Weak signal demodulation method for 3*3 coupler optical fiber hydrophone

Abstract

The invention relates to the technical field of underwater acoustic measurement, in particular to a 3 * 3 coupler optical fiber hydrophone weak signal demodulation method, which comprises the following steps: S1, preparation work and equipment positioning; s2, generating and adjusting a signal; s3, signal acquisition and storage; s4, performing ellipse fitting and parameter estimation; s5, signal demodulation and analysis; and S6, calibrating the phase shift sensitivity. According to the method, weak signal demodulation of the optical fiber hydrophone is realized by using an ellipse fitting parameter estimation method based on the Sampson distance, an experimental result shows that the weak signal calibration phase shift sensitivity is basically consistent with a strong signal calibration result, the effectiveness and accuracy of the method are verified, and the method is suitable for popularization and application. And the detection and identification capability of the optical fiber hydrophone on an underwater weak signal target can be further improved.

Description

Technical Field

[0001] The invention relates to the technical field of underwater acoustic measurement, and in particular to a 3×3 coupler optical fiber hydrophone weak signal demodulation method. Background Art

[0002] Fiber optic hydrophone is a new type of underwater acoustic sensor based on optoelectronic technology and fiber optic technology. Compared with traditional hydrophones, fiber optic hydrophones have many advantages, including high detection sensitivity, strong resistance to electromagnetic interference and signal crosstalk, large dynamic range, can be used for large-scale array multiplexing and long-distance monitoring, small size, light weight, etc. Therefore, fiber optic hydrophones are widely used in marine target detection, resource exploration, earthquake monitoring and other fields. In military confrontation, such as anti-submarine, underwater security, underwater weapon testing, etc., fiber optic hydrophones also play a vital role.

[0003] At present, the two main demodulation algorithms widely used in interferometric fiber optic hydrophones are the phase generated carrier algorithm and the 3×3 coupler algorithm. With the increasing demand for the application of fiber optic hydrophone technology, the demodulation results of the PGC algorithm are easily affected by factors such as light intensity disturbance, modulation depth, and associated amplitude modulation. At the same time, the demodulation dynamic range is also limited by the carrier frequency, which is becoming increasingly prominent. Compared with the PGC algorithm, the 3×3 coupler algorithm does not require carrier modulation, has a relatively simple principle, has a larger signal detection range, and has less impact when facing environmental noise. At the same time, the phase of the interferometer output signal of the demodulation system based on the 3×3 coupler has a specific relationship, and the demodulation process includes sine and cosine terms, which can avoid the occurrence of signal blanking. Therefore, the 3×3 coupler method is gradually receiving more attention and attention.

[0004] The 3×3 coupler method phase shift signal demodulation usually uses ellipse fitting based on the least squares method to achieve parameter estimation to eliminate the demodulation result deviation problem caused by the non-ideal 3×3 coupler. However, the least squares method has a high curvature deviation problem. When the fiber optic hydrophone receives a weak signal and the phase shift generated by the interferometer is small, the Lissajous figure ellipse formed is incomplete, resulting in a large deviation in the output phase shift demodulation result. Therefore, how to achieve weak signal demodulation of the fiber optic hydrophone and improve the detection and identification ability of the fiber optic hydrophone for underwater weak signal targets is the focus of research. Summary of the invention

[0005] In view of the above-mentioned shortcomings of the prior art, the first object of the present invention is to provide a weak signal demodulation method for a 3×3 coupler fiber optic hydrophone, which uses an ellipse fitting parameter estimation based on Sampson distance to realize weak signal demodulation of the 3×3 coupler fiber optic hydrophone and complete phase shift sensitivity calibration.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A 3×3 coupler optical fiber hydrophone weak signal demodulation method comprises the following steps:

[0008] S1. Preparation and equipment positioning;

[0009] S2, signal generation and conditioning;

[0010] S3, signal acquisition and storage;

[0011] S4, ellipse fitting and parameter estimation;

[0012] S5, signal demodulation and analysis;

[0013] S6. Phase shift sensitivity calibration.

