A heterodyne demodulation method for common-mode noise suppression of an interferometric fiber optic hydrophone

Abstract

A heterodyne demodulation method for common-mode noise suppression of an interferometric fiber optic hydrophone, comprising: 1. Building a heterodyne demodulation system; 2. Processing the optical signal to generate an optical pulse pair; the optical pulse passes through an optical fiber circulator, a reference hydrophone and a signal hydrophone and returns, outputting an interference optical pulse sequence; 3. Obtaining a corresponding sampling signal through an optoelectronic converter for the interference optical pulse sequence; obtaining the sampling signals of the signal hydrophone and the reference hydrophone through a signal processor; 4. Calculating the initial phase difference; 5. Extracting three-way phase shift heterodyne signals of the reference hydrophone; 6. Constructing a corrected sampling signal of the reference hydrophone using the three-way phase shift heterodyne signals of the reference hydrophone; 7. Calculating the demodulated phase signal; 8. Calculating the demodulated phase output. The present invention can obtain the three-way phase shift heterodyne signals of the reference interferometer under the condition that the signal, the reference hydrophone, etc. remain unchanged, thereby constructing a reference interference signal with the same initial phase as the signal interferometer, and realizing the suppression of the common-mode noise of the system.

Description

Technical Field

[0001] The present invention relates to the technical field of fiber optic sensing, and particularly to a heterodyne demodulation method for common mode noise suppression of an interferometric fiber optic hydrophone. Background Art

[0002] The interferometric fiber optic hydrophone is the most widely used fiber optic hydrophone at present. Its sensing core is a fiber optic interferometer. The measured acoustic signal is loaded in the interference signal of the fiber optic interferometer in the form of phase. Achieving stable detection of the phase of the interferometer is the key to fiber optic hydrophone technology.

[0003] The phase background noise is an important performance index of the fiber optic hydrophone system, and is closely related to key performances such as the minimum detectable signal and the dynamic range. A large number of studies have been carried out on the noise sources, conversion mechanisms and suppression methods of the fiber optic hydrophone system. Among them, the common mode noise has the same noise source and is shared by all hydrophone elements. Currently, it is one of the important factors limiting the noise performance of the fiber optic hydrophone system. In the interferometric fiber optic hydrophone system, the common mode noise mainly includes laser phase noise, relative intensity noise (RIN) and phase noise introduced by environmental disturbances through the transmission link. Since it is usually in the same frequency band as the measured acoustic signal, especially in the low frequency range, it is difficult to eliminate by frequency domain filtering methods.

[0004] Using a sound-insensitive reference hydrophone for noise cancellation is a common common-mode noise suppression method, and processing algorithms such as direct subtraction and adaptive filters are proposed based on this. However, the actual experimental results show that the suppression effect of common-mode noise is not stable in the long term, and the lower limit of the suppressed noise will change with the average phase difference, resulting in limited performance of existing common-mode noise suppression technologies. References [1] (G. Hechenblaikner, Common mode noise rejection properties of amplitude and phase noise in a heterodyne interferometer, J. Opt. Soc. Amer. A, vol. 30, no. 5, pp. 941-947, 2013.) and [2] (F. Liu et al., Common-Mode Noise Suppression Technique in Interferometric Fiber-Optic Sensors, J. Lightw. Technol, vol. 37, no. 21, pp. 5619-5627, 2019.) both studied the variation law of this noise suppression effect with the initial phase. The research shows that the common-mode noise suppression effect is related to the initial phase difference between the reference hydrophone and the signal hydrophone. The smaller the initial phase difference, the better the noise correlation and the better the noise suppression effect.

[0005] Reference [2] proposed a noise suppression scheme based on a 3×2 coupler reference interferometer. Using the three-path interference signals of the reference interferometer obtained by the 3×2 coupler, a reference interference signal with the same initial phase as the signal interferometer is constructed, thus effectively improving the noise cancellation effect. However, the problems of this scheme are as follows: ① It is necessary to add a relatively complex optical path structure in the array to access the three-path interference signals of the reference interferometer into the return optical pulse sequence of the array; ② Additional optical pulses will be added in the time-division pulse sequence, increasing the actual time-division multiplexing pulse number of the system, which will lead to an increase in array loss and a decrease in the system sampling frequency; ③ It is necessary to ensure that the three-path interference signals of the reference interferometer have the same signal amplitude, which requires precise loss control or amplitude compensation for the three paths of signals, increasing the complexity of the system. Summary of the Invention

[0006] The present invention provides a heterodyne demodulation method for common-mode noise suppression of an interferometric fiber optic hydrophone to solve the technical problem of the complex system structure used in existing heterodyne demodulation methods.

