A high-stability fiber-optic hydrophone array PGC demodulation system

Abstract

The present invention discloses a high-stability fiber optic hydrophone array PGC demodulation system, which belongs to the field of fiber optic hydrophones. The system includes: a signal sampling module for collecting interference light pulse signals returned by the array; an automatic delay calibration module for performing carrier phase calibration on the interference light pulse signals; a PGC demodulation module for performing mixing and filtering processing on the interference light pulse signals that have undergone carrier phase calibration to generate two orthogonal signals; a fast ellipse correction module for synthesizing the two orthogonal signals into a true ellipse, and then converting the true ellipse into a true circle to complete the carrier depth calibration of the interference light pulse signals; the PGC demodulation module is also used to continue the demodulation process to obtain a demodulated signal with high stability. The present invention supports large-scale fiber optic hydrophone arrays, has good demodulation result stability and consistency, can effectively suppress the impact of the environment on the array and signal demodulation system when the PGC demodulation algorithm is applied in engineering, and has high application and promotion value.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber hydrophones, and in particular to a high-stability optical fiber hydrophone array PGC demodulation system. Background Art

[0002] Fiber optic hydrophones have a series of advantages such as small size, light weight, resistance to electromagnetic interference, long transmission distance and high sensitivity. They have been widely used in ocean acoustic detection and have a wide range of military and civilian applications.

[0003] A fiber-optic hydrophone array system typically includes a laser, a phase modulator, an acousto-optic modulator, a fiber-optic amplifier transmitter, a fiber-optic hydrophone array, a fiber-optic amplifier receiver, a power equalizer, an optoelectronic converter, and a signal demodulator. The signal demodulator generates a periodic modulated pulse signal (TRIG) and a carrier signal (Fc). These signals act on the phase modulator and the acousto-optic modulator, respectively, to modulate the continuous light emitted by the laser into pulses of preset frequency, width, and period. The pulses are then amplified by the fiber-optic amplifier transmitter. After passing through the fiber-optic hydrophone array, they are returned as interference light pulses. The fiber-optic amplifier receiver amplifies the returned interference light pulses. The power equalizer adjusts the optical power of the amplified interference light pulses between different wavelengths in the array. The optoelectronic converter converts the adjusted interference light pulses into collectible interference light pulse signals (piezoelectric signals). These interference light pulse signals are then input into the signal demodulator for demodulation.

[0004] Currently, large-scale engineering applications of fiber optic hydrophone arrays all use an optical phase sensing mechanism. Therefore, it is necessary to recover the external signal sensed by the array from the optical phase. This process is called modulation and demodulation of the fiber optic hydrophone array signal.

[0005] Traditional phase demodulation technologies mainly include the 3×3 coupler method, the heterodyne method, and the Phase Generated Carrier (PGC). Among the three algorithms, the PGC algorithm is the most widely used.

[0006] The principle block diagram of the traditional PGC modulation and demodulation algorithm is as follows Figure 1As shown in the figure, I represents the interferometric optical pulse signal returned by the fiber optic hydrophone, cos(w0t) and cos(2w0t) represent the local mixing signals, w0 represents the carrier frequency, LPF (Low Pass Filter) represents the low-pass filter, DIFF (Differentiator) represents the differentiator, SUB (Subtracted) represents the subtractor, DIV (Divider) represents the divider, ∫ represents the integrator, HPF (High Hass Filter) represents the high-pass filter, ARTAN represents the inverse tangent, and Phase Unwarp represents the phase unwrapping and expansion. After mixing and low-passing, the two channels of data become two orthogonal signals. The PGC algorithm typically uses two methods to recover the phase information carried by these orthogonal signals: differential cross multiplication (DCM) and inverse tangent (ARTAN).

[0007] The traditional PGC demodulation algorithm operates as follows:

[0008] [1] The returned interference light pulse signal is photoelectrically converted and collected by ADC. The expression of the interference light pulse signal I is shown in formula (1):

[0009]

[0010] in is the phase shift caused by the carrier signal Fc, C is the carrier modulation amplitude (carrier depth), f0 is the carrier frequency, is the initial phase of the carrier, is the detection signal carried in the interference light pulse signal, A is the average light intensity, and B is related to the visibility of the interference light pulse signal.

[0011] [2] In order to extract the detection signal The carrier effect needs to be eliminated. The idea of ​​the PGC algorithm is to mix the interfering optical pulse signal I with the single-frequency cosine (w0t) and the double-frequency cosine (2w0t) and then low-pass filter, where w0 = 2πf0, to obtain a pair of mutually orthogonal cosine and sine terms (two orthogonal signals), as shown in Equations (2) and (3):

[0012]

[0013]

[0014] [3] Using the DCM method, equations (2) and (3) are subjected to differential cross-multiplication and subtraction, and then the integrated signal is shown in equation (4):

[0015]

[0016] In formula (4), J1(C) and J2(C) are the first-order and second-order Bessel functions of C value, respectively. and The initial phase The cosine function value of one and two times, B 2 It is related to the stability of light intensity, so if you want to obtain a highly stable demodulated signal (i.e. detection signal), it is necessary to ensure Stability, that is, eliminating C (carrier depth), (initial phase of the carrier) and the influence of the interference light intensity, otherwise the demodulated signal will be distorted.

