference optical fiber unit, a first cascade sensitivity enhancement channel, a second reference optical fiber unit, and a second cascade sensitivity enhancement channel;

[0014] The input end of the first reference unit is coupled with the first output of the optical modulation and beam splitting module in the same fiber, the input end of the first cascade sensitivity enhancement channel is coupled with the first output of the optical modulation and beam splitting module in a different fiber, and the first reference unit and the first cascade sensitivity enhancement channel respectively output interference optical signals I 1. I 2;

[0015] The input end of the second reference unit is coupled with the second output of the optical modulation and beam splitting module in the same fiber, and the input end of the second cascade sensitivity enhancement channel is coupled with the second output of the optical modulation and beam splitting module in a different fiber manner. The second reference unit and the second cascade sensitivity enhancement channel respectively output interference optical signals. I 3. I 4.

[0016] In one embodiment, the four-core fiber hydrophone specifically includes a four-core fiber cascade sensitivity enhancement unit, a fan-in module, a fan-out module, a first reference fiber R1, a second reference fiber R2, a fiber coupler C1, a fiber coupler C2, a fiber coupler C3, and a fiber coupler C4;

[0017] The four-core fiber cascade sensitivity enhancement unit is an elastic structure wound around four-core optical fibers, and the two ends of the four-core optical fiber cascade sensitivity enhancement unit are respectively connected to a fan-in module and a fan-out module, and the light waves of the four cores of the four-core optical fiber cascade sensitivity enhancement unit are coupled into four groups of conventional single-core optical fibers, that is, the fan-in module couples single-core optical fiber 1A, single-core optical fiber 2A, single-core optical fiber 3A, and single-core optical fiber 4A, and the fan-out module couples single-core optical fiber 1B, single-core optical fiber 2B, single-core optical fiber 3B, and single-core optical fiber 4B;

[0018] The single-core optical fiber 1B is fused with the single-core optical fiber 2A, and the single-core optical fiber 3B is fused with the single-core optical fiber 4A;

[0019] The optical fiber coupler C1, the optical fiber coupler C2, the optical fiber coupler C3, and the optical fiber coupler C4 all have a first port, a second port, a third port, and a fourth port, and in the optical fiber coupler C1, the optical fiber coupler C2, the optical fiber coupler C3, and the optical fiber coupler C4, the first port and the third port are homofiber coupled, and the first port and the fourth port are heterofiber coupled;

[0020] The first port of the fiber coupler C1 is connected to the first output of the optical modulation and beam splitting module, and the two ends of the first reference optical fiber R1 are respectively connected to the third port of the fiber coupler C1 and the first port of the fiber coupler C2, forming the first reference optical fiber unit;

[0021] The fourth port of the optical fiber coupler C1 is fused with the single-core optical fiber 1A, and the second port of the optical fiber coupler C2 is fused with the single-core optical fiber 2B to form the first cascade sensitivity enhancement channel;

[0022] The first port of the fiber coupler C3 is connected to the second output of the optical modulation and beam splitting module, and the two ends of the second reference optical fiber R2 are respectively connected to the fourth port of the fiber coupler C3 and the second port of the fiber coupler C4, forming the second reference optical fiber unit;

[0023] The third port of the optical fiber coupler C3 is fused with the single-core optical fiber 3A, and the first port of the optical fiber coupler C4 is fused with the single-core optical fiber 4B to form the second cascade sensitivity enhancement channel.

[0024] In one embodiment, the analog electrical signals output by the four photodetectors are specifically:

[0025] ;

[0026] in, V 1. V 2. V 3. V 4 is interference lightI 1. I 2. I 3. I 4Four analog voltage signals after photoelectric conversion, R 1. R 2. R 3. R 4 is the photoelectric conversion efficiency of the four photodetectors, ω 0. C are the frequency and amplitude required for phase-generated carrier modulation, I 01 , I 02 , I 03 , I 04 Interference light I 1. I 2. I 3. I The light intensity of 4, I n1 , I n2 , I n3 , I n4 Interference light I 1. I 2. I 3. I The light intensity noise term of 4, , , , Interference light I 1. I 2. I 3. I The initial phase of 4, , , , Interference light I 1. I 2. I 3. I The phase noise term of 4 is It is the phase shift introduced by the sound pressure signal in a single channel of a four-core optical fiber.

