1. Background introduction

　　Acoustic waves are the only form of energy known to humans that can be transmitted over long distances in seawater. Other forms of energy radiation, such as electromagnetic waves, cannot be transmitted over long distances in seawater, because their loss in seawater is about 3 orders of magnitude greater than that of sound waves. In the water medium, the attenuation coefficient of the sound wave is small, and it can travel long distances. Therefore, the sound wave is the best underwater information carrier. The hydrophone is a sensor that detects, locates and recognizes underwater targets by receiving acoustic signals.

　　The traditional hydrophone is a piezoelectric ceramic hydrophone. The sonar array composed of such piezoelectric ceramic hydrophone requires a large number of underwater electronic components for channel multiplexing and data transmission, as well as signal transmission cables and power supply cables. These electronic devices are expensive and heavy, and they often fail due to underwater sealing problems, which makes the reliability of the system very poor. With the continuous advancement of science and technology and the continuous development and maturity of the optical fiber communication industry, optical fiber hydrophones are currently a field of great concern in the field of underwater acoustic research, and have gradually moved from the early laboratory research stage to engineering applications. 

　　Optical fiber hydrophone is a new type of hydrophone based on optical fiber and optoelectronic technology. It converts underwater acoustic vibration into optical signal through high-sensitivity optical coherent detection, and transmits it to the signal processing system through optical fiber to extract acoustic signal information. It has sensitivity High frequency, wide frequency band response, anti-electromagnetic interference, resistance to harsh environment, light structure, easy to remotely measure and form a large-scale array. 

　　 Fiber optic hydrophones are mainly used to monitor sound propagation, noise, reverberation, seabed acoustic characteristics, target acoustic characteristics, etc. in the marine acoustic environment. It can be used for marine and terrestrial oil and gas exploration, as well as marine and terrestrial seismic wave detection and marine environment detection. It is also an advanced detection method for modern naval anti-submarine warfare, underwater weapon testing, offshore oil exploration and marine geological surveys. 

2. Working principle of optical fiber hydrophone

　　Fiber optic hydrophones usually consist of a transmitter, that is, an output light source, a hydrophone array, and a receiver. The light emitted by the laser needs to pass through an acousto-optic modulator to obtain pulses with a fixed time interval for time division multiplexing. The multiplexing method used by the hydrophone array can reduce the cost of the entire system. Multiple sensing channels use the same laser light source. The figure below is a schematic diagram of an optical fiber hydrophone system using wavelength division multiplexing/time division multiplexing:

Description of the devices in the system:

Laser：Narrow linewidth fiber laser;

AOM（Acoustic-Optic Modulator）;

WDM（Wavelength Division Multiplexing）;

OAM／ODM（Optical Add/Drop Multiplexing）：Optical add/drop multiplexer, three-port wavelength division multiplexer;

ISO（Isolator）;

Coupler;

EDF（Er-Doped Fiber）;

Pump;

FRM（Faraday Rotation Mirror）;

Attenuator;

Demodulator;

3. Matters needing attention

　　In the case of using a coupler to implement time division multiplexing, the main factors that limit the number of system channels are the light splitting ratio and sampling rate of the coupler;

      The position of the erbium-doped fiber will affect the noise characteristics of the system.