[0017] [4] Using the ARTAN method, equations (2) and (3) are divided, and the processed signal is shown in equation (5):

[0018]

[0019] Same as formula (4), if we want to obtain stable and accurate Also need to ensure However, compared with formula (4), B in formula (5) 2 is eliminated, that is, the PGC-ARTAN solution can suppress the stability problem caused by light intensity fluctuations, so that the stability of the demodulated signal is only related to C (carrier depth) and (initial phase of the carrier), but due to the introduction of division, the whole process is more sensitive to the change of C value.

[0020] The initial carrier phase value can cause sampling deviations in the interfering optical pulse signal due to variations in the optical path delay of the fiber optic hydrophone array. Specifically, a shorter optical path delay can lead to signal advance, while a longer optical path delay can lead to signal delay. Both signal advances and delays caused by variations in the optical path delay can lead to data sampling deviations, causing variations in the initial carrier phase value, which can distort the PGC algorithm during subsequent frequency mixing. The carrier depth can also vary due to changes in the power and temperature of the phase modulator, as well as the path difference between the array elements in the fiber optic hydrophone. Increasing or decreasing the carrier depth can lead to increased harmonics, distorting the demodulation results to a certain degree. Therefore, in practical engineering applications, real-time monitoring and calibration of the initial carrier phase and carrier depth of the interfering optical pulse signal are necessary to achieve highly stable fiber optic hydrophone phase modulation and demodulation.

[0021] In order to solve these problems, in 2010, HE et al. [1] proposed an arc tangent differential self-intersection multiplication algorithm (PGC-Arctan-DSM). Compared with the traditional PGC-DCM and PGC-ARTAN algorithms, its stability is greatly improved, but its operation is more complicated and its real-time performance is poor. Especially when the number of array channels increases on a large scale and there are certain differences in the path difference of the hydrophone units, this solution cannot meet the actual engineering application. In 2015, ZHANG [2] [3] et al. proposed to improve the baseband mixing and add a DC filter to the system to eliminate the influence of the DC component of the signal on the demodulation result. Later, they used the reference compensation PGC demodulation algorithm. However, both are not suitable for the engineering application of hydrophone arrays. In 2018, SUN [4] et al. proposed an improved phase generation carrier demodulation algorithm. Through signal mixing operation, it can improve the influence of the DC component on the system within the small signal range. However, the demodulation effect still depends on the modulation depth C, which is not conducive to the modulation and demodulation of large-scale engineering hydrophone array signals.

[0022] In summary, in actual engineering applications, real-time monitoring and calibration of the parameters PHASE and C are required to obtain high-stability fiber optic hydrophone phase modulation and demodulation, but the existing technology lacks relevant solutions.

[0023] References

[0024] [1]HE J, WANG L, LI F, et al. An ameliorated phase generated carrierdemodulation algorithm with low harmonic distortion and high stability[J]. Journal of Lightwave Technology, 2010, 28(22):3258-3265;

[0025] [2]ZHANGAL, WANGY, GONG MJ, et al.An improved algorithm of PGCdemodulation methodbased on fundamental frequency mixing[J].Acta PhotonicaSinica, 2014, 43(2):0206003(in Chinese);

[0026] [3]ZHANG S, ZHANG L, PAN HG, et al. Eliminating light intensitydisturbance with reference compensation in interferometers[J]. IEEE Optoelectronic Technology, 2015, 27(6):1888–1891;

[0027] [4] SUN W, YU M, CHANG TY, et al. Research and improvement based in PGCdemodulation method[J]. Acta Photonica Sinica, 2018, 47(8):806004(in Chinese). Summary of the Invention

[0028] In view of the problem in the prior art that when a phase-generated carrier (PGC) demodulation algorithm is used for modulation and demodulation of optical signals from a fiber optic hydrophone array, during actual operation, the fiber optic hydrophone array and demodulation system may experience demodulation distortion due to changes in the C value and PHASE value caused by factors such as the external environment and its own components. The present invention aims to provide a highly stable fiber optic hydrophone array PGC demodulation system.

