Fiber bragg grating type fiber hydrophone for inhibiting Rayleigh scattering

Abstract

The invention provides a fiber bragg grating type fiber hydrophone for inhibiting Rayleigh scattering, which comprises an inquiry laser pulse generation unit, a circulator, a sensing channel and a return light detection unit, and is characterized in that a pseudo-random code phase modulation technology is introduced into the inquiry laser pulse generation unit to adjust the phase of an inquiry laser pulse; the phase of the inquiry laser pulse is modulated into a plurality of discrete phases in pseudo-random distribution, so that the self-coherence of the inquiry laser pulse is reduced, and the Rayleigh scattering noise of the fiber bragg grating type fiber hydrophone is effectively suppressed. According to the method for adjusting the interrogation pulse coherence by using the pseudo-random code, the autocorrelation of the interrogation laser pulse can be reduced without influencing the detection signal, the Rayleigh scattering phase noise of an optical fiber hydrophone system is reduced, and the signal-to-noise ratio of the detected interference signal is improved.

Description

Technical Field

[0001] The present invention mainly relates to the technical fields of fiber optic sensing and spread spectrum communication, and in particular to a fiber optic grating type fiber optic hydrophone for suppressing Rayleigh scattering. Background Art

[0002] Since sound waves are the only information carriers that can propagate over long distances underwater, hydrophones that obtain underwater information through sound waves have become important equipment to support the national marine strategy. Fiber optic hydrophones, with their many superior characteristics such as high acoustic sensitivity, large dynamic range, wide detection bandwidth, high temperature and high pressure resistance, corrosion resistance, flexible structure design, and anti-electromagnetic interference, will have extremely important application prospects in the future underwater early warning and detection system.

[0003] After decades of development, the fiber optic underwater acoustic detection system has overcome a series of problems from basic theory to practical application. With the in-depth research, fiber optic hydrophones have entered the application stage in several fields. However, in the process of promoting practical application, basic scientific problems originating from the optical physics level but ignored in laboratory research have become new technical bottlenecks in engineering applications. One of the important problems is the Rayleigh scattering effect in fiber optic hydrophones. Rayleigh scattering will accumulate as the fiber transmission distance increases, which will in turn cause problems such as increased noise, limited multiplexing scale and transmission distance. The key to solving this problem lies in solving the contradiction between the narrow linewidth of high monochromatic laser and low coherence. Traditional methods for suppressing Rayleigh scattering include changing the coherence length of the light source or adding transmission fiber by means of a circulator and an isolator. However, the negative effects brought by these two methods are also obvious. The former will increase the background noise, and the latter will increase the complexity of the hydrophone and reduce the overall reliability, which is not suitable for large-scale sensing. Therefore, researching a method for suppressing Rayleigh scattering in fiber optic hydrophones that does not increase the background noise and system complexity and does not produce negative effects at all will undoubtedly have a huge promoting effect on the development of the entire fiber optic underwater acoustic detection system. Summary of the Invention

[0004] Aiming at the deficiencies in the prior art, the present invention proposes a fiber optic grating type fiber optic hydrophone for suppressing Rayleigh scattering.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] The present invention provides a fiber optic grating type fiber optic hydrophone for suppressing Rayleigh scattering, including a fiber optic grating type fiber optic hydrophone. The fiber optic grating type fiber optic hydrophone includes an interrogation laser pulse generation unit, a circulator, a sensing channel, and a backward light detection unit. The sensing channel is a sensing channel composed of fiber gratings;

[0007] The interrogation laser pulse generation unit is used to generate interrogation laser pulses. In the interrogation laser pulse generation unit, a pseudo-random code phase modulation technique is introduced to adjust the phase of the interrogation laser pulses, and the phase of the interrogation laser pulses is modulated into many discretized phases with pseudo-random distribution, so as to reduce the self-coherence of the interrogation laser pulses and effectively suppress the Rayleigh scattering noise of the fiber Bragg grating type fiber optic hydrophone.

[0008] The interrogation laser pulses generated by the interrogation laser pulse generation unit are input into the sensing channel through the circulator, and the return light of the sensing channel is transmitted to the return light detection unit after passing through the circulator.

[0009] Further, the interrogation laser pulse generation unit includes a laser light source, a fiber optic acousto-optic modulator, and a fiber optic phase modulator; the laser light emitted by the laser light source is modulated into pulsed laser light by the fiber optic acousto-optic modulator, and the pulsed laser light is phase-modulated by the fiber optic phase modulator to modulate the phase of the interrogation laser pulses into many discretized phases with pseudo-random distribution.

[0010] Further, the interrogation laser pulse generation unit further includes a fiber optic amplifier, and the fiber optic amplifier is used to amplify the power of the pulsed laser light.

[0011] Further, the fiber optic phase modulator is connected to a phase control unit, and the phase control unit includes a pseudo-random code generation unit, a high-speed arbitrary waveform generator, and a phase modulator driver. The pseudo-random code generation unit is used to generate a pseudo-random code and input the generated pseudo-random code into the high-speed arbitrary waveform generator. The high-speed arbitrary waveform generator generates randomly distributed high and low levels according to the pseudo-random code, and the randomly distributed high and low levels are input into the phase modulator driver, and the phase modulator driver modulates the fiber optic phase modulator to modulate the phase of the interrogation laser pulses according to the distribution law of the high and low levels.

[0012] Further, the pseudo-random code generation unit can generate pseudo-random codes of different orders or different types, and then modulate the interrogation laser pulses into different types of discrete distribution forms through the phase modulator.

[0013] Further, the pseudo-random code is a string of sequences containing randomly distributed 0s and 1s, or the pseudo-random code is a sequence with randomly distributed -1s and 1s.

[0014] Further, the pseudo-random code is different m-code sequences or different order Gold code sequences.

[0015] Further, the fiber Bragg grating type fiber optic hydrophone includes a fiber matching interferometer, which is connected in the optical path between the interrogation laser pulse generating unit and the circulator, or the fiber matching interferometer is connected in the optical path between the circulator and the sensing channel, or the fiber matching interferometer is connected in the optical path between the circulator and the backward light detection unit.

[0016] Further, the fiber matching interferometer includes a first fiber coupler, a first transmission arm, a second transmission arm, and a second fiber coupler. The laser input to the fiber matching interferometer is divided into two beams by the first fiber coupler. One beam is transmitted to the second fiber coupler through the first transmission arm, and the other beam is transmitted to the second fiber coupler through the second transmission arm. The lengths of the first transmission arm and the second transmission arm are different, and two pulsed lights with a certain time interval are combined and output by the second fiber coupler.

[0017] Further, the sensing channel is a single-channel fiber Bragg grating type sensing channel, multiple time-division channel fiber Bragg grating type sensing channels, or multiple wavelength-division channel fiber Bragg grating type sensing channels.

[0018] Further, it further includes a polarization switch. The interrogation laser pulse generated by the interrogation laser pulse generating unit passes through the polarization switch and then is input into the transmission link through the circulator, and is transmitted to the sensing channel through the transmission link. The backward light of the sensing channel is transmitted to the backward light detection unit through the circulator;

[0019] The frequency of polarization switching of the polarization switch is 1 / 4 of that of the acousto-optic modulator; when the previous pulse of the pulse pair output by the fiber coupler completely passes through the polarization switch, the polarization switch changes its polarization state once. Let the output polarization state of the first pulse pair be XX, the polarization state of the second pulse pair be XY, the polarization state of the third pulse pair be YY, the polarization state of the fourth pulse pair be YX, the polarization state of the fifth pulse pair be XX... and so on. Due to the presence of the polarization switch, the polarization states of the laser pulse pairs output to the fiber Bragg grating in the sensing channel are XX, XY, YY, YX, XX, XY, YY, YX, XX... in turn. Every four polarization states form a cycle. The polarization states of the pulse pairs of the backward light of the sensing channel transmitted to the data acquisition card through the circulator are also XX, XY, YY, YX, XX... After collecting the pulse pairs of the four polarization states, the polarization synthesis algorithm is used for demodulation, which can effectively solve the signal perturbation caused by polarization random perturbation.

[0020] The present invention provides a fiber Bragg grating type fiber optic hydrophone for suppressing Rayleigh scattering. Starting from the physical essence of light waves, inspired by fiber optic sensing and spread spectrum communication technologies, the high-efficiency suppression of Rayleigh scattering noise in fiber optic hydrophones is realized through signal modulation and demodulation means, providing important technical support for accelerating the engineering practicalization process of fiber optic hydrophones.

