What is a Hydrophone?

A transducer that converts acoustic signals into electrical signals, used to receive acoustic signals in water, is called a receiving transducer, and is often called a hydrophone. Hydrophones are widely used in underwater communication, exploration of the island, target positioning, tracking, etc., and are important components of sonar. Underwater detection, identification, communication, and development of marine environmental monitoring and marine resources are inseparable from water sound Transducer.

An underwater acoustic transducer is a device that converts electrical signals into underwater acoustic signals or converts underwater acoustic signals into electrical signals. Its position in sonar is similar to that of antennas in radio equipment. It transmits and receives sound waves underwater. Acoustic device. A transducer that converts electrical signals into underwater acoustic signals is used to radiate sound waves into the water, which is called a transmitting transducer. A transducer that converts acoustic signals into electrical signals, used to receive acoustic signals in water, is called a receiving transducer, and is often called a hydrophone.

According to the difference in the working principle, energy conversion principle, characteristics and structure, there are hydrophones such as sound pressure, vibration speed, non-directional, pointing, piezoelectric, magnetostrictive, electric (moving coil) and so on. Hydrophones and microphones have many similarities in principle and performance. However, due to the differences in sound media, hydrophones must have a solid watertight structure and must be made of impervious cables with anti-corrosive materials.

Sound pressure hydrophones detect underwater sound signals and noise sound pressure changes and produce a voltage output proportional to the sound pressure. Acoustic pressure hydrophone is an indispensable device in underwater acoustic measurement, and it is the core part of passive sonar system. According to the different sensitive materials used, sound pressure hydrophones can be divided into: piezoelectric ceramic sound pressure hydrophones, PVDF sound pressure hydrophones, piezoelectric composite sound pressure hydrophones, and optical fiber sound pressure hydrophones.

In the field of underwater acoustics, sensors are generally referred to as transducers, and receiving transducers mainly include scalar sensors and vector sensors, also called scalar hydrophones and vector hydrophones. In the sound field measurement, the traditional method is to use a scalar hydrophone (sound pressure hydrophone), which can only measure scalar parameters in the sound field. Typical scalar hydrophones such as the 810X series of B & K companies are often used as hydrophone standards use. The vector hydrophone can measure the vector parameters in the sound field, and its application is helpful to obtain the vector information of the sound field, which is of great significance to the function expansion of the sonar device.

In continuous media, the motion state near any point can be expressed by pressure, density, and media speed. At different locations in the sound field, these physical quantities have different values and have spatial variability. Moreover, for the same spatial coordinate point, these quantities change with time and have time variability. Therefore, the acoustic quantities describing the sound field, sound pressure, particle velocity, and compression are all functions of time and space. In an ideal fluid, there is no shear stress, so the sound pressure is scalar and the particle velocity is vector. Rich sound field information is included in both scalar and vector parameters. In the sound field measurement process, it is not enough to measure only the sound pressure parameter. Simultaneous measurement of scalar information and vector information, that is, sound pressure and particle velocity, can obtain complete sound field information. In this way, it can help the signal processing system to obtain more valuable information and make correct judgments. For example: the joint information processing system using a new combination sensor (sound pressure and vibration velocity) has better anti-coherent interference and line spectrum detection capabilities than the traditional pure sound pressure information processing system; a single small-scale combination sensor passes the joint through With signal processing, joint sound pressure and vibration velocity estimation of the target azimuth can be performed. In addition, from the perspective of energy detection, the use of vector hydrophones improves the system's ability to resist isotropic noise, and can realize multi-target recognition in the far field. The research work of vector hydrophone has received great attention. Therefore, multi-information detection, including vector information, is a development trend of sonar systems, and is increasingly valued by various naval powers.

With the continuous development of technology, more and more technical requirements are needed. In order to meet the needs of shore station construction, it serves the coast early-warning sonar system to achieve remote detection and identification, and low-frequency detection capabilities are increasingly important. In addition, due to the emergence of nuclear-powered submarines and the widespread adoption of new technologies such as submarine stealth, anti-submarine issues have received unprecedented attention from various countries. An effective method is to turn to test the low-frequency noise of the propeller. The intrinsic noise of quiet submarines and ships is in the low frequency band, which requires a low-frequency vector hydrophone. That is, the detection transducer is required to have a low-frequency detection capability. Low-frequency three-dimensional spatial omnidirectional vector detectors have become a new technical requirement. The successful development of such low-frequency vector hydrophones can be expected to solve the problem of detecting low-frequency signals transmitted at long distances. At the same time, as the target signal weakens, the problem of high-sensitivity detection becomes urgent ^[1]

Optical fiber hydrophone is a device that uses the optical fiber technology to detect underwater acoustic waves. Compared with traditional piezoelectric hydrophones, it has extremely high sensitivity, a sufficiently large dynamic range, essential resistance to electromagnetic interference, and no impedance matching requirements. The light weight of the system's "wet end" and the arbitrariness of the structure, etc., are sufficient to meet the challenges from the continuous improvement of submarine squelch technology, and meet the requirements of anti-submarine strategies of developed countries. ^[2] .