[0029] To achieve the above object, the technical solution of the present invention is:

[0030] A high-stability fiber optic hydrophone array PGC demodulation system, comprising:

[0031] A signal sampling module is used to collect the interference light pulse signal returned by the fiber optic hydrophone array at a preset sampling frequency;

[0032] An automatic delay calibration module is used to detect the characteristic light pulse in the interference light pulse signal and move the interference light pulse signal so that the detected characteristic light pulse is located at a preset sampling position, thereby completing the carrier phase calibration of the collected interference light pulse signal;

[0033] The PGC demodulation module is used to receive the interferometric optical pulse signal after carrier phase calibration and generate two orthogonal signals after frequency mixing and filtering.

[0034] A fast ellipse correction module is used to synthesize the two orthogonal signals generated by the PGC demodulation module into a perfect ellipse, and then convert the perfect ellipse into a perfect circle based on the proportional expansion of the major and minor axes, thereby completing the carrier depth calibration of the interfering optical pulse signal;

[0035] The PGC demodulation module is further used to receive two orthogonal signals after carrier depth calibration and continue to perform demodulation processing to obtain a demodulated signal with high stability.

[0036] In another preferred embodiment, a signal generation module is further included for synchronously generating a periodic modulation pulse signal TRIG and a periodic carrier signal Fc, so that the initial phases of the optical signal pulses contained in the interference optical pulse signal returned by the optical fiber hydrophone array are consistent, thereby enabling the PGC demodulation module to use signals of the same phase frequency to perform algorithmic mixing processing on all channel signals of the optical fiber hydrophone array.

[0037] In another preferred embodiment, the automatic delay calibration module implements carrier phase calibration of the collected interference light pulse signal through the following steps:

[0038] S1. Detecting the power of an interference light pulse signal and determining that a bottom pulse with zero power in the interference light pulse signal is the characteristic light pulse;

[0039] S2. Calculate and obtain a preset sampling position of the characteristic light pulse according to the sampling frequency, the frequency and period of the carrier signal Fc and the modulation pulse signal TRIG, and the physical length of the optical fiber hydrophone array;

[0040] S3. Move the collected interference light pulse signal so that the detected characteristic light pulse is located at a preset sampling position.

[0041] In another preferred embodiment, the bottom pulse is detected by the following steps:

[0042] S11, using a detection comb with a certain width to go through the entire sampling period of the interference light pulse signal from the sampling starting point;

[0043] S12, each time the detection comb slides over a sampling point, the sum of the data of all sampling points within the width of the detection comb is calculated;

[0044] S13. The sampling point corresponding to the minimum value of the sum of the data is the starting point of the bottom pulse.

[0045] In another preferred embodiment, the width of the detection comb is the same as the width of the characteristic light pulse, and the width of the characteristic light pulse is calculated based on the parameters obtained in step S2.

[0046] In another preferred embodiment, in S2, the sampling frequency of the signal sampling module is 120 MHz, the frequency of the carrier signal Fc is 4.8 MHz, the fiber optic hydrophone array includes 8 hydrophone units, the optical path between adjacent hydrophone units is 375 ns, and the modulation pulse TRIG period is 3125 ns; then the width of the characteristic light pulse is 15 sampling points.

[0047] In another preferred embodiment, the fast ellipse correction module implements carrier depth calibration of the interference light pulse signal by the following steps:

[0048] S10, acquiring two orthogonal signals, namely, an X-path signal and a Y-path signal, from the PGC demodulation module, wherein the two orthogonal signals are generated by the PGC demodulation module after performing mixing and filtering processing on the interference optical pulse signal after carrier phase calibration;

[0049] S20, combining the two orthogonal signals into a regular ellipse;

[0050] S30, extracting four eigenvalues ​​(Xmax, 0), (Xmin, 0), (0, Ymax), and (0, Ymin) from the regular ellipse, namely, the maximum and minimum values ​​of the X-channel signal when the Y-channel signal is 0, and the maximum and minimum values ​​of the Y-channel signal when the X-channel signal is 0;

[0051] S40, determining the major axis and minor axis of the perfect ellipse according to the four eigenvalues, and then converting the perfect ellipse into a perfect circle based on the scaling of the major and minor axes, thereby completing the carrier depth calibration of the interference light pulse signal.

[0052] By adopting the above technical solution, the beneficial effects of the present invention are as follows: the present invention introduces an automatic delay calibration module and a fast ellipse normalization module, wherein the automatic delay calibration module ensures that each hydrophone unit in the array maintains an accurate sampling position, thereby ensuring the stability of the carrier mixing, that is, ensuring the stability and consistency of the collected carrier phase; the fast ellipse correction module suppresses the ellipse of the orthogonal circle caused by the optical path difference and the change of the RF carrier modulation amplitude of each hydrophone unit in the fiber optic hydrophone array within a certain range, that is, it realizes the dynamic suppression of the C value change within a certain range, thereby ensuring the high stability of the demodulation output phase information. The present invention can support large-scale hydrophone arrays, has good demodulation result stability and consistency, high system operation reliability, effectively suppresses the impact of the environment on the array and signal demodulation system when the PGC demodulation algorithm is applied in engineering, and has high application and promotion value. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 This is the principle block diagram of the traditional PGC modulation and demodulation algorithm;