[0021] Specifically, by connecting an optical fiber phase modulator to an optical fiber hydrophone system to adjust the phase of the interrogation laser pulse, the self-coherence of the interrogation laser pulse is changed, thereby suppressing the influence of Rayleigh scattering on the interference signal. Since the self-correlation of the interrogation laser pulse is directly related to its own phase modulation situation, in order to minimize the self-correlation of each pulse sequence as much as possible, the present invention draws on the pseudo-random code technology in the field of spread-spectrum communication to achieve the adjustment of the phase of the interrogation laser pulse. An important characteristic of the pseudo-random code is its very low self-coherence. After introducing the pseudo-random code phase modulation technology, the present invention can modulate the phase of the original continuous interrogation laser pulse into many discretely distributed phases with pseudo-random distribution, thereby reducing the self-coherence of the interrogation laser pulse. Description of the Drawings

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on the structures shown in these drawings.

[0023] Figure 1 It is a schematic structural diagram of a fiber grating type fiber hydrophone for suppressing Rayleigh scattering provided by Embodiment 1;

[0024] Figure 2 It is a schematic structural diagram of a fiber grating type fiber hydrophone for suppressing Rayleigh scattering provided by Embodiment 2;

[0025] Figure 3 It is a schematic structural diagram of a fiber grating type fiber hydrophone for suppressing Rayleigh scattering provided by Embodiment 3;

[0026] Figure 4 It is a schematic structural diagram of a fiber grating type fiber hydrophone for suppressing Rayleigh scattering provided by Embodiment 4;

[0027] Figure 5 It is a schematic structural diagram of a fiber grating type fiber hydrophone for suppressing Rayleigh scattering provided by Embodiment 5;

[0028] Figure 6 It is a schematic structural diagram of a fiber grating type fiber hydrophone for suppressing Rayleigh scattering provided by Embodiment 6;

[0029] Figure 7 It is a principle block diagram for generating an m-code sequence;

[0030] Figure 8 It is a principle block diagram for generating a 7th-order Gold code sequence;

[0031] Figure 9 It is a schematic diagram of the pseudo-random code level output by a high-speed signal generator and the time domain of the passband of an acousto-optic modulator in an embodiment;

[0032] Figure 10 It is a schematic diagram of the phase of a pseudo-random code modulated interrogation laser pulse in an embodiment;

[0033] Figure 11 It is a schematic structural diagram of a fiber grating type fiber optic hydrophone for suppressing Rayleigh scattering provided in Embodiment 7;

[0034] Figure 12 It is a comparison diagram of the change in the background phase noise before and after the combination of the polarization switching technology and the pseudo-random code in a fiber grating type fiber optic hydrophone in an embodiment;

[0035] Figure 13 It is a comparison diagram of a fiber grating type fiber optic hydrophone without pseudo-random code modulation, using the same set of pseudo-random codes for modulation of four polarization states, and using pseudo-random codes with different sorting methods for four polarizers respectively under the condition of a 10-kilometer link length in an embodiment.

[0036] Reference numerals in the figure:

[0037] 1. Interrogation laser pulse generation unit; 101. Laser light source; 102. Fiber acousto-optic modulator; 103. Fiber phase modulator; 104. Fiber amplifier; 105. Pseudo-random code generation unit; 106. High-speed arbitrary waveform generator; 107. Attenuator; 108. Phase modulator driver; 109. Acousto-optic modulator driver;

[0038] 2. Circulator;

[0039] 3. Sensing channel; 301. Fiber grating; 302. Energy transmission fiber;

[0040] 4. Transmission link;

[0041] 5. Retroreflection detection unit;

[0042] 6. Fiber optic matching interferometer; 601. First transmission arm; 602. Second transmission arm; 603. First fiber optic coupler; 604. Second fiber optic coupler;

[0043] 7. Computer;

[0044] 8. Data acquisition card;

[0045] 9. Polarization switch. Detailed implementation manners

[0046] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0047] Figure 1 It is a schematic structural diagram of Embodiment 1 of the present invention. The present invention provides a fiber Bragg grating type fiber optic hydrophone for suppressing Rayleigh scattering, including a fiber Bragg grating type fiber optic hydrophone. The fiber Bragg grating type fiber optic hydrophone includes an interrogation laser pulse generation unit 1, a circulator 2, a transmission link 4, a sensing channel 3, and a backward light detection unit 5. The sensing channel 3 is a sensing channel composed of a series of fiber Bragg gratings 301, and the fiber Bragg gratings 3-1 are connected by an energy transmission fiber 302.

[0048] The interrogation laser pulse generation unit 1 is used to generate interrogation laser pulses. The pseudo-random code phase modulation technology is introduced into the interrogation laser pulse generation unit to adjust the phase of the interrogation laser pulses, and the phase of the interrogation laser pulses is modulated into many discrete phases with pseudo-random distribution to reduce the self-coherence of the interrogation laser pulses and effectively suppress the Rayleigh scattering noise of the fiber Bragg grating type fiber optic hydrophone.

[0049] The interrogation laser pulses generated by the interrogation laser pulse generation unit 1 are input into the transmission link 4 through the circulator 2, transmitted to the sensing channel 3 through the transmission link 4, and the backward light of the sensing channel 3 is transmitted to the backward light detection unit 5 through the circulator 2.

[0050] Generally, the optical path from the circulator 2 to the sensing channel 3 in the fiber optic hydrophone is called the transmission link 4. In a large-scale fiber optic sensing system, the transmission link 4 often reaches dozens or even hundreds of kilometers. The backward Rayleigh scattering generated when the optical pulse passes through the optical fiber of this part of the link, because Rayleigh scattering is elastic scattering, its wavelength is the same as the wavelength of the pulsed laser that triggers Rayleigh scattering, and the Rayleigh scattering light caused by each pulse will accumulate and interfere with the reflected interference light. This part of the interference is captured by the photodetector in the backward light detection unit 5 as the Rayleigh scattering phase noise of the interference signal.

[0051] Since the backward transmitted Rayleigh scattering noise and the returned interference signal have the same wavelength, that is, they can cause interference, the present invention modulates the phase continuity of the interrogation laser pulses. In this way, not only the phase continuity of the interrogation laser pulses is randomly modulated, but also the phase continuity of the Rayleigh scattering light caused by the interrogation laser pulses is disrupted into a positive and negative pseudo-random distribution.

[0052] The fiber Bragg grating type fiber optic hydrophone includes a fiber matching interferometer 6, such asFigure 1 As shown, the fiber optic matched interferometer 6 can be connected in the optical path between the interrogation laser pulse generation unit 1 and the circulator 2. It can be understood that the fiber optic matched interferometer 6 can also be connected in the optical path between the circulator 2 and the sensing channel 3, or the fiber optic matched interferometer 6 is connected in the optical path between the circulator 2 and the backward light detection unit 5.

[0053] The fiber optic matched interferometer 6 includes a first fiber optic coupler 603, a first transmission arm 601, a second transmission arm 602, and a second fiber optic coupler 604. The laser input to the fiber optic matched interferometer 6 is split into two beams by the first fiber optic coupler 603. One beam is transmitted to the second fiber optic coupler 604 through the first transmission arm 601, and the other beam is transmitted to the second fiber optic coupler 604 through the second transmission arm 602. The lengths of the first transmission arm 601 and the second transmission arm 602 are different, and the two pulsed light beams with a certain time interval formed are combined and output by the second fiber optic coupler 604.

[0054] The interrogation laser pulse generation unit 1 includes a laser light source 101, a fiber optic acousto-optic modulator 102, and a fiber optic phase modulator 103. The laser emitted by the laser light source 101 is modulated into pulsed laser by the fiber optic acousto-optic modulator 102, and the pulsed laser is phase-modulated by the fiber optic phase modulator 103 to modulate the phase of the interrogation laser pulse into a plurality of discrete phases with pseudo-random distribution.

[0055] The fiber optic phase modulator 103 is connected with a phase control unit. The phase control unit includes a pseudo-random code generation unit 105, a high-speed arbitrary waveform generator 106, and a phase modulator driver 108. The pseudo-random code generation unit 105 is used to generate a pseudo-random code and input the generated pseudo-random code into the high-speed arbitrary waveform generator 106. The high-speed arbitrary waveform generator 106 generates randomly distributed high and low levels according to the pseudo-random code. The randomly distributed high and low levels are input to the phase modulator driver 108, and the phase modulator driver 108 modulates the fiber optic phase modulator 103 to modulate the phase of the interrogation laser pulse according to the distribution law of the high and low levels, reducing the coherence of the interrogation laser pulse, thereby suppressing the Rayleigh noise of the transmission link optical fiber. The method of using the pseudo-random code to adjust the coherence of the interrogation pulse in the present invention can reduce the autocorrelation of the interrogation laser pulse while not affecting the detection signal, reduce the Rayleigh scattering phase noise of the fiber optic hydrophone system, and improve the signal-to-noise ratio of the detected interference signal. Refer to Figure 1, in this embodiment, the phase control unit further includes an attenuator 107. The pseudo-random code generation unit 105 generates a pseudo-random code and outputs it to the high-speed arbitrary waveform generator 106. The attenuator 107 can reduce the output voltage of the high-speed arbitrary waveform generator 106, and input the randomly distributed high and low levels after attenuation into the phase modulator driver 108. The phase modulator driver 108 modulates the fiber optic phase modulator 103 so that the phase of the interrogation laser pulse is modulated according to the distribution law of the high and low levels.