[0027] The model processing process of the phase demodulation and noise suppression processing module is as follows:

[0028] Will V 1 and V 2. V 3 and V 4 Perform optical differential subtraction between two signals to obtain two sets of differential signals , ,for:

[0029] ;

[0030] ;

[0031] ;

[0032] ;

[0033] ;

[0034] ;

[0035] in, R is the average photoelectric conversion efficiency of the four detectors, for I 1. I The average phase noise of 2, for I 1. I The light intensity noise difference of 2, for I 3. I The average phase noise of 4, for I 3. I 4 light intensity noise difference;

[0036] Will , Perform phase demodulation separately to obtain two sets of noisy phase terms , ,for:

[0037] ;

[0038] in, , for , Demodulated converted phase noise;

[0039] Then , Taking the average we get:

[0040] ;

[0041] ;

[0042] ;

[0043] in, Output results for signal and noise, is the phase noise output result, Output result for intensity noise.

[0044] The above noises belong to the A and B differential interference groups. and , and After incoherent superposition, the noise power spectral density is reduced to 1 / 2 of the original value, and the signal item detected by the system becomes twice of the original value, thereby achieving improved system sensitivity and noise suppression.

[0045] Compared with the prior art, the present invention has the following beneficial technical effects:

[0046] 1. The present invention proposes a cascade sensitivity enhancement scheme based on multi-core optical fiber, which greatly increases the acoustic sensitivity of the fiber-optic hydrophone under the condition of high hydrostatic pressure resistant structural constraints, and reduces the length of the wound optical fiber and the size of the element. A single optical fiber with multiple core optical channels is wound on the elastic sensitivity enhancement cavity or cylinder of the hydrophone, and multiple channels in the optical fiber can respond to the deformation of the elastic body at the same time. Utilizing this effect, the ends of different channels of the fan-out modules at both ends of the multi-core optical fiber are connected to form an ultra-long convoluted cascade sensitivity enhancement optical path, thereby achieving a significant improvement in the sensitivity of the fiber-optic hydrophone. Theoretically, a sensitivity enhancement of 6dB can be achieved by connecting two-core channels end to end, and a sensitivity enhancement of 12dB can be achieved by connecting all four-core channels end to end, and so on;

[0047] 2. The present invention proposes a multi-channel joint demodulation scheme based on a complementary interference optical path, which uses the correlation of the sensing signals of the multi-core optical fiber channel to suppress the phase noise of the optical fiber hydrophone. With two-core optical fibers as the minimum unit, a complementary interference channel with cascaded sensitivity enhancement is constructed. Through optical differential joint demodulation, the system signal-to-noise ratio can be improved while the sensitivity is improved, and the noise suppression effect is more than 3dB. Based on this, through the joint demodulation of more channels, better noise suppression effect can be achieved;

[0048] 3. The present invention makes full use of the multi-channel sensing characteristics of multi-core optical fibers to construct a fiber optic hydrophone system with high sensitivity and low noise. The test results of the four-core fiber optic hydrophone show that the present invention can achieve simultaneous improvement of the sound pressure sensitivity and phase noise indicators, so the equivalent noise sound pressure of the fiber optic hydrophone system can be reduced by more than 9dB, which is a significant improvement over the performance of existing systems, and has the advantages of simple optical structure, high fiber utilization, and high functional integration.

