A sound pressure sensitivity calibration device for distributed hydrophone optical cable based on compensation cavity

Abstract

The present invention belongs to the field of fiber optic hydrophone design, and specifically provides a sound pressure sensitivity calibration device for a distributed hydrophone optical cable based on a compensation cavity. The sound pressure around the optical cable is calculated by a compensation method, the sensing phase difference is obtained according to the distributed optical fiber sensor, and the sensitivity is calculated by a formula. The device can arbitrarily select calibration points. Since the sound pressure in the compensation cavity is stable and uniform, the calibrated sound pressure sensitivity has low uncertainty and high accuracy. At the same time, the device controls the movement of the optical cable through a roller machine, and can realize continuous calibration of the optical cable to be tested. The device can be directly calibrated without the need for a standard hydrophone as a reference, and can realize the automation of the calibration process.

Description

Technical Field

[0001] The invention belongs to the field of optical fiber hydrophone, and in particular relates to a sound pressure sensitivity calibration device of a distributed hydrophone optical cable based on a compensation cavity. Background Art

[0002] Distributed fiber optic hydrophones are developed from distributed acoustic sensor (DAS) technology based on phase optical time domain reflectometry (Φ-OTDR). The laser emitted by a high coherence narrow linewidth laser is modulated into pulse light by an acousto-optic modulator, and then amplified and enters the sensing fiber. Rayleigh scattered light is generated due to the unevenness of the microscopic materials and structures in the fiber. When there are sound waves or vibrations near the fiber, the phase of the Rayleigh scattered light in the fiber will change, and the amount of phase change is proportional to the amplitude of the vibration. The backscattered Rayleigh light is collected by a photodetector, and the phase difference of the backscattered Rayleigh light is obtained by signal processing. After demodulation, indicators such as sound pressure and frequency can be obtained, and then key information such as the angle of arrival of the sound wave and the target distance can be obtained. Due to the advantages of large array networking, large aperture, high sensitivity, and high packaging reliability of distributed fiber optic hydrophones, they are widely used in submarine detection, earthquake monitoring, oil and gas exploration, and marine hydrological monitoring. However, in practical applications, fiber optic hydrophones need to be calibrated to achieve accurate measurement of water acoustic parameters, so it is very important to calibrate the sensitivity of the hydrophone.

[0003] At present, the national standard "Low-frequency Calibration Method for Acoustic Hydrophones" (GB / T 4300-2017) specifies the calibration methods for hydrophones in the frequency range of 0.01Hz to 3.15kHz for traditional piezoelectric hydrophones, with a calibration size of about 10cm. However, these methods cannot meet the needs of the new generation of distributed fiber-optic hydrophones.

[0004] Although, in the patent: A device and method for rapid calibration of sensitivity of a fiber optic hydrophone array (110006516A), a device and method for rapid calibration of sensitivity of a fiber optic hydrophone array are disclosed, this method cannot accurately calibrate the sound pressure sensitivity of a certain portion of the optical cable fixed on the optical cable, is not suitable for continuous calibration of distributed fiber optic hydrophones, and the calibration of this method requires a standard hydrophone as a reference.

[0005] In summary, there is currently no method that can directly, continuously and accurately calibrate the sound pressure sensitivity of distributed fiber optic hydrophones. Summary of the invention

[0006] The purpose of the present invention is to overcome the deficiencies of the prior art and to provide a sound pressure sensitivity calibration device for a distributed hydrophone optical cable based on a compensation cavity, which can perform direct, continuous and high-precision calibration on the distributed hydrophone optical cable.

[0007] In order to achieve the above technical objectives, the solution proposed by the present invention is:

[0008] Provided is a sound pressure sensitivity calibration device for a distributed hydrophone optical cable based on a compensation cavity, characterized in that the device comprises: a distributed optical fiber sensor, a hydrophone optical cable to be tested, a roller machine, a fixed pulley, a bracket, a compensation cavity and a water tank; the compensation cavity is placed horizontally on the bracket and placed in the water tank, covered with water to ensure airtightness, and a circular hole is created at the center position of the top and bottom of the compensation cavity respectively, through which the hydrophone optical cable to be tested passes; the roller machine is a power device, which passes the optical cable passing through the compensation cavity around the fixed pulley and connects to the roller machine, so as to realize the pulling and continuous calibration of the hydrophone optical cable to be tested;

[0009] Furthermore, the distributed hydrophone optical cable can be calibrated section by section continuously, and the calibration method is as follows:

[0010] (1) selecting a starting calibration point of the hydrophone optical cable to be tested;