[0056] It can be understood that the pseudo-random voltage signal output by the high-speed arbitrary waveform generator 106 can be attenuated by the attenuator 107, or the voltage can be directly output to the fiber optic phase modulator 103 without passing through the attenuator 107.

[0057] It can be understood that the pseudo-random voltage signal output by the high-speed arbitrary waveform generator 106 can be transmitted to the phase modulator driver 108, and then the phase modulator driver 108 causes the fiber optic phase modulator 103 to generate a phase jump. The pseudo-random voltage signal output by the high-speed arbitrary waveform generator 106 can also be directly output to the fiber optic phase modulator 103 to cause phase modulation according to the law of the pseudo-random code.

[0058] The pseudo-random code generation unit 105 can generate pseudo-random codes of different orders or different types, and then modulate the interrogation laser pulse into different types of discrete distribution forms through the fiber optic phase modulator 103. The pseudo-random code is a digital sequence including randomly distributed 0s and 1s, or can also refer to a digital sequence including randomly distributed -1s and 1s. The pseudo-random code can be different m-code sequences or different order Gold code sequences.

[0059] Ideally, the amplitude of the high and low levels exactly causes the phase of the interrogation fiber pulse to change by π. In practical applications, the voltage amplitude of each chip of the pseudo-random code does not necessarily cause a phase change of π for the fiber optic phase modulation, and can also be an arbitrary phase change amplitude.

[0060] Preferably, the first fiber coupler 603 and the second fiber coupler 604 adopt 50% fiber couplers. The third fiber coupler is a 5:95 fiber coupler. A delay fiber is provided on the first transmission arm 601, and a PZT piezoelectric ceramic is provided on the second transmission arm 602, which can output a high-frequency sine signal to the PZT piezoelectric ceramic to realize carrier modulation of the phase of the transmitted light beam on the second transmission arm.

[0061] Refer to Figure 2 , Figure 2FIG. 0 is a schematic structural diagram of Embodiment 2 of the present invention. This embodiment provides a fiber grating type fiber hydrophone for suppressing Rayleigh scattering, including a fiber grating type fiber hydrophone. The fiber grating type fiber hydrophone includes an interrogation laser pulse generation unit 1, a circulator 2, a transmission link 4, a sensing channel 3, and a backward light detection unit 5. The sensing channel is a sensing channel composed of fiber gratings. The difference from Embodiment 1 is that the interrogation laser pulse generation unit further includes a fiber amplifier 104, and the fiber amplifier 104 is used to amplify the pulse laser power. Other setting requirements of Embodiment 2 are the same as those of Embodiment 1 and will not be elaborated here.

[0062] Refer to Figure 3 , Figure 3 FIG. 7 is a schematic structural diagram of Embodiment 3 of the present invention. This embodiment provides a fiber grating type fiber hydrophone for suppressing Rayleigh scattering, including a fiber grating type fiber hydrophone. The fiber grating type fiber hydrophone includes an interrogation laser pulse generation unit 1, a circulator 2, a transmission link 4, a sensing channel 3, and a backward light detection unit 5. The sensing channel is a sensing channel composed of fiber gratings. The difference from Embodiment 1 is that it further includes an acousto-optic modulator driver 109, a computer 7, and a data acquisition card 8. Among them, the first fiber coupler 603 and the second fiber coupler 604 are 50% fiber couplers. The third fiber coupler 301 is a 5:95 fiber coupler. A delay fiber is provided on the first transmission arm 601, and a PZT piezoelectric ceramic is provided on the second transmission arm 602. A high-frequency sine signal can be generated and output to the PZT piezoelectric ceramic to realize carrier modulation of the phase of the transmitted light beam on the second transmission arm. The data acquisition card 8 can generate a high-frequency sine signal and output it to the PZT piezoelectric ceramic to realize carrier modulation of the phase of the transmitted light beam on the second transmission arm. The acousto-optic modulator driver 109 is connected to the fiber acousto-optic modulator 102, and the data acquisition card 8 can also generate a pulse signal and output it to the acousto-optic modulator driver 109, thereby modulating the fiber acousto-optic modulator 102.

[0063] The pseudo-random code sequence is generated by the computer 7 and imported into the high-speed arbitrary waveform generator 106. The high-speed arbitrary waveform generator 106 will output a corresponding waveform according to the pseudo-random code sequence input by the computer 7. The data acquisition card 8 can generate a trigger signal to trigger the high-speed arbitrary waveform generator 106 to generate a pseudo-random signal. After receiving the trigger signal sent by the data acquisition card 8, the high-speed arbitrary waveform generator 106 starts to output a corresponding voltage waveform according to the characteristics of the pseudo-random code sequence. The output pseudo-random code voltage is transmitted to the phase modulator driver 108, and then the fiber phase modulator 103 is caused by the phase modulator driver 108 to generate a phase jump. Among them, there is a fixed time interval between the signal emitted by the data acquisition card 8 to the acousto-optic modulator driver 109 and the trigger signal output to the high-speed arbitrary waveform generator 106.

[0064] Further, the pseudo-random code used in the present invention is generated by a computer through a linear shift register, and is a string of number sequences with a random distribution of 0 and 1, and can also be modulated into a number sequence with a random distribution of -1 and 1. Further, the pseudo-random code generated by the computer in the present invention is input into a high-speed signal generator, and the high-speed signal generator generates high and low levels with a random distribution according to the pseudo-random code (which can be a sequence with a random distribution of 0 and 1, or a sequence with a random distribution of 1 and -1).

[0065] The other setting requirements of Example 3 are the same as those of Example 1 and will not be elaborated here.

[0066] Take Figure 3 as an example to specifically illustrate the specific implementation manner of suppressing Rayleigh scattering in the present invention. After the laser light source emits continuous narrow-linewidth laser, it is modulated into pulsed laser by an acousto-optic modulator in the optical fiber. After the pulsed laser passes through the phase modulator, it is split into two beams by the first optical fiber coupler and enters the fiber optic matched interferometer. Since the lengths of the two arms of the fiber optic matched interferometer are different, one arm contains delay optical fiber and the other arm contains a PZT piezoelectric ceramic. A single pulse is divided into two pulses with a certain time interval before and after, and then respectively transmitted to the third optical fiber coupler through the circulator and the transmission link. Each pulse is split into two beams and respectively reflected by the fiber Bragg grating to the fiber optic circulator. The reflected light passes through the circulator and then is transmitted to the photodetector of the backward light detection unit.

[0067] Figure 4 is the structural schematic diagram of Example 4. Example 4 is a multi-sensing channel multiplexing structure. Except for the increase in the number of sensing channels, the rest of the structure remains the same as that of the single-channel sensing system. Specifically, the sensing channel 3 is a single-channel fiber Bragg grating type sensing channel, multiple time-division channel fiber Bragg grating type sensing channels, or multiple wavelength-division channel fiber Bragg grating type sensing channels. Multiple time-division channels mean that the laser interrogation pulses of the same wavelength can realize sensing of multiple channels, and each channel is arranged in sequence in time. Wavelength division multiplexing means that different wavelengths of interrogation pulses are used for sensing, and each wavelength is transmitted to one or more channels, and each channel only acts on one wavelength.

[0068] Refer to Figure 5 , which is the structural schematic diagram of Example 5 of the present invention. The difference between the fiber Bragg grating type fiber optic hydrophone provided in Example 5 and Example 3 is that the fiber Bragg grating type fiber optic hydrophone in Example 5 is a hydrophone placed in the matched interferometer, and the fiber optic matched interferometer is connected in the optical path between the circulator and the sensing channel.

[0069] Refer to Figure 6, which is the structural schematic diagram of Embodiment 6 of the present invention. The fiber Bragg grating type fiber hydrophone for suppressing Rayleigh scattering provided by Embodiment 6 is different from that of Embodiment 3 in that the fiber Bragg grating type fiber hydrophone for suppressing Rayleigh scattering in Embodiment 6 is a matched interferometer rear hydrophone, and the fiber matched interferometer is connected in the optical path between the circulator and the retroreflective detection unit.

[0070] In any of the above embodiments: Since the Rayleigh scattering noise transmitted backward and the returned interference signal have the same wavelength, that is, they can cause interference, the present invention modulates the phase continuity of the interrogation laser pulse. In this way, not only the phase continuity of the interrogation laser pulse is randomly modulated, but also the phase continuity of the Rayleigh scattered light caused by the interrogation laser pulse is disrupted into a positive and negative pseudo-random distribution.