[0049] 4. The present invention has good scalability and can be extended to five-core, seven-core or even more-core optical fibers for hydrophone winding. In the specific implementation process, different cascaded sensitivity enhancement and joint demodulation channel allocation schemes can be designed according to the sensitivity and noise characteristic index requirements of the actual system, so it has strong adaptability. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] 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.

[0051] Figure 1 Schematic diagram of the structure of a high-sensitivity, low-noise hydrophone system based on four-core optical fiber in an embodiment of the present invention;

[0052] Figure 2 Schematic diagram of the basic structure of a four-core optical fiber hydrophone in an embodiment of the present invention;

[0053] Figure 3 A comparison diagram of the phase shift sensitivity of a four-core optical fiber and a single-core optical fiber hydrophone in an embodiment of the present invention;

[0054] Figure 4 This is a diagram showing the noise suppression results of the optical complementary differential channel in an embodiment of the present invention.

[0055] The realization of the purpose, functional features and advantages of the present invention will be further explained in conjunction with embodiments and with reference to the accompanying drawings. DETAILED DESCRIPTION

[0056] 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.

[0057] In the present invention, unless otherwise clearly specified and limited, the terms "connection", "fixation", etc. should be understood in a broad sense. For example, "fixation" can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection, an electrical connection, a physical connection, or a wireless communication connection; it can be a direct connection, or an indirect connection through an intermediate medium, or it can be the internal connection of two elements or the interaction relationship between two elements, unless otherwise clearly defined. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0058] In addition, the technical solutions between the various embodiments of the present invention can be combined with each other, but it must be based on the fact that ordinary technicians in the field can implement it. When the combination of technical solutions is contradictory or cannot be implemented, it should be deemed that such combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0059] This embodiment discloses a high-sensitivity, low-noise hydrophone system structure based on four-core optical fiber, which has the effects of efficient acoustic sensitization and background noise self-suppression, and can achieve effective suppression of the equivalent noise pressure of the optical fiber hydrophone system. The structure uses four-core optical fiber to wind the hydrophone to form an ultra-long gyroscopic sensitization physical channel, and combines the multi-channel coupling design to form a complementary interference channel. After optical differential joint demodulation, high-sensitivity, low-noise sensing information can be obtained. Compared with the deep-sea single-core optical fiber hydrophone of the same material thickness and the same size, the sensitivity of the optical fiber hydrophone of this embodiment can be improved by more than 6dB, the noise suppression can reach 3dB, and the comprehensive suppression effect of the equivalent noise pressure is better than 9dB. It has the advantages of simple structure, high optical fiber utilization, good optical compatibility, etc. It can be applied to deep-sea low-noise optical fiber hydrophone array systems, providing key technical support for solving problems such as high-sensitivity hydroacoustic detection resistant to deep water pressure and high-resolution seabed bottom exploration.

[0060] refer to Figure 1 In this embodiment, the high-sensitivity, low-noise hydrophone system structure based on four-core optical fiber specifically includes a laser, an optical modulation and beam splitting module, a four-core optical fiber hydrophone, four photodetectors D1-D4, four analog-to-digital converters AD1-AD4 and a phase demodulation and noise suppression processing module, wherein the laser preferably uses an ultra-narrow linewidth laser. Specifically,

[0061] After the ultra-narrow linewidth laser outputs laser light to the optical modulation and beam splitting module, the optical modulation and beam splitting module outputs two modulated equal-amplitude optical signals and connects to the two input ports of the four-core optical fiber hydrophone. C 1_1 and C 3_1 , the four output ports of the four-core fiber optic hydrophone C 2_3 , C 2_4 , C 4_3 , C 4_4 , output four-way interference optical signal I 1. I 2. I 3. I 4, and I 1 and I 2 constitutes a set of complementary interference light units, I 3 and I4 forms another set of complementary interference light units. Then the four-way interference light signal I 1. I 2. I 3. I 4 is connected to the photoelectric detectors D1~D4 and the analog-to-digital converters AD1~AD4 to obtain four digital signals, and finally connected to the phase demodulation and noise suppression processing module to achieve high-sensitivity signal demodulation and common-mode noise self-suppression.