[0011] (2) measuring the sound pressure around the optical cable in the cavity by a compensation method, wherein the distributed optical fiber sensor measures the sensing phase difference of the optical cable in the calibration area, thereby calculating the sound pressure sensitivity;

[0012] (3) The movement of the optical cable is controlled by a roller machine, so that the next section of the optical cable enters the compensation cavity, pauses briefly and performs calibration, and then continues to operate after completion;

[0013] Furthermore, the compensation cavity is a cylindrical sealed container, which mainly includes a source transducer, a compensation transducer, a displacement sensor and an elastic material; the source transducer is fixed at the bottom of the cylindrical sealed container to generate sound waves in the cavity; the side wall has a three-layer structure, which is respectively the compensation transducer, the elastic material and the displacement sensor from the inside to the outside, and is used to control the compensation state in the cavity;

[0014] Furthermore, the source transducer and the compensation transducer in the compensation cavity are driven by a signal source with dual output channels; the displacement sensor detects the output voltage signal via a filter and a phase-locked amplifier to determine the compensation state;

[0015] Compared with the prior art, the present invention has the following advantages:

[0016] In view of the fact that there is no method for continuous calibration of distributed optical fiber hydrophones in the current field of optical fiber hydrophones, the present invention proposes a sound pressure sensitivity calibration device for distributed hydrophone optical cables based on a compensation cavity. The device can arbitrarily select calibration points. Since the sound pressure in the compensation cavity is stable and uniform, the calibrated sound pressure sensitivity has low uncertainty and high accuracy. At the same time, the device controls the movement of the optical cable through a roller machine, and can realize continuous calibration of the optical cable to be tested. The device can be directly calibrated without the need for a standard hydrophone as a reference, and can realize the automation of the calibration process. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a structural schematic diagram of the sound pressure sensitivity calibration device of the distributed hydrophone optical cable of the present invention;

[0018] Figure 2 It is a structural schematic diagram of a cylindrical closed compensation cavity;

[0019] Figure 3 It is the connection control diagram of each unit in the device;

[0020] Figure 4 It is a flow chart of the sound pressure sensitivity calibration method based on the device of the present invention;

[0021] Figure 5 It is a schematic diagram of the structure of the characteristic constant measuring device of the compensation cavity;

[0022] Figure numerals: 1 is a distributed optical fiber sensor, 2 is a hydrophone cable to be tested, 3 is a roller machine, 4 is a fixed pulley, 5 is a bracket, 6 is a compensation cavity, and 7 is a water tank; 601 is a metal flange, 602 is a source transducer, 603 is a compensation transducer, 604 is an elastic material, 605 is a displacement sensor, and 606 is a shielding layer; 10 is a computer, 20 is a signal source, 30 is a power amplifier, 40 is a phase-locked amplifier, and 50 is a filter; 8 is a cylindrical open container, and 9 is a vibration exciter. DETAILED DESCRIPTION

[0023] The present invention is further described in detail below with reference to the accompanying drawings and specific examples. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention;

[0024] like Figure 1As shown, the present invention provides a sound pressure sensitivity calibration device for a distributed hydrophone optical cable based on a compensation cavity, specifically comprising: a distributed optical fiber sensor (1), a hydrophone optical cable to be tested (2), a roller machine (3), a fixed pulley (4), a bracket (5), a compensation cavity (6) and a water tank (7); the compensation cavity is a main calibration area, and a circular hole is made at the center position of the top and the bottom respectively, and the hydrophone optical cable to be tested passes through the hole, the compensation cavity is horizontally fixed on the bracket and placed in the water tank, and water is added to cover the compensation cavity to ensure airtightness, and the sound pressure around the optical cable in the cavity can be measured by the compensation method; the distributed optical fiber sensor is used to measure the sensing phase difference of the optical cable in the calibration area, and then the sensitivity is calculated according to the formula; the roller machine is a power device, and the optical cable passing through the compensation cavity is passed around the fixed pulley and connected to the roller machine, so as to realize the pulling and continuous calibration of the hydrophone optical cable to be tested;

[0025] The structure of the compensation cavity is as follows: Figure 2 As shown, it specifically includes a metal flange (601), a source transducer (602), a compensation transducer (603), an elastic material (604), a displacement sensor (605) and a shielding layer (606), forming a cylindrical closed container; the top and bottom of the container are metal flanges for fixing the internal structure; the top flange has a sealable filling port, and water is filled into the cavity when in use; the bottom flange fixes the source transducer; the side wall has a three-layer structure, which respectively includes a compensation transducer, an elastic material and a displacement sensor from the inside to the outside; the entire container is protected by a shielding layer;

