Acoustic detection is a basic way for human beings to perceive the environment. Hydrophone technologies are key means of underwater acoustic detection and play an important role in target detection, communication, navigation, resource exploration, and marine ecological monitoring. At present, the mainstream hydrophone technologies are mainly divided into two categories of piezoelectric hydrophone and fiber optic hydrophone (FOH). The former has been widely applied, and FOH rapidly developing in recent decades features high detection sensitivity, unpowered wet-end, and convenient networking. However, these conventional hydrophones have many disadvantages. First, they are in nodal type, the multiplexing scale and array size are limited, and the largest array number is far less than 1000. Second, their array parameters (array spacing, array aperture, etc.) cannot be changed after being determined, and the target type to be located is limited, which cannot meet the detection needs of various targets. Finally, the wet-end part needs to be prepared by hand due to the complex fiber connect relationship. Therefore, the existing hydrophone technologies are difficult to meet the strict requirements of advanced marine science and future underwater acoustic detection, such as large-scale detection arrays, rapid and flexible deployment, adaptive array reconstruction, and low-cost large-scale monitoring. Meanwhile, it is extremely important to develop new hydrophone technologies.

Distributed fiber optic hydrophone (DFOH) technology is a new underwater acoustic detection technology developed in recent years. In DFOH, the optical fiber is converted into thousands of acoustic transducers, and all acoustic information can be obtained along the fiber quantitatively and spatial-continuously from the backscattering of the inquiry laser pulse. DFOH has unique advantages including densely spatial sensing, flat frequency response, flexible array reconstruction in the digital domain, and large array (tens of kilometers). Additionally, in terms of engineering applications, the wet-end of DFOH can be mechanically produced with high efficiency and good consistency, which is essential on large-scale array construction and rapid mass production. In 2019, the Naval Research Laboratory in the United State publicly stated that research was being conducted on a new generation of hydrophone technology based on Rayleigh scattering, and afterward, DFOH technology attracted widespread attention and was rapidly developed.

In DFOH, the sensing fiber is converted into acoustic transducers by utilizing the laser phase response to the external sound field, and the external sound field is continuously detected in the spatio-temporal domain, with each channel separated in the way of optical time domain reflectometer (OTDR) or optical frequency domain reflectometer (OFDR). Thus, the fundamental principle is divided into laser phase response and signal demodulation. On the former, the DFOH response mechanism is consistent with that of conventional interferometric FOH, and fiber secondary coating and wed-end structured design (Fig. 1) are also effective in improving the DFOH response (sound pressure sensitivity). On signal demodulation, DFOH is quite different from FOH and channel separating is essential, with complex backscattering mixing along the fiber. The principle details are introduced by us.

The DFOH performance has been rapidly enhanced in recent decades. The preliminary foundation of DFOH is built from phase-sensitive OTDR. The first qualitative demodulation was proposed by Taylor in 1993, and the first quantitative demodulation (Fig. 2) was realized by the Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences in 2011. Soon afterward, many demodulation methods are proposed. The DFOH concept was first proposed in 2015, when the Shandong Academy of Sciences verified the feasibility of DFOH to detect underwater sound in the laboratory, with sound pressure sensitivity of -158 dB and phase noise of -56 dB. With the joint efforts of domestic and foreign scholars, the DFOH performance indexes are greatly improved, including sound pressure sensitivity (Table 1), system noise level (Table 2), system equivalent noise, dynamic range, and response directivity (Fig. 3). Meanwhile, the effective detection range (Table 3) of DFOH passive sonar system is theoretically evaluated, and the evaluation system of DFOH performance is gradually improved.

In recent years, the dry-end technology and wet-end cables keep optimizing, laboratory tests constantly improve, and the applications are explored in underwater suspended horizontal array, lightweight towed array, and hydrophone array with submarine communication cables. On the underwater suspended horizontal array, direction and localization of underwater target and lake tests are focused, and the representative groups are from SIOM (Figs. 4 and 5), Shanghai University, and Naval University of Engineering. The lightweight towing array application is still in the exploratory stage, the flow noise and channel crosstalk are studied, and Naval University of Engineering and Zhejiang Laboratory (Fig. 6) are the most representative groups. In terms of hydrophone array with submarine communication cables, the joint team of the Norwegian University of Science and Technology and Cornell University is the biggest concern, and they detect and track whales in the Arctic with existing submarine cables (Fig. 7), which is expected to provide a new means of all-weather monitoring for target detection in vast sea areas.

As a novel hydrophone technology, DFOH has unique advantages of continuous spatial detection, flexible array reconstruction, automatable wet-end production, light weight, and low cost. In recent years, DFOH has developed rapidly and has been verified in many application scenarios. We introduce the basic sensing principle and typical demodulation methods of DFOH and review the important performance indexes and research progress, including sound pressure sensitivity, system equivalent noise, response directivity, and dynamic range. Some representative applications are also introduced, such as underwater suspended horizontal array, lightweight towed array, and hydrophone array with submarine communication cables. Additionally, the existing problems and possible development trends are discussed. We believe that DFOH will play an important role in underwater target detection, marine communication and navigation, and environmental ecological monitoring.