[0071] The pseudo-random code used in the present invention is not an absolutely random sequence. Each element in the sequence is determinable and can be repeatedly generated and replicated according to a specific algorithm to better reduce the self-coherence of the interrogation laser pulse and achieve better suppression of the Rayleigh scattering noise of the fiber Michelson interferometer type fiber hydrophone. Preferably, the pseudo-random code is different m-code sequences or different order Gold code sequences. Among them, the Gold code sequence is composed of the compound of two preferred sequences of the m-code sequence. Therefore, compared with the pseudo-random code phase modulation based on the m-code sequence, the pseudo-random code phase modulation based on the Gold code sequence has a better suppression effect on Rayleigh scattering.

[0072] Without loss of generality, the m-code sequence is generated by a linear shift register, as Figure 7 shown, Figure 7 is the principle block diagram of the m-code sequence generation. Figure 7 In, the input clock signal, the currently generated element a i can be given by the recurrence relation formula, that is:

[0073]

[0074] When the initial values of the register network are not all zero, the element distribution in a single period of the generated sequence is only related to the feedback coefficient c i Therefore, the binary domain {0, 1} polynomial f(x) can be used to characterize a specific displacement register network, and there is:

[0075] f(x) = 1 + c 1 x + c 2 x 2 + ··· + c r x r c i ∈ {0, 1}

[0076] In the formula, f(x) is called the characteristic polynomial of the above r - order register network. Since the number of m - code sequences is relatively small, in practical applications, two m - code sequences of the same order are often combined to construct a new pseudo - random sequence, which is called a Gold - code sequence.

[0077] Taking the 7 - order Gold - code sequence as an example, the present invention refers to Figure 8 , Figure 8 which is the principle block diagram for generating the 7 - order Gold - code sequence. The preferred pair of m - code sequences can be simply described as: among the set of m - code sequences of a given order r, the two sequences with the closest maximum and minimum absolute values of the cross - correlation function. Figure 8 In [reference], the clock signal is input into the 7 - order m - sequence generator 1 and the 7 - order m - sequence generator 2. In the 7 - order m - sequence generator 1 and the 7 - order m - sequence generator 2, the polynomials f(x)=1 + x 3 +x 7 、f(x)=1 + x + x 2 +x 3 +x 7 corresponding 7 - order m - sequences constitute the preferred conditions. If the relative initial phases (or initial values) of the preferred pair of m - sequences are set differently, different Gold - code sequences can be output, that is, under the same topological structure, a much larger number of Gold - code sequences than m - sequences can be generated, which is called a Gold - code sequence cluster. The absolute value of the cross - correlation function between any two sequences in the Gold - code sequence cluster is significantly smaller than that of the m - code sequence, that is, it has better cross - correlation characteristics. The following takes the 9 - order Gold - code as an example to illustrate the distribution of pseudo - random codes, as the following {0, 1} sequences:

[0078] {000100101000111000010010100011100001001010001110000100101000111000011101011100010001110101110001000111010111000100 011101011100010001110101110001000111010111000100011101011100 010001110101110001000111010111000100011101011100010001110101 1100010001110101110001000100101000111000010010100011100001001010001110000100101000111000010010100011100001001010001110 00010010100011100001001010001110000100101000111000010010100 0111000010010100011100001001010001110000111010111000100011101011100010001110101110001000111010111000}

[0079] As Figure 9 shown Figure 9 in the figure, it is a schematic diagram of the pseudo-random code level output by the high-speed signal generator and the time domain of the passband of the acousto-optic modulator in an embodiment. The figure shows the high and low level time domains received by the acousto-optic modulator, the pseudo-random code trigger signal, and the high and low levels of the pseudo-random code arrangement. After the pseudo-random code is generated by a computer and transmitted to the high-speed arbitrary waveform generator, after receiving the pseudo-random code sequence, the high-speed arbitrary waveform generator will output the high and low levels of the pseudo-random code arrangement according to the pseudo-random code trigger signal. In the present invention, the trigger signal delay of the high-speed signal generator is modified according to the actual situation to make the output level of the high-speed arbitrary waveform generator consistent with the passband time of the acousto-optic modulator as much as possible.

[0080] Referring Figure 10 to Figure 10 is a schematic diagram of the phase of the pseudo-random code modulated interrogation laser pulse in an embodiment. Since the phase of the interrogation pulse light wave output by the laser light source is continuous before phase modulation (i.e., Figure 10 the continuous phase of the interrogation pulse in Figure 10After the pseudo-random code voltage shown, the phase of the laser light wave will be modulated according to the law of the level distribution, and the modulated phase is obtained. The magnitude of the phase change is related to the response of the phase modulator to the applied level. Ideally, the high and low level amplitudes exactly cause the phase of the interrogation optical fiber pulse to change by π.

[0081] In the present invention, the number of level sequences generated by pseudo-random codes of different orders is different, and the frequency of the high-speed arbitrary waveform generator can be set to adjust the duration of each chip.

[0082] The sensing element in the present invention is composed of a fiber grating. Compared with the traditional sensing structure based on a Faraday rotator mirror, the fiber grating structure is simpler, has fewer melting points, higher reliability, and a more convenient preparation process. However, the Faraday rotator mirror can make the polarization state of any reflected light orthogonal to the polarization state of the incident light, while the fiber grating cannot change the polarization state of the reflected light. Therefore, the sensing system of the fiber grating structure must consider the interference signal fading caused by the random polarization state of the optical fiber. Therefore, the present invention innovatively proposes to organically combine the pseudo-random code modulation technology and the polarization suppression technology to build a fiber grating fiber hydrophone that suppresses Rayleigh scattering. Specifically, a fiber grating fiber hydrophone that suppresses Rayleigh scattering is provided, which further includes a polarization switch 9. The interrogation laser pulse generated by the interrogation laser pulse generation unit 1 passes through the polarization switch 9 and then is input into the transmission link 4 through the circulator 2, and is transmitted to the sensing channel 3 through the transmission link 4. The return light of the sensing channel 3 is transmitted to the return light detection unit 5 through the circulator 2. The structure and design of the interrogation laser pulse generation unit 1 can adopt the structure described in any of the foregoing embodiments.

[0083] As Figure 11 shown, the polarization switching technology is adopted to suppress the random polarization fading of the optical fiber. Figure 11It is a schematic structural diagram of a fiber Bragg grating type fiber optic hydrophone for suppressing Rayleigh scattering provided by Embodiment 7. The interrogation laser pulse generated by the interrogation laser pulse generation unit 1 passes through the polarization switch 9 and then is input into the transmission link 4 through the circulator 2, and is transmitted to the sensing channel 3 through the transmission link 4. The return light of the sensing channel 3 is transmitted to the return light detection unit 5 through the circulator 2. The frequency of polarization switching of the polarization switch 9 is 1 / 4 of that of the acousto-optic modulator 102. After the previous pulse of the pulse pair output by the fiber coupler 604 completely passes through the polarization switch 9, the polarization switch 9 changes its polarization state once. Suppose the output polarization state of the first pulse pair is XX, the polarization state of the second pulse pair is XY, the polarization state of the third pulse pair is YY, the polarization state of the fourth pulse pair is YX, the polarization state of the fifth pulse pair is XX... and so on. Due to the presence of the polarization switch 9, the polarization states of the laser pulse pairs output to the fiber Bragg grating in the sensing channel are XX, XY, YY, YX, XX, XY, YY, YX, XX... in sequence, and every four polarization states form a cycle. Therefore, the polarization states of the pulse pairs transmitted to the data acquisition card 8 through the circulator 2 after being reflected by the fiber Bragg grating are also XX, XY, YY, YX, XX... After collecting the pulse pairs of the four polarization states, the polarization synthesis algorithm is used for demodulation, which can ensure that no matter how the polarization state changes due to the birefringence effect in the transmission link 4, the polarization synthesis algorithm can effectively solve the signal disturbance caused by the polarization random perturbation.

[0084] In this embodiment, to organically combine the pseudo-random code technology and the polarization switching technology, a group of pseudo-random codes needs to be prepared for each polarization state. Compared with the traditional fiber optic Michelson sensor based on a Faraday rotator mirror, the fiber optic hydrophone based on a fiber Bragg grating requires at least four different pseudo-random code sequences to prevent the increase of phase noise caused by the random phase consistency between different polarization states.

[0085] In this embodiment, it is necessary to precisely control the trigger delay between the acousto-optic modulator 102 and the fiber optic polarization switch 9 to ensure that after the previous pulse of the pulse pair output by the second fiber coupler 604 completely passes through the polarization switch 9, the polarization switch 9 switches to an orthogonal polarization state.