[0014] In the step S1, preparation and equipment positioning:

[0015] S1.1. Install the acoustic transducer into the water through a precise three-dimensional walking mechanism to ensure that its position is stable and aligned with the area to be tested. The selection of the transducer needs to have a suitable operating frequency range. Usually, a spherical transducer made of PZT material with high conversion efficiency is selected. Its frequency range is usually 1kHz-60kHz to cover the signal frequency required for the experiment. The fixation of the transducer needs to ensure that there will be no relative movement in the water. It can be adjusted using a precise adjustment platform or a floating bracket in the water;

[0016] S1.2. The fiber optic hydrophone is installed at a position opposite to the acoustic transducer, usually in the water body in the same direction as the sound wave propagation, to ensure that it receives the sound wave signal emitted by the transducer. The fiber optic hydrophone is firmly installed by a fixed bracket, and its position should be adjustable;

[0017] S1.3. Ensure that the transducer and the fiber optic hydrophone are in the best relative position through the precision adjustment of the three-dimensional walking mechanism control. The control system of the three-dimensional walking mechanism needs to be equipped with a high-precision servo drive and feedback control system to ensure that its position can be fine-tuned. The adjustment process not only needs to consider the distance between the transducer and the hydrophone, but also needs to ensure that the relative angle and incident path between the two have the least impact on signal acquisition. Combined with the laser alignment system, it can assist in determining the best distance and angle between the transducer and the hydrophone.

[0018] In the step S2, signal generation and adjustment:

[0019] S2.1. Use a signal generator to generate an accurate sine wave signal. The frequency of the signal should be set according to the experimental requirements. Usually, the frequency range is between 1kHz and 60kHz. The signal generator should have a high-precision frequency control function to ensure that the output signal is not affected by environmental factors such as temperature and humidity. The signal generator needs to be equipped with a phase locking function.

[0020] S2.2. The power amplifier should be suitable for the output signal range of the signal generator. Usually, a wide-band, low-distortion power amplifier is used to ensure that too much nonlinear distortion is not introduced during signal transmission. The output power of the power amplifier needs to be adjusted according to the requirements of the transducer, usually in the range of 1W to 10W, to ensure that the transducer can generate sufficient vibration response;

[0021] S2.3. The signal amplitude of the fiber optic hydrophone needs to be adjusted to an appropriate range, which is usually achieved by adjusting the power of the light source or the coupling efficiency of the fiber optic access end. The output optical signal amplitude of the fiber optic hydrophone needs to be maintained within a certain range, usually based on the linear response of the interference signal amplitude, to ensure that the signal strength is neither too large to cause saturation nor too small to cause excessive noise interference. During adjustment, a light intensity meter can be used to monitor the intensity of the output optical signal;

[0022] S2.4. After the output signal of the fiber optic hydrophone is converted by the photoelectric detector, the amplitude of the optical signal needs to be further adjusted to ensure that the amplitude of the interference signal after the photoelectric conversion remains consistent and to ensure that its signal-to-noise ratio is constant.

[0023] In the step S3, signal acquisition and storage:

[0024] S3.1. Connect the oscilloscope to the output end of the photodetector to ensure that the signal can be transmitted to the oscilloscope in real time. The oscilloscope needs to have sufficient bandwidth and sampling rate. Usually, an oscilloscope with a bandwidth greater than 5GHz is selected. When connecting, ensure the shielding performance of the signal line to avoid external electromagnetic interference affecting the signal quality.

[0025] S3.2. The oscilloscope collects the interference signal output by the fiber optic hydrophone in real time and displays its waveform. To ensure the accuracy of signal acquisition, the sampling rate of the oscilloscope should be high enough, at least 10 times the signal frequency. During the signal acquisition process, the trigger function needs to be used to ensure the consistency of the waveform collected each time and avoid trigger jitter or waveform distortion.

[0026] S3.3. Store the collected signals in the computer. During the collection process, regularly check the signals displayed on the oscilloscope to ensure that the collected data meets the expected standards. When checking, pay attention to the waveform shape, frequency stability and signal noise level of the signal. If the signal fluctuates abnormally or interferes, adjust the experimental equipment in time or re-collect the data.