[0007] To achieve the above object, the technical solution of the present invention is realized as follows:

[0008] The present invention provides a heterodyne demodulation method for common-mode noise suppression of an interferometric fiber optic hydrophone, comprising the following steps:

[0009] Step S1, construct a heterodyne demodulation system; the heterodyne demodulation system includes a laser, an optical pulse modulator, a pulse pair generation module, an optical fiber circulator, a reference hydrophone, and a signal hydrophone that are optically connected in sequence; it also includes a photoelectric converter and a signal processor; one end of the photoelectric converter is optically connected to the optical fiber circulator, and the other end is electrically connected to the signal processor, and the signal processor is electrically connected to the pulse pair generation module and the optical pulse modulator respectively;

[0010] Step S2, the optical signal output by the laser is modulated by the optical pulse modulator to generate optical pulses; the optical pulses are input into the pulse pair generation module to form a pair of optical pulses composed of two optical pulses; the optical pulses pass through the optical fiber circulator, the reference hydrophone, and the signal hydrophone in sequence, and return along the original path, and the returned interference optical pulse sequence is output through the optical fiber circulator;

[0011] Step S3, the returned interference optical pulse sequence is input into the photoelectric converter to obtain corresponding sampling signals; the sampling signals are input into the signal processor for de-time-division processing to obtain the sampling signal V s (k) of the signal hydrophone and the sampling signal V r (k) of the reference hydrophone, where k is the sampling point number;

[0012] Step S4, calculate the initial phase difference between the signal hydrophone and the reference hydrophone according to the sampling signal V s (k) of the signal hydrophone and the sampling signal V r (k) of the reference hydrophone

[0013] Step S5, for each sampling period, according to the obtained sampling signal V r (k) of the reference hydrophone, extract three phase-shifted heterodyne signals V r1 (k), V r2 (k), and V r3 (k) of the reference hydrophone respectively;

[0014] Step S6, utilize the three phase-shifted heterodyne signals V r1 (k), V r2 (k), and V r3 (k) of the reference hydrophone, and construct the corrected sampling signal V′ of the reference hydrophone with the same initial phase as the signal hydrophone r (k);

[0015] Step S7, for the sampling signal V s(k) and the corrected sampled signal V′ of the constructed reference hydrophone r Perform heterodyne phase demodulation calculations on (k) respectively to obtain the demodulated phase signals of the signal hydrophone and the reference hydrophone and

[0016] Step S8: For each sampling period, process the sampled signal according to steps S5 - S6 to obtain the demodulated phase signals of the signal hydrophone and the reference hydrophone corresponding to each sampling period and After performing high - pass filtering on the two signals and subtracting them, obtain the demodulated phase output after common - mode noise suppression

[0017] Furthermore, the pulse pair generation module in step S1 includes a first fiber optic coupler, an acousto - optic frequency shifter, a fiber optic delay line, and a second fiber optic coupler;

[0018] One end of the first fiber optic coupler is optically connected to the optical pulse modulator, and the other end is optically connected to the second fiber optic coupler through the fiber optic delay line and the acousto - optic frequency shifter respectively.

[0019] Furthermore, step S2 specifically includes the following steps:

[0020] Step S21: After the laser output optical signal is input into the optical pulse modulator, the optical pulse modulator generates optical pulses output at a certain repetition frequency;

[0021] Step S22: The optical pulses are fiber - split by the first fiber optic coupler, and after passing through the fiber delay of the fiber optic delay line or the acousto - optic frequency shift of the acousto - optic frequency shifter, they are fiber - combined by the second fiber optic coupler to generate an optical pulse pair composed of two optical pulses with a certain time delay. There is an optical frequency difference f m between the two optical pulses. Control the length of the delay fiber so that the time delay between the two optical pulses is τ, τ = 2nL / c, where L is the arm difference of the signal hydrophone interferometer, n is the refractive index of the fiber core, and c is the speed of light in vacuum;

[0022] Step S23: After passing through the fiber optic circulator, the optical pulse pair is successively input into the reference hydrophone and the signal hydrophone, and then returns through the signal hydrophone and the reference hydrophone. The returned interference optical pulse sequence is output by the fiber optic circulator to obtain the returned interference optical pulse signal.

[0023] Furthermore, the sampling frequency of the sampled signal in step S3 is f p = N·f m , and the sampling length of the sampled signal is N·M + N / 2, where N takes an integer value that is an integer multiple of 4, and M is the number of integral sampling periods.

[0024] Further, the step S4 specifically includes the following steps:

[0025] Step S41: Construct two orthogonal detection signals u s (k) and u c (k), and multiply them with the sampling signal V s (k) of the signal hydrophone and the sampling signal V r (k) of the reference hydrophone in sequence. After taking the average value, the two orthogonal detection output values SR s and CR s of the signal hydrophone and the two orthogonal detection output values SR r and CR r of the reference hydrophone are obtained respectively; r ;

[0026] Step S42: Repeat step S41 within different sampling periods, and take the average value of the multiple calculated two orthogonal detection output values SR s and CR s of the signal hydrophone and the two orthogonal detection output values SR r and CR r of the reference hydrophone to obtain the average values of the two orthogonal detections of the signal hydrophone r and the average values of the two orthogonal detections of the reference hydrophone ;

[0027] Step S43: Calculate the initial phase of the signal hydrophone and the initial phase of the reference hydrophone according to the average values of the two orthogonal detections of the signal hydrophone and the average values of the two orthogonal detections of the reference hydrophone and calculate the initial phase difference between the signal hydrophone and the reference hydrophone according to the initial phase of the signal hydrophone and the initial phase of the reference hydrophone

[0028]

[0028] Further, the orthogonal detection signals u s (k) and u c (k) in step S41 are respectively as follows:

[0029]

[0030]

[0031] The two orthogonal detection output values SR s and CR s of the signal hydrophone in step S41 and the two orthogonal detection output values SR r and CR of the reference hydrophoner Are as follows:

[0032]

[0033]

[0034] The initial phase of the signal hydrophone in step S43 and the initial phase of the reference hydrophone Are as follows:

[0035]

[0036]

[0037] Further, the three-channel phase-shifted heterodyne signals V r1 (k), V r2 (k) and V r3 (k) of the reference hydrophone in step S5 are as follows:

[0038] V r1 (k) = V r (k) (7)

[0039]

[0040]

[0041] Further, step S6 specifically includes the following steps:

[0042] Step S61, calculate the DC component amplitude A r :

[0043]

[0044] Step S62, remove the DC from the three-channel phase-shifted heterodyne signals V r1 (k), V r2 (k) and V r3 (k) of the reference hydrophone to obtain the three-channel phase-shifted heterodyne signals u r1 (k), u r2 (k) and u r3 (k) after DC removal:

[0045] u r1 (k) = V r1 - A r (11)

[0046] u r2 (k) = V r2 - A r (12)

[0047] u r3 (k)=V r3 -A r (13)

[0048] Step S63. Construct the corrected orthogonal interference signals vs r (k) and vc r (k):

[0049] vs r (k)=-u r2 (k) (14)

[0050]

[0051] Step S64. Construct the corrected sampling signal V′ of the reference hydrophone with the same initial phase as the signal hydrophone r (k):

[0052]

[0053] Furthermore, the step S7 specifically includes the following steps:

[0054] Step S71. Use the two-channel quadrature detection signals u s (k) and u c (k) to multiply with the sampling signal V s (k) of the signal hydrophone and the corrected sampling signal V′ r (k) of the reference hydrophone in turn, and take the average value to obtain the quadrature detection output values SR s , CR s of the signal hydrophone and the corrected quadrature detection output values SR′ r , CR′ r ;

[0055]

[0056]

[0057] Step S72. Use the arctangent algorithm to calculate the demodulation phase signal of the signal hydrophone and the demodulation phase signal

[0058]

[0059]

[0060] Furthermore, the demodulation phase output after common-mode noise suppression in the step S8 is specifically:

[0061]

[0062] Among them, HF(·) represents high-pass filtering.

[0063] Advantages of the present invention:

[0064] 1. The present invention ingeniously utilizes the relationship between the interferometer phase term 2πft introduced by heterodyne frequency shift and time. Through the design of integer-period sampling, it is possible to obtain three heterodyne interference signals with specific initial phase differences only by translating the sampling points, without adding any additional optoelectronic devices in the heterodyne demodulation system, and without increasing the complexity of the system; m 2. The technical solution proposed by the present invention does not require adding additional optical pulses in the time-division pulse sequence, does not increase the time-division multiplexing pulse number of the system, and the array loss and the system sampling frequency are not affected;

[0065] 3. The technical solution proposed by the present invention obtains three-phase-shifted heterodyne interference signals from the same sampling signal, with the same signal amplitude, without considering the power balance problem of the three signals, and without introducing additional errors due to the imbalance of the optical powers of the three signals;

[0066] 4. The technical solution proposed by the present invention constructs a reference interference signal with the same initial phase as the signal hydrophone by using the three-phase-shifted heterodyne signals of the reference hydrophone, improves the correlation of the common-mode noise, thereby effectively improving the effect and stability of common-mode noise suppression, and only using one reference hydrophone can simultaneously realize the noise compensation and suppression of multiple signal hydrophones, and can be conveniently applied to a multi-channel time-division multiplexing fiber optic hydrophone array system.

[0067] BRIEF DESCRIPTION OF THE DRAWINGS BRIEF DESCRIPTION OF THE DRAWINGS

[0068] Figure 1 It is a schematic structural diagram of the heterodyne demodulation system in the present invention;

[0069] Figure 2 It is a schematic diagram of the return optical pulse of the heterodyne demodulation system. DETAILED DESCRIPTION OF THE INVENTION

[0070] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0071] In the description of the present invention, it should be noted that, unless otherwise clearly specified and defined, the terms "installation", "connection", and "coupling" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood through specific situations.

[0072] In addition, the descriptions such as "first", "second", etc. in the present invention are only for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly indicating the quantity or order of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include at least one such feature. In the description of the present invention, the meaning of "a plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0073] Referring to Figure 1 , an external differential demodulation method for common mode noise suppression of an interferometric fiber optic hydrophone is provided in an embodiment of the present application, including the following steps:

[0074] Step S1: Build an external differential demodulation system; the external differential demodulation system includes a laser, an optical pulse modulator, a pulse pair generation module, an optical fiber circulator, a reference hydrophone, and a signal hydrophone that are optically connected in sequence; it also includes a photoelectric converter and a signal processor; one end of the photoelectric converter is connected to the optical fiber circulator, and the other end is electrically connected to the signal processor, and the signal processor is electrically connected to the pulse pair generation module and the optical pulse modulator respectively;

[0075] In this embodiment, the pulse pair generation module in step S1 includes a first optical fiber coupler, an acousto-optic frequency shifter, an optical fiber delay line, and a second optical fiber coupler; one end of the first optical fiber coupler is optically connected to the optical pulse modulator, and the other end is optically connected to the second optical fiber coupler through the optical fiber delay line and the acousto-optic frequency shifter respectively.

[0076] In this embodiment, the reference hydrophone in step S1 includes a third optical fiber coupler, a first Faraday rotation mirror, a reference hydrophone sensing optical fiber, a fourth optical fiber coupler, and a second Faraday rotation mirror;

[0077] Among them, the third optical fiber coupler, the reference hydrophone sensing optical fiber, and the fourth optical fiber coupler are optically connected in sequence, and the first Faraday rotation mirror and the second Faraday rotation mirror are optically connected to the third optical fiber coupler and the fourth optical fiber coupler respectively.