[0086] In a specific embodiment, the schematic structural diagram of the fiber Bragg grating type fiber optic hydrophone for suppressing Rayleigh scattering is as Figure 11As shown, the output wavelength of the laser light source 101 is 1539.77 nm, the modulation frequency of the acousto-optic modulator 102 is 250 kHz, the width of a single chip of the pseudo-random code sequence generated by the high-speed arbitrary waveform generator 106 is 2.4 ns, and the peak-to-peak voltage output by the phase modulator driver 108 is 5.5 V. A delay coil is provided on the first transmission arm 601, the optical fiber time delay of the delay coil is 500 ns, the round-trip time delay of the sensing optical fiber between adjacent fiber gratings is 500 ns, and the switching frequency of the polarization switch 9 is 62.5 kHz. After the previous pulse of the output pulse pair completely passes through the polarization switch 9, the polarization switch 9 changes its polarization state once. Let the output polarization state of the first pulse pair be XX, the polarization state of the second pulse pair be XY, the polarization state of the third pulse pair be YY, the polarization state of the fourth pulse pair be YX, the polarization state of the fifth pulse pair be XX, and so on. A group of pseudo-random codes corresponding to each polarization state is as follows:

[0087] The pseudo-random code corresponding to the XX polarization state is: {-1, 1, 1, 1, 1, 1, -1, 1, 1, -1, 1, 1, 1, -1, 1, 1, -1, -1, 1, 1, -1, -1, -1, 1, -1, 1, -1, 1, -1, 1, -1, 1, 1, -1, -1, 1, -1, -1, -1, -1, -1, 1, -1, 1, 1, -1, -1, 1, -1, 1, 1, 1, -1, 1, -1, -1, -1, 1, 1, 1, 1, -1, 1, 1, -1, -1, -1, 1, 1, -1, 1, 1, 1, -1, -1, -1, 1, 1, -1, -1, -1, -1, 1, 1, -1, 1, 1, 1, -1, 1, -1, 1, -1, 1, 1, -1, 1, -1, -1, -1, 1, 1, -1, -1, -1, 1, 1, 1, 1, 1, -1, 1, -1, -1, 1, -1, 1, -1, -1, 1, -1, 1, -1, 1, 1, -1, -1, -1, 1, -1, -1, -1, 1, -1, 1, -1, -1, 1, -1, 1, -1, -1, 1, 1, 1, -1, -1, 1, 1, -1, -1, 1, 1, 1, -1, -1, 1, -1, 1, -1, -1, 1, -1, 1, 1, -1, -1, -1, -1, 1, 1, -1, -1, -1, 1, 1, 1, 1, 1, -1, 1, 1, -1, 1, -1, -1, -1, -1, 1, 1, -1, -1, -1, 1, 1, -1, -1, 1, 1, 1, -1, 1, 1, -1, -1, 1, -1, -1, 1, 1, 1, 1, 1, -1, -1, -1, -1, 1, -1, -1, -1, 1, 1, 1, 1, 1, 1, -1, 1, 1, -1, -1, 1, 1, 1, 1, 1, -1, 1, -1, 1, 1, 1, -1, 1, -1, 1, -1, 1, 1, 1, -1, -1, 1, 1, 1, 1, -1, 1, -1, 1, 1, -1, 1, -1, 1, -1, -1, -1, -1, 1, -1, -1, -1, -1, -1, 1, 1, -1,-1,-1,-1,-1,1,-1,-1,1,1,-1,1,-1,-1,1,1,1,-1,-1,1,-1,-1,1,-1,1,-1,-1,-1,-1,1,1,-1,-1,-1,-1,1,-1,1,1,1,-1,-1,1,-1,-1,1,1,-1,-1,-1,1,1,-1,1,1,-1,1,-1,1,1,-1,-1,1,1,1,1,1,1,-1,-1,-1,1,1,-1,1,-1,1,-1,1,1,1,-1,-1,1,-1,-1,-1,1,-1,1,-1,-1,1,1,-1,-1,-1,1,-1,1,-1,1,-1,-1,-1,1,1,1,1,1,1,1,1,-1,-1,1,1,-1,1,-1,-1,-1,-1,1,-1,1,-1,1,-1,1,1,1,1,1,1,-1,-1,1,-1,-1,-1,1,-1,1,1,1,-1,-1,1,-1,-1,-1,-1,1,1,-1,-1,-1,1,1,-1,-1,1,1,1,-1,-1,1,-1,-1,-1,1,1,1,-1,-1,-1,-1,-1,1,1,1,1,1,1,1,1,-1,-1,-1,-1,-1,1,1,-1,-1,1,-1,-1,-1,-1,1,1,-1,1,1,-1,1,-1,-1,-1,1,1,1,1,1,-1,-1,-1,-1,1,-1,1,-1,-1,1,-1,1,-1,1,-1,1,-1,1,-1,-1,-1,1,-1,-1,1,1,-1,1,1,-1,1,-1,-1,1,1,-1,1,1,-1,-1,-1,-1,1,-1,1,1,1,-1,1,1,-1,-1,-1,1,1,-1,-1,1,1,-1,-1,1,1,1,1,-1,1,1,-1,1,1,-1,1,1,-1,1,1,1,1,-1,1,-1,1,1,-1,-1,1,-1,-1,1,-1,-1,-1,1,1,1,-1,1,-1,1,1,-1,1,1,1,-1,-1,1,-1,-1,1,-1,1,1,1,-1,1,-1,1,1,-1,1,1,-1,-1,1,-1,-1,-1,1,1,-1,1,1,-1,-1,-1,1,-1,1,-1,-1,1,1,-1,1,-1,1,1,1,1,1,-1,1,1,1,-1,-1,-1,-1,-1,-1,1,1,-1,1,1,1,1,1,-1,1,-1,-1,1,-1,-1,-1,1,1,1,1,1,1,-1,1,1,-1,-1,1,-1,1,-1,1,-1,1,1,-1,-1,1,-1,-1,1,-1,1,1,-1,1,1,1,1,1,1,-1,-1,-1,-1,-1,1,1,1,1,-1,-1,-1,1,-1,1,1,1,-1,1,-1,-1,1,1,-1,1,-1,-1,-1,-1,-1,1,-1,1,-1,-1,1,-1,-1,1,1,1,1,-1,1,-1,1,-1,-1,1,1,-1,-1,1,-1,-1,1,1,1,1,1,1,1,1,1,-1,1,-1,-1,-1,1,-1,-1,1,1,-1,-1,-1,-1,1,1,1,1,-1,1,-1,1,1,-1,-1,-1,1,-1,-1,-1,-1,-1,1,-1,1,-1,1,-1,-1,1,1,1,-1,1,-1,-1,-1,-1,-1,-1,-1,-1,-1,1,1,-1,1,1,1,-1,-1,1,1,-1,-1,1,1,1,-1,1,1,-1,-1,-1,1,-1,-1,1,1,-1,-1,1,-1,1,-1,1,1,1,-1,1,1,1,-1,1,1,-1,1,-1,1,-1,-1,-1,1,-1,-1,1,1,-1,1,1,-1,1,-1,1,-1},