Hydrophone principle

Optical fiber hydrophones can be divided into interference type, intensity type, and grating type according to the principle. The key technology of interference optical fiber hydrophones has gradually developed and matured, and products have been formed in some fields, and fiber grating hydrophones are the current hotspots of optical fiber hydrophones.

Fiber grating hydrophone is based on the principle that the resonant wavelength of the grating moves with the change of external parameters. Fiber grating hydrophones are generally based on fiber Bragg grating structures, as shown in Figure 1.

Fiber optic hydrophone

figure 1

When the output light of a broadband light source (BBS) passes through a fiber Bragg grating (CFBG), according to the mode coupling theory, it can be known that the wavelength meets the Bragg condition:

The light waves will be reflected back, and the remaining wavelengths will be transmitted. Where

Is the resonant coupling wavelength of FBG, that is, the central reflection wavelength,

Is the effective refractive index of the core, and n is the grating pitch. When the stress around the sensing grating changes with the sound pressure in the water, it will cause

Or n changes, resulting in a corresponding center reflection wavelength shift of the sensing grating, the shift amount is

It is determined that the wavelength modulation of the reflected signal light by the underwater acoustic pressure is achieved. Therefore, by detecting the center reflection wavelength shift in real time, and then according to the linear relationship between each parameter and the sound pressure, the information of the sound pressure change can be obtained.

Hydrophone features

(1) Low noise characteristics. Optical fiber hydrophones are constructed using optical principles and have high sensitivity. Due to their low self-noise characteristics, the minimum detectable signal is 2-3 orders of magnitude higher than traditional piezoelectric hydrophones, which makes weak signal detection possible.

(2) Large dynamic range. The dynamic range of piezoelectric hydrophones is generally 80-90dB, while the dynamic range of fiber-optic hydrophones can be 120-140dB.

(3) Strong resistance to electromagnetic interference and signal crosstalk. All-optical fiber-optic hydrophones use light as the carrier for signal sensing and transmission. The influence of electromagnetic interference below a few hundred megahertz is very small, and the signal crosstalk of each channel is also very small.

(4) Suitable for long-distance transmission and array. Optical fiber transmission loss is small, suitable for long-distance transmission. Optical fiber hydrophones are multiplexed using frequency division, wave division, and time division techniques, which is suitable for large-scale arrays of underwater arrays.

(5) Signal sensing and transmission are integrated to improve system reliability. The laser is emitted by the light source, transmitted to the fiber-optic hydrophone via the optical fiber, and after picking up the acoustic signal, it is transmitted back to the signal processing equipment on shore or on the ship through the optical fiber. There are no electronic equipment underwater. In addition, the optical fiber has low requirements for water tightness, high temperature resistance and corrosion resistance, which will greatly improve the reliability of the system.

(6) Engineering application conditions are reduced. The sonar system using all-optical fiber optic hydrophones, the detection cables and transmission cables are all optical cables, light weight and small size, and the system is easy to retract, which makes it impossible to implement solutions in the past, especially for towed arrays. Reduce and simplify many issues.

Hydrophone application

The main military applications of fiber optic hydrophones include: all-fiber hydrophone towed arrays, all-fiber submarine acoustic monitoring systems, all-fiber light submarines and surface ships conformal hydrophone arrays, ultra-low-frequency fiber gradient hydrophones, marine environments Noise and quiet submarine noise measurement. The optical fiber vibration speed type vector hydrophone can detect its "infrasound" peak noise, and it is suitable for coast guard sonar after arraying, detecting quiet submarines and tsunami early warning. It has technical advantages such as easy multi-unit multiplexing, ability to work passively electrically, and strong long-distance signal transmission capabilities. The micro-optical structure optical fiber hydrophone technology directly engraves the sensor on the optical fiber. It has the advantages of small size, easy wavelength division multiplexing, relatively simple manufacturing process, and reliable performance. It is suitable for large-scale shore-based sea defense and security systems and shipborne sound Nano array, marine noise monitoring array and other applications, especially hydrophone drag array applications.