[0078] In this embodiment, the signal hydrophone in step S1 includes a fourth fiber optic coupler and a second Faraday rotator mirror shared with the reference hydrophone, and also includes a signal hydrophone sensing fiber and a third Faraday rotator mirror; wherein, the fourth fiber optic coupler, the signal hydrophone sensing fiber, and the third Faraday rotator mirror are optically connected in sequence.

[0079] The reference hydrophone and the signal hydrophone form a fiber optic hydrophone array.

[0080] In step S2, the optical signal output by the laser is modulated by an optical pulse modulator to generate optical pulses; the optical pulses are input into a pulse pair generation module to form an optical pulse pair composed of two optical pulses; the optical pulses pass through the fiber optic circulator, the reference hydrophone, and the signal hydrophone in sequence, and return along the original path, and the returned interference optical pulse sequence is output through the fiber optic circulator.

[0081] In this embodiment, the specific steps of step S2 are as follows:

[0082] In step S21, after the optical signal output by the laser is input into the optical pulse modulator, the optical pulse modulator outputs optical pulses at a certain repetition frequency.

[0083] Specifically, the laser is a narrow linewidth laser, and emits narrow linewidth laser light input into the optical pulse modulator; the optical pulse modulator is an acousto-optic modulator or a semiconductor optical pulse amplifier, and the signal processor outputs modulation pulses to the optical pulse modulator to generate periodic optical pulses at a certain repetition frequency, as Figure 2 (a) shows.

[0084] In step S22, the optical pulses are fiber split through the first fiber optic coupler, and after the fiber delay of the fiber optic delay line or the acousto-optic frequency shift of the acousto-optic frequency shifter, they are fiber combined through the second fiber optic coupler to generate an optical pulse pair composed of two optical pulses with a certain time delay, and there is an optical frequency difference f m between the two optical pulses. The length of the delay fiber is controlled so that the time delay between the two optical pulses is τ, τ = 2nL / c, where L is the arm difference of the signal hydrophone interferometer, n is the refractive index of the fiber core, and c is the speed of light in vacuum;

[0085] Specifically, referring to Figure 2 , after the optical pulses are input into the fiber optic coupler, they are divided into two beams: one beam generates an optical frequency shift of f m through the acousto-optic frequency shifter and then is input into the first fiber optic coupler; the other beam is input into the second fiber optic coupler after passing through the fiber optic delay line. At this time, an optical pulse pair containing two optical pulses will be generated at the output port of the second fiber optic coupler. The optical frequency of the first optical pulse is f0, and the optical frequency of the second optical pulse is f0 + f m , where f0 is the optical frequency of the optical signal output by the laser, and f mLet \(f\) be the frequency shift of the acousto-optic frequency shifter, then there is an optical frequency difference \(f\) between two optical pulses. m ; Control the length of the delay fiber so that the time delay between the two optical pulses is \(\tau\), \(\tau = 2nL / c\), where \(L\) is the arm difference of the signal hydrophone interferometer, \(n\) is the refractive index of the fiber core, and \(c\) is the speed of light in vacuum. The optical pulse pairs will appear at a certain repetition frequency, and at the same time, the optical path loss needs to be controlled so that the peak powers of the two optical pulses are consistent. The optical pulse sequence output by the second fiber coupler is as Figure 2 (b) shown.

[0086] Step S23: After the optical pulse pairs pass through the fiber optic circulator, they are successively input into the reference hydrophone and the signal hydrophone, and then successively return through the signal hydrophone and the reference hydrophone. The returned interference optical pulse sequence is output by the fiber optic circulator to obtain the returned interference optical pulse signal.

[0087] Specifically, the optical pulse pairs are input into a port of the fiber optic circulator and output from another port of the fiber optic circulator to the fiber optic hydrophone array. The fiber optic hydrophone array consists of a reference hydrophone and a signal hydrophone, and both the reference hydrophone and the signal hydrophone are fiber optic interferometers.

[0088] After the optical pulse pairs are input into the third fiber coupler, they are divided into two beams. One beam of light is reflected by the first Faraday rotator mirror and then returns. The other beam is split by the reference hydrophone sensing fiber and the fourth fiber coupler and then reflected by the second Faraday rotator mirror and returns. The length of the reference hydrophone sensing fiber is \(L\), which is wound on an insensitive support structure. At this time, the second optical pulse of the optical pulse pair returned by the first Faraday rotator mirror will overlap with the first optical pulse of the optical pulse pair returned by the second Faraday rotator mirror in time, and these two optical pulses will interfere, as Figure 2 (c), (d) shown. Therefore, the third fiber coupler, the first Faraday rotator mirror, the reference hydrophone sensing fiber, the fourth fiber coupler, and the second Faraday rotator mirror actually form a fiber optic interferometer, which is the reference hydrophone. Since the reference hydrophone sensing fiber is insensitive to the external measured signal, the phase of this interferometer will be mainly affected by the system common mode noise.

[0089] One of the optical paths after the optical pulse pairs pass through the fourth fiber coupler will be input into the signal hydrophone sensing fiber and then reflected by the third Faraday rotator mirror and return. The length of the signal hydrophone sensing fiber is \(L\), which is wound on a sensitized structure to sense the external acoustic signal. At this time, the second optical pulse of the optical pulse pair returned by the second Faraday rotator mirror will overlap with the first optical pulse of the optical pulse pair returned by the third Faraday rotator mirror in time, and these two optical pulses will interfere, as Figure 2As shown in (d) and (e). Therefore, a fiber optic interferometer is actually formed by the fourth fiber optic coupler, the second Faraday rotation mirror, the signal hydrophone sensing fiber, and the third Faraday rotation mirror. This interferometer senses external acoustic signals through the signal hydrophone sensing fiber, which is the signal hydrophone.