[0062] In the specific implementation process, the four-core fiber hydrophone includes a first reference fiber unit, a first cascade sensitivity enhancement channel, a second reference fiber unit, and a second cascade sensitivity enhancement channel. Specifically, the input end of the first reference unit is coupled with the first output of the optical modulation and beam splitting module in the same fiber, the input end of the first cascade sensitivity enhancement channel is coupled with the first output of the optical modulation and beam splitting module in a different fiber, and the first reference unit and the first cascade sensitivity enhancement channel respectively output interference optical signals. I 1. I 2. The input end of the second reference unit is coupled with the second output of the optical modulation and beam splitting module by the same fiber, and the input end of the second cascade sensitivity enhancement channel is coupled with the second output of the optical modulation and beam splitting module by different fiber. The second reference unit and the second cascade sensitivity enhancement channel output interference optical signals respectively. I 3. I 4.

[0063] refer to Figure 2 The four-core fiber hydrophone specifically includes a four-core fiber cascade sensitivity enhancement unit, a fan-in module, a fan-out module, a first reference fiber R1, a second reference fiber R2, a fiber coupler C1, a fiber coupler C2, a fiber coupler C3, and a fiber coupler C4. Among them, the fiber couplers C1~C4 all have a first port, a second port, a second port, and a fourth port, and in the fiber couplers C1~C4, the first port and the third port are coupled in the same fiber, and the first port and the fourth port are coupled in different fibers. That is, Figure 2 As shown, in the first port C1-1, the second port C1-2, the second port C1-3 and the fourth port C1-4 of the optical fiber coupler C1, C1-1 and C1-3 are coupled with the same fiber, and C1-1 and C1-4 are coupled with the same fiber and with different fibers; in the first port C2-1, the second port C2-2, the second port C2-3 and the fourth port C2-4 of the optical fiber coupler C2, C2-1 and C2-3 are coupled with the same fiber, and C2-1 and C2-4 are coupled with the same fiber and with different fibers. In the first port C3-1, the second port C3-2, the second port C3-3 and the fourth port C3-4 of the optical fiber coupler C3, C3-1 and C3-3 are coupled with the same fiber, and C3-1 and C3-4 are coupled with the same fiber and with different fibers; in the first port C4-1, the second port C4-2, the second port C4-3 and the fourth port C4-4 of the optical fiber coupler C4, C4-1 and C4-3 are coupled with the same fiber, and C4-1 and C4-4 are coupled with the same fiber and with different fibers. Figure 2 As is known, C1-1 and C3-1 are two input ports of the four-core optical fiber hydrophone, and C2-3, C2-4, C4-3, and C4-4 are two output ports of the four-core optical fiber hydrophone.