[0054] Figure 2Schematic diagram of the system structure of the present invention;

[0055] Figure 3 Schematic diagram of demodulation of the optical fiber hydrophone array of the present invention;

[0056] Figure 4 Schematic diagram of optical signal operation when using fiber optic hydrophone array;

[0057] Figure 5 Schematic diagram of the optical path structure of the weakly balanced fiber optic hydrophone unit;

[0058] Figure 6 Schematic diagram of sampling offset of the interference light pulse signal returned by the array when the optical path delay changes;

[0059] Figure 7 Schematic diagram of sampling offset of the interferometric optical pulse signal returned by the array when the carrier depth changes;

[0060] Figure 8 Schematic diagram of the modulated pulse signal TRIG (top), carrier signal Fc (middle), and interfering light pulse signal returned by the array (bottom);

[0061] Figure 9 Schematic diagram of the theoretical sampling position (top) and actual sampling position (bottom) of the interference light pulse signal returned by the fiber optic hydrophone array;

[0062] Figure 10 A schematic diagram of the automatic delay calibration module locating the characteristic light pulse in the actually sampled interference light pulse signal;

[0063] Figure 11 Another schematic diagram of the automatic delay calibration module locating the characteristic light pulse in the actually sampled interference light pulse signal;

[0064] Figure 12 This is a schematic diagram of the carrier phase calibration of the interference optical pulse signal actually returned by the 8-time-division optical fiber hydrophone array in this embodiment;

[0065] Figure 13 Schematic diagram of the orthogonal circle ellipticalization caused by the change of carrier depth of the two orthogonal signals generated by the PGC demodulation module;

[0066] Figure 14 For the fast ellipse correction module Figure 13 Schematic diagram after the ellipse in is corrected;

[0067] Figure 15 Another correction diagram for the correct ellipse by the fast ellipse correction module;

[0068] Figure 16Schematic diagram of the phase demodulation result output by the PGC demodulation module after applying a 1 kHz sinusoidal signal to the fiber optic hydrophone unit;

[0069] Figure 17 Schematic diagram of the result of carrier phase calibration of the array return interference light pulse signal when the modulation pulse signal TRIG is synchronized with the carrier signal Fc. DETAILED DESCRIPTION

[0070] The following is a further description of specific embodiments of the present invention in conjunction with the accompanying drawings. It should be noted that the description of these embodiments is intended to facilitate understanding of the present invention and does not constitute a limitation of the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0071] Example 1

[0072] A high stability fiber optic hydrophone array PGC demodulation system, such as Figure 2 As shown, the system is used to demodulate the interference light pulse signal returned by the fiber optic hydrophone array and obtain an array signal demodulation result with high stability, that is, a demodulated signal.

[0073] Among them, such as Figure 3As shown, when the fiber optic hydrophone array is used: the output port of the semiconductor laser is connected to the input port of the lithium niobate crystal through a polarization-maintaining fiber to perform optical phase modulation; the output port of the lithium niobate crystal is connected to the input port of the acousto-optic modulation crystal through a polarization-maintaining fiber to perform continuous light pulse modulation; the output port of the acousto-optic modulation crystal is connected to the input port of the fiber optic amplifier and transmitter through a non-polarization-maintaining fiber to perform pulsed light power amplification; the output port of the fiber optic amplifier and transmitter is connected to the input port of the fiber optic hydrophone array through a transmission fiber; the output port of the fiber optic hydrophone array is connected to the input port of the fiber optic amplifier and receiver through a transmission fiber to perform power amplification of the interference light pulses (including multiple light pulses, the number of which corresponds to the number of fiber optic hydrophone units) returned by the array; the output port of the fiber optic amplifier and receiver is connected to the input port of the power equalization device through an optical fiber to adjust the optical power of each light pulse in the interference light pulse train between different wavelengths; the output port of the power equalization device is connected to the input port of the optoelectronic conversion device to convert the interference light pulse train returned by the array into a collectible voltage signal, i.e., the interference light pulse signal; the output port of the optoelectronic conversion device is connected to the input port of the signal demodulation device to realize the collection and algorithm processing of the interference light pulse signal. The signal demodulation device is used to generate a periodic modulated pulse signal TRIG and an adjustable sinusoidal carrier signal Fc. The carrier signal Fc and the modulated pulse signal TRIG are connected to the RF driver and RF driver respectively via RF lines, thereby achieving phase modulation and acousto-optic modulation of the continuous light emitted by the semiconductor laser. The power equalization device is composed of a multi-channel programmable attenuator (VOA), and the signal demodulation device is composed of an FPGA board and corresponding peripheral devices.