[0026] The connection control of each unit of the device is as follows Figure 3 As shown, a computer (10) is connected to and controls a signal source (20), a phase-locked amplifier (40) and a distributed optical fiber sensor (1), wherein the signal source has dual output channels, and the amplitudes and phases of the two output signals are independently adjustable, wherein one signal drives a source transducer (602) in a compensation cavity through a power amplifier (30) to generate sound waves in the cavity, and the other signal drives a compensation transducer (603) to achieve compensation; a displacement sensor (605) detects an output voltage signal through a filter (50) and a phase-locked amplifier (40) to determine whether a compensation state is reached; the computer (10) reads the output voltage signal of the signal source (20), the detection voltage signal of the phase-locked amplifier (40) and the sensing phase difference in the distributed optical fiber sensor (1) through a data acquisition card, and the sound pressure sensitivity of the optical cable can be calculated after data processing;

[0027] During measurement, the wavelength of the liquid sound wave in the compensation cavity must be greater than ten times the size of the compensation cavity to ensure that the sound pressure in the cavity is basically uniform. This limitation determines the upper limit of the sound wave frequency in the cavity. At the same time, in order to avoid a decrease in the signal-to-noise ratio at low frequencies, the lower limit of the frequency of the sound wave in the compensation cavity is 0.01Hz.

[0028] In this embodiment, the roller machine pulls the optical cable at a speed of 0.5m / s, stops every 10m and calibrates the optical cable in the area, and continues to operate after completion; in order to realize the automation of the continuous calibration process, the computer controls the operation of the roller machine through an automatic control switch;

[0029] Based on the above-mentioned sound pressure sensitivity calibration device of the distributed hydrophone optical cable, the following is a specific description of the sensitivity calibration method in this embodiment;

[0030] For a fiber optic hydrophone, its sound pressure sensitivity can be defined as: the change in the sensing phase of the fiber optic hydrophone caused by the free sound field The ratio of the actual sound pressure p acting on the fiber optic hydrophone is expressed in radians / Pa (rad / Pa), that is:

[0031]

[0032] The sensitivity of the fiber optic hydrophone is usually p With reference sound pressure sensitivity M ref Make a ratio and take the logarithm with base 10 and multiply it by 20 to get the sound pressure sensitivity level M of the hydrophone, that is:

[0033]

[0034] Among them, the unit of M is dB, M ref The value is usually 1rad / uPa.

[0035] like Figure 4 As shown, a flow chart of the calibration method is given, which specifically includes the following steps:

[0036] (1) Connect all instruments, fill the compensation chamber with degassed water, and ensure that the liquid level in the water tank is higher than the compensation chamber. Turn on all instruments and preheat them. Control the roller machine to adjust the hydroacoustic optical cable to the starting end.

[0037] (2) controlling the dual-channel signal source through a computer to drive the source transducer in the compensation cavity, and after generating sound waves in the cavity, driving the compensation transducer;

[0038] (3) The output voltage signal of the displacement sensor is detected by the filter and the lock-in amplifier, and the value of the output voltage U is read. d , adjust the drive voltage U of the compensation transducer c The phase and amplitude of the displacement sensor make the output voltage U d Approaching 0, reaching the compensation state; reading the driving voltage U of the compensation transducer at this time c , the sound pressure in the compensation cavity can be obtained, and its calculation formula is:

[0039] p=KUc

[0040] Where p is the sound pressure in the cavity, U c is the driving voltage of the compensation transducer, K is the characteristic constant of the compensation cavity;

[0041] (4) Read the sensing phase difference of the distributed optical fiber sensor at this time And record the calibration position of the optical cable, and calculate the sound pressure sensitivity of the section of optical cable. The formula is:

[0042]

[0043] It is expressed in sound pressure sensitivity level, M ref If the value is 1rad / uPa, the above formula becomes:

[0044]

[0045] in, is the sensing phase difference of the optical cable in the calibration area read by the distributed optical fiber sensor, and M(i) is the acoustic pressure sensitivity level of the optical cable;

[0046] (5) Control the roller machine to pull the hydrophone cable to be tested 10 m toward the end, so that the next section of the cable enters the calibration area and performs the measurement;

[0047] (6) Repeat steps (3) to (5) until all the hydroacoustic optical cables to be tested have been calibrated, record the data, turn off all instruments, and draw the calibration curve using a computer.