[0064] In this embodiment, the four-core optical fiber cascade sensitivity enhancement unit is an elastic structure wound with four-core optical fibers, and the optical fiber length is L. The action on the elastic body causes the optical fiber to expand and contract, modulating the length of the fiber cores 1 to 4 in the optical fiber to change synchronously and generate a phase difference ( ~ The two ends of the four-core optical fiber are connected to the fan-in module and the fan-out module to couple the light waves of the cores 1 to 4 into four groups of conventional single-core optical fibers, that is, the fan-in module couples the single-core optical fiber 1A, single-core optical fiber 2A, single-core optical fiber 3A, and single-core optical fiber 4A, and the fan-out module couples the single-core optical fiber 1B, single-core optical fiber 2B, single-core optical fiber 3B, and single-core optical fiber 4B. The lengths of the single-core optical fibers are l， The first group includes 1A, 1B and core 1, the second group includes 2A, 2B and core 2, the third group includes 3A, 3B and core 3, and the first group includes 4A, 4B and core 4. The single-core optical fiber 1B is fused with the single-core optical fiber 2A, and the single-core optical fiber 3B is fused with the single-core optical fiber 4A. The first port of the fiber coupler C1 is connected to the first output of the optical modulation and beam splitting module, and the two ends of the first reference optical fiber R1 are respectively connected to the third port of the fiber coupler C1 and the first port of the fiber coupler C2, forming a first reference optical fiber unit; the fourth port of the fiber coupler C1 is fused with the single-core optical fiber 1A, and the second port of the fiber coupler C2 is fused with the single-core optical fiber 2B, forming a first cascade sensitivity enhancement channel; the first port of the fiber coupler C3 is connected to the second output of the optical modulation and beam splitting module, and the two ends of the second reference optical fiber R2 are respectively connected to the fourth port of the fiber coupler C3 and the second port of the fiber coupler C4, forming a second reference optical fiber unit; the third port of the fiber coupler C3 is fused with the single-core optical fiber 3A, and the first port of the fiber coupler C4 is fused with the single-core optical fiber 4B, forming a second cascade sensitivity enhancement channel.

[0065] Ideally, the sound pressure The phase caused Both , then both cascade sensitization channels produce The phase change is doubled compared to the single-core fiber sensitization effect. In addition, in the fiber optic hydrophone element with a push-pull structure, the two reference fibers also use four-core fibers and are wound on the inner cavity structure of the elastic sensitization cavity. Figure 2 The reference optical fibers R1 and R2 correspond to two groups of cascaded sensitization channels of a single four-core optical fiber, that is, R1 corresponds to the 1-2 core combination of the four-core optical fiber, and R2 corresponds to the 3-4 core combination of the four-core optical fiber.

[0066] Figure 2 It also includes two sets of complementary interference optical units, in which the fiber coupler C1~C2, the first reference optical fiber R1, the 1~2 cores of the four-core optical fiber and the fan-in and fan-out optical fibers constitute the complementary interference optical unit A; the fiber coupler C1~C4, the reference optical fiber R2, the 3~4 cores of the four-core optical fiber and the fan-in and fan-out optical fibers constitute the complementary interference optical unit B. C1~C4 are all 4-port fused taper type 2×2 couplers with a splitting ratio of 50:50. The first reference optical fiber R1 is a single-core single-mode optical fiber. R1 is wound on a rigid cylinder to affect the sound pressure. P No response, forming a non-acoustically sensitive reference channel corresponding to the enhanced sensitivity channel. The length of R1 L R For 2 L+4l+ΔL , where Δ L is the difference of the unbalanced interference arms, and the length is 0.5m~1m. C1_3, C2_1 and both ends of R1 are fused, C1_4 and 1A, C2_2 and 2B are fused to form a Mach-Zehnder fiber interferometer. When the input light enters port C1_1, ports C2_3 and C2_4 output two sets of interference signals. According to the fiber coupling mode theory, hetero-fiber coupling introduces an additional phase shift of π / 2, so the phase shift of the input light from C1_1→reference fiber 1→output port C2_3 is 0, while the phase shift of C1_1→1A→2B→C2_3 is π / 2+π / 2, and the fixed phase shift after the two light paths form interference is π; the phase shift of the input light from C1_1→reference fiber 1→output port C2_4 is 0+π / 2, while the phase shift of C1_1→1A→2B→C2_4 is π / 2+0, and the phase shift after the two light paths form interference is π. For the fiber optic hydrophone with a push-pull structure, the reference optical fibers R1 and R2 are respectively the 1-2 core combination and 3-4 core combination of a single four-core optical fiber. Therefore, the corresponding channels of the fiber fan-out module need to be connected to the corresponding ports of the couplers C1~C4 respectively, and the phase shift of the formed complementary interference structure is still π.