[0074] The specific configuration is as follows:

[0075] [1] The semiconductor laser uses multiple wavelength semiconductor lasers in the 1550nm band;

[0076] [2] The phase modulator uses a lithium niobate modulator (EOM) crystal that uses longitudinal electro-optical phase modulation;

[0077] [3] The acousto-optic modulator uses an acousto-optic crystal (AOM) with a radio frequency carrier greater than 200 MHz;

[0078] [4] The optical amplification transmitting and receiving devices use 1550nm band EDFA fiber amplifiers;

[0079] [5] The fiber optic hydrophone array uses an 8-time hydrophone simulation array, and the array unit is a weakly balanced optical path structure;

[0080] [5] The optical fiber uplink and downlink transmission device uses an adjustable power balancing module, which includes two functions: optical power amplification and optical power balancing, and is used to adjust the optical power of the optical pulses returned by different array units;

[0081] [6] The photoelectric conversion device uses a low-noise, high-bandwidth photoelectric conversion (PINFET) module;

[0082] [7] The signal demodulation device uses a signal acquisition and processing circuit board based on an FPGA main control chip;

[0083] [8] The carrier signal Fc is generated by the carrier chip, which uses the AD9122 dual-channel DAC chip with a sampling rate of 1.2GHz;

[0084] [9] The RF driver device in the phase modulator uses a low-noise, high-amplification power amplifier.

[0085] like Figure 4 As shown, the continuous light (e.g. frequency f0) emitted by the semiconductor laser enters the phase modulator for optical phase modulation. The modulation frequency f c and modulation amplitude A c It is generated and controlled by the signal demodulation device; after the output light of the phase modulator passes through the acousto-optic modulator, the continuous light is modulated into a pulse light (frequency f0+f AOM ), where f AOM is the modulation frequency of the acousto-optic modulator, and the width and period of the pulse light are the same as the modulation pulse signal TRIG; the driving pulse signal of the acousto-optic modulator is generated and controlled by the signal demodulation device; the pulse light enters the fiber optic hydrophone array after optical power amplification, and the fiber optic hydrophone array is composed of multiple weakly balanced fiber optic hydrophone units. The interference light pulse signal finally returned by the fiber optic hydrophone array is as follows Figure 4 The optical path structure of the weakly balanced fiber optic hydrophone unit is shown in Figure 5 As shown in FIG, its operating principle is as follows: when a light pulse P0 of a certain width enters the incident port 1 of a 2×2 COUPLER (50:50 splitting ratio coupler), it is split into two light pulses P1 and P2 of equal amplitude by the coupler. The transmission path of light pulse P1 is: port 2→L1→FRM1→L1→2→4 (L1 is the reference arm fiber length), and the transmission path of light pulse P2 is: port 3→L2→FRM2→L2→3→4 (L2 is the sensing arm fiber length). The path difference between the two arms is ΔL=L2-L1, which is generally between 0.5m and 1m. When light pulses P1 and P2 return to coupler port 4, they overlap on the optical path, and the overlapping portion undergoes optical interference, generating an interference light pulse P3. After the returned interference light pulses enter the signal demodulation device through the photoelectric conversion device, they can be demodulated using the PGC algorithm to obtain the external signal detected by the fiber optic hydrophone array.

[0086] As described in the background technology, the core parameters of the PGC demodulation algorithm are carrier phase PHASE and modulation depth C. In actual operation, the fiber optic hydrophone array and demodulation system may change the C value and PHASE value due to external factors and its own components, which may cause demodulation distortion. The parameter PHASE value may cause sampling point deviation due to changes in optical path delay, thereby causing carrier mixing deviation. Figure 6 As shown in the figure, line 1 is the actual signal sampling position of the returned interference light pulse signal, line 2 is the interference light pulse signal advanced due to the shortening of the optical path delay, and line 3 is the interference light pulse signal delayed due to the lengthening of the optical path delay. The signal advance or delay caused by the delay change will lead to data sampling deviation, which in turn leads to changes in the PHASE value, causing the algorithm to be distorted during subsequent algorithm mixing. At the same time, parameter C will change due to changes in the power and temperature of the phase modulator, and will also change due to changes in the path difference of the array unit, as shown in the figure. Figure 7 As shown, line 1 represents the accurate carrier depth, line 2 indicates an increased carrier depth, and line 3 indicates a decreased carrier depth. Variations in carrier depth will increase harmonics and distort the demodulation results to a certain degree. Therefore, in practical engineering applications, real-time monitoring and calibration of the PHASE and C parameters are necessary to achieve highly stable fiber optic hydrophone phase modulation and demodulation.