[0027] In the step S4, ellipse fitting and parameter estimation:

[0028] S4.1. Draw a Lissajous ellipse diagram based on the two-way interference signals output by the fiber optic hydrophone, and fit the Lissajous ellipse diagram using an ellipse fitting method based on Sampson distance;

[0029] S4.2. Based on the fitting results, the DC term, AC term and phase of the interference signal are calculated, and the sine term and cosine term are calculated by eliminating the DC term, AC term and phase of the interference signal.

[0030] In the step S5, signal demodulation and analysis:

[0031] S5.1. Demodulate the phase-shifted signal from the two interference signals using the differential cross-multiplication method.

[0032] S5.2, extracting the frequency and amplitude information of the phase-shifted signal through the demodulated signal can be achieved by fast Fourier transform;

[0033] In the step S6, phase shift sensitivity calibration:

[0034] S6.1. Replace the fiber optic hydrophone with a sound-transmitting reflective film, and measure the film vibration velocity using a laser vibrometer under the same conditions to obtain the sound pressure value;

[0035] S6.2. Calculate the phase shift sensitivity of the fiber-optic hydrophone using the phase shift obtained by demodulation of the fiber-optic hydrophone and the sound pressure measured by laser vibrometer.

[0036] The present invention is further configured as follows: in the step S1, the preparation work and the equipment positioning, the transmitting transducer and the optical fiber hydrophone are fixed on the Z axis of the three-dimensional walking mechanism, the Z axis position is adjusted so that the two are located at the same depth, and it is ensured that there is enough distance between the two to meet the far field condition, that is, the distance d between the two is required to meet:

[0037]

[0038] d>5D1 and d>5D2

[0039] Where λ is the wavelength of the sound wave, D1 and D2 are the diameters of the transducer and the fiber optic hydrophone, respectively, and K f =1.3;

[0040] The far-field condition means that the distance between the hydrophone and the transducer is far enough so that the sound wave can be regarded as a plane wave.

[0041] The present invention is further configured as follows: in the step S3, signal acquisition and storage, the signal generator outputs a sinusoidal pulse signal, which is driven by a power amplifier to generate a sound wave through a transmitting transducer, and the amplitude of the optical signal is adjusted so that the amplitudes of the three-way interferometer output signals after conversion by the optical fiber hydrophone through the photoelectric detector are basically consistent, so as to ensure that the signal-to-noise ratio is basically consistent, and an oscilloscope is used to acquire and save the three-way output signals.

[0042] The present invention is further configured as follows: in the step S4, ellipse fitting and parameter estimation, the general ellipse equation can be expressed as:

[0043]

[0044] remember:

[0045] a=[a,b,c,d,e,f] T , D(X)=[x 2 ,xy,y 2 ,x,y,1],

[0046]

[0047] The above formula can be expressed as:

[0048]

[0049] The cost function based on the maximum likelihood estimation of Sampson distance can be defined as:

[0050]

[0051] Among them: A n =D(X n ) T D(X n ); α is a small positive number used to weigh the contribution of the constraint term; ||a|| is the Euclidean norm of a.

[0052] The above formula can be expressed in the form of least squares:

[0053] J Sampson =h(a)*h(a) T =||h(a)|| 2

[0054] Where: h(a) = [h1(a); h2(a); ...; h N (a); h N+1 (a)]=[h N (a); h N+1 (a)],

[0055] The optimal ellipse parameter estimate a = [a, b, c, d, e, f] can be obtained by performing multiple iterations of optimization to solve the minimum value of the above formula. T In each iteration, the parameter update rule of the nonlinear optimization algorithm is:

[0056]

[0057] Where: η is the update amount; J is the Jacobian matrix of h(a); I is the 6×6 unit matrix; the initial value a can be obtained by least squares fitting; λ is the adjustment factor.

[0058] The present invention is further configured as follows: in the step S5, signal demodulation and analysis, the collected three-way output interference signal of the optical fiber hydrophone 3×3 coupler can be expressed as:

[0059]

[0060] Among them, A1, A2, A3 are the DC components of the output signal; B1, B2, B3 are the AC components of the output signal; is the signal to be measured; φ1 and φ2 are the initial phase differences of the coupler output signals.