[0088] The pseudo-random code corresponding to the XY polarization state is: {-1, 1, -1, 1, 1, -1, -1, 1, -1, 1, -1, -1, -1, 1, -1, -1, 1, 1, 1, -1, -1, 1, -1, -1, 1, 1, -1, 1, -1, 1, -1, -1, 1, -1, -1, 1, -1, -1, 1, -1, -1, -1, -1, -1, 1, 1, -1, -1, 1, 1, 1, 1, -1, 1, -1, -1, -1, 1, -1, 1, 1, -1, 1, 1, 1, -1, -1, 1, 1, -1, 1, -1, 1, -1, 1, 1, -1, -1, -1, 1, 1, 1, 1, -1, 1, 1, -1, 1, -1, -1, -1, 1, 1, 1, -1, -1, -1, -1, 1, -1, -1, 1, -1, -1, 1, 1, 1, 1, 1, 1, -1, -1, 1, 1, 1, 1, -1, -1, -1, -1, 1, -1, 1, 1, 1, -1, 1, -1, -1, 1, 1, -1, 1, -1, 1, -1, -1, -1, 1, 1, 1, 1, 1, -1, 1, -1, 1, -1, 1, 1, -1, 1, -1, -1, 1, 1, 1, -1, -1, -1, -1, 1, 1, -1, 1, 1, 1, 1, -1, 1, 1, 1, 1, 1, -1, -1, -1, -1, -1, 1, 1, 1, 1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 1, -1, 1, -1, -1, 1, 1, 1, 1, 1, 1, 1, -1, -1, 1, -1, 1, -1, -1, -1, 1, 1, 1, 1, 1, 1, 1, -1, -1, -1, 1, 1, -1, 1, -1, -1, 1, 1, 1, 1, 1, 1, 1, -1, -1, 1, -1, -1, -1, 1, 1, 1, 1, 1, 1, -1, 1, -1, -1, -1, 1, 1, 1, -1, 1, 1, -1, -1, 1, 1, -1, -1, -1, 1, -1, 1, 1, -1, 1, 1, 1, 1, -1, -1, -1, -1, -1, -1, 1, -1, 1, 1, -1, -1, -1, 1, -1, 1, -1, -1, 1, 1, 1, -1, -1, 1, 1, -1, -1, -1, 1, -1, -1, -1, -1, -1, -1, -1, -1, 1, -1, 1, -1, -1, 1, 1, -1, -1, -1, 1, 1, 1, -1, 1, 1, -1, 1, -1, 1, 1, 1, -1, 1, -1, 1, 1, 1, -1, -1, -1, 1, 1, -1, 1, -1, -1, -1, 1, 1, 1, -1, -1, -1, -1, 1, -1, -1, 1, -1, 1, 1, 1, -1, 1, 1, 1, -1, 1,-1,1,1,-1,1,-1,1,-1,1,1,1,-1,-1,1,1,-1,-1,1,1,-1,1,1,1,-1,-1,-1,1,-1,1,-1,-1,1,1,1,-1,-1,-1,1,1,1,1,1,1,-1,-1,1,1,-1,-1,-1,-1,-1,1,-1,-1,1,-1,-1,-1,-1,-1,1,-1,1,1,1,1,1,1,-1,1,-1,-1,-1,-1,-1,1,-1,1,1,1,-1,-1,-1,1,-1,-1,-1,1,1,1,-1,1,-1,-1,1,-1,1,1,-1,-1,-1,-1,1,-1,1,-1,1,-1,-1,-1,-1,-1,1,-1,-1,1,-1,-1,-1,-1,1,-1,1,1,-1,-1,1,-1,-1,-1,1,1,-1,1,-1,-1,1,-1,-1,-1,1,-1,1,-1,-1,1,-1,1,1,1,1,-1,-1,-1,1,-1,1,-1,-1,1,1,1,-1,-1,-1,1,1,1,1,1,-1,1,1,1,1,-1,1,-1,1,1,-1,1,-1,1,1,1,-1,-1,1,1,-1,1,-1,1,1,-1,-1,1,-1,-1,-1,-1,1,-1,1,-1,-1,-1,-1,1,-1,1,-1,-1,1,-1,1,1,-1,-1,-1,1,-1,-1,1,1,-1,1,-1,1,1,1,-1,-1,1,1,-1,-1,-1,-1,-1,-1,-1,1,1,1,-1,1,-1,-1,1,1,-1,-1,-1,-1,-1,-1,-1,1,1,-1,1,-1,1,1,-1,-1,-1,1,-1,1,1,1,-1,-1,-1,1,-1,-1,1,1,-1,1,-1,-1,1,1,1,-1,-1,1,1,-1,1,-1,-1,1,-1,1,-1,-1,-1,-1,1,1,1,1,1,1,-1,-1,-1,-1,1,-1,1,-1,1,1,-1,1,-1,-1,1,-1,-1,1,-1,1,1,-1,1,1,-1,1,1,1,1,-1,1,-1,1,1,-1,1,1,-1,-1,1,1,1,-1,-1,1,1,-1,1,1,1,-1,-1,-1,-1,-1,1,-1,-1,1,1,-1,1,1,1,-1,-1,-1,-1,-1,-1,1,-1,-1,1,-1,-1,1,1,-1,1,-1,1,1,-1,-1,-1,-1,-1,1,1,-1,1,-1,1,1,1,-1,1,-1,-1,-1,1,-1,-1,-1,1,1,1,1,-1,-1,1,-1,1,1,-1,-1,-1,1,1,-1,1,-1,1,1,-1,-1,1,1,1,-1,-1,1,1,-1,1,1,-1,-1,-1,-1,-1,1,-1,1,1,-1,1,1,-1,1,-1,1,-1,-1,1,1,1,1,-1,1,-1,-1,-1,1,1,1,1,1,-1,-1,-1,1,-1,1,-1,-1,-1,-1,1,1,1,1,1,1,-1,-1,-1,-1,-1,-1,-1,1,-1,-1,-1,-1,1,-1,-1,1,-1,-1,-1,1,-1,-1,1,-1,1,1,-1,1,1,1,1,1,-1,-1,1,1,1,-1,1,-1,1,1,1,-1,1,1,-1,-1,-1,1,1,-1,1,-1,-1,-1,-1,1,-1,-1,1,1,-1,1,-1,1,1,-1,-1,1,1,-1,-1,-1,1,1,1,-1,-1,1,-1,-1,-1,1,-1,-1,1,1,1,-1,1,1,1,-1,1,-1,1,1,-1,-1,-1,-1,-1,1,-1,1,1,1,1},

[0089] The pseudo-random code corresponding to the YY polarization state is: {-1, 1, 1, 1, 1, 1, 1, -1, -1, 1, -1, 1, -1, 1, -1, 1, -1, 1, -1, -1, 1, -1, -1, 1, -1, 1, 1, 1, 1, -1, -1, -1, 1, -1, -1, -1, -1, -1, -1, -1, 1, 1, -1, -1, -1, -1, -1, 1, 1, -1, 1, -1, -1, -1, 1, 1, 1, -1, -1, -1, -1, 1, -1, -1, 1, -1, -1, -1, 1, 1, -1, 1, 1, -1, 1, 1, -1, 1, -1, -1, 1, -1, 1, 1, 1, 1, -1, 1, 1, -1, -1, -1, -1, -1, 1, 1, -1, 1, -1, -1, 1, -1, -1, -1, -1, 1, -1, 1, 1, -1, -1, 1, -1, 1, 1, -1, 1, -1, 1, -1, 1, -1, 1, 1, 1, 1, 1, -1, -1, -1, 1, -1, -1, 1, 1, -1, 1, -1, -1, -1, -1, -1, -1, -1, -1, 1, 1, -1, 1, 1, 1, 1, 1, -1, 1, 1, -1, -1, 1, -1, -1, 1, -1, -1, -1, -1, 1, -1, 1, 1, -1, 1, -1, 1, -1, 1, 1, -1, -1, 1, 1, 1, 1, -1, 1, -1, 1, -1, 1, 1, 1, 1, -1, 1, 1, 1, -1, 1, -1, -1, 1, 1, 1, 1, 1, 1, 1, 1, -1, -1, -1, 1, 1, -1, -1, -1, -1, 1, 1, 1, 1, 1, -1, -1, -1, 1, 1, -1, -1, -1, 1, 1, 1, -1, 1, 1, -1, 1, 1, -1, -1, -1, 1, -1, 1, -1, -1, 1, 1, 1, 1, 1, 1, -1, 1, -1, -1, -1, 1, 1, 1, 1, 1, 1, 1, -1, 1, -1, 1, 1, -1, 1, 1, 1, -1, 1, 1, 1, -1, -1, -1, -1, -1, -1, 1, -1, 1, -1, 1, 1, 1, 1, 1, -1, 1, -1, 1, 1, -1, -1, -1, 1, 1, 1, -1, -1, -1, -1, -1, -1, 1, -1, 1, 1, -1, -1, 1, 1, -1, 1, 1, 1, 1, -1, 1, 1, -1, -1, 1, 1, 1, 1, 1, -1, -1, -1, -1, 1, 1, -1, -1, -1, 1, -1, 1, -1, 1, 1, -1, -1, -1, -1, -1, -1, 1, -1, 1, 1, 1, 1, 1, 1, 1, -1, 1, 1, 1, 1, -1, 1, 1, 1, 1, -1, 1, 1, -1, 1, -1, -1, 1, 1, -1, 1, -1, -1, -1, -1, -1, -1, -1, -1, 1, 1, -1, -1-1,-1,1,-1,1,-1,1,1,1,1,1,1,-1,1,-1,-1,-1,-1,-1,-1,-1,-1,1,1,1,-1,1,1,1,-1,1,-1,1,1,1,-1,1,1,1,-1,-1,-1,-1,-1,-1,1,1,-1,1,1,1,-1,1,-1,1,1,-1,1,1,-1,-1,-1,-1,-1,1,-1,1,-1,1,1,1,-1,-1,-1,-1,-1,-1,-1,-1,-1,1,-1,-1,1,1,-1,-1,-1,-1,-1,-1,-1,1,1,1,-1,-1,-1,-1,-1,1,-1,-1,1,-1,-1,-1,-1,1,1,-1,1,-1,1,1,-1,-1,1,-1,-1,-1,-1,1,1,1,-1,1,1,-1,-1,1,1,-1,1,-1,1,1,-1,1,-1,-1,1,1,1,1,-1,-1,-1,-1,1,1,-1,-1,-1,1,-1,1,-1,1,1,1,-1,-1,1,1,-1,1,1,1,1,1,-1,-1,1,-1,-1,1,-1,1,-1,1,1,-1,-1,1,-1,1,1,-1,1,1,1,1,1,-1,1,1,1,1,1,1,-1,1,-1,-1,-1,-1,1,-1,1,1,1,1,-1,-1,1,-1,-1,-1,1,-1,1,-1,-1,-1,-1,1,1,-1,1,-1,-1,1,1,1,-1,1,1,1,1,-1,-1,1,-1,-1,-1,1,1,1,1,1,1,-1,-1,1,1,1,1,1,-1,-1,1,-1,1,1,-1,-1,1,1,1,1,-1,1,-1,1,1,1,-1,1,1,-1,-1,1,1,1,1,1,-1,1,1,-1,-1,1,-1,-1,1,1,1,-1,1,-1,1,-1,1,1,-1,-1,1,-1,1,1,1,1,1,1,-1,-1,1,-1,1,-1,1,1,-1,-1,-1,-1,1,1,-1,-1,1,-1,1,-1,1,-1,1,-1,-1,1,1,-1,1,-1,1,1,1,-1,1,1,-1,-1,-1,-1,1,-1,1,1,-1,1,-1,1,-1,1,-1,1,-1,-1,-1,1,1,1,1,-1,1,-1,-1,1,-1,1,-1,1,1,-1,-1,1,-1,-1,-1,-1,-1,1,1,1,-1,-1,1,-1,-1,1,1,-1,1,-1,1,1,-1,1,1,-1,-1,-1,-1,1,1,-1,1,-1,1,-1,-1,1,1,1,-1,1,-1,1,1,-1,1,1,-1,-1,-1,1,1,1,-1,-1,1,1,1,1,1,-1,1,1,1,1,1,1,-1,-1,-1,-1,1,1,-1,1,-1,-1,-1,-1,-1,-1,1,1,1,1,-1,-1,1,-1,1,1,-1,1,-1,1,1,-1,-1,1,1,1,1,1,1,1,1,-1,-1,-1,1,1,1,-1,1,-1,1,-1,1,1,-1,1,-1,-1,1,-1,-1,-1,1,1,1,-1,-1,1,-1,-1,-1,-1,1,1,-1,-1,-1,-1,1,-1,1,-1,-1,1,1,-1,1,1,1,-1,-1,-1,-1,-1,-1,-1,1,-1,1,1,-1,1,1,1,-1,-1,-1,-1,-1,1,-1,-1,-1,-1,1,-1,-1,-1,1,-1,1,1,-1,1,-1,1,1,1,-1,-1,-1,1,1,1,-1,1,1,-1,1,-1,1,1,-1,1,1,-1,-1,1,-1,1,1,-1},