[0090] Thus, the optical pulse sequence returned by the fiber optic hydrophone array will contain 4 optical pulses. The 2nd and 3rd optical pulses are the interference optical pulses of the reference hydrophone and the signal hydrophone respectively, as Figure 2 shown in (f). The returned optical pulse sequence is input to the second port of the fiber optic circulator and output through the third port of the fiber optic circulator.

[0091] Step S3: The returned interference optical pulse sequence is input into the optoelectronic converter to obtain the corresponding sampling signal; the sampling signal is input into the signal processor for de-time division processing to obtain the sampling signal V s (k) of the signal hydrophone and the sampling signal V r (k) of the reference hydrophone, where k is the sampling point number;

[0092] Specifically, the optical pulse sequence returned by the fiber optic hydrophone array is input into the optoelectronic converter. The optoelectronic converter is responsible for the optoelectronic conversion of the returned optical pulse sequence and completes the digital sampling of the converted electrical signal. The signal processor controls the digital sampling timing of the optoelectronic converter, performs digital sampling at the interference optical pulses of the signal hydrophone and the reference hydrophone to obtain the corresponding sampling signals, and the sampling frequency f p = N·f m , and the sampling length at each interference optical pulse is N·M + N / 2, where N takes an integer value that is an integer multiple of 4, M is the number of integral sampling periods, and the values of N and M need to be reasonably selected according to the actual requirements of the system.

[0093] The sampling signal is input into the signal processor for de-time division processing to obtain the sampling signal V s (k) of the signal hydrophone and the sampling signal V r (k) of the reference hydrophone, where k is the sampling point number, k = 1, 2,..., N·M + N / 2.

[0094] According to the interference principle of the fiber optic interferometer, V s (k) and V r (k) can be expressed as:

[0095]

[0096]

[0097] Among them, A s , A rThey are the DC component amplitudes of the sampling signals of the signal hydrophone and the reference hydrophone respectively, B s , B r They are the AC component amplitudes of the sampling signals of the signal hydrophone and the reference hydrophone respectively, t 0s , t 0r They are the initial sampling times of the sampling signals of the signal hydrophone and the reference hydrophone respectively, t p is the sampling time interval, is the phase difference between the two arms of the signal hydrophone, is the phase difference between the two arms of the reference hydrophone, is the measured signal phase introduced by the external measured acoustic signal in the sensing optical fiber of the signal hydrophone.

[0098] According to the sampling signal V s (k) of the signal hydrophone and the sampling signal V r (k) of the reference hydrophone can be simplified to:

[0099]

[0100]

[0101] For the signal hydrophone and the reference hydrophone, the initial sampling times t 0s , t 0r are determined values. For the sake of simplifying the discussion, is combined and denoted as the initial phase of the signal hydrophone. is combined and denoted as the initial phase of the reference hydrophone. s Then the sampling signal V r (k) of the signal hydrophone and the sampling signal V

[0102]

[0103]

[0104] Step S4, calculate the initial phase difference s between the signal hydrophone and the reference hydrophone according to the sampling signal V(k) of the signal hydrophone and the sampling signal V

[0105] In this embodiment, the step S4 specifically includes the following steps:

[0106] Step S41, construct two orthogonal demodulation signals u s (k) and u c(k), and successively multiply with the sampling signal V of the signal hydrophone s (k) and the sampling signal V of the reference hydrophone r (k), after taking the average value, respectively obtain the two-channel quadrature demodulation output values SR s 、CR s of the signal hydrophone and the two-channel quadrature demodulation output values SR r 、CR r of the reference hydrophone;

[0107] In this embodiment, the two-channel quadrature demodulation signals u s (k) and u r (k) in step S41 are respectively as follows:

[0108]

[0109]

[0110] The two-channel quadrature demodulation output values SR s 、CR s of the signal hydrophone in step S41 and the two-channel quadrature demodulation output values SR r 、CR r of the reference hydrophone are respectively as follows:

[0111]

[0112]

[0113] According to equations (26) and (27), it can be known that the two-channel quadrature demodulation output values SR s 、CR s of the signal hydrophone and the two-channel quadrature demodulation output values SR r 、CR r of the reference hydrophone can be expressed as:

[0114]

[0115]

[0116] It can be seen that the two-channel quadrature demodulation output values SR s 、CR s of the signal hydrophone and the two-channel quadrature demodulation output values SR r 、CR r of the reference hydrophone are related to the initial phase of the signal interferometer and the initial phase of the reference interferometer.