[0067] According to the above complementary interference phase shift analysis, add interference phase noise and intensity noise , we can get the interference light output from ports C2_3 and C2_4 , They can be expressed as:

[0068] (1)

[0069] In formula (1), I 01 , I 02 Interference light , The light intensity, , , , They are , The initial phase and phase noise terms, , is the light intensity noise term. Ideally , The same interference optical path is formed, and the light intensity I 01 = I 02 ,and = ;noise and , and are approximately equal, only the interference signal term is opposite, so , To form a set of complementary interference light units A.

[0070] Similarly, the complementary interference light unit B inputs light from C3_1, and the interference light output from ports C4_3 and C4_4 , They can be expressed as:

[0071] (2)

[0072] In formula (2), , Interference light , The light intensity, , , , for , The initial phase and phase noise terms, , is the light intensity noise term. Ideally I 03 = I 04 and = ; Noise term ~ , ~ ,and , An interference light unit B corresponding to the complementary interference light unit A is constructed.

[0073] The analog electrical signals output by the four photodetectors can be specifically expressed as:

[0074] (3)

[0075] In formula (3), V 1. V 2. V 3. V 4 is four-channel analog voltage signal, R 1. R 2. R 3. R 4 is the photoelectric conversion efficiency of the four photodetectors, ω 0. C are the frequency and amplitude required for phase-generated carrier modulation, and .

[0076] According to the complementary interference light unit A and complementary interference light unit B, V 1 and V 2. V 3 and V 4 are two sets of complementary interference terms A and B, which are optically subtracted from each other, and two sets of differential signals can be approximately obtained as follows:

[0077] (4)

[0078] In formula (4), , , , .

[0079] From formula (4), we can get V 1 and V Take 2 as an example, after optical differentiation, The interference signal term increases to 2 times of the original; for the intensity noise term, if the best case is considered, that is, and equal, Can be completely eliminated; worst case and is uncorrelated white noise, Increase to original times, Compared with V1 and V2, the signal-to-noise ratio is improved Times. Similarly, It also has the effect of improving the signal-to-noise ratio. Taking into account different noise conditions, the intensity noise suppression effect of formula (4) will be better than 3dB.

[0080] Furthermore, we can transform the formula (4) into , Perform phase demodulation respectively, and get two sets of noisy phase terms:

[0081] (5)

[0082] In formula (5), , for , Demodulated converted phase noise;

[0083] Will , Taking the average we get:

[0084] (6)

[0085] In formula (6), , , the above noises belong to the A and B differential interference groups, and , and After incoherent superposition, , The power spectral density is reduced by 3dB.

[0086] Finally, considering the equivalent noise sound pressure of the fiber optic hydrophone system , and its calculation formula is:

[0087] (7)

[0088] In formula (7), is the basic sensitivity of the fiber optic hydrophone, is the phase noise of the fiber optic hydrophone system, and the original phase noise term in (6) is and light intensity noise conversion term By comparing equation (1) and equation (6), it can be obtained that after adopting the scheme of this embodiment, under the same structure and winding optical fiber length, the signal item detected by the system becomes twice as much as before, that is, the basic unit sensitivity of the hydrophone is Increase by 6dB; according to the different proportions of phase noise and intensity noise, the noise term The reduction ranges from 3 to 6 dB. Under the superposition of the two coefficients, the equivalent noise sound pressure of the fiber optic hydrophone system The final reduction can be 9dB~12dB.

[0089] It is worth noting that the four-core fiber optic hydrophone sensitivity enhancement and optical differential noise suppression method proposed in this embodiment is a basic solution. By promoting multi-core optical fibers, such as 5-core and 7-core optical fibers, and utilizing the multi-physical channel combination sensing effect of more core optical fibers, longer cascade sensing sensitivity enhancement channels, push-pull cascade sensitivity enhancement channels and more optical differential interference combinations can be constructed, which can further improve the sensitivity of the primitive and reduce the background noise, and can have a better suppression effect on the equivalent noise sound pressure of the entire fiber optic hydrophone system.