[0087] This embodiment provides a high-stability fiber-optic hydrophone array PGC demodulation system capable of demodulating interferometric optical pulse signals returned by a fiber-optic hydrophone array and obtaining a highly consistent and stable array signal demodulation result, i.e., a demodulated signal. The system includes a signal sampling module, an automatic delay calibration module, a PGC demodulation module, and a fast ellipticity correction module.

[0088] Among them, the signal sampling module is used to collect the interference light pulse signal returned by the fiber optic hydrophone array at a preset sampling frequency; the automatic delay calibration module is used to detect the characteristic light pulse in the interference light pulse signal and move the collected interference light pulse signal so that the detected characteristic light pulse is located at the preset sampling position, thereby completing the carrier phase calibration of the collected interference light pulse signal; the PGC demodulation module is used to receive the interference light pulse signal that has undergone carrier phase calibration, and generate two orthogonal signals after mixing and filtering; the fast ellipse correction module is used to synthesize the two orthogonal signals generated by the PGC demodulation module into a true ellipse, and then convert the true ellipse into a true circle based on the proportional scaling of the major and minor axes, thereby completing the carrier depth calibration of the interference light pulse signal; on the other hand, the PGC demodulation module is also used to receive the two orthogonal signals after carrier depth calibration and continue to demodulate them to obtain a demodulated signal with high stability.

[0089] In this embodiment, the automatic delay calibration module implements carrier phase calibration of the collected interference light pulse signal through the following steps:

[0090] S1. Detecting the power of the interference light pulse signal and determining that the bottom pulse with zero power in the interference light pulse signal is a characteristic light pulse;

[0091] S2. Calculate the preset sampling position of the characteristic light pulse according to the sampling frequency, the frequency and period of the carrier signal Fc and the modulation pulse signal TRIG, and the physical length of the optical fiber hydrophone array;

[0092] S3. Move the collected interference light pulse signal so that the detected characteristic light pulse is located at a preset sampling position.

[0093] For example, the FPGA board in the signal demodulation device controls the AD9122 to generate a periodic non-continuous sinusoidal carrier signal Fc of a certain width and a frequency of 4.8MHz, and generates a modulated pulse signal TRIG with a frequency of 320kHz (i.e., a period of 3125ns). In addition, the sampling frequency of the signal sampling module is configured to be 120MHz, and the optical fiber hydrophone array includes 8 hydrophone units, and the optical path between adjacent hydrophone units is 375ns. Figure 8 As shown, the relationship between the modulation pulse signal TRIG (upper), the carrier signal Fc (middle) and the interference light pulse signal (lower) returned by the array is shown. Figure 8 The three types of signals in the are artificially biased.

[0094] The modulated pulse signal TRIG and the carrier signal Fc enter the acousto-optic modulator and phase modulator respectively through the RF cable, and each optical pulse output from the acousto-optic modulator carries the same initial phase. The carrier information of the periodic optical pulse is amplified by the EDFA fiber amplifier and then enters the fiber optic hydrophone array (8 hours). The array period is 3125ns, the internal optical path difference of each hydrophone is 0.7m, the optical path between hydrophone units is 375ns, the ADC sampling rate is 120MHz, and the carrier depth C of the actual returned interference optical pulse signal is approximately 2.5 to 3.0.

[0095] from Figure 8 It can be seen that the characteristic light pulse (i.e. bottom pulse) is obviously different from other light pulses. In the obtained sampling signal, we only need to find its position to know the position of the actual sampled interference light pulse signal, and then move it to complete the calibration operation. Figure 9As shown in the figure, the upper part is the accurate sampling position (calculated) set by the signal demodulation device under the given optical path delay. It can be seen that the interference light pulse signal should have no signal (bottom pulse) at the sampling points 360 to 375, which is obviously different from other signals, that is, the position of the characteristic pulse; Figure 9 The lower middle part is the actual sampling position. The starting position of the bottom pulse is located at the sampling point 85 on the X-axis. If it is not calibrated, the interference light pulse signal will produce deviations and errors. In actual application, the calibrated sampling point position will still cause the sampling point to shift due to environmental changes.

[0096] Specifically in S1, the bottom pulse (characteristic light pulse) is detected by the following steps:

[0097] S11, using a detection comb with a certain width to go through the entire sampling period of the interference light pulse signal from the sampling starting point;

[0098] S12, each time the detection comb slides over a sampling point, the sum of the data of all sampling points within the width of the detection comb is calculated;

[0099] S13. The sampling point corresponding to the minimum value of the sum of the data is the starting point of the bottom pulse.

[0100] Typically, the width of the detection comb is the same as the width of the characteristic light pulse, and the width of the characteristic light pulse is calculated based on the parameters obtained in step S2. For example, in this embodiment, the width of the characteristic light pulse is 15 sampling points.