[0117] Step S42. Repeat step S41 within different sampling periods, and average the two-channel quadrature detection output values SR s and CR s of multiple signal hydrophones and the two-channel quadrature detection output values SR r and CR r of the reference hydrophone to respectively obtain the two-channel quadrature detection output averages of the signal hydrophone and

[0118] As can be seen from equations (28) and (29), the two-channel quadrature detection output values SR s and CR s of the signal hydrophone are also related to the measured phase signal of the signal hydrophone. Since the measured phase signal caused by the externally measured acoustic signal is an alternating signal, by averaging the two-channel quadrature detection output values SR s and CR s obtained from different sampling periods, the influence of can be eliminated. Therefore, the averages of the two-channel quadrature detection output values SR s and CR s of the signal hydrophone are obtained through the following processing:

[0119]

[0120] The same averaging process can also be performed on the two-channel quadrature detection output values SR r and CR r of the reference hydrophone:

[0121]

[0122] Step S43. Calculate the initial phase of the signal hydrophone and the initial phase of the reference hydrophone based on the two-channel quadrature detection output averages of the signal hydrophone and and calculate the initial phase difference between the signal hydrophone and the reference hydrophone according to the initial phase of the signal hydrophone and the initial phase

[0123] The initial phase of the signal hydrophone and the initial phase of the reference hydrophone in step S43 are respectively as follows:

[0124]

[0125]

[0126] Step S5: For each sampling period, according to the acquired sampling signal V r (k) of the reference hydrophone, respectively extract three-phase shift heterodyne signals V r1 (k), V r2 (k) and V r3 (k) of the reference hydrophone;

[0127] In this embodiment, the three-phase shift heterodyne signals V r1 (k), V r2 (k) and V r3 (k) of the reference hydrophone in step S5 are as follows:

[0128] V r1 (k) = V r (k) (7)

[0129]

[0130]

[0131] where k = 1, 2, …, N·M.

[0132] According to Equation (27), the three-phase shift heterodyne signals V r1 (k), V r2 (k) and V r3 (k) of the reference hydrophone can be expressed as:

[0133]

[0134]

[0135]

[0136] It can be seen from Equations (32) to (34) that there is a phase difference of π / 2 between the three signals V r1 (k), V r2 (k) and V r3 (k), and thus three-phase shift heterodyne signals are obtained.

[0137] It can be seen that the present invention ingeniously utilizes the interferometer phase term 2πf introduced by heterodyne frequency shift mThe variation relationship of t with time, introducing the interference phase difference through the sampling time difference, and adopting the whole-cycle sampling design, so that only the sampling point translation is needed to obtain three-phase shift heterodyne interference signals with specific initial phase differences. For example, if the design is 16-fold sampling, that is, N = 16, f p = 16f m , then the interferometer will generate a phase shift of π / 2 every 4 sampling points, and the interferometer will generate a phase shift of π every 8 sampling points. Therefore, we only need to place the starting sampling points of the sampling signals of the reference interferometer at positions separated by 4 sampling points and 8 sampling points respectively to obtain interference signals with phase shifts of π / 2 and π. Compared with the existing method of using a 3×3 coupler to obtain phase-shifted interference signals, the technical solution proposed by the present invention does not require adding any additional optoelectronic devices to the system, will not increase the complexity of the system, and does not require adding additional optical pulses to the time-division pulse sequence. The array loss and the system sampling frequency are not affected. In addition, the three-phase shift heterodyne interference signals are obtained from the same sampling signal, and they naturally have the same signal amplitude. There is no need to consider the power balance problem of the three signals, and no additional error will be introduced due to the uneven optical power of the three signals.

[0138] Step S6, using the three-phase shift heterodyne signals V r1 (k), V r2 (k) and Vr 3( k) of the reference hydrophone, and constructing the corrected sampling signal V′ of the reference hydrophone with the same initial phase as the signal hydrophone according to the initial phase difference r (k);

[0139] In this embodiment, the step S6 specifically includes the following steps:

[0140] Step S61, calculating the DC component amplitude A r ;

[0141] According to equations (32) to (34), the three-phase shift heterodyne signals V r1 (k), V r2 (k) and V r3 (k) of the reference hydrophone can be simplified as:

[0142]

[0143]

[0144]

[0145] From this, A r can be calculated as:

[0146]

[0147] Step S62. Remove the direct current from the three-way phase-shifted heterodyne signals V r1 (k), V r2 (k), and V r3 (k) to respectively obtain the three-way phase-shifted heterodyne signals u r1 (k), u r2 (k), and u r3 (k) after direct current removal:

[0148]

[0149]

[0150]

[0151] Step S63. Construct the corrected quadrature interference signals vs r (k) and vc r (k):

[0152]

[0153]

[0154] Step S64. Construct the corrected sampling signal V′ r (k) of the reference hydrophone with the same initial phase as the signal hydrophone:

[0155]

[0156] According to Eqs. (41) and (42), V′ r (k) can be simplified to:

[0157]

[0158] It can be seen that V′ r (k) and the sampling signal V s (k) of the signal hydrophone will have the same initial phase.

[0159] Step S7. Perform heterodyne phase demodulation calculations on the sampling signal V s (k) of the signal hydrophone and the constructed corrected sampling signal V′ r (k) of the reference hydrophone to obtain the demodulated phase signals of the signal hydrophone and the reference hydrophone and

[0160] In this embodiment, the specific steps of Step S7 include the following steps:

[0161] Step S71. Use the two-way quadrature detection signals us (k) and u c (k) is successively multiplied with the sampling signal V of the signal hydrophone s (n) and the corrected sampling signal V′ of the constructed reference hydrophone r (n), and the average value is taken to respectively obtain the quadrature demodulation output values SR s 、CR s of the signal hydrophone and the corrected quadrature demodulation output value SR′ r 、CR′ r ;

[0162]

[0163]

[0164] According to equations (26) and (43), it can be known that SR s 、CR s and SR′ r 、CR′ r can be expressed as:

[0165]

[0166]

[0167] Step S72: Calculate the demodulation phase signal of the signal hydrophone using the arctangent algorithm and the demodulation phase signal of the reference hydrophone

[0168]

[0169]

[0170] In the foregoing description, for the sake of simplicity of discussion, the noise conversion of the common-mode noise in the phase demodulation process is not considered. In an actual system, in addition to the signal phase and the initial phase, the demodulation phase signal also includes a phase noise term formed by the conversion of the common-mode noise. Therefore, in an actual system and can be expressed as:

[0171]

[0172]

[0173] Step S8: For each sampling period, process the sampling signal according to steps S5 - S6 to obtain the demodulation phase signals of the signal hydrophone and the reference hydrophone corresponding to each sampling period and After performing high-pass filtering on two signals and subtracting them, a demodulated phase output after common-mode noise suppression is obtained.