[0048] The calculation of the characteristic constant K of the compensation cavity in steps (3) and (4) is based on the Pascal principle of fluid statics, as follows: Figure 5 As shown, a measuring device for the characteristic constant K is provided. A hole is opened in the side wall of a cylindrical open container (8) and connected to the filling port at the top of the compensation chamber (6) through a connecting pipe. The open container needs to be higher than the horizontal plane of the compensation chamber, and a vibration exciter (9) is fixed at the bottom. During measurement, water is injected from the open container to ensure that the compensation chamber is completely filled, and the water level in the open container is higher than the side wall opening. By controlling the bottom vibration exciter, the water in the open container is made to perform low-frequency sinusoidal vibration with a frequency lower than 1 Hz in the vertical direction, so that alternating static water pressure is generated in the compensation chamber. The amplitude and phase of the driving voltage of the compensation transducer (603) are adjusted at the same frequency to achieve a compensation state. At this time, the equation is:

[0049] ρgΔh=KU

[0050] The calculation formula for K is:

[0051]

[0052] Wherein, ρ is the density of water, g is the acceleration of gravity, Δh is the vibration amplitude of water in the open container, and U is the driving voltage of the compensation transducer in the compensation state;

[0053] Substituting the calculated K into the formulas in steps (3) and (4), the sound pressure sensitivity level of the hydrophone cable can be obtained.

[0054] In addition, a computer is used to write a program according to the above steps to connect and control each unit to realize the automation of the entire calibration process.

Claims

1. A sound pressure sensitivity calibration device for a distributed hydrophone cable based on a compensation cavity, the device comprising: Distributed fiber optic sensors, roller machines, compensation chambers and liquid tanks; The liquid tank is filled with liquid, the compensation cavity is located in the liquid tank, roller machines are arranged on both sides of the compensation cavity, and the hydrophone cable to be tested passes through the first roller machine, the compensator, and the second roller machine from the distributed optical fiber sensor in sequence, and the two roller machines drive the hydrophone cable to be tested to move; it is characterized in that the compensation cavity is composed of a tubular side wall, a top flange and a bottom flange to form a hollow sealed cavity; the tubular side wall includes three layers from the inside to the outside, respectively: a compensation transducer, an elastic material, and a displacement sensor, an active transducer is provided in the middle of the bottom flange, and a circular hole is provided at the center of the source transducer of the top flange and the bottom flange for the hydrophone cable to be tested to pass through; the compensation cavity is filled with liquid; the method for continuously calibrating the hydrophone cable to be tested is: (1) selecting a starting calibration point of the hydrophone optical cable to be tested; (2) The sound pressure around the hydrophone cable to be tested in the compensation cavity is measured by the compensation method, and the distributed optical fiber sensor measures the sensing phase difference of the hydrophone cable to be tested in the calibration area, thereby calculating the sound pressure sensitivity; (3) The movement of the hydroacoustic optical cable to be tested is controlled by a roller machine, so that the next section of the hydroacoustic optical cable to be tested enters the compensation cavity, pauses briefly and performs calibration, and continues to operate after the calculation is completed.

2. The sound pressure sensitivity calibration device of a distributed hydrophone optical cable based on a compensation cavity as claimed in claim 1, characterized in that: It also includes two fixed pulleys and a bracket. The two fixed pulleys are respectively arranged on both sides of the compensation cavity, each located between the compensation cavity and the corresponding roller machine. The fixed pulley is located below the liquid surface in the liquid tank, and the roller machine is located outside the liquid tank; the bracket is located at the bottom of the liquid tank and is used to support the compensation cavity.

3. The sound pressure sensitivity calibration device of a distributed hydrophone optical cable based on a compensation cavity as claimed in claim 1, characterized in that: The source transducer and the compensation transducer in the compensation cavity are driven by a signal source with dual output channels; the displacement sensor detects the output voltage signal via a filter and a phase-locked amplifier to determine the compensation state.

4. The sound pressure sensitivity calibration device of a distributed hydrophone optical cable based on a compensation cavity as claimed in claim 1, characterized in that: The wavelength of the liquid sound wave in the compensation cavity needs to be greater than ten times the size of the compensation cavity, and the lower limit of the frequency of the sound wave in the cavity is 1 Hz.

5. The sound pressure sensitivity calibration device of a distributed hydrophone optical cable based on a compensation cavity as claimed in claim 1, characterized in that: The roller machine pulls the hydrophone optical cable to be tested at a speed of 0.5 m / s, stops every 10 m and calibrates the hydrophone optical cable to be tested in the area, and continues to operate after completion.

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