[0101] like Figure 10 and Figure 11 As shown in the two figures, line 1 is the sampling data of the interference light pulse signal, and line 2 is the output result of the "automatic delay calibration module", corresponding to Figure 9 The upper and lower parts of the sample data. Figure 10 It can be seen that the starting point of the characteristic light pulse is located at 360 points on the X-axis. It can be seen that the automatic delay calibration module outputs a minimum point at this point (360 points on the X-axis), which shows that the module accurately finds the starting position of the characteristic light pulse; by changing the delay, the starting point of the characteristic light pulse is moved to the specified accurate sampling point, that is, at 360 points on the X-axis, to achieve subsequent accurate sampling and calculation, as shown in the figure. Figure 12 As shown. Similarly, Figure 11 In the figure, the starting point of the characteristic light pulse, that is, point 85 on the X-axis, is also accurately calibrated. At this time, it is only necessary to change the delay parameter in the signal demodulation device, such as shifting the samples right by 360-85=275 points, to realign and correct the offset data, thereby achieving subsequent accurate sampling and calculation.

[0102] In this embodiment, the fast ellipse correction module implements carrier depth calibration of the interference light pulse signal through the following steps:

[0103] S10, obtaining two orthogonal signals, namely, an X-path signal and a Y-path signal, from the PGC demodulation module. The two orthogonal signals are generated by the PGC demodulation module after mixing and filtering the interference optical pulse signal after carrier phase calibration;

[0104] S20, combining the two orthogonal signals into a regular ellipse;

[0105] S30. Extract four eigenvalues ​​(Xmax, 0), (Xmin, 0), (0, Ymax), and (0, Ymin) from the regular ellipse, i.e., the maximum and minimum values ​​of the X-channel signal when the Y-channel signal is 0, and the maximum and minimum values ​​of the Y-channel signal when the X-channel signal is 0;

[0106] S40, determining the major axis and minor axis of the right ellipse according to the four eigenvalues, and then converting the right ellipse into a perfect circle based on the scaling of the major and minor axes, thereby completing the carrier depth calibration of the interference light pulse signal.

[0107] The core idea of ​​the above-mentioned fast ellipse correction module is to obtain the values ​​of two orthogonal signals in real time and calculate the four eigenvalues ​​(Xmax, 0), (Xmin, 0), (0, Ymax), and (0, Ymin). Since the ellipses formed by the orthogonal signals are all true ellipses, the above-mentioned four values ​​represent the major axis and minor axis of the true ellipse. By scaling the major and minor axes proportionally, the conversion from ellipse to circle can be achieved. In contrast, in this embodiment, the PGC demodulation module Figure 7 The three cases of the carrier depth being unchanged, decreasing, and increasing (three lines) are processed separately, thus obtaining three types of two-way orthogonal signals. The fast ellipse correction module performs regular ellipse synthesis processing on the three types of two-way orthogonal signals, and the results are as follows: Figure 13 As shown, line 1 represents the calculated ellipse when the carrier depth remains unchanged, which is actually a perfect circle, with a carrier depth of C = 2.63; line 2 represents the calculated ellipse when the carrier depth decreases, with a carrier depth of C = 2.43; line 3 represents the calculated ellipse when the carrier depth increases, with a carrier depth of C = 2.83. This embodiment uses the major axis of the perfect ellipse as a reference to scale the perfect ellipse, that is, to achieve the conversion from an ellipse to a perfect circle, as shown in the following figure. Figure 14 The normalized circle is shown, where line 1, line 2, and line 3 are respectively Figure 13 In the figure, it can be seen that the elliptical shape of the circle caused by the change of the carrier depth C value has been corrected; of course, the elliptical expansion correction can also be performed based on the minor axis, such as Figure 15 shown.

[0108] In actual testing, a 1kHz sinusoidal signal is applied to the fiber optic hydrophone unit with a sampling frequency of 5kHz. After demodulation by the system provided in the embodiment of the present application, the phase demodulation result is as follows: Figure 16 As shown, the results are accurate and stable. At the same time, artificially changing the carrier Fc amplitude causes the carrier depth C value to change. In addition, artificially adding a section of optical fiber in the optical path causes a large offset of the sampling point. From the demodulation results, it can be seen that the phase output is stable and accurate. In actual engineering applications, this solution can be operated without changing the existing optical path connection structure or the assistance of any optoelectronic instruments. Through the precise coordination of the phase modulator, acousto-optic modulator, and FPGA demodulation board, high-stability phase modulation and demodulation of all hydrophone unit channels can be completed without adding optoelectronic devices, changing the optical power distribution in the existing optical path, or adjusting any optical path parameters in the existing optical path system.

[0109] Example 2

[0110] This embodiment also includes a signal generation module for synchronously generating a periodic modulation pulse signal TRIG and a periodic carrier signal Fc. Synchronization means that when one signal is generated, the other signal is also generated at the same time, so that the initial phases of the optical signal pulses contained in the interference optical pulse signal returned by the fiber optic hydrophone array are consistent, thereby enabling the PGC demodulation module to use signals of the same phase frequency to perform algorithmic mixing processing on all channel signals of the fiber optic hydrophone array, greatly reducing the complexity of signal processing.

