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New research findings presented in MDPI-Sensors, show the relationship between the directivity and array structure of a distributed fiber optic hydrophone (DFOH). Measurements of acoustic signals using the DFOH at Qingyang Lake in the province of Hunan were shown to agree with theoretical predictions. The group of Professor Zhou Meng performed the research at the College of Meteorology and Oceanography, the National University of Defense Technology in Changsha, China.

Study:  Directivity Dependence of a Distributed Fiber Optic Hydrophone on Array Structure . Image Credit: bluebay/Shutterstock.com

Acoustic signals are longitudinal waves that disturbances in materials can cause. These vibrations propagate through the materials over large distances. Detecting acoustic signals have many industrial and scientific applications. A non-exhaustive list includes:

Methods for acoustic sensing underwater are also pursued with great interest. This is because many aquatic species use sound waves for communication, reproduction and hunting. Gathering extensive data about the global ocean environment's vibrational signals is vital to determine and evaluate the effects of both human activity and natural processes.

A hydrophone is an instrument that can be used underwater to detect and record vibrations emanating from the ocean from all directions. A hydrophone picks up acoustic signals underwater like a microphone picks up sound in the air. Most hydrophones rely on a unique characteristic of some material that generates a tiny electrical current when exposed to variations in underwater pressure.

New developments in hydrophones have taken advantage of the unique abilities of fiber optic sensing technology. Building on fiber-optic hydrophones (FOH), integrating distributed acoustic sensors (DAS) has allowed long-distance spatially continuous acoustic signal measurements. DAS-based FOHs are known as distributed fiber optic hydrophones (DFOH).

A DFOH array has certain distinct advantages over a traditional FOH array. The DAS technology allows it to continuously gather undersea sound waves. A DFOH is primarily made of fibers. A FOH is constructed using numerous fibers and optical elements like Faraday rotators, polarizers, and fiber couplers. A DFOH is more convenient to implement than a traditional FOH due to its straightforward structure.

Professor Zhou Meng and his colleagues focused on how a DFOH's array structure affects the directivity function. In this study, simulations were conducted on the directivity function to theoretically analyze the dependence of the directivity function on the array structure. Later, experiments were conducted to verify the theoretical model.

A series of individual sensing channels construct a DFOH. Each channel consists of an elastic cylinder with a sensing fiber continually wrapped around it. Particular attention is given to the sensing fiber's length ratio to the elastic cylinder's length. The DFOH is divided into a discrete number of channels on the signal processing end with equal spacing between them. The length and the spacing of the sensing channel can be changed arbitrarily.

Vibrations underwater are detected by using the DAS technology within the DFOH. The channels on the DFOH measure the phase of backscattered optical beams. Each channel in a DFOH converts incoming acoustic signals by comparing them to a backscattered optical phase signal. Since the acoustic signals arrive randomly towards the sensor, a phase delay is measured by the DFOH.

The phase delay between two neighboring sensing channels determines the DFOH's directivity. In the theoretical description formulated by the group, specific parameters related to phase delay, and initial values were chosen carefully. The behavioral dependency of DFOH was simulated against different channel lengths and spacing.

Experimental verification of the DFOH was conducted at Qingyang lake in the province of Hunan. An acoustic signal generator was submerged 5 meters below the lake's surface as the source. The various initial values for the DFOH variables are elaborated in the MDPI-Sensors paper.

The directivity of a DFOH depends on particular features of the DFOH geometry according to the experimental findings. The directivity of the sensing channels influences the directivity of a DFOH. Channel length has a direct effect on DFOH sensitivity. But the spacing between the channel had no impact on the DFOH performance so long as the space length was less than the wavelength of the detected acoustic signal.

The results of the current DFOH study can be further optimized by improving the beamforming architecture. A more stringent simulation package accounting for weighting functions, super-directivity method and the directivity pattern of the DFOH can further enhance the results.

This research will support the DFOH in sound wave directionality and offer recommendations for the structure and design of SFOHs in signal processing.

Li, Wenmin, Yu Chen, Yan Liang, Yang Lu, and Zhou Meng. (2022) Directivity Dependence of a Distributed Fiber Optic Hydrophone on Array Structure. Sensors 22, no. 16: 6297. https://www.mdpi.com/1424-8220/22/16/6297

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Ilamaran Sivarajah is an experimental atomic/molecular/optical physicist by training who works at the interface of quantum technology and business development.

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