[0090] The pseudo-random code corresponding to the YX polarization state is: {1,1,1,1,1,-1,-1,-1,-1,-1,-1,-1,-1,1,1,1,-1,1,1,-1,1,-1,1,-1,1,-1,1,-1,1,-1,-1,1,-1,1,1,1,1,-1,-1,-1,1,1,-1,1,1,-1,-1,-1,1,1,-1,1,-1,1,1,1,1,-1,1,-1,-1,1,-1,-1,1,-1,1,-1,-1,1,1,-1,-1,-1,1,-1,-1,1,1,1,-1,1,1,1,1,-1,1,-1,-1,-1,-1,-1,1,1,-1,-1,-1,-1,-1,-1,1,1,1,-1,-1,-1,1,1,1,-1,-1,-1,1,1,-1,-1,-1,1,1,1,1,-1,-1,-1,1,-1,-1,-1,1,1,-1,1,1,1,-1,1,1,-1,-1,-1,1,-1,-1,-1,-1,1,-1,1,-1,-1,-1,-1,1,1,-1,1,-1,1,-1,1,-1,1,-1,-1,-1,1,-1,1,-1,-1,1,-1,-1,-1,1,1,-1,-1,1,-1,1,-1,-1,1,1,-1,1,1,1,1,1,-1,-1,-1,-1,-1,1,-1,1,1,1,-1,1,-1,-1,1,1,-1,1,-1,1,-1,1,1,1,-1,1,-1,1,1,-1,-1,1,1,1,1,-1,-1,-1,-1,-1,-1,1,1,1,1,1,-1,-1,1,-1,-1,1,-1,1,-1,1,-1,1,1,-1,-1,-1,1,1,-1,1,1,1,-1,1,-1,1,1,-1,-1,-1,1,-1,1,-1,-1,1,1,1,1,-1,-1,1,-1,1,-1,1,1,-1,-1,1,-1,-1,-1,1,-1,-1,1,-1,-1,-1,-1,1,1,1,-1,-1,1,-1,-1,-1,-1,1,-1,1,-1,-1,-1,1,1,1,-1,-1,-1,-1,-1,1,-1,1,1,-1,1,-1,-1,-1,1,1,1,-1,1,-1,1,1,1,-1,-1,1,1,-1,-1,-1,1,-1,-1,1,-1,-1,1,1,1,-1,-1,1,1,-1,-1,-1,-1,-1,-1,-1,-1,-1,1,-1,-1,1,1,-1,-1,-1,-1,1,-1,-1,1,1,-1,1,1,1,1,-1,1,-1,-1,-1,-1,-1,1,1,-1,1,-1,-1,1,-1,1,1,1,-1,-1,1,-1,1,-1,1,1,-1,1,1,1,-1,-1,1,-1,-1,1,1,1,1,-1,-1,-1,1,1,1,1,1,-1,-1,1,-1,-1,1,-1,-1,-1,1,-1,-1,-1,1,-1,-1,-1,-1,-1,1,1,1,1,1,-1,1,1,1,1,-1,-1,-1,1,-1,-1,1,-1,1,1,-1,1,1,-1,-1,1,-1,-1,1,-1,1,-1,-1,-1,-1,1,-1,1,-1,1,-1,1,-1,1,1,1,-1,-1,1,1,-1,-1,-1,1,1,-1,1,1,-1,-1,-1,-1,1,-1,-1,1,1,-1,1,1,1,-1,1,-1,1,-1,-1,-1,-1,-1,-1,1,1,1,-1,1,-1,1,-1,-1,1,-1,1,1,1,-1,-1,-1,1,1,1,1,1,1,1,-1,1,1,1,1,1,-1,-1,-1,-1,-1,-1,1,-1,1,1,-1,1,-1,1,1,-1,1,1,-1,1,-1,1,-1,1,1,-1,1,-1,1,1,-1,1,-1,-1,1,-1,1,1,-1,-1,-1,1,-1,1,-1,1,1,-1,-1,1,-1,1,1,1,-1,1,1,1,1,1,1,1,-1,-1,-1,-1,1,-1,-1,-1,-1,1,-1,-1,-1,-1,1,-1,1,1,-1,1,1,-1,1,-1,1,-1,-1,1,-1,-1,-1,-1,-1,1,1,1,1,-1,1,-1,1,-1,1,-1,1,-1,-1,-1,-1,1,-1,1,-1,1,-1,-1,1,1,1,1,1,1,-1,1,-1,-1,1,-1,-1,1,-1,-1,-1,1,1,-1,-1,-1,1,-1,-1,-1,1,1,1,-1,-1,-1,1,1,1,1,1,-1,1,1,1,-1,-1,-1,1,-1,1,1,-1,-1,1,1,-1,-1,1,1,-1,1,-1,-1,-1,-1,1,1,-1,-1,-1,-1,1,-1,1,1,1,-1,1,1,-1,1,1,-1,1,1,-1,-1,1,1,1,1,-1,-1,-1,1,1,-1,1,-1,-1,-1,1,-1,-1,1,-1,1,1,1,1,1,1,-1,1,1,1,-1,1,-1,1,-1,-1,1,-1,1,1,1,-1,1,-1,1,-1,1,-1,1,-1,-1,-1,1,1,-1,1,-1,-1,1,-1,1,-1,1,1,1,1,-1,-1,1,-1,1,-1,1,1,-1,-1,1,1,1,1,-1,1,-1,1,-1,1,1,-1,-1,1,-1,-1,1,1,1,1,1,1,-1,1,-1,1,1,-1,-1,-1,1,-1,-1,1,1,1,-1,-1,-1,1,1,1,1,-1,-1,-1,-1,-1,-1,1,1,-1,-1,1,-1,1,1,-1,1,1,1,-1,1,1,-1,1,1,1,-1,-1,-1,-1,1,1,-1,-1,-1,-1,-1,-1,-1,1,-1,1,-1,-1,1,-1,1,1,-1,1,1,1,-1,1,-1,1,1,-1,1,1,-1,-1,-1,1,1,-1,-1,-1,1,1,1,-1,-1,1,1,1,-1,1,1,1,1,-1,1,-1,-1,-1,-1,-1,1,-1,-1,-1,-1,1,-1,1,1,-1,-1,-1,1,-1,1,-1,1,-1,-1,-1,1,1,1,1,-1,1,1,-1}。,

[0091] Based on the above settings, in this embodiment, the change of the background phase noise before and after starting the technology that combines polarization switching technology and pseudo-random code is as follows Figure 12 shown. Before starting the technology that combines polarization switching technology and pseudo-random code, the background phase noise of the system is close to -60 dB@1 kHz. After starting the technology that combines polarization switching technology and pseudo-random code, the background phase noise of the system is reduced by about 20 dB.