[0111] like Figure 17 As shown in the figure, the triangles indicate the optical pulses returned by the eight hydrophone units in the fiber optic hydrophone array, and it can be seen that their initial phases are consistent.

[0112] The embodiments of the present invention are described in detail above with reference to the accompanying drawings, but the present invention is not limited to the described embodiments. It is apparent to those skilled in the art that various changes, modifications, substitutions, and variations to these embodiments may be made without departing from the principles and spirit of the present invention, and these changes and modifications still fall within the scope of protection of the present invention.

Claims

1. A high-stability fiber optic hydrophone array PGC demodulation system, characterized by: include: A signal sampling module is used to collect the interference light pulse signal returned by the fiber optic hydrophone array at a preset sampling frequency; An automatic delay calibration module is used to detect the characteristic light pulse in the interference light pulse signal and move the interference light pulse signal so that the detected characteristic light pulse is located at a preset sampling position, thereby completing the carrier phase calibration of the collected interference light pulse signal; The PGC demodulation module is used to receive the interferometric optical pulse signal after carrier phase calibration and generate two orthogonal signals after frequency mixing and filtering. A fast ellipse correction module is used to synthesize the two orthogonal signals generated by the PGC demodulation module into a perfect ellipse, and then convert the perfect ellipse into a perfect circle based on the proportional expansion of the major and minor axes, thereby completing the carrier depth calibration of the interfering optical pulse signal; The PGC demodulation module is further used to receive two orthogonal signals after carrier depth calibration and continue demodulation processing to obtain a demodulated signal with high stability; A signal generation module is used to synchronously generate a periodic modulation pulse signal TRIG and a periodic carrier signal Fc to ensure that the initial phases of the optical signal pulses contained in the interferometric optical pulse signal returned by the optical fiber hydrophone array are consistent, thereby enabling the PGC demodulation module to perform algorithmic mixing processing on all channel signals of the optical fiber hydrophone array using signals of the same phase frequency; The fast ellipse correction module implements carrier depth calibration of the interference light pulse signal through the following steps: S10, acquiring two orthogonal signals, namely, an X-path signal and a Y-path signal, from the PGC demodulation module, wherein the two orthogonal signals are generated by the PGC demodulation module after performing mixing and filtering processing on the interference optical pulse signal after carrier phase calibration; S20, combining the two orthogonal signals into a regular ellipse; S30, extracting four eigenvalues ​​(Xmax, 0), (Xmin, 0), (0, Ymax), and (0, Ymin) from the regular ellipse, namely, the maximum and minimum values ​​of the X-channel signal when the Y-channel signal is 0, and the maximum and minimum values ​​of the Y-channel signal when the X-channel signal is 0; S40, determining the major axis and minor axis of the perfect ellipse according to the four eigenvalues, and then converting the perfect ellipse into a perfect circle based on the scaling of the major and minor axes, thereby completing the carrier depth calibration of the interference light pulse signal.

2. The PGC demodulation system according to claim 1, wherein: The automatic delay calibration module implements carrier phase calibration of the collected interference light pulse signal through the following steps: S1. Detecting the power of an interference light pulse signal and determining that a bottom pulse with zero power in the interference light pulse signal is the characteristic light pulse; S2. Calculate and obtain a preset sampling position of the characteristic light pulse according to the sampling frequency, the frequency and period of the carrier signal Fc and the modulation pulse signal TRIG, and the physical length of the optical fiber hydrophone array; S3. Move the collected interference light pulse signal so that the detected characteristic light pulse is located at a preset sampling position.

3. The PGC demodulation system according to claim 2, wherein: The bottom pulse is detected by the following steps: S11, using a detection comb with a certain width to go through the entire sampling period of the interference light pulse signal from the sampling starting point; S12, each time the detection comb slides over a sampling point, the sum of the data of all sampling points within the width of the detection comb is calculated; S13. The sampling point corresponding to the minimum value of the sum of the data is the starting point of the bottom pulse.

4. The PGC demodulation system according to claim 3, wherein: The width of the detection comb is the same as the width of the characteristic light pulse, and the width of the characteristic light pulse is calculated based on the parameters obtained in step S2.

5. The PGC demodulation system according to claim 4, wherein: In S2, the sampling frequency of the signal sampling module is 120 MHz, the frequency of the carrier signal Fc is 4.8 MHz, the optical fiber hydrophone array includes 8 hydrophone units, the optical path between adjacent hydrophone units is 375 ns, and the modulation pulse TRIG period is 3125 ns; then the width of the characteristic light pulse is 15 sampling points.

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