[0061] Select any two of the three signals collected for demodulation. The phase difference between any two signals is about 120°, which can be expressed as:

[0062]

[0063] Expand to get:

[0064]

[0065] Simplifying, we get:

[0066]

[0067] Signals S1 and S2 are combined to obtain:

[0068]

[0069] The above formula is the expression of the ellipse equation, which can be expressed as follows by analogy with the general ellipse equation:

[0070]

[0071] The Lissajous ellipse formed by the two signals is fitted by the ellipse fitting method based on Sampson distance to estimate the ellipse parameters a~e, and A1, A2, B1, B2,

[0072]

[0073]

[0074] Given A1, A2, B1, B2, Calculation can be obtained

[0075] The signal to be measured can be demodulated by cross-multiplying and integrating the differentials.

[0076]

[0077] Beneficial Effects

[0078] Compared with the known public technology, the technical solution provided by the present invention has the following advantages:

[0079] Beneficial effects:

[0080] The present invention realizes weak signal demodulation of fiber optic hydrophone by using an ellipse fitting parameter estimation method based on Sampson distance. Experimental results show that the weak signal calibration phase shift sensitivity is basically consistent with the strong signal calibration result, which verifies the effectiveness and accuracy of the method of the present invention and helps to further improve the detection and identification capabilities of fiber optic hydrophone for underwater weak signal targets. BRIEF DESCRIPTION OF THE DRAWINGS

[0081] Figure 1 A schematic diagram of a fiber optic hydrophone measurement system for a 3×3 coupler fiber optic hydrophone weak signal demodulation method of the present invention;

[0082] Figure 2 A schematic diagram of a three-way output interference signal of a fiber optic hydrophone of a 3×3 coupler fiber optic hydrophone weak signal demodulation method of the present invention;

[0083] Figure 3 A schematic diagram of a Lissajous ellipse formed by 250mV excitation in a 3×3 coupler optical fiber hydrophone weak signal demodulation method of the present invention;

[0084] Figure 4 A schematic diagram of a Lissajous ellipse formed by 40mV excitation of a 3×3 coupler optical fiber hydrophone weak signal demodulation method of the present invention;

[0085] Figure 5 It is a schematic diagram of 250mV demodulation signal amplitude of a 3×3 coupler optical fiber hydrophone weak signal demodulation method of the present invention;

[0086] Figure 6 It is a schematic diagram of 40mV demodulation signal amplitude of a 3×3 coupler optical fiber hydrophone weak signal demodulation method of the present invention. DETAILED DESCRIPTION

[0087] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below.

[0088] The present invention will be further described below in conjunction with the embodiments.

[0089] Example 1

[0090] like Figure 1-Figure 6 As shown, the present invention provides a technical solution: a 3×3 coupler optical fiber hydrophone weak signal demodulation method. In this embodiment, a 3m×2m×2m anechoic water tank is used to perform a demodulation experiment:

[0091] First, the transmitting transducer and the optical fiber hydrophone are placed at a suitable position in the water through a three-dimensional walking mechanism. The two are located at the same depth and the horizontal distance is 0.6m, which meets the far-field condition. A 15kHz sinusoidal pulse signal is generated by a signal generator with a duration of 300μs. The peak-to-peak values ​​of the signal are 250mV and 40mV respectively. The signal drives the transducer through a power amplifier. The schematic diagram is shown in the figure. Figure 1 The interference signal output by the fiber optic hydrophone is collected using an oscilloscope, and three interference signals are collected as shown in Figure 2 As shown, the Lissajous diagrams formed by the two interference signals when the excitation is 250mV and 40mV are shown respectively as follows Figure 3 , Figure 4 As shown, the ellipse fitting parameters are estimated by the least square method and the ellipse fitting method based on Sampson distance respectively.

[0092] When the excitation is 250mV, the demodulation results of the two methods are basically the same. The demodulation result of the least square method is 3.8992rad, and the demodulation result based on the Sampson distance is 3.8983rad. Figure 5 The fiber optic hydrophone was replaced with a sound-transmitting reflective film, and the film vibration velocity was measured by a laser vibrometer under the same conditions. The sound pressure value obtained was 76.9296Pa, and the calibration sensitivity was -145.9dB.