[0092] Figure 13 is a comparison diagram of the fiber grating type fiber optic hydrophone under the condition of a 10-kilometer link length in an embodiment, without pseudo-random code modulation, using the same set of pseudo-random codes for four polarization states, and using pseudo-random codes with different sorting methods for four polarizers respectively. Refer to Figure 13 It can be seen that under the condition of a 10-kilometer transmission distance, the background noise suppression effects of not using pseudo-random code modulation, using the same pseudo-random code for four polarization states (XX, XY, YY, YX), and using different pseudo-random codes for four polarization states are compared. Among them, in the first case, without pseudo-random code modulation, based on the structure in Figure 3 where Figure 3However, the computer 7 does not generate a pseudo-random code sequence to be provided to the high-speed arbitrary waveform generator 106, that is, the fiber Bragg grating type fiber optic hydrophone does not use pseudo-random code modulation. In the second case, the same set of pseudo-random code modulation is used for the four polarization states. Based on Figure 3 the structure in, the computer 7 generates a pseudo-random code sequence {-1, 1} sequence and imports it into the high-speed arbitrary waveform generator 106. The high-speed arbitrary waveform generator 106 will output corresponding waveforms according to the pseudo-random code sequence input by the computer 7. By connecting the fiber optic phase modulator to the fiber optic hydrophone system to adjust the phase of the interrogation laser pulse, the self-coherence of the interrogation laser pulse is changed, thereby suppressing the influence of Rayleigh scattering on the interference signal. The third case is to add a polarization switch 9 to the structure shown in Figure 3 , and its structure is as shown in Figure 11 , and different sorting methods of pseudo-random code modulation are used for the four polarizers respectively. Specifically, the pseudo-random codes corresponding to the XX polarization state, the pseudo-random codes corresponding to the XY polarization state, the pseudo-random codes corresponding to the YX polarization state, and the pseudo-random codes corresponding to the YY polarization state adopt the specific forms provided above, which will not be elaborated here. In the three cases, except for the differences in whether to perform pseudo-random code modulation and whether to organically combine the pseudo-random code modulation technology and the polarization suppression technology, the selection of other devices and conditions are exactly the same. Under the condition of a 10-kilometer link length, the effects of the schemes of not performing pseudo-random code modulation on the fiber Bragg grating type fiber optic hydrophone, using the same set of pseudo-random code modulation for the four polarization states, and using different sorting methods of pseudo-random code modulation for the four polarizers are compared. The results are as shown in Figure 13 , and it can be seen from Figure 13 that when the same pseudo-random code is used for modulation of the four polarization states, not only the suppression effect is not as good as that when different pseudo-random codes are used for modulation of the four polarization states, but also virtual signals appear in the demodulation results.

[0093] Matters not covered in this invention are well-known technologies.

[0094] The technical features of the above embodiments can be combined arbitrarily. For the sake of brevity of description, 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, it should be considered as the scope described in this specification.

[0095] The above-described embodiments only represent several implementation manners of this application. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of this application, several modifications and improvements can still be made, and these all belong to the protection scope of this application. Therefore, the protection scope of this application should be subject to the appended claims.

[0096] 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 variations. 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. A fiber Bragg grating type optical fiber hydrophone for suppressing Rayleigh scattering, characterized in that: It includes a fiber grating type fiber optic hydrophone, which includes an interrogation laser pulse generating unit, a circulator, a sensing channel and a light return detection unit, and the sensing channel is a sensing channel composed of a fiber grating; The interrogation laser pulse generating unit is used to generate an interrogation laser pulse. A pseudo-random code phase modulation technique is introduced into the interrogation laser pulse generating unit to adjust the phase of the interrogation laser pulse, and the phase of the interrogation laser pulse is modulated into a plurality of pseudo-randomly distributed discrete phases to reduce the self-coherence of the interrogation laser pulse, thereby achieving effective suppression of Rayleigh scattering noise of the fiber Bragg grating type optical fiber hydrophone; The interrogation laser pulse generated by the interrogation laser pulse generating unit is input into the sensing channel through a circulator, and the return light of the sensing channel is transmitted to the return light detection unit after passing through the circulator.

2. The fiber Bragg grating type optical fiber hydrophone for suppressing Rayleigh scattering according to claim 1, characterized in that: The interrogation laser pulse generating unit comprises a laser light source, an optical fiber acousto-optic modulator and an optical fiber phase modulator; the laser light emitted by the laser light source is modulated into a pulse laser by the optical fiber acousto-optic modulator, and the pulse laser is phase-modulated by the optical fiber phase modulator to modulate the phase of the interrogation laser pulse into a plurality of discrete phases with pseudo-random distribution.

3. The fiber Bragg grating type optical fiber hydrophone for suppressing Rayleigh scattering according to claim 2, characterized in that: The interrogation laser pulse generating unit further comprises an optical fiber amplifier, which is used to amplify the pulse laser power.

4. The fiber Bragg grating type optical fiber hydrophone for suppressing Rayleigh scattering according to claim 2 or 3, characterized in that: The optical fiber phase modulator is connected to a phase control unit, which includes a pseudo-random code generation unit, a high-speed arbitrary waveform generator and a phase modulator driver. The pseudo-random code generation unit is used to generate a pseudo-random code, and the generated pseudo-random code is input into the high-speed arbitrary waveform generator. The high-speed arbitrary waveform generator generates randomly distributed high and low levels according to the pseudo-random code. The randomly distributed high and low levels are input into the phase modulator driver. The phase modulator driver modulates the optical fiber phase modulator so that the phase of the interrogation laser pulse is modulated according to the high and low level distribution rule.

5. The fiber Bragg grating type optical fiber hydrophone for suppressing Rayleigh scattering according to claim 4, characterized in that: The pseudo-random code generation unit can generate pseudo-random codes of different orders or types, and then modulate the interrogation laser pulse into different types of discrete distribution forms through a phase modulator.

6. The fiber Bragg grating type optical fiber hydrophone for suppressing Rayleigh scattering according to claim 5, characterized in that: The pseudo-random code is a sequence of numbers including 0 and 1 randomly distributed, or the pseudo-random code is a sequence of numbers including -1 and 1 randomly distributed.

7. The fiber Bragg grating type optical fiber hydrophone for suppressing Rayleigh scattering according to claim 5, characterized in that: The pseudo-random codes are different m-code sequences or different-order Gold code sequences.

8. The fiber Bragg grating type optical fiber hydrophone for suppressing Rayleigh scattering according to claim 1, 2, 3, 5, 6 or 7, characterized in that: The fiber grating type fiber optic hydrophone includes a fiber matching interferometer, which is connected in the optical path between the interrogation laser pulse generating unit and the circulator, or in the optical path between the circulator and the sensing channel, or in the optical path between the circulator and the return light detection unit.

9. The fiber Bragg grating type optical fiber hydrophone for suppressing Rayleigh scattering according to claim 8, characterized in that: The fiber matching interferometer comprises a first fiber coupler, a first transmission arm, a second transmission arm and a second fiber coupler. The laser input to the fiber matching interferometer is divided into two beams through the first fiber coupler, one beam is transmitted to the second fiber coupler through the first transmission arm, and the other beam is transmitted to the second fiber coupler through the second transmission arm. The first transmission arm and the second transmission arm have different lengths, and the two beams of pulsed light with a certain time interval are combined and output through the second fiber coupler.

10. The fiber Bragg grating type optical fiber hydrophone for suppressing Rayleigh scattering according to claim 1, 2, 3, 5, 6, 7 or 9, characterized in that: It also includes a polarization switcher, the interrogation laser pulse generated by the interrogation laser pulse generating unit is input into the transmission link through the polarization switcher and then through the circulator, and then transmitted to the sensing channel through the transmission link, and the return light of the sensing channel is transmitted to the return light detection unit after passing through the circulator; The polarization switching frequency of the polarization switcher is 1 / 4 of the acousto-optic modulator; when the previous pulse of the pulse pair output by the optical fiber coupler completely passes through the polarization switcher, the polarization switcher changes the polarization state once. Suppose the output polarization state of the first pulse pair is XX, the polarization state of the second pulse pair is XY, the polarization state of the third pulse pair is YY, the polarization state of the fourth pulse pair is YX, the polarization state of the fifth pulse pair is XX..., and so on. Due to the existence of the polarization switcher, the polarization states of the laser pulse pairs output to the optical fiber grating in the sensing channel are XX, XY, YY, YX, XX, XY, YY, YX, XX..., and each four polarization states form a cycle. The polarization states of the pulse pairs transmitted from the sensing channel to the data acquisition card via the circulator are also XX, XY, YY, YX, XX... After collecting the pulse pairs in the four polarization states, the polarization synthesis algorithm is used for demodulation, which can effectively solve the signal disturbance caused by random polarization disturbance.

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