A brief analysis of the working principle of optical fiber vector hydrophone

Infineon / Mitsubishi / Fuji / Semikron / Eupec / IXYS

A brief analysis of the working principle of optical fiber vector hydrophone

Posted Date: 2024-02-05

Working principle of fiber optic vector hydrophone

The optical fiber vector hydrophone is a new type of hydroacoustic measurement equipment. Its working principle is to convert underwater acoustic vibrations into optical signals through high-sensitivity optical coherent detection, and then transmit them to the signal processing system through optical fibers to extract the acoustic signal information.

Compared with traditional hydrophones, optical fiber vector hydrophones can more accurately measure underwater sound pressure, as well as sound field kinetic energy density and sound energy flow density (sound field Poynting vector), thereby providing richer data support.

The interference type fiber optic hydrophone is based on the high-performance Michelson interferometer structure. After the light emitted by the laser passes through the optical isolator, it is divided into two beams through the 3dB fiber coupler, and enters the two arms of the interferometer respectively. After passing through the optical fiber, After reflection by the mirror at the end, it returns to the fiber coupler for interference. The interference light signal is converted into an electrical signal by the photodetector, and then the acoustic wave information can be obtained by signal processing. The hydroacoustic signal acts on the elastomer, causing micro-strain of the elastomer, and the optical fiber wound around the elastomer is subsequently strained. The strain of the optical fiber then changes (modulates) the phase of the light wave propagating in the optical fiber. By demodulating the changes in the phase of the light waves, the magnitude of the underwater acoustic signal can be obtained.

Demodulation of the light wave phase is achieved through laser interference. Therefore, after the optical fiber is wound on the hydrophone elastomer to become the optical fiber hydrophone sensitive circle, the sensitive circle must be connected to the optical fiber Michelson interferometer. The optical path principle diagram of the fiber optic hydrophone is shown in Figure 1.

Schematic diagram of interference type fiber optic hydrophone

Various types of fiber optic vector hydrophones

There are many types of fiber optic vector hydrophones, the most common of which is the interference type fiber optic vector hydrophone. It uses the principle of an interferometer to convert underwater acoustic vibrations into phase difference changes in the interferometer, and then obtains underwater acoustic information by detecting the output light signal of the interferometer. The interference type fiber optic vector hydrophone has the advantages of high sensitivity, low signal transmission loss through optical fiber, immunity to electromagnetic interference, and no crosstalk, and can achieve long-term stable operation in harsh environments.

Another common fiber optic vector hydrophone is the micro-optical structure fiber optic hydrophone. This hydrophone technology engraves the sensor directly on the optical fiber, which 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 maritime defense and warning systems, ship-based sonar arrays, ocean noise monitoring arrays and other applications, especially for hydrophone towed array applications.

In addition, there are distributed fiber optic vector hydrophones. This hydrophone uses distributed fiber optic sensing technology (DAS) to detect underwater acoustic signals. Multiple optical fiber hydrophones form an array to spatially separate and pick up underwater sound waves. They can be used in underwater acoustic warning sonar, underwater thunder fuzes, fish detection, multi-base sonar, underwater navigation and other fields.

The above are several types of fiber optic vector hydrophones. They each have their own characteristics and are suitable for different application scenarios. In practical applications, the appropriate fiber optic vector hydrophone type can be selected according to specific needs.

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