[0174] As can be seen from Equations (46) and (47), and The signal contains components such as an initial phase, a signal phase, and a noise phase. Among them, the initial phase is a low-frequency component with slow variation and can be removed by high-pass filtering. The noise phase and Since V′ r (k) and V s (k) have the same initial phase, and will have good correlation. Therefore, and After the signals are high-pass filtered and subtracted, a demodulated phase output after common-mode noise suppression can be obtained.

[0175] In this embodiment, the demodulated phase output after common-mode noise suppression in step S8 is specifically:

[0176]

[0177] where HF(·) represents high-pass filtering.

[0178] It should be noted that:

[0179] (1) In the technical solution of the present invention, two orthogonal interference signals of a reference hydrophone can be obtained. Using these two signals, a corrected interference signal with an arbitrary initial phase can be constructed, thereby achieving common-mode noise suppression for a signal hydrophone with an arbitrary initial phase. Therefore, although the example given in this specification shows that the fiber optic hydrophone array only contains one reference hydrophone and one signal hydrophone, applying this technical solution can actually use only one reference hydrophone to achieve common-mode noise suppression for multiple signal hydrophones, thus being applicable to a multiplexed large-scale fiber optic hydrophone array system.

[0180] (2) In the technical solution of the present invention, there is no limitation on the specific form of the matched interference type fiber optic hydrophone. Therefore, although the example given in this specification shows that the fiber optic hydrophone array adopts a structure of a matched interference type fiber optic hydrophone based on a single sensing arm, this technical solution is also applicable to a matched interference type fiber optic hydrophone array system based on an independent interferometer type.

[0181] (3) The technical solution of the present invention can not only be used in an interferometric fiber optic hydrophone system, but also be applicable to other types of fiber optic hydrophone systems that adopt a matching interference method, such as fiber Bragg grating hydrophones, distributed fiber optic hydrophones, and interferometric fiber optic sensing array systems for sensing other types of physical quantities.

[0182] As described above, the above is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention can easily think of changes or substitutions, which should be covered within the protection scope of the present invention. Moreover, the technical solutions between various embodiments of the present invention can be combined with each other, but it must be based on the fact that those skilled in the art can implement it. When the combination of technical solutions appears to be contradictory or unable to be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the protection scope required by the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims

1. A heterodyne demodulation method for common-mode noise suppression of an interferometric fiber optic hydrophone, characterized in that: It includes the following steps: Step S1: Build a heterodyne demodulation system. The heterodyne demodulation system includes a laser, an optical pulse modulator, a pulse pair generation module, an optical fiber circulator, a reference hydrophone, and a signal hydrophone that are optically connected in sequence. It also includes a photoelectric converter and a signal processor. One end of the photoelectric converter is optically connected to the optical fiber circulator, and the other end is electrically connected to the signal processor. The signal processor is electrically connected to the pulse pair generation module and the optical pulse modulator respectively; Step S2: The optical signal output by the laser is modulated by the optical pulse modulator to generate optical pulses. The optical pulses are input into the pulse pair generation module to form an optical pulse pair composed of two optical pulses; The optical pulses pass through the optical fiber circulator, the reference hydrophone, and the signal hydrophone in sequence, and return along the original path. The returned interference optical pulse sequence is output by the optical fiber circulator; Step S3: The returned interference optical pulse sequence is input into a photoelectric converter to obtain corresponding sampling signals; the sampling signals are input into a signal processor for de-time-division processing to obtain the sampling signal V of the signal hydrophone s (k) and the sampling signal V of the reference hydrophone r (k), where k is the sampling point number; Step S4. Calculate the initial phase difference between the signal hydrophone and the reference hydrophone according to the sampling signal V s (k) of the signal hydrophone and the sampling signal V r (k) of the reference hydrophone Step S5: For each sampling period, based on the acquired sampling signal V of the reference hydrophone r (k), extract the three-channel phase-shifted heterodyne signals V r1 (k), V r2 (k) and V r3 (k) of the reference hydrophone respectively; Step S6: Use the three-channel phase-shifted heterodyne signals V r1 (k), V r2 (k) and V r3 (k) of the reference hydrophone, and construct the corrected sampling signal V ' (k) of the reference hydrophone with the same initial phase as the signal hydrophone according to the initial phase difference r '; Step S7: Perform heterodyne phase demodulation calculations on the sampled signal V s (k) of the signal hydrophone and the corrected sampled signal V r ′(k) of the constructed reference hydrophone respectively to obtain the demodulated phase signals of the signal hydrophone and the reference hydrophone and Step S8: For each sampling period, process the sampling signal according to Steps S5 - S6 to obtain the demodulation phase signals of the signal hydrophone and the reference hydrophone corresponding to each sampling period. and After performing high - pass filtering on the two signals and subtracting them, obtain the demodulation phase output after common - mode noise suppression.

