I. Introduction
It Has been about 30 years since the idea of a fiber optic hydrophone was first proposed [1]. Due to their high sensitivity, wide dynamic range, immunity to electromagnetic interference, and feasibility in multiplexing, fiber optic hydrophones have become one of the most promising acoustic detection devices for future operational sonar systems [2]. A great deal of research has been carried out concerning the design, fabrication, interrogation, and application of the fiber optic hydrophones. Conventional fiber hydrophones based on Mach–Zehnder or Michelson interferometers have been widely used since the 1980s. This type of hydrophone includes a reference arm and a sensing arm, which is usually wrapped on a plastic cylinder. But the multiplexing of the interferometric fiber hydrophone is complex [3], and reducing the size of the sensing head is difficult [4]. The development of the fiber laser sensors in the 1990s offered an alternative approach to fiber optic hydrophones. By using the interferometric interrogation technique, a strain resolution of can be achieved [5]. In the past several years, there have been a number of reports about fiber laser hydrophones. Most of these configurations are based on polymer coating on bare fiber laser [6]–[10], or shielded polymer coating on fiber laser [11]. However, such correction methods are likely to work better if the sensitivity is enhanced while the frequency response is flattened. In this paper, we report a new ultrathin fiber laser hydrophone with enhanced sensitivity by using a thin metal cylinder and a piston-like diaphragm. Owing to the greater deformation of the diaphragm with a hard core at its center, ultrathin dimensions and an ultrahigh sensitivity have been achieved.