[0093] When the excitation is 40mV, the Lissajous ellipse diagram formed by the interference signal output by the fiber optic hydrophone is incomplete, and the least squares method cannot accurately fit the estimated parameters. The demodulation result of the least squares method is 0.7880rad, and the demodulation result based on the Sampson distance is 0.6201rad. Figure 6 Under the same conditions, the vibration velocity of the film was measured by a laser vibrometer, and the sound pressure value obtained was 11.5901Pa. The least squares calibration sensitivity was -143.5dB, which was 2.4dB different from the strong signal calibration result. The calibration sensitivity based on the Sampson distance was -145.4dB, with a deviation of only 0.5dB.

[0094] The present invention realizes weak signal demodulation of fiber optic hydrophone by using an ellipse fitting parameter estimation method based on Sampson distance. Experimental results show that the weak signal calibration phase shift sensitivity is consistent with the strong signal calibration result, which verifies the effectiveness and accuracy of the method of the present invention and helps to further improve the detection and identification capabilities of fiber optic hydrophone for underwater weak signal targets.

[0095] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. Such modifications or replacements will not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A 3×3 coupler optical fiber hydrophone weak signal demodulation method, characterized in that: The transmitting transducer and the optical fiber hydrophone are placed on the same horizontal plane through a three-dimensional walking mechanism; the signal generating device includes a signal generator and a power amplifier, the signal generator generates a pulsed sine signal, which is amplified by the power amplifier and loaded to the transmitting transducer to transmit a pulsed sine sound wave; the optical fiber hydrophone outputs two interference signals which are collected by an oscilloscope and saved in a computer; the collected interference signal is estimated by ellipse fitting parameters based on Sampson distance to realize weak signal demodulation of the 3×3 coupler optical fiber hydrophone; and the underwater sound pressure is measured by laser vibrometer to calibrate the phase shift sensitivity of the optical fiber hydrophone; The specific steps are: S1. Preparation and equipment positioning: Install the acoustic transducer and the fiber optic hydrophone into the anechoic tank through the three-dimensional walking mechanism to ensure that their positions are stable and the sound centers of the two are located in the same horizontal plane; S2, signal generation and adjustment, connecting the transmitting transducer to the signal generating device, setting the voltage and frequency of the signal generator, and adjusting the power amplifier amplification factor to change the power; S3, signal acquisition and storage, connecting the output end of the optical fiber hydrophone to the photoelectric detector, connecting the interference signal after photoelectric conversion to an oscilloscope, and acquiring and storing the interference signal through the oscilloscope; S4, ellipse fitting and parameter estimation, ellipse fitting parameter estimation based on Sampson distance is performed on the Lissajous figure formed by the interference signal; S5, signal demodulation and analysis, obtain the sine and cosine terms through two interference signals and cross-differentiate and multiply them to realize the demodulation of the fiber optic hydrophone vibration signal, S6. Phase shift sensitivity calibration: The phase shift sensitivity of the fiber optic hydrophone is calculated using the phase shift obtained by demodulation of the fiber optic hydrophone and the sound pressure measured by laser vibrometer.

2. A 3×3 coupler optical fiber hydrophone weak signal demodulation method according to claim 1, characterized in that: In the step S4, ellipse fitting and parameter estimation, the general ellipse equation can be expressed as: remember: a=[a,b,c,d,e,f] T ,D(X)=[x 2 ,xy,y 2 ,x,y,1], The above formula can be expressed as: The cost function of the ellipse fitting parameter estimation method based on Sampson distance can be defined as: Among them: A n =D(X n ) T D(X n ); α is a small positive number used to weigh the contribution of the constraint term; ||a|| is the Euclidean norm of a, The above formula can be expressed in the form of least squares: J Sampson =h(a)*h(a) T =||h(a)|| 2 where: h(a) = [h1(a); h2(a); …; h N (a); h N+1 (a)] = [h n (a); h N+1 (a)], n = 1, 2, …, N, The optimal ellipse parameter estimate a = [a, b, c, d, e, f] can be obtained by performing multiple iterations of optimization to solve the minimum value of the above formula. T , in each iteration, the parameter update rule of the nonlinear optimization algorithm is: Where: η is the update amount; a is the Jacobian matrix of h(a); I is the 6×6 unit matrix; the initial value a can be obtained by least squares fitting; λ is the adjustment factor.

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