2. The heterodyne demodulation method according to claim 1, characterized in that The pulse pair generation module in step S1 includes a first optical fiber coupler, an acousto-optic frequency shifter, an optical fiber delay line, and a second optical fiber coupler. One end of the first optical fiber coupler is optically connected to the optical pulse modulator, and the other end is optically connected to the second optical fiber coupler through the optical fiber delay line and the acousto-optic frequency shifter respectively.

3. The heterodyne demodulation method according to claim 2, wherein Step S2 specifically includes the following steps: Step S21: After the optical signal output by the laser is input into the optical pulse modulator, optical pulses are output at a certain repetition frequency after passing through the optical pulse modulator; Step S22: The optical pulse is split by the first fiber coupler, and after the optical fiber delay of the optical fiber delay line or the acousto-optic frequency shift of the acousto-optic frequency shifter, it is combined by the second fiber coupler to generate an optical pulse pair composed of two optical pulses with a certain time delay, and there is an optical frequency difference f between the two optical pulses. m , control the length of the delay optical fiber so that the time delay between the two optical pulses is τ, τ = 2nL / c, where L is the arm difference of the signal hydrophone interferometer, n is the refractive index of the fiber core, and c is the speed of light in vacuum; Step S23: After the optical pulse pair passes through the optical fiber circulator, it is successively input into the reference hydrophone and the signal hydrophone, and returns after passing through the signal hydrophone and the reference hydrophone in sequence. The returned interference optical pulse sequence is output by the optical fiber circulator to obtain the returned interference optical pulse signal.

4. The heterodyne demodulation method according to claim 3, characterized in that The sampling frequency of the sampling signal in step S3 is f p = N·f m , and the sampling length of the sampling signal is N·M + N / 2, where N takes an integer value that is an integer multiple of 4, and M is the number of complete sampling periods.

5. The heterodyne demodulation method according to claim 3, characterized in that, Step S4 specifically includes the following steps: Step S41: Construct two orthogonal detection signals u s (k) and u c (k), and multiply them with the sampling signal V s (k) of the signal hydrophone and the sampling signal V r (k) of the reference hydrophone in sequence. After taking the average value, obtain the two-channel orthogonal detection output values SR s , CR s of the signal hydrophone and the two-channel orthogonal detection output values SR r , CR r of the reference hydrophone respectively; Step S42: Repeat step S41 within different sampling periods, and average the two-channel quadrature detection output values SR s , CR s of multiple signal hydrophones and the two-channel quadrature detection output values SR r , CR r of the reference hydrophone to respectively obtain the average two-channel quadrature detection outputs of the signal hydrophone and the average two-channel quadrature detection outputs Step S43: Calculate the initial phase of the signal hydrophone based on the average values of the two-channel quadrature demodulation outputs of the signal hydrophone and the average values of the two-channel quadrature demodulation outputs of the reference hydrophone Calculate the initial phase of the signal hydrophone and the initial phase of the reference hydrophone And based on the initial phase of the signal hydrophone and the initial phase of the reference hydrophone Calculate the initial phase difference between the signal hydrophone and the reference hydrophone 6. The heterodyne demodulation method according to claim 5, characterized in that, The two-channel quadrature detection signals u s (k) and u c (k) are respectively as follows: The two-channel quadrature detection output values SR of the signal hydrophone in step S41 s , CR s and the two-channel quadrature detection output values SR of the reference hydrophone r , CR r are respectively as follows: Initial phases of the signal hydrophone and the reference hydrophone in step S43 are as follows: respectively 7. The heterodyne demodulation method according to claim 5, characterized in that The three-channel phase-shifted heterodyne signals V r1 (k), V r2 (k) and V r3 (k) of the reference hydrophone in step S5 are as follows: V r1 (k) = V r (k) (7) 8. The heterodyne demodulation method according to claim 7, characterized in that Step S6 specifically includes the following steps: Step S61, calculate the amplitude A of the DC component r :[[]] Step S62: Remove the direct current from the three-way phase-shifted heterodyne signals V r1 (k), V r2 (k), and V r3 (k) to obtain the three-way phase-shifted heterodyne signals u r1 (k), u r2 (k), and u r3 (k) after removing the direct current, respectively: u r1 u(k) = V r1 -A r (11) u r2 k) = V r2 -A r (12) u r3 (k) = V r3 -A r (13) Step S63, construct the corrected orthogonal interference signals vs r (k) and vc r (k): vs r (k) = -u r2 (k) (14) Step S64, construct a corrected sampling signal V r ′(k) of a reference hydrophone having the same initial phase as the signal hydrophone:

9. The heterodyne demodulation method according to claim 8, wherein Step S7 specifically includes the following steps: Step S71: Use two orthogonal detection signals u s (k) and u c (k) to multiply with the sampling signal V s (k) of the signal hydrophone and the corrected sampling signal V r ′(n) of the reference hydrophone respectively, and take the average values to obtain the orthogonal detection output values SR s and CR s of the signal hydrophone, and the corrected orthogonal detection output values SR′ r and CR′ r of the reference hydrophone; Step S72: Calculate the demodulation phase signals of the signal hydrophone and the reference hydrophone using the arctangent algorithm and the demodulation phase signal of the reference hydrophone 10. The heterodyne demodulation method according to claim 9, wherein The demodulation phase output after common-mode noise suppression in step S8 Specifically: Wherein, HF(·) represents high-pass filtering.

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