[0090] The structure of the hydrophone system of this embodiment is further described below with reference to specific examples.

[0091] refer to Figure 3 , which is a comparison chart of the phase shift sensitivity test results of hydrophones wound with four-core optical fibers and single-core optical fibers of equal length using the same pressure-resistant and sensitivity-enhancing structure of the hydrophone skeleton. Figure 3 It can be seen that the single-core and four-core hydrophones have uniformly good sensitivity response flatness in the test frequency band of 10Hz~2000Hz, and after adopting the double-stage cascade sensitivity enhancement structure, the average sensitivity of the four-core fiber optic hydrophone is about 6.0dB higher than that of the single-core fiber optic hydrophone, and the sensitivity improvement is consistent with the expected results.

[0092] also, Figure 4 A comparison of the noise suppression effect of a single-core channel and a dual-core optical differential channel of the smallest module in the structure of the present invention is given. Figure 4 The results show that based on the designed dual-channel complementary interference optical unit structure, the noise suppression effect of the optical differential channel is 3.2dB, which has achieved the expected effect. Based on the minimum module, multi-channel expansion is expected to achieve more noise suppression effects.

[0093] The above description is only a preferred embodiment of the present invention, and does not limit the scope of the present invention. All equivalent structural changes made by using the contents of the present invention specification and drawings under the inventive concept of the present invention, or directly / indirectly applied in other related technical fields are included in the protection scope of the present invention.

Claims

1. A high-sensitivity, low-noise hydrophone system structure based on four-core optical fiber, characterized in that: include: A laser, used for outputting laser light; An optical modulation and beam splitting module, used for receiving the laser output by the laser device and outputting two modulated equal-amplitude optical signals; The four-core optical fiber hydrophone is used to receive the two optical signals output by the optical modulation and beam splitting module and output four interference optical signals. I 1. I 2. I 3. I 4, and I 1 and I 2 constitutes a set of complementary interference light units, I 3 and I 4 constitutes another set of complementary interference light units; Four photoelectric detectors, used to receive the four interference light signals respectively and output four analog electrical signals; Four analog-to-digital converters, used to receive the four analog electrical signals respectively and output four digital signals; A phase demodulation and noise suppression processing module is used to receive the four digital signals and output the signals after performing phase demodulation and noise suppression; The four-core optical fiber hydrophone comprises a first reference optical fiber unit, a first cascade sensitivity enhancement channel, a second reference optical fiber unit, and a second cascade sensitivity enhancement channel; The input end of the first reference unit is coupled with the first output of the optical modulation and beam splitting module in the same fiber, the input end of the first cascade sensitivity enhancement channel is coupled with the first output of the optical modulation and beam splitting module in a different fiber, and the first reference unit and the first cascade sensitivity enhancement channel respectively output interference optical signals I 1. I 2; The input end of the second reference unit is coupled with the second output of the optical modulation and beam splitting module in the same fiber, and the input end of the second cascade sensitivity enhancement channel is coupled with the second output of the optical modulation and beam splitting module in a different fiber manner. The second reference unit and the second cascade sensitivity enhancement channel respectively output interference optical signals. I 3. I 4; The four-core fiber optic hydrophone specifically includes a four-core fiber optic cascade sensitivity enhancement unit, a fan-in module, a fan-out module, a first reference fiber optic R1, a second reference fiber optic R2, a fiber optic coupler C1, a fiber optic coupler C2, a fiber optic coupler C3, and a fiber optic coupler C4; The four-core fiber cascade sensitivity enhancement unit is an elastic structure wound around four-core optical fibers, and the two ends of the four-core optical fiber cascade sensitivity enhancement unit are respectively connected to a fan-in module and a fan-out module, and the light waves of the four cores of the four-core optical fiber cascade sensitivity enhancement unit are coupled into four groups of conventional single-core optical fibers, that is, the fan-in module couples single-core optical fiber 1A, single-core optical fiber 2A, single-core optical fiber 3A, and single-core optical fiber 4A, and the fan-out module couples single-core optical fiber 1B, single-core optical fiber 2B, single-core optical fiber 3B, and single-core optical fiber 4B; The single-core optical fiber 1B is fused with the single-core optical fiber 2A, and the single-core optical fiber 3B is fused with the single-core optical fiber 4A; The optical fiber coupler C1, the optical fiber coupler C2, the optical fiber coupler C3, and the optical fiber coupler C4 all have a first port, a second port, a third port, and a fourth port, and in the optical fiber coupler C1, the optical fiber coupler C2, the optical fiber coupler C3, and the optical fiber coupler C4, the first port and the third port are homofiber coupled, and the first port and the fourth port are heterofiber coupled; The first port of the fiber coupler C1 is connected to the first output of the optical modulation and beam splitting module, and the two ends of the first reference optical fiber R1 are respectively connected to the third port of the fiber coupler C1 and the first port of the fiber coupler C2, forming the first reference optical fiber unit; The fourth port of the optical fiber coupler C1 is fused with the single-core optical fiber 1A, and the second port of the optical fiber coupler C2 is fused with the single-core optical fiber 2B to form the first cascade sensitivity enhancement channel; The first port of the fiber coupler C3 is connected to the second output of the optical modulation and beam splitting module, and the two ends of the second reference optical fiber R2 are respectively connected to the fourth port of the fiber coupler C3 and the second port of the fiber coupler C4, forming the second reference optical fiber unit; The third port of the optical fiber coupler C3 is fused with the single-core optical fiber 3A, and the first port of the optical fiber coupler C4 is fused with the single-core optical fiber 4B to form the second cascade sensitivity enhancement channel.

2. The high-sensitivity, low-noise hydrophone system structure based on four-core optical fiber according to claim 1 is characterized in that: The analog electrical signals output by the four photodetectors are as follows: ； in, V 1. V 2. V 3. V 4 is interference light I 1. I 2. I 3. I 4Four analog voltage signals after photoelectric conversion, R 1. R 2. R 3. R 4 is the photoelectric conversion efficiency of the four photodetectors, ω 0. C are the frequency and amplitude required for phase-generated carrier modulation, I 01 , I 02 , I 03 , I 04 Interference light I 1. I 2. I 3. I The light intensity of 4, I n1 , I n2 , I n3 , I n4 Interference light I 1. I 2. I 3. I The light intensity noise term of 4, , , , Interference light I 1. I 2. I 3. I The initial phase of 4, , , , Interference light I 1. I 2. I 3. I The phase noise term of 4 is It is the phase shift introduced by the sound pressure signal in a single channel of a four-core optical fiber.

3. The high-sensitivity, low-noise hydrophone system structure based on four-core optical fiber according to claim 2 is characterized in that: The signal processing process of the phase demodulation and noise suppression processing module is as follows: Will V 1 and V 2. V 3 and V 4 Perform optical differential subtraction between two signals to obtain two sets of differential signals , ,for: ； ； ； ； ； ； in, R is the average photoelectric conversion efficiency of the four detectors, for I 1. I The average phase noise of 2, for I 1. I The light intensity noise difference of 2, for I 3. I The average phase noise of 4, for I 3. I 4 light intensity noise difference; Will , Perform phase demodulation separately to obtain two sets of noisy phase terms , ,for: ； in, , for , Demodulated converted phase noise; Then , Taking the average we get: ； ； ； in, Output results for signal and noise, is the phase noise output result, Output result for intensity noise; The above noises belong to the A and B differential interference groups. and , and After incoherent superposition, the noise power spectral density is reduced to 1 / 2 of the original value, and the signal item detected by the system becomes twice of the original value, thereby achieving improved system sensitivity and noise suppression.

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